21CT-001

Technology Roadmap

for the

21st Century Truck Program

A Government-Industry Research Partnership

December 2000
This report is available via the DOE Information Bridge.

Web site http://www.doe.gov/bridge

This report was prepared as an account of work sponsored by an

agency of the United States government. Neither the United States

government nor any agency thereof, nor any of their employees,

makes any warranty, express or implied, or assumes any legal

liability or responsibility for the accuracy, completeness, or

usefulness of any information, apparatus, product, or process

disclosed, or represents that its use would not infringe privately

owned rights. Reference herein to any specific commercial product,

process, or service by trade name, trademark, manufacturer, or

otherwise, does not necessarily constitute or imply its endorsement,

recommendation, or favoring by the United States government or

any agency thereof. The views and opinions of authors expressed

herein do not necessarily state or reflect those of the United States

government or any agency thereof.
21CT-001

TECHNOLOGY ROADMAP FOR THE 21st CENTURY TRUCK PROGRAM

December 2000
GENERAL DISCLAIMER

This document may have problems that one or more of the

following disclaimer statements refer to:

! This document has been reproduced from the best copy furnished

by the sponsoring agency. It is being released in the interest of

making available as much information as possible.

! This document may contain data which exceeds the sheet

parameters. It was furnished in this condition by the sponsoring

agency and is the best copy available.

! This document may contain tone-on-tone or color graphs, charts

and/or pictures which have been reproduced in black and white.

! This document is paginated as submitted by the original source.

! Portions of this document are not fully legible due to the historical

nature of some of the material. However, it is the best reproduction

available from the original submission.
iii

CONTENTS

FIGURES............................................................................................................................................... v

TABLES ................................................................................................................................................ vii

ACRONYMS AND ABBREVIATIONS.............................................................................................. ix

ACKNOWLEDGMENTS..................................................................................................................... xiii

EXECUTIVE SUMMARY ................................................................................................................... xv

1. INTRODUCTION........................................................................................................................... 1-1

1.1 The 21st Century Truck Program............................................................................................. 1-1

1.2 Program Goals and Research Objectives ................................................................................ 1-1

1.3 Participants in the Technical Program and Roadmap.............................................................. 1-2

2. STRATEGIC IMPORTANCE OF THE 21st CENTURY TRUCK PROGRAM ........................... 2-1

2.1 Program Strategy..................................................................................................................... 2-2

2.2 Benefits of a Successful Program............................................................................................ 2-3

3. SELECTION OF VEHICLE PLATFORMS .................................................................................. 3-1

3.1 Fuel Consumption by Truck Class and Truck Use Patterns.................................................... 3-1

3.2 Representative Driving Cycles................................................................................................ 3-4

4. TECHNICAL PLAN....................................................................................................................... 4-1

4.1 Large Truck—Tractor-Trailer ................................................................................................. 4-1

4.1.1 Vehicle Efficiency....................................................................................................... 4-2

4.1.2 Emissions..................................................................................................................... 4-13

4.1.3 Truck-Related Safety (Truck-Trailer Systems) ........................................................... 4-16

4.2 Transit Bus .............................................................................................................................. 4-20

4.2.1 Vehicle Efficiency....................................................................................................... 4-22

4.2.2 Emissions..................................................................................................................... 4-25

4.2.3 Bus-Related Safety ...................................................................................................... 4-26

4.3 Medium Truck—Enclosed, Single-Axle Delivery Truck ....................................................... 4-26

4.3.1 Vehicle Efficiency....................................................................................................... 4-27

4.3.2 Emissions..................................................................................................................... 4-33

4.3.3 Truck-Related Safety (Medium Trucks)...................................................................... 4-33

4.3.4 Summary of Potential Efficiency Benefits .................................................................. 4-34

4.4 Small Truck—“Working” Pickup (>8500 lb GVW)............................................................... 4-35

4.4.1 Vehicle Efficiency....................................................................................................... 4-35

4.4.2 Emissions..................................................................................................................... 4-38

4.4.3 Truck Safety ................................................................................................................ 4-39

4.5 Military Vehicles..................................................................................................................... 4-39

4.5.1 The Systems Analysis Process .................................................................................... 4-39

4.5.2 Deriving System Requirements................................................................................... 4-40

4.5.3 Analysis of Selected Current Military Vehicle Fuel Efficiencies................................ 4-40

4.5.4 Impact of Aggressive Improvements on Military Truck Efficiencies ......................... 4-41

4.6 Crosscutting Technologies ...................................................................................................... 4-45

4.6.1 Alternative Fuels ......................................................................................................... 4-45

4.6.2 Internal Combustion Engine Technology.................................................................... 4-49
iv

4.6.3 Aftertreatment.............................................................................................................. 4-52

4.6.4 Hybrid Electric Propulsion Technologies.................................................................... 4-58

4.6.5 Mechanical Hybrid Truck Technology........................................................................ 4-71

4.6.6 Fuel Cells..................................................................................................................... 4-77

4.6.7 Auxiliary Power........................................................................................................... 4-82

4.6.8 Thermal Management.................................................................................................. 4-86

4.6.9 Materials...................................................................................................................... 4-87

4.6.10 More-Efficient and/or Lower-Emission Engine Systems............................................ 4-91

4.6.11 Vehicle Intelligence..................................................................................................... 4-96

4.6.12 Innovative, High-Payoff Technologies........................................................................ 4-102

5. PROGRAM SUMMARY ............................................................................................................... 5-1

5.1 Schedule and Milestones......................................................................................................... 5-1

6. REFERENCES................................................................................................................................ 6-1

APPENDIX A. CONTRIBUTORS TO THE 21st CENTURY TRUCK PROGRAM

TECHNOLOGY ROADMAP ........................................................................................................ A-1

APPENDIX B. 21st CENTURY TRUCK PROGRAM SUBTEAMS................................................... B-1

APPENDIX C. PARTICIPANTS IN THE OCTOBER 25, 2000, MEETING

OF THE 21st CENTURY TRUCK PROGRAM PARTNERSHIP

COORDINATING COMMITTEE ................................................................................................. C-1

APPENDIX D. INTERNAL COMBUSTION ENGINE TECHNOLOGY .......................................... D-1

APPENDIX E. AFTERTREATMENT/EMISSION CONTROL.......................................................... E-1

APPENDIX F. HYBRID ELECTRIC PROPULSION TECHNOLOGIES .......................................... F-1

APPENDIX G. MECHANICAL HYBRID SYSTEMS FOR 21st CENTURY TRUCKS.................... G-1

APPENDIX H. MORE-EFFICIENT AND/OR LOWER-EMISSION ENGINE SYSTEMS .............. H-1
v

FIGURES

2.1 The nation’s economy is linked to truck transport ....................................................................... 2-1

2.2 Trucks account for increasing highway transportation energy use............................................... 2-2

2.3 1997 inventory of commercial and military trucks covered by the 21st Century

Truck Program.............................................................................................................................. 2-3

3.1 Truck types by gross vehicle weight (GVW) ............................................................................... 3-1

3.2 Annual fuel use by commercial trucks (based on VIUS data)...................................................... 3-2

3.3 Fuel use (usual trips 100 miles or less vs greater than 100 miles) ............................................... 3-3

4.1 Class 8 truck energy audit ............................................................................................................ 4-2

4.2 Horsepower required to overcome aerodynamic drag and rolling friction/accessories

as a function of travel speed for a typical Class 8 tractor-trailer.................................................. 4-6

4.3 40-foot transit bus energy audit.................................................................................................... 4-20

4.4 Pathway for improving transit bus fuel economy......................................................................... 4-21

4.5 Common dry van .......................................................................................................................... 4-27

4.6 Typical medium truck—Class 6 enclosed van; distribution of energy requirements at

40 mph for a 26,000-lb (11,791 kg) vehicle................................................................................. 4-29

4.7 Friction power requirements of typical medium truck engine as a function of rpm..................... 4-31

4.8 Energy balance for the HMMWV ................................................................................................ 4-41

4.9 Energy balance for the FMTV...................................................................................................... 4-42

4.10 Energy balance for M916A2 ........................................................................................................ 4-43

4.11 System-level model for efficient engine design ........................................................................... 4-50

4.12 “Wired” integrated engine systems optimization ......................................................................... 4-51

4.13 The combination of engine-out emissions and emission control device performance

required to satisfy proposed EPA emission standards.................................................................. 4-53

4.14 Schematics for conventional and parallel mechanical hybrid vehicle designs............................. 4-72

4.15 Various ways that mechanical and electrical hybrid vehicles can process and

store energy ................................................................................................................................... 4-73

4.16 Classic and advanced diesel ideal cycles compared..................................................................... 4-92

4.17 Process for conducting vehicle research as borrowed from the Intelligent

Vehicle Initiative (IVI) ................................................................................................................. 4-96

4.18 Vehicle intelligence R&D plan for the 21st Century Truck Program ........................................... 4-98

F.1 Motor power for heavy-duty hybrid electric vehicles .................................................................. F-3

F.2 Battery pack capacity for heavy-duty hybrid electric vehicles..................................................... F-5

H.1 Schematic of free-piston-powered linear alternator ..................................................................... H-3

H.2 Schematic of free-piston hydraulic motor .................................................................................... H-4

H.3 Cylinder cycle for stationary-regenerator two-stroke sleeve-valve engine .................................. H-4

vii

TABLES

3.1 Representative truck models......................................................................................................... 3-3

4.1 Energy audit and potential fuel efficiency improvements for line-haul trucks ............................ 4-3

4.2 Engine efficiency projections ....................................................................................................... 4-4

4.3 Energy audit and potential fuel efficiency improvements for 40-ft transit bus............................ 4-21

4.4 Preliminary performance requirements for the 21st Century Truck Program

(Class 7) transit bus ....................................................................................................................... 4-23

4.5 Summary of technical targets and barriers for transit bus emissions ........................................... 4-26

4.6 Mission information for typical Class 6 vehicles ......................................................................... 4-28

4.7 Typical medium truck—Class 6 enclosed van distribution of energy requirements

for a fully loaded 26,000 lb (11,791 kg) enclosed van at 40 mph ................................................ 4-30

4.8 Medium truck power requirements [Class 6, 26,000 lb (11,791 kg) max GVW] ........................ 4-32

4.9 Design metrics for current and improved Class 6 medium trucks................................................ 4-34

4.10 Potential for fuel-efficiency improvements for medium trucks ................................................... 4-34

4.11 Distribution of energy requirements (kWh) for a typical small truck (Class 2b

commercial pickup), EPA combined driving cycle...................................................................... 4-36

4.12 Summary of technical targets and barriers for Class 2b truck emissions ..................................... 4-39

4.13 HMMWV M1097 A2 specifications ............................................................................................ 4-40

4.14 2.5-ton FMTV M1078 specifications ........................................................................................... 4-42

4.15 M916A2 specifications................................................................................................................. 4-43

4.16 Summary of technical targets and barriers for natural gas transit buses ...................................... 4-47

4.17 Comparison of energy-storage options......................................................................................... 4-62

4.18 Summary of current performance and technical targets for batteries........................................... 4-63

4.19 Efficiencies of diesel mechanical hybrid Class 2b trucks (small) ................................................ 4-74

4.20 Efficiencies of diesel mechanical hybrid Class 6 trucks (medium).............................................. 4-75

4.21 Truck auxiliary power requirements............................................................................................. 4-83

4.22 Air brake compressor power requirements................................................................................... 4-84

4.23 Estimates of air-conditioning compressor power requirements ................................................... 4-84

4.24 Alternator power requirements for day and night constant-load operation.................................. 4-84

5.1 21st Century Truck schedule and milestones ................................................................................ 5-1

F.1 Type of energy storage used in heavy-duty hybrid electric vehicle projects................................ F-5

F.2 Hazard risk categories .................................................................................................................. F-8

ix

ACRONYMS AND ABBREVIATIONS

A/C air conditioning

ABS anti-lock brake system

AC alternating current

ADVISOR Advanced Vehicle Simulator

AFC alkaline fuel cell

ANL Argonne National Laboratory

ANOVA analysis of variance

APTA American Public Transportation Association

APU auxiliary power unit

BBW brake-by-wire

BEA boundary element analysis

BMEP brake mean effective pressure

BOI beginning of injection

BSFC brake-specific fuel consumption

CAN Communication Area Network

CARAT Cooperative Automotive Research for Advanced Technologies

CARB California Air Resource Board

CB-DPF catalyst-based diesel particle filters

CBD central business district

CDPF catalytic diesel particle filter

CFD computational fluid dynamics

CI compression ignition

CNG compressed natural gas

CR-DPF continuously regenerating-diesel particle filter

CVT continuously variable transmission

Cyl cylinder

DARPA Defense Advanced Research Projects Agency

DC direct current

DECSE Diesel Emission Control Sulfur Effects

DOC diesel oxidation catalyst

DOC U.S. Department of Commerce

DOD U.S. Department of Defense

DOE U.S. Department of Energy

DOT U.S. Department of Transportation

DPF diesel particle filter

DVECSE Diesel Vehicle Emission Control Sulfur Effects

EGR exhaust gas recirculation

EMA Engine Manufacturers Association

EMI electromagnetic interference

EPA U.S. Environmental Protection Agency

ESR equivalent series resistance

EV electric vehicle

FHWA Federal Highway Administration

FTA Federal Transit Administration

FUDS Federal Urban Driving Schedule

GDP gross domestic product

GM General Motors Corporation

GPS global positioning system

GTO gate-turnoff thyristor

GVW gross vehicle weight
x

GVWR gross vehicle weight rating

HC hydrocarbon

HC SCR hydrocarbon selective catalytic reduction

HCCI homogenous charge compression ignition

HDE heavy-duty engine

HEV hybrid electric vehicle

HVAC heating, ventilation, and air conditioning

HWFET Highway Fuel Economy Test

IC internal combustion

IFC International Fuel Cells

IGBT insulated-gate bipolar transistor

IITRI IIT Research Institute

ISE ISE Research Corporation

IVI Intelligent Vehicle Initiative

LE55 Low Emission, 55% Efficient

LNG liquefied natural gas

MCFC molten carbonate fuel cell

MCT MOS-controlled thyristor

MECA Manufacturers of Emission Controls Association

MOS metal-oxide semiconductor

MOSFET metal-oxide semiconductor field effect transistor

NASA National Aeronautics and Space Administration

NFESC Naval Facilities Engineering Service Center

NG natural gas

NiCad nickel cadmium

NiMH nickel-metal hydride

NIST National Institute of Standards and Technology

NMHC non-methane hydrocarbons

NREL National Renewable Energy Laboratory

NVH noise-vibration-harshness

OBD on-board diagnostics

OHVT Office of Heavy Vehicle Technologies

OMB Office of Management and Budget

ORNL Oak Ridge National Laboratory

OSTP Office of Science and Technology Policy

PACCAR PACCAR, Inc., formerly Pacific Car and Foundry

PAFC phosphoric acid fuel cell

PCC Partnership Coordinating Committee

PEBB power electronics building block

PEM proton exchange membrane

PEMFC proton exchange membrane fuel cell

PM particulate matter

PNGV Partnership for a New Generation of Vehicles

PNNL Pacific Northwest National Laboratory

PSAP Public Safety Answering Point

PWM pulse-width modulation

R&D research and development

SAE Society of Automotive Engineers

SCCI stratified charge compression ignition

SCR selective catalytic reduction

SEA statistical energy analysis

SI spark ignition

SNL Sandia National Laboratories
xi

SOC state of charge

SOFC solid oxide fuel cells

SOP statement of principles

STICK Stimulate Trucking Innovative Concepts and Knowledge

SUV sport utility vehicle

SwRI Southwest Research Institute

TACOM Tank-Automotive and Armaments Command

TBD to be determined

TMA Truck Manufacturers Association

TWC three-way catalytic

U.S. United States

VDC vehicle dynamic control

VGT variable geometry turbocharger

VIUS Vehicle Inventory and Use Survey

VVA variable valve actuation

Chemical compounds

CO carbon monoxide

CO2 carbon dioxide

CO(NH2)2 urea

H2 hydrogen

NO nitric oxide

NO2 nitrogen dioxide

NOx nitrogen oxide

Units of measure

Af frontal area

°C degrees celsius

°F degrees fahrenheit

A ampere

avg average

bhp brake horse power

Btu British thermal unit

Cd drag coefficient

Crr rolling resistance coefficient

fhp frictional horsepower

ft foot

g gram

g/bhp-h grams per brake-horsepower

hour

g/mi grams per mile

gal gallon

h hour

hp horsepower

hph horsepower hour

I

2

R watt

kg kilogram

kHz kilohertz

km kilometer

kW kilowatt

kWh kilowatt hour

l liter

lb pound

m meter

m

2 square meter

max maximum

min minimum, minute

MPa megapascals

mph miles per hour

ms millisecond

Nm newton-meter

Pa pascal

ppm parts per million

psi pounds per square inch

rms root mean square

rpm revolutions per minute

s second

V volt

VAC volts alternating current

VDC volts direct current

W/kg watts per kilogram

W/l watts per liter

Wh/l watt-hour per liter

Wh/kg watt-hour per kilogram

xiii

ACKNOWLEDGMENTS

This Technology Roadmap for the 21st Century Truck Program is the product of the tireless effort of

many people during the past six months. If I were to attempt to mention the names of all those who

contributed to the Roadmap through participation in meetings or teleconferences, by writing or

contributing text to its various sections, or by reviewing and providing constructive comments on its

successive drafts, I would violate one of the rules that we established when we began—to keep it short.

I would be remiss, however, if I did not express my personal appreciation to all the members of the

Roadmap Team, whose names are provided in Appendix A, for their cooperation, support, and

enlightening comments during meetings and for their individual contributions to this report.

Special thanks go to the following team members, who chaired the Roadmap subteams and/or who served

as principal authors of the section(s) for which their subteam was responsible:

John Woodrooffe, consultant to TMA

Bill Siegel, U.S. DOT

Tom Biros, International Truck and Engine Co.

Charles Roehm, ANL

Ron York, GM

Steve Zimmer, DaimlerChrysler

Scott McBroom, SwRI

Pat Dessert, Oakland University

Nabil Hakim, Detroit Diesel

Ron Graves, ORNL

Keith Vertin, NREL

John Kargul, U.S. EPA

Shang Hsiung, U.S. DOT

Jud Virden, PNNL

John Hull, ANL

Bill Corwin, ORNL

John Clarke, Caterpillar

Ray Resendes, U.S. DOT

Each subteam chair was supported by subteams members, whose names are provided in Appendix B, and

by numerous colleagues in their own organizations, who, unfortunately, are not named here.

The Roadmap Team is indebted to Dan Santini and his colleagues from ANL, who worked nights and

weekends in the early stages of the Roadmap process to provide the analyses of fuel use by truck

categories that led to the selection of the truck platforms to be addressed in the 21st Century Truck

Program. I particularly want to thank René Abarcar, of Energetics, Inc., for his major contribution in

writing Sections 1 through 3, and Pat Flaherty, of DaimlerChrysler and SAE Fellow at DOE, for her

insightful comments at many junctures of the roadmapping process.

Several of my colleagues at ORNL played visible or invisible roles in the preparation of this Roadmap.

David O’Kain, Bill Corwin, Ron Graves, and Bill Knee were members of the Roadmap Team; they also

served as a sounding board and kept me headed in the right direction. Dick Ziegler, Ray Johnson, Phil

Sklad, and Arvid Pasto helped behind the scenes by reviewing early drafts and by pointing out where

additional work was needed. My secretary, Pam Wenzel, made a major contribution to the entire

Roadmap process through her organizational skills in arranging meetings, her persistence in working with

team members to obtain the text of this document, and her tireless efforts in reviewing the several drafts

of the report for completeness, consistency, and accuracy. The ORNL Creative Media Solutions group
xiv

provided editing, electronic publishing, graphics, library, and reproduction services and worked whenever

needed to keep this report on schedule.

Finally, I want to thank the senior executives of the companies and of the federal agencies that have

supported the preparation of this Roadmap. Many of these, but I’m sure not all, are listed in Appendix C.

I particularly want to thank Dr. James Eberhardt and Mr. Thomas Gross for giving me the opportunity to

lead the development of the Technology Roadmap for the 21st Century Truck Program and for their

support throughout this process.

Ron Bradley

Oak Ridge, Tennessee

December 2000
xv

EXECUTIVE SUMMARY

The 21st Century Truck Program will support the development and implementation of commercially

viable technologies that will dramatically cut fuel use and emissions of commercial trucks and buses

while enhancing their safety and affordability as well as maintaining or enhancing performance. The

innovations resulting from this program will reduce dependence on foreign oil, improve our nation’s air

quality, provide advanced technology for military vehicles, and enhance the competitiveness of the U.S.

truck and bus industry while ensuring safe and affordable freight and bus transportation for the nation’s

economy.

The 21st Century Truck Program was announced on April 21, 2000, in Romulus, Michigan, at a gathering

of U.S. truck and supporting industries, concerned environmentalists, and federal agency representatives.

The program’s goals and research objectives are to

• improve fuel efficiency,

• reduce emissions,

• enhance safety,

• reduce total owning and operating costs, and

• maintain or enhance performance.

Making progress in each of these goals simultaneously is a major challenge. The federal government and

the trucking and supporting industries will work actively together to develop a balanced portfolio of

research aimed at achieving all these goals, coordinate their research activities as appropriate, and make

effective use of the nation’s research universities and national laboratories. Proprietary research

agreements between individual companies and federal agencies, which cannot be shared with industrial

competitors, will continue to be appropriately funded.

This research partnership will support research aimed at developing production prototype vehicles that

achieve all of the following objectives:

• Improve fuel efficiency of heavy-duty trucks and buses, specifically, by 2010

– double the Class 8 line-haul truck fuel efficiency on a ton-miles-per-gallon basis,

– triple the Class 2b and 6 truck (delivery van) fuel efficiency on a ton-miles-per-gallon basis, and

– triple the fuel efficiency of heavy-duty transit buses on a miles-per-gallon basis.

• Reduce emissions:

– throughout the Program, meet prevailing standards for oxides of nitrogen, particulate matter,

carbon monoxide, and hydrocarbons.

• Enhance safety:

– In 1998, truck-related crashes resulted in 5,374 fatalities and 127,000 injuries. The U.S.

Department of Transportation is committed to reducing truck-related fatalities by 50% by 2010.

The 21st Century Truck program will contribute to the goal of improving truck and bus safety by

fostering advancements in vehicle design and performance.

• Enhance affordability:

– maintain or enhance performance.

A central goal of this initiative is to develop cost-effective, heavy-duty vehicles for truck operators that

are fully competitive in prevailing markets.

A strong partnership between the United States truck and bus industry and their supporting industries and

the federal government has been formed to conduct the 21st Century Truck Program. The government and

industry partners are committed to cooperatively coordinate the needed research and development and

share its costs.
xvi

The partnership’s federal component is led by the U.S. Department of Energy in cooperation with the

Office of Science and Technology Policy and the Office of Management and Budget. Other participating

federal agencies are the U.S. Department of Transportation, the U.S. Department of Defense (represented

by the Army), and the U.S. Environmental Protection Agency. The federal government brings to the table

its resources for research and development, including the capabilities resident in government laboratories.

The industrial participants of the partnership are truck and bus manufacturers, their suppliers, and their

trade associations, including

Allison Transmission Eaton NovaBus

BAE SYSTEMS Controls Freightliner Oshkosh Trucks

Caterpillar General Motors PACCAR

Cummins Engine Honeywell International Volvo Trucks North America

DaimlerChrysler International Truck & Engine Engine Manufacturers Association

Detroit Diesel Mack Trucks Truck Manufacturers Association

In addition to their contributions to the research and development required to meet the program’s goals,

the industrial participants will ensure that the technology developed will be pertinent to the needs of the

trucking industry and its customers, thereby guaranteeing its widespread incorporation into the

marketplace.

On October 25, 2000, the first meeting of the government-industry Partnership Coordinating Committee

(PCC) was held. At that meeting, agreements were reached concerning the PCC’s membership and

operating approach. This meeting cemented the partners’ relationship and established the overall technical

direction of the program.

To guide the development of the technical advancements that will enable needed improvements in

commercial truck fuel economy, emissions, and safety, the current Technology Roadmap for the 21st

Century Truck Program has been drafted. The three types of trucks that use the greatest portion of fuel

were identified and, along with transit buses and military vehicles, were examined to define the greatest

needs and opportunities for the improvements needed to meet the goals of the program. Based on the

analysis of those vehicles, the program goals and technical targets were established.

Fuel efficiency multiplier Vehicle

Technical target Program goal

Large truck 1.6 2

Transit bus 2.6 3

Medium truck 2.4 3

Small truck, >8500 lb 1.5–1.75 3

The technical targets represent aggressive development and implementation of technologies currently

being considered but not yet commercially viable. The technical target shown in the table for the large

truck does not yet include gains believed to be available from vehicle weight reduction and information

technologies.

The goals described are long-term stretch goals for which technology breakthroughs will be required.
xvii

These program goals and research objectives are aggressive, and there is no certainty that they can be

achieved. Fully aware of the magnitude of the challenge and the importance of meeting the partnership’s

objectives, the parties commit their best efforts to the achievement of the goals.

Accordingly, the PCC endorsed the approach described within this Roadmap and agreed to jointly

undertake such actions as are needed to meet the goals of the 21st Century Truck Program.

1-1

1. INTRODUCTION

1.1 THE 21st CENTURY TRUCK PROGRAM

The 21st Century Truck Program is a partnership between the U.S. truck and bus industry and its

supporting industries and the federal government for research and development (R&D) of commercially

viable technologies that will dramatically cut the fuel use and emissions of commercial trucks and buses

while enhancing their safety and affordability and maintaining or enhancing performance. The

innovations resulting from this partnership will reduce U.S. dependence on foreign oil, improve our

nation’s air quality, provide advanced technology for military vehicles, and enhance the competitiveness

of the U.S. truck and bus industry while ensuring safe and affordable freight and bus transportation to

benefit the nation’s economy.

The Program was announced on April 21, 2000, in Romulus, Michigan, at a gathering of the U.S.

trucking and supporting industries, concerned environmentalists, and federal agency representatives.

1.2 PROGRAM GOALS AND RESEARCH OBJECTIVES

The program goals and research objectives of the 21st Century Truck Program are to develop and

demonstrate commercially viable technologies to

• improve fuel efficiency,

• reduce emissions,

• enhance safety,

• reduce total owning and operating costs, and

• maintain or enhance performance.

Making progress in each of these goals simultaneously is a major challenge. The federal government and

the trucking and supporting industries will work actively together to develop a balanced portfolio of

research aimed at achieving all these goals, coordinate their research activities as appropriate, and make

effective use of the nation’s research universities and national laboratories. Proprietary research

agreements between individual companies and federal agencies, which cannot be shared with industrial

competitors, will continue to be appropriately funded.

This research partnership will support research aimed at developing production prototype vehicles that

achieve all of the following objectives:

• Improve fuel efficiency of heavy-duty trucks and buses, specifically, by 2010

– double the Class 8 line-haul truck fuel efficiency on a ton-miles-per-gallon basis,

– triple the Class 2b and 6 truck (delivery van) fuel efficiency on a ton-miles-per-gallon basis, and

– triple the fuel efficiency of heavy-duty transit buses on a miles-per-gallon basis.

• Reduce emissions:

– throughout the Program, meet prevailing standards for oxides of nitrogen, particulate matter,

carbon monoxide, and hydrocarbons.

• Enhance safety:

– In 1998, truck-related crashes resulted in 5,374 fatalities and 127,000 injuries. The U.S.

Department of Transportation (DOT) is committed to reducing truck-related fatalities by 50% by

2010. The 21st Century Truck program will contribute to the goal of improving truck and bus

safety by fostering advancements in vehicle design and performance.

• Enhance affordability:

– maintain or enhance performance.
1-2

A central goal of this initiative is to develop cost-effective, heavy-duty vehicles for truck operators that

are fully competitive in prevailing markets.

These program goals and research objectives are aggressive, and there is no certainty that they can be

achieved. Fully aware of the magnitude of the challenge and the importance of meeting the partnership’s

objectives, the parties commit their best efforts to the achievement of the goals.

The federal government will promote the introduction of innovative truck technologies developed in the

initiative through its own purchases of these trucks and buses and encourage state government and other

purchasers to take similar actions.

1.3 PARTICIPANTS IN THE TECHNICAL PROGRAM AND ROADMAP

Development of the advanced technology needed to achieve the ambitious goals of the 21st Century Truck

Program will require a “teaming” effort among truck and bus manufacturers, their suppliers, federal and

private research laboratories, and universities.

The partnership’s federal component is led by the U.S. Department of Energy (DOE) in coordination with

the Office of Science and Technology Policy (OSTP) and the Office of Management and Budget (OMB).

Other federal agencies involved are DOT; the U.S. Department of Defense (DOD), represented by the

Army; and the U.S. Environmental Protection Agency (EPA). The federal government brings to the table

R&D resources, including the capabilities resident in government laboratories. Government and industry

will coordinate R&D and share the costs. University participation also will be encouraged.

This 21st Century Truck Program Technology Roadmap describes the R&D needed to achieve the vision

of tomorrow’s cleaner, safer, and more efficient trucks and buses. The Technology Roadmap was

prepared by a team of government and industry engineers and scientists who met in Chicago, Seattle, San

Diego, and Washington D.C., during the period from July through October 2000. The members of the

Roadmap Team are shown in Appendix A.

The Technology Roadmap is organized by vehicle platform (large, medium, and small trucks, transit

buses, and military vehicles) and by crosscutting technologies that are expected to be applicable to more

than one vehicle platform. Subteams of engineers and scientists conferred by telephone and e-mail to

establish the goals and technical targets for each vehicle platform and crosscutting technology and to

prepare the written text for the Technology Roadmap. Members of the subteams are shown in

Appendix B. Those who participated in, or were invited to, the October 25, 2000, meeting of the

21st Century Truck Partnership Coordinating Committee are shown in Appendix C. The Technology

Roadmap was reviewed and discussed at this meeting and those in attendance endorsed its direction.

Two drafts of the Technology Roadmap were distributed to the Roadmap Team and other representatives

of the industry and government participants in the 21st Century Truck Program for their review and

comment, the first in early October 2000 and the second in early November 2000. Comments received on

both those drafts have been addressed in this current version of the Roadmap. It is expected that the

Technology Roadmap is a “living document” and will be revised as new technologies evolve and

programmatic imperatives change.
2-1

2. STRATEGIC IMPORTANCE OF THE 21st CENTURY TRUCK PROGRAM

The vision of the 21st Century Truck Program as shared by industry and government the as follows:

A productive, innovative U.S. trucking and supporting industry is essential for the economic prosperity of

every American business. Innovation is also needed to ensure that truck and bus manufacturers and

suppliers located in the U.S. remain competitive in world markets and continue to provide rewarding

employment opportunities for large numbers of Americans. U.S. manufacturing facilities face stiff

worldwide competition. New truck and bus technologies will help truck and bus owners and operators and

their customers cut fuel and operating costs and increase safety. The Department of Defense, a major

owner and operator of trucks, would share these gains and also benefit from reduced logistics costs

associated with transporting fuel during operations. The truck and bus manufacturing and supporting

industries face a range of new challenges: increasingly stringent emissions standards, new concerns about

the threat of global warming, concerns about U.S. fuel supplies, increased expectations about safety, and

more. The truck and bus industry’s future depends on its ability to produce affordable, high-quality, safe,

environmentally sensitive products.

The new challenges can be met best if government, industry, and universities work together to develop an

improved generation of commercial trucks and buses for our nation’s commercial and military truck fleet.

Trucks are the mainstay for trade, commerce, and economic growth in the United States. The gross

domestic product (GDP) of the United States, and hence the country’s economic activity, is strongly

related to freight transport (see Fig. 2.1). It is estimated that currently as much as 80% of the total quantity

of goods is transported by trucks; therefore, meeting truck transport energy demands for movement of

goods and for services is critical to the economy.

Within the U.S. transportation sector, truck energy use has been increasing at a faster rate than that of

automobiles. Since the 1973 oil embargo, all of the increase in highway transportation fuel use has been

due to trucks (see Fig. 2.2), mainly because of their extensive use in trade and commerce and in providing

essential services. In recent years, another contributor to the increasing highway transportation energy use

has been the popularity for personal use of low-fuel-economy pickup trucks, vans, and sport utility

Fig. 2.1. The nation’s economy is linked to truck transport.

Source: Argonne National Laboratory.
2-2

vehicles (SUVs). Only the light trucks with gross vehicle weight (GVW) of greater than 8500 lb

(Class 2b) used for commercial purposes are included within the scope of the 21st Century Truck Program.

The 1997 Vehicle Inventory and Use Survey (DOC 1999) reports that there are as many as 68 million light

trucks [Class 1 and 2 trucks up to 10,000 lb (4,535 kg) in GVW], almost 3 million medium trucks [Class

3–6 trucks between 10,001 and 26,000 lb (11,791 kg) GVW], and about 2.5 million heavy trucks [Class

7–8 trucks between 26,001 and 130,000 lb (56,550 kg) GVW] registered in the United States. About

21 million of these trucks are registered for commercial use. There are about 9 million Class 2b trucks

[8,501 to 10,000 lb (4,535 kg) GVW] included in this 21st Century Truck Program, as well as the

5.5 million Class 3–8 trucks. Also included are the 246,000 tactical wheeled military trucks [DOD/Army

Tank Automotive and Armament Command (TACOM)] that constitute the logistical backbone of the

Army. The truck populations covered by the 21st Century Truck Program and their respective levels of

energy consumption are shown in Fig. 2.3.

Wartime operation is expected to increase military truck energy demands to sustain a military force on the

battlefield. It is estimated that military operation at the same level experienced during World War II could

potentially contribute as much as 6% to total commercial and military truck energy use. The 21st Century

Truck program will strengthen our national security by dramatically reducing operational support costs

and increasing combat effectiveness through a lighter, more mobile military force resulting from rapid

integration of advanced, commercially viable technologies into military trucks.

2.1 PROGRAM STRATEGY

Government and industry will coordinate R&D efforts and will share costs. The federal agencies will

build on existing research and will assign high priority to major new research identified in this technology

roadmap. DOE has been assigned to lead the federal R&D component of this program because of the

close alignment of the stated 21st Century Truck Program goals and research objectives with DOE’s

mission “to foster a secure and reliable energy system that is environmentally and economically

sustainable....” Since early 1996, DOE’s Office of Transportation Technologies, in collaboration with

Fig. 2.2. Trucks account for increasing highway transportation energy use.

Sources: EIA. 1999. EIA Annual Energy Outlook 2000, DOE/EIA-0383 (2000),

December 1999. Davis, Stacy C. 1999. Transportation Energy Data Book:

Edition 19, DOE/ORNL-6958, September 1999.
2-3

trucking industry partners and their suppliers, has been funding and conducting a customer-focused

program to research and develop technologies that will enable trucks and other heavy vehicles to be more

energy-efficient and able to use alternative fuels while simultaneously reducing emissions. DOT brings to

this program its mission-oriented intelligent transportation systems and highway transportation safety

programs. DOD, as a major owner and operator of trucks, will define the military mission performance

requirements and will fund appropriate dual-use and military-specific technologies so that national

security will benefit by innovations resulting from this Program. R&D will be closely coordinated with

EPA so that critical vehicle emissions control breakthroughs cost-effectively address the increasingly

stringent future EPA standards needed to improve the nation’s air quality.

Industry will move research achievements into production vehicles rapidly when their commercial

viability has been demonstrated. The partnership will work closely with fuel producers to accelerate the

development and production of new fuels required by new engine designs to meet the program goals.

2.2 BENEFITS OF A SUCCESSFUL PROGRAM

A successful 21st Century Truck Program will enable the trucking and bus industry and its supporting

industries to face new challenges, specifically, increasingly stringent emissions standards, concerns about

the threat of global climate change, concerns about U.S. fuel supplies, and increased expectations

regarding highway safety. These new challenges will be addressed as government and industry R&D

teams work together to develop improved technology for our nation’s commercial and military truck fleet.

Major advances and breakthroughs are expected toward achievement of the goals set to achieve cleaner,

safer, and more efficient trucks and buses.

Fig. 2.3. 1997 inventory of commercial and military trucks covered by the 21st

Century Truck Program. Source: Commercial Trucks: U.S. DOC, Bureau of Census,

1997 Vehicle Inventory and Use Survey, January 2000. Military Trucks: DOD/TACOM.
2-4

Over the past 8 to 10 years, typically about 10 to 12% of the total fatalities from vehicle crashes have

involved medium and heavy trucks. In 1998, truck-related crashes resulted in 5,374 fatalities and 127,000

injuries. The majority of those killed were occupants of other motor vehicles. Most fatal crashes occurred

on rural roads and involved tractor-trailers, the most common large truck configuration. DOT is

committed to reducing truck-related fatalities by 50% by 2010. It is expected that the technology

developed through the 21st Century Truck Program will contribute significantly to meeting this goal.

The Program will also strengthen U.S. national security by dramatically reducing operational support

costs and increasing the combat effectiveness of military vehicles. Fuel cost for the Army, as a major

owner and operator of military trucks, is more than 20% of the cost of operating and maintaining its truck

fleet. In addition, more than 70% of the bulk tonnage needed to sustain the Army during a conflict is fuel.

As the Army transforms itself into a lighter, more mobile force, the rapid introduction of advanced,

commercially viable technologies into military trucks is vital in reducing the logistics cost associated with

transporting fuels during wartime operation.
3-1

3. SELECTION OF VEHICLE PLATFORMS

One of the expected outcomes of the 21st Century Truck Program is a reduction in U.S. dependence on

imported oil. Focusing the R&D on technologies applicable to those trucks that use most of the fuel will

have the biggest impact on oil imports. To identify the trucks that use most of the fuel, the Vehicle

Inventory and Use Survey (VIUS) data collected in the 1997 economic census were examined (DOC

1999, DOC 2000).

Trucks are classified by GVW into the eight truck classes shown in Fig. 3.1.

3.1 FUEL CONSUMPTION BY TRUCK CLASS AND TRUCK USE PATTERNS

Estimates of fuel use by commercial trucks, by GVW class, as determined from the VIUS data base, are

shown in Fig. 3.2. These estimates have been cross-checked against U.S. DOT estimates (FHWA 1998).

The truck weight class is based on the owner’s registered GVW, which for a small minority of owners

could be higher than the manufacturer’s GVW rating. Of the Class 2 trucks, only those with GVW greater

than 8500 lb (Class 2b) are included in the 21st Century Truck Program. Because the VIUS data base does

not separate Class 2b trucks from other Class 2 trucks, the engine size was used to estimate the number of

Class 2b trucks and their fuel use. Based on Truck Index data (Truck Index 1994–1997), all Class 2 trucks

Fig. 3.1. Truck types by gross vehicle weight (GVW). Source: Commercial Carrier Journal

(http://www.ccjmagazine.com).
3-2

with gasoline engines larger than 5.9 liters and all commercial-use Class 2 diesel trucks are assumed to be

Class 2b.

Figure 3.2 shows that the fuel used by Class 8 trucks, 18 billion gallons per year, far exceeds that used by

commercial trucks in any other weight class. Class 6 and Class 2b have the next largest fuel consumption.

These three classes, 2b, 6, and 8, were selected as the focus for technology development to meet fuel

efficiency goals of the 21st Century Truck Program and are referred to as small (Class 2b), medium

(Class 6), and large (Class 8) trucks in this roadmap.

The VIUS data base provides data on the percentage of time that respondents’ trucks are used for trips in

the following categories:

• off-road (minimal use of public roads),

• less than 51 miles,

• 51–100 miles,

• 101–200 miles,

• 201–500 miles, and

• greater than 500 miles.

Trips of greater than 100 miles are very likely to be trips that involve interstate driving outside of major

metropolitan areas. Trips of less than 51 miles are likely to be in urban driving, while trips from 51 to

100 miles may sometimes involve both interstate and intrastate travel, but would often involve extensive

driving within urban areas.

Figure 3.3 shows a comparison of fuel use among Class 2b, Class 6, and Class 8 trucks, with the latter

subdivided into those with trips of less than 100 miles and those with trips of greater than 100 miles. Of

the Class 8 trucks, those with trips of greater than 100 miles use the most fuel. Most of the Class 8 trucks

used for trips of 100 miles or less are vocational and urban trucks. Medium and light trucks are used

mostly for trips of less than 100 miles and mostly in urban and suburban areas.

Further analysis of VIUS data showed that of the Class 2b vehicles, pickup trucks consume by far the

most fuel. The next largest fuel consumers of Class 2b trucks are the panel, multi-stop, and step vans, at

Fig. 3.2. Annual fuel use by commercial trucks (based on VIUS data).

0

4,000

8,000

12,000

16,000

20,000

Class 3 Class 4 Class 5 Class 6 Class 7 Class 8

Truck Class

0

4,000

8,000

12,000

16,000

20,000

Truck Class

Fuel Use (106 Gallons)

(Commercial Trucks)

Class 2b
3-3

about one-fourth the fuel consumption of pickup trucks. The pickup truck is, therefore, selected as

representative of Class 2b trucks.

Of the Class 6 trucks, the single-unit trucks (local delivery trucks) in urban (less than 100 miles) driving

are the most typical. They account for well over half of the fuel consumption in Class 6.

Of the Class 8 trucks, combination trucks such as the semitrailer and the truck trailer units consume the

most fuel. These trucks are operated by private businesses (i.e., department stores, grocery chains,

utilities, and others) and for-hire operators. Most are for-hire combination trucks with usual trips of more

than 100 miles. The 40-foot transit bus is representative of most of the Class 7 and 8 buses.

The specific truck models that account for the largest sales volume for the different truck manufacturers

were selected to be the representative platforms and are shown in Table 3.1.

Table 3.1. Representative truck models

Platform Model

Large—Class 8 line-haul Freightliner FLD120

International 9200/9400

Kenworth T800

Mack CH613

Peterbilt 379

Volvo WC

Transit bus 40-ft Transit Bus

Medium—Class 6 delivery van International 4000 Series

GM “C” Series

Small—Class 2b pickup Dodge 250 (8,600 GVW)

GM 2500 (8,600 GVW)

Fig. 3.3. Fuel use (usual trips 100 miles or less vs greater than 100 miles).

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

Class 2B Class 6 Class 8 <=100 Mi Class 8 >100 Mi

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

Class 2B Class 6 Class 8 <=100 Mi Class 8 >100 Mi

6

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

Class 2B Class 6 Class 8 <=100 Mi Class 8 >100 Mi

Fuel Use (10 Gallons)
3-4

3.2 REPRESENTATIVE DRIVING CYCLES

Appropriate representative driving cycles are necessary in analyzing, on a common basis, fuel efficiency

and productivity improvements between the baseline (current) truck and the “advanced” 21st Century

Truck vehicle. Several existing driving cycles for the “urban” portion of commercial truck use are

candidates for use in this program. Three “chase” cycles developed for DOE by West Virginia University

were evaluated in detail (Clark et al. 1999). These driving cycles were developed by following (chasing)

trucks and recording speed. The faster two of the three cycles, when used in National Renewable Energy

Laboratory’s (NREL’s) Advanced Vehicle Simulator (ADVISOR) model (Wipke et al. 1998) to estimate

fuel consumption, gave most reasonable results. The faster of the two cycles, which averaged 34 mph,

included driving on an urban interstate during a significant portion of the cycle. This cycle is termed the

“Urban High-Speed” cycle. The slower of the two, which averaged 14 mph, did not include any urban

interstate driving. This was termed a “Suburban” cycle. The Urban High-Speed cycle has a top speed of

61 mph. Only 9% of the time of the cycle is idling, and only 12% can be regarded as steady speed. The

remainder is acceleration and deceleration. For the Suburban cycle, top speed is 45 mph, time idling is

25%, and time at steady speed is only 7%. The Urban High-Speed and Suburban driving cycles are

considered as “City” driving for heavy trucks. Further studies will be done early in the program to

establish an appropriate driving cycle to determine the baseline fuel efficiency for like-haul trucks.

The transit bus will use a representative urban driving cycle that focuses on operation in the central

business district (CBD). This operation is stop and go to a top speed of 20 mph. The CBD consists of four

segments: (1) a 10-second acceleration phase from 0 to 20 mph, (2) an 18.5-second cruise phase at

20 mph, (3) a 4.5-second deceleration phase from 20 to 0 mph, and (4) a 7-second phase at idle. This

cycle is repeated seven times per mile traveled with a total for fourteen repetitions for a 600-second test,

making a CBD-14 drive cycle. The CBD cycle was first used to define vehicle operation expectations in

the late 70s. It has been part of the bus specification guidelines used by transit authorities in defining

vehicle performance for all of the transit buses (more than 70,000) in operation today.
4-1

4. TECHNICAL PLAN

Commercial trucks and tractors cover a wide spectrum of applications and vocations, and range by size

from the smaller pickup truck and van to the large, over-the-road, line-haul tractor trailer. Loosely

categorized by actual physical size, the entire spectrum can be broken down to simply small, medium, and

large vehicle types.

4.1 LARGE TRUCK—TRACTOR-TRAILER

Class 8 trucks consume approximately 68% of all commercial truck fuel used. The Class 8 trucks (mostly

tractor semitrailers) operating on the open road (trip lengths greater than 100 miles) consume about 70%

of this fuel. Therefore, the line-haul tractor-trailer offers the best benefit for any technology that will

improve fuel efficiency. Nonetheless, the use of Class 8 trucks for trips of less than 100 miles (vocational

and urban delivery trucks) still consumes more fuel than all Class 6 and 2b trucks combined; therefore,

technology to improve fuel efficiency for Class 8 vocational and urban delivery trucks should be

addressed. Most of the vocational vehicles operate in the vicinity of urban areas. Their operational

characteristics are significantly different from those of most tractor semitrailers. The principal variables

defining difference are operating speed, number of brake applications and acceleration cycles per unit

time, and idling time. The power-train technology for this application is expected to be similar to that for

Class 6 trucks; therefore, it is addressed in the section on medium trucks.

The goal of the 21st Century Truck Program with respect to Class 8 trucks is to develop, by 2010, enabling

technology for line-haul trucks that (1) will have a fuel efficiency of 2× ton-mpg at 65 mph, fully loaded

on a level road, and (2) will meet prevailing emission standards using petroleum-based diesel fuel. An

additional goal is to improve the safety of heavy trucks through the implementation of truck-based

technology.

Maximum benefit will be achieved if some of the technology is also adaptable to retrofit/rebuild

processes. Adaptable technologies may include emissions aftertreatment, fuel processors, auxiliary power

unit (APU) additions, vehicle intelligence, engine rebuilding technologies, brakes, and replacement of

body/cab panels with lighter and more aerodynamic components/assemblies. This strategy will ensure, to

the extent possible, that the benefits will accrue to the existing fleet, thereby ensuring an early and widely

based implementation of the 21st Century Truck Program.

Analysis of the truck as a system will allow a comprehensive evaluation of how the various components

and subsystems relate and interact with each other, and how improvements in the efficiency of various

components lead to overall efficiency improvement of the whole system. Synergisms among the various

subsystems can be exploited so that the overall improvement can potentially be larger than the product of

the improvements of the individual subsytems. Systems analysis will be used to investigate various

technology pathways to achieving the aggressive goals of the 21st Century Truck Program and to provide

guidance in the direction of R&D. The following criteria will be used for the analysis of the vehicle

platforms that have been identified: (a) vehicle functional requirements; (b) duty, driving, or operational

cycles; and (c) performance requirements and expectations. Objective evaluation of performance, cost,

benefit, and risk for the various technology pathways will be facilitated by systems analysis.

Modeling and simulation software that includes a wide number of vehicle subsystem and system

configurations will be used in vehicle systems analysis for preliminary design studies as well as for

performance trade-offs and cost comparisons for alternative vehicle configurations. Vehicle systems

analysis will also provide the technical engineering teams with performance targets and reasonable tradeoff parameters.
4-2

4.1.1 Vehicle Efficiency

Heavy truck fuel efficiency is influenced by several factors, including basic vehicle design, zone of

operation, driver technique, and weather factors. Extending the definition of fuel efficiency to include the

productivity measure “ton-mile of payload transported” presents a more meaningful measure. Some of the

new technologies, such as aerodynamic treatments, will require flexibility in the application of size and

weight regulations, as will some of the operational strategies that benefit fuel efficiency.

The nature of heavy-truck energy use can be better appreciated if it is summarized in an energy audit.

Figure 4.1 and Table 4.1 contain an energy audit of a typical Class 8 vehicle operating on a level road at a

constant speed of 65 mph with a GVW of 80,000 lb (36,280 kg).

Engine losses, aerodynamic losses, and tire-rolling resistance account for approximately 94% of the

energy used to sustain vehicle speed at 65 mph. Because these factors are all dependent on vehicle speed,

terrain, traffic conditions, etc., the expected benefits to fuel economy will be highly dependent on zone of

operation. Driveline friction and engine-based accessories, such as compressors and alternators, account

for the remaining 6%. It follows, therefore, that improvements in engine efficiency, aerodynamic drag,

and tire-rolling resistance will have a significant impact on fuel efficiency; improvements in driveline and

accessory efficiency will have a small influence on fuel efficiency. Nevertheless, any improvement in

efficiency should be actively pursued if the cost-benefit relationship is favorable.

Vehicle systems analysis software such as the ADVISOR or PSAT models will be used to investigate

technology pathways to meeting the 21st Century Truck vehicle fuel efficiency and productivity

improvement targets defined in this roadmap. The ADVISOR model, with the input assumptions

developed for this analysis, estimated 4.3 mpg for a typical Class 8 truck on the “Suburban” cycle and

6.3 mpg on the “Urban High-Speed” cycle (see Sect. 3.2). For the analysis done to date for the tractor

Total Energy Used per Hour

(65 mph, fully loaded, level road for one hour)

Base = 400 kWh (6.6 mpg) ⋅ Target = 255.5 kWh (10.3 mpg)

Engine Losses

Base = 240 kWh

Target = 143 kWh

Engine Efficiency

Base = 40%

Target = 44%

Aerodynamic Losses

Base = 85 kWh

Target = 68 kWh

Rolling Resistance

Base = 51 kWh

Target = 30.6 kWh Drivetrain

Base = 9 kWh

Target = 6.3 kWh

Auxiliary Loads

Base = 15 kWh

Target = 7.5 kWh

Woodrooffe & Associates

Fig. 4.1. Class 8 truck energy audit.
4-3

Table 4.1. Energy audit and potential fuel efficiency improvements for line-haul trucksa

Energy loss sources Baseline Improvement

(%) Target

10b Engine losses per hour (kWh) 240

30c

144.0

Auxiliary loads (kWh) 15 50 7.5

Drivetrain losses (kWh) 9 30 6.3

Aerodynamic losses (kWh) 85 20 68.0

Rolling resistance losses (kWh) 51 40 30.6

Total energy used per hour (kWh) 400 256

Fuel consumption at constant 65 mph (mpg) 6.6 56 10.3

Fuel economy multiplier 1.0 1.56

Vehicle tare weight reduction 15–20

Total fuel economy (ton-miles/gal) multiplier 1.0 71

 a

Fully loaded on level road at 65 mph for 1 h.

 b

10% net engine efficiency improvement after losses in efficiency due to emissions requirements.

 c

Due to reduced power needs.

trailer truck used as the base for large trucks for this roadmap, “Highway” driving has been approximated

as steady-speed interstate driving at 65 and 70 mph. Grades and speed variations due to traffic have been

ignored. The ADVISOR model estimated 6.7 mpg for a typical Class 8 heavy truck at a steady 65 mph

and 6.1 mpg at a steady 70 mph, illustrating the importance of aerodynamic drag at highway speeds.

Heavy truck fuel economy from ADVISOR simulations agreed reasonably well with VIUS estimates of

fuel economy for Class 8 heavy duty trucks. This was done using a combination of the Suburban (§ 11%)

and Urban High-Speed (§  3FLW\ ́ F\FOHV VWHDG\  §  DQG  PSK § 41%) “highway” cycles,

with some overnight idling fuel consumption (§  DGGHG LQ WKH ORQJHVW WULSV 7KH UHVXOWLQJ HVWLPDWHG

mpg for a “typical” Class 8 truck was 5.8 mpg.

4.1.1.1 Power-Train Efficiency

Status of Technology

The power train includes the engine and associated components such as the alternator, air compressor,

and hydraulic pump. Engine losses in the form of waste heat contained in the exhaust and rejected from

the engine and radiator account for 60% of the energy content of the fuel burned. These losses are

associated with the thermodynamic engine cycle, and reductions in this loss through engine design

changes are possible within the reciprocating diesel engine platform.

To date, new engine technologies capable of replacing reciprocating diesel engines for use in Class 8

trucks have not been forthcoming. However, we expect that the results of the 21st Century Truck Program

activities will lead to breakthrough technologies that would significantly improve overall power-train

efficiencies (see Sects. 4.6.4, 4.6.5, and 4.6.10). Some new power source technologies, such as fuel cell

systems, may be capable of providing efficient auxiliary power to Class 8 trucks to sustain the cab

environment during stationary periods (see Sects. 4.6.6 and 4.6.7).

Peak thermal efficiencies of the best current production diesel engines typically used in the Class 8 linehaul trucks are in the 45 to 46% range. Thermal efficiency of 46% translates to 0.30 lb/hph brake-specific

fuel consumption. On a typical Class 8 line-haul cycle, the overall fuel efficiency would run closer to

about 40% (0.35 lb/hph brake-specific fuel consumption). This corresponds to the energy audit illustrated
4-4

in Fig. 4.1, with the engine at 40% thermal efficiency and, therefore, the remaining 60% of the fuel

energy lost as heat.

Technical Targets

Several areas of the engine system provide fertile ground for improvements in fuel efficiency. Table 4.2

contains a summary of engine R&D areas and rough estimates of realistic targets for a 10-year leveraged

program.

Table 4.2. Engine efficiency projections

Development activity Efficiency gain (%)

Exhaust heat recuperation and improved thermal management 7

More electric accessories and system optimization 6

Peak cylinder pressure 4

Reduced engine friction 1

More efficient combustion 2

New engine concepts 25

Given the technical potential, a 10 to 20% improvement target in engine efficiency, building on the

traditional diesel engine reciprocator platform, is reasonable for a 10-year leveraged program. This

increase in thermal efficiency, nominally from 45% to the range of 50 to 54% and would build on much

R&D groundwork conducted in the DOE Office of Heavy Vehicle Technologies (OHVT) Low Emission,

55% (LE55) program. For more detail on potential improvements in diesel engines, see Sect. 4.6.2.

New engine concepts involving significant departures from the traditional diesel engine platform hold

even greater potential benefit. A target of 25% improvement (45–56%) in thermal efficiency would not be

unreasonable in this category.

For more detail on potential improvements in advanced engine technology, see Sect. 4.6.10.

Barriers

The barriers to improved diesel engine fuel efficiency fall into several general classes:

• poor cost-effectiveness of known exhaust-heat-recovery devices;

• NOx reduction in-cylinder and aftertreatment;

• material limits (temperature capability and strength);

• tribological limits of current materials and lubricants;

• cost of advanced materials and their processing;

• lack of adequate combustion understanding and simulation capability;

• lack of full electronic management (i.e., smart motors in place of belts and gears to drive accessories,

flywheel starter motor/generator, etc.);

• lack of investment in the traditional reciprocator platform and in advanced engine concepts; and

• limitations of air-handling components and systems.

Diesel engine efficiency is limited by the peak combustion temperatures and cylinder pressures that the

engine materials can withstand. Commercially viable advanced materials for the combustion chamber

exhaust valves, for example, limit the cycle efficiency. Friction, wear, and lubrication (tribology) limit

engine efficiency. Higher in-cylinder temperatures necessary for higher efficiency require significant

advances in tribology (e.g., piston ring and cylinder liner wear life, exhaust valve life). The industry’s
4-5

understanding of the fundamentals involved in the diesel combustion process is impressive but still

limited. An accurate simulation capability (from first principles) with predictive capability for emissions

is lacking.

Technical Approach

The 21st Century Truck Program presents an opportunity to address the key barriers to cleaner, higherefficiency diesel engines. The facilities and expertise found in the government laboratories and

universities are well suited to participation in collaborative projects with industry. The following R&D to

improve engine efficiency should be performed:

• Optimize mechanical design and combustion system for increased peak firing pressure and EGR.

• Develop and integrate auxiliary drives (electric, variable speed, for example) that have less parasitic

load on the engine.

• Develop and integrate cost-effective exhaust-heat-recovery technologies into the engine system.

• Develop commercially viable advanced high-temperature, high-strength materials for combustion

components.

• Develop a better understanding of frictional effects, and develop materials and lubricants with

enhanced tribological properties to extend the life of piston rings/liners, exhaust valves, and cam

shafts.

• Develop accurate simulation capability for combustion processes with predictive capability for

emissions.

• Develop materials and technologies for improved thermal management.

• Develop improved sensors for control systems.

For additional details, see Sects. 4.6.2, 4.6.3, 4.6.7, 4.6.8, 4.6.9, and 4.6.10.

4.1.1.2 Vehicle-Related Losses

Vehicle-related losses include aerodynamic resistance, rolling resistance, and driveline and accessory

losses.

Aerodynamic Resistance

At highway speeds, the fraction of fuel expended to overcome aerodynamic drag is approximately half of

the fuel not expended in engine losses. Reducing aerodynamic drag by 25% results in savings in fuel

consumption for steady highway travel in the range of 10 to 15%.

All vehicles will benefit from aerodynamic drag reduction, and the higher the operating speed and the

longer the drive duration, the greater the benefit will be. Figure 4.2 illustrates the estimated horsepower

associated with aerodynamic drag compared with the power required to overcome rolling resistance and

to supply needed auxiliary power, plotted as a function of speed for a modern Class 8 tractor-trailer truck

weighing 80,000 lb (36,280 kg) and possessing a wind-average drag coefficient of Cd = 0.6.

To reduce aerodynamic drag, the vehicle design should be optimized for minimum forebody and base

drag. For Class 8 trucks, the trailer and tractor should have an integrated design with an optimum gap

distance, and the height and width mismatch between tractor and trailer should be eliminated. Undertrailer turbulence should also be minimized. The external turbulent flow can also be controlled with body

shaping and/or active flow devices. It is also important to improve internal (under-hood) airflow to

provide the flexibility needed in tractor front-end and hood design to minimize aerodynamic drag while

providing the required engine and accessory cooling. However, large reductions in aerodynamic wake

turbulence may put increased demands on brake systems, with resultant safety implications.
4-6

Status of technology. Industry currently determines the aerodynamic characteristics of a truck design by

using several techniques, including wind tunnel testing on reduced-scale models, full-scale trucks, and

vehicle components (e.g., mirrors). Experience is critical to help guide the designs. Modeling with

experimental validation to evaluate design changes is also important. Several generic resources are critical

to the execution of any applied aerodynamic project:

• Experience—Involving highly experienced individuals with a strong empirical data base early in the

process is the key to the successful planning and execution of any technical project, particularly in the

complex and elusive world of aerodynamics.

• Empirical data—Using empirical data is the most cost-effective means of directing the first-order

improvements in design. It is also useful for the prioritization of tasks that warrant more complex

analysis techniques.

• Computational fluid dynamics (CFD)—This technique produces a continuum of data at exact

conditions allowing for the exploration of unexpected issues at reasonable cost. This method is

limited by assumptions and approximations, and the boundary conditions must be known to the same

degree of accuracy as is desired from the solution.

• Wind tunnel testing—This method is the next natural step following CFD analysis. The results of

wind tunnel tests reflect “real physics,” and once a model is built, many data points can be generated

quickly. Wind tunnel testing is expensive, and the resulting data are largely limited to those which are

anticipated and instrumented in advance.

• Field testing—The ultimate means of proving a design. Field test results are real and reflect the

operating environment. Field testing is not normally a cost-effective means to guide design

optimization; rather, it is most effective as the final step in the development process.

The DOE OHVT Aerodynamic Design of Heavy Vehicles Project Team has an R&D program under way

to evaluate truck aerodynamic designs through advanced CFD development, experimentation, and

application with industry partners. The DOE team is a multi-laboratory, multi-university collaboration. In

addition to the national laboratory consortium, commercial performers experienced in the development of

improved Class 8 truck aerodynamic designs will be an important component of the final development

team.

Fig. 4.2. Horsepower required to overcome aerodynamic drag and

rolling friction/accessories as a function of travel speed for a typical Class 8

tractor-trailer.
4-7

Technical targets. Reducing fuel consumption hinges upon the availability of trucks having greater

aerodynamic efficiency. It is estimated that the drag coefficient (Cd) for a typical tractor semitrailer

(assuming a conventional tractor) is in the range of 0.65–0.70.

It is estimated that an aggressive program could result in a 20% reduction in the drag coefficient. This

would have a very significant effect on the fuel efficiency of line-haul highway trucks.

Barriers. Perhaps the greatest barrier to reducing aerodynamic drag is the fact that trailers are fully

interchangeable (i.e., a tractor does not always pull the same trailer); therefore, tractor-trailer aerodynamic

optimization must include compatibility among the fleet as opposed to optimization of particular vehicles.

In addition, it is difficult to change prevailing attitudes regarding cab or trailer shape, or add-on control

devices. DOT regulations prohibit the use of some control devices, such as boat tails, that have

demonstrated positive enhancements.

Technical approach. To foster rapid deployment of emerging technologies resulting from the 21st

Century Truck Program, the early construction of prototype tractor semitrailer platforms is seen as an

important strategic initiative. The construction of such vehicles could be initiated shortly, and the

prototypes would serve as a platform for the new technologies. Aerodynamic drag and tire-rolling

resistance have been identified as the two dominant factors influencing fuel economy. First-generation

prototype tires could be available early in the program, and early prototype aerodynamic treatments could

be implemented to demonstrate their effectiveness and operational practicality. It is clear that a successful

21st Century Truck technology option must be “practical” in order for it to be accepted. An early

prototype vehicle will provide the ability to investigate new concepts, help guide their development, and

ensure that they can be successfully incorporated into the heavy truck “system.” New concepts for safety

improvements would be ideal candidates for the prototype vehicles because their effectiveness could be

clearly demonstrated.

The early prototypes will also provide concrete evidence of progress and will serve as continuity

platforms for the life of the project. This will allow accurate and fair reference comparisons of

incremental design improvements and new technologies that emerge during the life of the program.

The challenge of reducing Class 8 truck aerodynamic drag will require a highly directed systems

approach to the engineering task. The tractor-trailer is the vehicle that will gain the most benefit from

aerodynamic improvement; thus it is imperative that trailer manufacturers be part of this program.

Vocational vehicles will not benefit as much from aerodynamic improvements given their zone of

operation (lower speed) and comparatively smaller number of operating units; however, some of the

benefits that are expected to arise out of the tractor-trailer effort will be transferable to vocational

vehicles.

The areas in which improvement in aerodynamic drag of Class 8 trucks can be realized are

• Front-end development;

• tractor-trailer interface;

• underbody drag and skirt; and

• trailer treatments such as boat tailing, collapsible roof lines, and active control.

The goal of reducing aerodynamic drag must be considered in light of other vehicle requirements. In

particular, the addition of exhaust gas recirculation (EGR) systems will put additional requirements on

cooling systems, including pumps, fans and radiators. These components impact the underhood space

requirements and work in opposition to the need to reshape the front-end of the vehicle for drag reduction.

In addition, the competition for space between cooling systems and front end shaping may also affect the

need for improved front-end energy-absorption systems. These seemingly contradictory requirements
4-8

underscore the need for a systems approach. (For more discussion of thermal management see Sect. 4.6.8;

for more discussion of safety-related issues, see Sect. 4.1.3.)

Rolling Resistance

A truck tire is largely a pneumatic load-carrying device that fulfills many functions including load

support, transmission of acceleration and braking forces, transmission of guiding forces, and absorption of

vertical dynamic shock loads. The truck tire generates energy losses as it performs these functions.

Typical loss origins and their magnitude relative to total rolling resistance loss are as follows:

• hysteresis losses, 85–90%;

• tire/ground friction, 5–10%;

• aerodynamic losses, 3–5%; and

• tire/wheel friction, less than 1%.

The largest energy losses are created by hysteretic material losses within the tire structure as it operates,

and for practical purposes, it equates to “tire-rolling resistance.” All of these energy-related concerns must

be balanced against safety imperatives for greater longitudinal and lateral traction in braking, and the

possibility that smaller diameter tires would reduce cargo center-of-gravity heights, thereby providing a

substantial enhancement of roll stability.

Status of technology. For a line-haul Class 8 type vehicle, tire-rolling resistance can account for a

significant fraction of the total fuel consumed. At a given tread depth, a truck tire’s total energy losses

will vary as the operational conditions change (speed, loading, and duration). However, this variation will

be of secondary importance compared with the total vehicle fuel consumption, which will vary greatly,

depending upon operating conditions (stop-and-go delivery vs interstate line-haul transport). Typical

ratios of tire-rolling resistance to total vehicle fuel consumption are as follows:

• line-haul Class 8 trucks, 1: 3 to 4;

• regional usage Class 8 trucks, 1: 5 to 6; and

• urban bus usage, 1: 8.

Industry experience indicates that for a typical Class 8 tractor-trailer combination running on an interstate

circuit, a 30% decrease in total vehicle tire-rolling resistance would improve fuel consumption by

approximately 10%.

The total tire-rolling resistance contribution is equal to the sum of the individual tires’ rolling resistance

(steer, drive, and trailer axle) at their given load. An approximation of the losses for a tractor semitrailer,

assuming new tires on a fully loaded typical Class 8 vehicle, would be about 15% of the total tire energy

loss on the steer axle [12,000 lb (5,442 kg) max load], about 50% on the drive axles [34,000 lb

(15,470 kg) max load] and about 35% on the trailer axles [34,000 lb (15,470 kg) max load]. As a tire

wears, tire-rolling resistance will normally decrease relative to the change in tread depth. A well-worn

drive tire may approach the rolling resistance value of an unworn trailer tire. A loaded tire dissipates more

energy than an unloaded tire; however, energy dissipated per ton of transported cargo will be lower for

more heavily loaded vehicles.

Tire optimization vs customer expectations. The tire-rolling resistance is only one of several key

performance criteria that affect tire selection. Other important performance criteria include tread wear

life, traction, noise, durability and retreadability, and cost.

The tire must meet the customer’s performance and cost expectations. Each market segment values the

various performance criteria differently. A single performance parameter can easily be enhanced, but an
4-9

optimal balance of all the criteria must be maintained. In particular, this balance requires simultaneous

reduction in rolling resistance and increased tire traction available for braking and cornering. Unlike

passenger car tires, truck tires are most often axle-specific in relation to these different performance

criteria. For example, a low-tread-depth, rib-type tire may be desirable for a trailer axle application;

however, the same tire would be commercially unacceptable for a drive axle application because of

traction and tread wear life performance.

Technical targets. Total tire energy losses can be reduced primarily through rolling resistance reduction.

Improvements in total tire aerodynamic losses can offer second-order improvements. The proposed target

for reducing the tire’s contribution to energy loss is

• Decrease “best in class” tire-rolling resistance values for steer tires to 4.5 kg/1,000 kg, for drive tires

to 4.5–5.0 kg/1,000 kg range, and for trailer tires to 4.0 kg/1,000 kg while simultaneously increasing

longitudinal and lateral tire traction to support a 30% reduction in stopping distance and improved

handling. These values represent an approximate additional 15% reduction in tire-rolling resistance

values vs today’s best-in-class standards. This would translate to an additional 4 to 5% fuel savings

for a line-haul Class 8 vehicle configuration.

Much more ambitious tire-rolling resistance targets below 4.0 kg/1,000 kg will not be reached across a

wide range of tires before 2010 unless there are major breakthroughs in material dissipation properties

and types of construction, or breakthroughs in the optimization for wear, traction, and tire-rolling

resistance.

Barriers. There is great variation in tire-rolling resistance values; however, the ability to reduce this

variation will be limited by the commercial acceptance of “optimized” low-rolling-resistance tires if other

performance criteria are compromised. Reducing recapped tire-rolling resistance values to approximate

new tire-rolling resistance values is desirable, given that there are more recapped tires on the road than

new tires. The use of wide-based single tires can reduce rolling resistance by 10 to 30%. However,

pavement loading at the tire/roadway interface is an important consideration for highway infrastructure

protection (as measured by axle loads and tire/pavement surface area loads) and must be taken into

account as design options are considered.

Lower-hysteresis rubber compounds for internal components and the tread area will reduce rolling

resistance; however, a significant development effort will be required.

Technical approach. Apart from overcoming the barriers listed, other development activities leading to

reduced tire-rolling resistance include the following:

• Optimize tread design to reduce energy losses while maintaining or improving tire traction. Lowertread-void-volume designs offer improved rolling resistance values but generally reduce wet traction.

This reduction would have to be offset by innovations in tread pattern design and tread rubber

characteristics.

• Use alternative tire configurations to reduce aerodynamic drag. Single tire fitments on drive and

trailer positions offer slight aerodynamic reductions. These gains will probably be second order.

• Optimize tire construction to reduce losses in load-bearing areas such as the bead and shoulder area.

• Increase tire inflation pressure to reduce tire-rolling resistance. Research is required to ensure that

higher pressures do not negatively impact other criteria such as ride comfort or pavement damage.

• Develop simple tire/vehicle total life energy models to calculate the “energy balance” for different

tire system proposals. This total life model would need to include the three essential components of

total tire energy: energy to produce (and recap) the tire, energy consumed in the form of rolling

resistance and other losses throughout its entire life, and energy recovered or expended during

recycling/disposal.

• Develop and use analytical models to study tire-related vehicle fuel efficiency.
4-10

• Conduct tire-related aerodynamic drag studies to quantify the benefits of tire shape and configuration

optimization.

• Research tire materials that offer the same advantages as current rubber compounds while reducing

energy dissipation. This has been an area of ongoing research within tire companies and material

suppliers for many years. Achievements in this area will be difficult and will require considerable

effort; however, this area has the greatest potential benefit since the material dissipative losses are by

far the dominant contributor to rolling resistance.

• Continue the ongoing work related to the impact of tire alignment on vehicle fuel efficiency.

• Continue to emphasize the need to maintain tire pressure to reduce energy loss. This is a real area of

improvement because few fleets maintain tire pressures at recommended levels for optimal rolling

resistance performance.

Driveline Losses

The driveline includes the transmission, drive shaft, differential, and wheel bearings. The driveline system

is a mature system achieving better than 95% efficiency at high-torque applications. At highway speeds,

the driveline efficiency approaches 98%. The efficiency drops under low-torque conditions because of

constant drag torque and viscous losses. The three areas of technical development that may lead to

improvements in efficiency include gear meshing, lubrication, and continuously variable transmissions

(CVTs).

Gear tooth profiles have evolved to a point where little benefit is anticipated from further research

activity. However, gear contact friction is an area where improvements are possible. Improvements would

include the reduction of surface roughness and the use of low-friction coatings, new gear materials, and

lubricants.

The second area of potential benefit is in the development of improved lubricants. Viscosity has long

been the key to reducing gear-tooth sliding friction and increasing durability, but viscosity is also a

contributor to reduced efficiency through viscous churning and damping.

The third area of potential benefit is the development of CVTs. To date, CVT technology has not been

widely accepted by the heavy truck industry because it has not been demonstrated that CVT design

concepts are practical in terms of cost or durability. However, newly emerging technologies are causing

manufacturers of heavy transmissions to rethink the possibility of offering CVT functionality to line-haul

trucks. This capability makes it possible for engines to operate at more nearly optimized speed/load

conditions as a means of improving efficiency. In addition, CVT functionality may represent an enabling

technology with respect to certain diesel aftertreatment technologies required to meet EPA regulations

beginning in 2007.

Technical targets. It is anticipated that improvements in transmission driveline efficiency may be as high

as 30%, which would yield a 1.5% improvement in fuel efficiency. The benefits associated with CVTs are

more difficult to quantify because of the interaction with the driver and the engine. It is anticipated that

the benefits of the CVT would lead to significant reductions in emissions and fuel consumption.

Barriers. The ability to develop additives and chemical formulations that can produce low friction and

wear characteristics as well as low viscous losses at the full range of operating temperatures is the most

significant barrier. New lubricants must be compatible with friction materials in clutches and brakes. The

creation of improved lubricants would have universal benefit for all vehicles. Treatments developed to

reduce gear tooth and bearing friction are not yet practical or cost-effective. The principal barrier

preventing the successful introduction of a CVT for the heavy truck industry has been the availability of

suitable technology that can withstand the high torque loads over extended operating periods at a

reasonable price. However, recent encouraging developments in this area have raised the probability of a

commercially viable CVT.
4-11

Technical approach. Driveline efficiency improvement work has the advantage of being highly focused

on three primary objectives. The implications of successful development in this research domain are clear

and compelling and have enormous crosscutting potential that extends well beyond motor vehicles. For

this reason, industry is highly motivated to find a solution. Any projects under consideration for

deployment will require a high level of industry involvement and a very critical analysis of the costbenefit profile of the proposed research effort.

To foster the development of CVTs, vehicle systems-level modeling should be conducted to simulate the

effect of CVT functionality and potential synergies that are created in other subsystems, such as exhaust

aftertreatment.

Accessory Loads

Accessory loads common to Class 8 trucks include engine-based components such as the alternator, air

compressor, air-conditioning compressor, hydraulic pump, and engine oil and fuel pumps; cab-related

accessories including heating and cooling fans, lighting, windows, and mirror heaters; and living

accessories such as refrigerators, microwaves, coffee pots, and entertainment devices. The power

requirements for Class 8 trucks vary between 3 and 30 kW, depending on the truck drive cycle and the

type of work function. At highway speed, it is estimated that the accessory load for a tractor semitrailer

accounts for about 4% of the fuel consumed by the vehicle. The technologies that might be considered to

reduce accessory loads are described in Sect. 4.6.7. It is expected that development of these technologies

could result in a 50% reduction in auxiliary loads.

4.1.1.3 Mass Reduction

Reduction of the tare weight mass of heavy vehicles translates into a direct efficiency gain on the basis of

increased payload productivity potential. However, the maximum benefit of increased payload efficiency

is not realized for loads that occupy the maximum volumetric capacity of the vehicle unless the maximum

allowable weight is reached. Given that a significant portion of freight transported by heavy trucks is

volume-limited, particularly back-haul freight, estimates of the benefits of reduced tare weight need to

reflect this reality. Items such as white goods and furniture are examples of volumetrically sensitive

freight. Load-sensitive freight that would benefit fully from reduced tare weight includes bulk

commodities such as petrochemicals, forest products, and structural steel. However, mass reduction can

also benefit volume-limited trucks. It has been estimated that a 4,000-lb (1,814 kg) reduction in tare

weight will reduce rolling resistance by a minimum of 5% and will enhance braking efficiency as well.

Status of Technology

The Class 8 truck industry has a history of being sensitive to vehicle tare weight because of its

relationship to productivity. Some truck purchasers aggressively select vehicles on the basis of tare

weight; others are less sensitive to this issue. Heavy truck manufacturers have addressed this need by

considering mass when selecting components and by balancing tare weight, vehicle durability, and

performance. To take this effort to the next level will require a systems approach that optimizes

subcomponents to minimize mass. Heavy truck frame structures can be improved to reduce tare weight

through high-level engineering, materials selection, and technology development. Trailers are completely

independent of the tractor; therefore, they will require unique approaches to weight reduction.

Technical Targets

It is anticipated that the mass of tractor-trailer assemblies can be reduced by about 15 to 20%. Some of

this benefit may be lost as a result of the additional mass requirements of anticipated 21st Century Truck

technologies such as aftertreatment emission devices, hybrid power trains, fuel cells, and regenerative

braking units.
4-12

Assuming that the tare weight of a typical tractor-trailer is approximately 27,000 lb (12,245 kg), and the

GVW is limited to 80,000 lb (36,280 kg) by regulation, then the payload component would be

approximately 53,000 lb (24,036 kg). A 15 to 20% reduction in tare weight will translate into a 7.6 to

10% improvement in productivity efficiency, assuming that the vehicle is operating at the fully loaded

state. Factoring in the net benefits that would accrue to the entire fleet, assuming that 30 to 50% of

vehicle miles traveled are at maximum allowable loads, the net improvement in efficiency would be

approximately 2 to 5%. Of course, any changes in materials and/or structures to reduce tare weight must

not sacrifice vehicle durability or crash survivability and must not compromise cargo containment.

Barriers

The barriers limiting progress in tare weight reduction include

• the sensitivity of the marketplace to minimum capital cost for vehicles,

• the need to ensure that durability will not be compromised,

• the arms-length relationship with component suppliers,

• lack of experience in joining new lightweight materials,

• lack of an appropriate data base on lightweight materials for use by design engineers,

• lack of experience in repairability and maintenance, and

• the design flexibility requirements that define the largely custom-built truck market.

Technical Approach

To accomplish meaningful reduction in tare weight will require a concerted effort from both the vehicle

manufacturer and the trailer manufacturer along with technologists in advanced materials and

manufacturing processes. A systems approach will be required to improve component design, with an

emphasis on reduced mass and system integration. A number of projects are under way within the DOE

OHVT High Strength Weight Reduction Materials Program. Close collaboration between national

laboratories, universities, truck manufacturers, and component and materials suppliers must be

maintained. Development of lighter frame structures is a practical goal, but attention must be given to

ensuring continued durability and vertical dynamic response. Goals may be achieved by a near-term use

of better design and use of high-strength steel, followed by increased use of aluminum and incorporation

of carbon fiber polymer matrix composites as technology to manufacture cost-effective components

becomes available. See Sect. 4.6.9 for more details.

The following specific R&D projects should be included:

• Develop low-cost processes for manufacturing large components in order to reduce part count and

assembly steps.

• Develop a materials design methodology and manufacturing technologies to reduce the weight of

structural components of trailers. Components must exhibit strength, stiffness, and durability to meet

the duty cycle and maintain crash energy management capabilities.

• Develop manufacturing processes to take advantage of the high specific strength of magnesium and

titanium alloys. Develop cost-effective forming processes for aluminum and high-strength steels.

• Develop reliable joining techniques for lightweight materials.

• Develop predictive analytical computer models for dimensional management of full assemblies.

• Assess the potential of lightweight, high-strength polymer matrix composite materials to meet

performance and cost targets for structural applications.
4-13

4.1.2 Emissions

4.1.2.1 Status of Technology

Emission reduction in large tractor-trailer combination trucks must focus on the diesel engine. The diesel

engine presently dominates this sector of commercial trucks because of its efficiency, durability, and

torque/speed characteristics. In the foreseeable future, no other type of power plant is expected to be

ready for this application in spite of years of research on alternatives.

Over the past 20 years, diesel-engine manufacturers have achieved remarkable reductions in nitrogen

oxide (NOx) and particulate matter (PM) emissions in response to regulations. When the EPA first began

regulating diesel emissions in the mid- to late 1970s, trucks typically had emission values of 10 to

15 g/bhp-h of NOx and 1 g/bhp-h of PM. Emissions reductions have been achieved by optimizing

electronic control, retarding fuel-injection timing, increasing injection pressures, improving air-handling

systems, using oxidation catalysts, and implementing EPA’s mandate for low-sulfur diesel fuel (no

greater than 0.05% sulfur content) for on-highway vehicles in the early 1990s. Today’s heavy-duty diesel

engines are regulated to 4.0 g/bhp-h of NOx and 0.10 g/bhp-h of PM (less than 0.05 g/bhp-h for transit

buses), and substantially lower emissions have been achieved in research engines.

In 1996, the EPA, the state of California, and major engine manufacturers prepared a Statement of

Principles (SOP) that requires further reduction to 2.4 g/bhp-h of NOx plus non-methane hydrocarbons

(NMHC) or 2.5 g/bhp-h of NOx plus NMHC with a maximum of 0.5 g/bhp-h of NMHC by 2004. An

action by the EPA and the U.S. Department of Justice resulted in a consent decree with the diesel-engine

manufacturers that moves the SOP requirements to October 2002 and places caps on emissions at all

operating conditions. The requirement for the diesel-engine manufacturers to meet these lower emissions

standards will likely be met with implementation of cooled EGR, resulting in reduced engine efficiency

and perhaps less durability. Numerous preproduction engines have achieved the SOP emissions levels.

The SOP and various state programs are spurring the use of emission control technologies in retrofit. In

particular, diesel particle filters (DPFs) and oxidation catalysts are applicable to older engines in the fleet.

In May 2000, EPA unveiled proposed emissions regulations for heavy-duty engines to begin in 2007.

EPA is proposing a PM emission standard for new heavy-duty engines of 0.01 g/bhp-h, to take full effect

in the 2007 heavy-duty engine model year. The proposed standards for NOx and NMHC are 0.20 g/bhp-h

and 0.14 g/bhp-h, respectively. These NOx and NMHC standards for diesel engines would be phased in

together between 2007 and 2010.

It is widely held that the emissions levels in these proposed rules could be met only with the integration of

robust NOx and PM exhaust emission control devices with the engine. The lower limit of engine-out

emissions for direct-injection diesels is estimated to be about 1.5 g/bhp-h NOx. Only the realization of

high-risk technologies such as homogenous charge compression ignition (HCCI) engines would change

this perspective. The mature and highly effective three-way catalyst (TWC) systems in today’s gasolinefueled automobiles are not applicable to diesel or other lean-burn engines. In TWC systems, both

reduction of NOx and oxidation of carbon monoxide (CO) and hydrocarbon (HC) gases can be

accomplished in a single catalyst bed; sufficient reducing gases are present to reduce NOx, and enough

oxygen is available to oxidize the CO and hydrocarbons through precise control of air-fuel ratio near

stoichiometry. However, because diesel engines operate under lean-fuel conditions (i.e., excess oxygen),

conventional catalysts are not effective; therefore new approaches to NOx control are required.

The most promising NOx emission control technologies include the following:

• NOx adsorber-catalysts,

• selective catalytic reduction (SCR) systems using urea,
4-14

• SCR systems using hydrocarbon reductants, and

• plasma-assisted SCR with hydrocarbon reductants.

Development and optimization work with NOx adsorber technology is progressing. In programs utilizing

very-low-sulfur fuel, NOx reduction levels of more than 90% have been achieved for fresh devices in both

engine test cells and experimental vehicle systems. However, on representative heavy-duty cycles, the

experience has been 60 to 70% conversion; and in the presence of even low amounts of sulfur,

performance degrades dramatically within tens of hours. To improve transient performance, extensive

R&D work is still needed in the areas of optimizing the NOx adsorption/desorption and conversion

functions, defining and optimizing sulfur removal (“desulfurization”) techniques and strategies, and

examining the use of sulfur traps upstream of the catalyst.

SCR technology is being developed for commercial application and will be available for some motor

vehicles in the very near future. The urea-based SCR technology is achieving NOx reductions on the order

of 80 to 90% and is also capable of reducing hydrocarbon emissions and PM.

Plasma devices are being explored in conjunction with hydrocarbon SCR systems to convert NOx to NO2

and to modify the hydrocarbons used as reductants. They are generally in the early prototype scale.

Control technologies for PM have seen significant progress in recent years to the point of limited

commercial application. Catalyst-based DPFs used on engines operated on low-sulfur diesel fuel can

achieve PM and toxic hydrocarbon reductions well in excess of 90%. Indeed, when very-low-sulfur diesel

fuel is used, the level of particulate emissions is almost undetectable. Where diesel fuel containing less

than 10 ppm sulfur has been used, filter technology has demonstrated impressive durability, in some

applications continuing to provide excellent particulate removal at 600,000 km of vehicle operation.

Diesel oxidation catalysts (DOCs) are often utilized in complete NOx and PM control systems. Their

functions can include oxidizing NO to NO2, eliminating ammonia slip in SCR systems, and generating

heat. DOCs are a mature technology and have been widely used commercially.

Certain fuel characteristics are critical to achieving emissions targets. In the early 1990s EPA mandated

an approximate 90% reduction in diesel fuel sulfur (on-highway use) to assist with PM control. It is now

evident from rigorous test programs, many of which have been jointly conducted by DOE and industry,

that fuel sulfur will need to be substantially lowered again for the emission control devices and systems to

be effective and durable. Therefore, EPA has proposed a sulfur cap of 15 ppm in diesel fuel beginning in

2006 (typical levels today are 200–300 ppm). Performance enhancement of emission control systems may

be achievable through tailoring other fuel properties or through additives. Engine-out emissions can also

be reduced by a reasonable degree through fuel reformulation.

Emission control technologies, their challenges, and recommended R&D are described in more detail in

Sect. 4.6.10.

4.1.2.2 Technical Targets

The emissions targets are as follows: 0.1 g/bhp-h PM and 2.4 g/bhp-h of NOx plus NMHC, or 0.1 g/bhp-h

PM plus 2.5 g/bhp-h of NOx plus NMHC with a maximum of 0.5 g/bhp-h of NMHC or less by October

2002, while achieving the efficiency goals. (Test cycles are defined in the Consent Decree.) The target for

2007 is compliance with emissions regulations at that time. The final EPA rulemaking is expected to be in

December 2000. Proposed rules are 0.20 g/bhp-h NOx and 0.01 g/bhp-h PM and a maximum 5% fuel

economy penalty from the emission control system.
4-15

4.1.2.3 Barriers

The following are key barriers to achieving the technical targets for emissions from diesel engines for

tractor-trailer trucks:

• NOx/PM trade-off—that is, maintaining efficiency and low NOx while keeping PM down:

– limitations of air-handling system,

– limitations of fuel-injection technology,

– incomplete optimization of cooled EGR and its durability issues, and

– limited effectiveness of cost-effective fuel additives and reformulation;

• unproven durability and transient performance of NOx aftertreatment technology:

– degradation from sulfur in fuel,

– temperature extremes, and

– Inadequate methods of introducing reductants;

• undeveloped infrastructure for urea SCR; and

• immature systems integration and optimization of PM and NOx control devices because of inadequate

simulation capability for aftertreatment devices.

4.1.2.4 Technical Approach

Meeting the technical targets for emissions will require a three-pronged diesel engine emission-control

strategy:

1. understanding and optimizing in-cylinder combustion processes,

2. optimizing fuel formulation, and

3. further developing exhaust aftertreatment technologies such as improved catalysts.

The following are recommended R&D paths (additional details can be found in Sect. 4.6.10):

• Apply advanced diagnostics to describe and quantify (when possible) the in-cylinder formation of

NOx and PM.

• Further develop advanced, highly flexible fuel-injection systems and engine control strategies to

apply the knowledge gained from the preceding path.

• Reduce or eliminate particulate and sulfur contributions from lube oil by development of improved

liquid lubricants and advanced solid lubrication technology where applicable.

• Optimize cooled EGR for maximum NOx reduction without PM increase, and mitigate durability

concerns through materials engineering and operational controls.

• Improve the scientific foundation of NOx control absorber and catalyst performance and degradation

mechanisms. Similarly, expand the foundation of understanding of plasma processes as they pertain

to NOx and PM control.

• Identify and exploit fuel properties that reduce overall tailpipe emissions through lower engine-out

emissions and/or enhancement of aftertreatment system performance.

• Utilize the preceding accomplishments to improve the materials, components, and system designs for

emission controls. Improve and apply emission control simulation tools for system design and

optimization.

• Develop better methods for generating and introducing effective reducing species to NOx catalysts

• Develop desulfurization processes or sulfur sequestration technology for emission control devices.

• Devise suitable technologies and procedures for urea supply for SCR.

• Develop and apply sensors in controls and diagnostics of engine and emission control processes.

• In development of emission control aftertreatment devices, include the necessary features to make the

devices suitable for retrofit of the existing fleet.
4-16

4.1.3 Truck-Related Safety (Truck-Trailer Systems)

Medium/heavy trucks account for approximately 3% of vehicles in use on the nation’s highways and

accumulate 7% of all the vehicle miles traveled, while being involved in 8% of all fatal crashes and 3% of

all crashes. The relative proportional involvement of medium/heavy trucks in fatal crashes has decreased

over the past 8 to 10 years; they typically accounted for 10 to 12% of the total 10 years ago.

Each year, more than 4,000 people die in crashes involving combination trucks (defined as tractor-trailers,

bobtail tractors, and single-unit trucks towing trailers). This number represents about 74% of the fatalities

resulting from crashes involving all types of medium/heavy trucks. Over 80% of these fatal crashes are

multiple-vehicle crashes, and the vast majority of the fatalities (about 80%) are occupants of other

vehicles. About 13% are truck occupants, and 7% are not vehicle occupants (pedestrians, bicyclists, etc.).

In about two-thirds of two-vehicle crashes involving combination trucks, the point of impact on the truck

is the front. Nearly half of these involve the front portion of the truck being struck or striking some

portion of another (typically smaller) vehicle. The second most prevalent crash type is the front of the

truck impacting the side of another vehicle.

DOT has established a long-term goal of a 50% reduction in fatalities resulting from medium/heavy truck

crashes by 2010. That number stood at 5,374 in 1998 (the benchmark year); thus the goal is to experience

no more than 2,687 fatalities in 2010. This is a daunting challenge that will require simultaneous action

on many fronts. Technologies developed in the 21st Century Truck Program will contribute to meeting this

challenge.

In developing programs to improve vehicle safety, it is essential to consider the multiple factors

contributing to the cause or enabling of truck crashes. These include

• motor carrier management commitment to safety and their safety management practices;

• driver skill, performance, and behavior;

• driver distraction and driver fatigue;

• roadway design and condition;

• traffic volumes and density;

• vehicle design, performance, and condition; and

• institutional issues such as motor carrier regulations and enforcement.

Vehicle design, performance, and condition obviously represent only one of these factors. Nevertheless, it

is the focus of this program, and improvements made in this area can yield significant crash prevention/

mitigation improvements.

4.1.3.1 Crash Avoidance

Safety Issues and Status of Technology

Among the many factors leading to truck crashes, vehicle design and performance characteristics play a

critical, if somewhat unrecognized and underreported, role. In many cases, these attributes, if they do not

directly cause a crash to occur, make it more difficult for a truck driver to recover from an error or avoid

an unforeseen conflict. Once a crash occurs, the way trucks are designed can affect the severity of trauma

sustained by the occupants of all the vehicles involved. Significant components of the basic vehicle

design, such as overall dimensions, weight, and axle location, are controlled by government size and

weight regulations. These regulations are primarily focused on infrastructure preservation and not

necessarily on vehicle safety. Issues aimed at improving commercial vehicle safety through vehicle

design and/or configuration change will most certainly require cooperation from the policy sector.
4-17

It is widely recognized that other factors, principally the roadway type on which the truck is operated and

the behavior/performance of both truck drivers and other vehicle drivers, have a large influence on crash

causation. Nevertheless, vehicle design and performance attributes are important concerns that, if

optimized, can enhance large truck safety and help reduce truck crash-related fatalities. However, it is

important to balance optimization efforts. For example, design enhancements that reduce aerodynamic

drag may adversely affect braking capability. On the other hand, such design enhancements might be used

to reduce the severity of car-truck impacts.

With this information as background, it is essential that measurable safety goals be established for the 21st

Century truck platforms. In many cases, it will be beyond the scope of this program to link performance

improvements to estimates of fatalities/injuries that might be prevented as a result of incorporating the

improvement; therefore, reasonable surrogate goals will be used—namely, direct engineering

measurements of various safety-relevant vehicle performance attributes.

The safety goals will relate to the crash avoidance and crash protection performance characteristics of

each vehicle platform and will include, but not be limited to, the following:

• straight-line, controlled stopping performance (stopping distance);

• retention of braking capacity during grade descents, or other situations involving sustained brake

applications (brake thermal capacity);

• roll stability in a steady-state turning maneuver (static roll stability threshold);

• roll and directional stability in crash-avoidance steering maneuvers (load transfer ratio, and, in the

case of multi-trailer combinations, rearward amplification of steering-induced lateral acceleration);

• low-speed offtracking in a 90° turn;

• enhancement of driver capability to avoid collisions (various measures of advanced technology

collision avoidance system performance, e.g., roll stability warning, side and rear blind spot

elimination, forward collision avoidance systems, run-off road warning systems);

• truck occupant protection [as measured by existing Society of Automotive Engineers (SAE)

Recommended Practices]; and

• reduction of truck frontal and side structural aggressivity in multi-vehicle collisions (vehicle/vehicle

dimensional compatibility and truck structural kinetic energy absorption/deflection performance).

Each of these performance measures needs to be gauged comparatively with the most prevalent in-use

truck represented by each platform.

The 21st Century Truck Program will include only those aspects of truck safety that can be addressed

through on-board truck technologies.

Technical Targets

Several high-technology tractor-trailer demonstrators have already been built that have shown a reduction

on the order of 30% in stopping distance compared with current production designs. This has been

accomplished by a combination of air disc brakes throughout the tractor-trailer combination, much more

powerful front axle brakes, and electronic control. Electronic control of braking offers better brake control

and balance because the braking action can be modulated at each individual wheel of the combination. It

also offers reduced application times, which is especially important in multiple-trailer combinations. A

performance target of a 30% reduction in stopping distances is reasonable for this program. To achieve

this goal, the frictional characteristics of tires will also have to be improved from current production

designs. This may have a significant impact on the ability to reduce rolling resistance for tires because

increasing braking traction typically also increases rolling resistance.

The use of disc brakes on both tractors and trailers will also improve the thermal capacity (fade

resistance) for new Class 8 foundation brake systems. The biggest challenge will be to provide disc brake
4-18

designs that are economically feasible and not at odds with energy-saving goals because current disc

brake designs are both much heavier and much more expensive than drum brakes. The size of currently

available disc brakes inhibits their adoption in North America. New, lightweight friction materials will

have to be developed for both rotors and brake pads. In order to obtain sufficient stopping power with

smaller-diameter wheels, designs employing multiple discs will be necessary. Incompatibility of brakes

between old trailers and new tractors, or vice-versa, will present a major problem during phase-in, which

may be, at best, addressed through the use of electronically controlled brake systems.

Engine braking can be a significant additional factor. Today, with the use of variable-geometry

turbochargers, the power absorption capability of engine brakes may exceed the power rating. With the

use of electrically assisted turbochargers, additional braking capability is possible.

Vehicle stability characteristics such as static roll stability and load transfer ratio can be improved by

reducing the center-of-mass height of the vehicle and by such vehicle design improvements as increasing

the tractor to 102 inches in overall vehicle width.

Advanced technology collision avoidance systems—measures and performance targets—are also areas of

activity where improvements can be expected.

Barriers

Size and weight regulations, both state and federal, can have a constraining effect on vehicle design and

configuration choices. For the purposes of this program, however, they need not be absolute barriers to

exploring new technological approaches if it can be demonstrated that employing them will enhance all

the program’s goals simultaneously and that the resulting vehicle will be reasonably sized for the highway

and traffic environments in which it will operate. Other barriers include brake compatibility issues,

tractor-trailer compatibility, material limitations, tire friction characteristics, systems reliability

constraints, and prohibitions on the use of engine brakes because of noise considerations.

Technical Approach

It will be necessary to examine the effectiveness of various safety-related initiatives with respect to such

matters as vehicle system effects and infrastructure impact. For example, smaller wheels and tires may

also improve stability by reducing the center-of-mass height and may decrease aerodynamic drag, but

they may cause a disproportionate increase in pavement wear. Such an initiative would also require

fundamental changes in foundation brake design.

• Disc brakes hold great promise in terms of brake torque and thermal capacity performance, but they

could be significantly enhanced by using technologically advanced materials in the discs and frictionpad materials.

• Electronically controlled brakes represent an essential enabling technology for monitoring brake

condition/status and for advanced systems to augment control of directional stability.

• Tire longitudinal traction performance must be improved if stopping performance is to be improved,

but it needs to be optimized relative to rolling resistance and wear.

• Static roll stability, load transfer ratio, and low-speed offtracking would all be substantially enhanced

by the use of an articulated trailer with axles at mid-length. Work would be needed to design such a

trailer to be capable of backing without jackknifing. Vehicle stability characteristics such as static roll

stability and load transfer ratio can be improved by focusing on reducing the center-of-mass height of

the vehicle and by such vehicle design improvements as increasing the tractor to 102 inches in overall

vehicle width and developing steerable trailer wheels to improve low-speed offtracking.

• On-board vehicle system status/condition monitoring, as well as driver performance monitoring and

warning systems, offer significant potential for reducing maintenance and operational costs and for

enhancing safety performance.
4-19

• An electrically assisted turbocharger could deliver more air to the engine, causing it to become a more

rigorous air pump and increase brake power absorption capability more than a variable-geometry

turbocharger can.

4.1.3.2 Crash Protection

Safety Issues and Status of Technology

Work to improve crash protection for truck occupants has been under way within the truck manufacturing

industry for approximately the last 10 years. That work includes improvements to occupant restraint

systems, rollover protection, and cab structural integrity. Progress in that area can be incorporated and

expanded upon in this program.

Until recently, activities to reduce the structural aggressivity of trucks in collisions with other vehicles

have been limited to the rear structures of trailers. The incorporation of aerodynamic shapes/designs in

tractor-trailers offers the possibility of making truck frontal and side structures complementary and

compatible with the increasingly advanced crash protection features/capabilities of passenger cars, light

trucks, and SUVs, thereby improving the likelihood that occupants of vehicles involved in collisions with

trucks will survive.

Technical Targets

Due to the subjective and complex nature of road vehicle safety, it is difficult to quantify anticipated

benefits in terms of percentage improvements. Professional opinion strongly supports systems approach

initiatives for improved safety. It is expected that an industry/government initiative in crash protection

will lead to substantial gains in safety for passenger car/truck conflicts.

Barriers

In addition to cost and operational practicability/durability considerations, the proposed technology must

not significantly increase vehicle tare weight and should not significantly increase vehicle length.

Technical Approach

Fundamental research will be required to fully understand the dynamics of collisions between smaller

vehicles and large trucks. Research to achieve improvements in protection and survivability would

consider various options for vehicle redirection and crash energy management:

• Development of advanced technology and materials for cab, frontal, and side structures.

• Development of computer methods to verify energy absorption capabilities for heavy-duty vehicles.

• Reduction of side collision aggressivity—performance similar to that of roadside guard rails; that is,

the ability to successfully redirect a 3,000-lb (1,365 kg) passenger car when it impacts the side of a

trailer at a 10 to 15° angle from the longitudinal centerline of the truck at 60 mph.

• Studies on front structure aggressivity—the ability to manage the kinetic energy of a 3,000-lb

(1,365 kg) passenger car impacting the front structure of the truck in a 50% offset collision, at a

closing speed of 60 mph, without exceeding existing car-occupant-trauma thresholds.

• Finite element and occupant kinematic analyses of candidate structural designs to identify optimized

designs quickly and inexpensively.

• Determination of the capacity of sandwich, cored, and foam materials for energy absorption

applications.

No safety performance measure would be reduced as a result of design changes related to energy savings.
4-20

4.2 TRANSIT BUS

Another segment of vehicles to be evaluated is the 40-ft transit bus used in most urban areas for public

transportation. These vehicles are predominately purchased by public or quasi-public agencies or

authorities. Public funds from either special taxes or general revenue taxes are used to support operation

of these services. The public nature of this transportation and the highly visible service it provides result

in a great demand for innovation in reducing emissions and improving passenger service. The capital cost

for new transit buses is predominantly funded (80%) by the federal government, which greatly

encourages new technology that can reduce operating costs, reduce emissions, or provide better customer

service. The number of transit buses is small (fewer than 100,000) compared with the number of Class 8

tractor-trailer vehicles; however, this segment by its nature works in very close proximity to the public. A

new transit bus may be in service 20 hours a day. The stop-and-go operation prevalent in many urban

passenger services leads to comparatively low fuel economy in operation.

The goal for 40-ft transit buses is to achieve a 3× improvement in fuel economy measured in miles per

gallon. Achieving this goal will require significant improvements in several technology areas, including

improved power-train efficiency, reduction in parasitic losses, and reduction in vehicle mass. Many

variables affect the fuel economy of transit buses. Factors such as traffic congestion, hilly terrain, and the

number of passengers have a major impact on fuel economy; therefore, it is difficult to describe a

“typical” drive cycle for a transit bus. One cycle that is frequently used is the central business district

(CBD) cycle. The CBD cycle consists of four segments: (1) a 10-second acceleration phase from

0 to 20 mph, (2) an 18.5-second cruise phase at 20 mph, (3) a 4.5-second deceleration phase from

20 to 0 mph, and (4) a 7-second phase at idle. This cycle is repeated seven times per mile traveled with a

total of fourteen repetitions for a 600 second test. The estimated distribution of energy loss in typical

operation of a transit bus over the CBD cycle is shown in Fig. 4.3.

The specific goals are to achieve an increase in fuel economy from 3 mpg to 6 mpg by 2004 and to lay the

foundation to achieve 9 mpg by 2010. Transit bus models were developed to use test data and industry

experience to estimate energy use over the CBD-14 driving schedule (see Table 4.3). An ADVISOR

systems analysis was subsequently performed to investigate pathways to reach the 3× fuel economy goal.

A preliminary ADVISOR analysis indicates that a 2.6× improvement in fuel economy can be realized

mainly through transit bus hybridization, weight reduction, and auxiliary load reduction (see Fig. 4.4).

Fig 4.3. 40-foot transit bus energy audit.

Total Energy and Fuel Economy over CBD-14 Drive Cycle

(at 1/2 Seated Load Weight, Air Conditioning On

Base = 3.20 mpg (1x), 24.16 kWh • Future = 8.22 mpg (2.6x), 9.37 kWh

Drivetrain Losses

Base = 0.87 kWh

Future = 0.26 kWh

Drivetrain Losses

Base = 0.87 kWh

Future = 0.26 kWh

Generator Losses

Base = NA

Future = 0.19 kWh

Generator Losses

Base = NA

Future = 0.19 kWh

Motor/Controller

Losses

Base = NA

Future = 0.36 kWh

Motor/Controller

Losses

Base = NA

Future = 0.36 kWh

Engine Losses

Base = 14.46 kWh

Future = 5.32 kWh

Engine Losses

Base = 14.46 kWh

Future = 5.32 kWh

Vehicle Weight

Base = 14515 kg

Future = 11158 kg

Vehicle Weight

Base = 14515 kg

Future = 11158 kg

Energy Storage

System Losses

Base = NA

Future = 0.43 kWh

Energy Storage

System Losses

Base = NA

Future = 0.43 kWh

Au xiliary Loads

Base = 6.06 kWh

Future = 1.26 kWh

Au xiliary Loads

Base = 6.06 kWh

Future = 1.26 kWh

Friction Braking

Base = 1.37 kWh

Future = 0.71 kWh

Friction Braking

Base = 1.37 kWh

Future = 0.71 kWh

Rolling Resistance

Base = 1.20 kWh

Future = 0.64 kWh

Rolling Resistance

Base = 1.20 kWh

Future = 0.64 kWh
4-21

Table 4.3. Energy audit and potential fuel efficiency

improvements for 40-ft transit busa

Energy loss sources Baselineb Targetc

Engine losses (kWh) 14.46 5.32

Auxiliary loads (kWh) 6.06 1.26

Drivetrain losses (kWh) 0.87 0.26

Generator losses (kWh) d 0.19

Energy storage system losses (kWh) d 0.43

Motor/controller losses (kWh) d 0.36

Friction braking (kWh) 1.37 0.71

Aerodynamic losses (kWh) 0.22 0.22

Rolling resistance losses (kWh) 1.20 0.64

Transit bus weight (kg) 14,515 11,158

Total energy used over CBD-14 (kWh) 24.16 9.37

Fuel consumption over CBD-14 (mpg) 3.20 8.22

Total fuel economy multiplier 1.0 2.6

 a

Over the CBD-14 drive cycle, operating at 1/2 seated load weight with A/C on.

 b

Conventional power train.

 c

Future series hybrid.

 d

Not applicable.

Fig. 4.4. Pathway for improving transit bus fuel economy.
4-22

Many other pathways are possible, and more analysis is needed to study design options, controls, and

complex system trade-offs.

As discussed in Sect. 4.1 for large trucks, systems analysis will be used to guide the R&D for transit

buses.

4.2.1 Vehicle Efficiency

Transit bus power requirements can be broken into four categories: power required to accelerate the

vehicle to speed (a function of weight), power required to operate auxiliary systems (“hotel load”), power

required to overcome aerodynamic drag, and power required to overcome rolling resistance (a function of

weight) and drivetrain losses. Meeting the efficiency-improvement goals will require optimized vehicle,

auxiliary, and energy-storage systems that are advanced beyond those currently employed on today’s

hybrid electric vehicles (HEVs). Consequently, the twin pillars of lightweight vehicle structure and hybrid

electric propulsion will form the foundation on which to build the technology base for transit buses of the

future. An opportunity exists to improve the efficiency of transit buses because little improvement in

power-train design or efficiency has occurred in the past decade. To increase overall propulsion efficiency

and reduce emissions, engine transient operating conditions must be minimized. To accomplish this,

engine speed and load must be independent of drive-wheel speed and required tractive effort. By

optimizing vehicle design, together with power-train efficiency improvements, the transit bus can be

significantly improved in overall operational efficiency. Hybrid electric or mechanical hybrid propulsion

provide a practical means of decoupling engine power demand from the drive wheel power required. For

a detailed discussion of hybrid technology, see Sects. 4.6.4 and 4.6.5.

4.2.1.1 Power-Train Efficiency

Hybrid electric propulsion functions comprise a critical path to achieve major improvements in efficiency

and systems operation. Within a series configuration, in addition to decoupling the engine from the drive

wheels, auxiliary systems can also be decoupled from the engine, adding to overall efficiency. Within a

parallel configuration, the electric motor supplements the engine to provide a broader, high-efficiency

operating zone and to provide the possibility to optimize the system globally. Power-split configurations

similar to the Toyota Prius system should also be taken into account. The electric traction motor is also

designed to function as a generator during deceleration, converting the vehicle’s kinetic energy into

electrical energy and slowing the vehicle without using friction brakes. Hybrid electric power trains also

make it possible to use other energy-conversion devices, such as gas turbines or fuel cells.

The data in Table 4.4 are preliminary performance requirements that can in turn can be used to identify

the propulsion system power, torque, and energy-storage requirements for the 21st Century Truck

Program transit bus. Many of the requirements were either taken directly from the APTA guidelines

(APTA 2000b) (environment, acceleration, and braking) or are modified APTA requirements (top speed

and gradeability). Others were derived from existing 21st Century Truck Program simulation data/goals

and/or reasonable estimates of performance conditions.

4.2.1.2 Parasitic Losses

Parasitic losses include all power requirements on the bus, including aerodynamic resistance, rolling

resistance, driveline losses, and accessory loads.

Aerodynamic Resistance

The urban driving cycle consists of stop-and-go operation with a maximum vehicle speed of 20 mph. This

reality, as well as the need to maintain interior space, is reflected in the boxy shape of conventional transit

buses. Although aerodynamic losses may be minimal in the project driving cycle, the thermal loads of
4-23

Table 4.4. Preliminary performance requirements for the 21st Century Truck Program (Class 7) transit bus

Environment, normal operationa

 Ambient temperature range, °F

 Relative humidity, %

 Altitude, ft above sea level

–10 to 115

5 to 100

Up to 3,000

Weights

 Max estimated GVWR, lb (kg)

 Min capacity, lb (kg)

 Max estimated CW, lb (kg)

33,000 (14,966)

11,000 to 12,000 (4,989 to 5,442)

20,000 to 22,000 (9,070 to 9,977)

Top Speed @ GVWR, 0 % grade, full accessoriesb,c

 Intermittent, mph

 Continuous, mph

65 for 600 seconds (10.8 miles)

55

Acceleration @ GVWR, 0% grade, full accessories,b,c

mph

 5 seconds

 10.8 seconds

 20 seconds

 31 seconds

0–10

0–20

0–30

0–40

Gradeability @ GVWR, full accessoriesb,c Intermittent

 40 mph on 2.5% grade for 270 seconds (3.0 miles)

 7 mph on 16% grade for 260 seconds (0.5 miles)

Continuous

 35 mph on 2.5% grade

 7 mph on 16% grade

Startability @ GVWR, full accessoriesb,c 20% grade

Estimated accessory load, kW

 Max

 Min

 High avg

 Low avg

35–40

2–3

10–18

3–10

Braking 16% stops at 3 ft/second2

50% stops at 6 ft/second2

26% stops at 9 ft/second2

8% stops at 12 ft/second2

 aSpeed, gradeability, and acceleration performance will be met at, or corrected to, 77°F, 29.31 in. Hg, dry air.

 b

Components included in the “full accessories” condition need to be determined for each operating scenario.

 c

Initial condition. Performance assumes energy storage @ 75% of useable energy.

new technology are becoming an increasing concern. Some aerodynamic modeling to improve airflow

and air quality to the cooling systems will improve the commercial viability of these technologies.

Rolling Resistance

Rolling resistance is a function of axle, wheel, tire and drive shaft mass, together with sealing and

lubricating these components. Special attention must be paid to the force necessary to make the bus move.

For example, this may require research on complete brake shoe release after releasing the brake pedal.

Developing seals that are durable but do not leak and do not contribute to the bearing torque would also

be desirable. Power requirements attributable to rolling resistance can also be further reduced by the use

of radial tires and “super-single” tires in place of the rear duals. There has already been a major shift

toward use of radial tires instead of bias-ply tires in transit buses and trucks. The super single offers less

rolling resistance than duals. It is an available technology offering fuel savings of a few percent. Among

the concerns with the super single is the lack of redundancy in the event of a failure. The super singles are
4-24

also taller than other radials, thus reducing the interior volume of the bus and passenger capacity. They

were first used on experimental buses.

Driveline Losses

Driveline losses in a series hybrid configuration are mostly electrical and result in component heating.

Many of the new engine emission reduction strategies such as EGR will require more heat rejection.

Typically, the overall heat rejection through the radiator and the charge air cooler is about

10,000 Btu/min. The engine manufacturers are forecasting a 25 to 30% increase in heat rejection to the

coolant and a 10 to 13% increase in the charge air cooler.

Accessory Loads

Accessory power requirements contribute significantly to overall energy usage of buses. The actual power

requirement is in the 30- to 40-kW range for a heavy-duty, 40-ft transit bus, depending on which

auxiliaries are in use; however, it is a constant draw on the propulsion system, consuming substantial total

energy. In some urban duty operations, energy for accessory load can surpass energy required for

propulsion over the course of a day. One analysis indicated that for a Class 7 vehicle, an additional 5-kW

accessory load is equivalent to a 20% decrease in propulsion efficiency. This accessory load may increase

when engine-driven pumps and fans are electrically driven.

On conventional transit vehicles the engine directly drives all major vehicle auxiliary systems, generally

through mechanical belt drives or through a power take-off. Consequently, the functionality, design,

sizing, and efficiency of the auxiliary systems are directly dependent on the operating speed of the engine.

Auxiliary systems must be able to operate over the relative speed range of the engine, from idle to redline.

The widely variable speed of operation means that auxiliary systems (pumps, compressors, alternators,

and fans) must be designed to perform adequately at all operating speeds with the common design point

being engine idle. This demand has created an operating mode of high engine idle that is used by drivers

when in heavy traffic or idling at the curb for extended periods. This mode improves bus compartment

cooling and heating and may be necessary to meet the air pressure recovery times in the brake system.

Low engine speed operation forces designers to make compromises, resulting in larger, heavier, and less

efficient components compared to operation at optimum speed, discrete speeds, or zero speed if they are

not needed.

Engine-driven accessories have remained essentially unchanged for several decades. Air compressors and

refrigerant compressors, which are of a reciprocating type, are large and heavy, and have poor noise and

vibration characteristics. None of these attributes is considered desirable by the public transit community.

Power steering is provided by an engine-driven hydraulic pump with the pressurized fluid routed to the

power steering unit in the front of the vehicle and then returned to the pump. Hydraulic systems are

generally inefficient and prone to leakage.

Technical targets. To consider achieving the efficiency goal of 3× current fuel economy, a hybrid

propulsion system must be used. Both mechanical and electrical hybrids are being considered, the electric

hybrid being the predominant choice. With an electric hybrid, all accessories can be driven electrically—

power steering pump, air compressor, battery cooling fans, electronic cooling pumps and fans, and

traction motor cooling pumps and fans. All are constantly energized and operating, with the exception of

the air-conditioning system, which is demand-responsive.

Electric-accessory drives afford designers the capability to develop optimally efficient designs for

auxiliary components and the flexibility to mount and package the systems away from the engine, in less

hostile environments, and in locations that provide easy access for maintenance. Electric drive also

provides the capability to operate the accessories independently of the engine in a demand-responsive

mode, which saves considerable energy. Typical accessory technical targets are discussed below. In
4-25

addition, a typical target would be to reduce hotel energy load by 25% through the use of optimized

electrically driven auxiliaries.

Fans and pumps. The propulsion system already monitors temperatures in traction batteries, power

controls for traction, the auxiliary power system, and the traction motor. These temperature data would be

used to provide control inputs to solid-state relays that would switch fans “on” or “off” as cooling demand

increases or decreases, saving approximately 2 kW of power and lowering noise emissions. Demandresponsive variable-speed control of the electronics-coolant pump would save an additional 1 kW.

Air compressor. The existing constant-operation, cast-iron reciprocating air compressor would be

replaced with a lightweight, quiet, low-vibration rotary-vane air compressor that is 25% more efficient

when working. The compressor would also be operated with a demand-responsive on/off control, saving

an additional 0.75 kW over the reciprocating device when not working.

Power steering. The existing constant-operation power-steering pump would be replaced with an

electric-assist demand-responsive unit. This type of unit is available for light automotive applications, but

no such unit currently exists for heavy-duty vehicles. This could be due to the fact that the electrical

power requirements for such a device cannot be met by conventional vehicle electrical systems. Two

companies currently produce automotive electric-assist steering and would be likely candidates for the

development of heavy-duty units. The working power savings of this electric-assist unit are unknown at

this time. However, when the unit is performing no work (i.e., no steering input when the vehicle is

driving straight or at idle), the power savings is in excess of 0.90 kW.

Accessory drive motors. Individual direct-drive motors sized for each accessory’s power

requirement would mitigate low-load efficiency effects experienced with the large single motors currently

used.

4.2.1.3 Mass Reduction

Power required to accelerate a vehicle and overcome rolling resistance is directly proportional to vehicle

weight. The power required for acceleration and gradeability decreases commensurately with decreasing

weight, thus reducing fuel consumption and emissions. This relationship between weight and propulsion

power is of major significance in increasing fuel efficiency from the current 3 mpg to 6 mpg. However, in

the past, far too little emphasis has been placed on vehicle weight control for such reasons as policies and

regulations, available funding and incentives, institutional reluctance, a risk-averse industry, and the

heavy vehicle industry’s limited knowledge and experience in alternative construction and materials. The

goal is to reduce the gross vehicle weight rating (GVWR) of an existing 40-ft urban transit bus by about

20% from 32,000 lb (14,512 kg) to the target weight of 24,600 lb (11,156 kg). This weight reduction

alone will result in at least a 20% improvement in fuel efficiency at gross weight.

4.2.2 Emissions

Emission requirements for automobiles are determined on a gram per mile (g/mi) or “vehicle” basis while

transit bus and truck emissions are determined on a gram per brake horsepower-hour (g/bhp-h) or

“component” basis. See Sect. 4.1.2 for more detailed information on engine emissions regulations.

The currently used component protocol does not account for significant emission benefits that are

achievable through the implementation of advanced vehicle systems such as hybrid electric propulsion

and lightweight vehicle structures and components. To ensure credit for the real-world emission

reductions provided by advanced vehicle systems, an administrative change in the EPA certification

procedure may be required to permit the emissions measurements to be vehicle-based, rather than enginebased. Technical targets for transit bus emissions are provided in Table 4.5. DOT, under its Advanced

Vehicle Program and in conjunction with the Northeast Advanced Vehicle Consortium, has initiated an

effort to address the issue of certification of hybrid electric transit buses. A working group comprising
4-26

Table 4.5. Summary of technical targets and barriers for transit bus emissions

Goals Technical targets Barriers

Establish EPA hybrid

emissions protocol

Develop hybrid engine duty cycle for

EPA emissions certification of hybrid

vehicle engines.

Consensus among the

industry and EPA on dutycycle. Institutional

bureaucracy

Demonstrate NOx and

PM aftertreatment w/

fuel reformulation

EPA proposal: max of 0.2 g/bhp-h NOx

and 0.01 g/bhp-h particulate matter in

2007

CARB Public Transit Bus Fleet Rule and

Emissions Standards for New Urban

Buses

Cost, durability, of lean

combustion NOx catalyst/

PM filter. Cost, lubricity,

availability of reformulated

fuels

government regulators, including EPA and the California Air Resource Board (CARB), and industry

participants, has agreed on an interim engine test cycle for engines used for hybrid transit buses. The

working group will continue to address the issue of a specific hybrid engine test cycle for certification and

a chassis test procedure as well.

4.2.3 Bus-Related Safety

The greatest risk involved with the operation of transit buses is hitting pedestrians. Technology to assist

the driver in avoiding pedestrians will be considered in the program. Fatality rates and injury rates for

drivers and passengers are extremely low for transit buses because buses normally travel at low speeds.

Efforts to improve crash avoidance and crash protection will likely be given low priority.

Safety issues concerning the use of alternative fuels, particularly those related to pressure vessels for

compressed natural gas, must be considered.

The introduction of new technology, such as hybrid electric power trains, brings new hazards that must be

addressed. Safety issues related to hybrid electric power trains are discussed in Sect. 4.6.4.4.

4.3 MEDIUM TRUCK—ENCLOSED, SINGLE-AXLE DELIVERY TRUCK

The medium truck category consists of vehicles whose size is larger than a pickup truck but significantly

smaller than a tractor-trailer type rig. With load-carrying capability roughly proportional to size and

number of axles, the medium class has more capability than a pickup truck but considerably less

capability than a tractor-trailer.

As the large tractor-trailer class is the prime choice in the interstate movement of product, the medium

truck class vehicle is the best fit for the pickup and delivery of goods in the urban areas. Its medium size

makes it maneuverable in congested areas, and it has significantly more load-carrying capacity than a

pickup truck or van. In addition to pickup and delivery service, the medium truck also serves as the prime

building platform for vocational trucks such as utility and crane trucks, beverage trucks, fuel haulers, oil

field rigs, dump and refuse trucks, and even school buses. Also, the medium truck is usually the base

platform for military truck applications of the 2.5-ton and select 5-ton classifications.

For the 21st Century Truck Program, the specific vocation being considered for the medium truck is the

common dry van as pictured in Fig. 4.5. Truck classification terminology defines this specific product as

a Class 6 vehicle with gross weight carrying capability of 19,000 to 26,000 lb (8,617 to 11,791 kg). The

rationale for selecting this group of vehicles for the 21st Century Truck Program is discussed in Sect. 3.1.
4-27

The goals of the 21st Century Truck Program with respect to medium trucks are to develop by 2010,

enabling technology for medium-size delivery trucks that will result in an increased fuel efficiency

approaching a factor of three over a typical drive cycle, meet prevailing emission standards while using

petroleum-based diesel fuel, and simultaneously improve their safety. Achieving these goals will require

significant improvements in several technology areas, including improvement of power-train efficiency,

reduction in parasitic losses, and reduction in vehicle mass. As discussed for large trucks (see Sect. 4.1),

systems analyses will be conducted to guide the R&D for medium trucks.

Many variables affect the fuel economy of delivery trucks. For example, factors such as traffic

congestion, hilly terrain, and variations in loads (both among vehicles and for a given vehicle during the

course of the day) have a major impact on fuel economy; therefore, it is difficult to describe a “typical”

drive cycle for a delivery van. However, for establishing a baseline fuel economy for medium trucks, a

driving and use cycle often used by manufactures and operators of medium-sized trucks was used. The

estimated distribution of energy losses for such operations is discussed in Sect. 4.3.1.

4.3.1 Vehicle Efficiency

4.3.1.1 Truck Description

Medium Truck Fuel Usage

The Class 6 medium truck group is the second largest consumer of diesel fuel in the United States

according to the VIUS data base, behind only the Class 8 group. It is for this reason that the Class 6

vehicle was chosen as one of the target groups for the 21st Century Truck Program.

Bureau of Census VIUS information shows that 1.75 million Class 6 medium type trucks were registered

in the United States as of 1997; this number is conservative with respect to today, considering that the

truck business has seen close to record sales for the last 3 years in a growing national economy. Given the

popularity of these medium models and the recent growth trends, total registrations for Class 6 medium

trucks are estimated currently to be more than 2 million units. Total quantity of diesel fuel consumed in

1997 for the Class 6 national fleet was approximately 3.5 billion gallons. Factoring in growth trends, the

fuel usage figures as of this writing are even higher.

Fig. 4.5. Common dry van.
4-28

The VIUS data base shows the average fuel consumption for the entire national Class 6 fleet to be

7.2 mpg. The range of data is broad and extends from 6.6 mpg for the off-highway segment to 12.8 mpg

for the very light load segment. For the purpose of this analysis, the baseline figure is 7.2 mpg; this value

agrees closely with computer model results for vehicles with typical medium truck missions and

specifications.

Typical Mission of the Medium Truck

Understanding the mission of the typical medium truck is important in determining the key parameters for

setting the roadmap to the future. To accomplish this, a computer model was used to simulate the mission

of a typical medium truck; experience has shown this model to be very accurate because the mission

profiles in the system are based on real-world data from actual truck users. All truck and engine

manufacturers regularly use these types of models; they are considered state-of-the-art analysis tools for

designers and engineers. Analysis of the model information, combined with the wealth of practical

experience and expertise that truck manufacturers have with this vehicle segment, provides an accurate

picture of the operation of the typical Class 6 medium truck in urban pickup and delivery service.

Mission information from the computer model for a typical Class 6 vehicle under different driving cycles

is shown in Table 4.6.

Table 4.6. Mission information for typical Class 6 vehicles

City Suburb Highway

Mission time (min) 30 52 173

Fuel use (mpg) 6.7 8.7 8.5

Average speed (mph) 18.6 39.5 54.6

Load factor on engine (%) 24.2 39.8 56.3

Average hp for mission 47 78 110

Specifications of the Typical Medium Truck

The hardware specifications for medium trucks vary considerably and depend on the specific vocation,

geographic location, and customer preferences. Generally speaking, all or most Class 6 vehicles are

diesel-engine powered, with horsepower ratings ranging from 175 to 250 hp. Transmission usage is

equally split between manual and automatic, but the clear market trend is toward the automatic. Empty

weight for the Class 6 medium truck is typically 12,000 lb (5,442 kg), which means that the maximum

payload for this class is 14,000 lb (6,349 kg). Middle-of-the-road specifications for the Class 6 dry van

target application is given in the following list:

• Class 6 dry van with 19,000 to 26,000 lb (8,617 to 11,791 kg) GVW;

• power train typically includes six- to eight-cylinder diesel engine;

• both manual and automatic transmissions are common; automatic usage is rising;

• single rear axle;

• city pickup and delivery, stop and go driving;

• transient engine operation;

• typical 25 mph average speed; 30,000 miles/year;

• empty weight ~12,000 lb (5,442 kg); and

• average mpg = 7.2 (from 1997 VIUS).
4-29

Energy Usage for Medium Trucks

Medium truck fuel efficiency is influenced by several factors, including basic vehicle design, mode of

operation, driver technique, and weather factors. Figure 4.6 and Table 4.7 contain an energy audit, a

breakdown of energy requirements, of a typical Class 6 delivery truck. For first-order considerations, the

vehicle is operating at a steady speed of 40 mph, typical of the average speed in a suburban environment,

with a GVW of 26,000 lb (11,791 kg). A comparison of baseline fuel economy for a steady 40 mph

(10 mpg) with the VIUS data (7.2 mpg) representing actual usage shows that the stop-and-go driving

typical of the medium truck’s normal pickup-and-delivery cycle is much less fuel efficient.

The medium delivery truck presents an opportunity for significant improvement in efficiency. To increase

overall propulsion efficiency and reduce emissions, engine transient operating conditions must be

minimized. To accomplish this, engine speed and load must be independent of drive-wheel speed and

required tractive effort. By optimizing vehicle design, together with improvements in power-train

efficiency, the delivery truck can be significantly improved in overall operational efficiency. Given the

duty cycle typical for these vehicles, hybrid electric or mechanical hybrid propulsion is a practical means

of decoupling engine power demand from the drive-wheel power required and should yield significant

increases in vehicle efficiency. For a detailed discussion of hybrid technology, see Sects. 4.6.4 and 4.6.5.

Engine losses, inertial resistance, and tire-rolling resistance, have significant effects on vehicle efficiency,

whereas drivetrain friction is less significant. Aerodynamics, a major factor for line-haul trucks, plays

only a minor role for delivery trucks because they have much slower operating speeds. However, relative

energy usage for engine-based accessories, such as compressors and alternators, can be a major factor for

medium truck operations. For low-power operations, such as the city missions (see Table 4.6), the

auxiliary loads (ranging up to 15 hp) can represent a fairly large percentage of overall mission

Auxiliary Loads

Base = 7.5 kWh

Target = 3.8 kWh

Drivetrain Losses

Base = 3.0 kWh

Target = 1.9 kWh

Friction Braking

Base = Tbd kWh

Target = Tbd kWh

Aerodynamic Losses

Base = 15.7 kWh

Target = 14.3 kWh

Engine Losses

Base = 71.8 kWh

Target = 64.6 kWh

Rolling Resistance

Losses

Base = 9.7 kWh

Target = 6.7 kWh

Empty Vehicle

Base = 5,450 kg

Target = 3,360 kg

Hybrid Powertrain

Generator Losses*

Energy Storage System

Motor/Controller Losses*

*Individual Losses For Hybrid

System To Be Determined.

Total Benefit On Fuel Efficiency

Estimated To Be 30–50%.

Medium Truck Total Energy and Fuel Economy

Base = 107.7 kWh, 10.0 mpg

Target = 55.2 kWh, 24.0 mpg

Fig. 4.6. Typical medium truck—Class 6 enclosed van; distribution of energy requirements at 40 mph

for a 26,000 lb (11,791 kg) vehicle.
4-30

Table 4.7. Typical medium truck—Class 6 enclosed van distribution of energy requirements

for a fully loaded 26,000 lb (11,791 kg) enclosed van at 40 mph

Baseline Improvement (%) Target

Engine losses (kWh) 71.7 10 64.6

Auxiliary loads (kWh) 7.5 50 3.8

Drivetrain losses (kWh) 3.0 37 1.9

Aerodynamic losses (kWh) 15.7 9 14.3

Rolling resistance losses (kWh) 9.7 31 6.7

Hybrid power traina TBD TBD TBD

Total energy used (kWh) 107.7 48.7 55.2

Fuel consumption at 40 mph (mpg) 10.0 19.5

Fuel economy multiplier 1.0 1.95

Empty vehicle weight (kg) 5,450.0 28 3,600.0

Effect of weight reduction on fuel economy 1.0 1.22

Total fuel economy multiplier 1.0 2.4

 a

Individual losses for hybrid system to be determined; benefit of hybrid power train on fuel efficiency

estimated to be 30 to 50%.

requirements. It follows, therefore, that improvements in engine efficiency, weight reduction, tire-rolling

resistance, and accessory efficiency can have a significant impact on medium truck fuel efficiency while

improvements in its drivetrain and aerodynamics will have a smaller influence. Nevertheless, any

improvement in efficiency should be actively pursued if the cost-to-benefit relationship is favorable.

4.3.1.2 Power-Train Efficiency

Analysis of Operation, Typical Class 6 Medium Truck

Mission information for the typical urban/suburban cycle for the Class 6 medium truck shows that only

50 to 80 hp, on the average, is required from the engine for the vehicle to perform the mission. Actually,

more horsepower is required to cover the peak demands of the vehicle, such as when accelerating or

passing, but on the average, the power requirements are quite low relative to what is required to sustain

speed for continuous highway operation.

A comparison of the average power requirements for the city cycle to the friction horsepower for the

typical medium truck diesel engine (Fig. 4.7) shows that roughly the same horsepower is required to

perform its mission (47 hp) as is required to overcome the engine’s friction at 2,000 to 2,500 rpm (typical

for city driving). This means that half the fuel burned in a typical city duty cycle is used simply to

overcome the engine’s friction. This suggests that from a fuel economy standpoint, the engines in today’s

current medium trucks, which are sized to provide the maximum power required only occasionally, are

too large relative to their typical mission, thereby wasting a lot of fuel just in overcoming the engine’s

friction during normal operation.
4-31

For a delivery truck on an urban cycle, torque—not horsepower—is what is really required to accelerate

the mass of the vehicle to perform the mission. Today, this torque is produced primarily by the engine; in

the future, to save fuel by downsizing the engine, the torque must come from something other than a large

engine.

It is anticipated that a hybrid power system will be required to simultaneously reduce fuel economy and

provide high power and torque when needed. Some of these needs may be met by using a CVT to

maximize torque with either an optimized diesel engine or full hybrid power system. The engine can then

be optimized to operate at a very specific speed and load point for best fuel economy and emissions.

Given the typical mission for the medium delivery truck, it is anticipated that a 3- to 4-L diesel will be

sufficient as a source of primary power in a hybrid system. It may be possible to reduce the size and

weight of the engine even further while boosting its torque and overall efficiency by advanced

turbocharging that utilizes electrically assisted, high-pressure ratio, wide-flow-range systems.

The engine can be decoupled from the drive wheels in hybrid systems; in addition, auxiliary systems can

be decoupled from the engine, permitting only on-demand usage and adding to overall efficiency.

Moreover, the electric traction motor in a hybrid system is also designed to function as a generator during

deceleration, thus converting a portion of the vehicle’s kinetic energy back into electrical energy, and

slowing the vehicle in conjunction with downsized friction brakes. Typical propulsion power

requirements that any new power train would have to provide for a medium truck under varying

conditions are shown in Table 4.8.

Further information on barriers to and technical approaches for optimization of the engine, hybrid drive

trains, and auxiliary power sections can be found in sections on internal combustion engine (Sect. 4.6.2),

hybrid electric power trains (Sect. 4.6.4), mechanical hybrids (Sect. 4.6.5), and auxiliary power

(Sect. 4.6.7).

4.3.1.3 Vehicle-Related Losses

Vehicle-related losses include aerodynamic resistance losses, rolling resistance losses, driveline losses,

and accessory losses.

 Frictional horsepower comparison

Oil temperature 200°F

0

10

20

30

40

50

60

70

80

500 1000 1500 2000 2500 3000 3500

crank speed (rpm)

 current

 future

Fig. 4.7. Friction power requirements of typical

medium truck engine as a function of rpm.
4-32

Table 4.8. Medium truck power requirements

[Class 6, 26,000 lb (11,791 kg) max GVW]

Power requirement (hp) Speed (mph)

100% load 80% load

Steady speed

30 28 25

40 51 47

50 85 80

60 133 127

Maximum horsepower for reserve, passing,

grades, etc.

185 175

Aerodynamic Resistance

Mission analysis also tells us that for urban pickup and delivery, the average vehicle speed is less than

20 mph. This suggests that improvements to aerodynamics (drag) will not improve fuel economy much as

compared to the Class 8 highway application that operates at high vehicle speeds for long periods of time.

Instead, improvements should focus on reduction of rolling resistance (tires).

Rolling Resistance

Because rolling resistance is a major contributor to parasitic truck losses even at low speeds, it is an

important factor to address for medium trucks. Further information on barriers to and technical

approaches for minimizing rolling resistance are described in Sects. 4.1.1.2 for large trucks and 4.2.1.2 for

buses. The technology developed to reduce rolling resistance in large trucks and buses will be applicable

to the medium trucks.

Driveline Losses

The issues concerning driveline losses for the medium truck are very similar to those for the large tractor

trailer as modified by concerns for inclusion of hybrid power systems. These issues are described for the

line-haul and bus platforms in Sects. 4.1.1.2 and 4.2.1.2 and in the hybrid electric power-train technology

crosscutting section (Sect. 4.6.4).

Accessory Loads

Accessory loads common to medium trucks include primarily engine-based components such as the

alternator, air compressor, air-conditioning compressor, hydraulic pump, engine-oil and fuel pumps, and

cab-related loads for heating and cooling. The maximum accessory power requirements for medium

trucks typically range from 12 to 15 hp. At highway speeds, the accessory load for a medium deliverytype truck accounts for about 7% of fuel consumed by the vehicle, but in typical in-city use, consumption

can climb beyond 30%.

Approaches to reducing these accessory loads involve decoupling accessories from dependence on direct

engine speed and driving them electrically at variable speeds most conducive to energy-use reduction.

The approaches described for the line-haul trucks (Sect. 4.1.1.2) and buses (Sect. 4.2.1.2) and in the

auxiliary power technology crosscutting section (Sect. 4.6.7), will be applicable to medium trucks.
4-33

Mass Reduction

Overall weight reduction is a must for medium trucks, but neither structural integrity nor vehicle

affordability can be compromised. Reduction of weight will improve fuel economy by increasing payload

capability and by improving both ton-miles and mpg under empty and part-load conditions. The benefit of

increased payload efficiency does not occur for loads that occupy maximum volumetric capacity of the

vehicle before the maximum allowable weight is reached. In this case, only supplementary benefits are

realized. Given that a significant portion of payload transported by medium trucks is limited by volume,

particularly during multiple-stop delivery runs, estimates of the benefits of reduced tare weight need to

reflect this reality. The goal is to reduce the unloaded weight of a typical medium truck delivery van by

33% from 12,000 lb (5,442 kg), to the target weight of 8,000 lb (3,628 kg). This weight reduction alone

will result in approximately a 30% improvement in fuel efficiency.

The approaches to reducing vehicle mass along with their barriers and potential solutions, described for

the line-haul trucks (Sect. 4.1.1.3) and buses (Sect. 4.2.1.3), and in the materials technology crosscutting

section (Sect. 4.6.9), will be applicable to reducing vehicle mass in medium trucks.

4.3.2 Emissions

Emission reduction concerns, requirements, and approaches for solutions in medium trucks are very

similar to those for line-haul trucks. See Sects. 4.1.2 and 4.6.3 for discussion of this topic.

4.3.3 Truck-Related Safety (Medium Trucks)

Compared to the number for Class 8 heavy-duty trucks, the number of people killed each year in crashes

involving medium-duty single-unit trucks is fairly small (about 300 for Classes 5 and 6 combined). This

is primarily due to the fact that these trucks typically operate in a lower-speed urban, daylight setting.

About 20% of those fatalities are occupants of the truck, 70% are occupants of other vehicles involved in

the same crashes, and 10% are nonoccupants. Even though the operational use patterns of this platform

differ from that of the tractor-trailer platform, the crash avoidance safety issues are similar. The primary

focus should be on braking, visibility, and rollover. This sector of the market is usually not large enough

to support the development of separate safety technologies; however, the improvements made in light

vehicles and heavy trucks will also benefit medium-duty trucks. In braking, this category of trucks uses

mostly hydraulic brakes, but some of the heavier ones use air or air-over-hydraulic brakes. Although discbrake technologies exist for this platform, they are seldom used because of their cost. Usage for these

trucks is more urban and at slower speeds than for tractor-trailers, so aerodynamic braking would

probably not be worthwhile. However, electric and hybrid power plants will allow regenerative braking to

decrease the burden on the foundation brakes. The potential for additional electrical hazards associated

with hybrid electric power trains will exist for those medium trucks with such systems. A discussion of

the electrical safety issues associated with hybrid electric power trains can be found in Sect. 4.6.4.4.

As discussed in Sect. 4.1.3 for large trucks, the primary crash survivability emphasis will be to improve

the likelihood that occupants of other vehicles involved in collisions with the trucks will survive. Many of

the same technologies and approaches used for heavier trucks could also be applied. Because the mass

ratio between the truck and other smaller vehicles will not be quite as great, there is a better chance of

success. Although truck occupant fatalities for this type of truck are very low, truck occupant

crashworthiness improvements achieved on the tractor trailer platform could be applied here because the

chassis and cabs are often the same or similar. In addition to cost and operational practicability/durability

considerations, weight as well as vehicle length increases need to be either minimized or counterbalanced

to offset cargo capacity losses.
4-34

4.3.4 Summary of Potential Efficiency Benefits

To meet the aggressive 21st Century Truck Program fuel economy targets set forth by the Committee,

major changes are required in the Class 6 medium truck as we know it today. To make these changes

possible, the industry must commit to significant paradigm changes in the major systems of the vehicle,

and then implement these changes via new and breakthrough technologies. By system, these high-level

changes are as follows:

• The current power train must be replaced by a hybrid system employing a smaller engine

supplemented by an electric motor and/or a new type of transmission with continuously variable ratio.

• Empty chassis weight must be reduced significantly to improve payload and reduce power

requirements during part-load and empty operation; system structural integrity cannot be

compromised.

• Vehicle rolling resistance must be improved to reduce system power requirements; this requires

breakthroughs in tire technology.

• Engine emissions must be reduced using aftertreatment and very low sulfur content fuel. Engine

brake-specific fuel consumption (BSFC) and emissions must be optimized at or near constant speed

consistent with hybrid drive.

Baseline values and targets for improvements of aspects of medium trucks related to fuel efficiency and

emissions for medium trucks on realistic operating cycles are summarized in Table 4.9. The effects of

those target changes on vehicle fuel efficiency are shown in Table 4.10.

Table 4.9. Design metrics for current and improved Class 6 medium trucks

Current Future target

Fuel consumption (mpg) 7.2 typical 17.0 (corrected to 21.6 with payload)

Weight, unloaded, lb (kg) 12,000 (5,442) 8,000 (3,628)

Engine displacement (l) 6.0 to 8.0 3.0 to 4.0 with hybrid drive

Engine weight, lb (kg) 1,000–1,300 (454–590) 600 max (272)

Aero drag coefficient (Cd) 0.6 0.5 or better

Rolling resistance, lb (kg) 4.9/1,000 (2.2/454) avg 3.0/1,000 (1.4/454)

Overall driveline eff (%) 90 95

BSFC, minimum 0.340 <0.300

Engine emissions (g/hp-h)

 NOx <4.0 2.0 g max engine out w/90% aftertreat

 PM <0.1 95% aftertreat filtration; <15 ppm sulfur fuel

 HC <1.3

Table 4.10. Potential for fuel-efficiency improvements for medium trucks

Change Reduction in energy usage

(%)

Fuel efficiency

multiplier

Engine improvements 10 1.10

Reductions in aerodynamic, rolling resistance,

drivetrain, and auxiliary load losses

26 1.26

Hybrid power train 30–50 1.40

Mass reduction and payload improvements 28 1.22

Total 2.4
4-35

4.4 SMALL TRUCK—“WORKING” PICKUP (>8500 lb GVW)

For the purposes of the 21st Century Truck Program the Small Truck platform is defined as Class 2b

vehicles used in commercial applications and does not include personal-use vehicles. In this vehicle class,

the predominantly produced vehicle is the common gasoline-powered pickup truck that is utilized as a

small business work truck. Also included in this class are trucks (e.g., panel vans, pickup truck cabchassis, cutaways) that would have third-party, task-specific bodies installed. Although diesel engines are

available, they represent less than one-quarter of total production.

The original stated goal for the small truck platform is to achieve a 3× improvement in fuel economy by

the year 2010. The ability to achieve this aggressive goal based upon available or known technology has

not been demonstrated. It is important to note that overall vehicle performance requirements must not be

sacrificed because doing so would adversely affect the vehicle’s marketability. To meet this aggressive

goal in a cost-effective way will require major scientific breakthroughs in several technology areas,

including improved power-train efficiency, reduction in parasitic losses, and reduction in vehicle mass

that are not known at this time. The proposed 21st Century Truck small truck research program may bring

about the necessary breakthrough technologies.

Many variables can affect the fuel economy of small trucks. Factors such as overall basic vehicle

performance requirements, trailer-towing capability, and off-road capability have major impacts on basic

vehicle design and therefore significantly influence the vehicle’s overall potential fuel economy.

Currently, no fuel-economy requirements are mandated for Class 2b vehicles, and emissions certification

is done by engine dynamometer test utilizing the Federal Heavy Duty cycle. This emissions certification

requirement is expected to stay the same for Class 2b diesel-powered trucks within the 21st Century Truck

time frame. For the purpose of this roadmap, the Federal Urban Driving Schedule (FUDS) and the

Highway Fuel Economy Test (HWFET) on a chassis dynamometer were used to compute fuel economy.

When subjected to this test regimen via modeling, today’s typical gasoline-powered Class 2b truck would

achieve approximately 12.1 mpg city, 15.4 mpg highway, and 13.6 mpg combined.

The specific technical targets for the small truck platform are expected to achieve an increase in combined

fuel economy from 13.6 mpg to 20.4 mpg by 2007 (1.5×), to achieve an increase in fuel economy to

26.0 mpg by 2010 (1.9×) and to lay the foundation to achieve a long-term-stretch goal of 30.0 mpg. Based

on a parallel hybrid power train, the projected aggressive improvements in the various technology areas

required to approach the 2010 (1.9×)fuel economy goal are listed in Table 4.11. (All individual

improvements are not necessarily cumulative.)

As discussed in Sect. 4.1 for large trucks, systems analyses will be conducted to guide the R&D for small

trucks.

4.4.1 Vehicle Efficiency

The small truck platform has seen continuing improvement in fuel efficiency in the past decade as a spinoff of that achieved in Class 1/2a trucks. To increase overall propulsion efficiency and reduce emissions,

transient engine operating conditions must be minimized. To accomplish this, engine speed and load must

be independent of drive-wheel speed and required tractive effort. By optimizing vehicle design, together

with improving power-train efficiency, the small truck can be improved significantly in overall

operational efficiency. Hybrid propulsion is a practical means of decoupling engine power demand from

the required drive wheel power. For a detailed discussion of hybrid technology, see Sects. 4.6.4 and 4.6.5.

4.4.1.1 Power-Train Efficiency

Significant gains in fuel efficiency can be made by switching from gasoline engines to diesel engines in

Class 2b vehicles. Coupled with hybrid propulsion, the diesel engine is considered to be the technology
4-36

Table 4.11. Distribution of energy requirements (kWh) for a typical small truck

(Class 2b commercial pickup), EPA combined driving cycle

Base line (SI) Improvement (%) Target

Engine losses (kWh) 16.76 56.2 7.35

Auxiliary load losses (kWh) 0.25 57.4 0.10

Drivetrain losses (kWh) 2.66 13.1 2.31

Aerodynamic losses (kWh) 1.08 22.4 0.84

Rolling resistance losses (kWh) 0.69 31.8 0.47

Friction braking (kWh) 0.43 96.3 0.02

Hybrid power train (parallel)

 Generator losses (kWh)

 ESS losses (kWh) 0.09

 Motor/controller losses (kWh) 0.54

Total energy used (kWh) 15.81 46.4 11.71

Vehicle weight (empty) (kg) 2,650 (2,514+136) 2,147 (2,011+136)

mpg over EPA combined drive cycle 13.4155 91.4 25.6823

Fuel economy multiplier 1.0 1.9a

 aIncludes 5% improvement in fuel economy resulting in 20% reduction in vehicle weight.

that will yield the most cost-effective solution to achieving the vehicle fuel efficiency targets in the 2010

time frame. Hybrid propulsion is a critical-path technology, providing for other major improvements in

efficiency and systems operation in the small truck. In addition to the engine’s being decoupled from the

drive wheels, auxiliary systems can also be decoupled from the engine, adding to overall efficiency.

4.4.1.2 Regenerative Braking

The hybrid motors are also designed to function as regenerators during deceleration, thus recovering the

vehicle’s kinetic energy into stored energy, slowing the vehicle without using friction brakes. There is,

however, a practical deceleration limit in normal driving situations to which the driver and passengers can

be subjected without discomfort or adversely affecting vehicle safety. In city driving, braking can waste

up to 50% of the useful energy that the engine is able to provide to the wheels. Efficient regenerative

braking can recover a significant portion of this energy for storage and later use.

4.4.1.3 Parasitic Losses

Parasitic losses include all power requirements on the small truck, including aerodynamic resistance,

rolling resistance, driveline losses, and accessory loads.

Aerodynamic Resistance

Aerodynamics is important when considering the overall power requirements of the small truck platform.

Also, the small truck will be subjected to FUDS and HWFET. These chassis dynamometer tests require

coastdown testing to determine the road load horsepower of the vehicle at 50 mph in order to properly set

the absorber. At this speed aerodynamics plays a significant role. Additionally, the thermal loads of new

technology are becoming an increasing concern, especially the power electronics portion of an electrical

hybrid drivetrain. Some aerodynamic modeling to improve airflow and air quality to the cooling systems

will also improve the commercial viability of these technologies. The current pickup truck that is typical
4-37

of the many models available has a Cd of approximately 0.47. Because the basic nature of pickup truck

design does not allow for very clean aerodynamic shapes, it is felt that an achievable stretch goal would

be to reduce the Cd to 0.375, or a 20% reduction, in the 2010 time frame. It is felt that a longer-term

stretch goal could be as much as a 30% reduction.

Rolling Resistance

Rolling resistance is a function of the interaction of the tire with the road surface and the characteristics of

the tire itself that impact overall vehicle fuel efficiency. Tires that are used on current pickup trucks have

a typical Crr of 0.0069. Tire manufacturers are continually improving their product, and a reduction in tire

Crr to 0.0055, or 20% is considered an achievable stretch goal without impacting vehicle safety in the

2010 timeframe. It is felt that a longer-term stretch goal could be as much as a 40% reduction.

Driveline Losses

Driveline losses are a summation of clutch or torque converter, transmission, drive shaft, axle, and brake

drag losses. Developing seals that are durable but do not leak and do not contribute to the bearing torque

would also be desirable. More efficient transmissions such as CVTs are very desirable. Research on

complete brake-shoe release after releasing the brake pedal is needed. For four-wheel-drive-configured

vehicles the need for two-speed transfer cases requires evaluation because this feature, which is rarely

used by most consumers, needlessly contributes to upsizing of drive shafts and drive axles and increases

its own internal losses. Improved lubrication is also an area that can contribute to significant reduction in

driveline losses. A 20% reduction in driveline losses is considered a realistic target for the 2010

timeframe.

Accessory Loads

The power requirement for accessories is a significant contributor to overall energy usage of vehicles.

Although the actual power requirement is in the 1.5- to 25-kW range for a pickup truck, depending on

which auxiliaries are in use, it is a constant draw on the propulsion system, consuming substantial total

energy. On conventional vehicles the engine directly drives all major vehicle auxiliary systems, generally

through mechanical belt drives. Consequently, the functionality, design, sizing, and efficiency of the

auxiliary systems are directly dependent on the operating speed of the engine. Auxiliary systems must be

able to operate over the relative speed range of the engine, from idle to redline. The widely variable speed

of operation means that auxiliary systems (e.g., pumps, air-conditioning compressors, alternators, and

cooling fans) must be designed to perform adequately at all operating speeds with the common design

point being engine idle. This demand has created an operating mode of higher engine idle, which also aids

in improving passenger compartment cooling and heating. The operation at engine idle forces designers to

make compromises, resulting in larger, heavier, and less-efficient components compared to operation at

optimum speed, discrete speeds, or zero speed if they are not needed. The use of hybrid power-train

technologies creates the opportunity to shut the engine off when its power is not required. This feature

would require accessories to be driven from the hybrid’s alternative electrical or mechanical power

source.

To minimize power requirements and to optimize component designs, all accessories (power-steering

pump, cooling fans, cooling pump, and air-conditioning compressors) could be independently driven. The

accessories themselves are simply electrically or mechanically driven versions of common engine-driven

components with some reduction in size to reflect the higher operating speed. Independent accessory

drives afford designers the capability to develop optimally efficient designs for auxiliary components and

the flexibility to mount and package the systems away from the engine, in less-hostile environments and

in easier-to-maintain locations. Independently driven accessories also provides the capability to operate

the accessories independently of the engine in a “demand-responsive” mode, which saves considerable

energy. Incorporating a 42-VDC vehicle electrical system instead of today’s 12-VDC system will provide
4-38

additional efficiency improvements. A typical target would be to reduce engine accessory energy load by

35% through the use of optimized electrically or mechanically driven auxiliaries. The following are

typical accessory technical targets.

Fans and pumps. The power-train system already monitors the temperatures of engine coolant and

transmission oil. These temperature data would be used to provide control inputs to solid-state relays that

switch fans “on” or “off” as cooling demand increases or decreases, saving approximately 2 kW of power

and lowering noise emissions. For electric hybrids, demand-responsive variable-speed control of the

hybrid electronics coolant pump would save an additional 1 kW.

Power steering. The existing constant-operation power-steering pump would be replaced with an

electric-assist demand-responsive unit. This type of unit is available for light automotive applications

today. The total working power savings of this electric assist unit is unknown at this time. However, when

the unit is performing no work (i.e., no steering input when the vehicle is driving straight or at idle), the

power saving is in excess of 0.90 kW.

4.4.1.4 High-Performance Thermal Management

High-performance thermal management focuses on minimizing the auxiliary load requirements for

heating, ventilation, and air-conditioning (HVAC) systems while maintaining the thermal comfort of the

vehicle occupants. Additional benefits in fuel efficiency can be achieved through the development of

high-performance heat exchangers and cooling media (fluids), which will reduce the need for high-output

engine water pumps. Numerous technologies have been identified, including direct heating and cooling of

the vehicle occupants, eliminating in-dash venting systems, reducing vehicle peak and steady-state

thermal loads, and employing heat-generated cooling techniques. Technologies for reducing the vehicle

thermal (solar) loads include advanced window glazings, thermal insulation, and ambient cooling and

ventilation systems. Additionally, heat generated in the vehicle cabin can be used in various cooling

techniques, including metal hydride systems, absorption, desiccant systems, and exhaust-heat wasterecovery systems.

4.4.1.5 Mass Reduction

Vehicle acceleration and rolling resistance are linked through the common physical property of mass, or

weight. The power required to accelerate the vehicle and overcome rolling resistance is directly

proportional to vehicle weight. By decreasing weight, the power required for acceleration and gradeability

decreases commensurately, thus reducing fuel consumption and emissions. This relationship between

weight and propulsion power is of major significance in increasing fuel efficiency. However, in the past,

far to little emphasis has been placed on vehicle weight control for such reasons as policies and

regulations, available funding and incentives, institutional reluctance, a risk-averse industry, and the

industry’s limited knowledge and experience in alternate construction and materials. The goal is to reduce

the curb weight of typical Class 2b pickup trucks by 20%, from 5,540 lb (2,512 kg) to the target weight of

4,430 lb (2,009 kg). This weight reduction alone will result in at least a 15% improvement in fuel

efficiency.

4.4.2 Emissions

Emission requirements for automobiles and light-duty trucks are determined on a gram per mile (g/mi) or

“system” basis while emission requirements from medium- and heavy-duty trucks and buses are

determined on a gram-per-brake-horsepower-hour (g/bhp-h) or “component” basis. This component

protocol does not account for significant emission benefits that are achievable through the implementation

of advanced vehicle systems such as hybrid propulsion and lightweight vehicle structure and components.

To ensure credit for the real-world emission reductions provided by advanced vehicle systems, an

administrative change in the EPA certification procedure would be required to permit the emissions
4-39

measurements to be vehicle-based, rather than engine-based. It is contemplated that the emissions test

protocol for Class 2b vehicles will change within the program time frame to a vehicle basis. Issues

associated with hybrid vehicle testing and fuel composition still need to be addressed (see Table 4.12).

Table 4.12. Summary of technical targets and barriers for Class 2b truck emissions

Goals Technical targets Barriers

Establish EPA hybrid

 emissions protocol

Develop hybrid vehicle duty cycle for

EPA emissions certification of hybrid

vehicles engines

Consensus among the industry

and EPA on duty-cycle.

Institutional bureaucracy

Demonstrate NOx and

 PM aftertreatment

 w/fuel reformulation

EPA proposal:

max of 0.2 g/bhp-h NOx and

0.01 g/bhp-h particulate matter in 2007

CARB diesel engine emissions

standards

Cost, durability, of lean

combustion NOx catalyst/PM

filter

Cost, lubricity, availability of

reformulated fuels

4.4.3 Truck Safety

4.4.3.1 Vehicle Design

The greatest improvement in vehicle safety for Class 2b vehicles will result from vehicle design that,

where practical, incorporates all safety features required of passenger cars and light-duty vehicles.

Additionally, to make the Class 2b vehicle more crash-friendly with passenger cars and other vehicles on

the road, bumper heights should be required to be compatible.

4.4.3.2 Electrical Safety

The introduction of new technology, such as hybrid electric power trains, brings new hazards that must be

addressed. Safety issues related to hybrid electric power trains are discussed in Sect. 4.6.4.4.

4.5 MILITARY VEHICLES

This section provides additional information regarding military requirements for the light, medium, and

heavy truck platforms. These vehicles represent significant portions of the U.S. Army tactical truck fleet.

The representative military light truck is the HMMWV M1097A2. For the medium truck, the

representative military version is the 2-ton FMTV M1078. The military equivalent for the heavy truck is

the M916A2 tractor-trailer. Preliminary analysis indicates that the military vehicles will derive benefits in

fuel economy similar to those that commercial cousins would derive.

4.5.1 The Systems Analysis Process

The systems analysis process should encompass the following:

• deriving system requirements,

• using system requirements to set subsystem requirements,

• requirements review and prioritization, and

• requirements validation/verification.

This process is described in the sections that follow.
4-40

4.5.2 Deriving System Requirements

The process of deriving system requirements begins with the “voice of the customer.” The end user of any

product from this initiative will essentially stipulate the criteria for purchasing the product upon the

completion of the program. The user’s needs must be identified at the beginning of the process for it to be

effective. A user might be willing to make a higher capital investment for truck having higher fuel

economy, for example, if the return on investment is faster than a “standard” truck purchase. Typical user

requirements might focus on vehicle performance, such as the use of a standard slack-adjusting hitch or

the capability of hauling a 20,000-lb (9,070 kg) load up a 4% grade while maintaining a speed of 65 mph.

4.5.3 Analysis of Selected Current Military Vehicle Fuel Efficiencies

4.5.3.1 Driving Schedule

In the absence of any military dynamometer driving cycles other than the Munson Standard Fuel Course,

FUDS was chosen for a preliminary analysis of the military vehicles. The Army provided data for the

purpose of validating the vehicle models. After validation the models were used to predict fuel economy

of the military trucks on the FUDS.

4.5.3.2 Light Truck

The military vehicle chosen to represent the Class 2b light trucks for this analysis is the HMMWV. The

baseline characteristics of the HMMWV are listed in Table 4.13.

Table 4.13. HMMWV M1097 A2 specifications

Configuration 4×4 cargo/shelter/troop carrier

Engine manufacturer GM IDI Diesel

Aspiration Natural

Engine displacement (L) 6.5

Engine peak power (kW) 119 @3,400 rpm

Transmission GMPT automatic

Number of gears 4

 1st gear 2.48

 2nd gear 1.48

 3rd gear 1

 4th gear 0.75

Final drive ratio 1.92 h 2.73 d 1.01 tc 5.29 overall

Empty vehicle weight (curb) (kg) 2,676

Gross vehicle weight (kg) 4,672

Frontal area (m2

) 3.58

Coefficient of drag 0.5

Wheel base (m) 3.3

Tire type Goodyear radial 37×12.5 R16.5

Rolling radius (m) 0.4558

Coefficient of rolling resistance 0.013 paved/0.045 off road

Acceleration 0–30 mph (second) 10

Acceleration 0–50 mph (second) 29

Idle speed (rpm) 700

On the basis of using validated models, a simulated weight of curb plus 2/3 maximum cargo, and the

FUDS drive cycle, the resulting fuel economy in ton mpg was 13.4 (9.1 mpg). Figure 4.8 shows an energy

balance/distribution of the simulation. Simulated accessory loads were only to those required to run the

engine.
4-41

4.5.3.3 Medium Truck

The military vehicle chosen to represent the Class 6 medium trucks for this analysis is the FMTV 2.5 ton.

The baseline characteristics of the FMTV are listed in Table 4.14.

Based on validated models, a simulated weight of curb plus 2/3 maximum cargo and the FUDS drive

cycle, the resulting fuel economy in ton mpg was 9.8 (5.9 mpg). Figure 4.9 shows an energy balance of

the simulation. Simulated accessory loads were only those required to run the engine.

4.5.3.4 Heavy Truck

The military vehicle chosen to represent the Class 8 heavy trucks for this analysis is the M916A2. The

baseline characteristics of the M916A2 are listed in Table 4.15.

On the basis of using validated models, a simulated weight of curb plus 2/3 maximum cargo, and the

FUDS drive cycle, the resulting fuel economy in ton mpg was 66 ton mpg (3.3 mpg). Figure 4.10 shows

an energy balance of the simulation. Simulated accessory loads were limited to those required to run the

engine.

4.5.4 Impact of Aggressive Improvements on Military Truck Efficiencies

As a preliminary approach to achieving the fuel economy goals for the year 2010, the models for the three

military vehicles were simulated with aggressive improvements assumed to be available in the future.

This is a preliminary analysis because the candidate technologies are yet to be determined through the

systems engineering process. The technologies discussed below are to provide the 21st Century Truck

Program Technology Roadmap with some direction for military vehicles.

Fig. 4.8. Energy balance for the HMMWV.
4-42

Table 4.14. 2.5-ton FMTV M1078 specifications

Configuration Standard cargo

Engine type CAT 3116 6-cyl diesel

Aspiration Turbocharged, aftercooled

Engine (L) 6.6

Engine peak power (kW) 220 @2600 rpm

Transmission Allison MD-D7

Number of gears 7

 1st gear 5.64

 2nd gear 3.45

 3rd gear 1.84

 4th gear 1.39

 5th gear 1

 6th gear 0.76

 7th gear 0.66

Final drive ratio 7.8

Empty vehicle weight (curb) (kg) 7484

Gross vehicle weight (kg) 9752

Frontal area (m2

) 5.46

Coefficient of drag 0.75

Wheel base (m) 3.9

Tire type 395 × 85 R20 XML

Rolling radius (m) 0.5738

Coefficient of rolling resistance 0.008 paved/0.045 off road

Acceleration 0–30 mph (second) 8.2

Acceleration 0–50 mph (second) 20

Acceleration 0–58.5 mph (second) 33.4

Idle speed (rpm) 600

Fig. 4.9. Energy balance for the FMTV.
4-43

Table 4.15. M916A2 specifications

Configuration 14 ton tractor truck 6×6

Engine type DDC Series 60 6-cyl diesel

Aspiration Turbocharged, aftercooled

Engine (L) 12.7

Engine peak power (kW) 298 @2100 rpm

Transmission Allison HT-740

Number of gears 4

 1st gear 3.692

 2nd gear 2.021

 3rd gear 1.383

 4th gear 1

Final drive ratio 4.9 diff, 0.98tc, 4.8 overall

Empty vehicle weight (curb) (kg) 12583

Gross vehicle weight (kg) 30844

Gross combination weight (GCW) 58967

Frontal area (m2

) 9.87

Coefficient of drag 0.75

Wheel base (m) 4.445

Tire type 315/80 R 22.5 tubeless radial

Rolling radius (m) 0.5238

Coefficient of rolling resistance 3.5 static, 0.0495 dynamic

Acceleration 0–30 mph (second) 30 at GCW

Acceleration 0–50 mph (second) 99 at GCW

Idle speed (rpm) 600

Fig. 4.10. Energy balance for M916A2.
4-44

First, improvements were made in the engines, including reduced friction, reduced heat loss, addition of

EGR, advance injection timing, turbocompounding for the FMTV and M916A2, and better aftertreatment.

Second, a continuously variable transmission (CVT) replaced the automatic transmissions, and a parallel

hybrid power train was implemented. Third, aerodynamics, rolling resistance, and vehicle weight

improvements were evaluated.

These improvements were targeted for peacetime operations of the three classes of military vehicles.

However, during wartime, meeting emissions requirements may be less important than fuel savings.

Engine improvements that hinder engine efficiency but improve emissions, such as SCR, lean NOx traps,

or retarded injection timing, may not be necessary during wartime. Without such improvements, the fuel

economy in wartime would be measurably higher than the results have shown. Features that can be

“turned off” in wartime, such as SCR, are of great interest to the military.

An analysis of variance (ANOVA) was performed on the three vehicles by means of the FUDS cycle. All

possible combinations of the baseline values and improvements were simulated to determine the effect of

each improvement, as well as the combined effect of various improvements. The ANOVA shows that,

although a vehicle improvement may have little impact on fuel economy alone, the effect is enhanced

when combined with other improvements.

4.5.4.1 Future Class 2 Light Truck

Improvements were applied to the GM 6.5-L engine (see Table 4.13). With the hybrid power train,

however, the engine was scaled down to 75% of its original power. The aerodynamic coefficient of drag

was reduced by 20% on the basis of predictions from the 21st Century Truck working group meetings.

According to the Parasitic Losses team, rolling resistance was reduced by about 31%. The vehicle curb

weight was reduced by 30%. The cargo weight was increased by 2/3 of the difference in curb weights.

With all the improvements applied to the vehicle, the FUDS simulation resulted in a fuel economy of

2.8 times the baseline value, or 38 ton mpg. Simulated accessory loads were limited to those required to

run the engine.

In the ANOVA, which was based on the average fuel economy from the simulations, the hybrid power

train had the most significant effect on fuel economy, increasing it by 48%. The next most effective

changes were the reduced rolling resistance and improved engine. Reduced weight and improved

aerodynamics had smaller positive effects. The addition of the CVT showed no improvement, unlike in

the results of the commercial truck analysis. Further calibration of the CVT model for the HMMWV is to

be examined.

The two-factor ANOVA reveals the combined effect that various pairs of improvements have on fuel

economy. The pairing of hybrid drivetrain and reduced rolling resistance show the greatest effect,

followed by improved engine paired with hybrid drivetrain, hybrid drivetrain paired with reduced weight,

and hybrid drivetrain paired with improved aerodynamics. Although improved aerodynamics alone adds

only a minor advantage, the hybrid drivetrain combined with the improved aerodynamics has a significant

effect.

The three-factor ANOVA revealed that the three improvements that work best as a combination are the

hybrid drivetrain, improved aerodynamics, and reduced rolling resistance.

4.5.4.2 Future Class 6 Medium Truck

Improvements were applied to the CAT 3116 engine (see Table 4.14). With the hybrid power train,

however, the engine was scaled down to 75% of its original power. The aerodynamic coefficient of drag

was reduced by 15%, and the rolling resistance was reduced by about 19%. The vehicle curb weight was

reduced by 25%. The cargo weight was increased by 2/3 of the difference in curb weights. With all the
4-45

improvements applied to the vehicle, the FUDS simulation resulted in a fuel economy of 3.4 times the

baseline value, or 33.5 ton mpg. Again, simulated accessory loads were limited to those required to run

the engine.

An ANOVA was performed on the FMTV FUDS simulations with the average fuel economy taken from

the simulations. The hybrid power train most significantly improved fuel economy, followed by the

improved engine and reduced rolling resistance. Reduced weight and improved aerodynamics had smaller

positive effects. The addition of the CVT showed no improvement, unlike in the results of the commercial

truck analysis. As with the HMMWV, further calibration of the CVT model for the FMTV may be

necessary.

In the two-factor ANOVA, the pairing of a hybrid drivetrain with reduced rolling resistance show the

most improvement, followed by the reduced weight paired with hybrid drivetrain, followed by the hybrid

drivetrain paired with improved aerodynamics. The improved engine had less of a two-factor impact on

fuel economy for the FMTV. However, in the three-factor ANOVA, the three improvements that work

best as a combination are the hybrid drivetrain, improved engine, and reduced rolling resistance.

4.5.4.3 Future Class 8 Heavy Truck

Improvements were applied to the DDC Series 60 engine (see Table 4.15). With the hybrid power train,

however, the engine was scaled down to 75% of its original power. The aerodynamic coefficient of drag

was reduced by 25%, and the rolling resistance was decreased by about 19%. The vehicle curb weight

was reduced by 20%. The cargo weight was increased by 2/3 of the difference in curb weights. With all

the improvements applied to the vehicle, the FUDS simulation resulted in a fuel economy of 1.9 times the

baseline value, or 125 ton mpg. Again, simulated accessory loads were limited to those required to run the

engine.

An ANOVA was also performed on the M916A2 FUDS simulations, with the average fuel economy

taken from the simulations. The hybrid power train most significantly improved fuel economy, followed

by the improved engine and reduced rolling resistance. Reduced weight and improved aerodynamics had

smaller positive effects. The addition of the CVT showed no improvement, unlike in the results of the

commercial truck analysis. As with the HMMWV and FMTV, further calibration of the CVT model for

the M916A2 may be necessary.

In the two-factor ANOVA, as with both the FMTV and the HMMWV, the hybrid drivetrain paired with

reduced rolling resistance show the most improvement. The next most successful improvements were

from the hybrid drivetrain paired with improved aerodynamics and reduced weight paired with hybrid

drivetrain. The improved engine had less of a two-factor impact on fuel economy for the M916A2.

However, in the three-factor ANOVA, the three improvements that work best as a combination are the

hybrid drivetrain, improved engine, and improved aerodynamics.

4.6 CROSSCUTTING TECHNOLOGIES

4.6.1 Alternative Fuels

4.6.1.1 Status of Technology

The use of alternative fuels in future commercial trucks and buses will facilitate achievement of national

goals related to fuel diversity, use of domestic energy resources, energy efficiency, and lowering of

exhaust emissions.

DOE and DOT have established programs to promote research, development, and deployment of

alternative-fuel vehicle technology. Although alternative fuels are most readily used by fleets having

central refueling and maintenance facilities, renewable fuel such as ethanol and biodiesel may find
4-46

application in large vocational trucks as well. Refueling station corridors are being established in some

U.S. cities to promote the use of alternative fuels on a broader scale

Fleet studies have been conducted to compare the performance of transit buses operating on natural gas,

ethanol, methanol, and biodiesel with buses operating on conventional diesel fuel (NREL 1996, NREL

2000a). These fleet studies have shown that alternative-fuel buses generally have higher operating and

maintenance costs than conventional diesel-powered vehicles have. The operating costs can vary widely

because the fuel price is strongly dependent on the location of the fleet operation. Maintenance costs are

generally higher for alternative-fuel bus fleets since the technology is less mature than the mass-produced

diesel engine technology.

Natural gas has emerged as the most widely used alternative fuel for transit buses in the United States.

Buses powered by natural gas accounted for 22% of the 4,225 transit buses built in 1998 (Inform 2000).

In 1999, about 92.5% of the nation’s transit bus fleet were diesel buses and 6.2% were natural gas buses

(APTA 2000a). The remaining 1.3% of buses were electric, hybrid electric, or fueled by alcohol, gasoline,

propane, or other alternative fuel.

Natural gas buses have penetrated the marketplace in part due to legislative incentives and mandates, but

also due to their lower NOx and PM emissions and favorable environmental image. Transit agencies and

commuters perceive natural gas buses to be less polluting than diesel buses due to their quieter operation

and absence of diesel odor and smoke. Natural gas transit buses typically cost 15 to 25% more than diesel

transit buses due to the higher cost of the engine and fuel storage and delivery systems. Refueling and

maintenance facility costs may significantly limit market penetration of natural gas vehicles. The cost of

typical natural gas fueling stations can range from $600,000 to $1.5 million for a fleet of 80 to 160 natural

gas buses (NYC 2000). In addition to this cost, facility modification for some existing garages in dense

urban areas has been reported to cost more than $18 million.

Substantial progress has been made to develop natural gas technology for commercial trucks and buses:

Cummins, Detroit Diesel, Deere, and Mack offer factory-built, spark-ignited, lean-burn natural gas

engines for commercial vehicles. A wide range of engine displacements and power ratings are now

available. The engine manufacturers have cited the need for larger production volumes to justify their

continued investment in natural gas engine development and production. MVE, Lincoln Composites, and

other firms have developed fuel storage and delivery systems to safely handle compressed natural gas

(CNG) and liquefied natural gas (LNG) on-board vehicles.

Natural gas engines have been certified to meet the California Optional Low-NOx Standard of 2.5 g/bhp-h

NOx. Moreover, engine-out NOx emissions of 1.0 g/bhp-h have been demonstrated on a productionfeasible, lean-burn natural gas engine in the engine laboratory (NREL 2000b). Emissions tests conducted

on the chassis dynamometer have confirmed that natural gas vehicles emit substantially lower NOx and

PM compared with closely matched diesel vehicles (NREL 1996, NREL 2000a, Inform 2000). Natural

gas engines will require advancements in emissions control technologies to comply with future stringent

emissions standards such as the CARB Public Transit Bus Fleet Rule and Emissions Standards for New

Urban Buses, and the proposed 2007 EPA emissions standards for heavy-duty engines.

Experiments and field tests are under way with blended alternative fuels such as vegetable oils (biodiesel)

and ethanol (oxydiesel). Both types of fuels are typically blended 10 to 20% in conventional petroleum

diesel fuel, although biodiesel can be used in neat form. The blended fuels offer a displacement of

petroleum and modest emissions benefits, especially reduction of PM. Diesel fuels produced from natural

gas feedstocks, such as Fischer-Tropsch fuel, are attractive because they are free of sulfur and aromatics.

Numerous engine experiments have shown substantial reductions in PM emissions when Fischer-Tropsch

diesel is used in unmodified engines.
4-47

Alternative-fuel vehicle research and development is being directed toward next generation natural gas

vehicles, “gas-to-liquid” fuels that can be used in unmodified diesel-powered vehicles, and hydrogen

technology. These fuels may also be used in combination with HEV technology.

4.6.1.2 Technical Targets

The technical targets for alternative-fuel vehicles are outlined in Table 4.16. The technical targets

emphasize gaseous-fueled vehicle technology because they are currently the most widely used alternative

fuels in the transit bus market.

Table 4.16. Summary of technical targets and barriers for natural gas transit buses

Engine parameter Current practice Target Barrier

Cost The capital cost of a natural

gas bus is 15 to 25% higher

than costs for a comparable

diesel bus. Operating costs and

facility costs are generally

higher than those for diesel

vehicles

No more than 20%

higher cost than

comparable diesel

vehicle by 2010

Current production volumes are

low. Fuel storage system costs

are high. Added components and

complexity add cost. Fuel

economy is usually lower than

that for diesel. Additional

facility costs

Efficiency Dedicated natural gas buses

average about 3.2 miles per

diesel equivalent gallon over

CBD cycle [40,000 lb

(18,140 kg) GVW, 40-ft bus].

Part-load efficiency of SI

engines poorer than diesel

Dedicated gaseousfueled bus: match

current diesel fuel

economy of 3.75 mpg

over the CBD cycle by

2010

Alternative-fuel hybrid

bus: 3× efficiency goal

from 21st Century

Truck by 2010

Same barriers as for a highefficiency diesel engine, plus

low cetane rating of fuel makes

diesel cycle difficult. May need

low-cost sensors for knock

detection and ignition system

improvements

Chassis weight

Emissions Current dedicated lean-burn

NG spark-ignited engine:

NOx = 2.5 g/bhp-h

NMHC = 0.5 g/bhp-h

CO = 6.0 g/bhp-h

PM < 0.05 g/bhp-h

Prevailing emissions

standards: EPA

proposal: max of

0.2 g/bhp-h NOx and

0.01 g/bhp-h PM in

2007

CARB Public Transit

Bus Fleet Rule and

Emissions Standards

for New Urban Buses

Maintaining low emissions

while increasing efficiency

Transit bus emissions standards

are more stringent than truck

standards and may force

development of alternative

power plants and fuels

Reliability Variable. Poor reliability at

some sites, while others have

achieved parity with diesel

reliability.

Same as diesel Spark plug life (if SI), fuel

delivery system reliability,

valve/valve seat wear. New

power plant technologies

The costs associated with alternative-fuel vehicles are significantly higher compared with costs for diesel

vehicles. Operating and maintenance costs can be affected through product design and development. The

incremental cost of the natural gas vehicles could also be improved if production volumes increased.

Infrastructure, refueling and maintenance facilities costs will be difficult to change.
4-48

Spark-ignited engine efficiency is significantly lower than diesel engines due to pumping and throttling

losses and lean air/fuel ratio limits needed to ensure ignition without misfire. Research is under way to

improve the energy efficiency of the spark-ignited alternative-fuel engines to approach diesel engine

efficiency.

The overall emissions target is to meet prevailing emissions certification standards. These standards are

stringent for transit buses and may force significant changes to power train technology. Near-term

development efforts are to focus on emissions controls for lean-burn gaseous engines, including active

NOx aftertreatment devices. Research efforts should focus on near-zero emission power plants, including

the fuel cell.

The reliability of all alternative-fuel vehicles should be comparable to that of diesel vehicles to promote

market penetration and customer acceptance. The storage and containment of alternative fuels in the event

of a collision must present no greater hazard to vehicle occupants or the surrounding environment than

those presented by current diesel-powered vehicles.

4.6.1.3 Barriers

The primary barriers for alternative-fuel vehicles are cost, market acceptance, and deployment because a

variety of proven technologies are already commercially available. Because of their additional cost and

complexity, alternative gaseous-fueled vehicles may be limited to vocational use and niche applications

unless further incentives or legislative mandates are established. Safety issues related to on-board storage

of CNG must be considered if CNG is to be used more widely.

The barrier to wider use of oxydiesel, biodiesel, and Fischer-Tropsch liquids is their higher cost.

Compatibility of these fuels with existing engine materials and systems has generally been determined to

be satisfactory.

Further emissions R&D will be critical if alternative-fuel power plants are to meet proposed emissions

standards such as the CARB Public Transit Bus Fleet Rule and Emissions Standards for New Urban

Buses and proposed 2007 EPA emissions standards for heavy-duty engines. The status, technical targets,

and barriers for alternative fuels are summarized in Table 4.16.

4.6.1.4 Technical Approach

Natural gas transit buses are commercially available and have been proven in service. However,

alternative-fuel buses are more costly and somewhat less reliable than comparable diesel-powered buses.

The stringent emissions standards proposed for transit buses will significantly influence research

directions and the development of alternative power trains.

Alternative fuels research in the 21st Century Truck Program will focus on three pathways to meet

technical targets:

• Near-term engine development to improve lean-burn spark-ignited engines: Improve part-load

thermal efficiency of the spark-ignited engines to reduce operating costs. Develop emission control

devices for lean-burn gaseous engines to meet future emissions standards, including development of

active NOx aftertreatment devices.

• Promote small-fleet demonstration of diesel and natural gas hybrid electric or fuel-cell-powered

vehicles to build experience in the new technology, making commercial viability possible.

• Mid-term development of gaseous-fueled engine systems for use in HEVs: Natural gas-hybrid or

hydrogen-hybrid transit bus configurations offer opportunities to greatly improve fuel efficiency

while simultaneously reducing exhaust emissions. Optimize the spark-ignited engine design for use in

HEVs. In series hybrid applications, explore control strategies to operate the engine at high power, to
4-49

minimize the inefficiencies attributed to low-load operation. Explore other engine modifications,

including the use of electrically driven accessories in place of mechanical drives. Investigate design,

cost, and packaging of gaseous fuel storage and delivery systems for HEV applications in

collaboration with the Materials Crosscut subteam.

• Long-term research of other high-efficiency, alternative-fuel power plants such as fuel cells.

4.6.2 Internal Combustion Engine Technology

4.6.2.1 Background and Status of Technology

The selection of engine type and optimization of its efficiency is the most critical element of the vehicle

power train in meeting the aggressive Program targets. The engine is responsible for nearly 60% of the

energy inefficiency we seek to minimize in the vehicle system. A 10% increase in engine thermal

efficiency has a direct 1:1 impact on achieving the broader initiative goals. The diesel combustion cycle

(i.e., direct-injection compression-ignition) is the engine system of choice for large trucks because of its

inherent thermal efficiency, high power delivery, and advanced state of development. It is the most

efficient transportation power plant available today. The engine is responsible for the production of

exhaust emissions as well as the inlet conditions for aftertreatment devices thus affecting the overall

efficiency of emission reduction. The engine is critical to the safety of the heavy vehicle by providing a

burst of power to avoid traffic incidents, and also via braking power. Already a key safety ingredient, the

importance of the engine brake will increase as aerodynamic and drivetrain enhancements reduce the

parasitic drag in future vehicles. The diesel engine is a mature, state-of-the-art transportation technology,

offering the lowest possible life cycle costs (see Appendix D).

4.6.2.2 Technical Targets

Fuel Economy

Diesel engines derive high efficiency by both emulating high-efficiency thermodynamic cycles and

minimizing mechanical losses. Diesel high efficiency is due to high compression (expansion) ratio, high

rate of combusting lean mixtures, and use of air-fuel ratio (instead of throttling) for load control, thus

avoiding the part-load pumping losses associated with throttled engines. Turbocharging increases engine

power density and recaptures some of the exhaust heat energy to improve net efficiency. Achieving high

power density primarily through high brake mean effective pressure (BMEP), diesel engines operate at

relatively low speeds, which helps to minimize mechanical friction losses. Other design features, such as

strategic cooling, serve to minimize thermal energy losses and also augment overall power plant power

density. Due to its fuel economy, reliability, and low life-cycle cost, the diesel engine is the power source

for commercial surface transportation and buses in the United States and worldwide.

Modern highway truck diesel efficiency approaches 45%; production gasoline engines have an efficiency

of 30%. This is approximately a 40% improvement relative to the late 1970s diesel engines. Thermal

efficiency can be increased to 50% within the next few years in research designs but will not be

sufficiently developed for commercial production. For example, turbocompounding is a proven

technology for exhaust heat recovery, but is not utilized because it is not cost-effective. A brake thermal

efficiency of 50% for the engine has been set as an aggressive but achievable objective. To achieve the

thermal efficiency target while meeting proposed emission-reduction efficiencies of 90 to 95%+ presents

a much more complex challenge.

The optimization of in-cylinder combustion and heat transfer characteristics to enhance thermal efficiency

acts in opposition to recent developments intended to reduce in-cylinder emissions. Further increases in

fuel efficiency will necessarily force emission controls technology toward the use of aftertreatment

devices that will utilize chemical reactions and catalysts to convert criteria pollutants into less harmful

constituents after the combustion event has concluded. This will lead to the design of an unprecedented
4-50

amount of engine-aftertreatment integration that may still elicit an associated potential of compromising

engine performance to assist the achievement of ultra-low emissions.

Further advances in thermal efficiency will be achieved with improvements in components and operating

characteristics of engines similar in overall architecture to those in use today. In addition, an effective

exhaust heat recovery system is critical to meeting the 50% efficiency target. Traditional implementation

of such energy recovery systems significantly reduces exhaust gas temperatures, a result that is

inconsistent with aftertreatment that requires higher exhaust temperatures. The use of “low-heatrejection” strategies and materials can increase the available exhaust temperature for both recovery and

aftertreatment operation. Furthermore, less heat rejection to the engine coolant reduces demands on heat

exchangers, permitting more flexibility in truck aerodynamics.

Exhaust Emissions

Reductions of diesel engine emissions must be achieved in addition to improvements in efficiency. Over

the past 20 years, diesel engine manufacturers have achieved remarkable reductions in NOx and PM

emissions by modifying their engines. Today’s heavy-duty diesel engines are regulated to 4.0 g/bhp-h of

NOx and 0.10 g/bhp-h of PM (less than 0.05 g/bhp-h for transit buses), and substantially lower emissions

have been achieved in research engines. To address these challenges one can consider three approaches:

(1) minimizing engine-out emissions, (2) exhaust aftertreatment, and (3) fuel reformulation with or

without additives.

Optimizing fuel combustion has led to significant reductions in emissions through strategies including

injection rate control, increased injection pressure, and lower temperatures. However, further reductions

are required to meet future standards. The key is an improved understanding of the process of diesel

combustion and emissions formation and the development of design tools (i.e., models) that incorporate

improved understanding and allow engine designers to rapidly explore alternative combustion system

designs (see Fig. 4.11). The level of detailed understanding of the mechanisms controlling combustion

and emissions that is needed by engine designers to make further improvements is not available. As new

diagnostics advance the understanding of in-cylinder processes in diesel engines, more advanced concepts

such as HCCI may emerge (see Sect. 4.6.10.4).

System Level Building Blocks

EGR VGT Electronics Fuel

Injection

Valve

Actuation

Aftertreatment

Flow Robust

Control

Rate

Shaping

Robust

EGR

Integrated

Strategy

An Engine

that

Thinks

Smart, Clean Efficient Engine

Courtesy of Detroit Diesel Corporation

Possibilities

Courtesy of Detroit Diesel Corporation

Fig. 4.11. System-level model for efficient engine design.
4-51

4.6.2.3 Technical Approach

Elements of the diesel engine technology roadmap include the following (see Fig. 4.12):

• Define baseline engine designs in sufficient detail to delineate the areas of required technology

advancement. This would be a guide for enabling technology projects. Conduct, on a continuing

basis, analysis and supporting validation tests to assess progress toward goals. Develop advanced

combustion-chamber components for high peak and brake mean effective pressures, utilizing new

architectures for components, advanced materials, thermal barriers, and novel cooling strategies.

• Perform materials evaluation to support engine design targets, pre-component tests, performance and

durability tests of new components, and tests of complete engine systems.

• Develop fuel-injection and combustion technologies that will provide better efficiency without

increasing NOx with modeling and simulation as an integral component of the system design strategy.

• Develop and integrate sensors, controls, diagnostics, and enabling experimental tools.

• Develop improved turbocharger and/or air-handling systems and controls, and trade-offs between

efficiency and transient response. Develop new low-inertia materials and response-enhancing

technologies.

• Pursue new exhaust heat-recovery technologies. Develop materials and designs for improved

insulation of exhaust systems.

• Develop effective thermal-management systems to better protect the engine and to reduce losses.

Refine analysis of benefits of cooling and thermal-barrier strategies and support with experiments.

Continue development of thermal-barrier designs and enabling materials.

• Continue refinement of piston/cylinder designs, valve trains and other mechanical components for

reduced friction losses. Carry out R&D of low-friction materials and lubricants.

• Optimize effectiveness of EGR for NOx reduction.

• Exploit fuel reformulation for overall emissions reductions.

• Core Engine System Design

– Mechanical

– Air System

– Combustion System

– Fuel System

– Cooling System

– Lubrication System

– NVH

• Aftertreatment System Design

– NOx: (Lean-NOx, NOx Traps, SCR)

– PM: (Oxidation Catalyst, CRT)

• Vehicle Powertrain

• Control / System Integration

– Model Based Controls & Adaptive

Systems

– Sensors

“Wired” Integrated Engine Systems Optimization

Vehicle

Powertrain

Aftertreatment

System

Control / System Integration

Core Engine

System

Courtesy of Detroit Diesel Corporation

Fig. 4.12. “Wired” integrated engine systems optimization.
4-52

4.6.3 Aftertreatment

4.6.3.1 Requirements for Emission Control Devices

The requirements for exhaust-treatment emission controls are tied to upcoming emissions regulations

currently in the proposal stage. The 21st Century Truck Declaration of Intent requires compliance with

emissions regulations in force in 2010. Although not spelled out in 21st Century Truck Program goals,

significant improvement in air quality in certain regions could be accelerated by retrofitting existing

vehicles with emission control devices developed on this program, and this is certainly within the spirit of

the Declaration of Intent.

Today’s heavy-duty diesel engines are regulated to 4.0 g/bhp-h of NOx and 0.10 g/bhp-h of PM (less than

0.05 g/bhp-h for transit buses). Only urban buses and some delivery-type vehicles use any aftertreatment

device, and they use a diesel oxidation catalyst.

In 1996, the EPA, the state of California, and major engine manufacturers prepared a Statement of

Principles(SOP) (National Archives 1996) that requires further reduction to 2.4 g/bhp-h of NOx plus

NMHCs or 2.5 g/bhp-h of NOx plus NMHCs with a maximum of 0.5 g/bhp-h of NMHC by 2004. An

action by the EPA and the U.S. Department of Justice resulted in a consent decree with the diesel-engine

manufacturers that moved the SOP requirements to October 2002 and placed caps on emissions at all

operating conditions. Diesel-engine manufacturers will likely approach meeting the consent decree

requirements with implementation of cooled EGR, resulting in reduced engine efficiency and perhaps less

durability. Pre-production engines using cooled EGR have achieved the SOP emissions levels without the

use of aftertreatment devices.

In May 2000, the EPA issued proposed emissions regulations for heavy-duty engines to begin in 2007.

The EPA is proposing a PM emission standard for new heavy-duty engines of 0.01 g/bhp-h, to take full

effect in the 2007 heavy-duty engine model year. The proposed standards for NOx and NMHC are

0.20 g/bhp-h and 0.14 g/bhp-h, respectively. These NOx and NMHC standards would be phased in

together between 2007 and 2010 for diesel engines and fully required for gasoline engines in 2007. (For

complete details of the proposed rule see EPA 2000.)

It is widely held that the emissions levels in these proposed rules could be met only with a robust,

integrated engine and aftertreatment system along with certain key fuel properties. The required

combination of engine-out emissions and emission control device performance is illustrated in Fig. 4.13.

The lower limit of engine-out emissions for direct-injection diesels is estimated as about 1.5 g/bhp-h NOx,

so the effectiveness of aftertreatment technology must be on the order of 90%. The addition of

aftertreatment devices is expected to reduce fuel efficiency by at least 5%. Only the realization of highrisk technologies such as HCCI engines would change this perspective. The mature and highly effective

TWC systems in today’s gasoline-fueled automobiles are not applicable to diesel or other lean-burn

engines.

Similarly, PM control by aftertreatment must function at 80 to 90% effectiveness if the proposed

standards become law.

For the foreseeable future, engines used in commercial vehicles of over 8500 lb GVWR will be certified

to federal standards according to the engine dynamometer test procedure for heavy-duty engines.

California LEV II regulations require certification of all vehicles of 8,500 to 14,000 lb (3,855 to 6,349 kg)

GVWR by chassis dynamometer procedures to a specific set of standards. The aftertreatment performance

requirement (e.g., 90% removal) is essentially the same for both chassis and engine dynamometer

certification, although the exhaust flows and temperatures over the certification cycles are quite different.
4-53

4.6.3.2 Status of Emission Control Technology

NOx Control

NOx adsorber catalysts. A NOx adsorber catalyst consists of two principal components: a NOx adsorbent

and a three-way conversion catalyst, both on the same substrate material. Periodically, NOx stored by the

adsorbent is released and reduced to N2. This process requires a momentary exhaust gas composition that

contains CO and HCs but is depleted in oxygen.

An engine management system must determine when the NOx adsorbent is approaching saturation and

then trigger the change in engine operation that results in generation of the rich condition required for

release and reduction of the stored NOx. The duration and “richness” are critical to avoid excessive fuel

use and HC breakthrough while still accomplishing complete regeneration (DECSE 1999).

The NOx adsorber-catalyst is very sensitive to sulfur, its effectiveness dropping quickly with fuels

containing 16 ppm or more sulfur. In programs utilizing 3-ppm sulfur fuel, NOx reduction levels of more

than 90% have been achieved for fresh devices in both engine test cells and experimental vehicle systems,

the latter over a transient cycle (DECSE 1999, DVECSE 2000). In heavy-duty engine transient tests,

experience has shown conversion efficiencies of about 60%. Although these results provide

encouragement, extensive R&D work is still needed on optimizing the NOx adsorption/desorption and

conversion functions, defining and optimizing sulfur removal (“desulfurization”) techniques and

strategies, and examining the use of sulfur traps upstream of the catalyst. The interactions with DPFs in a

full system are in the early stages of exploration. The NOx adsorber-catalyst carries a substantial fuel

economy penalty due to the frequent NOx regeneration, although further optimization is expected to bring

Fig. 4.13. The combination of engine-out emissions and emission control device

performance required to satisfy proposed EPA emission standards.
4-54

this to a tolerable level (a few percentage points). Furthermore, durability of the adsorber-catalyst is far

from established.

Urea SCR. SCR of NOx using ammonia or urea has been used for many years in stationary diesel engine

applications. In the SCR process, NO reacts with the ammonia, which is injected into the flue gas stream

before the catalyst. Because of the toxicity and handling problems with ammonia, the most widely

accepted and commercialized reductant is urea, CO(NH2)2 (Ecopoint 2000). Water solutions appear to be

the preferred form of urea. SCR technology, which utilizes an oxidation catalyst to facilitate NOx

reduction to achieve high control efficiencies, requires the same low sulfur levels as the NOx adsorber

technology (MECA 2000). SCR systems would require an anti-defeat system to ensure that the vehicle is

not operated without a urea supply.

Numerous SCR experiments and demonstrations are in progress. NOx control efficiency has been

recorded at 80 to 90%, with 70% being more representative in transient operation. Durability is being

determined by the numerous field tests, primarily in Europe but now getting under way in the United

States (Miller et al. 2000).

HC SCR. Also known as “lean NOx catalysis,” HC SCR typically utilizes diesel fuel or an HC readily

derived from fuel as the reducing agent for NOx in the presence of a catalyst. Other HCs, ethanol for

instance, have been used in commercial SCR systems for stationary power plants. After many years of

experiments and tests of thousands of catalyst formulations, the probability of HC SCR achieving 90%

NOx conversion over a sufficiently wide temperature range is presently low. The keys to success appear

to be getting the optimum HCs manufactured from on-board fuel, and greatly improving the

understanding of the HC utilization/NOx reduction mechanisms. Lean NOx catalysts exhibit a moderate

degree of sensitivity to sulfur compounds from fuel.

Plasma-assisted NOx catalysis. Non-thermal plasma-assisted catalytic reduction of NOx is a relatively

new technology that has shown promise for enhanced NOx reduction. Up to 80% NOx reduction has been

observed on simulated exhaust and up to 55% in real exhaust (SAE 1999).

The plasma is believed to enhance NOx reduction over catalysts via a two-step process (Penetrante 1997).

First, the plasma is a strongly oxidizing environment in which NO is converted to NO2 with some partial

oxidation of the HC reductants, if present. The second step is reduction of NO2 to N2 by the HC over a

catalyst. Plasma-assisted catalyst NOx control is unproved in transient test cycles. Limited testing on

light-duty vehicles has been performed for PM removal. The energy penalty (from electrical energy and

reductant addition) is possibly higher than for other NOx reduction technologies (about 5% total). While

both NOx and particulate removal by plasmas have been demonstrated separately, a system with

combined function for NOx and PM removal has not been developed or tested.

Particulate Matter Emission Controls

Catalyst-based DPF. Control technologies for PM have seen significant progress in recent years to the

point of limited commercial application. Catalyst-based DPFs used on engines operated on low-sulfur

diesel fuel can achieve PM and toxic HC reductions well in excess of 90%. Where diesel fuel containing

less than 10-ppm sulfur has been used, filter technology has demonstrated impressive durability, in some

applications continuing to provide excellent particulate removal at 600,000 km of vehicle operation

(Warren et al. 2000). The ability of catalyst-based DPFs to reduce HC emissions by more than 95% has

also been clearly established (Letavec et al. 2000).

Two types of DPFs are well-developed and engaged in field trials—the continuously regenerating DPF

(CR-DPF) and the catalytic DPF (CDPF). For both, PM is removed from the exhaust stream by collecting

it on a ceramic wall-flow filter element.
4-55

The CR-DPF accomplishes regeneration by continuously converting engine-out NO to NO2 over an

oxidation catalyst placed upstream of a DPF (in this case, the DPF has no active catalyst on it). The NO2

completes the regeneration. Sulfur in the exhaust, however, can be oxidized over the CR-DPF, forming

sulfates, which are measured as PM. Sulfur oxides also compete for the critical NO and NO2 reaction,

making the regeneration characteristics less effective (Liang et al. 2000).

The CDPF regenerates by using a catalyst coating on the DPF element that uses oxygen available in the

diesel exhaust to promote oxidation of the collected PM. Sulfur in the exhaust can be oxidized over the

CDPF to form sulfates.

Exhaust-gas temperature and fuel-sulfur level are critical factors that affect the performance of both CRDPFs and CDPFs. The poor regeneration at low temperatures, and filter plugging by ash (mostly from

engine lubricating oil) over time, are among the few remaining shortcomings of the technology. The ash

can be removed by backflushing with air, but the required frequency is not fully established.

The catalyst-based DPF is additionally attractive because it is a self-contained passive device that can be

retrofitted to diesel-powered vehicles that exhibit sufficient exhaust heat to ensure regeneration.

Diesel oxidation catalysts. The DOC employs technology that dates back to the early stages of gasoline

vehicle emission control in the early 1970s. When formulated for use in diesel vehicles, these catalysts are

effective in removing HC, CO, and the soluble organic fraction of PM, which can be on the order of 30%

or more of the total PM. They also diminish the usual pungent odor of diesel engine exhaust. DOCs are

used on off-road vehicles, diesel-powered trucks, and cars worldwide.

Plasma reduction of PM. Plasma devices for PM removal have been the subject of numerous

experiments, and full-scale prototypes are emerging in test programs. In laboratory experiments, they

have been very effective in PM control (Fanick 1995).

Non-catalytic DPF. Numerous non-catalytic systems have been proposed for PM control that relied on a

trapping process and then heating by external means. Typically, extra fuel was burned to raise the

temperature of the trap to burn the stored carbon. The fuel economy penalty is rather prohibitive for most

applications. Other systems use a fuel additive that deposits a material on the filter to lower the light-off

temperature (Psaras and Summers 1995) and promote regeneration. Finally, a filter made of a ceramic

paper has been developed whose material of construction couples efficiently with microwave energy. The

filter can be regenerated on demand with externally supplied microwave energy. Prototypes of this device

have been evaluated in engine test cells and on vehicles and have shown good filtering efficiency (over

80%) and the ability to regenerate as intended (Nixdorf 2000).

Enablers for Emissions Controls

Sulfur traps. A sulfur trapping system is attractive to protect the NOx or PM control devices that exhibit

high sensitivity. Even the EPA-proposed 15-ppm fuel sulfur cap may not be low enough to ensure the

needed durability of devices such as NOx adsorbers. Furthermore, sulfur compounds in lubricants also

give rise to notable SO2. Sulfur traps or “guard beds” are similarly used in fuel refining to protect the

process catalysts. Sulfur traps for diesel emission control systems are being developed and at least in one

case are being integrated with NOx adsorbers (Parks et al. 1999).

Reductant generation and deposition systems. NOx catalysts and adsorber-catalysts function with

higher efficiency if the reducing agent is a prescribed compound other than diesel fuel. Mixtures of

hydrogen, CO, and specific HCs have been found to be among the best reductants. Devices such as diesel

fuel reformers could provide a higher level of NOx control by generating reductants on the vehicle.

Reformer technology is mature for some applications using natural gas but is not well developed for

reforming diesel fuel. There has been little investigation of optimizing fuel constituents for this purpose.

Experiments with in-cylinder late fuel injection have been conducted to achieve a similar effect; that is,
4-56

they produce a more tailored exhaust for NOx reduction. These have shown trends in the right direction,

yet not a large enough effect.

Sensors. NOx and PM sensors are critical for emission and aftertreatment control. Either sensor, with an

adequate response time, can aid in the control of engine-out emissions, depending on the control strategy

of the engine manufacturer. Unfortunately, a PM sensor does not currently exist and NOx sensors are

inadequate in the current configuration.

Current electrochemical NOx sensors have many shortfalls (e.g., slow response time ~500 ms, poor

poison resistance, inadequate selectivity) but are based on a proven, robust technology. This type of

sensor could be considerably faster and more dependable with continued research. However, the

electrochemical NOx sensor will not be able to meet the response times necessary to be used as a control

sensor (less than 15 ms). For aftertreatment systems such as SCR, NOx sensors are being integrated for

control and diagnostics. Their durability remains less than desired.

For a more detailed discussion of the status of aftertreatment and emission control technology, see

Appendix E.

4.6.3.3 Technical Barriers

The barriers to successful NOx and PM control are described in two categories: technology deficiencies,

and R&D process deficiencies.

Technology Deficiencies

• Unproven durability and transient performance of NOx aftertreatment technology.

• Inadequate NOx conversion efficiency in full-scale, integrated systems, especially in combination

with DPFs.

• Current technology that is too sensitive to contaminants present in the exhaust (e.g., sulfur). The

contaminants can reduce performance to well below the needed levels.

• Sulfur levels in fuel too high for certain emission control devices. Sulfur levels in lubricants are of

concern as well.

• A temperature range within which catalyst performance is acceptable that is either too high for

practical use or too narrow to perform effectively during all operating conditions (especially HC

SCR), including cold-start.

• Incomplete understanding and optimization of catalysts for plasma-assisted systems.

• Regeneration temperature for DPF higher than desired for light-load applications.

• Incomplete proof of durability for DPFs—periodic de-ashing required.

• No demonstration of technologies that eliminate or neutralize contaminants such as sulfur.

• Device cost.

• Possible generation of unregulated toxic emissions.

• Reduction in fuel economy through several mechanisms.

• Inadequate methods for introducing effective reductants (e.g., reformers and related devices).

• Undeveloped infrastructure for urea SCR.

• Inadequate sensors for process control or diagnostics.

• Difficult packaging of aftertreatment devices in limited space of heavy truck applications.

Deficiencies in the R&D Process

Up to the present, development of new catalytic converter technology has relied almost totally on

empirical, incremental developments. Catalysts and converter designs have been initially selected on the

basis of educated guesses and long-term testing to scale up from laboratory scale to device scale.
4-57

However, it is expected that models of the dominant physical and chemical processes involved could

greatly speed the development of new generations of catalytic converter technology. It may be possible to

reach the stage of the so-called “designer catalysts,” in which totally new catalyst formulations are

proposed on the basis of a detailed model relating catalyst composition and morphology to on-the-road

performance. A summary list of process deficiencies include the following:

• Systems integration is immature. Optimization of PM and NOx control devices is hindered by

inadequate simulation capability for aftertreatment devices.

• The understanding and tools needed for predicting catalyst behavior are insufficient, limiting the

ability to address technology shortfalls to a highly empirical procedure.

• Rapid-aging test methodologies are insufficient for research screening of new technologies.

• Rapid screening test methods are inadequate or unreliable, leading to slow, highly empirical catalyst

development.

• Collaboration on precompetitive subjects is less than optimal among engine companies, emission

control suppliers, and government scientific laboratories.

4.6.3.4 Technical Approach

To meet the challenges and deficiencies described, concurrent efforts at the system, component, and

scientific foundation levels should proceed. System research is required to optimize the total emission

control package while providing data on deficiencies in components as well as gaps in the technical

understanding of phenomena in catalysts, adsorbers, filters, sensors, and controls. Systems optimization,

and component performance, can be accelerated through the application of computer simulations; but for

aftertreatment systems, these simulations must be substantially improved to be on a par with engine/

combustion simulations. Finally, durability must be established and tested in real-world environments

during the R&D process.

An unprecedented level of engine and aftertreatment integration will be required to achieve engine system

durability simultaneously with future required emission-reduction and thermal-efficiency targets. There is

a strong need to accurately model the flow, mass transport, and heat transport in the exhaust gases

flowing to the converter from the engine, and to model the coupling between these macroscopic flow

effects and the chemical processes occurring on the catalyst surface. This connection between the

macroscopic flow field and chemistry is critical because either of these general regions can dominate the

overall converter performance, depending on operating conditions.

Models for the surface chemistry and appropriate kinetics models that will accurately reflect the effects of

individual exhaust-gas species and local variations in temperature require development. Integrating

engine system thermal, chemical species, and flow effects with the aftertreatment device surface kinetics

simultaneously in high detail should offer the most accurate predictions about the impact of design

changes or changes to the catalyst properties. However, it will require teraflop levels of computational

effort similar to those for detailed modeling of in-cylinder combustion. Low-order aftertreatment models

offer another approach for situations where computational speed (as opposed to detail) is essential for

modeling the complete engine and aftertreatment system over short transient events (such as for real-time

diagnostics and control). In this second case, it would be advantageous to develop simplified versions of

the detailed models that can still produce the correct overall system dynamic response.

A third major category of modeling needed for diesel emissions control is simulation of the mechanisms

for catalyst regeneration, degradation, and poisoning by engine behavior. Specifically, if alternative

methods are not found for reducing sulfur’s rapid inhibition of lean NOx adsorber-catalysts, diesel fuels

will have to be processed to extremely low sulfur levels, and potentially new approaches to ring/liner

lubrication could be required. A detailed model for the physical and chemical processes involved in sulfur

poisoning could potentially lead to improvements in catalyst design and/or operation that would
4-58

significantly increase the sulfur tolerance of catalysts and would lessen the need for expensive exhaust

gas processing or complex catalyst regeneration or desulfurization processes.

The technologies of higher priority are NOx adsorbers, urea SCR, and the related technologies that will

support and optimize these systems while the development of DPFs for the complete system is completed.

Certain features of plasma-assisted catalysis and HC SCR are highly attractive and also justify a viable

program of R&D. The retrofit development of DPFs also should receive emphasis.

With regard to the R&D process, multi-industry and government collaboration will be expanded in

precompetitive areas. These areas would include, in particular, development of simulation tools,

fundamental mechanisms (degradation, for example), and experimental tools and methods. Federal

laboratories will be partnered with industry and universities in these efforts.

In summary, the key components of the technical approach for aftertreatment emission control include the

following:

• Identify and exploit fuel properties that reduce overall tailpipe emissions through lower engine-out

emissions and/or enhancement of aftertreatment system performance (such as through a NOx reducing

agent).

• Improve the scientific foundation of NOx control adsorber and catalyst performance and degradation

mechanisms. Similarly, expand the foundation of understanding of plasma processes as they pertain

to NOx and PM control and to catalysts used specifically with plasmas.

• Utilize the above advancements in fundamental understanding to improve the materials, components,

and system designs for emissions controls.

• Improve and apply emission control simulation tools for accelerating system design and optimization.

• Conduct continuing work at the system level to discover subtle interactions between NOx and PM

control devices and how to deal with them.

• Develop better methods and technologies for generating and introducing effective reducing species to

NOx catalysts and adsorbers, including fuel constituents.

• Develop desulfurization processes or sulfur sequestration technology for emission control devices. In

parallel, develop and implement lubricants and fuels with the minimum feasible sulfur content.

• Devise suitable technologies and procedures for urea supply for SCR.

• Develop and apply sensors in controls and diagnostics of engine and emission control processes.

• Examine and exploit the advantages offered by hybrid drivetrains in NOx adsorber operation and

regeneration.

• In development of emission control aftertreatment devices, include the necessary features to make the

devices suitable for retrofit to the existing fleet.

4.6.4 Hybrid Electric Propulsion Technologies

Hybrid electric propulsion systems may be needed to meet performance and efficiency goals for both

commercial and military vehicles. Hybrid electric vehicles (HEVs) feature a power plant in combination

with an electric motor(s) and electrical energy storage system. Many series, parallel, and power-split

hybrid propulsion system configurations are possible. The optimum propulsion system configuration is

dependent on the vehicle performance goals, efficiency goals, duty cycle, and other practical

considerations, including manufacturing cost, serviceability, market differentiation, and customer

acceptance of the technology.

Hybrid electric vehicles have the potential to have greater energy efficiency than vehicles with

conventional power trains. In a HEV, the power plant can be utilized at its most efficient operating

condition. Moreover, electric components can be used to brake the vehicle and to recover and store

braking energy that can be used to propel the vehicle during subsequent accelerations. Hybrid electric
4-59

propulsion systems could replace engine retarders and hydraulic driveline retarders that dissipate braking

energy as waste heat. Other mechanical energy storage devices could also be adopted, as discussed in

Sect. 4.6.5.

Many of the heavy HEVs built to date have used components that are commercially available, but most of

these components were not designed or optimized for use in on-road HEVs. Electric components can be

costly because precision manufacturing tools are needed to produce the components and production

volumes are low. There is tremendous potential to improve the performance and efficiency of these

components through computer-aided redesign, systems optimization, and improved manufacturing

techniques.

A new generation of components is needed for commercial and military HEVs. It is envisioned that the

availability of improved components would promote the use of hybrid electric propulsion systems in a

wider variety of vehicle applications.

A “crosscut” R&D effort is needed to develop enabling technologies for hybrid electric propulsion

systems. Electric motors, electrical energy storage, power electronics, electrical safety, regenerative

braking, and power-plant control optimization have been identified as the most critical technologies

requiring further research to enable the development of higher efficiency hybrid electric propulsion

systems. Development of improved electrical energy storage systems and power electronics is especially

important due to the high cost and limited availability of new components and subsystems. Vehiclespecific optimization of the propulsion system will be performed as discussed in the vehicle platform

sections of the roadmap.

4.6.4.1 Electric Motors and Generators

Status of Technology

Several types of motors and generators have been proposed for hybrid-electric drive systems, many of

which merit further evaluation and development. Certain types of motors may work better for specific

vehicle applications or performance requirements than others.

Motor generators can be configured before or after the transmission. Series HEVs typically have larger

motors with higher power ratings because the motor alone propels the vehicle. In parallel hybrids, the

power plant and the motor combine to propel the vehicle. Motor and engine torque blending is usually

accomplished through couplings and planetary gear sets.

Motor subsystems such as gear-reductions and cooling systems must be considered when comparing the

specific power, power density, and cost of the motor assemblies. Gear reductions may be needed for highspeed motors and can significantly increase weight. Air-cooled motors are simpler and generally less

expensive than liquid-cooled motors. Liquid-cooled motors may also require more cooling-system

maintenance than air-cooled versions. However, liquid-cooled motors are generally smaller and lighter for

a given power rating. Various coolant options exist for liquid-cooled motors, including water, waterglycol, and oil.

The current status of electric motor technology is further discussed in Appendix F.

Technical Targets

To meet the performance requirements of the heaviest Class 8 trucks , the motive drive system must be

capable of providing approximately 2,100 Nm of peak torque, with a peak power requirement of 300 to

400 kW and a continuous power requirement in the range of 150 to 200 kW. In series hybrid-electric

drive systems, the main drive motor(s) must be sized to meet these torque and power requirements. In
4-60

parallel hybrid configurations, the motor(s) are assisted by a power plant (typically an internal

combustion engine), so their peak power requirements are generally much lower.

The next generation of electric traction motors must have higher specific power and lower cost to achieve

efficiency goals and customer expectations. Motors, gear reduction systems, and cooling systems must be

improved through the use of advanced materials, improved cooling systems, novel packaging, and

advanced assembly techniques. Motors with higher voltages and speeds may be needed to meet specific

power and power density expectations.

Motor and generator subsystems, including gear reductions and cooling systems, should be designed for

minimum cost, weight, and parasitic losses, and must have extremely high reliability for commercial and

military vehicle applications. The motor itself should last the life of the vehicle without any major

maintenance or servicing events, and the gear-reduction system should have reliability superior to that of

conventional heavy-duty vehicle transmissions, which are inherently more complex.

Barriers

There are few, if any, inherent technical barriers to meeting the technical targets. Prototype heavy-duty

hybrid vehicles have been successfully demonstrated with main drive motors with continuous power

ratings ranging from as little as 37 kW to as much as 388 kW, an order of magnitude span. The main

barriers relate to weight and cost reduction.

Most motors capable of powering a large Class 8 truck or transit bus [40,000 lb (18,140 kg) and greater]

weigh more than 1,000 lb (454 kg). When gear reduction and cooling systems are included, total motor

subsystem weight can approach 2,000 lb (908 kg). Lighter-weight motors have been demonstrated in

commercial applications. Smaller, liquid-cooled motors with exotic designs are comparatively expensive

and have not yet demonstrated durability in commercial vehicle applications.

Drive motors presently represent one of the most expensive components of heavy-duty HEVs, but also

one of the greatest cost-reduction opportunities. Supplying the 300 to 400 kW of peak power required for

large Class 8 vehicles requires the use of very large single motors with prices typically in the range of

$15,000 to $50,000, or multiple smaller motors costing anywhere from $10,000 to $20,000. Hence, the

total drive motor package for a large Class 8 truck or transit bus, including gear reduction, is unlikely to

be procured at today’s prices for less than $30,000 to $40,000 per vehicle. However, larger production

volumes should lead to substantial reductions in parts costs. Achievement of the recent DOE cost target of

$4/kW (continuous) would require a reduction in motor production costs to less than $1,000 per vehicle.

Historically, breakthroughs in the electrical motor design are rare.

Technical Approach

Several advanced technologies can be employed to improve the specific power and reduce the cost of

electric traction motors. These include advanced winding methods, advanced assembly techniques, use of

highly conductive or lightweight materials, development of optimized cooling systems, and novel

packaging. Various motor-control methods can also be employed. (See Sect. 4.6.4.6.)

Systems analysis is needed in the early stages of the program to identify the best opportunities for cost

and weight reduction. Results from the systems analysis will guide the R&D approach taken for each

vehicle platform. In some cases the best design may include a combination of different motors and

technologies.

A broad technical effort is recommended, focusing on traction motor systems and generators optimized

for hybrid-electric drive systems. In the short term, emphasis should be placed on developing motors

sized such that single or tandem motors can meet vehicle torque and power requirements.
4-61

Longer-term research should also consider the series wheel motor configurations. Means of reducing

motor manufacturing costs should also be explored, including employment of automated production

techniques.

Results from the systems studies will allow selection of available electric motor technology and will

guide the specifications used for each truck application. In some cases the best design may include a

combination of different motors and motor technology.

4.6.4.2 Electrical Energy Storage

An electrical energy storage system is needed to capture energy from the generator, to store energy

captured during vehicle braking events, and to return energy when power is demanded by the driver.

Whereas pure electric vehicles require a high-energy storage system, HEVs require high-power storage

systems. Electrical energy storage systems currently consist of battery packs that have electrical, thermal,

and safety control features.

The three major electrical energy storage systems that are being considered for hybrid electric propulsion

systems are electrochemical batteries, ultra-capacitors, and electric flywheels. Over the past six years, the

Partnership for a New Generation of Vehicles (PNGV) initiative has supported R&D of electrical energy

storage systems for light-duty vehicles. PNGV has directed most of its resources to batteries because of

the better potential for short-term commercialization. PNGV has established technical targets for the

program’s hybrid battery development efforts for power-assist and dual-mode HEVs. Ultra-capacitors and

electric flywheels warrant further consideration in the 21st Century Truck Program because commercial

vehicles have much different performance, cost, and service life requirements than light-duty vehicles.

A comparison of energy-storage options for heavy vehicles has been published by the Transportation

Research Board (TCRP 2000) and is reproduced in Table 4.17.

Batteries

Status of technology. Although a few production HEVs with advanced batteries have been introduced

into the market, improvements in life cycle economics, power, and energy efficiency are needed,

especially for commercial and military vehicles that have longer service life requirements than light-duty

vehicles.

Desirable attributes of high-power batteries for HEV applications are high-peak and pulse-specific power,

high specific energy at pulse power, a high charge acceptance to maximize regenerative braking

utilization, and long calendar and cycle life. Developing designs and methods to balance the packs

electrically and thermally, developing accurate techniques to determine a battery’s state of charge (SOC),

developing abuse-tolerant batteries, and improving recycleability are additional technical challenges.

The heavy-duty hybrid system design offers a unique engineering opportunity to design the system so that

batteries only assist at peak power demands or provide full power when the vehicle is used as an electric

vehicle in urban areas with the internal combustion engine being a range extender. In the first application

the depth of discharge may be only a few percent, 65% to 60% SOC, whereas in the second application

the range of operation would be 65% to 20% SOC. Data on battery life are readily available for

conventional electric vehicle applications but are far less certain for hybrid application.

Lead acid batteries are currently used in many electric vehicles and have been used in hybrid transit bus

applications. Lead acid batteries can be designed for high power and are inexpensive, safe, and reliable. A

recycling infrastructure is in place for them. But low specific energy, poor cold-temperature performance,

and short calendar and cycle life are still impediments to their use. Advanced high-power lead acid

batteries are being developed for HEV applications.
4-62

Table 4.17. Comparison of energy-storage optionsa

ElectroSource

lead-acidb

Ovonics

NiMHc

Saft advanced

NiCad

Lithium

iond

3M Hydro Quebec

lithium metal

polymer

Maxwell

Supercapacitor Flywheele

Model H12N85 HEV 60

Ah

STX-600 d 119Ah Pc2500 e

Specific power

(W/kg)

240 500 350 300 315 3,305 600–5,600

Specific energy

(Wh/kg)

42 70 35 90 155 248 15–132

Calendar life

(months)

24 60f 36f 20–120g Lifetime of

bus

Cycle life (full

discharge cycle)

700 1,000 2,000 1,000 600 20–120g Lifetime of

bus

Cost ($/kWh) 295h 200–2,000 833 1,000–3,000 10,526 N/A 150–2,000

(projected)

Maintenance None None Distilled water None None None None

Recycling (%) 100 100 100 100 100 None 100

 a

Main source: Transit Cooperative Research Program. (TCRP) 2000. Hybrid Electric Transit Buses: Status, Issues, and

Benefits. Report No. 59. National Academy of Sciences, Washington, D.C.

 b

Batteries currently used in Lockheed-Orion hybrid electric bus.

 c

Specifications for Ovonics 60-Ah battery model; cost range represents target ($200/kWh) and current prototype costs

($2,000/kWh).

 d

Various manufacturers and models. Source: California Air Resources Board (CARB). 1998. 1998 ZeroEmission Vehicle Bienniel Program Review.

 e

Various manufacturers and models. Source: U.S. Department of Energy (DOE). 1996. An Assessment of Flywheel Energy

Storage Technology for Hybrid and Electric Vehicles. U.S. Department of Energy, Washington, D.C.

 f

Source: Transportation Research Board (TRB). 2000. Review of the Research Program for a New Generation of Vehicles.

Sixth Report. Transportation Research Board, National Research Council, Washington, D.C.

 g

Lifetime of the supercapacitors depends on operating conditions such as temperature and operating voltage.

 h

Cost is based on purchases of single module; discounts are available for volume purchases.

Nickel-cadmium, nickel-metal hydride (NiMH), and lithium ion batteries have also been used in

commercial and prototype HEVs. The current status of these batteries is further discussed in Appendix F.

NiMH batteries are used in the Toyota Prius and Honda Insight.

Technical targets. Technical targets for energy storage systems depend on the type of hybrid

configuration and type of vehicle. The technical targets in Table 4.18 are mostly drawn from the PNGV

technical targets.

One of the most important targets is to achieve high power density while having adequate energy.

Calendar life, cycle life, reliability, and safety are also very important issues that need to be addressed to

enable successful commercialization.

Barriers. The primary barriers for electrical energy storage systems are achieving high power densities

with high available energy, reliability, safety, and cycle life. Battery cost and life cycle costs are critical

issues that could influence market acceptance for heavy vehicle applications. Many battery materials are

currently too expensive. The chemicals used in many types of batteries need to be more stable to avoid

self-discharge. Long, shallow discharges can cause chemical instability. Lithium ion batteries have

potential safety issues. Other barriers are proper integration of batteries in a pack within the vehicle,

thermal management, and proper control systems.
4-63

Table 4.18. Summary of current performance and technical targets for batteriesa

Current performance Technical targets Parameter

Li-Ion NiMH Power assistb Duel modec

Energy efficiency (%) 90 88 >90 >95

Calendar life (years) 3 5 6d 6d

Available specific energy (Wh/kg) 23 15 7.5 23.1

Specific power (18 s) (W/kg) 625 400 625 615

Cost/available energy ($/kWh) 1,041 1,064 1,00 333

Available energy density (Wh/L) 29 23 9.4 37.5

 aSource: Transportation Research Board (TRB). 2000. Review of the Research Program for a New

Generation of Vehicles. Sixth Report. Transportation Research Board, National Research Council,

Washington, D.C.

 b

Power assist is a type of parallel hybrid electric vehicle.

 c

Dual mode is a type of series hybrid electric vehicle.

 d

All technical targets are from PNGV except for the calendar life, which is for battery replacement at

midlife of the vehicle (at 6 years for transient bus).

Technical approach. Research and development needs to meet the technical targets include the

following:

• Near-term focus on battery development and demonstration of system performance is in the following

tasks:

– monitor battery electrochemistry research progress by PNGV and U.S. Advanced Battery

Consortium;

– use system simulation tools to develop system control strategies and charging algorithms;

– battery electrochemistry research;

– investigate a hybrid energy storage system that use either different types of batteries or a

combination of ultra-capacitors, batteries and electric flywheels to determine the potential for

load acceptance and system performance; and

– integrate the energy storage system, including packaging, thermal management, and electrical

management (monitoring, balancing, and control), using computer aided design tools, prototype

fabrication and testing on bench scales, and limited heavy vehicle tests.

• Mid-term and long-term focus will be on development of advanced batteries and system validations

via vehicle demonstrations. Safety, reliability, and cost issues will be the major focus.

Ultra-Capacitors

Ultra-capacitors are higher-specific-energy and higher-power versions of electrolytic capacitors—devices

that store energy as an electrostatic charge. They are electrochemical systems that store energy in a

polarized liquid layer at the interface between an ionically conducting electrolyte and a conducting

electrode. Energy storage capacity increases by increasing the surface area of the interface. Ultracapacitors have been developed as primary energy devices for power assist during acceleration and hill

climbing as well as for recovery of braking energy. They are also potentially useful as secondary energy

storage devices in HEVs, providing load-leveling power to chemical batteries. Current R&D aims to

create ultra-capacitors with capabilities of 1,000 W/kg. Additional electronics are required to maintain a

constant voltage because voltage drops as energy is discharged.

Maxwell Technologies has developed large ultra-capacitor cells up to 2700 Farads (10 Wh/l at 600 W/l).

PowerStor has developed Aerogel cells with power of up to 4 kW/kg at 1 Wh/kg. JMK, Inc., has used

ultra-capacitors for starting a transit bus and a truck with capacity of 1.6 Wh/kg at 1.3 kW/kg. Los
4-64

Alamos National Laboratory is conducting research on materials that can lead to capacitors with

0.9 Wh/kg at 3.2 kW/kg. Panasonic and Saft are also actively developing ultra-capacitors.

Further research is needed to investigate the potential for using ultra-capacitors in commercial and

military vehicles. Ultra-capacitors may have better acceptance of regenerative braking energy than

batteries and therefore could be a key enabling technology for improving energy efficiency. Ultracapacitors may be used alone or in combination with battery systems. New ultra-capacitor concepts need

to be developed and tested in the laboratory. Upon proof of concept, packaging, safety, energy

management, and thermal management of ultra-capacitor banks must be further investigated for

commercial and military vehicle applications.

Electric Flywheels

Modern high-performance flywheels offer several attractive features for use in hybrid vehicles. They have

outstanding power handling capabilities, with low-to-moderate specific energy; therefore, flywheels are

best fitted for applications that demand high power levels and relatively low energy storage, such as a

“power assist” parallel hybrid vehicle. Flywheels provide significant advantages over batteries in the

areas of calendar life, cycle life, efficiency, consistent performance at different temperatures and different

ages, and ease of measurement of state of charge.

In an electric flywheel, the input and output energy is in the form of electrical energy. A motor/generator

is installed inside with the high-speed flywheel rotor inside a vacuum housing. Operation of the rotor in

vacuum greatly reduces the windage drag on the surface of the rotor. The motor rotor is attached to the

flywheel rotor; permanent magnet motors are often chosen because they match well with the high

operating speed of the flywheel (typically 40,000 to 70,000 rpm). Power electronics are necessary to

provide the variable frequency input to the flywheel and to convert the variable frequency output to direct

current.

A key design issue will be structural properties of the rotor to permit high-speed operation necessary for

reasonable specific energy and minimization of risks associated with rotor failure. The rotor is normally

fabricated with materials that have a high strength-to-density ratio, such as a carbon fiber/epoxy

composite. Safety and containment are major issues being addressed by flywheel developers.

Understanding of the safety issues (rotor stress design margin, rotor integrity, control of rotor failure

modes, and containment system design) is a key barrier to the commercialization of flywheel technology.

Flywheels can be operated with ball bearings or magnetic bearings. Magnetic bearings provide the

greatest efficiency, but their cost is higher.

Flywheels will use the same technical targets as batteries, with the understanding that the specific power

target can be easily achieved, but the specific energy target is challenging. Due to their likely higher

initial cost and longer life, flywheels will be most attractive when cost comparisons are made on a life

cycle cost basis.

4.6.4.3 Power Electronics

Status of Technology

U.S. industries currently supply power electronic products for commercial and military HEV applications;

however, no manufacturers in the United States can supply the high-power transistors required for such

power electronic products. The power electronics system plays a crucial role in the conversion and

distribution of power and energy in automotive applications. The selection of power semiconductor

devices, converters/inverters, control and switching strategies, packaging of the individual units, and the

system integration are very important for the development of an efficient and high-performance truck of

the future. The current status of power electronics is further discussed in Appendix F.
4-65

Technical Targets

The power electronics system must meet the continuous operating requirements of the vehicle; therefore,

it needs to be highly reliable and durable. The unit should be efficient to improve the fuel economy.

Technical barriers relating to better power semiconductors, improved power management, reduced power

loss, reduction of leakage currents, and packaging limitations must be overcome to achieve low cost, high

reliability, and market-accepted systems. The power electronics and the rest of the system should be

developed to reduce the operating costs of the vehicle. The system needs to be fault tolerant and needs to

provide “limp-home” capability.

Several technical challenges need to be overcome, and new developments are needed from device level to

system level. There is a strong need for a power device that combines the metal-oxide semiconductor

(MOS) gate control characteristics with the current carrying capability and voltage drop characteristics of

a thyristor-type structure. The device forward voltage drop, at even currents greater than 400 A, must be

less than 2 V. The first device to meet these requirements was MCT, but it did not meet the expectations

of the power electronics community. In addition to the switching device, there is a need for development

of a new power diode with superior dynamic characteristics.

Wire bonding and device interconnections are barriers to development of high-current-density power

units. Hence technologies related to device packaging need to be investigated for developing a power

switch. Some of these technologies are topside power connection without wire bonds, minimizing wire

bonds, topside probing for dynamic matching, heat-sinking both sides of the die, direct bond copper on

alumina and aluminum-nitride substrates, interconnect solutions for large-scale manufacturing and

reliable operation of power modules, and other related packaging technologies.

Capacitors with high-frequency and high-voltage operation, low ESR, and high ripple current capability

need to be further developed. The technology of laminated bus bars with high isolation voltage and low

inductance needs further work for use in truck-operating environments. To meet the packaging goals, the

devices must be designed to operate over a much higher temperature range. The research needs to be

focused on high-temperature capacitors, semiconductors, and sensors. The research on silicon carbide

needs to be accelerated to enable its application to high-power switching devices. A novel way of cooling

the entire unit should be examined to quickly transfer heat from the devices.

Although soft-switching inverters have the advantage of lower switching losses and low electromagnetic

interference (EMI), they need more components compared with hard-switched inverters and may require

higher operating device voltages and complicated control. Hence the soft-switched inverter application is

limited to very special types of needs. There is a need to develop an inverter topology to achieve the

performance of the soft-switched inverter with fewer components and simplified control. In the area of

DC/DC converters, further development is needed to obtain 12 V from higher voltages (300 V and

above), and manufacturing techniques need to be advanced to obtain reliable operation.

Methods to eliminate the motor speed/position sensors and inverter current sensors, have been under

investigation for several years. These technologies have not yet proven to be practical for automotive

applications. The technology development effort needs to be focused on sensorless operation of electric

machines and reduction or elimination of current sensors in the inverters. The controllers need to be

developed for robust operation of all vehicle subsystems.

The power electronics required for accessory loads must be individually evaluated to obtain the maximum

efficiency under all operating conditions.

Barriers

The barriers for introducing improved power electronic systems for truck applications are the cost,

complexity, reliability, and the operating environment. The PNGV cost target for the propulsion inverter
4-66

is $7.00 per kW. This cost target is also applicable for commercial truck applications. For specialized

civilian and military applications, the cost could be about $9/kW.

The forward voltage drop of the power-switching device is a barrier to operating efficiency. The device

forward voltage drop needs to be less than 2 V and at the same time needs to be able to be operated at

switching frequencies higher than 10 kHz. Unavailability of low ESR, high-ripple-current capability,

high-temperature, and high-voltage capacitors is a major barrier.

The other barriers are thermal system for fast removal of heat from the junction of the devices, control of

EMI generated due to switching of the devices, and achieving a low-inductance package for the power

inverter.

The cost of developing new manufacturing processes and packaging techniques are prohibitive for low

production volumes.

All technical barriers can be overcome if sufficient resources are made available. The cost of developing

new manufacturing processes and packaging techniques can be prohibitive for low production volumes.

Technical Approach

Power electronic components and subsystems need to be further developed to meet the reliability, service

life and cost expectations for commercial and military vehicles.

In order to reduce the total system cost, the effort should be focused on the following:

• integration of the inverter and the motor as one system by using a design that can be tailored to meet

specific vehicle requirements;

• low-cost manufacturing methods for the motor and inverter; and

• development of low-cost, high-temperature magnets. This would lead to widespread use of permanent

magnet motors. Permanent magnet motors have higher efficiency and need less current to obtain the

same torque as other motor types. This may also reduce the cost of related power devices.

The following technologies need to be investigated to develop an improved production-viable system for

truck applications:

• Improved bus bar design with low inductance and high-voltage isolation capability. Methods to lower

the bus bar resistance and integration of bus bar into the electronics packaging.

• Efficient cooling system.

• Improvement in connector technology.

• Capacitor technology needs to be significantly developed: high ripple current capability, low ESR,

high voltage, and smaller size. Improved dielectric components need to be investigated.

• Inverters with very minimal use of capacitors.

• Power electronic building blocks.

• Topologies with two or more integrated functions such as inverter, charger, OTDC/DC converter.

• Integrated EMI filters.

• New soft-switching topologies with a reduced number of components, having simplicity similar to

hard-switching converters.

• Lightweight enclosures that are waterproof and are immune to EMI.

• Reduction or elimination of speed/position, current, temperature, and voltage sensors.

• Fault-tolerant, fail-safe designs.
4-67

4.6.4.4 Electrical Safety

Status of Technology

Electrical safety requirements must encompass acceptable design practice, accessibility, durability of

safety provisions, human factors and risk management. Electrical vehicle technology has led the way for

development of hybrid vehicle safety technology to a substantial extent. However, the greater extent and

complexity of high-voltage components and cabling in HEVs requires extension of safe practices. (For

purposes herein “high voltage” shall be considered to be any voltage exceeding 50 volts DC or 50 volts

rms AC.) Electrical safety can be considered in three subcategories: functional, personnel, and hazard

identification and mitigation.

Functional safety includes establishing a product safety checklist and design practice, ensuring

crash/rollover isolation, integrating of low-voltage accessories, and conducting failure effects and sneakpath effects analysis. Personnel safety includes consideration of emergency disconnects, access

door/cover/power interlocks, high-voltage cable/harness routing, high voltage cable/harness unique

identification, maintenance and emergency personnel training, and warning labels. Hazard analysis,

tracking and mitigation is a formal process that has been used to define and improve functional and

personnel safety objectives, and is further discussed in Appendix F.

Technical Targets

Technical targets include development of safety standards and codes for new hybrid electric propulsion

systems and compliance with existing requirements and best practices:

• SAE Surface Vehicle Information Report J2344, Guideline for Electrical Vehicle Safety, June, 1998.

• SAE Surface Vehicle Recommended Practice J1766 – Recommended Practice for Electric and

Hybrid Electric Vehicle Battery Systems Crash Integrity Testing, June, 1998.

• SAE Surface Vehicle Standard J1654, High Voltage Primary Cable, June, 1994.

• U.S. DOT Rules and Regulations.

• New York State DOT, Title 17, Official Compilation of Codes, Rules, and Regulations, State of New

York, Part 720.9, Electric Powered Motor Vehicles, effective July 18, 1999.

• Underwriters Laboratory practices.

Barriers

Safety risks may be higher for prototype HEVs that have not been subjected to rigorous hazard analysis.

Care must be taken to not overlook possible safety hazards.

Technical Approach

Participation in safety task forces and working groups will be essential to comply with existing safety

standards, to comply with best practices, and to develop new safety standards if appropriate.

Special precautions must be taken to ensure the safe testing of prototype vehicles.

4.6.4.5 Regenerative Braking

Status of Technology

A conventional heavy vehicle relies on friction brakes at the wheels, in combination with an optional

engine retarder or hydraulic transmission retarder to reduce vehicle speed. Kinetic energy is converted to

heat primarily by the friction brakes. This conventional configuration has a large brake system, heavy
4-68

brake heat sinks, high infrared signature at the wheels, audible brake squeal, and consumable components

requiring maintenance and replacement.

Hybrid electric propulsion systems recover some of the vehicle’s kinetic energy through regenerative

braking, where motive energy is captured and directed to an energy storage device. The remaining kinetic

energy is dissipated through conventional wheel brakes or brake retarders. The regenerative braking

system requires an air-pressure input from the friction brake system to function. Because the brake pedal

is linked to the friction braking system, application of the brake pedal causes the friction brakes to

engage. Consequently, there is always simultaneous operation of both the electric regenerative system and

the friction system, allowing only some of the kinetic energy to be captured electrically. This type of

hybrid braking system helps fuel economy, emissions, brake heat, and wear; however, further benefits

may be realized by the development of an electric regenerative braking system decoupled from the wheel

brake friction system.

Technical Targets

A brake-by-wire (BBW) braking system provides the ability to capture the maximum amount of the

vehicle’s kinetic energy during deceleration, thereby reducing fuel consumption, emissions, weight, noise,

heat loss, and brake maintenance. In concept, the brake pedal would provide an electronic signal to a

brake system processor, the pedal would not be directly connected to the braking system as in current

practice air brake treadle. From the pedal signal the processor would determine the amount of braking

torque requested by the operator. The processor would then command the propulsion control system to

provide regenerative braking as required, up to the maximum capacity of the energy storage and

propulsion system. The brake processor would command the pneumatic or hydraulic brakes to actuate and

assist in slowing the vehicle only when the braking torque required were to exceed the regenerative

capacity of the propulsion system.

Maximizing regenerative braking energy recovery has benefits in the areas of fuel economy, emissions,

weight, heat, noise, and maintenance. Today, hybrid drive systems typically activate regenerative braking

as the brake pedal is depressed to a maximum comparable to an aggressive transmission. As the brake

pedal is further depressed, the mechanical brake pressure continues to rise until the brakes reach full

brake pressure in a hard brake application [about 45 lb (20 kg) force]. Based on the capacity of a typical

energy storage device the amount of regenerative braking can increase by as much as 75%. Currently the

regenerative braking recovers 15% to 19% of the inertial energy during a CBD14 test cycle. This test

cycle represents urban stop-and-go operation, from 0 to 20 to 0 mph 14 times, maintaining the 20 mph for

10 seconds and having a 7-second idle between cycles. The potential improvement from changes in the

regenerative brake control algorithm translates into a 7 to 13% fuel economy and emissions savings. New

electrical storage devices could raise this even further by allowing more power to be stored without

causing the voltage to rise beyond existing limits. Because most of the braking can be accomplished

electrically through the propulsion system the need for massive friction brakes is lessened, decreasing

overall vehicle weight and particularly unsprung weight for improved ride quality. The friction brakes

will not have to work as hard, thereby minimizing heat generation and reducing tire degradation and

brake maintenance. Also, because they work less, the friction brakes will create less noise.

Barriers

Technical barriers can be overcome if sufficient resources are made available. The cost of designing,

developing, and testing prototype regenerative braking systems could govern the rate of technology

development.
4-69

Technical Approach

Development of improved regenerative braking systems will involve cooperation among key component

suppliers and vehicle manufacturers to identify system requirements. Simulation will be performed to

estimate possible energy-savings benefits that will be realized for candidate vehicle platforms. Improved

energy storage systems will be needed. System design and prototype development will be carried out to

prove the concept.

Resistance braking will also be examined as an approach to dissipate braking energy and extend wheel

brake life.

4.6.4.6 Power Plant and Control System Optimization

Status of Technology

Most heavy-duty HEVs produced to date use commercially available internal combustion engines for onboard power generation. The engine’s displacement and power rating is generally lower for HEVs

because the energy storage system provides stored braking energy during accelerations or peak power

demands. Diesel-electric and natural gas-electric hybrids have operated successfully in commercial fleets.

Engine operating conditions are substantially different for conventional vehicles and HEVs. There are

opportunities to design a purpose-built engine for use in hybrid electric propulsion systems to improve

fuel efficiency. For instance, electronic controls can be used to “switch on” the engine such that it

generates electrical power near peak efficiency (at peak torque conditions), and does not operate at lowload and high-speed conditions, where efficiency is low.

Gas turbines and fuel cells have also been installed in prototype HEVs. High-speed gas turbines are wellsuited to series hybrid vehicles because there is synergism with the electrical generators. These generators

can be very small and efficient when designed for the turbine’s high rotational speed. Reformer-equipped

fuel cell engines, which may have limited response rates, can use the hybrid’s energy storage system to

augment acceleration power. Aside from such major departures in vehicle engines, large benefits are still

envisioned from hardware changes which capitalize on specific operating zones and limited rates of

change, which are the cornerstones of hybrid engine operation.

First-generation heavy-duty HEVs have met or exceeded expectations for fuel economy and emissions

reductions. These vehicles were built with predominantly “off-the-shelf” commercial components,

including the engine, battery, and generator. Although these components have worked in the new hybrid

application, further energy-efficiency gains may be realized when components and controls are designed

with the hybrid system in mind. Cost and efficiency gains may be realized if components can be

combined into fewer, more integrated packages.

Technical Targets

Current series-hybrid transit buses operating in New York City have about 1.25 times greater fuel

economy than conventional diesel buses. Further development of on-board power-generation systems and

optimization of those systems is expected to improve the fuel economy of the vehicle by an additional

20%.

Barriers

Alternative power plants, such as fuel cells and gas turbines, are less mature than mass-produced internal

combustion engine technology. These alternative power plants will require extensive R&D to match

diesel engine efficiency, reliability, and operating cost.
4-70

Technical Approach

Power plant performance, emissions and efficiency may be improved by pursuing these research

activities:

• Internal combustion engines further optimized for efficient operation in hybrid vehicle applications.

This may also include reduction of parasitic loads by replacing gear-driven accessories with higherefficiency electrically driven accessories.

• Novel internal combustion engine concepts that may be inappropriate for conventional drivetrains.

HCCI, for example, has the potential to overcome the current-day NOx, PM, and fuel-economy tradeoffs without extensive exhaust aftertreatment systems. Other combustion concepts may be utilized to

greater advantage in hybrid systems as well.

• Development of alternative power plants, such as fuel cells and gas turbines.

• Exhaust aftertreatment devices, which may have higher emission conversion efficiencies in a hybrid

electric propulsion system with smart controls.

Several key areas of innovation are available for control hardware and software that can bring about

further improvements in the safety, emissions and fuel economy of HEVs.

On-board diagnostics. Hybrid vehicles employ energy storage devices to reduce emissions. Some form

of on-board diagnostics (OBD) can be used to monitor and flag conditions that could upset the emissionssaving effect of the energy storage system. Currently, industry uses OBD sensors on the engine alone.

Research is required to invent monitors for energy storage devices.

Global positioning systems. Interface for Energy Storage Scheduling: Many have proposed that globalpositioning systems (GPSs) will allow hybrid controls to take into account the road situation before it is

encountered. Anticipating stops, starts, hills and dales will allow for gradual adjustment of the state of

charge (SOC) of the energy storage system. In turn, the engine will be buffered from abrupt or large

departures from optimum conditions.

Advanced control algorithms. Today’s controls rely on predetermined truth tables and fixed or adaptive

calibration tables for actuation of operations. Fixed logic is poor because it does not handle component

aging or unexpected operating conditions. Adaptive controls alleviate these concerns but may not be

sufficiently robust for hybrid systems. Researchers are considering “fuzzy logic” and neural networks as

tools to capitalize on the control flexibility of hybrid propulsion. However, there is no clear path choice of

this or any other alternative algorithm strategy.

Improved energy management strategies. Hybrid vehicles require operating and control strategies that

coordinate the allocation of energy flow through a hybrid system. Energy allocation is the key to the

emissions, fuel economy, and performance of the vehicle. Improved strategies are required to adapt to

various driving cycles, driving patterns, and other requirements to optimize the overall system

performance. These strategies will also be required to coordinate braking implementation from multiple

sources such as regeneration, service brake, retarder, engine exhaust brake, and electrical grid. With all of

the choices available to the controls engineer, simulation will be the key to good vehicle strategy.

Control feature development. Control algorithm development is required to define and implement

features such as trap regeneration, hill-hold, anti-lock braking system (ABS) coordination, traction

control, and other safety and driveability improvements.

High-speed communication networks. Current heavy-duty vehicles predominantly utilize the SAE

J1939 Communication Area Network (CAN). This system has been adequate for conventional enginetransmission systems and current hybrid prototypes, but the number of distributed controls for safety,

economy, and emission controls on future trucks will stress the data communication of CAN systems.
4-71

EPA requirements for on-board diagnostics must also be considered for future vehicle communication

technology.

Hybrid tools and standards commonality. It would benefit the hybrid drive industry to have common

diagnostic tools for service and data acquisition between various components such as engines, APUs,

batteries, power electronics, and system controller.

System architecture optimization. Trade-off studies are required to develop and optimize the hybrid

vehicle system architecture for both series and parallel configurations. This will include combining

cooling systems to reduce weight and components, techniques for dissipating excess braking energy,

evaluation of auxiliary power systems, integration of the power electronics and vehicle control functions,

simplification of the overall system to improve reliability and reduce cost, and evaluation of the control

impact of alternative energy storage systems.

4.6.5 Mechanical Hybrid Truck Technology

4.6.5.1 Status of Technology

Hybrid technology offers a new way to design and build trucks in order to double and triple the fuel

economy under the 21st Century Truck Program. The basic components in the design of mechanical

hybrid trucks are functionally similar to the components in electric hybrids; however, the detailed design

and implementation varies substantially. There are at least two critical advantages that make mechanical

hybrids worth considering along with electric hybrids as a technological solution to meet the Program

goals. First, mechanical hybrids are likely to be able to capture significantly more of the regenerative

braking energy because the mechanical storage system generally has higher specific power than a battery

system. Second, mechanical hybrid storage components may last through several rebuilds of a truck,

whereas batteries are likely to be replaced two to four times during the life of the truck.

Figure 4.14 illustrates simple schematics for conventional and “parallel” mechanical hybrid trucks. The

defining characteristic of a mechanical hybrid, of course, is that it has two power sources. Here, the

“advanced engine” is the primary power source. The mechanical and electric storage systems are

analogous in concept. For example, a mechanical design could use a hydraulic pump for storing

pneumatic energy in an accumulator or a flywheel; an electric design would use a generator to store

energy in batteries or an ultra-capacitor. Finally, the pneumatic system would use a hydraulic motor to

retrieve the mechanical stored energy; the electric system would use an electric motor to retrieve energy

from batteries and deliver it to the drive shaft.

Hybrid propulsion systems provide a “buffer” between the power required to propel the truck and the

power produced by the engine; this buffer moderates the variation of power demand experienced by the

engine. The buffer also allows regenerative braking (recovering much of the energy otherwise lost in the

brakes) because it can receive and store energy from both the engine and other sources as well. One of the

significant benefits of mechanical hybrids over electrical hybrids is that mechanical systems have a much

greater capacity to store the recovered energy. The overall effectiveness of a mechanical hybrid

propulsion system depends on its ability to operate the engine at peak efficiencies and on the capacity and

efficiency of the buffer medium.

Many mechanical hybrid designs are possible. Major contenders for the “primary” power source are

piston engines and gas turbines. Major options for the “secondary” power source include flywheels,

hydraulic accumulators, and heat batteries. By pairing up the various primary and secondary power

sources, it is possible to identify a large number of potential mechanical hybrid truck designs.
4-72

Figure 4.15 illustrates the various ways that hybrid trucks can process and store energy. Energy stored

chemically in a fuel can be “transformed” to heat energy on the mechanical side (e.g., through a

combustion engine) or to electric energy on the electrical side (e.g., through a fuel cell). A heat engine

could be used as the primary power source, either for mechanical hybrid storage or for electric hybrid

storage. Likewise, a fuel cell could be used as a power source for either type of hybrid storage, but it is

much more likely to be used in an electric hybrid.

4.6.5.2 Potential Efficiency of Mechanical Hybrid Trucks—Technical Targets

This discussion will focus on changes to three primary design areas to achieve the 3× fuel efficiency goal;

engine efficiency, drivetrain efficiency, and regenerative braking efficiency.

The first key source of energy loss in a conventional truck is simply an inherent aspect of today’s internal

combustion engine, and how the engine and its drivetrain are operated in the truck. The efficiency of an

engine is limited by thermodynamics, but the best engines today only achieve peak efficiencies in the

Fig. 4.14. Schematics for conventional and parallel mechanical hybrid vehicle designs.
4-73

40 to 45% range, and their average efficiency is significantly lower. Average engine efficiency in current

trucks is the result of a poor match between engine power capacity and average power demand. The peak

efficiencies of an engine occur at high loads, yet many trucks operate much more frequently at low loads,

where the engine efficiencies are much lower. When the engine is the only source of power on board a

truck, it must be sized to meet the highest power requirements (such as when commanding maximum

acceleration from 0 to 65 miles per hour or towing a load up an extended grade) even if those high power

levels are needed only occasionally. However, the transient nature of truck operation, particularly in

urban driving, results in very inefficient operation of the engine. The bottom line is that, based on these

and other factors, an engine that might have a peak efficiency of over 40% might only average 15 to 20%

efficiency in typical urban driving.

The second source of energy loss is inherent in the drivetrain used in conventional trucks, which have

drivetrain efficiencies in the low 70% during urban driving. A mechanical hybrid drivetrain can have

efficiency improvements near 10%.

A third critical source of energy loss in trucks is braking. In contrast to acceleration, which delivers

energy to the wheels, braking removes energy from the wheels. Because an internal combustion engine

can only produce and not reclaim energy, a conventional power train is a one-way energy path. Braking is

achieved by a friction braking system, which renders useless the temporarily unneeded kinetic energy of

the truck by converting it to heat. In city driving, braking can waste up to one-half of the useful energy

that the engine is able to provide to the wheels.

Tables 4.19 and 4.20 illustrate what fuel efficiency could be achieved by utilizing mechanical hybrid

components in a typical Class 2b or Class 6 truck. Case 1 shows the baseline efficiencies for a

conventional diesel Class 2b or Class 6 truck. Cases 2, 3 and 4 reveal improvements that are directly

related to the engine and mechanical hybrid systems. Cases 5 through 10 illustrate some of the additional

efficiencies that are possible by reducing the energy demands resulting from aerodynamic drag, rolling

resistance, and weight. Other cases could be added to illustrate designs that factor in improvements such

as accessory power and exhaust heat recovery. All the modeling that was completed had assumed EPA’s

Fig. 4.15. Various ways that mechanical and electrical hybrid vehicles can

process and store energy.
Table 4.19. Efficiencies of diesel mechanical hybrid Class 2b trucks (small)

Efficiency (%) Configurationa Engine Drivetrain Regen brake Fuel economy (mpg) Factor increase Configuration Case

Af Cd Crr lb City Hwy City Hwy City Hwy City Hwy Comb City Comb

Conventional gasolineb 0 3.7 0.47 0.01 9,000 20 28 70 79 0 0 10.3 18.0 12.8 0.7 0.7Conventional dieselc 1 3.7 0.47 0.01 9,000 29 33 70 79 0 0 15.0 21.2 17.2 1.0 1.0 Diesel mech. hybridd 2 3.7 0.47 0.01 9,000 37 39 76 76 67 66 30.2 25.9 28.1 2.0 1.6Diesel mech. hybride 3 3.7 0.47 0.01 9,000 42 42 76 76 67 66 34.3 27.8 31.0 2.3 1.8Diesel mech. hybridf 4 3.7 0.47 0.01 9,000 42 42 85 85 80 80 41.6 31.8 36.5 2.8 2.1 Diesel mech. hybridg 5 3.7 0.38 0.01 9,000 42 42 85 85 80 80 44.1 35.6 39.8 2.9 2.3Diesel mech. hybridh 6 3.7 0.38 0.0075 9,000 42 42 85 85 80 80 51.1 40.2 45.5 3.4 2.6Diesel mech. hybridI 7 3.7 0.38 0.0075 8,200 42 42 85 85 80 80 54.5 41.8 47.9 3.6 2.8Diesel mech. hybridj 8 3.7 0.34 0.0075 8,200 42 42 85 85 80 80 56.4 44.5 50.3 3.8 2.9Diesel mech. hybridk 9 3.7 0.34 0.0075 7,400 42 42 85 85 80 80 60.6 46.5 53.3 4.0 3.1 Diesel mech. hybridl 10 3.7 0.34 0.006 7,400 42 42 85 85 80 80 66.7 50.2 58.1 4.5 3.4 

aAf = frontal area; Cd = drag coefficient; Crr = rolling resistance coefficient. bGasoline baseline.cDiesel baseline.dEngine running at peak with secondary storage and regenerative braking.eAdding advanced high-efficiency engine. fImproving drivetrain and recovery from braking.gLowering Cd by 20%.hLowering Crr by 25%.iLowering weight by 2,000 lb (10% of unloaded weight).jLowering Cd by an additional 10%.kLowering weight by 2,000 lb more (20% of unloaded weight). lLowering Crr by an additional 20%.

4-74
Table 4.20. Efficiencies of diesel mechanical hybrid Class 6 trucks (medium)

Efficiency (%) Configurationa Engine Drivetrain Regen brake Fuel economy (mpg) Factor increase Configuration Case

Af Cd Crr lb City Hwy City Hwy City Hwy City Hwy Comb City Comb

Conventional gasolineb 0 7.1 0.6 0.008 22,000 20 28 70 79 0 0 4.4 7.9 5.5 0.7 0.7Conventional dieselc 1 7.1 0.6 0.008 22,000 29 33 70 79 0 0 6.4 9.3 7.5 1.0 1.0 BDiesel mech. hybridd 2 7.1 0.6 0.008 22,000 37 39 76 76 67 66 13.6 11.5 12.5 2.1 1.7Diesel mech. hybride 3 7.1 0.6 0.008 22,000 42 42 76 76 67 66 15.4 12.3 13.9 2.4 1.9Diesel mech. hybridf 4 7.1 0.6 0.008 22,000 42 42 85 85 80 80 18.9 14.1 16.4 2.9 2.2 HDiesel mech. hybridg 5 7.1 0.48 0.008 22,000 42 42 85 85 80 80 20.3 16.1 18.1 3.1 2.4Diesel mech. hybridh 6 7.1 0.48 0.006 22,000 42 42 85 85 80 80 23.1 17.9 20.4 3.6 2.7Diesel mech. hybridI 7 7.1 0.48 0.006 20,000 42 42 85 85 80 80 24.6 18.5 21.5 3.8 2.9Diesel mech. hybridj 8 7.1 0.43 0.006 20,000 42 42 85 85 80 80 25.6 19.8 22.6 4.0 3.0 EDiesel mech. hybridk 9 7.1 0.43 0.006 18,000 42 42 85 85 80 80 27.4 20.7 23.9 4.3 3.2 R

a

Af = frontal area; Cd = drag coefficient; Crr = rolling resistance coefficient. bGasoline baseline.

cDiesel baseline.dEngine running at peak with secondary storage and regenerative braking.eAdding advanced high-efficiency engine.fImproving drivetrain and recovery from braking.gLowering Cd by 20%. hLowering Crr by 25%. iLowering weight by 2,000 lb (17% of unloaded weight).jLowering Cd by an additional 10%. kLowering weight by 2,000 lb more (33% of unloaded weight).

4-75
4-76

standard transient driving cycle for light-duty vehicles. When used in an urban setting, comparable

efficiencies are possible for a Class 8 transit bus, and a Class 8 mechanical hybrid truck.

4.6.5.3 Barriers to Implementation

This section summarizes the key implementation changes and potential barriers for mechanical hybrids to

achieve the Program goals. Additional details of these challenges are described in the mechanical hybrid

Appendix G.

Secondary Energy Storage/Release

Accumulators and flywheels perform a similar function to a battery in an electric hybrid. They are able to

quickly absorb and release large amounts of energy (high specific power). A 95% efficiency is needed to

meet the fuel economy improvement goal.

Important design parameters for accumulators include weight, specific energy (energy storage density),

cycle efficiency (charge/discharge efficiency), cost, safety, and optimum storage pressure. A 30-gal

accumulator (for a Class 6 truck) made of steel weighs approximately 600 lb (272 kg). The challenge is to

reduce weight, most likely with high-technology materials, without increasing cost. Specific energy will

be improved by reducing accumulator weight and by increasing system pressure. Cost reduction will be

accomplished by utilizing advancements in compressed natural gas storage containers and by high

production volumes. Safety will be enhanced by new materials, such as Kevlar and other strong light

weight fibers, acting as a “blanket” allowing the release of gas pressure but inhibiting the release of highpressure oil or vessel fragments. Trade-offs between weight, cost and specific energy will determine the

optimum storage pressure.

The efficiency of flywheels may be improved by evacuation of the cage and air bearings. The trade-off

between low speed/high mass and high speed/low mass will be investigated. Structural integrity will be a

major design issue.

Power Management and Transformation

Power must be efficiently transferred from the source(s) to the road. Hydraulic hybrid designs include

control valves, actuators and clutches. Flywheel hybrid designs use CVTs, clutches, and conventional

transmissions. Overall, power management efficiency needs to be between 90 and 95%. Power

transformers with an unlimited number of “gear” ratios (CVTs, pumps/motors) will allow the engine to

operate at its most efficient point. Systems will be complicated to control, especially when regenerating

energy.

Regenerative Braking

In urban driving, braking can waste one-half or more of the total energy to the wheels. In a hydraulic

hybrid, the energy is recovered by using a hydraulic pump to store braking energy in the accumulator for

reuse to power the truck or for other purposes. In a mechanical hybrid, the energy is recovered by using a

CVT to spin up the flywheel. The CVT is then used in reverse fashion to transfer the energy back to the

vehicle. Efficiency of energy recovery, safety, and cost are the most important issues. Based on hardware

currently available, none of which has been optimized for automotive

applications, overall net efficiency is around 60%. The long-term goal is to increase it to 80%. Ultimately,

regenerative braking will have to be fully integrated with anti-lock systems.
4-77

Control Technology

Hybrid systems will require more complex control systems. The control problem is complicated by the

need to coordinate and optimize the utilization of two different power sources (the engine and the

flywheel or hydraulic system) as well as the additional components associated with the regenerative

braking. The challenge will be to minimize the added complexity.

Accessories

Today accessories are powered inefficiently. Because they are sized for low engine speeds, a lot of power

is wasted at high engine speeds. In a mechanical hybrid, the energy for the accessories could come from

the accumulator or the flywheel. This will allow the engine to be shut down when the power demand is

low. The Program designs will consider the incremental fuel economy benefit against the cost of changing

accessory components.

4.6.5.4 Technical Approach

• Develop designs and materials for safe, high-density-energy storage accumulators and flywheels for

secondary energy storage.

• Develop very high-efficiency power transformer devices.

• Develop very high-efficiency (greater than 80%) regenerative braking systems that are safe and

dependable.

• Develop hybrid control technology to optimize power management, including auxiliary devices.

4.6.6 Fuel Cells

A fuel cell is an electrochemical device that generates electricity by harnessing the reaction of hydrogen

and oxygen. Because it converts the chemical energy of a fuel into electrical energy without combustion,

the process is highly efficient and extremely clean. There are many types of fuel cells—proton exchange

membrane (PEMFC), molten carbonate (MCFC), solid oxide (SOFC), phosphoric acid (PAFC), alkaline

(AFC)—distinguished by the electrolytes that are used and the temperatures at which they operate

(Appleby 1989, Larminie and Dicks 2000). All fuel cells can use pure hydrogen as a fuel or can use

reformed hydrocarbon fuels such as gasoline, diesel, methanol, ethanol, or natural gas.

Three fuel cell types, PEMFC (Chalk 1999), SOFC (Solid State Workshop 2000), and AFCs, are

applicable for use as prime power or auxiliary power units for vehicles. The PEMFC capitalizes on the

simplicity of the cell. The electrolyte is a solid polymer in which protons are mobile. PEMFCs operate at

low temperature (90 to 110°C) and use noble metal catalysts on the electrodes to provide acceptable

reaction rates. All fuel cells are composed of the same core building block, a single cell, which when

layered makes a stack. Each component of the single cell serves a unique purpose, without which it could

not function properly. In the PEMFC, hydrogen gas ionizes at the anode, releasing electrons and creating

protons. At the cathode, oxygen ionizes and reacts with protons to form water.

The SOFC operates at high temperature (600 to 1,000°C), which allows the electrochemical reactions to

proceed without the need for noble metal catalysts (Minh and Takahashi 1995). The high operating

temperature also allows certain fuels to be internally reformed in the fuel cell stack, eliminating the need

for an external reformer. Although the SOFC is inherently simple, the ceramic materials and

manufacturing methods can be expensive. The cathode is the electrode on the air side of the fuel cell that

provides the electrocatalytic activity and the surface on which molecular oxygen is ionized. The

electrolyte conducts the oxygen ions to the anode. The anode serves to catalyze the fuel to form H2, and

CO or CO2 and serves as the place where oxygen ions, conducted through the electrolyte, react with

hydrogen to form steam and electricity.
4-78

The AFC has two porous electrodes, which are separated by a circulating potassium hydroxide electrolyte

operating at low temperature (70°C). Hydrogen is passed over the outside of one electrode, and air is

passed over the other. AFC systems operate well at room temperature; they yield the highest voltage of all

fuel cell systems, and cell and electrodes can be built from low-cost carbon and plastics.

Fuel cells may offer unique performance benefits that may be integrated into heavy-duty vehicles.

Because fuel cells are electric power plants, there will be additional opportunities to more easily integrate

electronic components such as GPS tracking, laptop access to the Internet, paging/fax devices, traffic

information devices, climate-controlled seats, and more, into vehicles. Electrically driven accessories that

can enhance the overall fuel efficiency of an electric vehicle will also enhance the overall efficiency of a

fuel cell vehicle.

4.6.6.1 Status of Technology

A.D. Little recently completed a study for DOE to assess the current state of fuel cell development and

the capability to meet the fuel cell technology goals established by PNGV (A. D. Little 2000). Although

there are significant differences between the requirements for light-duty and heavy-duty vehicle

applications, the wealth of information developed for fuel cells as part of the PNGV program and other

automotive efforts provides a valuable starting point. The study assessed the cost of a 50-kW PEMFC

system for transportation applications, including a multi-fuel-capable reformer, the fuel cell stack, and

balance-of-plant components. The study concluded that, using currently available technology, the 50-kW

fuel cell system (if it were factory produced, assuming volumes of 500,000 vehicles) would cost $14,700,

or $294 per kilowatt.

It has been estimated that about $1 billion a year is now being spent on fuel cell technology development,

with transportation applications accounting for the lion’s share of this support. DaimlerChrysler alone has

stated that it will spend $1.4 billion over the next four years to bring fuel cell vehicles to the marketplace.

It is important to note that the automotive effort is focused on PEMFC technology.

While most of the attention has focused on fuel cell applications for passenger vehicles, the basic fuel cell

power plant potentially can be used across a range of light- and heavy-duty vehicles. A significant

difference between light-duty and heavy-duty applications is the current durability of automotive engines

(gasoline) and truck engines (diesel). The useful life of an automotive engine is 5,000 hours/100,000

miles. In contrast, truck or bus engines have expected useful lives of 20,000 to 25,000+ hours/500,000 to

750,000 miles. It is anticipated that the useful mileage will grow to 1,000,000 miles in the future.

Additionally, the current fuel of choice for heavy-duty vehicles is diesel, contrasted with gasoline for

light-duty vehicles.

Whether the fuel cell technology being developed for the automotive market, PEMFC, will also satisfy

the truck and bus market is not certain at this point. However, SOFCs have some characteristics that

warrant their consideration for truck and bus applications. These include (1) high efficiencies (as high as

70% for a simple cycle); (2) lower electrode polarization/ohmic losses without use of catalysts; (3) ability

to use fuels containing CO and inorganic impurities, such as sulfur; (4) fuel flexibility using direct and

indirect reforming of logistic, common, and complex liquid fuels; and (5) operation over a wide range of

temperatures from 600 to 1,000°C.

There are two major transportation applications for SOFCs: (1) auxiliary power and (2) prime power to

replace diesel engines, particularly those that run at varied power levels and in continuous operation

(idling).

A central issue in transportation applications is power density, or energy per unit volume, which

determines how well a fuel cell plant will fit into vehicles. Preliminary calculations indicate that as fuel

cell power densities approach three times the current state of the art, the size of a SOFC power unit would
4-79

equal the size of an internal combustion engine on a comparable power-out basis. However, the current

SOFC would easily fit into a large diesel or locomotive without any changes. A transportation engine of

nearly 70% efficiency would greatly reduce CO2 emissions (by 36%) and reliance on imported petroleum.

Relative to internal combustion engine technology, a multistage fuel cell engine may increase cruising

ranges dramatically.

Fuel cell transit bus demonstrations are ongoing in the United States, Canada, and Germany. Running on

fuels such as natural gas, hydrogen and methanol, these buses are much cleaner than even the most

advanced diesels. For example, tests performed on Georgetown University’s second-generation fuel cell

bus—featuring a methanol-powered PAFC fuel cell power plant built by International Fuel Cells (IFC)—

showed zero emissions of PM and hydrocarbons and near-zero emissions of CO and NOx.

Ballard Power Systems has used fuel cells to power ten transit buses, six of which have been used in

revenue service. Ballard has established an alliance with DaimlerChrysler and Ford to form EXCELLSiS

Fuel Cell Engines, Inc., a company dedicated to developing and commercializing PEMFC systems for

cars, buses and trucks. EXCELLSiS plans to begin commercial production of its Phase 5 fuel cell engine

by the end of 2002. The company plans to produce transit bus fuel cell engines by 2007 that are costcompetitive with buses using CNG.

Thor Industries, IRISBUS, and NovaBus (Volvo) have received or are working with IFC to develop

commercial vehicle fuel cell power plants for transit bus applications. IFC power plants used in these

activities are fueled with hydrogen, California Reformulated Phase II Gasoline, or methanol. Delphi

Automotive Systems also is developing PEMFC power plants for traction applications. In addition, IFC

and Freightliner are developing PEMFCs for auxiliary power applications, and Delphi is focusing on

SOFCs as auxiliary power units.

Several companies are working on fuel processing systems for fuel cell vehicle applications. Nuvera Fuel

Cells announced plans to ship complete gasoline-powered fuel processing systems to four major

automotive companies in the United States, Europe, and Japan for testing in demonstration vehicles.

McDermott Technology and Catalytica Advanced Technologies are developing a compact catalytic fuel

processor capable of processing multiple fuels into a hydrogen-rich gas. General Motors and Exxon have

developed a gasoline fuel processor that puts 80% of the hydrogen it generates into the fuel cells. IFC has

developed and is delivering to auto makers a fuel processor that generates hydrogen from California

Phase II reformulated gasoline. IFC and Shell Hydrogen are establishing a company to develop and sell

fuel processors for automobiles, buses, power generators and hydrogen filling stations.

Companies such as IMPCO Technologies are developing on-board storage tanks for hydrogen that can be

fueled to 5,000 to 10,000 psi. Hydrogen Burner Technology has introduced a skid-mounted system

designed for on-site hydrogen generation that can be coupled with compression, storage, and delivery

systems to provide high-purity hydrogen as part of a hydrogen refueling station. The company was

recently awarded a contract with the California Air Resources Board to build a hydrogen fueling station to

serve fuel cell buses built by EXCELLSiS and operated under the California Fuel Cell Partnership.

Although much R&D is under way and tremendous progress has been made, a number of hurdles remain.

Improvements are needed in a number of areas, including cost, durability and reliability, operating and

maintenance performance, hydrogen carrier or fuel of choice, and related infrastructure.

4.6.6.2 Technical Targets

The 50-kW fuel cell power plants being developed for passenger vehicles will be adequate for many truck

applications. For larger trucks and buses, two or more 50-kW fuel cell power plants can be modularly

integrated for higher power output. Therefore, it is useful to consider the technology targets already

developed for the PNGV program as applicable for the heavy-duty market. Currently, the light- and
4-80

heavy-duty markets are focusing on PEMFC technology because of system efficiencies and low operating

temperatures. However, SOFCs can operate at higher temperatures and are much more compatible with a

broader range of hydrocarbon fuels, including diesel; therefore, they might be applicable in the heavyduty markets.

The PNGV year 2004 goal for fuel cell system cost is $50 per kW or $2,500 for the entire 50-kW system.

The system cost can be broken into several sub-systems: fuel cell (60%), fuel processor (29%), balanceof-plant (3%), and assembly and indirect (8%). There are also several PNGV performance targets,

including power density, 300 W/l; specific power, 300 W/kg; durability, 5,000 hours; energy efficiency,

48% at 25% of peak power; transient response, 10 seconds from 10 to 90% power; and start-up to full

power, 30 seconds.

While these costs and performance targets may be applicable to most heavy-duty applications, there may

be truck markets and duty cycles that require significant adjustments to these targets. For example, the

durability target for PNGV vehicles may not be adequate for the heavy-duty market where transit buses

may be sold with a 500,000-mile, 12-year warranty. On the other hand, fleet trucks may not be as

demanding as personal transportation vehicles in requirements for start-up to full power.

The PNGV program also has required that fuel processors for fuel cell systems be capable of processing a

number of potential fuels. While reforming gasoline has been a principal focus for light-duty applications,

the heavy-duty market is likely to be more interested in the reforming of diesel. Further, the on-board

storage of hydrogen may be more conducive to fleet applications, where vehicles operate on fixed routes

and return to a central facility that could include a hydrogen fueling station.

4.6.6.3 Barriers

PEMFC

Many of the technical barriers that have been identified by the PNGV program are consistent with the

challenges facing the application of fuel cells in trucks and buses. These barriers for PEMFC technology

can be roughly divided into five areas: efficiency, thermal air/water management, fuel processing/storage,

durability, and cost.

The A. D. Little study (A. D. Little 2000) found that current PEMFC technology produced energy

efficiencies of 34% at 25% of peak power, significantly below the PNGV target of 48% energy

efficiency. More recent results from IFC’s ambient-pressure PEM power plants tested by OEMs are more

promising: 48% efficiency at 100% power and 60% efficiency at 20% power. Preliminary results from

IFC’s 50-kW power plant are more promising, with 48% efficiency at 100% power and 60% efficiency at

20% power. Current production diesel engines have peak efficiencies of 42% and could potentially

advance to 50%. This presents an even greater challenge.

To improve overall energy efficiency, higher cell voltage is required. Fuel cell stacks currently operate at

about 0.6 to 0.7 volts and must attain voltages of approximately 0.9 volts to meet PNGV’s target of 40%

energy efficiency at 25% of peak power. There is a trade-off, however, between increased efficiency and

power density. In obtaining higher operating voltages to increase efficiency, the fuel cell must operate at

lower power densities. Consequently, the stack size (i.e., system size) will have to be increased to meet

vehicle power demands. One limitation on raising the cell voltage is the polarization losses at the cathode.

If the operating voltage is to be raised to improve efficiency, oxygen augmentation should be examined.

A host of balance-of-plant issues also must be resolved. For example, the use of off-the-shelf compressors

results in significant parasitic power losses that reduce overall system performance.

Water management is a significant issue for PEMFC systems, which must be hydrated to optimal levels

using closed-loop control systems and pure (deionized) water. Compact, lightweight, and efficient water

condensation and recovery systems must be devised to provide deionized water. For military and other
4-81

remote vehicle applications, this source of pure water could be used for logistics purposes. Since the

overall power requirement for truck and buses is not greatly different from that of cars, but much more

volume is available; therefore, thermal and water management problems in truck and bus applications are

greatly reduced.

The technical and practical challenges of using hydrogen in a fuel cell system have largely been

successfully overcome. However, the storage of hydrogen aboard the vehicles is a continuing barrier,

given the molecular structure of hydrogen. As previously mentioned, storage tanks are being developed to

accommodate hydrogen at 5,000 to 10,000 psi. New approaches for metal hydride storage are also very

promising.

One of the principal challenges of developing fuel processors for hydrocarbon fuels (gasoline and diesel)

is reducing sulfur levels in the output hydrogen. Up to now, sulfur removal has not been made a priority;

however, sulfur can adversely impact the operation of a fuel cell. While companies like IFC and Nuvera

have been successful in removing sulfur from gasoline, there has been much less effort to reduce the

sulfur levels of diesel fuel in an on-board fuel processor. Being able to use logistic fuels is critical for

military applications. The capability to use readily available fuels within local theaters of operations is

essential for the military.

Additional barriers associated with the use of fuel processors include reducing their size and weight,

cutting start-up times, improving transient response, improving catalyst durability, and improving CO

cleanup. CO, which can degrade the performance of the fuel cell stack, is produced during fuel

processing. Researchers are now developing gas cleanup systems that can reduce CO to levels that can be

tolerated by the fuel cell stack.

Urban buses and trucks may operate in very harsh environments that could pose daunting challenges to

the durability of advanced fuel cell systems. The electrochemical reaction performed by fuel cells is

initiated by catalysts on both the cathode and anode. These catalysts are likely to degrade over time, as

will the high-temperature catalysts found in fuel processors. Designing catalyst loadings that can

withstand repeated operation over extended periods of time is a continuing challenge for fuel cell

developers. The heavy-duty market will be much more demanding on durability than will light-duty

applications.

Significant progress has been made in reducing the costs of PEMFC systems and their subcomponents,

but much more remains to be done to reduce costs from today’s $300/kW to the 2004 target of $50/kW.

The fuel cell stack itself is made of catalyst-coated, solid graphite plates surrounding a thin, membrane

film. Companies such as 3M, DuPont, and W. L. Gore are working to develop effective membranes that

can be mass-produced at a low cost (the PNGV cost target is $10/kW). Most PEMFC stacks use

expensive platinum as the catalyst, accounting for 20% of the total system cost. Reducing platinum

loading is a key challenge in the commercialization of PEMFC technologies.

SOFC

Several challenges and barriers must be overcome for SOFCs to be used for stationary and transportation

power applications. The main barriers for use of SOFCs relate to integration of improved design,

materials, and fabrication processes to increase performance and lower fabrication and manufacturing

costs. Alternative configurations for SOFCs, such as advanced planar designs, are required to achieve

high-power generation and lower fabrication costs. The materials issues that must be addressed include

the development of low-temperature mixed conducting electrodes, metallic interconnects with a

conductive coating, and seal technology. The largest barrier to SOFC technology is cost, which needs to

be addressed by developing new fuel cell designs and carrying out in-depth costing at production levels

approaching those required for heavy vehicles.
4-82

SOFC technology requires a change of emphasis from large SOFC power generation to small generators

that can be scaled to larger size. Reduction in cold-start-up times and the ability of the device to cycle

thermally may also be barriers. Fuel processing and reforming of the more complex liquid fuels, and

stability in fuel impurities, must be addressed. One promising area for overcoming fuel reforming

problems involves internal reforming of fuels in SOFCs, which offers the promise of minimizing size,

mass, and complexity. This possibility is particularly attractive for military applications that require highperformance systems with long mean time between failures.

R&D must be directed toward solving these issues specifically as they relate to lower fabrication costs

and high performance. The issues are challenging and will require an integrated approach.

AFC

The technical barriers that need to be addressed for AFCs are Pt catalyst loading in the anode, CO2

contamination of the liquid electrolyte, and increasing the power density.

4.6.6.4 Technical Approach

Three critical areas of research have been identified for the heavy-duty vehicle market. One area would

focus on the advancement of PEMFC technologies that would build upon the PNGV and other

automotive fuel cell efforts. The focus of this path would be the issues of power requirement, durability,

cost, fuel of choice, operating and maintenance performance requirements as applied to heavy-duty

vehicles, water recovery, on-board sulfur trap development, and oxygen augmentation. For example,

where the PNGV program is primarily focused on the development of gasoline reformers for passenger

vehicles, this path would direct more attention to the reforming of diesel and Fischer-Tropsch diesel fuels

or other alternative fuels that are compatible with truck and bus requirements.

The second critical research area focuses on fuel reformation, on-board storage of H2, and infrastructure.

The heavy-duty market already has seen the use of CNG advance in the transit bus market. This

experience with compressed gaseous fuels can easily be translated into the use of hydrogen gas. The

transit industry’s experience with gaseous refueling stations and the on-board storage of gaseous fuels has

not been matched in the passenger vehicle market. For certain fleet applications, on-board storage of

hydrogen may be a viable option. Efforts to improve the on-board storage of hydrogen for these fleet

vehicles should be addressed.

A third research area is other fuel cell technologies that are more compatible with diesel fuel. Efforts

focused on SOFC technologies need to address lowering the operating temperatures, increasing power

densities, reducing start-up times, reducing the volume and weight of such systems as well as reducing

costs, developing or exploiting new SOFC designs that address these issues, and investigating internal

reforming. Efforts on AFC need to address the development of a regenerable CO2 scrubber for air and

reformed fuels, removing the Pt from the anode, increasing power densities, and reducing the weight and

volume of such systems.

4.6.7 Auxiliary Power

4.6.7.1 Status of Technology

“Auxiliary power” is a crosscutting technology that addresses the efficient and practical management of

both electrical and thermal requirements for all classes of trucks and buses in the 21st Century Truck

Program. Auxiliary power requirements are derived from many vehicle functions, including hotel loads,

engine and fuel heating, HVAC, lighting, auxiliary components (e.g., pumps, starter, compressors fans),

computers, entertainment systems, and on-board appliances (refrigerator, microwave, coffee pot, hot pad),

as well as work function loads such as trailer refrigeration and the operation of power lifts and pumps for

bulk fluid transfer.
4-83

Truck auxiliary power requirements by truck class are shown in Table 4.21. The range of power needs for

each truck class represents both current and future auxiliary power requirements.

Table 4.21. Truck auxiliary power requirements

Type of truck Potential load Power required

(kW)

Small truck HVAC, coolant pump, water pump, starter, oil pump, compartment

fans, catalyst heater, valve control, direct injection, electric

suspension, computer, communications, electric power steering,

heated windshield, lights.

1–5

Power site equipment (compressor, lights). 5

Medium truck HVAC, lights, coolant pump, starter, oil pump, water pump,

compartment fans, computer, communications, lights, heated

windshield.

1–5

Transit bus HVAC, lights, coolant pumps, air compressor, hydraulic pump/power

steering, doors, multiplex system, fuel pumps, starters, oil pump,

water pump, compartment fans, computer, farebox, communications.

30–40

Vocational Base electrical loads, lights, HVAC, battery charging,

communications, computer.

1–5

Power for task at idle (i.e., mixer, pumps, lifts). TBD

Tractor trailer Base electrical loads, lights, battery charging, communications,

computer.

1–5

Hotel loads: lighting, simple HVAC, computer, appliances. 3–5

Full truck electrification: all of the above, plus water and oil pumps,

starter, cooling fans, transmission and hydraulic system, brake

compressors, and fuel and air-handling systems.

5–15

Trailer refrigeration, other external power. Up to 30

Although all classes of trucks share similar auxiliary power functions, such as powering lights and

HVAC, the total required power load can differ significantly based on truck size and function. In general,

trucks that fall in the small-to-medium truck category require auxiliary power in the range of 1 to 5 kW

because of limited thermal loads (i.e., smaller passenger compartments lead to reduced HVAC

requirements), no hotel loads, and limited power requirement for work functions. On the other hand,

transit buses require large amounts of power, up to 40 kW, to drive auxiliaries that meet HVAC, braking,

and other functional needs. Class 8 trucks exhibit a range of power requirements from 3 to 30 kW,

depending on truck drive cycle and the type of work function considered.

The overwhelming majority of trucks on the road today derive auxiliary power from belt- or gear-driven

systems. These systems convert fuel energy to mechanical and electrical energy. Mechanical energy is

used to operate mechanical-based auxiliaries (such as pumps and compressors); electrical energy is used

for lights, ignition, fans, radio, and other electrical components. Although they are reliable, durable, and

commercially cost-competitive, belt- and gear-driven systems inefficiently convert fuel energy to

electrical or mechanical energy and tend to have constant outputs rather than supplying power on demand.

Estimates of current average power requirements for truck and bus air-brake and air-conditioning

compressors are shown as a function of duty cycle in Tables 4.22 and 4.23, respectively (SAE 1988). The

alternator power requirements for selected accessories for day and night constant-load operation are

shown in Table 4.24 (SAE 1988). As seen in Tables 4.22, 4.23, and 4.24, estimates of component power

requirements vary considerably based on truck function.
4-84

Table 4.22. Air brake compressor power requirements

Type of operation Type of engine Compressor intake Duty cycle (%) Average power

(kW)

Line-haul Gas NA 10 1.3

Diesel NA 10 1.5

Diesel Turbocharged 5 2.3

Short-haul Gas NA 20 1.5

Diesel NA 20 1.8

Diesel Turbocharged 10 2.8

Local-haul Gas NA 60 2.2

Diesel NA 60 3.0

Diesel Turbocharged 30 3.5

Table 4.23. Estimates of air-conditioning compressor power requirements

Duty cycle (%) Power (kW)

Line-haul tractor and trucks 50 2.2

Short-haul tractor and trucks 50 2.2

Local-haul tractors and trucks 50 2.2

Long-haul buses 50 11.2

Short-haul buses 80 17.9

Local-haul buses 80 17.9

Table 4.24. Alternator power requirements for day and

night constant-load operation

Includes headlights, high-beam indicators, taillights, clearance lights,

identification lights, marker lights, license plate lights, instrument lights,

instruments, ignition, field current alternator, and heater defroster

fan with air conditioner

Power required (kW)

Daytime constant load operation 1.2

Nighttime constant load operation 1.8

Auxiliary power needs and requirements are also strongly influenced by power requirements of line-haul

Class 8 trucks. An estimated 458,000 Class 8 trucks travel farther than 500 miles from their home base

each day and are likely to be idling during overnight stopovers (Stodolsky, Gaines, and Vyas 2000).

Assuming an average of six hours of idle time per day, this represents 566 million truck-hours of idle time

per year. Engine idling allows the driver to heat or cool his cab/sleeper, as well as operate appliances and

communications, computer, and entertainment equipment. Idling also keeps the fuel and engine warm

during cold weather, thus avoiding problematic engine cold starts. Idling to generate auxiliary power has

a significant impact on overall vehicle fuel efficiencies and emissions. For example, an estimated

838 million gallons of fuel is consumed and 9.6 million tons of CO2 is emitted annually for line-haul

trucks idling overnight to heat or cool the cab/sleeper, operate electrical accessories, and keep the fuel and

engine warm in the winter (Stodolsky, Gaines, and Vyas 2000). U.S. government hours-of-service

regulations that are pending could require Class 8 line-haul drivers to decrease the number of hours on the

road per day and increase frequency and length of rest periods. If enacted, proposed regulations could

increase the amount of time that drivers spend stationary in their trucks with the engine idling to provide

cabin heating or cooling and to operate accessories.
4-85

One technology being developed to reduce engine idle of Class 8 trucks during rest periods and to meet

future truck electrical power requirements is the auxiliary power unit (APU) (Stodolsky, Gaines, and

Vyas 2000). Auxiliary power units for Class 8 tractor-trailers are available today that are capable of

generating electrical and mechanical energy, independently of the truck engine operation. Existing APUs

are usually small internal combustion engines, equipped with a generator and heat recovery system to

provide electricity and heat. These APUs, which are typically externally mounted on the truck cab or

sleeper, supply upwards of 6 kW of power, weigh between 300 and 350 lb (136 to 159 kg) and occupy

approximately 8 cubic feet. Existing APU units can be utilized to heat and cool the truck cab or sleeper as

well as keep the battery charged and the engine coolant warm. Commercially available APUs have the

advantage of integrating with current truck components and are based on reliable and proven

technologies. Market penetration of APUs has been limited due to high initial cost ($500–$1,000 per

kilowatt) and potential loss of payload because of weight of the APU unit. High market penetration of

auxiliary power units, to reduce Class 8 truck engine idle during rest periods would have a dramatic effect

on fuel usage and emissions. It has been estimated that APUs could reduce auxiliary fuel usage and

emission by upwards of 80% (Stodolsky, Gaines, and Vyas 2000).

Fuel cells are also being developed as APUs for cars and trucks. The primary advantage of fuel cells is the

potential of high system efficiencies (up to 50%) in converting fuel energy to electrical energy, as well as

reduced emissions, weight, and noise. DOT has initiated under its Advanced Vehicle Program a project

effort with WestStart-CALSTART and a project team of freightliner and EXCELLSiS to develop a PEM

fuel cell APU system for Class 8 trucks. A demonstrator unit that uses on-board hydrogen has been

completed, and work is under way to develop this unit to operate on reformate.

Truck electrification involves removing the thermal and electrical loads from the truck engine by

transitioning from today’s belt- or gear-driven technology to an electrical “power on demand” system.

Managing where and when power is needed can provide many benefits, such as fuel savings, emissions

reductions, and productivity enhancements. In addition, the overall system derives a number of benefits

from the ability to provide flow, pressure, or power where needed for an engine function and from

continuous adjustment to different operating modes. For example, varying speed control of both the fan

and water pump during some operating conditions can result in improved net fuel efficiency as well as

noise reduction. Controlling an accessory independently of engine speed can reduce accessory drag

during engine cranking to enhance cold weather starting, or starting an electric engine oil pump just prior

to engine cranking can provide bearing lubrication. Truck electrification involves the use of a generator to

provide electrical power to drive auxiliaries independently of engine speed. Auxiliaries that will become

electrically driven include engine water and oil pumps, starter, radiator cooling fans, transmission and

hydraulic system pilot and scavenge pumps, air-conditioning and brake system compressors, and fuel and

air systems. The transition from belt- or gear-driven auxiliaries will occur with those emerging

electrically driven auxiliary components that yield the most benefit, reliability, and durability at a

commercially competitive cost.

Another option to reduce truck idling is truck-stop electrification. Truck-stop electrification would enable

trucks to plug into electrical outlets at truck stops during rest periods. This would allow truckers to power

electrically driven engine block heaters, fuel heaters, cab and sleeper heaters and air conditioners, marker

lights, entertainment equipment, communication equipment, and appliances. Truck-stop electrification

would require modifying both the truck and the truck stops. (Trucks would have to be modified to operate

on electrical inputs.)

Currently, only a limited number of truck stops supply plug-in power for trucks, although pilot tests are

planned to evaluate this concept.
4-86

4.6.7.2 Barriers

There are several barriers to the development of auxiliary power technologies that will efficiently meet

current power needs, address anti-idling issues, and meet future truck and bus power requirements. The

trucking industry operates on small profit margins. Fuel costs and payload weights are important factors

that directly affect profitability in the industry. Although many existing technologies have been

demonstrated, the technology development process must focus on technology options that ultimately can

be commercially viable. This includes the development of cost-competitive, safe, reliable, and durable

technologies. Existing technologies, such as a small combustion-engine APU, can play a significant role

in reducing fuel usage and emissions only if they are utilized by the trucking industry. Technologies must

be developed to reduce fuel utilization, minimize weight, and meet all current codes and regulations.

Complete electrification of the truck will require the development of energy-efficient and costcompetitive technologies as the industry transitions from belt- or gear-driven technologies to electrically

driven components. This same technology is also directly applicable to and will benefit many other

markets that utilize the same basic engines, such as buses, construction equipment, marine equipment, and

military equipment.

4.6.7.3 Technical Approach

The technical approach to addressing current auxiliary power requirements will include the following

steps:

• Conduct system analysis to evaluate potential technologies that support the electrification of

auxiliaries and reduce electrical requirements.

• Develop and evaluate low-cost and reduced-weight APU systems that will reduce idle time and that

can be integrated with current truck infrastructure.

• Develop and demonstrate cost-effective technologies that will enable the electrification of auxiliaries

by means of stationary power sources.

• Support development of industry standards for electrical system designs for heavy-duty vehicles to

assist in establishing criteria such as uniform voltage levels.

• Assist in establishing industry standards for uniform connector and power level for electrical power

connections at truck stops.

4.6.8 Thermal Management

4.6.8.1 Background and Status of Technology

Thermal management, a crosscutting technology focused on managing heat rejection, can have an

important impact on fuel economy and emissions, as well as the reliability and safety of all classes of

trucks.

Many thermal-management issues are common between present-day vehicles and the advanced concepts

under consideration. For example, on most vehicles, and especially on large trucks, the size of radiators

and coolers dictates that the front-end design contributes significantly to the drag coefficient, and thus to

fuel economy. Exhaust gas recirculation (EGR), which is the most probable near-term strategy for

reducing NOx emissions, is expected to add a 20 to 50% heat load to heat-rejection systems.

Unfortunately, many conventional cooling-system components such as radiators, oil coolers, and airconditioner condensers, are already at or are approaching their maximum practical size and functional

limits.

The trend toward hybrid and fuel-celled vehicles is expected to further increase the demand on coolant

heat-rejection systems. In fuel-cell vehicles, the exhaust of the fuel cell contains water vapor that needs to

be recovered to reduce the amount of water inventory that is carried. Minimizing the size of the heat
4-87

exchanger to accomplish this is a challenge. In diesel hybrids, there may be up to five separate cooling

systems (for engine, batteries, motors, electronics, and charge air), and optimization of this design is a

complex task. Many thermal management issues are also specifically associated with advanced concepts

or with military applications. For military operations, any increases in radiator size will not only affect

aerodynamics and parasitic energy losses, but also limit any decrease in cab size, a decrease that is

desirable for space savings in airlift operations. All of these demands have created a need for new and

innovative thermal management technologies that will require long-term R&D.

4.6.8.2 Technical Approach

Several research areas identified by industry and government researchers can provide both near-term and

long-term solutions to many of the next management problems. The research areas are identified as

follows:

• Intelligent thermal management systems

– Thermal management related to use of higher electrical bus voltage

– Variable speed pumps and fans

– Variable shrouding

– Integration of thermal management components into vehicle structure

• Advanced heat exchangers and heat-transfer fluids

– Innovative, enhanced airside heat-rejection concepts

– New materials, such as carbon foams, for cooling-system components

– Nanofluid technologies for improving heat transfer properties of coolants and engine oils

– Fundamental understanding of fouling mechanisms and mitigation

• Advanced thermal management concept development

– Heat pipes

– Cooling by controlled nucleate-boiling

– Waste-heat recovery technologies (e.g., thermo-electric generators)

• Simulation-code development

– Comprehensive CFD module for airflow and temperatures to include power train, underhood

aerodynamics and airflow, lubricant cooling, vehicle-load predictions, cooling systems, and

control systems

– Experimental data base

• Sensors and control components development

– Accurate, reliable, robust, and real time

– NOx, engine temperatures, pressures, coolant flow, airflow

– Combination with computer control

• Thermal signature management

– Masking technologies to mask overall signature

– Masking technologies to mask specific cargoes

• Electronics cooling

– Power electronics in hybrids and fuel cells

– Cooling for communications and battlefield integration equipment

– Auxiliary military equipment

4.6.9 Materials

Materials needs for heavy vehicles fall into four categories: propulsion materials, lightweight materials,

power transmission materials, and functional materials. In each case, materials development includes

identifying the most appropriate material for the application in terms of both performance and price and

then developing the materials processing, characterization, rig and engine/vehicle testing, and costeffective manufacturing technology. In most cases both improved performance and lower cost are
4-88

expected. Cost effectiveness can be ensured by using economic cost modeling to predict the cost of the

component at every stage in the manufacturing process and then as a guide in development of the material

and process.

4.6.9.1 Propulsion Materials

Diesel Engines

The development of advanced diesel engines imposes greater mechanical, thermal, and tribological

demands on materials. Higher-efficiency engines will require higher peak and brake mean effective

pressures, higher stresses on components, higher temperatures, greater precision, and lighter weight.

Requirements include materials for advanced combustion-chamber components, cylinder heads, engine

blocks, and EGR and exhaust systems.

Future regulatory reductions in allowable exhaust emissions will require improved catalysts, better PM

traps, alternative aftertreatment technologies, better lubricant control, and improved fuel-injection

systems. Associated materials requirements include highly durable catalyst materials, catalyst supports,

and wash coats for NOx reduction; durable materials for effective, regenerating particulate traps;

improved materials for lubricant control to reduce PM emissions, and high-strength wear- and fatigueresistant materials and precision manufacturing for high-pressure fuel-injection systems (OHVT 1998).

Engines that incorporate alternative fuels (such as natural gas) require materials that are chemically

compatible with the fuels and the altered combustion chemistry and are durable in the presence of lowlubricity fuels. Materials requirements include stable, corrosion-resistant materials for glow plugs and

durable components such as wear- and corrosion-resistant intake valves, valve seats, and valve guides.

Safer storage of gaseous fuels for longer driving range will also require materials that reduce storage

pressures and permit conformable tank fabrication (OHVT 1997).

Materials R&D for propulsion systems will include materials for fuel and air-handling systems, exhaust

aftertreatment, valve-train components, structures and insulation, friction and wear reduction, and thermal

management. Candidate materials include high-temperature alloys, intermetallic alloys, ceramic-metal

composites (cermets), structural ceramics, bulk amorphous alloys, ceramic and metal-matrix composites,

thermal-barrier coatings, and wear coatings (OHVT 1999a).

Hybrid Propulsion

The major materials needs for hybrid propulsion systems fall into two principal categories: energy storage

and power electronics, including electric motors. Energy storage needs include batteries, ultra-capacitors,

flywheels, and/or hydraulic-pneumatic-mechanical storage systems. Reducing the cost and increasing the

energy storage capacity, life, and reliability are the most significant challenges common to both batteries

and ultra-capacitors. Challenges for flywheels are reduced cost, reduced complexity, and improved

containment safety. Power electronics needs include improved electric motors, capacitors, inductors,

power modules, and packaging. Improved motors must overcome limitations related to capacitive

coupling, high-performance electrical insulation and corona resistance, magnet costs and performance,

eddy current losses, and structural performance. Materials for capacitors and inductors must be developed

to provide improved performance, better durability, longer life and higher temperature capabilities. Highvolume manufacturing must be developed for silicon carbide or eventually diamond-based power devices

to reduce cost and increase quality, durability, inspectability, and temperature capability. Power module

packaging must be improved, including the development of materials for more affordable, robust, and

efficient power and voltage sensors, improved high-temperature solders, and more durable and

manufacturable interconnects (OHVT 1999b).
4-89

Fuel Cells

Fuel cells will likely play multiple roles in reducing the emissions and increasing the fuel economy of

trucks and buses—as the prime source of motive power in a hybrid propulsion system and as a source of

auxiliary power. The power levels for the various applications will be significantly different, but the

materials challenges, similar. They can be broken down as follows: PEMFCs, SOFCs, and reformers.

The highest priority of the materials challenges that must be met in the near term for PEM fuel cells is to

develop affordable catalysts. Affordable catalysts must be developed that have high activity at low

loading levels and are durable and tolerant of both sulfur and CO. Other materials barriers that must be

overcome include development of lower cost membranes that can operate at higher temperatures more

efficiently; more durable and affordable bipolar plates that are lighter weight and higher strength; and

improved packaging with better seals, clamping, and electrical connections.

A wide range of materials challenges will need to be met to realize the very high potential efficiencies of

SOFCs. Anodes that are sulfur tolerant and that provide integral reforming must be developed, as must

durable, mixed ionic conductive cathodes and fabrication methods to produce affordable, pore-free thinfilm, solid electrolytes. All of the electrode and electrolyte materials must be made to operate efficiently

at lower temperatures. Materials and fabrication methods for SOFC assembly and packaging must be

developed to provide corrosion-resistant interconnects for bipolar plates as well as their high-strength,

conductive oxide coatings. Insulating seals that are compatible with all components and gas streams as

well as being durable over wide temperature ranges will be required, along with high-temperature bus

bars and gas connections. High-temperature metallic housings will also need to be developed.

One of the greatest materials challenges to make PEMFCs and SOFCs commercially viable is to provide

adequate reforming of the fuel on which they run. Different types of fuel cells and fuels require different

amounts of reforming. PEMFCs that use diesel fuels require the greatest amount of reforming, SOFCs on

gaseous fuels, such as natural gas, require the least. Nonetheless, a combination of barriers will need to be

overcome for all systems to provide adequate materials for reformer components. Sulfur-tolerant,

selective catalysts are needed in steam reformers, autothermal reactors, and CO scrubbers, as are durable,

low-thermal-mass supports. Durable, corrosion-resistant materials with high surface areas and high

thermal conductivity are needed to manage heat transfer in preheaters and radiators.

4.6.9.2 Lightweight Materials

The use of alternative, lightweight materials will reduce weight significantly and thus will yield

substantial benefits in fuel efficiency and emissions reduction by both reducing the inertial loading and

increasing the available payload of the vehicles. Lightweight materials must be developed that are costeffective, stronger, more reliable, and safer. Affordable, efficient manufacturing processes to make the

materials available to the ground transportation market must also be developed (Sklad 2000).

The principal barriers to overcome in reducing the weight of trucks and buses include the following:

• the inherently higher cost of alternative materials;

• the lack of understanding of medium- to high-volume manufacturing methods as applied to new

materials;

• insufficient experience in joining and fastening;

• the lead time to bring new materials and processes into the manufacturing cycle;

• the lack of appropriate data bases and design tools for use by design engineers, particularly for

composite structures;

• the lack of experience in repairability and maintenance; and

• a limited supplier base.
4-90

Lightweight materials R&D in the 21st Century Truck Program will focus on developing technologies that

are aimed at addressing these barriers for lightweight materials to permit their accelerated development

and introduction into the trucking industry. Materials development focused on weight reduction for trucks

and buses will address three key elements:

• development of technologies for enhanced manufacturability of lightweight components for trucks

and buses;

• development of design concepts and material data bases to provide design engineers the flexibility to

consider lightweight materials in vehicle design; and

• development of technology in support of advanced materials, joining, maintenance, and repair.

The greatest weight reductions are foreseen through the use of high-strength steel, aluminum alloys, and

polymer matrix composites in frames and bodies and, in lesser quantities, in wheels, cabs, transmission

housing and shafts, and suspension components. Ultra large, thin-wall aluminum and steel castings will

reduce part count and thereby weight. Other weight reduction opportunities include stainless steel in

frames, reinforced aluminum blocks in light-duty engines; sandwich, cored, and foam materials for body

panels; and metal matrix composites, titanium, and magnesium alloys for specialized components.

4.6.9.3 Power Transmission Materials

Up to 40% of the power generated by the engine is lost due to parasitic loses. When these are reduced, a

corresponding increase is required in the performance of the braking system. Hence, improvements in

power transmission materials—tires, drivetrain components, and brakes—are required for reduction in

emissions and improvements in both vehicle efficiency and safety.

Several factors must be addressed to improve tire safety: tire reinforcement material strength and

stiffness, temperature stability and damage tolerance, dynamic alignment materials and sensors that detect

heat, pressure, and imbalance. Advanced materials will enable higher performance and longer-lived

traditional drive trains. Development of improved understanding and modeling of friction and wear

interactions and failure mechanisms will enable incorporation of the improved materials and surface

treatments that are needed for more efficient, longer-life clutches, gears, bearings, and solid lubricants.

Advanced braking systems will need improved thermal management materials and braking surfaces that

are more heat tolerant, wear resistant, and corrosion resistant, and lighter weight. Improved lubricant

control and imbedded sensors will be needed as well (Blau 1999, OHVT 1999c).

4.6.9.4 Functional Materials

Advanced sensors to improve engine and emission controls and safety devices, improved thermal

management devices, and enhanced corrosion control are needed to meet the goals of the program. These

applications all depend on development of durable, low-cost functional materials. R&D will be required

to develop improved functional materials and low-cost manufacturing.

Improved smart materials, such as piezoelectric, electrostrictive, and magnetostrictive materials; hightemperature shape memory alloys; and active electro-optical glazing materials are needed as sensors and

actuators for fuel-injection, valve actuation, intelligent braking, traction control, drag reduction, and

HVAC load-reduction systems. Improved heat management materials will

• enable smaller, more efficient radiators, significantly reducing aerodynamic drag;

• enhance engine efficiency with improved recuperators in the air-handling and exhaust systems;

• improve durability and performance of electronic components essential for hybrid propulsion and

regenerative braking systems; and

• provide super insulation needed for exhaust, HVAC, fuel cell, and reforming systems.
4-91

Reducing vehicle corrosion with improved modeling, sensors, and coatings will reduce required corrosion

allowances and thereby reduce vehicle weight and maintenance costs.

4.6.10 More-Efficient and/or Lower-Emission Engine Systems

4.6.10.1 Introduction

Many people believe that the efficiency potential of the practical diesel engine has now been reached.

This opinion, although wrong, influences debate. The following paragraphs argue first that the potential

of the current reciprocating combustion engine for further efficiency improvement is seriously

underestimated and then highlights the engine design areas that can be modified to approach the much

higher ultimate potential of the reciprocating internal combustion engine. The discussion intentionally

centers on technology stretch concepts. Emission reduction methods are then outlined in the areas of

pretreatment, aftertreatment, and in-cylinder methods.

4.6.10.2 Current Efficiency—Practical and “Theoretical”

Diesel engines achieve on-the-road efficiency up to about 44%, which, although the highest among

vehicle engines, has room for much improvement. The engines typically use a compression ratio of about

16. The classic ideal cycle formula for the efficiency of a diesel cycle with this compression ratio yields

an “efficiency upper limit,” assuming perfect processes, of 60% even for lean (0.4 equivalence ratio)

operation. Indeed, if the actual engine remains in its present form, its efficiency is not likely to exceed

about 50% through a continuing process of refinement. Even if such refinement is achieved, it is unlikely

to overcome the burden of additional constraints imposed by emission reduction measures. So, even to

retain current efficiency levels, a review is needed of the opportunities for basic engine cycle

improvement.

Better Use of Available Pressure Ratio

Current diesel engine performance is limited structurally to a maximum cylinder pressure of about

20 MPa. Ambient pressure is 0.1 MPa, so the available overall pressure ratio is 200. To take advantage of

this available energy, a piston engine would need to have a volume expansion ratio of about 44, and

would have a corresponding ideal efficiency of 78%. This is a 30% increase in ideal cycle efficiency. For

various reasons this volume ratio cannot be achieved in practice in a single cylinder. However, by

combining turbo compression and expansion (turbocharging) with in-cylinder compression and

expansion, this ideal process can be approached today in experimental turbocompound engines. Using

cylinders in series to provide the compression and/or expansion in more than one stage is also an option.

Indeed several potentially efficient but untested designs, such as those of Assanis, Karvounis and Bell

1993 and Clarke and Berlinger 1999, feature gas exchange between cylinders.

Thermal Recycling

The engine thermodynamic cycles mentioned above do not use regenerative heat exchange. This is a new

opportunity for reciprocating engines. Very significant additional gains are obtained in principle by

thermal regeneration, as outlined by Clarke 1990 and Ferrenberg 1990. (This principle is already applied

in recuperative gas turbines, where it lowers the compression pressure requirement and helps part load

operation, but has only a second-order effect on efficiency because it does not overcome the fundamental

turbine temperature limitation.) Extensive one-dimensional transient analysis by Ferrenberg et al. 1993 of

in-cylinder regeneration has shown that temperature ratios across the regenerator can be as high as 1.6.

With the same overall pressure limit and equivalence ratio, this raises the ideal cycle efficiency to 86%.

This is a 43% improvement in ideal cycle performance. This improvement is due to increased combustion

temperature and less net heat rejection. The heating in the regenerator is done without compression and

this allows much higher temperature within the pressure limit. In other words, the pressure limit, a

principal reciprocating engine efficiency constraint, becomes less restrictive. Finally, this very efficient
4-92

cycle still has hot exhaust and does not employ intercooling. An approach to extract all the available work

from the exhaust is to use isothermal compression followed by a recuperation of exhaust heat. The cycle

then has an ideal thermal efficiency of 89%. This is a 48% improvement in ideal cycle performance. (See

Fig. 4.16.)

This cycle satisfies the following “acceptable” thermal and mechanical properties:

• maximum cylinder pressure = 20 MPa,

• maximum regenerator temperature = 1500 K,

• maximum recuperator temperature = 462 K,

• maximum recuperator pressure difference = 0.36 MPa,

• boost pressure ratio = 4.6, and

• cylinder exhaust temperature = 711 K.

4.6.10.3 Practical Implementation

The advanced cycle applies the well established rules for increasing heat engine efficiency, namely,

minimize internal irreversibilities, maximize heat-input temperature and minimize heat-rejection

temperature. In practical terms it requires the following:

• Avoiding blow-down energy losses by compounding using either turbines or reciprocating units to

achieve the necessary efficient extra expansion. (The technology of applying motor/generator units

directly on turbomachine shafts is likely to help meet this need; see for instance Hofbauer 2000.) If

turbocompounding is adopted as the best practical means to improve the overall expansion efficiency,

then the use of high-speed electric motor/generators becomes a mechanically simple way to extract

useful energy from the expansion. This approach has synergy with vehicles that have electrically

driven auxiliary systems (see Sect. 4.6.7) and the need to add power to the compressor shaft (hence

increase airflow) during transients. It also provides an option to optimize the share of power output

between the crankshaft and the turboshaft.

Fig. 4.16. Classic and advanced diesel ideal cycles compared.
4-93

• Adjusting compression ratio and scheduling fuel injection so that the combustion approaches constant

pressure at the cylinder pressure limit (the technology of electronically scheduled fuel rates and the

use of lean mixtures will help to meet this need).

• Applying efficient turbomachinery to improve mechanical efficiency by raising BMEP and to more

nearly approach isentropic performance for the whole pressure ratio (modern CFD designs can help

meet this need).

• Applying regenerative heat exchange as part of the cylinder processes (this is a novel feature of the

compression ignition cycle and it is made possible only by the properties of newly developed porous

foams of high melting point metals or ceramics (e.g., chemical vapor deposited silicon carbide as

described by Sherman, Tuffias, and Kaplan 1991).

• Using intercooling to approach isothermal compression thus recovering exhaust energy and reducing

the work required for the first stage of compression (there is technology from multi-stage intercooling

of stationary engines and/or liquid vaporization during compression).

• Avoiding heat losses as much as practical by using low surface-to-volume ratios and allowing nonlubricated surfaces to approach their adiabatic temperature (there is technology from the low heat

rejection programs of recent years).

There has never been a concerted effort to exploit all the above features, although they can all add to

engine efficiency.

The extensive DOE-funded program for improved in-cylinder components and reduced heat rejection

optimized the design by means of simulations restricted to relatively small changes from current engines.

Even so, single cylinder tests (with simulated turbomachinery) exceeded 50% thermal efficiency, and

49% was achieved by the multi-cylinder version of this turbo-compound engine.

4.6.10.4 Exemplary Developments

Two near-term benefits are likely to come from the relatively conventional approaches that do not employ

heat exchange to enhance the cycle: (1) going for an optimum combination of high pressure and

controlled pressure (near constant pressure) combustion by using an optimized (ramp up) fuelinjection/combustion rate and (2) improving the compression and expansion efficiencies.

The longer term results stem from the opportunity to incorporate more thermal recycling (e.g., in-cylinder

regeneration) or chemical recycling (e.g., endothermic fuel reforming).

Although not intended to be all-inclusive, the following four research areas illustrate some important

emerging directions.

Homogeneous Charge Compression Ignition

Homogeneous charge compression ignition (HCCI) is an alternative engine combustion process that can

provide high, diesel-like efficiencies, very low NOx emissions, and very low particulate emissions.

In addition, HCCI could offer dividends in terms of reduced cost (less expensive fuel-injection equipment

than a diesel) and fuel flexibility.

HCCI engines operate on the principle of having a dilute, premixed charge that reacts and burns

volumetrically as it is compressed by the piston. The charge may be made dilute by being very lean, by

mixing with EGR, or a combination of the two. Because the charge is very dilute, combustion

temperatures are low, and little NOx is produced. Particulate emissions are also very low because the

premixed charge is lean, or at most, stoichiometric. The foremost barrier to HCCI is in controlling the

ignition timing, heat-release rate, and amount of unburned hydrocarbons across the load-speed map of the

engine. For a given fuel, the HCCI combustion process is controlled by three main parameters: time,
4-94

temperature, and mixture. Partial stratification of the charge temperature, charge mixture, or both has a

strong potential to alter these parameters to control ignition timing and heat-release rate, and its potential

has been largely unexplored. Controlled charge stratification also has the potential of increasing the

power density to a level comparable with that of a diesel engine, whereas traditional fully homogeneouscharge HCCI engines have typically had lower power densities. Accordingly, investigation of the effects

of partial charge stratification (stratified charge compression ignition, SCCI) should be included in the

R&D efforts. Additionally, hybrid power train configurations that accommodate a limited engine

operating range may assist HCCI application.

Experimental efforts will include additional mapping of the operating space and fuel-type effects, the

investigation of potential control strategies, comparisons of engine performance with detailed kinetic-rate

calculations. (t will also include detailed in-cylinder measurements using advanced laser diagnostics to

determine such things as the nature of HCCI/SCCI combustion, the source of unburned hydrocarbon

emissions (e.g., wall quench and crevices), the effects of fuel/air/residual mixture and partial stratification

of this mixture, and the effect of charge-temperature stratification.

Numerical modeling efforts will include advancements in chemical-kinetics modeling (e.g., reduced

mechanism for application in CFD codes), surrogate blends to simulate real fuels, efforts to develop and

apply detailed CFD codes to HCCI processes, and algorithms for control strategies.

Free Piston Engine Configurations

Goldsborough and Van Blarigan 1999 and Goertz and Peng 2000 have simulated free piston engines that

couple a reciprocating piston directly to a linear generator. (See Appendix H, Fig. H.1.) Such devices

contain no crankshaft or camshaft. Freed from some of the kinematic and bearing load constraints of the

slider-crank mechanism, they appear well suited for application to the HCCI combustion process. They

are also well suited because they have more operating degrees of freedom, which may be vital for control

of practical HCCI combustion. This design offers the attractive possibility that the mechanical simplicity

will lead to cost-effective, efficient, clean, and more nearly direct conversion of combustion energy to

electric power. Achten 1996 has built and tested a free piston engine in which the diesel 2-stroke working

chamber is directly attached to a hydraulic pump. (See Appendix H, Fig. H.2.) This is also suitable for

HCCI operation for the same reasons as described in the previous section. Applications would obviously

focus on vehicles for which hydraulic transmissions are appropriate. It is not yet clear whether the

transmission and energy storage required for hybrid vehicles should be based on electric or hydraulic

technologies. Either way, the free piston engine with HCCI may be an attractive prime mover.

In-Cylinder Regeneration

Ferrenberg has simulated the thermodynamic performance of cylinders in which the working gas picks up

heat prior to combustion by passing through a thin disc of porous material (Farrenberg 1990, Ferrenberg

et al. 1993). (See Appendix H, Fig. H.3.) The disc is reheated when the expanded gas passes through it

prior to exhaust. Simulated efficiency of such an engine is up to 58%. The reasons for this high efficiency

are discussed in an earlier section.

Chemical Recovery of Exhaust Heat

When the fuel is preprocessed by an endothermic reaction with exhaust heat, the heating value of the fuel

is increased and thermal efficiency can potentially be improved. Experiments with an exhaust-driven

methanol dissociation system were conducted some time ago (Karpuk 1989), but the control of the overall

process was very complex and hydrogen in the fuel gave rise to pre-ignition. Recently, Kawamura and

Ishida 2000 reported on a concept to achieve a system thermal efficiency of 68% by using this technique

among others for natural gas. They used ceramic insulation to increase the exhaust heating potential and
4-95

then drive a reaction that changes a mixture of methane and carbon dioxide into carbon monoxide and

hydrogen. The products add about 30% more heating value to the original fuel.

4.6.10.5 Lower Emissions

Emission Reduction Through Pretreatment

Historically, fuel preparation has been very important to the fuel and engine symbiotic relationship, from

raising octane to cutting lead in gasoline to lowering ash and sulfur in diesel fuel. Our current focus on

liquid hydrocarbons for transport stems primarily from economy and convenience rather than the “ideal”

nature of the fuels. It has been shown repeatedly, and recently by Nurun et al. 1999, that increased oxygen

content (such as in alcohol or dimethyl ether) greatly improves the ability to avoid soot in compression

ignition engines. With less in-cylinder soot production, the options for both aftertreatment and in-cylinder

control of NOx and PM are greatly increased. If biomass, which contains sufficient oxygen to reduce soot

production emerges as a major long-term energy source (NREL 2000) and acidic or enzymatic hydrolysis

can reduce cellulosic material economically, prospects for clean and efficient engines are much improved.

Similarly, if natural gas emerges as a major long-term energy source, perhaps even from hydrates

(Gornitz and Fung 1994), and economic conversion into oxygen rich liquid fuel is possible, then

prospects for clean and efficient mobile engines are much improved. Exhaust gas recirculation reduces

oxygen (gas) to fuel ratio and serves to greatly reduce NOx production. Other ways to pretreat the air are

also worth exploring. Fuel desulfurisation is aimed primarily at helping aftertreatment processes. But by

reducing exhaust acidity, it also helps prospects for practical recovery of exhaust heat in heat exchangers.

Lower Emissions Through Current In-Cylinder Processes

Current measures reducing NOx and soot include recirculation of cooled exhaust gas, and the use of highpressure, electronically controlled fuel injection. Numerous unconventional fuel-injection systems such as

air-blast atomizers, two-fluid injection, electric field enhancement, and piezoelectric atomization, have

been attempted. These measures are at best neutral, but often negative on efficiency, and typically

negative on cost, complexity and bulk. It is thought that these measures by themselves may not be

sufficient for meeting projected emissions regulations. So either there needs to be an even more effective

aftertreatment technology, or the problem must be handled at the source by a different mode of

combustion, such as follows here. (See also Sect. 4.6.2.)

Lower NOx and Particulates Through Homogeneous Charge Compression Ignition (HCCI)

It has become clear that it is the flame front that forms the NOx as it propagates the combustion reaction.

However, at sufficiently low fuel concentration there is insufficient energy to propagate a flame. So, using

conventional combustion, there is a minimum to both flame temperature and associated NOx as discussed

by Flynn (Flynn et al. 2000). HCCI appears to be an attractive alternative worthy of consideration to meet

the diesel emissions challenge (see also Sect. 4.6.10.4). Because of this, industry has a significant interest

in developing a practical engine of this type. Both diesel-engine and automobile manufacturers have

established, or are establishing, HCCI/SCCI engine development efforts. Further HCCI/SCCI

development efforts are required, and to support these industrial engine development efforts, additional

research is required on the fundamentals of HCCI/SCCI combustion and the advancement of HCCI/SCCI

simulation models. Several researchers have shown NOx below 10 ppm and zero smoke. Some production

2-stroke engines (see for example, Ishibashi and Asai 1996), use this combustion mode to improve light

load performance. Research continues in this area and this may become the preferred combustion mode

for efficient, clean, reciprocating engines. The potential of various HCCI concepts and approaches needs

to be fully explored.
4-96

4.6.11 Vehicle Intelligence

4.6.11.1 Technical Approach Overall Plan

The vehicle intelligence activities will be advanced primarily through research but will also include

program assessment and activities that may support the development of standards. To achieve the

program objectives, vehicle intelligence R&D is focused in four areas: identification and definition of

problem areas (services), selection of services for development, system design and development, and

operational test and evaluation. This process will be carried out in each problem area. The work in each

problem area is focused on solving specific problems. To have a cohesive process, each research area is

tied to the vehicle intelligence crosscutting activities. The end result is envisioned to be market driven,

deployable products that enhance roadway safety, overall mobility, and system efficiency.

Figure 4.17 illustrates the process for conducting research. This process was borrowed from the

Intelligent Vehicle Initiative (IVI) (DOT 1997, DOT 2000). The progression of activities generally moves

from left to right on the diagram. The level of cooperative involvement with the motor vehicle industry

increases from left to right. At the current time, the majority of the work within the IVI program is in, and

to the right of, the middle column. This represents a significant U.S. DOT effort to develop an

understanding of the details of system performance that describe effective safety-enhancing systems. A

more precise description of status for each problem area is provided in the next section. The status of

various problem areas is also a function of the state of production-readiness of commercially available

systems.

Fig. 4.17. Process for conducting vehicle research as borrowed from the Intelligent Vehicle

Initiative (IVI).
4-97

The state of production readiness is generally described by the generation of the system in question.

Generation 0 systems are expected to be ready for production planning by 2003, Generation 1 systems are

expected to be ready for production planning by 2010, and Generation 2 systems are expected to be ready

for production planning by 2015. For the purposes of the 21st Century Truck Program, only Generations

0 and 1 are included. The majority of the DOT-initiated work has focused on problem areas for which

Generation 1 systems appear to be the most realistic solution. However, in some cases, for example

crashes at intersections, Generation 2 systems may be the most realistic expectation for effective

countermeasures. The planning of the program in each problem area began with a thorough analysis of

crash data files. This work took place in the early 1990s and is reflected in the left column in Fig. 4.16.

Subsequently, decisions were made on which problem areas should be emphasized in the U.S. DOT work.

This corresponds to the second column from the left in Fig. 4.16. The Generation concept illustrates the

iterative nature of the IVI R&D process. If we use the rear-end collision problem area as an example,

testing of a commercially available system for commercial vehicles is being conducted under

Generation 0. Under Generation 1, performance specifications are being developed, and testing will be

conducted of a more advanced system for light vehicles.

The problem areas that were selected are discussed in more detail later in this section. The work in some

of these problem areas has progressed to the point that it is feasible to design and fabricate prototype

systems that would be available for use in operational tests, most notably, rear-end crashes. As work

progresses in other problem areas, additional operational tests of Generation 1 systems will be initiated.

Deployment of effective intelligent vehicle systems is the domain of the motor vehicle industry, so the

right column on Fig. 4.16 reflects design and production by vehicle manufacturers and suppliers. The

concept of Generation 0 systems is a relatively new concept in the IVI program. The idea behind this

concept is that safety-related systems nearing production could benefit from an evaluation based on

results of an operational test. A Request for Application, published in December 1998, has resulted in

operational tests of such systems. These projects enter the IV Program at the operational test level. Two

other aspects of the program are significant. The crosscutting block on Fig. 4.17 represents those activities

that cut across platforms (e.g., the development of general-purpose research tools and the assessment of

institutional issues). Another perspective of consolidating activities is reflected in the integration box at

the bottom of Fig. 4.17. Both of these activities are incorporated in the problem area lines of the GANT

chart shown in Fig. 4.18. These activities are associated with combinations of two or more proven

systems, usually within a single platform. A key question that comes up is whether the combination

system, and especially the driver-vehicle interface for the combination, is more or less effective than the

individual systems. Within this same box are studies of the impact on safety of combinations of systems

that, by themselves, have little or no impact on safety.

Figure 4.18 illustrates the R&D plan that was specifically developed for the 21st Century Truck Program.

Activities that are or will be funded by the IVI program are illustrated in black. Activities that are not

funded are illustrated in red. This R&D plan stretches the IVI program goals, which are primarily safety

focused, to cover mobility, efficiency, and productivity goals as well.

4.6.11.2 Program Objectives

• Accelerate product introduction of driving information, driver assistance, and control systems that

will improve significantly the safety of motor vehicle operations.

• Develop and validate performance specifications and design guidelines for IVI systems that will be

deployed in motor vehicles in the next 10 years.

• Recognizing the complexity of the driving task and issues such as risk compensation and workload,

demonstrate systems and evaluate their impact on driving safety.

• Reach agreement on the basic functional requirements for driver-assistance features and target those

functionalities for which industry investment has developed the basis for working prototypes.
4-98

Pre

2001 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Generation 0

Research

 Transit Rear-end Collision Warning

 Transit Rear-Impacting Collision Warning

 Transit Lane Change Collision Warning

Operational Tests

 Mack

 Freightliner

 Volvo

 MN DOT

Generation 1

Research

 Driver Condition Monitoring

 Vehicle Stability/Rollover Warning Control

 Human Factors

 Benefits

 Safety Event Recorders

 Driving Baseline Data Collection

 Prognostics/Diagnostics

 Collision Warning Systems

 Rear-end

 Lane Change/Merge

 Road Departure

 Intersections

 Rollover

 Vision Enhancement

 Road Condition Monitoring

 Enhanced Digital Maps

 Transit Precision Docking

 Transit Automated Control

 Driver Condition Monitoring Electrically

 Controlled Brakes

 Vehicle Stability

 Transit Collision Warning System

Operational Tests

 System Integration Test

Fig. 4.18. Vehicle intelligence R&D plan for the 21st Century Truck Program.

• Recognizing the need for balance between public benefit and private incentive, ensure that reasonably

achievable safety benefits are identified.

• Identify refined, more-detailed estimates of benefits to assess which cooperative infrastructure

deployment investments can be justified and to stimulate new safety products.

• Identify and conduct the R&D required to achieve increased levels of system capability and fuel

efficiency on a ton-mile per gallon basis.
4-99

4.6.11.3 Safety-Related Problem Areas

The following problem areas are prime candidates for improvement through application of advanced invehicle or cooperative technology. The R&D plan (Fig. 4.18) is annotated to show which services are

currently the subject of DOT-funded research under IVI.

Rear-End Collision Avoidance

This feature would sense the presence and speed of vehicles and objects in front of the equipped vehicle

and would provide warnings and limited control of the vehicle speed (coasting, downshifting, or braking)

to minimize risk of collisions with vehicles and objects in the vehicle’s lane of travel. It is expected that

the first implementation of this service would be through autonomous in-vehicle systems. These systems

would monitor the motion and location of vehicles and other objects in front of the vehicle and would

advise the driver, through an appropriate driver-vehicle interface, of imminent rear-end crashes. These

systems may share some elements of adoptive cruise control systems and are expected to complement

their performance. Adaptive cruise control systems are expected to precede collision avoidance systems

as a commercial product. Later versions of these systems may include automatic braking in the event of

an impending crash. The performance of these systems may be enhanced through future combination with

other systems, such as other collision avoidance systems, route guidance-navigation systems with

enhanced map data bases, and cooperative communication with the highway infrastructure to set adaptive

cruise control systems at safe speeds.

Road Departure Collision Avoidance

This feature would provide warning and control assistance to the driver through lane or road edge

tracking and by determining the safe speed for road geometry in front of the vehicle. It is expected that

the first implementation of this service would be through autonomous in-vehicle systems. These systems

would monitor the lane position, motion relative to the road edge, and vehicle speed relative to road

geometry and road conditions and would advise the driver, through an appropriate driver-vehicle

interface, of imminent unintentional road departure. Later versions of these systems may include

cooperative communication with the highway infrastructure to automatically provide safe speeds for

upcoming road geometry and conditions. The performance of these systems may be enhanced through

future combination with other systems, such as other collision avoidance systems, drowsy driver advisory

systems, and route guidance-navigation systems with enhanced map data bases.

Lane Change and Merge Collision Avoidance

It is expected that the first implementation of this service would be through in-vehicle systems that may

be augmented with vehicle-to-vehicle communications. These systems would monitor the lane position,

relative speed, and position of vehicles (including motorcycles) beside and to the rear of the vehicle and

would advise the driver during the decision-phase of a lane-change maneuver, through an appropriate

driver-vehicle interface, of the potential for a collision. Later versions of these systems may provide

additional advice of an imminent crash to the driver during the action-phase of the lane change or entryexit maneuver. The performance of these systems may be enhanced through future combination with

other systems, such as other collision avoidance systems and roadside communication and sensing

systems.

Intersection Collision Avoidance

The first implementation of this service is expected to be through in-vehicle systems that are augmented

by information from enhanced map data bases or from cooperative communication with the highway

infrastructure. These systems would monitor position relative to intersection geometry and relative speed

and position of other vehicles in the vicinity of the intersection, and would advise the driver through an

appropriate driver-vehicle interface of appropriate action to avoid a violation of right-of-way or to avoid
4-100

an impending collision. Complexities of providing this service include the need to sense the position and

motion of vehicles and to determine the intent of these vehicles to turn, slow down, stop, or violate

right-of-way. A fully autonomous in-vehicle system would probably not be capable of providing this

service.

Vision Enhancement

The first implementation of this service is expected to be through autonomous in-vehicle systems. These

systems would use infrared radiation from pedestrians and roadside features to provide the driver with an

enhanced view of the road ahead. Later versions of these systems may include additional information

from improvements in the highway infrastructure, such as infrared reflective lane edge markings.

Automatic Collision Notification

The first implementation of this service is expected to be through in-vehicle systems that are augmented

by communication links to Public Safety Answering Points (PSAPs). These systems would monitor

position of the vehicle and severity of the crash. This information would be transmitted automatically to

the appropriate PSAP for the location of the crash. These systems may also be combined with manually

activated systems for requesting roadside assistance. The research in this area is sufficiently mature such

that no additional work is needed to ensure that this capability is available for the 21st Century Truck.

Vehicle Stability Warning and Assistance

An early version of this service would assist drivers in maintaining safe speeds on curves by measuring

the rollover stability properties of a typical heavy vehicle as it is operated on the roadway and by

providing the driver with a graphical depiction of the vehicle’s loading condition relative to its rollover

propensity. More advanced services would employ an active brake control system coupled with electronic

brake system technology and infrastructure-provided information to selectively apply brakes to stabilize

the vehicle and thus reduce the incidence of rear trailer rollover in double- and triple-trailer combination

vehicles during crash avoidance or other emergency steering maneuvers.

Driver Condition Warning

This service would provide a driver monitoring and warning capability to alert the driver to problems,

such as drowsiness or other types of impairments. The first implementation of this service is expected to

be on commercial and transit vehicles.

Vehicle Diagnostics

The vehicle diagnostic information service would be an extension of current vehicle monitoring and

self-diagnostic capabilities such as oil pressure and coolant temperature gauges. This service would

monitor the vehicle’s safety-related functions. Examples of conditions to be monitored include braking

system integrity, tire pressure, sensor and actuator performance, and the communication system. This

information is intended to be useful to the driver, as well as to assist and support fleet maintenance and

management functions.

Safety Event Recorder

This feature would record selected driver and vehicle parameters to support the reconstruction of

conditions leading to a critical safety event. Data from this recorder could provide input to the crash

notification subsystem for transmission of collision data to the emergency service provider.
4-101

Road Condition Warning

This service would initially warn the driver of reduced traction, but in advanced configuration, would also

provide control assist capabilities to assist the driver in regaining control of the vehicle. Sensors on-board

the vehicle would detect when the tire-to-road surface coefficient of friction is reduced due to water, ice,

or road surface condition.

Fleet Management and Automated Transactions

This feature would make use of such technology as transponders and “smart cards” to implement

productivity-enhancing capabilities for electronic transactions (e.g., electronic toll collection, parking fee

payment, and transit fare payment) and additional commercial vehicle-related functions (e.g., credentials

and permit verification).

4.6.11.4 Fuel-Efficiency-Related Problem Areas

Significant technological achievements have been experienced over the past decade in the area of

communications, electronics, and tracking technologies. Commonly referred to as “Telematics,” such

technologies have been widely applied to highway and automotive safety. Such technologies also hold

great promise for enhancing the productivitiy and efficiency of our nations automotive and trucking

industries.

Through the innovative use of such technologies, trucking carriers can more effectively manage the

whereabouts of vehicles in their fleet, providing more timely service and saving fuel in the process.

Merging vehicle-location technologies with software for logistics will allow more timely delivery of

goods with reduced fuel consumption.

Automotive e-business technologies are beginning to demonstrate how drivers can more effectively

utilize their in-vehicle time. Such technologies, if designed and implemented in a way that does not

negatively affect safety, can be just the edge that is needed to boost productivity.

Although it is expected that much of this technology will be implemented by the private sector without

assistance from this government-industry partnership, the following specific tasks will be considered in

the 21st Century Truck Program. Other tasks to improve fuel efficiency on a ton-mile per gallon basis will

be considered in the future.

Tight Maneuver/Precision Docking

This service would position the bus or commercial vehicle very precisely relative to the curb or loading

platform. The driver would maneuver the bus into the loading area and then turn it over to automation.

Sensors would continually determine the lateral distance to the curb, front and rear, and the longitudinal

distance to the end of the vehicle loading area. The driver would be able to override at any time by

operating brakes or steering, and would be expected to monitor the situation and take emergency action if

necessary (for example, if a pedestrian steps in front of the vehicle). When the vehicle is properly docked,

it would stop and revert to manual control. In freight or bus terminals this service could increase facility

throughput as well as safety.

Fully Automated Control at Certain Facilities

This service would enhance efficiency and productivity by providing automated movement of vehicles in

dedicated facilities. Initial applications may include automated bus movement in maintenance areas and

automated container movement within a terminal area. The transit bus application could be a preliminary

use of automation in a low-speed, controlled environment. The automated container movement

application would consist of using vehicle automation technologies to move containers within rail depots,
4-102

truck terminals, ship yards, or other centralized facilities. This service could be expanded to include

platooning capabilities such as electronic tow bars.

4.6.12 Innovative, High-Payoff Technologies

As the R&D plan for the 21st Century Truck Program is defined in more detail, provisions will be made

for periodically seeking out new, innovative, potentially high-payoff technologies that could result in

significant improvements in truck safety, fuel efficiency, and/or reduction in emissions. A major source

for the leading-edge technology breakthroughs will come from ongoing programs sponsored by the

government and industry, including research at the national laboratories, universities, and companies

involved in the trucking industry. However, it will also be very valuable to track all of the latest relevant

advances at the National Aeronautics and Space Administration (NASA), the Defense Advanced

Research Projects Agency (DARPA), and the National Institute of Standards and Technology (NIST);

from industry at large; and from foreign research activities.

Areas in which particularly relevant advances can be expected include the following:

• fundamental understanding of engines, including breakthrough advances in combustion and

aftertreatment;

• new materials technology breakthroughs;

• new analytical and computational methods, particularly those resulting from supercomputing

initiatives;

• advances in motors and power electronics, including new materials and models, superconductivity,

and high-temperature superconductivity;

• advances in energy storage, including advanced batteries, ultra-capacitors, flywheels, and other novel

storage media;

• the integration of mechanical-hybrid technology into heavy vehicles;

• radical breakthroughs in tires, aerodynamics, auxiliaries, and other sources of parasitic losses; and

• breakthroughs in vehicle intelligence for improved efficiency and safety.

In addition, programs such as the Cooperative Automotive Research for Advanced Technologies

(CARAT) Program or the Program to Stimulate Trucking Innovative Concepts and Knowledge (STICK)

in the DOE Office of Transportation Technologies, as well as similar programs in other participating

agencies, should be monitored as a way of mining advanced technologies from innovative small

businesses.
5-1

5. PROGRAM SUMMARY

5.1 SCHEDULE AND MILESTONES

The overall program schedule and milestones developed by the Roadmap subteams are given in

Table 5.1. At the time of this printing, the milestones are those developed by each platform and crosscut

team working independently. Since the platform and crosscut teams have some members in common,

there has been some coordination between the platform and crosscut teams, but not as much as will be

needed as more detailed R&D plans are developed in the coming months. The dates by which technology

will be available from the crosscut activities must be reconciled with the dates planned to demonstrate

technologies on vehicle platforms. This has not yet been done, but will be the focus of Roadmap activities

in the next few months.

Based on input from most of the platform and crosscut technology subteams, it is estimated that a budget

of $300 to $350 million per year for 10 years will be required to achieve the 21st Century Truck Program

goals. This reflects only the federal government share, assuming that the program will be 50-50 cost share

with industry over the 10-year life of the program. This budget estimate will be refined as more detailed

R&D program plans are developed.

Table 5.1. 21st Century Truck schedule and milestones

01 02 03 04 05 06 07 08 09 10

4.1 Large Truck—Tractor Trailer

4.1.1 Waste Heat—Develop technology to reduce engine waste heat

from 240 kWh to 220 kWh by 2002, to 175 kWh by 2006, and to

141 kWh by 2009

4.1.2 Auxiliary Loads—Develop technology to reduce auxiliary load

energy losses from 15 kWh to 12.5 kWh by 2003, to 10 kWh by

2006, and to 7.5 kWh by 2009

4.1.3 Drivetrain Efficiency—Develop technology to reduce drivetrain

energy losses from 9 kWh to 6 kWh by 2003, to 5 kWh by 2006,

and to 4.5 kWh by 2009

4.1.4 Rolling Resistance—Develop tire technology to reduce rolling

resistance energy loss from 70 kWh to 60 kWh by 2003, to 50 kWh

by 2005, and to 40 kWh by 2008

4.1.5 Aerodynamic Drag—Develop and validate tools to enable new

designs to reduce aerodynamic drag energy losses from 85 kWh to

80 kWh by 2003, to 75 kWh by 2005, and to 68 kWh by 2010

4.1.6 Mass Reduction—Complete project to demonstrate capability for

10% tare-weight reduction in each of 2005 and 2009

4.1.7 Collison Avoidance—Complete demonstration of collision

avoidance systems

4.1.8 Crashworthiness—Complete laboratory tests and field trials of

systems to reduce destructive effects of crashes

4.1.9 Brake Performance—Initiate development of electronic brake

systems

4.1.10 Demonstrate electronic brake systems on tractors and trailers

4.1.11 Develop technology for rollover avoidance

4.1.12 Complete operational test and evaluation of vehicle intelligence

and communication system

4.1.13 Implement a practical retrofit program to greatly reduce PM

4.1.14 Establish feasibility of practical engine and aftertreatment for 2010

emissions compliance

4.1.15 Identify fuel and lubricant requirements for success of the complete

emissions control (EC) systems

4.1.16 Develop the enabling technology (materials, lubes, sulfur traps,

turbomachinery, etc.) to ensure performance and durability of EGR,

aftertreatment, and air-handling systems
5-2

Table 5.1 (continued)

01 02 03 04 05 06 07 08 09 10

4.1.17 Develop and implement advanced engineering simulations for

integrated engine and aftertreatment systems to use in optimizing

the system configuration and operating protocols

4.1.18 Confirm adequate durability of complete EC systems

4.1.19 Complete field tests of EC systems

4.1.20 Complete construction of prototype tractor-trailer platforms

4.2 Transit Bus

4.2.1 Investigate state of manufacture of hybrid electric technology and

the individual components

4.2.2 Develop innovative vehicle designs and materials usage to achieve

target weight reduction

4.3 Medium Truck

4.3.1 Identify detailed power-train architectures and specifications

required to reduce engine and accessory losses and to enable hybrid

propulsion

4.3.2 Identify detailed opportunities and designs for weight reduction

4.3.3 Develop control strategies for optimizing power-train performance

for typical duty cycles

4.3.4 Demonstrate Phase I mass, rolling resistance, and accessory load

reduction

4.3.5 Demonstrate Phase I power-train improvements

4.3.6 Demonstrate Phase II advanced mass, rolling resistance, and

accessory load reduction

4.3.7 Demonstrate Phase II advanced power-train improvements

4.4 Small Truck (TBD)

4.5 Military Vehicles (TBD)

4.6 Crosscutting Technologies

4.6.1 Alternative Fuels (TBD)

4.6.2 Internal Combustion Engine

4.6.2.1 Achieve 2002 emissions levels with engine efficiency maintained

at approximately 44%

4.6.2.2 Demonstrate, in a dynamometer environment, the potential (stretch)

engine efficiency achievable with integration of high peak firing

pressure engine design, feasible exhaust-heat recovery, advanced

controls and other improvements in auxiliary drives, lowmechanical-friction features, air handling, and thermal management

4.6.2.3 Achieve 2010 emissions levels with a production-feasible engine

system with 10% higher efficiency over 2002 levels, including any

losses from emission-control devices

4.6.3 Aftertreatment

4.6.3.1 Improve the performance and durability of NOx reduction

technology through improved understanding of basic mechanisms

4.6.3.2 Improve and apply emission-control simulation tools

4.6.3.3 Develop better methods and technologies for generating and

introducing NOx reductants to the NOx control device

4.6.3.4 Establish the influence of fuels and lubricants on emission-control

technologies

4.6.3.5 Determine the best system configuration for NOx and PM control

devices through examination of their interdependence in full

systems

4.6.3.6 Establish a feasible infrastructure/supply strategy for SCR systems

that use non-fuel reductants

4.6.4 Hybrid Electric Power Trains

4.6.4.1 Develop a new generation of electric traction motor systems that

have higher specific power, lower cost, and durability matching the

service life of the vehicle

4.6.4.2 Develop advanced electrical energy-storage systems having higher

specific power, improved power acceptance, and lower cost.

Demonstrate improved system performance and service life in

commercial vehicles
5-3

Table 5.1 (continued)

01 02 03 04 05 06 07 08 09 10

4.6.4.3 Develop power-electronics building blocks needed to lower the cost

and improve the performance of hybrid propulsion control systems

4.6.4.4 Determine "best practices" for hybrid electric vehicle electrical

safety, disseminate safety information, and promote safety

awareness

4.6.4.5 Design and test a brake-by-wire regenerative braking system on a

prototype vehicle that is capable of capturing more than 50% of the

wheel braking energy over the CBD cycle

4.6.4.6 Develop application-specific power plants and customizable

system-controller interfaces for commercial and military hybrid

electric vehicles

4.6.5 Mechanical Hybrid

4.6.5.1 Systems Analysis/Architecture—Coordinate with platform teams to

analyze and determine the systems architecture for mechanical

hybrid trucks and buses

4.6.5.2 Safety Standards—Coordinate with platform teams and with

safety/standards organizations to develop safety standards

associated with mechanical hybrid components and systems

4.6.5.3 Mechanical Hybrid Drivetrain Technology—Demonstrate

mechanical hybrid drivetrain enabling technologies, such as

efficient accumulators/flywheels, valve blocks, transmissions,

actuators, clutches, pumps, motors, and regenerative braking

components

4.6.5.4 Mechanical Hybrid Engine Technology— Demonstrate advanced

engines that will enable mechanical hybrid technologies

4.6.5.5 Mechanical Hybrid Components for Small Truck—Demonstrate

components for a mechanical hybrid to achieve 3× fuel efficiency

improvement in a small truck

4.6.5.6 Technical System Integration for Small Truck—Demonstrate

integrated production-intent vehicle for a mechanical hybrid to

achieve 3× fuel efficiency improvement in a small truck

4.6.5.7 Mechanical Hybrid Components for Medium Truck—Demonstrate

components for a mechanical hybrid to achieve 3× fuel efficiency

improvement in a medium truck

4.6.5.8 Technical System Integration for Medium Truck—Demonstrate

integrated production-intent vehicle for a mechanical hybrid to

achieve 3× fuel efficiency improvement in a medium truck.

4.6.5.9 Mechanical Hybrid Components for Transit Bus—Demonstrate

components for a mechanical hybrid to achieve 3× fuel efficiency

improvement in a transit bus.

4.6.5.10 Technical System Integration for Transit Bus—Demonstrate

integrated production-intent vehicle for a mechanical hybrid to

achieve 3× fuel efficiency improvement in a transit bus.

4.6.6 Fuel Cells

4.6.6.1 Design and develop reformer systems for logistics fuels

4.6.6.2 System design and integration for fuel cells for primary propulsion

power

4.6.6.3 System design and integration for APUs

4.6.6.4 Develop gaseous fuel storage technology

4.6.7 Auxiliary Power

4.6.7.1 Complete truck electrification technology roadmap

4.6.7.2 Initiate heavy-vehicle electrification program

4.6.7.3 Complete component and APU demonstration and evaluation

4.6.7.4 Initiate component and APU integration projects

4.6.7.5 Complete prototype demonstration and evaluation of truck

electrification

4.6.7.6 Initiate durability and cost-reduction projects

4.6.7.7 Complete demonstration and evaluation of full truck electrification
5-4

Table 5.1 (continued)

01 02 03 04 05 06 07 08 09 10

4.6.8 Thermal Management

4.6.8.1 Develop intelligent sensors, controls, and managment algorithms to

optimize thermal-management systems

4.6.8.2 Develop advanced designs for heat exchangers, taking advantage of

newly developed heat-exchanger materials and heat-transfer fluids

4.6.8.3 Develop thermal management systems for advanced power trains

(e.g., power electronics and fuel cells)

4.6.8.4 Perform comprehensive computational fluid dynamic simulations

of truck airflow to optimize cooling

4.6.8.5 Develop innovative thermal-management concepts

4.6.8.6 Design, develop, and demonstrate new thermal-management

systems in multiple-truck platforms

4.6.9 Materials

4.6.9.1 Diesel Engines: Develop cost-effective materials solutions for

critical problems in fuel systems, air handling, exhaust

aftertreatment, high-pressure cylinder heads and engine blocks, and

thermal management

4.6.9.2 Hybrid Propulsion: Develop cost-effective materials for power

storage (batteries, ultra-capacitors, flywheels, hydraulic-pneumatic

mechanical storage systems) and power electronics for hybrid

electric propulsion systems

4.6.9.3 Fuel cells: Develop cost-effective SOFC materials technology for

hybrid propulsion systems and for APUs

4.6.9.4 Develop cost-effective manufacturing technology for lightweight

truck components

4.6.9.5 Develop advanced design concepts and materials data bases for

lightweight components

4.6.9.6 Develop joining, maintenance, and repair technology for

lightweight truck components

4.6.9.7 Develop advanced materials for truck braking systems to enhance

vehicle safety and reliability

4.6.9.8 Develop advanced tire systems to reduce rolling resistance while

improving truck safety and reliability

4.6.9.9 Develop improved lubricant and bearing materials to reduce powertransmission losses and to improve component durability

4.6.9.10 Develop advanced sensors for intelligent engine control and

emission reduction of diesel engines

4.6.9.11 Develop advanced piezoelectric materials for smart actuators in

fuel and valve-train systems of diesel engines

4.6.9.12 Develop long-life corrosion-control materials and coatings to

minimize required corrosion allowances and to enable the use of

high-strength weight-reduction materials

4.6.10 More-Efficient and/or Lower-Emission Heat Engine Systems

(TBD)

4.6.11 Vehicle Intelligence (See Fig. 4.18)

Indicates beginning

Indicates completion
6-1

6. REFERENCES

Achten, P. A. J. 1996. “The End of a Revolution,” Ph.D. Thesis, Eindhoven University of Technology,

Eindhoven, The Netherlands.

American Public Transportation Association (APTA). 2000a. APTA 2000 Public Transportation Fact

Book. American Public Transportation Association, Washington, D.C.

American Public Transportation Association (APTA). 2000b. Standard Bus Procurement Guidelines.

http://www.apta.com/services/procurement/index.htm.

Appleby, J. A. 1989. Fuel Cell Handbook, Van Nostrand, New York.

Arthur D. Little, Inc. (A. D. Little). 2000. Cost Analysis of Fuel Cell System for Transportation: Baseline

System Cost Estimate—Report to the U.S. Department of Energy. Ref. No. 49739, SFAA No.

DE-SCO2-98EE50526. Arthur D. Little, Inc., Cambridge, Massachusetts.

Assanis, D. N., E. Karvounis, and J. A. E., Bell. 1993.”Design Optimization of the Piston Compounded

Adiabatic Diesel Engine Through Computer Simulation,” SAE 930986, Society of Automotive

Engineers, Warrendale, Penn.

Blau, P. 1999. Energy Efficiency in Heavy Vehicle Tires, Drivetrains, and Braking Systems: Draft MultiYear Program Plan for Running Resistance. December 22, 1999.

California Air Resources Board (CARB). 1998. 1998 Zero-Emission Vehicle Bienniel Program Review.

California Air Resources Board.

Chalk, S.1999. Progress Report for Fuel Cells for Transportation. U.S. Department of Energy, Office of

Advanced Automotive Technologies, Washington, D.C.

Clark, N. N., J. J. Daley, R. D. Nine, and C. M. Atkinson. 1999. “Application of the New City-Suburban

Heavy Vehicle Route (CSHVR) to Truck Emissions Characterization.” Topic in General Emissions.

Special Publication SP-1459, Paper 1999-01-1467. SAE International, Warrendale, Penn. 1–15.

Clarke, J. M. 1990. “Ideal Cycles Incorporating Heat Exchange,” SAE 900913, Society of Automotive

Engineers, Warrendale, Penn.

Clarke, J. M., and W. G. Berlinger. 1999. “A New Compression Ignition Engine Concept for High Power

Density.” Journal of Engineering for Gas Turbines and Power 121 (2): 211.

Davis, Stacey C. 1999. Transportation Energy Data Book: Edition 19. DOE/ORNL-6958, Oak Ridge

National Laboratory, Oak Ridge, Tenn.

Diesel Emission Control Sulfur Effects Program (DECSE). 1999. DECSE Program Phase I Interim Data

Report No. 2: NOx Adsorber Catalysts. U.S. Department of Energy, Office of Transportation

Technologies, Washington, D.C.

Diesel Vehicle Emission Control Sulfur Effects Program (DVECSE). 2000. Diesel Vehicle Emission

Control Sulfur Effects Program. (DVECSE) Project, Phase 1 Final Report. Oak Ridge National

Laboratory, March 2000.

Ecopoint. 2000. Selective Catalytic Reduction, DieselNet Technology Guide. Ecopoint Inc. Revision

2000.08.
6-2

Energy Information Administration (EIA) 1999. EIA Annual Energy Outlook 2000. DOE/EIA-0383

(2000). Energy Information Administration, Washington, D.C., December.

Fanick, Robert, et al. 1995. “Reduction of Diesel NOx/Particulate Emissions Using a Non-Thermal

Plasma.” In Proceedings of the 1995 Diesel Engine Emission Reduction Workshop, July 24–27, 1995.

U.S. Department of Energy, Washington, D.C.

Federal Highway Administration (FHWA). 1998. Highway Statistics ’97. FHWA-PL-98-020. U.S.

Department of Transportation, Washington, D.C.

Ferrenberg, A. 1990. “The Single Cylinder Regenerated Internal Combustion Engine.” SAE 41st

Earthmoving Industry Conference. Society of Automotive Engineers, Warrendale, Penn.

Ferrenberg, A., et al.1993. “The Low Heat Rejection Regenerated Diesel Engine,” 28th. Intersociety

Energy Conversion Engineering Conference (IECEC), August 1993, Atlanta, Georgia.

Flynn, P. F., R. P. Durrett, G. L. Hunter, W. C. Akinyemi, and L. A. Farrell. 2000. “Minimum Engine

Flame Temperature Impacts on Diesel and Spark-Ignition Engine NOx Production.” SAE

2000-01-1177. Society of Automotive Engineers, Warrendale, Penn.

Goertz, M., and L. Peng. 2000. “Free Piston Engine: Its Application and Optimization,” SAE

2000-01-0996. Society of Automotive Engineers, Warrendale, Penn.

Goldsborough, S. S., and P. Van Blarigan. 1999. “A Numerical Study of a Free Piston IC Engine

Operating on Homogeneous Charge Compression Ignition Combustion.” SAE 1999-01-0619, Society

of Automotive Engineers, Warrendale, Penn.

Gornitz, V., and I. Fung. 1994. “Potential Distribution of Methane Hydrates in the World Oceans.”

Global Biogeochemical Cycles 8(3): 335–347.

Hofbauer, P. 2000. “Principles of Operation of an Electrically Assisted Turbocharger with Electronic

Speed Modulation.” September 2000.

Inform, Inc. 2000. Bus Futures—New Technologies for Cleaner Cities. Inform, Inc.

Ishibashi, Y., and M. Asai. 1996. “Improving the Exhaust Emissions of Two-Stroke Engines by Applying

the Activated Radical Combustion.” SAE 960742. Society of Automotive Engineers, Warrendale,

Penn.

Karpuk, M. E. 1989. Design and Testing of a Dissociated Methanol Vehicle. DOE/CH/10093-H5. U.S.

Department of Energy, Washington, D.C.

Kawamura, H., and A. Ishida. 2000. “Development of Ceramics Adiabatic Engine Having 68% Thermal

Efficiency.” ASME 2000-ICE-334, ASME ICE Fall Technical Conference. American Society of

Mechanical Engineers, Warrendale, Penn.

Larminie, J., and A. Dicks. 2000. Fuel Cell Systems Explained. John Wiley and Sons, New York.

Letavec, C. A., J. S. Degal, and J. P. Uihlein. 2000. “EC-Diesel Technology Validation Program Interim

Report.” CEC/SAE Spring Fuels & Lubricants Meeting & Exposition, June 2000, Paris, France.

2000-01-1854. Society of Automotive Engineers, Warrendale, Penn.

Liang, C. Y., et al. 2000. “Effects of Diesel Fuel Sulfur Level on Performance of a Continuously

Regenerating Diesel Particulate Filter and a Catalyzed Particulate Filter.” CEC/SAE Spring Fuels &
6-3

Lubricants Meeting & Exposition, June 2000, Paris, France. 2000-01-1876. Society of Automotive

Engineers, Warrendale, Penn.

Manufacturers of Emission Controls Association (MECA). 2000. MECA-Written Statement of the

Manufacturers of Emission Controls Association on the U.S. Environmental Protection Agency’s

Proposed Heavy-Duty Engine and Vehicle Standards and Highway Diesel Sulfur Control. Docket

No. A-99-06, August 14, 2000.

Miller, William, et al. 2000. “The Development of Urea-SCR Technology for U.S. Heavy Duty Trucks.”

SAE Paper No. 2000-01-0190. Society of Automotive Engineers, Warrendale, Penn.

Minh, N. Q., and T. Takahashi. 1995. Science and Technology of Ceramic Fuel Cells. Elsevier,

Amsterdam.

National Archives. 1996. Federal Register. Vol. 61, 33421, June 26, 1996.

National Renewable Energy Laboratory (NREL). 1996. Alternative Fuel Transit Buses, Final Results

from the NREL Vehicle Evaluation Program. National Renewable Energy Laboratory, Golden, Colo.

National Renewable Energy Laboratory (NREL). 2000a. Alternative Fuel Transit Buses, DART’s LNG

Bus Fleet Final Data Report. National Renewable Energy Laboratory, Golden, Colo.

National Renewable Energy Laboratory (NREL). 2000b. Development of an Ultra-Safe, Ultra-Low

Emissions Natural Gas Fueled School Bus: Final Report. NREL/SR-540-23842. National Renewable

Energy Laboratory, Golden, Colo.

National Renewable Energy Laboratory (NREL). 2000. “Biomass: Nature’s Renewable Storehouse of

Solar Energy and Chemical Resources.” http://www.nrel.gov/research/industrial_tech/biomass.html.

Nixdorf, Dick. 2000. “Microwave-Regenerated Ceramic Filter for Diesel Exhaust Particulate Control,”

CIDI Engine Combustion, Emission Control, and Fuels Review. May 2000.

Nurun, N. M., et al. 1999. “Ultra Low NOx and Smokeless Diesel Combustion with Highly Oxygenated

Fuel,” 15th International Combustion Engine Symposium, Seoul.

NYC. 2000. NYC Transit Report.

Office of Heavy Vehicle Technologies (OHVT). 1997. Comprehensive Program Plan for Natural Gas

Vehicle Research. Office of Heavy Vehicle Technologies, Office of Transportation Technologies,

Energy Efficiency and Renewable Energy, U.S. Department of Energy, Washington, D.C.

Office of Heavy Vehicle Technologies (OHVT). 1998. Diesel Engine Combustion Research, Program

Plan FY 1998–2002. June 1998 Draft, Office of Heavy Vehicle Technologies, Office of

Transportation Technologies, Energy Efficiency and Renewable Energy, U.S. Department of Energy,

Washington, D.C.

Office of Heavy Vehicle Technologies (OHVT). 1999a. Heavy Vehicle Propulsion Materials Draft MultiYear Program Plan. Revision 4. Office of Heavy Vehicle Technologies, Office of Transportation

Technologies, Energy Efficiency and Renewable Energy, U.S. Department of Energy.

Office of Heavy Vehicle Technologies (OHVT). 1999b. Heavy Vehicle Hybrid Propulsion Systems R&D,

Program Plan FY 1999–2003. Draft, Office of Heavy Vehicle Technologies, Office of Transportation

Technologies, Energy Efficiency and Renewable Energy, U.S. Department of Energy.
6-4

Office of Heavy Vehicle Technologies (OHVT). 1999c. Multi-Program Plan—Reducing Friction and

Wear in Heavy Vehicles. Office of Heavy Vehicle Technologies, Office of Transportation

Technologies, Energy Efficiency and Renewable Energy, U.S. Department of Energy, Washington,

D.C.

Parks, J. E. II, G. J. Wagner, W. S. Epling, M. W. Sanders, and L. E. Campbell. 1999. “NOx Sorbate

Catalyst System with Sulfur Catalyst Protection for the Aftertreatment of No. 2 Diesel Exhaust,” SAE

Technical Paper 1999-01-3557. Society of Automotive Engineers, Warrendale, Penn.

Penetrante, B. M. “Plasma-Assisted Heterogeneous Catalysis for NOx Reduction in Lean-Burn Engine

Exhaust.” Proceedings of the 1997 Diesel Engine Emission Reduction Workshop.

Psaras, Dimitri, and Jerry Summers. 1995. “EOLYS (trademark), an Effective System Approach for

Achieving Low Emissions in Advanced Diesel Engines,” Proceedings of the 1995 Diesel Engine

Emission Reduction Workshop, July 24–27, 1995. U.S. Department of Energy, Washington, D.C.

Sherman, A. J., R. H. Tuffias, and R. B. Kaplan. 1991. “Refractory Ceramic Foams: A Novel, New High

Temperature Structure.” American Ceramics Society Bulletin 70 (6): 1025–29.

Sklad, P. 2000. High-Strength Weight Reduction Materials Draft Multi-Year Program Plan. Revision 1.

February 24, 2000 (Draft).

Society of Automotive Engineers (SAE). 1988. “Truck and Bus Engine and Vehicle Performance

Subcommittee, Information Relating to Duty Cycles and Average Power Requirements of Truck and

Bus Engine Accessories.” SAE J1343, Society of Automotive Engineers, Warrendale, Penn.

Society of Automotive Engineers (SAE). 1999. “Non-Thermal Plasma for Exhaust Emission Control:

NOx, HC, and Particulates.” SAE Special Publication SP-1483. Edited by M. L. Balmer, G. Fisher,

and J. Hoard. Society of Automotive Engineers, Warrendale, Penn.

Solid State Energy Conversion Alliance. 2000. Workshop Proceedings. Baltimore, Md.

Stodolsky, F., L. Gaines, and A. Vyas. 2000. Analysis of Technology Options to Reduce the Fuel

Consumption of Idling Trucks. ANL/ESD-43. Center for Transportation Research, Argonne National

Laboratory, Argonne, Ill.

Transit Cooperative Research Program (TCRP). 2000. Hybrid Electric Transit Buses: Status, Issues, and

Benefits. Report No. 59. National Academy of Sciences, Washington, D.C.

Transportation Research Board (TRB). 2000. Review of the Research Program for a New Generation of

Vehicles. Transportation Research Board, National Research Council, Washington, D.C.

Truck Index. 1994. Gasoline Truck Index and Diesel Truck Index. Truck Index, Inc., Santa Ana, Calif.

—. 1995. Gasoline Truck Index and Diesel Truck Index. Truck Index, Inc., Santa Ana, Calif.

—. 1996. Gasoline Truck Index and Diesel Truck Index. Truck Index, Inc., Santa Ana, Calif.

—. 1997. Gasoline Truck Index and Diesel Truck Index. Truck Index, Inc., Santa Ana, Calif.

U.S. Department of Commerce (DOC). 1999. 1997 Economic Census: Vehicle Inventory and Use Survey.

EC97TV-US, U.S. Census Bureau, Department of Commerce, Washington D.C.
6-5

U.S. Department of Commerce (DOC). 2000. 1997 Vehicle Inventory and Use Survey: Microdata File.

U.S. Census Bureau, U.S. Department of Commerce, Washington, D.C.

U.S. Department of Defense (DOD). Army Tank-Automotive and Armaments Command (TACOM).

http://www.tacom.army.mil/.

U.S. Department of Energy (DOE). 1996. An Assessment of Flywheel Energy Storage Technology for

Hybrid and Electric Vehicles. U.S. Department of Energy, Washington, D.C.

U.S. Department of Transportation (DOT). 1997. Intelligent Vehicle Initiative Business Plan. U.S.

Department of Transportation, Intelligent Transportation Systems Joint Program Office, Washington,

D.C.

U.S. Department of Transportation (DOT). 2000. Intelligent Vehicle Initiative Business Plan. U.S.

Department of Transportation, Intelligent Transportation Systems Joint Program Office, Washington,

D.C.

U.S. Environmental Protection Agency (EPA). 2000. Proposed Heavy Duty Engine and Vehicle

Standards and Highway Diesel Fuel Sulfur Control Requirements. EPA420-F-00-022, U.S.

Environmental Protection Agency, Washington, D.C.

Warren, J. P., et al. 2000. “European Experience of High Mileage Durability of Continuously

Regenerating Diesel Particulate Filter Technology.” SAE 2000 World Congress, March 2000, Detroit,

Mich. 2000-01-0480. Society of Automotive Engineers, Warrendale, Penn.

Wipke, K., M. Cuddy, D. Bharathan, S. Burch, V. Johnson, T. Markel, and S. Sprik. 1998. “ADVISOR

2.0: A Second-Generation Advanced Vehicle Simulator for Systems Analysis.” In Conference

Proceedings from North American EV & Infrastructure Conference and Exposition, Phoenix,

Arizona.

APPENDIX A

CONTRIBUTORS TO THE 21st CENTURY TRUCK PROGRAM TECHNOLOGY ROADMAP

A-3

APPENDIX A

CONTRIBUTORS TO THE 21st CENTURY TRUCK PROGRAM TECHNOLOGY ROADMAP

Rene Abarcar, Energetics

Ed Bass, Allison Transmission

Kevin Beaty, Eaton

Rich Bergstrand, PACCAR

Tom Biros, International Truck and Engine

(Subteam Chair)

Ron Bradley, ORNL (Team Leader)

Fred Cartwright, Allison Transmission

John Clark, Caterpillar (Subteam Chair)

Roger Clark, General Motors

Bob Clarke, U.S. DOT

Patrick Dessert, Oakland University

Lee Dodge, SwRI

Kate Drakos, EMA

James Eberhardt, U.S. DOE

Pat Flaherty, U.S. DOE

Christina Gikakis, U.S. DOT

Bill Gouse, Freightliner

Ron Graves, ORNL (Subteam Chair)

Charles Gray, EPA

Nabil Hakim, Detroit Diesel (Subteam Chair)

Katherine Hartman, U.S. DOT

Norm Helgeson, Navy/NFESC

Dean Helms, TMA

Greg Hofmann, BAE SYSTEMS Controls

Shang Hsiung, U.S. DOT (Subteam Chair)

Caitlin Hughes, U.S. DOT

John Hull, ANL (Subteam Chair)

John Kargul, EPA (Subteam Chair)

Glenn Keller, EMA

Jack Klein, Mack Trucks

Bill Knee, ORNL

Linda Lawson, U.S. DOT

William Leasure, TMA

Scott Lockledge, OSTP

Bart Mancini, Nova Bus

Scott McBroom, SwRI

Jim McDowell, Nova Bus

Oliver McGee, U.S. DOT

Don Moore, Western Star Trucks

Dave O’Kain, ORNL

Jim Patten, Battelle-Columbus

Duane Perrin, U.S. DOT

Rob Piacesi, SwRI

David Powell, IITRI

Ray Resendes, U.S. DOT (Subteam Chair)

Guy Rini, Mack Trucks

Charles Roehm, ANL

Dan Santini, ANL

S.M. Shahed, Honeywell

Bill Siegel, U.S. DOT (Subteam Chair)

Mike Simon, ISE Research

Gurpreet Singh, U.S. DOE

Paul Skalny, TACOM

Dave Smith, Allison Transmission

David Stafford, Michelin

Randy Steer, OMB

Frank Stodolsky, ANL

Tom Vachon, Caterpillar

Kathy VanLenten, Energetics

Keith Vertin, NREL (Subteam Chair)

Jud Virden, PNNL (Subteam Chair)

John Woodrooffe, TMA Canada

(Subteam Chair)

Skip Yeakel, Volvo

Ron York, General Motors (Subteam Chair)

Steve Zimmer, DaimlerChrysler

(Subteam Chair)

APPENDIX B

21st CENTURY TRUCK PROGRAM SUBTEAMS

B-3

APPENDIX B

21st CENTURY TRUCK PROGRAM SUBTEAMS

Large Truck—Tractor Trailer

John Woodrooffe (Chair), TMA Canada

Rich Bergstrand, PACCAR

John Bishop, SwRI (Army rep)

Bob Clarke, U.S. DOT

Bill Corwin, ORNL

Dan Farmer, PACCAR

Bill Gouse, Freightliner

Ron Graves, ORNL

Nabil Hakim, Detroit Diesel

Kate Hartman, U.S. DOT

Scott McBroom, SwRI (Army rep)

Rich Nellums, Eaton

Guy Rini, Mack Trucks

Dan Santini, ANL

S.M. Shahed, Honeywell

Dave Smith, Allison Transmission

David Stafford, Michelin

Tom Vachon, Caterpillar

Keith Vertin, NREL

Steve Weissner, International Truck and Engine

Chris Yakes, Oshkosh Trucks

Skip Yeakel, Volvo

Transit Bus

Bill Siegel (Chair), U.S. DOT

Ed Bass, Allison Transmission

Bill Corwin, ORNL

Vinod Duggal, Cummins Engine

Mary Gerstner, Honeywell

Greg Hofmann, BAE SYSTEMS Controls

Jim McDowell, Nova Bus

Dave O'Kain, ORNL

Mike Simon, ISE Research

Jerry Trotter, APTA

Keith Vertin, NREL

Medium Truck

Tom Biros (Chair), International Truck

and Engine

Ed Bass, Allison Transmission

John Bishop, SwRI (Army rep)

Shubhayu Chakraborty, Eaton

Bill Corwin, ORNL

Bill Gouse, Freightliner

Greg Hofmann, BAE SYSTEMS Controls

Scott McBroom, SwRI

Duane Perrin, U.S. DOT

Charles Roehm, ANL

Small Truck

Ron York (Chair), General Motors

Steve Zimmer (Chair), DaimlerChrysler

Kevin Beaty, Eaton

John Bishop, SwRI (Army rep)

Roger Clark, General Motors

Bill Gouse, Freightliner

Greg Hofmann, BAE SYSTEMS Controls

Scott McBroom, SwRI (Army rep)

Duane Perrin, U.S. DOT

Charles Roehm, ANL

John Stang, Cummins Engine

Jud Virden, PNNL

Internal Combustion Engine

Nabil Hakim (Chair), Detroit Diesel

Tom Asmus, DaimlerChrysler

Brian Bolton, Detroit Diesel

Bob Carling, SNL

Christina Gikakis, U.S. DOT

Shang Hsiung, U.S. DOT

Gary Hunter, Cummins Engine

Yury Kalish, Detroit Diesel

Glenn Keller, EMA

Pete Schihl, TACOM

Tom Vachon, Caterpillar

Aftertreatment

Ron Graves (Chair), ORNL

Mari Lou Balmer, PNNL

Lee Dodge, SwRI (Army rep)

Christina Gikakis, U.S. DOT

Bill Gouse, Freightliner

Nabil Hakim, Detroit Diesel

Shang Hsiung, U.S. DOT

Dale McKinnon, MECA
B-4

George Muntean, Cummins Engine

Jim Patten, Battelle-Columbus

Tom Vachon, Caterpillar

Jud Virden, PNNL

Hybrid Electric Power Trains

Keith Vertin (Chair), NREL

Ed Bass, Allison Transmission

Kevin Beaty, Eaton

Bill Corwin, ORNL

Vinod Duggal, Cummins Engine

Mike Duoba, ANL

Mary Gerstner, Honeywell

Greg Hofmann, BAE SYSTEMS Controls

Shang Hsiung, U.S. DOT

Bill Kahn, Peterbilt Motors

Scott McBroom, SwRI (Army rep)

Jim McDowell, Nova Bus

Dave O'Kain, ORNL

Kaushik Rajashetara, Delphi

Bill Siegel, U.S. DOT

Mike Simon, ISE Research

Dave Smith, Allison Transmission

Jerry Trotter, APTA

Mechanical Hybrid

John Kargul (Chair), EPA

Tom Biros, International Truck and Engine

Dwight Holloway, Caterpillar

Scott McBroom, SwRI (Army rep)

Dave O'Kain, ORNL

Tom Vachon, Caterpillar

Fuel Cells

Shang Hsiung (Chair), U.S. DOT

Tim Armstrong, ORNL

Ed Bass, Allison Transmission

Vince Callaghan, International Fuel Cells

Steve Chalk, U.S. DOE

Ken Koyoma, California Energy Commission

Nguyen Minh, Honeywell

Alan Montemayor, SwRI (Army rep)

Robert Nowak, DARPA

Ric Pow, EXCELLSiS Fuel Cell Engines

Bob Rose, U.S. Fuel Cell Council

Jules Routbort, ANL

Jim Wood, Delphi

Jiemin Zhu, Detroit Diesel

Auxiliary Power

Jud Virden (Chair), PNNL

Tim Armstrong, ORNL

Jeff Corey, Peterbilt

Lee Dodge, SwRI (Army rep)

Mary Gerstner, Honeywell

Christina Gikakis, U.S. DOT

Bill Gouse, Freightliner

Shang Hsiung, U.S. DOT

Dave Orr, Caterpillar

Thermal Management

John Hull (Chair), ANL

Lee Dodge, SwRI (Army rep)

Christina Gikakis, U.S. DOT

Shang Hsiung, U.S. DOT

Moe Khaleel, PNNL

Rick Kleine, Cummins Engine

James Klett, ORNL

Mark Moeckel, Caterpillar

Eric Poulson, PACCAR

Freidoon Rastegar, Honeywell

Marty Wambsganss, Consultant

Materials

Bill Corwin (Chair), ORNL

Paul Becker, Cummins Engine

Rich Bergstrand, PACCAR

Christina Gikakis, U.S. DOT

Shang Hsiung, U.S. DOT

Caitlin Hughes, U.S. DOT

Bart Mancini, Nova Bus

Phil McCluskey, Caterpillar

Mark Smith, PNNL

Tom Yonushonis, Cummins Engine

More-Efficient and/or Lower-Emission

Heat Engine Systems

John Clarke (Chair), Caterpillar

Mark Aramli, Capstone Turbine

Lee Dodge, SwRI (Army rep)

Ron Graves, ORNL

Karl Hellman, EPA

Glenn Keller, EMA

Tom Vachon, Caterpillar
B-5

Vehicle Intelligence

Ray Resendes (Chair), U.S. DOT

Bill Gouse, Freightliner

Bill Knee, ORNL

Guy Rini, Mack Trucks

Ron Temple, Cummins Engine

Skip Yeakel, Volvo

APPENDIX C

PARTICIPANTS IN THE OCTOBER 25, 2000, MEETING OF THE

21st CENTURY TRUCK PROGRAM PARTNERSHIP COORDINATING COMMITTEE

C-3

APPENDIX C

PARTICIPANTS IN THE OCTOBER 25, 2000, MEETING OF THE

21st CENTURY TRUCK PROGRAM PARTNERSHIP COORDINATING COMMITTEE

Industry Members

René Allen, Vice President, Product Planning,

NovaBUS Incorporated

Randy Allran, Executive Vice President,

Engineering and Operations, Detroit Diesel

Corporation

Keith Brandis, Vice President, Marketing, Volvo

Trucks North America

Eduardo Castro-Wright, President and CEO,

Honeywell Transportation Systems,

Honeywell, Inc.

Philip J. Christman, Vice President, Core

Engineering and Product Support,

International Truck and Engine Corporation

Denyse LeFever, Vice President, HEV Power

and Drive Systems, BAE SYSTEMS Controls

Stephen Homcha, Executive Vice President,

Class 8 Program, Mack Trucks, Inc.

Tim Morscheck, Vice President and General

Manager, Trucking Electronic Systems

Division, Eaton Corporation

David M. Piper, Director of Engineering,

Allison Transmission Division, General

Motors Corporation

Bernard I. Robertson, Senior Vice President,

Engineering Technologies, DaimlerChrysler

Corporation

Margaret Sullivan, Director of Advanced

Technology, PACCAR Inc.

Don Verhoff, Vice President, Technology,

Oshkosh Truck Corporation

Michael von Mayenburg, Senior Vice President,

Engineering and Technology, Freightliner

LLC

Christine Vujovich, Vice President,

Environmental Policy and Product Strategy,

Cummins, Inc.

Sherril K. West, Vice President, Technical

Services Division, Caterpillar, Inc.

Ron York, Director, Partnership for a New

Generation of Vehicles, GM Advanced

Technology Vehicles, General Motors

Corporation

Government Members

Roger Ballentine, Deputy Assistant to the

President, Executive Office of the President

Mortimer L. Downey (Vice-Chair), Deputy

Secretary, Office of the Secretary, S-2, U.S.

Department of Transportation

T. J. Glauthier (Chair), Deputy Secretary, U.S.

Department of Energy

Thomas Gross, PCO Executive Director and

Deputy Assistant Secretary for

Transportation Programs, Energy Efficiency

and Renewable Energy, EE-30, U.S.

Department of Energy

Paul J. Hoeper (Vice-Chair), Assistant Secretary

of the Army for Acquisition, Logistics, and

Technology, U.S. Department of Defense

Duncan T. Moore, Associate Director for

Technology, White House Office of Science

and Technology Policy

Robert Perciasepe (Vice-Chair), Assistant

Administrator for Air and Radiation, U.S.

Environmental Protection Agency

Dan Reicher,(Alternate Chair), Assistant

Secretary, Energy Efficiency and Renewable

Energy, EE-1, U.S. Department of Energy

Wesley P. Warren, Associate Director for

Natural Resources, Energy and Science

Office of Management and Budget

Morley A. Winograd, Senior Policy Advisor and

Director, Partnership for Reinventing

Government, Office of the Vice President

Observing Members

Glenn F. Keller, Executive Director, Engine

Manufacturers Association

Bill Leasure, Executive Director, Truck

Manufacturers Association

APPENDIX D

INTERNAL COMBUSTION ENGINE TECHNOLOGY

D-3

APPENDIX D

INTERNAL COMBUSTION ENGINE TECHNOLOGY

D.1 IN-CYLINDER COMBUSTION AND EMISSIONS

The in-cylinder combustion and emissions processes are central to the performance, efficiency, and

engine-out emissions of diesel engines. Our understanding of these processes have improved dramatically

over the past decade through the application of advanced optical diagnostics and modeling tools.

However, much still remains to be learned, and modeling improvements are required. Moreover, incylinder processes will need to be optimized to work efficiently with aftertreatment systems.

D.1.1 Combustion Event and Emissions Processes

Diesel engine combustion and emissions formation processes are driven by a very complex set of

interacting fluid-mechanic and chemistry processes. The current understanding of these processes

provides a general picture of the structure of a diesel spray during injection for moderate-load, quiescent

conditions and the scaling of many of the features of the diesel spray with engine and injector parameters.

This picture includes when and where fuel breaks up and is vaporized, where combustion occurs, and

when and where soot and NOx are formed. The knowledge of the spatial evolution of NOx and soot after

the end of injection is also emerging.

This understanding provides a foundation on which to build a more comprehensive picture of the diesel

combustion process for a wide range of in-cylinder conditions. The goal is to provide the understanding

needed to enhance fuel-air mixing during injection, tailor and control heat release, lower peak in-cylinder

temperatures, and enhance late-cycle mixing. These comprise the basic pathways for improving

combustion performance and reducing emissions. To achieve this goal, improved understanding is needed

in the following main areas: in-cylinder flow prior to fuel injection, fuel-injection spray interaction,

ignition/premixed burn (fuel effects), combustion and emissions formation during injection, post-injection

combustion, and emissions development.

D.1.2 Exhaust Gas Recirculation (EGR)

Current understanding of the impact of EGR on in-cylinder diesel processes is very limited. Models and

data must be developed to understand the effects of EGR on all the various in-cylinder diesel processes.

Mixing uniformity of EGR with fresh intake air in the engine cylinders needs to be studied.

D.1.3 Bowl Geometry and Fluid-Mechanics Effects

Bowl geometry influences the diesel combustion process through several paths, including the interaction

of the vaporized fuel jet with the bowl wall; bowl shape influences on the turbulent energy that promotes

fuel-air mixing; and bowl aspect ratio and re-entrancy influence on the strength of the squish flows, which

help large-scale vortical structures within the bowl (a critical factor in late-cycle mixing).

Predictive models of bowl geometry effects, pinned in the underlying physics, are presently inadequate. A

detailed understanding is needed and can only be achieved through comprehensive in-cylinder

measurements of flow and composition, supported by detailed modeling.

D.1.4 Fuel Types and Effects

Chemical and physical properties of fuels are known to influence pollutant emissions through several

mechanisms. A complete understanding is lacking of the influence of fuel properties on in-cylinder
D-4

processes. In-cylinder diagnostics can provide a knowledge base, enabling new engines to be developed

that use new fuels to their full potential. Three general fuel classes require further study: improved

“traditional” petroleum-based diesel fuel and how a tailored set of fuel properties can help achieve fuel

efficiency and emissions targets; oxygenated (renewable) fuels, which also contain oxygen that can be

produced domestically to reduce dependence on foreign oil; and Fischer-Tropsch fuels that can be

produced from natural gas and coal.

D.1.5 Fuel-Injection System

Recent research strongly indicates that further fuel-injection system improvements will yield significant

enhancements to engine performance and emissions. Briefly, technology areas requiring further R&D

include pilot injection, injection-rate shaping, and multiple injections; increased and controlled injection

pressures; injector tip/orifice geometry; and advanced actuators such as piezoelectric for more precise

control of injection parameters.

D.1.6 Water Emulsions and Other Water-Injection Techniques

Water injection technology holds the promise of breaking the soot-NOx trade-off conundrum that often

limits the design of diesel engine combustion strategies. Water-emulsified fuels require no special

injection equipment. However, they have several drawbacks, including the supply infrastructure and

possible separation of the water and fuel. A fixed water concentration eliminates the possibility of

“tuning” the water fraction to obtain an optimal reduction in soot formation without inhibiting the final

soot burnout at each operating condition. Alternative water supply methods provide greater control and

cycle-by-cycle flexibility. Although the benefits of water (and other oxygenates) addition have been well

documented, the cost of cosolvents or additional injection equipment has been prohibitive so far.

D.1.7 Advanced Fuel-Injection and Mixing Techniques

Advanced, nonconventional fueling techniques and auxiliary in-cylinder mixing offer the potential of

reducing diesel emissions while maintaining or even improving fuel consumption. An evaluation of

potential techniques would be followed by a down-selection to the most promising methods for more indepth R&D. Examples include

• variable orifice size;

• high-frequency injection-rate modulation to increase air entrainment and spreading the angle of the jet

in the near-orifice region, leading to reduced soot formation;

• ultrasonic dual-fluid mixing injectors as means for supplying a water-fuel mixture that can be varied

on demand;

• late-cycle air-gas mixing; and

• possibly gaseous fumigation techniques for HCCI-like combustion of gaseous-diesel fuel

combinations.

D.1.8 Heat Transfer

Heat transfer influences both global and spatially localized aspects of the combustion process. Globally,

heat transfer affects the energy available in the in-cylinder gases to perform useful work, while local heattransfer rates can affect processes as diverse as film vaporization rates and the kinetics of chemical

reactions. Improvements in predictive capabilities are required for convective and radiative heat transfer

in both models and correlations.
D-5

D.1.9 Advanced Modeling Development and Techniques

The current spray-modeling approach is inappropriate for the high-pressure injectors used in modern

diesel engines. Areas of improvement for the computational fluid dynamics codes include improved spray

and jet-mixing models, variable-density and one-dimensional turbulence models, fuel injector orifice-flow

modeling, and soot burnout.

Understanding chemical kinetics is critical to the complete understanding of diesel combustion, including

an experimental program to measure reaction rates at high pressure, the development of chemical kinetic

mechanisms for a small set of molecules that make up the surrogate diesel fuel, and the development of

new techniques to minimize the number of reaction mechanisms needed to adequately describe the incylinder chemistry.

D.1.10 Advanced Diagnostic Development for Combustion and Emissions Measurements

Developing an understanding of the foregoing requires advanced diagnostic tools for the measurement of

parameters such as temperature, molecular species, particulates, and liquid-vapor fuel quantification.

D.2 ENGINE SUBSYSTEM TECHNOLOGY

D.2.1 Parasitic Losses

Some of an engine’s useful work is dissipated through the friction losses or is expended through pumping

losses. Significant fuel savings can be realized by addressing these parasitic losses.

The major portion of total engine mechanical friction can be traced to the cylinder kit, bearings, and valve

train. Further advancement in the tribology of these interfaces can yield improvements of several percent

in overall engine fuel economy, especially through the introduction of new materials and coatings.

Longer-term concepts include vapor-phase lubrication and advanced lubricants.

Energy expended on driving the engine fuel, oil, and water pumps is an unavoidable part of the engine

energy balance, but its reduction is necessary and indeed possible. The highest potential in this area lies

with the introduction of electronically controlled pumps. The development of novel pump designs is an

important priority.

Currently, turbocharged engines capture part of their available exhaust energy. Additional utilization of

the exhaust energy is possible. Significant development is needed to create cost-effective and reliable

turbocompounding designs or other concepts.

A significant portion of the crankshaft energy is still expended in driving the engine-cooling fan,

generator/alternator, air compressor, and air conditioning. New approaches need to be developed to

reduce this energy expenditure. Ways of reducing the time of engine idling can be helpful.

D.2.2 Heat Rejection

Heat rejection is an important consideration for fuel economy. It is also a significant application issue.

High heat rejection implies the need for larger coolers and radiator, which is contradictory to truck design

trends requiring smaller frontal areas and smaller engine compartments for better aerodynamics as well as

improved visibility and safety. Areas of lower heat rejection include strategic cooling, EGR cooling,

charge cooling, radiator design, and measurements.

Not all engine components require the same level of cooling, which calls for strategically routing the

coolant. This would provide more coolant flow to the areas of the engine that need it most and reduce the

coolant flow to other areas, thus minimizing parasitic fan and pump use.
D-6

Addition of EGR counteracts heat rejection reduction. Lowering the heat rejection will include evaluating

EGR to air coolers, improving the efficiency of EGR coolers, and routing the EGR. EGR cooler

development will continue targeting durability, erosion, corrosion, and fouling. It is also important to

continue development of lower-cost, more-efficient and less-bulky radiator and charge air coolers.

Experimental measurements of engine heat rejection in a truck are cumbersome and expensive.

Measurement of engine heat rejection in the laboratory exhibits significant variability. Heat-rejection

measurement methods need improvement, and reliable analytical means of predicting heat rejection

require continual development.

D.2.3 Controls

Early electronically controlled diesel engines had two control parameters: beginning of injection (BOI)

and pulse width. Newer engines will feature many other controlled subsystems, such as turbochargers,

EGRs, aftertreatment devices, and possibly coolant and lubricating pumps. By itself, a common rail fuelinjection system, in addition to BOI and pulse width of the main event, requires control of rail pressure,

duration of pilot injections and post-injections, and separations between sub-injections.

All of these changes will require significant new developments in control strategies, rapid prototyping of

controls, auto-calibration, and on-board diagnostics (OBD). Always a large part of the control

development process, engine calibration (populating numerous maps in the engine controls software) will

invariably become more cumbersome on future engines with many more maps to fill. Automatic

calibration, if successfully developed, will shorten the calibration time and will arrive at global optima

without the tedious experimental calibration process. Approaches include design of experiments and

neural networks and application of genetic algorithms.

Significant groundwork needs to be initiated for the truck industry to be prepared for future OBD

requirements. Novel sensors need to be developed and applied as part of the controls strategies. Some

may be virtual sensors, requiring model based controls development.

D.2.4 Noise-Vibration-Harshness (NVH)

Attention to all three noise steps—generation, propagation through the engine structure, and radiation—is

necessary for adequate engine noise abatement. The characteristic diesel engine combustion sound is a

product of the sharp pressure rise in the combustion chamber during fast premixed burn. Further research

will allow reducing combustion noise without affecting engine performance or emissions. Mechanical

noise is related to piston slap and impacts in the bearings, valve train, and gear train. Research into the

exact mechanisms of mechanical noise generation will provide further reductions. Other sources include

high-frequency turbocharger whine and airborne noise of the gas in the EGR circuit.

Prior studies into noise propagation have identified that main paths of vibration energy travel through the

engine structure. Developments of statistical energy analysis (SEA) and boundary element analysis

(BEA) should be expanded to yield proper ways of engine structure design for low noise.

BEA may also be applied to structure-borne noise radiation. The shape of engine surface and location of

engine accessories need to be selected with noise radiation in mind. Acoustical materials such as

laminated steels, foams, and viscoelastic damping layers require further development. Also, enginemounting innovations will contribute to the vibration isolation from the engine to the vehicle.

D.2.5 Tribology

Tribology benefits engine fuel economy through friction reduction, engine durability through wear

reduction, and engine performance-emissions trade-offs by enabling a broader-temperature engine
D-7

operation. Tribological issues requiring further development include cylinder kit, bearings, EGR

condensation, and fuel-injection system wear.

The cylinder kit is a source of significant friction losses and often becomes the limiting factor for engine

reliability (due to scuffing) and for engine durability (due to wear). It is also the major contributor to

engine oil consumption and related particulate emissions formation. Cylinder kit tribological

improvement will produce large benefits. Better understanding is required of physical processes through

analytical modeling and experimental investigations and the correlation of fixture results to the engine.

Also, low-friction coatings, and durable-wear and scuff-resistant coatings require development.

Innovative lubrication systems and oil formulations, especially the additives package, also require further

improvement. In particular, alternatives to today’s sulfur-laden anti-wear additives may be needed to

prolong the life of aftertreatment devices.

EGR is an effective means of reducing NOx but acidic condensation of EGR gas is a serious threat to the

reliable operation of an engine. Better protection from EGR condensation requires new corrosion-resistant

materials and coatings for application to intake manifold and in-cylinder components.

Fuel-injection systems lubricated with low-lubricity diesel fuel present some of the most challenging

tribological problems. Better understanding of the fuel-system tribology with its very high pressures is

needed. New materials and coatings will be enabling technologies.

D.2.6 EGR—Multiple-Cylinder Effects

Many EGR technical areas still require improvement and optimization, including EGR layout, EGR

drivers, actuation and sensing, mixing, condensation and integration with aftertreatment. Several EGR

options exist, such as cooled variable-geometry turbocharger (VGT)-driven, high-pressure-loop EGR.

Many analytical and experimental investigations are needed to sort out all options, reveal technical

barriers, and identify possible trade-offs.

Modulated EGR in transient operation requires innovative controls. Improvements should also come from

developing better flow-measurement and flow-calculation methodologies.

EGR effectiveness for a multiple-cylinder engine is dependent on even distribution between the cylinders.

Technology development in this area will provide significant improvement in NOx reduction. Additional

research into EGR-based acid formation and behavior under in-cylinder conditions is needed. Effective

ways to prevent EGR condensation under all ambient and engine operating conditions must be developed.

Future proposed emission levels forcing particulate aftertreatment will make it important to reconsider the

viability of the low-pressure loop EGR. Temperature requirements for some aftertreatment devices will

require careful optimization of EGR rates across engine speeds and loads.

D.2.7 Engine Components and Subsystems

Improvements of individual engine components and subsystems, especially the injection system and

turbocharging, will provide significant contribution to overall engine performance and emissions. Fuelinjection development includes achieving higher pressure for all speeds and loads and greater flexibility

with the injection event. Ways of reducing manufacturing variability need further investigation. Fast

actuators need to be developed, including more powerful solenoids and smart materials.

Transient response of the turbocharger is an important design characteristic and will be improved through

the introduction of lightweight wheel materials and shaft bearings with lower friction. More efficient

blade shapes need to be developed. Development is also needed for cost-effective, reliable designs of

VGTs and dual-stage turbochargers and alternate boosting machinery. Approaches that provide some
D-8

additional flexibility in boosting, such as electric-assisted machines, can help with EGR control as well as

air handling.

D.2.8 Aftertreatment

While development and selection of the exhaust gas aftertreatment package is a subject of another section,

its success is only possible within an integrated engine/aftertreatment system. Specific engine-related

issues include selection of the proper exhaust temperature regimes, NOx/soot ratio, reductant and

emission spatial and temporal distribution, tuning of the late cycle fuel injection as a reductant,

implications for the selection of the EGR approach and packaging.

D.2.9 Variable Valve Actuation (VVA)

Heavy-duty diesel engines with their relatively narrow speed range and tough durability requirements will

not be the initial application for VVA; smaller engines may achieve significant, positive trade-offs in fuel

economy vs emissions. VVA approaches include electrohydraulic, electromagnetic, and hydromechanical

systems. Research would include VVA’s parasitic losses, energy consumption, reliability, and production

cost.

D.2.10 Materials

Technological improvement in future diesel engines will require better or new materials. A general need

is the improvement of material-characterization tools. In many cases, research would be improved with

the introduction of on-line material-characterization methods and tools to be used during engine testing.

D.2.11 Sensors

The use of new engine sensors is addressed in the controls section. The development of novel sensors is

an important subject on its own. New, more robust, more accurate, environmentally friendly, durable,

reliable, and less-expensive ways of measuring temperature (especially exhaust), pressure (especially high

cylinder pressure), and concentration of chemical species are needed. Self-diagnostics of sensors is

another important area of research.

D.2.12 Vehicle Integration

Many research areas are related to power-train and vehicle integration. These include interactions

between engine braking and vehicle braking systems, trends toward engine weight and size reduction,

implications of required storage space for additional fluids on board, and much more.

D.2.13 Manufacturing

Manufacturing processes require improvement to realize the potential of many new technologies for

future diesel engines. Precision and reliable machining is needed for fuel injector tip holes and for a wider

range of hole shapes. The engine-block and cylinder-head casting process must be improved to allow for

thinner walls, elimination of core shift, strategic cooling, and more aggressive intake port shapes. Ceramic

manufacturing, especially ceramic machining, needs to be improved to deliver on the promise of ceramic

structural components. Cost-effective piezoelectric stack manufacturing may be a barrier to introduction

of novel fuel-injection systems.

D.3 ENGINE SYSTEM INTEGRATION VIA VIRTUAL METHODOLOGY DEVELOPMENT

Future aggressive engine performance targets require an integrated test/simulation “Wired” approach,

incorporating sophisticated virtual (simulation) and experimental methodologies to define a pervasive

roadmap. Integral pieces of the overall engine system simulation toolbox include engine structural design,
D-9

charge-air, fuel-injection, combustion, cooling and lubrication systems. Enhancements are paramount as

physical-process understanding improves with tool integration and refinement to allow realistic modeling

of system interactions. To achieve these performance targets, the integrated tools will focus on individual

components as well as the total engine system. Diesel-cycle simulation, materials and structural analysis,

engine fluid system, air-handling system, fuel-injection system, in-cylinder combustion, engine and

aftertreatment system, and underhood thermal-management modeling are all individual simulation tools.

D.3.1 Diesel Engine Cycle Simulation

Cycle simulation is perhaps the most powerful available system tool for addressing basic engine design

and engine calibration parameters as related to overall engine performance. Deficient current submodels

include combustion, heat transfer, flow dynamics and distribution at complex junctions, turbocharger

turbine modeling, and idealized actuator control as well as a lack of control strategy models. Integrating

more complex software [higher-resolution computational fluid dynamics (CFD), heat transfer, chemical

kinetics, or control] to reconcile these deficiencies will improve cycle simulation resolution and predictive

capabilities.

D.3.2 Engine Fluid System Modeling

Intelligent control of fluid systems is implied by the competing demands to achieve the fuel efficiency,

emissions, safety, and life-cycle cost targets. An integrated fluid management system balances available

cooling capacity against cooling requirements within the engine and points toward intelligent cooling

resource utilization. This tool is essential for properly addressing engine thermal management and its

impact on EGR cooling, lube oil temperature, and required pump and fan loads with strong implications

concerning system controls and strategic heat rejection. This will require integration and special attention

on proper interfacing with vehicle- and engine-level boundary and initial conditions.

D.3.3 Materials and Structural Modeling

Fuel efficiency, life-cycle costs and reduced size and weight pressures have driven the use of lighterweight structures of steel, aluminum, magnesium, and plastics. These lightweight materials and improved

designs mandated improved engineering analysis. Rapid-prototyping capability of the forward-engineered

design is critical to accelerate commercialization potential. The greater thermal-mechanical loading

associated with achieving future engine performance targets will require much greater model resolution

than is standard today to ensure eventual convergence toward a robust design.

D.3.4 Air-Handling System Modeling

The engine’s air throughput system has direct impact on combustion air utilization and EGR effectiveness

toward meeting both high thermal efficiency requirements and associated emission standards. Exhaust

and induction systems require tuning for delivery of proper in-cylinder EGR distribution, necessary incylinder flow distribution, and proper residual gas distribution while controlling in-cylinder heat transfer.

Many complex interactions between the induction, exhaust, EGR, and reciprocator need to be addressed

at a system level. Corresponding modeling tools that address associated issues with each subsystem and

its interfaces will provide a mechanism for optimization of the engine.

D.3.5 Fuel-Injection System Modeling

The historical improvement in power and efficiency of diesel engines is due, to some extent, to advances

in fuel-injector technology. Adequate in-cylinder mixing between fuel and air is strongly dependent on

the characteristics of the fuel injector. Because fuel-spray characteristics such as velocity, spray angle,

and droplet size and dispersion are critical factors in fuel-air mixing, it is important to understand the

relationship between these characteristics and injector design. CFD modeling of the flow inside and at the

exit of injectors could provide important information about how design parameters actually relate to spray
D-10

characteristics. Control strategy and actuator-modeling capability extend the analysis to the entire fuelinjection system, allowing analysis of cylinder-to-cylinder differences caused by calibration or concept

designs. There are still wide gaps in the understanding of the physical processes inside the injector and

how they relate to the subsequent formation of liquid sheets and drops beyond the injector exit. Highresolution CFD models of a fuel injector could provide both fundamental and specific design information

for the next generation of high-efficiency, low-emission engines.

D.3.6 In-Cylinder Combustion Modeling

This aims at dissecting root causes of emission formation in evolutionary combustion systems and

evaluation of promising revolutionary, novel systems without initial hardware development. Current

models offer tremendous insight of the combustion event but require continual tuning to maintain

predictive capability. Beyond the injector, diesel-combustion performance is determined to a large extent

by the efficiency with which the liquid fuel droplets evaporate and mix with the air. Thus it is important

to have computational models that simulate the impact of droplet characteristics and airflow features. At a

minimum, this type of simulation requires consideration of the three-dimensional continuum turbulence

associated with both flows. Furthermore, the heat released during combustion can modify the turbulence

in the flow, resulting in a fully coupled evolution of the fluid-flow and chemical processes. Current

understanding of this type of flow field is limited, and future augmentation in engine performance will

require significant further improvement in our understanding of how the flow and combustion processes

couple together.

D.3.7 Engine and Aftertreatment System Modeling

Often, the simultaneous goals of high fuel efficiency and low emissions are strongly at odds. This is

especially true for diesel combustion, which is carried out under lean conditions that do not favor the

subsequent reduction of NOx in a catalytic converter. An unprecedented level of engine and aftertreatment

integration will be required to achieve engine system durability simultaneously with future required

emission reduction and thermal efficiency targets. Models of the catalytic converter’s dominant physical

and chemical processes could greatly speed the development of new generations of catalyst technology.

There is also a strong need to accurately model the flow, mass transport, and heat transport in the exhaust

gases flowing to the converter and the coupling between these macroscopic flow effects and the chemical

processes occurring on the catalyst surface.

Integrating engine system thermal, chemical specie, and flow effects with the aftertreatment device

surface kinetics simultaneously in high detail should offer the most accurate predictions about the impact

of design changes or changes to the catalyst properties. Low-order aftertreatment models offer another

approach for modeling the complete engine and aftertreatment system over short transient events. Such

information could also be valuable for assessing the effectiveness of various types of on-board emissions

sensors. A third major modeling need for diesel emissions control is to simulate the mechanisms for

catalyst regeneration, degradation, and/or poisoning by engine behavior.

D.3.8 Underhood Thermal Management

The use of cooled EGR is an example of the approach to maintain thermal efficiency while reducing NOx

trade-off. Often, increasing the amount of EGR to achieve the transient and steady-state NOx emissions

targets is a significant step toward improving thermal efficiency and vehicle fuel economy. However, this

yields higher heat rejection that conflicts with aerodynamic enhancements via reducing frontal area and

the airflow to the underhood area. Designing the underhood systems and components such as engine,

fans, radiators, heat exchangers, and intake manifolds requires optimal location and shapes as well as

optimization of the thermal performance of the power system. This complicated analysis requires

integration of high-fidelity models of thermal-hydraulic processes that stretch the state of the art in CFD

and high-performance computing. The computational model should integrate thermal models for
D-11

convective, conductive, and radiative heat transport as well as integrate models for critical heatmanagement system components, including cooling fans and radiators.

D.4 EXPERIMENTAL METHODOLOGY DEVELOPMENT

Experimental testing is one of the most expensive and time-consuming aspects of the engine R&D

process. Truck engine companies maintain test facilities that typically include dozens of performance test

cells, durability test stands, specialized test facilities, and technical support personnel. There is a constant

demand for providing more accurate and higher-value data in a shorter time.

For example, future regulations call for a tenfold reduction in emissions. Current measurement is not

capable of measuring these low levels reliably. In fact, the general accuracy of the measurements is the

same order of magnitude as the absolute levels that will need to be measured. Accuracy of the

instrumentation needs to be improved, and types of the instruments and methodology of emissions testing

may need to be invented.

A more direct, cost-effective real-time measurement of particulates is needed. Another issue related to

particles is particulate size distribution. Instrumentation capable of analyzing exhaust particle size in a

practical fashion needs to be developed.

Modern engine performance test facilities rely on real-time combustion analysis equipment. Integration

and comparison of the processed data sets is a time-consuming and tedious task for the test engineer.

Valuable development time may be reduced and errors may be eliminated if a major initiative is

undertaken to integrate test cell data acquisition and post-processing equipment into combustion analysis

software. Significant reductions in the duration of the test programs will be realized with better-automated

test protocols. Another benefit of this is the improvement in the accuracy of data and test-to-test

variability. The most immediate need is in the area of performance development testing.

The advent of virtual testing is changing the nature of the experimental testing. Instead of providing large

amounts of data for further analysis by a development engineer, experimental testing is becoming a tool

for model-input parameters and validation. Only carefully designed experiments covering few hardware

configurations are run on a limited number of test conditions. This paradigm shift will undoubtedly create

new requirements for the experimental testing.

Most engine systems and components developments are tested as part of the overall engine package,

expending valuable resources and risking failures of expensive prototype engines. The alternative

approach is the development of test rigs and fixtures for evaluation of individual engine subsystems. The

current use of this approach is limited due to the lack of full correlation between the performance of a

component on a fixture and on an engine. Development of new, better methodologies for subsystems such

as fuel-injection systems, turbochargers, cylinder kits, bearings, and valve trains will remove that barrier.

Examples of other areas of engine development processes that could benefit from better experimental

methodologies include heat rejection measurement, EGR condensation detection and quantification,

reliable means of data transmission from moving components, real-time wear measurements, and motion

measurements of fuel-injection components.

APPENDIX E

AFTERTREATMENT/EMISSION CONTROL

E-3

APPENDIX E

AFTERTREATMENT/EMISSION CONTROL

E.1 STATUS OF EMISSION CONTROL TECHNOLOGY

E.1.1 NOx Control

E.1.1.1 NOx Adsorber-Catalysts

A NOx adsorber-catalyst is a flow-through emissions control device that has the potential to significantly

lower NOx, HC, and CO emissions from diesel engine exhaust. A NOx adsorber catalyst consists of two

principal components: a NOx adsorbent and a three-way conversion catalyst (the two components are

actually on the same substrate material). Periodically, NOx stored by the adsorbent is released and

reduced to N2. This process requires a momentary exhaust gas composition that is depleted of oxygen but

contains CO and HC.

An engine management system is thus critical to the operation of a NOx adsorber. The system must

determine when the NOx adsorbent is approaching saturation and then trigger the change in engine

operation that results in generation of the rich condition required for release and reduction of the stored

NOx. The duration and “richness” are critical to avoid excessive fuel use and HC breakthrough while still

accomplishing complete regeneration (DECSE 1999).

The NOx adsorber-catalyst is very sensitive to sulfur, its effectiveness dropping quickly with fuels

containing 16 ppm or more of sulfur. In programs utilizing 3-ppm sulfur fuel, NOx reduction levels of

over 90% have been achieved for fresh devices in both engine test cells and experimental vehicle systems,

the latter over a transient cycle (DECSE 1999, DVECSE 2000). In heavy-duty engine transient tests,

experiences have shown conversion efficiencies of about 60%. While these results provide

encouragement, extensive R&D work is still needed in optimizing the NOx adsorption/desorption and

conversion functions, and defining and optimizing sulfur removal (“desulfurization”) techniques and

strategies, as well as examining the use of sulfur traps upstream of the catalyst. The interactions with

DPFs in a full system are in the early stages of exploration. The NOx adsorber-catalyst carries a

substantial fuel-economy penalty due to frequent NOx regeneration, although further optimization is

expected to bring this to a tolerable level (a few percent). Furthermore, durability of the adsorber-catalyst

is far from established.

E.1.1.2 Urea Selective Catalytic Reduction (SCR)

SCR of NOx using ammonia or urea has been used for many years in stationary diesel engine applications.

In the SCR process, NO reacts with the ammonia, which is injected into the flue gas stream before the

catalyst. Due to the toxicity and handling problems with ammonia, the most widely accepted and

commercialized reductant is urea, CO(NH2)2 (Ecopoint 2000). Water solutions appear to be the preferred

form of urea. Different SCR catalyst systems based on platinum, vanadium oxide, or zeolites have

different operating temperature windows and can be selected for a particular SCR applications. SCR

catalysts have not been used commercially for diesel vehicles due to their complexity, large size, safety

concerns, and ammonia/urea injection control issues (Ecopoint 2000). SCR systems would further require

an anti-defeat system to ensure that the vehicle was not operated without a urea supply. SCR technology,

which utilizes an oxidation catalyst to facilitate NOx reduction to achieve high control efficiencies,

requires the same low sulfur levels as the NOx adsorber technology (MECA 2000). Certain SCR

technology designs are less sensitive to sulfur, but very low sulfur fuel allows even these technologies to

achieve the highest NOx reductions and allows for the full optimization of the engine/exhaust control

technology system and potential fuel economy improvements.
E-4

Numerous SCR experiments and demonstrations are in progress. NOx control efficiency has been

recorded at 80 to 90%, with 70% being more representative in transient operation. Durability is being

determined by the numerous field tests, largely in Europe, but now getting under way in the U.S. (Miller

2000).

E.1.1.3 Hydrocarbon (HC) SCR

Also known as “lean NOx catalysis,” HC SCR typically utilizes diesel fuel or a HC readily derived from

fuel as the reducing agent for NOx in the presence of a catalyst. Other hydrocarbons, ethanol for instance,

have been used in commercial SCR systems for stationary power plants. After many years of experiments

and tests of thousands of catalyst formulations, the probability of HC SCR achieving 90% NOx

conversion over a sufficiently wide temperature range is presently low. NOx reductions of 80% or more

have been achieved in research environments in narrow temperature ranges, but the scale-up systems have

exhibited only 30% conversion at best over test cycles. The technology remains attractive because of its

passiveness, requiring no regeneration and likely no second fluid to replenish. The keys to success appear

to be getting the optimum HCs manufactured from on-board fuel, and greatly improving the

understanding of the HC utilization/NOx reduction mechanisms. Lean NOx catalysts exhibit a moderate

degree of sensitivity to sulfur compounds from fuel.

E.1.1.4 Plasma-Assisted NOx Catalysis

Non-thermal plasma-assisted catalytic reduction of NOx is a relatively new technology that has shown

promise for enhanced NOx reduction. Up to 80% NOx reductions have been observed on simulated

exhaust and 55% in real exhaust (SAE 1999).

The plasma is believed to enhance NOx reduction over catalysts via a two step process (Penetrante 1997).

First, the plasma is a strongly oxidizing environment in which NO is converted to NO2, as well as

accomplishing some partial oxidizing of the HC reductants, if present. The second step is reduction of

NO2 to N2 by the HC over a catalyst. It is widely understood that in lean NOx catalysts, NO2 is the

primary species that reacts with HCs, so the catalyst must typically achieve both steps unless a plasma is

present. By using plasma one can separate the oxidation and reduction function of the catalyst to open the

door for several new lean-NOx catalyst systems.

Plasma-assisted catalyst NOx control is unproven in transient test cycles. Limited testing on light-duty

vehicles has been performed for PM removal. The energy penalty (from electrical energy and reductant

addition) is possibly higher than other NOx reduction technologies (about 5% total). Although both NOx

and particulate removal by plasmas have been demonstrated separately, a system with combined function

for NOx and PM removal has not been developed or tested.

E.1.2 Particulate Matter (PM) Emission Controls

E.1.2.1 Catalyst-Based DPF

Control technologies for PM have seen significant progress in recent years to the point of limited

commercial application. Catalyst-based DPFs used on engines operated on low-sulfur diesel fuel can

achieve PM and toxic HC reductions well in excess of 90%. Where diesel fuel containing less than

10 ppm sulfur has been used, filter technology has demonstrated impressive durability, in some

applications continuing to provide excellent particulate removal at 600,000 km of vehicle operation

(Warren et al. 2000). The ability of catalyst-based DPFs to reduce hydrocarbon emissions by greater than

95% has also been clearly established (Letavec et al. 2000).

Two types of DPFs are well-developed and are engaged in field trials: the CR-DPF and the catalytic DPF

CDPF. For both, PM is removed from the exhaust stream by collecting on a ceramic wall-flow filter

element. Unlike other diesel emissions control devices, primary removal of the targeted pollutant (PM) is
E-5

fixed by the physical characteristics of the filter medium and is relatively unaffected by the engine

operating conditions. The critical issue, instead, is the cleaning or regeneration of the DPF (by oxidation

of the collected PM) to prevent the DPF from plugging.

The CR-DPF accomplishes regeneration by continuously converting engine-out NO to NO2 over an

oxidation catalyst placed upstream of a DPF (here, the DPF has no active catalyst on it). The NO2, which

is a more effective low-temperature oxidizing agent for diesel PM than oxygen, completes the

regeneration. Sulfur in the exhaust however, can be oxidized over the CR-DPF, forming sulfates, which

are measured as PM. Sulfur oxides also compete for the critical NO and NO2 reaction, making the

regeneration characteristics less effective (Liang et al. 2000).

The CDPF regenerates by using a catalyst coating on the DPF element to promote oxidation of the

collected PM using available oxygen in the diesel exhaust. Sulfur in the exhaust can be oxidized over the

CDPF to form sulfates.

Exhaust-gas temperature and fuel-sulfur level are critical factors that affect the performance of both types

of DPFs (CR-DPF and CDPF). The poor regeneration at low temperatures, and filter plugging by ash

(mostly from engine lubricating oil) over time are among the few remaining shortcomings of the

technology. The ash can be removed by backflushing with air, but the required frequency is not fully

established.

The catalyst-based DPF is additionally attractive because it is a self-contained, passive device that can be

retrofitted to diesel-powered vehicles that exhibit sufficient exhaust heat to ensure regeneration.

E.1.2.2 Diesel Oxidation Catalysts (DOCs)

DOCs employ technology that dates back to the early stages of gasoline vehicle emission control in the

early 1970s. When formulated for use in diesel vehicles, these catalysts are effective in removing HC,

CO, and the soluble organic fraction of PM, which can be on the order of 30% or more of the total PM.

They also diminish the usual pungent odor of diesel engine exhaust. DOCs are used on off-road vehicles,

diesel-powered trucks, and cars worldwide. For the future, additional uses may be as components within

total NOx and PM control systems where oxidation or temperature rise is needed. DOCs, of course, will

convert SO2 to sulfate PM, this being their most significant sensitivity to fuel sulfur. Sulfur, as well as

phosphorous and zinc from the lubes and fuel, will tend to poison and deactivate a DOC over time; but

the durability of DOCs has been acceptable for widespread use in vehicles using 500-ppm sulfur fuel.

E.1.2.3 Plasma Reduction of PM

Plasma devices for PM removal have been the subject of numerous experiments, and full-scale prototypes

are emerging in test programs. In laboratory experiments, they have been very effective in PM control

(Fanick et al. 1995).

E.1.2.4 Non-Catalytic DPF

There have been numerous proposed non-catalytic systems for PM control that relied on a trapping

process and then heating by external means. Typically, extra fuel was burned to raise the temperature of

the trap to burn the stored carbon. The fuel-economy penalty is rather prohibitive for most applications.

Other systems use a fuel additive that deposits a material on the filter to lower the light-off temperature

(Psaras and Summers 1995) and to promote regeneration. In addition, a filter made of a ceramic paper has

been developed whose material of construction couples efficiently with microwave energy. The filter can

be regenerated on demand with externally supplied microwave energy. It has particular attractiveness in

applications like small passenger cars, where the exhaust temperature may not always reach the

regeneration temperature required by passive devices. Prototypes of this device have been evaluated in
E-6

engine test cells and on vehicles and have shown good filtering efficiency (over 80%) and the ability to

regenerate as intended (Nixdorf 2000).

E.1.3 Enablers for Emissions Controls

E.1.3.1 Sulfur Traps

A sulfur-trapping system is attractive to protect the NOx or PM control devices that exhibit high

sensitivity. Even the 15-ppm fuel sulfur cap proposed by EPA may not be low enough to ensure the

needed durability of devices such as NOx adsorbers. Furthermore, sulfur compounds in the lubricant also

give rise to notable SO2. Sulfur traps or “guard beds” are similarly used in fuel refining to protect the

process catalysts. Sulfur traps for diesel emission control systems are being developed, and at least in one

case are being integrated with NOx adsorbers (Parks et al. 1999).

E.1.3.2 Reductant Generation and Deposition Systems

NOx catalysts and adsorber-catalysts function with higher efficiency if the reducing agent is a prescribed

compound other than diesel fuel. Mixtures of hydrogen, CO, and specific hydrocarbons, have been found

to be among the best reductants. Devices such as diesel fuel reformers could provide a higher level of

NOx control by generating reductants on the vehicle. Reformer technology is mature for some

applications using natural gas but is not well developed for reforming diesel fuel. Optimizing fuel

constituents for this purpose has received little investigation. Experiments with in-cylinder late fuel

injection have been conducted to achieve a similar effect, that is, producing a more tailored exhaust for

NOx reduction. These have shown trends in the right direction, yet not a large enough effect.

E.1.3.3 Sensors

NOx and PM sensors are critical for emission and aftertreatment control. Either sensor, with an adequate

response time, can aid in the control of engine out emissions depending on the control strategy of the

engine manufacturer. Unfortunately, a PM sensor does not currently exist and NOx sensors are inadequate

in the current configuration.

Current electrochemical NOx sensors have many shortfalls (slow response time, approximately 500 ms;

poor poison resistance; inadequate selectivity) but are based on a proven, robust technology. This type of

sensor could be considerably faster and more dependable with continued research. However, the

electrochemical NOx sensor will not be able to meet the response times necessary to be used as a control

sensor (less than 15 ms). For aftertreatment systems such as SCR, NOx sensors are being integrated for

control and diagnostics. Their durability remains less than desired.
APPENDIX F

HYBRID ELECTRIC PROPULSION TECHNOLOGIES

F-3

APPENDIX F

HYBRID ELECTRIC PROPULSION TECHNOLOGIES

Hybrid electric propulsion systems may be needed to meet performance and efficiency goals for both

commercial and military vehicles. A “crosscut” R&D effort is needed to develop enabling technologies

for hybrid electric propulsion systems. The status of technology, technical targets, barriers, and technical

approach have been summarized in the 21st Century Truck Roadmap.

This appendix provides additional information about the current status of hybrid electric propulsion

system technologies. The current status of electric motors, batteries, power electronics, and electrical

safety are further discussed to establish a baseline for improving performance and efficiency of hybrid

electric components and systems.

F.1 STATUS OF ELECTRIC MOTOR TECHNOLOGY

Electric motors capable of driving

heavy-duty HEVs are presently

offered by numerous suppliers,

including Allison, Emerson Electric,

General Electric, Hughes, ISE

Research, Kaman, Lockheed Martin,

Northrop-Grumman, Reuland

Electric, SatCon, Siemens,

Solectrica, Systronix, and Unique

Mobility. Rated motor power for a

variety of commercial HEVs are

illustrated in Fig. F.1.

Several types of motors have been

proposed for hybrid electric drive

systems, many of which merit further

evaluation and development. Certain

types of motors may work better for

specific vehicle applications or

performance requirements than

others.

At least five major electrical design options exist, including classical DC-commutator type, permanent

magnet, switch reluctance, AC-synchronous, and AC-induction motor designs. Each of these options has

advantages and disadvantages with respect to size, weight, efficiency, cost, complexity, and other

considerations.

Motor generators can be configured before or after the transmission. Series HEVs typically have larger

motors with higher power ratings because the motor alone propels the vehicle. In parallel hybrids, the

power plant and the motor combine to propel the vehicle. Motor and engine torque blending is usually

accomplished through couplings and planetary gear sets.

Air-cooled motors are simpler and generally less expensive than liquid-cooled motors. Liquid-cooled

motors may also require more cooling-system maintenance than air-cooled versions require. However,

liquid-cooled motors are generally smaller and lighter for a given power rating. Various coolant options

exist for liquid-cooled motors, including water, water-glycol, and oil.

0

50

100

150

200

250

300

350

400

450

- 10,000 20,000 30,000 40,000

G ross V e hicle W e ight (kg)

M a x im um M otor Pow e r (kW )

Fig. F.1. Motor power for heavy-duty hybrid electric vehicles.
F-4

Wheel motors, in which at least one wheel on each side of the vehicle is driven directly by its own motor,

require more extensive changes to the vehicle but offer the potential benefits of increasing motive drive

efficiency, reducing vehicle weight, and lowering the chassis. Wheel motors allow independently

controllable four-wheel traction control to potentially improve vehicle performance and safety. Fail-safe

design of wheel motors must be considered such that operational problems with any one motor does cause

a loss of vehicle control. Wheel motors have been used in a small number of heavy-duty HEV designs,

but further systems analysis is needed to investigate efficiency, weight, and packaging issues.

Heavy-duty vehicles are driven with motors operating at voltages ranging from less than 300 VDC to

more than 600 VDC. Higher operating voltages enable lower operating currents, reducing I2

R losses and

generally reducing inverter costs. However, higher-voltage systems require more batteries and more

complex DC-to-DC conversion if the main battery pack is used to drive lower-voltage (e.g., 12 VDC or

24 VDC) subsystems.

F.2 STATUS OF BATTERY TECHNOLOGY

Although a few production HEVs with advanced batteries have been introduced in the market,

improvements in life-cycle economics, power, and energy efficiency are needed, especially for

commercial and military vehicles that have longer service life requirements than light-duty vehicles.

Desirable attributes of high-power batteries for HEV applications are high-peak and pulse-specific power,

high specific energy at pulse power, a high charge acceptance to maximize regenerative braking

utilization, and long calendar and cycle life. Developing designs and methods to balance the packs

electrically and thermally, developing accurate techniques to determine a battery’s state of charge,

developing abuse-tolerant batteries, and improving recycleability are additional technical challenges.

Lead acid batteries are currently used in many electric vehicles and have been used in hybrid transit bus

applications. Lead acid batteries can be designed for high power and are inexpensive, safe, and reliable. A

recycling infrastructure is in place for them. But low specific energy, poor cold temperature performance,

and short calendar and cycle life are still impediments to their use. Advanced high-power lead acid

batteries are being developed for HEV applications.

Although nickel-cadmium batteries, used in many electronic consumer products, have higher specific

energy and better life cycle than lead acid batteries, they have lower specific power for HEV applications.

The toxicity of cadmium is of concern and may limit the widespread use of these batteries.

Nickel-metal hydride (NiMH) batteries, used routinely in computer and medical equipment, offer good

specific energy and specific power capabilities. Their components are recyclable, but a recycling structure

is not yet in place. NiMH batteries have a much longer life cycle than lead acid batteries and are safe and

abuse-tolerant. These batteries have been used successfully in production electric vehicles and recently in

low-volume production HEVs. The main challenges with nickel-metal hydride batteries are their high

cost, high self-discharge and heat generation at high current, the need to control losses of hydrogen, and

their low cell efficiency.

Lithium ion batteries are rapidly penetrating into laptop and cell-phone markets because of their high

specific energy. They also have high specific power, high energy efficiency, good high-temperature

performance, and low self-discharge. Components of lithium ion batteries could also be recycled. These

characteristics make lithium ion batteries suitable for HEV applications. However, to make them

commercially viable for HEVs, further development is needed, including improvement in calendar and

cycle life, a higher degree of cell and battery safety, abuse tolerance, and cost reduction.

Lithium polymer batteries with high specific energy, initially developed for electric vehicle applications,

also have the potential to provide high specific power for HEV applications. The other key characteristics
F-5

of the lithium polymer batteries are safety and good cycle and calendar life. The battery could be

commercially viable if the cost is lowered and batteries having higher specific power are developed.

ZEBRA batteries, developed in Europe, have been demonstrated in more than 50 grid-connected hybrid

and diesel HEVs. They have a high specific energy (more than 90 Wh/kg) and a specific power of 160

W/kg. The battery modules have built-in monitoring systems for each battery pack, and the cells fail

similarly to NiCd batteries in a low-resistance mode, which does not initiate a string problem. This

characteristic makes the batteries reliable and tolerant to abuse. The battery temperature must be

maintained, which causes a self-discharge rate that varies depending on climate. (Cost of batteries is

currently $840/kWh, the target is $420/kWh CDN or $560/kWh and $280/kWh.)

Among the batteries available today, lead acid and nickel cadmium batteries are inexpensive, available,

and reliable. From a recent survey, most heavy hybrid vehicles use these two battery types in their

vehicles (Table F.1). The capacity of the battery packs for heavy-duty commercial vehicles typically

ranges from 20 kWh to 70 kWh (Fig. F.2). A small number of prototype vehicles have used ultracapacitors. Hybrid electric buses with flywheel energy storage are in use in several services in Germany.

A prototype hybrid electric bus with a flywheel energy storage unit is being built for Houston Metro.

Table F.1. Type of energy storage used in heavy-duty

hybrid electric vehicle projects

Energy storage type Europe and Asia USA

Lead acid 8 12

Nickel cadmium 7 5

Other (including ultra-capacitors and

flywheels)

2 2

Total projects surveyed 17 19

A summary of the current status of these battery types is shown in Table 4.16. HEV batteries need to have

high power and adequate specific energy (energy per weight) and energy density (energy per volume),

0

10

20

30

40

50

60

70

- 5,000 10,000 15,000 20,000 25,000

Gross Vehicle Weight (kg)

Battery Capacity (kWh)

Fig. F.2. Battery pack capacity for heavy-duty hybrid

electric vehicles.
F-6

low cost (per Wh, W, or unit), and longevity (years) and cycle life (number of cycles). Safety and

reliability must also be considered. Lead acid and lithium ion batteries could deliver a very high power,

although NiMH batteries may also have adequate power and life. Both Honda Insight and Toyota Prius

use NiMH for their energy storage systems. NiCad batteries are reliable, provide good energy and

adequate power, and have reasonable cost. Lithium ion batteries can provide good energy, power, and

eventually low cost, however, life and safety issues remain. More data are needed on cycle life for

discharge between 65 to 60% SOC.

A significant amount of engineering effort is needed to package the electrical energy storage system in

vehicles. The system must be continuously monitored to ensure proper performance. In addition an

electronic control system and a thermal management system are needed for controlling and balancing the

pack to ensure optimum performance and safe operation. In the last few years, battery and vehicle

manufacturers and system integrators have worked together on the development of packaging and

management systems using computer-aided design tools and electrical simulation software. Most of the

energy storage systems in current prototype or production vehicles have a management system.

F.3 STATUS OF POWER ELECTRONICS

The U.S. industry is currently in need of suppliers and power electronic products for commercial and

military HEV applications. The power electronics system plays a crucial role in the conversion and

distribution of power and energy in automotive applications. The selection of power semiconductor

devices, converters/inverters, control and switching strategies, packaging of the individual units, and

system integration are very important for the development of an efficient, high-performance truck of the

future.

The advancement of the technology of power semiconductor devices led to the development of

lightweight, small-size, high-efficiency power conversion systems. Thyristors, gate turn-off thyristors

(GTOs), power metal-oxide semiconductor field-effect transistors ( MOSFETs), and insulated-gate

bipolar transistors (IGBTs) are being used at different power levels and voltages depending on the

application. Thyristors are mainly used for AC to DC power conversion and have low voltage drop

compared with other devices. GTOs, MOSFETs, and IGBTs are used for conversion of DC to AC power

because they can be turned off more easily than thyristors can. GTOs, are being used in high-voltage,

high-power applications and they need an elaborate gate drive circuit for turn-off. MOSFETs are being

used mainly up to voltages of about 200 V. Higher-voltage devices have higher on-resistance. Also, as the

current increases, the on-resistance of the device increases, leading to higher power dissipation at higher

currents. IGBTs have the high input impedance and high-speed characteristics of MOSFETs with the

conductivity characteristics of bipolar transistors. IGBTs are being widely used up to about 1,000 V; in

limited applications, higher than 1,000-V devices are used.

The inverter strategies for controlling AC motors are being mainly classified as hard-switched inverters

and soft-switched inverters. They are also classified as voltage-source and current-source inverters,

depending on the type of the input source. The hard-switched inverters are simple and easy to control.

However, unlike soft-switched inverters they have more switching losses and generate electomagnetic

interference (EMI). The commonly used three-phase hard-switched inverter is the three-phase bridge

voltage source inverter. There are several versions of the three-phase soft-switched inverters based on the

resonant power conversion. Soft-switched inverters have low switching losses and generate less EMI than

the hard-switched inverters do. The inverters need to be operated at as high a frequency as possible to

obtain less acoustic noise and to generate low ripple current in the motor current waveforms. But higher

switching frequency results in higher switching losses and hence reduces the efficiency of the system. The

switching frequency is also limited by the switching times of the power devices.

The stability and dynamic performance of any system depends on the closed-loop control strategy. The

closed loop control strategy depends on the type of the AC machine and the required control
F-7

characteristics. Field orientation control (or vector control) has become the standard strategy for highperformance systems. Field orientation control enables independent control of torque and flux in all AC

machines. There are various versions of the field orientation control, such as rotor flux orientation, stator

flux orientation, and air gap flux orientation. The pulse-width modulation (PWM) strategies and vector

control strategies are implemented by using high-performance microprocessors or digital signal

processors. To have intelligent control systems, fuzzy logic and neutral network technologies are being

incorporated into the machine control and are being interfaced with the rest of the complete vehicle

system controllers. Sine-triangle modulation techniques and space vector modulation techniques are the

commonly used PWM strategies to generate three-phase variable voltage and frequency at the output of

the inverters. These strategies allow for smooth control of the output power, lower harmonics at the

inverter output, and simpler implementation.

In addition to the power devices and controllers, capacitors, inductors, bus bars, thermal systems, and

other components form a major portion of the power electronics unit. In the past ten years, the technology

of magnetic components and capacitors has significantly advanced to be used in high-frequency power

electronics applications. The packaging of all these units as one system has significant challenges. The

U.S. Department of Energy, U.S. Navy, and other organizations have funded the development of Power

Electronics Building Blocks (PEBBs), to develop modular types of power electronics systems ranging

from 10 kW to several megawatts of power.

The DC-DC converter technologies have significantly advanced over the years, and now they have

become commodity units for many types of applications. However, they are still under development stage

for automotive applications.

F.4 STATUS OF ELECTRICAL SAFETY

Electrical safety requirements must encompass acceptable design practice, accessibility, durability of

safety provisions, human factors, and risk management. Electrical vehicle technology has led the way for

development of hybrid vehicle safety technology to a substantial extent. However, the greater extent and

complexity of high-voltage components and cabling in HEVs requires extension of safe practices. (For

purposes herein, “high voltage” shall be considered to be any voltage exceeding 50 volts DC or 50 volts

rms AC.) Electrical safety can be considered in three subcategories: functional, personnel, and hazard

identification and mitigation.

Functional safety includes establishing a product safety checklist and design practice, ensuring

crash/rollover isolation, integrating of low-voltage accessories, and conducting failure effects and sneakpath effects analysis. The guiding principle here is, “No normally operating system, or system having a

single-point electrical failure shall present risk of injury or death from electrical contact.” Such a

requirement has vast implications for design decisions affecting use of chassis ground/double insulation

or floating ground/single insulation circuit architectures, architecture of ground-fault monitoring and

architecture of safety-related failure monitoring.

Personnel safety includes consideration of emergency disconnects, access door/cover/power interlocks,

high-voltage cable/harness routing, high-voltage cable/harness unique identification, maintenance and

emergency personnel training, and warning labels. The guiding principles here are “Prevent personnel

casual contact with high voltage”; and “Provide training, interlocks, and labeling to guide authorized

personnel safely through maintenance and emergency procedures involving high voltage.”

Hazard analysis, tracking, and mitigation are means by which safety objectives may be accomplished.

They represent a continuous process. Hazard analysis, tracking, and mitigation are especially effective

when applied to technologies in transition from laboratory to marketplace as is the case for HEV

technology. Hazard analysis consists of identifying, to the extent possible, all hazards that may result in

outcomes of injury, death, or significant loss of property. Analysis proceeds by ranking hazards by a
F-8

severity index that relates to the seriousness of the outcome and the frequency with which that outcome is

expected to occur. An example matrix for assigning a severity index to (each) outcome is given in

Table F.2. The severity index ranges from 1 to 20; 1 represents the most severe risk/loss level and 20

represents the least severe risk/loss level. The severity index is useful for prioritizing risk-mitigating

activities, but eventually all risks must be mitigated as required to achieve acceptable loss levels.

Hazard tracking is the formal process by which hazards are recorded as they occur, are recognized, or are

identified; progress on their mitigation is monitored, and the ultimate actions for mitigation are

documented.

Hazard mitigation is the process of reducing risk/loss to acceptable levels. Mitigation is accomplished by

design, protective devices/construction, warning labels, definition of formal safety procedures, and

personnel training.

Table F.2. Hazard risk categories

Hazard risk index Hazard risk category

Catastrophic

(CAT I)

Critical

(CAT II)

Marginal

(CAT III)

Negligible

(CAT IV)

Death or

system loss

Severe injury,

occupational

illness, or major

system damage

Minor injury,

occupational

injury, minor

system damage

Less than minor

injury,

occupational

illness, or system

damage

Frequent (A)

Likely to occur frequently

1

(highest risk)

3 7 13

(medium risk)

Probable (B)

Will occur several times in

life of system

25 9

(medium risk)

16

Occasional (C)

Likely to occur sometime

in life of system

4 6 11

(medium risk)

18

Remote (D)

Unlikely, possible to occur

in life of system

8 10

(medium risk)

14 19

Improbable (E)

So unlikely, assume it may

never occur

12

(medium risk)

15 17 20

(lowest risk)
APPENDIX G

MECHANICAL HYBRID SYSTEMS FOR 21st CENTURY TRUCKS

G-3

APPENDIX G

MECHANICAL HYBRID SYSTEMS FOR 21st CENTURY TRUCKS

G.1 PRIMARY POWER SOURCE—ENGINE AND FUEL CELLS

G.1.1 Engines

The problem with state-of-the-art piston engine designs is that such designs have been refined and

optimized for decades for those parameters that have been most important to the consumer, such as low

cost, reliability, durability, and power. Unfortunately, there has been little motivation to redesign piston

engines from a “clean sheet of paper” for high efficiency or minimum greenhouse gas emissions. The key

feature of the engine used in a mechanical hybrid truck is that when it runs, it only runs at or near its peak

efficiency throughout its typical driving cycle.

For a mechanical hybrid truck to meet the 21st Century Truck Program fuel-economy goals, the piston

engine must be extremely efficient. The goal in the near term is to demonstrate an engine with peak

efficiencies in the 40 to 45% range and overall urban cycle efficiency, when used in a hybrid

configuration, of around 35 to 40%. The long-term goal is peak efficiency in the 45 to 55% range and an

urban cycle efficiency of 40% or greater.

Another desirable goal would be to ensure that the engine could operate on renewable fuels, which will

help to minimize total greenhouse emissions even more.

In addition, the piston engine must be extremely clean with respect to conventional exhaust emissions.

For most pollutants, such as hydrocarbons and carbon monoxide, this should not present a major problem

in that there is some (though not a proportional) correlation between efficiency and lower emissions (the

one exception to this is the possibility of multiple restarts with a hybrid truck). The pollutant of most

concern, however, is the class of compounds that are referred to as oxides of nitrogen, or NOx. A small,

efficient engine operated at or near peak power can emit high levels of NOx because of the elevated

combustion temperatures and pressures. A major objective of any hybrid engine project should be to

identify engine designs that yield maximum efficiency and that meet the proposed 2007 NOx emission

standard of 0.020 grams per brake horsepower.

Affordability is a goal of the 21st Century Truck Program; therefore, the engine must cost as little as

possible. This is, of course, an advantage of utilizing a piston engine because, fore example, it will not

require as many changes relative to current designs as would a fuel cell. The fact that a piston engine for

hybrid application (in urban use trucks) will be smaller than today’s truck engines is helpful in this regard.

The cost reduction that will accrue based on the size reduction will help offset the increased cost of other

hybrid components that are not necessary with current truck designs.

G.1.2 Fuel Cells

As discussed in the “Mechanical Hybrid” section of the Roadmap (see Sect. 4.6.5), a fuel cell could be

used as a power source for either type of hybrid storage as well, but it is much more likely to be used in a

hybrid electric truck. The discussion of fuel cells is contained in Sect. 4.6.6.

G.2 ENERGY STORAGE—SECONDARY POWER SOURCES (ACCUMULATORS,

FLYWHEELS)

Mechanical hybrids store excess energy in a mechanical storage system so it is available to be retrieved

and use during other times when the truck needs energy in addition to that provided by the engine.
G-4

Mechanical hybrids using pneumatic pressure and hydraulic oil as the working fluid use accumulators to

store that energy. Flywheels represent another mechanical energy storage system.

G.2.1 Accumulators

Accumulators are vessels that store energy in mechanical hybrid designs. They perform a similar function

to that of a battery in a battery-electric hybrid. Because accumulators are characteristically high-specificpower/low-specific-energy devices, they are best suited to the “load-leveling” or “power-trimming” role

they play in the mechanical hybrid design. When the hydraulic system is acting as a pump (running off

the engine or recovering braking energy), hydraulic fluid is pumped into the accumulator, where it is

pressurized against nitrogen gas in the accumulator, up to many thousands of pounds per square inch. The

nitrogen gas acts as a gas spring, storing energy to be utilized later. When the truck needs energy in

addition to that provided by the engine, the pressurized hydraulic fluid is released from the accumulator

and through the hydraulic motor, producing mechanical power.

Important design parameters for accumulators include weight, specific energy (energy storage density),

cycle efficiency (charge/discharge efficiency), cost, safety, and optimum storage pressure. Today,

accumulators are used predominantly in stationary applications and industrial mobile applications, such as

large earth-moving equipment. Accordingly, little effort has gone into optimizing for certain parameters,

particularly weight, specific energy, efficiency, and optimum storage pressure. When more energy storage

capacity was needed for a stationary application, the easiest thing to do was to simply make the tank

larger, rather than to try to optimize for these other parameters. Consequently, current accumulator

designs are heavy, thick-walled steel vessels; therefore, there is great opportunity to optimize accumulator

designs for commercial truck applications.

There are many potential trade-offs between the various accumulator design parameters that must be

examined. For example, using advanced lightweight materials for the accumulator vessel will reduce the

weight of the vessel and will increase the specific energy, but doing so may also increase cost. Higher

pressures within the accumulator will reduce the size of the vessel as well as the amount and weight of oil

needed for a given level of stored energy but will also require a stronger accumulator, which could tend to

increase weight and cost. The considerable ongoing research on improved methods for storing

compressed natural gas (CNG) on board trucks should be very helpful in finding answers to these

questions about accumulators because the issues are very similar.

For an accumulator with a capacity of about 30 gal (approximately the size that may be needed for a

Class 6 hybrid truck), today’s steel designs typically weigh approximately 600 lb (272 kg) empty [the

hydraulic fluids will add approximately 150 lb (68 kg)]. The near-term goal is to work with contractors to

design and build a filament-reinforced composite accumulator, which could lower the weight down to

approximately 400 lb (181 kg) empty. In the long term, the use of ultralight materials in novel

accumulator designs suggests the possibility of reducing accumulator weight down to 150 lb (68 kg)

empty.

A second critical criterion for accumulator performance is specific energy. Specific energy is directly

related to the weight of the accumulator, so one way to increase specific energy is to reduce the

accumulator shell weight. A second option is to increase system pressure, which will decrease the amount

of oil necessary, again reducing weight. A third option is to increase accumulator capacity by retrieving a

higher percentage of energy stored in the accumulator. Today’s designs produce about 0.0004 hp-h/lb and

short and long-term goals are 0.0006 and 0.0012 hp-h/lb, respectively.

A third important design parameter is the accumulator cycle efficiency (this is a measure of the energy

that is discharged from the accumulator expressed as a percentage of the energy that is provided to the

accumulator). Cycle efficiencies for current accumulator designs are approximately 80%. Near-term and

long-term goals for accumulator efficiency are 95% and 98%, respectively.
G-5

A fourth critical design criterion is cost. The near-term goal is approximately $1,600 and the long-term

goal is to reduce it to less than $1,000 for high production volumes. The goals of reducing weight and

increasing efficiency may require the use of advanced materials and construction, which might drive cost

up, but efficiency gains and high production volumes suggest the potential for lowering costs overall. The

considerable research now devoted to bringing down the cost of tanks to store CNG should be directly

transferable to accumulator design as well.

Safety is an important design parameter as well. Accumulators should not present much of a fire hazard

because the nitrogen gas is relatively inert compared, for example, with natural gas stored under similar

pressures in CNG vehicles; likewise, the mineral oil used in the accumulators is relatively less flammable

than the gasoline stored in conventional vehicles. However, accumulator fluids are under high pressures,

and the potential exists for tank ruptures. Fortunately, safety has always been an important design

parameter with accumulators. We will investigate ways of segregating the nitrogen gas/mineral oil

mixture so that the effects of any possible rupture could be minimized (the energy is stored in the

compressed nitrogen gas, not the hydraulic fluid). Certain new materials such as Kevlar and other strong,

lightweight fibers may be able to act as a “blanket,” allowing the release of gas pressure but inhibiting the

release of high-pressure oil or any vessel fragments. Again, the ongoing research with CNG tanks will

provide important assistance in this regard.

A final fruitful area for future research will involve determining the optimum storage pressure. Higher

pressures will reduce the size of the accumulator vessel and the amount and weight of oil needed for a

given level of stored energy, but will also require stronger accumulators, which would increase vessel

weight and cost. An evaluation of the overall impacts on specific energy and cost is essential. The trend

with natural gas storage technology has been toward higher pressures based on improvements in design

and the use of advanced materials. Just a few years ago CNG was generally stored at pressures of around

2,000 psi, but now CNG tanks are being designed to hold pressures of 3,600 psi and industry researchers

have discussed the likelihood of further increases in the future. We will investigate the desirability of

accumulator pressures of 5,000 psi and possibly even higher.

To meet the efficiency goal, accumulators should demonstrate 95% efficiency (charge plus discharge)

within 2 years and 98% efficiency within 4 years. Weight considerations and heat loss from the

compressed nitrogen after regeneration are key barriers to reaching the technical targets.

G.2.2 Flywheels

Modern high-performance flywheels offer several attractive features for use in hybrid vehicles. (For a

discussion of electric flywheels, see Sect. 4.6.4.) Flywheels offer outstanding power-handling capabilities

with low-to-moderate specific energy; therefore, flywheels are best fitted for applications that demand

high power levels and relatively low energy storage, such as a “power-assist” parallel hybrid vehicle.

Flywheels provide significant advantages over batteries in the areas of calendar life, cycle life, efficiency,

consistent performance at different temperatures and different ages, and ease of measurement of state of

charge.

A key design issue will be structural properties of the rotor to permit the high-speed operation necessary

for high specific energy and minimization of risks associated with rotor failure. Safety and containment

are major issues being addressed by flywheel developers. Understanding of the safety issues (rotor stress

design margin, rotor integrity, control of rotor failure modes, and containment system design) is a key

barrier to the commercialization of flywheel technology. Selection of rotor material will likely involve the

use of fiberglass or carbon-fiber composite materials. The designer will face a trade-off of price and

performance.

When used in a mechanical hybrid, the flywheel rotor will be attached to a continuously variable

transmission (CVT). Minimization of losses will be a key issue. It will be necessary to operate the
G-6

flywheel in a vacuum environment in order to reduce windage drag losses on the rotor surface. Low-drag

seals and bearings are essential; magnetic bearings are options.

The energy storage device in a hybrid vehicle needs to have high input and output efficiency to maximize

the percentage of energy available for reuse. A round-trip efficiency of 90% within 2 years and 95%

within 4 years are targets for achieving the overall power-train efficiency goals.

G.3 POWER MANAGEMENT—VALVE BLOCKS, TRANSMISSIONS, ACTUATORS, AND

CLUTCHES

For mechanical hybrid trucks to work efficiently, they must manage power stored in the secondary power

source. In many hydraulic hybrid designs components (such as control valve blocks), actuators and

clutches are used to manage the flow of power. For hybrid flywheel designs, components such as CVTs,

clutches, and conventional transmissions are used to move power. These are examples for general

mechanical components commonly seen in hybrid designs. In the future other power management

components may be added based on additional research.

To reach the 21st Century Truck Program efficiency goal, overall power management efficiency would

need to reach 90% within 2 years and 95 % within 4 years.

G.3.1 Hydraulic Valve Blocks

Hydraulic valve blocks are used to manage high and low pressure. Mechanical valve noise, leakage, low

efficiency, and cost are the key barriers to achieving the efficiency goal. The technical approach will

begin with investigation of new multifunction control valves.

G.3.2. CVT for Flywheel System

A toroidal system is an example of a power management system used to transfer speed from a flywheel

back into the drive shaft. Durability, low efficiency, and cost are the key barriers that must be overcome

to reach the efficiency goal. The technical approach will begin with evaluating new CVT systems like the

one proposed by Southwest Research Institute (SwRI) for electric hybrids.

G.3.3 Transmissions

Transmissions transfer engine power to the axle and wheels. One of the great advantages of a hybrid

truck, with respect to efficiency, is that at any given time the power output of the engine can be

independent of the power needed by the truck; the corresponding drawback is that the transfer of power

among the various system components becomes more complicated.

In its mechanical hybrid design activities, the 21st Century Truck Program will consider a CVT with the

usual movable pulley of variable effective diameter (or other multiple-gear-ratio transmission). The most

valuable benefit of a CVT is that by permitting an unlimited number of gear ratios, the engine can be

operated at or near its peak efficiency for any given power demand. CVTs have been under development

for many years and are now used in a limited number of production trucks. The project will develop a

unique CVT configuration that is hydraulically activated and electronically (computer) controlled and that

can be integrated with the mechanical hybrid propulsion systems. In parallel with CVT development,

hydrostatic drive designs will also be investigated that could be easily integrated with the hydraulic

hybrid design and could replace the need for a separate transmission.

The critical criteria for transmission design for a hybrid truck are efficiency, weight, responsiveness, and

cost. Of course, trade-offs are involved in optimizing a design for a particular truck. The objective of the

transmission project will be to design a transmission that is more efficient, lighter, and lower cost than

current designs while retaining excellent responsiveness.
G-7

G.3.4 Actuators and Clutches

Actuators and clutches are particularly important in a hybrid truck because the dual power sources make

power transmission, shifting, and management much more complex. Because hybrid truck design is in its

infancy, little work has been done in this area, particularly with mechanical hybrids. The ultimate goal is

to make the existence of two power sources completely transparent to the driver of the truck. Of course,

this must be done without adding much weight or cost to the truck. There are many different options for

the use of clutches in a hybrid truck, and the first objective of the work in this area will be to investigate

the advantages and disadvantages of each.

One interesting option with the hydraulic hybrid is to not utilize any clutching between the engine, the

hydraulic pump/motor, and the transmission. The advantage of this, of course, is that it minimizes weight

and cost. The drawback is that the engine may occasionally be motoring while the pump/motor is

charging the accumulator during regenerative braking or when delivering small amounts of power by

itself. This creates a small drag on the power train and therefore decreases its efficiency. Given the small

displacement of the hybrid engine and the low frequency for the motoring mode, it may well be that the

motoring is preferable from a cost-effectiveness standpoint. In any case, the trade-offs associated with

utilizing or not utilizing a clutch in this regard must be examined in detail.

G.4 POWER TRANSFORMERS—PUMPS AND MOTORS, CVT

For mechanical hybrid trucks to work efficiently, they must transform power from the primary and

secondary power sources. Mechanical systems have a distinct advantage over most electrical systems to

be able to provide very high specific power for a given weight.

Hydraulic hybrid designs typically utilize pumps and motors. Flywheel hybrid designs can use CVT,

electric motor/generators (for an electric flywheel), and hydraulic motors (for a hydraulic-based

flywheel). These are examples for general mechanical components commonly seen in hybrid designs. All

of these components have the advantage of being well demonstrated. In the future, other power

management components, based on additional research, may be added.

To reach the 21st Century Truck Program efficiency goal, overall power management efficiency (power

retrieved from power stored) of at least 90% would be needed within 2 years and at least 95% within

4 years.

G.4.1 Hydraulic Pumps and Motors

Hydraulic pumps and motors are essential in hydraulic hybrid truck design. They assist in the powering of

the truck, they allow the engine to be utilized as much as possible and at peak efficiency, they permit the

recovery and reuse of energy otherwise lost in braking, and they allow the deletion of some systems

otherwise required on conventional trucks.

Hydraulic pumps and motors are used, of course, in a wide range of industrial applications. However,

they have never been optimized for use in hybrid trucks. Conventional hardware can be used for initial

proof-of-concept evaluations, but there is considerable opportunity for optimization for truck applications.

Consequently, they will need considerable development to yield the speed, torque, and power ranges

necessary for hybrid application but with maximum efficiency and minimum weight and cost

implications. Key performance criteria for hydraulic pumps and motors are efficiency, weight, cost,

noise/vibration, and responsiveness.

Current designs can be 90% or more efficient over a wide range of their operating maps. A near-term goal

is to develop systems that can be 95% efficient over 80% of the operating map. A long-term goal is a

design that has an efficiency of 97% or greater over 90% of the operating map. Of course, weight is a

critical design parameter as well; a very efficient pump/motor would be worthless if its weight were
G-8

prohibitive. Cost is also critical and related to efficiency and weight. Today’s most efficient prototype

pumps/motors can cost thousands of dollars when custom ordered in small numbers. A near-term goal

will be to lower cost to approximately $1,000 per unit; the long-term goal will be less than $500 per unit

for high production volumes. Noise and vibration will be much more important in a truck application than

in many other uses. In addition, the hydraulic unit must be able to instantaneously change from supplying

power for truck acceleration (acting as a motor) to receiving power from the kinetic energy during truck

braking (acting as a pump).

One interesting area that must be studied is to evaluate the relative advantages and disadvantages of

utilizing one pump/motor or two pump/motor systems. The latter would add hardware, but it would allow

the use of a smaller transmission and a smaller primary pump/motor system, which would increase the

efficiency of both of these components. This important trade-off must be investigated. Key barriers in

meeting the efficiency goal are basic cost, efficiency, and mechanical noise of the pump motors.

G.4.2 CVT

When a flywheel system is used, there are several variations to transform power. For example, if a

flywheel hybrid used a CVT to get power to and from the flywheel it would need a transmission to deliver

the power to the wheels. However, if it used an electric motor or a hydraulic motor to get power from the

flywheel, then it could just send electric or hydraulic power directly to the drive wheels. Key barriers in

meeting the efficiency goal are cost and efficiency.

G.5 REGENERATIVE BRAKING

Regenerative braking, or recovering and storing at least some of the energy otherwise lost in truck

braking, is utilized in most hybrid truck designs. For conventional trucks in urban driving, braking can

waste one-half or more of the total useful energy provided to the wheels. Recovering and reusing as much

of this energy as possible is a highly desirable feature of strategies to fulfill the “factor of three” fuel

economy goal. In a hybrid truck that already has a secondary energy storage system, the addition of

regenerative braking is much less complex and costly. In a hydraulic mechanical hybrid truck, the energy

is recovered by using a hydraulic pump to store braking energy in the accumulator for reuse to power the

truck or for other purposes. In a flywheel mechanical hybrid truck, the energy is recovered by using a

CVT to spin up the flywheel, thereby transferring the vehicle’s kinetic energy to flywheel kinetic energy

during braking. The CVT is then used in reverse fashion to transfer the flywheel kinetic energy to vehicle

kinetic energy for the next vehicle acceleration.

Efficiency of energy recovery, safety, and cost are the most important issues in regenerative braking

system design. The overall net efficiency (energy received from the wheels, stored, and then redirected to

the wheels) of a regenerative braking system is critical and is dependent on the individual efficiencies of

several components. Based on hardware currently available, none of which has been optimized for

automotive applications, overall net efficiency is around 60%. The long-term goal is to increase it to 80%.

Braking systems for conventional cars are currently in a transitional state with respect to safety, moving

toward widespread industry adoption of anti-lock braking systems (ABSs). Ultimately, regenerative

braking will have to be fully integrated with ABSs.

G.6 CONTROL TECHNOLOGY

Sophisticated on-board electronic control systems will be essential for hybrid trucks to provide the driver

the “feel” that he or she is accustomed to with a conventional truck drivetrain. Control systems on

conventional trucks have become much more sophisticated in recent years, but hybrid systems involving

two power sources and several additional power-train components will necessarily require much more

complex systems.
G-9

A control system for a mechanical hybrid truck design will have to include all the variables included in

control systems for conventional engines and trucks, such as engine speed, engine temperature, oil

pressure, coolant temperature, fuel rate, injection timing, airflow, exhaust gas temperature, and exhaust

oxygen levels. Additionally, the control problem is greatly complicated by the need to coordinate and

optimize the utilization of two different power sources (the engine and the flywheel or hydraulic system)

as well as the additional components associated with the regenerative braking.

G.7 ACCESSORIES

There could also be some consideration for how some of the truck accessories are powered. For example,

in a hydraulic hybrid, the power steering pump could be powered from the hydraulic system, thus leaving

the control system able to shut the engine down during low power modes. The 21st Century Truck

Program designs will consider the incremental fuel-economy benefit against the cost of changing

accessory components.

APPENDIX H

MORE-EFFICIENT AND/OR LOWER-EMISSION ENGINE SYSTEMS

H-3

APPENDIX H

MORE-EFFICIENT AND/OR LOWER-EMISSION ENGINE SYSTEMS

H.1 FREE PISTON ENGINE CONFIGURATIONS

Goldsborough and Van Blarigan 1999 and Goertz and Peng 2000 have simulated free piston engines that

couple a reciprocating piston directly to a linear generator. (See Fig. H.1.) Such devices contain no

crankshaft or camshaft. Freed from some of the kinematic and bearing load constraints of the slider-crank

mechanism, they appear well suited for application to the HCCI combustion process. They are also well

suited because they have more operating degrees of freedom, which may be vital for control of practical

HCCI combustion. This design offers the attractive possibility that the mechanical simplicity will lead to

cost-effective, efficient, clean, and more nearly direct conversion of combustion energy to electric power.

Achten 1996 has built and tested a free piston engine in which the diesel 2-stroke working chamber is

directly attached to a hydraulic pump. (See Fig. H.2.) This is also suitable for HCCI operation for the

same reasons as described in the discussion of HCCI in Sect. 4.6.10.4. Applications would obviously

focus on vehicles for which hydraulic transmissions are appropriate. It is not yet clear whether the

transmission and energy storage required for hybrid vehicles should be based on electric or hydraulic

technologies. Either way, the free piston engine with HCCI may be an attractive prime mover.

H.2 IN-CYLINDER REGENERATION

Ferrenberg has simulated the thermodynamic performance of cylinders in which the working gas picks up

heat prior to combustion by passing through a thin disc of porous material (Farrenberg 1990, Ferrenberg

et al. 1993). (See Fig. H.3.) The disc is reheated when the expanded gas passes through it prior to exhaust.

Simulated efficiency of such an engine is up to 58%. The reasons for this high efficiency are discussed in

an earlier section.

Fig. H.1. Schematic of free-piston-powered linear alternator.
H-4

Fig. H.2. Schematic of free-piston hydraulic motor.

Fig. H.3. Cylinder cycle for stationary-regenerator two-stroke sleeve-valve engine.