Work Performed Under Contract No. A101-78CS54209

August 31, 1984

LIBRARY COpy

LANGLEY RESEARCH GENTI;R

LIBRARY, NASA

H;"r'"~ToN, VIRGIt-\IA

By

H. W. Schneider

DOE/CS/54209-15

(DE85004585)

EVALUATION OF HEAT ENGINES FOR HYBRID VEHICLE

APPLICATION

NASA-CR-174163 B~E!,"S

'OS I~ 3JifU . . ,

\~g'5 a00 qerS'!

Jet Propulsion Laboratory

Pasadena, California

Technical Information Center

Office of Scientific and Technical Information

United States Department of Energy
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DOE/CS-5420~jl~o CNT;:

NAS7-918 DE-AI01-78CS-54209 84/08/31 72 PAGES UNCLASSIFIED DOCUMlNT

UTTL: Evaluation of heat engjne for hYbrid vehicle apPli·cation AUTH: A/SCHNEIDER, H: W~

CORP: Jet.Propulsion Lab:~ California Inst: of Tech:, Pasade~a:

SAP:HC A04/MF AOI

EXHAtJ:3T FUEL

AB:3; Ti1E: statt~E~ ()f ()f19() i r"%9 f·!eat,-erf~.i ne cie\!e I oPfner!ts~ inc 1\Jci i n9 spar~.. - i 9rl iti ()fl,

compression-ignition! internal-combustion, and external-combustion en91nes

is preserlte(~~ Trle pc-terlt,ial ()f err~irle c:()r~cept~3 t~n(ier c:or!~3i(lerati()r) f~)r

hYbri-d -veh1cle use is evaluated, using self-imposed criteria for

selection: The deficiencies of the engines curre~tly being evaluated in

hybrlQ vehicles are-discussed: Focus 18 on recent research with two-stroKe~ rotary, and free-piston eu91nes= It is concluded tha th~se

engine concepts have the most promising potentj-ai for future 3PP lcation

in hYbrid vehlcles~ Recommendations are made for analysis and -Ef~TER:
.•
5030-568

Electric & Hybrid Vehicle System

Research & Development Project

DOE/CS/54209·15

(DE85004585)

Distribution Category UC·96

Evaluation of Heat Engines for

Hybrid Vehicle Application

H.W. SChneider

August 31, 1984

Prepared for

U.S. Department of Energy

Through an Agreement with

National Aeronautics and Space Administration

by

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California

JPL Publication 84-14

ABSTRACT

The status of ongoing heat-engine developments, including spark-ignition,

compression-ignition, internal-combustion, and e<ternal-combustion engines is

presented in this report. The potential of engine concepts under consideration for hybrid vehicle use is evaluated, using self-imposed criteria for

selection. The deficiencies of the engines currently being evaluated in

hybrid vehicles are discussed. The study focuses on recent research with

two-stroke, rotary, and free-piston engines and concludes that these engine

concepts have the most promising potential for future application in hybrid

vehicles. Recommendations are made for analysis and experimentation to

evaluate stop-start and transient emission behavior of recommended engine

concepts.

ACKNOWLEDGMENTS

The author wishes to express appreciation to Barbara Bonzo and

Thedra McMillian, members of the EHV team, and Catherine Edwards, editor,

for their valuable contributions to this report.

iii

PART ONE:

A.

CONTENTS

EXECUTIVE SUMMARY

INTRODUCTION AND SCOPE 1

B.

C.

CONCLUSIONS • • •

RECOMMENDATIONS •

• 3

8

D. SUMMARY OF CURRENT HEAT-ENGINE RESEARCH

AND DEVELOPMENT • • • • • • • • • • • • . . . . . . . . 8

PART TWO:

I.

EVALUATION OF HEAT ENGINES

METHODOLOGY FOR HYBRID HEAT-ENGINE EVALUATION . . . 1-1

A.

B.

C.

PRIMARY ELECTRIC CAR WITH AUXILIARY HEAT ENGINE

PRIMARY HEAT-ENGINE-DRIVEN CAR WITH AUXILIARY

ELECTRIC POWERTRAIN ••• • • • • • •

HEAT-ENGINE RECHARGEABLE PRIMARY ELECTRIC CAR

• •

1-1

1-1

1-1

D. TRUE HYBRID HEAT-ENGINE/ELECTRIC CAR . . 1-2

I I. STATUS REVIEW OF VARIOUS HEAT-ENGINE CONCEPTS 2-1

A. SPARK-IGNITION ENGINES . . . . 2-1

1.

2.

3.

Four-Stroke Piston Engines

Four-Stroke Rotary Engines

Two-Stroke Piston Engines •

2-1

2-3

2-4

B. COMPRESSION-IGNITION ENGINES (DIESEL) 2-15

L

2.

Indirect-Injection Diesel Engines

Direct-Injection Diesel Engines

2-15

2-18

C. EXTERNAL COMBUSTION ENGINES • • • •

Brayton Engines (Gas Turbine)

2-18

2-18

2.

3.

Stirling Engines •••••••

Rankine Engines (Steam Engine)

v

•

•

2-20

2-22
D. FUEL-INJECTION SYSTEMS • • • • • • • • • • • • • • • Z-ZZ

E. FREE-PISTON ENGINES • • • • • • • • • • Z-Z3

III. POTENTIAL FOR HYBRID APPLICATION • • • • • • • • • • • • • 3-1

A. BRAYTON ENGINE • • • • • • • • • • • • • • • • • • • • • 3-1

B. STIRLING ENGINE • • • • • • • • • • • • • 3-3

C. DIESEL ENGINE • • • • • • • • • • • • • • • • • • 3-3

D. HIGH-COMPRESSION, FOUR-STROKE, SPARK-IGNITION

PISTON ENGINE • • • • • • • • • · · · • • • • · 3-3

E. lWO-STROKE, SPARK-IGNITION PISTON ENGINE • • • • • • • 3-5

F. FOUR-STROKE, SPARK-IGNITION ROTARY ENGINE • • • • • 3-7

G. FREE-PISTON ENGINE • • • • • • • • • • • 3-7

VI. SU!~Y AND CONCLUSIONS • • • • • • • • • • • • • • 4-1

V. RECOMMENDATIONS • • • • • • • • • • • • • • • 5-1

REFERENCES . . 6-1

APPENDIX A. CONTACTS MADE IN CONJUNCTION WITH HYBRID VEHICLE

HEAT-ENGINE EVALUATION TASK • • • • • • • • • •

vi

• • • A-I
Part One

Executive Summary

EXECUTIVE SUMMARY

A. INTRODUCTION AND SCOPE

The objective of the U.S. Department of Energy (DOE) Electric Hybrid

Vehicle (EHV) Program is to evaluate the potential of electric vehicles (EV)

and/or hybrid vehicles (HV) to significantly reduce petroleum consumption.

The Jet Propulsion Laboratory (JPL) supports the DOE by performing EHV System!<

research and development (R&D). In this role, JPL is responsible for the

development and evaluation of hybrid-vehicle concepts. Previous HV activities

included JPL assessments (References 1 and 2) and a DOE/JPL contract to

General Electric Company (GE) to demonstrate HV feasibility through development of a complete vehicle (References 3 and 4). A limitation of previous

assessments and vehicle development act ivities was that only off-the-shelf

engines were considered.

Having demonstrated the technical feasibility of the electric/gasoline

engine hybrid-vehicle concept (see Reference 3), JPL initiated a small-scale

study to evaluate advanced engines by assessing their attributes when applied

to unique HV requirements. Based on a literature search and personal communications with a variety of sources that are actively engaged in heat-engine R&D

work, this study evaluates the potential of advanced heat-engine concepts for

future hybrid application, using a system-prioritized rating criteria (Table

1) that builds on the findings established in the aforementioned references.

In addition to fuel efficiency and emissions, these criteria emphasize stopstart capability, low motoring-power requirements, weight, packaging flexibility, service needs, and accessibility. While most of the criteria are

conventional, added emphasis has been placed on weight and packaging flexibility as a result of the experience gained during the GE contract to build

an HV. In addition, a recent assessment of HVs (Reference 5) indicates the

strong influence of these criteria to the viability of the HV.

Included in the evaluation (Table 2) are advanced versions of internalcombustion, four-stroke, spark-ignition piston engines; Wankel-type rotary

engines; spark-ignition, two-stroke piston engines; and external-combustion

engines such as Brayton (gas turbine) and Stirling engines. Production sparkignition piston and indirect-injection diesel engines of current design are

included for comparison. The status and potential of direct-injection, highspeed diesel engines and free-piston engines are briefly discussed. All of

the engines evaluated are atmospherically aspirated. Turbocharged engines

were not considered because of their incompatibility with rapid stop-start

operations (lag) and because they are too costly for hybrid applications in

which the heat engine is only an accessory.

The original task plan prOVided for an in-depth, fact-finding follow-up

to fill information gaps and for an analytical drive-cycle evaluation of the

most promising candidate heat-engine concepts. This portion of the task

could not be completed because funding was discontinued beyond FY 1983.

1
• Table 1. Priority Rating of Hybrid-Vehicle Engine Properties

Separate System In-Series Parallel

Heat-Engine Properties

Powertrain

Concepts

Primary

Electric

Power

Auxiliary

Heat-Engine

Power

Primary

Heat-Engine

Power

Auxiliary

Electric

Power

Primary

Electric

Power

Heat-Engine

Battery

Charging

Combined

Electric

Heat-Engine

Power

Low Brake-Specific Fuel Consumption 2 __--=5~ ---=4::....- -.::.4 _

Low Brake-Specific Emissions

Low Weight

Packaging Advantages

Low Cost

Low Service Needs

Stop:Start Capability

Instant Full-Power Ca

Starting Power Requirements

3 5

3 4

4 4

4 4

5 3

1 1

3 3

Conventional

Manual starting I electric

desirable I starter

I

3

3

4

4

5

3

3

Cranking to

firing with

generator

4

3

5

4

5

5

5

Cranking up to

50% nominal speed

with motor generator desirable

Ratings: 1

2

Unimportant

Less Important

3 - Desirable

4 - Important

5 - Very Important
As originally planned, this heat-engine evaluation task consisted of

three activities:

(1) Literature search and review of on-going R&D.

(2) Hardware evaluation.

(3) Hybrid-vehicle simulations using data from Steps (1) and (2).

This report presents the findings of the first of three subtasks.

A summary of the R&D status for various heat-engine concepts is presented

at the end of this section to prOVide insight into the study's conclusions

and recommendations. An in-depth discussion on the topics summarized below

are presented in the technical discussions beginning in Section I of this

report.

B. CONCLUS IONS

Figure 1 compares the relative ratings for various engine concepts

against three parameters that are key for HV applications. Figure la shows

relative fuel efficiency rankings for engine types while Figure lb shows

overall specific volume rankings, and Figure lc compares the specific weight

characteristics. From these figures it can be concluded that the unique

requirements for engines in HVs make it likely that non-conventional engines

offer the opportunity for substantial improvements in both performance and

marketing of HVs. Furthermore, it seems that the use of off-the-shelf engines

has placed too many constraints on previous HV developments and studies. The

following engine-specific conclusions support the preceding general conclusions.

(1) The advanced, fuel-injected, two-stroke engine is the most attractive choice of a heat-engine concept for use in a hybrid vehicle

for the following reasons:

(a) Its fuel efficiency is excellent, exceeding that of a direct

injection diesel at part load (see Figure la) while exhibiting

emissions that are acceptable without after treatment (Figure

2) and are excellent if used in conjunction with a thermal

reactor.

(b) Having no valves, the advanced two-stroke engine is simple in

design and its service needs and production cost are low. It

is also eKtremely attractive from the packaging standpoint because it occupies only one-third of the volume (see Figure lb)

and is more than 50% lighter (see Figure lc) than a conventional or advanced four-stroke engine of comparable performance.

3
Table 2. Comparison of Engine Design and Performance Criteria

tlternd eo.bUltion

Engln..

Oiuels

Indirect Injection'

Naturaly A.pirated Iraytoa 5tirUna:

(TurbochU8"d)

Rabbit cr-lOO Hod 1

YW 0" 1m Production "0 Re...rch En '" 37/5000 75/68000 54/4000

I.H.)

22 4.5 Pl 17.5

'" 200 ]0' 260/30 3200/<100 2H/6lJ

1.20.05) 1.0 • 3.54

1.4(1.0 <1.0 1.64

10 498 .fA .fA

Direct injection 2350" turbtne 15 KPa

1l1prove. liSle inlet te.perature Hydrogen

and power output 2-,haft reSenerated (2117 p.U

by 15% Lo" NClII: cOI'buIUon

including g"aul

... Suzuki

OEC 10. Re.e.reb En

ATAC

'CAL

le.elrell En

Inter""t Co-eu.Uon En tne.

1Wt-871

.su

..0

Naturally A.plnted Four-Stton

Span-hnitton Engine

Audl 1700

Audl/WV

Production

55/5000 88/6000 121/6.500 16/4S0Qlo 80/6000

1.7(4) 2.0(4 ) ).0(2) 0.372(1) 1.2(3) 8.' II .., J. , '/A

m ]0' ]00

268/6) 239/67 248/75

32.0 42.7 66.3

1.0 .fA 0.' 1.0 '/A 0.32

11 978 11 339 17 925

ContinuOUI Fuel In eetioR Pulle Fuel In ecHon

Currently in I Highly I Stratified I Rel1dud g•• StratUled duel

ule in etlperi-! turbulent I cOlIIbuttion! controlled vortex cOIlbua-

_ntal hybricl I c(llllbuatloni I tion vehicles I I Dud fuel I Fuel injection

I I injection I PneulUtlc '~l

I I I Unthrottled Injel:.tlon

I I O~l I operation

I I ignition I

I I I

I I .1

I !Prediction I

Engine Concept

Criteria

Relative Weight to Power bUo:

Relative VoluM to Power Ratio

Engtne Du1&natlo-n

DevClloper. Relleareher

StatuI

Maxi..... Power 8SFC. a1kWh

Opt. BSFC. &!kWh/l power

Power Concentration kW/

HaxhlUlll Power Output. kW/rfN/-.1n I

DhplaC::Ment, (No. Cyl1nden. RotOf,b>l

Co. re.,lon beio

Seed Ca ,bUlt kW x rev/lIlin

I Naturally Aapluted. Two-Stroke

I I.oury Spuk-lanlt1on Enaine

i + --'P"."•.!.tjl'o'- !-'<I!W!!.""'''."-'L)-f------,------ --"-===="---

I MIIy I

I k1cardo I

R"

-Relative to copventlonal lov-c(lIIIpre,.ion-r.tlo, four-stroke. spark-ignition ell&ine.

bRotary (Wankel) Engine.
z

r---. z

_ _ o o

z ." _ zO_ "

oz o _ = <_ _ __

§ -

>.7 _o _ oz__o _,o =Z e= -'_ _ 7 ,. _ <a z_ z o o,

m _- < _ _ _,_ -- _ 0 0

_: _ E z _ _ _ -- 8 _ _ o

0_ _ z: _ _ o__o _-:zZO_o. ,_ °X_ - _- _" X o o

_o_ __ ? - , :_ .- .-. ,

. 1 o o

o.__ ___,u_ _- _< _ '_._ - __ _ < <> _Z _ < o

_> Z I I < {3 u

0.1-- - "--i '< --

I

-,.4

0.0 _ _ . 0 - r._

(o) (b) (c)

OPTIMUMBRAKE-SPECIFIC SPECIFICOVERALLVOLUME,JVkW SPECIFICWEIGHT,kg/kW

FUELCONSUMPTION, kg/kWh
10 20

(c) POWER OUTPUT

PER CYLINDER, kW

10 20

(b) POWER OUTPUT

PER CYLINDER, kW

0~__-''''£J4..<..i<..''-L'''~--:-'

o 10 20

(0) POWER OUTPUT

PER CYLINDER, kW

300 30

..<: ..<:

3: 3: 200 20 Cl

"I'

"I

"I' Z

Z Z 400 0

0 Q V;

"I "I VI

"I VI

2 .... w .... X

u 100 0 0 10

:r U 200 Z

'"

LEGEND:

o CARBURETED TWO-STROKE

• FUel-INJECTED TWO-STROKE

'\l CARBURETED FOUR-STROKE

... FUEL-INJECTED FOUR-STROKE

• FUEL-INJECTED TWO-STROKE

WITH THERMAL REACTOR

AREA 1 PRODUCTION TWO-STROKE ENGINES

2 PRODUCTION FOUR-STROKE ENG INES

3 ADVANCED RESEARCH TWO-STROKE ENGINES

Figure 2. Emission vs. Power Output per Cylinder
(c) The stop-start capability of the advanced two-stroke engine is

excellent, delivering full power instantly without a warmup.

Because of its low compression ratio (7.5:1) and the absence

of valve mechanisms, its motoring power is less than half that

required for four-stroke engines, which translates into low

transient energy losses and improved vehicle fuel economy. In

conjunction with an advanced load-dependent metered injection

of the lubricant, its low compression ratio (7.5:1) translates

also into a long service life under stop-start operating

conditions. Combining this low compression ratio with a

high degree of internal turbulence allows this engine to be

tolerant of poor fuel quality (octane).

(2) The (Wankel-type) rotary engine should also be considered for

hybrid-vehicle application, despite the fact that its fuel efficiency is not competitive with other candidate engines (see Figure

la) because its packaging advantages (see Figure lb) are excellent. Its high speed capability (see Table 2) would permit it to

be coupled directly to and/or integrated with a high-speed alternator. In this case, the elimination of the gear losses would

compensate for its lack of fuel efficiency when compared with

alternatives that must be geared to the alternator.

(3) Advanced high-compression, four-stroke piston engines have good

fuel efficiency (see Figure la), but they are expected to have NOx -

emission problems because of higher process tern.peratures and impaired catalytic converter efficiency. In hybrid stop-start

operation the catalytic converter does not reach its full steadystate efficiency (see Reference 3). The motoring power requirements

of high-compression-ratio engines are estimated to be 50% higher

than those of current four-stroke engines and 300% higher than

those of advanced two-stroke and rotary engines. This fact will

reduce their otherwise excellent fuel efficiency in stop-start

operation. The effects of stop-start operation on engine service

life will also be more pronounced with a high-compression-ratio

piston engine.

(4) Free-piston, spark-, and compression-ignition engines (see Figure

la) are attractive in terms of fuel efficiency, packaging, and

vibration when coupled with a linear alternator for battery

charging. Because of their capability to produce high compression

ratio at low friction losses, they are well suited for the implementation of diesel combustion and of spark-ignited combustion

with the use of alcohols or ammonia. If linear alternators show

potential for HV application, additional development of free-piston

engines should be considered because of their ability to enhance

fuel econ any.

(5) Based on established criteria, external-combustion Brayton (gas

turbine) and Stirling engines have no potential for most hybridvehicle applications because of their inherent inability to be used

7
in rapid start-stop operation. Because of small-size effects,

Brayton engines cannot be considered for power outputs below 75 kW;

otherwise, they would have a potential for battery charging.

(6) The indirect-injection diesel loses out against other candidate

engines in all areas (see Figure 1), including that of fuel

efficiency. With direct injection, the diesel will be the most

fuel-efficient, internal-combustion engine available when compared

in terms of grams per kilowatt hour. However, because of its need

for preheating prior to start and its high cranking power requirements, the diesel engine needs to idle instead of being turned

off. Although its idle fuel-flow rate is only 20 to 25% that of

spark-ignition engines, this requirement will reduce its actual

fuel efficiency to some degree in most hybrid applications. The

idling diesel is an attractive alternative because of hybrid

accessory power and heating, which are still unresolvE'd problems

with heat-engine stop/start operation.

C. RECOMMENDATIONS

Because of the great potential for improvements in. HV fuel economy,

weight, and other factors affecting overall HV Viability, further research

is recommended. The follOwing efforts consider the unique HV requirements

in a prioritized sequence:

(1) Based on available information and the conclusions drawn by this

study, it is recommended that heat-engine development be pursued

as indicated in Table 3, which is grouped by hybrid powertrain

concepts. The two-stroke engine is the primary choice of all

powertrain concepts considered, wi th the exception of a primary

heat-engine-powered hybr id car with an auxiliary electric powertrain that requires more than 50 kW of heat-engine power. The

low-compression, four-stroke, spark-ignition engine is the best

backup choice if it is equipped with a fuel-injection system and

accessories developed to cope with the special requirements for

stop-start operation. The free-piston diesel and the alcoholfueled, spark-ignition, free-piston engines have an at tractive

potential for battery charging with a linear alternator. This

potential should be re-evaluated, taking the latest technology

and innovative concepts into account.

(2) The study further recOClmends follow-on analysis and experimentation

with candidate engine concepts that are believed necessary to

support the selection of a heat-engine concept for future hybrid

vehicles. This effort should concentrate on the determination and

evaluation of properties of specific interest to HVs that usually

cannot be supplied by the heat-engine developer. The rationale and

objectives of these recommendations are outlined in detail in

Section V of this report.

8
Table 3. Engine Concepts Rated Most Promising for Hybrid-Vehicle Application

Powertrain

Concepts

Separate Systems In-Series Parallel

Fuel-Injected Two-Strokea

Fuel-Injected Rotaryb

Recommendations

(in order of

preferance)

Heat-Engine Concept

Cooling System

Primary Electric

Power

Auxiliary Heat

Engine Power

Carbureted

Two-Stroke

Carbureted LowCompression

Four-Stroke

Forced Air

Primary Heat

Engine Power

Auxiliary (Urban)

Electric Power

Fuel-Injected

High-Compression

Four-Stroke

Primary Electric

Power

Heat-Engine Battery

Charging

Direct-injection

diesel or alcoholfueled spark ignition, free-piston

engined

Liquid

Combined Electric

Heat-Engine Power

(Power Blending)

Fuel-Injected

Low-Compression

Four-Strokec

Starting Method

Electric Starter

Manual starting

capability

desirable

Electric Starter Hybrid Motor/Generator, AlternatorC

Exhaust After Treatment None Two-Stroke and Rotary:

Four-Stroke Piston Engine:

Diesel:

Thermal Reactore

Catalytic Converter

None

aFor power outputs up to 50 kW.

bWhere a direct coupling with alternator is applicable.

CEquipped with stop-start proof lubrication and fuel-injection system.

dFree-piston start: Electric excitation into boot-strapping resonance or pneumatic latch-release starter system.

eWaiver can possibly be obtained for hybrid with low-emission transients.
D. SUMMARY OF CURRENT HEAT-ENGINE RESEARCH AND DEVELOPMENT

1. Four-Stroke, Spark-Ignition Piston Engines

Concentrated efforts are being made by well-known developers to

improve the fuel efficiency of the atmospherically aspirated, four-stroke,

spark-ignition engine by raising its compression ratio and introducing highly

turbulent, squish-flow agitated combustion to inhibit knocking. Fuel efficiency improvements of up to 15% and increased power output per unit displacement of up to 30% have been demonstrated. Developers are hopeful that highcompression engines can be marketed by 1985 as an option. Flame erosion of

the pistons, closer head tolerances, and associated higher production cost

are the pacing problems to be resolved.

2. Four-Stroke, Spark-Ignition (Wankel) Rotary Engines

State-of-the-art, rotary, four-stroke, spark-ignition engines are

equivalent to four-stroke piston engines of current low-compression (8.5 to

8.7:1) design in fuel efficiency and emissions. This has been achieved

primarily by improving the rotor seals, raising the compression ratio to 9:1,

and introducing stratified combustion brought about by dual fuel injection

and ignition. However, with these measures the fuel efficiency potential of

rotary engines is essentially exhausted, with rotor sealing and combustion

chamber configuration (high surface to volume ratio) being the primary limiting factors. They are, therefore, incapable of competing with high-compression four-stroke piston engines in the future. A certain potential still

exists to improve the already excellent speed potential of rotaries (see Table

2) by means of lift-off seals, which still remain to be developed. The major

developers of advanced fuel-injected, rotary (Wankel) engines (Audi-NSU, and

Curtiss-Wright), have reassessed their goal of (some day) dominating the automotive market and are now looking for other fields of application where the

packaging advantages, high-speed capability, and smoothness of the rotary

concept would be of overriding importance. According to dealer information,

Mazda will market a fuel-injected rotary engine as an option in 1984 model

cars.

3. Diesel Engines

The uncertainties associated with diesel emission legislation in

Europe and the United States as well as declining diesel sales because of

the diminishing pr ice advantage of diesel fuel relative to gasoline strongly

inhibit the incentive to further improve the small automotive indirect injection diesel engine. A proposed quiet, direct-injection, high-speed diesel

engine with an improved (by another 15%) fuel efficiency was initiated by BMW

and Steyer in the early 1980s but was shelved because of economical reasons

and the technical difficulties encountered.

4. Two-Stroke, Spark-Ignition Piston Engines

Because they were forced out of the vehicular market more than ten

years ago by emission legislation, the developers of two-stroke engines are

10
making a quiet effort to regain their place in the market. All vehicular

applications-oriented research with two-stroke engines is concentrated in

Japan, Switzerland, Ireland, and Australia and has produced significant

results. The emission problem of two-stroke engines has been essentially

eliminated by introducing cylinder fuel injection and stratified combustion

techniques that allow the engine to be operated unthrottled over a wide

operating range. The part-load fuel efficiencies independently demonstrated

with advanced two-stroke engines in Australia, Japan, and Europe are equivalent to or better than those of an automotive 'diesel of current design. The

pacing problem is that 'of a high-speed, fuel-injection system capable of

reliably handling small amounts of fuel. Developers are confident that the

two-stroke engine in the advanced form will make a comeback in the smallvehicle market in 3 to 5 years. Ongoing R&D work with two-stroke engines in

the United States is primarily concentrated on improving the performance of

carbureted outboard marine and utility engines.

5. External-Combustion Engine

R&D work with automotive Brayton and Stirling engines is concentrated in the United States but is progressing slowly because of reduced

government funding. The use of high-temperature materials (ceramics) in

the Brayton engine, and of high-pressure hydrogen with its associated sealing difficulties in the Stirling engine, are the pacing problems. There is

little hope that any of the external combustion engines will mature by the

end of this .decade.

6. Free-Piston Engine

After a long period of dormancy, sporadic project work and experimentation with diesel, Otto and Stirling free-piston engines have resurfaced

in Europe and the United States, using new technology and design approaches.

A power output of 75 kW at 5000 cycles per minute has recently been demonstrated by Stelzer in Germany with a spark-ignited, carbureted, free-piston

engine.

In conjunction with a f1irectly coupled linear alternator, free-piston

engines are believed to have potential for the generation of electrical

energy over a wide range of power outputs from 3 to 700 kW because of their

simplicity and capability of producing high compression ratios with low

internal friction losses. If the predicted speeds can be achieved (up to

30,000 cyCles per minute), a free-piston, linear alternator power unit w11l

be very attractive for the generation of electrical energy in hybrid vehicles

in terms of weight and packaging. According to predictions, free-piston

engines are capable of delivering up to 30% more power at the same fuel

consumption as a crankshaft engine using an identical thermodynamic cycle.

However, none of the recent efforts has progressed beyond proving the validity

and feasibility of the concept.

11

Part Two

Evaluation of Heat Engines

SECTION 1

METHODOLOGY FOR HYBRID VEHICLE ENGINE EVALUATION

Hybrid vehicles use a heat engine in combination with an electrical

powertrain that derives its energy from a rechargeable battery. The design

objective is to reduce petroleum fuel consumption and air pollution. The

possible combinations and uses of a heat engine' in a hybrid vehicle (HV) are

many and depend on mission objectives, drive cycle, and minimum vehicle performance requirements. The rationale for heat engine selection is, therefore,

difficult to define in general terms, and each individual case must be judged

separately. The scenario of hybrid-vehicle missions, powertrain arrangements, and related rationale for the selection of heat engines can be

divided into four major categories:

A. PRIMARY ELECTRIC CAR WITH AUXILIARY HEAT ENGINE

The car is used primarily in the electric mode for short-range urban

use while the heat engine is only used when the battery has been completely

discharged. In this case, simplicity and low cost, engine reliability,

low service needs, and manual starting capability have priority over fuel

efficiency and emissions.

B. PRIMARY HEAT-ENGINE-DRIVEN CAR WITH AUXILIARY ELECTRIC POWERTRAIN

Hybrid-vehicle developers in Europe (Reference 1-1) envision a vehicle

that is pr imarily heat-engine-driven. It is designed to depend on battery

power only when operating in metropolitan areas where the use of heat engines

may be banned in the future due to pollution constraints. The heat-engine

design objectives and selection rationale for a hybrid vehicle of such a

concept are essentially the same as those that govern the engine design and

selection of conventional, primarily heat-engine-powered cars, i.e., compliance with minimum performance, drive-cycle fuel economy and emissions

requirements, packaging advantages, weight, and noise are the primary criteria

for the selection of the heat engine. In hybrids of this type, the engine is

sized to provide for the power needed to meet vehicle performance requirements

without the help of the electric motor.

C. HEAT-ENGINE RECHARGEABLE PRIMARY ELECTRIC CAR

For an in-series arrangement in which the heat engine has only the task

of charging the battery whenever the state of charge has dropped to a tolerable minimum, the rationale for the selection of a heat engine is in essence

the same as those for a mobile, standby generator, i.e, sure-start capability,

low service needs, low noise, weight, and a low hourly fuel consumption at a

constant speed are the primary criteria for heat-engine selection. The

engine must be sized to meet maximum charge power requirements in an operating

point near, or at its best, specific fuel consuQption.

1-1
D. TRUE HYBRID HEAT-ENGINE/ELECTRIC CAR

Maximum petroleum fuel efficiency and versatility to adapt to various

mission and vehicle performance requirements can be achieved with a parallel

powertrain arrangement (Figure 1-1). A heat engine and an electric motor

can act in unity or alone on the wheels of the car through a transmission.

This study addresses primarily the aspects of heat-engine selection relating

to the parallel powertrain concept.

Depending on the type of transmission used, the heat engine for a

parallel powertrain must basically meet the requirements for cars that are

primarily powered by heat engines. However, because the heat engine operates

only part of the time, engine service life and power-related emission requirements necessary to meet vehicle-emission standards in terms of grams per mile

are not as stringent as for cars that are powered primarily by heat engines.

The control strategy usually applied uses power blending above a ,prescribed

vehicle speed. If the vehicle starts up with the electric motor alone, the

operating range to be covered by the heat engine is narrower; and the requirements for low-speed, part-load fuel economy are less demanding than those

in a conventional, primarily heat-engine-powered car. Future hybrid vehicles

may take advantage of a continuously variable transmission (CVT) to further

improve petroleum fuel efficiency. In this case, the engi~e characteristics

on the line of opticum fuel efficiency are of primary interest.

ACCESSORIES

I

I I

OVERRUNNING OVERRUNNING

CLUTCH CLUTCH

I I

HEAT ELECTRIC

ENGINE MOTOR

I I

CLUTCH MODULATED

CLUTCH

I I

I

TRANSMISSION

I

WHEELS I DIFFERENTIAL I WHEELS

I I

Figure 1-1. Schematic of a Hybrid Parallel Powertrain

1-2
The efficient use of the parallel powertrain concept requires that the

heat engine be fired up on driver or battery demand and be shut off if drivecycle vehicular power demands can be satisfied with the electric motor alone,

and when no battery charging is required. To satisfy this requirement, the

heat engine must have the capability of starting and delivering full power

almost instantly. It must be compatible with a repeated stop-start operation

in short time intervals (seconds) at low ambient temperatures throughout the

engine service life.

To keep transient emissions to the lowest possible level, the fuel

induction system of a hybrid engine unit must have the capability of starting

and shutting off fuel flow instantly at the source of injection to avoid

the dumping and dribbling of raw fuel into the exhaust. Practical experience

(Reference 1-3) has shown that a continuous-flow (K-Jetronic-type) fuelinjection system has a definite handicap. From the standpoint of start and

transient energy losses and to minimize transient emissions, it is desirable

to start fuel flow no earlier than safe firing can be achieved. To accomplish

this, the cranking (motoring) power requirements of the engine must be low.

It is desirable to have a hot exhaust to get converters (catalytic or thermal)

up to temperature and efficiency quickly. During frequent start-stop operation the catalytic converter rarely reaches the efficiency produced by a

continuously operated heat engine (Figure 1-2).

t

>- U

Z

w

u

u.

u.

w

STEADY-STATE CONVERSION EFFICIENCY - --;..:::::::::::--

TIME •

ENGINE - ON ENGINE - OFF

Figure 1-2. Catalyst Behavior with On/Off Engine Operations (see Reference 3)

1-3
Practical experience with the hybrid test vehicles currently tested has

also shown that packaging advantages, access to the engine, and low service

needs are extremely important factors to ensure that the engine performance,

fuel economy, and emission criteria can be sustained throughout the vehicle

service life. As can be seen from Figure 1-3, because of the size of the

battery pack and the large number of required components, there is considerably

less space available for the engine in a hybrid vehicle than in a primarily

heat-engine-driven car of comparable performance. The possiblity of a flexible, decentralized arrangement of engine-associated systems and accessories

is desirable from the overall integration and packaging standpoint.

To avoid gearing losses and to reduce dynamic interactions to a possible

minimum, the engine should have a speed capability that allows for an ungeared

direct-in-line drive with a motor generator or alternator. It is also desirable

that the engine be sufficiently short to make a tranverse in-line arrangement

of the heat engine and the motor generator possible. In current test vehicles

(see Figure 1-3), the heat engine and the motor generator are parallel to each

other and are inter-connected by a chain drive. A low engine inertia facilitates clutch modulation and keeps the size and weight of clutches to a minimum. According to current test experience (References 4 and 1-3), it is

difficult to avoid overheating the clutches without introducing jerky shifts.

Noise is an important factor in a hybrid vehicle because of the drastic

changes frequently occurring between the relatively low road and air noises

dominant during electric driving and the entirely different noise spectrum

suddenly produced when the engine is engaged.

PROPU LS ION BAITERY

HEAT ENGINE

FUEL TANK

ELECTRIC MOTOR

Figure 1-3. General Electric test vehicle (see Reference 4)

1-4
SECTION II

STATUS REVIEW OF VARIOUS HEAT ENGINE CONCEPTS

Engine concepts under consideration are internal and external combustion

engines of advanced design that have been primarily conceived with vehicular

use in mind. Only those concepts most representative for a specific concept

are discussed. The major design and rating criteria for these engines are

summarized in Table 2-1. The data and discussions are based on open literature and information obtained from personal communication with the indicated

sources. Over 100 papers were reviewed and 19 sources were contacted for

consultation. The development status, projections, potential for improvement,

and pacing problems associated with concepts under consideration are discussed

in the following subsections.

A. SPARK-IGNITION ENGINES

1. Four-Stroke Piston Engines

For the projected ten-year time frame the three-way, catalystcontrolled, continuous-flow electronically fuel-injected, naturally aspirated, four-stroke, spark-ignition piston engine will dominate the market

for passenger cars that are primarily powered by heat engines. Multi-source

fuel injection has brought an improvement of about 15% over the single-source

carbureted or fuel-injected engine by significantly reducing the uneven

distribution to the cylinders.

A few developers (Saab, Buick, Audi, and others) offer turbocharged,

electronically knock-controlled power plants. The relative high cost for the

maintenance and repair of turbocharged engines, however, inhibit the broad

acceptance of turbocharging by the general public. The high speed and power

boost potential of the turbocharged engine is only attractive to a small

number of users. Usually there are no economical advantages although some

developers claim that fuel economy advantages can be shown with wastegated

turbochargers sized for maximum torque at medium engine speeds, with a reduced

rear-axle ratio and an undersized, lighter engine. Although turbocharger lag

has been greatly improved during recent years due to lightweight rotor design

and the application of wastegate control strategy, costs and upkeep remain a

deterrent for the users of a car that operates predominantly at low to medium

speeds.

Besides general improvements that are currently being made in engine

design, materials, components, and production technology, some major developments are being made in the improvement of the naturally aspirated, sparkignition engine by increasing its breathing capability and compression ratio.

While the potential to further improve the breathing capability obtainable

with two poppet valves appears to be fairly exhausted, a sizable potential to

improve partload fuel economy still exists by increasing the cocpression

ratio. Special precautions, however, have to be taken to inhibit knocking.

2-1
Table 2-1. Comparison of Engine Design and Performance Criteria

N

I

N

Engine Concept

Criteria

Engine Duign.tion

Developer, Researcher

StatuI

Hax1lllum Power Output, kWlrev/min I

Displacement. (No. Cylinders, Rotorsb )I

Co resaion Ratio

H.uigum Power 8SFC, g/kWh

Opt. 8SFC. g/kWh/:f: power

Power Concentration kW/

Relative Weight to Power Ratio:

Relative Volume to Power Ratio

Seed Ca abilit kW x rev/min

Remark.

InterMI Collbultion En lnea b: t ernal Combuat iob.

Na.turally Aaplrated Four-Stroke Ena:ine.

Sparlt-Ignition Engine Dteull

I Naturally Alpirated TlIo-Stroke Indirect InjectionI lotny Spark-Ignition Engine Naturaly .bpirated Brayton SUrUna

Phton I

(Wanhl) (Turbocharged)

I I Audl 1700 I lI.oy I 1tIQt-81l ATAC I Suzuki Rabbit Gr-lOO Hod [

Audl/WV I Ricardo I .su .eAL I DEC VW ODA H"

Production I ",0 ... le.earch En I.. leeeareh En In. Production ... R.llurdt tn In•

55/5000 I 88/6000 121/6500 16/45001' 80/6000 37/5000 75/68000 54/4000

1.1(4 ) I 2.0(4) ].0(2) 0.372(1) 1.2(] ) 1.5(4 )

••• n .., 7.' .,. 22 4.S PR 17.5

Jl2 '" 360 3D' 300 31' 200 lo. 268/61 240/67 305/15 239/67 248/15 260/30 3200/<100 225/60

)2.0 .. 40.4 42.7 66.3

1.0 .,. 0.' 1.2(1.05) l.O. 3.54

1.0 .,. 0.32 1.4(1.0) (1.0 1.64

11 978 11 339 17925 10 498 .,. Nt•

ContlnllOUl Fuel In eetion Pulle Fuel In eetion

Currently in I Highly I Stratified I Realdual gaa Stratified dual Direct injection 2)~O·F turbine IS HPa

USl1l in ex peri-I turbulent I cOIllbusrion I controlled vortell cOIIIbus- illprovea 8St'C inlet telllperature Hydrogen

melle"l hybl"1d I cOlIIbustioni , tion and power output 2-ahatt regenerated (2117 pit)

vehicles I I Dual fuel I Fuel injection by IS% Low NOK cOIIIbustion

I I injection I Pneumatic f~l includina gearsa

I 1 I Unthrottled injection

I I D~I I operation

I 1 ignition I

I I I

I I .1

I IPredlctlon I

IlReiative to conventional lov-cOGlpression-r,tio. four-stroke, spark-ignition engine.

bRoury (Wankel) Engine.
The most successful approach taken to date is that of highly turbulent combustion brought about by high-velocity squish flow between the piston surface

and the cylinder head.

According to research completed at Volkswagen (Reference 2-1) optimum

fuel economy was obtained with a compression ratio of 13:1, representing an

improvement of 8 to 15% over the conventional low-compression engine. An

improvement of twice that order could be demonstrated for the part-load fuel

efficiency. The improvement of effective compression ratios at low engine

speeds that are strongly affected by valve overlap are primarily responsible.

Torque was increased up to 25% at full power and up to 13% at full torque,

compared with the low-compression engine. The leaning capability of the

engine also improves with increasing compression ratios.

An unresolved problem with implementing high-compression ratios and

squish techniques in production engines is that of flame erosion due to high

hot gas velocity flow and of piston-to-head clearance tolerances. The Volkswagen (VW) high-compression engine, for example, requires a piston-to-head

clearance of 0.6 mm (0.024 in.) under hot-operating conditions to obtain

optimum results with RON 92 gasoline without knocking. A disadvantage of a

high compression ratio is an increase in NO>< emission due to higher process

temperature, especially at part load. Research engines that use high compression ratios in the range of 12:5 to 15:1 and squish techniques to inhibit

knocking are GIlder development at Ricardo in England (Reference 2-2), at

Porsche and VW in Germany (References 2-1 and 2-3), and at Nissan in Japan.

Engines with a Ricardo May Fireball high compression cylinder head were

offered by Jaguar at a "reduced" compression ratio of 12.5 instead of 15 as

an option. Porsche claims that they will be ready to market their highcompression TOP-engine in 1984. Volkswagen is currently using a highly turbulent chamber in l.05-liter engines of their alpha-experimental series cars.

Despite some unresolved problems, it is not improbable that engines with a

canpression ratio of 13:1 and higher will penetrate the market on a broader

basis beginning in 1985.

2. Four-Stroke Rotary Piston Engines

The outstanding advantages of four-stroke rotary engines in smoothness, weight, and size have spurred further development for special applications in which the Wankel engine has been very competitive with other candidate piston engines. A typical field in which Wankel engines are still

believed to have a future, for example, is that of general aviation. The R&D

work carried on during recent years at Audi/NSU in Germany and at Curtiss

Wright in the United States is strongly aviation-oriented. The inherent disadvantages of the Wankel-type, four-stroke rotary engine are (1) a relatively

high heat-rejection rate, (2) high production cost, and (3) high HC emissions,

which have inhibited its widespread use in the automotive field.

State-of-the~art representat ives of advanced Wankel engine design that

has resulted from an approximately 20-year R&D effort are the research engines

NSU-KKM-87l and CW-RC-75, which, as their developers claim (Reference 2-4 and

2-3
2-5), are equivalent to currently designed fuel-injected piston engines in

fuel economy, emissions, and service life. The improvements have been achieved

primarily by the introduction of dual ignition and two-stage, continuous-flow

fuel ignition, stratified combustion, and by an improved seal design capable

of producing higher compression ratios (up to 9: 5) at a reduced low-speed

blow-by. Housing coolant flow has been improved, rotors are now oil-cooled,

and advanced metallurgical methods are used to treat housing interior surfaces

for increased wear resistance.

With these measures, however, the potential of the Wankel engine for

further thermodynamic improvements is essentially exhausted, which makes it

difficult for the Wankel engine to compete with its high-compression piston

engine contenders. According to Curtiss Wright (Reference 2-6), an attractive

potential still exists for the improvement of the speed capability of the

Wankel engine, which is already impressive when compared with piston engines

(Figure 2-1; also see Table 2-2 for identification of data source number in

Figure 2-1). An improvement of the rotary speed potential by a factor of two

or more is believed possible by using gas-balanced seals pressed against the

housing surface at low engine speeds' that will lift off the surface at high

speeds. The problem of an inherently high heat-rejection rate due to a high

surface-to-volume ratio could be resolved with ceramic coatings. However,

further development will be required, and results are uncertain at this time.

3. Two-Stroke Piston Engines

One of the features that drove the conventional carbureted fuel-oil

mixture scavenged two-stroke engine out of the vehicular field during the last

30

M

'0 20

15

6

4

2

70 __"-- "__ 12 "--"--'- LEGEND:

o CARBURETED TWO-STROKE

• FUEL-INJECTED TWO-STROKE

FUEL-INJECTED FOUR-STROKE

o CARBURETED ROTARY

• FUEL-INJECTED ROTARY

t::. INDIRECT-INJECTION AUTOMOTIVE DIESEL

o~O-.../'-~5!--~10:---;f;20:---4:':0:--6;':;0:-:8:'::071O~0:-:2:-:-:00

POWER OUTPUT PER CYLINDER, (ROTOR), kW

Figure 2-1. Comparison of Speed capabilities of Engines and Engine Concepts

under Consideration (see Legend Table 2-2).

2-4
Table 2-2. Identification of Data Points by Number

in Figure 2-1, 2-3, 2-4

No.

1

2

3

4

5

6

7

8

9

10

11

12

Researcher or Developer

Yamaha Uotor Company

Audi NSU

Audi NSU

Curtiss Wright Corporation

Audi

!lay Fireball

Porsche

Swiss Instutute of Technologya

Nippon Clean Air Lab

Fuji Heavy Industry

Volkswagen

Curtiss Wright Corporation

Location

Japan

West Germany

West Germany

United States

West GerIJany

Swi tzerland

West Germany

Swit zerland

Japan

Japan

West Germany

Un ited States

Engine Type

Racing

RC2-60, Production

K~~-B77, Research

2-75, Research

1.74-1, Production

Research

TOP (Thermodynamically Optimized

Porsche) R&D

Research

Research

Research

Research

Production

Rotary Research

with Lif t-off

Seals (Estimate)

a Eidgenoessische Technische Hochschule.

decade was an extremely high emission of carbon mo[)ox ide and unburned hydrocarbons (Figures 2-2 and 2-3) which produced sr:loke and an unpleasant odor.

The scavenging of large amounts of fuel into the exhaust (short-circuiting)

and the relatively large amounts of oil (1:20) mixed into fuel to satisfy

worst-case lubrication requirements is primarily responsible for these conditions in carbureted engines. As shown in Figure 2-4(a,b), these conditions

also resulted in the poor fuel economy of carbureted two-stroke engines compared to four-stroke engines. Fuel economy and hydrocarbons emissions are

closely related.

These deficiencies, which have been considered "inherent" for a long

time, have now been overcome as the result of research conducted in Japan,

Switzerland, and Australia. Better fuel efficiencies than those for indirect

injection diesels as well as emissions comparable to those for four-stroke

engines have been demonstrated with research fuel-injected, two-stroke engines

(see Figures 2-2 and 2-4).

According to publications and personal communications with Japanese

researchers (Reference 2-7), improvements demonstrated with Japanese research

engines have been achieved primarily by using cylinder fuel injection, carefully metering and minimizing the amounts of lubricating oil, and controlling

cylinder charge with the amounts of residual gases in the cylinder with an

2-5
10 20

(c) POWER OUTPUT

PER CYLINDER, kW

30

10 20

(b) POWER OUTPUT

PER CYLINDER, kW

o!:-------4IlLLI~~~t_---J

o 10 20

(0) POWER OUTPUT

PER CYLINDER, kW

.c .c .c

3: 3: 3:

200 20 Cl

",' ",' '"Z

Z Z 400 Q 0 Q '" '" '" '" '" '" w

w 100 w x 10 u 0 0

N :I: U 200 Z

I

aLEGEND:

o CARBURETED TWO-STROKE

• FUEL-INJECTED TWO-STROKE

'\1 CARBURETED FOUR-STROKE

... FUEL-INJECTED FOUR-STROKE

• FUEL-INJECTED TWO-STROKE

WITH THERMAL REACTOR

AREA 1 PRODUCTION TWO-STROKE ENGINES

2 PRODUCTION FOUR-STROKE ENGINES

3 ADVANCED RESEARCH TWO-STROKE ENGINES

Figure 2-2. Emission vs. Power Output per Cylinder
104

10

LEGEND:

E o CARBURETED TWO-STROKE

Q.

103 • FUEL-INJECTED TWO-STROKE Q.

o "UEL-INJECTED TWO-STROKE V>-

Z WITH THERtML REACTOR

0 '9' FUEL-INJECTED FOUR-STROKE

V>

V> ... FUEL-INJECTED FOUR-STROKE

:!: WITH CATALYTIC CONVERTER w

102 --AT OPTIMUM BRAKE-SPECIFIC u

J: FUEL CONSUMPTION

A T MAXIMUM POWER

100 101 102

CO EMISSION, %

Figure 2-3. Effects of Fuel Injection and Exhaust Aftertreatment on HC and

CO Emissions of Two- and Four-Stroke Engines for Optimum BrakeSpecific Fuel Consumption and Maximum Power (see Table 2-2 for

identification of data points by number)

exhaust throttle that permits reduction of the torque level over a wide speed

range without throttling the inlet. These improvements are largely responsible for the achievement of diesel-equivalent fuel efficiencies with the sparkignition engine. A research engine implementing this concept is the Active

Thermo Atmosphere Combustion (ATAC) engine (Reference 2-8) under development

at the Nippon Clean Air Laboratory in Japan. Figure 2-5 shows the performance

and emission maps produced with this engine, which are still unpublished at

this time.

A strong contender to the Japanese engine is a two-stroke engine under

development at the Orbital Engine Company (DEC) in Australia (Reference 2-9).

Figure 2-6 shows a front and side view comparing the DEC three-cylinder

research engine with a four-stroke engine of comparable performance. Figures

2-7 through 2-10 show fuel efficiency and emission dal:a generated with this

2-7
10

500r----,---,---,.----,---,

8

400

;E 300 ..c 300 3: ..lI.

'dl 11

U U· u.. "- VI 200 VI 200 '" '"

N

I

00

100

o~-----=':_--f:_---;'::----:o:--___:_~

o 20 40 60 80 100

(0) POWER CONCENTRATION FOR

MAXIMUM POWER, kW/l

LEGEND:

• FUEL-INJECTED ROTARY

6. INDIRECT-INJECTION AUTOMOTIVE DIESEL

o CARBURETED TWO-STROKE

• FUEL-INJECTED TWO-STROKE

A FUEL-INJECTED TWO-STROKE WITH THERMAL REACTOR

'Y FUEL-INJECTED FOUR-STROKE

100

O~-_::::_-____;"';:_--_;';:_--_;!;:::-____;~ o 20 40 60 80 100

(b) POWER CONCENTRATION FOR

OPTIMUM FUEL EFFICIENCY, kW/l

AREA A - CARBURETED SINGLE-STROKE

AREA B - FUEL-INJECTED TWO-STROKE

AREA C - FUEL-INJECTED FOUR-STROKE

Figure 2-4. Engine Brake-Specific Fuel Consumption vs. Power Output per Liter Displacement (see Table 2-2 for identification of data points by number)
engine that have also not yet been published. As can be seen from Figure 2-11,

the two-cycle engine's inherent instability at low speed is nonexistent,

and the cycle-to-cycle variation achieved is comparable to those of four

eight-stroke engines at lean conditions (>4). According to a presentation

given by OEC at JPL on August 8, 1983, the "secret" of this extraordinary

performance is a new dual-vortex stratified combustion process and a pneumatic

fuel-injection system, described in Reference 2-10, that ensures proper

metering and atomization of the fuel over a wide range of fuel flow rates and

engine speeds. These measures together permit the operation of the engine.

unthrottled, controlling load primarily with the fuel-to-air ratio. As is the

case with the Japanese ATAC engine, this improvement explains the achievement

of a specific fuel consumption comparable to or better than that of a prechamber, indirect-injection, four-stroke diesel engine.

Similar results were also obtained with a cylinder fuel-injected, twostroke engine in Switzerland (References 2-11 and 2-12). The Swiss engine uses

a production Motosacoche snowmobile engine modified for fuel injection and

equipped with a separate blower instead of the piston to scavenge the engine.

This modification makes it possible to lubricate the engine in a conventional

manner and to improve the emission quality of the two-stroke engine, which,

however, does not seem to be necessary. The lube oil consumption achievable

with a carefully metered two-stroke mixture lubrication system (Figure 2-12)

is already as good as or better than that of a conventional four-stroke lubrication system. A definite advantage of a blower-scavenged engine, however, is

its charge characteristics. As can be seen in Figure 2-13, because of the

square pressure-speed relationship of the blower, the blower-scavenged engine

produces an amazingly flat torque output over all its speed range. If variable

in speed, the blower could be used to control the engine by controlling the

scavenging ratio. On the other hand, the required blower, drive mechanism,

and required external ducting contribute to unwanted bulk and significantly

impair the inherent simplicity of the two-stroke engine.

The pacing problems with the cylinder fuel-injected, two-stroke engine

are that of high-speed fuel injection and system reliability. The pulse frequency of two-stroke fuel injection systems is twice as high as that needed

for four-stroke engines while the total fuel-injected per unit time is the

same. For research purposes Swiss researchers have used a conventional fourstroke, jerk-pump system running (as with four-stroke engines) at one-half

the crankshaft speed with two plungers working alternately on one cylinder.

The Japanese (see Reference 2-8) have modified a conventional Bosch fourstroke injection pump to work at two-stroke crankshaft speed, which is possible under laboratory conditions for a limited time. Efforts to solve the

high-speed, fuel-injection problem for two-stroke and for diesel engines are

underway (see subsection II-D).

The inherent physics of the two-stroke engine is sensi t ive to dynamic

interactions between the engine cylinder cavity and the external flow systems,

the inlet, and the exhaust. The possibility of scavenging a two-stroke engine

with resonance effects alone was demonstrated by Cadinnacy more than 50 years

ago. Since then the exhaust system of conventional carbureted two-stroke

engines have been primarily tuned to boost low end torque and to improve midrange fuel economy by plugging the exhaust port with the returning pressure

2-9
HC EMISSIONS, ppm

<? 0-

BREAK MEAN- BREAK MEANEFFECTIVE PRESSURE, kg/em2 EFFECTIVE PRESSURE, kg/em2

0 N W .... '" 0- " 0 N W .... '" 0- "

0 0

0>

0

0 m m Z N Z N G'l 0 G'l 0

m

Z Z

m

'" 0

'" ."m ."

m 0- m

m a

,0 W 8 0 .0 m

0 ." W

0 0 m m ."

Z m m Z Z Z 3 3 5· .... 5· :I: x x 0'" 0 .... .... 0 =I 0 0 0 0 N r- N m

CO EMISSIONS, %

BREAK MEANEFFECTIVE PRESSURE, kg/em2

-

/ -.....

"

\

\\

6

m

0 0

;", ."

m

Z

....

:I:

0'"

=l

rm

o

N

o

....

o

'"

o

m

Z

G'l

Z

m N

I ."'" .... m 0 m

,0

m

Z

3

5·

x

0 0

N

BRAKE-SPECIFIC FUEL

CONSUMPTION, g/kWh

Figure 2-5. Maps Generated from Active Thermal Atmospheric Combustion

Two-Stroke Research Engine (see Reference 2-7)
FRONT VIEW SIDE VIEW

LEGEND:

WHITE: 1.2-1 THREE-CYLINDER, TWO-STROKE ENGINE

BLACK: 1.6-1 FOUR-CYLINDER, FOUR"STROKE ENGINE

Figure 2-6. Size Comparison of TWo- and Four-Stroke, Fuel-Injected

Engines of Comparable Performance (see Reference 2-9)

.

Z

o

>=0-

V>

Z

o

u

-'

:::>

u

u:

U

0-

V>,

'"

'"

n

t..::J

1.2-1, TWO-STROKE

SPARK-IGNITION ENGINE

(ORBITAL ENGINE CO.)

1.6-1, FOUR-STROKE

SPARK-IGNITION ENGINE

(GENERAL MOTORS)

1.5-1, FOUR-STROKE

SPARK-IGNITION ENGINE

(FORD)

1.5-1, INDIRECT-INJECTION

DIESEL

(VO LKSWAGEN)

Figure 2-7. Comparison of 25% Partload Brake-Specific Fuel

Consumption at 2000 rev/min (Reference 2-9)

2-11
100

800

1\ \

600 I. \ \\

as-'" \\~

\

"......-

--

u' 400 • V>

u.. ~. _.----- '" 200

oL---l...._--l.__.J.-_--l._

a 25 50 75

LEGEND:

--CARBURATED BASELINE

TWO-STROKE

---FUEL-INJECTED TWO-STROKE

RESEARCH (ORBITAL ENGINE CO.)

---1.5-1, FOUR-STROKE (FORD)

----1.6-.P, (GM WORLD CAR CO.)

FOUR-STROKE

PART LOAD, %

Figure 2-8. Comparison of Engine Partload Brake-Speciic Fuel Consumptions

in Spark-Ignition Engines (see Reference 2-10)

'"

w 0

Oww z"" ..... ::::iO~ >-"'-,

u:;;z ",,,,, , ,- w

o""

NO He EMISSIONS X EMISSIONS

- ..... ... "i' '" :::>:::>-' v>, 0 ..... OO:J 0 '" v>

0 u.. u.. u..

1: - - 0 - , r- - 1: ---- ... , ... 0 ... , ... ": , ,

- - ": "l

r- - - rr- - -

Figure 2-9. Comparison of 25% Partload Brake-Specific Exhaust Emissions

in Spark-Ignition Engines at 2000 rev/min (see Reference 2-9)

2-12
800

0l......-_....c.:=:..=..:J==~_---l_-

o 25 50 75 100

LEGEND:

___ CARBURETED TWO-STROKE

--- FUEL-INJECTED TWO-STROKE

(ORBITAL ENGINE CO.)

200

400

600 -z

Q

V'l

V'l

::EuJ

U

J:

!:!:

UuJ

c..

Vl

I

uJ

;2

'"

PART LOAD, %

Figure 2-10. Comparison of Partload Brake-Specific HG Emissions at

2000 rev/min in Spark Ignition Engines (see Reference 2-10)

Figure 2-11. Combustion Pressure Trace of Fuel-Injected, Two-Stroke Research

Engine (Orbital Engine Gocpany, Australia) at 2000 rev/min and

67:1 air/fuel Ratio (see Reference 2-9)

2-13
OIL CONSUMPTION, % FUEL

o

o

•

lJ1 •

o .-lJ1

.

o

ADVANCED

TWO-STROKE

SPARK-IGNITION ENGINE

AT INDICATED

FUEL- TO-OIL MIXTURE RATIO

TYPICAL

FOUR-STROKE

SPARK-IGNITION ENGINE

ED

LACED

I I I I

OIL BURN

OIL REP

200 : 1

100 : 1

Figure 2-12. Comparison of Engine Oil Consucption Relative to Fuel Consucption

5,------,-----,----r----,---,------,

.B

• 4 w

::>'"

w

'" 3 w

>

;::

u

w

::; 2

Z

::;

WIDE OPEN e-e THROTTLE e e~ -e_.-

0.20

0.15

0.10

o~---l...-____;~--.l.--_;;_;';;:;;----l...-__:_:_=

o 1200 2400 3600

ENGINE SPEED, rev/min

Figure 2-13. Perforcance Hap of (Eidgenoessische Technische lIoschschule,

Switzerland) Fuel-Injected, Blower-Scavel~ed, Two-Stroke

Research Engine (see Reference 2-11)

2-14
,

'/

i

,

wave as necessary to reduce short-circuiting of the scavenging flow, which was

the primry cause of high HC emissions and power fuel efficiency. Broad-based

systematic research and experimentation addressing the problems of exhaust

tuning was conducted during the last decade at the University of Belfast

(Reference 2-13). The exhaust design of most two-stroke engines in existence

is based on the conclusions drawn from this research.

The tuning objectives for the cylinder fuel-injected, two-stroke engines

are different from those that apply to the carbureted engine because the need

to plug the exhaust port to stop short-circuiting of the scavenging flow does

not exist. The fuel-injected, two-stroke engine can be tuned for maximum

charge at any selected speed. The tuning rationale for an advanced two-stroke

engine using the ATAC combustion concept (see Reference 2-8) may be entirely

different because exhaust throttling is used to control the cylinder aircharge with the amounts of residual gases remaining in the cylinder. The

improvement potential of the ATAC-engine concept with exhaust tuning has not

yet been explored.

The inlet of conventional two-stroke engines is usually too short to

produce noticeable resonance effects. The Yamaha Motor Company (Reference

2-14) solves this problem, for example, by providing an extra "Ton-Raum"

chamber branched off from the inlet duct between the carburetor and the engine

inlet (Figure 2-14). The Swiss (ETR) engine uses a resonating scavenging tube

between adjacent cylinders (Figure 2-15) that boosts caximum power torque by

as much as 25%.

B. COMPRESSION-IGNITION ENGINES (DIESEL)

1. Indirect-Injection Diesel Engines

Aspects of the automotive diesel engine, covering the development

and market status through 1981, have been studied preViously (Reference 2-15)

at JPL. There have been no significant changes since that study that would

change the conclusions drawn from this report.

In terms of miles per gallon the indirect-injection automotive (prechamber) diesel engine is, as an average, 40% more fuel-efficient than a

gasoline engine of current design but only 20 to 25% if compared with base

petroleum fuel. In addition, the fuel efficiency margin of the diesel narrows

quickly·l£ compared with the contending spark-ignition engine developments.

Turbocharging, indirect-injection diesels have improved diesel driveabUity

approaching the driveabUity expected from gasoline-powered cars. Fuel

efficiency has also been slightly improved with turbocharging because of a

high torque output that permits the reduction of gearing, engine speed, and

friction at the same power output. With these measures, however, the improvement potential of the indirect-injecion diesel engine achievable with current

technology is almost exhausted. The uncertainty of emission legislation in

regard to particulates (which is still an unresolved problem), rising diesel

fuel prices, and the declining sales of·diesel cars have been a deterrent for

producers in making large investments in future R&D work. It can be expected

that no significant improvements of the indirect-injected diesel will be made

2-15
VOLUME

8 12 16

1.2

/0 w 0

Z

C> 1.1 Z 0 w

I >t

U

0

... 1.0 CONSTANT: D = 8 mm V>

0

... 00 100 200 300 400

;:: VOLUME OF RESONANCE CHAMBER, em3

:5

w

'" (b) w

...

9...

V> 1.2 V>

... 0-0-0

0 1.1 0'" ... 0" V> 0"" 0 ,/ 0

'" ,/J CONSTANT: L =200 mm 1.0

DIAMETER OF RESONAI"CE PIPE, mm

(e)

Figure 2-14. Boost Capability of Yamaha Energy Induction System at

1/4 Open Throttle and 2500 rev/min (a) System Schematic

(b) Effect of Chamber Volume (c) Effect of Line Diameter.

(see Reference 2-"14)

2-16
1

BOTTOM DEAD

CENTER

HOT-CO LD INTERFACE

SHOCKWAVE

.. INLET

1

TOP DEAD

CENTER

(0)

6...---r---.,--..,----r---r-----.

.

w

::>""

VI

VI

W

""

0-

W

>

i=

u

w

u..

u..

w

5

4

3

2

e-e, _e-e/

...-_..~--.--.....--.......' '

LEGEND:

-e-WITH TUBE

--e--WITHOUT TUBE

O'--_--L__--L__--,L,__--'-__.1-_--'

o 10 20 30 40 50 60

COMBUSTION CYCLES/,

(b)

Figure 2-15. Intercy1inder Resonating Scavenging Tube (ETH): (a) System

Schematic (b) Boost Capability (see Reference 2-11)

2-17
within the projected ten-year· time frame that will alter the conclusions

drawn at this time.

2. Direct-Injection Diesel Engines

A still untapped potential for the improvement of the small. highspeed diesel is that of direct-fuel injection. which would improve its power

output and fuel economy by approximately 15%. This will be necessary to

maintain the lead against gasoline engine contenders of advanced design. The

pacing problems are those of short-injection times. waterhammer line effects.

and diesel knock due to ignition delay. In a joint R&D effort between the

Bavarian Motor Works (BMW) and Austro-Steyer (Reference 2-16). the basic

feasibility of a direct-injection diesel for passenger cars was demonstrated

at BMW/Steyer with a design approach that provides for single-plunger injection pumps for each individual cylinder and for an accoustically attenuating

engine structure. Because of the uncertain future of diesel powered passenger

cars and technical difficulties encountered. this engine project was discontinued.

Efforts to improve the direct-injection diesel engine by turbocompounding and the introduction of ceramic materials are being done under u. S.

Government contract at Cummins. However. these efforts cannot be expected

to have a bearing on the design of passenger-car-sIze diesels within the

projected (ten-year) time frame.

All of the R&D work on automotive diesels is exclusively oriented toward

the four-stroke engine. Two-stroke. direct-injection diesels are successfully

used to power trucks. railroad locomotives. and ships. There is no noticeable

R&D work in progress to use the potential of the two-stroke diesel for automotive applications. As in the case of the small four-stroke diesel engines.

fuel injection is the pacing problem with two-stroke. direct-injection diesel

engines.

C. EXTERNAL COMBUSTION ENGINES

1. Brayton Engines (Gas Turbine)

The development of automotive gas turbines has made significant

progress during the last decade in regard to component efficiency. low NOx

combustion. and the development of a usable regenerator. The inherent problems inhibiting automotive application are those of an inherently poor

part-load fuel efficiency and the high operating temperatures needed to produce a competitive fuel efficiency over a wide speed and power range.

While part-load fuel efficiency can be improved to a certain extent with

variable geometry. high operating temperatures from 1300 to l400·C (2300 to

2500·F) are needed to achieve canpet1tive fuel efficiency (Figure 2-16.

Reference 2-17). The compatibility of the hot-path components with these

temperatures requires the introduction of ceramic materials. The strength

of structural ceramics at high temperatures has be'i'n significantly improved

during the last 10 years. but their brittleness and notch sensitivity still

2-18
20 40 60 80

NET POWER OUTPUT, kW

871°C (1600 of)

320

280

$

260 u'

....

'"

<D

240

180

0

0

Figure 2-16. Effect of Combustion Temperature on Brake-Specific Fuel Consumption (Using Diesel No. 2 Fuel) Achievable with 7S-kW (lOO-hp)

Regenerated Automotive Gas Turbine at Standard Atmospheric

Conditions (see Reference 2-17)

260 '---"'T""---,.---r--"'T""---r---,---r----,

240

180

SIZED FOR

37.3 kW (SO hpJ

NOM. POWER

SIZED FOR

74.4 kW (100 hpJ

oI:--....L.--:';,------'---+.----!----;l::---..l--,,,,!

o 20 40 60 80

NET POWER OUTPUT, kW

Figure 2-17. Effect of Engine Size and Power Output on Brake-Specific Fuel

Consumption Achievable (at ISoC. Sea Level) with Regenerated

Automotive Gas Turbine ad Standard Atmospheric Conditions

(see Reference 2-19)

2-19
represent a major design probletl, especially lolhen frequent temperature changes

are encountered. Differential thermal expansions of the materials make the

design of unavoidable metallic-ceramic interfaces and joints problematic.

A problem also for the automotive Brayton engine is size (Figure 2-17,

Reference 2-18). The efficiency of Brayton engine compressors and turbines

deteriorate significantly with diminishing size due to (1) Reynolds number

effects, (2) increased turbine and' compressor clearance losses, (3) poor

aerodynamic quality of the blading, and (4) increased leakage across the

regenerator hot-cold interfaces., Unfortunately, a higher operating temperature leads also to a smaller engine for a given power output. This 1n turn

offsets some of the thermodynamic efficiency gains achievable with a higher

operating temperature. /

/

State-of-the-art automotive gas turbines are the GT 100, under development at the Detroit Diesel Allison (DDA) Division of General Motors (Reference

2-19) and the GT 101 >see Reference 2-17), under development at the Garrett

AiResearch Corporation. According to specifications of the United States

Department of Energy~ both turbines are designed to develop a maximum power of

75 kW (100 hp) at the driveshaft, lolhich is considered the minimum power output

producible without intolerable, small-size penalties. Using a rotary heat

exchanger, bot';lengines are basically of the same thermodynamic concept, but

entirely different design approaches are taken to comply with the 100-hp / requirement./ With an all-concentric design and a single-shaft radial turbine,

AiResearch 1s aiming at an optimum achievable minimum overall size and

interior aerodynamic cleanliness and symmetry. With a conventionally angledoff combustion chamber feeding into a convolute and separate axial power

turbine, the DDA design is more conservative and compromising for the benefit

of development flexibility.

Despite optimistic projections that predicted the market penetration of

automotive Brayton engines to start in 1985, it will probably take 10 to 15

years for the automotive Brayton engine to mature into a production engine.

As in the case of the diesel engine, it also becomes difficult for the Brayton

engine to compete with the development of advanced versions of potential

candidate engines.

2. Stirling Engines

Theoretically, the Stirling engine is the most efficient heatengine concept. As shown in Figures 2-18 and 2-19, fuel efficiencies equivalent to and better than that of an indirect injection-diesel over a wide

operating range have already been demonstrated with a state-of-the-art

automotive Stirling engine jointly designed and developed by Mechanical

Technology, Inc, and United Stirling in Sweden (Reference 2-20).

Unfortunately, the Stirling concept has twO inherent problems that

inhibit its development into a competitive automotive engine. Changing the

power output requires a change of the mass of the working fluid that participates in the heat cycle. For an increase in power, more mass of working fluid

must be bled into the system from a high-pressure source. The other inherent

problem 1s that of sealing. To size a Stirling engine down into a package

2-20
1.2

1.0

w

=>

8

....

o 0.8

Z

8

0.6

i=

:s

0.4 w

=>

8 0.2

LEGEND:

---STIRLING

---DIESEL

~////// 8SFC <275 e/kWh

O.OL-....LL--_L--_..l..-_...l.-_...,J

0.0 0.2 0.4 0.6 0.8 1.0

SPEED RELATIVE TO NOMINAL SPEED

Figure 2-18. Compariaon of Torque Speed Relationships of the Stirling

and Indirect - Injection Automotive Diesel Engines (see

Reference 2-20)

LEGEND,

••• REPEAT DATAPOINTS

o ESTIMATED IDLE POINT

700

600

SOO

400

u·

300 ...

m

200

100

0

0 1000 2000 3000 4000

ENGINE SPEED, rev/min

Figure 2-19. Model I Stirling Engine Brake-Specific Fuel Consumption vs.

Engine Speed and Working Gas Pressure (see Reference 2-20)

2-21
that is acceptable for the designer of an automobile, hydrogen must be used as

a working fluid, and high operating pressures of about 15 MPa (2175 psi) must

be applied. The combination of both of these necessities imposes seal ing

problems that to date have not been satisfactorily resolved. As in the case

of the Brayton engine, it is uncertain when the Stirling engine will mature

into an automotive engine within the projected (la-year) time period.

3. Rankine Engines (Steam Engine)

The efficient use of the Rankine process requires deep expansion

into the vacuum generated by a condenser, which is impractical for a passenger-car engine. For piston eKpanders the inherent problem is that of dead

volume. To overcome this problem, the Carter automotive steam engine (Reference 2-21) uses high pressures of about 17 MPa (2500 psi) and a pistonactivated poppet valve in the cylinder head, which is a concept of questionable integrity if used at high engine speeds. A variety of eKpander concepts

have been conceived and tested but have not succeeded in producing sufficiently deep e><pansion without unwanted complexity and bulk.

To achieve fuel efficiencies that only appro~imate those of conventional

spark-ignition engines, the carter engine also applied superheating to temperatures up to 540·C (lOOO·F). These high temperatures impose cylinder lubrication problems that have not been resolved. Lear had the same problems with

his automotive steam-engine project. Because the automotive steam-Rankine

engine offers no potential for competition with current automotive designs,

government supported R&D was discontinued several years ago.

D. FUEL-INJECTION SYSTEMS

Four-stroke, spark-ignition engines of state-of-the-art design are

usually equipped with a single- or multi-source, continuous-flow, fuelinjection system. The fuel flow is primarily controlled by an air-flow meter

in conjunction with an oxygen probe positioned in the exhaust. Engines that

depend on a cyclic injection of fuel into the cylinder are (1) diesels; (2)

advanced, two-stroke, spark-ignition engines; and (3) programmed combustion

(Ford-PROCO), four-stroke, spark-ignition engines.

For small automotive engines, the pacing problems with cyclic cylinder

fuel injection are (1) the short time available for injection, (2) the small

amount of fuel to be injected per cycle, and (3) the achievement of a good

fuel atomization over a wide range of engine speeds and fuel flow rates. The

high-speed, fuel-injection problem becomes especially pronounced in two-stroke

engines because the number of pulses to be delivered per unit time is twice

as high, whereas the time available for injection and the amounts of fuel to

be injected per cycle are only one half that of a four-stroke engine.

The research with two-stroke engines in Japan and Switzerland (see

References 2-8 and 2-11), featured modified production, four-stroke, fuelinjection systems of the Bosch jerk-pump type. These were adequate for laboratory tests but were limited in the time engines could be operated at full power

and speed. Both of the indicated researchers agree that new solutions to the

two-stroke, fuel-injection problem must be found.

2-22
Reportedly (see Reference 2-12), the Federal Swiss Institute of Technology (ETH) is developing an electronically controlled cylinder, fuelinjection system applicable to diesels and spark-ignition engines that is

capable of producing the typical programmed injections at high speeds (Figure

2-20). The system is said to be capable of producing 100+ pulses per second

at pressures up to 600 atm. The system is an ETH .proprietary item that has

not been published.

The Australian researchers have taken an approach to the high-speed,

fuel-injection problems of their own (described in detail in Reference 2-10).

As shown in Figure 2-21, the concept uses a combination of compressed air and

plunger-displacement metering to inject an extemely rich fuel-air mixture into

the cylinder. This system has been successfully used in the OEC-research,

two-stroke engine described in subsection A-3. It produced an extremely wide

range of total air-to-fuel ratios so that partload could be controlled by

means of air-to-fuel variation alone without throttling the engine.

According to press publications such as WARDS Engine Update and Automotive News, several prominent producers of fuel-injection systems such as

Bosch and Lucas are making progress in the development of electronically

controlled, cylinder fuel-injection systems for high-speed diesels. It can,

therefore, be reasonably assumed that the fuel-injection problems of the twostroke engine can be resolved in the future.

E. FREE-PISTON ENGINES

Because force actions are primarily of an inertial, dynamic nature,

free-piston engines have a particularly attractive potential for diesels and

for alcohol-fueled, spark-ignition, combustion cycles. There is no crankshaft,

and high compression ratios can be produced without combustion-preosurerelated friction penalties. Figure 2-22 compares the predicted friction

losses and the power output of a resonating free-piston diesel engine against

data obtained with a conventional two-stroke crankshaft diesel. As can be

seen, the free-piston friction losses are nearly constant whereas the friction

losses of the crankshaft engine rise steeply with speed by a factor of more

than six. This translates into a power and fuel efficiency gain of up to 40%.

The problem is how to efficiently extract the power generated by a free-piston

engine.

Uniflow-scavenged, two-stroke, diesel free-piston engines of the opposed

piston type were first developed by Junkers in Germany and Pescara in France

between 1930 and 1950. Because of their compactness, narrow width, and smooth

vibration-free operation, diesel free-piston compressors were for many years

the preferred source of energy to power jackhammers on construction sites and

to pressurize torpedo systems on submarines. Pescara experimentally used two

or more free-piston diesel compressors in parallel as ·a gasifier for gas

turbines to power trucks and ships. The Pescara free-piston engine (Figure

2-23, Reference 2-22) had a power output of 90 kW at 1800 cycles per minute

and (including power turbine and gears) a weight of 278 kg!kW, which is light

for a diesel engine. However, including the turbine and reduction gears, the

Pescara power system was not competitive with diesels of conventional crank2-23
(a)

LEGEND:

1 - FUEL PUMP

2 - PRESSURE CONTROL VALVE

3 - ORIFICES

4 - SOLENOID VALVE

5 - DIFFERENTIAL PISTON

6 - PINTlE

7 - COMBUSTION CHAMBER

I!:!

3:

(b) 9

.u-

...!.

w

:J

UCRANK ANGLE

Figure 2-20. Swiss Institute of Technology Fuel-Injection System (a) System

Schematic (b) Typical Fuel-Injection Pattern Characteristics

Producible with the System (see Reference 2-12)

2-24
600 kPa

ENGINE CYCLE

INJECTION METERING

PHASE PH S

FUEl

TANK

AIR

FIlTER

ENGINE

COMPRESSOR

~FUEl

ID1I!GAS

,

.'_~~

INJECTION

lOW LOAD

METERING ROD (FIXED, EXCEPT DURING)

LOAD CHANGES)

INJECTIO

PHASE

J,

l:{l:::::l

.....

METERING

-

HIGH lOAD

(a) (b) (e)

Figure 2-21. Orbital Engine Company (Australia) Pneumatic Fuel-Injection System (a) Fundamental Metering

Principle (b) System Schematic (c) Relationship Metering Phase and Injection Pulse-to-Engine

Cycle (see Reference 2-10)
PRICTION

LOSSES

POWER

OUTPUT

o

w

w

....

'"

Q

""

o....

LEGEND:

--CRANKSHAFT ENGINE

---FREE PISTON ENGINE

oI,-....L.---Jl-----l---,L_--:.."J

o 25 50 75 100

NOMINAL ENGINE SPEED, %

Figure 2-22. Comparison of Predicted Power Output and Friction Losses of

Uniflow Scavenged, Free-Piston Diesel Engine with GM-type

Blower Scavenged Crankshaft Two-Stroke Diesel Engine

(see Reference 22)

BOUNCE

CHAMBER

COMPRESSOR

CYLINDER

AIR INLET

FUEL INJECTOR

GAS OUTLET POWER

CYLINDER

INLET PLENUM

CHAMBER

COMPRESSOR

CYLINDER

BOUNCE

CHAMBER

Figure 2-23. Schematic of Uniflow Scavenged Diesel Compressor/Gasifier

of Conventional Junker/Pescara Design (see Reference 2-22)

2-26
shaft design in weight, bulk, and fuel efficiency. The system exhibited an

optimum specific fuel consumption as high as 335 g/kWh because of the poor

partload efficiency of the turbine and reduction gear losses. The approach

using free-piston engines to generate shaft power was therefore abandoned.

The idea of integrating a free-piston engine with a linear alternator

is an old one, but it was never realized because of the limited speed potential of free-piston engines of conventional design. The free-piston engines

discussed above developed a maximum speed of approximately 1800 cycles per

minute. The minimum possible weight of the oscUlating masses, the dynamics

of the synchronizing mechanism, and the capability of the mechanical fuel

injection systems used were the limiting factors. Some research work with

turbocharged spark-ignited, gasoline-fueled, high-speed, free-piston engines

to generate electrical power on an aircraft was in progress in Germany during

World War II. These efforts were discontinued because of difficulties encountered with piston and alternator cooling and because of other war-related

priorities. However, to the best knowledge of the author, the feasibility

of such a concept was successfully demonstrated.

After three decades of dormancy, sporadic research and project work with

free-pIston engines has reappeared in various places in the belief that the

inherent problems of the past can now be overcome with advanced materials and

technology. According to recent studies (References 2-23 and 2-24) linearalternator, free-piston engine combinations have a potential for the generation of electrical power over a wide range (between 3 and 700 kW). A power

output of 75 kW at 5000 cycles per minute has recently been demonstrated with

a new spark-ignited, two-stage compression free-piston engine schematically

shown in Figure 2-24. The developer, Stelzer, believes that the concept has

the potential for oscUlating speeds up to 30,000 cycles per minute. Resonance Motors in Irvine, California (see Reference 2-25), is currently studying

the feasib ility of a 668-cc, free-piston engine capable of oscUlating at 42

to 47 Hz and developing a power output of 28.6 kW. The engine projected is

shown schematically in Figure 2-25. Resonance Motors does not reveal in their

report how the engine is scavenged and was not available for questioning.

The use of a freely oscUlating Stirling engine without a crankshaft in

conjunction with a linear alternator has been studied in concept by Mechanical

Technology, Inc., under government contract (Reference 2-26). It eliminates

the external hydrogen leakage problem with dynamic seals and results in a compact configuration that is more tolerable from the packaging standpoint than a

crankshaft Stirling engine. The results of· a recent project study in this direction are presented in Figure 2-26 and include the following specifications:

(1) Bore

(2) Stroke

(3 ) Dis plac ement

(4) Compression Ratio

(5) Engine Power Output

(6) Engine-Specifi~ Fuel Consumption with Diesel No.2 Fuel

(7) Alternator Power Output

(8) Unit Overall Volume

(9) Unit Weight

2-27

76.2 mm

66.7 mm

0.3 1

17:1

5.4 kW

310 g/kWh

4.5 kW

36.3 1

59 kg
N

I

N

(Xl

EXHAUST

EXHAUST

CARBURETOR

POWER

TAKE OFF

LEGEND:

.:.:.:.: COMPRESSED FUEl/AIR MIXTURE

- 000 UNCOMPRESSED FRESH MIXTURE

'000 PRECOMPRESSED MIXTURE ENTERING

000 COMBUSTION SPACE

•••••• COMBUSTION PRODUCTS

EXHAUST

COMBUSTION

SPACE

COMBUSTION

SPACE

Figure 2-24. Schematic of Stelzer Free-Piston, Spark-Ignition

Engine (References 2-23 and 2-24)
6

3

4

4

3

LEGEND:

1 - INLET VALVES

2 - PISTONS

3 - COMBUSTION SPACE

4 - OUTLET PORTS

5 - RETURN SPRINGS

6 - LINEAR ALTERNATOR

2-25. Schematic of Free-Piston Linear Alternator Unit by

Resonance Motors, Inc. (modified from Reference 2-25)

j.-------------62.5cm-------------.j

DISPLACER

GAS 8EARINGS

HEATER TU8ES

DISPLACER

E

u

'"

COOLER

POWER

PIS,ON/ALTERNATOR

PLUNGER

Figure 2-26. Sealed Stirling Free-Piston Power Unit

(see Reference 2-26)

2-29

SECTION III

POTENTIAL FOR HYBRID VEHICLE APPLICATION

Figure 3-1 generically compares the key properties of the engine concepts under consideration. These concepts are discussed in the following

suhsections.

A. BRAYTON ENGINE

Fuel efficiencies that theoretically can be obtained with a regenerated

Brayton engine (see Figures 2-16 and 3-1a) are very competitive, provided the

projected component efficiencies and operating temperatures can be achieved

with production hardware. As shown in Figure 3-lb there are no striking size

or packaging advantages of the Brayton engine over its piston-engine counterparts because of the bulkiness of the heat exchanger, associated ducts, drive

mechanisms, and the housings required for gearing down from the turbine to the

output power shaft.

Because of its multi-fuel capability, the Brayton engine would be the

ideal heat engine for a hybrid, but the gas turbine does not lend itself to

hybrid operations that require rapid stop-start capability. It takes a minimum

of 5 s to crank the compressor into the boot-strapping range and another 5 s

or more to accelerate the rotor, gears, and the heat eJ<changer mass to full

maximum speed. It also takes a certain time to achieve the hardware thermal

equilibrium necessary to obtain good fuel efficiencies typical of a regenerated Brayton engine. Special precautions to obtain an acceptable stop-start

capabiity could be taken but still remain undeveloped. The compressor inlet,

for example, could be shut off during cranking and opened instantly upon

reaching boot-strapping speed to reduce compressor and turbine work and to

prevent heat exchanger coolout. Low-inertia shutters that can be operated

quickly would probably be needed across the inlet and outlet plane of the

heat exchanger to prevent undesirable heat losses during engine shutdowns.

The Brayton engine would have a potential for battery charging if scaled

to the desired power output (20 to 30 kW). Because of small-size effects

this would not be possible without strong fuel-efficiency penalties (see

Figure 2-17). A direct power take-off from the turbine rotor in the form of

electrical energy would be the ideal way to charge the battery of a hybrid

vehicle. Assuming that a down-scaling at constant Mach-numbers without an

impairment of the engine thermodynamics is possible in the future, with

single-shaft Brayton engines of advanced design, for the power output of

interest (20 to 50 kW), the shaft speeds would be in the order of 100,000 to

165,000 rev/min.

Unfortunately, the low level of funding currently provided for the development of automotive gas turbines, as well as the dependence on ceramic

technology, makes it difficult to predict at what time an economical gas turbine might be commercially available. It seems probable that no mature Brayton

3-1
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OPTIMUMBRAKE-SPECIFIC SPECIFICOVERALLVOLUME,J_/kW SPECIFICWEIGHT,kgikW

FUELCONSUMPTION, kg/kWh
engine will be available within the 10-year time frame under consideration

for this study.

B. STIRLING ENGINE

Superior fuel efficiency, low NOx emissions, low noise level, and the

capabilty to adapt to a variety of alternative fuels are attractive features

for any application in which an inherent relatively slow response and bulkiness do not matter. Because packaging considerations are of primary importance in hybrid-vehicle design, the Stirling engine does not seem to be suited

for a hybrid powertrain concept where most (two-thirds) of the power must

still be generated by the heat engine. The necessity of using hydrogen and

relatively high operating pressures (>2000 psi) as well as the hazard of

hydrogen leakage are of great concern in the use of a Stirling engine within

the confines of a passenger car. In addition, it takes a minimum of 30 s

for a cold Stirling engine to develop full power, which precludes its use in

a rapid stop-start manner.

A more practical use of the Stirling concept is as a sealed free-piston

engine linear-alternator unit to charge batteries (see Figure 2-26). This

application would eliminate e<ternal leakage (the sealing problem with dynamic

seals) and result in a better configuration for packaging. Approaches in this

direction merit special attention although it is believed that they will have

no bearing on any hybrid development work that will take place within the

projected 10-year time frame.

C. DIESEL ENGINE

Compared with other candidate internal-combustion advanced engines

(Figure 3-1), the indirect-injection diesel is less efficient in all categories, including that of fuel efficiency. If the problem of high-speed direct

injection can be resolved, an improvement of about 15% over the indirectinjection engine would result. However, the diesel engine's inherently poor

start-up ability in cold weather (requiring preheating) and its high starting

torque and· motoring power requirements would still deter its use in s hybrid

vehicle. Idling would have to be permitted instead of a cutoff. Although the

idle fuel-flow rate of diesels is only 20 to 25% that of spark-ignition

engines, this characteristic reduces their actual fuel efficiency to some

degree in hybrid operations. The idling diesel is an attractive option when

viewed from the practical standpoint. However, hybrid accessory power and

heating are still unresolved problems with heat-engine, stop-start operations.

D. HIGH-COMPRESSION, FOUR-STROKE, SPARK-IGNITION PISTON ENGINE

High-compression, four-stroke, spark-ignition piston engines are most

likely to penetrate the market in the second half of the 1980s. The advanced

four-stroke engine will satisfy hybrid-vehicle requirements in fuel economy,

weight, and size (see Figure 3-1). A number of disadvantages and inherent

problems with high-compression-ratio engines, however, should be taken into

3-3
consideration when weighing the four-stroke concept against other engine

concepts.

Comparing an advanced engine having a compression ratio of 15:1 with the

conventional engine currently in use in hybrid test vehicles (8.5 :1), the

torque required to crank the engine increases approximately by a factor of 1.5

or more (Figure 3-2). The service life of high-compression engines is also

more affected by lubrication deficiencies, which are present in hybrid engines

that must operate under frequent stop-start cpnditions. At this time it is

still uncertain how frequent stop-start operations affect the wear pattern of

conventionally designed, four-stroke gasoline engines in the long run.

Although no evidence has been produced to date, Volkswagen (see Reference 1-1)

has been keeping a low profile with their stop-start R&D work because of the

overwhelming complex of open questions that must still be answered to satisfy

product reliability and service-life guarantee requirements. The concensus

among conservative engine designers is that the lub rication system must be

specifically designed and developed to cope reliably with frequent and prolonged stop-start operations under worst-case environmental and driving

conditions. For example, an electrically driven oil pump that allows the oil

system to be pressurized prior to engine turnover or keeps the oil system

pressurized all the time is believed necessary•

PRODUCTION

ENGINES

}

HIGH-COMPRESSION

RESEARCH ENGINES

LEGEND:

o DIESEL }

• SPARK-IGNITION ENGINE

!'>. MAY FIREBAll

... PORSCHE TOP

OPEN

CHAMBER

SQUISH /

FlOW~,

,

/ ~'

o

.----,---.....--.,....--0 2.0

""

w

:;:

o 1.5 0-

Cl

""o....

1.0 ~:.---:-l:---.....,J'---~--'

8 12 16 20

COMPRESSION RATIO

Figure 3-2. Estimate of Motoring Power Requirements Relative to Conventional

8.5:1 Compression-Ratio, Four-Stroke Engine

3-4
The No", emission must also be taken into account. High-compression,

four-stroke engines are higher in NOx than low-compression engines and depend

more on efficient catalytic converters to meet eQission requirements. Unfortunately, as discussed earlier and shown in Figure 1-2, the converter rarely

achieves its best possible efficiency under frequent stop-start operating

conditions.

Another factor to consider is that all recent four-stroke engine developments, including rotaries, use a continuous-flow, fuel-injection system like

the Bosch K-Jetronic (or similar) system. As discussed earlier (see Reference

1-2), tests with hybrid vehicles using a low-compression, four-stroke engine

equipped with a continuous fuel-flow injection system have exhibited undesirably high HC emission transients duririg engine start and CO-emission spikes

dur ing engine shutdown. The system' s inherent tendency to overshoot at a

sudden opening of the throttle produces a fuel-rich mixture. At engine

cutoff, the manifold vacuum is broken to affect fuel cutoff by opening of a

special (VW-introduced) vent valve. But with the engine still turning over

for a fe'" times before it comes to a full stop, the system remains partially

open, and small amounts of fuel dribble into the engine until the system

pressure has decayed below nozzle-opening pressure. This causes a lean tlixture

and high CO emission during every engine cutoff. The built-in, fuel-enrichment

features that provide for a timed injection of extra fuel to facilitate cold

start are also responsible for the high HC emission during engine start.

These deficiencies can be eliminated but will require special development. Instead of developing a stop-start, transient, emission-proof,

continuous-flow, fuel-injection system, it would be more straightforward to

use a pulse-flo", system that controls fuel· flo'" directly at the injection

nozzle with a solenoid valve. An example is the Bosch K-Jetronic system.

Based on the analytical studies conducted at General Electric (see Reference

1) a K-Jetronic-equipped, 1.7-1, four-stroke, spark-ignition production Audi

engin·e was chosen for use in experimental hybrid vehicles because it was

available and because the developer (Volks",agen) already had some stop-start

test experience with this engine. The aforementioned vent valve that breaks

the manifold vacuum at engine cutoff was added later to stop the fuel flow

faster at engine cutoff.

E. TWO-STROKE, SPARK-IGNITION PISTON ENGINE

For the near future, the advanced cylinder, fuel-injected, two-stroke,

spark-injection piston engine seems to be the most attractive engine concept

for use in smaLl automobiles and hybrid vehicles. It has fewer moving parts,

is mechanically simple, is most tolerant to poor fuel quality, and ranks

highest among all other concepts in service needs and reliability. Its fuel

efficiency (see Figures 2-7, 2-8, and 3-la) is competitive with highcanpression, four-stroke, spark-ignition engines and is considerably better

than that of current four-stroke engines, including the indirect-injection

diesel engine.

The relatively large amount of residual gases still present at the start

of each new cycle represents a built-in EGR that keeps NOx emissions low (see

3-5
Figure 3-la), despite the fact that cocrbustion temperatures are higher than

those encountered in four-stroke engines. The higher combustion and exhaust

temperatures of the two-stroke engines help to burn off HC emissions to an

extent that makes the use of a separate external thermal reactor almost

unnecessary (see Figures 2-3 and 3-la). The volume of a tuned two-stroke

exhaust system is large enough to accomplish an efficient burn-off of unburned

hydrocarbons. The two-stroke, low speed, and idling instability that has

been considered inherent in the two-stroke engine has been largely eliminated

(see Figure 2-11). The fuel-to-oil ratio of advanced two-stroke engines (see

(Figure 2-12) ranges from 100 to 200, and the HC emissions generated by oil

mixed into the airflow for lubrication (see Figure 2-10) is now extremely low.

The motoring requirements for two-stroke engines (Figure 3-3) are considerably lower than those for conventional four-stroke engines because there

are no moving valves, and the compression ratio is low (7.5:1). This advantage can translate into subtantial energy savings in drive cycles that require

frequent stop-start of the engine. Current hybrid drive-cycle analysis does

not take differences in cranking power requirements into consideration when

comparing engines. Another advantage of the two-stroke engine is that it

requires a minimal warm-up period, and full power can be drawn from the engine

upon demand. The striking weight and size advantages of the advanced twostroke engine over other candidate engines are also significant (see Figure

2-6 and 3-1).

All of the engines discussed are liquid cooled. Liquid-cooled engine

systems are usually slightly heavier but, as a system, they are more spaceefficient and more versatile from the packaging standpoint than forced-air2-1, 4-CYLINDER

FOUR-STROKE

1000 2000 3000

DUE TO BRAKING

.........

~/////FRICTION

"..,.

..................

____...:-::..-

--

-- ---BRAKING I1.2-~, 3-CYLINDER

TWO-STROKE

FRICTION

-------

200

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w

u..

u..

w

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00

ENGINE SPEED, rev/min

Figure 3-3. Comparison of Motoring Mean-Effective Pressures of Two- and FourStroke Spark-Ignition Engines (see Reference 2-9)

3-6
cooled engine systems. Liquid-cooled engines are also more (=10%) fuelefficient than air-cooled engines. Forced-air cooling, however, seems to be

the better choice from the reliability and cost standpoint, particularly

in case of an auxiliary heat engine for a primary electric car.

F. FOUR-STROKE, SPARK-IGNITION ROTARY ENGINE

Despite the fact that the fuel economy of the advanced rotary engine is

not competitive with advanced two- and four-stroke engines, the rotary engine

concept is still an attractive option for hybrid application because of its

speed, weight, and size advantages (see Table 2-1, and Figures 3-1 and 3-4).

However, if a single-rotor configuration must be chosen to meet hybrid-vehicle

power requirements, the packaging and weight advantages of the rotary engine

over its competitors are greatly reduced (Figure 3-5) because of larger

bearings and balancing weights.

As shown in Figure 3-6, the motoring requirements for rotary engines are

considerably lower (up to 50%) than those of four-stroke piston engines. Because it is effectively mixture-lubricated in critical wear areas, the fuelinjected rotary engine requires little, if any, warmup before full power can

be demanded. Another positive feature is its inherent potential for smoothness and speed (see Figure 2-1), which offers the possibility of an integral

single-shaft design with a high-speed alternator. A shorter service life

than piston engines may be acceptable for hybrid application because the

engine works only part of the total time the vehicle is driven. Advanced

rotary engines are expected to be more costly than other advanced candidate

engines. However, the cost factor may be acceptable in a hybrid vehicle if

the advantages it has to offer are of overriding importance.

G. FREE-PISTON ENGINE

The integration of a free-piston engine with a linear alternator is

potentially attractive for battery charging in hybrid vehicles because of its

compactness and smooth operation. Because of its capability of producing high

compression ratios with relatively low friction losses, the free-piston concept lends itself favorably to the implementation of diesel as well as sparkignited (Otto-like) combustion with alternative fuels that tolerate high

compression ratios, such as alcohols and ammonia. Squish techniques as applied

to high-compression gasoline crankshaft engines cannot be applied because the

piston stroke and the points of piston reversal are not fixed in a free-piston

engine. The free-piston diesel has a superior start capability because the

piston stroke is not limited, and a high compression ratio can be produced by

electrically exciting the oscillating masses into resonance. The idle flow

rate should also be considerably lower than that of crankshaft engines because

less friction must be overcome for minimum sustained operation at zero load.

Many of the inherent problems of the free-piston,

engine of conventional design that limit its potential to

shaft engines can be overcome with modern technology.

complex fuel-injection and timing requirements needed for

3-7

internal-combustion

compete with crankFor example, the

optimum free-piston
LEGEND:

BLACK - VB PISTON ENGINE

WHITE - ROTARY ENGINE

Figure 3-4. Size Comparison of Twin Rotor Wankel with v-a Piston Engine of

Equivalent Performance (Reference 3-1)

4 ROTORS

3 ROTORS

2 ROTORS

SINGLE ROTOR

I I

0.0 0.5 1.0

VOLUME- TO-DISPLACEMENT RATIO

RELATlVE TO TWO-ROTOR ENGINE

1.5

Figure 3-5. Effect of Number of Rotors on Overall Volume to Displacement

Ratio of Rotary Engine Relative to Two-Rotor Design

(Reference 3-2).

3-8
--- ROTARY ENGINE, TWO ROTORS

FOUR-CYLINDER}

__ SIX-CYLINDER . PISTON ENGINES

(a) O-BAR MEP (b) 4 BAR-MEP (c) a-BAR MEP

2.5 .B

• w

:>"'" w VI 2.0

I VI

'" W

"'"

Q.

W

> 1.5 ;::

u

w

u..

u..

w 1.0 .. Z

<l:

w "~ROTARY :E

Z 0.5

0

;::

u

"'"

u.. 0

1500 3500 5500 1500 3500 5500 1500 3500 5500

ENGINE SPEED, rev/min

Figure 3-6. Comparison of Rotary Engine Friction Mean-Effective Pressure with

Four-Stroke Piston Engine Data (Reference 3-3)
operation can be met today with electronic techniques to sense piston speed

and reversal points, and to ensure resonance at various load levels by controlling fuel-injection timing, fuel flow, and pressure in the bounce chamber.

The application of trip- and miss-control techniques is also possible.

Piston heating, which has been a problem with small, high-speed, freepiston engines, can be overcome with modern high-temperature metallic materials and, perhaps, with ceramics. The free-piston engine lends itself ideally

to the application of ceramic materials and gas lubrication because of its

simple configuration, its relatively low dynamic forces, and the absence of

lateral force action on the piston. For battery charging in hybrid passenger

cars (15-20 kW), a single-mass, turbocompounded, ceramic, free-piston diesel

or alcohol-fueled Otto engine with opposing cylinders (Figure 3-7) represents

an attractive concept from the fuel efficiency, weight, and packaging standpoint.

As previously mentioned, the free-piston, linear-alternator concept is

also an attractive option for the Stirling engine (see Figure 2-26). It would

eliminate the external sealing problem with high-pressure hydrogen through

dynamic seals and make the Stirling concept more acceptable for hybrid battery

charging as far as packaging and safety are concerned.

3-10
7. @'10%j 7

6 @S :m S~\"4'1 6

1'-1 ....

I 4 3 ...

... FP//M

LEGEND:

I - BLOWER

2 - INLET PLENUM

3 - INLET PORTS

4 - COMBUSTION SPACE

5 - PISTONS

6 - FUEL INJECTION NOZZLE

SPARK PLUG (ALCOHOL ENGINE)

7 - EXHAUST PORTS

8 - EXHAUST PLENUM

9 - TURBINE

10 - LINEAR ALTERNATOR, STARTER SOLENOID

11 - ALTERNATOR, BLOWER STARTER MOTOR

Figure 3-7. Schematic of TurbocOClpounded Free-Piston Power Unit Concept

SECTION IV

SUMMARY AND CONCLUSION

Based on the information presented and considering the self-imposed

rating criteria and restraints for the selection of hybrid-vehicle engines,

the study concludes as follows:

(1) Among the advanced engine concepts under consideration, the watercooled cylinder fuel-injected, crankcase-scavenged, two-stroke,

spark-ignition piston engine emerges as the best choice for future

hybrid vehicles of the parallel powertrain concept. Because of

its low-compression ratio and large internal turbulence this engine

is tolerant of poor quality (low-octane) fuels: The two-stroke

inherent lubrication physics prevent long-term wear damage due to

stop-start operations. Its specific fuel consumption and the power

required to crank the engine are lowest. The number of firings

encountered per unit cranking energy and the chance to fire up

during a given number of turns are highest. In summary, the fuelinjected, two-stroke engine is most cost- and energy-efficient,

and it satisfies all the other HV needs. A carbureted, air-cooled

version of an advanced two-stroke engine equipped with an up-todate oil-spray lubrication system may satisfy the requirements for

an aUXiliary engine for a primarily electrically driven vehicle,

depending upon area emission restraints.

(2) The spark-ignition, four-stroke engine of current low-compression

design is the best backup choice for a hybrid vehicle of the parallel powertrain concept. However, it should be equipped with a

fuel-injection system that is positively controlled at the point

of injection to reduce to a minimum transient emissions due to

dribbling after engine cutoff. It should also be equipped with an

electrical, externally powered oil pump that allows pressurization

of the lubrication system prior to engine start. This should

eliminate the concern about damages due to long-term wear effects

that potentially result from frequent stop-start operation.

(3) Rotary four-stroke, spark-ignition engines are not competitive in

regard to fuel economy, but their small-volume and high-speed

capability may be a decisive factor where the need for these

properties exists. The rotary engine's high-speed capability

permitting it to run directly coupled with a high-speed alternator,

as well as its low starting and motoring power requirements, should

translate into fuel savings that must be taken into account when

the rotary engine is compared with four-stroke engines in an

analysis of hybrid vehicle performance.

(4) The high-compression, four-stroke, spark-ignition engine is not

well-suited for hybrid operations requiring stop-start capahility

because of the inherent problems associated with high-compressionratio engines. These are mainly high starting torque and energy

4-1
losses due to high cranking power requirements, high NOX emissions

in combination with a low catalytic converter efficiency, and an

increased susceptibility to the lubrication deficiencies brought

about by stop-start operations.

(5) The indirect-injection automotive diesel is not competitive at all

for hybrid vehicle use. The direct-injection diesel could be an

alternative to the spark-ignition, two-stroke engine in applications in which its size and weight do not matter and when it is

allowed to idle instead of requiring an engine cutoff. An idling

Dr-diesel may have advantages over the two-stroke engine where

prolonged air conditioning and heating times are involved.

(6) A free-piston, direct-injection diesel or a spark-ignition engine

fueled with alcohol and coupled directly to a linear alternator

are attractive for future use in battery charging, from the standpoint of packaging, fuel efficiency, low vibration, and noise.

(7) Because of the inherent physical laws governing external combustion

engines such as the Brayton, Stirling, and steam engines, they are

not suited for use in hybrid-vehicle, stop-start operations at all

without special measures that remain to be developed. The large

number of still-omresolved basic development problems as well as

an omcertain funding situation in this area of research precludes

the application of external combustion engines for hybrid uses

within the considered (lO-year) time period.

4-2
SECTION V

RECOMMENDATIONS

This study recommends follow-on analysis and experimentation with candidate engine concepts and components that are necessary to support a final

selection of a heat-engine concept for future hybl:id vehicles. Heat engines

are usually not conceived and developed to meet the specific requirements of

hybrid-vehicle operation. Heat engines must be adapted to meet these requirements. Developers usually cannot provide all the data necessary to evaluate

a heat-engine concept in regard to its potential for hybrid use and to assess

the modifications necessal:y. Specific hybrid-related analysis and tests other

than vehicular tests alone are therefore needed to generate the data base

necessary for hybrid-engine selection and development planning. Listed in

order of priority are the following recommendations resulting from this

study.

(1) Conduct a comparative driving cycle analysis with most promising

advanced engine concepts, including an assessment of the transient

energy losses resulting from the motoring, inertial, and starting

characteristics of each individual concept known to date. Current

hybrid-vehicle models do not realistically account for transient

energy losses because of high-speed motoring, delayed starts, and

clutching during frequent and l:apid stop-start operations.

(2) Conduct evaluation bench tests with the high-speed, fuel-injection

systems discussed in this report to verify the claims made by their

developers, to understand how they function and to determine how to

assess their true potential for hybrid use.

(3) Simulate hybrid typical heat-engine opel:ations on the dynamometer.

The capability of determining tl:ansient powel: output, cranking

power input, fuel flow, and emissions must be prOVided. Listed in

ordel: of priority, the following engine concepts should be tested:

(a) A low-compression, four-stroke, spark-ignition engine (Audi

1.7) identical to those installed in the hyb rid vehicles

currently tested. This is necessal:y to better understand the

transient emission behavior of the vehicles tested and to

take measures to l:educe the transient emission of the vehicles using this engine to a pOssible minimum. A test program

of this nature that uses hardware and equipment on hand was

proposed in September 1982 (Reference 5-1).

(b) A two- or three-cylinder, two-stroke Marine engine developed

in the United States modified to meet the combustion and

fuel-injection requirements of the OEC research engine (see

Reference. 2-10) discussed in sub sect ion II-A3. The engine

should be equipped initially with a conventional dual plunger

Bosch-jerk pump, unless suggested otherwise as the result

of the bench tests described in V-(2). Two of the sources

•

5-1
contacted (see References 2-7 and 2-11) have offered to conduct a task of this nature at cost with their own research

hardware.

(c) A fuel-injected rotary engine has been recently marketed by

Mazda that develops 100 kW (130 hpJ. The power of this

engine is more than twice the power output projected for

hybrid engines, but data could· be reduced to single-rotor

configuration after the basic friction and pu",ping losses

of the two-rotor engine have been determined.

(d) A high-compression, spark-ignition, four-stroke engine. A 2-

1 engine with a 13:1 compression ratio may possibly be made

available from the discussed sources after their own tests

with this engine are completed.

(4) Conduct analysis as described in V-(l). Use the transient data

generated in tests recommended in V-(3).

(5) Conduct a project design study of an alcohol-fueled spark-ignited,

free-piston engine coupled to a linear alternator. Use of the

latest technology and innovative design concepts.

(6) Keep abreast with ongoing developments through literature and

personal communications. Considerable effort has been made in the

course of this study to locate and approach competent sources and

to encour.age their cooperation. It is important to maintain these

contacts. The evaluation and demonstration of foreign advanced

heat-engine technology represents a unique opportunity to gain

experience and to make contributions to the solution of pacing

problems for the benefit of the DOE Hybrid Vehicle Program as

well as for United States industry in general.

5-2
REFERENCES

1. Near-Term Hybrid Vehicle Program, Final Report, General Electric Report

SRD-79-l34l4, October 8, 1979.

2. Hardy, K. S., et al., Advanced Vehicle Subsystem Technology Assessment,

JPL Internal Document D-230, Jet Propulsion Laboratory, Pasadena,

Calif., September 1982.

3. Hybrid Vehicle Program, Test Report, Test Bed Mule, General Electric Report SRD-81-o91, November 20, 1981.

4. Hybrid Vehicle Program, Final Report, JPL Publication 9950-912, Jet Propulsion Laboratory, Pasadena, Calif., June 30, 1984.

5. Levin, R., et al., Hybrid Vehicle Assessment, JPL Publication 84-15, Jet

Propulsion Laboratory, Pasadena, Calif., (in press).

1-1. Personal communication with Volkswagen AG., Wolfsburg, Federal Republic

of Germany. April 1981.

1-2. Trummel, M. C" and Burke, A. F., "Development History of the

Test Vehicle," IEEE Transactions on Vehicular Technology, Vol.

No.1, February 1983.

Hybrid

VT-32 ,

1-3. Hybrid Vehicle Program, Interim Report, General Electric Report SRD-8l012, June 30, 1981.

2-1. Lee, Wenpo and Schaefer, H.T., "Verbrauchsreduzierung am Ottor:1otor durch

Optimieren von Brennaumform und Verdichtungsverhaeltnis," MTl Motortechnische Zeitschrift 43 (1982) 7/8.

2-2. May, M. G., Lower Specific Fuel Consumption with High Compression Lean

Burn Spark Ignited Four-Stroke Engines, SAE 790386

2-3. Gruden, D.O., and Hahn,R., "T.O.P. - The Thermodynamically Optimized

Porsche Engine," 5th Int'l Automotive Propulsion Systems Symposium,

April 14-18, 1980.

2-4. Basshuysen, R. V., and Wilmers, G., An Update of the Development of the

New Audi NSU Rotary Engine Generation, SAE 780418, February 27 to March

3, 1978.

2-5. Jones, C., An Update of Applicable Automotive Engine Rotary Stratified

Charge Development, SAE 820347, February 22-26, 1982.

2-6. Jones, C" A Review of Curtiss Wright Rotary Engine Developments with

Respect to General Aviation Potential, SAE 790621, April 3-6, 1979.

2-7. Personal Communication with Nippon Clean Air Laboratory, Ishikawa-Ken,

Japan, 1982-83.

6-1
2-8. Onishi, Shigero, et al., Active Thermo-Atmosphere Combustion

New Combustion Process for Internal Combustion Engines, SAE

February 26 to March 2, 1979.

(ATAC): A

79-501,

2-9. Personal Communication with Orbital Engine Co., Balacatta, Australia,

Visit and Subject Presentation of OEC at Jet Propulsion Labora tory,

Pasadena, Calif., August 1983.

2-10. McKay, M. L., Pneumatic Fuel Metering - A New Approach to Advanced

Fuel Control, SAE 820351, February 22-26, 1982.

2-11. Berchtold, M. and Eggard. C., "A New Two-Cycle Spark-ignited Engine,"

Coof. Rep. 800419, Vol. 2, Fifth Symposium on Autom. Prop. System,

Dearborn, Michigan, April 14, 1980.

2-12. Personal Communication with Prof. H. Berchtold, Eidgenoessische Technische Hochschule, Zuerich, Switzerland, at JPL, February 1983.

2-13. Blair, G. P., and Johnson, M. B., Simplified Design

Expansion Chambers for 2-Cycle Gasoline Engines, SAE

12-16, 1970.

Criteria for the

700123, January

2-14. Rata, Noriyuki, et al., Modification of 2-Stroke Engine Intake System

for Improvement of Fuel Consumption Performance through the Yamaha

Energy Induction System (YEIS), SAE 810923, 1981.

2-15. Schneider, H•. W., Conventional Engine Techology - Vol II: Status of

Diesel Engine Technology, JPL Publication 81-65, Jet Propulsion Labora

tory, Pasadena, Calif., 1981.

2-16. Personal Communication with Bavarian Motor Works, Munich, Federal

Republic of Germany, visit in April 1982.

2-17. "Garrett/Ford Advanced Gas Turbine Program Sunnnary," Garrett-AiResearch

Brochure MS636l.

2-18. Klann, T. L. and Johnsen, R. L., "Automotive Gas Turbine Downsizing

Study", NASA - LeRC Presentation, Contractor Coordination Meeting,

Dearborn, Michigan, October 26-29, 19B1.

2-19. Advanced Gas Turbine Powertrain System Development Project, General

Motors - Detroit Diesel Allison Progress Report, NASA Contract DEN3-

168, Dearborn, Michigan, October 26-29, 1983.

2-20. Dowdy, M. W., Automotive Stirling Engine Development Program, MTI Rep.

No. 8l-ASE 224 PR17.

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Powered Automobile, SAE 750071.

6-2
2-22. Taylor, Charles F., The Internal Combustion Engine, 2nd Edition, Vol.

7, p. 481, The MIT Press, Cambridge, Massachusetts, 1966.

2-23. "Der Motor Arbeitet Ohne Mechanik," VDI, Nachrichten Nr. 39/26, p. 4,

Sept ember 1980.

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12, 1981.

2-25. Internal Report, Resonance Motors, Inc., Irvine, California; communication with W. K. Hardy, JPL, February 5, 1980.·

2-26. Rosenguist, K., Lia, T., and Goldwater, B., The Stirling Engine for the

Automotive Application, SAE 790329, February 26 to March 2, 1979.

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Reibungsverhalten zwischen Kreiskolbenmotor und Hubkolbenmotoen von

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224. September 28, 1982.

6-3

APPENDIX A

CONTACTS MADE IN CONJUNCTION WITH THE HYBRID JEHICLE

HEAT-ENGINE EVALUATION TASK

Prof. Helmuth Hieke1

Fachhochschule Angsburg

D8900 Augsburg

Baumgartner Strasse 16

Fed. Republic of Germany

Mr Yasunari Hoshino

Sr. Research Engineer

Nissan Motor Co. Ltd.

Central Eng. Laboratory

1, Natsushima-cho

Yokosuka 237, Japan

Dr. Eberhardt Wesnes

Dr. Wenpo Lee

Mr. Paulus Heidemeier

Mr. R. Illersch

Prof. Heiland

Volkswagen A. G.

3180 Wolfsburg 1

Fed. Republic of Germany

Prof. Stuart A. Hoenig

The University of Arizona

Tucson, Arizona 85721

Dr. S. H. Jo

Nippon Clean Air Laboratory

CO 205-3 Kitaysane Machi

Kanizawa-shi Ishikawa-keu

Japan 920

Dr. Roy Choudehory

Dept. of Mech. Eng.

University of Southern California

Olin Hall 200

University Dr., LA 90007

Mr. Max Grainer

Robert Bosch Corp.

26877 N.W. Hwy

Suite 220

Southfield, MI 48034

Mr. Sadamachi Sasaki

President

Fuji Heavy Industries

7-2 l-Chome Nishi-Shinjuku

Shinjuku-ku Tokyo, Japan 160

A-I

Ilo-Werke GmbH

Postfach 1620

D-2080 Pinneberg

Fed. Republic of Germany

Fa. Fichtel Sachs A.G.

Postfach 1140

D-8720 Schweinfurt

Fed. Republic of Germany

Dr. W. Gross, Prog. Manager

Robert Bosch Corp.

26877 N.W. Hwy, Suite 220

Southfield, MI 48034

Mr. H. Po11ari

Outboard Marine Corp.

Seashore Dr.

Waukegan, IL

Mr. H. Brookhaus

Mgr. Tech. Devel.·

Audi NSU Auto Union AG

Postfach 220

8070 Ingolstadt

Fed. Republic of Germany

Mr. R. van Basshuysen

Audi NSU Auto Union AG

Postfach 1144

D-7107 Neckarsulm

Fed. Republic of Germany.

Mr. T.W. Mohr, Dir Research

OMC Research Center

4109 N. 27th St.

P.O. Box 663

Milwaukee, WI 53201

Dr. T. R. Sarich

Orbital Engine Compo Pty. Ltd.

4 Whipple St.

Balaccatta

Western Australia 6021

Fa-Croco Ltd.

CH-5043 Holziken-Heliport

Switzerland
Dr. Max Berchtold

Swiss Fed. Institute of Technology

ETH Centrum

Zuerich 8092, Switzerland

·U. S. GOYERNMtNT PRINTING OFFICE: 1985-544-063/10414

A-2

Mr. C. Johnes

Director of Research Rotary Combustion

Curtiss Wright Corp.

Passaic St.

Woodridge, NJ 07075



J

DOE/CS/54209-15 EVALUATION OF HEAT ENGINES FOR HYBRID VEHICLE APPLICATION DOE

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