Recent Analysis of UCAPs in M

Ahmad Pesaran

Jeff Gonder

National Renewable Energy Labo

Golden, Colorado

6th Advanced Automotive Battery Conf

Baltimore, Maryland

May 17-19, 2006 NREL/PR-540-39731
Disclaimer and Government 

This work has been authored by Midwest Research InstituContract No. DE-AC36-99GO10337 with the U.S. Departm“DOE”). The United States Government (the “Governmenpublisher, by accepting the work for publication, acknowleGovernment retains a non-exclusive, paid-up, irrevocablepublish or reproduce the published form of this work, or afor Government purposes.Neither MRI, the DOE, the Government, nor any other agof their employees, makes any warranty, express or impliliability or responsibility for the accuracy, completeness, oinformation, apparatus, product, or process disclosed, or use would not infringe any privately owned rights. Referespecific commercial product, process, or service by trade manufacturer, or otherwise does not constitute or imply its recommendation, or favoring by the Government or any aviews and opinions of the authors and/or presenters expr

necessarily state or reflect those of MRI, the DOE, the Goagency thereof. 
Outline• Background and objectives

• Review of ucap-mild HEV analysis • NREL’s results with mild HEV with simple s

• Ucap roles in hybrid vehicles 

• Energy requirements for various functions• What applications could make ucaps attrac

• Summary

• Proposed future analysis and testing
Background• Last year’s analysis by Burke (AABC-05) su

using a relatively small high-voltage, ultracamidsize car, without any engine downsizing

the city fuel economy improvement by:

– 60% over conventional vehicle, and – 20% over the same hybrid using NiMH batteries.

• This created interest in evaluating high-volt

ultracapacitors for mild/moderate hybrid ap

• Analysis and discussions were initiated am

stakeholders.

– We performed analysis using the assumptions in – A recently proposed “sawtooth” control strategy w

up to 72% (city) and up to 24% (highway) fuel ecoover the conventional vehicle the same engine siz

• The potential roles of ultracapacitors in mee

requirements of hybrid vehicles were revisit
Objectives• Review the fuel economy improvement tren

HEVs with respect to degree of hybridizatio

• Perform analysis to see the extent of fuel e

improvement possible with various strategiemild/moderate HEVs, with no engine downs

either batteries or ultracapacitors.

• Identify energy requirements of various driv

events/functions – what matches a limited u

• Discuss potential roles for high-voltage ultra

HEVs, if any.
BackgroundToday’s Commercial or Concept Hybrid Veh

with ucaps

Prius 

Civic HEV

Accord HEV

Escape HEV

Vue 42V HEV

GMC Sierra 42 HEV

BMW Concept X3

0%

20%40%60%80%

100%

0.00 0.10 0.20 0.30 0.40 

Degree of Hybridization 

[Ratio of Motor Power to (Motor+Engine) Po

 EPA City Fuel Economy Increase (%) 

Compared to Conventional Counterpart Fuel economy improvem

hybridization, but could bsuch as improved aerodyresistance, cylinder deacengine downsizing, use olower accessory loads, eThe HEV and conventionequivalent performance. 

Mild Hybrid Moderate Hybrid Fu
Ultra 

capacitor

Fuel 

Cell 

BackgroundOne Example of Ucap Use in a Mild Fuel Ce

Item Specification Note

Volume (L) 36.8Weight (kg) 34.3Max. voltage (V) 216Max. power (kW) 28Energy (Wh) 80 216-Æ 108VNumber 80

carbon-carbon with propylene carbonate electrolyte 

Sources: AABC-03 and 2004 SAE Congress

EPA city/highway FE 62/51 miles per kg of hydrogen 
• 6-cyl engine 190kW • Motor 30kW (peak power 60kW)• Max. 600 Nm torque, 400 Nm from electric motor• Start/Stop and regen functionality• Ucap capacity 53 Wh • Estimated acceleration 0-60 mph in 6.7 S • Estimated 20% fuel economy increase

BMW X3 Efficient Dynamics MilConcept

Deg. of Hybr. = 0.16
Configuration ESS Technology ESS kg

Unadj

(% mpg 

UDDS 

Conv. CVT

120 kW

N/A N/A 21.7 

HEV CVT

(120 + 15/27) kW

NiMH 

(previous 

generation)

20

32.9

(52%)

HEV CVT

(120 + 15/27) kW

Ultracap 

(advanced 

generation)

7

36.1

(66%)

* Andrew Burke – “The Present and Projected Performance and Cost of Double-LUltracapacitors for Hybrid Vehicle Applications,” AABC, Honolulu, Hawaii, June 

Conventional and Hybrid Vehicle Assumptions: Mass = ~1500 kg; Engine = 120 kW peak; Motor =15 kW continuous/27 kUltracapacitor = 7 kg, ~35 Wh “available”; previous gen. NiMH Battery = 

Control Strategy: Typical control parameters for full assist hybrids

Ucap-Mild HEV AnalysisResults of Analysis by Burke (AABC-05)*
0 1000 2000 3000 4000 5000 6000

0

50

100150200250300

18.1

22.1

26.1

30.1

30.1 32.1

32.1

33.1

33.1

Speed (rpm)

Torque (Nm)

max torque curveoutput shaftop. pts(includes inertia & accessories)

0 1000 20

0

50

100150200250300

26.1

30.1

Torque (Nm)

Conventional Vehicle - 21.7 mpg (city) Ultracapacitor Mild H

Avg Eff = 16% Av

0 200 400 

-40-20

0

204060

Power (kW) or Speed (mph)

0 200 400 600 800 1000 1200 1400

-10

0

102030405060

Cycle Time (s)

Power (kW) or Speed (mph)

Engine (kW )Speed (mph)

Engine Efficiency and Engine/Motor Power Based on our simulations similar to Burke’s 2005 a

A close
870 875 880 885 890 

-30-20-10

0

102030405060

Cycle Tim e (s)

Power (kW) or Speed (mph)

Closer Look at 2005 Analysis Showed RaOn/Off Oscillation for Ucap Hybrid

870 875 880 

0

0.5

1

1.5

2

2.5

3

Fuel rate (g/s)

870 875 880 

Power (kW)

and Speed (mph)

Cycle Time (S)

Fuel Rate (g/s)

Speed (mph)

The high fuel economies comes from many f

engine/fuel cycling off and on (5-10 Seco

Cycling off-on engines quickly may not be practical from operating, noise, durability, and drivability perspectives so stated high fuel economy predictions would not be achievable.
How Much Fuel Economy Is Possible wRelatively Aggressive Mild Hybrid Strategy Ass

Engine is OFF when vehicle has stopped, ON above elect

OFF during decel below electric decel speed. Regen ene

extensive fuel cycling during higher speed driving.

No fuel 

consumed 

during electric 

launch (up to 

~4.5 mph)

or idle stop

1260 1280 1300 1320 1340 1360 

05

10152025

Time (s)

Speed (mpha); Fuel Rate (g/s*10)

Vehicle Speed

Fuel Rate

Idealized Low-Speed Engine Off Assumption: no added drivelin
Configuration ESS ESS kg Unadju

UDDS U

Conv. 5-spd

120 kW

N/A N/A 26.0 2

HEV 5-spd

(120 + 15/27) kW

Advanced 

NiMH 20 31.8 3

HEV 5-spd

(120 + 15/27) kW

Current 

Li-Ion 20 32.1 3

HEV 5-spd

(120 + 15/27) kW

Ultracap 7 32.2 3

Strategy: Practical engine OFF/ON and Regen Capture. Componestudy except using 5 speed manual transmission: no downsizing o

Assumed that all runs are with hot engine, ignoring cold start loss

Fuel economy comparison with conventional• Gains: 23% on UDDS, 13% on US06, 3% on HWFET• More in line with hybridization benefits from commercial mild hyFuel economy comparison between hybrids• No appreciable difference between ucap and battery hybrids in th

NREL Simulation Results – Mild Hybr
Categorization of Mild Hybrid Fuel (no significant mpg difference between Li-Ion and Ucap hybri

530 540 550 560 570 580 05

1 0

U c a p H yb rid F ue l R a te C o m p a ris o n -- U S 0 6

C yc le tim e (s )

Fuel rate (g/s)

530 540 550 560 570 580 

C o nv F ue l U c a p H yb ri

Driving Condition 

Category

UDDS HWFET 

Ucap Li-ion Ucap Li-ion 

Stationary - Stop 47% 48% 8% 7% 

Decelerating 42% 44% 77% 82% 

Accelerating 10% 9% 15% 10% 

Accel savings from 

electric launch

Decel savings from low-speed 

idle-off & removal of mech 

accessories during idle

St

(Cycle mpg gain) (~23% over conv.) (~13% over conv.) 
Ucap & Battery Energy Use Comp(Drive-cycle characteristics dictate the amount of ene

0 200 400 600 800 100

-20-10

0

1 02 03 04 0

E S S E nergy Use C om parison -- UD D S

ESS Use Profile (Wh)

0 200 400 600 800 100

0 100 200 300 400 

-80-60-40-20

0

20406080

E S S E nergy Use C om parison -- US 06

C ycle tim e (s )

ESS Use Profile (Wh)

0 100 200 300 400 

40 Wh 33 Wh

41 108 Wh

Wh

C
Comments on Results with Simple MiStrategy and No Engine Downsizing

• Differences Between the Drive Cycles

• Larger number and duration of stops on UDDS 

Æ larger 

from idle-off

• Comparing Hybrids’ Performance

• Efficiency gains of Ucap hybrid is offset by restricted reg

SOC ceiling (limited energy)

• Mild hybrid control strategy with no engine downsizing

• Æ Small power flow through ESS relative to engine

• Æ Diluted fuel economy impacts of ESS changes

• Modeled batteries larger than needed – used <50% of av

for given control strategy and drive cycles. 

• Can this energy be used better if engine is downsized ?

• Fuel economy improvements (23% for city and 13% for U

high end due to idealized low-speed engine off assumptipenalty or added driveline friction)

• No significant difference in fuel economy between Ucap 
Comments on Recent Sawtooth ConStrategy for Mild HEVs*

• Fuel economy of baseline conventional vehicle seems too 

high mpg improvement (74% in city with no engine losses)

• Many engine/fuel on/off cycles (e.g. – engine alternately O

OFF for 30 seconds over ~1/2 hour of highway driving).

– Critical to properly model how this engine operation could be im– Drivability and emissions must be considered.

• If not de-clutching the engine, repeatedly blending torque O

driveline requires either idling the engine or accounting for torque/friction on the driveline when powering the wheels e

– Underestimating the engine idle or restart fuel requirement or d

in over prediction of the fuel savings.

• Such outside-the-box HEV control strategies (pairing nove

with frequent but shallow cycling of the ESS) could create a Ucaps to provide significant FE improvements.

– Requires practical treatment of drivability, noise, engine wear, 

– Will compete with advanced engine strategies not as reliant up

cylinder deactivation/variable displacement) that provide high e

requested, rather than alternately providing excess power at hiswitching the engine off.

* Burke (UCAP 2006)
Potential of Mild Hybrids with Fuel Economy 

Burke's Sawtooth

P

Civic HEV

Accord HEV

Escape HEV

Vue 42V HEV

GMC Sierra 42 HEV

BMW Concept X3

Burke's Mild HEV

NREL Simple Control

0%

20%40%60%80%

100%

0.00 0.10 0.20 0.30 0.40 

Degree of Hybridization 

[Ratio of Motor Power to (Motor+Engine) 

 EPA City Fuel Economy Increase (%) 

Compared to Conventional Counterpart Fuel economy improv

hybridization, but cousuch as improved aeresistance, cylinder dengine downsizing, ulower accessory loadThe HEV and convenequivalent performan

with ucaps 

??? Practical considerations Advanced controls 2005 2006
Rationale for Using Ultracapacitors wi

Taking advantage of ultracapacitor’s strengths (+) while minits weaknesses (-) if the COST was comparable to batteries

+ High specific power+ Efficient charge acceptance+ Low resistance+ Quick response (short time constant)+ Long anticipated calendar and cycle lifes+ High specific power at low temperatures (cold starts)

– Low specific energy 

Engine as

Regen 

Low cool

Suppor

Limited power dra

- Acceleration

- Engine assist, sp

- Running auxiliar

The best use for ucaps are strategies that make engine tmore efficiently (idle off, load leveling), and recapturing r
0 50 100 150 200 250 

UDDS

HWFET

US06

2% grade4% grade

1 kW accessory3 kW accessory

20 kW for 15 s

Prius

Accord

Vue

Event or Vehicle

Energy (Whr)

Available Estimate Total Est

Ucap is Energy Limited.How Much Energy Needed for Vario

20% of total energy 

in current NiMH 

packs

Energy to provide con

accessory load fo

75% of en

mph for 

g

50% of 

cyc

decele

Total Energy 

(at wheels) 

calculated for 

1520 kg 

vehicle

Energy for 15 s of c

20 kW electrical a

Cold-start capability is expected to dictate the size of bat
Summary - 1

• The high fuel economies reported last yea

with use of relatively small, high-voltage upack in mild hybrids are high and could nin practice.

• For Ucaps used with a simple mild hybrid 

strategy utilizing idle-off and regen capturengine downsizing) in a midsize vehicle, w

– The fuel economy improvement could be at best ~

driving.

– There is little fuel economy difference with a hybri

batteries.

– If cost is the same, and without taking advantage 

from Li-Ion, the superior life and low-temperature p

Ucaps make them more attractive.
Summary - 2

• Ucaps provide opportunity for application

and quick response needs.

• Many of the functions required from an en

device in mild hybrids could be met with lfor a midsize car.

• Practical engine management and contro

“key” in enabling Ucaps (or batteries) to a

economy improvements much higher thamild hybrid approaches.

• Some advanced engine and control strate

may match especially well with particular 

such as quick response, high power, and 
Future Work for High-Voltage U

• Need to go beyond “no downsized” engine m

applications.

• Evaluate different component sizes (engine, m

and control parameter values over a realistic the optimum vehicle design for a set of drive 

• Apply constraints to define what is “optimum” 

performance constant and maximize fuel ecompg/$-increment vehicle; maximize combined performance, cost and fuel economy; consideetc.).

• Study could also output relationship between 

energy and fuel economy improvement (given control parameters at each Wh size incremen

• Evaluate concepts by perform experiments us

but practical control and engine management 
AcknowledgmentsDOE and FreedomCAR Program Suppo

• Tien Duong• Dave Howell

USABC Technical Guidance

• Franco Leonardi • Cyrus Ashtiani• Harshad Tataria