SAE TECHNICAL
`PAPER SERIES
`
`
`
`920447
`
`Hybrid/Electric Vehicle Design Options and
`Evaluations
`
`A.F. Burke
`EG&G Idaho, Inc.
`
`Reprinted from: Electric and Hybrid Vehicle Technology
`(SP-915)
`
`The Engineering Society
`For Advancing Mobility
`Land Sea Air and Space®
`I N T E R N A T I O N A L
`
`lnternational Congress & Exposition
`Detroit, Michigan
`February 24-28, 1992
`
`400 COMMONWEALTH DRIVE, WARRENDALE, PA 15096-0001 U.S.A.
`
`Page 1 of 27
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`FORD EXHIBIT 1021
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`ISSN 0148-7191
`Copyright 1992 Society of Automotive Engineers, Inc.
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`FORD EXHIBIT 1021
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`920447
`
`Hybrid/Electric Vehicle Design Options and
`Evaluations
`A.F. Burke
` EG&G Idaho, Inc.
`
`ABSTRACT
`
`V a r i o u s a s p e c t s o f t h e d e s i g n a n d
`evaluation of hybrid/electric vehicles are
`considered
`e m p h a s i s o n
`the
`w i t h
`consequences of utilizing advanced electric
`d r i v e l i n e
`components
`such
`as
`AC
`motors/electronics
`a n d u l t r a c a p a c i t o r s .
`Special attention is given to series hybrid
`drivelines, because they benefit much more
`d i r e c t l y t h a n p a r a l l e l h y b r i d d r i v e l i n e s
`from the recent large improvements in the
`s p e c i f i c w e i g h t a n d v o l u m e o f e l e c t r i c
`d r i v e m o t o r s / e l e c t r o n i c s . T h e r e s u l t s o f
`t h e p r e s e n t s t u d y i n d i c a t e t h a t s e r i e s
`hybrid vehicles with an electric range of
`90-100 km and good acceleration performance
`(0-88 km/h acceleration times of less than
`12 seconds)
`can
`be
`designed with
`a
`powertrain weight and volume comparable to
`t h a t o f a p a r a l l e l h y b r i d o f t h e s a m e
`performance. The driveline efficiencies of
`t h e s e r i e s a n d p a r a l l e l d e s i g n s f o r b o t h
`c i t y a n d h i g h w a y d r i v i n g d i f f e r b y l e s s
`than 15 percentage ponts. The control of
`the series hybrid driveline is expected to
`b e s i g n i f i c a n t l y s i m p l e r t h a n t h a t o f t h e
`p a r a l l e l h y b r i d s y s t e m a n d i n a d d i t i o n ,
`m e e t i n g t h e C a l i f o r n i a U L E V e m i s s i o n
`standards should be less difficult for the
`series hybrid design, because the start of
`its engine can be delayed until the
`
`Work supported by the U.S. Department of
`Energy Assistant Secretary for Conservation
`and Renewable Energy (CE), under DOE Idaho
`Field Office, Contract DE-AC07-76ID01570.
`
`catalyst is warm without affecting vehicle
`d r i v e a b i l i t y .
`Simulation results for series hybrid
`v e h i c l e s o n t h e F U D S a n d t h e F e d e r a l
`H i g h w a y c y c l e s i n d i c a t e t h a t t h e i r f u e l
`economy (miles per gallon) operating in the
`hybrid mode will be 25-50% greater than
`conventional ICE vehicles of comparable
`i n t e r i o r s i z e .
`H y b r i d / e l e c t r i c v e h i c l e s
`u s i n g u l t r a c a p a c i t o r s t o l o a d l e v e l t h e
`e n g i n e i n t h e d r i v e l i n e s h o w e d e v e n a
`i m p r o v e m e n t i n f u e l
`g r e a t e r p o t e n t i a l
`Load leveled operation of the
`economy.
`e n g i n e m a y m a k e i t l e s s d i f f i c u l t t o u s e
`high specific power engines, such as two-
`stroke and gas-turbine engines, in light
`d u t y v e h i c l e s h a v i n g s t r i n g e n t e m i s s i o n
`control requirements.
`
`INTRODUCTION
`
`which
`vehicles,
`H y b r i d / e l e c t r i c
`utilize both an electric driveline and an
`engine to provide the power and energy for
`propulsion, have been studied for the last
`Hybrid propulsion systems are
`20 years.
`u s e d p r i m a r i l y t o o v e r c o m e t h e r a n g e
`l i m i t a t i o n o f p u r e e l e c t r i c v e h i c l e s
`powered by batteries alone. A number of
`hybrid vehicles have been built and tested
`to demonstrate the viability of various
`hybrid powertrain approaches. Much of the
`e n g i n e e r i n g a c t i v i t y o n h y b r i d v e h i c l e s
`occurred between 1978 and 1984 as part of
`the response of the United States to the
`oil crises of 1973 and 1979.
`In recent years,
`i n t e r e s t i n h y b r i d
`vehicles has been relatively low and most
`o f t h e w o r k o n v e h i c l e s u s i n g e l e c t r i c
`
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`drivelines has been concerned with pure
`e l e c t r i c d e s i g n s .
`Some work on hybrid
`vehicles was continued after 1984 and that
`w o r k w i l l b e i n c l u d e d i n t h e r e v i e w o f
`hybrid vehicles given in the next section.
`S i n c e 1 9 9 0 , t h e r e h a s b e e n i n c r e a s e d
`interest in hybrid vehicles due primarily
`to the California Initiative that requires
`2% of the vehicles sold in California to be
`electric vehicles by 1998.
`I n l i g h t o f
`this renewed interest in hybrid vehicles by
`the auto industry worldwide, this paper is
`i n t e n d e d t o
`hybrid
`v e h i c l e
`update
`development and design options reflecting
`t h e r e c e n t a d v a n c e s i n e l e c t r i c d r i v e l i n e
`component technology and the requirement
`for ultra-low-emission vehicles (ULEV) in
`C a l i f o r n i a .
`B o t h s e r i e s a n d p a r a l l e l
`h y b r i d d r i v e l i n e c o n f i g u r a t i o n s w i l l b e
`considered. The basic features of each are
`i d e n t i f i e d i n s u b s e q u e n t s e c t i o n s o f t h i s
`paper.
`
`REVIEW OF PAST/PRESENT HYBRID VEHICLE
`PROJECTS
`
`T h i s r e v i e w w i l l i n c l u d e b o t h p a s t
`a n d p r e s e n t p r o j e c t s a n d s e r v e a s a n
`introduction to the more detailed
`discussion of hybrid vehicle design options
`in
`later sections of
`the paper. A summary
`of hybrid
`vehicle projects
`is given
`in
`Table 1.
`
`PAST PROJECTS - The term
`"past
`p r o j e c t s "
`means
`v e h i c l e
`studies
`and
`f a b r i c a t i o n / t e s t a c t i v i t i e s t h a t h a v e b e e n
`c o m p l e t e d a n d a r e n o t p a r t o f o n g o i n g
`programs.
`The review will be concerned
`primarily with U.S. Department of Energy
`programs, as they are completely documented
`and can be easily referenced.
`Much of
`the
`DOE-supported work in the 1978-1984
`p e r i o d w a s d o n e b y t h e J e t P r o p u l s i o n
`Laboratory (JPL), JPL/ General Electric
`C o . , and the Aerospace Corporation. As
`presented
`in References 1-6,
`JPL and
`Aerospace performed detailed studies of
`various hybrid vehicle missions and design
`options,
`including
`in-depth
`computer
`simulations of vehicle operation on complex
`representations
`o f u r b a n a n d h i g h w a y
`cycles. The hybrid vehicle designs treated
`had acceleration performance comparable to
`diesel engine-powered conventional ICE
`vehicles.
`That
`performance
`was
`considerably better than that of the pure
`
`electric vehicles which were being designed
`The JPL/Aerospace studies
`a t t h a t t i m e .
`c o n c l u d e d t h a t t h e p a r a l l e l e l e c t r i c / h e a t -
`engine driveline approach yielded much
`lighter, smaller, and less expensive hybrid
`d r i v e l i n e s t h a n t h e s e r i e s u t i l i z i n g a
`heat-engine-driven generator and thus the
`parallel hybrid driveline was recommended
`for the relatively high performance hybrid
`It was recognized that
`vehicles studied.
`the parallel drivelines were more complex
`a n d m o r e d i f f i c u l t t o c o n t r o l t h a n t h e
`s e r i e s d r i v e l i n e s .
`Various approaches to the design of
`hybrid vehicles were also evaluated as part
`of the JPL/General Electric (GE) Near-Term
`H y b r i d V e h i c l e p r o g r a m c o n d u c t e d i n
`1978-1982. The
`results of
`those studies
`f a b r i c a t i o n , a n d t e s t
`and vehicle design,
`activities are given in References 7-12.
`The JPL/GE studies done in Phase I (prior
`to the vehicle design and fabrication phase
`o f t h e p r o g r a m ) i n d i c a t e d t h a t f o r t h e
`(0.04375 kW/kg)
`p o w e r - t o - w e i g h t r a t i o
`r e q u i r e d t o m e e t t h e a c c e l e r a t i o n t i m e
`g o a l s o f t h e p r o g r a m , t h e H y b r i d T e s t
`V e h i c l e ( H T V ) s h o u l d u t i l i z e a p a r a l l e l
`The
`d r i v e l i n e
`c o n f i g u r a t i o n .
`s t a t e - o f - t h e - a r t o f e l e c t r i c d r i v e l i n e s
`(motors and electronics) in 1978 precluded
`t h e p a c k a g i n g o f t h e 8 0 - 9 0 k W e l e c t r i c
`d r i v e l i n e r e q u i r e d b y a s e r i e s h y b r i d
`design in the space available in the HTV.
`In addition, the weight of the 90 kW series
`h y b r i d d r i v e l i n e w o u l d h a v e b e e n m u c h
`g r e a t e r t h a n t h a t o f t h e p a r a l l e l h y b r i d
`driveline that utilized a 33 kW DC motor
`and a 55 kW, 4-cylinder gasoline engine.
`A sketch of the HTV hybrid driveline
`is shown in Figure 1 (taken from Reference
`8 ) . As discussed in References (8 and 9),
`the HTV was built and dynamometer-tested on
`t h e F U D S a n d F e d e r a l H i g h w a y c y c l e s
`demonstrating electric only, engine only,
`and load-shared combined operation under
`JPL
`The
`micro-processor
`c o n t r o l .
`dynamometer test results (Reference 9)
`showed that the HTV had a high potential
`for large petroleum savings f o r r e a l i s t i c
`user missions
`and
`had
`acceleration
`c o m p a r a b l e t o c o n v e n t i o n a l
`performance
`diesel-powered ICE vehicles. The emissions
`test data (Reference 9) indicate that the
`HTV could have been engineered to meet the
`1980 emission standards, but likely not the
`u l t r a l o w e m i s s i o n s s t a n d a r d s o f t h e l a t e
`1990s in California.
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`As discussed in References 13-15,
`t h e r e h a v e a l s o b e e n s t u d i e s d i r e c t e d
`toward the design and fabrication of series
`Such vehicles
`h y b r i d / e l e c t r i c v e h i c l e s .
`had the relatively low performance typical
`of the pure electric vehicles of 1975-1985
`and an engine-generator sized to provide
`the average power (5-10 kW) needed for the
`Those designs yield
`FUDS or C cycles.
`relatively small range extension at freeway
`and highway speeds that require much higher
`power (see Figure 2). The weight and size
`of those series hybrid drivelines confirmed
`t h e e s t i m a t e s o f t h e J P L / A e r o s p a c e / G E
`studies cited previously.
`
`- There are
`PRESENT PROJECTS
`of
`active
`presently
`a
`number
`the world,
`hybrid/electric projects around
`including both parallel
`(References 16-19)
`and
`series
`(References 20-23) designs.
`Some of
`these projects
`(see Table 1)
`utilize
`state-of-the-art
`electric
`driveline
`components,
`engines,
`and microprocessor
`controllers. The
`presently
`active
`hybrid
`projects
`represent
`the base
`from which
`future projects utilizing advanced electric
`driveline
`and
`engine
`technologies will
`evolve. Such design options
`for hybrid
`vehicles are discussed
`in
`later sections of
`this paper.
`
`USER-PATTERN CONSIDERATIONS
`
`T h e k e y u s e r - p a t t e r n i n f o r m a t i o n
`required to design a hybrid vehicle is the
`statistics of daily usage (fraction of days
`f o r w h i c h t h e t o t a l d a i l y t r a v e l i s l e s s
`than selected values), as that permits the
`s p e c i f i c a t i o n o f t h e a l l - e l e c t r i c r a n g e o f
`t h e v e h i c l e o n a r a t i o n a l b a s i s .
`The
`energy (kWh) required by the vehicle to
`travel this distance is an important factor
`in sizing the battery. The second factor
`in sizing the battery is the maximum power
`( k W ) r e q u i r e d f r o m t h e e l e c t r i c d r i v e l i n e
`during
`vehicle
`acceleration.
`If
`the
`e l e c t r i c r a n g e
`s p e c i f i e d f o r a h y b r i d
`v e h i c l e i s s i g n i f i c a n t l y l e s s t h a n t h a t
`u s e d f o r a p u r e e l e c t r i c v e h i c l e , i t i s
`l i k e l y t h a t t h e b a t t e r y i n t h e h y b r i d
`vehicle will be sized by peak power, not
`energy storage, requirements. This is even
`more likely to be the case in the future
`than
`it was
`in
`the past, because
`the
`acceleration
`time
`requirements
`for
`
`electric/hybrid vehicles are becoming more
`s t r i n g e n t .
`The results of a study of the impact
`o f u s e - p a t t e r n o n t h e d e s i g n o f e l e c t r i c
`and hybrid vehicles are given in Reference
`24.
`The daily vehicle use was analyzed
`using a Monte Carlo random-trip generator
`model for various percentiles of car owners
`Calculated
`b a s e d o n a n n u a l m i l e a g e .
`t r a v e l
`cumulative
`p r o b a b i l i t y
`d a i l y
`statistics for percent of days and percent
`of vehicle miles on electricity are given
`in Figure 3. For example, note from the
`figure
`that
`for
`the 50th-percentile owner,
`if the useable electric range of a vehicle
`is 64 km (40 miles), the vehicle would be
`used as a pure electric vehicle on 90% of
`the days representing about 90% of the
`miles traveled per year; further, a 96 km
`(60 mile) range on electricity would permit
`t h e 9 0 t h - p e r c e n t i l e o w n e r t o o p e r a t e t h e
`v e h i c l e o n e l e c t r i c i t y a l o n e f o r 8 0 % o f
`the days and 80% of the total miles per
`F o r a h y b r i d v e h i c l e , i t s e e m s
`year.
`reasonable to define useable electric range
`a s t h e d i s t a n c e t h e v e h i c l e c a n t r a v e l
`primarily on electricity before the battery
`reaches 80% depth-of-discharge (DOD).
`Prior to 80% DOD, the engine would not be
`needed to recharge the battery.
`
`HYBRID DRIVELINE CONFIGURATION OPTIONS
`
`There are three basically different
`h y b r i d d r i v e l i n e o p t i o n s : ( 1 ) t h e s e r i e s
`hybrid (Figure 4) in which all the torque
`t o t h e w h e e l s i s f r o m t h e e l e c t r i c m o t o r
`a n d t h e e n g i n e p o w e r s a g e n e r a t o r f o r
`r e c h a r g i n g t h e b a t t e r i e s a n d s u p p l y i n g
`electrical energy after the batteries are
`d i s c h a r g e d t o a s p e c i f i e d l e v e l , ( 2 ) t h e
`parallel hybrid (Figure 5) in which both
`the electric motor and the engine provide
`torque to the wheels either separately or
`together and the motor can be used as a
`generator to recharge the batteries when
`the engine can produce more power than is
`needed to propel the vehicle, (3) the split
`hybrid (Figure 6) in which the front wheels
`(or rear wheels) are driven by an electric
`driveline and the other wheels are driven
`by torque from the engine. This paper is
`concerned primarily with the series and
`configurations
`a l t h o u g h , a s
`p a r a l l e l
`indicated in Table 1, split hybrid designs
`have been built and operated successfully.
`The split hybrid can be considered a
`
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`s p e c i a l c a s e o f t h e p a r a l l e l h y b r i d i n
`which the electric and engine drivelines
`are completely separate.
`
`SERIES HYBRID - In simplest terms, a
`s e r i e s h y b r i d v e h i c l e i s a n e l e c t r i c
`vehicle with an engine/generator to supply
`e l e c t r i c a l
`energy w h e n t h e v e h i c l e ' s
`battery is sufficiently discharged that it
`can no longer provide the energy and power
`t o p r o p e l t h e v e h i c l e .
`T h e p r i m a r y
`f u n c t i o n o f t h e e n g i n e / g e n e r a t o r i s t o
`e x t e n d t h e r a n g e o f t h e e l e c t r i c v e h i c l e
`beyond that possible on batteries alone.
`Many
`s e r i e s h y b r i d d e s i g n s t o d a t e
`(Reference 25) were intended to have a
`s i g n i f i c a n t
`range
`extension
`only
`in
`relatively low speed city driving and thus
`used
`engine/generator
`u n i t s ( g e n s e t s )
`having low power (5-10 kW). The gensets
`used in those vehicles were large and heavy
`(70 -150 kg) and higher power units could
`not be packaged in the space available in
`the vehicles.
`The series hybrid vehicles considered
`this
`paper
`are
`in
`i n t e n d e d t o b e
`m u l t i - p u r p o s e v e h i c l e s w i t h a c c e l e r a t i o n
`performance and city and highway ranges
`comparable to conventional ICE vehicles
`whose range is limited only by the size of
`the fuel tank. Specification of the output
`p o w e r o f t h e e n g i n e / g e n e r a t o r u n i t s i n
`multipurpose hybrids depends on the desired
`maximum vehicle cruising speed at which the
`g e n s e t i s t o s u p p l y e l e c t r i c a l e n e r g y
`c o n t i n u o u s l y t o t h e e l e c t r i c d r i v e l i n e .
`The range at speeds less than the maximum
`cruising speed then depends only on the
`size of the fuel tank as in conventional
`ICE vehicles. As will be discussed later,
`recent improvements in electric motor and
`power electronics technologies have greatly
`reduced
`the weight
`and
`size
`of
`the
`e l e c t r i c a l
`c o m p o n e n t s i n
`e l e c t r i c
`drivelines, permitting the peak power from
`t h e m o t o r a n d t h e g e n s e t t o b e g r e a t l y
`increased compared to the earlier designs.
`T h e a c c e l e r a t i o n
`and
`t o p
`s p e e d o f
`state-of-the-art series hybrids will now be
`limited primarily by the peak power density
`of the battery and the size and weight of
`the engine used to power the generator.
`Series hybrids comparable in performance
`and range to parallel hybrid designs are
`now possible.
`This was not the case ten
`y e a r s a g o w h e n p r a c t i c a l d e s i g n s o f
`multipurpose,
`h i g h p e r f o r m a n c e h y b r i d
`
`v e h i c l e s r e q u i r e d t h e u s e o f a p a r a l l e l
`driveline configuration.
`The key considerations in designing a
`hybrid
`driveline
`are
`the
`series
`specification of the maximum motor torque
`and power, the maximum generator power, and
`the useable electric range.
`For a given
`battery technology, the battery is sized by
`either the peak power required to meet the
`motor output power or the energy required
`t o m e e t t h e v e h i c l e r a n g e s p e c i f i c a t i o n .
`For a hybrid with a useable range of about
`80 km and a 0 to 96 km/h acceleration time
`of 15 sec or less, it is likely the battery
`will be sized by the power requirement for
`I n f a c t , i t i s p o s s i b l e t h a t
`acceleration.
`the acceleration performance of the vehicle
`will be limited by the space available for
`the battery and not by the weight/volume of
`the motor/electronics that can be packaged
`in the vehicle.
`packaging
`m a j o r
`A
`second
`consideration will be the size of the en-
`g i n e / g e n e r a t o r u n i t t h a t c a n b e u t i l i z e d
`w i t h o u t s i g n i f i c a n t l y c o m p r o m i s i n g t h e
`u t i l i t y o f t h e v e h i c l e .
`I n o r d e r t o
`achieve good gradeability and a top speed
`of at least 100 km/h, a generator output of
`20-30 kW (see Figure 2) is required. In
`all likelihood, this requirement precludes
`t h e u s e o f 4 - s t r o k e g a s o l i n e o r d i e s e l
`engines, w h i c h a r e r e l a t i v e l y l a r g e a n d
`heavy, and will instead require the use of
`engines with higher specific power, such as
`rotary engines, 2-stroke engines, and small
`Exhaust emission
`gas-turbine engines.
`control, especially HC and CO emissions, is
`n o t l i k e l y t o b e a p r o b l e m i n a s e r i e s
`a
`hybrid
`for
`any
`of
`the
`engines
`if
`preheated catalyst is used. In the series
`hybrid, the electrical energy to heat the
`catalyst is readily available from the main
`battery and the activation of the engine
`can be delayed for a short time while the
`catalyst is being heated without effecting
`the driveability of the vehicle.
`e f f i c i e n c y i s
`Engine/generator
`i m p o r t a n t , b u t n o t t h e c r i t i c a l f a c t o r , i n
`the series hybrid application, because the
`v e h i c l e w i l l b e o p e r a t e d o n t h e b a t t e r y
`using wall-plug electricity for 80-90% of
`T h e c r i t i c a l
`v e h i c l e
`m i l e s .
`the
`e n g i n e / g e n e r a t o r c h a r a c t e r i s t i c s w i l l b e
`s p e c i f i c p o w e r ( k W / l i t e r a n d k W / k g ) a n d
`emissions after appropriate post-treatment
`of the exhaust.
`The control strategy for
`o p e r a t i n g t h e e n g i n e / g e n e r a t o r i n t h e
`
`Page 6 of 27
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`

`series hybrid is likely to be rather simple
`with the engine being turned on when the
`b a t t e r y s t a t e - o f - c h a r g e d e c r e a s e s t o a
`The
`specified value (for example, 20%).
`engine might be turned off when the battery
`increased
`to
`a
`state-of-charge
`has
`specified value (for example, 30%). Using
`t h i s c o n t r o l s t r a t e g y , t h e f r a c t i o n o f t h e
`time the engine is on would depend on the
`r a t i o o f t h e a v e r a g e p o w e r r e q u i r e d t o
`propel the vehicle and the power output of
`the engine/generator. At maximum cruising
`t h e e n g i n e w o u l d b e o p e r a t e d
`speed,
`continuously.
`T h e p r e v i o u s b r i e f d i s c u s s i o n o f
`series-hybrid-vehicle design considerations
`indicates that if the components needed to
`meet the vehicle performance specifications
`can be packaged in the space available in
`t h e v e h i c l e , t h e o p e r a t i o n o f t h e s e r i e s
`hybrid is simple, and the system is not
`difficult
`to
`control.
`In
`addition,
`emission
`control
`for
`the
`series hybrid
`should
`not
`present
`serious
`difficulty,
`because the engine can be activated as
`d e s i r e d b y
`the
`system
`c o n t r o l l e r
`t h e
`i n d e p e n d e n t o f
`d r i v e r ' s
`power
`requirements.
`T h e c r i t i c a l
`i s s u e o f
`component sizing and driveline packaging
`will be considered in a later section.
`
`PARALLEL HYBRIDS - A schematic of a
`p a r a l l e l h y b r i d d r i v e l i n e w a s s h o w n i n
`Figure 5.
`In designing a parallel hybrid
`driveline, it is not likely one would start
`with an electric driveline and modify it as
`i n t h e c a s e o f t h e s e r i e s h y b r i d .
`The
`p a r a l l e l h y b r i d i n v o l v e s l o a d s h a r i n g
`between the electric and engine drivelines,
`even when
`the
`battery
`is
`at
`a
`high
`state-of-charge, with the combined maximum
`p o w e r f r o m t h e e l e c t r i c a n d e n g i n e
`drivelines being effectively equal to that
`o f t h e e l e c t r i c d r i v e l i n e i n t h e s e r i e s
`hybrid.
`In addition, there is a need for
`variable ratio gearing between the engine
`and the main driveshaft. Figure 5 shows a
`continuously
`v a r i a b l e t r a n s m i s s i o n t o
`perform the function of matching the engine
`and driveshaft speeds.
`I n t h e p a r a l l e l
`hybrid, load sharing control can be based
`on vehicle speed, power demand by the
`d r i v e r ,
`a n d / o r b a t t e r y s t a t e - o f - c h a r g e .
`Control of this driveline is more complex
`t h a n t h e s e r i e s h y b r i d , b u t i t i s t h e
`c o n t r o l o p t i o n s a v a i l a b l e t h a t o f f e r t h e
`
`possibility for meeting the hybrid vehicle
`acceleration and range specifications with
`l e s s c o s t l y d r i v e l i n e
`a l i g h t e r , s m a l l e r ,
`than is possible using the series hybrid
`approach.
`In the parallel hybrid, the maximum
`power rating of the electric driveline will
`be smaller and that of the engine will be
`larger than in the series hybrid, and the
`e l e c t r i c d r i v e l i n e i s s i z e d s u c h t h a t a
`large fraction of the total energy required
`to drive the vehicle on the FUDS cycle can
`be provided from the battery. The results
`o f t h e s t u d i e s r e p o r t e d i n R e f e r e n c e 7
`i n d i c a t e t h a t t h i s r e q u i r e s t h a t t h e
`e l e c t r i c d r i v e p r o v i d e a b o u t 4 0 % o f t h e
`hybrid
`p e a k p o w e r
`o f t h e p a r a l l e l
`There is no need in the paral-
`d r i v e l i n e .
`l e l h y b r i d f o r a s e p a r a t e g e n e r a t o r ,
`because the drive motor is also used as a
`generator to recharge the battery, using
`excess power from the engine.
`S i n c e t h e d e m a n d s o n t h e e l e c t r i c
`d r i v e l i n e c o m p o n e n t s a r e l o w e r f o r t h e
`parallel hybrid than for the series hybrid,
`the peak power required from the battery is
`also less, resulting in a lower peak power
`density requirement for the battery in the
`Because meeting the
`p a r a l l e l h y b r i d .
`battery peak power density requirement will
`be a
`key
`issue
`in designing a
`series
`hybrid, the reduction in battery peak power
`i n t h e p a r a l l e l h y b r i d i s a s i g n i f i c a n t
`advantage of this configuration.
`transmission
`The
`engine
`and
`requirements are more demanding for the
`parallel hybrid than for the series hybrid.
`I n t h e p a r a l l e l h y b r i d , t h e e n g i n e m u s t
`operate over a wider range of power and
`speed, and the torque from the engine must
`be combined smoothly and efficiently with
`the torque from the electric motor to meet
`requirements
`f o r v e h i c l e
`t h e t o r q u e
`The engine may be turned off
`propulsion.
`and on frequently,
`in a
`fraction of a
`second, in response to the system control
`The transmission and gearing
`strategy.
`that combines the engine and electric motor
`torques must operate over a wide range of
`motor speed and be capable of quickly and
`smoothly decoupling the engine and motor
`f r o m t h e d r i v e s h a f t w h e n t h e y a r e n o t
`needed. The operation of the engine in the
`p a r a l l e l h y b r i d i s m u c h l i k e t h a t i n a
`c o n v e n t i o n a l I C E v e h i c l e e x c e p t t h a t i t
`will operate much less frequently at low
`power, because the electric driveline will
`
`Page 7 of 27
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`57
`
`

`

`provide the power at low vehicle speeds and
`l i g h t l o a d s .
`Achieving ultralow vehicle emissions
`from the engine in the parallel hybrid will
`be more difficult than in a series hybrid,
`because in the parallel hybrid, the engine
`is turned off and on more frequently and
`r a p i d e n g i n e r e s p o n s e i s r e q u i r e d t o
`achieve smooth system operation and good
`driveability even when the engine is cold
`or
`being
`warmed-up.
`Under
`these
`conditions, it is less likely that the gas
`turbine, rotary, and two-stroke engines,
`w h i c h a r e p r i m e c a n d i d a t e s f o r u s e i n
`series hybrids, because of their small size
`and light weight, can be adapted for use in
`the parallel hybrid.
`
`THE CHOICE BETWEEN SERIES AND
`PARALLEL HYBRID CONFIGURATIONS - Whether a
`series
`or
`a
`p a r a l l e l
`d r i v e l i n e
`c o n f i g u r a t i o n will
`be
`utilized
`in
`a
`particular hybrid vehicle application will
`d e p e n d o n s e v e r a l f a c t o r s .
`T h e f i r s t
`consideration is packaging space.
`I f a n
`electric driveline, including a high-power
`battery, meeting the maximum power demands
`of the vehicle being designed can be pack-
`a g e d i n t o t h e s p a c e a v a i l a b l e , t h e n a
`s e r i e s h y b r i d d e s i g n i s a p o s s i b i l i t y .
`O t h e r w i s e t h e v e h i c l e m u s t b e d e s i g n e d
`using the parallel driveline approach.
`I f
`a n e l e c t r i c d r i v e s y s t e m f o r a s e r i e s
`hybrid is a possibility, it should then be
`p o s s i b l e t o d e s i g n a s m a l l , l i g h t - w e i g h t
`g e n e r a t o r a n d i t s e l e c t r o n i c c o n t r o l f o r
`coupling with a high-speed engine to obtain
`the engine/generator unit for the series
`hybrid.
`A s e c o n d c o n s i d e r a t i o n t h a t c o u l d
`preclude the use of a series configuration
`i n
`a
`hybrid
`vehicle
`is
`low
`system
`e f f i c i e n c y .
`In Reference 17, it is stated
`that "the efficiency of the entire (series)
`d r i v e i s n o t s a t i s f a c t o r y " a n d t h a t " a
`(from
`the
`total power of over 3 Pmax
`engine)
`is
`required
`to drive at Pmax
`(at
`the
`axle)".
`These
`statements
`from
`Reference 17 seem to greatly exaggerate the
`losses
`in
`the
`series
`hybrid
`driveline
`compared to those in the parallel hybrid
`d r i v e l i n e .
`C o n s i d e r f i r s t t h e e f f i c i e n c y
`f o r c i t y d r i v i n g m o d e s i n w h i c h m o s t l y
`e l e c t r i c a l e n e r g y i s u s e d t o p r o p e l t h e
`vehicle, even in the parallel hybrid. When
`the battery is being recharged by the en-
`t h e
`series
`and
`p a r a l l e l
`gine,
`
`configurations (see Figures 4-5) encounter
`the
`same
`losses
`-
`the
`e s s e n t i a l l y
`generator, battery charge/discharge, and
`the motor/electronic losses- and the system
`e f f i c i e n c i e s f o r t h e t w o d e s i g n s w i l l b e
`For highway driving, a
`nearly the same.
`s i g n i f i c a n t f r a c t i o n o f t h e e l e c t r i c a l
`energy from the engine/generator in the
`s e r i e s c o n f i g u r a t i o n i s u s e d d i r e c t l y t o
`energize the motor to propel the vehicle.
`If both the generator and motor/electronics
`have an efficiency of 90%, there is then a
`2 0 % l o s s f r o m t h e e n g i n e o u t p u t t o t h e
`wheels.
`I n t h e c a s e o f t h e p a r a l l e l
`the only loss between the
`configuration,
`engine output and the wheels occurs in the
`transmission and/or other gearing, which
`has an efficiency of about 95%. Hence the
`difference in the losses in highway driving
`is only about 15%.
`Since the engine in the series hybrid
`is smaller than in the parallel hybrid and
`can be operated closer to its minimum bsfc
`point than is likely to be the case in the
`parallel hybrid, it is not a certainty that
`the efficiency of the parallel hybrid will
`be higher than that of the series hybrid
`even for highway driving modes. A more
`c r i t i c a l p o i n t t h a n s y s t e m e f f i c i e n c y i s
`likely to be differences in the bsfc maps
`for the engines used in the two types of
`hybrid driveline configurations.
`T h e c r i t i c a l f a c t o r s i n c h o o s i n g
`b e t w e e n t h e s e r i e s a n d p a r a l l e l h y b r i d
`approaches are likely to be those which are
`t h e m o s t d i f f i c u l t t o q u a n t i f y b e f o r e
`e i t h e r s y s t e m i s d e v e l o p e d - t h a t i s t h e
`time and cost of component and driveline
`development, and overall system complexity
`a n d r e l i a b i l i t y .
`T h e s e r i e s h y b r i d
`d r i v e l i n e w i l l m o s t l i k e l y b e s o m e w h a t
`h e a v i e r , l a r g e r , and more expensive than
`the parallel driveline, but it will also be
`less complex, more reliable, and have lower
`e m i s s i o n s t h a n t h e p a r a l l e l s y s t e m . I n
`addition, since the series hybrid is a more
`d i r e c t e x t e n s i o n o f t h e p u r e e l e c t r i