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`SA E TECHNICAL
`PAPER SERIES
`
`The Energy Flow Management and Battery
`Energy Capacity Determination for the Drive
`Train of Electrically Peaking Hybrid Vehicle
`
`Yimin Gao, Khwaja M. Rahman, and Mehrdad Ehsani
`Texas A & M Univ.
`
`Reprinted from: ElectricIHybrid Vehicles: Alternative Powerplants,
`Energy Management, and Battery Technology
`(S P-1284)
`
`mA =For Advancing Mobiiity
`The Engineering Society
` and Sea Air and S~ace-
`
`AUG 0 6 1997
`
`Future Transportation
`Technology Conference
`San Diego, California
`August 6-8,1997
`
`400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (412)776-4841 Fax:(412)776-5760
`Page 1 of 8
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`ISSN01487191
`Copyright 1997 S o c i i o f Automotive Engineers, Inc.
`
`Positions and opinions advanced in this paperarethose of the author@) and not necessarily
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`The Energy Flow Management and Battery Energy
`Capacity Determination for the Drive Train of
`Electrically Peaking Hybrid Vehicle
`Yimin Gaol, Khwaja M. Rahman, and Mehrdad Ehsani
`Texas A & M Univ.
`
`Copyright 1997 Society of Automotive Engineers, Inc.
`
`ABSTRACT
`
`In this paper, the configuration of a parallel hybrid
`vehicle, called electrically peaking hybrid (ELPH) vehicle
`is introduced. Several operation modes of the engine and
`electric motor and different control strategies are
`analyzed. The results show that, with proper selection of
`the drivetrain parameters, the vehicle can satisfy
`the
`urban and highway driving with a small internal
`combustion engine, a small battery pack and a single gear
`transmission. Moreover, the vehicle does not need to
`charge the battery pack from the electricity network for
`keeping its battery SOC at a reasonable level.
`
`INTRODUCTION
`
`In recent years, increasing concern over air pollution,
`caused by tailpipe emissions of petroleum-based vehicles,
`and the dwindling petroleum resources have lead the
`automotive engineers and automakers to probe the
`the zero-emission (ZE) and ultra-low
`possibility of
`emission (ULE) vehicles. Among all kind possible
`schemes, electric vehicle @V) seems to be the most
`attractive due to their zero emission, petroleum-free
`energy supply, control flexibility, and simple construction.
`However, pure electric vehicles suffer from other
`disadvantages. [ $61
`
`1. The heavy and bulky battery pack, with very limited
`energy storage, makes the EV limited in range, and load
`carrying capacity.
`
`2. Long charging time limits the EV's availability.
`
`1 Visiting Scholar from Jilin University of Technology, China
`
`Therefore, commercial success of the EV depends
`entirely on development of advanced high energy batteries.
`However, progress in batteries over the past several decades
`has not been adequate.
`
`Hybrid configurations, in which two power sources are
`applied to propel the vehicles, are now holding the
`greatest promise. The hybrid electric-internal combustion
`engine drive train, if properly configured, can combine the
`advantages of both EV and ICE vehicles with no
`drawbacks.
`
`The configuration of a parallel hybrid vehicle, called
`electrically peaking hybrid (ELPH) vehicle is shown in
`Fig. 1.[7,8,9,10] The internal combustion engine (ICE)
`and the electric motor are coupled by a set of match gear
`(or chain) into the input shaft of the transmission. The
`transmission would be multi-speed or single-speed
`depending completely on the performance requirement
`and drivetrain parameters selected.
`
`When the vehicle operates on level road with constant
`cruising speed, relatively low power is required, but large
`amount of energy is consumed in a long trip. In this case,
`a small ICE alone is used to power the vehicle, resulting
`in an excellent fuel economy due to its operating point
`being close to the optimal point. When the vehicle
`experiences an acceleration or a steep hill climbing, the
`electric motor, functioning as a load leveling device,
`supplies supplementary power to the drive train to meet
`the performance requirement. The ELPH configuration
`has the ability to recover braking energy with the electric
`motor functioning in regenerating mode. Furthermore,
`when the vehicle operates with light load, such as at a
`relatively low constant speed or going down a slight hill,
`
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`Clutch
`
`1
`
`Differential
`
`Motor
`
`Match Gear
`
`J
`
`1
`
`the engine can recharge the battery pack to maintain
`adequate state-ofsharge. More beneficially, this enhances
`where, P,= power output of engine,
`Pm = power output of electric motor,
`the engine load, for operation close to its optimal point. A
`well designed ELPH vehicle may never use a wall plug to
`P, = load power of the vehicle,
`charge its battery pack and can obtain an excellent fuel
`P b = discharge power of the battery pack,
`economy.
`q p discharge efficiency of the battery pack,
`flm- efticiency of the motor. n
`In this operating mode. All the required energy must
`be supplied by battery pack.
`F H g i 1 { Trans.
`Battew Pack Charging Mode - When the load power of
`-
`the vehicle is less than the engine power with wide open
`-
`throttle, engine has the extra power to charge the battery
`pack, if necessary. The electric motor functions in the
`1
`regenerating mode to convert the engine power into
`electric power to recharge the battery pack. The electric
`Wheel
`motor power (as a generator), P, and battery pack
`recharging power, Pb, are
`
`Fig. I The Conjiguration of the ELPH Vehicle
`
`MANAGEMENT OF MOTOR AND
`ENGINE POWER
`
`The operation of a vehicle can be divided into three
`basic modes: constant speed (cruising), acceleration (peak
`power) and deceleration (regeneration). In each mode, the
`engine and the motor operate with the appropriate
`behavior to meet the load power requirement and keep
`proper SOC on the battery.
`CRUISING MODE - The cruising mode is the
`operation mode of the vehicle in which the engine alone
`can meet the load power requirement, such as operating
`on a level road with constant speed or with a slight
`acceleration or a slight hill climbing with constant speed.
`In this operation mode, the engine and electric motor have
`several operating status.
`Motor-onlv Tractive Mode - When the speed of the
`vehicle is less than the speed that is limited by the
`minimum rpm of engine or greater than the speed that is
`limited by the engine maximum rpm, all the power
`required by the load of the vehicle is supplied by the
`electric motor. The engine must remain at standstill or in
`idling. In these cases, we have
`
`where; qk= the battery pack charging efficiency .
`
`Negative Pm means electric motor functioning as
`generator.
`Engine-onlv Mode - If the battery pack is not required
`to be recharged, for example, the SOC of the battery pack
`reaches its top line, the electric motor is idling and the
`engine power is equal to the load power of the vehicle,
`that is
`
`PEAK POWER MODE - When the vehicle
`experiences an acceleration or a steep hill climbing, the
`load power is much greater than that the engine can
`produce. Consequently, the motor must work together
`with the engine to produce enough power to meet the
`requirement. In this case, the motor power output and
`battery power output are
`
`(3)
`
`REGENERATING MODE - When the vehicle
`experiences a deceleration or a hill descending, the engine
`is turned off or idles. The electric motor functions in
`
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`regenerating mode. The electric motor (generator) power
`Pm, and battery charging power, Pb, are
`
`Pm =
`Pb = Pmfltmflmflbc
`
`(9)
`
`(10)
`
`where a = hctional factor of power recovery,
`= efficiency from motor to the drive wheels.
`
`In equations (1) to (lo), positive P, Pm and Pb mean
`that the engine, motor and battery pack supply powers to
`the vehicle. In contrast, negative Pm and Pb means that
`motor and battery pack absoh power from the engine or
`regenerating braking.
`
`THE ENERGY CHANGE IN THE
`BATTERY PACK .
`
`As explained above, in the peaking power mode, the
`battery pack must supply energy
`to
`the vehicle.
`Consequently, the stored energy in the battery pack is
`decreased. On the other hand, when the vehicle operates
`at low load or in the braking mode, the battery pack
`absorbs energy from engine or regenerative braking. In a
`whole drive cycle, if the consumd energy and absorbed
`energy are balanced, the battery pack will never have to
`get energy from wall plug. Therefore the range of the
`vehicle is only limited by the fuel tank as in a
`conventional vehicle.
`
`The amount of energy change in the battery pack at
`time t in the drive cycle ( t-0 represents the beginning of
`the drive cycle) is expressed by
`
`be applied by brake system of the vehicle). In the actual
`operation, the control system of the drivetrain can
`determine the power output of each power unit in many
`ways, provided
`the total power output meets the
`requirement Different control strategies will obtain
`different fuel economies and different battery energy
`capacities.
`
`MAXIMUM BATTERY SOC CONTROL
`STRATEGY - The maximum battery state-of-charge
`control strategy is consistent with the principle that, at
`any time, except the battery SOC reaching its top line, the
`engine should operate with full load (wide-open throttle)
`to produce maximum power. One part of the engine power
`is used to counterbalance the vehicle load power, and the
`remainder is used to charge the battery. This control
`strategy is illustrated in Fig. 2. In this figure, the segments
`a and a' represent the battery charging power for high-
`speed and
`low-speed gears of
`the
`transmission
`respectively. Similarly, b and b' represent the battery
`discharging power for higher and lower speed gear of the
`transmission. Fig. 2 also implies that a multi-speed
`transmission is helpful to reduce the size of battery pack
`However the penalty is a complicated construction and
`control system.
`
`60
`
`Load Pow& on
`6% GradeiRoad , i
`
`I
`
`The negative sign means that when Pb is positive
`(battery pack supplies power to the vehicle), the energy in
`the battery pack is decreased. If, at the end of the drive
`cycle, the value of E is the same as that at the beginning
`of the driving cycle, the battery SOC will be kept the same
`as at the beginning of the drive cycle. Consequently the
`vehicle will not need wall plug to charge the battery.
`
`CONTROL STRATEGIES OF THE
`DRIVETRAIN
`
`As explained above, at any time in driving, the sum of
`the engine power, Pe, and the electric motor power, Pm,
`should be equal to the vehicle load power PI, (except in
`the braking mode, in which external braking power may
`
`Vehicle speed (Kmh)
`Fig. 2 Battery Charging and Discharging Power
`
`OPTIMAZ, CONTROL STRATEGY - Fig. 3 shows
`the fuel consumption map of a typical SI engine with its
`optimal fuel economy operating line. If operating point of
`the engine is just on the optimal fuel economy operating
`line, ;he e n h e has a -optimal operating- efficiency.
`to the
`Fortunately, the power output corresponding
`optimal operating line is just a little smaller than the
`power output with a full load (wide-open throttle). This
`implies that, if the control system controls the engine
`operating on the optimal operating line, the vehicle can
`not only maintain the battery SOC at a certain level, but
`
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`also have a good fuel economy, and generally, good
`emission characteristics.
`
`Generally, the optimal operating line of engine is quite
`difficult to obtain analytically. It is approximately
`assumed that power on the optimal fuel economy
`operating line is proportional to the power output of
`engine with a wide-open-throttle in a large speed range
`(see Fig. 3) . Thus
`
`kept within the range of 40?? to 60%, the cycle efficiency
`is optimal[l]. Therefore, we set the highest SOC of the
`and the lowest 4m..
`battery pack being
`
`Thus, we have
`
`where P,, = power output corresponding to the optimal
`fuel economy operating line,
`fl= fractional factor,
`P,,
`= power of the engine with wide open throttle.
`
`where, Ca= the energy capacity of the battery pack with
`Kw.h.
`
`I
`01
`1000 1500 2000 2500 5000 3500 40(30 4500 5000
`Crankshaft Speed RPM
`Fig. 3 Specific Fuel ~onsl(m~tion Map Of a Typical
`Sl Engine with Its Optimal Operating Line
`
`DETERMINATION OF BATTERY
`ENERGY CAPACITY
`
`Proper selection for energy capacity of the battery
`pack is crucial for the design of the ELPH vehicle.
`Oversized battery pack would cause vehicle overload, and,
`undersized battery pack can not supply adequate energy
`and power for the needs of the vehicle.
`
`The minimal value of E in equation (1 1) within a drive
`cycle represents the lowest SOC of the battery, E=O
`represents the highest SOC of the battery pack
`
`Fig. 4 shows a charge and discharge efficiency of a
`typical lead-acid battery along with its state sfcharge.
`This figure suggests that if the SOC of the battery pack is
`
`Fig. 4 Battery Eficiency Respect with Battery
`State-of-Charge [I]
`
`A NUMERICAL EXAMPLE
`
`The specification of the example ELPH vehicle
`prototype which is being developed at Texas A&M
`University is listed as below:
`
`Weight
`Rolling resistance coeficient of tire
`Aero-dynamic drag coeficient
`Frontal area
`Wheel radius
`
`1 700 Kg,
`0.013,
`0.29,
`2.13 m2
`0.2794 m.
`
`The following parameters are used in the calculations.
`
`Engine power capacib
`Diflerential gear ratio
`Single gear transmission, gear ratio
`Transmission eficiency @om engine and
`motor to drive wheels
`Motor eflciency
`Battery charge and discharge eficiency
`
`30 Kw
`4.23
`1.0
`
`0.9
`0.85
`0.85
`
`The engine speed-power characteristic is shown in
`Fig. 5.
`
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`CONCLUSION
`
`The electrically peaking hybrid vehicle has a parallel
`configuration in which a small internal combustion
`engine and an electric power peaking motor cooperate.
`When the vehicle is operating with a light load, the
`engine can charge the battery pack with the remaining
`power. When the vehicle operates with high load , the
`battery pack can supply energy to the drivetrain to meet
`the requirement of the load power.
`
`The calculation results show that, for a 1700 Kg
`passenger car ,the combination of a 30 Kw power capacity
`engine and a small battery pack with a single-gear
`transmission will satisfy the requirement in both urban
`and highway driving conditions. With the engine
`operating point being controlled on the optimal operating
`line by the optimal control strategy, the vehicle will
`achieve an excellent fuel economy, and generally good
`emissions characteristics.
`
`ACKNOWLEDGMENT
`
`The financial support of Texas Higher Education
`Coordinating Board and Texas Transportation Institute
`for the ELPH project is gratefully acknowledged.
`
`REFERENCE
`
`1. Clark G. Hochgraf, Michael J. Ryan, and Herman
`L. Wiegman , Engine Control Strategy for a Series Hybrid
`Electric Vehicle Incorporating Load Leveling and
`Computer Controlled Energy Management, SAE 960230,
`19%
`2. Martin Endar and Philipp Dietrich, Duty Cycle
`Operation as a Possibility to Enhance the Fuel Economy
`of an SI Engine at Part Load, ASE 960229.
`3. AF. Burke, Battery Availability for Near Term (1998)
`Electric Vehicle, SAE 9 1 19 14.
`4. J.Y. Wong , The Theory of Ground Vehicle , John
`Wiley & Sons Inc. Press, 1978.
`5. MEhsani, K. M. Rahman, and H. Toliyat, Propulsion
`System Design of Electric & Hybrid Vehicle, accepted for
`publication in the IEEE Tran. of Industrial Electronics.
`6. & A Toliyat, K. M. Rahrnan, and M. Ehsani, Electric
`Machine in Electric & Hybrid Vehicle Application,
`Proceeding of ICPE, 95, Seoul, pp 627435
`7. M.Ehsani, Electrically Peaking Hybrid System And
`Method, U.S. Patent Granted 1996
`8. J. Howze, M. Ehsani and D.buntin, Optimizing Torque
`Controller for a Parallel Hybrid Electric Vehicle, U.S.
`Patent granted, 19%
`
`Fig. 5 The Speed-power Characteristic of Engine
`Best Batterv SOC Control Strategy - Fig. 6 shows the
`engine power, motor power, battery power and the change
`of battery energy along with driving time in the urban
`driving cycle FPT75.
`
`Fig. 6 indicates that E has the same value at the
`beginning and end of the driving cycle. Therefore, the
`vehicle does not requires a wall plug to charge the battery
`pack in urban driving. Fig. 6 also gives a minimal E equal
`to 0.1421 Kw.h. Using equation (14) , the energy capacity
`of the battery pack, Ck=O. 7105 Kw.h, is obtained. This
`result means that, from the energy point of view, only a
`very small battery pack is needed.
`
`While operating with highway driving cycle, the
`engine power, motor power, battery power and energy
`change in the battery pack are shown in Fig. 7. This
`figure indicates that engine alone can almost satisfy the
`requirement except for the transient acceleration at the
`beginning of the drive cycle. The minimal E is equal to
`0.0431 Kw.h. So, the Ck=0.2155 Kw. h is enough for this
`driving cycle.
`Outirnal Control Strategy - Fig. 8 and Fig. 9 show the
`time history of the engine power, motor power, battery
`power and the change in the battery storage energy
`corresponding to urban and highway driving cycle of F P
`75. The $ (see equation (12)) is 0.87 The results show
`that, even with the frequent start-stop urban driving mode,
`the vehicle does not need a wall plug to charge its battery
`pack. The minimal E is equal to 0.1850Kw.h So, the
`C&= 0.925 Kw.h This m e . that, with this control
`strategy, the battery size is also quite small.
`
`The situation of highway driving with optimal control
`is quite similar to that with best battery SOC control. The
`engine alone can almost satisfy the power requirement
`
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`9. Proceedings of the ELPH Conference, Texas A&M
`University, College Station, Texas, Ckt. 1994
`10. Yimin Ciao, Khwaja URahman and Mehrdad
`Ehsani, Parametric Design of The Drive Train of An
`Electrically Peaking Hybrid (ELPH) Vehicle, ASE
`970294.
`
`1000
`
`1200
`
`1
`1400
`
`-0.2
`0
`
`200
`
`400
`
`800
`600
`T i c (sco.)
`Fig 6. The Time History of Engine Power, Motor Power,
`Battery Power and Change of Battery Storage
`Energy Corresponding to FTP7.5 Urban Driving
`Cycle with Best Battery SOC Control Strategy
`
`Tune (stc.)
`Fig. 8 The Time History of Engine Power, Motor Power,
`Battety Power and Change of Battery Storage
`Energy Corresponding to FTP7.5 Urban Driving
`Cycle with Optimal Control Strategy)
`
`Baacly +md-) a
`
`(+I p e w )
`
`1
`
`Fig. 9 The Time History of Engine Power, Motor Power,
`Battery Power and Change of Battery Storage
`Energy Corresponding to FTP7.5 Highway Driving
`Cycle with Optimal Control Strategy
`
`i%mgy -6
`
`in b W q (Kwh)
`
`-0.2
`0 1 0 0 2 0 0 M O 4 0 0 U W ) 6 0 0 7 0 0 8 0 0
`T i (see.)
`Fig. 7 7 e Time History of Engine Power, Motor Power,
`Battery Power and Change of Battery Storage
`Energu Corresponding to FTP7.5 Highway Driving
`Cycle with Best Battery SOC Control Strategy
`
`1
`
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