Proceedings of the Institution of Mechanical
`Engineers, Part D: Journal of Automobile
`Engineering
`
`http://pid.sagepub.com/
`
`
`A Hybrid Internal Combustion Engine/Battery Electric Passenger Car for Petroleum Displacement
`I Foster and J R Bumby
`Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering
`DOI: 10.1243/PIME_PROC_1988_202_155_02
`
` 1988 202: 51
`
`The online version of this article can be found at:
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`BMW1016
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`

`51
`
`A hybrid internal combustion engine/battery electric
`passenger car for petroleum displacement
`
`1 Foster, BSc, PhD and J R Bumby, BSc, PhD, CEng, MIEE
`School of Engineering and Applied Science, University of Durham
`
`This paper examines the potential of the hybrid electric vehicle in substituting petroleum fuel by broad-based electrical energy. In
`particular a hybrid car is considered. The way in which the powertrain can be controlled and the effect component ratings have on
`achieving the petroleum substitution objective are described. It is shown that a hybrid vehicle can be designed that can achieve a
`petroleum substitution of between 20 and 70 per cent of the equivalent internal combustion engine vehicle, be capable of entering
`environmentally sensitive areas and yet be capable of a range at high and intermediate speeds that is limited only by the size of itsfuel
`tank.
`
`NOTATION
`El energy input to the i.c. engine (J)
`E , energy input to the electric traction system (J)
`F objective function
`9, gear ratio
`L , petroleum energy weighting factor
`L, electrical energy weighting factor
`torque split ratio
`x
`
`defined, for example in urban delivery duty. Indeed, it
`has been in such vehicles as the urban milk delivery
`vehicle or milk float that relatively low performance
`electric traction drives have been traditionally applied
`with a great deal of success. Currently the demand is for
`urban electric vehicles to be developed with greater
`traffic compatibility in terms of speed and range. It is in
`such vehicles as the ‘Freight Rover Sherpa’ and the
`‘Bedford CF‘ one tonne vans that higher performance
`electric drives have started to appear (2).
`Although urban delivery vehicle applications will
`help to reduce the dependence of the road transport
`sector on petroleum-based fuels, the major part of this
`market requires vehicles that are not limited in range
`and have a performance compatible with today’s inter-
`nal combustion (i.c.) engine vehicles. The use of
`advanced traction battery technology to overcome the
`range limitation of electric vehicles is one possible solu-
`tion. However, this would still result in a vehicle limited
`in range and may in itself create additional problems.
`For example, due to the much greater on-board stored
`energy, the charging time required will be greater than
`at present while the higher continuous rating of the
`traction components required to achieve high-speed
`performance compatible with today’s vehicles will need
`careful attention.
`The range limitations of the pure electric vehicle can
`be overcome by using a hybrid i.c. engine/electric drive
`which incorporates both an i.c. engine and an electric
`traction system. Although such a vehicle can be
`designed to meet a number of end objectives, it has been
`argued (3) that a vehicle which seeks to remove the
`range limitation of the electric vehicle while substituting
`a substantial amount of petroleum fuel by electrical
`energy is the vehicle most worth pursuing. With the
`emphasis of the vehicle design biased towards the elec-
`tric drivetrain the intention may be to operate in an
`all-electric mode under urban conditions and to use the
`i.c. engine for long-distance motorway driving. The
`hybrid mode could then be used for extending urban
`improving vehicle accelerative per-
`range and/or
`formance on accelerator kick-down.
`The concept of a hybrid electric vehicle capable of
`substituting petroleum fuel is not new, Bosch (4) and
`Volkswagen (5) having built vehicles in the 1970s. More
`0265-1904/88 $2.00 + .05
`Proc Inst Mech Engrs Vol202 No DI
`
`1 INTRODUCTION
`The disruption to the world’s oil supplies that occurred
`in 1973 and 1978 helped focus international attention
`upon the finite nature of this particular energy resource.
`It also prompted the adoption of energy conservation
`policies and emphasized the need to transfer energy
`demand away from oil to other sources of energy, such
`as natural gas, coal and nuclear. As a result of such
`policies the inland deliveries of petroleum products,
`expressed as a percentage of the total energy consumed,
`dropped from 50 to 40 per cent over the period 1973-85
`(1).
`Although such an energy transfer is feasible in many
`energy sectors, it is more difficult in the transport area,
`due to the operational requirements placed on many of
`the vehicles. Some energy transfer has been achieved in
`the rail system by the use of increased electrification but
`in the road sector such an energy transfer is more diffi-
`cult as the ‘free ranging’ nature of the majority of vehi-
`cles requires the energy store to be on-board the vehicle
`itself. Consequently, in the period 1973-85, the use of
`petroleum in the road transport sector, expressed as a
`percentage of the total inland deliveries of petroleum
`products, increased from 12 to 39 per cent (1).
`A transfer of energy from oil to electricity, with its
`associated broad fuel base, can be achieved to a limited
`extent in the road transport sector by the increased use
`of electric vehicles. However such vehicles are limited in
`range due to the amount of energy that can be realisti-
`cally stored on-board the vehicle without unduly affect-
`ing payload. As a consequence of this, electric vehicles
`must be used in situations where daily usage is well
`
`The MS was received on 7 April 1987 and was accepted for publication on 25
`August 1987.
`22/88 0 IMechE 1988
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`52
`
`I FORSTER AND J R BUMBY
`
`
`
`- -
`
`
`
`- -
`
`I
`
`U
`
`
`
`
`
`Engine Engine
`
`
`
`t t
`
`
`
`Transmission Transmission
`
`
`Final Final
`
`drive drive
`
`
`
`I I
`
`
`
`I I
`
`
`
`- -= - -=
`
`
`
`L L
`
`Accelerator
`
`Brake
`
`Controller
`Controller
`
`Motor
`Motor
`
`I
`
`recently, the advent of the Electric and Hybrid Vehicle
`Research, Development and Demonstration Pro-
`gramme in the United States of America initiated the
`design and construction of a Near Term Hybrid Vehicle
`(NTHV) with the principal aim of substituting pet-
`roleum fuel by ‘wall plug’ electricity (6, 7).
`As part of the NTHV programme a large number of
`conceptual studies were conducted but on vehicles
`aimed at the American passenger car market. In this
`paper optimization studies were conducted, but now on
`a vehicle suitable for the European medium-sized pass-
`enger car market. Such optimization studies are impor-
`tant as, with two sources of traction power available,
`the way in which they are controlled, and their relative
`sizing, is fundamental to the way the vehicle performs.
`In order to optimize the control and component
`rating of the hybrid drivetrain, the performance and
`energy consumption of
`the vehicle over standard
`driving cycles is assessed using the road vehicle simula-
`tion program Janus (8). Janus is a flexible road vehicle
`simulation program capable of predicting the energy
`use and performance of vehicles with a variety of
`powertrain configurations and has been used previously
`to study the performance of advanced i.c. engine vehi-
`cles (9) and hybrid electric vehicles (3).
`Before examining in detail the optimum control strat-
`egy for the drivetrain, Section 2 defines the hybrid
`arrangement under study. A description of the opti-
`mization process using an appropriate cost function is
`then presented in Section 3 followed by a method of
`translating the resulting control structure into a sub-
`optimum algorithm capable of being implemented in
`real time. Using the optimum control structure the
`effect of component ratings on the vehicle’s performance
`is evaluated in Section 4, while Section 5 discusses the
`practical implementation of an overall vehicle control
`algorithm. Finally, in Section 6, an indication of the
`vehicle’s potential for substituting petroleum fuel by
`electricity is given.
`Proc Instn Mech Engrs Vol 202 No D1
`
`2 THE BASE HYBRID VEHICLE
`Although a large number of hybrid i.c. engine/electric
`traction drive system arrangements are possible the
`configuration selected as having the greatest potential
`for use in a passenger car application is shown in Fig. 1.
`In this parallel arrangement both the electric traction
`motor and the i.c. engine are capable of driving the road
`wheels directly, and independently, through a common
`transmission. Such an arrangement offers the potential
`for maximizing
`the overall
`transmission efficiency
`between either prime mover and the road wheels,
`although, when both prime movers are operative, a
`compromise must be achieved. Although minor gains
`are possible if each power source is fed through its own,
`independent, transmission the efficiency benefit of such
`an arrangement must be carefully balanced against the
`added complexity, weight and cost.
`To examine the potential of the hybrid drive as a
`means of displacing petroleum fuel a set of vehicle
`parameters typical of a mid-range European passenger
`car are defined in Table 1. Included in this table is a
`comparable parameter set for a conventional i.c. engine
`vehicle. Because the hybrid arrangement has two on-
`board energy sources the selection of the engine and
`traction motor rating must be carefully balanced. In this
`base hybrid vehicle specification the i.c. engine is sized
`to meet the maximum level road speed and is capable of
`sustaining 120 km/h on a 2 per cent gradient. The elec-
`tric traction motor is rated so as to give adequate all-
`electric performance and also, when combined with the
`i.c. engine, provides the required acceleration demand
`placed on the vehicle. This value is comparable with
`that of a similar i.c. engine vehicle. The performance
`curves for the i.c. engine and the traction motor selected
`are shown in Fig. 2. Also shown in Fig. 2 is the com-
`bined torque capability of the drive system and the road
`load requirement in top gear. Throughout this paper,
`whenever any changes are made to the size of a com-
`ponent the installed power capacity of the vehicle is
`
`@ IMechE 1988
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`53
`A HYBRID INTERNAL COMBUSTION ENGINEIBATTERY ELECTRIC PASSENGER CAR
`Table 1 Base vehicle data
`severe discharge condition on the battery during hard
`acceleration at high speed. However, during normal
`Parallel hybrid
`operation battery discharge rates are substantially
`reduced and are within acceptable bounds.
`
`Conventional
`
`1640 kg
`1900 kg
`
`950 kg
`1200 kg
`
`0.35
`0.01
`1.95 m 2
`
`0.35
`0.01
`1.95 m2
`
`3 CONTROL OF THE HYBRID ELECTRIC
`DRIVETRAIN
`
`55 kW, 5000 r/min
`-
`
`3.1 General
`
`35 kW, 5000 r/min
`35 kW, shunt
`lead-acid EV2- 13
`E, = 150 kJ/kg
`(42 W-h/kg)
`wt = 300 kg
`3.5 : 1
`4 speed automatic
`gear ratios
`1st 3.5 : 1
`2nd 2.4 : 1
`3rd 1.3 : 1
`4th 1.0: 1
`
`14 s
`
`130 km/h
`145 km/h
`
`3.5 : 1
`4 speed manual
`
`3.5 : 1
`2.4 : 1
`1.3 : 1
`1.0: 1
`
`12 s
`
`145 km/h*
`
`Vehicle weights:
`kerb weight
`test weight
`Vehicle parameters:
`
`CR
`A
`Component sizes:
`i.c. engine
`traction motor
`battery
`
`final drive
`transmission
`
`Performance:
`0-60 mile/h
`(driver only)
`Maximum speed :
`i.c. engine only
`hybrid
`* at 5000 r.p.rn.
`
`With the hybrid arrangement shown in Fig. 1 it is pos-
`sible to operate the drive in a number of different ways,
`or modes. As the use of these different operating modes
`is fundamental to the operation and control of the
`hybrid drive it is important to clearly define what these
`modes are. This is done in Table 2. Although it is clear
`that the regenerative braking mode should be used to
`recover vehicle kinetic energy during braking, how the
`other modes should be utilized is much less obvious.
`This statement is particularly true of the hybrid mode
`when both the i.c. engine and electric traction motor
`drive the road wheels together. The picture is further
`complicated as the battery charge mode, the primary i.c.
`engine mode and the primary electric mode are all
`essentially hybrid modes. However, they differ from the
`true hybrid mode in that one or other of the two prime
`movers is the principal power source and is only aug-
`mented by the other when it, the principal source, is
`unable to provide the requested output torque. In con-
`trast in the true hybrid mode either one of the two
`prime movers is capable of providing all the necessary
`torque but to improve drivetrain efficiency the load is
`shared in some way between the two prime movers.
`How the load should be shared, or indeed if this mode
`should be used at all, is unclear.
`
`a Combined maximum torque line
`b Traction motor torque
`c Engine full-throttle torque
`A Constant-speed road load
`
`modified so as to maintain an acceleration performance
`and level road speed similar to those values defined in
`Table 1.
`The battery system selected for use in the base hybrid
`vehicle is a high-performance lead-acid battery. In any
`hybrid electric vehicle a major limitation in sizing the
`battery is the allowable maximum discharge power
`density the battery can withstand. The selection of a
`300 kg Iead-acid battery for the base vehicle imposes a
`
`250 r
`
`-
`
`200
`
`150
`
`501
`
`100 I-
`
`\
`\
`\
`
`I- /.=
`
`I50 km/h
`
`0 1
`
`I
`1000
`
`I
`2000
`
`I
`3000
`
`I
`4000
`
`I
`5000
`
`Engine speed
`rlmin
`Fig. 2 Base hybrid electric vehicle performance curves
`
`@ IMechE 1988
`
`Proc Instn Mech Engrs Vol 202 No D1
`
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`

`54
`
`I FORSTER AND J R BUMBY
`
`i.c. engine mode
`
`Primary electric mode
`
`Primary i.c. engine mode
`
`Table 2 Possible operating modes
`Mode
`Description
`All propulsion power supplied by the
`Electric mode
`electric traction system
`All propulsion power supplied by the
`i.c. engine
`The electric traction system provides
`the principal torque but when
`necessary its maximum torque is
`augmented by the i.c. engine
`The i.c. engine provides the principal
`torque but when necessary its
`maximum torque is augmented by
`the electric traction system
`Both the i.c. engine and the electric
`traction system together, in some
`way, provide the propulsion power
`The i.c. engine provides both the
`propulsion power and power to
`charge the batteries with the traction
`motor acting as a generator
`During braking the vehicle kinetic
`energy is returned to the battery with
`the traction motor acting as a
`generator
`Essentially a primary i.c. engine mode
`when increased torque is provided
`to give acceleration
`
`Hybrid mode
`
`Battery charge mode
`
`Regenerative braking
`
`Accelerator 'kick-down'
`
`/
`
`Acceleration
`
`\"' rake
`
`Parameter
`
`A
`
`Driving cycle
`B
`C
`
`D
`
`Maximum speed
`Vc (mileih)
`Acceleration time
`f a 6)
`Cruise time
`1, (s)
`Coast time
`t,, 6)
`Brake time
`f b (s)
`Idle time
`t , 6)
`Total time
`fs)
`
`102 I
`
`2 0 2
`
`30 '.
`
`45 5 1
`
`4 a l
`
`192
`
`0
`
`1 9 2
`
`18 2
`
`20 2
`
`2 2 1
`
`4 2
`
`8 2
`
`28 2 2
`
`50 2 2
`
`I02 I
`
`3 - t l
`
`5 ? 1
`
`9 2 1
`
`9'. 1
`
`3 0 5 2
`
`2 5 ? 3
`
`2 5 2 2
`
`2 5 k 2
`
`3 9 2 2
`
`7 2 2 2
`
`8 0 2 2
`
`12222
`
`Fig. 4 5227 schedule 'a' series of driving cycles
`
`and to provide any vehicle acceleration, is determined
`at discrete (typically one second) intervals. Over each
`discrete time interval the power and speed are assumed
`to be constant. These values are then reflected back
`through the drivetrain to the on-board energy sources.
`At each drivetrain component full account is taken of
`efficiency, which may vary with both torque and speed,
`so that the calculated energy consumed accounts for
`both the road load requirement and the system losses.
`At each time interval the optimization process com-
`putes the energy consumptions E l and E , associated
`with every possible combination of gear ratio and
`torque split. For a predefined set of weighting factors,
`L , and L, , a three-dimensional surface is therefore
`obtained with the objective function as the dependent
`
`3.2 Optimum control of the hybrid electric drivetrain
`In order to examine the hybrid control problem an
`optimization process has been developed whereby, for a
`given power and speed demand at the road wheels, that
`transmission ratio and torque split between the two
`prime movers which minimizes an objective function
`F(gr9 X) = L,E,(gr> XI + L, E,(gr, X)
`(1)
`is found. In this expression El and E , are the energy
`inputs to the i.c. engine and the electric traction system
`respectively, while L , and L, are weighting factors that
`allow total energy use to be biased towards one or
`other of the two on-board energy sources. The gear
`ratio gr and the torque split ratio x are two, indepen-
`dent, control variables that can be varied to minimize
`the objective function F(gr, x).
`To implement this optimization process over an
`urban driving cycle such as the ECE-15 (Fig. 3) or the
`J227a-D (Fig. 4) the torque required at the road wheels
`to overcome both vehicle drag and rolling resistance,
`
`?"I
`
`Time
`S
`Fig. 3 ECE-15 urban driving cycle
`
`~
`
`Proc Instn Mech Engrs Vol 202 No DI
`
`0 IMechE 1988
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`

`A HYBRID INTERNAL COMBUSTION ENGINEtBATTERY ELECTRIC PASSENGER CAR
`
`55
`
`a Fuel consumption of the base i.c. engine vehicle
`b 80 km range, 95% of all journeys
`c 50 km range, 90% of all journeys
`d 30 km range, 80% of all journeys
`
`L I ’L2
`Fig. 5 The influence of weighting factor ratio on the performance
`of the base hybrid vehicle (ECE-15)
`
`variable and torque split and gear ratio as the two
`control variables. A direct search for the minimum
`value of
`the objective function then identifies the
`optimum combination of gear ratio and torque split
`that minimizes the objective function F at that time
`instant. The time interval is then updated and the calcu-
`lation repeated. Such a process leads to a minimum
`control trajectory for the objective function F through
`the cycle. A more detailed explanation of the opti-
`mization process can be found in reference (3).
`When the base hybrid vehicle is simulated over an
`urban driving cycle, for example the ECE-15, with dif-
`ferent ratios of weighting factors, L J L , , the vehicle
`range to battery discharge and the i.c. engine fuel con-
`sumption can be expressed as a function of L J L , as in
`Fig. 5. At low values of L J L , greater emphasis is
`placed upon the i.c. engine while at higher values
`greater emphasis is placed upon the electric traction
`system. Thus, when L J L , = 0 the vehicle would com-
`plete the cycle solely on the i.c. engine while for
`L,/L, % 0 the vehicle would tend to operate as an elec-
`tric vehicle over the same cycle. In between these two
`extremes, hybrid operation occurs and, as L J L ,
`increases, greater emphasis is placed on the electric trac-
`tion system, so that range and fuel consumption
`decrease, with energy from the battery being used to
`substitute petroleum fuel.
`At L,/L, = 0.35-0.4 the range is infinite as no net
`energy is removed from the battery and yet fuel con-
`sumption is reduced relative to the ic. engine-only
`extreme. In this condition the energy recovered by the
`electric traction system during braking is used during
`the acceleration phase of the vehicle after accounting for
`system losses. This results in an increase in the average
`it. engine efficiency and a corresponding reduction in
`fuel consumption. This point also corresponds to the
`condition of minimum energy use (3). In this condition
`
`Q IMechE 1988
`
`the hybrid vehicle can be described as operating in the
`‘energy-saving’ hybrid mode.
`Analysis of the UK Travel Survey Data (10) shows
`that 95 per cent of all journey lengths are under 80 km
`and, although this urban range cannot be achieved by
`the base hybrid vehicle when in its all-electric mode, it
`would satisfy 90 per cent of all journeys. This implies
`that the vehicle could undertake most urban journeys
`in an electric mode leaving the hybrid mode, with
`0.4 < L,/L2 < 1.0, for longer urban journeys and the i.c.
`engine mode for high-speed cruise. Such a control
`scheme has substantial potential for substituting pet-
`roleum fuel by ‘wall plug’ electricity. In the i.c. engine
`mode the fuel consumption of the hybrid at 90 km/h
`would be 5.5 litres/100 km and 7.8 litres/100 km at
`120 km/h.
`Inspection of the i.c. engine usage maps in Fig. 6 for
`two values of L J L , shows that the use of the i.c. engine
`tends to be restricted to an area surrounding the point
`of maximum efficiency, the size of which depends upon
`the emphasis placed on this particular power source.
`This implies that the operation of the i.c. engine tends
`to be the dominant factor in determining the drivetrain
`control. It should be noted that the optimization
`process is constrained in that i.c. engine clutch slip is
`not allowed such that the electric traction system is
`used to move the vehicle from rest. This operating pro-
`cedure results in efficient operation of the i.c. engine and
`simplifies the drivetrain control in that no automated
`clutch control is necessary. To further improve driveline
`efficiency the i.c. engine is assumed to be switched off
`and decoupled from the driveline when not in use. The
`traction motor is permanently connected to the drive-
`line to ensure that regenerative braking capability is
`always immediately available.
`The torque variations experienced by the i.c. engine
`and the electric motor throughout the cycle are shown
`Proc Instn Mech Engrs Vol 202 No D1
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`56
`
`-
`90
`-
`81
`-
`12
`-
`3 63
`g
`5 4 -
`’ E
`.- 8 2 4 5 -
`-
`Do
`6
`36
`-
`27
`-
`18
`9 -
`01
`0
`
`I
`
`1000
`
`(a) L,IL, = 0.4
`
`d
`
`l
`
`
`
`I FORSTER AND J R BUMBY
`
`701
`
`60
`
`I
`1
`3000
`2000
`Engine speed
`rlmin
`
`I
`
`4000
`
`L
`5000
`
`150
`
`200
`
`L 1
`100
`Time __
`S
`
`(a) Internal combustion engine
`
`100
`
`20
`
`- 20
`
`,% - 630 -
`540 -
`5 450 -
`360 -
`2 7 0 -
`
`U
`
`$
`
`E
`
`8
`
`0
`
`1000
`
`3000
`2000
`Engine speed
`r/min
`
`4000
`
`5000
`
`Fig. 6
`
`0.6
`(b)L,/L,
`Usage data for the i.c. engine for two values of L J L ,
`(ECE-15) (the figures indicate the percentage of the
`cycle time the i.c. engine spent in each area of the fuel
`map)
`in Fig. 7 for the hybrid with L J L , N 0.6. These results
`show that sharing of the load between the two energy
`sources is only resorted to when either alone cannot
`meet the power demand and then it is the primary i.c.
`engine mode that is used. When in this mode the i.c.
`engine torque is limited to about 90 per cent of full-
`throttle output in order to maximize the i.c. engine eff-
`ciency. The true hybrid mode defined in Table 2
`whereby torque is proportioned between the two prime
`movers is never selected and nor is the primary electric
`made. The torque variations also show that with the
`optimization process used here battery charging from
`the i.c. engine is avoided if at all possible due to the low
`overall conversion effciency associated with this route.
`However, on long journeys, should the state of charge
`of the battery become unacceptably low, then battery
`charging from the i.c. engine would be adopted.
`Although Fig. 5 shows that potential exists for the
`hybrid drive in substituting petroleum fuel by ‘wall
`plug’ electricity, the so-called ‘petroleum substitution’
`hybrid, no account has yet been taken of the effect
`variations in the rating of the driveline components may
`have. In addition,
`the optimal control algorithm
`described above uses a direct search method to deter-
`mine the minimum of a discontinuous and highly non-
`Proc Instn Mech Engrs Vol 202 No D1
`
`Time __
`S
`
`(b) Electric motor
`Fig. 7 Torque variations over the ECE-15 urban driving
`cycle for L J L , ‘v 0.6
`
`linear objective function. As a result, a large amount of
`computing time is required, so making implementation
`of the algorithm in real time difficult. However, the i.c.
`engine usage data presented in Fig. 6 suggest that a
`control strategy that seeks to limit the operation of the
`i.c. engine to a specified region of high effciency could
`produce comparable results.
`
`3.3 Sub-optimum control
`To develop a control algorithm that can be implement-
`ed on an actual vehicle a sub-optimal control algorithm
`is postulated that seeks to restrict the operation of the
`i.c. engine to the high-efficiency region. This algorithm
`accepts demand power as its control variable and, by
`sensing road speed, transforms this power to a torque at
`the output of the transmission. Demand power, as far as
`the simulation is concerned, is simply transmission
`output power, but in reality would be driver-demand
`power, expressed as a function of accelerator pedal posi-
`tion. Knowing the fixed transmission ratios available, a
`set of torque and speed values at the torque split point
`can be defined, the number of which will correspond to
`the number of discrete gear ratios available.
`By defining an operating region or ‘box’ around the
`i.c. engine maximum effciency region as shown in Fig. 8
`then a region of acceptable engine performance is
`defined. The control algorithm always seeks to place the
`i.c. engine operating point within the ‘box’ using the
`available transmission ratios. If no points occur in the
`@ IMechE 1988
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`BMW1016
`Page 7 of 15
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`

`

`A HYBRID INTERNAL COMBUSTION ENGINEjBATTERY ELECTRIC PASSENGER CAR
`
`57
`
`A Electric operation
`B Internal combustion engine operation
`C Hybrid operation
`
`D Operation not allowed A E Reduced sub-optimum region
`
`I
`
`- - - - - \
`
`\
`
`\
`
`\ \
`‘ , Upper torque bound
`
`IS0 k
`I
`
`100
`
`so
`
`O l
`0
`
`I
`I
`I
`I
`( E l
`I
`I
`
`I
`I
`
`I
`1000
`
`
`
`B
`
`D
`
`speed bound
`
`2
`
`I
`4000
`
`Lower
`torque bound
`
`I
`5000
`
`. _
`I
`I
`3000
`2000
`speed
`r/min
`Fig. 8 Sub-optimum control operating regions
`
`box and all points fall below or to the left of the box,
`then the electric mode of operation is selected. If all
`points occur above the box then primary i.c. engine
`operation is selected with the i.c. engine set at about 90
`per cent throttle (to maximize engine efficiency) with the
`electric traction system making up the additional torque
`requirement. Should more than one point occur inside
`the region, then the box is ‘shrunk’ towards the point of
`maximum efficiency and the test repeated.
`During the electric mode of operation a transmission
`ratio is selected that places the traction motion speed
`closest to its ‘break speed’ as it is in this region that
`maximum traction motor efficiency occurs.
`The result of applying such a control algorithm to the
`hybrid drive designed for petroleum substitution is
`shown in Fig. 9 where the i.c. engine fuel usage map and
`the variation in i.c. engine and traction motor torque
`and speed over the cycle are shown. As can be seen, i.c.
`engine usage is successfully restricted to the high-
`efficiency region and is comparable with the usage data
`for the optimally controlled case shown in Fig. 6b. A
`comparison of the i.c. engine and electric motor torques
`with Fig. 7 shows a similar profile but with a reduced
`number of gear changes. This results because the
`optimum algorithm always seeks the minimum of the
`objective function F, and therefore may make rapid
`changes in the operating strategy that are practically
`unacceptable. Such changes are avoided with the sub-
`optimum algorithm, making for a more practical
`control policy.
`
`4 OPTIMIZATION OF THE HYBRID DRIVE
`4.1 The influence of battery size
`To ensure that satisfactory electric mode performance is
`obtained from the hybrid drive, designed for ‘petroleum
`0 IMechE 1988
`
`substitution’, it is necessary that sufficient energy is
`stored on-board the vehicle to give a range of about
`60 km. To achieve acceptable range in electric vehicles
`it is necessary to use a battery that is typically 25-30
`per cent of the gross vehicle weight (GVW). In order to
`understand why this is so it is necessary to consider
`how the power density of a lead-acid traction battery
`varies with energy density, a typical variation of
`which is shown in the Ragonne plots of Fig. 10. This
`figure shows that not only is the maximum energy
`density available relatively low [typically 144162 kJ/kg
`( 4 M 5 W h/kg) for an advanced lead-acid cell] but also
`as the power density discharge increases the available
`energy density decreases. Therefore, in order to improve
`the discharge efficiency of the battery, it is necessary to
`limit the maximum power density excursions of the
`battery by employing a large battery weight. However,
`as the battery weight increases an increasing proportion
`of the battery energy is being used to propel the addi-
`tional battery weight and not the payload, even though
`vehicle range may still be increasing. To quantify how
`well the battery is being utilized the range per kilogram
`of battery weight (the specific range) can be expressed as
`a function of the ratio of battery weight to total vehicle
`weight (the specific battery weight or battery fraction).
`Such a variation is shown in Fig. 11 for the hybrid car
`in the electric mode for several different driving cycles
`and battery types.
`When using a lead-acid battery and simulating the
`hybrid vehicle over
`the ECE-15 urban cycle the
`optimum battery weight for maximum battery uti-
`lization occurs at about 25 per cent of vehicle weight
`while for a constant 90 km/h cruise it increases to about
`30 per cent of vehicle weight. At battery fractions below
`the optimum, power discharges are high, and so limit the
`available energy from the battery while with too large a
`
`Proc Instn Mech Engrs Vol 202 No D1
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`BMW1016
`Page 8 of 15
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`

`

`-
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`0'5 2'o
`
`0.5
`
`I FORSTER AND J R BUMBY
`r
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`I<
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`C
`
`58
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`
`1000
`-
`900
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`7 0 0 -
`-
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`u1 g,
`h
`600
`8 m
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`200-
`
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`
`1
`
`I
`3000
`2000
`Engine speed
`dmin
`
`I
`4000
`
`1
`5000
`
`(a) Internal combustion engine usage data (the figures indicate the
`percentage of cycle time the i.c. engine spent in each area of the fuel
`map)
`
`it
`
`EV-106
`
`Improved state of the art } Lead acid
`7
`
`EV2- 13
`-
`d Nickel iron
`e Nickel zinc
`
`1
`3.6
`
`J
`
`I
`36
`
`\ \\\ I
`'
`\
`
`I
`360
`
`Energy density
`kJ/kg
`Fig. 10 Ragonne plots for various battery types
`
`charge efficiency is increasing. For a pure electric
`vehicle with a lead-acid battery, typical battery weights
`are 25-30 per cent of the total vehicle weight and are
`close to the maximum utilization as defined by Fig. 11.
`In practice, selected battery fractions may err to the
`right of the maximum to ensure adequate range and
`maintain driver confidence.
`With the hybrid electric vechicle designed particularly
`for petroleum substitution battery utilization can be
`maximized with a battery fraction of 2&25 per cent.
`Choosing a battery fraction slightly below the optimum
`will help to minimize vehicle weight and is acceptable as
`driver confidence from a range point of view is achieved
`
`w
`
`ECE- 15:Ni/Zn
`
`90 km/h:Ni/Fe
`
`ECE- I5 : Ni/Fe
`
`90 km/h
`
`150 a
`200
`
`L
`L
`100
`Time
`S
`
`(b) Internal combustion engine torque
`
`0.3.
`
`T
`
`