D Optimisation and control of a hybrid electric car
`
`J.Ft. Bumby, BSc, PhD, CEng, MIEE
`I. Forster, BSc, PhD
`
`Indexing terms: Optimal control, Optimisation, Algorithm, Mathematical techniques
`
`Abstract: The paper examines the potential of
`hybrid electric vehicles and, in particular, a hybrid
`electric passenger car. Two operating objectives
`are identified, one for energy saving and the other
`for
`substituting petroleum fuel
`by
`electrical
`energy. The way in which the power train control
`and component rating can be optimised to meet
`these particular operating objectives is discussed.
`In the final part of the paper the perfonnance of
`the optimised hybrid vehicles are compared with
`both [C engine and electric vehicles and the pet-
`roleum substitution design is shown to warrant
`
`further development.
`
`1
`
`Introduction
`
`A hybrid vehicle can be defined as a vehicle which utilises
`two or more energy storage mediums within its drive
`train, any of which is capable of driving the vehicle when
`connected to the road wheels through a suitable prime
`mover. In this paper, the hybrid electric vehicle is con-
`sidered with energy being stored in petroleum fuel and an
`electric traction battery. The associated prime movers are
`
`engine power and speed to be controlled independently
`of the power and speed demand at the road wheels. In
`contrast,
`the second basic arrangement connects both
`prime movers in parallel, as shown in Fig. 2. In such a
`parallel arrangement, both prime movers are capable of
`driving the road wheels directly with the ‘mix’ of power
`at any instant being controlled.
`The first hybrid vehicles appeared in the early part of
`this century when the electric traction motor was used to
`augment the power output of the then limited IC engine
`[1]. However, the rapid development of the IC engine
`soon made the electric traction motor unnecessary and
`hybrid vehicles were not again considered seriously until
`the 1960s. At this time, concern was being expressed at
`the level of exhaust emissions from conventional
`IC
`
`engine vehicles, and the hybrid emerged as one possible
`way in which exhaust emissions could be dramatically
`reduced [2]. However, with the oil crises of 1974 and
`1979 there became immediate concern about the depen-
`dence of the Western World on oil-based fuels. The
`
`hybrid vehicle now became one possible way of reducing
`the dependency of the transport sector on petroleum-
`based fuels, either by reducing the amount of energy used
`[3] or by substituting petroleum fuel by broader based
`electrical energy [4]. These latter objectives are equally
`wheels
`
`accelerator
`
`brake
`
`Fig. 1
`
`Series hybrid electric vehicle drive train
`
`
`
`an internal combustion (IC) engine and electric traction
`motor,
`respectively. To utilise the prime movers, a
`number of difl"erent power train configurations are pos-
`sible, but, in general, fall into one of two basic categories.
`In the series arrangement of Fig. 1, the IC engine does
`not drive the road wheels directly, but is connected indi-
`rectly through an electric generator and electric traction
`motor. Introducing a traction battery between the gener-
`ator and motor buffers the engine output, allowing the
`
`Paper 562lD(Cl2, SI) received 24th February 1986
`The authors are with the Department of Engineering, University of
`Durham, Science Laboratories, South Road, Durham DH]
`JLE,
`United Kingdom
`
`IEE PROCEEDINGS, Vol. I34, Pt. D, No. 6, NOVEMBER I987
`
`Page 1 of 15
`
`accelerator
`
`controller
`
`
`
` brake
`
`Fig. 2
`
`Parallel hybrid electric vehicle drive train
`
`373
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`is this particular aspect of reducing
`valid today, and it
`the dependency of road vehicles on petroleum-based fuels
`that this paper addresses.
`To achieve the objectives outlined so far, both series
`and parallel hybrid arrangements have been evaluated in
`the past. Although the series arrangement allows great
`flexibility in component positioning, it is not a generally
`favoured arrangement. The main reason for this is that
`the acceleration and maximum continuous speed require-
`ments of most vehicles necessitates the use of a large,
`heavy and expensive traction motor rated to meet both
`the maximum torque and maximum continuous power
`demand of the vehicle. Consequently, when the vehicle is
`used over mild urban driving cycles, the traction motor
`operates at part
`load with a relatively low efficiency.
`When this is combined with the efficiency of the gener-
`ator, controller and IC engine,
`the result
`is reduced
`energy performance when compared with the parallel
`arrangement [5]. However, if a vehicle is to be designed
`for a specific duty cycle, for example a city bus, then the
`potential of the series arrangement increases. Indeed, it is
`for this particular use that the series hybrid has seen most
`development [6, 7].
`is the parallel
`As a result of these considerations, it
`hybrid arrangement that offers most potential. However,
`even within this framework, the component arrangement,
`rating and control offers innumerable possible alterna-
`tives. To develop a suitable control strategy, and decide
`on the appropriate component sizing and arrangement, a
`number
`of
`studies
`have
`been
`commissioned
`and
`
`published, for example see References 8 and 9. The most
`recent of the hybrid vehicle studies have been conducted
`during the feasibility stage of the Near Term Electric and
`Hybrid Vehicle Programme commissioned by the US
`Department of Energy [10]. The tendancy in all of the
`most recent studies is to use some form of computer
`simulation to assess the performance of the vehicle over a
`predefined driving cycle. Parametric studies are then con-
`ducted to show how modifications in control strategy,
`component sizing and arrangement effect the vehicle per-
`formance. Typical of these studies are those conducted by
`the General Electric Company (USA) [ll, 12], where
`computer simulation methods were used to evaluate and
`design a hybrid vehicle suitable for the American car
`market. The aim of this vehicle being to substitute pet-
`roleum fuel by ‘wall plug electricity’.
`In this paper, rather than postulating a number of
`control options and exploring their relative benefits, an
`optimisation process is used which seeks to minimise an
`energy-based objective function, the aim of which is to
`reduce the dependence of the vehicle on petroleum-based
`fuels. This process then leads to the definition of a
`control algorithm that can be used in a vehicle suitable
`for the European car market. Parametric studies are then
`conducted to optimise component ratings and further
`improve the vehicle performance. The final part of the
`paper compares the optimised hybrid design(s) with an
`IC engine vehicle and electric vehicle, and their relative
`features are discussed.
`
`2
`
`Base vehicle parameters
`
`A previous analysis of the UK national energy statistics
`has
`shown that,
`in the road transport
`sector, cars
`between 1000cc and 2000cc (ccEcm3) are the major
`users of petroleum fuel, consuming approximately 40%
`of all the energy used [13]. They also represent a large
`market in potential sales, with approximately one million
`374
`
`vehicles being sold per annum. Consequently, if either a
`reduction in the amount of petroleum used, or a transfer
`from petroleum to electrical energy could be achieved
`within this market,
`there is significant potential for a
`reduction in the UK dependence on petroleum-based
`fuel.
`
`Analysis of the usage pattern of the type of vehicle
`described here shows that 95% of all car journeys are less
`than 80 km and could, therefore, be satisfied by an elec-
`tric vehicle. However, the usage pattern also shows that
`this type of vehicle is used regularly for journeys in excess
`of 80 km, for example at weekends and holidays. Conse-
`quently, although 95% of all journeys are under 80 km,
`any vehicle restricted to 80 km or less may only be useful
`80% of the time [14] and would be unlikely to find
`general
`acceptance. For any hybrid vehicle
`to be
`accepted, it must achieve a reduction in the petroleum
`fuel used, while not suffering the range limitation of the
`electric vehicle.
`
`An initial set of parameters representative of a parallel
`hybrid car able to meet these needs are outlined in Table
`1. The size of the IC engine has been selected to give a
`maximum level cruise speed in excess of 120 km/h, while
`the electric traction motor augments this to provide the
`necessary acceleration performance and low-speed elec-
`tric operation. The torque/speed envelope for the com-
`bined system is shown in Fig. 3. A set of parameters for
`250
`
`200
`
`150
`
`50
`
`torque,Nm S O
`
`1000
`
`2000
`
`3000
`
`1.000
`
`5000
`
`engine speed, rev/min
`
`Base hybrid electric performance curves
`Fig. 3
`a Combined maximum torque line
`b Traction motor torque
`c Engine full throttle torque
`A constant speed road load
`
`an equivalent IC engine car are also included in Table 1.
`In both cases, performance is comparable and, through-
`out
`the paper, vehicles will be compared assuming a
`similar performance criterion with component weight
`changes being automatically included. A weight propaga-
`tion factor of 1.35 is used throughout
`the study. To
`achieve uniformity in terms of body perfonnance, both
`the hybrid and the IC engine vehicle are assumed to have
`the same drag and rolling resistance characteristics. This
`ensures that any benefits accruing from the hybrid are
`identified as coming from the power train. The values
`used are typical of good present day vehicles.
`In assessing the performance of the hybrid vehicle,
`the standard European urban driving cycle, ECE-15,
`is
`used along with 90 km/h and 120 km/h cruise results.
`
`IEE PROCEEDINGS, Vol. I34, P1. D, No.6, NOVEMBER 1987
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`Page 2 of 15
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`Table 1 2 Base vehicle data
`
`Parallel hybrid
`
`Conventional
`
`is necessary to
`it
`before any control can be attempted,
`define precisely the purpose of the control. For example,
`
`1640 kg
`1880 kg
`300 kg
`
`0.35
`0.01
`1.95 m’
`
`945 kg
`1185 kg
`
`0.35
`0.01
`1.95 m’
`
`Vehicle weights:
`kerb weight
`test weight
`battery
`Vehicle parameters:
`C,
`C,
`A
`Component sizes:
`IC engine
`traction motor
`battery
`
`final drive
`transmissions
`
`35 kW, 5000 rev/min
`35 kW, shunt
`lead/acid EV2-13
`E5 = 150 kJ/kg
`(42 Wh/kg)
`3.5 : 1
`4-speed automatic
`gear ratios
`1st
`3.521
`2nd
`2.4 :1
`3rd
`1.3:1
`4th
`1.0:1
`
`
`
`cruise
`
`
`
`
`lo
`1:
`lco
`lb
`ts
`
`acceleration
`
`Fig. 43
`
`J22? schedule ‘a’ series of driving cycles
`
`Parameter
`
`A
`
`Driving cycle
`C
`
`B
`
`D
`
`Max. speed
`V,,miIe/h
`Accel. time
`r_,s
`Cruise time
`r,,s
`Coast time
`(ms
`Brake time
`t,.s
`Idle time
`t,,s
`Total time,
`5
`
`1011201] 30:!
`
`45:1
`
`431 I911 l8j:2
`
`2312
`
`0
`
`I911 2011
`
`5012
`
`2:1 4:1 811
`
`I03!
`
`31f St! 9:1
`
`9:1
`
`3012 25:2 251% 25:2
`
`39:2 7212 80:2 l22:2
`
`the main objective of the control may be to maximise the
`accelerative performance of the vehicle, minimise exhaust
`emissions or
`to minimise energy use. An alternative
`objective, and the subject of this paper,
`is to examine
`ways
`in which the dependence of
`the vehicle on
`petroleum-based fuels can be reduced. This objective can
`be achieved either by improving the overall energy con-
`sumption of the vehicle, or by transferring some of the
`energy demand to the electrical system. To examine the
`type of control policy that would achieve this, an energy-
`based objective function:
`
`F = 1,5, + A, 15,
`
`(1)
`
`is defined, where E 1 and E, are the onboard petroleum
`and electrical energy requirements, respectively, and A,
`and 2., are weighting factors.
`The two energies E, and E, depend on the way in
`which the load is divided between two prime movers.
`Examination of Fig. 2 suggests that the demand torque
`could be supplied either by the IC engine or the electric
`motor alone, or from some appropriate combination of
`the two. Indeed, the IC engine could supply torque in
`excess of the value demanded at the road wheels, such
`that the excess energy is used to charge the traction bat-
`teries. These various possibilities can be rationalised by
`defining a torque split fraction, or ratio, which defines
`the percentage of the road torque supplied by the IC
`engine as shown in Fig. 5. At torque split values in excess
`of one, the [C engine supplies the full torque demand,
`and additional energy is used to charge the traction bat-
`teries.
`
`Although E, and E2 depend directly on the ratio in
`which the demand torque is divided between the two
`prime movers, they are also dependent on the efficiency
`of the prime movers and associated equipment. As the
`375
`
`3CoT
`
`»>
`
`55 kW, 5000 r.p.m.
`-
`—
`
`3.5 : 1
`4-speed manual
`
`3.5:1
`2.4: 1
`1.3:1
`1.0:1
`
`12 s
`
`145 km/h‘
`
`Performance
`0-60 mile/h
`(driver only)
`Max. speed:
`IC engine only
`hybrid
`
`’ at 5000 rev/min
`
`14 s
`
`130 km/h
`145 km/h
`
`The ECE-15 cycle is shown in Fig. 4A and, besides defin-
`ing precisely the velocity time profile, gear change points
`are also defined. As will be seen in Section 6, the use of a
`different gear change schedule, optimised to the actual
`vehicle, can significantly reduce the urban fuel consump-
`tion. As the hybrid vehicle necessitates total control of
`the drive train, the ECE-15 gear change schedule will not
`be adhered to. This is similar to current testing practice
`for vehicles with automatic transmissions.
`
`40
`
`E 30
`3
`E
`>-
`
`o
`5 2°
`E
`>
`
`10
`
`60
`
`5° 5
`E
`40 -“_
`>
`
`o
`30 '5
`E
`20 >
`
`10
`
`20
`
`40
`
`60
`
`80
`
`I00
`
`120
`
`11.0
`
`160
`
`I80 200
`
`time _s
`
`Fig. 4A
`
`ECE-I5 urban driving cycle
`
`The ECE-l5 cycle is a relatively mild cycle and,
`because of this, some aspects of hybrid vehicle per-
`formance can be difficult to interpret clearly, while when
`a slightly more severe cycle is used these performance
`aspects are clarified. In such situations, ECE-15 results
`are augmented by simulations over the .l227a-D urban
`cycle shown in Fig. 4B.
`
`3
`
`Hybrid vehicle control
`
`3.1 Control optimisation
`When two or more power sources are used in a vehicle
`power train, the way in which they are controlled is fun-
`damental
`to the performance of the vehicle. However,
`
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`the traction motor
`efficiency of either the engine or
`depends strongly on its operating torque and speed, then
`
`torque
`
`0.5
`
`torque split ratio
`
`
`
`Eflecl of torque split ratio on the torque distribution
`Fig. 5
`a [C engine torque
`b Electric motor torque
`in + b Demand torque
`
`torque split ratio and transmission gear ratio are the two
`control variables within the drive train that vary the
`value of the objective function at any given demand
`load. The aim of the control optimisation is therefore to
`minimise the objective function defined in eqn. 1, with
`torque split and gear ratio as the control variables.
`During a driving cycle, the torque demand and oper-
`ating speed of the prime movers is continually changing,
`and therefore it
`is necessary to minimise the objective
`function on a continuous basis. To implement the opti-
`misation process, the hybrid vehicle is simulated over a
`defined driving cycle using the Janus road vehicle simula-
`tion program [15]. This program calculates the torque
`and speed requirement at the road wheels, at each second
`of the driving cycle, and then reflects this demand back
`through the power
`train to the energy source(s)
`to
`compute the net
`input energy required over that one
`second interval. Throughout this process, full account is
`taken of the losses associated with each of the drive train
`
`components. These losses vary both with load and speed
`and, within the prime movers, can be particularly
`complex. For these components, efficiency maps of actual
`components are used. By calculating the energy supplied
`from both the IC engine and the battery at each time
`instant, over a range of torque splits and for all gear
`ratios, a three-dimensional map can be generated with
`torque split ratio and gear ratio as the two independent
`variables and the objective function as the dependent
`variable. A direct search technique is then employed to
`find the minimum of the objective function. Repeating
`this process at each second throughout the cycle allows a
`minimum energy path through the driving cycle to be
`obtained.
`
`1 shows that E, and E,
`Further examination of eqn.
`are dependent on the efficiency and operating character-
`istics of all the power train components, while 1., and 1.,
`allow a weighting to be placed on the relevant impor-
`tance of the two energy sources. By varying the ratio of
`weighting factors, }.,/A2,
`the effect on the control of
`placing greater emphasis on one energy source relative to
`the other can be assessed. By selecting the correct ratio of
`weighting factors, the effect of the conversion efficiency to
`the raw energy source (e.g. power station), the effect of
`376
`
`different pricing policies, the influence of fuel supply or
`the influence of government policy towards electrical
`energy use on the hybrid control can be qualitatively
`assessed.
`
`is greater than unity, battery
`When the torque split
`charging is initiated and the flow of electrical energy
`becomes negative with all the energy being supplied from
`the petroleum fuel, E2 now represents that proportion of
`the petroleum derived energy,
`less any conversion loss,
`that is converted to chemical energy and stored in the
`battery. To utilise this energy at a later time, account
`should be taken of the power train efficiency when this
`chemical energy is reconverted and appears at the torque
`split point. This reconversion efficiency includes
`the
`battery discharge efficiency, motor and controller effi-
`ciency and the gear efficiency of the torque split. As this
`net efficiency varies with load and speed, an optimistic
`value of 90% is assumed implying a 3., value during this
`operating mode of 1.1 regardless of the value set on 2,.
`Throughout
`the optimisation process, although E,
`is
`directly related to the petroleum fuel used, E, is depen-
`dent on the rate at which the battery is discharged. E, is
`therefore calculated as the product of the incremental
`change in the battery state of charge and the battery five-
`hour energy capacity.
`The optimisation process must also take into account
`any physical constraints imposed by the drive train. Fig.
`2 shows a clutch between the engine and transmission
`and, although an operating condition where this clutch is
`continually slipped or modulated can be conceived, it is
`not particularly attractive. The optimisation process is
`therefore constrained not
`to allow this mode of oper-
`ation.
`
`3.2 Optimal control of the base hybrid vehicle
`Implementing the above optimisation with the base
`hybrid vehicle parameters of Table 1 allows vehicle range
`(i.e. distance travelled until the battery is discharged) and
`petroleum use to be plotted against the ratio 2.,//lz for the
`ECE-l5 driving cycle as shown in Fig. 6. The full curve
`in this Figure refers to the base vehicle with a standard
`300
`
` E
`
`x‘
`§~
`9
`
`too
`
`0
`
`0
`
`0.25
`
`0.5
`
`0.75
`
`l.O
`
`l.Z5
`
`I5
`
`\ fuel consumption
`\
`
`9 E
`3 E
`7
`E
`s
`5,
`
`5 §
`
`4
`3
`2
`1
`0
`
`2
`3
`§
`_
`g
`“
`
`A I I A2
`Influence of weighting factor ratio on the performance of the
`Fig. 6
`base hybrid vehicle
`4-speed transmission
`-— — Continuously variable transmission
`- --
`a Fuel consumption of base IC engine vehicle
`b 80 km range. 95% of all journeys
`c 50 km range. 90% of all journeys
`d 30 km range. 80% of alljoumeys
`
`.
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`centage of the cycle time that the engine spent in different
`portions of the map is shown. The optimal control policy
`maximises engine efficiency by moving each operating
`100
`
`90
`80
`70
`60
`50
`
`1.0
`
`20
`,0
`
`B.
`E,
`g
`gr
`'
`'0
`
`O"
`
`8 _
`U
`" -
`
`5
`‘Si? 5
`too
`so
`Egg
`time, s
`at
`Variation of torque split ratio (%) during the ECE-I5 urban
`Fig. 7A
`driving cyclefor the optimally controlled energy saving hybrid
`
`0
`
`20
`
`1.0
`
`so
`
`120140160
`
`I80
`
`200
`
`1.
`
`3
`
`'5 20
`U5
`
`1 N
`
`0
`
`20
`
`40
`
`60
`
`80
`
`100
`time, s
`
`I20
`
`I40
`
`I60
`
`I80 200
`
`Variation of year during the ECE-15 urban driving cycle for
`Fig. 73
`the optimally controlled energy saving hybrid
`
`100
`
`90
`
`80
`\l O
`0‘ O
`
`
`
`engineloadtoctor_'/. N(JU‘COO
`
`1‘ 0
`
`O
`
`0
`
`20
`
`40
`
`60
`
`80
`
`00
`
`120
`
`11.0
`
`I60
`
`180 200
`
`times
`Variation of engine load factor during the ECE-I5 urban
`Fig. 70
`driving cycle for the optimally controlled energy saving hybrid
`torque
`maximum torque available
`
`engine load factor =
`
`x l00%
`
`70
`
`10 motor
`loadfoctorfl.
`
`40
`
`I20
`I00
`time-,5
`
`11.0
`
`zoo
`
`60
`50
`
`1.0
`
`30
`
`20
`
`°
`-,o
`-20
`
`Variation of motor load factor during the ECE-I5 urban
`Fig. 7D
`driving cyclefor the optimally controlled energy saving hybrid vehicle
`
`377
`
`four-speed transmission, while the dashed lines show the
`effect of introducing a continuously variable transmission
`(CVT). At low ratios of A,/1., vehicle range is infinite, as
`the energy drained from the battery during motoring is
`replaced
`by
`energy
`recovered
`during
`regenerative
`braking. As A,/A2 increases, the petroleum consumption
`reduces with range remaining infinite. When }.,/}.2 2 0.3
`to 0.35, all
`the energy recuperated during regenerative
`braking is used during the motoring phase, and the state
`of charge of the battery is the same at the end of the cycle
`as at the beginning. At the other extreme when 2.,/2.,
`is
`greater than one,
`the vehicle essentially operates as an
`electric vehicle with no petroleum fuel being used. In
`between these two extremes, as /1, /.12 is increased, greater
`emphasis is placed on the electrical system with increas-
`ing substitution of petroleum fuel by electricity.
`These observations lead to the specification of two
`types of hybrid vehicle. In the first vehicle, the battery
`state of charge is the same at the end of the cycle as at
`the beginning, with all the energy being supplied directly
`from the petroleum fuel. The electrical system now seeks
`to accept regenerative braking energy and provide pro-
`pulsion power when the IC engine efficiency is low. This
`has the effect of increasing the average engine load factor
`and efficiency. Battery weight should be minimised to
`improve the hybrid performance, and, as a result, no sig-
`nificant all-electric range should be anticipated. Such a
`vehicle can be termed an ‘energy saving hybrid’. In con-
`trast, the second type of vehicle sacrifices urban range for
`reduced petroleum fuel consumption, thereby achieving
`significant petroleum displacement. This would ideally
`require a battery weight above that of the base vehicle, to
`achieve a range as an electric vehicle of typically 60 km.
`Such a vehicle may be described as a ‘petroleum substi-
`tution hybrid’.
`
`3.3 Energy saving hybrid
`From an overall energy point of view /1, and 2., can be
`selected to represent minimisation of raw energy. If an
`efficiency of 25% is assumed, for power generation, trans-
`mission and battery charging and an efficiency of 90%
`for the petroleum production process, 11/112 2: 0.28. This
`implies energy minimisation will be achieved with the
`energy-saving hybrid. However, should petroleum be
`produced from coal,. with a conversion efficiency of 60%,
`then energy minimisation would be obtained with
`).,//“.z »_~_« 0.42. This now points to mild hybrid operation.
`Although the above argument
`indicates the type of
`hybrid design that would minimise energy use, an overall
`energy saving would only be achieved if a comparable IC
`engine vehicle had a higher fuel consumption. The urban
`fuel consumption for the base IC engine vehicle described
`in Table l is shown in Fig. 6, and is substantially higher
`than the fuel consumption of the energy-saving hybrid
`with optimal control.
`The variation of the two control variables, torque split
`and gear ratio throughout the cycle are shown in Fig. 7
`for the energy-saving hybrid with ).,/).2 20.3 to 0.35.
`These results suggest that charging of the batteries from.
`the IC engine is not a favoured option owing to the low
`conversion and reconversion efficiency associated with
`this route, while operation on one or other of the two
`energy sources is generally favoured. When operating on
`the IC engine, the lowest gear ratio (highest gear) is sel-
`ected to increase the engine output torque, reduce engine
`speed and hence increase engine efficiency. Greater detail
`of how the IC engine is used during the cycle is shown on
`the engine usage map of Fig. 8. In this Figure, the per-
`
`IEE PROCEEDINGS, Vol. I34, Pt. D, No. 6, NOVEMBER I987
`
`Page 5 of 15
`
`FORD 1492
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`

`
`region as shown in Fig. 9, with increased emphasis being
`placed on the electric traction system.
`The distribution of torque between the two prime
`movers is shown in Fig. 10, for the base vehicle of Table 1
`with }.,/A, z 0.6 and subject to the constrained optimised
`
`0
`
`2o
`
`1.0
`
`so
`
`100 120140160180 zoo
`so
`limo;-,5
`
`ICX)
`
`90
`
`so
`70
`so
`50
`
`Z
`
`E
`0'
`g
`§
`1:)
`
`C ‘
`
`E’
`
`40
`30
`2'68 20
`E"3‘gun I0
`."'>~
`13§§
`II|0.0.
`33’:
`
`Variation of torque split ratio (%) during the ECE-I5 urban
`Fig. 10A
`driving cyclefor the optimally controlled petroleum substitution hybrid
`
`0
`
`20
`
`L0
`
`60
`
`100
`80
`tirne,s
`
`120140 160
`
`180
`
`200
`
`Variation of gear during the ECE-I5 urban driving cycle for
`Fig. 103
`the optimally controlled petroleum substitution hybrid
`
`I00
`
`90
`QC
`
`\IO
`O! 0
`(J1 O
`l\0
`
`G—-N)(A)Oooo
`
`
`
`engineloadtoctorfl.
`
`20
`
`1.0
`
`60
`
`80
`
`100
`
`I20
`
`I40
`
`I60
`
`180
`
`200
`
`lime,s
`
`Variation of engine load factor during the ECE-I5 urban
`Fig. 10C
`driving cycle for the optimally controlled petroleum substitution hybrid
`70
`
`60
`
`50
`
`J‘ O
`(4)O
`
`IQ
`
`oo
`
`L. o
`
`time,s -20
`
`
` 8motorloadtactorfl.
`
`100
`
`120
`
`140
`
`900 . 800
`~—/'6},
`roo
`0.5 1.o(5f‘57
`/----\
`600
`’q.§__
`~..» 1.5
`to 1:0
`"
`1.5
`1.5
`-_i_.o
`1.5
`_,
`500
`~ “
`,—v""
`
`2,0
`\
`
`_/
`
`I’
`
`
`
`brokemeanettectivepressure,kPa
`
`point as close to the maximum efficiency region as the
`available transmission ratios will allow. Thus there is a
`
`1000
`
`1.0
`
`
`
`0
`
`1000
`
`2000
`
`3000
`
`1.000
`
`5000
`
`engine speed , revlmin
`IC engine usage data for the optimally controlled energy saving
`Fig. 8
`hybrid over the ECE-I5 urban cycle
`Percentage |ime engine off = 53.6%
`
`tendency to use low gear ratios (high gears) as much as
`possible when the IC engine is selected as the power
`source. The use of the electric drive is also shown in Fig.
`7 and, during this cycle,
`is used only for regenerating
`braking and initial movement of the vehicle. Torque
`transfers to the IC engine when the engine speed and
`load is sufficiently high to give acceptable elliciency.
`During a more severe cycle, the electric traction system
`may augment the IC engine torque to give the required
`performance.
`These results suggest that the IC engine can be regard-
`ed as the principle power source, when the aim of the
`optimal control is to maintain the efficiency of this com-
`ponent as high as possible. This is achieved by allowing
`operation only in the most efficient part of the engine fuel
`map and by switching off and decoupling the engine
`when not in operation. In addition, a proportion of the
`accelerative energy is recovered by regenerating into the
`battery. For the energy-saving hybrid, as the battery state
`of charge is the same at the end of the cycle as at the
`beginning, this policy can be regarded as essentially being
`an improved power train energy management system.
`
`3.4 Petroleum-substitution hybrid
`from the
`The petroleum-substitution design differs
`energy-saving aim primarily in the emphasis placed on
`the electric traction system. When the ratio of weighting
`factors is increased such that AM, 2 0.6, the use of the
`IC engine is further restricted towards the high-efficiency
`
`if
`I 1000
`‘ 900
`
`/
`
`aoo
`mo
`soo
`
`soo
`400
`
`300
`200
`
`
`
`
`
`brakemeaneffectivepressure
`
`_5— —"'
`15 t..6‘/"7
`Z-9-.°-5“"'
`.’_Q.s ‘\
`0.5 0.5
`I
`I
`- \ \
`____ --—-—-‘
`
`I
`
`1000
`
`2000
`
`3000
`
`4000
`
`\ O
`
`IC engine usage data for the optimally controlled petroleum
`Fig. 9
`substitution hybrid over the ECE-I5 urban cycle
`
`engine speed, rev Imin
`
`Variation of motor load factor during the ECE-I5 urban
`Fig. 10D
`driving cycle for the optimally controlled petroleum substitution hybrid
`vehicle
`
`378
`
`Page 6 of 15
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`IEE PROCEEDINGS, Vol. I34. Pt. D, No.6, NOVEMBER 1987
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`FORD 1492
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`
`obtained by considering rating variations about this base.
`Inevitably, factors such as battery size and the relative
`rating of the power sources will differ if a vehicle is to
`meet either the energy saving or petroleum substitution
`aims. This is particularly true during urban operation,
`when the specific requirements for each type of hybrid
`must be examined separately.The effect on the two hybrid
`strategies of varying the component sizing is examined in
`the following Sections, assuming the vehicle to be opti-
`mally controlled.
`
`4.1 Energy-saving hybrid : battery size and IC engine
`power fraction
`the role of the electric
`In the energy-saving objective,
`traction system is to maximise petroleum fuel economy,
`with the battery state of charge being approximately the
`same at the end of the cycle as at the beginning. When
`operating in such a mode, the fuel economy returned by
`the optimally controlled base vehicle defined in Table 1,
`when simulated over the ECE-l5 and J227a-D urban
`
`cycles,
`
`is shown in Fig. ll. The results show the fuel
`
`8 ;
`
`5
`
`7
`
`fuelconsumption,litre/l00km 8
`
`A-—-/‘“..,re_./°//*‘*'
`5
`5 J
`
`
`
`
`
`/
`
`J227a-D
`I
`
`control. The increased use of the electric traction system
`is evident and is used for initial vehicle movement, as in
`the energy-saving hybrid, but now also during the cruise
`periods, when the torque and speed loading imposed on
`the engine would give a low efficiency. One exception to
`this is the first low-speed cruise, when the efficiency of
`both systems is low and, for this particular drive train,
`the IC engine operation is preferred. To further maximise
`engine efficiency, engine torque loadings above about
`90% are avoided and, if the engine cannot provide the
`necessary power at high efficiency, it is augmented by the
`electric traction motor. An example of this is during the
`acceleration phase between 128 and 142 s. It
`is inter-
`esting to note that, for the energy-saving hybrid,
`this
`acceleration was driven solely on the IC engine, but in a
`lower gear (higher gear ratio). This operating condition is
`not selected in this case, as with the different ratio of
`weighting factors it would now lead to an increase in the
`value of the objective function being minimised.
`One particular requirement of the petroleum substitu-
`tion vehicle is to extend the use of the electric traction
`
`system, to enable the complete urban cycle to be driven
`in an all-electric mode, thereby eliminating the high fuel
`penalty associated with a cold engine.
`It
`is therefore
`necessary that this vehicle has adequate electric range. As
`discussed in Section 2, 95% of all car journeys are under
`80 km and, although the base hybrid design cannot
`satisfy this in an electric mode, it can achieve a range of
`40 km. This would satisfy approximately 90% of all jour-
`neys, with the vehicle having to resort to hybrid oper-
`ation for urban journeys in excess of this. Such range
`demands on the vehicle are shown in Fig. 6 and indicate
`the degree of hybrid operation, and the emphasis that
`must be placed on the IC engine to meet a particular
`range requirement. Consequently, the petroleum substi-
`tution hybrid exhibits all
`the benefits of an electric
`vehicle, but has no range limitation due to the presence
`of the IC engine, and is also capable of high-speed, long-
`distance cruise performance.
`
`3.5 General considerations
`
`Discussion so far has centred on urban use of the hybrid,
`with no comment being made on cruise perfonnance. At
`90 km/h and 120 km/h, the efficiency of the IC engine in
`the conventional vehicle is high, typically 20-25%, and
`here the aim of the hybrid is not
`to compromise this
`good fuel economy due to the increased vehicle weight.
`The ways in which this can be achieved are discussed in
`Section 4.
`
`Further consideration of the optimal policy described
`earlier points to a number of factors which limit its prac-
`tical application. First, the implementation of the optimal
`algorithm requires substantial computation time because
`of the direct search technique used. As a result, it cannot
`be implemented in real
`time. Other optimisation tech-
`niques have been explored, but
`the highly non-linear
`nature of the loss variations make these difficult to use
`
`reliably. Secondly, some of the operating conditions
`imposed on the system are unacceptable, for example the
`number of gear changes being made. However, a sub-
`optimal policy that overcomes these problems can be
`developed, the effect of which is described in Section 5.
`
`4
`
`Optimisation of power train component sizing
`
`Although the component ratings described in Table 1
`form a good initial estimate of those appropriate to a
`hybrid car, further performance improvements can be
`
`IEE PROCEEDINGS, Vol. I34, Pt. D, No.6, NOVEMBER I987
`
`0 2
`
`0.3
`
`0.1.
`
`0.5
`
`0.6
`
`0.7
`
`08
`
`1C engine power traction
`
`Urban performance of the energy saving hybrid with optimum
`
`Fig. 11
`control
`O--O 200 kg, lead acid battery
`x — x
`300 kg, lead acid battery
`