Page 183 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Hybrid vehicle design CHAPTER 7.1

AC types, which are generally lighter than DC ones, and conditions. Essentially, the ‘split’ drive system divides the
94% overall efficiency could be expected, the researchers output from the engine using a planetary gear, Fig. 7.1-11.
maintain. It is suggested that one motor per drive wheel Instead of using a switching system, between series
be used with differential speed action being geared to the and parallel drive, (a), the split system acts as a series and
position of the steering wheel and an anti-spin system is parallel system at all times, the planetary gear dividing
envisaged. the drive between the series path of engine to generator
The Rover Group has been involved in the TETLEI and parallel path of engine to drive wheels. As parallel-
Euro-taxi project and K. Lillie, with Warwick University path engine speed increases in proportion to vehicle
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co-workers , has also gone into print on the gas turbine speed, output energy from the engine also increases with
series hybrid concept. The taxi will be based on the latest vehicle speed, as is normally required. At high speeds
Range Rover and the series hybrid mode allows the gas most of the engine output is supplied by the parallel
turbine to be decoupled from the wheels so as to operate path and a smaller generator for the series system can
at its optimum speed and load and avoid the classic therefore be employed.
limitation of this type of power unit, poor fuel economy The dual system, (b), is an optimized arrangement of
at light loads, poor dynamic response and a high rota- the split system and thus far has been applied to a Toyota
tional speed required, over 60 000 rpm. A sophisticated Corollawith an all-up weightof 1345 kg, involving a 660 cc
control system is required to run the turbine in on–off engine adapted to drive-by-wire throttle control and giving
mode according to power demand. One of the vehicle 90–100 kph cruising speed. In the Toyota the dual system
schematics under consideration is seen at (d) which engine is mounted, for front wheel drive, onto a transaxle
will be computer modelled to assess its effectiveness. (c) of just 359 mm overalllength, which is shorter than the
The researchers argue that the development of a valid production automatic transmission installation and 30 kg
simulation requires that a number of factors are fully lighter than the engine/transaxle assembly of the standard
considered. As all of the data stems from an initial cal- model. The transaxle is of four shaft configuration, with
culation of the battery current, it is important that this compactness achieved by mounting motor and engine on
value is accurate. Small variations in the internal resis- separate shafts, each having optimized gear reduction, of
tance of the battery can cause large variations in the rail 4.19 overall for the engine and 7.99 for the motor. The
voltage (square of variation). It is important to have an planetary splitter gearhas a carrier connected to the engine
accurate model of the battery which considers both the and ring gear to the output shaft; it also acts as a speed
variation of internal resistance and open-circuit voltage of increasing and torque reducing device for the motor/gen-
the battery at different states of charge. Modelling the erator, with a 3.21 reduction ratio.
drive cycle at the two extremes of battery operation The motor/generator is a brushless 8 pole DC ma-
(80% and 20% discharge) gives a good indication of the chine with 6 kW output, having generator brake and
range of values over which the voltage and currents in the planetary gear installed within the coil ends for com-
system may vary for specification purposes. The next pactness. The generator functions as a starting device,
step is to introduce a more realistic battery model and clutch and form of CVT. The 40 kW traction motor is
practical limits on the power sourced from regenerative a 4 pole brushless motor which functions as a torque
braking. During electrical regenerative braking, there is levelling device of the parallel hybrid system. Under low-
a practical upper limit to the voltage permitted across load cruising conditions, the system uses parallel hybrid
a battery in order to avoid ‘gassing’. This restriction may mode with the brake engaged, preventing the motor/
be overcome by the use of an additional form of power generator from causing energy conversion losses. Brake
sink if the alternator is to continue operating at a fixed cooling oil is also used for cooling the motor coils. Twenty-
load point. four lead–acid batteries are used, of 25 Ah capacity each,
to give a total voltage of 288 V, the type being Cyclon-
25C VRLA. Overall control strategy is seen at (d).
7.1.3.7 Dual hybrid system

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Japanese researchers from Equos Research have de- 7.1.3.8 Flywheel addition to hybrid drive
scribed the dual system of hybrid drive which differs
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from the more familiar series–parallel drives and their According to Thoolen , the problem of providing peak
combinations. It allows free control of the IC engine power for acceleration, and recuperation of braking
while keeping mechanical connection between it and the energy, in an efficient hybrid-drive vehicle can be over-
drive wheels; a compact transaxle design integrates the come with an electromechanical accumulator. Such sys-
two electric drive motors, to simplify the conversion of tems also admirably suit multiple stop–start vehicles such
conventional vehicles, and use of the generator as a motor as city buses by using flywheel and electric power
in combination permits flexible adaptation to driving transmission.

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