Page 186 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 7 .1 Hybrid vehicle design

Fig. 7.1-13 Toyota Prius and its THS inverter.

7.1.4.1 Toyota prius systems system, which can steplessly effect the optimum power
flow to suit the driving conditions encountered at any
The Toyota Hybrid System (THS), (a), has two motive one moment. One of the output shafts of the power-
power sources, which are selectively engaged, depending split device is linked to the electric generator, while the
on driving conditions: (1) A 1.5 litre petrol engine, other is linked to the electric motor and road wheels.
developing 42.5 kW at 4000 rpm and a peak torque of The complex transmission system (c), which also in-
102 Nm at 4000 rpm; (2) A battery-powered permanent cludes a reduction gear, is electronically controlled, with
magnet synchronous electric motor with a maximum the power flow allocation constantly being reviewed by
output of 30 kW over the speed range of 940–2000 rpm the special control unit. This means that the in-
and peak torque of 305 Nm from standstill to 940 rpm. formation, which has been gathered by a number of key
The petrol engine is the hybrid’s main power source. It is sensors, is compared with the target values encoded in
a 1.5 litre DOHC 16 valve, 13.5:1 compression ratio, the ECU, the system’s brain. This ECU ensures that the
engine with Variable Valve Timing: Intelligent (VVT-I, appropriate elements in the epicyclic transmission are
a continuously variable valve mechanism) and electronic being braked or released, so that the respective speed of
fuel injection, using the highly heat-efficient Miller cycle, the petrol engine, the electric generator, and the electric
that, in turn, is a further development of the high ex- motor are held within the optimum performance band.
pansion Atkinson cycle. In this cycle the expansion con- This power flow allocation split will depend on whether
tinues for longer than in the conventional 4-stroke the car is being driven at a steady rate, accelerated or
engine, thereby extracting more of the thermal energy of slowing down. The distribution of the petrol engine’s
the burning gases than can be achieved in either power, which is so regulated that it will generally operate
a 2-stroke or 4-stroke engine of conventional design. mainly in its optimum fuel efficiency band, the high
Engine revolutions are restricted to 4000 rpm maximum, torque zone, is determined by such factors as throttle
and the engine is electronically controlled to run always opening, vehicle speed, and state of battery charge. The
within a relatively narrow band of engine speed and load, portion that is used to turn the wheels is balanced
corresponding to optimum fuel efficiency. Toyota claim against that which is used to generate electric power.
that this sacrifice of a wide span of engine speed is more Electric power created by the largish generator may then
than made up for by the greater flexibility of the epicyclic be used to power the electric motor, to help drive the
drive system and the power split between the two vehicle. There are a number of systems operating
motive-power units (Fig. 7.1-14). conditions.
THS functions as a continuously variable transmission In ‘normal driving’ the engine power is divided into
and combines power from the petrol engine and the two power-flow paths by the power-split device, one
electric motor, to give smooth power delivery with little route will directly power the road wheels, and the other
lag between the driver depressing the accelerator pedal will drive the electric generator. Electric current from
and vehicle response. The innovative features of the the generator may be used to power the electric motor,
Prius are in the design details of the power sources and to assist in driving the road wheels. Electric current may
the power split device in the hybrid transmission that also flow into the traction battery pack, to top up its
allocates power from the petrol engine either directly to charge. The power-split electronic control system de-
the vehicle’s front wheels or to the electric generator. termines the ratio of power flow to these outlets in such
The power-split device, (b), employs a planetary gear a manner that optimum fuel efficiency and responsive

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