Page 188 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 7 .1 Hybrid vehicle design

periods of high speed motoring. The vehicle is being capacity, thanks to the newly developed rear suspension
promoted as a car that produces only half the amount of which has no internal protrusions into the luggage com-
CO 2 of conventionally powered compact-size cars and partment. The slanted, short bonnet covers the trans-
only one-tenth of the amount of HC, CO, and NO x versely mounted and very compact THS power train
permitted under current Japanese emission regulations. assembly. With a height of 1.49 m, the car stands taller
Despite the vehicle having a kerb weight of 1.5 tonne, than others in its class. The blending of the three box
Toyota claim that Prius will accelerate from standstill to layout into a good aerodynamic shape has resulted in
400 m in 19.4 seconds and reach a top speed of 160 km/h. a drag factor of C d ¼ 0.30. Considerable weight saving,
In ‘starting from rest and light load’ mode (moving at (e), without any sacrifice in passive safety, has been
low speed or descending a slight gradient) the electric achieved in the body-in-white. The platform is based on
motor drives the vehicle and the petrol engine is stopped. Toyota’s Global Outstanding Assessment (GOA) con-
The high torque characteristic of the electric motor helps cept study of an impact-absorbing body and high in-
to get the car moving and will sustain it during low load tegrity occupant cabin design, developed to meet 1999
demand slow speed progress in urban centres. Should US safety standards. Ribs made of energy-absorbing
additional power be required from the petrol engine, the materials are embedded inside the pillars and roof side
computer control system will ensure that the engine will rails. GOA also features strong cross members, several
play its part, either by charging the traction battery pack produced in higher-tensile-strength sheet steel, linking
or by some direct contribution to driving the road the various body frame elements. These provide strength
wheels. But when coming to rest at traffic lights, the fuel and stiffness, particularly in potential collision damage
supply to the engine is cut off and the engine is auto- zones, and also spread the impact loading, thereby min-
matically stopped. imizing intrusion into the cabin, the occupant safety cell.
During ‘deceleration and braking’ mode, the kinetic Air conditioning and power-assisted steering are fea-
energy of the moving mass of the vehicle passes from the tured. The automatic air conditioner creates a double
road wheels through the epicyclic transmission gearing of layer of air, recirculating only internal air around the leg
the power-split device to the electric motor. This then areas, even when the fresh air intake mode has been se-
acts as an electric generator, delivering this energy as lected. The glass in the side and rear windows is of a type
a charging current to top up the traction battery pack. which inhibits heating up of the cabin space, by blocking
This feature of regenerative braking comes into play, most of the sun’s ultraviolet rays. Insulating materials in
regardless of whether the operator applies the foot brake the roof and floor panels also contribute to maintaining
or relies on engine braking to slow down the car. A com- a comfortable cabin atmosphere. They also offer good
plex but compact full power inverter and control unit sound insulation. Steering has a power-assist system
ensures that the traction battery pack is being maintained using an electric motor, which consumes power only
at a constant charge. When the charge is low, the electric during steering operations. The front suspension has
generator routes power to the battery. In most instances, MacPherson struts with L-arms for locating their lower
this energy will come from the internal combustion ends. In the semi-trailing-arm rear suspension, the
engine rather than the energy recovered during braking. combined coil spring and hydraulic damper units are
The system has been so designed that the batteries do not much shorter. Their lower attachment is to a trailing arm
require external charging, which means that there is no each, which, in turn, is attached to an innovative type of
practical restriction to the operating range of the vehicle. torsion beam, of an inverted channel section. It in-
Toyota have compensated for the dual drive and bat- corporates toe-control links, to improve handling stabil-
tery weight penalty with a number of ingenious mea- ity and the double-layer anti-vibration mounts joining the
sures: since the petrol engine has been restricted to suspension to the chassis suppress much of the road
a maximum of 4000 rpm, key components have been noise. Passenger comfort is appropriate to a car which
pared down to save weight. Compared with an engine of retails at around 27–30% above a comparable, but con-
comparable size but 5600 rpm maximum speed, the in- ventionally propelled model.
ternal dynamic loadings on many of the moving parts are For the power-split device, Fig. 7.1-15, which is a key
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halved. Consequently there is scope for reducing the part of the system, company engineers have provided
dimensions of, for instance, crankshaft journals and also the diagram at (a) to show how the engine, generator and
pistons, which have remarkably short skirts; the overall motor operate under different conditions. At A level
effect of paring down of individual components is re- with the vehicle at rest, the engine, generator and motor
duced weight of the built-up assembly. Overall length is are also at rest; on engine start-up the generator produces
only 4.28 m, but the car provides an interior space equal electricity acting as a starter to start the engine as well as
to that of many medium-class cars, by having a relatively operating the motor causing the vehicle to move off as at
long wheelbase of 2.55 m. a 1.7 m wide body, and short B. For normal driving the engine supplies enough power
overhangs front and rear. The Prius boot has a reasonable and there is no need for the generation of electricity, C.

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