Lightweight Electric/Hybrid Vehicle Design

Viable energy storage systems 51
70 mph on a motorway. The engine produces 55 kW of waste heat. If a third is converted to
electricity 18.33 kW is made available and perhaps 15 kW of this is additional mechanical power.
Consequently the engine only has to produce 60% of the mechanical power to give the same
output of 20 kW and the electrical system could be reduced to 10 kW. To summarize, the 20 kW
requirement could be met by using 12 kW of engine power and 8 kW of waste heat recovery
power. Future schemes could possibly improve on these figures.
2.5.2 THE TURBINE RECOVERY SCHEME
In this scheme the concept is to use a small turbine system in association with an electric generator.
Figure 2.21 illustrates the conceptual realization of the idea. Some modelling of the turbine shows
that, for reasonable efficiency, speeds of 150 000 rpm are necessary. If such speeds can be achieved
the sizes of the components will be tiny. For example, for 10 kW, the rotor of the generator will be
25 mm diameter by 15 mm long. The generator needs to be kept separate from the turbine stages
due to the high temperatures involved in the thermodynamic processes. The turbine bearings will
be hydrodynamic gas bearings – the back of the turbine rotors and the shaft will be plated with a
zig-zag pattern, microns thick, designed to created turbulence at high speeds. Dynamically the
system will operate below the first critical speed.
The generator bearings will be angle contact bearings (6 mm) using ceramic balls (RHP-INA)
and Kluber Isoflex Super LDS 18 grease. The rear bearing is preloaded and free to slide by means
of a crinkle spring. The coupling between generator and motor can be a tongue and fork, permitting
easy removal, as the torque is below 1 Nm.
The generator losses preheat the air entering the first compressor stage. The generator has
laminations of 0.2 mm radiometal with powder coat insulation applied to ensure minimum eddy
current losses. The rotor has a one-piece tubular magnet, 22 mm diameter and 15 mm long by
3.5 mm thick, of ‘one-five’ samarium cobalt. This is glued onto a stainless steel shaft of high
AIR
EV
DRIVE SYSTEM
COMPRESSOR
216 V
BATTERY
43 AMPS
Fig. 2.21 Block diagram of turbine waste heat.
ENGINE
EXHAUST
WASTE HEAT
HEAT
EXCHANGER
TAIL PIPE
INVERTOR
TURBINE WASTE HEAT
150 000 RPM
3O 10KW
GENERATOR
165 V 4 POLE
35 AMPS 5 kHZ