Lightweight Electric/Hybrid Vehicle Design

Fig. 1.18 Gas turbine technology.
Current EV design approaches 21
1.3.7 HGV
The heavy goods vehicle is an articulated truck which weighs 40 tonnes. Often omitted from clean
air schemes on the grounds of low numbers they travel intercontinental distances every year and
are major emitters of NO and solid particles. Their presence is felt where there are congested
x
urban motorways, and each one typically deposits a dustbin-full of carbon alone into the atmosphere
every day, the industry declining to collect and dispose of this material! What is the solution? Use
hybrid drivelines based on gas turbine technology; these vehicles would be series hybrids.
A gas turbine/alternator/transistor active rectifier, Fig. 1.18, provides a fixed DC link of 500 V.
This is backed up by a battery plus DC/DC converter. A battery of 220 V (totally insulated) is used
for safety. High quality thermal management would be vital to ensure long battery life; 2 tonnes of
lead–acid units would be needed (144 × 6 V × 110 Ah) to be able to draw 400 bhp of peak power.
It is likely that capital cost would be offset by fuel cost savings. Another benefit is that the gas
turbine can be multifuel and operation from LNG could be especially beneficial. The drive wheels
are typically 1 metre in diameter giving 683 rpm at 80 mph. Usually there are 3:1 hub reductions
in the wheels and a 2:1 ratio in the rear axle, giving a motor top speed of 4000 rpm. Translated into
torque speed this means 2866 Nm at 1000 rpm, falling to 716 Nm at 4000 rpm. All motors are
viable at this power; however, two factors dominate: (a) low cost and (b) low maintenance. DC
brushed motors with 3000 hour brush life are unlikely contenders! PM brushless DC is unlikely
on cost grounds, requiring 36 kg of magnets for 2900 Nm of torque. Both induction motors and
switched reluctance are viable contenders but switched reluctance wins on efficiency and weight.
The contenders are tabulated at Fig. 1.17(c).
In the above review of four motor technologies for three vehicle categories, there is no clear
winner under all situations but a range of technologies is evident which are optimal under specific
conditions. Continuing development should improve the electronically commutated machines
especially brushless DC and switched reluctance types. The relative success of these machines
will be determined by improvements in magnet technology, especially plastic magnets, and cost
reduction with volume of usage. On the device front, development is approaching a near ideal
with 1/2 micron line width insulated gate bipolar transistors (40 kHz switching/l.5 V VCE saturated)
but reduction in packaging cost must be the next major goal.
1.4 Inverter technology
Inverters are one area where progress is being made in just about every area 3 : silicon, packaging,
control, processors and transducers. The task is to find a way down the learning curve as quickly
as possible. Polaron believe the lowest cost will come from packaging motor and inverter as a
single unit. The major development this year is that of reliable wire bond packaging for high