Lightweight Electric/Hybrid Vehicle Design

Fig. 4.7 GM’s PEM fuel-cell
stack capable of powering a
1500 kg vehicle.
Process engineering and control of fuel cells, prospects for EV packages 91
overcome the problem of drag due to brake hang-on with conventional systems when the singleacting
piston fails to withdraw, and some 2 kW can typically be lost at motorway speeds. Of
course, hydro-mechanical brakes only operate rarely on EVs, which can rely on regenerative
braking for light-duty work. The Precept is an order-of-magnitude change in technology compared
with ordinary American-produced vehicles, but of course would cost considerably more than the
$10 000 dollars now charged.
4.6.3 FUEL-CELL VEHICLE PROSPECTS
The real future is with fuel-cell cars because the Precept version so fitted will have a fuel-cell
stack volume of just 1.3 ft3 and produce 70 kW continuously, 95 V at 750 A, Fig. 4.7. Plate current
density is 2 A/cm2 ; the cell is currently world leader in PEM-stack design and if used intelligently
has higher power density than a thermal engine. This is, then, the turning point, and it is underlined
by Mercedes whose work on the Necar IV is showing that 38% efficiency is obtained from hydrogen
in the fuel tank to power at the road wheels. This compares 13–15% for a standard thermal-engine
car. With this three times improvement in fuel economy GM believes that they are going to develop
fuel-cell vehicles. The major challenge over the next five years is making it at a low enough cost
to be attractive.
4.6.4 BATTERY ELECTRICS/HYBRIDS IN THE INTERIM
EV technology will be needed for the future fuel-cell car but currently it cannot be proven
with current traction batteries which are too heavy, too expensive and made from materials
that are not in plentiful enough supply. Most good quality batteries are based on nickel
technology and the reason for being in this difficulty is not because the battery research