Lightweight Electric/Hybrid Vehicle Design

50 Lightweight Electric/Hybrid Vehicle Design
2.5 Waste heat recovery, key element in supercar efficiency
In the longer term vehicles will be electrically propelled using flywheel storage, hydrogen fuel
cells or both. These systems potentially offer high cycle efficiencies, and low emissions vital to
improving air quality in our big cities 11 .
Bill Clinton’s initiative for US family cars to achieve 100 miles per gallon by 2003 is a
serious challenge to the engineering industry; see Chapter 4. Current solutions include
lightweight structures, reduced running losses and small engines. However, the author believes
that the target can be achieved using conventional vehicle platforms with low drag floorpans
and instead attention is focused on high efficiency drivetrains. Small engines give poor
acceleration so to achieve acceptable performance hybrid technology is required. The internal
combustion engine car achieves 28% efficiency under motorway conditions and half that on
an urban cycle. The key problem is how to convert more of that energy into useful work. The
account below will investigate two schemes: (a) turbine recovery system and (b) thermoelectric
recovery system.
In both schemes the energy produced is converted into electricity. This is because such an
arrangement provides a plausible method for matching the power into the electrical drive. It is
accepted that all mechanical solutions are also viable in a hybrid vehicle.
2.5.1 HYBRID ELECTRIC DRIVE
Figure 2.20 illustrates a parallel hybrid driveline using a Wankel engine and a brushless DC Motor.
The package can produce 70 kW peak power and 20 kW average. This combination provides
excellent acceleration using energy stored in a small flat plate lead-acid battery. Tests to date show
this battery still delivers 100% peak power of 45 kW and 80% capacity after 22 000 cycles to 30%
depth of discharge. Thermal management is vital to achieving these figures.
The engine operates on a two stroke cycle and produces high quality waste heat at the exhaust.
The temperature of the gas is around 1000°C. This gas contains 72% of the energy in the fuel. If
we can convert a third of this energy into electricity we can nearly double the NTG of the vehicle
under motorway conditions.
Why is this important? Hybrid solutions are very effective at improving efficiency under urban
cycle conditions but make no impact under motorway conditions. Here is a system that can begin
to solve this problem. Supposing in the existing scheme we require 20 kW to operate a vehicle at
MOTOR
AND REDUCTION
Fig. 2.20 Parallel hybrid drive Wankel
engine and brushless DC motor.
C
B A
ENGINE