lllllCar weight (total) would be 580 to 400 kg, less than half of what is estimated by OTA even with carbon
fiber construction.Drag co-efficients are reduced to 0.14 to 0.10, about half the best levels forecast by OTA.Switched reluctance motors that drive the wheels directly are assumed to have an average efficiency over
the EPA test cycle of 93 percent. This is an unusally high average value for a motor.Accessory loads on the engine would be reduced to zero.Regenerative braking efficiency is assumed to have very high (>75%) recovery of inertia losses.If these input assumptions were used by OTA in our analysis, we would obtain fuel economy
levels of over 100 mpg. However, the above analysis does not specify the size and power of the
motors or engine, and it is unclear what such a vehicle’s performance would be with any payload.
Aside from theoretical analyses, some actual hybrid vehicles have been built and tested. For
example, a number of series hybrid vehicles have been developed by universities. These vehicles
have been reported to have achieved high fuel efficiencies, but OTA’s examination of the actual
data showed that the efficiencies achieved were not unusually high. At a constant speed (40 to 50
mph), the best car showed about 60 mpg, while many cars achieved 20 mpg or lower. The best
series hybrid vehicle (Michigan State) was a converted Ford Escort that had low performance
relative to our benchmark of 50 kW per ton of weight plus payload; its power rating was only
22.8 kW per ton, implying that it had less than half the power level required to be equivalent to an
average car in today’s fleet.^64 In addition, the constant speed 40 mph mode is one where even a
conventional Escort can attain 50 mpg (the Escort’s highway fuel economy on the EPA test is
over 45 mpg) while providing much better performance. Rather than proving the potential for
high fuel economy, these early hybrid demonstrations have shown how difficult it is to gain
any benefit in fuel economy from shifting to a hybrid drivetrain.
Parallel Hybrids
In a parallel hybrid, both the engine and the motor can drive the wheels. The close coupling
between engine and motor duty cycles makes the parallel hybrid difficult to analyze without a
detailed simulation model that computes efficiencies as a function of operating speed/load for
each of the two prime movers. Conceptually, however, the general strategy of a parallel hybrid is
to downsize the engine, so that the maximum power requirement of the vehicle is satisfied by
having both engine and motor operate simultaneously. The motor size required in a parallel hybrid
is much smaller than that required in a series hybrid, because in the latter, the motor is the only
source of power driving the wheels.