31 mpg to about 50 mpg.^42 Further, reducing the peak power requirement by 20 percent (to 40
kW/ton) would save an additional $1,000.Hybrid-Electric Vehicles.
Hybrids are vehicles that combine two energy sources (for example, an IC engine and a battery)
in a single vehicle, and use electric motors to provide some or all of the vehicle’s motive force.
The hybrid drivetrain offers several advantages: limited range becomes less of a problem, or no
problem; a portion of inertia losses can be recovered through regenerative braking; and the engine
can be operated near its optimum (most efficient) point.^43 A key disadvantage can be the added
weight, cost, and complexity of the hybrid’s multiple components.
A number of proponents have claimed that a hybrid configuration can yield fuel economy
improvements of as much as 100 percent over an otherwise-identical conventional vehicle, and a
number of experimental vehicles, including winners of DOE’s “Hybrid Challenge” college
competition, have claimed very high levels of fuel economy, up to 80 mpg. An examination of the
actual vehicle results indicates, however, that the conditions under which high fuel economies
were achieved are conditions that typically lead to high levels of fuel economy with conventional
vehicles, and the test vehicles typically had limited performance capability. In OTA’s view, the
results reveal little about the long-term fuel economy potential of hybrids that could compete with
conventional vehicles in the marketplace.
There are numerous powertrain and energy management strategy combinations for hybrid
drivetrains, though many are ill-suited for high fuel economy or for the flexible service
characteristic of current vehicles. OTA examined a limited set of hybrids designed to achieve a
close performance match with conventional vehicles, combining IC engines with battery, flywheel,
and ultracapacitor storage (see box 1-4) in series and parallel combinations (see box 1-5).OTA found that hybrids of this sort could achieve 25 to 35 percent fuel economy
improvement over an otherwise-identical vehicle with conventional drivetrain and similar
performance if very good performance could be achieved from the storage devices and
other electric drivetrain components. The importance of improving electric drivetrain
components is paramount here. For example, a series hybrid without improved storage, that is,
using an ordinary lead acid battery, would achieve lower fuel economy than the conventional
vehicle, because the battery’s lower specific power (power per unit weight) requires a larger,
heavier battery for adequate performance, and because more energy is lost in charging and
discharging this battery than would be lost with a more advanced battery. This latter result agrees
with results obtained by several current experimental vehicles built by European manufacturers.
Perfecting high power density/high specific power^44 batteries or other storage devices is