analysis, the ultracapacitor size is tripled from the size needed for power. The result--peak
acceleration capability of 24 seconds and EV range of 2.4 miles--still seems inadequate, however,
because the ultracapacitor will not be able to support long, repeated accelerations, which maybe
necessary on the highway, and on most trips the engine would have to be shut down and restarted
several times, which may adversely affect emissions.
If flywheel storage becomes commercially practical by 2005, the composite fuel economy
of an ICE/flywheel hybrid will be similar to that of the ultracapacitor-based hybrid--about
60 mpg. With the flywheel sized to provide the necessary 40 kW of peak power, it can provide
this power level for about 54 seconds or allow travel in an EV mode for about five miles. The
peaking capability may be on the margin of acceptability, though it is doubtful whether there will
be enough power for rapidly repeated accelerations. In OTA’s analysis, the flywheel size is
doubled from the size required just to meet peak power requirements.
By 2015, the use of a lightweight aluminum body with low drag and low rolling resistance tires,
and the use of a high-efficiency engine permits the HEV with a bipolar battery to be 280 lbs
lighter than the advanced conventional vehicle, although the engine must be a 0.7 litre, two-
cylinder engine with the attendant noise and vibration problems of such engines. The advanced
bipolar lead acid battery, rated at 500 W/kg of specific power, weighs only 82 kg. Even so, the
fuel efficiency of the vehicle at 65.3 mpg is less than 23 percent better than the equivalent
2015 advanced vehicle with a conventional drivetrain. The ultracapacitor and flywheel-
equipped vehicles are estimated to be even lighter and more fuel efficient at 71 to 73 mpg,
but the problems of energy storage still persist. Assuming that the ultracapacitor meets the
DOE long-term goal of a specific energy storage capacity of 15 Wh/kg, it can still provide peak
power for only about 25 seconds starting from a fully charged condition, if sized for peak power.
Similarly, a flywheel sized for peak power can provide this peak power for only 65 seconds. Such
low values makes it impossible for a vehicle to have repeatable acceleration characteristics, if they
are subjected to two or three hard accelerations in the duration of a few minutes. As done in
OTA’s analysis for 2005, the flywheel capacity is doubled and the ultracapacitor size is tripled to
provide sufficient energy storage, with resulting cost and weight penalties. At their expected
levels of energy storage, ultracapacitor’s would have to be substantially oversized (with
respect to their power capability) to be used with an HEV, as even a tripling of
ultracapacitor size provides peak power for only about one minute from a fully charged
state. At this time, a high peak-power lead-acid battery appears to be a better storage
technology for a series HEV than an ultracapacitor or flywheel, although the battery will be
less efficient If developers can substantially increase the specific energy storage capability
of ultracapacitors and flywheels, however, they will become far more practical as hybrid
vehicle energy storage devices.
The estimated fuel economies attained by the hybrids are sensitive to the assumptions about the
efficiency of the electric drivetrain components. Although the component efficiencies assumed in
the above analysis are superior to the best current values, the PNGV is aiming at still higher
efficiencies. A sensitivity analysis of the results displayed in table 4-12 indicates that improving
motor/generator efficiencies by increments of 2 percent will boost fuel economy by a similar
percentage. For example, for the 2015 lead acid hybrid, a 2 percent boost in engine efficiency
raises vehicle fuel economy from 65.3 to 66.9 mpg; an additional 2 percent boost raises it to 68.5