vehicle’s power needs are below the engine output, the excess energy goes to recharge the storage
device.
A careful examination of the vehicle’s energy requirements and the characteristics of the
available power sources is necessary to show whether the popular vision will work in practice.
First, examining an engine’s power characteristics does make it clear that the engine should be
used to provide the total energy for driving, while the battery or other storage device should be
sized to provide peak power. Although an ICE does have high specific power (power output per
kilogram of engine weight) under normal operation, keeping the engine at its peak efficiency point
sharply limits specific power. That is, a typical engine operating at its best efficiency point
produces only about 40 percent of its peak output.^54 Such an engine, combined with a generator,
radiator, and other engine components, would weigh 7.5 to 8.5 kg/kW and have specific power
about 117 to 130 W/kg.^55 In contrast, advanced lead acid batteries of the semi-bipolar or bipolar
type provide specific power of over 300 W/kg for a 30-second rating, while ultracapacitors and
flywheels can provide 2,000 W/kg or more. That is, the storage devices can have higher specific
power than the engine itself.
Second, the storage mechanisms are limited in the amount of power they can provide, which
has important implications for engine sizing and operations. The battery, for example, is capable
of providing peak power in short bursts only, because of heat removal requirements.
Ultracapacitors are limited by their low specific energy; they would have to be very large to
provide high power for a long period. Consequently, while the storage devices can be used to
satisfy high-power requirements that last a short period, the engine itself must be sized large
enough to take care of any high-power requirements that may be of long duration.
Consistent with the analysis for EVs OTA has imposed the requirement that the vehicle be
capable of sustaining a long climb of a 6 percent grade at 60 mph.^56
Sizing the hybrid’s engine in this manner--to provide enough power to climb a long hill--implies
that the engine, when operating at its most efficient speed, is providing a higher average power
output than needed for most driving. This means that much of the time the engine is operating, it
will be charging the battery or other storage device. When the storage device becomes fully
charged, the engine must be turned off and the vehicle operated in the following manner:
. As long as power demands are moderate, the vehicle operates as an EV, until the storage is drawn down
far enough to allow the engine to be turned on again. Depending on the energy storage capacity of the
buffer, then, the engine might be turned off and on several times (for low-energy storage, such as with an
ultracapacitor) or possibly just once during an average drive (with battery storage). The engine must be
turned on well before the buffer is drained of its energy, however, because the buffer must still be
available to provide a power boost, if needed.
. During the period when the engine is turned off, it will have to be restarted, if there is a demand for
power that exceeds the capacity of the buffer. In a hilly area, the engine may need to be restarted often.