called series and parallel hybrids. In a series hybrid, the power generated by the ICE is always
converted to electricity, and either stored (in a battery, flywheel, or ultracapacitor) or used
directly to drive a motor, which is connected to the vehicle’s wheels. In a parallel hybrid, the
engine or the motor, or both, can drive the wheels directly. The two design types are shown
schematically in figure 4-3. Although both systems have advantages and disadvantages, most
manufacturers who have displayed prototype hybrid vehicles have selected the series design. The
exception is VW, and its engineers believe that series designs are being displayed largely because
they are very easy to develop, but are inefficient for reasons explained later. Another classification
method is according to whether the vehicles require externally supplied electrical power (as an EV
does), or can operate solely on gasoline, and these are labeled as nonautonomous and
autonomous hybrids, respectively..For either the series or parallel type hybrid, the ICE and the electrical system can be of widely
different sizes. In both hybrid types, one extreme would be to have the engine act as a “range
extender” by charging the battery (or other electricity storage device) while the electric drivetrain
is quite similar in size to that of a pure EV. With this type of setup, sizing the engine’s maximum
output close to the vehicle’s average power demand during highway cruise (e.g., 15 to 20 kW/ton
of vehicle weight) would allow the range of the vehicle to be similar to that of a conventional car.
Moreover, unless there were an abnormally long hill climb, the battery state of charge could be
maintained at near constant level. At the other end of the spectrum, an engine could be large in
size and the battery or power storage device made relatively small, so that the engine could be
employed to provide peak power for acceleration and battery recharging capability. Obviously,
there are infinite combinations in between the two extremes. The amount of energy stored in the
battery or other storage device, as well as the device’s peak-power capability, are key
determinants of how the engine and storage device will interactively supply power to the
drivetrain under any arbitrary driving cycle. Autonomous hybrids of either the parallel or series
type usually utilize larger engines than nonautonomous ones.The hybrid vehicle concept is neither new nor revolutionary. The earliest hybrids were built in
1917, and DOE funded a large research program in the late-1970s and early 1980s. Many of the
same arguments and analyses in vogue now in support of hybrid powertrains were voiced after the
two oil crises of the 1970s.^44 The Jet Propulsion Laboratory and General Electric developed
studies, published in 1980, that estimated that a mid-sized car could attain 33 mpg on the city
cycle, which was about 40 to 50 percent better than vehicles of that era. A prototype in the early
1980s demonstrated about 50 percent improvement in fuel economy relative to a early-1980s
conventional vehicle of the same size, though it had lower performance.^45More recently, several papers^46 have claimed that hybrid vehicles using lightweight body
construction, can provide a fuel economy increase of about 100 percent, while one paper claims
an improvement potential of several hundred to several thousand percent for a hybrid
configuration with a carbon fiber body, superb aerodynamics, and improved tires.^47 Moreover,
PNGV contractors have discussed charts where some form of hybrid powertrain (undefined) was
(^44) General Electric, "New Term Hybrid Vehicle Program,” report No. SRD-79-134, 1979.
(^45) General Electric, "Hybrid Vehicle Program: Final Report,” report No. SRD-83-031, November, 1993.
(^46) E.g., A. F. Burke, "Hybrid Vehicles,” Encyclopedia of Energy and Technology (New York, NY: John Wiley, 1995) pp. 1709-1723.
(^47) A.B. Lovins et a1., "Supercars: The Coming Light Vehicle Revolution," ECEEE Conference Proceedings, June 1993.