The efficiency of an internal combustion engine can be increased if the engine is combined with an electric motor that enables it to reduce the standby energy lost while the engine is idling or running below full power. The flow chart in the figure at right compares the 100 units of fuel needed in a typical internal combustion engine (up left) to 50 units needed by a "2X electric hybrid" (up right) to produce the same amount of drive power in a car with twice the efficiency. Such efficient hybrids are now commercially available. As seen in the figure, even today's relatively efficient gasoline spark-ignition internal combustion engine loses 84 units of power per 100 units of input fuel, primarily to the exhaust, radiator, engine friction, and accessories. The most easily reduced of these losses are the standby losses, which eat up roughly 11% of the fuel; they are high in part because the engine is oversized to allow for acceleration. The 16 units of power delivered to the drive train are eventually all dissipated in braking and in overcoming rolling and wind resistance.

The 2X electric hybrid suffers no standby engine losses because it features a less powerful, more fuel-efficient engine-generator that is either on at full load or off. The hybrid uses a battery-powered electric motor to boost acceleration, so that the internal combustion engine is not oversized. When the engine is off, the battery powers these motors. When the engine is on, the batteries are either being recharged or boosting acceleration. The hybrid configuration shown in the figure also uses regenerative braking rather than friction braking: The electric motor is run as a generator during braking to recapture energy and charge the battery.

Prototypes of 2X vehicles have been produced by three US automakers--DaimlerChrysler, Ford, and General Motors--in conjunction with the Department of Energy. In 1993, these three companies joined with DOE in a government-industry venture called the Partnership for a New Generation of Vehicles (PNGV) whose long-term goal is to come up with a 3X car that can triple the fuel efficiency of a 1993 car while preserving safety, performance, amenities, and the potential for recycling--all while holding down costs.¹⁸ Meeting this challenge will require continued intensive R&D. A major obstacle to reaching the PNGV affordability goal is the high cost of advanced lightweight body and tire materials.

The PNGV is also investigating other paths besides the hybrid to eliminate standby losses; these include advanced diesels, direct-injection stratified-charge gasoline engines, and fuel cells. As a result of PNGV, federal government R&D in advanced automotive technologies has been reorganized and redirected toward this ambitious goal.

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© 2001 American Institute of Physics