efficiency gain needed for the PNGV Goal 3 vehicle but increases the size, weight, complexity, and cost of the complete powertrain. This kind of first-order qualitative analysis has resulted in the powertrain technologies listed below. These technologies are currently being pursued by PNGV for Goal 3 vehicles, all of which will operate as hybrid systems.

The powertrain technologies being pursued by the PNGV for Goal 3 vehicles are as follows:

  • four-stroke DICI engines

  • gas turbines

  • stirling engines

  • fuel cells

  • reversible energy-storage devices (namely, batteries, flywheels and ultracapacitors)

  • electrical and electronic power-conversion devices

Hybrid powertrain systems are attractive to increase powertrain efficiency for two reasons. When combined with a suitable energy-storage device, these systems allow the possibility of recovering a significant portion of the kinetic energy of the vehicle as it decelerates. They also allow the primary energy converter (engine or fuel cell) to be smaller and to operate under load and speed conditions that are independent of the vehicle's immediate needs. This reduces its size and permits its efficiency to be optimized. In addition, this arrangement allows an engine to operate at a speed and load that are independent of the vehicle, and increases the feasibility of using powerplants that would otherwise be unsuitable for passenger vehicles. Emissions can also be reduced significantly, especially at startup when the car can start without the engine.

Both series and parallel hybrid configurations are being considered. In the series configuration, all of the engine power is transmitted to the wheels through electric machines. In a parallel configuration, the engine supplies some power directly to the drive wheels through a mechanical transmission, and this is supplemented by electrical machines and an electrical power source. Continuously variable transmissions (CVTs) allow the relationship between engine speed and vehicle speed to be changed at will and are candidates for the parallel hybrid application. It appears that little or no work with respect to CVTs is being conducted on behalf of the PNGV program in the United States. However, foreign firms are continuing to develop such transmissions (Dancey, 1995; Liebrand, 1995). The committee, therefore, believes that these developments should continue to be incorporated into the PNGV agenda.



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