Review of the Research Program of the Partnership for a New Generation of Vehicles: Third Report (1997)

The following is an excerpt from Chapter 6, Powertrain Developments, of the committee's second report, to provide the reader background on powertrains and series and parallel hybrid vehicle configurations (NRC, 1996).

Even when combined with reductions in vehicle mass, aerodynamic drag, tire rolling resistance, and other energy-saving vehicle design parameters, the PNGV technical team estimates that achieving the Goal 3 fuel economy target (up to three times fuel efficiency of today's comparable vehicle) will require a power plant with at least 40 percent thermal efficiency (PNGV, 1996). Achieving this efficiency by incremental improvements to current gasoline engines is unlikely. Therefore, a variety of alternative energy conversion devices and drivetrain components are being considered by the PNGV. None of these alternatives is, at present, suitable for passenger car application without further development. Moreover, many combinations are possible; therefore, system tradeoff analyses must be performed to fully understand the fuel efficiency potential of each. For instance, adding hybrid and regenerative braking driveline¹ components reduces the power plant 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 power plants 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 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 continuously variable transmissions is being conducted on behalf of the PNGV program in the United States. However, foreign firms are continuing to develop such transmissions. The committee, therefore, believes that these developments should continue to be incorporated into the PNGV agenda.

NRC. 1996. Review of the Research Program for a New Generation of Vehicles, Second Report. Board on Energy and Environmental Systems and Transportation Research Board. Washington, D.C.: National Academy Press.