0.01 g/mile for emissions of particulate matter for the CIDI engine. The current target is 0.04 g/mile. Meeting the stretch research objective presents new challenges to the candidate CIDI engine, which would require expanded technology development to meet the PNGV goals. PNGV would also need to reevaluate other power plants relative to CIDI engines.

The hybrid electric vehicle (HEV), which is the PNGV power train of choice, uses an energy storage device to decrease the fluctuations in the demands on the primary power plant. This reduction allows for a decrease in the peak power output required from the primary energy conversion system and an opportunity to improve efficiency both by restricting the power fluctuations and by recovering a significant fraction of the vehicle's kinetic energy during braking operations. The PNGV is sponsoring research on batteries, flywheels, and ultracapacitors as energy storage devices.

Achieving the high fuel economy levels for the Goal 3 vehicle will require more than improving the energy conversion efficiency of the power train (including energy converters and transmissions) and reducing other energy losses in the vehicle. Vehicle weight reduction through the use of new vehicle designs and lightweight materials will be extremely important in achieving the very ambitious fuel economy targets.

The committee re-evaluated the candidate energy conversion and energy storage technologies, as well as candidate electrical and electronic systems, that were considered last year and addresses them in this chapter. This chapter also reviews progress on advanced structural materials for the vehicle body, a subject that was not addressed by the committee in its third review. The technologies evaluated in this chapter are listed below:

  • four-stroke CIDI engines

  • continuous combustion systems

  • fuel cells

  • electrochemical storage systems

  • electro-mechanical storage systems

  • electrical and electronic power-conversion devices

  • materials

The committee reviewed R&D programs on each of these technologies to assess the progress that has been made and the developments required for the future. The PNGV Technical Roadmap, which has been updated for most of these technologies, provided a good summary of the program goals (PNGV, 1997). In the committee's opinion, the PNGV has made substantial progress in assessing the potential of most candidate systems and identifying critical technologies that must be addressed to make each system viable. A few exceptions are noted in the sections describing specific technologies.

The committee has also described some international developments in the various technology areas, based both on its own knowledge and experience and



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