devote substantial additional resources to the DICI hybrid powertrain in view of its relatively high potential to meet PNGV Goal 3 objectives.

From the standpoint of automotive applications, fuel cells offer the potential for high efficiency and low emissions. However, fuel cells are the least developed option under consideration for a power source, and they present the most barriers to a commercially viable system. Despite a number of significant PNGV achievements in fuel-cell technology, fuel-related barriers remain. Several major inventions are needed for the successful development of hydrogen storage systems or efficient, compact fuel processors for hydrocarbon fuels. Although fuel cells have outstanding long-term potential, the PNGV may find it necessary to restructure the fuel-cell program, extending the schedule and revising funding priorities, depending on progress over the next two years.

Gas turbine engines have many potential advantages for hybrid vehicles, including very high specific power, but the main barriers continue to be the difficulty of increasing thermal efficiency and finding satisfactory materials and manufacturing techniques to achieve the necessary performance at acceptable cost. The research challenges for gas turbine engines are very demanding, but the PNGV has made some progress. In the committee's view, the pace of progress over the next two years will be a critical determinant of whether the gas turbine engine remains a viable PNGV option.

Energy storage systems recapture otherwise wasted vehicle kinetic energy and reduce peak load requirements for the main propulsion system, thereby increasing operating efficiency. Current PNGV support is concentrated on three candidates—batteries, flywheels, and ultracapacitors. Although substantial progress has been made in virtually all areas of battery technology critical to battery hybrid vehicles, present high-power batteries cannot simultaneously satisfy the power density, energy density, and cycle-life requirements for the PNGV energy-storage subsystem. Therefore, a combined battery/ultracapacitor subsystem is under consideration. The performance of ultracapacitors has shown significant improvement in laboratory demonstrations, with increased energy and power densities, but major barriers remain with respect to total energy storage and cost. The cost of power electronics is a barrier to the integration of all power components, including batteries, ultracapacitors, and flywheels. R&D on flywheels has resulted in significant performance improvements; but cost remains a barrier, and safety concerns need to be addressed.

Goal 3 vehicles will make significant use of lightweight structural materials. Notable PNGV achievements in this area include the demonstration of high-volume fabrication processes for steel and aluminum. The committee recommends that USCAR continue to pursue these very promising developments in steel and aluminum materials made by materials suppliers and



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