individual vehicles or trucks), although this chapter does at times point to some of the system-level requirements, such as changes in land-use patterns and density, that may be needed to support such improvements.

In examining opportunities for energy efficiency in transportation, the panel considered the three time periods set out in the America’s Energy Future (AEF) project:

1. Early deployment: through 2020;

2. Medium-range deployment: 2020 through 2030–2035;

3. Longer-range deployment: beyond 2030–2035.

Current technologies offer many improvements in fuel economy that become increasingly competitive and attractive as fuel prices rise. For the early period of its assessment (through 2020), the panel focused primarily on opportunities to improve the energy efficiency of mainstream power trains and vehicles. Reductions in fleet fuel consumption through 2020 are likely to come primarily from improving today’s spark-ignition engine, compression-ignition (diesel) engine, and hybridelectric vehicles fueled with petroleum, biofuels, and other nonpetroleum hydrocarbon fuels.¹ Annual, incremental improvements in engines and transmissions are expected to continue. When coupled with changes in the deployment fractions of these propulsion systems, as well as substantial vehicle weight reductions, these improvements could reduce average vehicle fuel consumption steadily over this time period.

In the medium-term (2020 through 2030–2035), changes in power-train and vehicle technologies that go beyond incremental changes become feasible. Plugin hybrid-electric vehicles using electricity plus any of the above fuels may well become a significant fraction of new-vehicle sales. Their deployment may be followed by substantial numbers of (fully) battery-electric vehicles.

Over the longer term (beyond 2030–2035), major sales of hydrogen fuel-cell vehicles and the necessary hydrogen supply and distribution infrastructure may develop.