FIGURE 4.2 U.S. hybrid electric vehicle sales through 2006. Note that the order of vehicles in the key matches the order indicated in the bars of the graph from top to bottom. SOURCE: DOE (2007). Available at

Potential for Reducing Oil Use in Conventional Light-Duty Vehicles

This section examines options for improving fuel economy in conventional gasoline, diesel, and hybrid power train vehicles. Although not discussed explicitly in this chapter, CO2 reductions directly follow improvements in fuel efficiency in these types of vehicles. CO2 estimates are presented in Chapter 6. Various transportation demand reduction efforts are under way, but are not considered in this report.

Gasoline and diesel power train vehicle technologies are considered mature, but they will continue to advance and improve for the foreseeable future. The following material reviews the areas in which improvements can be expected to continue, evaluates the potential extent of these improvements, and projects their impact on fuel economy. Potential cost increases are also estimated, although these are more qualitative. For improved vehicle technology to have a significant effect on CO2 emissions of the light-duty fleet and on oil imports, it must be directed to fuel efficiency improvements and must be included in a large fraction of all vehicles manufactured and sold in a given year; then it, or further improved versions, must continue to be manufactured for the time required to turn over a majority of the vehicles in the existing fleet. This is a decadal process.

Technical Improvements in Gasoline and Diesel-Powered Light-duty Vehicles

The approaches to improving the fuel economy for gasoline and diesel vehicles are well understood. Each of these areas offers considerable potential fuel efficiency benefits. The methods considered here include the following:

  • Efficiency improvements

  • Transmission evolution

  • Vehicle weight reduction

  • Aerodynamic improvements, reduced rolling resistance

Efficiency Improvements—Spark-ignition Engine. Technical improvements that can be applied to spark-ignited internal combustion engines include the following:

  • Variable valve timing (VVT) and variable valve lift offer improvements in part-load engine efficiency. Engines only operate at full load during hard acceleration and hill climbing; the remainder of the time, engine operation is at part load, when much less power or a smaller engine is all that is needed. VVT and lift allow the engine to supply the reduced power requirement with improved fuel efficiency. The next major change is camless valve actuation (CVA), which can vary lift and timing and also allow strategies such as cylinder deactivation at light loads while simultaneously reducing valve-train friction.

  • Cylinder deactivation (also called cylinder cut-out) can also reduce fuel consumption under part-load vehicle operation. Efficiency is improved by having fewer cylinders working at higher load. This technology is already being implemented, especially in V8 engines, but constitutes a small fraction of the market to date. Application to four- and six-cylinder engines is more difficult because of noise and vibration problems.

  • Gasoline direct injection (GDI) and GDI combined with turbocharging allow higher compression ratio operation because of the cooling effect of in-cylinder fuel evaporation, which protects against knock. Raising the compression ratio increases efficiency (which is why diesels are relatively efficient), but conventional spark-ignition engines are susceptible to pre-ignition (knock) if the compression is too high. With GDI, the cooling effect counters the heat from the high compression. This technology benefits from variable-geometry turbines for turbo boosting and variable compression ratio. This produces more horsepower from a smaller engine, allowing weight reduction and fuel savings. This technology option is in limited production in Europe and Japan and is on a few models in the United States.

  • Homogeneous-charge compression ignition (HCCI) enabled by CVA might decrease fuel consumption more than GDI with turbocharging. HCCI involves the introduction of a homogeneous air-fuel mixture, where the fuel is a gasoline-range hydrocarbon, into the cylinder. HCCI uses the same kind of “charge” as a spark-ignition engine but with higher compression, whereas a classical compression-ignition engine uses a stratified charge with a higher-boiling fuel that is injected directly into highly compressed air in the cylinder. Although HCCI has typically been considered a separate technology, its components will most likely be implemented as engine technology advances providing additional reductions in fuel consumption.

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