FIGURE 2.2 Percent decrease in fuel consumption (FC) as a function of percent increase in fuel economy (FE), illustrating the decreasing benefit of improving the fuel economy of vehicles with an already high fuel economy.

FIGURE 2.2 Percent decrease in fuel consumption (FC) as a function of percent increase in fuel economy (FE), illustrating the decreasing benefit of improving the fuel economy of vehicles with an already high fuel economy.

literature, the data were converted to fuel consumption, using the curve of either Figure 2.1 or 2.2 for changes in fuel economy. Because of this, the committee recommends that the fuel economy information sticker on new cars and trucks should include fuel consumption data in addition to the fuel economy data so that consumers can be familiar with this fundamental metric since fuel consumption difference between two vehicles relates directly to fuel savings. The fuel consumption metric is also more directly related to overall emissions of carbon dioxide than is the fuel economy metric.


Motor vehicles have been powered by gasoline, diesel, steam, gas turbine, and Stirling engines as well as by electric and hydraulic motors. This discussion of engines is limited to power plants involving the combustion of a fuel inside a chamber that results in the expansion of the air/fuel mixture to produce mechanical work. These internal combustion engines are of two types: gasoline spark-ignition and diesel compression-ignition. The discussion also addresses alternative power trains, including hybrid electrics.

Basic Engine Types

Gasoline engines, which operate on a relatively volatile fuel, also go by the name Otto cycle engines (after the person who is credited with building the first working four-stroke internal combustion engine). In these engines, a spark plug is used to ignite the air/fuel mixture. Over the years, variations of the conventional operating cycle of gasoline engines have been proposed. A recently popular variation is the Atkinson cycle, which relies on changes in valve timing to improve efficiency at the expense of lower peak power capability. Since in all cases the air/fuel mixture is ignited by a spark, this report refers to gasoline engines as spark-ignition engines.

Diesel engines—which operate on “diesel” fuels, named after inventor Rudolf Diesel—rely on compression heating of the air/fuel mixture to achieve ignition. This report uses the generic term compression-ignition engines to refer to diesel engines.

The distinction between these two types of engines is changing with the development of engines having some of the characteristics of both the Otto and the diesel cycles. Although technologies to implement homogeneous charge compression ignition (HCCI) will most likely not be available until beyond the time horizon of this report, the use of a homogeneous mixture in a diesel cycle confers the characteristic of the Otto cycle. Likewise the present widespread use of direct injection in gasoline engines confers some of the characteristics of the diesel cycle. Both types of engines are moving in a direction to utilize the best features of both cycles’ high efficiency and low particulate emissions.

In a conventional vehicle propelled by an internal combustion engine, either SI or CI, most of the energy in the fuel goes to the exhaust and to the coolant (radiator), with about a quarter of the energy doing mechanical work to propel the vehicle. This is partially due to the fact that both engine types have thermodynamic limitations, but it is also because in a given drive schedule the engine has to provide power over a range of speeds and loads; it rarely operates at its most efficient point.

This is illustrated by Figure 2.3, which shows what is known as an engine efficiency map for an SI engine. It plots the engine efficiency as functions of torque and speed. The plot in Figure 2.3 represents the engine efficiency contours in units of brake-specific fuel consumption (grams per kilowatt-hour) and relates torque in units of brake mean effective pressure (kilopascals). For best efficiency, the engine should operate over the narrow range indicated by the roughly round contour in the middle; this is also referred to later in the chapter as the maximum engine brake thermal efficiency (ηb,max). In conventional vehicles, however, the engine needs to cover

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement