The Sport-Utility Vehicle: Debating Fuel-Economy Standards in Thermodynamics

Article excerpt

The environmental challenges we face in the 21st century are scientifically complex, multidisciplinary problems. The next generation of scientists and engineers will be the repository of the technical expertise, as well as the source of technological innovation; it is essential that they be key contributors in the debate and formulation of national and international science policy. An exciting challenge we have as science educators is to investigate how to develop in our science and engineering students the ability to ask the types of questions, both personally and on a larger scale, that maximize constructive involvement in these critical conversations.

This paper describes a debate about national fuel-economy standards for sport-utility vehicles (SUVs) used as a foundation for exploring a public policy issue in the physical science classroom. The subject of automobile fuel economy benefits from a familiarity with thermodynamics, specifically heat engines, and is therefore applicable to a broad range of courses found in the disciplines of physics, chemistry, and engineering. This topic could also be discussed, with somewhat less technical sophistication, in a physical science course for nonscience majors.

Thermal and statistical physics

This discussion of SUV fuel-economy standards has been used in Thermal and Statistical Physics, a semester-long, upper-division course for physics majors and minors. Typical enrollment is approximately 10 students. The primary course objective is to examine the fundamental principles of thermal physics from both statistical and classical viewpoints and to apply these principles to thermodynamic systems. A secondary goal of the course is to provide students with activities in which they can apply their developing technical expertise in thermodynamics to societally relevant issues. Practical curricula related to these topics are incorporated into the course, supplementing the traditional course materials (Mayer 2007; Mayer 2006).

The SUV fuel-economy debate was used following a discussion of heat engines. The traditional course content included a description of the heat engine, the Carnot cycle, and a description and comparison of different theoretical engine designs (internal combustion [i.e., Otto cycle] and diesel) and their efficiencies. Students were also given a brief introduction to the science of hydrogen fuel cells. Material related to the heat engine was supplemented with curricula related to automobile efficiencies and a description of some advanced automotive technologies (described below); this material provided both a practical and interesting application of the topic of heat engines and a technical foundation for the fuel-economy debate.

Background: U.S. fuel-economy standards

In response to the Arab oil embargo of 1973--1974, Congress passed the Energy Policy and Conservation Act in 1975, which, among other things, established corporate average fuel economy (CAFE) standards for new passenger automobiles sold in the United States. New car fuel economy had decreased from a fleet average of 14.8 mpg in 1967 to 12.9 mpg in 1974 (Bamberger 2002). The 1975 CAFE legislation called for a fuel-economy standard of 18 mpg for new passenger vehicles in model year (MY) 1978 and a rise to 27.5 mpg by MY 1985. The CAFE standard remained at this level for MY 2007.

Fuel-economy standards were later set separately for light-duty trucks (gross vehicle weight rating [GVWR] less than 8,500 lb.). A summary of fuel-economy standards for both new passenger cars and light-duty trucks is shown in Figure 1 (NHTSA 2007). The automobile fuel economy for a sample of MY 2007 vehicles is shown for reference in Table 1. Light-duty trucks that exceed 8,500 GVWR are not required to comply with CAFE standards. Manufacturer compliance with the standards is determined by calculating a sales-weighted average fuel economy for the company's product line. …