In response to the Clean Air Act of 1970 and concerns about oil prices and availability, automobile manufacturers instituted a number of engineering improvements in order to control emissions and increase fuel economy. These refinements include greater use of lightweight materials, development of emissions control technologies such as catalytic converters, and use of computer-sensing equipment to monitor engine performance. As a result, average miles per gallon for passenger vehicles has increased significantly since the 1970s and a new, well-maintained automobile releases about 95 percent fewer harmful emissions than cars of two decades ago. Nonetheless, certain emissions remain a concern, including nitrous oxides, which contribute to formation of acidic precipitation in areas where traffic is heavy, and carbon dioxide, a greenhouse gas, which is a by-product of the combustion of fossil fuels. Transportation accounts for about 30 percent of all human-produced contributions of greenhouse gas in the United States.
Research continues, therefore, to develop transportation technologies that reduce emissions.
One strategy is continuing improvements in engine performance. For example, only about 12 percent of the energy produced in a typical car is converted into usable kinetic energy. Design improvements in this area would increase the number of miles per gallon. Other technologies are discussed in more detail in the resources below. How Stuff Works page offers a complete guide to how gasoline engines work.