Professor Anna Stefanopoulou (left) examines a V8 internal combustion engine with students Jacob Larimore and Xinfan Lin at the University of Michigan's Automotive Research Center. The researchers model engine performance to improve efficiency. Courtesy Automotive Research Center hide caption

itoggle caption Courtesy Automotive Research Center

Attendees at the Los Angeles Auto Show look at the BMW i8 plug-in hybrid concept car. Toyota's Prius is the best-selling hybrid on the market, but almost every carmaker has some form of hybrid technology. Jae C. Hong/AP hide caption

itoggle caption Jae C. Hong/AP

Morris and Salom Electrobats are seen in front of the Old Metropolitan Opera House on 39th Street in the Manhattan borough of New York, 1898. Museum of Modern Art/AP hide caption

itoggle caption Museum of Modern Art/AP

A Nissan Leaf charges at a station in Portland, Ore., that can recharge an electric car in 30 minutes. Electric cars could be an integral part of meeting 55-mpg fuel standards by 2025, but many consumers are put off by the vehicles' higher price and what some call "range anxiety." Rick Bowmer/AP hide caption

itoggle caption Rick Bowmer/AP