Diesel cars are not new. They've been widely popular in Europe where they secured 50 percent of the overall market last year. But they've been slow to catch on in the United States, capturing only 2.3 percent of the market.

J. D. Power and Associates predicts that by the year 2015, they will slightly outsell hybrids, 1.5 million to 1.45 million. But to fulfill that prediction, diesels will have to overcome their environmentally unfriendly image. For decades, it was tough for car manufacturers to meet tighter U.S. emission standards for pollutants—specifically nitrogen oxides, or NOx.