In practical terms, incremental improvements to conventional gasoline-powered cars may offer the quickest way to reduce air pollution from automobiles.
Hyundai Motor America, for instance, unveiled a turbocharged, four-cylinder engine with direct injection in Los Angeles for its six-passenger HED-5 concept car. Among U.S. manufacturers, Ford and GM are also increasingly using turbocharging and direct injection to get more power out of smaller engines.
Better aerodynamics, innovative transmissions and lightweight materials can also help achieve better mileage, without having to rely on unproven technologies.
Here's a slideshow look at the "green" vehicles displayed in Los Angeles, ranging from the theoretical to the practical:
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While gas prices have dropped dramatically, the sales of diesel cars has continued to grow, rising 13.5 percent in 2014 over 2013, according to the latest data compiled by IHS Automotive for the Diesel Technology Forum.
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