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: