LOS ANGELES — The red car humming quietly along this four-lane suburban road looks pretty much like your average four-door sedan.

But take a close look at the latest model of Honda's FCX Clarity, unveiled at the Los Angeles Auto Show in November, and you'll spot one key difference: There's no tailpipe at the back.

This car doesn't need one. As a hydrogen fuel-cell vehicle, it releases only water vapor from an outlet on its underside, instead of carbon dioxide, carbon monoxide and other unsavory exhaust produced by conventional gasoline-consuming cars.