'Green' concept cars of 2007

A 500-horsepower diesel SUV isn't the sort of thing you'd expect to see in a gallery on "green" cars. But this one represent another attempt by Audi to dispel the notion that diesels, which are more fuel-efficient than ordinary gasoline engines, are slow and boring to drive.

Diesel engines, which use high compression and heat to combust diesel fuel, offer much better fuel economy than gasoline engines. Diesel engines have also improved greatly from the noisy, smoky units that powered some cars back in the 1970s.

The downside is that diesel engines also produce more pollution than gasoline. For that reason, states that follow California's lead on car pollution rules restrict the sale of diesel passenger vehicles.

Audi already makes a V6-powered diesel version of the Q7 SUV. This one doubles the number of cylinders, producing 500 horserpower and promising a zero-to-62 time (that's zero to 100 kilometers per hour) of about 5.5 seconds. Still, fuel consumption is about 20 miles per gallon. Not bad, considering.