In 1900, when Rudolf Diesel rolled out his namesake engine, he said, "The use of vegetable oils for engine fuels may seem insignificant today. But such oils may become, in course of time, as important as petroleum." That time hasn't arrived yet. But high gas prices and fears about dwindling oil reserves are garnering mainstream attention for vegetable-oil auto fuel (or biodiesel).

About 400 retail outlets nationwide sell the clean-burning fuel, which powers virtually any diesel engine. Until recently, biodiesel was too expensive to be popular, but its price has dropped by up to a dollar per gallon this year due to a federal tax credit; in Boulder, Colo., biodiesel sold for $2.89 per gallon last week, while conventional diesel was $2.35. Though it's pricier, biodiesel "takes some of the guilt out of driving," says Marcus Cole of Denver, part of the 2 percent of car owners who drive diesel vehicles. Supporters (including Willie Nelson and Darryl Hannah) say the price will come down more in 2007 when an EPA rule cracking down on conventional diesel soot takes effect.

Touring a Virginia processing plant recently, President George W. Bush gave biodiesel a boost, calling it "one of the nation's most promising alternative fuel sources." Several states have passed laws to spur the clean fuel's use in recent years, and a Minnesota bill that goes into effect this summer will require all diesel sold to contain at least 2 percent biodiesel. Government institutions are the largest clean-fuel users, with more than 500 auto fleets running on the stuff; Babylon, N.Y., officials announced recently that the town will run its 100 municipal vehicles on biodiesel to reduce smog. Which probably makes the "Granola Ayatollah of Canola," a rapper who travels the country singing biodiesel's praise, happy. "I got that veggie fuel burnin' now I'm rollin' with ease," he raps. "Is that the scent of french fries I'm smellin' on the breeze?"