Every few weeks, Etta Kantor goes to a Chinese restaurant and fills a couple of five-gallon pails with used cooking oil. Back in her garage, the 59-year-old philanthropist and grandmother strains it through a cloth filter and then pours it into a custom-made second fuel tank in her 2003 Volkswagen Jetta diesel station wagon. Once the car is warmed up, she flips a fuel toggle on the dashboard to switch to the vegetable oil. Wherever she drives, she’s trailed by the appetizing odor of egg rolls.

Sean Parks of Davis, California, collects his cooking oil from a fish-and-chips restaurant and a corn-dog shop. He purifies it chemically in a 40-gallon reactor that he built himself for about $200. The processed oil can be used even when his car's engine is cold, at a cost of about 70 cents a gallon. Parks, 30, a geographer for the U.S. Forest Service, makes enough processed oil to fuel his family's two cars.

Kantor and Parks are willing to go the extra mile to reduce their dependence on petroleum and cut down on pollution. But these days environmentalists are not the only ones banking on biodiesel, as diesel-engine fuel made from vegetable oil is known. Entrepreneurs and soybean farmers are creating a new biodiesel industry, with some 300 retail biodiesel pumps nationwide so far. Commercial production of biodiesel grew 25 percent in 2004, making it the fastest-growing alternative fuel in the United States. Even the singer Willie Nelson recently started a company to market the fuel at truck stops.

The greening of the diesel engine is a return to its roots. Rudolf Diesel, the German engineer who in 1892 invented the engine that bears his name, boasted that it ran on peanut and castor oil. "Motive power can be produced by the agricultural transformation of the heat of the sun," he said. The inventor foresaw a future of virtually unlimited renewable energy from plants, but the idea slipped into obscurity because petroleum was so much cheaper than vegetable oil.

A century later, customers for commercial biodiesel include the U.S. Postal Service, the U.S. Army, the Forest Service, the city of Denver and numerous private truck fleets. Almost all use blends of 2 to 20 percent biodiesel mixed with standard petroleum diesel. The mixture helps federal and state agencies comply with a 2000 executive order by President Clinton mandating less petroleum consumption. Minnesota recently became the first state to require that all diesel fuel sold there be 2 percent biodiesel. Daimler-Chrysler's 2005 diesel Jeep Liberty comes off the production line with its tank filled with a 5 percent biodiesel mixture.

The major obstacle to wider use is price. Pure biodiesel sells for $2.50 to $3 a gallon, about 50 cents to $1 more than petrodiesel. To spur biodiesel's use, some European nations levy no taxes on it, and in October 2004, President Bush signed into law a 50 cent to $1 credit to fuel manufacturers for every gallon of biodiesel blended into petrodiesel. Diesel engines differ from gasoline engines in their use of high pressure rather than a spark plug to ignite the fuel and drive the pistons. Diesel engines can run on fuel that is heavier than gasoline, making it possible to substitute filtered waste grease for petrodiesel. Both used and virgin vegetable oil contain glycerin—a syrupy liquid used in hand lotions. It burns well in a hot engine, as in Etta Kantor's retrofitted diesel, but clogs a cold one. Removing the glycerin yields biodiesel, which is suitable for even a cold engine.

Skeptics have questioned whether it takes more fossil fuel to produce biodiesel—to fertilize crops, transport them and press them for their oil—than the resulting biodiesel replaces. But Jim Duffield, an agricultural economist with the U.S. Department of Agriculture (USDA), says the "few lone voices" who still make that point have not kept up with improvements in agriculture and biodiesel technology. Indeed, a study by the U.S. Departments of Agriculture and Energy in 1998 and another in 2002 for the French government show that soybeans and canola oil yield three to four times more energy than is needed to make the fuel. (Similar skepticism has also dogged ethanol, a corn-based fuel mixed with gasoline to create gasohol. But USDA and other studies show that today's ethanol provides up to 30 percent more energy than it takes to make it.)

Another benefit of burning biodiesel is cleaner air. Compared with fossil fuels, it emits less carbon monoxide and hydrocarbons, as well as sulfur compounds related to acid rain. Pure biodiesel also substantially reduces overall emission of carbon dioxide, a major contributor to climate change, because the plants from which the oil was extracted absorbed atmospheric carbon dioxide while they were growing. A bus running on pure biodiesel would emit 32 percent less particulate matter, which has been implicated in the dramatic increase in asthma cases in cities. The only air pollution downside of pure biodiesel, according to the 1998 U.S. study, is a slight increase of smog-inducing nitrogen oxides.

The inspiration for the do-it-yourself biodiesel movement came from Joshua Tickell, 29, of Baton Rouge. While studying in Germany in 1996, he was astonished to see a farmer using canola oil to run his tractor. Back in the States, Tickell used his last student loan check to help buy a 1986 diesel Winnebago. He painted sunflowers on his "Veggie Van" and, for two years beginning in 1997, toured the country, towing a simple reactor that turned restaurant oil into biodiesel. In 2000, he coauthored what would become the biodiesel bible, From the Fryer to the Fuel Tank. "My goal is very simply to make OPEC obsolete," he says.

Vegetable power also appeals to 50-year-old Marty Borruso, a chemist and partner in Environmental Alternatives in New York City, who insists he's no "environmental crazy." He produces biodiesel for a generator that makes electricity and hot water for an 87-family apartment house. He also sells the fuel to a tow truck fleet and anyone who comes to a pump he operates next to his production facility in Staten Island. In a 7,000-gallon reactor, Borruso processes out-of-date virgin vegetable oils, which he buys at a steep discount, and free grease from a fried chicken emporium. But he spurns grease from a seafood restaurant. "It smells like calamari," he says. "I love calamari, but I don't know if I want to drive it."

On average, fast-food restaurants in any major U.S. city generate about 22 pounds of waste grease each year per city resident, according to a 1998 study by the National Renewable Energy Laboratory (NREL). The National Biodiesel Board, a trade group in Jefferson City, Missouri, estimates that more than 2.5 billion pounds of waste cooking grease are available annually—enough to make 100 million gallons of biodiesel.

Of course, America's appetite for petroleum is huge: 2004 consumption was nearly 315 billion gallons, including 139 billion in gasoline and 41 billion diesel. Robert McCormick, a fuels engineer at NREL, says that biodiesel could displace 5 percent of the petrodiesel used in the United States within ten years. To replace more will require growing vegetable crops specifically for fuel—and America's soybean farmers are standing by. Some proponents envision growing aquatic algae—richer in oil than any other plant—in pools next to electric power stations. In an ecological two-for-one, the smokestack carbon dioxide would feed the algae, which would churn out biodiesel.

Grass-roots fans aren't waiting. Kantor, who paid $1,400 to outfit her VW diesel with a second fuel tank, says she gets nearly 200 miles per petrodiesel gallon. "This is not about money," says Kantor, who speaks at schools about protecting the environment. "I'm doing this to set an example."