Grease Mobile.

On their two-month road trip last summer, recent college grad Justin Carven, 23, and pal Skill) Wrightson, 22, refueled regularly at fast-food restaurants. They loaded up on burgers, fries, and a tank of grease--yes, grease--for Carven's retooled 1982 VW camper van.

The van runs not on gas but on smelly, discarded frying grease. And the van's converted diesel engine (an engine that uses the heat of highly compressed air to ignite a spray of fuel) averages 25-30 miles per gallon--about the same mileage it coaxed out of diesel fuel. Not only does Carven's grease van emit less pollution than gas and diesel fuels, it's also economical, since waste grease is usually free. The major drawback: "The grease gives off kitchen smells. Once it was distinctly grilled chicken," says Carven.

How did he turn a diesel-guzzling VW into a bacon-grease and salad-oil machine? First he welded a special fuel tank for the grease onto the van's bottom. Next he wrapped an exhaust pipe around the tank to heat and liquefy the grease, so it would flow freely through the engine's pipes and filters.

The challenge, explains Carven, is that diesel engines demand small amounts of diesel fuel for ignition. So Carven had to create a "dual fuel system." Once diesel fuel ignites the engine, he simply flips a dashboard switch and--voila!--the engine burns vegetable oil.

Sound radical? Well it's not. The first diesel engine, which debuted at the 1900 World's Fair, ran on peanut oil. By design, diesel engines can churn on most greasy liquids, not just the petroleum-based fuel sold at gas stations. "What I'm doing is flipping back to Rudolf Diesel's original idea," Carven says.

Carven has recently begun selling an $800 grease-car conversion kit. That may sound pricey, but as fuel prices soar, your grease kit could pay for itself in about a year. And french-fry grease is a lot better for your car than for your body.