SOUTH KINGSTOWN, R.I. — Please forgive Nick Russell if he drives by with a wry little smile on his face as you fill your gas guzzler. It's the kind of smile that says, as he himself put it, "Hey, I don't have to do that."

As you stand at the pump, you'll quickly find out why the grin. As Nick's 1985 tan Mercedes passes, you'll see large letters in the rear window that spell out: POWERED BY VEGETABLE OIL. It's above the sticker on the trunk lid that reads: KILL YOUR TELEVISON.

That full tank you just put in your car or truck probably cost you anywhere from $25 to $60. Know that when Nick drives by, his full tank cost him nothing. Zilch. Zero.

And besides the obvious cost benefit, "It doesn't pollute and you're not dependent on foreign oil," Russell said.

Russell, 19, the son of Joel and Lois Russell of Hopkinton, R.I., is a sophomore at the University of Rhode Island, where he is a member of Student Action for Sustainability, a division of URI's Renewable Energy Club.

He and his fellow club members have a simple mission: "We try to get people to use alternate fuels and alternate transportation."

In November 2005, while a freshman, Russell decided to practice what he preaches.

He spent $800 at greasecar.com for a kit that allowed him to convert his diesel-powered Mercedes to vo-power (as in vegetable oil). "It seems like a lot of money, but I should be getting it back by April or May," Russell said. "I should be in the positive by then."

It took him and his father, working in their spare time, until March 2006, to finish the conversion.

Conversion kits are available only for diesel-powered cars and trucks, like Nick's Mercedes.

Russell's kit allowed him to put in a second fuel tank, for the vo. When he gets in his car, he makes sure his new three-switch system is on diesel.

He runs on diesel-power for five minutes or so, allowing the engine to warm up enough to be able to handle the vo, which needs warmth to keep it from congealing. To aid with the heating process, hoses were installed in the Mercedes' coolant system to warm the fuel lines.

Once the engine warms, he flips the switch to a second setting, which cuts off the diesel tank and lets the vo flow from its tank. At the end of the journey, he flips the switch to a third setting — the "purge" setting. That clears the lines of the vo, which, if left in the lines, would congeal.

He gets his used soybean oil every other month from the Two Little Fish restaurant.

"It was a pretty casual thing," said Tim Brennan, the owner of Two Little Fish. "They came in and introduced themselves, and we were glad to help."

Russell fills up a 50-gallon drum of oil, then heads back to his family's home in Hopkinton, where he transfers the used oil, using the pump, to a big trash can. He then places the trash can, containing the congealed oil, over a fire in a pit. After boiling away any water that got in the oil, he takes the soybean oil and strains it back into the 50-gallon plastic drum.

Once cool, the vo is ready to be pumped into the Mercedes' tank.

The process, including the drive to the restaurant, takes a minimum of three hours, but it's not something you have to stand over.

"I've looked at it as a fun thing to do," Russell said.

His Mercedes has 237,000 miles on the odometer, but he didn't hesitate to install the conversion kit. A Mercedes lasts. "This car is worth $3,800 and that's pretty amazing for a 20-year-old vehicle," Russell said. The Mercedes gets 28 miles to the gallon with vegetable oil, as compared with the 29 miles he gets with diesel.

What he doesn't get is the smell of burning carbon fuel. "It doesn't smell exactly like French fries," he joked, before adding: "But it's not like a heavy diesel smell, either. It's more a sweet smell."

"I'm pretty sure I want to do a career in something like this," Russell said. "I think someone could make a lot of money with something like this."

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