HEBER CITY — A faint odor of restaurant grease lingers in the back of a steel welding shop off U.S. 40, but no burgers or fries are served here.

Over the vrooms of diesel-powered trucks driving through the Heber Valley, Bill Hartlieb explains how two 55-gallon drums are filtering waste vegetable oil, converting it into fuels that can power cars and trucks, as well as farm and construction equipment.

"This is water and funk and Lord knows what," Hartlieb says, referring to vegetable oil in one of the drums that he got from a pub in Park City. "Chicken bits and all that crap. And so it's not glamorous. I wouldn't recommend every garage have a biodiesel-making plant."

Hartlieb's setup is a refinery of sorts, small scale and relatively low tech — and also illegal, according to federal law.

But he disputes that his hobby violates the Clean Air Act, since fuels derived from vegetable oil have fewer emissions than petroleum.

"I think it's really kind of a gray issue," he says.

Even so, he says he can't make most of the fuel he needs, and he ends up buying 80 percent of his fuel at a Park City station that sells biodiesel.

High fuel prices and global warming have made fuels derived from vegetable oil more attractive. Hartlieb is among a growing number of Utahns who are using and making biodiesel and straight vegetable oil fuel, or SVO.

For Hartlieb, the process of making fuel from soybean and canola oils is about finding a renewable alternative to petroleum.

"The big difference is this comes from a farmer's field," he says. "This doesn't require an Iraq war."

It's also about experimentation and the scientific process. "I just love to be able to tinker with this stuff," says Hartlieb, who has a degree in aerospace engineering from St. Louis University and works in product development at Backcountry.com.

But the growing popularity of tinkering with fuels derived from vegetable oil may be fraught with legal consequences. The Environmental Protection Agency has a list of companies that are certified to convert vehicles to alternative fuel. No companies for SVO kits are listed.

"Basically, all diesel fuels and gasoline fuels and fuel additives are required to be registered with the EPA," says Jeff Kimes, a Denver-based EPA environmental engineer. "Let's say you want to start making biodiesel and introduce it into commerce. You have to register it with the EPA."

At least one SVO conversion-kit company is in the process of testing with the EPA. But the fuel has not been approved by the EPA.