Sherry Dean has a secret she has been keeping from her drivers since March: Their Upshur County school buses are running on an alternative fuel made of vegetable oil and diesel.

"I wanted to run it without my drivers or mechanics knowing," she said. "That way I can have a true feeling for how it's doing."

So far, she said, the results have been great.

Dean is among a slowly growing number of county transportation directors in West Virginia and across the nation who are switching from straight diesel to a mixture of diesel and biodiesel, a fuel based primarily on vegetable oils.

Biodiesel use has been growing since 1992, when Congress passed the Energy Policy Act in a move to reduce U.S. dependency on foreign oil. It has since been approved by the Environmental Protection Agency as an alternative fuel.

Since 1999, biodiesel use nationwide has grown from 500,000 gallons to 25 million gallons in 2004. It is estimated that usage will surpass 50 million gallons this year, said Amber Pearson with the Missouri-based National Biodiesel Board. The board was established in 1992 by soybean commodity groups to promote the use and research of biodiesel.

School systems have contributed to the growth. About 100 systems nationwide have made the switch, "and that number is growing all the time," Pearson said.

Nevada's Clark County started using the fuel after lawmakers in that state required the phased-in purchase of alternative-powered vehicles. The school system, which includes Las Vegas, uses about 3 million gallons a year, said Frank Giordano, who oversees the county's fleet of 1,300 school buses and 1,600 other vehicles.

"The good part of that is we've displaced 600,000 gallons of petroleum fuel," Giordano said. "That's something we're pretty proud of."

Although county and school officials say they have not noticed any appreciable increase in miles per gallon, they say biodiesel does not require expensive modifications to their diesel-engine buses. Also, the fuel produces fewer emissions and is healthier for the 24 million schoolchildren who ride buses daily.

Perhaps more important, recent petroleum price increases -- coupled with state and federal incentives -- make biodiesel an economical choice.

The EPA announced a $7.5 million grant program over the summer to help school systems reduce emissions from older buses through replacing engines or switching to alternative fuels.

Kentucky officials expect to announce a $48,000 grant program this month. The state's 176 county and municipal school systems can use the money to pay the difference between regular diesel and biodiesel.

New Jersey also offers biodiesel on a statewide contract for local and state agencies to use. The state also has a biodiesel rebate program to offset the additional cost.

Four of West Virginia's 55 counties are using the fuel, and several more plan to make the switch. Those that do use biodiesel are eligible for increased state transportation funding.

"I was banking on fuel continuing to rise," said Buster Nicholson, Jefferson County's operations director. "It's kind of a hedge against inflation."

The county, which is about 90 miles from the District, began using the fuel in May. Nicholson said the change may generate about $60,000 in additional funding from the state Department of Education.

Under the state school aid formula, counties receive about 85 cents for every dollar in transportation costs. By switching to biodiesel, the reimbursement increases to 95 cents. Although it takes as long as two years for the state to begin paying at the higher rate, "once that cycle kicks in, it just keeps coming," Nicholson said.

Increased consumer interest and state and federal tax incentives for producers mean there are now about 1,500 biodiesel distributors nationwide.

"It's a matter of economics," said Rich Cregar with the National Alternative Fuels Training Consortium at West Virginia University. "I don't think anybody is arguing we're going to see cheap oil again.

"Alternative fuels will be economically viable."

As the operations manager for the nation's longest running user of biodiesel in school buses, Joe Biluck Jr. has seen the industry evolve.

"Today, I'm happy to say there has been a tremendous amount of interest," said Biluck, director of operations for the Medford Township, N.J., school system. "The supply chain is improving."

When Biluck started using biodiesel, the closest distributor was in Massachusetts. He now can buy fuel from a supplier in nearby Philadelphia.

Biodiesel supporters refer to the fuel as a renewable energy source because it is partly based on soybeans or other oil-producing vegetables. The common mixture is 20 percent biodiesel, 80 percent diesel fuel.

The fuel requires less technology to produce than ethanol, and Cregar said studies have shown that fuels based on soybeans and other crops could produce up to 3.5 million barrels of fuel daily.

An improperly mixed batch, however, can cause problems with a bus, or even make it stop running, said Marion County Transportation Director Tim James. Marion was the first county in West Virginia to begin using the fuel in its 79 buses.

"Just make sure when it's delivered, it's mixed," James said.

The EPA is requiring refiners to produce lower-sulfur diesel fuel beginning next year, and diesel engine manufacturers to build cleaner-burning engines by 2010. The agency estimates the new rule will cut emissions of smog-forming nitrogen oxides, soot, carbon monoxide, acid-rain-causing sulfur dioxide and other air pollutants, preventing 8,300 premature deaths, more than 9,500 hospitalizations and 1.5 million lost workdays.

"I consider it a very effective transition technology," Biluck said. "Five to eight years from now, we'll be talking about something different."

In the interim, using biodiesel "shows people we are trying to do everything we can to offset energy costs and at the same time, be more sustainable," he said.