AUGUSTA – Fans of the vegetable-oil fuel known as biodiesel pleaded their case before the state Board of Environmental Protection on Thursday, asking why, in the interest of environmental protection, the state of Maine has banned the sale of fuel-efficient diesel cars.

Beginning with the model year 2004, Volkswagen diesel Jettas, Golfs and Beetles – the only cars on the market in the United States that can run on biodiesel – cannot be sold in this state because they do not meet the California emission standards Maine has adopted.

Used cars, defined as having more than 7,500 miles, are not affected.

William Lord, a biodiesel devotee from Cape Porpoise, petitioned the BEP in October for an exemption for the diesels, and a well-attended public hearing on his request was held Thursday in Augusta.

Diesels, particularly when they’re run on pure biodiesel or a fuel blend, are among the most environmentally sound vehicles available, with a fuel efficiency of up to 50 miles per gallon, Lord said.

The Volkswagen diesel cars fail the regulations because their emissions – when running on regular diesel – exceed a threshold for nitrous oxides, one of four pollutants identified in the strict air quality standard used in Maine, California, Vermont and Massachusetts. Nitrous oxides are a major component of the ground-level ozone known as smog that many blame for the air quality problems and high asthma rates in southern and coastal Maine.

However, even on regular diesel, the cars have lower overall greenhouse gas emissions than many cars on the market – 41 percent lower than the Chrysler Concorde, and 61 percent lower than the Toyota Land Cruiser, both of which can be sold in Maine, Lord said, citing federal Environmental Protection Agency data.

“This is a vehicle that will, on balance, do far less damage to Maine’s environment than many of those on the road today,” he said. “Greenhouse gases are the ultimate environmental time bomb.”

However, the DEP on Thursday opposed Lord’s petition. The federal Clean Air Act allows states, like Maine, to adopt the strict California emission standards, but it requires that states accept the standards in their entirety.

Ron Severance of the DEP’s Air Quality Bureau presented board members Thursday a letter from the EPA stating that a diesel exemption would run afoul of federal law.

Fighting the EPA on this issue could stand in the way of Maine’s efforts to convince the federal government that the state is doing all it can to address air quality violations, Severance said. The federal government is already considering raising the status of air quality problems in southern Maine from “moderate” to “severe,” Severance said. Such a rating might be followed by federal enforcement of the Clean Air Act and potential fines against the state.

The EPA letter also questioned Maine’s current practice of looking the other way when state residents cross the state line to buy diesels in New Hampshire, then register them in Maine. All other states that use California emission standards refuse to register new cars that don’t meet the standard, Severance said.

That the diesel emissions are high in nitrous oxides but lower in some other pollutants doesn’t change the law, Severance said. “We don’t believe it’s appropriate to trade off pollutants,” he said.

The conflict will be resolved in 2006, when ultralow-sulfur diesel comes onto the market nationwide. The clean fuel might be available now if not for an extension that the EPA granted to oil refineries, and this new, cleaner fuel, is expected to allow diesel cars to meet emission standards.

But in the meantime, Maine car dealers say they’re losing money. Last year, 19.6 percent of the Volkswagens that Bill Sowles sold at his Brunswick and Falmouth dealerships were diesel. With two diesel models becoming available next year, Sowles predicted a third of his customers would be forced to seek diesels in New Hampshire.

“You can see the passion,” he said. “They’ll drive anywhere to buy a diesel.”

Tom Brown, president of the Maine Automotive Dealer Association, added that this problem could be expected to grow. Next year, Jeep, Mercedes and BMW are expected to introduce diesel cars or trucks to the U.S. market, he said.

“This is an economic penalty to the dealers in the state … without any environmental benefits,” Brown said.

The BEP will vote on the petition at a future meeting.