Environmentalists this month are in the unfamiliar, and possibly uncomfortable, position of praising the Bush administration.

The issue that has elicited unusual positive statements from usual Bush critics is the clean-air benefits of new rules for diesel-powered vehicles issued by the Environmental Protection Agency.

The latest regulations, issued earlier this month, will mandate reduced emissions from off-road equipment such as bulldozers, farm tractors, tugboats and locomotives - through a combination of low-sulfur diesel fuel and new, advanced technology diesel engines. An earlier EPA order targeted diesel engines in tractor-trailers and municipal buses.

While nearly invisible to many people, the heavy-duty diesel equipment run on construction sites, in major harbors, on farms and railroads contribute an estimated 25 percent of smog-causing nitrogen oxide and about half of the soot from diesel sources.

One official noted that the puff of black smoke from a diesel-powered truck, bulldozer or train will be a thing of the past.

Taken together, the coming clean-up of diesel-powered equipment, both on the road and off, will have a major impact in helping to further clean the air Americans breath. The mandated low-sulfur diesel fuel, which is already produced in Europe, will be flowing through U.S. refineries by 2007, with a further reduction in sulfur coming by 2010. The low-sulfur fuel will in turn allow engine manufacturers to use catalytic-converters and other after-burn technologies to further clean the exhaust emissions of big diesel engines.

The new regulations are, according to the EPA, the result of cooperation between federal regulators, the oil industry, environmentalists and engine manufacturers.

Today's diesel fuel contains as much as 3,000 parts per million of sulfur. The EPA-mandated 2007 level will be 500 parts per million. By 2010, the sulfur will be reduced to 15 parts per million.

Bush administration officials have compared clean-air impact of the coming clean-up of diesel fuel and diesel engines to the environmental and health benefits of the elimination of lead from regular gasoline decades ago.

The coming production of clean diesel fuel by U.S. refiners will not only help clean up significant air pollution from big rigs, tractors, locomotives and bulldozers. It will also encourage more American motorists to switch to newly developed diesel-powered cars, which will take advantage of the cleaner fuel while at the same time produce 30 percent to 40 percent better fuel economy.

Until recently diesel fuel attracted little attention - either positive or negative. In recent years, however, the spotlight has been shining on diesel both for the pending clean-up of telltale puffs of soot, but also for the significant fuel savings available.

Diesel fuel, including emerging bio-diesel blends, may well turn out to be the best transition fuel to bridge from today's gasoline engines to the fuel-cell systems of tomorrow.