The new environmental rules require a 97 percent reduction in the sulfur content of highway diesel fuel, from its current level of 500 parts per million (ppm), to 15 ppm. Refiners were required to start producing this cleaner fuel as of Thursday, and it should be available nationwide by October 15.

In addition to the obvious benefit of reducing emissions from the 13 million heavy trucks using diesel fuel, the new cleaner fuel could make it easier for automakers to introduce more passenger diesel cars that meet strict US emissions standards.

Only about 3.6 percent of new passenger vehicles registered in the US were powered by diesel, according to the Diesel Technology Forum, an industry group.

That is a sharp contrast to Europe, where about half the cars use diesel fuel.

But the US market is expected to rise, in part because of the better fuel economy from diesel.

The new cleaner diesel fuel "opens up the door of opportunity" to sell more diesel cars and light trucks in the US market, says Allen Schaeffer, executive director of the Diesel Technology Forum.

"Diesel has not had a great public image as a technology but that is changing. The new diesel engines have no visible exhaust, no smoke, the noise is equivalent to a gasoline model, and it offers 20 to 40 percent better fuel economy."

Additionally, Schaeffer said, a diesel engine offers more "torque," or acceleration capability, compared with an equivalent gasoline engine.

But automakers still need to do more to tweak their diesel engines to meet US emissions standards. New diesel engines will use filters for particulates -- black soot particles -- but will need further reductions of nitrogen oxides.

David Cole, chairman of the Center for Automotive Research in Ann Arbor, Michigan, said greater use of diesel is possible but not guaranteed.

"A low-sulfur diesel fuel is absolutely necessary if we are able to meet emissions standards," he said, but added that "nitrogen oxide standards are very difficult even with the cleaner fuel."

Cole said there is "a real horse race" among three different technologies in which automakers are trying to improve fuel efficiency: improvements to the traditional gasoline engine, the hybrid gasoline-electric powertrain and the diesel engine.

Each of these can offer some improvement, Cole said, but it remains unclear now which will end up being the most economical.

"Diesel is in the game and, I think we're going to see some interesting technology," he said.

"Everyone is working on these technologies and they can't pick the winners."

Automakers have to determine how much additional costs will be borne by consumers for the cars in order to save on fuel. For example, diesel engines may add 3,000 dollars to the cost of a car and save 30 percent on fuel. Hybrids can cost 4,000 to 6,000 dollars extra and save 30 percent or more, depending on driving habits.

"I don't see any problem with consumer acceptance, but the issue is really economics. Can they be competitive with existing technology with the cost penalty they have?" Cole said.

Currently, only a handful of diesel-powered passenger cars are sold in the US market by Volkswagen and DaimlerChrysler, along with several large pickup trucks from General Motors and Ford.

But on Thursday Chrysler Group said it will offer a diesel engine option for its 2007 Jeep Grand Cherokee, making it the first diesel-powered, full-size sport-utility vehicle to be offered in the United States.

Notwithstanding the automobile industry, the new regulations will have a major environmental benefit by cutting emissions, including a number of cancer-causing pollutants, from 13 million big diesel trucks on US roads.

"Diesels are the workhorse of American transportation, and now diesel engines can be much much cleaner," said Bruce Hill, scientist for the Clean Air Task Force.

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