Steering the big E320 sedan, Mercedes-Benz engineer Rudolf Thom comes around a corner and gently presses down on the accelerator, revealing the faint yet familiar sound of a diesel engine.

"If you compare this diesel to the diesels of the past, can you hear this diesel?" he said. "Can you smell this diesel? It is not like the bad diesels of the past."

The E320 gets 35 miles per gallon, 30 percent better than the comparable gasoline version of the car. Because of favorable tax policies in Europe, diesels have taken off there in the past 15 years, accounting for 50 percent of the new models sold each year. In the United States, development of the market has lagged because of government concerns over what the fuel does to air quality; diesel vehicles make up just 3.2 percent of the market.

The dynamics began to shift this week. On Sunday, the Environmental Protection Agency began requiring refiners and fuel importers to reduce the sulfur content in diesel fuel by 97 percent. The low-sulfur fuel opens the door to a new generation of clean diesel cars, and automakers are moving to bring out more models in the U.S. market.

The change promises to significantly cut air pollution caused by diesel emissions. Regulators say high concentrations of sulfur in the old diesel fuel poison the engine systems that clean exhaust of harmful pollutants. The biggest concern is particulate matter, one of the byproducts of engine combustion, said Margo Oge, director of the EPA's office of transportation and air quality. The particles are a fraction of the size of a human hair. Public health advocates have described the particles as tiny spaceships that dive into the respiratory system when people inhale, damaging the lining of the lungs.

Particles from diesel emissions are classified by the government as a potential carcinogen and are linked to premature deaths, heart attacks and respiratory illness.

Mark MacLeod, director of special projects for the advocacy group Environmental Defense, said the new EPA rules are expected to prevent about 8,000 premature deaths each year, 1.5 million lost workdays and 360,000 asthma attacks.

Detroit automakers have pledged to expand diesel offerings, particularly in pickup trucks. J.D. Power and Associates projects that the diesel share of light-vehicle sales is expected to increase to more than 10 percent by the middle of the next decade from 3.2 percent in 2005. Japanese automakers are also stepping up development of diesel technology.