Diesel Cars Come Cleaner

HAVE you followed a diesel car or truck down the road lately, wondering about the black smoke belching out the tail pipe?

Environmental regulators are working to set standards to clean up diesel exhaust: Next year, the Environmental Protection Agency will require cars to produce less than 0.08 gram per mile of their heavy black soot, known as particulates. Currently, the EPA allows no more than 0.2 gram per mile. The cars will also have to meet tighter standards for nitrous-oxide emissions. And strict standards for heavy-duty diesels, based on emission standards California passed in 1989, will take effect in 1994.

"The EPA felt safe in passing tighter standards because five engines already met our standards," says Jerry Martin of the California Air Resources Board. Exit from California

After the stricter regulations went into effect in California,

which has worked to clean up the dirtiest air in the United States, several diesel-car manufacturers pulled their automobiles out of the California market.

Mercedes-Benz, the largest seller of diesel cars in the US, had been working since the mid-1980s to develop technology to clean up diesel emissions, but it stopped selling its diesel products in California when the stricter regulations were passed. Almost no new diesel cars have been sold in the state since.

Now diesels are starting to make a comeback as manufacturers develop new technologies to meet the tighter controls.

Two carmakers, Mercedes-Benz and Volkswagen, plan to introduce diesels that use catalytic converters and new combustion technologies to meet the stringent California requirements.

"Diesel owners are some of our most loyal customers," says Tony Fouladpour of Volkswagen US. "People come in here with diesel Rabbits with 200,000 miles on them and want to know when they can get a new one."

Volkswagen's Eco-Diesel is the only diesel-powered car certified for sale in California under the state's new emission requirements, but the car hasn't been sold there because of a model change that has delayed introduction of the chassis for the engine. Volkswagen did sell the Eco-Diesel in the rest of the US in 1992 and claims the car has less of the notorious diesel stench.

Volkswagen uses a turbocharger to boost airflow into and out of the engine, leaving enough oxygen in the exhaust for a three-way catalyst to remove unburned exhaust. The engine also produces about 20 percent more power than the engine Volkswagen used before.

The engine in the upcoming Mercedes E300 also boosts airflow by using high-performance technology but doesn't take advantage of the potential power increase. Instead, it uses increased air flow through the system to run a catalytic converter. The engine uses four valves per cylinder, a first for a diesel engine, says Fred Heiler, spokesman for Mercedes-Benz North America.

In the mid-1980s, Mercedes tried using particulate traps to meet California emissions requirements but abandoned the effort when the devices proved unreliable.

"We don't see much future in particulate traps," says Bob Jorgensen, product environmental manager for Cummins Engine Company of Columbus, Ind., which builds heavy-duty diesel engines for trucks and buses. He calls particulate traps "not very cost effective" because they clean up only diesel soot, not the hydrocarbon and nitrous-oxide emissions regulated on all cars. Injection techniques

Mr. Jorgensen is focusing on solutions like using higher pressure to inject heavy diesel fuel into the cylinders in smaller droplets, which produce smaller particles in the exhaust, and varying injection timing to burn the fuel thoroughly.

Following heavy-duty diesel manufacturers, Volkswagen is developing an engine that abandons the prechamber design typical in automotive diesels, Mr. Fouladpour says. Volkswagen's sister company, Audi, already has a direct-injection engine on the market in Europe, and Volkswagen's engine could be on the market in the US by the 1995 model year. Direct injection has long been shunned for automotive use because of the harsh vibrations it generates, the expense of a high-pressure injection pump, and the more difficult cold starting.

In addition to using a high-pressure injection pump, Mercedes calls its new engine a "direct injection" diesel, Mr. Heiler says.

But the engine will use some type of prechamber, probably located between the valves at the top of the combustion chamber, he says, because "Mercedes customers would probably not put up with the roughness of most direct-injection engines."