Diesel engines are efficient and burn a variety of fuels. But as any motorist stuck behind a diesel-powered vehicle knows, they also can be dirty. Richard Sapienza of the Brookhaven National Laboratory pointed out recently at a meeting of the American Chemical Society, "A car with a diesel engine will get 25 percent more miles per gallon than the same car with a gasoline engine. At the same time, however, the diesel car emits about 13 pounds of soot every 10 ,000 miles."
Therein lies the challenge of this fuel-saving engine. Cleaning it up isn't going to be easy.
Traps and filters now used to treat diesel exhaust aren't all that satisfactory, Sapienza says. He and his colleagues at Brookhaven are coming at the problem another way by trying to prevent soot formation in the first place.
Chemists think it arises in the fuel-air mixture and along the sides of the combustion chamber during the fuel burning process. The Brookhaven scientists are trying to prevent this by adding alcohol to the fuel and by treating the engine wall with platinum. Adding alcohol cuts soot by 40 percent, while the platinum brings another 40 percent reduction. However, there are snags. Sapiensa reports that the platinum comes off after an engine has run for more than 8 hours.
Alcohol may present a more subtle problem. When alcohol (in this case, methanol) is burned directly with the diesel fuel, it makes the exhaust particles more "biologically active." They more readily cause genetic mutations in bacteria -- a property that raises questions about hazards to higher life forms.
As Prof. Samuel S. Lestz of Pennsylvania State University points out, this also raises a serious question about present efforts to develop an engine that burns both diesel fuel and methanol.
He notes that the problem seems to lie in the way the fuels are burned. Unlike gasoline and ethanol -- the form of alcohol added to gasoline to make gasohol -- diesel fuel and methanol don't readily mix. Moreover, while the diesel fuel auto-ignites, the methanol has to be ignited by a spark. These problems now are circumvented by having the fuels enter the combustion chamber separately. Diesel fuel goes in through the usual injection system while methanol is sprayed into the intake air before it enters the chamber.
Handled in this way, the methanol appears to make exhaust particulates more active biologically. This, says Lestz, suggests that a new kind of diesel-like engine needs to be designed to burn methanol effectively and in an environmentally safe way.
Diesels can bring substantial energy savings -- a 6 percent drop in national oil use if 25 percent of the US automobile fleet were to use them with the typical 25 percent gain in mileage. Yet as the work both at Brookhaven and Penn State illustrates, realizing that advantage probably means designing a new type of diesel engine.