In the late '80s and early '90s, demand for diesel-engined cars dropped off. Peugeot had left the U.S. market, and General Motor's abortive attempts at converting gasoline engines into diesels soured many Americans to the idea of an oilburner. In 1999, Mercedes-Benz sold only 3,473 E300 Turbodiesel models. This model was sold in all 50 states and boasted fuel economies of 36 mpg on the highway and 26 mpg around town, outstanding for such a large car. The E300 was fairly conventional when it came to diesel engine technology. When the E-Class was face-lifted for the 2000 model year, this older technology wouldn't fit under the new car's lower hoodline, and Mercedes-Benz made plans to skip a year and then offer a new-technology, common-rail direct injection diesel engine in the E-Class in 2001.
If interest in diesel technology in the U.S. had waned, in Europe and Asia it was and is still going strong. The application of new and innovative technology has helped to create diesel engines for everything from the tiny VW Lupo to large sport utility vehicles. The biggest innovation has been the development of common-rail direct fuel injection. Traditionally, diesels have made use of an auxiliary pre-chamber where combustion is initiated by injecting fuel into a compressed charge of air. The fuel spontaneously ignites and transfers out of the small chamber into the cylinder, igniting the rest of the air and fuel mixture. The advantage of using a pre-chamber is the low fuel pressure needed for injection, which is less costly. This old diesel technology has some significant downsides that are often associated with the American diesel experience. The engines are slow to start when cold, make significant noise and smoke and have limited power output.
Enter common-rail direct injection. Instead of initiating combustion in a prechamber, diesel fuel is injected directly into the combustion chamber as it nears the top of its compression stroke. This requires significantly higher fuel pressures of up to 28,000 psi and the timing of the fuel pulse down to one millisecond. In a common-rail system, a continuous high-pressure pump maintains fuel pressure, and fast-acting electronic solenoid valves precisely meter the amount of fuel. The rewards are great. Fuel economy is 10- to 15-percent better than a conventional diesel engine and 20- to 30-percent better than that of a gasoline engine. What's more, emission control is better, cold starting is improved through electronic control, and even noise can be improved by pre-injecting a tiny milliliter of fuel into the combustion chamber just ahead of the main injection. Add in a turbocharger for an acceptable performance level, and these European diesel engines are serious competitors to the gasoline engine.
Fiat and Volkswagen were the first automakers in Europe to introduce direct-injection diesel cars in Europe, causing a rebirth of diesel cars on that continent. Diesel cars now account for more than 47 percent of the overall market in Europe.
So why aren't diesels popular over here in America? Heavy trucks and buses certainly use diesel engines, and the U.S. manufacturers offer diesels in the largest of their pickup trucks for people who need to carry heavy loads. A limited number of VW models are equipped with the excellent 1.9-liter TDI engine from Europe, whose 50-mpg highway mileage and quick response is re-introducing the concept of a diesel-engined car.
But, there are problems looming on the horizon, making European and domestic auto manufacturers wary of diesels for the U.S. market. To meet national emissions standards that will come into effect in 2004 requires a big reduction in the amount of NOx (nitrides of oxygen). To do this with a diesel will require a special NOx catalytic converter. But this converter is poisoned if sulfur levels greater than 30 parts per million (ppm) are present in the diesel fuel. Current U.S. diesel fuels have about 200 ppm sulfur. The sulfur levels are determined by the geographical location the crude oil comes from, and it costs a lot of money for the oil companies to make low-sulfur-content fuel from high-sulfur crude.
If that isn't enough, the problem for diesel engine cars is worse in California. The California Air Resources Board (CARB) has identified particulate emissions from diesel fuel as toxic air contaminants. Diesel particulate filters and traps can reduce particulate emissions by up to 85 percent, but only when used with 15 ppm low-sulfur fuels. Just as with the NOx converter, higher sulfur levels poison the particulate traps and filters. CARB has dictated that by 2006 all diesel fuel in California must be at 15 ppm sulfur or less, but nobody, least of all the oil companies, knows how this is going to happen in an economical way. Heavy trucks are not exempt from these problems, but powerful lobbying from the trucking industry may give them some reprieve.
Even more difficult, however, is CARB's insistence that passenger cars with diesel engines must meet the same exhaust emissions standards as those powered by gasoline engines before the end of 2003. The technical community is pretty much in agreement that this is impossible, given the level of sulfur in current U.S. diesel fuels. CARB's regulation may just be a death sentence for diesel-engined passenger cars. Mercedes-Benz has already suspended its plans to introduce an E300 with a diesel engine to the U.S. market.
The irony in all of this is that the new turbocharged common-rail direct-injection diesel engines are so much better than the old-fashioned diesels. Anyone who's driven a Volkswagen with the 1.9-liter TDI engine knows it is fast, quiet and economical. And this engine is actually at least a generation behind the latest and greatest common-rail diesel engines sold in Europe. Forty-one percent of the cars Mercedes-Benz sold last year in Europe were powered by diesel engines. Research continues in Europe into making even more gains, and industry experts predict another 5- to 10-percent improvement in diesel fuel economy by 2005 with even less noise and better performance.
In the American market, as the industry and government ponder increases in corporate average fuel economy (CAFE) and the use of hybrid vehicles and fuel cells in vehicles to reduce emissions and increase fuel economy, the diesel engine that could provide at least part of the answer seems destined for the history books.