Diesel Revisited

Bosch's American subsidiary recently held a special event in Detroit for U.S. journalists called Diesel Day. On hand were about 15 diesel-powered models from Audi, BMW, DaimlerChrysler, Ford, General Motors, and Volkswagen. When I turned the key of the Mercedes-Benz S400, its powerful V8 engine started up immediately. The car propelled powerfully and smoothly through the gears as I depressed the pedal, and was nearly silent. The same was true for most of the other cars, especially the large, luxury models, which emitted virtually no smell, smoke, or noise, and delivered powerful acceleration from a dead stop to highway speeds.



What has made such diesels possible? The most significant breakthrough has been the development of the common rail, a reservoir that runs alongside the cylinder heads in which diesel fuel is stored under pressure. When the engine requires fuel for combustion, a high-pressure pump forces the fuel from the rail straight into the injectors, which quickly open and close, spraying fuel into the cylinders. In older diesel engines, by contrast, fuel had to travel from the pump to each individual cylinder in separate pipes.



The common rail enables fuel to be injected into the engine's combustion chamber at very high pressures, so the fuel and air mix more thoroughly and burn more efficiently. And because the pump constantly replenishes the common rail with pressurized fuel, high fuel pressure is maintained throughout the engine's range of speeds -- ending the problem of hesitation on acceleration. "Common rail is the biggest single thing that has civilized the diesel engine," says Bernard Robertson, senior vice president of engineering technologies at DaimlerChrysler.

A second recent advancement in diesel engine technology is direct fuel injection. Diesel engines of the past relied on indirect injection, a system in which fuel was sprayed into a small chamber in the cylinder head, where a glow plug sparked combustion. With direct injection, fuel is sent directly into the combustion chamber. The result is even more fuel economy and lower emissions than with a traditional diesel engine.



Engineers are striving to further refine the diesel engine. They're developing injectors that will more precisely deliver fuel into the combustion chamber for an even more thorough burn. One intriguing new fuel injector is the piezoactuator. Launched by the German automotive supplier Siemens, it is based on technology borrowed from the company's high-speed inkjet printers. Piezoactuators distribute fuel faster -- about five times during a single combustion stroke -- than a traditional solenoid valve can open and close. At Bosch, meanwhile, engineers are increasing the pressure of the fuel that's propelled from the common rail into the injectors -- they've recently pushed it to 23,500 pounds per square inch, up from 19,500.



It'll still be a struggle, however, for today's diesel engines to meet upcoming U.S. emissions regulations. The EPA is introducing restrictions known as Tier 2 that are specifically aimed at reducing nitrogen oxides and particulates, among other things. Those rules will be phased in beginning in 2004, with full compliance required by 2009. Additionally, California -- a state that's large enough to influence automakers' planning -- aims to dramatically reduce smog-forming emissions, especially in Los Angeles, with its stringent Low Emission Vehicle II regulations, which will be phased in from 2004 to 2010.