For automakers to bring the innate fuel efficiency of a diesel engine with peppy performance and low emissions to the U.S. market, it required a national change-over to ultra-low-sulfur diesel fuel, the development of precisely controlled engines, and extensive treatment of exhaust gases. With these goals accomplished, there is a wave of diesel-powered vehicles coming to America despite the price premium on diesel fuel.
Mercedes-Benz has been leading the movement with its 50-state-legal "clean diesels" featuring "BlueTec" technology that mark the culmination of years of scientific research, powertrain engineering, and market preparation. The resulting BlueTec diesels burn cleaner than most gasoline engines and exhibit superior fuel economy at the same time.
This month, Mercedes announced that it would offer a clean-diesel option for three of its 2009-model SUVs: the midsized ML- and R-Class, and the big GL-Class. All will use the same 210-hp, 3.2-liter diesel V6. These should be some of the most fuel-efficient SUVs on the road. Mercedes has estimated that the GL320 BlueTec will record an average of 24 mpg, some 20 to 40 percent better than a comparable gasoline engine. It should also be able to travel 600 miles on a single tank of fuel.
There may have been nothing inevitable about Mercedes’ particular choices for creating a clean diesel. Conceivably a roster of alternative technologies could have accomplished the same thing. Honda, Nissan, and Toyota are all developing clean-diesel technology and they’re apt to use their own strategies to bring them to fruition. But the Mercedes BlueTec system just happens to be the first practical mass-market application available in the United States.
What is BlueTec?
It’s the proprietary name for a series of engine technologies, filters, and catalysts that result in low emissions and high fuel economy. It starts with a high-pressure common-rail turbo direct injection diesel engine. "Common rail" refers to the method for injecting precisely the right amount of fuel directly into each cylinder at exactly the right moment. The turbocharger boosts horsepower, and the system recirculates the exhaust gas that powers it in order to consume any unburned fuel.
When exhaust gas leaves the engine, a multi-phase treatment process begins. By their nature, diesels tend to produce high levels of carbon monoxide, oxides of nitrogen known collectively as NOx, and particulate matter, also known as soot. Oxidation catalysts minimize the carbon monoxide and further reduce unburned hydrocarbons. A maintenance-free particulate filter, or soot trap, then reduces soot to nearly undetectable levels.
NOx emissions are the trickiest to deal with. NOx forms in the first place as a result of burning atmospheric air, and every lean-burning engine generates NOx as the ratio of fuel to air in the combustion cycle decreases. Current approaches to trapping or destroying NOx emissions all use catalysts that are vulnerable to degradation in the presence of sulfur. That’s why it was necessary to put the whole country on a diet of ultra-low-sulfur fuel.
Mercedes uses two strategies to deal with NOx, one for cars and one for SUVs. The E320 BlueTec sedan, which we tested last year, attacks NOx with a storage catalyst and a second catalyst called SRC (Selective Catalytic Reduction). Periodically the engine runs rich for a little while, which cleanly purges the NOx storage catalyst.
For heavier vehicles, such as SUVs, Mercedes has adopted a urea-injection system called AdBlue. That’s because the NOx trap technology used in cars would require too-frequent purging and thus reduce fuel economy.
The AdBlue system uses the SRC catalyst and a tank of liquid urea, which is misted into the exhaust stream. When the urea comes in contact with the hot exhaust gas, it releases ammonia, which in turn reacts with NOx and emerges as harmless nitrogen and water. The urea supply is said to be good for 10,000 miles, so it only needs to be refilled at the vehicle’s normal service intervals. Mercedes says that the AdBlue system reduces NOx emissions by 80 percent.
The Mercedes-Benz BlueTec SUVs are slated to go on sale this fall as 2009 models.
Also read "A clean diesel' sedan tops a performance-tuned hybrid" (available to online subscribers).