The new ones are virtually indistinguishable from an equivelent gasoline car and pollute less than a Prius.
The Case for Diesel: Clean, Efficient, Fast Cars (Hybrids Beware!)Merging with northbound traffic on Interstate 75 just outside Auburn Hills, Mich., I punch the accelerator, quickly swing left into the passing lane and pull forcefully ahead of the cars around me. In any other ride, on any other gray morning, it’d be just another Interstate moment. But this rush hour, I’m behind the wheel of a preproduction 2009 Volkswagen Jetta, which is powered by a 2.0-liter turbo-charged, direct-injected diesel engine that, even as I leave the speed limit in tatters, is averaging nearly 50 mpg. Equally important, what’s coming out of the tailpipe is no dirtier than the emissions from the 35-mpg econoboxes I can now see in my rearview mirror. Speed, fuel efficiency and minimal emissions? These aren’t characteristics usually associated with diesel-powered vehicles. But they will be.
Most Americans have a bad impression of diesel cars. We think of them as loud, hard to start and foul-smelling. We sneer at them for lacking the get-up-and-go of their gasoline-powered cousins. And we dislike them for their perceived environmental sins, chiefly the polluting brew of sulfur and nitrogen compounds that they emit into the atmosphere. All those complaints were fair a generation ago, when the twin energy crises of the 1970s propelled diesels into national popularity and kept them there for a decade. Back then, many drivers ignored diesel’s faults, or were unaware of them, because diesel cars ran 30 percent farther on a gallon of fuel than similar gasoline-powered cars. It felt savvy to buy a diesel, even daring. Then fuel prices dropped in the mid-1980s, and drivers abandoned their clattering, odoriferous fuel sippers. They went back to gasoline.
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“If you told me 10 years ago that I’d be putting ‘clean’ and ‘diesel’ in the same sentence, I’d have said you were out of your mind,” says Margo Oge, director of the Office of Transportation and Air Quality at the Environmental Protection Agency. However, in response to EPA mandates that went into effect in late 2006, oil refineries are now producing what’s called ultra-low-sulfur diesel (ULSD). By definition, this “clean diesel” has sulfur concentrations of no more than 15 parts per million (ppm). That’s 98.5 percent cleaner than the sludge that coursed through the fuel delivery systems in those disco-era rides, and 97 percent less sulfur than was allowed under a 500-ppm standard instituted in 1993. The cut in sulfur means that less sulfur dioxide goes into the atmosphere, where it can combine with water to produce sulfuric acid—and thus, acid rain. There are further beneficial effects of the sulfur-light fuel, ones that could make the advent of clean diesel as environmentally momentous as the introduction of unleaded gasoline in 1974.
“Sulfur clogs emission-control devices in diesel-powered cars the same way lead impeded catalytic converters in gasoline systems,” Oge says. “Removing the lead from gasoline enabled engineers to develop a new generation of emission-control technologies that helped reduce noxious exhaust emissions by 98 to 99 percent.“ Carmakers have already started building exhaust-scrubbing systems for engines that burn ULSD—so that not just sulfur but a rogue’s gallery of other pollutants are kept out of the air. Though they differ in design, the systems share some basic components. In a Mercedes-Benz BlueTec system, for example, exhaust from the engine is first filtered through a device that lowers carbon monoxide and hydrocarbon levels. Then it runs through an apparatus that removes soot and other particulates. Finally, the remaining exhaust gas is sprayed with a urea-based substance that helps convert harmful nitrogen oxides—NOx is the shorthand for this group of compounds—into harmless nitrogen gas and water vapor. According to Mercedes-Benz, the system reduces the total output of harmful emissions by 80 to 90 percent. The process even eliminates diesel’s bad odor.
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