I recently returned from an extended stay in Europe, where most new cars run on diesel. Those cars are typically a lot more fuel-efficient than our gas guzzlers, which makes me wonder why there aren't more diesels on American roads. I know that diesel has a reputation for causing dirtier tailpipe emissions than petrol, but isn't that a bygone problem?
Technological wizardry has, indeed, made diesel-powered vehicles vastly cleaner than in olden days. As a result, lots of gearheads are touting diesels as finally safe enough for American motorists, who will dig the cars' impressive fuel-economy numbers. There's considerable excitement on these shores, for example, over the impending arrival of the 2009 Volkswagen Jetta TDi, a "clean diesel" vehicle that purportedly gets 50 miles per gallon on the highway yet spews out far less soot than the diesels of yore, which wreaked havoc on air quality. So will the erstwhile environmental boogeyman of diesel fuel end up saving us all? The Lantern is still far from convinced.
Diesel, named after German engineer Rudolf Diesel, has traditionally been simpler to refine than gasoline, although making it also requires more crude oil per gallon. The end result is a fuel that boasts much greater energy density than gasoline, which explains why diesel cars get up to 40 percent more miles per gallon than their petrol counterparts. The higher energy density also means that burning a gallon of diesel emits more greenhouse gases than burning a gallon of gasoline—about 15 percent more, to be specific. But due to the appreciable fuel-economy savings, diesel cars usually emit less of these gases per mile driven.
There's a more disturbing difference between diesel and gasoline: Burning diesel also emits nasty particulates and smog-forming nitrogen oxides, as should be apparent to anyone who's ever gotten a mouthful of bus or tractor exhaust.
The good news is that today's diesel contains significantly less sulfur than in years past, resulting in much less harmful soot. On top of that, new diesel cars are outfitted with ingenious emissions-control systems such as BlueTec, which treats exhaust with a urea-based solution to reduce its toxicity.
But these improvements have come with costs. According to the Union of Concerned Scientists (PDF), manufacturing a gallon of the new, low-sulfur diesel requires even more crude oil than the old diesel. Also, diesel engines are more complicated than petrol engines and thus require more energy and materials to manufacture.
Still, a diesel car's improved fuel economy can offset these drawbacks. The UCS recommends that car shoppers revise a diesel vehicle's miles-per-gallon rating downward by 20 percent in order to get a more accurate picture of the overall impact on oil consumption.
Fans of the forthcoming Jetta TDi point out that the car's tailpipe emissions are clean enough to pass muster in California, a state with exceptionally tough emissions regulations. Yet the diesel TDi still lags behind many other vehicles that meet California's stringent requirements, including the gas-powered 2008 Jetta, which qualifies as a partial zero-emissions vehicle.
The relative dirtiness of even the most advanced diesels worries some researchers, who argue that the resulting soot (which they term "black carbon") may be a key factor in global warming. According to a 2002 Stanford University study, even if all diesels were designed to meet California's emissions standards, diesel cars could still warm the globe more than petrol cars over the next half-century.
None of this is to imply that gasoline is necessarily more eco-friendly than diesel—the two fuels just have different pluses and minuses. European regulators seem to care more about reducing the continent's greenhouse-gas emissions than its particulate emissions and so have favored policies that prop up diesel. As you probably learned during your foreign sojourn, diesel is cheaper than petrol in virtually all of Europe, largely due to its being more lightly taxed (though maybe not for long). The opposite is true here in the United States, where diesel tends to cost significantly more than regular gasoline—in part because our new, low-sulfur diesel is more expensive to manufacture, but also because of a higher federal per-gallon tax.