Dark Clouds Over Clean Diesels

Soot's health impacts and global warming potential dilute diesel's fuel-efficiency benefit

PHOTO: Jens Lucking/Getty Images

11 June 2008—Diesel vehicles are increasingly presented as an equal alternative to gas-electric hybrids, thanks to their up to 5.88-liter-per-100-kilometer (40-mile-per-gallon) fuel economy advantage over conventional gasoline-powered cars and trucks and new emissions-control devices that vastly reduce their exhaust. ”We think that’s a win-win for both the environment and the climate,” says Allen Schaeffer, executive director of the Diesel Technology Forum, a Frederick, Md.–based trade group that promotes diesels.

But mounting evidence on the pernicious effects of particulate pollution—including a potentially serious contribution to climate change—suggests that diesel’s promise may be oversold. Some regulators are getting the message. California is implementing new fuel standards to reduce the carbon content of transportation fuels, and draft rules for the state’s Low-Carbon Fuel Standard, to be finalized by the end of this year, seem to explicitly avoid giving automakers and consumers an incentive to switch from gasoline cars to diesels as a solution to climate change.

”We expect the Low-Carbon Fuel Standard to introduce more alternative fuels: biofuels, electric vehicles, hydrogen vehicles,” says Renee Littaua, manager of the fuel section at the California Air Resources Board (CARB). ”We’re not looking to give credit to dieselization.”

California Governor Arnold Schwarzenegger issued an executive order last year calling for a 10 percent reduction by 2020 in the greenhouse-gas pollution associated with the production and burning of motor fuels sold in the state. But CARB staffers hope to keep fuel suppliers from selling more diesel instead of gasoline to meet this commitment. Fuel producers and importers would have to reduce the carbon content of both fuels to comply.

Understanding why California is ambivalent on diesel requires a closer look at diesel performance and emissions. First, while diesel vehicles go farther on a liter of fuel than gasoline-powered vehicles, it takes more energy to produce a liter of diesel. ”In terms of barrels of oil, there’s a greater amount of petroleum per gallon in diesel than there is in gasoline,” says Don Anair, a senior analyst with the Union of Concerned Scientists, an advocacy organization in Cambridge, Mass.

More oil per liter means that diesel’s fuel efficiency translates to a somewhat smaller climate change benefit. CARB estimates that when corrected for diesel’s higher energy and carbon content per liter, the average diesel vehicle will generate 19.8 percent less carbon dioxide than the same vehicle running on gasoline would.

The bigger knock on diesels is higher tailpipe emissions of nitrogen oxides (NO x ), which contribute to photochemical smog and soot. Anair is the first to admit that diesels have come ”a very long way” thanks to particulate traps and other equipment, which capture about 95 percent of the engine’s soot. It is thanks to these advances that over half a dozen major European, U.S., and Japanese automakers will be offering diesel cars across the United States starting with the 2009 model year. Nevertheless, says Anair, these ”clean diesels” will still have higher tailpipe emissions than comparable gasoline-powered cars.

Under federal tailpipe standards for pollutants such as soot and NO x , automakers can qualify a given vehicle for a range of different performance levels, called bins, that run from Bin 1 (equivalent to a battery-powered vehicle with no tailpipe) to Bin 10 (the highest pollution level allowable, which many SUVs will be shooting for). ”The new diesels are generally going to meet the Bin 5 standard, whereas there are gasoline vehicles that have been meeting the lower bins for a while now,” says Anair. Hybrids are among the best, with Toyota’s Prius and Camry Hybrid hitting Bin 3 and the Honda Civic Hybrid qualifying for Bin 2.

That extra pollution from diesel is important to CARB. At its May board hearing, agency staff presented an updated estimate of the public health impacts of black carbon (of which diesel soot is the state’s top source). The new analysis found black carbon to be 70 percent more toxic than previously believed and suggests increasing the estimated annual mortality caused by fine-particle pollution in California from 8900 to as much as 24 000.

Black carbon is also under scrutiny as a contributor to climate change. In contrast to sulfate particles from power plants, which cool the earth locally by reflecting away sunlight, black carbon particles absorb sunlight and release heat. Estimates of black carbon’s warming effect have varied widely, prompting the Intergovernmental Panel on Climate Change (IPCC) to downplay it. As a result, regulators—including CARB—have left it out of their analyses of diesel emissions.

But recent research could change regulators’ minds. A report in Nature Geoscience this March by atmospheric scientist V. Ram Ramanathan, at the Scripps Institution of Oceanography, in San Diego, and University of Iowa chemical engineering professor Greg Carmichael, measured black carbon’s warming effect at three to four times as much as the range of estimates recognized by the IPCC. They conclude that emissions of diesel soot and other forms of black carbon may have a warming impact on Earth’s atmosphere second only to carbon dioxide. The result affirms above-average estimates from researchers at Stanford, Caltech, and NASA that clashed with the IPCC view.

Mark Jacobson, the Stanford professor of civil and environmental engineering who first identified black carbon’s warming potential in 2000, says that factoring this potential in could eliminate most of diesel’s apparent carbon advantage. ”It’s just total nonsense to think that diesel cars actually reduce carbon emissions,” says Jacobson.

In a hearing before the U.S. Congress’s House Committee on Oversight and Government Reform this past fall, Jacobson testified that a diesel emitting 10 milligrams of black carbon per mile (the limit for Bins 2 through 6 under the federal emission standard; 6.25 mg/km) will cause more warming than a typical gasoline-powered car even if it is 30 percent more fuel efficient. Jacobson’s conclusion: ”The conversion of gasoline to diesel vehicles is a poor strategy for addressing global warming.”

About the Author

Contributing Editor Peter Fairley has reported for IEEE Spectrum from Bolivia, Beijing, and Paris. In May 2008 he wrote for us about China’s rapid gains in wind power.

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