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.