A visitor looks at a Citroen diesel engine at the Geneva Motor Show in Switzerland in 2010. Diesel is popular in Europe but not the U.S.; now, automakers are giving the technology more consideration as a means to meet new fuel economy standards.
Automakers are working in many ways to meet new tough fuel economy standards — both national goals set by the Obama administration and state standards, like California's new rules passed Friday requiring that 15 percent of new vehicles sold in the state must produce little or no air pollution by 2025.
But carmakers aren't just turning to new technologies like hybrid and electric cars. Many in the auto industry are taking a second look at an old technology: the diesel engine.
University of Michigan professor Margaret Wooldridge explains how an engine running on diesel fuel differs from a gasoline-powered engine.
"The strategy for introducing the fuel into the [diesel] engine is you inject it directly into the engine," she says. "For a gasoline engine it needs a spark plug to ignite the fuel; a diesel engine doesn't have a spark plug."
The spark plug is not the only difference between the two engines.
"Pound for pound in terms of fuel economy, the diesel engine wins," Wooldridge says. "That's the primary advantage. If you sit down and do the math and look at the fuel cost — and it really depends on where diesel fuel is relative to gasoline prices — you can come out with a benefit."
More Fuel-Efficient, But A Higher Cost
Diesel fuel is a cousin of gasoline; it's still a fossil fuel. Both come from the same sweet light crude, says Wooldridge.
"That's what we like most. ... It's just a different refining process to get you to a different mixture, a different set of hydrocarbons," she says.
U.S. automakers largely abandoned diesel after engines manufactured during the 1970s oil crisis gained a reputation for being dirty and unreliable. European carmakers kept at the technology, and more than half of new cars sold there are diesel-powered. Those engines are now around 20 percent more fuel-efficient than gasoline.
Diesel cars, as a percentage of the total consumer market
Most trucks use diesel already. In passenger cars, the difference in technology between diesel and gasoline is essentially cost: Diesel engines cost more to engineer and build, and right now, the cost of diesel fuel in the U.S. is higher. Their exhaust also contains more soot.
Woolridge says diesel has several advantages to gas-powered cars, too. They go from zero to 30 mph faster, they tend to last longer and there's the fuel-efficiency advantage.
A 'Bad Taste In Their Mouth' For Diesel
If diesel has all these good qualities, why are there so few diesel passenger vehicles in the U.S.? The answer is history. After the oil crisis in the 1970s, car companies looked to diesel to help solve the fuel economy problem.
John O'Dell with Edmunds.com says diesel got such a bad reputation in part because General Motors took an internal combustion gasoline engine and heavily modified it to make a diesel out of it very quickly, with limited success.
"It was an absolute disaster of an engine: It broke, it smelled bad, it was noisy, it was unreliable," he says. "And it left most Americans with an incredibly bad taste in their mouth ... [for] diesel."
The U.S. essentially gave up on diesel cars. European carmakers, however, keep making them better. In Europe, diesel is one of the primary ways of getting more miles to the gallon — up to 20 percent more.
A lot has changed since GM first launched its diesel engine. The kind of diesel fuel sold for passenger cars in the U.S. is now cleaner, and diesel could be a way for carmakers to get higher fuel economy. The Obama administration has proposed aggressive new fuel standards and gives incentives for hybrid and electrics — but not diesel.
Incentives For Alternative Fuels
Volkswagen is one manufacturer that's bet a lot on diesel. David Geanacopoulos, general counsel with Volkswagen Group of America, says the new fuel standards shouldn't favor one technology over another.
"Let the customers and the marketplace and future technical and scientific developments determine which are the winners," he says. "In a technology-neutral approach, the regulations can maximize innovation and improve our chances of achieving efficiency throughout the product range."
Meanwhile, Gina McCarthy with the Environmental Protection Agency says diesel engines don't get the same incentives because they're already in the marketplace.
"They are available [now] and the infrastructure's there to support them," McCarthy says. "We want to give the customers an ability to get these other advanced vehicles as well and get them into the market sooner, and that's the reason for the incentives."
Even without government incentives, it will soon be more common to see more diesel cars on the road. The biggest car company in the world, General Motors, is making a diesel version of its best-selling car, the Chevy Cruze. This time, the executives say, they'll get it right.