For some car buyers, diesel vehicles are attractive because they promise greater fuel efficiency than gas-powered cars and provide a greener footprint.
But with Volkswagen facing a charge from the Environmental Protection Agency that it had created software that allowed its diesel cars to cheat on air pollution standards, that's raising a host of questions about not only VW's diesel vehicles but the industry as a whole. Consumers interested in diesel-fueled cars are typically attracted by their fuel efficiency and peppy feel when on the road, said Andre Boehman, professor of mechanical engineering at University of Michigan and a self-described diesel-vehicle fan.
"It's a shame when someone tries to cheat this test because it's an important yardstick for us to make sure the car will provide cleaner driving," Boehman said. "It would make me a little more careful in my shopping for which vehicle to purchase, although it may be hard for a consumer to know if vehicles are operating the way" the automaker states it should be.
For some diesel enthusiasts, the idea of driving a car that's easier on the environment -- diesel-engine fuel efficiency can be as much as 30 percent higher than gas-powered cars, and diesel cars can also use biodiesel fuel -- is the main draw. The cars were not only billed as virtuous to drive, since they consume less fuel than gas-powered cars, but fun, too.
The downside to diesel engines is their exhaust, which includes larger particulates than gas-powered cars. The gas and particulates that are emitted by diesel engines are considered a health hazard, with the EPA noting that acute exposure can lead to eye, throat and lung irritation, exacerbate asthma, and create some neurological effects.
Diesel vehicle makers have been creating better methods of controlling emissions, partly in response to government regulations that required cleaner diesel engines in 2007. That helped spark interest in diesel vehicles among American drivers, especially since the higher standards came at a time when prices at the gas pump were rising, making fuel-efficient vehicles more attractive.
"That ushered in the era of clean diesel technology, which is the catchphrase the diesel industry uses," noted Boehman.
Except when the "clean diesel" is anything but, as in the case of Volkswagen. Researchers at the International Council on Clean Transportation (ICCT) alerted the EPA after finding that, when on the road, VW's cars released emissions that were up to 35 times higher than the government limit.
VW's cheat may throw doubt into consumers' minds as to whether any diesel car is actually providing the benefits its manufacturer claims, but Boehman noted that consumers really have no recourse except to "rely on the EPA to validate the emissions of the vehicles."
For Volkswagen owners, the scandal is only growing bigger. On Tuesday, VW said the software at the center of the emissions scandal is fitted in 11 million of its cars across the globe. The automaker had previously said the software was installed in 482,000 passenger cars sold in the U.S.
Those numbers illustrate diesel's much wider popularity outside America's borders. More questions may come from European buyers, said Martin Zimmerman, the Ford Motor Company clinical professor of business administration at University of Michigan's Ross School of Business.
"The big issue is, is this limited to VW or does it affect the European industry, where the bulk of the world's diesel is produced?" he said. "If the industry can show it's limited to VW, then it's primarily VW's issue."
If consumers can be convinced that "clean diesel" really is clean, then the long-term impact should be small, he added.
Despite sales growth during the past decade, diesel vehicles remain only about 3 percent of the total U.S. vehicle market, according to Allen Schaeffer, the executive director of the Diesel Technology Forum, a non-profit that promotes the use of diesel engines. The group's members include automakers including VW and Ford, and Schaeffer said he couldn't discuss individual automakers.
So far this year, sales of diesel vehicles have declined 2 percent, which Schaeffer said was likely due to the lower price of gasoline, which is making gas-powered cars more attractive to some buyers. Still, automakers are rolling out additional models of diesel cars, and Schaeffer said he believes diesel could double or triple its market share in the next few years.
Current owners of Volkswagen diesel vehicles are feeling burned, with some saying that they bought their cars for their fuel efficiency and minimal impact on the environment, which now appears to be a lie.
"I am very disappointed that VW willfully decided to lie not only to the government, but to its customers," one car owner wrote on Volkswagen's Facebook page. "Many of us have been fans of the brand through thick and thin for entire lifetimes. I currently own two diesels and I feel betrayed and cheated."
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