Gone are the days of riding in the family station wagon, inhaling smelly, sooty fumes from a noisy diesel engine.

Diesel engines have always been more efficient than their gasoline counterparts, but over the last decade, the autos and the fuel they run on have had to come a long way to shed their dirty image. Today, diesel cars offer Americans another fuel-efficient vehicle option that could also ease the pain at the pump.

"The great thing about diesel is that it's coming in at the right time," said Allen Schaeffer, executive director of the Diesel Technology Forum. "It's proven itself as a technology, and it has the credentials to compete with policymakers and customers that want to drive a car that's good for the environment and low in carbon dioxide emissions."

Heavy-duty diesel engines found in big trucks, machinery and agricultural equipment have long been the workhorses of the American economy, contributing hundreds of billions of dollars to the country's gross domestic product each year. Diesel cars, however, make up about 1 percent of new car sales in the United States.

But recent regulatory and technological advancements at a time of high gas prices have spurred a revival in the U.S. diesel passenger car market.

According to sales information compiled by Baum & Associates and HybridCars.com, U.S. sales of clean diesel autos increased 39.6 percent last month over March 2011. Diesel auto sales also went up 35 percent in just the first quarter of 2012.

With more clean diesel cars set to hit the market, the trend is expected to continue.

"There's still a perception from the past where in consumers' minds, diesel engines are for trucks or are dirty and stinky," said Torsten Karnahl, general manager for product strategy at Volkswagen, which recently upped production of the diesel-powered Passat to meet rising demand. "But that's just not the case anymore, and customers are now realizing it."

GM's earlier attempt faltered

To improve air quality and cut down pollution from the nation's existing fleet of 11 million diesel engines, U.S. EPA announced $20 million in grant funding yesterday to support clean diesel projects and another $9 billion through direct state allocations.

While diesel cars are cleaning up their act, they have endured a bumpy road since they were first introduced to the United States in the late 1960s, in a sector dominated by the European brands Volkswagen and Mercedes.

U.S. auto manufacturers dabbled in the diesel auto industry during the oil embargo in the late 1970s, when there was a strong push to bring more efficient vehicle technologies to market. In an attempt to roll out a new product quickly, General Motors ended up creating a loud, unreliable car that would turn the American public off diesel for decades.

Europe, meanwhile, did not give up on diesel automobiles. European car companies turned diesels from old clunkers into peppy rides. Thanks also to a fuel tax on gasoline, diesel cars in Europe have been steadily catching up to their gasoline counterparts. Over the last decade, diesel passenger cars accounted for more than half of all new sales in Europe, according to Schaeffer.

Diesel technology in the United States today has also been transformed, and stringent emissions standards set by EPA in 1999 have made diesel engines more than 90 percent cleaner.

The introduction of ultra low-sulfur diesel fuel in late 2006, which cut down harmful sulfur emissions by 97 percent, also gave manufacturers an opportunity to reconsider how diesel cars would fit into their strategic plans for the U.S. market. Both government and industry leaders have described the new fuel as a "game changer."

Now, U.S. automakers are getting back in the diesel game.

GM tries again with a small diesel

"We're definitely seeing a niche market that's growing ... and that's a niche that we've decided to play in," said Tom Read, powertrain spokesman for Detroit-based General Motors.

Earlier this year, GM announced it would draw on European expertise to develop a diesel-powered version of the popular Chevrolet Cruze, set to hit showrooms in 2013. Down the line, GM also plans to offer a diesel version of the Cadillac ATS.

"I think we're the first domestic automaker to say that we'll offer a small displacement diesel in a passenger car, and I think that reflects the push for innovation in propulsion systems that GM is willing to get into," Read said.

The outlook on diesel cars in the United States looks bright.

The sticker price on a diesel car is still $2,000 to $3,000 higher than on a comparable gasoline vehicle. But today's high gasoline prices mean U.S. diesel owners can enjoy short payback periods and even some additional savings. Diesel cars are also prime candidates to help meet the Obama administration's proposed 54.5 mpg fleetwide fuel economy standard by 2025.

"From an engineering standpoint, the diesel fuel and the way we use it allow us to be more thermodynamically efficient," said Margaret Wooldridge, a professor of mechanical engineering at the University of Michigan.

Diesel engines have a higher compression ratio than gasoline ones do, and more compression means more power out of the same amount of fuel, she said.

With the addition of particulate filters and nitrous oxide (NOx) catalytic converters to help meet EPA clean air regulations, new diesels are also just as clean as gasoline cars. In a recent study, researchers found that trucks in compliance with the 1999 EPA standards produced 95 percent less particulate emissions and 98 percent less NOx emissions.

"What's next for diesel? It's about taking what diesels currently have and enhancing it," said Byron Bunker, a director in the EPA Office of Transportation and Air Quality. "It's going to be the same story: Improve the particulate filter, make your NOx catalyst better, make your electronics better, and [the vehicle] will get cleaner and cleaner."

There is definitely still room for improvement.

Better engines for the climate

On average, about 85 percent of the particle matter emitted from diesel trucks is black carbon, said Christopher Frey<, a professor of environmental engineering at North Carolina State University.

"That carbon has certain physical properties where it tends to absorb infrared energy, which is part of the greenhouse effect," he said. "These particles also tend to be very small and can be carried a long distance in the air."

Studies show that deposits in the polar region could be linked back to trucks in the Americas, Frey said. Those deposits make snow grayer, causing it to absorb more sun and consequently melt faster.

Scientists are still trying to develop technologies to further reduce fine particulate matter, which can also have negative effects on human health (ClimateWire, April 23).

But in terms of long-term consequences, diesel engines are better for the climate than a gasoline-powered car, said Katsumasa Tanaka, scientific researcher at ETH Zürich university in Switzerland.

"When you look at the carbon dioxide emissions per kilometer driven, diesel has about 10 to 15 percent higher fuel efficiency than a gasoline engine," said Tanaka, who led a study published in Environmental Science & Technology last month on the climate effects of gasoline versus diesel cars in line with European emissions standards.

The application of hybrid powertrains and renewable fuels on diesel platforms will further reduce the vehicles' greenhouse gas emissions and could breathe still more life into what was a very hazy U.S. industry not long ago.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500