Volkswagen's New Beetle includes a diesel version that scored a 6 on the EPA's 0-10 air quality index -- the first diesel to get more than a 1.
By Reporter
updated 4/21/2004 12:29:33 PM ET 2004-04-21T16:29:33

If gas prices have got you looking at your auto options, European carmakers and even the U.S. Environmental Protection Agency would like you to consider this: a cleaner, quieter and even higher mileage generation of diesels that leave their smoky 1970s relatives in the dust.

Clean air and energy efficiency advocates won't be the first to climb aboard since they figure gasoline-electric hybrids are a better way to go, polluting less while getting high mileage.

Diesel advocates counter that hybrids are limited to a few models so far, whereas diesels have the potential to boom in the United States because it's already happened in Europe, where 40 percent of new passenger cars are diesel thanks to tax credits. The U.S. market share for new cars is less than one percent.

The 2005 Mercedes-Benz E320 CDI looks a lot like its gasoline E-class cousins but gets much better mileage.
Germany's Volkswagen and Mercedes-Benz, a division of DaimlerChrysler, lead the European pack, and are offering several diesel models in the United States. DaimlerChrysler also intends to roll out a diesel Jeep Liberty for 2005, but other U.S. carmakers have limited their diesel presence to pickups.

Listing the benefits
Diesel's advantages over gasoline have traditionally been price — it's around a dime a gallon cheaper today — as well as significantly higher mileage. The latter is due in part to the fact that diesel's chemistry gives it a 10 percent higher energy content than gasoline.

Diesels also emit 20-30 percent less carbon dioxide, the gas many scientists fear is warming the Earth. In fact, that advantage is the driving force behind European governments' use of tax credits to promote diesel cars.

In recent years, improved technology has also made diesels quieter, smoother, cleaner and up to 50 percent more fuel efficient than gasoline-powered cars.

That combination is reflected in diesels like VW's 2004 Beetle TDI. The manual version is rated at 38/46 mpg for city and highway driving. It's also the highest scoring diesel ever on the EPA's 0-10 air pollution index — garnering a 6, compared to a 1 the previous year. Its Jetta and Golf diesel cousins scored 4 this year and 1 in 2003.

The Mercedes E320 engine uses a technology called common rail injection, or CDI, to improve diesel performance and emissions.

Next up for diesel cars in the United States is the Mercedes E320 CDI sedan. Available this spring, Mercedes' first U.S. diesel in six years is expected to get 27/37 mpg for city and highway driving, compared with 19/27 mpg for its gasoline sister.

The EPA and the Energy Department have touted the diesel advances, noting on their Web site that modern diesels are "substantially more energy efficient and considerably cleaner" than those sold 20 or 30 years ago.

They're are also much more responsive, notes David Greene, a fuel economy researcher at the Energy Department's Oak Ridge National Lab. They have higher torque than gasoline engines, he notes, and turbocharged diesels have virtually no lag time. "You step on the accelerator and get pressed against the seat," he says.

Listing the drawbacks
As notable as the advances are, obstacles remain.

For one, diesels are more expensive. VW's diesels, for example, cost about $1,200 more than their gasoline sisters.

Greene expects that to remain the case because diesels rely on high-pressure fuel injection to power a car. Gasoline engines, on the other hand, use less expensive low-pressure fuel injection. Moreover, says Greene, treating diesel exhaust for pollutants is also more expensive.

There's also the fact that modern diesels still can't pass the most stringent air standards — those enforced by California and four other states — because of their emissions of nitrogen oxides, or NOx, a key ingredient for smog.

"NOx control is the key issue," says Greene. "Current NOx control technology is expensive and cumbersome. Better solutions are very likely, but not a done deal yet."

The American Council for an Energy Efficient Economy, which each year rates new cars for their green quotient, noted that while the Jetta TDI gets 40 percent better mileage than its gasoline sister, it emits more than eight times the amount of NOx.

The manual Beetle TDI did fare better, says council research associate Jim Kliesch, getting an "above average" class ranking while other VW diesels got "below average" or "inferior." That's because the manual transmission "puts less of a load on the vehicle's engine, resulting in less formation of nitrogen oxides," Kliesch says.

He adds that while the manual Beetle TDI reached "the cleanest level a diesel passenger vehicle has ever been certified to" that level "is not a remarkably clean standard. Today's cleanest hybrid-electric and conventional gasoline vehicles meet emission standards much cleaner. ... the Toyota Prius (hybrid) meets a NOx level 10 times cleaner."

Looking to the future
The next big step for diesels in the United States is the EPA requirement that low-sulfur fuel be available across the country as of 2006. Until that happens, carmakers won't be able to add catalytic converters to greatly reduce NOx emissions. That's because sulfur poisons the converters.

Mercedes, for one, expects that requirement will allow its diesels to meet every states' air quality standards.

U.S. automakers have also experimented with a diesel-electric hybrid, which Greene notes offers the added bonus over gas-electric hybrids of much lower carbon dioxide emissions.

And the energy council points out that Ford is testing a diesel Focus sedan with a device that sprays urea and water onto a catalytic converter to reduce NOx from the exhaust stream.

But it adds that "widespread use of such systems is still some years away, particularly if a new chemical such as urea needs to be widely distributed along with ultra-clean diesel fuel."

That's not to say much cleaner diesel is impossible, however.

One of the industry's toughest critics has been Frank O'Donnell, executive director of the Washington, D.C.-based Clean Air Trust. Yet even O'Donnell is hopeful.

"The truth about diesel engines is that the jury is still out," he says. "Historically, they've not been as clean as gasoline engines, but that's partly because they've never been required to be.

"I tend to be optimistic," he adds. "Companies have shown tremendous ingenuity in meeting tough environmental standards — if and when they are required to do so."

"Advanced pollution controls such as particle filters, coupled with low-sulfur diesel fuel, hold the promise that the phrase 'clean diesel' could be more than just a slogan," he says. "But if we back away from requiring tough pollution standards — as some car companies like General Motors have hinted they want — diesel may never escape its 'dirty' image."

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