In the next few years it seems every manufacturer will reintroduce an existing car, but newly out-fitted with a diesel engine. Diesels are the next big thing, but they’re not new at all.

So where’s the story?

Maybe this time people will actually buy them.

Diesels have come a long way since Rudolf Diesel’s invention of the internal combustion engine alternative. More efficient than gasoline-powered engines, they're powerful, deliver more low-end torque, and offer on an average up to 30 percent better fuel economy than their comparable gas-consuming siblings.

But diesels are also notorious for being uncomfortably loud and unforgivably smoky-- carpet bombing passengers, innocent bystanders and generally anyone unfortunate enough to be traveling behind them with suffocating black clouds of noxious fumes.

It’s an outdated image that manufacturers are trying to shake.

Modern diesel engines are 95 to 99 percent cleaner than their predecessors, according to Chrysler spokesman Nick Cappa. Cappa is an enthusiastic supporter of diesels, and to hear him talk about their upcoming 2010 Dodge Ram 1500 equipped with a clean BluTec diesel engine (it does not use urea injection) you’d think Chrysler had just figured out sliced bread.

“People still need trucks,” he explains. “They just want a way to make them more fuel efficient.”

And diesels are an easy way to do that.

Diesel engines provide better fuel economy without sacrificing performance. For example, the 2008 Jeep Grand Cherokee available with a 3.0-liter diesel V-6 gives you the same torque as a V-8 engine.

“Torque is what really matters,” Cappa says of the 376 pound-feet that you get with the diesel.

Torque may be what propels a vehicle forward, but what makes these cars the majority of the auto population in Europe is their high gas mileage.

But is this the sole measure of environmental friendliness?

The further you go on a gallon of gas, the less fuel you burn. The less you burn, the less you pollute the environment. What made diesels environmentally harmful in the past was not poor gas mileage – they were paired with sloppy emissions systems and ran on sulfur-rich diesel fuel. The roll-out of low-sulfur fuel in the United States has taken care of one part of the problem, and carmakers now have better engineered emissions systems that trap more of the smog-causing NOx pollutants that are also linked to asthma.

But few carmakers can solve the California variable.

California enforces stricter emissions standards than the federal government, making most new diesel vehicles ineligible for sale in California and the seven so-called “clean states” that have adopted California’s vehicle regulations, including Maine, Massachusetts, New York, Vermont, Connecticut, Pennsylvania and Rhode Island.

The first to clear this hurdle is Mercedes-Benz with its BluTEC diesel engines, purporting to be the cleanest diesels in the world. Their diesels when coupled with their AdBlue urea-injection technology will make the R320, ML320 and GL320 SUVs available for sale in all 50 states starting October 2008, according to Mercedes-Benz spokesperson Nicole Weiss. The E320 is equipped with a BluTEC diesel engine, but it does not include the AdBlue technology and therefore is not available for sales in “clean states,” including California.

These powerful and more fuel efficient luxury SUVs are hardly “cars for the masses,” but it’s a start. With expansion of clean diesel technology and manufacturers competing to meet increasingly stringent fuel economy standards, hopefully we’re seeing a new wave of high gas-mileage vehicles.

But delaying diesel proliferation is their higher costs. Diesel engines are more expensive because they’re built stronger, the advantage being that they last longer. That longevity is ironically the reason why diesels have such a bad rap in the states – keeping decrepit diesels on the road long after their sell-by date, and consequently off the consumer's radar when it comes to shopping for fuel-efficient cars.

Also, there aren’t many new diesels to choose from. Apparently it’s the classic chicken and egg problem: Customers don’t want diesels because they’re still under the impression that they’re dirty, while manufacturers are reluctant to introduce new clean diesel engines in the market until there’s enough customer demand.

Compounding the problem is the price of diesel fuel, which has risen steadily over the past few years and is now more expensive than gasoline.

However, many carmakers believe that U.S. consumers are finally ready to give diesel another shot. And auto manufacturers are racing to comply with the recently passed new CAFE standards.

So what are the “new” diesels consumers will be seeing?

Volkswagen will make their 2009 Jetta TDI and Jetta TDI SportWagen available for sale in all 50 states beginning August 2008. And they’ll be doing it without urea. The Jetta TDI manual gets 30 MPG in the city and 40 MPG on the highway compared to 21/29 for the 2008 2.5L Jetta. But what fans of das auto are really waiting for is the news that the VW Polo BluMotion (the EU version of the Golf with a diesel engine) will hop the pond and knock the Prius off the “most fuel efficient” podium. VW spokesperson Sean Maynard says that the Polo BluMotion diesel achieves an average of 60.3 MPG.

And it’s not just German manufacturers who are bullish on diesels. Subaru is examining the best way to bring to the United States its new boxer diesel engine, which recently debuted in the EU versions of the Legacy and Outback. But Subaru is noncommittal about which products it will be introducing and when.

In 2009, Acura will deliver a diesel option vehicle based on the 2.2L i-DTEC diesel engine used in the EU, but with further enhancements in emissions controls to meet California’s strict standards. Urea will not be used in Acura’s vehicles, so there won't be another component that consumers will need to maintain. A diesel option is also rumored to become available in the Honda Civic in the not too distant future, which is already offered in the EU along with the Accord and CRV.

These engines are hoping to give the popular hybrids a run for their money. But is it a question of one or the other? If diesels are good, wouldn’t a diesel hybrid be even better?

“Diesels and hybrids yield the most impressive fuel economy combo, but it’s also the most expensive,” says Nick Cappa.