One easy way to reduce both carbon-dioxide emissions and oil imports is to switch to diesel engines in cars and trucks, since they’re inherently more efficient than gasoline engines. In fact, diesel engines are almost as efficient as gas-electric hybrids, without the need for hybrid technology.

But to date, consumer diesel vehicles have not been widespread in the United States, where tight emissions controls made them more expensive to develop than in diesel-loving Europe. What’s more, U.S. drivers’ historical indifference to fuel economy–along with their perception that diesel engines are smelly and dirty–convinced automakers that Americans wouldn’t buy them anyway.

Starting on October 15, however, ultra-low-sulfur diesel will be available throughout the United States at the pump, as a result of EPA regulations originally devised by the Clinton administration.

By itself, the new diesel fuel will cut soot emissions by 10 percent–but it also opens the way for affordable technologies that can reduce emissions by 90 to 95 percent. The reason sulfur is so significant is that it forms organic sulfates, which create soot, clog emissions filters, and render ineffective catalysts that help convert the soot to harmless materials.

The new U.S. fuel standards slash sulfur levels in diesel from 500 parts per million to 15, making practical the kinds of emissions controls already used in Europe, as well as better treatments for nitrogen oxides, which are key components in smog. This improvement should make meeting toughening emissions standards far easier–and could pave the way for a new diesel era in the United States.

“The cleaner diesel fuel opens the door to diesel cars that can be as clean as gasoline cars, yet offer 20 to 40 percent better fuel economy,” says Richard Kassel, senior attorney at the National Resources Defense Council. Such efficiency gains approach those of gasoline-electric hybrid vehicles and, as with hybrids, the clean diesel vehicles would also cut carbon-dioxide emissions by reducing the total fuel consumed.

While a few diesel cars have been available in the United States, more automakers are poised to enter the market. Allen Schaeffer, executive director of Diesel Technology Forum, a not-for-profit educational group representing diesel equipment manufacturers, says Honda will have a diesel vehicle (probably an Accord) for the United States in three years. GM has announced a light-truck engine for 2010, as has Cummins. Meanwhile, Daimler-Chrysler is introducing a Mercedes diesel vehicle into the United States next week, and a new Jeep Grand Cherokee next year. Volkswagen also has several diesel vehicles on the market, he says.

“With the cleaner fuel, manufacturers will have the greatest possibility they’ve had in quite a long while for bringing new models into the market,” Schaeffer says. “This clean fuel is spurring new interest in diesel.”

To be sure, advanced diesel engines and the turbochargers that typically accompany them will cost more than conventional gasoline engines. But they’ll probably cost less than the extra batteries, motor, and other components in hybrids.

An additional benefit of diesel engines is that they can burn biodiesel, which can be derived from crops such as soybeans, using less energy than is needed to create another currently popular biofuel: ethanol from corn.

Farther down the road may be hybrid diesel cars, which could potentially get fuel economies of 50 to 70 miles per gallon, according to Kassel. Already, hybrid diesels are a boon for companies such as UPS, since their delivery vehicles are used for the stop-and-go urban driving that hybrids do best (see “Heavy Duty Hybrids”).

Nevertheless, the initial higher costs may deter American consumers, say experts. Diesel hybrids may have more of a chance in Europe, where diesel consumer vehicles have been popular for years, now constituting about half of the sales of such vehicles, whereas just a few percent of cars, SUVs, and light-and medium-duty trucks sold in the United States are diesels. In Europe, the popularity of diesels is in large part due to tax incentives, Kassel says, but also because their historically lower sulfur levels in fuel have allowed better exhaust treatments.

Bill Van Amburg, senior vice president of Weststart-Calstart, a not-for-profit organization that supports cleaner, more efficient vehicle technologies, says the improved emissions controls made possible by the new fuel are just part of advances in diesel vehicles in recent years, transforming them from noisy, dirty vehicles to quiet, clean, high-performance vehicles. These changes, which include advanced computer controls, are also making it possible to optimize engines “on the fly” to burn a variety of fuels.

One European concept car, says Van Amburg, can run on five different fuels, including gasoline, ethanol, and propane, without sacrificing efficiency or performance. Now that concerns about fuel prices, energy security, and global warming are all on the rise, Van Amburg says we will abandon the “mono-fuel” system dominated by gasoline for a “poly-fuel” system, including clean diesel, in which consumers can choose the fuels that make the most sense to them.