For as long as people have cared what wafts from vehicle tailpipes, diesel motors have had the rap as the dirtiest, smelliest, noisiest engines on the road. That could soon change.
In a move that may presage diesel's Cinderella-like transformation, the Environmental Protection Agency on Thursday required US refineries to begin making ultra-low-sulfur diesel (ULSD), a fuel with 97 percent less sulfur than ordinary diesel that, as a result, slashes soot emissions.
The rule, which mandates that 80 percent of the diesel produced for highway use be ULSD-compliant, was just the first step. By Oct. 15, all filling stations now selling diesel will be required to sell ULSD instead of or in addition to diesel.
All who drive diesel vehicles - which account for only about 3 percent of sales of light-duty vehicles - will immediately emit about 10 percent less pollution upon shifting to ULSD. But the biggest pollution abatement - as much as 90 percent cleaner - will come with the EPA-mandated debut of "clean diesel" engines, probably late next year or early in 2008.
That moment, say environmentalists, is the transformation everyone is waiting for.
The new EPA rule "is the biggest step toward cutting vehicle pollution since lead was taken out of gasoline two decades ago," says Richard Kassel, director of the Clean Fuels and Vehicles Project at the Natural Resources Defense Council, an environmental group based in Washington.
Though environmentalists say they've not had much to cheer about during the Bush administration, they give credit to the EPA for pushing past political obstacles to cement a major antipollution rule.
Heavy-duty trucks and buses will be first to take full advantage of the new fuel, with new clean-diesel engines slated to hit the road in early 2007 - cutting soot and nitrous-oxide (smog-forming) emissions by 90 percent, Mr. Kassel says. Some city bus fleets, like New York's, already use ULSD.
Off-road diesel vehicles - including farm and construction equipment - won't be required to shift to ULSD immediately. Their emissions requirements will be phased in from 2007 to 2010.
Diesel engines have always had their virtues: high torque, durability, and good mileage. They also produce less greenhouse gases per mile than do gasoline-powered engines because of their greater fuel efficiency, says Kassel.
But the clouds of soot particles that spew from buses, garbage trucks, and 18- wheelers have been an environmental and health hazard, officials say. The new clean-diesel vehicles are expected to be no louder or dirtier than gasoline engines, but they get 20 to 40 percent better mileage per gallon.
Will that put more diesels on the road? Perhaps.
They may account for 7 to 15 percent of vehicle sales by 2010, some analysts say. But diesel vehicles no longer have a fuel-cost advantage, the factor that propelled more Americans to buy them after the 1970s oil embargo. Today, diesel fuel sells for about what gasoline does.
As the number of clean diesels on the road grows, so might the number of drivers filling up with even cleaner biodiesel fuel from organic sources. The hope is that more fuel options will help put a big dent in US dependence on foreign oil.
"We're learning that we need all these options - ethanol, hybrids, fuel cell, and a clean-diesel option," says Allen Schaeffer of the Diesel Technology Forum, an industry group. "There's no silver bullet for reducing reliance on imported oil, but there are silver pellets - and clean diesel is one of those."
One caveat is that because diesel motors are long-lasting, it may take decades for older, more-polluting vehicles to be replaced by new clean-diesel ones. Another is that old-technology diesel vehicles are now seeing a sales spurt. It all adds up to slow-motion change.
Once the new diesel rule is fully implemented in 2030, it is expected to yield a 90 percent cut in pollution from the nation's 13 million diesel trucks and buses. That would mean more than 8,000 premature deaths averted each year and about $70 billion annually in health benefits as a result of cleaner air, the EPA estimates.
For some, such as Robert Issem of Roanoke, Va., the move to clean-diesel engines can't come soon enough.
The tennis-tournament administrator drives a diesel Volkswagen Golf that gets 60-plus miles per gallon on the highway - and he's been going out of his way to make it burn clean by using biodiesel. The trouble is that the nearest biodiesel outlet is three hours from home, so Mr. Issem fills five-gallon jugs with the fuel and totes them in his back seat.
Now, with clean-diesel fuel coming to nearby filling stations, he plans to use a mix of biodiesel and ULSD for a lower- pollution ride. He also plans to be at the showroom to take a look when clean-diesel cars arrive in a year or so. "We'll definitely be looking at clean diesel."