For one, they suffer from an image problem that dates back to an ill-conceived effort by GM in the 1970s to convert a gasoline engine design to diesel. The result was clattering, smoky and unreliable -- and made diesels anathema to a generation of car buyers. Polishing the image will simply take time and exposure to modern diesels, which are far cleaner and more reliable than those of the past.
Diesels also cost more than gas engines: Automakers typically charge about $1,000 more for a diesel than a comparable gasoline-powered vehicle. Diesel backers believe the new tax credit should help alleviate the cost barrier.
Indeed, even without the credit, diesel sales in the U.S. were up 33% last year -- although still accounting for a scant 3% of the passenger vehicle market. The vast majority of those sales were diesel pickups, which can be sold in all 50 states because of lesser emission restrictions on trucks. Diesel cars represented only 0.2% of the passenger car market.
Market researchers at J.D. Power & Associates predict that diesels, including pickups, will account for about 7.5% of the U.S. passenger vehicle market by 2012.
Although diesel fuel prices have soared recently in step with crude oil prices, diesel is still more economical than gasoline. Diesel engines deliver 20% to 30% better fuel economy than gasoline engines, so even paying $3.10 a gallon for diesel versus $2.70 for regular gas, a driver could still save up to 2 cents per mile.
That extra mileage also makes diesel a potential ally in California's fight against emissions of carbon dioxide, which are linked to global warming.
Rather than using electric sparks to ignite their fuel like gasoline engines, diesels use very high compression to superheat the intake gases, which ignite the fuel as it is sprayed into the cylinders. This burns less fuel than a gas engine per mile traveled and therefore creates less carbon dioxide.
But this combustion method also leads to diesel's own particular pollution problems, which occur because the extremely high temperatures in the cylinders form oxides of nitrogen while sulfur residue from unburned fuel forms particulate matter, or soot.
Filters and high-pressure diesel fuel injection that ensures that most of the fuel is burned have pretty much cured the soot issue. And several automakers are developing chemical treatments to deal with the nitrogen oxides. Volkswagen's technology, for example, includes filters for particulate matter and a "storage catalyst" that collects and destroys the oxides before they can be emitted.
California air quality regulators, once openly anti-diesel, say it now appears likely that the industry can meet the state's emission standards with a variety of particulate filters and chemical treatments now being used in Europe.
"We have seen evidence ... they can probably meet our standards," said Steve Albu, assistant chief of the state air board's Mobile Source Control Division.
Albu and other California air board managers said they didn't expect to see California-approved diesel cars until 2008 or 2009. But Volkswagen expects "to begin selling our Jetta and Golf diesel models at the end of 2006" in all 50 states, said spokesman Tony Fouladpour.
Meantime, California diesel enthusiasts wait.
DesRoches, who drives a Honda Odyssey minivan, says that with the federal tax credit easing the diesel premium, he'd buy one again if they could be sold here. The Simi Valley resident gets just 15 miles to the gallon in his gasoline-using Honda when towing his sailboat to Castaic Lake, about one-third less than he got 15 years ago with his diesel-powered Mitsubishi mini-pickup.
"It's not just the [economic] payback," said DesRoches. "Part of it is that I feel good using less fuel, and part is having the best motor to do the job. If it also saves enough money to pay back the extra cost of the diesel [engine] then that's just gravy."