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Most makers of diesel cars will not honor warranties on cars that burn biodiesel in higher concentrations than B5 because it can eat away at seals in the fuel system.
According to statistics published by the U.S. Department of Energy's National Renewable Energy Laboratory, the U.S. will soon have the capacity to produce about 900 million gallons a year, or enough to supplant less than 1 percent of the gasoline used in this country.
In Consumer Reports' own tests, a car running on biodiesel produced slightly less pollution than the same car running on conventional diesel but achieved slightly fewer mpg.
A variation on biodiesel is straight vegetable oil (SVO), which can also burn in diesel engines without modification. Cars running on SVO, however, need a separate fuel tank for the veggie oil and a preheating system to keep it from congealing. Conversion kits to add the fuel tank and other hardware to existing diesels cost about $800, and the components reduce a vehicle's cargo space.
In our tests, the car running on vegetable oil posted almost the same acceleration and similar emissions as it did on petroleum diesel. Many people who go this route get free recycled fryer oil from restaurant kitchens. But it has to be filtered thoroughly before being put in the tank.
Diesels: Cleaning up their act
Pros Good fuel economy, typically lower fuel prices, lower CO2emissions.
Cons Higher NOx and particulate emissions, noisy, usually slower acceleration.
Diesel engines are up to 30 percent more efficient than gasoline engines. But drawbacks have kept them from gaining general acceptance in the U.S. Historically, they've suffered from slow acceleration, a loud clatter, smoky exhaust, and an oily smell. Compared with gasoline engines, diesels also emit higher levels of two hard-to-manage pollutants: oxides of nitrogen (NOx), which forms smog and contributes to acid rain, and particulates (soot), which have been linked to respiratory problems. Because of this, emissions laws in five states--California, Maine, Massachusetts, New York, and Vermont--effectively ban the sale of diesel cars. (Trucks such as pickups and SUVs are allowed.)
That's beginning to change, however. With turbocharging, ultra-precise, high-pressure "common rail" fuel injection and better exhaust-system catalysts, modern diesel cars are quicker, quieter, and much cleaner than their predecessors.
Most important in the resurgence of diesels, the federal government has mandated a switch to low-sulfur fuel, which is supposed to go on sale in October 2006. This will allow automakers to lower NOx emissions by using special catalytic converters, which now would be easily contaminated by sulfur. According to the latest EPA regulations, this would likely allow diesels to be sold in all states. The de-sulfurizing process is estimated to add about five to seven cents per gallon to the cost of diesel fuel.
In March 2006, the Environmental Protection Agency loosened proposed diesel emissions regulations to allow more diesels to be sold. Honda, Nissan, Subaru, Toyota, and others are developing diesel models to join those offered by Mercedes-Benz and Volkswagen.
Chet France, lab director at the EPA's Office of Transportation and Air Quality, predicts that "the new-generation diesels will be more than 90 percent cleaner than current models."
Unlike hybrids, which achieve fuel mileage gains mainly in stop-and-go city traffic, diesels are more efficient on the highway.
Some environmentalists, however, claim that the efficiency advantage of diesel is overstated because more oil goes into refining diesel than gasoline.
Hydrogen: A long wait
Pros Low or virtually no harmful emissions.
Cons Fuel-supply problems, expensive fuel-cell technology, onboard fuel storage, range, cold-temperature driveability.
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