Diesel vs. biodiesel vs. vegetable oil

We test homegrown fuel alternatives to gasoline

With gasoline prices volatile and the Obama administration committed to easing the United States' addiction to oil, Americans seem to be taking more interest in alternative fuels, including those derived from farm crops and other renewable organic sources. Among the most widely available are biodiesel and vegetable oil, both of which can be used to power a diesel engine.

Developed from vegetable or animal fats, biodiesel is functionally identical to petroleum diesel. Adherents claim it pollutes much less than regular diesel.

Biodiesel is most commonly sold in blends with normal diesel. B5, which is 5 percent biodiesel and 95-percent petroleum diesel, sold for a penny a gallon more than regular diesel in January 2009 on nationwide average, according to the U.S. Department of Energy. In the same report, pure biodiesel (B100) sold for $1.03 more per gallon than regular diesel. That price differential tends to shrink, however, as petroleum diesel gets more expensive than it is today.

A relative of biodiesel is plain, edible cooking oil. But because it's not financially practical to fuel a car with cooking oil from grocery store shelves—since a gallon costs about $8—some people are modifying diesel engines to run on the used deep-fryer oil that restaurants often throw away. Discarded oil is sometimes available free, though more restaurants are now charging for it.

To see how biodiesel—both B5 and B100—and fryer grease compare with conventional petroleum diesel fuel, we converted a diesel-powered 2002 Volkswagen Jetta TDI so that it could operate on all three. We found that they all allowed the car to perform well but differed in price and convenience. We experienced the best overall results using B5, the 5 percent biodiesel blend. It provided the best balance of performance, emissions, fuel economy, and convenience. B5 will run in any diesel engine without making vehicle modifications, and it is pumped into the tank just like any standard fuel. Because it is 95 percent petroleum diesel, however, it does little to wean drivers off fossil fuels.

In checking with a fuels engineer at a major automaker, we heard that modern clean diesels with high-pressure direct fuel injection, catalytic converters and other sophisticated emission controls may be less tolerant of impurities found in a lot of biodiesel.

Our Jetta ran well on the used cooking oil, but the inconvenience of locating fuel sources and preparing the oil for use in the engine limits its appeal and offsets its low price.