Sure, we've all heard the old story: "You don't need to pay OPEC for diesel, just get the leftover french-fry oil from McDonald's." Does this sound crazy? Well, maybe it's not, this could be a cheap and environmentally friendly way to fuel diesel trucks, but few have thought it worth the trouble or risk to try it. That was until the huge increases in fuel prices in 2005 and questions arose about fuel availability in some areas.

It's possible to fuel a diesel engine with filtered cooking oil, but that isn't what people mean by the word "biodiesel." And if it were as simple as pouring in filtered deep-fry oil, we'd have done it long ago.

Raw vegetable oil will probably clog your truck's fuel system in the short-term and dissolve rubber hoses and seals and corrode metal parts over the long haul. Even carefully processed 100-percent biodiesel fuel oil, called B100, has many of the same problems. It may be too soon for biodiesel to be your main fuel, but it's quickly gearing up to be a significant component when blended with petroleum-based diesel. Those blends are referred to by the percentage of biodiesel in the mix, commonly two, five, or 20 percent.

Biodiesel is a renewable fuel produced from sources like soybean oil, which has been processed to remove injector-clogging glycerin. It not only has the benefit of being domestically produced, it also emits less net carbon dioxide. That's because while the engine still produces CO2 as the byproduct of combustion, the plants grown to make the fuel feed on carbon dioxide, which they pull from the atmosphere. Some additional CO2 is released while the fuel is created and transported, so it's not truly carbon neutral, but it's close.

Biodiesel also produces less sooty particulate material for reasons that still elude experts. But even without understanding the mechanism, we can enjoy the benefit of reduced pollution. "It's a no-brainer to use biodiesel," Ric Hiller, Bureau Chief for Arlington County, Virginia, explains to the National Biodiesel Board. "By using the biodiesel we've eliminated the particulate and the big black plume. We're doing good for the environment and we're reducing our petroleum use by 20 percent." It also smells better when burned.

Biodiesel is cleaner, with none of the sulfur that fouls emissions controls, so in an age of movement toward clean diesel, biodiesel is an obvious part of the solution. It has a higher cetane rating, a significant improvement over the low-quality diesel typically available here. "Third-world countries have better-quality diesel than the U.S.," groans one Mercedes diesel engineer.

Biodiesel has better lubricity, which keeps all parts sliding smoothly. It's so good, in fact, that a mere one percent blend with petrodiesel boosts lubricity by 65 percent.