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.

One problem: Biodiesel is a solvent that releases gunk accumulated in the fuel tank or system. Fill the tank with it, and that gunk will probably clog the truck's fuel filter. Change the filter, though, and the truck should be ready for continued use. A bigger problem is that pure biodiesel attacks natural and butyl rubbers, so manufacturers will have to eliminate those materials from fuel systems if they're going to certify their diesels for use with B100. Blends of B20 and lower don't have that problem.

Also, biodiesel, like any other vegetable oil, will go bad if left to sit long enough. You need to be sure you have a supply of fresh oil, as old oil can spoil. It also absorbs moisture, which can corrode fuel-system components.

Biodiesel is even more prone to gelling in cold weather, making its use a challenge in winter. Blended B20 fuel may plug the truck's filter at two to 10 degrees higher temperatures than regular diesel would, according to the National Biodiesel Board. Pure biodiesel gels sooner than petrodiesel, and the solutions are similar as for #2 diesel: blending with #1 diesel, using fuel heaters and parking indoors, and using a cold-flow improvement additive. The Rocky Mountain National Park already has addressed the cold-weather issue, though. Its government vehicles burn B20 even in winter, points out Loren Beard, senior manager for environmental and energy planning at Daimler-Chrysler.

The possibility of depending less on petroleum is appealing, so burning B100 would be ideal. Unfortunately, it's tough to find a suitable supply of pure biodiesel, and your truck's manufacturer isn't going to support your environmentalism. "We strongly recommend against [using B100]," says Beard. "To do it, you would have to replace a lot of elasomers and make some changes to the fuel pump. B20 is a small enough departure that the vehicle can deal with it." And, while stories are common of running on B100 without any trouble, a fact Beard acknowledges, "I wouldn't do it if it were my truck." Richard Baker, senior technical leader on combustion and emissions in powertrain research and advanced engineering at Ford Motor Co., explains what can happen on B100. "The kinds of things that'll show up are clogged fuel filters, water pickup, and corrosion in fuel pumps and lines. The fuel system is intricate, and [it] can lead to a significant, expensive overhaul."

We may have to be satisfied burning blended biodiesel, and even then probably the lower B2 and B5 mixtures for a while until better B20 guidelines have been established. Manufacturers are solidly behind use of B5, and Chrysler even shipped new Jeep Liberties from the Toledo, Ohio, plant with that fuel. "Using B100 takes an awful lot of handholding and precautions such as changing out hoses," says Steve Howell, technical director of the National Biodiesel Board. If you want to convert, he explains, "be absolutely sure the supplier is using ASTM-certified products. We know that bad biodiesel can cause problems. The best way to feel confident is to use suppliers who've gone through NBB certification."

What about B20? The trouble is that there's no standard agreed-upon specification for the fuel as there is for B5, because of the need for careful handling to make sure it isn't too old.

But we can get away with using B20, can't we? "You can have a good experience using B20," concedes Baker. "You can also have a very bad experience. It depends on the situation." Because manufacturers can't know ahead of time whether you're getting good or bad fuel, they have to err on the side of caution. "We need tighter standards around what the product is going to be so we don't get surprises," says Baker. "What can be perceived as reluctance on the manufacturer's part is really a question of getting the standards in place."

Where does that leave us if we do try it? Is our warranty void? Here, the manufacturers hem and haw. Most don't support use of B20 (except the Chrysler Group, approved use of military-grade B20 in 2007 Dodge Rams), but admit that it would be tough to deny claims for much other than a vehicle littered with fuel receipts from the local soybean grower's co-op, with a clogged filter or corroded fuel pump. In any case, the warranty on non-fuel-system parts should be unaffected.

When is the industry going to get its collective act together and provide standardized, fully or mostly renewable fuel and the vehicles built with the necessary fuel-system components to use it without problems? It'll be at least five years before manufacturers will sell vehicles approved for B20 industrywide, even if an agreeable B20 specification is approved this year, says Baker, because of the need for durability testing of the new parts with the fuel.

Meanwhile, the NBB predicts biodiesel production will accelerate from the 25 million gallons sold nationally in 2004 and approximately 75 million gallons sold in 2005 to a billion gallons a year by 2015.

To find the certified biodiesel source nearest to you, check the NBB's Web site at www.NBB.org. And buy some spare fuel filters.

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