Why Can't 'Flex-Fuel' Clean Diesels Run Entirely On Biodiesel?

By John Voelcker

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Grille - 2010 Ford Super Duty F-450 4WD Crew Cab Lariat

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Our article last week on Ford's doubling of its flex-fuel models included a mention that diesel models of the new 2011 Ford Super Duty pickup trucks will run on either pure diesel fuel or a mix of up to 20 percent biodiesel, known as B20.

Reader Suzanne Johnson responded with the following question:

I live near a biodiesel production facility and have learned from an engineer there that not all new diesels will run on pure biodiesel. Audi and Volkswagen only allow a max of 10 percent biodiesel mixed in with regular diesel.

Given the severity of the Gulf oil spill, how about including info on biodiesel use for the new diesels?

2010 Volkswagen Jetta Sportwagen TDI

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2010 Audi A3 TDI

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Diesel Fuel Price Atlanta 2/09

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Cleaner diesel fuel is here and now

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Carmaker concerns

We know that modern clean diesel engines are much more sensitive to fuel content than older ones, but to learn more, we chatted with our former colleague (and "rabid diesel enthusiast") Colin Mathews.

Manufacturers, he pointed out, are rightly scared of exposing themselves to warranty claims from consumers burning unapproved or unexpected fuels in highly calibrated, very low-emission clean diesel engines.

What IS biodiesel, exactly?

Among car fans, the term "biodiesel" may encompass anything from refinery-produced products through 5-gallon cans of peanut oil to recycled fryer grease from your local fast-food joint.

More accurately, biodiesel is a blend of lipids (from vegetable oil or animal fat) and alcohol. In the U.S., the general definition says it is domestically derived from renewable oils. There's also a technical definition, as there are for both gasoline and low-sulfur diesel fuel.

Some U.S. states have specific requirements for diesel fuel, as do various European countries. So the definition of permissible biodiesel can vary from place to place.

Even if the fuel being discussed is perfectly consistent, engines have to be tweaked to handle two problems: higher viscosity at low temperatures, and the effect of biodiesel on rubber engine and fuel-system components.

Too thick at cold temperatures

First, viscosity. If you've ever had olive oil turn to a cloudy solid in your fridge, you know that vegetable oils solidify at low temperatures. This happens at much warmer temperatures than it does standard # 2 diesel, and the temperature varies depending on the lipid used.

Fuel in clumps plays havoc with modern common-rail fuel systems, whose injectors fire vaporized fuel into the cylinder at the astounding pressure of 23,000 pounds per square inch.

The problem is solved today by permitting only a small amount of vegetable-based oil in anything sold as "diesel" fuel (B5 has 5 percent biodiesel; B20 has 20 percent). Blending it into a larger amount of regular diesel solves the problem of fuel gelling.

Eats rubber, too

The problem of biodiesel degrading natural rubber over time is not much of an issue today. All engine components that could come into contact with the fuel must be made of synthetic rubber. But that's required for any biodiesel at all, even today's permitted low levels.

Diesels sold in North America have largely made this switch already. Accommodating B100 could require yet more engineering, new parts again, and more durability testing, e.g. higher costs.

Up to B20 at most

While the advocacy site Biodiesel.org says that "blends of up to 20 percent" biodiesel that meets the proper specific (D 6751) will "work in any diesel engine with no modifications to the engine or the fuel system," not every diesel manufacturer warrants its engine for B20.

Volkswagen and Audi don't for their TDI engines, for example, though in the heavy-duty U.S. pickup segment--as with Ford's recent announcement and for new GM heavy-duty diesels--B20 is becoming the standard.

Biodiesel.org also notes that, "Biodiesel has a cleansing effect that may release deposits accumulated on tank walls and pipes from previous diesel fuel usage," which end up trapped by the car's fuel filter. That means fuel filters should be checked more often.

Chicken and egg?

In part, Mathews concludes, biodiesel use can be viewed as a chicken-and-egg situation. If diesel engines for passenger vehicles grow in popularity, demand for diesel fuel will grow, and biodiesel may offer a new fuel source that could keep down higher diesel prices.

But until diesel penetration rises, and manufacturers see more demand for biodiesel, we may stay about where we are now.