Biodiesel seems like the ideal way to unify two German passions: fast food and getting the most mileage out of their fuel money.

The possibility of turning deep-fryer oil left over from schnitzels and French fries into a fuel to help cars race down the autobahn as a replacement for conventional, carbon-based diesel has many in Germany excited.

Even though biodiesel's price follows diesel's highs and lows, it generally sells at between 10 and 15 euro cents per liter ($0.12 and $0.18 per quarter gallon) less than regular diesel and as it is currently exempt from Germany's petroleum tax, the organic fuel is becoming especially attractive. But reaching for the green pump isn't a viable alternative for every diesel car owner.

Not for newer diesel cars

"No one should use biodiesel in a car without a manufacture's release," said Karin Retzlaff of the Association of the German Biofuel Industry. "Using biodiesel in cars that have not been approved can invalidate the car's warranty."

Ironically, it's newer, more efficient cars that are least likely to be approved for biodiesel use. Rust particle filters, which come equipped in many new diesel automobiles, do not tolerate biodiesel, and the fuel also does not meet updated European emissions standards to curb air pollution, which come into effect in January 2006.

While reducing carbon dioxide, burning biodiesel emits more nitrous oxide than allowed by the looming EU regulations. For these reasons, "few new diesel cars are receiving approval to use biodiesel," Andrea Gärtner, a technician for Germany's largest auto club, told the DPA news agency.

Consumers have two options

There are ecological advantages and disadvantages to using biological fuels, according to Guido Reinhardt of the Institute for Energy and Environmental Research in Heidelberg.

"The advantages of the currently used biofuels will probably increase as compared to conventional fuels," he wrote in a 2004 study before concluding that there is still much more research to be done.

Until additional answers on the climate effect of biodiesel are found, drivers are left with two options, according to Retzlaff.

"Consumers looking to do something good for the environment, and for their wallets, have to buy a car that's been approved or settle for filling up with the diesel-biodiesel mixes," she said.

Biological blends already in tanks

Biodiesel blends have been permitted at German gas stations without special labeling requirements since January 2004. The automobile industry has agreed biodiesel can account for up to five percent of "regular" diesel without adversely affecting performance. Pure biodiesel, called B100, is available at about 12 percent of Germany's 15,000 gas stations.

Though it is possible to convert fatty oils and household garbage, with the exception of metal and ceramic, the products of these processes tends to clog fuel filters. To avoid some of these problems, German biodiesel is made exclusively by processing rapeseed oil with methane gas.

The organic fuel it produces is important in helping Germany reach European Union goal of using biological fuels to cover two percent of energy needs by 2005 and 5.75 percent by 2010. Biodiesel currently makes up four percent of Germany's total diesel sales.

Even taking into account that fewer passenger cars will be able to fill up with biodiesel, Retzlaff said Germany "using its own sources of raw materials, would be able to meet the EU goals."

Transport companies interested in biodiesel

Biodiesel may have problems in the consumer market, but it plays an important role for trucking and delivery companies looking to save on fuel costs. Transportation companies have found switching to biodiesel to be especially profitable over the long-term, according to Retzlaff.

"Biodiesel will meet the updated European standards for trucks and commercial vehicles," she said. "Also because the companies can go directly to the provider, they can get a price that's better than paying at the pump."

Germany in good position

Thanks in part to increased commercial use, Retzlaff said she expects the market for biodiesel to continue growing and that German companies, which were among the initial adopters of commercial biodiesel in the early 1990s, are in a good position to take advantage of the growth.

"Germany has technological head start, and there is a demand for experience in running both biodiesel operations and facilities," she said.