Put a soybean in your tank

Do not be surprised if, in the not too distant future, you spot a four-wheeled vehicle rumbling by that sounds like a Volkswagen, runs like a diesel truck, and smells something like hot oil in a frying pan.

Last month, Volkswagen of Brazil announced completion of a year and a half of road tests on four 13-ton diesel trucks that ran for 75,000 miles on 100 percent soybean oil without mechanical failures and performed as well as diesel-fueled vehicles.

The project, coordinated by five automobile companies, may be doubly good news for debt-squeezed Brazil, which is trying to reduce its expensive imported petroleum diet and which is also the second largest soybean producer in the world.

The four Volkswagen trucks tested showed that this purified soy oil burns cleanly, is as energy rich as diesel, and gives off only an innocuous smell rather like oil sizzling in a frying pan. Only a minor adjustment of the diesel's injection nozzle was needed to adapt diesel engines to soy oil.

Actually, there's nothing new about this sort of experiment. France and other nations were experimenting with vegetable oil fuel substitutes during World War II, when oil was expensive and embargoes frequent.

But after the war, when cheap petroleum flowed abundantly, most of these vegetable fuel oil projects were left fallow. With the oil shocks of the 1970s, however, interest in these so-called ''biomass energy'' projects was revived in earnest.

Today, in the Philippines, some 5 percent of auto fuel comes from coconut oil. African countries are working on palm oil fuels, and a number of tropical countries are experimenting on deriving fuel from such diverse sources as peanuts, sunflowers, and grape seeds.

Brazil started using fuel alcohol distilled from sugar cane back in 1923 (as a measure to boost sagging sugar prices). And now Brazilian gasoline contains some 17 to 20 percent alcohol.

Thanks to rich soils and financing from the World Bank, Brazil has built up a booming, 12.8-million-ton-a-year soybean harvest that is fully mechanized. Scientists here and elsewhere began experimenting on crude, salad-grade soybean oil, but most abandoned the projects when car motors ended up clogged with impurities and sediments.

It was Volkswagen's team of Drs. Rolf Wilhelm Siekmann, a chemist, and Georg H. Pischinger, a mechanical engineer, who rediscovered the process of refining soy oil into a manageable, efficient fuel oil.

Siekmann and Pischinger estimate it will be seven to 10 years before commercial vehicles start filling up with soy. Soy oil is so compatible with diesel fuel vehicles, says Pischinger, ''We can sneak it into the pumps without anyone having to know.''