It has been known for more than 50 years that some diesel engines could be fueled for short periods with vegetable oils, either neat or with hydrocarbon fuel additives. World overproduction of soybean oil is increasing its potential as an economical diesel fuel extender. The subject test program was undertaken to determine long-term effects of this alternate fuel on a modern, high-speed diesel engine.The choice of a vegetable oil (soybean oil) as an alternative diesel engine fuel or fuel extender rather than the other major biomass motor fuel (ethanol) is related to the relative properties of these fuels. The common U.S. vegetable oils are much closer to hydrocarbon (No. 2D) diesel fuel than is ethanol in both cetane rating and volumetric energy content. Unlike ethanol, the vegetable oils can be blended 1:1 with No. 2D fuel to produce engine power and volumetric fuel consumption levels practically identical to those obtained with 100% No. 2D fuel. However, engine operation and laboratory bench tests demonstrated that some fuel blends were unsatisfactory for continuous use. The reasons for these difficulties were determined and a satisfactory fuel blend was proven through prolonged testing.The results of the soybean oil fuel extender tests are considered to be particularly significant because: ○ Soybean oil is the principal U.S. grown vegetable oil, representing about 85% of possible current national production. ○ The total duration of the tests (over 1,000 hours) is the longest known U.S. operation of a single engine on any type of vegetable oil fuel. ○ Successful operation was obtained under standard engine test conditions without any engine modification or adjustment. ○ From an economic standpoint, soybean oil is closer to the cost of No. 2D fuel than any common U.S. vegetable oil.