Using Unmodified Vegetable Oils as a Diesel Fuel Extender

This paper is a review of literature concerning using vegetable oils as a replacement for diesel fuel. The term vegetable oils as used in this paper refers to vegetable oils which have not been modified by transesterification or similar processes to form what is called biodiesel. The oils studied include virgin and used oils of various types including soy, rapeseed, canola, sunflower, cottonseed and similar oils. In general, raw vegetable oils can be used successfully in short term performance tests in nearly any percentage as a replacement for diesel fuel. When tested in long term tests blends above 20 percent nearly always result in engine damage or maintenance problems. Some authors report success in using vegetable oils as diesel fuel extenders in blends less than 20 percent even in long term durability studies. Degumming is suggested by one author as a way to improve use of raw oils in low level blends. It is apparent that few, if any, engine studies using low-level blends of unmodified vegetable oils, < 20%, have been conducted.

Many studies have been done at the University of Idaho and elsewhere involving vegetable oils as a primary source of energy. Particularly, during the early 1980's, studies were completed that tested the possibility of using unmodified vegetable oils as a replacement for diesel fuel.
There is no question that vegetable oil can be placed in the tank of a diesel powered vehicle and the engine will continue to run and deliver acceptable performance. Some vegetable oils, such as rapeseed oil, have very high viscosity and thus may starve the engine for fuel when operated at 100 percent. Most studies show that power and fuel economy, when compared to operation on diesel, are proportional to the reduced heat of combustion of the vegetable oil fuel.
Despite the success when diesel engines are operated on vegetable oil for short term performance tests, the real measure of success when using vegetable oil as a diesel fuel extender or replacement depends primarily on the performance of vegetable oils in engines over a long period of time. Thus many researchers have been involved in testing programs designed to evaluate long term performance characteristics. Results of these studies indicated that potential hazards such as stuck piston rings, carbon buildup on injectors, fuel system failure, and lubricating oil contamination (Pratt, 1980) existed when vegetable oils were used as alternative fuels. This effect diminishes as the blend of vegetable oil in diesel is decreased. The question of this literature review is to determine if there is a blend level at which vegetable oil in the unmodified form can be used as a diesel fuel extender. Throughout this paper when the term vegetable oil or the name of a particular vegetable oils is used, such as canola, it refers to the unmodified form.