An excellent review of literature concerning the use of various kinds of plant oils as a replacement for diesel fuel was made by Jones and Peterson of the University of Idaho. Pryde (1982) also reviewed the results of a number of studies on the use of plant oils as a diesel substitute and reported that whole short-term engine tests showed promising results, long-term engine tests revealed serious problems resulting from carbon build up and lubricating oil contamination. It was concluded that plant oils must either be chemically altered or blended with diesel fuel to prevent premature engine failure. Quick (1980) also surveyed the work on over 30 different plant oils that had been used to operate compression engines. It was found that while plant oils showed great potential as diesel fuel substitutes, extended operation resulted in serious carbonization of critical engine components. Blending the plant oil with diesel fuel was found to yield better results, reducing coking and extending engine life.
Seddon (1942) studied the performance of different plant oils as fuel using a Perkins P6 diesel engine with good results. The experiments showed that plants oils could be used to power diesel engines under normal operating conditions. It was noted, however, that much more work would be needed to fully ascertain the reliability of plant oils as substitute for diesel fuel since problems tend to surface as the trials lasted longer periods. Bruwer et al (1980) examined the use of 100% sunflower seed oil as a diesel fuel substitute in running a farm tractor. A power loss of 8% occurred after 1,000 hours of operation, which was corrected by replacing the fuel injectors and injector pump. After 1,300 hours of operation, the carbon deposits in the engine were measured to be equivalent to an engine fueled with 100% diesel except for the injector tips, which exhibited excessive carbon build-up.
Goering et al (1981) studied the performance of eleven plant oils to determine which oils could be used as an alternative fuel source. Of the eleven plant oils used in the experiments, corn, rapeseed, sesame, cottonseed, and soybean oils showed the most favorable fuel properties. Similarly, Bacon et al (1981) evaluated the use of several plant oils as diesel fuel substitute. Short 2-hour tests were conducted to visually compare the effects of using different plant oils in place of diesel fuel. The initial engine performance tests showed acceptable performance although there was noticeable carbon build up in the combustion chamber. As the tests progressed with the continuous running of the diesel engine at part-load and mid-speeds, it was observed that carbon deposits accumulated rapidly on the injector tips. Although short-term engine test results were promising, it was recommended that long-term engine testing would be necessary to ascertain the overall effects on diesel engines of using plant oils as fuel.
Schoedder (1981) obtained mixed results using rapeseed oils as a diesel fuel replacement in a series of studies. Although short-term engine tests indicated that rapeseed oil had similar energy outputs compared to diesel fuel, the results of long-term engine tests revealed operating difficulties arising from deposits on piston rings, valves and injectors, particularly after 100 hours of continuous operation. The study recommended further long-term testing to determine how these difficulties could be avoided. Nearly similar results were obtained by Yarbrough et al (1981) when testing six sunflower oils as diesel fuel replacements. The tests showed that raw sunflower oils were unsuitable for use as diesel fuel substitute but refined sunflower oil was found to give acceptable performance. They further showed that degumming and dewaxing of the plant oils would be required to prevent engine failure.
Reid et al (1982) conducted injection studies and engine tests to evaluate the chemical and physical properties of plant oils related to their use as alternative fuels. The injection studies showed that the plant oils dispersed differently compared to diesel fuel due to their much higher viscosities. The engine tests showed that the level of carbon deposit varied even for plant oils with nearly similar viscosities, indicating that oil composition was also an important factor.
The tests also revealed that pre-heating the oil prior to injection could reduce the amount of carbon deposits in the engine. Auld et al (1982) evaluated the suitability of rapeseed oil as an alternative fuel in diesel engines and showed that there is a relationship between viscosity and fatty acid chain length. Although the engine power and torque results using rapeseed oil were similar to that of diesel fuel during short-term tests, the study clearly indicated the need for further long-term testing to better evaluate engine durability.
Bettis et al (1982) evaluated sunflower, safflower and rapeseed oils as possible sources for liquid fuels. These plant oils were found to contain approximately 95% of the energy content of diesel fuel but were about 15 times more viscous. As with earlier findings, short-term engine tests indicated that the plant oils could deliver power nearly equal to that of diesel fuel, but longterm durability tests revealed serious problems related to carbonization of the combustion chamber. Tahir et al (1982) tested sunflower oil in farm tractors as a diesel fuel substitute. Although the viscosity of sunflower oil was 14% higher than diesel fuel at 370C, the test showed engine performance comparable to that of diesel fuel, but with slightly higher fuel consumption. Furthermore, the oxidation of the sunflower oil left heavy gum and wax deposits on test equipment that, with continued use, could lead to engine failure.
Engler et al (1983) used raw sunflower and cottonseed oils as fuel in a series of engine performance tests and obtained poor results. However, when processed or refined sunflower and cottonseed oils were used, the tests produced results slightly better than similar tests for diesel fuel. There was, however, the occurrence of carbon deposits and lubricating oil contamination problems, indicating that these oils were acceptable only for short-term use as diesel fuel substitute. Pryor et al (1983) conducted short-term and long-term engine performance experiments using 100% soybean oil in a small diesel engine. The results confirmed the findings of similar earlier studies – short-term use gave performance equivalent to that of diesel fuel, but long-term use would result in severe power loss due to carbon buildup in the injectors.
The studies on the use of various types of plant oils as replacement for diesel fuel indicate that a diesel engine can be successfully run with 100% refined or processed plant oil but only on a short-term basis. The long-term use of 100% plant oil can cause engine failure due to engine oil contamination, stuck piston rings, and excessive carbon build-up on internal engine components. It can, therefore, be concluded that 100% unmodified plant oils are not acceptable diesel fuel replacements.