Plant Oil-Diesel Blends as Fuel

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With the preponderance of evidence from numerous studies showing the unsuitability of using pure or straight plant oil for long-term use as diesel fuel substitute in unmodified modern CI engines, researchers have focused on using various blends of different plant oils with diesel fuel. There are basically two parameters that can be changed or adjusted: first, the type of plant oil; and second, the ratio of plant oil to diesel fuel in the blend. A summary of the results of a number of tests and field trials is presented here using varying blends of various plant oils such as soybean, rapeseed, sunflower, peanut, palm and others.

Engelman et al (1978) conducted a series of performance tests using 10% to 50% soybean oil blended with diesel fuel in diesel engines with initially encouraging results. The carbon build-up in the combustion chamber was minimal at the end of the 50-hour test run and the power delivered was only slightly lower compared to 100% diesel fuel. However, fuel blends containing 60% or higher concentrations of plant oil caused the engine to sputter due to fuel filter plugging. The tests showed that among the plant oils used, waste soybean oil gave the most promising results.

Sims et al (1981) showed that rapeseed oil-diesel fuel blends could be used as a replacement for diesel fuel. Short-term engine tests showed that a 50:50 rapeseed oil-diesel fuel blend had no adverse effects although long-term tests resulted in injector pump failure and cold starting problems. The amount of carbon deposits on combustion chamber components was found to be nearly the same as that found in engines operated on 100% diesel fuel. Worgetter (1981) used a 50:50 blend of rapeseed oil and diesel fuel to operate a 43-kW tractor. Initial results were good but after 400 hours of continuous operation the test had to be aborted due to serious engine problems resulting from heavy carbon deposits on the injector tips and pistons.

Van der Walt and Hugo (1981) examined the long-term effects of using sunflower oil-diesel fuel blends as a replacement for 100% diesel fuel in directand indirect-injected diesel engines. The indirect-injected diesel engines were run for over 2,000 hours using varying blends of de-gummed, filtered sunflower oil with no adverse effects. However, the direct-injected engines were not able to complete 400 hours of operation using a 20:80 sunflower oil-diesel fuel blend due to severe power loss resulting from severely coked injectors, carbon buildup in the combustion chamber, and stuck piston rings. There was also considerable wear of the piston, liner and bearing as indicated by the analysis of the lubricating oil. Barsic and Humke (1981) also studied the effects of mixing sunflower oil and peanut oil with diesel fuel in a single cylinder engine2 The fuel blends were found to have lower heating value compared to diesel fuel and were observed to increase the amount of carbon deposits on the combustion side of the injector tip. In addition, there was serious fuel filter plugging when crude sunflower oil and crude peanut oil were used as diesel fuel extenders. In a much later study, Fuls (1983) reported similar findings for indirect- and direct-injection engines using 20:80 sunflower oil-diesel fuel blends where injector coking occurred in directinjected diesel engines.

McCutchen (1981) compared engine performance of direct-injection engines to indirect-injection engines when fueled with a 30:70 soybean oildiesel fuel blend. The results showed that the indirect-injection engine could be successfully operated on this fuel blend but the direct-injection engine could not without severe engine problem occurring due to injector coking and piston ring sticking. Bartholomew (1981) reported that plant oils mixed with diesel fuel in small amounts did not cause engine failure. Short-term tests of plant oil-diesel fuel blends of up to 50% plant oil yielded acceptable results but reducing the blend to only 20% plant oil gave better and more consistent engine performance. Other studies by Hofman et al (1981) and Peterson et al (1981) confirmed earlier findings that while plant fuel blends yielded encouraging results in short-term testing, problems occurred in long-term performance tests. The most common problems encountered were carbon build-up, ring sticking, and lubricating oil Contamination.

Fort et al (1982) reported that cottonseed oil-diesel fuel blends functioned reasonably well during short-term performance and emissions tests, particularly in terms of power, fuel consumption, and quality of emissions. However, long-term engine durability was always a concern during extended use due to the formation of carbon deposits and clogging of the fuel delivery system. Similar studies by Baranescu and Lusco (1982) using three blends of sunflower oil and diesel fuel indicated that the sunflower oil caused premature engine failure due to carbon buildup and that cold weather operation caused fuel system malfunction. Engine testing by Ziejewski and Kaufman (1982) using a 50:50 blend of sunflower oil and diesel fuel was unsuccessful due to significant carbon buildup on the injectors, intake ports and piston rings.

Wagner and Peterson (1982) reported severe engine damage during shortterm engine testing using 100% rapeseed oil as a substitute fuel and heating the oil prior to combustion exhibited no measurable improvement. However, a long-term test using a 70:30 rapeseed oil-diesel fuel blend was successful for 850 hours with no significant wear, contamination of lubricating oil, or loss of power. Peterson et al (1982) also used a 70:30 rapeseed oil-diesel oil blend to successfully operate a small single cylinder diesel engine for 850 hours. In a much later study, McDonnell et al (2000) used a 25:75 semi-refined rapeseed oil-diesel fuel blend. The results showed that the injector life was shortened due to carbon buildup but there were no signs of significant internal engine wear or lubricating oil contamination.

Ryan et al (1984) showed that the atomization and injection characteristics of several plant oils were significantly different from that of diesel fuel due primarily to the higher viscosity of the plant oils. A series of engine performance tests also showed that power output slightly decreased when using blends of plant oils and diesel fuel and that injector coking and lubricating oil contamination were the dominant problems associated with the use of plant oil-based fuels with high viscosities. Pestes and Stanislao (1984) used a 50:50 plant oil-diesel fuel blend to study the formation of piston ring deposits and found that the most severe carbon deposits occurred on the thrust face of the first piston ring. It was suggested that to reduce piston ring deposits a fuel additive could be used or the concentration of plant oil in the blend could be lowered.

German et al (1985) used six farm tractors averaging 1,300 hours of operation to study the formation of carbon deposits. It was found that carbon deposits on the internal engine components were greater for the tractors using a 50:50 sunflower oil-diesel fuel blend than for those using a 25:75 sunflower oil-diesel fuel blend. And all test engines using plant oil blends had more carbon buildup than the engine using 100% diesel fuel. The results of the study indicated that plant oil-diesel fuel blends could not be used to completely replace petroleumbased fuels on a long-term basis without adversely affecting engine life.

Nag et al (1995) conducted studies in India using fuel blends as high as 50:50 of oil from the Indian Amulate plant and diesel fuel and found no loss of power, knock-free performance, and no significant carbon deposits on the functional parts of the combustion chamber. Sapaun et al (1996) reported that studies in Malaysia with palm oil-diesel fuel blends exhibited encouraging results. Short-term performance tests indicated that power outputs were nearly the same for various blends of palm oil and diesel fuel and 100% diesel fuel with no signs of adverse combustion chamber wear, increase in carbon deposits, or lubricating oil contamination.

The various studies on the use of plant oil-diesel fuel blends indicate that they can be used in diesel engines for short periods with no significant decline in performance provided that the concentration of the plant oil in the blend is less than 20%. Long-term engine performance tests show that plant oil concentrations higher than 20% can have adverse effect on the engine due to accumulation of carbon deposits, fuel line clogging, and lubricating oil contamination. Degumming is suggested as a way to improve the characteristics of plant oils in low level blends. The use of suitable additives has also been suggested to overcome many of the problems associated with the use of higher concentrations of plant oil in the fuel blends.

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