VEHICLE TECHNOLOGIES OFFICE Straight Vegetable Oil as a Diesel Fuel? Biodiesel, a renewable fuel produced from animal fats or vegetable oils, is popular among many vehicle owners and fleet managers seek- ing to reduce emissions and support U.S. energy security. Questions sometimes arise about the viability of fueling vehicles with straight vegetable oil (SVO), or waste oils from cooking and other processes, without intermediate processing. But SVO and waste oils differ from biodiesel (and conventional diesel) in some important ways and are generally not considered acceptable vehicle fuels for large-scale or long-term use. Straight vegetable oil is not the same as biodiesel and is generally not Performance of SVO consistent across a substantial body of recommended for long-term vehicle use. Photo from iStock 8561648 Research has shown that there are sev- technical literature, including an SAE eral technical barriers to widespread technical paper that reviewed the pub- use of SVO as a vehicle fuel. lished data on the use of SVO in diesel engines.2 The SAE paper states: increasing proportions of vegetable oil The published engineering literature blended into the fuel.3 strongly indicates that the use of SVO Compared to No. 2 diesel fuel, all of the vegetable oils are much more Viscosity (the measure of a fuel’s leads to reduced engine life,1 caused by viscous, are much more reactive to the buildup of carbon deposits inside resistance to flow) is another impor- oxygen, and have higher cloud point the engine and the buildup of SVO in tant consideration related to the use and pour point temperatures. the engine lubricant. These issues are of SVO. The viscosity of SVO is much higher than that of diesel fuel at nor- attributable to SVO’s high viscosity Diesel engines with vegetable oils offer 1 and high boiling point relative to the acceptable engine performance and mal operating temperatures. Figure required boiling range for diesel fuel. emissions for short-term operation. 2 (on p. 2) illustrates the viscosities of The carbon buildup doesn’t neces- Long-term operation results in opera- diesel fuel and of 100% sunflower oil sarily happen immediately upon use tional and durability problems. over a range of temperatures. High of SVO; it typically takes place over fuel viscosity can cause premature the long term. These conclusions are Some investigators have explored wear of the fuel pumps and injectors. modifying vehicles to preheat SVO It can also dramatically alter the struc- prior to injection into the engine. Oth- ture of the fuel spray coming out of 1.8 ers have examined blends of vegetable the injectors: increasing droplet size, oil with conventional diesel. These decreasing spray angle, and increasing 1.6 techniques may mitigate the problems to some degree but don’t eliminate 1.4 them entirely. Studies show that car- 1 Sidibé, S.S.; Blin, J.; Vaitilingom, G.; Azoumah, Y. “Use of Crude Filtered Vegetable Oil as a Fuel in bon buildup (coking) continues over Diesel Engines State of the Art: Literature Review.” 1.2 time, resulting in higher engine main- Renew Sust Energ Rev 14(9):2748-2759 (2010). tenance costs and/or shorter engine 2 Babu, A.K.; Devaradjane, G. “Vegetable Oils 1.0 life. Figure 1 shows that the tendency and Their Derivatives as Fuels for CI Engines: An Overview.” SAE Technical Paper No. to form carbon deposits increases with 2003-01-0767. Coking Index Relative to Diesel to Relative Index Coking 0.8 3 0% 3% 5% 10% 15% 20% Figure 1 adapted from Jones, Samuel T.; Peterson, Charles L.; Thompson, Joseph C. Biological and Percent Oil in Fuel Agricultural Engineering Department, University of Idaho, Moscow, Idaho. “Used Vegetable Oil Fuel Blend Comparisons Using Injector Coking in a DI Figure 1. Buildup of carbon deposits Diesel Engine.” Presented at 2001 ASAE Annual in the engine as a function of the International Meeting, Sacramento, Calif., July 30– proportion of oil in the fuel. August 1, 2001. SAE Paper No. 01-6051. cleancities.energy.gov • January 2014 VEHICLE TECHNOLOGIES OFFICE • January 2014 • Page 2 100 than SVO and results in better engine 90 performance. In particular, biodiesel has a lower boiling point and viscosity 80 than SVO. 70 Biodiesel is most commonly used as a /second) 60 2 blend with petroleum diesel fuel. All 100% Sunflower Oil 50 manufacturers of diesel vehicles and engines have approved the use of B5 40 (a blend containing 5% biodiesel and 30 95% petroleum diesel), and some approve the use of blends up to B20 Viscosity (mm Viscosity 20 Diesel (20% biodiesel and 80% petroleum 10 diesel) or higher. 0 To ensure good performance in 18 28 38 48 58 68 78 88 engines, biodiesel must meet quality Temperature in °C specifications developed by ASTM International. ASTM Specification Figure 2. Viscosities of sunflower oil and conventional diesel fuel as a D6751 is for pure biodiesel (B100), function of temperature. used for blending with petroleum diesel fuel. Biodiesel that meets spray penetration. These effects tend to The long-term effect of using SVO in ASTM D6751 is legally registered increase wetting of the engine’s internal diesel engines equipped with modern with the U.S. Environmental Protec- surfaces, thereby diluting the engine emission control systems is also a mat- tion Agency. Blends up to B5 may lubricant and increasing the tendency ter of concern. Buildup of fuel in the be found in conventional diesel fuel for coking. lubricant is more significant in these without additional labeling at the engines—even for petroleum diesel— pump. Properties of these low-level and would likely be severe with SVO. blends are covered by the diesel fuel Where can I get more In general, these systems were not ASTM Specification D975. For blends originally designed to accommodate information? of biodiesel ranging from B6 to B20, the properties of SVO, and they can be a separate specification exists, ASTM ■ The U.S. Department of Energy’s seriously damaged or poisoned by out- D7467, and pump labeling is required Alternative Fuels Data Center, at of-spec or contaminated fuel. to inform the consumer that a bio- afdc.energy.gov, contains a vast diesel blend is being sold. collection of information on alter- native fuels and alternative fuel Biodiesel: Fuel Made For a complete list of ASTM biodiesel vehicles. From SVO requirements, see the Biodiesel Han- Biodiesel is an alternative fuel that can dling and Use Guidelines at www.nrel. ■ The National Biodiesel Board is the national trade association be made from SVO or other fats in a gov/vehiclesandfuels/pdfs/43672.pdf. In representing the biodiesel indus- chemical process called transesterifica- addition, the biodiesel industry has try. Its website, biodiesel.org, tion, which involves a reaction with instituted a quality assurance program serves as a clearinghouse of methanol, using caustic soda (sodium for biodiesel producers and market- biodiesel-related information. hydroxide) as a catalyst. Biodiesel ers. To learn more about the BQ-9000 has substantially different properties program, visit bq-9000.org. Clean Cities Technical Response Service 800-254-6735 • [email protected] DOE/GO-102014-3449 • January 2014 Prepared by the National Renewable Energy Laboratory (NREL), a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy; NREL is operated by the Alliance for Sustainable Energy, LLC. Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 10% post consumer waste..