Viscosity Graph


Clean Cities Fact Sheet, April 2006, prepared by The National Renewable Energy Laboratory (NREL). NREL is a U.S. Department of Energy National Laboratory Operated by Midwest Research Institute

This chart shows the differences in viscosity between vegetable oil and diesel fuel over a range of temperatures. It stresses the importance of heating vegetable oil to a proper and safe temperature. In order to use vegetable oil, it is extremely important to know the heat-producing capability of your system. Make sure you know that the system you use (or plan to use) can produce sufficient temperatures to use this technology safely. 

As shown on the chart, traditional (petroleum based) diesel is substantially thinner than vegetable oil. In order to reach the optimum viscosity and safe lubrication properties vegetable oil must be well over 120 degrees to be used safely. We recommend operational temperatures of 160 degrees minimum and if 180 to 220 degrees can be reached, the vegetable oil will spray out of the injector in an even more efficient manner. These higher temperatures will allow for a more complete combustion in the cylinder.  It will reduce the chances of vegetable oil getting past the piston rings and winding up in the engine oil, or worse yet, building up as carbon deposits and coking up the piston rings or valve seats. These problems can totally ruin a diesel engine which is designed to run on compression (Carnot cycle). In addition, if the vegetable oil accumulates in the engine oil it can slowly damage the crankshaft and connecting rod bearings from polymerization. 

Three thousand mile intervals are recommended for all oil changes when using vegetable oil. This is a major concern to us at V.O. Tech due to the quality of used vegetable oil available here in the United States.  While soy oil is readily available and used for fuel, it is a substantially dryer oil than others and has a greater chance of producing polymerization. It has a much higher Iodine value and because it is such a dry oil, it can cause damage if the proper criteria is not met. Currently we do not know of any testers to test for IV (Iodine value) so all safeguards must be taken to insure proper and complete combustion. The most important of this criteria is oil temperature for better atomization of the fuel. Hence the chart. Again, we cannot stress strongly enough the importance of heating the vegetable oil to a proper and safe temperature. What temperature is your oil?