Vegetable oil contains a large amount of hydrocarbons and on a molecular level is very similar to diesel fuel. Because of this, it should be no surprise that a diesel engine can run on straight vegetable oil (SVO). There are a few car companies that produce diesel vehicles, including Ford, Audi, Jeep, Volvo and Volkswagen. If you search the net there are hundreds of pages discussing using vegetable oil as a source of fuel on all these vehicle types. In March 2009 I purchased a 2001 Golf TDI that ran vegetable oil for about 40,000 miles. There are a number of sites that say running a direct injection style diesel (Like TDI VWs) on vegetable oil will destroy the engine. I decided to write this page to illustrate exactly what can happen to a VW TDI when it is run on vegetable oil.
The first thing to mention about SVO is that it has a higher viscosity than diesel. At room temperature, SVO has a viscosity about six times times greater than diesel. If high viscocty SVO is pumped into the engine of a TDI there can be dramatic and serious consequences. First, diesel fuel filters are designed to filter out particulates to a size of 5-10 microns. This is smaller than cold SVO so most likely the filter will clog and the engine will stall and the first part of the fuel system will need to be cleaned out. If a larger filter (or no filter) is used and high viscosity SVO gets to the engine it can clog up the fuel injector pump and fuel injectors. Since the SVO will not spray into the cylinder chamber correctly (if at all) carbon deposits will build up in the cylinder.
To overcome the high viscosity of SVO many people heat the oil to the temperature of the running engine; about 190 degrees Fahrenheit. At 190F SVO still has a higher viscosity than diesel fuel, however it will flow through most modern fuel systems. While not as sudden as cold SVO, the same damage to an engine can happen with warm SVO. Engine parts are designed to operate within certain limits including fuel viscosity. Since even warmed SVO is thicker than diesel the injector pump that feeds high pressure fuel to the engine can become clogged up with SVO and other deposits. Diesel injectors are also designed with tight tolerances. Small holes in the tip of injectors (as small as 0.170″) are designed to spray diesel in a specific pattern that will ensure most of the fuel will be burned. When warm SVO is pushed through these holes it is almost guaranteed that the spray pattern will not be ideal. A non-ideal fuel spray will result in incomplete combustion and cause sooty carbon deposits to form in the cylinder and exhaust system.
[Stopping an engine on SVO]
Pretty much, when SVO is used in a TDI engine carbon deposits can form. Now some might wonder what can happen because of carbon build up in the cylinder or exhaust system. Filters and injectors are not the only precision systems in an engine, the pistons that drive the engine crank shaft are designed to fit snugly in the cylinder. To achieve this snug fit there are rings that move along the cylinder walls with the piston. One of these rings is an oil ring which is designed to allow some oil to reach the other rings and lubricate them. Carbon deposits in the cylinder can clog up this oil ring and starve the piston of much needed lubrication. Without this lubrication heat will build up from the friction of the rings on the cylinder walls. The heat and constant motion of this un-lubricated friction can gouge the walls of the cylinder which destroys the tight seal the piston rings provide. As one can assume when you loose the pressure seal in the cylinder the propulsion force of the fuel combustion is decreased and the vehicle looses power. Pretty much the only way to fix this problem is to remove, disassemble the engine and bore the cylinders in the block larger. This requires new parts like pistons, a lot of work and most likely a lot of money.
Loss of power from combustion “blow by” caused by score cylinder is not the only problem that can plague an engine. Lets say only one cylinder is damaged and the loss of power goes un-noticed. The combustion blown by the pistons causes gases and unburned fuel to enter the oil pan and mix with the engine oil lubricant. It has been well documented that when vegetable oil is mixed with engine oil, the SVO polymerizes the oil causing it to thicken. This thickening of the oil robs the entire rest of the engine from lubrication causing untold damage almost anywhere where there are moving parts.
To compound the problem of a loss of compression, carbon deposits don’t only form on pistons and rings, they can build up on the exhaust valves and exhaust port. Since all burned fuel passes the exhaust valves, large amounts of carbon build up can form, upwards of a quarter of an inch. This build up can also rob an engine of power since less exhaust is removed from the engine due to the smaller diameter of the exhaust port.
How do you avoid these nightmares?
There is a lot of discussion on how to lower the viscosity of SVO to the point of diesel. One method is to heat SVO used as fuel to about 220F using an in-line fuel heater. Other methods include blending SVO with some diesel or kerosene. Both these methods will be discussed deeper at a later date.