The Case for Conversion
By Rick Da Tech
Vegetable oil will absolutely burn in diesel engines. Study after study finds no short-term effects from adding up to 50% vegetable oil to diesel fuel. The long-term studies, however, indicate engine life is shortened due mostly to carbon buildup in the engine.
Modern diesel engines use fuel injection systems designed to deliver and properly atomized diesel fuel with a viscosity of approximately eight centistokes. Common vegetable oils are 10-20 times more viscous (thick) and cannot properly atomize in stock systems. Thicker more viscous fuels have larger droplets out of the injectors than fuels with the correct viscosity. These larger droplets do not burn completely. Indirect injection (IDI) engines have a little pre-combustion chamber where the fuel gets sprayed first before the fireball expands into the cylinder. Big droplets of fuel just splatter against the very hot walls of the pre-combustion chamber, vaporize and eventually burn completely. Direct Injection (DI) engines run the risk of accumulating partially-burnt fuel on the cylinder walls. If the fuel droplets are too big to finish burning before hitting the cylinder walls, the oil can stick to the cylinder wall because the cylinder walls are not hot enough to vaporize it like is done in the pre-combustion chamber. The rings scrape off this partially-burnt goo, which bakes itself into the ring grooves, gluing the rings in place. It is a process called "piston ring coking." The result of piston ring coking is a loss of compression and cylinder wall scoring if left uncorrected. The only way to repair the damages is to tear down and re-ring the engine.
Ring coking can allow fuel to blow by the seized rings diluting the crankcase oil with fuel. Once in the crankcase, vegetable oil polymerizes making the lubricating oil thicker and ineffective. The result is a catastrophic engine failure. Loss of lubrication has been known to cause loud knocking noises followed by the appearance of large holes in the engine block.
Carbon can also build up on injectors, clogging them causing fuel to dribble out of the injector, not completely shutting off and resulting in poor spraying patterns. Poor spraying patterns cause incomplete combustion. Fuel can also drip from a partially clogged injector at the wrong time. Fuel dripping from the nozzle can cause hot spots on the piston head which can lead to a damaged piston or even a hole in the piston. The fuel can seep past the piston and mix with the lubricating oil, diluting it and interfering its ability to lubricate critical engine components. With ultra-high pressure direct injection systems, it can be nearly impossible to repair an injector once it has clogged up with carbon buildup.
The increased stresses caused by pumping cold vegetable oil to high pressures can also damage injector pumps. The injector pump is designed to pressurize “diesel” viscosity liquids and pump them towards the injectors. Since cold vegetable oil is thicker than diesel, the pump sees higher stresses. If it is a CAV pump, like a Land Rover has, it uses a cam ring to work the injection pistons. Higher stresses can result in cracking the cam ring, after a while. Some vehicles like the Mazda pickup use a stronger Japanese-licensed version of the most common Bosch IP. Your engine might also use the Bosch “rotary” pump. The other (and more desirable) kind of IP is the Bosch in-line pump, as commonly found on Mercedes, Caterpillar, Nissan, and Toyota. They use four rotating cams to push four injection pistons, one for each cylinder and are usually considered bomb-proof.
Filter clogging is also a problem with using vegetable oil as a fuel in an unmodified engine. Saturated fats and oils have a higher melting point than unsaturated oils. When cold saturated oils hit a cold filter, they catch the saturated oils in the filter element causing it to clog resulting in the engine leaning out and sputtering. When the engine is cold, the coking accelerates. Combustion is even more incomplete than when the engine is hot, making it hard to crank, produce less power and smoke excessively.
As an emergency fuel, vegetable oil in an unmodified engine is a good choice, but as a regular fuel, it shortens the engine life making teardown and rebuilds of the engine more frequent. The good thing is that all of these “problems” are addressed by heating the fuel and thereby reducing its viscosity. To reduce vegetable oil’s viscosity requires modifying your fuel system. See the Related Links section below for companies and forums that specialize in WVO fuel systems for your diesel engine.