LO 10-7 Outline lubrication system maintenance and service procedures

Engine Oil Contamination with Biodiesel

For years biodiesel has been considered a fuel lubricity additive to keep moving parts operating smoothly inside fuel injection systems. The dilution of engine oil with biodiesel is another matter. Biodiesel promotes fuel dilution of engine oil to a much greater extent than mineral diesel fuel. Oil dilution with biodiesel occurs when a common rail fuel system uses a postcombustion injection of fuel into the cylinders to regenerate the particulate trap or NOx absorber. All the fuel is expected to vaporize in the cylinder and not combust until it reaches the exhaust catalysts. Heavier, less volatile fractions of fuel, however, do not vaporize during postcombustion injection, and liquid fuel clings to the cylinder walls before it is scraped into the crankcase.

Because biodiesel has a higher distillation temperature and boiling point, it does not burn away or boil out of engine oil. The result is a tendency to dilute the engine oil disproportionately to its blend ratio in the fuel. Biodiesel blends higher than B20 (20% biosource) cause larger amounts of unburned fuel to slip by the piston rings. Fuel passes by the pistons and accumulates in the crankcase. Once there, fuel polymerizes (joins together to form long, complex-shaped molecules) with oil, producing thick sludge. Engine damage can result from oil passageways being blocked by sludge. Deposits also form in the ring belt area, causing the rings to stick and crankcase blowby to increase, which further aggravates oil contamination with increased levels of soot.

Raw or refined vegetable oils produce the greatest harm, which may not become evident until a significant amount of damage has occurred over an extended period. Engine oil change intervals may need to be shortened significantly if using blends of biodiesel higher than B20. Note that detergents and specific antiwear additives do not work as well when oil is diluted with biodiesel either.