Analysis of Oil Sump Contamination with Ethanol in Flex Fuel Engines after Crank and Warm-up

A flex fuel engine which operates with high ethanol content fuel can show a significant level of oil sump contamination with ethanol depending on startup coolant temperature and driving behavior. In general, the lower the coolant and the more aggressive the driving pattern the higher the oil sump contamination. As the engine warms up and the oil temperature increases, the liquid ethanol mixed in oil sump evaporates (boil-off phenomenon) and returns into the induction system through the engine venting system. Oil contamination and boil-off phenomena have to be properly considered by the engine management controller so that closed loop fuel control, fuel trim diagnosis and ethanol learning are not fooled by the extra fuel coming by blow-by. This paper presents the results of an analysis conducted in a GMPT 1.4L Flex Fuel engine running with E85 and E100 fuels. A method for quantifying the different levels of oil contamination is proposed based on measured exhaust air-fuel ratio during oil decontamination. A correlation is developed between the unburned fuel during cold engine operation and the vaporized ethanol during boil-off. It is shown that warm-up driving mode is more important for oil sump contamination than the engine crank. Therefore, an oil contamination algorithm should be capable of identifying the differences in driving behavior in order to provide a consistent contamination index and prepare the fuel injection management system for the corrective actions that should be taken during boil-off.