Fuel losses to the crankcase and hydrocarbon return with recirculated oil and ventilation flow

Abstract

Gasoline fuel losses to the crankcase, fuel/oil interactions and fuel return as unburned hydrocarbons (HCs) in the crankcase ventilation flow and recirculated oil films have been investigated for a contemporary spark ignition (SI) engine. The results of engine tests under motored and firing conditions have been used to develop a detailed description of the fuel transport path. This has then been applied to predict the level and rate of fuel accumulation in the oil sump. Fuel transported past the piston with blow-by is the major source of HCs in the crankcase ventilation flow. During cold operation, a large proportion of this is retained by oil films along the blow-by path, thus contributing to bulk oil dilution. At elevated engine temperatures, this absorption mechanism becomes considerably less effective, and the majority of blow-by fuel is transported directly through to the crankcase vent. The return of fuel hydrocarbons from the oil takes place via desorption into the crankcase gas, which makes a further small contribution to the HC concentration in the ventilation flow, and desorption from oil recirculated to the cylinder wall films. Crankcase ventilation flow generally accounts for the majority of the desorbed fuel, which is then recirculated to the cylinders and assimilated into the fresh charge. Fuel desorption from the liner oil films consists of smaller flowrates but can contribute directly to feedgas HC emissions. The evidence strongly suggests that the in-cylinder desorption primarily involves the heavier fractions of the injected fuel.