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

Gasoline fuel losses to the crankcase, fuel/oil interactions and fuel retur n as unburned hydrocarbons (HCs) in the crankcase ventilation flow and reci rculated oil films have been investigated for a contemporary spark ignition (SI) engine. The results of engine tests under motored and firing conditio ns have been used to develop a detailed description of the fuel transport p ath. This has then been applied to predict the level and rate of fuel accum ulation in the oil sump. Fuel transported past the piston with blow-by is t he major source of HCs in the crankcase ventilation flow. During cold opera tion, a large proportion of this is retained by oil films along the blow-by path, thus contributing to bulk oil dilution. At elevated engine temperatu res, 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 desorpti on into the crankcase gas, which makes a further small contribution to the HC concentration in the ventilation flow, and desorption from oil recircula ted to the cylinder wall films. Crankcase ventilation flow generally accoun ts 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 t o feedgas HC emissions. The evidence strongly suggests that the in-cylinder desorption primarily involves the heavier fractions of the injected fuel.