Effects of Fuel Absorbed in Oil Film on Unburnt Hydrocarbon Emissions from Spark Ignition Engines : Influence of Oil added on Piston Crown on Total Hydrocarbon Concentration in Exhaust Gas

Absorption and desorption of fuel by motor oil that coats the side wall of the cylinder with a thin oil film is thought to be one of the important sources of unburnt hydrocarbon emissions from spark ignition engines. In order to determine the effect of oil layers on exhaust hydrocarbon emissions, the total hydrocarbon concentration (THC) in the exhaust gas is measured by a flame ionization detector when a constant amount of motor oil or castor oil is placed on the piston crown. Experiments show that the THC in the exhaust gas increases in proportion to the amount of added oil, and that the addition of castor oil causes a higher increase of the THC than the addition of motor oil. A numerical model which describes the process of fuel absorption and desorption in the additional oil layer on the piston crown is developed. Calculated results confirm that the increase of measured THC occurs because the absorbed fuel in the oil layer on the piston crown is released in the cooling cylinder gas during the expansion and exhaust strokes.