Hydrocarbon Emissions of SI Engines as Influenced by Fuel Absorption-Desorption in Oil Films

In-cylinder absorption and desorption of fuel by lubricating oil films is assumed to be among the main sources of unburned hydrocarbon emissions from spark ignition engines. As this phenomenon is mainly conditioned by the solubility of fuel in lubricant, differences in hydrocarbon emission levels were measured running an automotive engine, with various couples of singular constituent fuels and lubricants. In this way it was found that lowering the solubility by a factor of 40 caused an average HC emission reduction of 30%.

It was found too that with commercial unleaded fuels the lubricant constitution had little influence on the emissions. As this fact was difficult to explain, and because the gasoline solubility values are not available, a single cylinder engine with an extended piston was specially designed to run with or without oil between the cylinder liner and the piston rings. The experimental method has been validated using an insoluble gaseous fuel (propane). Other experiments carried out with this set-up confirmed the results obtained with the automotive single cylinder engine, and showed once again the relatively low interaction between commercial fuels and lubricants.