Investigation of the Source of Increased Hydrocarbon Emissions Over the Life Cycle of Small Utility Engines

Small utility engines typically experience an increase in hydrocarbon emissions as they age. This is likely due to the formation of combustion chamber deposits (CCD) in the cylinders. The CCD originate primarily from oil entering the cylinder. This paper presents results from a study designed to determine the relative importance of various oil introduction mechanisms to the increase in hydrocarbon emissions and CCD formation. In the engines studied, it was found that early in the life cycles, approximately 20% of the increase is due to oil entering through the breather element, and approximately 45% is due to oil entering down the valve stems. The remaining increase is attributable to oil bypassing the piston rings and fuel effects.