Initial Estimation of the Piston Ring Pack Contribution to Hydrocarbon Emissions from a Small Engine

The contribution to the engine-out hydrocarbon (HC) emissions from fuel that escapes the main combustion event in piston ring crevices was estimated for an air-cooled, V-twin utility engine. The engine was run with a homogeneous pre-vaporized mixture system that avoids the presence of liquid films in the cylinder, and their resulting contribution to the HC emissions. A simplified ring pack gas flow model was used to estimate the ring pack contribution to HC emissions; the model was tested against the experimentally measured blowby. At high load conditions the model shows that the ring pack returns to the cylinder a mass of HC that exceeds that observed in the exhaust, and thus, is the dominant contributor to HC emissions. At light loads, however, the model predicts less HC mass returned from the ring pack than is observed in the exhaust. Time-resolved HC measurements were performed and used to assess the effect of combustion quality on HC emissions. It was shown that the both the HC concentration and the HC mass emitted per cycle, from a single-cycle gas flow model, correlated with low IMEP and late combustion phasing. A statistical analysis showed that the correlation was significant. Thus, at light load incomplete combustion was found to be a significant contributor to engine-out emissions.