Ring Pack Crevice Effects on the Hydrocarbon Emissions from an Air-Cooled Utility Engine 2008-32-0004
The effect of the ring pack storage mechanism on the hydrocarbon (HC) emissions from an air-cooled utility engine has been studied using a simplified ring pack model. Tests were performed for a range of engine load, two engine speeds, varied air-fuel ratio and with a fixed ignition timing using a homogeneous, pre-vaporized fuel mixture system. The integrated mass of HC leaving the crevices from the end of combustion (the crank angle that the cumulative burn fraction reached 90%) to exhaust valve closing was taken to represent the potential contribution of the ring pack to the overall HC emissions; post-oxidation in the cylinder will consume some of this mass. Time-resolved exhaust HC concentration measurements were also performed, and the instantaneous exhaust HC mass flow rate was determined using the measured exhaust and cylinder pressure. At high load the model predicts that the ring pack returns approximately three times as much HC mass to the cylinder as is measured in the exhaust, indicating that the HC emissions are dominated by the ring pack contribution. At the lightest load condition tested, the ring pack model predicts significantly less mass returning to the cylinder from the ring pack than is observed in the exhaust, clearly indicating that another HC mechanism is significantly contributing to the exhaust HC emissions. The integrated exhaust HC mass from the time-resolved HC measurement was found to correlate inversely with the IMEP on a cycle-by-cycle basis for light load condition, which strongly suggest that incomplete combustion is materially contributing to the exhaust HC emissions. The intermediate load condition represents a combination of these two extremes. The ensemble-average ring pack model results indicate that the mass returned to the cylinder from the ring pack is slightly higher than the amount measured in the exhaust. But, a conditional sampling analysis indicates that there are sub-groups, i.e. late-burning cycles, for which this is not true. There is expected to be some in-cylinder post-oxidation of the ring pack HC mass at this condition, and the late burning cycles were not found to excessively contribute to the HC emissions, which both strongly suggests that there are other mechanisms besides the ring pack that are significantly contributing to the HC emissions at this condition. The most likely mechanism is incomplete combustion.