Modeling of HC Emissions Due to Crevice Flows in Premixed-Charge Engines 892085
Multi-dimensional computations were made to assess the effect of crevice flows through piston-cylinder-ring crevices on combustion and engine-out hydrocarbon emissions. The computations were made using the KIVA code with a characteristic time combustion sub model that accounts for laminar kinetics and turbulent-mixing effects. The crevice-flow boundary conditions were specified using a phenomenological crevice-flow model. A central-ignition pancake-chamber engine was considered, and the effects of top-land crevice design and engine operating condition were examined.
The computed peak cylinder pressure was found to be 6 to 8 percent lower in cases with crevice flow than without because the crevice flow reduced the effective in-cylinder charge mass by similar percentages during the main stages of combustion. However, the details of combustion were essentially unchanged by the crevice flow. The amount of engine-out unburned fuel was found to be determined by competing factors such as the strength of the annular wall jet that emerges from the top-land crevice during the expansion and exhaust strokes, the fuel oxidation rate, the detailed history of the crevice flow, including blow by, and the exhaust-valve position and timing. Engine-out unburned fuel increased with increasing engine load and increasing top-land crevice gap and was relatively insensitive to the top-land crevice volume. The engine-out unburned fuel increased with decreasing speed and decreasing mixture strength and could be reduced by minimizing the crevice flow that re-enters the combustion chamber during the later stages of the expansion stroke, when low gas temperature leads to low fuel-oxidation rates.