Effect of Ring Dynamics and Crevice Flows on Unburned Hydrocarbon Emissions

A significant source of unburned hydrocarbon emissions from internal combustion engines originates from the flow of unburned fuel/air mixture into and out of crevices in the piston-cylinder-ring assembly. During compression, fuel vapor flows into crevice regions. After top dead center, the trapped fuel vapor that returns into the cylinder escapes complete oxidation and contributes to unburned hydrocarbon emissions. In this work, the crevice flow model developed by Namazian and Heywood is implemented into KIVA-II, a multidimensional, reacting flow code. Two-dimensional, axisymmetric simulations are then performed for a 2.5 liter gasoline engine to investigate the effects of engine speed and selected piston-ring design parameters on crevice flows and on unburned hydrocarbon emissions. Results suggest that engine-out unburned hydrocarbon emissions can be reduced by optimizing the ring end gap area and the piston-cylinder side clearance.