Development and Application of an Improved Ring Pack Model for Hydrocarbon Emissions Studies 961966
Because only the unburned gases in the crevices can contribute to hydrocarbon emissions, a model was developed that can be used to determine the temporal and spatial histories of both burned gas and unburned gas flow into and out of the piston-liner crevices. The burned fraction in the top-land is primarily a function of engine design. Burned gases continue to get packed into the inter-ring volume until well after the end of combustion and the unburned fuel returned to the chamber from this source depends upon both the position of the top ring end gap relative to the spark plug and of the relative positions of the end gaps of the compression rings with respect to each other. Because the rings rotate, and because the fuel that returns to the chamber from the inter-ring crevice dominates the sources between BDC and IVO when conditions are unfavorable to in-cylinder oxidation, these represent two sources of variability in the HC emissions. A model for unburned fuel emissions due to exhaust valve leakage is included as is a model for the contributions by the head crevices (head gasket, spark plug, and valve seat crevices). At EVO, the fuel remaining in the exhaust valve seat crevice gets swept into the exhaust producing a spike of unburned fuel emissions of several thousand ppm for a few crank angles. At IVO, the fuel remaining in the intake valve seat gets swept into the backflow and, for the present conditions, does not return to the cylinder until after EVC. Predictions are made for a variety of operating conditions for the 2-valve engine that was the subject of extensive hydrocarbon emissions measurements by Kaiser and coworkers at Ford. Comparisons between the model predictions and the experimental measurements spatially-resolved at the cylinder exit - of unburned fuel and non-fuel HCs are used to draw conclusions regarding the effects of operating conditions and fuel structure on the two sources of unburned fuel, the contributions by the other sources, the fraction retained, the fraction totally oxidized, and the fraction partially oxidized.