In this paper the effect of in-cylinder crevices formed by the piston cylinder clearance, above the first ring, and the spark plug cavity, on the entrapment of unburned fuel air mixture during the late compression, expansion and exhaust phases of a spark ignition engine cycle, have been simulated using the Computational Fluid Dynamic (CFD) code KIVA II. Two methods of fuelling the engine have been considered, the first involving the carburetion of a homogeneous fuel air mixture, and the second an attempt to simulate the effects of manifold injection of fuel droplets into the cylinder.
The simulation is operative over the whole four stroke engine cycle, and shows the efflux of trapped hydrocarbon from crevices during the late expansion and exhaust phases of the engine cycle.
The simulated data have been evaluated against experimental measurements of unburned hydrocarbon (HC) obtained with a rapid sampling valve and fast response ionisation detector for two locations, one in the cylinder and the other in the exhaust port of the engine. The manifold injection option results in late combustion due to poor mixing and evaporation of fuel droplets, thereby increasing both in-cylinder and exhaust hydrocarbon concentrations. Fuel droplets were observed to be trapped on the exhaust side of the topland crevice even after exhaust valve opening (EVO). The overall trends predicted in the simulation study are similar to those observed in an experimental study conducted elsewhere, though quantitative agreement is less satisfactory. Areas in which the modelling might be improved have been identified.