Effect of Piston Crevices on 3D Simulation of a Heavy-Duty Diesel Engine Retrofitted to Natural Gas Spark Ignition

3D CFD IC engine simulations that use a simplified combustion model based on the flamelets concept can provide acceptable results with minimum computational costs and reasonable running times. More, the simulation can neglect small combustion chamber details such as valve crevices, valve recesses and piston crevices volume. The missing volumes are usually compensated by changes in the squish volume (i.e., by increasing the clearance height of the model compared to the real engine). This paper documents some of the effects that such an approach would have on the simulated results of the combustion phenomena inside a conventional heavy-duty direct-injection CI engine, which was converted to port-fuel injection SI operation. 3D IC engine simulations with or without crevice volumes were run using the G-equation combustion model. A proper parameter choice ensured that the simulation results agreed well with the experimental pressure trace. The results show that including the crevice volume affected the mass of unburned mixture inside the squish region, which in turn influenced the flame behavior and heat release during late-combustion stages.