Modeling the effects of geometry generated turbulence on HCCI engine combustion

The present study uses a numerical model to investigate the effects of flow turbulence on premixed iso-octane HCCI engine combustion. Different levels of in-cylinder turbulence are generated by using different piston geometries, namely a disc-shape versus a square-shape bowl. The numerical model is based on the KIVA code which is modified to use CHEMKIN as the chemistry solver. A detailed reaction mechanism is used to simulate the fuel chemistry. It is found that turbulence has significant effects on HCCI combustion. In the current engine setup, the main effect of turbulence is to affect the wall heat transfer, and hence to change the mixture temperature which, in turn, influences the ignition timing and combustion duration. The model also predicts that the combustion duration in the square bowl case is longer than that in the disc piston case which agrees with the measurements. The results imply that it is preferable to incorporate detailed chemistry in CFD codes for HCCI combustion simulations so that the effect of turbulence on wall heat transfer can be better simulated. On the other hand, it is also found that the onset of combustion is also very sensitive to the initial conditions so that an accurate estimate of initial mixture conditions is essential for combustion simulations.