Prediction of carbon monoxide and hydrocarbon emissions in iso-octane HCCI engine combustion using multizone simulations

Homogeneous charge compression ignitions (HCCI) engines show promise as an alternative to Diesel engines, yet research remains; development of practical HCCI engines will be aided greatly by accurate modeling tools. A novel detailed chemical kinetic model that incorporates information from a computational fluid mechanics code has been developed to simulate HCCI combustion. This model very accurately predicts many aspects of the HCCI combustion process. High-resolution computational grids can be used for the fluid mechanics portion of the simulation, but the chemical kinetics portion of the simulation can be reduced to a handful of computational zones. (For all previous work, 10 zones have been used.) While, overall, this model has demonstrated a very good predictive capability for HCCI combustion, previous simulations using this model have tended to underpredict carbon monoxide emissions by an order of magnitude. A factor in the underprediction of carbon monoxide may be that all previous simulations have been conducted with 10 chemical kinetic zones. The chemistry that results in carbon monoxide emissions is very sensitive to small changes in temperature within the engine. The resolution in temperature is determined directly by the number of zones. This paper investigates how the number of zones (i.e., temperature resolution) affects the model's prediction of hydrocarbon and carbon monoxide emissions in an HCCI engine. Simulations with 10, 20, and 40 chemical kinetic zones have been conducted using a detailed chemical kinetic mechanism (859 species, 3606 reactions) to simulate an iso-octane-fueled HCCI engine. The results show that 10 zones are adequate to resolve the hydrocarbon emissions, but a greater number of zones is required to resolve carbon monoxide emissions. Results are also presented that explore spatial sources of the exhaust emissions within the HCCI engine combustion chamber.