A 3D-Simulation with Detailed Chemical Kinetics of Combustion and Quenching in an HCCI Engine 2008-01-1655
A 3D-CFD model with detailed chemical kinetics was developed to investigate the combustion characteristics of HCCI engines, especially those fueled with hydrogen and n-heptane. The effects of changes in some of the key important variables that included compression ratio and chamber surface temperature on the combustion processes were investigated. Particular attention was given, while using a finer 3-D mesh, to the development of combustion within the chamber crevices between the piston top-land and cylinder wall. It is shown that changes in the combustion chamber wall surface temperature values influence greatly the autoignition timing and location of its first occurrence within the chamber. With high chamber wall temperatures, autoignition takes place first at regions near the cylinder wall while with low surface temperatures; autoignition takes place closer to the central region of the mixture charge. The presence with n-heptane of some hydrogen, methane, or carbon monoxide is shown to slow down the reaction rates delaying to varying extents the onset of autoignition and increasing the overall equivalence ratio and the resulting knock free power output. It was also found that increasing the combustion chamber wall surface temperature reduces the extent of flame quenching and emissions. Through analysis of the temporal and spatial distribution variations in both the fuel and oxygen concentrations and temperature within the mixture, the extent of quenching of combustion within the piston crevices was determined. The contribution of such quenching to emissions and overall fuel consumption are to be highlighted and discussed.