A Study on the Performance of Combustion in a HCCI Engine Using n-Heptane by a Multi-Zone Model

A study of n-heptane combustion in an HCCI engine was carried out by a multi-zone numerical simulation that covers a complete engine cycle. A reaction mechanism that includes 177 chemical species and 1638 reactions was used. The results of the numerical simulations were compared to existing experimental data for a range of air/fuel ratios, compression ratios and engine speeds. It is shown that the numerical simulation is able to reasonably capture the experimental cylinder pressure data over a wide range of operation conditions. It also provides a qualitative trend of CO emissions. The numerical simulation overpredicted the combustion at some operating conditions, such as at extremely high air/fuel ratios and higher engine speeds. Some differences were observed between the experimental and numerical data for NO_X emissions. The numerical simulation predicted a monotonic decrease in NO_X emissions as air/fuel ratio increased or compression ratio decreased, while an increase in NO_X emissions was observed experimentally when combustion became very weak at extremely high air/fuel ratios or low compression ratios. It is suggested that further experiments and numerical simulations should be performed to explain this discrepancy.