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Internal combustion engines have been required to achieve even higher thermal efficiency and cleaner exhaust emissions in recent years in order to comply with increasingly tighter environmental regulations every year owing to global warming and other environmental issues. Another factor involved here is that global energy demands have prompted a quest for alternatives to liquid fuels such as gasoline, diesel fuel and other petroleum-derived fuels. Homogeneous Charge Compression Ignition (HCCI) engines, featuring higher compression ratios and uniform, lean combustion, are a promising technology for improving the efficiency and reducing the emissions of internal combustion engines. However, it is difficult to control the ignition timing of HCCI engines[1],[2] because they lack any physical means of controlling ignition.

This study focused on autoignition behavior and knocking characteristics. Using an optically accessible engine, autoignition behavior was observed over the entire bore area, and the relationship between autoignition behavior and knocking characteristics was clarified on the basis of visualized combustion images and frequency analysis of the in-cylinder pressure waveform. In addition, chemical kinetic simulations were used to investigate the effects of different fuel chemical compositions on combustion and autoignition characteristics under equivalent octane ratings. The results showed that the rate of autoignition development has a significant effect on knocking intensity. In addition, the ρ1,0 mode is the dominant vibration mode caused by knocking, regardless of the location of autoignition. It can be inferred that strong knocking is caused by multiple vibration modes.