Understanding the correlation between auto-ignition, heat release and knocking characteristics through optical engines with high compression ratio

Abstract

Improving compression ratio is an effective way to meet higher thermal efficiency and lower CO2 emissions for internal combustion engines. However, the severe thermodynamic condition under low speed and high road may promote the possibility of auto-ignition and damage engine body if it is uncontrollable. Therefore, further study on the correlation between auto-ignition and knocking characteristics are necessary for modern engines with high compression ratio. In this study, combined with high-speed direct photography and simultaneous pressure acquisition, flame images and in-cylinder pressures characterizing auto-ignition, heat release rate and knocking combustion were studied based on optical engine experiments. The results show that auto-ignition occurs in all engine cycles under current critical knocking conditions, and fast SI flame speed cycles is more likely to facilitate end-gas auto-ignition. However, the auto-ignition flame speed is comparable to the SI flame and auto-ignition is not a sufficient condition for knock: for some cycles multipoint auto-ignitions occur near the SI flame front and result in two peak heat release rate, which is beneficial to engine thermal efficiency. Further analysis shows that there is no obvious pertinence between knock intensity and auto-ignition onset while knock intensity strongly depends on the peak HRR per unit volume. High peak HRR per unit volume always results in knocking combustion. The current study shall give insights into utilizing auto-ignition in high compression ratio SI engines.