LAUSR

Abstract Engine knock essentially involves the interplay between end-gas auto-ignition and primary flame propagation. However, literature shows inconsistent correlations between turbulent flame speed and knocking intensity. In this study, different levels of turbulent intensity were achieved by modifying in-cylinder swirl ratios, and the effect of turbulent flame speed on knocking characteristics was firstly investigated by using an optical engine under critical knocking conditions. In-cylinder swirl ratios were controlled by a swirl control valve (SCV) and particle image velocimetry (PIV) experiments were employed to quantify the turbulent flow velocity under different swirl ratio conditions. High-speed photography and instantaneous pressure acquisition were synchronously employed for auto-ignition and knocking measurement. The experimental results show that end-gas auto-ignition is a sufficient condition for engine knock and engine knock is strongly associated with the peak heat release rate when auto-ignition occurs. Under critical knocking conditions, fast turbulent flame propagation always results in advanced auto-ignition timing and concentrated heat release and thereby severe auto-ignition. Consequently, the higher flame speed promotes heavier knocking combustion under enhanced turbulent intensity conditions. The current study shall provide useful insights into the nature of engine knock and the regulation of engine combustion.