Abstract Flame-induced auto-ignition (FIAI), where spark-triggered flame propagation causes auto-ignition without engine knock, is one of the promising high-efficiency combustion modes. FIAI and SI knocking combustion have several similarities; however, the former is beneficial to thermal efficiency, and the latter should be avoided for engine safety. This study investigates the flame structures, intermediate species and pressure evolution during FIAI and SI knocking combustion using an optical engine with ultra-high speed imaging of CH2O and OH chemiluminescence. The flame structures are analyzed together with cylinder pressure and intermediate species to understand their dynamic interactions. Findings reveal that auto-ignition in FIAI begins from the outer rim of the SI flame front; the reaction front propagates at a speed of approximately 160m/s, which is more than one order higher than the speed of normal flame propagation. The reaction front propagates in subsonic speed. The propagation regime changes from normal flame propagation to reaction front propagation at approximately 795K for n-heptane. The temperature of the unburned zone and the remaining energy are the key factors for the occurrence of knocking; they also differentiate FIAI from typical knocking.