Knock is one of several different well-known abnormal combustion processes in spark-ignition engines; most of these abnormal combustion issues have been addressed with improved design or control schemes as well as fuel specifications. However, emerging advanced SI engines—e.g., those with turbocharging and direct injection—can exhibit a new phenomenon: super-knock, or violent engine knock.

Super-knock can damage engine components within only a few cycles. Because the events are sporadic and uncontrolled, it has been difficult to assess the causes and devise solutions.

In a new paper in the journal Fuel, a team from Tsinghua University has used optical diagnostics to analyze the progression from pre-ignition to super-knock. This work is a follow-on to the team’s earlier study distinguishing between pre-ignition and super-knock.

By utilizing a rapid compression machine, events of pre-ignition and super-knock in a closed system under high temperature and high pressure were captured by synchronous high-speed direct photography and pressure measurement, with the results demonstrating that the mechanism of super-knock is constituted by hotspot-induced deflagration to detonation followed by high-pressure oscillation (DDP).