Cyclic variations of in-cylinder pressure and turbulent flame propagation in an optical direct-injection spark-ignition engine

The direct injection spark ignition (DISI) engines, while showing the extended controllability in operation conditions over port-injection engines, have a cyclic variation issue. For the past decade, many studies have been conducted to identify the causing factors of the cyclic variation and mitigate its negative effects. However, the fundamental knowledge of cyclic variation in DISI engines is still lacking. This experimental study measures cyclic variations in the in-cylinder pressure, early-stage burn duration and flame growth rate in a single-cylinder optical DISI engine and evaluates their correlations. While the in-cylinder pressure was recorded by the pressure sensor mounted in the spark plug, a high-speed camera was used to capture the flame propagation for over 100 individual cycles. The results show significant cyclic variations in both pressure derived data and flame image derived data; however, a clear positive correlation is observed between the burn duration and flame growth rate. Further analysis of the flames indicates higher level of flame front wrinkling measured in some individual cycles is a cause of the higher flame growth rate.