Effects of enhanced in-cylinder flow on knock reduction in a direct injection spark-ignition engine

This study experimentally investigates the effects of intake valve closing (IVC) timing and start of injection (SOI) on in-cylinder flow structures, turbulence characteristics, and knock possibility in a spark-ignition (SI) engine equipped with a continuously variable valve duration (CVVD) system. Miller cycle operation was realized using early and late intake valve closing (EIVC and LIVC) strategies, and optical engine experiments were combined with computational fluid dynamics (CFD) simulations to analyze the interaction between valve timing control and single-injection strategies. The results show that under EIVC conditions, the early intake flow was intensified due to the increased valve opening rate; however, the shortened intake duration weakened tumble sustainability and turbulent kinetic energy (TKE) near the end of the compression stroke compared to LIVC conditions. Depending on injection timing, the spray–flow interaction exhibited either counter-flow or co-flow characteristics. Early start of injection (ESOI, BTDC 300–280) mainly formed counter-flow, resulting in limited tumble enhancement, whereas late start of injection (LSOI, BTDC 270–240) promoted co-flow and effectively strengthened the tumble motion. Under LIVC conditions, excessive injection retard strengthened the initial tumble prior to valve closing but induced reverse flow toward the intake during the subsequent compression process, resulting in a loss of effective rotational momentum. This led to reduced TKE near ignition and increased knock possibility, indicating the existence of an optimal injection timing.