The Study of the Fundamental Characteristics of Tumble in a Spark-Ignition Engine via Numerical Analysis 2021-01-0408

A spark-ignition engine commonly induces tumble flow because it generates high turbulence, which is a crucial factor in determining the flame propagation speed. Since tumble affects not only the flame propagation speed but also the various in-cylinder phenomena, it predominantly determines the performance of the engine. In that sense, many studies have been conducted to investigate tumble. Although various studies have revealed the characteristics of tumble numerically and experimentally, there has been no research to identify the physical mechanisms of these characteristics. Although some studies specified the mechanisms from an angular momentum perspective, the theory was insufficient to explain the entire phenomena of tumble. Hence, this study attempts to comprehend the fundamental causes of tumble phenomena such as ‘spinning up’ and ‘vortex breakdown’ from the perspective of kinetic energy. The movement characteristics of the tumble center during the compression stroke are also identified. Although this study addresses the formation of tumble, it primarily focuses on the compression stroke, when the influence of the piston on tumble is significant. To simplify this analysis work, the in-cylinder velocity vector is assumed to be divided into two velocity components: tumble velocity and piston-induced velocity. With this assumption, the abovementioned features of tumble are elucidated with a physics-based analysis. The tumble behaviors depend greatly on the timing of intake valve closing. To check the validity of the predicted behaviors, the various intake valve operation strategies and connecting rod length results obtained from 3D computational fluid dynamics were considered. Consequently, this study provides insight into tumble, which can be used to more accurately predict the flow variances according to different engine conditions.