The study of fundamental characteristics of tumble with variable valve operation strategies by numerical analysis 2020-01-1108
Spark-ignition engine commonly uses tumble flow in that it generates high turbulence which is a crucial factor determining the flame propagation speed. Since tumble affects not only the flame propagation speed but also the various in-cylinder phenomena such as heat transfer and air-fuel mixing, it determines the performance of the engine considerably. In that sense, many studies were conducted to investigate the tumble. Although various researches revealed the tumble characteristics numerically and experimentally, there has been no research identifying the reasons for those characteristics. Even though some studies specified the reasons using angular momentum perspective, the theory was insufficient to explain the entire phenomena of tumble. Hence, this study attempts to figure out the fundamental causes for the tumble phenomena such as ‘spinning up’ and ‘vortex breakdown’ with kinetic energy analysis. The movement characteristic of tumble center during the compression stroke is also identified. Though the study deals with the formation of tumble, it primarily focuses on the compression stroke where the piston exerts its influence to tumble significantly. For the simple analysis, the in-cylinder velocity vector is assumed to be divided into the two velocity components: tumble velocity and piston induced velocity. With the assumption, abovementioned features of tumble are elicited with physics-based analysis. The various valve operation strategy and engine geometry variance results obtained from 3D computational fluid dynamics (CFD) were used for the validation. Consequently, the study is expected to provide the insight of tumble which helps to predict the tumble variances according to the different engine conditions.