Investigation of Cycle-to-Cycle Variation of Turbulent Flow in a High-Tumble SI Engine

The thermal efficiency of a spark-ignition (SI) engine must be improved to reduce both environmental load and fuel consumption. Although lean SI engine operation can strongly improve thermal efficiency relative to that of stoichiometric SI operation, the cycle-to-cycle variation (CCV) of combustion increases with the air dilution level. Combustion CCV is caused by CCVs of many factors, such as EGR, spark energy, air-fuel ratio, and in-cylinder flow structure related to engine speed. This study focuses on flow structures, especially the influence of a tumble structure on flow fluctuation intensity near ignition timing. We measured the flow field at the vertical center cross section of an optically accessible high-tumble flow engine using time-resolved particle image velocimetry. There are many factors considered to be sources of CCV, we analyzed three factors: the intake jet distribution, distribution of vortex core position and trajectory of the fluid particle near the spark plug. To estimate the trajectory of fluid particle, the flow structure near the spark plug is tracked backward from the ignition timing to the intake stroke. The CCV of the fluid particle trajectory has two modes, and it is induced by the different of horizontal velocity component. The more detail study is needed to detect the cause of this two-mode behavior. Furthermore, the relationships between CCV factors were analyzed by correlation coefficient and multiple regression analysis. It was not confirm that the CCV of intake jet flow and the pressure wave affect directly the following CCV factors. The CCV of the flow field is mainly generated in the compression stroke. The key part for the CCV at ignition timing is not only the vortex core position, but also the area around the vortex core.