Experimental Investigation of Cyclic Variation of the In-Cylinder Flow in a Spark-Ignition Engine with a Charge Motion Control Valve

The cycle-to-cycle variations (CCV) have a substantial impact on the improvement of thermal efficiency and the expansion of operational limitations in internal combustion engines. For spark ignition engines, the variation of the in-cylinder flow field, especially the CCV of flow near the spark plug at the ignition timing, is a key factor causing the CCV of combustion. However, the physical mechanisms behind the CCV control of the in-cylinder flow field are still not well understood. The objective of this study is to determine how different tumble intensities induced by manipulating the opening and shutting of a tumble flap influence the flow CCV at the spark plug position at the ignition timing. High-speed particle image velocimetry (PIV) measurements were performed in an optically accessible single-cylinder, spark-ignited engine at a constant engine speed of 800 rpm. The frequency distributions of the velocity magnitude and flow angle are more concentrated under the high tumble intensity, indicating that the CCV of flow at the spark plug position at the ignition timing can be effectively reduced by closing the tumble flap. To gain a deeper insight into the mechanism of flow CCV alleviation, a correlation map analysis was employed, which can determine the relationship between the flow at the spark plug position and the flow distribution during the intake and compression stroke in time and space. To enhance the correlation between the above two, the proper orthogonal decomposition (POD) method was employed to extract the large-scale coherent structures and then the flow fields were reconstructed. The results demonstrated that the factors influencing the flow CCV under the tumble flap opening condition are primarily attributed to the CCV of the collision region position of the two intake jet flows in the later stage of the intake stroke and flow shear with the combustion chamber wall at the late compression stroke, while the factor influencing the flow CCV under the tumble flap closing condition is mostly connected to the CCV of tumble vortex position. Besides, closing the tumble flap can markedly increase the averaged kinetic energy and turbulent kinetic energy of the flow field in the vicinity of the spark plug position in the late compression stroke.