Investigation of the Effect of the In-Cylinder Tumble Motion on Cycle-to-Cycle Variations in a Direct Injection Spark Ignition (DISI) Engine Using Large Eddy Simulation (LES)

Cycle-to-cycle variations (CCVs) limit the extension of the operating range by inducing load variations and even misfire and/or knock for direct injection spark ignition (DISI) engines and hence need to be controlled. One of the effective and flexible ways to reduce CCV is to employ a charge motion control valve. This study is aimed to analyze the flow characteristics and CCV using large eddy simulation (LES) and fast Fourier transform (FFT) in a non-reacting, DISI engine equipped with a tumble flap (i.e., a specific type of charge motion control valve) inside the intake port. The in-cylinder flow characteristics are analyzed in detail, and the possible effects of multi-scale structures of the fluid field on the subsequent ignition and combustion processes are also discussed. Computational results indicate that closing the tumble flap helps enhance the intensity of the coherent structures and increase the total integral length scale (ILS) while decreasing the Kolmogorov scale and stabilizing the flow field by suppressing the CCV of tumble ratio and tumble center. Furthermore, based on a newly developed FFT triple decomposition, each instantaneous flow field is decomposed into three subfields, termed ensemble mean part and low- and high-spatial frequency parts, respectively. It is found that switching the tumble flap position greatly affects the first two subfields, but it has negligible effect on the last part. With the closed tumble flap, the energy portion of the mean part increases, the rate of energy decay reduces, and the CCV of the low- and high-spatial frequency parts decreases.