Abstract
In-cylinder air motion is one of the most important factors that control the degree of mixture preparation and thus is fundamental to improvements in the combustion process and overall engine performance. The major aim of this paper is to elucidate, through a predictive study, the main features of in-cylinder flow fields in a motored homogeneous charge compression ignition (HCCI) engine cylinder with variable negative valve overlapping (NVO). A commercial finite-volume computational fluid dynamics (CFD) package was used in the programme of simulation. The computational model was validated through a qualitative comparison between CFD results and the available experimental data. Thus one of the main developments presented in this study is the investigation of the intake process of the HCCI engine with various valve strategies, and it is perhaps the first time (to the current authors' best knowledge) that a direct comparison has been made of the results obtained in the same HCCI NVO motored engine using modelling and experimental approaches. The comparison illustrated a fair agreement between both sets of results, with some differences. A parametric predictive study of the effects of variable valve timings on the in-cylinder air motion has then been carried out. Three different sets of valve timings have been applied to the intake and exhaust valves to generate NVO of 70, 90, and 110 degrees of crank angle (°CA). The NVO was controlled by adjusting the times of exhaust valves closing (EVC) and intake valves opening (IVO) while keeping the times of exhaust valves opening (EVO) and intake valves closing (IVC) unchanged. The predicted results show a noticeable modification of the strength and the global direction of the in-cylinder charge motion as a result of increasing the magnitude of NVO. Modifications of in-cylinder swirl and tumble motions obtained by applying higher degrees of NVO are expected to have a considerable effect on the air-fuel mixture preparation process as well as the actual in-cylinder conditions at the end of the compression stroke.