Effects of charge motion on knocking combustion under boosted high load condition of a medium-duty gasoline engine

Due to higher power density and strong swirl flow field, knock tendency is more severe in medium duty (MD) gasoline engines derived from diesel engine platforms. How to design the charge motion reasonably, so as to increase the thermal efficiency and suppress knock simultaneously needs to be further discussed in combination with the actual combustion boundary conditions. The investigations in this study focus on the charge motion roles on knock and thermal efficiency under a high load/low speed operating condition using numerical simulations. The results show that the mechanisms of inducing knocking combustion and the effects of tumble ratio (TR) on knock intensity (KI) are different under swirl, inclined swirl and tumble motion conditions. With strong swirl configurations, weaker turbulence intensity at the bottom of the combustion chamber reduces the convective heat transfer between the fresh charge and hot exhaust gas, resulting in an obvious high temperature region, and inducing the first hot spot. Increasing TR can effectively suppress knock by shortening the residence time of end gas. With inclined swirl and strong tumble configurations, the position of hot spots is determined by the asymmetrical shape of the flame front, which is induced by large-scale tumble motion around the spark plug close to TDC. The end gas reactivity is higher and the temperature distribution in the end gas region is more uniform with the increase of TR, resulting in a sharp increase in auto-ignition velocity and thus KI. Due to the suppression of knock, reduction of heat losses as well as promotion of flame speed, an optimized inclined swirl motion based on the flat cylinder head is more appropriate for MD gasoline engines.