Fuel economy improvement of a downsized engine through the reduction of knocking in transient conditions

Abstract

Due to energy crisis and greenhouse gas problems, researchers have tried to improve the efficiency of gasoline engines. Among various strategies for the efficiency improvement of the gasoline engines, downsizing has been highlighted. Downsizing is the technology that replaces the large displacement engine with the small displacement one that can produce same power through the use of forced inductions. Downsizing, since the engine operates in the high load region where the specific fuel consumption is low, is considered as efficient technology. Despite these strong advantages, the downsized engines’ benefit is limited by the phenomenon called knocking. Knocking is the abnormal combustion phenomenon where the air and fuel mixture auto-ignites before the flame front initiated by the spark discharge arrives the piston. The high in-cylinder pressure and temperature are the main causes of knocking. The downsized engines become even more vulnerable to knocking under transient conditions, mainly due to the sudden increase in the intake flow. Thus, in this research, through the parametric study, the effects of intake cam, exhaust cam, and spark ignition timing, and the tumble flap on the knocking combustion are analyzed. Moreover, using the analysis and the experimental result, the transient operation of the engine is assessed from the knocking combustion perspective. From this analysis, it was found that the residual gas fraction is increased from the sudden closure of wastegate during the transient acceleration, and this aggravates the knocking propensity. Thus, by applying the transient engine parameter control that aggressively advances the exhaust valve timing and the intake valve timing during this region, the wastegate closing level was reduced. Thus, the exhaust backpressure was reduced, and this increased the ignition advance margin without aggravating the knocking propensity. By applying this engine parameter control, the engine torque output and the fuel consumption level are improved by 3.3% and 2.4%, respectively.