Effect of Compression Ratio and Spray Injection Angle on HCCI Combustion in a Small DI Diesel Engine

Abstract

An experimental investigation was performed on a small direct injection (DI) diesel engine equipped with a common-rail injection system to find the optimal operating conditions of a homogeneous charge compression ignition (HCCI) engine. It is generally agreed that NOx is formed within the stoichiometric diffusive flame and may continue to form in the hot combustion products and that particulate matter is formed under a rich fuel/air mixture condition. Therefore, if an adequately diluted fuel/air mixture is formed before the start of ignition by prolonging the ignition delay, a homogeneous lean mixture is burned at a low temperature. In this way, simultaneous reduction of NOx and soot can be achieved. To realize this fundamental concept and find the optimal operating conditions, injection timing was varied from top dead center (TDC) to 80° before TDC and up to 45% of exhaust gas recirculation (EGR) was tested. The features of the base engine were modified as follows. First, the geometric compression ratio was reduced from 17.8 of the base diesel engine to 15 for expanding ignition delay by modification of the piston shape. Second, the injection angle was reduced from 156° of the base engine to 60° to reduce fuel deposition on the wall of the combustion chamber when using early injection timing. Experimental results showed that clean operating conditions existed either at an early start of injection timing with/without EGR or at a very late start of injection timing near TDC with high rates of EGR.