Effects of water direct injection on a combustion phase and performance of six-stroke gasoline homogeneous charge compression ignition engine

An experimental study was conducted to examine how direct water injection (WI) can optimize the combustion phases of a six-stroke gasoline homogenous charge compression ignition (HCCI) engine. With six-stroke HCCI combustion, the limitations of four-stroke HCCI combustion under high loads and the stability of combustion under low loads can be resolved by adding additional compression and expansion stroke. Previous six-stroke HCCI combustion research suggested that the time loss could be reduced by delaying the combustion phase to the top dead center (TDC) with additional cooling. Thus, in this study, the effects of WI on the overall pressure curve, as well as the first and second combustion phases, were studied from a thermal and chemical kinetics perspective. The performance, thermal efficiency, and pressure rise rate were also examined. It was found that WI retarded the first and second combustion. The first and second combustion phases were located after and before TDC, respectively, resulting in a decrease in the first gross mean effective pressure (GMEP) (180-540 CA) and an increase in the second GMEP (540-900 CA). Because the increase of second GMEP was dominant, the indicated thermal efficiency also monotonically increased (2.7-3.5%p). In addition, the maximum pressure rise rate decreased by 23.1% to 65.2%. Thus, the WI can be used to control the combustion characteristics of sixstroke gasoline HCCI engine.