Effects of spark-assistance on controlled auto-ignition combustion at different injection timings in a multicylinder direct-injection gasoline engine

Controlled auto-ignition (CAI) combustion, also known as homogeneous charge compression ignition (HCCI) can be achieved by trapping residuals with early exhaust valve closure in a direct fuel injection in-cylinder four-stroke gasoline engines. CAI combustion is achieved by auto-ignition independent of spark discharge. However, it is found that, at loads with reduced trapped residuals, the presence of spark influences combustion. Therefore the effects of spark timing on the CAI combustion process were investigated through the introduction of spark. The effect on engine performance and the emission specific values were evaluated. The engine speed was maintained at 1500 r/min and lambda was kept constant at 1.2. It was found that with spark-assisted CAI, indicated mean effective pressure (IMEP), and indicated specific oxides of nitrogen (ISNO(x)) values increased as compared with CAI without spark. ISHC and ISCO values were lower for spark-assisted CAI as compared with CAI without spark. Heat release data were analysed to better understand this phenomenon.