The CAI/HCCI combustion was achieved through residual gas trapping using the negative valve overlap method in a direct injection single-cylinder gasoline engine with optical access. Effects of injection timing and spark discharge on the engines performance and emission were investigated by means of simultaneous in-cylinder heat release analysis and the high-speed chemiluminescence imaging technique. The presence and magnitude of fuel reformation and heat release associated with the fuel injection into lean-burn residual gases were identified. Furthermore, the effect of quantity of directly injected air from an air-assisted injector into stoichiometric burned residual gas was studied. The in-cylinder heat release and imaging showed that spark discharge had much less effect on the heat release process in the lean-burn CAI operation than that in the stoichiometric operation because of the less ignitability of the leaner mixture. Direct injection into the hot residual gas was found to produce minor heat release and led to chemiluminescence. This process resulted in higher charge temperature and earlier autoignition followed. Furthermore, it was found that small quantity of air injection included in the air-assisted injection process during the negative valve overlap period could be used to promote the fuel reforming and minor heat release reactions in order to control the main combustion phasing.