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.

Over the last decade, controlled autoignition (CAI) combustion, or homogeneous charge compression ignition (HCCI), has been the subject of numerous studies and has shown its potential to reduce fuel consumption and NO x emissions simultaneously from gasoline engines. However, there are still some fundamental questions that are yet to be addressed, such as combustion control and underlying physical and chemical processes. In the present study, a single-cylinder optical gasoline engine was operated with negative valve overlap to investigate CAI and hybrid spark ignition (SI)–CAI combustion. The effects of direct fuel injection and spark timings were studied by means of a simultaneous in-cylinder heat release study and high-speed visualization of total chemiluminescence, OH and CHO radicals.