An experimental study of the extent of the operating region and emission characteristics of stratified combustion using the controlled autoignition method

Abstract

Controlled autoignition (CAI) combustion, also known as homogeneous charge compression ignition (HCCI), offers the potential to simultaneously improve fuel economy and reduce emissions. CAI combustion was achieved in a single-cylinder gasoline direct-injection (DI) engine with a cylinder running in CAI mode. Standard components were used in the camshafts, which had been modified to restrict the gas exchange process. We investigated the effects of the air/fuel ratio, residual exhaust gas recirculation (EGR) rate, and injection timing such as early and late injections on the attainable CAI combustion region. The effect of injection timing on factors such as the start of combustion, its duration, and its heat release rate was also investigated. The results show that early injection caused the mixture to ignite earlier and to burn more quickly because of the exothermic reaction during the recompression and gave rise to thorough mixing of the fuel and air. Late injection extended the operation region more than early injection, but the emissions of HC and NOx were slightly increased over early injection.