This paper compares the combustion characteristics of CNGDI by pilot homogeneous charge compression ignition of gasoline and combustion of CNGDI by spark ignition. Homogeneous charge of gasoline is created by injecting the fuel in the manifold and the charge is heated by an air heater in order to achieve HCCI combustion of gasoline that ignites the direct injected and stratified CNG at 80 BTDC. The combustion characteristics obtained are compared with that of CNG by spark ignition. The results show that there are significant effects on HC, CO and NOx emissions. HCCI combustion has higher fuel conversion efficiency compared SI and less emissions within certain range of loads. © 2011 Asian Network for Scientific Information.