Direct injection gasoline engines have the potential for improved fuel economy through principally the engine down-sizing, stratified charge combustion, and Controlled Auto Ignition (CAI). However, due to the limited time available for complete fuel evaporation and the mixing of fuel and air mixture, locally fuel rich mixture or even liquid fuel can be present during the combustion process of a direct injection gasoline engine. This can result in significant increase in UHC, CO and Particulate Matter (PM) emissions from direct injection gasoline engines which are of major concerns because of the environmental and health implications. In order to investigate and develop a more efficient DI gasoline engine, a camless single cylinder DI gasoline engine has been developed. Fully flexible electro-hydraulically controlled valve train was used to achieve spark ignition (SI) and Controlled Autoignition (CAI) combustion in both 4-stroke and 2-stroke cycles. In this paper, the heat release and performance characteristics of CAI and SI combustion at the same engine load conditions are studied and analysed. The effect of load and valve timings on the gaseous and Particulate Matter (PM) emissions is presented for both SI and CAI combustion in 4-strokes. Within the CAI operational range achieved at engine speeds of 800rpm to 3000rpm, particle emissions from DI gasoline engines are found to be dominated by smaller particles. Hotter charge and better mixing are the main parameters affecting the soot particles in the exhaust irrespective of the combustion modes and valve timings.