One of the main drawback of the two stroke engine is the unstable running especially at light load which involves high emission of unburned hydrocarbons. In this case, the scavenging process has a low efficiency and the high quantity of residual gas in cylinder leads to incomplete combustion in certain cycles. The use of controlled auto-ignition combustion or ATAC, which is initiated by auto-ignition instead of a spark plug, provides a consistent increase of the thermal efficiency and a lower ignition of the fresh charge. Its main characteristic is a great cyclic stability. The experimental work described in this paper is focused on the analyses of the auto-ignition phenomenon. A FORCE outboard engine 1230 cc three cylinders is used in this research work with extra special throttles in the tranfer duct. Large windows are mounted in the cylinder head and allow us a complete chamber visualization in firing conditions at many crankangle position. The engine is entirely instrumented (pressure, exhaust gas analysis, cylinder gas analysis by a sampling valve …). The parametric study and its interpretation reveal that the change of combustion process can be explained in term of aerodynamic and of the enthalpy of the residual gas fraction. Control of the combustion process diminishes knocking apparition and allows us a wide range of auto-ignition running conditions. Response of energy release has been studied and shows a cycle regulation of the start of ignition and combustion speed. To complete the parametric study, images obtained in the two modes are presented and reveal important differences as for example a random ignition location for auto ignition combustion. A video film comparing the two combustion modes has been realised.