The improvement of the exhaust emission and fuel consumption in the conventional two-stroke engines would be urgent. Our previous papers have suggested that the timing controlled auto-ignition, namely Activated Radical Combustion(AR combustion) could be a solution for that.In this time, the AR combustion was applied to a 250 cm3 motorcycle for the intention of commercialization of the AR engine. The alternating phases between AR combustion and SI combustion were analyzed and successfully improved the typical pinking noise. The AR combustion finally decreased the HC emission by approximately 60% in the EC 40 emission evaluation mode.As the power units for the small motorcycles or outboards, two-stroke engines are yet majority. That is because they have advantages such as higher power output, simpleness and compactness of the structure, at the same time, their drawbacks in fuel consumption and exhaust emissions are also pointed out in the issues of preserving the environment.It is known that these drawbacks result from the new mixture shortcut and irregular combustion in the part load operation. Both are caused by the structure of the conventional two-stroke engines, whose scavenging process overlaps with the exhaust process. Several researches on improving two-stroke engines have been published.(1)(2) A stratified charge concept by using an in-cylinder direct fuel injection system could be a solution. However from a view of the commercialization, the solution should maintain the original advantages of two-stroke engines and it must be low cost.It was known that the auto-ignition could be a solution for the irregular combustion.(3) In the light load operating conditions of two-stroke engines, the flame propagation initiated from the ignition plug is disturbed by the large amount of residual gas, which causes the irregular combustion. While the auto-ignition has no flame propagation process, therefore it should improve the irregular combustion.Our intention to put this auto-ignition into practical use required to distinguish it from other abnormal combustion such as pre-ignition or knocking. As it was revealed that the flashing timing in the auto-ignition process could be controlled by the cylinder pressure at the beginning of the compression stroke. A concept of auto-ignition whose timing are controlled to be optimum, namely, Activated Radical Combustion (AR combustion) are proposed in our first paper.(4) The AR combustion was successfully applied to a 400 cm3 motorcycle engine while maintaining the original advantages. The benefits and practicality of the engine are explained in the second paper.(5)In an engine applying AR combustion, the ignition automatically alternates between the spark ignition (SI) and AR combustion corresponding to the engine's operating load. For its smooth alternation, the combustion in the alternating phases were analyzed. This paper will discuss about the clean emission potential and breakthroughs for a commercialization of the AR combustion by using a 250 cm3 motorcycle engine with an exhaust valve.