The regulator system, which maintains constant engine power output in spite of the change of parameter value with optimalization, was mainly studied as a first step of the investigation. The regulator is a closed loop system which has an indicated work detector as a feed back signal, a digital filter, and a pulse motor throttle actuator. The results of the experiments for air fuel ratio control showed normal working of the regulator under a certain engine operation condition. Moreover one optimalization algorithm was tried using the regulator and an increase of the thermal efficiency was obtained. The regulator is not tuned optimumly in this step, but it has a high speed performance of time response, essentially. Therefore, there are good possibilities for the next step of the investigation.