Air-fuel ratio control system for internal combustion engines capable of controlling air-fuel ratio in accordance with degree of warming-up of the engines

摘要

While an exhaust gas sensor having an output characteristic linear with respect to the concentration of a specific component in exhaust gases from an internal combustion engine, the opening of an automatic choke valve is controlled in response to the degree of warming- up of the engine. From the time of activation of the exhaust gas sensor to the time of completion of warming-up of the engine, the automatic choke valve and an air-fuel ratio control valve arranged in an air passage bypassing a throttle valve in an intake passage of the engine, are driven so as to achieve a desired air-fuel ratio, respectively, when the difference between the desired air-fuel ratio and actual one is larger than a predetermined value, and when the difference is smaller than the predetermined value. After completion of warming-up of the engine, the air-fuel ratio control valve is driven in response to operating conditions of the engine so as to achieve the desired air-fuel ratio.