Exhaust gas purifying method of an internal combustion engine

Method of purifying exhaust gas of an internal combustion engine having an air adjusting means for controlling the amount of secondary air supplied into an exhaust system of the engine, a sensor for detecting concentration of a particular component contained in the exhaust gas after having been mixed with said supplied secondary air, and a catalytic converter for reducing noxious components contained in the exhaust gas after having been mixed with said supplied secondary air, said method comprising the steps of:

• generating, in response to a sensed signal from said sensor, an air-fuel ratio discrimination signal having two voltage levels depending upon whether the air-fuel ratio of said engine is rich or lean in comparison with a predetermined air-fuel ratio;
• gradually increasing or decreasing, in response to the level of said air-fuel ratio discrimination signal, the amount of secondary air supplied into the exhaust system from said air adjusting means;
• and,
• additionally supplying the secondary air to the exhaust system from said air adjusting means, in an amount which changes depending upon the change of the average value of said air-fuel ratio discrimination signal, so as to bias the supplied secondary air amount.