METHOD AND EQUIPMENT FOR MONITORING SECONDARY AIR SYSTEM IN COOPERATION WITH EXHAUST GAS SYSTEM OF AUTOMOBILE

PURPOSE: To perform highly reliable monitoring of the introduction of secondary air, by judging the functioning of a secondary-air system by forming a secondary air stream signal from a specified air-fuel ratio. CONSTITUTION: After starting an engine, a circuit of a secondary-air pump 120 is closed and a valve 122 is opened. Thus, a catalyst 116 is quickly heated up to an operating temperature, and harmful substances-components in exhaust gas at the stage of warming up of the engine 100 are reduced. Next, the air mass flow rate in an intake path 102 is determined based on output signals from an air mass measuring device 106. Successively, the fuel mass weighed per unit time is determined based on the injection time and the engine speed, and further the air-fuel ratio of a mixture drawn in the engine 100 is determined. On the other hand, the air-fuel ratio prevailing in an exhaust gas passage 104 on the downstream side of the opening of a secondary- air conduit pipe 118 is determined. Furthermore, the secondary-air flow rate is obtained based on the air-fuel ratio prevailing in an exhaust gas passage 104 and the air fuel ratio prevailing in an air intake path 102. The functioning is judged with respect to the determined secondary-air flow rate.