Patent ID: 7266440

Claim:
An air-fuel ratio control system for an internal combustion engine, said system comprising: a catalytic device installed in an exhaust pipe of an internal combustion engine; a first air-fuel ratio sensor installed in the exhaust pipe upstream of said catalytic device to measure an air-fuel ratio of an exhaust gas flowing through the exhaust pipe; a second air-fuel ratio sensor installed in the exhaust pipe downstream of said catalytic device to measure an air-fuel ratio of the exhaust gas flowing through the exhaust pipe; a feedback circuit working to determine a preselected controlled parameter based on the air-fuel ratios, as measured by said first and second air-fuel ratio sensors, and perform air-fuel ratio feedback control; wherein said feedback circuit multiplies the air-fuel ratios measured by said first and second air-fuel ratio sensors by given weighting factors, respectively, to determine the controlled pararneter; and a deterioration detector working to detect a deterioration of said catalytic device, and wherein said feedback circuit determines the weighing factors based on a degree of the deterioration of said catalytic device, as detected by said deterioration detector.