Air-fuel ratio control system for internal combustion engines having a plurality of cylinder groups

In an air-fuel ratio control system for an internal combustion engine having a plurality of cylinders divided into a first group of cylinders and a second group of cylinders, first and second exhaust systems extending respectively from said first and second groups of cylinders, said first and second exhaust systems having downstream end portions thereof joined together into a confluent portion, and first and second catalytic converters arranged respectively in said first and second exhaust systems, said air-fuel ratio control system including first and second upstream oxygen sensors arranged respectively in said first and second exhaust systems upstream of said first and second catalytic converters, and a single downstream oxygen sensor arranged in said confluent portion of said first and second exhaust systems downstream of said first and second catalytic converters,

• the improvement comprising:
• first air-fuel ratio control means operable when said engine is in a predetermined operating condition, for controlling the air-fuel ratio of an air-fuel mixture supplied into said first group of cylinders by the use of a first air-fuel ratio control amount based on outputs from said first upstream oxygen sensor and said downstream oxygen sensor, and for controlling the air-fuel ratio of an air-fuel mixture supplied into said second group of cylinders by the use of a second air-fuel ratio control amount based on outputs from said second upstream oxygen sensor and said downstream oxygen sensor;
• second air-fuel ratio control means operable when said engine is in an operating condition other than said predetermined operating condition, for controlling the air-fuel ratio of an air-fuel mixture supplied into one of said first and second groups of cylinders By the use of a third air-fuel ratio control amount based solely on an output from said downstream oxygen sensor, and for controlling the air-fuel ratio of an air-fuel mixture supplied into the other of said first and second groups of cylinders by the use of a predetermined value;
• and
• catalyst deterioration-detecting means for detecting deterioration of said first catalytic converter or said second catalytic converter, based on an output from said second air-fuel ratio control means.