Air-fuel ratio control system for internal combustion engines

An air-fuel ratio control system for an internal combustion engine having an exhaust system, a catalytic converter arranged in said exhaust system, a first air-fuel ratio sensor arranged upstream of said catalytic converter, and a second air-fuel ratio sensor arranged downstream of said catalytic converter, comprising:

• air-fuel ratio control variable-calculating means for calculating an air-fuel ratio control variable, based on an output from said second air-fuel ratio sensor;
• delay means for delaying timing of determination as to inversion of an output from said first air-fuel ratio sensor by a first predetermined period of time from a time a first inversion occurs in said output from said first air-fuel ratio sensor, said first inversion being in a direction of change of said output from said first air-fuel ratio sensor from a leaner side to a richer side, and for delaying said timing of determination as to inversion of said output from said first air-fuel ratio sensor by a second predetermined period of time from a time a second inversion occurs in said output from said first air-fuel ratio sensor, said second inversion being in a direction of change of said output from said first air-fuel ratio sensor from the richer side to the leaner side;
• air-fuel ratio correction amount-calculating means for calculating an air-fuel ratio correction amount, based on an output from said delay means and said air-fuel ratio control variable;
• air-fuel ratio control means for controlling the air-fuel ratio of said mixture supplied to said engine, based on said air-fuel ratio correction amount;
• catalytic converter deterioration-detecting means for detecting deterioration of said catalytic converter;
• and
• delay time-changing means for changing at least one of said first and second predetermined periods of time such that a ratio of said first predetermined period of time to said second predetermined period of time increases, when deterioration of said catalytic converter is detected.