Air-fuel ratio control system for internal combustion engines

In an air-fuel ratio control system for an internal combustion engine having an exhaust system, and exhaust gas-purifying means arranged in said exhaust system, including an oxygen concentration sensor arranged either in said exhaust system at a location downstream of said exhaust gas-purifying means or in said exhaust gas-purifying means, and air-fuel ratio perturbation means for periodically changing an air-fuel ratio of a mixture supplied to said engine alternately to a rich side and to a lean side with respect to a stoichiometric air-fuel ratio,

• the improvement comprising:
• purification capacity-determining means for determining purification capacity of said exhaust gas-purifying means, based on an output from said oxygen concentration sensor;
• and
• repetition period-changing means for shortening a repetition period of said periodical changing of said air-fuel ratio by said air-fuel ratio perturbation means as said purification capacity determined by said purification capacity-determining means is deteriorated, wherein said repetition period-changing means prolongs at least one of a rich-side duration over which said air-fuel ratio is continuously deflected to said rich side by said air-fuel ratio perturbation means and a lean-side duration over which said air-fuel ratio is continuously deflected to said lean side by said air-fuel ratio perturbation means when a rich-side duration correction coefficient for correcting said rich-side duration and a lean-side duration correction coefficient for correcting said lean-side duration both exceed a predetermined value, said repetition period-changing means shortening at least one of said rich-side duration and said lean-side duration when said rich-side duration correction coefficient and said lean-side duration correction coefficient are both below said predetermined value.

In an air-fuel ratio control system for an internal combustion engine having an exhaust system, and exhaust gas-purifying means arranged in said exhaust system, including an oxygen concentration sensor arranged either in said exhaust system at a location downstream of said exhaust gas-purifying means or in said exhaust gas-purifying means, and air-fuel ratio perturbation means for periodically changing an air-fuel ratio of a mixture supplied to said engine alternately to a rich side and to a lean side with respect to a stoichiometric air-fuel ratio,

• the improvement comprising:
• purification capacity-determining means for determining purification capacity of said exhaust gas-purifying means, based on an output from said oxygen concentration sensor;
• and
• repetition period-changing means for shortening a repetition period of said periodical changing of said air-fuel ratio by said air-fuel ratio perturbation means based upon a degree of deterioration of said purification capacity which is determined by said purification capacity-determining means.