Air/fuel ratio control system for lean combustion engine using three-way catalyst

A control system for feedback control of the air/fuel ratio of an air-fuel mixture supplied to an internal combution engine which uses a three-way catalyst for purifying the exhaust gas, the control system comprising:

• air/fuel ratio detection means for detecting actual values of air/fuel ratio in the engine;
• load detection means for detecting the load under which the engine is operating;
• temperature detection means for detecting the temperature of the three-way catalyst;
• and
• control means for performing feedback control of the feed of fuel or air to the engine based on the detected actual values of air/fuel ratio, the control means comprising target value setting means for determining a variable target value of the air/fuel ratio according to information obtained by said load detection means such that the target value becomes higher than the stoichiometric air/fuel ratio at least in a portion of a steady zone of operating conditions of the engine and modification means for varying the feed of fuel or air to the engine when the temperature of the three-way catalyst is above a predetermined temperature while the target value of feedback control of air/fuel ratio is higher than the stoichiometric ratio such that the actual air/fuel ratio decreases to a predetermined value which is optimum for the activities of the three-way catalyst or to a still lower value.