Air-fuel ratio control method for internal combustion engines

摘要

A method of controlling the air-fuel ratio of a mixture of fuel supplied to an internal combustion engine. Feedback control is effected to response to an output from an exhaust-gas concentration sensor to bring the air-fuel ratio to a predetermined value when the engine is in a predetermined medium-load operating region. It is determined whether or not the engine is in a predetermined high-load operating region in which the feedback control is interrupted for bringing the air-fuel ratio to a value smaller than the predetermined value, and whether or not the engine is in a predetermined low-load operating region in which the feedback control is interrupted for bringing the air-fuel ratio to a value larger than the predetermined value. The feedback control is effected even when the engine is in the predetermined low-load operating region, if the engine continually stayed over a first predetermined time period in the predetermined high-load operating region, and has shifted to the low-load operating region within a second predetermined time period after leaving the predetermined high-load operating region.