Patent ID: 7200988

Claim:
An air-fuel ratio control system for an internal combustion engine having an exhaust passage, the system comprising: an upstream catalyst provided in the exhaust passage; a downstream catalyst provided downstream the upstream catalyst in the exhaust passage; a first exhaust sensor provided upstream the upstream catalyst in the exhaust passage for detecting a first air-fuel ratio thereat; a second exhaust sensor provided between the upstream catalyst and the downstream catalyst in the exhaust passage for detecting a second air-fuel ratio thereat; a third exhaust sensor provided downstream the downstream catalyst in the exhaust passage for detecting a third air-fuel ratio therat; and a target control means for determining a target second air-fuel ratio to be attained between the upstream catalyst and the downstream catalyst based on the third air-fuel ratio detected by the third exhaust sensor, and for determining a target first air-fuel ratio to be attained upstream the upstream catalyst based on the target second air-fuel ratio and the second air-fuel ratio detected by the second air-fuel ratio sensor; a feedback control means for feedback controlling an air-fuel ratio of mixture based on the target first air-fuel ratio and the first air-fuel ratio detected by the first exhaust sensor; and correcting means for correcting the target first air-fuel ratio based on the third air-fuel ratio detected by the third exhaust sensor, wherein the target control means controls a change in the target first air-fuel ratio to be smaller in a predetermined range than that in a range other than the predetermined range based on the third air-fuel ratio detected by the third exhaust; and wherein the target control means gradually increases the change in the target first air-fuel ratio as the third air-fuel ratio becomes leaner.