Patent ID: 6941745

Claim:
An exhaust gas cleaning system for an internal combustion engine having an upstream catalyst and a downstream catalyst arranged in series in an engine exhaust, the system comprising: first feedback control means for correcting a quantity of fuel supplied to the internal combustion engine based on a first air-fuel ratio on an upstream side of an upstream catalyst and a first target air-fuel ratio on the upstream side of the upstream catalyst; second feedback control means for correcting a first target air-fuel ratio based on a second air-fuel ratio on a downstream side of the upstream catalyst and a second target air-fuel ratio on a downstream side of the upstream catalyst; third feedback control means for correcting the second target air-fuel ratio based on the second air-fuel ratio and a third air-fuel ratio on a downstream side of a downstream catalyst arranged downstream of the upstream catalyst; and operating condition detecting means for determining an operating condition of the internal combustion engine, wherein the third feedback control means corrects the second target air-fuel ratio based on an operating condition detected by the operating condition detecting means, the second air-fuel ratio and the third air-fuel ratio, wherein the operating condition detecting means includes flow rate detecting means for determining an exhaust gas flow rate in the engine exhaust, and wherein the third feedback control means assigns weight on the second air-fuel ratio determined by the second air-fuel ratio detecting means and the third air-fuel ratio determined by the third air-fuel ratio detecting means based on the exhaust gas flow rate detected by the flow rate detecting means, and corrects the second target air-fuel ratio based on the second air-fuel ratio and the third air-fuel ratio on which weight is assigned, wherein the operating condition detecting means includes temperature detecting means for determining a temperature in the exhaust gas passage, and wherein the third feedback control means corrects the second target air-fuel ratio based on the temperature in the exhaust gas passage detected by the temperature detecting means, the second air-fuel ratio and the third air-fuel ratio.