Abstract: In order to suppress the quantity of the nitrogen oxide to be released to the atmosphere when the operation state is shifted from the engine operation with lean air fuel ratio to that with stoichiometric air fuel ratio, the exhaust gas purification apparatus is arranged such that by forced rich operation, when the operation state is shifted from the engine operation with lean air fuel ratio to that with stoichiometric air fuel ratio, the operation state is shifted to the engine operation with stoichiometric air fuel ratio after performing that with rich air fuel ratio.

Abstract: In order to suppress the quantity of the nitrogen oxide to be released to the atmosphere when the operation state is shifted from the engine operation with lean air fuel ratio to that with stoichiometric air fuel ratio, the exhaust gas purification apparatus is arranged such that by means of the forced rich operation means, when the operation state is shifted from the engine operation with lean air fuel ratio to that with stoichiometric air fuel ratio, the operation state is shifted to the engine operation with stoichiometric air fuel ratio after performing that with rich air fuel ratio.

Abstract: An exhaust gas purifier for an internal combustion engine which is capable of detecting the deterioration degree of an NOx catalyst making use of a time elapsed from the time when the air-fuel ratio of exhaust gas flowing into an NOx absorber catalyst is made rich to the time when the air-fuel ratio of exhaust gas flowing out from the NOx absorber catalyst becomes rich. When the time required for a change in the output level of an air-fuel ratio sensor is shorter than a predetermined time, the time required for the change in the output level of the air-fuel ratio sensor is lengthened by reducing the rich degree at the time of rich operation, thereby judging the deterioration degree of the NOx absorber catalyst with accuracy.

Abstract: An exhaust gas purifier for an internal combustion engine which is capable of detecting the deterioration degree of an NOx catalyst making use of a time elapsed from the time when the air-fuel ratio of exhaust gas flowing into an NOx absorber catalyst is made rich to the time when the air-fuel ratio of exhaust gas flowing out from the NOx absorber catalyst becomes rich.

Abstract: In order to improve estimation accuracy of estimating the value of the NOx exhaust concentration, the air fuel ratio control apparatus is adapted to estimate the NOx exhaust concentration from at least either one of a value depending on the air fuel ratio or a value depending on the exhaust gas recirculation quantity and a value depending to the ignition timing by means of the ECU 230.