| abstract |
An NO x absorber (18) is arranged in an exhaust passage of an internal combustion engine. The NO x absorber (18) absorbs the NO x when the air-fuel ratio of the exhaust gas flowing into the NO x absorber (18) is lean while releases the absorbed NO x when the air-fuel ratio of the exhaust gas flowing into the NO x absorber (18) becomes the stoichiometric air-fuel ratio or rich. An air-fuel ratio sensor (22) is arranged in the exhaust passage downstream of the NO x absorber (18). When the air-fuel ratio detected by the air-fuel ratio sensor (22) is switched from lean to rich after the air-fuel ratio of the exhaust gas flowing into NO x absorber (18) is switched from lean to rich, it is decided that the releasing action of NO x from the NO x absorber (18) is completed. |