Patent ID: 8141349

Claim:
An exhaust emission control device for an internal combustion engine, for purifying exhaust gases emitted from the engine into an exhaust system, comprising: a catalyst that is disposed in the exhaust system, for purifying exhaust gases; a NOx catalyst that is disposed in the exhaust system at a location downstream of the catalyst, for trapping NOx contained in the exhaust gases under an oxidizing atmosphere, and reducing the trapped NOx under a reducing atmosphere to thereby purify the trapped NOx; an air-fuel ratio sensor that is disposed between said catalyst and said NOx catalyst in the exhaust system, for detecting an air-fuel ratio of the exhaust gases; NOx reduction control means for causing said NOx catalyst to carry out an operation for reducing NOx, by supplying a reducing agent to an upstream side of said catalyst to control the exhaust gases flowing into said NOx catalyst such that the exhaust gases form the reducing atmosphere; purified NOx amount parameter-calculating means for calculating a purified NOx amount parameter indicative of an amount of NOx purified in said NOx catalyst, so as to determine a time at which reduction control by said NOx reduction control means should be terminated; exhaust gas flow amount-calculating means for calculating a flow amount of exhaust gases emitted from the engine; total exhaust gas inflow amount-calculating means for calculating, based on the calculated exhaust gas flow amount, a total amount of exhaust gases flowed into said NOx catalyst in a state where the detected air-fuel ratio converges within a vicinity of a predetermined target air-fuel ratio, as a total exhaust gas inflow amount; and calculation start means for causing calculation of the purified NOx amount parameter to be started by said purified NOx amount parameter-calculating means upon lapse of a predetermined time period after the detected air-fuel ratio converges within the vicinity of the predetermined target air-fuel ratio.