Exhaust emission control system for internal combustion engine

Abstract

An exhaust emission control system for an internal combustion engine amounted on an automotive vehicle. The exhaust emission control system comprises a first catalyst disposed in an exhaust gas passage of the engine. The first catalyst functions to store NOx in an atmosphere having an air-fuel ratio leaner than a stoichiometric level and to release NOx in an atmosphere having an air-fuel ratio ocher than the stoichiometric level and reduce released NOx in presence of HC and CO. A second catalyst is disposed in the exhaust gas passage upstream of the first catalyst and having an oxygen absorbing ability. A control unit is provided including a first section for accomplishing an enrichment treatment by enriching stepwise an air-fuel ratio of an air-fuel mixture to be supplied to the engine to an enrichment degree at a timing at which a lean operation is changed to a stoichiometric operation in the engine, and recovering the enriched air-fuel ratio to the stoichiometric level at a recovery rate immediately after the enriching the air-fuel ratio. A second section of the control unit is for estimating the oxygen absorbing ability of the second catalyst. A third section of the control unit is for correcting the enrichment degree to decrease in the stepwise enriching as the estimated oxygen absorbing ability decreases.