Patent ID: 6185929
Filing Date: 2001-02-13
Classification: B01D,F01N,F02D

Abstract:
An exhaust emission control system for an internal combustion engine, comprising:a first catalyst disposed in an exhaust gas passage of the engine, said first catalyst functioning to trap 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 richer than the stoichiometric level and reduce released NOx in presence of HC and CO;a second catalyst disposed in the exhaust gas passage upstream of said first catalyst and having an oxygen trapping ability;means 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 enriching the air-fuel ratio;means for estimating the oxygen trapping ability of said second catalyst;means for correcting the enrichment degree in the stepwise enriching to decrease in correlation to a decrease in the estimated oxygen trapping ability.