Exhaust emission control system for internal combustion engine

An exhaust emission control system for an exhaust system for an internal combustion engine, having a nitrogen oxide removing means for absorbing nitrogen oxides contained in exhaust gases in an exhaust lean condition;

• said control system comprising:
• first and second oxygen concentration sensors respectively provided upstream and downstream of said nitrogen oxide removing means, said oxygen concentration sensors detecting the oxygen concentration in the exhaust gases;
• deterioration-determination enriching means for changing the air-fuel ratio of the air-fuel mixture to be supplied to said engine from a lean region to a rich region with respect to a stoichiometric ratio;
• reducing component amount calculating means for calculating an amount of reducing components flowing into said nitrogen oxide removing means from the time when the output value from said first oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio after the enrichment executed by said deterioration-determination enriching means has started;
• deterioration determining means for determining deterioration of said nitrogen oxide removing means according to the amount of reducing components calculated by said reducing-component amount calculating means and the output value from said second oxygen concentration sensor;
• and
• reduction enriching means for intermittently enriching the air-fuel ratio to reduce nitrogen oxides absorbed by said nitrogen oxide removing means, wherein said deterioration-determination enriching means executes enrichment of the air-fuel ratio with a degree of enrichment smaller than a degree of enrichment executed by said reduction enriching means over a time period longer than a time period of enrichment executed by said reduction enriching means.