System and method for diagnosing deterioration of NOx-occluded catalyst

Method of diagnosing deterioration of a NOx-occluded catalyst installed in an exhaust stream of an internal combustion engine, said catalyst occluding NOx in the exhaust gas when the engine exhaust gas led to the catalyst shows a leaner air/fuel ratio and releasing NOx therefrom when said engine exhaust gas shows a stoichiometric and/or richer air/fuel ratio, said step comprising:

• estimating NOx concentration at an inlet part of the catalyst;
• detecting NOx concentration at an outlet part of the catalyst;
• deriving a rate of the minimum value of the NOx concentration detected for a predetermined period to a value of NOx concentration estimated at the time when said minimum value is detected, said predetermined period being a period between a time when the engine exhaust gas shows the stoichiometric and/or richer air/fuel ratio and a subsequent time when the engine exhaust gas shows the stoichiometric and/or richer air/fuel ratio;
• and
• judging a deterioration degree of the catalyst in accordance with the derived rate.

A system for diagnosing deterioration of a NOx-occluded catalyst installed in an exhaust stream of an internal combustion engine, said catalyst occluding NOx in the exhaust gas when the engine exhaust gas led to the catalyst shows a leaner air/fuel ratio and releasing NOx therefrom when said engine exhaust gas shows a stoichiometric and/or richer air/fuel ratio, said system comprising:

• first means for estimating NOx concentration at an inlet part of said catalyst;
• second means for detecting NOx concentration at an outlet part of said catalyst;
• and
• third means for judging a deterioration degree of the catalyst in accordance with a rate of the minimum value of NOx concentration detected by said second means for a predetermined period to a value of NOx concentration estimated by said first means at the time when said minimum value of NOx concentration is detected, said predetermined period being a period between a time when the engine exhaust gas shows the stoichiometric and/or richer air/fuel ratio and a subsequent time when the engine exhaust gas shows the stoichiometric and/or richer air/fuel ratio.