Abnormality diagnosis system for an exhaust gas purification apparatus

An abnormality diagnosis system for an exhaust gas purification apparatus which includes:

• an NOx selective reduction catalyst that is arranged in an exhaust passage of the internal combustion engine, and reduces NOx in an exhaust gas by using ammonia as a reducing agent;
• and
• an ammonia supply device that is arranged in the exhaust passage at the upstream side of the NOx selective reduction catalyst, and supplies ammonia to the NOx selective reduction catalyst;
• wherein the abnormality diagnosis system is applied to the exhaust gas purification apparatus and comprises a controller comprising at least one processor configured to carry out an abnormality diagnosis of the NOx selective reduction catalyst;
• wherein
• the controller
• detects a concentration of ammonia in the exhaust gas at the downstream side of the NOx selective reduction catalyst;
• estimates a first estimated adsorption amount which is an amount of adsorption of ammonia in the NOx selective reduction catalyst at the time when the NOx selective reduction catalyst is assumed to be in a predetermined abnormal state;
• estimates a second estimated adsorption amount which is an amount of adsorption of ammonia in the NOx selective reduction catalyst at the time when the NOx selective reduction catalyst is assumed to be in a predetermined normal state;
• carries out diagnostic supply control to supply ammonia by the ammonia supply device at the time of carrying out the abnormality diagnosis of the NOx selective reduction catalyst, in such a manner that when an amount of adsorption of ammonia in the NOx selective reduction catalyst at the time when ammonia begins to slip from the NOx selective reduction catalyst in the case where the NOx selective reduction catalyst is in the predetermined abnormal state is defined as a first slip development adsorption amount, and when an amount of adsorption of ammonia in the NOx selective reduction catalyst at the time when ammonia begins to slip from the NOx selective reduction catalyst in the case where the NOx selective reduction catalyst is in the predetermined normal state is defined as a second slip development adsorption amount, the first estimated adsorption amount becomes equal to or more than a first predetermined adsorption amount which is equal to or more than the first slip development adsorption amount, and the second estimated adsorption amount becomes smaller than a second predetermined adsorption amount which is equal to or less than the second slip development adsorption amount;
• carries out the abnormality diagnosis of the NOx selective reduction catalyst based on the concentration of ammonia detected by the ammonia detection unit when the diagnostic supply control is carried out;
• and
• carries out decreasing control to decrease the amount of adsorption of ammonia in the NOx selective reduction catalyst after the end of the execution of the diagnostic supply control, in such a manner that a difference between the second estimated adsorption amount and the second predetermined adsorption amount is made larger than a difference between the first estimated adsorption amount and the first predetermined adsorption amount.