Exhaust gas purifying device for internal combustion engine

An exhaust gas purifying device to be applied to an exhaust gas purifying system for an internal combustion engine,

• the exhaust gas purifying system comprising:
• an exhaust gas pipe through which an exhaust gas emitted from the internal combustion engine flows;
• a unit having NOx catalyst placed in the exhaust gas pipe;
• a NOx sensor placed at a downstream side of the unit having NOx catalyst;
• and
• a reducing agent addition unit, placed at an upstream side of the unit having NOx catalyst, for adding reducing agent into the inside of the exhaust gas pipe, where the NOx catalyst in the unit adsorbs the reducing agent, and the reducing agent selectively purifies NOx contained in the exhaust gas,
• the exhaust gas purifying device comprising:
• a control unit configured to instruct the reducing agent addition unit to adjust an addition amount of the reducing agent, and configured to execute abnormality diagnosis on the basis of a detection result by the NOx sensor when the addition amount of the reducing agent is changed,
• wherein the control unit is configured to execute the abnormality diagnosis when a first condition (a) and a second condition (b) are satisfied:
• (a) the detection result of the NOx sensor indicates that the exhaust gas flowing in the exhaust gas pipe contains NOx which is not less than a predetermined value;
• and
• (b) there is no abnormality caused by the reducing agent adsorbed to the NOx catalyst,
• wherein the control unit is further configured to calculate an adsorption amount of the reducing agent on the NOx catalyst on the basis of series data of a difference between an addition amount of the reducing agent added to the NOx catalyst by the reducing agent addition unit and an amount of the reducing agent consumed by the reduction reaction using the NOx catalyst;
• and
• the control unit is configured to initialize the adsorption amount of the reducing agent to the NOx catalyst, and
• wherein the second condition (b) is one of the following conditions (b-1) and (b-2):
• (b-1) no NOx purifying rate is increased even if the control unit initializes the adsorption amount of the reducing agent;
• and
• (b-2) there are cases of not less than predetermined times, in which the NOx purifying rate is increased when the adsorption amount of the reducing agent is initialized.