Exhaust gas cleaning device

Abstract

An exhaust cleaning device includes a NOx storage reduction catalyst storing NOx when an air-fuel ratio of an inflowing exhaust gas is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas is rich; an air-fuel ratio changing unit; a reducing agent supplying unit which adds a reducing agent to the NOx storage reduction catalyst; a temperature detection unit which detects a temperature of the NOx storage reduction catalyst; a poisoning recovery timing judging unit which judges a timing for executing a SOx poisoning recovery process of releasing SOx from the NOx storage reduction catalyst; and a control unit.

1 . An exhaust cleaning device comprising: 
 a NOx storage reduction catalyst, being disposed in an exhaust passage of a lean-combustion-enabled internal combustion engine and, the Nox storage reduction catalyst storing NOx when an air-fuel ratio of an inflowing exhaust gas is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas is rich;    an air-fuel ratio changing unit which changes the air-fuel ratio;    a reducing agent supplying unit which adds a reducing agent to the NOx storage reduction catalyst;    a temperature detection unit which detects a temperature of the NOx storage reduction catalyst;    a poisoning recovery timing judging unit which judges a timing for executing a SOx poisoning recovery process of releasing SOx from the NOx storage reduction catalyst; and    a control unit making the reducing agent be added from the reducing agent supplying unit when the poisoning recovery timing judging unit judges that the timing for executing the SOx poisoning recovery process has arrived, and the control unit controlling the air-fuel ratio changing unit to put the air-fuel ratio in a rich state if the detected temperature from the temperature detection unit is lower than a thermal dissociation temperature of the NOx storage reduction catalyst and is higher than a NOx reduction reaction enabling temperature.    
   
   
       2 . The exhaust gas cleaning device according to  claim 1 , wherein, when the poisoning recovery timing judging unit judges that the timing for executing the SOx poisoning recovery process has arrived, the control unit makes the added amount of the reducing agent which is added from the reducing agent supplying unit, increase gradually and, 
 if the detected temperature from the temperature detection unit is lower than the thermal dissociation temperature of the NOx storage reduction catalyst and is higher than the NOx reduction reaction enabling temperature, the control unit controls the air-fuel ratio changing unit to put the air-fuel ratio in a rich state.    
   
   
       3 . The exhaust gas cleaning device according to  claim 1 , further comprising a control unit, which, in the SOx poisoning recovery process of releasing SOx from the NOx storage reduction catalyst, controls the air-fuel ratio changing unit and the reducing agent supplying unit to perform a first reducing agent addition by the reducing agent supplying unit so that the detected temperature from the temperature detection unit become a temperature in a range that is higher than the reduction reaction enabling temperature of the NOx storage reduction catalyst and lower than the thermal dissociation temperature of the NOx storage reduction catalyst and, when the detected temperature from the temperature detection unit becomes a temperature in the range, sets the air-fuel ratio to a rich state by the air-fuel ratio changing unit and executes a second reducing agent addition by the reducing agent supplying unit so that the detected temperature from the temperature detection unit becomes equal to a SOx poisoning recovery target temperature for the NOx storage reduction catalyst.  
   
   
       4 . The exhaust gas cleaning device according to  claim 3 , wherein, when the detected temperature from the temperature detection unit is lower than a reaction enabling temperature of the catalyst, the control unit performs temperature raising control of the internal combustion engine so that the detected temperature from the temperature detection unit becomes equal to the reaction enabling temperature.  
   
   
       5 . The exhaust gas cleaning device according to  claim 3 , wherein the added amount of the reducing agent is higher in the second reducing agent addition than in the first reducing agent addition.