Exhaust gas purifying apparatus for internal combustion engine

Abstract

An exhaust pipe is provided with an oxidation catalyst, a SCR catalyst (ammonia selective reduction catalyst), and an ammonia slip catalyst. In the exhaust pipe, the urea water adding valve is provided between the oxidation catalyst and the SCR catalyst. A NOx sensor which detects the NOx quantity in an exhaust gas is provided downstream of the SCR catalyst. An ECU controls the urea water adding valve to add the urea water to the exhaust gas. While the urea water is added to the exhaust gas, the ECU successively obtains a NOx sensor output, and computes an adding quantity command value in which the NOx sensor output becomes minimum.

1 . An exhaust gas purifying apparatus for an internal combustion engine which includes a NOx reduction catalyst provided in an exhaust gas pipe, a reducing agent adding means for adding a reducing agent upstream of the NOx reduction catalyst, and a NOx sensor detecting a NOx quantity downstream of the NOx reduction catalyst, the exhaust gas purifying apparatus performing a reducing agent addition by means of the reducing agent adding means according to an adding quantity command value of a target value, the exhaust gas purifying apparatus comprising;
 an adding quantity control means for controlling an adding quantity of the reducing agent added by the reducing agent adding means; and   a command value computing means for computing the adding quantity command value in which a sensor output of the NOx sensor becomes minimum while obtaining the sensor output with respect to every adding quantity of the reducing agent controlled by the adding quantity control means.   
   
   
       2 . An exhaust gas purifying apparatus according to  claim 1 , wherein
 the adding quantity control means varies an adding quantity of the reducing agent at least into increasing side or decreasing side relative to the adding quantity command value as a reference.   
   
   
       3 . An exhaust gas purifying apparatus according to  claim 1 , wherein
 the adding quantity control means performs at least three steps of reducing agent additions while the adding quantity of the reducing agent is varied by a predetermined variation width, and   the adding quantity control means performs the reducing agent addition again while the variation width is made small in a case that the sensor output becomes minimum with respect to a medium quantity of reducing agent among at least three steps of reducing agent additions   
   
   
       4 . An exhaust gas purifying apparatus according to  claim 1 , further comprising
 an estimation means for estimating whether an adding quantity of the reducing agent for obtaining minimum value of the sensor output is in a decreasing side or increasing side based on the sensor output before varying the adding quantity and the sensor output after varying the adding quantity, wherein   the adding quantity control means increases or decreases the adding quantity of the reducing agent based on an estimation by the estimation means.   
   
   
       5 . An exhaust gas purifying apparatus according to  claim 1 , wherein
 the adding quantity control means sets a variation width of the adding quantity of the reducing agent based on the output value of the NOx sensor.   
   
   
       6 . An exhaust gas purifying apparatus according to  claim 1 , wherein
 the command value computing means does not newly compute the adding quantity command value when a difference between a minimum value of the sensor output and a maximum value of the sensor output, which are obtained due to a variation in adding quantity of the reducing agent is within a specified value.   
   
   
       7 . An exhaust gas purifying apparatus according to  claim 1 , wherein
 the adding quantity control means decreases the adding quantity in a specified injection interval which is longer than another injection interval in which the adding quantity is increased.   
   
   
       8 . An exhaust gas purifying apparatus according to  claim 1 , further comprising a learning means for storing the adding quantity command value in a back-up memory as a learning value and for updating the learning value as needed. 
   
   
       9 . An exhaust gas purifying apparatus according to  claim 8 , wherein
 the learning means stores the adding quantity command value along with a driving condition of the internal combustion engine at a time of controlling the adding quantity of the reducing agent.   
   
   
       10 . An exhaust gas purifying apparatus according to  claim 1 , further comprising:
 an oxidation catalyst arranged downstream of the NOx reduction catalyst for purifying the reducing agent; and   a determination means for determining whether the oxidation catalyst is active or not, wherein   when the determination means determines the oxidation catalyst is inactive, the command value computing means performs a computation of the adding quantity command value.   
   
   
       11 . An exhaust gas purifying apparatus according to  claim 1 , wherein
 the NOx sensor is provided with a sensor element which includes a solid electrolyte and a NOx detecting electrode provided on the solid electrolyte.