Catalyst monitoring system and monitoring method

Abstract

A catalyst monitoring system diagnoses deterioration of an NO X catalyst ( 18 ) which is arranged in an exhaust passage ( 12 ) of an internal combustion engine ( 10 ). An NO X sensor ( 25 ) is arranged downstream of the NO X catalyst ( 18 ). An output integrated value of the NO X sensor ( 25 ) is calculated by integrating an output from the NO X sensor ( 25 ) during at least a period near the end of air-fuel ratio control. Deterioration of the NO X catalyst ( 18 ) is diagnosed based on the output integrated value of the NO X sensor ( 25 ).

1 . (canceled) 
   
   
       2 . The catalyst monitoring system according to  claim 16 , wherein
 the NOX sensor is capable of detecting a concentration of NH3 as well as the concentration of NOX.   
   
   
       3 . The catalyst monitoring system according to  claim 16 , wherein
 the NOX sensor is a limiting current NOX sensor.   
   
   
       4 . The catalyst monitoring system according to  claim 16 , wherein
 the period near the end of the air-fuel ratio control is a period during which the output from the NOX sensor temporarily increases abruptly.   
   
   
       5 . The catalyst monitoring system according to  claim 16 , further comprising
 reducing agent amount calculating portion that calculates an amount of reducing agent that has flowed into the NOX catalyst during the air-fuel ratio control,   wherein the air-fuel ratio controlling means ends the air-fuel ratio control when the reducing agent amount reaches a predetermined amount.   
   
   
       6 . The catalyst monitoring system according to  claim 16 , further comprising
 reducing agent amount calculating portion that calculates an amount of reducing agent that has flowed into the NOX catalyst during the air-fuel ratio control,   wherein the diagnosing portion diagnoses deterioration of the NOX catalyst based on the output integrated value of the NOX sensor and the reducing agent amount.   
   
   
       7 . The catalyst monitoring system according to  claim 16 , further comprising
 reducing time measuring portion that measures a reducing time of NOX according to the air-fuel ratio control,   wherein the diagnosing portion diagnoses deterioration of the NOX catalyst based on the output integrated value of the NOX sensor and the reducing time.   
   
   
       8 . The catalyst monitoring system according to  claim 16 , wherein
 the air-fuel ratio controlling portion starts the air-fuel ratio control when the amount of NOX that has flowed into the NOX catalyst reaches a predetermined value.   
   
   
       9 . The catalyst monitoring system according to  claim 16 , further comprising
 an O2 sensor which is arranged downstream of the NOX catalyst and detects a concentration of O2,   wherein the air-fuel ratio controlling portion ends the air-fuel ratio control when an output from the O2 sensor while the air-fuel ratio control is being executed becomes the rich air-fuel ratio.   
   
   
       10 . The catalyst monitoring system according to  claim 16 , wherein
 the diagnosing means starts to diagnose deterioration of the NOX catalyst when a predetermined executing condition for determining deterioration is satisfied, and the predetermined executing condition for determining deterioration includes: i) a condition that the internal combustion engine operate at a predetermined operating condition while the air-fuel ratio control is executed; and ii) a condition that a temperature of the NOX catalyst be within a predetermined temperature range while the air-fuel ratio control is executed.   
   
   
       11 . The catalyst monitoring system according to  claim 10 , wherein
 the predetermined operating condition includes a condition that at least one from among a speed of the internal combustion engine, a throttle opening amount of the internal combustion engine, and an intake air amount of the internal combustion engine be within a predetermined range.   
   
   
       12 . The catalyst monitoring system according to  claim 4 , wherein the period near the end of the air-fuel ratio control includes a period during which the output from the NOX sensor temporarily increases abruptly due to NH3 that is discharged from the NOX catalyst. 
   
   
       13 . The catalyst monitoring system according to  claim 16 , wherein
 the calculating portion calculates the output integrated value of the NO X  sensor by integrating the product of a physical quantity corresponding to an amount of intake air flowing into the NOX catalyst multiplied by the output from the NOX sensor.   
   
   
       14 . The catalyst monitoring system according to  claim 16 , wherein
 the air-fuel ratio controlling portion ends the air-fuel ratio control when the output current from the NOX sensor while the air-fuel ratio control is being executed corresponds to the rich air-fuel ratio.   
   
   
       15 . A catalyst monitoring method comprising:
 performing air-fuel ratio control that temporarily switches an air-fuel ratio of exhaust gas during operation of the internal combustion engine at a lean air-fuel ratio to a rich or a stoichiometric air-fuel ratio;   calculating an output integrated value of an NOX sensor which is arranged downstream of an NOX catalyst in an exhaust passage of the internal combustion engine by integrating an output from the NOX sensor at least during a period near the end of the air-fuel ratio control; and   diagnosing deterioration of the NOX catalyst based on the output integrated value of the NOX sensor.   
   
   
       16 . A catalyst monitoring system comprising:
 an NOX catalyst which is arranged in an exhaust passage of an internal combustion engine;   an NOX sensor which is arranged downstream of the NOX catalyst and detects a concentration of NO X ;   an air-fuel ratio controlling portion that temporarily switches an air-fuel ratio of exhaust gas during operation of the internal combustion engine from a lean air-fuel ratio to a rich or a stoichiometric air-fuel ratio;   a calculating portion that calculates an output integrated value of the NOX sensor by integrating an output from the NOX sensor at least during a period near the end of the air-fuel ratio control; and   a diagnosing portion that diagnoses deterioration of the NOX catalyst based on the output integrated value of the NOX sensor.