Patent ID: 7555895

Claim:
An exhaust gas purifying method for an internal combustion engine, the internal combustion engine comprising an NOx removing catalyst that absorbs nitrogen oxides in an exhaust gas of the engine when an air-fuel ratio of the exhaust gas streaming thereinto is lean and that releases and reduces the absorbed nitrogen oxides therefrom when the air-fuel ratio of the exhaust gas streaming thereinto is rich, the method comprising: providing an exhaust gas atmosphere varying section that varies a ratio between an oxidizing agent in the exhaust gas and a reducing agent therein; providing a first exhaust gas atmosphere detecting section disposed in an upstream side of an exhaust passage with respect to the NOx removing catalyst to detect the ratio between the oxidizing agent in the exhaust gas and the reducing agent therein; providing a second exhaust gas atmosphere detecting section disposed in a downstream side of the exhaust passage with respect to the NOx removing catalyst to detect the ratio between the oxidizing agent in the exhaust gas and the reducing agent therein; and executing an abnormality determination of the NOx removing catalyst on the basis of output values of both of the first exhaust gas atmosphere detecting section and the second exhaust gas atmosphere detecting section from a time at which the output value of the first exhaust gas atmosphere detecting section is varied to a first predetermined value to a time at which the output value of the second exhaust gas atmosphere detecting section reaches-a second predetermined value when the exhaust gas atmosphere varying section increases the ratio between the reducing agent and the oxidizing agent in the exhaust gas; calculating an integration quantity with respect to time of a difference between the output values of the first exhaust gas atmosphere detecting means and the second exhaust gas atmosphere detecting means and executing the abnormality determination of the NOx removing catalyst on the basis of the calculated integration quantity of the difference; and suspending the abnormality determination of the NOx removing catalyst when the difference of the output values of the first exhaust atmosphere detecting section and the second exhaust gas atmosphere detecting section is larger than a third predetermined value when the output value of the second exhaust gas atmosphere detecting section reaches the second predetermined value.