Exhaust gas purifying apparatus and method for internal combustion engine

An exhaust gas purifying apparatus for an internal combustion engine, comprising:

• NOx removing catalyst means for absorbing 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;
• exhaust gas atmosphere varying means for varying a ratio between an oxidizing agent in the exhaust gas and a reducing agent therein;
• first exhaust gas atmosphere detecting means disposed in an upstream side of an exhaust passage with respect to the NOx removing catalyst means for detecting the ratio between the oxidizing agent in the exhaust gas and the reducing agent therein;
• second exhaust gas atmosphere detecting means disposed in a downstream side of the exhaust passage with respect to the NOx removing catalyst means for detecting the ratio between the oxidizing agent in the exhaust gas and the reducing agent therein;
• and
• abnormality determining means for executing an abnormality determination of the NOx removing catalyst means on the basis of output values of both of the first exhaust gas atmosphere detecting means and the second exhaust gas atmosphere detecting means from a time at which the output value of the first exhaust gas atmosphere detecting means is varied to a first predetermined value to a time at which the output value of the second exhaust gas atmosphere detecting means is reached to a second predetermined value when the exhaust gas atmosphere varying means increases the ratio of the reducing agent in the exhaust gas.

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;

• 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 is reached to a second predetermined value when the exhaust gas atmosphere varying section increases the ratio of the reducing agent in the exhaust gas.