The disclosure of Japanese Patent Application No. 2001-305424 filed on Oct. 1, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to an exhaust gas purifying apparatus for an internal combustion engine and a control method thereof.
2. Description of the Related Art
As an exhaust gas purifying apparatus for purifying an exhaust gas in an internal combustion engine, there is known an art in which plural catalysts for purifying the exhaust gas are arranged in parallel in exhaust passages. In such an exhaust gas purifying apparatus for an internal combustion engine, catalysts having an oxidation function such as nitrogen oxides (NOx) catalysts may be arranged in parallel.
The NOx catalyst can purify the exhaust gas discharged from angas is absorbed in the NOx catalyst. However, if the exhaust gas at a lean air-fuel ratio continues to be supplied to the NOx catalyst, the amount of the NOx absorbed in the NOx catalyst reaches a saturation amount, and as a result, the catalyst cannot absorb the NOx any further and the NOx passes through the catalyst. Thus, it is required that, in the NOx catalyst, the oxygen concentration of the exhaust gas should be decreased at predetermined timing before the amount of the NOx absorbed in the NOx catalyst reaches a saturation amount, and a component amount of hydrogen carbide (HC) in the exhaust gas should be increased, thereby emitting the NOx absorbed in the NOx catalyst and reducing the NOx into nitrogen (N2) in order