Air-fuel ratio control device for internal combustion engine

Abstract

An air-fuel ratio control apparatus for an internal combustion engine according to an embodiment of the invention (the present control apparatus) sets a target air-fuel ratio to a target rich air-fuel ratio when it is determined on the basis of the output value Voxs of a downstream air-fuel ratio sensorthat the oxygen adsorption amount of a catalysttends to be excessive, and sets the target air-fuel ratio to a target lean air-fuel ratio when it is determined on the basis of the output value Voxs that the oxygen adsorption amount of the catalysttends to be insufficient. Further, the present control apparatus determines whether or not an operation state in which a large amount of nitrogen oxide flows into the catalystis reached on the basis of 'whether a predetermined condition is fulfilled', and makes the target rich air-fuel ratio obtained when the predetermined condition is fulfilled less than the target rich air-fuel ratio obtained when the predetermined condition is not fulfilled. As a result, the concentration of a reducing agent inside the catalystcan be increased before a large amount of NOx flows into the catalyst. Therefore, when a large amount of NOx flows into the catalyst, most of the NOx can be eliminated.