Air-fuel ratio control apparatus for an internal combustion engine

An air-fuel ratio control apparatus for an internal combustion engine, comprising:

• a catalyst, which is disposed in an exhaust air system for the internal combustion engine, for removing a toxic substance from an exhaust gas;
• a first oxygen concentration sensor for detecting a first oxygen concentration in the exhaust gas at a position upstream of the catalyst;
• a second oxygen concentration sensor for detecting a second oxygen concentration in the exhaust gas at a position downstream of the catalyst;
• an injector for supplying a fuel to the internal combustion engine;
• injector drive means for adjusting an amount of the fuel supplied from the injector to the internal combustion engine;
• first air-fuel ratio control means for controlling the injector drive means such that a first air-fuel ratio corresponding to a detection signal from the first oxygen concentration sensor matches a first target air-fuel ratio;
• second air-fuel ratio control means for setting the first target air-fuel ratio such that a second air-fuel ratio corresponding to a detection signal from the second oxygen concentration sensor matches a second target air-fuel ratio;
• and
• target air-fuel ratio setting means for setting the second target air-fuel ratio, wherein:
• the target air-fuel ratio setting means stores a first predetermined value based on NOx purification rate characteristics of the catalyst depending on the air-fuel ratio in the exhaust air system for the internal combustion engine;
• and
• the target air-fuel ratio setting means sets the second target air-fuel ratio at the first predetermined value on a rich side of a stoichiometric air-fuel ratio such that an NOx purification rate is a maximum when the catalyst is in a deteriorated condition.