Air/fuel ratio control apparatus and method for internal combustion engine and engine control unit

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

• air/fuel ratio sensing means for outputting a detection signal indicative of an air/fuel ratio of exhaust gases at a location downstream of a catalyst in an exhaust passage of said internal combustion engine;
• target air/fuel ratio setting means for setting a target air/fuel ratio for converging an output of said air/fuel ratio sensing means to a predetermined target value such that the output of said downstream air/fuel ratio sensing means fluctuates over a predetermined amplitude and at a predetermined frequency higher when the output is near said predetermined target value than when the output is not near said predetermined target value;
• and
• air/fuel ratio control means for controlling the air/fuel ratio of an air/fuel mixture supplied to said internal combustion engine in accordance with the set target air/fuel ratio.

An air/fuel ratio control method for an internal combustion engine comprising the steps of:

• detecting an output of upstream air/fuel ratio sensing means indicative of an upstream an air/fuel ratio of exhaust gases at a location upstream of a catalyst in an exhaust passage of said internal combustion engine;
• detecting an output of downstream air/fuel ratio sensing means indicative of a downstream air/fuel ratio of the exhaust gases at a location downstream of said catalyst in said exhaust passage;
• setting a target air/fuel ratio for converging the output of said downstream air/fuel ratio sensing means to a predetermined target value such that the output of said downstream air/fuel ratio sensing means fluctuates over a predetermined amplitude and at a predetermined frequency higher when the output of said downstream air/fuel ratio sensing means is near said predetermined target value than when the output of said downstream air/fuel ratio sensing means is not near said predetermined target value;
• and
• controlling the air/fuel ratio of an air/fuel mixture supplied to said internal combustion engine based on the output of said upstream air/fuel ratio sensing means to match the upstream air/fuel ratio of the exhaust gases upstream of said catalyst with the set target air/fuel ratio.

An air/fuel ratio control method for an internal combustion engine comprising the steps of:

• detecting an output of an air/fuel ratio sensing means indicative of an air/fuel ratio of exhaust gases at a location downstream of a catalyst in an exhaust passage of said internal combustion engine;
• setting a target air/fuel ratio for converging the output of said air/fuel ratio sensing means to a predetermined target value such that the output of said air/fuel ratio sensing means fluctuates over a predetermined amplitude and at a predetermined frequency higher when the output of said air/fuel ratio sensing means is near said predetermined target value than when the output of said air/fuel ratio sensing means is not near said predetermined target value;
• and
• controlling the air/fuel ratio of an air/fuel mixture supplied to said internal combustion engine in accordance with the set target air/fuel ratio.