Air/fuel ration control system of internal combustion engine

In an internal combustion engine having a catalyst located in an exhaust passage for adsorbing oxygen contained within exhaust gases entering the catalyst, and an air/fuel ratio sensor located in the exhaust passage upstream of the catalyst for detecting an air/fuel ratio based on a percentage of oxygen contained within the exhaust gases flowing through the exhaust passage and entering the catalyst, an air/fuel ratio control system for controlling an air/fuel mixture ratio of the internal combustion engine at as close to stoichiometric as possible, comprising:

• (a) an arithmetic-calculation means for calculating a quantity of oxygen stored in the catalyst on the basis of a deviation of the air/fuel ratio detected by the air/fuel ratio sensor from a stoichiometric air/fuel ratio, to produce informational data indicative of a calculated value of the quantity of oxygen stored,
• (b) a control means for controlling an air/fuel mixture ratio so that the calculated value of the quantity of oxygen stored is adjusted to a desired value, and
• (c) a limiter means for preventing the calculated value of the quantity of oxygen stored from exceeding a predetermined limit value of the quantity of oxygen stored in the catalyst, said limiter means comprising
• (1) a determination means for determining whether the engine operates at a fuel cutoff operating mode, and
• (2) an update means for terminating arithmetic-operation for the quantity of oxygen stored, executed by the arithmetic-calculation means and for updating the calculated value by the predetermined limit value, when the calculated value of the quantity of oxygen stored is above the predetermined limit value during the fuel cutoff operating mode.

A method for controlling an air/fuel mixture ratio of an internal combustion engine, wherein the engine includes a catalyst located in an exhaust passage for adsorbing oxygen contained within exhaust gases entering the catalyst, and an air/fuel ratio sensor located in the exhaust passage upstream of the catalyst for detecting an air/fuel ratio based on a percentage of oxygen contained within the exhaust gases flowing through the exhaust passage and entering the catalyst, the method comprising:

• calculating a quantity of oxygen stored in the catalyst on the basis of a deviation of the air/fuel ratio detected by the air/fuel ratio sensor from a stoichiometric air/fuel ratio, to produce informational data indicative of a calculated value of the quantity of oxygen stored,
• controlling an air/fuel mixture ratio so that the calculated value of the quantity of oxygen stored is adjusted to a desired value,
• determining whether the engine operates at a fuel cutoff operating mode,
• terminating arithmetic-operation for the quantity of oxygen stored and updating the calculated value by a predetermined limit value of the quantity of oxygen stored in the catalyst, when the calculated value of the quantity of oxygen stored is above the predetermined limit value during the fuel cutoff operating mode.