Air/fuel ratio control system of internal combustion engine

An air/fuel ratio control system of an internal combustion engine, comprising:

• a catalyst located in an exhaust passage, for adsorbing oxygen contained within exhaust gases entering the catalyst;
• an upstream 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;
• a downstream air/fuel ratio sensor located in the exhaust passage downstream 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 leaving the catalyst;
• and
• a control unit configured to be electronically connected to the upstream and downstream air/fuel ratio sensors, for controlling an air/fuel mixture ratio at as close to a stoichiometric air/fuel ratio as possible, said control unit comprising:
• (a) an arithmetic-calculation section which calculates a quantity of oxygen stored in the catalyst on the basis of a deviation of the air/fuel ratio detected by the upstream air/fuel ratio sensor from the stoichiometric air/fuel ratio, to produce informational data indicative of a calculated value of the quantity of oxygen stored,
• (b) a control section which controls 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 which prevents the calculated value of the quantity of oxygen stored from exceeding a specified level, said limiter being operable in response to both of first and second determinations, in which:
• (i) the first determination is made when an air/fuel control with respect to the stoichiometric air/fuel ratio is inhibited, and
• (ii) the second determination is made when the air/fuel ratio detected by the downstream air/fuel ratio is indicated to be leaner than a predetermined lean air/fuel criterion,
• (d) a calculator which calculates a desired air/fuel ratio based upon the specified level in response to a third determination, in which
• (iii) the third determination is made when the air/fuel control with respect to the stoichiometric air/fuel ratio is resumed.