Patent ID: 7694508

Claim:
An air-fuel ratio control system for an internal combustion engine having a catalyst provided in an exhaust system of said engine for purifying exhaust gases, said air-fuel ratio control system controlling an air-fuel ratio of an air-fuel mixture supplied to said engine and comprising: intake air flow rate detecting means for detecting an intake air flow rate of said engine; a first oxygen concentration sensor disposed upstream of said catalyst; a second oxygen concentration sensor disposed downstream of said catalyst; inflowing oxygen amount calculating means for calculating an amount of oxygen flowing into said catalyst according to an oxygen concentration detected by said first oxygen concentration sensor and the intake air flow rate detected by said intake air flow rate detecting means; air-fuel ratio switching control means for alternately controlling the air-fuel ratio to a lean air-fuel ratio which is leaner than the stoichiometric ratio and a rich air-fuel ratio which is richer than the stoichiometric ratio according to a result of comparison between the inflowing oxygen amount calculated by said inflowing oxygen amount calculating means and target values of the inflowing oxygen amount; rich operation oxygen amount calculating means for calculating a rich operation oxygen amount which indicates an amount of oxygen flowing into said catalyst during a time period from the time when the oxygen concentration detected by said first oxygen concentration sensor changes to a value indicative of a rich air-fuel ratio which is richer than the stoichiometric ratio to the time when an output of said second oxygen concentration sensor changes to a value indicative of the rich air-fuel ratio; lean operation oxygen amount calculating means for calculating a lean operation oxygen amount which indicates an amount of oxygen flowing into said catalyst during a time period from the time when the oxygen concentration detected by said first oxygen concentration sensor changes to a value indicative of a lean air-fuel ratio which is leaner than the stoichiometric ratio to the time when the output of said second oxygen concentration sensor changes to a value indicative of the lean air-fuel ratio; and correction amount calculating means for calculating a correction amount for correcting the oxygen concentration detected by said first oxygen concentration sensor, based on the rich operation oxygen amount, the lean operation oxygen amount, and an accumulated value obtained by accumulating the intake air flow rate detected by said intake air flow rate detecting means, wherein said inflowing oxygen amount calculating means, said rich operation oxygen amount calculating means, and said lean operation oxygen amount calculating means use the oxygen concentration corrected with the correction amount to respectively calculate the inflowing oxygen amount, the rich operation oxygen amount, and the lean operation oxygen amount.