Patent ID: 8726637

Claim:
An air-fuel ratio control system for an internal combustion engine having a plurality of cylinders, comprising: air-fuel ratio detecting means for detecting an air-fuel ratio in an exhaust passage of said engine; oscillation signal generating means for generating an oscillation signal to oscillate the air-fuel ratio with a set frequency which is different from a 0.5th-order frequency which is half of the frequency corresponding to a rotational speed of said engine; air-fuel ratio oscillating means for oscillating the air-fuel ratio according to the oscillation signal; 0.5th-order frequency component intensity calculating means for calculating an intensity of the 0.5th-order frequency component contained in an output signal of said air-fuel ratio detecting means; set frequency component intensity calculating means for calculating an intensity of the set frequency component contained in the output signal of said air-fuel ratio detecting means during operation of said air-fuel ratio oscillating means; determination parameter calculating means for calculating a determination parameter applied to determining an imbalance degree of air-fuel ratios corresponding to the plurality of cylinders according to the 0.5th-order frequency component intensity and the set frequency component intensity; imbalance failure determining means for determining an imbalance failure that the imbalance degree of the air-fuel ratios exceeds an acceptable limit, using the determination parameter; predicted imbalance value calculating means for calculating a predicted imbalance value indicative of a predicted value of the imbalance degree; and amplitude setting means for setting an amplitude of the oscillation signal according to the predicted imbalance value.