DEVICE FOR DETERMINING IMBALANCE IN AIR-FUEL RATIO BETWEEN CYLINDERS OF INTERNAL COMBUSTION ENGINE

An inter-cylinder air-fuel ratio imbalance determination apparatus (determination apparatus) according to the present invention obtains, based on an output value Vabyfs of an air-fuel ratio sensor 67, an air-fuel ratio fluctuation indicating amount AFD which becomes larger as an air-fuel ratio fluctuation of an exhaust gas passing through a position at which the air-fuel ratio sensor 67 is disposed becomes larger, during a parameter obtaining period. The determination apparatus estimates an air-fuel ratio sensor element temperature Temps having a strong relation with responsiveness of the air-fuel ratio sensor 67 during the parameter obtaining period, and obtains a corrected air-fuel ratio fluctuation indicating amount by correcting the air-fuel ratio fluctuation indicating amount AFD based on the estimated air-fuel ratio sensor element temperature Temps. The determination apparatus adopts the corrected air-fuel ratio fluctuation indicating amount as the imbalance determination parameter X, and determines whether or not an inter-cylinder air-fuel-ratio imbalance state has been occurring based on a comparison between the imbalance determination parameter X and the imbalance determination threshold Xth.