Patent ID: 8589055

Claim:
An inter-cylinder air/fuel ratio imbalance determination apparatus, comprising: an air/fuel ratio sensor that is a limiting current type sensor that is disposed in an exhaust confluence portion of an exhaust passageway of a multi-cylinder internal combustion engine in which flows of exhaust gas discharged from a plurality of cylinders of the engine meet, or is disposed downstream of the exhaust confluence portion; a plurality of fuel injection valves that inject a fuel that is contained in a mixture that is supplied into a combustion chamber of each of the cylinders; an injection command signal send-out device that is configured to send out an injection command signal to the fuel injection valves so that each of the fuel injection valves injects an amount of the fuel that is commensurate with a predetermined commanded fuel injection amount; a fuel-cut device that is configured to execute a fuel-cut operation by stopping fuel injection performed by the fuel injection valves when a predetermined fuel-cut condition is satisfied; an air/fuel ratio imbalance index value acquisition device that is configured to acquire an air/fuel ratio imbalance index value that increases with increase in degree of non-uniformity between the cylinders in a cylinder-by-cylinder air/fuel ratio that is an air/fuel ratio of the mixture supplied into the combustion chamber of each of the cylinders; an imbalance determination device that is configured to determine whether or not an inter-cylinder air/fuel ratio imbalance state has occurred, based on a result of comparison between the air/fuel ratio imbalance index value acquired and a predetermined imbalance determination threshold value, wherein the air/fuel ratio imbalance index value acquisition device is configured to acquire a correction-purpose output value that increases with increase in output value of the air/fuel ratio sensor during a period of execution of the fuel-cut operation, and to acquire as the air/fuel ratio imbalance index value an air/fuel ratio fluctuation index quantity that increases with increase in fluctuation of the output value of the air/fuel ratio sensor and that decreases with increase in the correction-purpose output value, based on the output value of the air/fuel ratio sensor and the correction-purpose output value.