INTER-CYLINDER AIR/FUEL RATIO IMBALANCE JUDGMENTAL DEVICE FOR INTERNAL-COMBUSTION ENGINE

An apparatus for determining an air-fuel ratio imbalance among cylinders according to the present invention comprises "an upstream air-fuel ratio sensor 67, a catalytic converter 53, and a downstream air-fuel ratio sensor 68" disposed at positions downstream of an exhaust gas aggregated portion, calculates a sub feedback amount to have an output value of the downstream air-fuel ratio sensor coincides with a value corresponding to the stoichiometric air-fuel ratio, and performs an air-fuel ratio feedback control to have an air-fuel ratio of a mixture supplied to an engine based on the sub feedback amount and the output value of the upstream air-fuel ratio sensor. The apparatus for determining obtains a parameter for imbalance determination based on a learning value of the sub feedback amount, and determines that the air-fuel ratio imbalance among cylinders is occurring when the parameter for imbalance determination is equal to or larger than a threshold. When the number of times of update opportunity for an evaporated fuel gas concentration learning value after a start of the engine is smaller than a predetermined number of times, the parameter for imbalance determination is affected by an evaporated fuel gas purge, and therefore, the apparatus prohibits the determination of the air-fuel ratio imbalance among cylinders.