Patent ID: 8249794

Claim:
In a multi-cylinder internal combustion engine, having a plurality of combustion chambers, fuel injectors arranged corresponding to said combustion chambers, respectively and an air-fuel ratio sensor arranged for detecting an air-fuel ratio of an exhaust gas in one of an exhaust passage converging portion where exhaust passages in communication with said corresponding combustion chambers converge and an exhaust passage portion downstream of said exhaust passage converging portion, wherein an amount of fuel injected from each fuel injector is controlled such that an air-fuel ratio of a mixture formed in each combustion chamber becomes a target air-fuel ratio on the basis of an output value of said air-fuel ratio sensor, an inter-cylinder air-fuel ratio imbalance judgment device for the multi-cylinder internal combustion engine for judging whether there is an inter-cylinder air-fuel ratio imbalance condition where a difference in the air-fuel ratio of the mixture between said combustion chambers is greater than a predetermined air-fuel ratio difference, wherein assuming that an amount of air introduced into said combustion chamber is referred to as an intake air amount, an amount of change of the air-fuel ratio of the exhaust gas detected by said air-fuel ratio sensor per unit time is referred to as a unit air-fuel ratio change amount, and the unit air-fuel ratio change amount when the difference in the air-fuel ratio of the mixture between said combustion chambers is smaller than or equal to said predetermined air-fuel ratio difference and the intake air amount is a specific intake air amount is referred to as a base unit air-fuel ratio change amount, one of said base unit air-fuel ratio change amount and the unit air-fuel ratio change amount greater than said base unit air-fuel ratio change amount by a predetermined value is set as a judgment value, wherein the unit air-fuel ratio change amount is calculated during the operation of said internal combustion engine, wherein when the intake air amount is smaller than said specific intake air amount, the unit air-fuel ratio change amount calculated during the operation of said internal combustion engine is corrected such that the unit air-fuel ratio change amount increases in the condition that said base unit air-fuel ratio change amount is an upper limit for the corrected unit air-fuel ratio change amount, wherein when the intake air amount is greater than said specific intake air amount, the unit air-fuel ratio change amount calculated during the operation of said internal combustion engine is corrected such that the unit air-fuel ratio change amount decreases in the condition that said base unit air-fuel ratio change amount is a lower limit for the corrected unit air-fuel ratio change amount, and wherein when the corrected unit air-fuel ratio change amount is greater than said judgment value, it is judged that the inter-cylinder air-fuel ratio imbalance condition occurs.