Inter-cylinder air-fuel ratio imbalance determination apparatus for an internal combustion engine

An inter-cylinder air-fuel ratio imbalance determination apparatus for an internal combustion engine, applied to a multi-cylinder internal combustion engine having a plurality of cylinders, comprising;

• an air-fuel ratio sensor, which is disposed in an exhaust merging portion of an exhaust passage of said engine into which exhaust gases discharged from at least two or more cylinders among a plurality of said cylinders merge or disposed in said exhaust passage at a position downstream of said exhaust merging portion, and which includes an air-fuel ratio detection section having a solid electrolyte layer, an exhaust-gas-side electrode layer which is formed on one of surfaces of said solid electrolyte layer, a diffusion resistance layer which covers said exhaust-gas-side electrode layer and which said exhaust gases reaches, and an atmosphere-side electrode layer which is formed on the other one of said surfaces of said solid electrolyte layer and is exposed to an atmosphere chamber, wherein, when a predetermined voltage is applied between said exhaust-gas-side electrode layer and said atmosphere-side electrode layer, said air-fuel ratio sensor outputs, based on a limiting current flowing through said solid electrolyte layer, an output value corresponding to an air-fuel ratio of an exhaust gas passing through said position at which the air-fuel ratio sensor is disposed;
• a plurality of fuel injection valves, each of which is disposed in such a manner that it corresponds to each of said at least two or more of said cylinders, and each of which injects fuel contained in an air-fuel mixture supplied to each of combustion chambers of said two or more of said cylinders;
• air-fuel ratio fluctuation indicating amount obtaining means for obtaining, based on said output value of said air-fuel ratio sensor, an air-fuel ratio fluctuation indicating amount whose absolute value becomes larger as a fluctuation of said air-fuel ratio of said exhaust gas passing through said position at which said air-fuel ratio sensor is disposed becomes larger;
• and
• imbalance determining means for comparing an imbalance determination parameter which becomes larger as said absolute value of said obtained air-fuel ratio fluctuation indicating amount becomes larger with a predetermined imbalance determination threshold, and for determining that an inter-cylinder air-fuel ratio imbalance state has been occurring when said imbalance determination parameter is larger than said imbalance determination threshold;
• wherein,
• said imbalance determining means includes imbalance determination parameter obtaining means for obtaining, based on said output value of said air-fuel ratio sensor, a parameter obtaining period average air-fuel ratio which is an average value of said air-fuel ratio of said exhaust gas passing through said position at which said air-fuel ratio sensor is disposed during a period in which said air-fuel ratio fluctuation indicating amount is being obtained, and for obtaining, as said imbalance determination parameter, a value obtained by correcting said air-fuel ratio fluctuation indicating amount based on said parameter obtaining period average air-fuel ratio in such a manner that said air-fuel ratio fluctuation indicating amount is made larger as said parameter obtaining period average air-fuel ratio is closer to the stoichiometric air-fuel ratio.