Patent ID: 8401766
Filing Date: 2013-03-19
Classification: F02D

Abstract:
1. 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 at a position 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 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 reach, an atmosphere-side electrode layer which is formed on the other one of said surfaces of said solid electrolyte layer, and a heater which heats a sensor element section including said solid electrolyte layer, said exhaust-gas-side electrode layer, and said atmosphere-side electrode layer, 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 said 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, in an amount in accordance with an instructed fuel injection amount; imbalance determining unit which is configured to: control a temperature of said sensor element section to a first temperature using said heater during a parameter-obtaining-period in which a predetermined parameter obtaining condition is satisfied, and obtain, as a usual temperature air-fuel ratio fluctuation indicating amount, a value corresponding to an air-fuel ratio fluctuation indicating amount which becomes larger as a fluctuation of an air-fuel ratio of said exhaust gas passing through said position at which said air-fuel ratio sensor is disposed becomes larger; control said temperature of said sensor element section to a second temperature higher than said first temperature using said heater during said parameter-obtaining-period, and obtain, as an elevated temperature air-fuel ratio fluctuation indicating amount, said value corresponding to said air-fuel ratio fluctuation indicating amount which becomes larger as said 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 obtain, based on said elevated temperature air-fuel ratio fluctuation indicating amount and said usual temperature air-fuel ratio fluctuation indicating amount, a value which becomes larger as a degree becomes larger of a difference between said elevated temperature air-fuel ratio fluctuation indicating amount and said usual temperature air-fuel ratio fluctuation indicating amount, as an imbalance determination parameter, and determine that an inter-cylinder air-fuel-ratio imbalance state has occurred when said obtained imbalance determination parameter is larger than a predetermined imbalance determination threshold, and determine that said inter-cylinder air-fuel-ratio imbalance state has not occurred when said obtained imbalance determination parameter is smaller than said imbalance determination threshold.