Patent ID: 8401765
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 of 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 detecting section including 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, 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, 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, owing to an application of a predetermined voltage between said exhaust-gas-side electrode layer and said atmosphere-side electrode layer; 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 perform an imbalance determination by obtaining, based on said output value of said air-fuel ratio sensor, an air-fuel ratio fluctuation indicating amount which becomes larger as a fluctuation of an air-fuel ratio of an exhaust gas passing through said position at which said air-fuel ratio sensor is disposed becomes larger, in a parameter obtaining period which is a period in which a predetermined parameter obtaining condition is being satisfied; by performing a comparison between an imbalance determination parameter obtained based on said obtained air-fuel ratio fluctuation indicating amount and a predetermined imbalance determination threshold; and by determining that an inter-cylinder air-fuel ratio imbalance state has occurred when said imbalance determination parameter is larger than said imbalance determination threshold and determining that said inter-cylinder air-fuel ratio imbalance state has not occurred when said imbalance determination parameter X is smaller than said imbalance determination threshold; wherein, said imbalance determining unit includes: an element temperature estimating portion configured to estimate an air-fuel ratio sensor element temperature which is a temperature of said solid electrolyte layer during said parameter obtaining period; and a pre-comparison preparation portion configured to perform at least one of determinations before performing said comparison between said imbalance determination parameter and said imbalance determination threshold, wherein one of said determinations being to obtain a corrected air-fuel ratio fluctuation indicating amount by performing, on said obtained air-fuel ratio fluctuation indicating amount, a correction to decrease said obtained air-fuel ratio fluctuation indicating amount as said estimated air-fuel ratio sensor element temperature becomes higher with respect to a specific temperature, and/or, a correction to increase said obtained air-fuel ratio fluctuation indicating amount as said estimated air-fuel ratio sensor element temperature becomes lower with respect to said specific temperature, and to determine, as said imbalance determination parameter, a value corresponding to said corrected air-fuel ratio fluctuation indicating amount; and the other of said determinations being to determine, based on said estimated air-fuel ratio sensor element temperature, said imbalance determination threshold, in such a manner that said imbalance determination threshold increases as said estimated air-fuel ratio sensor element temperature becomes higher.