Patent ID: 8600647
Filing Date: 2013-12-03
Classification: F02D

Abstract:
1. An air-fuel ratio control apparatus applied to a multi-cylinder internal combustion engine having a plurality of cylinders, comprising: a catalytic converter disposed in an exhaust passage of said engine and at a position downstream of an exhaust gas aggregated portion into which exhaust gases discharged from combustion chambers of at least two or more of a plurality of said cylinders merge; fuel injectors, each injecting a fuel to be contained in a mixture supplied to each of said combustion chambers of said two or more of said cylinders; a downstream air-fuel ratio sensor, which is disposed in the exhaust passage and at a position downstream of the catalytic converter, and which outputs an output value according to an air-fuel ratio of a gas passing through said position at which said downstream air-fuel ratio sensor is disposed; first feedback amount updating means for updating, every time a predetermined first update timing arrives, a first feedback amount to have said output value of said downstream air-fuel ratio sensor coincide with a value corresponding to a target downstream-side air-fuel ratio, based on said output value of said downstream air-fuel ratio sensor and said value corresponding to the target downstream-side air-fuel ratio; learning means for updating, every time a predetermined second update timing arrives, a learning value of said first feedback amount in such a manner that said learning value brings in a steady-state component of said first feedback amount, based on said first feedback amount; air-fuel ratio control means for controlling an air-fuel ratio of an exhaust gas flowing into said catalytic converter by controlling an amount of said fuel injected from said fuel injectors, based on at least one of said first feedback amount and said learning value; expedited learning means for inferring whether or not an insufficient learning state is occurring in which a second error which is a difference between said learning value and a value on which said learning value is supposed to converge is equal to or larger than a predetermined value, and for performing an expedited learning control to increase a changing speed of said learning value when it is inferred that said insufficient learning state is occurring as compared to when it is inferred that said insufficient learning state is not occurring; and prohibiting expedited learning means for inferring whether or not a disturbance which transiently varies said air-fuel ratio of said mixture supplied to said combustion chambers of said at least two or more of said cylinders occurs, and for prohibiting said expedited learning control when it is inferred that said disturbance occurs; and wherein, said air-fuel ratio control means includes: an upstream air-fuel ratio sensor, which is disposed at said aggregated exhaust gas portion or between said aggregated exhaust gas portion and said catalytic converter in said exhaust passage, which outputs an output value according to an air-fuel ratio of a gas passing through a position at which said upstream air-fuel ratio sensor is disposed, and which includes a diffusion resistance layer with which said exhaust gas which has not passed through said catalytic converter contacts and an air-fuel ratio detecting element which outputs said output value; base fuel injection amount determining means for determining a base fuel injection amount to have said air-fuel ratio of said mixture supplied to said combustion chambers of said at least two or more of said cylinders coincide with a target upstream-side air-fuel ratio, based on an intake air amount of said engine and said target upstream-side air-fuel ratio; second feedback amount updating means for updating, every time a predetermined third update timing arrives, a second feedback amount to correct said base fuel injection amount, based on said output value of said upstream air-fuel ratio sensor, said first feedback amount, and said learning value, in such a manner that said air-fuel ratio of said mixture supplied to said combustion chambers of said at least two or more of said cylinders coincides with said target upstream-side air-fuel ratio; and fuel injection instruction means for instructing said fuel injectors to inject said fuel of a fuel injection amount obtained by correcting said base fuel injection amount by said second feedback amount; said air-fuel ratio control apparatus comprises: parameter for imbalance determination obtaining means for obtaining, based on said learning value, a parameter for imbalance determination which increases as a difference between an amount of hydrogen included in said exhaust gas which has not passed through said catalytic converter and an amount of hydrogen included in said exhaust gas which has passed through said catalytic converter becomes larger; and air-fuel ratio imbalance among cylinders determining means for determining that a non-uniformity is occurring among individual cylinder air-fuel ratios of mixtures, each supplied to each of said at least two or more of said cylinders, when said obtained parameter for imbalance determination is equal to or larger than an abnormality determination threshold.