Air-fuel ratio imbalance among cylinders determining apparatus for an internal combustion engine

摘要

An air-fuel ratio imbalance among cylinders determining apparatus (a determining apparatus) according to an aspect of the present invention, obtains a parameter X for imbalance determination which becomes larger as a variation of an air-fuel ratio of an exhaust gas passing through a position at which the air-fuel ratio sensor is disposed becomes larger, based on an output value Vabyfs of an air-fuel ratio sensor 67. At the same time, the determining apparatus changes a target air-fuel ratio which is a target value of an air-fuel ratio of a mixture supplied to an engine to “an air-fuel ratio (a target rich air-fuel ratio AFrich or a target lean air-fuel ratio AFlean) other than a stoichiometric air-fuel ratio” from the stoichiometric air-fuel ratio. Accordingly, the determining apparatus can obtain the parameter X for imbalance determination in a state in which a responsivity of the air-fuel ratio sensor 67 is not low. As a result, it can make a determination on the air-fuel ratio imbalance among cylinders with high accuracy.

主权项

1. An air-fuel ratio imbalance among cylinders determining apparatus, applied to a multi-cylinder internal combustion engine having a plurality of cylinders comprising:
an air-fuel ratio sensor, disposed at an exhaust-gas-aggregated-portion of an exhaust gas passage to which exhaust gases discharged from at least two or more cylinders of said plurality of cylinders aggregate or at a position downstream of said exhaust-gas-aggregated-portion in said exhaust gas passage, having an air-fuel ratio detection section including a solid electrolyte layer, an exhaust-gas-side electrode layer formed on one of surfaces of said solid electrolyte layer, a diffusion resistance layer which said exhaust gases reach and which covers said exhaust-gas-side electrode layer, an atmosphere-side electrode layer formed on the other one of surfaces of said solid electrolyte layer so as to be exposed to an air in an atmosphere chamber, said air-fuel ratio sensor outputting an output value varying depending on an air-fuel ratio of said exhaust gases passing through a position at which said air-fuel ratio sensor is disposed, based on a limiting-current flowing through said solid electrolyte layer caused by applying a predetermined electrical voltage between said exhaust-gas-side electrode layer and said atmosphere-side electrode layer; a plurality of fuel injectors, each of said injectors provided in accordance with each of at least said two or more cylinders and injecting a fuel whose amount is in accordance with an instructed fuel injection amount, said fuel being contained in a mixture supplied to each of combustion chambers of said at least two or more cylinders; and an electric controller operatively connected to said plurality of fuel injectors, said electric controller configured to: (a) control said instructed fuel injection amount in such a manner that an air-fuel ratio of said mixture supplied to said combustion chambers of said at least two or more cylinders coincides with a target air-fuel ratio; (b) obtain, using said output value of said air-fuel ratio sensor, a parameter for imbalance determination which becomes larger as a variation of said air-fuel ratio of an exhaust gas passing through said position at which said air-fuel ratio sensor is disposed becomes larger; (c) determine that an air-fuel ratio imbalance state among cylinders is occurring when said obtained parameter for imbalance determination is larger than a predetermined threshold value for imbalance determination, and to determine that said air-fuel ratio imbalance state among cylinders is not occurring when said obtained parameter for imbalance determination is smaller than said predetermined threshold value for imbalance determination; (d) set said target air-fuel ratio at a non-stoichiometric air-fuel ratio which is an air-fuel ratio other than a stoichiometric air-fuel ratio and obtain said parameter for imbalance determination in a period in which a predetermined condition for obtaining a parameter for imbalance determination is satisfied, and to set said target air-fuel ratio at said stoichiometric air-fuel ratio in a period in which said predetermined condition for obtaining a parameter for imbalance determination is not satisfied; (e) maintain said target air-fuel ratio at said stoichiometric air-fuel ratio and obtains said parameter for imbalance determination as a tentative parameter based on the output value of the air-fuel ratio sensor before it sets said target air-fuel ratio at said non-stoichiometric air-fuel ratio in said period in which said predetermined condition for obtaining a parameter for imbalance determination is satisfied; (f) determine that said air-fuel ratio imbalance state among cylinders is occurring when said obtained tentative parameter is larger than a predetermined high side threshold value, (g) determine that said air-fuel ratio imbalance state among cylinders is not occurring when said obtained tentative parameter is smaller than a low side threshold value which is smaller than said high side threshold value by a predetermined amount; (h) defer a determination as to whether or not said air-fuel ratio imbalance state among cylinders is occurring when said obtained tentative parameter is smaller than said high side threshold value and larger than said low side threshold value; (i) set said target air-fuel ratio at said non-stoichiometric air-fuel ratio and obtains said parameter for imbalance determination based on said output value of said air-fuel ratio sensor as a final parameter, in a period in which said predetermined condition for obtaining a parameter for imbalance determination is satisfied in a case in which said determination as to whether or not said air-fuel ratio imbalance state among cylinders is occurring is deferred; (j) determine that said air-fuel ratio imbalance state among cylinders is occurring when said obtained final parameter is larger than said threshold value for imbalance determination, and determines that said air-fuel ratio imbalance state among cylinders is not occurring when said obtained final parameter is smaller than said threshold value for imbalance determination; and (k) control said plurality of fuel injectors in response to said determination of said air-fuel ratio imbalance state among cylinders.