JP2010180746A - Air-fuel ratio inter-cylinder imbalance determining device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide " an air-fuel ratio inter-cylinder imbalance determining device of an internal combustion engine of high practical use ". <P>SOLUTION: This air-fuel ratio inter-cylinder imbalance determining device includes " an upstream side air-fuel ratio sensor 55, a catalyst 53 and a downstream side air-fuel ratio sensor 56 " arranged in order on the downstream side of an exhaust gas collecting part of an exhaust passage, and calculates a sub-feedback quantity for making an output value of the downstream side air-fuel ratio sensor coincide with a value corresponding to the stoichiometric air-fuel ratio, and performs air-fuel ratio feedback control for making the air-fuel ratio of an air-fuel mixture supplied to the engine coincide with the stoichiometric air-fuel ration based on a sub-feedback quantity and an output value of the upstream side air-fuel ratio sensor. At this time, a determining device obtains a hydrogen quantity difference indicating parameter based on a learning value of the sub-feedback quantity, and obtains a rotating speed variation parameter of indicating a rotational variation in the engine. The determining device determines that an air-fuel ratio inter-cylinder imbalance is caused when the hydrogen quantity difference indicating parameter is a threshold value or more, and corrects its threshold value so as to become small as the rotating speed variation parameter becomes large. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is applied to a multi-cylinder internal combustion engine, and an air-fuel ratio imbalance of an air-fuel mixture supplied to each cylinder (air-fuel ratio imbalance among cylinders, air-fuel ratio variation among cylinders, air-fuel ratio non-uniformity among cylinders). The present invention relates to an “air-fuel ratio imbalance among cylinders determination apparatus for an internal combustion engine” that can determine (monitor / detect) that the engine has increased.

  Conventionally, a three-way catalyst disposed in an exhaust passage of an internal combustion engine, an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor disposed in the exhaust passage and upstream and downstream of the three-way catalyst, An air-fuel ratio control device including the above is widely known. This air-fuel ratio control device is configured to output the upstream air-fuel ratio sensor output value and the downstream air-fuel ratio sensor output value so that the air-fuel ratio of the air-fuel mixture supplied to the engine (engine air-fuel ratio) matches the stoichiometric air-fuel ratio. Based on the above, the air-fuel ratio of the engine is feedback-controlled.

  Such an air-fuel ratio control apparatus controls the air-fuel ratio of the engine using a control amount (air-fuel ratio feedback amount) common to all cylinders. That is, the air-fuel ratio control is executed so that the average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine matches the stoichiometric air-fuel ratio.

  For example, when the measured or estimated value of the intake air amount of the engine deviates from the “true intake air amount”, the fuel injection amount deviates from “the amount necessary to obtain the theoretical air-fuel ratio”. As a result, the air-fuel ratio of each cylinder is uniformly shifted to the “rich side or lean side” with respect to the theoretical air-fuel ratio. In this case, in the conventional air-fuel ratio control, the air-fuel ratio of the air-fuel mixture supplied to the engine is shifted to the “lean side or rich side”. As a result, the air-fuel ratio of the air-fuel mixture supplied to each cylinder is corrected to an air-fuel ratio near the stoichiometric air-fuel ratio. Therefore, the combustion in each cylinder is close to complete combustion (combustion when the air-fuel ratio of the mixture is the stoichiometric air-fuel ratio), and the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is close to the stoichiometric air-fuel ratio or near the stoichiometric air-fuel ratio. It becomes an air fuel ratio. Therefore, deterioration of emission is avoided.

  Incidentally, in general, an electronic fuel injection type internal combustion engine includes one fuel injection valve in each cylinder or an intake port communicating with each cylinder. Accordingly, when the characteristic of the fuel injection valve of a specific cylinder becomes “a characteristic of injecting an amount of fuel that is larger than the instructed fuel injection amount”, the air-fuel ratio of the air-fuel mixture supplied to that specific cylinder (that Only the air-fuel ratio of the cylinder) greatly changes to the rich side. That is, the non-uniformity of air-fuel ratio among cylinders (air-fuel ratio variation among cylinders, air-fuel ratio imbalance among cylinders) increases. In other words, an imbalance occurs between the air-fuel ratios of the air-fuel mixture supplied to each of the plurality of cylinders (air-fuel ratios for each cylinder).

  In this case, the average of the air-fuel ratio of the air-fuel mixture supplied to the engine becomes an air-fuel ratio richer than the stoichiometric air-fuel ratio. Therefore, the air-fuel ratio of the specific cylinder is changed to the lean side so as to approach the stoichiometric air-fuel ratio by the air-fuel ratio feedback amount common to all cylinders. Further, the air-fuel ratios of the other cylinders are changed to the lean side so as to be away from the stoichiometric air-fuel ratio. At this time, the air-fuel ratio of the specific cylinder is still richer than the stoichiometric air-fuel ratio, and the air-fuel ratios of other cylinders are slightly leaner than the stoichiometric air-fuel ratio. As a result, the average of the overall air-fuel ratio of the air-fuel mixture supplied to the engine is made substantially coincident with the theoretical air-fuel ratio.

  However, the air-fuel ratio of the specific cylinder is still richer than the stoichiometric air-fuel ratio, and the air-fuel ratios of the remaining cylinders are leaner than the stoichiometric air-fuel ratio. The combustion state becomes a combustion state different from complete combustion. As a result, the amount of emissions discharged from each cylinder (the amount of unburned matter and the amount of nitrogen oxides) increases. For this reason, even if the average air-fuel ratio of the air-fuel mixture supplied to the engine is the stoichiometric air-fuel ratio, the three-way catalyst cannot completely purify the increased emission, and as a result, the emission may be deteriorated. Therefore, it is important to detect that the non-uniformity of the air-fuel ratio between cylinders is excessive and to take some measures so as not to deteriorate the emission.

  Conventional apparatus for determining whether or not such "non-uniformity of air-fuel ratio among cylinders (air-fuel ratio imbalance among cylinders, imbalance between cylinder-specific air-fuel ratios)" has become excessive (between air-fuel ratio cylinders) One of the imbalance determination devices analyzes the output of a single air-fuel ratio sensor disposed in the exhaust collecting portion, thereby obtaining an estimated air-fuel ratio that represents the air-fuel ratio of each cylinder. And this conventional apparatus determines whether "the non-uniformity of the air-fuel ratio between cylinders" has become excessive using the estimated air-fuel ratio of each cylinder (for example, see Patent Document 1). reference.).

JP 2000-220489 A

  However, the above-mentioned conventional apparatus must detect the air-fuel ratio of the exhaust gas that fluctuates with the rotation of the engine with an air-fuel ratio sensor every short time. For this reason, a highly responsive air-fuel ratio sensor is required. Furthermore, since the responsiveness decreases when the air-fuel ratio sensor deteriorates, there arises a problem that the air-fuel ratio of each cylinder cannot be accurately estimated. In addition, it is not easy to separate fluctuations in the air-fuel ratio from noise. In addition, a high-performance CPU with high-speed data sampling technology and high processing capability is required. As described above, the conventional apparatus has many problems to be solved.

  Accordingly, one of the objects of the present invention is to determine whether or not the “non-uniformity of air-fuel ratio among cylinders” has become excessively high. The object is to provide a “balance determination device”.

  For this reason, the air-fuel ratio imbalance among cylinders determining apparatus according to the present invention provides an output of a “downstream air-fuel ratio sensor that outputs an output value corresponding to the air-fuel ratio of exhaust gas that has been disposed downstream of the catalyst and has passed through the catalyst”. Based on the above, a “hydrogen amount difference indicating parameter” for determining whether or not the air-fuel ratio imbalance among cylinders has occurred is acquired. This hydrogen amount difference indicating parameter is a parameter that increases as the difference between "the amount of hydrogen contained in the exhaust gas before passing through the catalyst" and "the amount of hydrogen contained in the exhaust gas after passing through the catalyst" increases. It is. The hydrogen amount difference indicating parameter changes in accordance with the degree of air-fuel ratio imbalance among cylinders, as will be described in detail later. Therefore, if the hydrogen amount difference indicating parameter can be obtained with high accuracy, it is possible to accurately determine whether or not an air-fuel ratio imbalance among cylinders has occurred.

  More specifically, the air-fuel ratio imbalance among cylinders determination apparatus according to the present invention is applied to a multi-cylinder internal combustion engine (10) having a plurality of cylinders as shown in the schematic configuration of the apparatus in FIG. The air-fuel ratio imbalance among cylinders determination apparatus includes a catalyst (43), a fuel injection valve (25), an upstream air-fuel ratio sensor (55), and a downstream air-fuel ratio sensor (56).

  The catalyst is an exhaust passage of the engine, and an “exhaust collecting portion” collects exhaust gas discharged from “a combustion chamber of at least two cylinders (preferably, three or more cylinders) of the plurality of cylinders” (HK) ”is disposed at a downstream site. This catalyst is a catalyst that oxidizes at least hydrogen among the components contained in the exhaust gas. Therefore, this catalyst may be a three-way catalyst, an oxidation catalyst, or the like. Furthermore, this catalyst may be a catalyst element provided so as to cover the downstream air-fuel ratio sensor.

  The fuel injection valve is disposed corresponding to each of the at least two cylinders. Each fuel injection valve injects the fuel contained in the air-fuel mixture supplied to the respective combustion chambers of the two or more cylinders.

  The upstream air-fuel ratio sensor is disposed between the exhaust collecting portion or the exhaust collecting portion of the exhaust passage and the catalyst. The upstream air-fuel ratio sensor includes a diffusion resistance layer in contact with the exhaust gas before passing through the catalyst, and an output corresponding to the air-fuel ratio of the exhaust gas that is covered by the diffusion resistance layer and that has reached through the diffusion resistance layer An air-fuel ratio detection element that outputs a value.

  An example of the upstream air-fuel ratio sensor is disclosed in, for example, “A wide-range air-fuel ratio sensor having a diffusion resistance layer” disclosed in JP-A-11-72473, JP-A-2000-65782, JP-A-2004-69547, and the like. It is. That is, an example of the upstream air-fuel ratio sensor includes a solid electrolyte layer, an exhaust-side electrode layer, an atmosphere-side electrode layer exposed to the space where the atmosphere is introduced, and a diffusion resistance layer, and the same as the exhaust-side electrode layer. The air-fuel ratio sensor is formed on both surfaces of the solid electrolyte layer so that the air electrode layer is opposed to the air electrode layer and the exhaust electrode layer is covered with the diffusion resistance layer. . In this case, the solid electrolyte layer, the exhaust-side electrode layer, and the atmosphere-side electrode layer constitute “the air-fuel ratio detection element”.

  Such an air-fuel ratio sensor passes through the diffusion resistance layer and the exhaust-side electrode layer (the air-fuel ratio detection element) when the air-fuel ratio of the gas to be detected is an air-fuel ratio leaner than the stoichiometric air-fuel ratio. An output value that changes in accordance with the “oxygen concentration” of the gas that has reached 1 is output. Further, such an air-fuel ratio sensor passes through the diffusion resistance layer when the air-fuel ratio of the gas to be detected is richer than the stoichiometric air-fuel ratio, and passes through the diffusion resistance layer (the air-fuel ratio detection). An output value that changes in accordance with the “unburned substance concentration” of the gas that has reached the device is output. That is, in such an air-fuel ratio sensor, the component (the air-fuel ratio) of the exhaust gas that has passed through the diffusion resistance layer and reached the air-fuel ratio detection element regardless of whether the air-fuel ratio of the detection target gas is lean or rich. The output value according to is output.

  The downstream air-fuel ratio sensor outputs an output value corresponding to the air-fuel ratio of the exhaust gas after passing through the catalyst. The downstream air-fuel ratio sensor is disposed, for example, at a position downstream of the catalyst in the exhaust passage.

  The air-fuel ratio feedback control means injects from the fuel injection valve so that “the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor” matches “theoretical air-fuel ratio (upstream target air-fuel ratio)”. The “fuel injection amount” that is the amount of fuel to be performed is “feedback controlled”.

  The imbalance determination means is configured to determine whether or not an imbalance has occurred between “cylinder air-fuel ratio” that is “the air-fuel ratio of the air-fuel mixture supplied to each of the at least two cylinders”. Execute “imbalance imbalance”. Here, the principle of determining the air-fuel ratio imbalance among cylinders by the air-fuel ratio imbalance among cylinders of the present invention will be described.

  As described above, the air-fuel ratio feedback control means is the air-fuel mixture supplied to the combustion chambers of the two or more cylinders so that the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor matches the stoichiometric air-fuel ratio. The air-fuel ratio (in this case, the amount of fuel injected from each fuel injection valve) is feedback-controlled. Therefore, “the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor” is “the true average value of the air-fuel ratios of the air-fuel ratio supplied to the combustion chambers of the two or more cylinders (the true time of the air-fuel ratio). If it is equal to “average value)”, the “true average value of the air-fuel ratio of the air-fuel mixture supplied to the two or more cylinders” matches the stoichiometric air-fuel ratio by the feedback control. In the following, “the mixture supplied to the two or more cylinders” is also referred to as “the mixture supplied to the entire engine” for convenience.

  However, in reality, when the non-uniformity of the air-fuel ratio among the cylinders becomes excessive, the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine becomes the air-fuel ratio leaner than the stoichiometric air-fuel ratio (theoretical The air-fuel ratio is greater than the air-fuel ratio). The reason for this will be described below.

The fuel supplied to the engine is a compound of carbon and hydrogen. Therefore, when the air-fuel ratio of the air-fuel mixture supplied for combustion is an air-fuel ratio richer than the stoichiometric air-fuel ratio (an air-fuel ratio smaller than the stoichiometric air-fuel ratio), “hydrocarbon HC, carbon monoxide CO and hydrogen H _2. Etc. "unburned material is produced as an intermediate product. In this case, as the air-fuel ratio of the air-fuel mixture used for combustion is richer than the stoichiometric air-fuel ratio and farther from the stoichiometric air-fuel ratio, the probability that the intermediate product encounters oxygen and combines during the combustion period is increased. It decreases rapidly. As a result, the amount of unburned matter (HC, CO, and H ₂ ) in the exhaust gas becomes abrupt (for example, as a quadratic function) as the air-fuel ratio of the air-fuel mixture supplied to the cylinder becomes richer. Increase (see FIG. 13).

  Now, it is assumed that only the air-fuel ratio of the specific cylinder is greatly shifted to the rich side. Such a situation occurs, for example, when the injection characteristic of the fuel injection valve provided for the specific cylinder becomes “a characteristic for injecting a fuel amount much larger than the instructed fuel injection amount”. . Such an abnormality of the fuel injection valve is also referred to as “rich abnormality of the fuel injection valve”.

In this case, the air-fuel ratio of the air-fuel mixture supplied to the specific cylinder (the air-fuel ratio of the specific cylinder) is larger than the air-fuel ratio of the air-fuel mixture supplied to the remaining cylinders (the air-fuel ratio of the remaining cylinders). It changes to the rich air-fuel ratio. That is, an air-fuel ratio imbalance among cylinders occurs. At this time, an extremely large amount of unburned matter (HC, CO, H ₂ ) is discharged from the specific cylinder.

By the way, hydrogen H ₂ is a small molecule compared to hydrocarbon HC, carbon monoxide CO, and the like. Accordingly, hydrogen H ₂ diffuses more quickly in the diffusion resistance layer of the upstream air-fuel ratio sensor than other unburned substances (HC, CO). For this reason, when a large amount of unburned material composed of HC, CO and H ₂ is contained in the exhaust gas, selective diffusion (preferential diffusion) of hydrogen H ₂ occurs in the diffusion resistance layer. That is, the hydrogen H ₂ reaches the surface of the air-fuel ratio detection element in a larger amount than “other unburned substances (HC, CO)”. As a result, the balance between the concentration of hydrogen H _{2 and} the concentration of other unburned substances (HC, CO) is lost. In other words, the proportion of hydrogen H ₂ to all of the unburnt substances contained in the exhaust gas reaching the air-fuel ratio sensing element of the upstream air-fuel ratio sensor, the hydrogen H ₂ to all of the unburnt substances contained in the exhaust gas discharged from the engine Will be greater than the percentage.

  For example, when the amount (weight) of air sucked into each cylinder of a four-cylinder engine is A0 and the amount (weight) of fuel supplied to each cylinder is F0, the air-fuel ratio A0 / F0 is theoretically empty. Assume a fuel ratio (eg, 14.5).

  In this case, it is assumed that the amount of fuel supplied (injected) to each cylinder is equally 10% excessive. That is, it is assumed that 1.1 · F0 fuel is supplied to each cylinder. At this time, the total amount of air supplied to the four cylinders (the amount of air supplied to the entire engine while each cylinder completes one combustion stroke) is 4 · A0, and is supplied to the four cylinders. The total amount of fuel (the amount of fuel supplied to the entire engine while each cylinder completes one combustion stroke) is 4.4 · F0 (= 1.1 · F0 + 1.1 · F0 + 1.1 · F0 + 1. 1 · F0). Therefore, the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine is 4 · A0 / (4.4 · F0) = A0 / (1.1 · F0). At this time, the output value of the upstream air-fuel ratio sensor becomes an output value corresponding to the air-fuel ratio A0 / (1.1 · F0). Accordingly, the air-fuel ratio of the air-fuel mixture supplied to the entire engine is made to coincide with the theoretical air-fuel ratio A0 / F0 that is the upstream target air-fuel ratio by the air-fuel ratio feedback control. In other words, the amount of fuel supplied to each cylinder is reduced by 10% by air-fuel ratio feedback control. That is, 1.0 · F0 fuel is supplied to each cylinder, and the air-fuel ratio of each cylinder matches the theoretical air-fuel ratio A0 / F0.

  Next, the amount of fuel supplied to one particular cylinder is an excess amount by 40% (ie, (1.4 · F0)), and the amount of fuel supplied to the remaining three cylinders It is assumed that the amount is an appropriate value (the amount of fuel necessary to obtain the theoretical air-fuel ratio, in this case 1.0 · F0). At this time, the total amount of air supplied to the four cylinders is 4 · A0. On the other hand, the total amount of fuel supplied to the four cylinders is 4.4 · F0 (= 1.4 · F0 + F0 + F0 + F0). Therefore, the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine is 4 · A0 / (4.4 · F0) = A0 / (1.1 · F0). In other words, the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine in this case is the same as “when the amount of fuel supplied to each cylinder is equally 10% excessive”. Value.

However, as described above, the amount of unburned matter (HC, CO, and H ₂ ) in the exhaust gas increases rapidly as the air-fuel ratio of the air-fuel mixture supplied to the cylinder becomes richer. In addition, exhaust gas mixed with exhaust gas from each cylinder reaches the upstream air-fuel ratio sensor. Therefore, “the total amount of hydrogen contained in the exhaust gas in the above case where only the amount of fuel supplied to a specific cylinder is an excess amount by 40%” is “the amount of fuel supplied to each cylinder”. When the amount is equally excessive by 10%, it becomes significantly larger than the “total amount of hydrogen contained in the exhaust gas”.

As a result, due to the above-mentioned “selective diffusion of hydrogen H ₂ ”, the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor is “the true average of the air-fuel ratio of the air-fuel mixture supplied to the entire engine” The air-fuel ratio is richer than the value (A0 / (1.1 · F0)). In other words, even when the average value of the exhaust gas air-fuel ratio is the same rich-side air-fuel ratio, when the air-fuel ratio imbalance among cylinders is occurring than when the air-fuel ratio imbalance among cylinders does not occur The concentration of hydrogen H _{2 in} the exhaust gas that reaches the air-fuel ratio detection element of the upstream air-fuel ratio sensor increases. Therefore, the output value of the upstream air-fuel ratio sensor becomes a value indicating the richer air-fuel ratio than the true average value of the air-fuel ratio of the air-fuel mixture.

  As a result, the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine is calculated theoretically by feedback control that attempts to make the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor coincide with the stoichiometric air-fuel ratio. The air-fuel ratio is controlled to be leaner than the air-fuel ratio. The above is the reason why the true average value of the air-fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio when the non-uniformity of the air-fuel ratio between the cylinders becomes excessive.

On the other hand, hydrogen H ₂ contained in the exhaust gas discharged from the engine is oxidized (purified) in the catalyst together with other unburned substances (HC, CO). Further, the exhaust gas that has passed through the catalyst reaches the downstream air-fuel ratio sensor. Therefore, the output value of the downstream air-fuel ratio sensor becomes a value corresponding to the average value of the true air-fuel ratio of the air-fuel mixture supplied to the engine. As a result, when only the air-fuel ratio of the specific cylinder is greatly shifted to the rich side, the output value of the downstream air-fuel ratio sensor is “corrected to the excessively lean side” by feedback control based on the output value of the upstream air-fuel ratio sensor. It becomes a value corresponding to “true air-fuel ratio”.

  In other words, as the amount of hydrogen contained in the exhaust gas before passing through the catalyst increases as the air-fuel ratio of the specific cylinder shifts to the rich side, the “selective diffusion of hydrogen” and the “output value of the upstream air-fuel ratio sensor” The “true air-fuel ratio of the air-fuel mixture supplied to the engine” is controlled to “more lean side” due to the “feedback control based on”, and the result appears in the output value of the downstream air-fuel ratio sensor. Therefore, the output value of the downstream air-fuel ratio sensor changes according to the difference between the amount of hydrogen contained in the exhaust gas before passing through the catalyst and the amount of hydrogen contained in the exhaust gas after passing through the catalyst. become.

Therefore, the imbalance determination means includes a hydrogen amount difference instruction parameter acquisition means.
This hydrogen amount difference indicating parameter acquisition means is configured to determine, based on “the output value of the downstream air-fuel ratio sensor when the feedback control is being executed”, “the amount of hydrogen contained in the exhaust gas before passing through the catalyst”. And a “hydrogen amount difference indicating parameter” that increases as the “difference between the amount of hydrogen contained in the exhaust gas after passing through the catalyst” increases.

  FIG. 2 is a graph showing a “sub feedback amount learning value (sub FB learning value)” calculated based on the output value of the downstream air-fuel ratio sensor by a curve C1. The horizontal axis in FIG. 2 represents the imbalance ratio. The imbalance ratio is “the ratio (Y / X) of the difference Y (= X−af) between the theoretical air-fuel ratio X and the rich air-fuel ratio af of the cylinder with respect to the theoretical air-fuel ratio X”. .

  The sub feedback amount is a feedback amount of an air fuel ratio (fuel injection amount) for making the output value of the downstream air fuel ratio sensor coincide with a value corresponding to the theoretical air fuel ratio. In other words, the sub-feedback amount is an amount that compensates for the “true air-fuel ratio corrected excessively to the lean side” by feedback control based on the output value of the upstream air-fuel ratio sensor. It becomes a value corresponding to the “corrected true air-fuel ratio”. Accordingly, the sub-feedback amount is an amount that changes according to the difference between the amount of hydrogen contained in the exhaust gas before passing through the catalyst and the amount of hydrogen contained in the exhaust gas after passing through the catalyst. Further, the learning value of the sub feedback amount is a value updated so as to approach the steady component of the sub feedback amount. Therefore, the sub feedback amount or the learned value of the sub feedback amount can be adopted as the hydrogen amount difference indicating parameter.

  Therefore, the learning value of the sub feedback amount increases as the imbalance ratio increases, as indicated by the curve C1 in FIG. The imbalance determination means is configured to determine that an air-fuel ratio imbalance among cylinders has occurred, for example, when the “learning value of sub-feedback amount” as the hydrogen amount difference indicating parameter is larger than the determination threshold Athbase. Can be done.

  Further, even when an air-fuel ratio imbalance among cylinders in which only the air-fuel ratio of one specific cylinder is greatly shifted to the lean side occurs, as shown in the region where the imbalance ratio in FIG. 2 is negative, the amount of hydrogen The “learning value of the sub feedback amount” as the difference instruction parameter increases. Such a situation occurs, for example, when the injection characteristic of the fuel injection valve provided for the specific cylinder becomes “a characteristic for injecting a fuel amount considerably smaller than the instructed fuel injection amount”. . Such an abnormality of the fuel injection valve is also referred to as “an abnormality of lean deviation of the fuel injection valve”.

  Hereinafter, the reason why the learning value of the sub feedback amount increases even when the air-fuel ratio imbalance among cylinders in which only the air-fuel ratio of one specific cylinder is greatly shifted to the lean side occurs will be briefly described. Also in the following description, it is assumed that the amount of air (weight) taken into each cylinder of the engine is A0. Further, it is assumed that the air-fuel ratio A0 / F0 matches the stoichiometric air-fuel ratio when the fuel amount (weight) supplied to each cylinder is F0.

  Now, the amount of fuel supplied to one specific cylinder (for convenience, “first cylinder # 1”) is an excessive amount (that is, 0.6 · F0) by 40%, and the remaining amount. The amount of fuel supplied to each of the three cylinders (second cylinder # 2, third cylinder # 3 and fourth cylinder # 4) is such that the air-fuel ratio of these cylinders matches the stoichiometric air-fuel ratio. Is assumed (ie, F0). In this case, it is assumed that no misfire occurs.

  In this case, the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine is 4 · A0 / (3.6 · F0) = A0 / (0.9 · F0), and the stoichiometric air-fuel ratio A0 / The air-fuel ratio is leaner than F0. Therefore, the amount of fuel supplied to the first cylinder # 1 to the fourth cylinder # 4 is increased by feedback control based on the output value of the upstream air-fuel ratio sensor. As a result, the amount of fuel supplied to the first cylinder # 1 is 0.7 · F0, and the amount of fuel supplied to each of the second cylinder # 2 to the fourth cylinder # 4 is 1.1 · F0. Assume that

  In this state, the total amount of air supplied to the four cylinders is 4 · A0, and the total amount of fuel supplied to the four cylinders is 4 · F0 (= 0.7 · F0 + 1.1 · F0 + 1.1 · F0 + 1). .1 · F0). Therefore, the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine 10 is 4 · A0 / (4 · F0) = A0 / F0, which is the stoichiometric air-fuel ratio.

  In this state, since the air-fuel ratio of the first cylinder # 1 is an air-fuel ratio (A0 / 0.7 · F0) that is leaner than the stoichiometric air-fuel ratio, almost no hydrogen is generated. Further, the air-fuel ratios of the other cylinders (second cylinder # 2 to fourth cylinder # 4) are richer than the stoichiometric air-fuel ratio (A0 / 1.1F0). An amount of hydrogen corresponding to the fuel ratio (A0 / 1.1F0) is generated.

  On the other hand, when the air-fuel ratio imbalance among cylinders does not occur and the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine is the stoichiometric air-fuel ratio (A0 / F0), Since the fuel ratio is also the theoretical air fuel ratio (A0 / F0), the “total amount of hydrogen contained in the exhaust gas” is extremely small.

  As a result, when the air-fuel ratio imbalance among cylinders due to the “lean deviation abnormality of the fuel injection valve” occurs, the “true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine” is the stoichiometric air-fuel ratio. Even so, the total amount of hydrogen in the exhaust gas is larger than “when the air-fuel ratio of all cylinders is the stoichiometric air-fuel ratio”.

  Therefore, even in this case, the influence of selective diffusion of hydrogen appears in the output value of the upstream air-fuel ratio sensor. As a result, feedback control based on the output value of the upstream air-fuel ratio sensor is further executed, and the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine is overcorrected to the lean side from the stoichiometric air-fuel ratio. End up.

  Further, as the air-fuel ratio of the specific cylinder shifts to the lean side, the air-fuel ratio of the remaining cylinders becomes “richer air-fuel ratio” than the stoichiometric air-fuel ratio by “feedback control based on the output value of the upstream air-fuel ratio sensor”. To be moved to. Therefore, the “true air-fuel ratio of the air-fuel mixture supplied to the engine” is derived from “selective diffusion of hydrogen” and “feedback control based on the output value of the upstream air-fuel ratio sensor”. Converges to "fuel ratio". The result appears in the output value of the downstream air-fuel ratio sensor.

  Accordingly, the “sub-feedback amount and the learned value of the sub-feedback amount” based on the output value of the downstream air-fuel ratio sensor are “the output of the upstream air-fuel ratio sensor” even when “lean deviation abnormality of the fuel injection valve” occurs. It increases so as to compensate for “overcorrection of the air-fuel ratio to the lean side” by “feedback control based on the value”.

  Therefore, for example, when the “learning value of the sub feedback amount” as the hydrogen amount difference indicating parameter is larger than the determination threshold value Athbase, the imbalance determination means determines that the “empty fuel injection valve lean deviation abnormality” It can be detected that the "fuel ratio imbalance among cylinders" has occurred.

  Incidentally, as described above, the upstream air-fuel ratio sensor (55) is disposed in the “exhaust collecting portion (HK)” or the “exhaust passage between the exhaust collecting portion (HK) and the catalyst (43)”. . However, as shown in FIG. 1, the flow of exhaust gas is indicated by an arrow, the upstream air-fuel ratio sensor (55) has a specific cylinder (FIG. 1) depending on the shape of the exhaust manifold and the position of the upstream air-fuel ratio sensor. In this example, the exhaust gas discharged from the first cylinder # 1) may hit a larger amount and / or directly than the exhaust gas discharged from each of the other cylinders. That is, depending on the engine (10), the way in which the exhaust gas strikes the upstream air-fuel ratio sensor (55) may not be uniform among the cylinders. In this case, the upstream air-fuel ratio sensor (55) reacts more sensitively to the exhaust gas from a certain cylinder (for example, the first cylinder # 1) than to the exhaust gas from each of the other cylinders.

Assume that the upstream air-fuel ratio sensor reacts more sensitively to the exhaust gas from the first cylinder # 1 than to the exhaust gas from each of the other cylinders. At this time, if a “rich deviation abnormality” occurs in the fuel injection valve of the first cylinder # 1, the output value of the upstream air-fuel ratio sensor is “the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder. It becomes a value corresponding to “a richer air-fuel ratio” than “case”. This is because the upstream air-fuel ratio sensor in order susceptible to the first cylinder # 1 of the exhaust gas is to sensitively react with a large amount of hydrogen H ₂ contained in the first cylinder # 1 of the exhaust gas.

  As a result, by the feedback control based on the output value of the upstream air-fuel ratio sensor, the “air-fuel mixture supplied to the engine” is compared with “the case where the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder”. Since the “true air / fuel ratio” is to be controlled “more lean”, the sub-feedback amount (and hence the hydrogen amount difference indicating parameter based on the learned value of the sub-feedback amount) increases “further”. That is, the sub feedback amount learning value (hydrogen amount difference indicating parameter) shifts from the curve C1 to the curve C2 in the region where the imbalance ratio in FIG. 3 is positive. The curve C1 is the same curve as the curve C1 shown in FIG.

  Further, when the upstream air-fuel ratio sensor reacts more sensitively to the exhaust gas from the first cylinder # 1 than to the exhaust gas from each of the other cylinders, the fuel injection valve of the first cylinder # 1 “lean shift” When "abnormal" occurs, the output value of the upstream air-fuel ratio sensor is "richer air-fuel ratio closer to the stoichiometric air-fuel ratio than when" the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder " (The air-fuel ratio with a small degree of richness) ”. This is because, as a result of the feedback control based on the output value of the upstream side air-fuel ratio sensor, the upstream side air-fuel ratio sensor becomes “the other cylinder (second cylinder # 2) controlled to an air-fuel ratio richer than the theoretical air-fuel ratio”. This is because it is insensitive to hydrogen contained in “exhaust gas from 2 to 4th cylinder # 4)”.

  As a result, the hydrogen amount difference indicating parameter acquired based on the learned value of the sub feedback amount is not as large as “when the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder”. That is, the learning value of the sub feedback amount shifts from the curve C1 to the curve C2 in the region where the imbalance ratio in FIG. 3 is negative.

  Next, the upstream air-fuel ratio sensor responds to the exhaust gas from the first cylinder # 1 with respect to the exhaust gas of any cylinder other than the first cylinder # 1 (that is, the second cylinder # 2 to the fourth cylinder # 4). Assuming the case of sensitive reaction. Here, for convenience of explanation, the upstream side air-fuel ratio sensor reacts more sensitively to the exhaust gas from the fourth cylinder # 4 than the exhaust gas from each of the other cylinders (first cylinder # 1 to third cylinder # 3). Assume that.

At this time, if a “rich deviation abnormality” occurs in the fuel injection valve of the first cylinder # 1, the output value of the upstream air-fuel ratio sensor is “the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder. It is a value corresponding to “the air-fuel ratio on the rich side closer to the stoichiometric air-fuel ratio (the air-fuel ratio with a small degree of richness)” than “when to do”. This is because the upstream air-fuel ratio sensor is insensitive to a large amount of hydrogen H ₂ contained in the exhaust gas of the first cylinder # 1. Therefore, the degree of overcorrection of the air-fuel ratio to the lean side based on the output value of the upstream air-fuel ratio sensor is smaller than “when the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder”. .

  As a result, the hydrogen amount difference indicating parameter acquired based on the learned value of the sub feedback amount is not as large as “when the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder”. That is, the learning value of the sub feedback amount shifts from the curve C1 to the curve C3 in the region where the imbalance ratio in FIG. 3 is positive.

  Further, when the upstream air-fuel ratio sensor reacts more sensitively to the exhaust gas from the fourth cylinder # 4 than to the exhaust gas from each of the other cylinders, the fuel injection valve of the first cylinder # 1 “lean shift” When the "abnormality" occurs, the output value of the upstream air-fuel ratio sensor becomes "the richer air-fuel ratio (the degree of richness is higher) than" when the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder ". The value corresponds to “large air-fuel ratio)”. This is because the upstream air-fuel ratio sensor is controlled to a richer air-fuel ratio than the theoretical air-fuel ratio as a result of the “feedback control based on the output value of the upstream air-fuel ratio sensor”. This is because it is sensitive to hydrogen contained in "exhaust gas from".

  As a result, the hydrogen amount difference indicating parameter acquired based on the learned value of the sub feedback amount becomes larger than “when the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder”. That is, the learning value of the sub feedback amount shifts from the curve C1 to the curve C3 in the region where the imbalance ratio in FIG. 3 is negative.

  As described above, when “the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder”, when the hydrogen amount difference indicating parameter is larger than the determination threshold Athbase, the air-fuel ratio imbalance between cylinders is It can be determined that it has occurred. However, if the upstream air-fuel ratio sensor responds more sensitively to "exhaust gas from one specific cylinder" than to "exhaust gas from each of the other cylinders", the degree of air-fuel ratio imbalance among cylinders is very high. Even if it increases, the hydrogen amount difference indicating parameter may not reach the determination threshold Athbase (see the curve C3 in the region where the imbalance ratio is positive and the curve C2 in the region where the imbalance ratio is negative in FIG. 3). . That is, there is a case where the determination of the air-fuel ratio imbalance among cylinders cannot be performed with high accuracy only by simple comparison between the hydrogen amount difference indicating parameter and the determination threshold value Athbase.

  In view of this, the inventor has focused on fluctuations in the engine speed when the air-fuel ratio imbalance among cylinders occurs (hereinafter, also simply referred to as “rotational fluctuation”). The “rotational speed fluctuation parameter” representing this rotational fluctuation is, for example, the time required for an arbitrary cylinder to reach from the compression top dead center to the expansion bottom dead center (this time is required for the crank angle to rotate 180 degrees). Since this is the time required, hereinafter, it is also referred to as “time T180”.) ”And the absolute value of the difference between the time T180 of the other cylinders that have finished the expansion stroke immediately before the expansion stroke of the arbitrary cylinder, It can be obtained by averaging over a number of expansion strokes. Of course, the rotational speed variation parameter is not limited to this.

  As described above, when an air-fuel ratio imbalance among cylinders occurs due to the rich deviation abnormality or the lean deviation abnormality of the fuel injection valve of the specific cylinder, the difference between the air-fuel ratio of the specific cylinder and the air-fuel ratio of other cylinders becomes large. Become. Further, the time T180 changes according to the air-fuel ratio of the air-fuel mixture supplied to the cylinder. Accordingly, when the air-fuel ratio imbalance among cylinders occurs, the difference between the time T180 of the specific cylinder and the time T180 of the other cylinders increases, and thus the rotational speed variation parameter increases. That is, as shown in FIG. 4, as the imbalance ratio (the absolute value of the imbalance ratio) increases, the rotational speed variation parameter (rotational variation) increases. In FIG. 4 and FIG. 5 to be described later, the rotational speed variation parameter is indicated by a relative amount using “predetermined unit time t”.

  Therefore, the “imbalance determination means” of the air-fuel ratio inter-cylinder imbalance determination apparatus according to the present invention includes, in addition to the above-described hydrogen amount difference instruction parameter acquisition means, rotational speed variation parameter acquisition means, and imbalance determination execution means. .

  The rotational speed fluctuation parameter acquisition means acquires a rotational speed fluctuation parameter that changes in accordance with a fluctuation in the rotational speed of the engine.

  The imbalance determination execution means executes the air-fuel ratio imbalance determination between cylinders by using “both” of “the acquired hydrogen amount difference indicating parameter” and “the acquired rotation speed fluctuation parameter”.

  As a result, an air-fuel ratio imbalance among cylinders has occurred, but the sensitivity to exhaust gas from a specific cylinder of the upstream air-fuel ratio sensor is different from the sensitivity to exhaust gas from other cylinders. Even if it is not possible to determine that an air-fuel ratio imbalance among cylinders has occurred only by comparing the command parameter with a predetermined determination threshold value, by using the rotational speed fluctuation parameter, it is possible to It becomes possible to detect the balance.

For example, one aspect of the imbalance determination execution means is:
When the acquired hydrogen amount difference indicating parameter is larger than a predetermined first imbalance determination threshold, it is determined that the air-fuel ratio imbalance among cylinders is occurring, and the larger the acquired rotation speed variation parameter, the more the first It is comprised so that 1 imbalance determination threshold value may be made small.

  As is clear from FIG. 3, when the hydrogen amount difference indicating parameter changes as shown by the curves C1 and C2 in the region where the imbalance ratio is positive, and in the region where the imbalance ratio is negative, the hydrogen amount difference indicating parameter is When the curves C1 and C3 change, the hydrogen amount difference instruction parameter is compared with the first imbalance determination threshold (determination threshold Athbase in FIG. 3), and the hydrogen amount difference instruction parameter is greater than the first imbalance determination threshold. Can also be determined that an air-fuel ratio imbalance among cylinders has occurred.

  On the other hand, when the hydrogen amount difference indicating parameter changes as shown by a curve C3 in a region where the imbalance ratio is positive, and when the hydrogen amount difference indicating parameter changes as shown by a curve C2 in a region where the imbalance rate is negative, Since the hydrogen amount difference instruction parameter does not become larger than the first imbalance determination threshold value (determination threshold value Athbase in FIG. 3), depending on the comparison between the hydrogen amount difference instruction parameter and the first imbalance determination threshold value, “air-fuel ratio imbalance among cylinders” "Cannot occur.

  Therefore, as can be understood from FIGS. 4 and 5, the imbalance determination execution unit configured as described above decreases the first imbalance determination threshold Ath as the acquired rotational speed variation parameter increases. According to this, even if the upstream air-fuel ratio sensor reacts sensitively to the exhaust gas from the specific cylinder, if the imbalance ratio increases, the hydrogen amount difference indicating parameter becomes the first imbalance determination threshold value Ath. (See the curve C3 in the region where the imbalance ratio is positive and the curve C2 in the region where the imbalance ratio is negative). Therefore, it is possible to reliably detect that the air-fuel ratio imbalance among cylinders has occurred.

In particular, in this case, the imbalance determination execution means
When the acquired rotational speed fluctuation parameter is within a predetermined range including 0 (for example, in the example shown in FIG. 5, when the rotational speed fluctuation parameter is about 100 t or less, in other words, the imbalance ratio is about -10% to + 40%), the first imbalance determination threshold value is maintained at a constant reference value (Athbase), and when the obtained rotational speed fluctuation parameter is outside the predetermined range, the same rotational speed fluctuation It is preferable that the first imbalance determination threshold is reduced as the parameter increases.

  According to this, when the degree of air-fuel ratio imbalance among cylinders is relatively small (that is, when the imbalance ratio is not large enough to be determined as “air-fuel ratio imbalance among cylinders”). It is possible to avoid erroneously determining that the air-fuel ratio imbalance among cylinders has occurred.

Alternatively, the imbalance determination execution means is
The acquired hydrogen amount difference indicating parameter is corrected so as to increase as the acquired rotational speed fluctuation parameter increases, and the corrected hydrogen amount difference indicating parameter is determined based on a predetermined second imbalance determination threshold (Athbase). It can also be configured to determine that the air-fuel ratio imbalance among cylinders is occurring when it is large.

  According to this, instead of changing the determination threshold, the hydrogen amount difference indicating parameter is corrected so as to increase as the rotational speed variation parameter increases. Therefore, even when the hydrogen amount difference indicating parameter changes like the curve C3 in the region where the imbalance rate is positive and the curve C2 in the region where the imbalance rate is negative, the imbalance rate increases to a certain extent. At this time, the corrected hydrogen amount difference indicating parameter has a value exceeding the second imbalance determination threshold value. Therefore, it can be reliably determined that the air-fuel ratio imbalance among cylinders has occurred.

Even in this case,
When the obtained rotational speed fluctuation parameter is within a predetermined range including 0 (for example, when the rotational speed fluctuation parameter is about 100 t or less in the example shown in FIG. 5), the hydrogen amount difference indicating parameter is rotated. When the acquired rotational speed fluctuation parameter is outside the predetermined range without correcting with the speed fluctuation parameter, it is desirable to correct the hydrogen amount difference indicating parameter so as to increase as the rotational speed fluctuation parameter increases.

  According to this, when the degree of air-fuel ratio imbalance among cylinders is relatively small (that is, when the imbalance ratio is not large enough to be determined as “air-fuel ratio imbalance among cylinders”). It is possible to avoid erroneously determining that the air-fuel ratio imbalance among cylinders has occurred.

Furthermore, as an alternative, the imbalance determination execution means
When the acquired hydrogen amount difference indicating parameter is greater than a predetermined third imbalance determination threshold (Athbase), it is determined that the air-fuel ratio imbalance among cylinders is occurring, and the acquired hydrogen amount difference indicating parameter is Even when the rotation speed fluctuation parameter is greater than a fourth imbalance determination threshold value (see threshold value Bth in FIG. 4), the air-fuel ratio is smaller than a predetermined third imbalance determination threshold value (Athbase). It can also be configured to determine that an inter-cylinder imbalance has occurred.

  According to this, in the example shown in FIG. 5, when the imbalance ratio is in a predetermined range including 0 (a range where the imbalance ratio is about −10% to + 40%), the hydrogen amount difference indicating parameter is used. Thus, it is possible to reliably detect that the air-fuel ratio imbalance among cylinders has occurred. Further, when the imbalance ratio is outside the predetermined range, the rotational speed fluctuation parameter that is not directly related to the output value of the upstream air-fuel ratio sensor (usually obtained based on the output value from the crank angle sensor). Therefore, even if the upstream air-fuel ratio sensor reacts sensitively to hydrogen contained in the exhaust gas from a specific cylinder, it is determined whether or not an air-fuel ratio imbalance between cylinders has occurred. It is possible to reliably determine that imbalance has occurred.

  In the above-described air-fuel ratio imbalance determining apparatus of the present invention, the air-fuel ratio feedback control means is configured to include sub-feedback amount update means, fuel injection amount correction means, and sub-feedback amount learning means. obtain.

The sub feedback amount updating means includes
Every time the predetermined first update timing arrives, the sub feedback amount for making the output value of the downstream air-fuel ratio sensor coincide with the value corresponding to the theoretical air-fuel ratio is updated based on the output value of the downstream air-fuel ratio sensor. To do.

The fuel injection amount determining means includes
The fuel injection amount is determined based on at least the output value of the upstream air-fuel ratio sensor and the sub feedback amount each time a predetermined second update timing arrives.

The sub feedback amount learning means includes:
It is means for updating the “learned value of the sub feedback amount” based on the sub feedback amount every time a predetermined third update timing arrives, so that the learned value approaches the steady component of the sub feedback amount. The same learning value is updated.

Furthermore, in this case, the hydrogen amount difference indicating parameter acquisition means includes
The hydrogen amount difference indicating parameter may be calculated based on the learned value of the sub feedback amount.

  As described above, the sub-feedback amount is an air-fuel ratio (fuel injection amount) feedback amount for matching the output value of the downstream air-fuel ratio sensor with a value corresponding to the theoretical air-fuel ratio. Therefore, the sub feedback amount is an amount that compensates for the “true air / fuel ratio that has been excessively corrected to the lean side” by feedback control based on the output value of the upstream side air / fuel ratio sensor. It becomes a value corresponding to the “true air / fuel ratio”. Further, since the learning value of the sub feedback amount is a value that is updated so as to approach the steady component of the sub feedback amount, the “feedback control based on the output value of the upstream air-fuel ratio sensor” is set to “excessively lean side”. It becomes a value corresponding to the “corrected true air-fuel ratio”. Therefore, by calculating the hydrogen amount difference indicating parameter based on the learned value of the sub feedback amount, it is possible to easily obtain the accurate hydrogen amount difference indicating parameter.

1 is a schematic diagram of an internal combustion engine to which an air-fuel ratio imbalance among cylinders determination apparatus according to a first embodiment of the present invention is applied. 5 is a graph showing a relationship between an air-fuel ratio imbalance ratio between cylinders and a hydrogen amount difference indicating parameter. 5 is a graph showing a relationship between an air-fuel ratio imbalance ratio between cylinders and a hydrogen amount difference indicating parameter. 4 is a graph showing the relationship between an air-fuel ratio imbalance ratio between cylinders and a rotational speed fluctuation parameter. 6 is a graph showing a relationship between a hydrogen amount difference indicating parameter and a determination threshold with respect to an air-fuel ratio imbalance ratio between cylinders and a rotational speed fluctuation parameter. 1 is a configuration diagram of an internal combustion engine to which an air-fuel ratio imbalance among cylinders determination apparatus according to an embodiment of the present invention is applied. It is a schematic sectional drawing of the upstream air-fuel ratio sensor shown in FIG. It is a figure for demonstrating the action | operation of an upstream air-fuel-ratio sensor in case the air-fuel ratio of waste gas (to-be-detected gas) is an air-fuel ratio leaner than a stoichiometric air-fuel ratio. It is the graph which showed the relationship between the air fuel ratio of waste gas, and the limiting current value of an upstream air fuel ratio sensor. It is a figure for demonstrating the action | operation of an upstream air-fuel-ratio sensor in case the air-fuel ratio of waste gas (to-be-detected gas) is an air-fuel ratio richer than a theoretical air-fuel ratio. It is the graph which showed the relationship between the air fuel ratio of waste gas, and the output value of an upstream air fuel ratio sensor. It is the graph which showed the relationship between the air fuel ratio of waste gas, and the output value of a downstream air fuel ratio sensor. 3 is a graph showing a relationship between an air-fuel ratio of an air-fuel mixture supplied to a cylinder and unburned components discharged from the cylinder. It is the flowchart which showed the fuel-injection control routine which CPU of the electric control apparatus shown in FIG. 6 performs. It is the flowchart which showed the routine which CPU of the electric control apparatus shown in FIG. 6 performs in order to calculate the main feedback amount. It is the flowchart which showed the routine which CPU of the electric control apparatus shown in FIG. 6 performs in order to calculate a sub feedback amount and a sub FB learning value. FIG. 7 is a flowchart showing a routine executed by the CPU of the electric control device shown in FIG. 6 for “calculating a time required for the crank angle to rotate 180 degrees”. It is the flowchart which showed the routine performed in order that CPU of the electric control apparatus shown in FIG. 6 may calculate a rotational speed fluctuation parameter. It is the flowchart which showed the routine performed in order that CPU of the electric control apparatus shown in FIG. 6 may perform the air fuel ratio cylinder imbalance determination. It is the figure which showed a part of routine performed by CPU of the air-fuel ratio imbalance among cylinders determination apparatus which concerns on 2nd Embodiment of this invention. It is the figure which showed a part of routine performed by CPU of the air-fuel ratio imbalance among cylinders determination apparatus which concerns on 3rd Embodiment of this invention.

  Hereinafter, an air-fuel ratio imbalance among cylinders determination apparatus (hereinafter simply referred to as “determination apparatus”) of a multi-cylinder internal combustion engine according to a first embodiment of the present invention will be described with reference to the drawings. This determination device is a part of an air-fuel ratio control device that controls the air-fuel ratio of the internal combustion engine. Further, the air-fuel ratio control device is also a fuel injection amount control device that controls the fuel injection amount.

(Constitution)
FIG. 6 shows a schematic configuration of the internal combustion engine 10 to which the determination device is applied. The engine 10 is a four-cycle / spark ignition type / multi-cylinder (four cylinders in this example) / gasoline fuel engine. The engine 10 includes a main body 20, an intake system 30, and an exhaust system 40.

  The main body portion 20 includes a cylinder block portion and a cylinder head portion. The main body portion 20 includes a plurality (four) of combustion chambers (first cylinder # 1 to fourth cylinder # 4) 21 including a piston top surface, a cylinder wall surface, and a lower surface of the cylinder head portion.

  In the cylinder head portion, an intake port 22 for supplying “a mixture of air and fuel” to each combustion chamber (each cylinder) 21, and an exhaust gas (burned gas) from each combustion chamber 21 are discharged. An exhaust port 23 is formed. The intake port 22 is opened and closed by an unillustrated intake valve, and the exhaust port 23 is opened and closed by an unillustrated exhaust valve.

  A plurality (four) of spark plugs 24 are fixed to the cylinder head portion. Each spark plug 24 is disposed such that its spark generating part is exposed at the center of each combustion chamber 21 and in the vicinity of the lower surface of the cylinder head part. Each spark plug 24 generates an ignition spark from the spark generating portion in response to the ignition signal.

  A plurality (four) of fuel injection valves (injectors) 25 are further fixed to the cylinder head portion. One fuel injection valve 25 is provided for each intake port 22. In response to the injection instruction signal, the fuel injection valve 25 injects “the fuel of the indicated injection amount included in the injection instruction signal” into the corresponding intake port 22 when it is normal. Thus, each of the plurality of cylinders 21 includes the fuel injection valve 25 that supplies fuel independently from the other cylinders.

  Further, an intake valve control device 26 is provided in the cylinder head portion. The intake valve control device 26 has a known configuration that adjusts and controls the relative rotation angle (phase angle) between an intake camshaft (not shown) and an intake cam (not shown) by hydraulic pressure. The intake valve control device 26 operates based on an instruction signal (drive signal), and can change the valve opening timing (intake valve opening timing) of the intake valve.

  The intake system 30 includes an intake manifold 31, an intake pipe 32, an air filter 33, a throttle valve 34, and a throttle valve actuator 34a.

  The intake manifold 31 includes a plurality of branch portions connected to each intake port 22 and a surge tank portion in which the branch portions are gathered. The intake pipe 32 is connected to the surge tank portion. The intake manifold 31, the intake pipe 32, and the plurality of intake ports 22 constitute an intake passage. The air filter 33 is provided at the end of the intake pipe 32. The throttle valve 34 is rotatably attached to the intake pipe 32 at a position between the air filter 33 and the intake manifold 31. The throttle valve 34 changes the opening cross-sectional area of the intake passage formed by the intake pipe 32 by rotating. The throttle valve actuator 34a is formed of a DC motor, and rotates the throttle valve 34 in response to an instruction signal (drive signal).

  The exhaust system 40 includes an exhaust manifold 41, an exhaust pipe (exhaust pipe) 42, an upstream catalyst 43, and a downstream catalyst 44.

  The exhaust manifold 41 includes a plurality of branch portions 41a connected to each exhaust port 23, and a collection portion (exhaust collection portion) 41b in which the branch portions 41a are gathered. The exhaust pipe 42 is connected to a collective portion 41 b of the exhaust manifold 41. The exhaust manifold 41, the exhaust pipe 42, and the plurality of exhaust ports 23 constitute a passage through which exhaust gas passes. In the present specification, the collecting portion 41b of the exhaust manifold 41 and the exhaust pipe 42 are referred to as “exhaust passage” for convenience.

The upstream catalyst 43 is a three-way catalyst that supports “noble metal as a catalyst material” and “ceria (CeO 2)” on a support made of ceramic and has an oxygen storage / release function (oxygen storage function). The upstream catalyst 43 is disposed (intervened) in the exhaust pipe 42. When the upstream catalyst 43 reaches a predetermined activation temperature, it exhibits a “catalytic function for simultaneously purifying unburned substances (HC, CO, H _2, etc.) and nitrogen oxides (NOx)” and “oxygen storage function”. . The upstream catalyst 43 can also be expressed as having “a function of purifying at least hydrogen H ₂ by oxidizing” in order to detect an air-fuel ratio imbalance among cylinders. That is, the upstream catalyst 43 may be another type of catalyst (for example, an oxidation catalyst) as long as it has a “function of purifying by oxidizing hydrogen H ₂ ”.

  The downstream catalyst 44 is a three-way catalyst similar to the upstream catalyst 43. The downstream catalyst 44 is disposed (intervened) in the exhaust pipe 42 downstream of the upstream catalyst 43.

  The determination device includes a hot-wire air flow meter 51, a throttle position sensor 52, a crank angle sensor 53, a water temperature sensor 54, an upstream air-fuel ratio sensor 55, a downstream air-fuel ratio sensor 56, an accelerator opening sensor 57, and an intake cam position sensor 58. It has.

  The hot-wire air flow meter 51 detects the mass flow rate of the intake air flowing through the intake pipe 32 and outputs a signal representing the mass flow rate (intake air amount per unit time of the engine 10) Ga.

  The throttle position sensor 52 detects the opening degree of the throttle valve 34 and outputs a signal representing the throttle valve opening degree TA.

  The crank angle sensor (crank position sensor) 53 outputs a signal having a narrow pulse every time the crankshaft of the engine 10 rotates 10 degrees and a wide pulse every time the crankshaft rotates 360 °. It has become. This signal is converted into an engine speed NE by an electric control device 60 described later.

  The water temperature sensor 54 detects the temperature of the cooling water of the internal combustion engine 10 and outputs a signal representing the cooling water temperature THW.

  The upstream air-fuel ratio sensor 55 is disposed in either the exhaust manifold 41 or the exhaust pipe 42 (that is, the exhaust passage) at a position between the collecting portion 41 b of the exhaust manifold 41 and the upstream catalyst 43. The upstream air-fuel ratio sensor 55 is disclosed in, for example, “limit current type wide area air-fuel ratio including a diffusion resistance layer” disclosed in JP-A-11-72473, JP-A-2000-65782, JP-A-2004-69547, and the like. Sensor ".

  As shown in FIG. 7, the upstream air-fuel ratio sensor 55 includes a solid electrolyte layer 55a, an exhaust gas side electrode layer 55b, an atmosphere side electrode layer 55c, a diffusion resistance layer 55d, a partition wall portion 55e, a heater 55f, , Including.

The solid electrolyte layer 55a is an oxygen ion conductive oxide sintered body. In this example, the solid electrolyte layer 55a is a “stabilized zirconia element” in which CaO is dissolved in ZrO ₂ (zirconia) as a stabilizer. The solid electrolyte layer 55a exhibits well-known “oxygen battery characteristics” and “oxygen pump characteristics” when its temperature is equal to or higher than the activation temperature. As will be described later, these characteristics are characteristics that should be exhibited when the upstream air-fuel ratio sensor 55 outputs an output value corresponding to the air-fuel ratio of the exhaust gas. The oxygen battery characteristic is a characteristic that generates an electromotive force by allowing oxygen ions to pass from a high oxygen concentration side to a low oxygen concentration side. The oxygen pump characteristic means that when a potential difference is applied to both ends of the solid electrolyte layer 55a, oxygen ions in an amount corresponding to the potential difference between the electrodes from the cathode (low potential side electrode) to the anode (high potential side electrode). It is a characteristic that moves

  The exhaust gas side electrode layer 55b is made of a noble metal having high catalytic activity such as platinum (Pt). The exhaust gas side electrode layer 55b is formed on one surface of the solid electrolyte layer 55a. The exhaust gas side electrode layer 55b is formed by chemical plating or the like so as to have sufficient permeability (that is, in a porous shape).

  The atmosphere-side electrode layer 55c is made of a noble metal having high catalytic activity such as platinum (Pt). The atmosphere-side electrode layer 55c is formed on the other surface of the solid electrolyte layer 55a so as to face the exhaust gas-side electrode layer 55b with the solid electrolyte layer 55a interposed therebetween. The atmosphere-side electrode layer 55c is formed to have sufficient permeability (that is, in a porous shape) by chemical plating or the like.

The diffusion resistance layer (diffusion rate limiting layer) 55d is made of a porous ceramic (a heat resistant inorganic substance). The diffusion resistance layer 55d is formed by, for example, a plasma spraying method so as to cover the outer surface of the exhaust gas side electrode layer 55b. The diffusion rate of the hydrogen H _{2 having} a small molecular diameter in the diffusion resistance layer 55d is larger than the diffusion rate in the diffusion resistance layer 55d of “hydrocarbon HC, carbon monoxide CO, etc.” having a relatively large molecular diameter. Therefore, due to the presence of the diffusion resistance layer 55d, the hydrogen H ₂ reaches the “exhaust gas side electrode layer 55b” more rapidly than the hydrocarbon HC and carbon monoxide CO. The upstream air-fuel ratio sensor 55 is arranged so that the outer surface of the diffusion resistance layer 55d is “exposed to exhaust gas (exhaust gas discharged from the engine 10 contacts)”.

  The partition wall 55e is made of alumina ceramic that is dense and does not allow gas to pass therethrough. The partition wall 55e is configured to form an “atmosphere chamber 55g” that is a space for accommodating the atmosphere-side electrode layer 55c. Air is introduced into the atmospheric chamber 55g.

  The heater 55f is embedded in the partition wall 55e. The heater 55f generates heat when energized, and heats the solid electrolyte layer 55a.

  The upstream air-fuel ratio sensor 55 uses a power supply 55h as shown in FIG. The power source 55h applies the voltage V so that the atmosphere side electrode layer 55c side has a high potential and the exhaust gas side electrode layer 55b has a low potential.

  As shown in FIG. 8, when the air-fuel ratio of the exhaust gas is an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the air-fuel ratio is detected by utilizing the above-described oxygen pump characteristics. That is, when the air-fuel ratio of the exhaust gas is an air-fuel ratio leaner than the stoichiometric air-fuel ratio, oxygen molecules contained in a large amount in the exhaust gas reach the exhaust gas-side electrode layer 55b through the diffusion resistance layer 55d. The oxygen molecules receive electrons and become oxygen ions. Oxygen ions pass through the solid electrolyte layer 55a, emit electrons at the atmosphere-side electrode layer 55c, and become oxygen molecules. As a result, a current I flows from the positive electrode of the power source 55h to the negative electrode of the power source 55h through the atmosphere side electrode layer 55c, the solid electrolyte layer 55a, and the exhaust gas side electrode layer 55b.

  The magnitude of this current I is “the exhaust gas passing through the diffusion resistance layer 55d among oxygen molecules contained in the exhaust gas reaching the outer surface of the diffusion resistance layer 55d when the magnitude of the voltage V is set to a predetermined value Vp or more. It changes in accordance with the amount of “oxygen molecules reaching the side electrode layer 55b by diffusion”. That is, the magnitude of the current I changes according to the oxygen concentration (oxygen partial pressure) in the exhaust gas side electrode layer 55b. The oxygen concentration in the exhaust gas side electrode layer 55b changes according to the oxygen concentration of the exhaust gas that has reached the outer surface of the diffusion resistance layer 55d. As shown in FIG. 9, the current I does not change even if the voltage V is set to a predetermined value Vp or more, and is therefore called a limit current Ip. The air-fuel ratio sensor 55 outputs a value corresponding to the air-fuel ratio based on the limit current Ip value.

On the other hand, when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio, as shown in FIG. 10, the air-fuel ratio is detected by utilizing the above-described oxygen battery characteristics. More specifically, when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio, unburned substances (HC, CO, H _2, etc.) contained in a large amount in the exhaust gas are diffused resistance layer 55d. And reaches the exhaust gas side electrode layer 55b. In this case, since the difference (oxygen partial pressure difference) between the oxygen concentration in the atmosphere-side electrode layer 55c and the oxygen concentration in the exhaust gas-side electrode layer 55b becomes large, the solid electrolyte layer 55a functions as an oxygen battery. The applied voltage V is set to be smaller than the electromotive force of this oxygen battery.

  Accordingly, oxygen molecules present in the atmospheric chamber 55g receive electrons in the atmospheric electrode layer 55c and become oxygen ions. The oxygen ions pass through the solid electrolyte layer 55a and move to the exhaust gas side electrode layer 55b. And an unburned substance is oxidized in the exhaust gas side electrode layer 55b, and an electron is discharge | released. As a result, a current I flows from the negative electrode of the power source 55h to the positive electrode of the power source 55h through the exhaust gas side electrode layer 55b, the solid electrolyte layer 55a, and the atmosphere side electrode layer 55c.

  The magnitude of the current I is determined by the amount of oxygen ions that reach the exhaust gas side electrode layer 55b from the atmosphere side electrode layer 55c through the solid electrolyte layer 55a. As described above, the oxygen ions are used to oxidize the unburned material in the exhaust gas side electrode layer 55b. Therefore, as the amount of unburned matter that reaches the exhaust gas side electrode layer 55b through the diffusion resistance layer 55d by diffusion increases, the amount of oxygen ions that pass through the solid electrolyte layer 55a increases. In other words, the smaller the air-fuel ratio (the richer the air-fuel ratio than the stoichiometric air-fuel ratio and the greater the amount of unburned matter), the larger the magnitude of the current I. However, since the amount of unburned matter reaching the exhaust gas side electrode layer 55b is limited by the presence of the diffusion resistance layer 55d, the current I becomes a constant value Ip corresponding to the air-fuel ratio. The upstream air-fuel ratio sensor 55 outputs a value corresponding to the air-fuel ratio based on the limit current Ip value.

  As shown in FIG. 11, the upstream air-fuel ratio sensor 55 based on such a detection principle outputs according to the air-fuel ratio (upstream air-fuel ratio abyfs) of the exhaust gas flowing through the position where the upstream air-fuel ratio sensor 55 is disposed. Outputs the value Vabyfs. The output value Vabyfs is obtained by converting the limit current Ip into a voltage. The output value Vabyfs increases as the air-fuel ratio of the detection gas increases (lean). The electric control device 60 described later stores the air-fuel ratio conversion table (map) Mapabyfs shown in FIG. 11 and applies the output value Vabyfs to the air-fuel ratio conversion table Mapabyfs, so that the actual upstream air-fuel ratio abyfs is obtained. To detect. This air-fuel ratio conversion table Mapabyfs is created in consideration of selective hydrogen diffusion. In other words, the table Mapabyfs displays the “upstream air-fuel ratio when the air-fuel ratio of the exhaust gas reaching the upstream air-fuel ratio sensor 55 is set to the value x by setting the air-fuel ratio of each cylinder to the same air-fuel ratio x. It is created based on the “actual output value Vabyfs of the sensor 55”.

  Referring again to FIG. 6, the downstream air-fuel ratio sensor 56 is disposed in the exhaust pipe 42 (that is, the exhaust passage) at a position between the upstream catalyst 43 and the downstream catalyst 44. The downstream air-fuel ratio sensor 56 is a well-known concentration cell type oxygen concentration sensor (O2 sensor). The downstream air-fuel ratio sensor 56 has a configuration similar to that of the upstream air-fuel ratio sensor 55 shown in FIG. 7, for example (except for the power supply 55h). Alternatively, the downstream side air-fuel ratio sensor 56 is exposed to the test tube solid electrolyte layer, the exhaust gas side electrode layer formed outside the solid electrolyte layer, and the atmosphere chamber (inside the solid electrolyte layer) and the solid electrolyte chamber layer. Diffusion resistance that covers the exhaust gas side electrode layer and is in contact with the exhaust gas (disposed to be exposed to the exhaust gas), which is formed on the solid electrolyte layer so as to face the exhaust gas electrode layer across And a layer. The downstream air-fuel ratio sensor 56 outputs an output value Voxs corresponding to the air-fuel ratio (downstream air-fuel ratio afdown) of the exhaust gas flowing through the position where the downstream air-fuel ratio sensor 56 is disposed.

  As shown in FIG. 12, the output value Voxs of the downstream air-fuel ratio sensor 56 becomes the maximum output value max (for example, about 0.9 V) when the air-fuel ratio of the detected gas is richer than the stoichiometric air-fuel ratio. When the air-fuel ratio of the detection gas is leaner than the stoichiometric air-fuel ratio, the minimum output value min (for example, about 0.1 V) is obtained. When the air-fuel ratio of the detected gas is the stoichiometric air-fuel ratio, the maximum output value max and the minimum output value min Voltage Vst (intermediate voltage Vst, for example, about 0.5 V). Further, this output value Voxs suddenly changes from the maximum output value max to the minimum output value min when the air-fuel ratio of the gas to be detected changes from an air-fuel ratio richer than the stoichiometric air-fuel ratio to a lean air-fuel ratio. When the air-fuel ratio of the detection gas changes from an air-fuel ratio leaner than the stoichiometric air-fuel ratio to a rich air-fuel ratio, it suddenly changes from the minimum output value min to the maximum output value max.

  The accelerator opening sensor 57 shown in FIG. 6 detects the operation amount of the accelerator pedal AP operated by the driver, and outputs a signal indicating the operation amount Accp of the accelerator pedal AP.

  The intake cam position sensor 58 outputs one pulse every time the intake cam shaft rotates 90 degrees, 90 degrees, and 180 degrees from a predetermined angle. Based on the signals from the crank angle sensor 53 and the intake cam position sensor 58, the electric control device 60 acquires the absolute crank angle based on the compression top dead center of the specific cylinder (for example, the first cylinder # 1). It has become.

  The electric control device 60 is a “well-known microcomputer” including “a CPU, a ROM, a RAM, a backup RAM (or a nonvolatile memory such as an EEPROM), and an interface including an AD converter”.

  The backup RAM is supplied with electric power from a battery mounted on the vehicle regardless of the position of an ignition key switch (not shown) of the vehicle on which the engine 10 is mounted (any one of an off position, a start position, an on position, etc.). It is like that. When receiving power from the battery, the backup RAM stores data according to an instruction from the CPU (data is written) and holds (stores) the data so that the data can be read. The backup RAM cannot retain data when the power supply from the battery is interrupted, for example, when the battery is removed from the vehicle. Therefore, when the power supply to the backup RAM is resumed, the CPU initializes (sets to the default value) data to be held in the backup RAM.

  The interface of the electric control device 60 is connected to the sensors 51 to 58 and supplies signals from the sensors 51 to 58 to the CPU. Further, the interface sends an instruction signal (drive signal) or the like to the ignition plug 24 of each cylinder, the fuel injection valve 25 of each cylinder, the intake valve control device 26, the throttle valve actuator 34a, etc. in accordance with an instruction from the CPU. It is like that. The electric control device 60 sends an instruction signal to the throttle valve actuator 34a so that the throttle valve opening TA increases as the acquired accelerator pedal operation amount Accp increases.

(Principle of air-fuel ratio imbalance determination)
Next, the principle of “air-fuel ratio imbalance determination” will be described. Air-fuel ratio imbalance determination is whether or not the non-uniformity of the air-fuel ratio between the cylinders exceeds a predetermined value, in other words, a significant imbalance between the cylinder-by-cylinder air-fuel ratio (that is, the air-fuel ratio between cylinders). It is to determine whether or not (imbalance) has occurred.

The fuel of the engine 10 is a compound of carbon and hydrogen. Therefore, in the process in which the fuel burns and changes into water H ₂ O and carbon dioxide CO ₂ , unburned substances such as “hydrocarbon HC, carbon monoxide CO and hydrogen H ₂ ” are generated as intermediate products. .

As the air-fuel ratio of the air-fuel mixture used for combustion becomes smaller than the stoichiometric air-fuel ratio (that is, as the air-fuel ratio becomes richer than the stoichiometric air-fuel ratio), the amount of oxygen necessary for complete combustion of the fuel And the actual amount of oxygen increases. In other words, as the air-fuel ratio becomes richer, the shortage of oxygen in the middle of combustion increases and the oxygen concentration decreases, so the probability that the intermediate product (unburned material) encounters oxygen and combines (oxidizes) with oxygen. It decreases rapidly. As a result, as shown in FIG. 13, the amount of unburned matter (HC, CO, and H ₂ ) discharged from the cylinder increases as the air-fuel ratio of the air-fuel mixture supplied to the cylinder becomes richer. (In a quadratic function). Note that points P1, P2, and P3 in FIG. 13 indicate that the amount of fuel supplied to a cylinder is 10% of the amount of fuel when the air-fuel ratio of the cylinder matches the stoichiometric air-fuel ratio. It shows the points that are excessive by (= AF1), 30% (= AF2) and 40% (= AF3).

Furthermore, hydrogen H ₂ is a small molecule compared to hydrocarbon HC and carbon monoxide CO. Therefore, hydrogen H ₂ diffuses more quickly in the diffusion resistance layer 55d of the upstream air-fuel ratio sensor 55 than other unburned substances (HC, CO). For this reason, when a large amount of unburned material composed of HC, CO, and H ₂ is generated, selective diffusion (preferential diffusion) of hydrogen H ₂ occurs remarkably in the diffusion resistance layer 55d. That is, hydrogen H ₂ reaches the surface of the air-fuel ratio detection element (exhaust gas side electrode layer 55b formed on the surface of the solid electrolyte layer 55a) in a larger amount than “other unburned substances (HC, CO)”. become. As a result, the balance between the concentration of hydrogen H _{2 and} the concentration of other unburned substances (HC, CO) is lost. In other words, the ratio of hydrogen H ₂ to all unburned components contained in “the exhaust gas that has reached the air-fuel ratio detection element (exhaust gas side electrode layer 55b) of the upstream air-fuel ratio sensor 55” is “the exhaust gas discharged from the engine 10”. The ratio of hydrogen H ₂ to the total unburned components contained in “

  Incidentally, the determination device is a part of the air-fuel ratio control device. The air-fuel ratio control apparatus matches “the upstream air-fuel ratio abyfs (the air-fuel ratio corresponding to the output value Vabyfs) represented by the output value Vabyfs of the upstream air-fuel ratio sensor 55” with “the upstream target air-fuel ratio abyfr”. “Air-fuel ratio feedback control (main feedback control)”. In general, the upstream target air-fuel ratio abyfr is set to the stoichiometric air-fuel ratio stoich.

  Further, the air-fuel ratio control device converts the output value Voxs of the downstream air-fuel ratio sensor 56 (or the downstream air-fuel ratio afdown represented by the output value Voxs of the downstream air-fuel ratio sensor) to the downstream target value Voxsref (or downstream). The sub-feedback control of the air-fuel ratio is performed so as to match the downstream target air-fuel ratio represented by the side target value Voxsref. In general, the downstream target value Voxsref is set to a value (0.5 V) corresponding to the theoretical air-fuel ratio.

  Assume that the air-fuel ratio of each cylinder is uniformly shifted to the rich side in a state where no air-fuel ratio imbalance among cylinders has occurred. Such a situation occurs, for example, when the “measured value or estimated value of the intake air amount of the engine”, which is the basic amount for calculating the fuel injection amount, becomes larger than the “true intake air amount”. To do.

In this case, for example, it is assumed that the air-fuel ratio of each cylinder is AF2 shown in FIG. When the air-fuel ratio of a certain cylinder is AF2, more unburned matter (and hence hydrogen H ₂ ) is discharged into the exhaust gas than when the air-fuel ratio of a certain cylinder is the air-fuel ratio AF1 that is closer to the theoretical air-fuel ratio than AF2. Included (see point P1 and point P2). Accordingly, “selective diffusion of hydrogen H ₂ ” occurs in the diffusion resistance layer 55 d of the upstream air-fuel ratio sensor 55.

However, in this case, the true average value of the air-fuel ratio of “the air-fuel mixture supplied to the engine 10 while each cylinder completes one combustion stroke (a period corresponding to a crank angle of 720 degrees)” is also AF2. . Furthermore, as described above, the air-fuel ratio conversion table Mapabyfs shown in FIG. 11 is created in consideration of “selective diffusion of hydrogen H ₂ ”. Therefore, the upstream air-fuel ratio abyfs expressed by the actual output value Vabyfs of the upstream air-fuel ratio sensor 55 (the upstream air-fuel ratio abyfs obtained by applying the actual output value Vabyfs to the air-fuel ratio conversion table Mapabyfs) is: This coincides with the “true average value AF2 of the air-fuel ratio”.

  Therefore, by the main feedback control, the air-fuel ratio of the air-fuel mixture supplied to the entire engine 10 is corrected to coincide with “the theoretical air-fuel ratio that is the upstream target air-fuel ratio abyfr”, and the air-fuel ratio imbalance among cylinders is generated. Therefore, the air-fuel ratio of each cylinder also substantially matches the stoichiometric air-fuel ratio. Therefore, the sub feedback amount (and the learned value of the sub feedback amount described later) does not become a value that greatly corrects the air-fuel ratio. In other words, when the air-fuel ratio imbalance among cylinders does not occur, the sub-feedback amount (and the learned value of the sub-feedback amount described later) does not become a value that greatly corrects the air-fuel ratio.

  Another description will be given below of the behavior of each value in the above-mentioned case “when the air-fuel ratio imbalance among cylinders does not occur”.

  For example, when the air amount (weight) taken into each cylinder of the engine 10 is A0 and the fuel amount (weight) supplied to each cylinder is F0, the air-fuel ratio A0 / F0 is the stoichiometric air-fuel ratio (for example, 14.5).

  Then, it is assumed that the amount of fuel supplied (injected) to each cylinder is excessively increased by 10% due to an estimation error of the intake air amount. That is, it is assumed that 1.1 · F0 fuel is supplied to each cylinder. At this time, the total amount of air supplied to the engine 10 which is a four-cylinder engine (the amount of air supplied to the entire engine 10 while each cylinder completes one combustion stroke) is 4 · A0. Further, the total amount of fuel supplied to the engine 10 (the amount of fuel supplied to the entire engine 10 while each cylinder completes one combustion stroke) is 4.4 · F0 (= 1.1 · F0 + 1.1 · F0 + 1.1 · F0 + 1.1 · F0). Therefore, the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine 10 is 4 · A0 / (4.4 · F0) = A0 / (1.1 · F0). At this time, the output value of the upstream air-fuel ratio sensor becomes an output value corresponding to the air-fuel ratio A0 / (1.1 · F0).

  Accordingly, the amount of fuel supplied to each cylinder is reduced by 10% by the main feedback control (1 · F0 fuel is supplied to each cylinder), and the amount of fuel supplied to the entire engine 10 is reduced. The air-fuel ratio is made equal to the theoretical air-fuel ratio A0 / F0.

  On the other hand, it is assumed that only the air-fuel ratio of the specific cylinder is greatly shifted to the rich side. Such a situation is, for example, when the injection characteristic of the fuel injection valve 25 provided for the specific cylinder becomes “a characteristic for injecting a fuel amount much larger than the instructed fuel injection amount”. Arise. Such an abnormality of the fuel injection valve 25 is also referred to as “rich abnormality of the fuel injection valve”.

  Now, the amount of fuel supplied to one specific cylinder is an excess amount (ie, 1.4 · F0) by 40%, and the amount of fuel supplied to the remaining three cylinders is It is assumed that the amount of fuel is equal to the stoichiometric air-fuel ratio (ie, 1 · F0). In this case, the air-fuel ratio of the specific cylinder is “AF3” shown in FIG. 13, and the air-fuel ratio of the remaining cylinders is the stoichiometric air-fuel ratio.

  At this time, the total amount of air supplied to the engine 10 which is a four-cylinder engine (the amount of air supplied to the entire engine 10 while each cylinder completes one combustion stroke) is 4 · A0. On the other hand, the total amount of fuel supplied to the engine 10 (the amount of fuel supplied to the entire engine 10 while each cylinder completes one combustion stroke) is 4.4 · F0 (= 1.4 · F0 + F0 + F0 + F0). ).

  Therefore, the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine 10 is 4 · A0 / (4.4 · F0) = A0 / (1.1 · F0). In other words, the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine 10 in this case is as described above “when the amount of fuel supplied to each cylinder is equally excessive by 10%”. It becomes the same value.

However, as described above, the amount of unburned matter (HC, CO, and H ₂ ) in the exhaust gas increases rapidly as the air-fuel ratio of the air-fuel mixture supplied to the cylinder becomes richer. Therefore, according to FIG. 13, the total amount SH1 of hydrogen H ₂ contained in the exhaust gas when “only the amount of fuel supplied to the specific cylinder is an excess amount of 40%” is SH1 = H3 + H0 + H0 + H0. = H3 + 3 · H0. On the other hand, according to FIG. 13, the total amount SH2 of hydrogen H ₂ contained in the exhaust gas when “the amount of fuel supplied to each cylinder is uniformly increased by 10%” is SH2 = H1 + H1 + H1 + H1. = 4 · H1. At this time, the amount H1 is slightly larger than the amount H0, but both the amount H1 and the amount H0 are extremely small. That is, it can be said that the amount H1 and the amount H0 are substantially equal to each other when compared with the amount H3. Therefore, the total hydrogen amount SH1 is extremely larger than the total hydrogen amount SH2 (SH1 >> SH2).

  In this way, even if the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine 10 is the same, the total amount SH1 of hydrogen contained in the exhaust gas when the air-fuel ratio imbalance among cylinders occurs is When the imbalance between cylinders does not occur, the total amount SH2 of hydrogen contained in the exhaust gas becomes significantly larger.

Therefore, when only the amount of fuel supplied to the specific cylinder becomes an excessive amount by 40%, the upstream side empty space is caused by the “selective diffusion of hydrogen H ₂ ” in the diffusion resistance layer 55 d described above. The air-fuel ratio represented by the output value Vabyfs of the fuel-fuel ratio sensor is richer than “the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine 10 (A0 / (1.1 · F0))”. (Small air-fuel ratio). That is, even if the average value of the air-fuel ratio of the exhaust gas is the same, when the air-fuel ratio imbalance among cylinders is occurring, the upstream air-fuel ratio is higher than when the air-fuel ratio imbalance among cylinders is not occurring. Since the concentration of hydrogen H ₂ in the exhaust gas side electrode layer 55b of the sensor 55 is increased, the output value Vabyfs of the upstream air-fuel ratio sensor 55 is a value indicating an air-fuel ratio richer than the “true average value of air-fuel ratio”. It becomes.

  As a result, the true average of the air-fuel ratio of the air-fuel mixture supplied to the entire engine 10 is controlled to be leaner than the stoichiometric air-fuel ratio by the main feedback control.

On the other hand, the exhaust gas that has passed through the upstream catalyst 43 reaches the downstream air-fuel ratio sensor 56. Hydrogen H ₂ contained in the exhaust gas is oxidized (purified) in the upstream catalyst 43 together with other unburned substances (HC, CO). Therefore, the output value Voxs of the downstream air-fuel ratio sensor 56 is a value corresponding to the true air-fuel ratio of the air-fuel mixture supplied to the entire engine 10. Therefore, the control amount of the air-fuel ratio (sub-feedback amount or the like) calculated by the sub-feedback control is a value that compensates for the overcorrection of the air-fuel ratio to the lean side by the main feedback control. The true average value of the air-fuel ratio of the engine 10 is made to coincide with the stoichiometric air-fuel ratio by such a sub-feedback amount and the like.

  Thus, the control amount of the air-fuel ratio (sub-feedback amount) calculated by the sub-feedback control is “to the lean side of the air-fuel ratio due to the rich deviation abnormality (air-fuel ratio imbalance between cylinders) of the fuel injection valve 25. It is a value that compensates for “over-correction”. The degree of overcorrection to the lean side is such that the fuel injection valve 25 that has caused the rich deviation abnormality injects a larger amount of fuel than the “instructed injection amount” (that is, It increases) as the air-fuel ratio of the specific cylinder becomes richer.

  Accordingly, in the “system in which the air-fuel ratio of the engine is corrected to a richer side” as the sub feedback amount is a positive value and the magnitude thereof is larger, “a value that changes according to the sub feedback amount (actually Is a sub-feedback amount learning value incorporating a steady component of the sub-feedback amount) ”, for example, is a value indicating the degree of air-fuel ratio imbalance among cylinders.

  Based on this knowledge, this determination apparatus acquires a value that changes according to the sub feedback amount (in this example, “sub FB learning value” that is a learning value of the sub feedback amount) as an imbalance determination parameter. That is, the imbalance determination parameter is “the larger the difference between the amount of hydrogen contained in the exhaust gas before passing through the upstream catalyst 43 and the amount of hydrogen contained in the exhaust gas after passing through the upstream catalyst 43, , A value that increases. Therefore, the imbalance determination parameter is also referred to as a hydrogen amount difference instruction parameter.

  When the hydrogen amount difference indicating parameter is equal to or greater than the “abnormality determination threshold” (that is, the value that increases or decreases in accordance with the increase or decrease of the sub FB learning value is “the engine air / fuel ratio is richer than the abnormality determination threshold value”). When the value becomes “a value indicating correction to the side”), it is determined that an air-fuel ratio imbalance among cylinders has occurred.

A curve C1 in FIG. 2 shows the sub FB learning value when the air-fuel ratio imbalance among cylinders occurs and the air-fuel ratio of a certain cylinder deviates from the stoichiometric air-fuel ratio to the rich side and the lean side. The horizontal axis of the graph shown in FIG. 10 is the “imbalance ratio”. The imbalance ratio is “the ratio (Y / X) of the difference Y (= X−af) between the theoretical air-fuel ratio X and the rich air-fuel ratio af of the cylinder with respect to the theoretical air-fuel ratio X”. . As described above, as the imbalance ratio increases, the influence of selective diffusion of hydrogen H ₂ increases rapidly. Therefore, as shown by the curve C1 in FIG. 2, the sub FB learning value (and hence the hydrogen amount difference indicating parameter that is an imbalance determination parameter) increases in a quadratic function as the imbalance ratio increases. .

  As shown by the curve C1 in FIG. 2, even when the imbalance ratio is a negative value, the sub FB learning value increases as the absolute value of the imbalance ratio increases. That is, for example, even when an air-fuel ratio imbalance among cylinders occurs in which only the air-fuel ratio of one specific cylinder is greatly shifted to the lean side, the sub-FB learning value (sub-FB learning value as the hydrogen amount difference indicating parameter) is generated. The corresponding value) increases. Such a situation is, for example, when the injection characteristic of the fuel injection valve 25 provided for the specific cylinder becomes “a characteristic for injecting a fuel amount considerably smaller than the instructed fuel injection amount”. Arise. Such an abnormality in the fuel injection valve 25 is also referred to as “an abnormality in lean deviation of the fuel injection valve”.

  Hereinafter, the reason why the sub FB learning value increases even when the air-fuel ratio imbalance among cylinders in which only the air-fuel ratio of one specific cylinder is greatly shifted to the lean side occurs will be briefly described. Also in the following description, it is assumed that the amount of air (weight) taken into each cylinder of the engine 10 is A0. Further, it is assumed that the air-fuel ratio A0 / F0 matches the stoichiometric air-fuel ratio when the fuel amount (weight) supplied to each cylinder is F0.

  Now, the amount of fuel supplied to a specific cylinder (for convenience, the first cylinder # 1) is an amount that is too small (ie, 0.6 · F0) by 40%, and the remaining 3 The amount of fuel supplied to the cylinders (second, third and fourth cylinders) is assumed to be the amount of fuel such that the air-fuel ratio of those cylinders matches the stoichiometric air-fuel ratio, that is, F0. To do. In this case, it is assumed that no misfire occurs.

  In this case, it is assumed that the amount of fuel supplied to the first cylinder # 1 to the fourth cylinder # 4 is increased by the same predetermined amount (10%) by the main feedback control. At this time, the amount of fuel supplied to the first cylinder is 0.7 · F0, and the amount of fuel supplied to each of the second to fourth cylinders is 1.1 · F0.

  In this state, the total amount of air supplied to the engine 10 which is a four-cylinder engine (the amount of air supplied to the entire engine 10 while each cylinder completes one combustion stroke) is 4 · A0. is there. Further, as a result of the main feedback control, the total amount of fuel supplied to the engine 10 (the amount of fuel supplied to the entire engine 10 while each cylinder completes one combustion stroke) is 4 · F0 (= 0.7 · F0 + 1.1 · F0 + 1.1 · F0 + 1.1 · F0). Therefore, the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine 10 is 4 · A0 / (4 · F0) = A0 / F0, that is, the stoichiometric air-fuel ratio.

However, “the total amount SH3 of hydrogen H ₂ contained in the exhaust gas” in this state is SH3 = H4 + H1 + H1 + H1 = H4 + 3 · H1. However, H4 is the amount of hydrogen generated when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio A0 / (0.7 · F0), and is substantially equal to H0. Accordingly, the total amount SH3 is at most (H0 + 3 · H1).

On the other hand, when the air-fuel ratio imbalance among cylinders does not occur and the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine 10 is the stoichiometric air-fuel ratio, “hydrogen H ₂ contained in exhaust gas” The total amount SH4 ”is SH4 = H0 + H0 + H0 + H0 = 4 · H0. As described above, H1 is slightly larger than H0. Accordingly, the total amount SH3 (= H0 + 3 · H1) is larger than the total amount SH4 (= 4 · H0).

  Accordingly, when the air-fuel ratio imbalance among cylinders due to “lean deviation abnormality of the fuel injection valve” occurs, the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine 10 is obtained by the main feedback control. Even when the air-fuel ratio is shifted to the stoichiometric air-fuel ratio, the influence of the selective hydrogen diffusion appears in the output value Vabyfs of the upstream air-fuel ratio sensor 55. That is, the upstream air-fuel ratio abyfs obtained by applying the output value Vabyfs to the air-fuel ratio conversion table Mapabyfs is “richer (smaller) air-fuel ratio” than the stoichiometric air-fuel ratio that is the upstream target air-fuel ratio abyfr. . As a result, the main feedback control is further executed, and the true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine 10 is corrected to be leaner than the stoichiometric air-fuel ratio.

  Therefore, the control amount of the air-fuel ratio calculated by the sub-feedback control is “overcorrection of the air-fuel ratio to the lean side by the main feedback control due to the lean deviation abnormality (air-fuel ratio imbalance among cylinders) of the fuel injection valve 25. ”To compensate. Therefore, the “sub-FB learning value” acquired based on “the control amount of the air-fuel ratio calculated by the sub-feedback control” has a negative imbalance ratio and the absolute value of the imbalance ratio increases. It increases.

  As a result, the determination apparatus can detect the hydrogen amount difference indicating parameter (for example, increase / decrease in the sub FB learning value) not only when the air-fuel ratio of the specific cylinder is shifted to the rich side but also when the air / fuel ratio is shifted to the lean side. When the imbalance determination parameter that increases or decreases in response to the value becomes equal to or greater than the “predetermined threshold Athbase”, it is determined that an air-fuel ratio imbalance among cylinders has occurred (see curve C1 in FIG. 2).

  By the way, as described above, the upstream air-fuel ratio sensor 55 is disposed in the exhaust passage between the exhaust collecting portion 41 b of the exhaust manifold 41 and the catalyst 43. Therefore, in general, the upstream air-fuel ratio sensor 55 has substantially the same sensitivity to the exhaust gas discharged from each cylinder.

  However, depending on the shape of the exhaust manifold 41 and the arrangement position of the upstream air-fuel ratio sensor 55, the way in which exhaust gas strikes the upstream air-fuel ratio sensor 55 may not be uniform among the cylinders. In this case, the upstream side air-fuel ratio sensor 55 determines that the “other cylinders (for example, the second cylinders), for example, the“ exhaust gas (particularly, hydrogen contained in the exhaust gas) from a certain cylinder (for example, the first cylinder # 1) ”. It reacts more sensitively than "exhaust gas from # 2 to fourth cylinder # 4)".

Assume that the upstream air-fuel ratio sensor 55 reacts more sensitively to “exhaust gas from the first cylinder # 1” than to “exhaust gas from each of the other cylinders”. At this time, if the “rich deviation abnormality” occurs in the fuel injection valve of the first cylinder # 1, the output value Vabyfs of the upstream air-fuel ratio sensor 55 is “the upstream air-fuel ratio sensor 55 is equivalent to the exhaust gas from each cylinder” It is a value corresponding to “a richer air-fuel ratio” than “when reacting to”. This is because the upstream air-fuel ratio sensor 55 is sensitive to the large amount of hydrogen H ₂ contained in the exhaust gas of the first cylinder # 1 because it is easily affected by the exhaust gas of the first cylinder # 1. .

  As a result, the above-described main feedback control based on the output value Vabyfs of the upstream side air-fuel ratio sensor 55 enables the “engine” to be compared with “when the upstream side air-fuel ratio sensor 55 reacts equally to the exhaust gas from each cylinder”. The “true air-fuel ratio of the air-fuel mixture supplied to the entire engine 10” tends to be controlled to “more lean side”. As a result, the amount of sub-feedback (and hence the learned value of the amount of sub-feedback) increases “further”. That is, the learning value of the sub feedback amount shifts from the curve C1 to the curve C2 in the region where the imbalance ratio in FIG. 3 is positive. A curve C1 in FIG. 3 shows the learning value of the sub feedback amount when “the upstream air-fuel ratio sensor 55 responds equally to the exhaust gas from each cylinder”, and is the same curve as the curve C1 shown in FIG. It is.

  Further, when the upstream air-fuel ratio sensor 55 reacts more sensitively to “exhaust gas from the first cylinder # 1” than to “exhaust gas from each of the other cylinders”, the fuel injection of the first cylinder # 1 When the “lean deviation abnormality” occurs in the valve 25, the output value Vabyfs of the upstream air-fuel ratio sensor 55 is “more theoretical than the case where the upstream air-fuel ratio sensor 55 reacts equally to the exhaust gas from each cylinder”. The value corresponds to the rich air-fuel ratio close to the air-fuel ratio (the air-fuel ratio with a small degree of richness). This is because, as a result of the “main feedback control”, the upstream side air-fuel ratio sensor 55 “other cylinders (second cylinder # 2 to fourth cylinder # 4) controlled to an air-fuel ratio richer than the theoretical air-fuel ratio”. This is because it is insensitive to hydrogen contained in “exhaust gas from”.

  As a result, the learning value of the sub feedback amount is not as large as “when the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder”. That is, the learning value of the sub feedback amount shifts from the curve C1 to the curve C2 in the region where the imbalance ratio in FIG. 3 is negative.

  Next, the upstream air-fuel ratio sensor 55 detects “from the first cylinder # 1 to the exhaust gas in any cylinder other than the first cylinder # 1 (that is, any one of the second cylinder # 2 to the fourth cylinder # 4)”. Suppose that the reaction is more sensitive than “exhaust gas”. Here, for convenience of explanation, the upstream side air-fuel ratio sensor 55 has exhaust gas from each of “other cylinders (first cylinder # 1 to third cylinder # 3)” with respect to “exhaust gas from fourth cylinder # 4”. Assume that it reacts more sensitively.

At this time, if the “rich deviation abnormality” occurs in the fuel injection valve 25 of the first cylinder # 1, the output value Vabyfs of the upstream air-fuel ratio sensor 55 becomes “the upstream air-fuel ratio sensor 55 is in response to the exhaust gas from each cylinder. Therefore, the value corresponds to “the air-fuel ratio on the rich side closer to the theoretical air-fuel ratio (the air-fuel ratio with a small degree of richness)”. This is because the upstream air-fuel ratio sensor 55 is insensitive to a large amount of hydrogen H ₂ contained in the exhaust gas of the first cylinder # 1. Therefore, the degree of overcorrection of the air-fuel ratio to the lean side based on the upper main feedback control is smaller than “when the upstream air-fuel ratio sensor 55 responds equally to the exhaust gas from each cylinder”.

  As a result, the learning value of the sub feedback amount is not as large as “when the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder”. That is, the learning value of the sub feedback amount shifts from the curve C1 to the curve C3 in the region where the imbalance ratio in FIG. 3 is positive.

  Further, when the upstream air-fuel ratio sensor 55 reacts more sensitively to “exhaust gas from the fourth cylinder # 4” than to “exhaust gas from each of the other cylinders”, the fuel injection of the first cylinder # 1 When the “lean deviation abnormality” occurs in the valve, the output value Vabyfs of the upstream air-fuel ratio sensor 55 is “richer than the case where the upstream air-fuel ratio sensor 55 reacts equally to the exhaust gas from each cylinder”. The air-fuel ratio (the air-fuel ratio with a high degree of richness) ”. This is because the upstream air-fuel ratio sensor 55 is included in the "exhaust gas from the fourth cylinder # 4" that is controlled to a richer air-fuel ratio than the theoretical air-fuel ratio as a result of "main feedback control". It is because it is sensitive to.

  As a result, the learning value of the sub feedback amount becomes larger than “when the upstream air-fuel ratio sensor reacts equally to the exhaust gas from each cylinder”. That is, the learning value of the sub feedback amount shifts from the curve C1 to the curve C3 in the region where the imbalance ratio in FIG. 3 is negative.

  As described above, when “the upstream air-fuel ratio sensor 55 responds equally to the exhaust gas from each cylinder”, the hydrogen amount difference indicating parameter that changes according to the learning value of the sub feedback amount is determined from the determination threshold Athbase. Can be determined that an air-fuel ratio imbalance among cylinders has occurred. However, when the upstream air-fuel ratio sensor 55 responds more sensitively to “exhaust gas from one specific cylinder” than to “exhaust gas from each of the other cylinders”, the degree of imbalance between the air-fuel ratios is extremely high. However, there are cases where the hydrogen amount difference indicating parameter does not reach the determination threshold Athbase (see the curve C3 in the region where the imbalance ratio is positive and the curve C2 in the region where the imbalance ratio is negative in FIG. 3). ). That is, there is a case where the determination of the air-fuel ratio imbalance among cylinders cannot be performed with high accuracy only by simple comparison between the hydrogen amount difference indicating parameter and the determination threshold value Athbase.

  Therefore, the determination device uses the rotation fluctuation of the engine 10 for determining the air-fuel ratio imbalance among cylinders. This rotational fluctuation is represented by a rotational speed fluctuation parameter. An example of the rotational speed variation parameter is an average value of the variation amount at time T180. The time T180 is “a time required for an arbitrary cylinder to reach from the compression top dead center to the expansion bottom dead center. When the fuel injection valve 25 of the specific cylinder has a rich shift abnormality or a lean shift abnormality, Since the difference between the air-fuel ratio and the air-fuel ratio of the other cylinders increases, the torque generated by the combustion in the specific cylinder decreases, so that the time T180 of the specific cylinder greatly differs from the time T180 of the other cylinders. For this reason, when the air-fuel ratio imbalance among cylinders occurs, the rotational speed fluctuation parameter (the fluctuation range of the time T180) increases as shown in FIG.

  Therefore, when the “hydrogen amount difference indicating parameter acquired based on the learned value of the sub feedback amount” is larger than the “predetermined first imbalance determination threshold value Ath”, the determination device determines that an air-fuel ratio imbalance among cylinders has occurred. And the correction is performed so that the “first imbalance determination threshold Ath” becomes smaller as the acquired rotational speed variation parameter becomes larger.

  As described above, as shown in FIG. 3 and FIG. 5, when the hydrogen amount difference indicating parameter changes like the curves C1 and C2 in the region where the imbalance ratio is positive, and in the region where the imbalance ratio is negative. When the hydrogen amount difference indicating parameter changes as shown by the curves C1 and C3, the determination device compares the hydrogen amount difference indicating parameter with the first imbalance determination threshold value (the determination threshold value Athbase in FIG. 3), and determines the hydrogen amount difference. When the instruction parameter becomes larger than the first imbalance determination threshold, it is determined that an air-fuel ratio imbalance among cylinders has occurred.

  Further, when the hydrogen amount difference indicating parameter changes as shown by a curve C3 in a region where the imbalance ratio is positive, and when the hydrogen amount difference indicating parameter changes as shown by a curve C2 in a region where the imbalance rate is negative, However, if the imbalance ratio increases, the hydrogen amount difference indicating parameter exceeds the “first imbalance determination threshold value Ath corrected by the rotational speed fluctuation parameter” (the imbalance ratio in FIG. (Refer to curve C3 in the area of (2) and curve C2 in the area where the imbalance ratio is negative.). Therefore, “the case where the upstream air-fuel ratio sensor 55 does not react equally to the exhaust gas from each cylinder, that is, the case where the upstream air-fuel ratio sensor 55 is strongly influenced by the exhaust gas from the specific cylinder. Even if the determination device compares the “hydrogen amount difference indicating parameter” with the “first imbalance determination threshold value Ath corrected by the rotational speed fluctuation parameter”, the air-fuel ratio imbalance among cylinders has occurred. Can be reliably detected.

(Actual operation)
Next, the actual operation of this determination apparatus will be described.
<Fuel injection amount control>
The CPU performs the routine for calculating the fuel injection amount Fi and instructing the fuel injection shown in FIG. 14 every time the crank angle of a predetermined cylinder becomes a predetermined crank angle before the intake top dead center (for example, BTDC 90 ° CA). In addition, the process is repeatedly performed on the cylinder (hereinafter also referred to as “fuel injection cylinder”). Accordingly, when the predetermined timing is reached, the CPU starts the process from step 1400, sequentially performs the processes of steps 1410 to 1440 described below, proceeds to step 1495, and once ends this routine.

  Step 1410: The CPU determines “in-cylinder intake air” which is “the amount of air sucked into the fuel injection cylinder” based on “the intake air amount Ga measured by the air flow meter 51, the engine rotational speed NE, and the lookup table MapMc”. The quantity Mc (k) ”is acquired. The in-cylinder intake air amount Mc (k) is stored in the RAM while corresponding to each intake stroke. The in-cylinder intake air amount Mc (k) may be calculated by a well-known air model (a model constructed according to a physical law simulating the behavior of air in the intake passage).

  Step 1420: The CPU obtains the basic fuel injection amount Fbase by dividing the in-cylinder intake air amount Mc (k) by the upstream target air-fuel ratio abyfr. The upstream target air-fuel ratio abyfr is set to the stoichiometric air-fuel ratio stoich except in special cases as described later.

  Step 1430: The CPU calculates the final fuel injection amount Fi by correcting the basic fuel injection amount Fbase with the main feedback amount DFi (more specifically, adding the main feedback amount DFi to the basic fuel injection amount Fbase). . The main feedback amount DFi will be described later.

  Step 1440: The CPU instructs the fuel injection valve 25 to inject the fuel of the final fuel injection amount (instructed injection amount) Fi from the “fuel injection valve 25 provided corresponding to the fuel injection cylinder”. Send a signal.

  Thus, the amount of fuel injected from each fuel injection valve 25 is uniformly increased or decreased by the main feedback amount DFi common to all the cylinders.

<Calculation of main feedback amount>
The CPU repeatedly executes the main feedback amount calculation routine shown in the flowchart of FIG. 15 every elapse of a predetermined time. Accordingly, when the predetermined timing comes, the CPU starts processing from step 1500 and proceeds to step 1505 to determine whether or not the main feedback control condition (upstream air-fuel ratio feedback control condition) is satisfied.

The main feedback control condition is satisfied when all of the following conditions are satisfied.
(Condition A1) The upstream air-fuel ratio sensor 55 is activated.
(Condition A2) The engine load (load factor) KL is equal to or less than the threshold KLth.
(Condition A3) Fuel cut is not in progress.

Here, the load factor KL is obtained by the following equation (1). Instead of the load factor KL, an accelerator pedal operation amount Accp, a throttle valve opening degree TA, or the like may be used as the engine load. In the equation (1), Mc is the in-cylinder intake air amount, ρ is the air density (unit is (g / l)), L is the exhaust amount of the engine 10 (unit is (l)), and “4” is the engine. The number of cylinders is 10.
KL = (Mc / (ρ · L / 4)) · 100% (1)

  If the description continues assuming that the main feedback control condition is satisfied, the CPU makes a “Yes” determination at step 1505 to sequentially perform the processing of steps 1510 to 1540 described below, and then proceeds to step 1595. This routine is temporarily terminated.

Step 1510: The CPU acquires the feedback control output value Vabyfc according to the following equation (2). In equation (2), Vabyfs is the output value of the upstream air-fuel ratio sensor 55, Vafsfb is the sub-feedback amount calculated based on the output value Voxs of the downstream air-fuel ratio sensor 56, and Vafsfbg is the learning value of the sub-feedback amount (sub FB learning value). These values are all values obtained at the present time. A method of calculating the sub feedback amount Vafsfb and the sub FB learning value Vafsfbg will be described later.
Vabyfc = Vabyfs + (Vafsfb + Vafsfbg) (2)

Step 1515: The CPU obtains the feedback control air-fuel ratio abyfsc by applying the feedback control output value Vabyfc to the air-fuel ratio conversion table Mapabyfs shown in FIG. 11, as shown in the following equation (3).
abyfsc = Mapabyfs (Vabyfc) (3)

Step 1520: The CPU “in-cylinder fuel supply amount Fc (k−N)” which is “the amount of fuel actually supplied to the combustion chamber 21 at a time point N cycles before the current time” according to the following equation (4): " That is, the CPU divides “the in-cylinder intake air amount Mc (k−N) at a point N cycles before the current point (ie, N · 720 ° crank angle)” by “the feedback control air-fuel ratio abyfsc”. Thus, the in-cylinder fuel supply amount Fc (k−N) is obtained.
Fc (k−N) = Mc (k−N) / abyfsc (4)

  Thus, in order to obtain the in-cylinder fuel supply amount Fc (k−N), the in-cylinder intake air amount Mc (k−N) N strokes before the current stroke is divided by the feedback control air-fuel ratio abyfsc. This is because “a time corresponding to the N stroke” is required until “the exhaust gas generated by the combustion of the air-fuel mixture in the combustion chamber 21” reaches the upstream air-fuel ratio sensor 55. In practice, however, the upstream air-fuel ratio sensor 55 arrives after the exhaust gas discharged from each cylinder is mixed to some extent.

Step 1525: The CPU, according to the following equation (5), “target in-cylinder fuel supply amount Fcr (k) which is“ the amount of fuel that should have been supplied to the combustion chamber 21 at the time N cycles before the current time ”. -N) ". That is, the CPU obtains the target in-cylinder fuel supply amount Fcr (k−N) by dividing the in-cylinder intake air amount Mc (k−N) N strokes before the current time by the upstream target air-fuel ratio abyfr.
Fcr = Mc (k−N) / abyfr (5)

The upstream target air-fuel ratio abyfr is set to the stoichiometric air-fuel ratio stoich during normal operation. On the other hand, the upstream target air-fuel ratio abyfr is set to a leaner air-fuel ratio than the stoichiometric air-fuel ratio when a predetermined lean setting condition is established for the purpose of preventing the generation of exhaust odor due to sulfur or the like. . When any one of the following conditions is satisfied, the upstream target air-fuel ratio abyfr is set to an air-fuel ratio richer than the stoichiometric air-fuel ratio.
-When the elapsed time since the start of the engine 10 is less than the elapsed time after the threshold start.
The cooling water temperature THW is equal to or lower than the threshold cooling water temperature THWth.
-The current time is within a predetermined period after the end of fuel cut (fuel supply stop) control.
-When it is the driving | running state (high load driving | running state) which should prevent the overheating of the upstream catalyst 43

Step 1530: The CPU acquires the in-cylinder fuel supply amount deviation DFc according to the following equation (6). That is, the CPU obtains the in-cylinder fuel supply amount deviation DFc by subtracting the in-cylinder fuel supply amount Fc (k−N) from the target in-cylinder fuel supply amount Fcr (k−N). This in-cylinder fuel supply amount deviation DFc is an amount representing the excess or deficiency of the fuel supplied into the cylinder at the time point before the N stroke.
DFc = Fcr (k−N) −Fc (k−N) (6)

Step 1535: The CPU obtains the main feedback amount DFi according to the following equation (7). In this equation (7), Gp is a preset proportional gain, and Gi is a preset integral gain. Further, the “value SDFc” in the equation (7) is “an integral value of the in-cylinder fuel supply amount deviation DFc”. That is, the CPU calculates the “main feedback amount DFi” by proportional-integral control for making the feedback control air-fuel ratio abyfsc coincide with the upstream target air-fuel ratio abyfr.
DFi = Gp · DFc + Gi · SDFc (7)

  Step 1540: The CPU adds the in-cylinder fuel supply amount deviation DFc obtained in the above step 1530 to the integral value SDFc of the in-cylinder fuel supply amount deviation DFc at that time, so that a new in-cylinder fuel supply amount deviation DFc is obtained. An integral value SDFc is obtained.

  Thus, the main feedback amount DFi is obtained by proportional integral control, and this main feedback amount DFi is reflected in the final fuel injection amount Fi by the processing of step 1430 of FIG. 14 described above.

  By the way, the “sum of the sub feedback amount Vafsfb and the sub FB learning value Vafsfbg” on the right side of the equation (2) is smaller than the output value Vabyfs of the upstream air-fuel ratio sensor 55 and is smaller. It is limited to be. Therefore, “the sum of the sub-feedback amount Vafsfb and the sub-FB learning value Vafsfbg” is set so that “the output value Voxs of the downstream air-fuel ratio sensor 56” is “a downstream target that is a value corresponding to the theoretical air-fuel ratio”. It can be considered as an “auxiliary correction amount” for matching the value “Voxsref”. As a result, it can be said that the feedback control air-fuel ratio abyfsc is a value substantially based on the output value Vabyfs of the upstream air-fuel ratio sensor 55. That is, the main feedback amount DFi is a correction amount for making “the air-fuel ratio of the engine represented by the output value Vabyfs of the upstream air-fuel ratio sensor 55” coincide with “the upstream target air-fuel ratio abyfr (theoretical air-fuel ratio)”. Can be said.

  On the other hand, if the main feedback control condition is not satisfied at the time of determination in step 1505, the CPU determines “No” in step 1505 and proceeds to step 1545 to set the value of the main feedback amount DFi to “0”. To do. Next, in step 1550, the CPU stores “0” in the integral value SDFc of the in-cylinder fuel supply amount deviation. Thereafter, the CPU proceeds to step 1595 to end the present routine tentatively. Thus, when the main feedback control condition is not satisfied, the main feedback amount DFi is set to “0”. Accordingly, the basic fuel injection amount Fbase is not corrected by the main feedback amount DFi.

<Calculation of sub feedback amount and sub FB learning value>
The CPU executes the routine shown in FIG. 16 every elapse of a predetermined time in order to calculate the “sub feedback amount Vafsfb” and the “learning value (sub FB learning value) Vafsfbg of the sub feedback amount Vafsfb”. Therefore, when the predetermined timing comes, the CPU starts processing from step 1600 and proceeds to step 1605 to determine whether or not the sub feedback control condition is satisfied.

The sub-feedback control condition is satisfied when all of the following conditions are satisfied.
(Condition B1) The main feedback control condition is satisfied.
(Condition B2) The downstream air-fuel ratio sensor 56 is activated.
(Condition B3) The upstream target air-fuel ratio abyfr is set to the stoichiometric air-fuel ratio stoich.

  The description will be continued assuming that the sub-feedback control condition is satisfied. In this case, the CPU makes a “Yes” determination at step 1605 to sequentially perform the processing from step 1610 to step 1630 described below to calculate the sub feedback amount Vafsfb.

Step 1610: The CPU obtains an “output deviation amount DVoxs” which is a difference between the “downstream target value Voxsref” and the “output value Voxs of the downstream air-fuel ratio sensor 56” according to the following equation (8). That is, the CPU obtains “output deviation amount DVoxs” by subtracting “current output value Voxs of downstream air-fuel ratio sensor 56” from “downstream target value Voxsref”. The downstream target value Voxsref is set to a value Vst (0.5 V) corresponding to the theoretical air-fuel ratio.
DVoxs = Voxsref−Voxs (8)

Step 1615: The CPU obtains a sub feedback amount Vafsfb according to the following equation (9). In this equation (9), Kp is a preset proportional gain (proportional constant), Ki is a preset integral gain (integral constant), and Kd is a preset differential gain (differential constant). SDVoxs is an integral value (time integral value SDVoxs) of the output deviation amount DVoxs, and DDVoxs is a differential value of the output deviation amount DVoxs.
Vafsfb = Kp · DVoxs + Ki · SDVoxs + Kd · DDVoxs (9)

  Step 1620: The CPU obtains a new output deviation amount integrated value SDVoxs by adding “the output deviation amount DVoxs obtained in step 1610” to “the integrated value SDVoxs of the output deviation amount at that time”.

  Step 1625: The CPU obtains a new value by subtracting “the previous output deviation amount DVoxsold, which is the output deviation amount calculated when this routine was executed last time” from “the output deviation amount DVoxs calculated in Step 1610”. Find the differential value DDVoxs of the output deviation amount.

  Step 1630: The CPU stores “the output deviation amount DVoxs calculated in step 1610” as “the previous output deviation amount DVoxsold”.

  Thus, the CPU calculates the “sub feedback amount Vafsfb” by proportional / integral / differential (PID) control for making the output value Voxs of the downstream air-fuel ratio sensor 56 coincide with the downstream target value Voxsref. This sub-feedback amount Vafsfb is used to calculate the feedback control output value Vabyfc, as shown in the above-described equation (2).

  Next, the CPU calculates the “sub FB learning value Vafsfbg” by sequentially performing the processes of steps 1635 to 1655 described below, and then proceeds to step 1695 to end the present routine tentatively.

  Step 1635: The CPU stores the current sub FB learning value Vafsfbg as the pre-update learning value Vafsfbg0.

Step 1640: The CPU updates the sub FB learning value Vafsfbg according to the following equation (10). The left side Vafsfbg (k + 1) of the equation (10) represents the updated sub FB learning value Vafsfbg. The value α is an arbitrary value from 0 to less than 1.
Vafsfbg (k + 1) = α · Vafsfbg + (1−α) · Ki · SDVoxs (10)

  As is clear from the equation (10), the sub FB learning value Vafsfbg is a value obtained by performing “filter processing for noise removal” on the “integral term Ki · SDVoxs of the sub feedback amount Vafsfb”. In other words, the sub FB learning value Vafsfbg is a value corresponding to the stationary component (integral term) of the sub feedback amount Vafsfb, and is the primary delay amount (smooth value) of the integral term Ki · SDVoxs. That is, it is a value corresponding to the stationary component (integral term Ki · SDVoxs) of the sub feedback amount Vafsfb. Thus, the sub FB learning value Vafsfbg is updated so as to approach the steady component of the sub feedback amount Vafsfb. The updated sub FB learning value Vafsfbg (= Vafsfbg (k + 1)) is stored in the backup RAM.

Step 1645: The CPU calculates a change amount (update amount) ΔG of the sub FB learning value Vafsfbg according to the following equation (11).
ΔG = Vafsfbg−Vafsfbg0 (11)

Step 1650: The CPU corrects the sub feedback amount Vafsfb with the change amount ΔG according to the following equation (12).
Vafsfb = Vafsfb−ΔG (12)

  The processing of step 1645 and step 1650 will be described. As shown in the above equation (2), the CPU adds the “sub feedback amount Vafsfb and the sub FB learning value Vafsfbg” to the “output value Vabyfs of the upstream air-fuel ratio sensor 55”, thereby providing the feedback control output value Vabyfc. Get. The sub FB learning value Vafsfbg is a value obtained by incorporating a part of the integral term Ki · SDVoxs (stationary component) of the sub feedback amount Vafsfb. Therefore, when the sub FB learning value Vafsfbg is updated, if the sub feedback amount Vafsfb is not corrected according to the updated amount, double correction is performed by the “updated sub FB learning value Vafsfbg and the sub feedback amount Vafsfb”. . Therefore, when the sub FB learning value Vafsfbg is updated, the sub feedback amount Vafsfb is corrected according to the update ΔG of the sub FB learning value Vafsfbg.

  Therefore, when the CPU updates the sub FB learning value Vafsfbg so as to increase by the change amount ΔG, as shown in the equations (11) and (12), the CPU decreases the sub feedback amount Vafsfb by the change amount ΔG. In the equation (11), Vafsfbg0 is the sub FB learning value Vafsfbg immediately before the update. Accordingly, the change amount ΔG is a positive value or a negative value.

Step 1655: The CPU corrects the integral value of the output deviation amount DVoxs based on the change amount ΔG according to the following equation (13). Note that step 1655 may be omitted. Steps 1645 to 1655 may be omitted.
SDVoxsnew = SDVoxs−ΔG / Ki (13)

  Through the above processing, the sub feedback amount Vafsfb and the sub FB learning value Vafsfbg are updated every time a predetermined time elapses.

  On the other hand, if the sub-feedback control condition is not satisfied, the CPU makes a “No” determination at step 1605 in FIG. 16, performs the processing of step 1660 and step 1665 described below in order, and proceeds to step 1695 to execute this routine. Is temporarily terminated.

Step 1660: The CPU sets the value of the sub feedback amount Vafsfb to “0”.
Step 1665: The CPU sets the integral value SDVoxs of the output deviation amount to “0”.

  Thereby, as is apparent from the above equation (2), the feedback control output value Vabyfc is the sum of the output value Vabyfs of the upstream air-fuel ratio sensor 55 and the sub FB learning value Vafsfbg. That is, in this case, “update of the sub feedback amount Vafsfb” and “reflection of the sub feedback amount Vafsfb to the final fuel injection amount Fi” are stopped. However, at least the sub FB learning value Vafsfbg corresponding to the integral term of the sub feedback amount Vafsfb is reflected in the final fuel injection amount Fi.

<Air-fuel ratio imbalance determination between cylinders>
Next, a process for executing the “air-fuel ratio imbalance determination” will be described with reference to FIGS.

  The CPU executes the “time T180 acquisition routine” shown in the flowchart of FIG. 17 every time the crank angle rotates by 30 degrees from the compression top dead center of any cylinder. By this routine, time T180 serving as basic data for obtaining the rotation speed variation parameter is calculated. Time T180 is the time required for an arbitrary cylinder to reach from the compression top dead center to the expansion bottom dead center.

  Therefore, when the predetermined timing is reached, the CPU starts processing from step 1700 and proceeds to step 1710 to determine whether or not the current time point is the compression top dead center of any cylinder. At this time, if the current time is not the compression top dead center of any cylinder, the CPU makes a “No” determination at step 1710 to directly proceed to step 1795 to end the present routine tentatively.

  On the other hand, when the CPU proceeds to step 1710, if the time is the compression top dead center of any cylinder, the CPU makes a “Yes” determination at step 1710 to proceed to step 1720. “Current time T180new”, which is the latest time T180 stored when the routine was last executed, is stored as “previous time T180old, which is the previous time T180”.

  Next, the CPU proceeds to step 1730 and subtracts “the time (previous time) FRold when this routine was executed last time” from the current time FRnow, so that it is necessary from the previous time FRold to the current time FRnow. Time T180 is obtained and stored as current time T180new.

  Next, the CPU proceeds to step 1740 to store the current time FRnow as “previous time FRold” for the next processing of this routine. Thereafter, the CPU proceeds to step 1795 to end the present routine tentatively.

  Through the above processing, the time T180 is obtained as “current time T180new every time any cylinder reaches compression top dead center”. In addition, the time T180 required for the crankshaft to rotate from the time when the crank angle is 360 degrees before the compression top dead center to the time when the crank angle is 180 degrees before the compression top dead center is stored as the previous time T180old. Is done. Note that step 1710 may be replaced with a step of determining whether any cylinder is at an expansion bottom dead center.

  Further, the CPU executes a “rotational speed fluctuation parameter acquisition routine” shown by a flowchart in FIG. 18 every time a predetermined time elapses.

  Accordingly, when the predetermined timing comes, the CPU starts the process from step 1800 in FIG. 18 and proceeds to step 1805 to determine whether or not the current time is immediately after the engine 10 is started. At this time, if the current time is immediately after the start of the engine 10, the CPU makes a “Yes” determination at step 1805, executes the processing of steps 1810 to 1820 described below, and then proceeds to step 1825. On the other hand, if the current time is not immediately after the engine 10 is started, the CPU makes a “No” determination at step 1805 to directly proceed to step 1825.

Step 1810: The CPU sets the value of the rotation speed variation parameter acquisition number counter CKH (hereinafter also simply referred to as “counter CKH”) to “0”.
Step 1815: The CPU sets the value of the rotation speed fluctuation parameter acquisition completion flag XKH to “0”.
Step 1820: The CPU sets the rotation speed fluctuation parameter integrated value SΔT180 to “0”.

  Next, the CPU proceeds to step 1825 to determine whether or not a condition for acquiring a rotation speed fluctuation parameter (hereinafter also referred to as “rotation fluctuation acquisition condition”) is satisfied. This rotation fluctuation acquisition condition is satisfied when, for example, all of the following (condition C1) to (condition C4) are satisfied.

(Condition C1) The cooling water temperature THW is equal to or higher than the threshold water temperature THWth. The threshold water temperature THWth is set to a water temperature when the engine 10 is completely warmed up (for example, 75 ° C.).
(Condition C2) The throttle valve opening degree TA is “0”. That is, the throttle valve 34 is fully closed.
(Condition C3) The engine rotational speed NE is substantially in the rotational speed region that can be regarded as the idling rotational speed. Specifically, the engine rotation speed NE is equal to or less than a value (for example, 1200 rotations) obtained by adding a predetermined value (for example, 500 rotations) to a normal idle rotation speed (for example, 700 rotations).
(Condition C4) The air conditioning device mounted on the vehicle (not shown) is off.

  That is, the rotation fluctuation acquisition condition is set to a condition that is satisfied when the operation state of the engine 10 is a “stable idle operation state”. Therefore, for example, other conditions such as a condition that a predetermined electric load device (not shown) is off and a condition that the battery voltage is equal to or higher than a predetermined voltage may be added.

  At this time, if the rotation fluctuation acquisition condition is not satisfied, the CPU makes a “No” determination at step 1825 to directly proceed to step 1895 to end the present routine tentatively.

  On the other hand, if the rotation fluctuation acquisition condition is satisfied, the CPU makes a “Yes” determination at step 1825 to proceed to step 1830, where the current time is 30 ° crank angle (after the compression top dead center of any cylinder) It is determined whether or not the time is immediately after (ATDC 30 degrees).

  At this time, if the current time is not 30 ° ATDC of any cylinder, the CPU makes a “No” determination at step 1830 to directly proceed to step 1895 to end the present routine tentatively.

  On the other hand, if the current time is ATDC 30 degrees of any cylinder, the CPU proceeds to step 1835 to increase the value of the counter CKH by “1”. Next, the CPU proceeds to step 1840 to add “the absolute value of the difference between the current time T180new and the previous time T180old | T180new−T180old |) to the rotational speed fluctuation parameter integrated value SΔT180 at that time to obtain a new rotational speed fluctuation parameter. An integrated value SΔT180 is obtained.

  Next, the CPU proceeds to step 1845 to determine whether or not the counter CKH is equal to or greater than a predetermined threshold CKHth. At this time, if the counter CKH is smaller than the predetermined threshold CKHth, the CPU makes a “No” determination at step 1845 to directly proceed to step 1895 to end the present routine tentatively.

  On the other hand, if the counter CKH is equal to or greater than the predetermined threshold CKHth, the CPU makes a “Yes” determination at step 1845 to proceed to step 1850, where the rotational speed fluctuation parameter integrated value SΔT180 is divided by the threshold CKHth. The rotational speed fluctuation parameter ΔT180AVE is acquired. That is, the rotational speed fluctuation parameter ΔT180AVE is an average value for the number CKHth of the absolute value | T180new−T180old | of the difference representing the fluctuation amount of the engine rotational speed.

  If the air-fuel ratio imbalance among cylinders does not occur and the air-fuel ratios of the cylinders are equal to each other, the rotational speeds of the cylinders are substantially equal, and the absolute value | T180new−T180old | of the difference is substantially zero. Therefore, if the air-fuel ratio imbalance among cylinders does not occur, the rotational speed fluctuation parameter ΔT180AVE becomes an extremely small value. On the other hand, when the air-fuel ratio imbalance among cylinders occurs and only the air-fuel ratio of a specific cylinder changes to the rich side or the lean side compared to the air-fuel ratio of other cylinders, the time of the specific cylinder T180 becomes larger than time T180 of the other cylinders. Therefore, when the air-fuel ratio imbalance among cylinders occurs, the rotational speed fluctuation parameter ΔT180AVE increases.

  Thereafter, the CPU proceeds to step 1855 to set the value of the rotation speed fluctuation parameter acquisition completion flag XKH to “1”. That is, the rotational speed fluctuation parameter acquisition completion flag XKH indicates that the rotational speed fluctuation parameter ΔT180AVE is not yet acquired after the current start of the engine 10 when the value is “0”, and the value is “1”. It is shown that the acquisition of the rotational speed fluctuation parameter ΔT180AVE is completed after the start of the engine 10 at this time. Then, the CPU proceeds to step 1895 to end the present routine tentatively.

  Further, the CPU repeatedly executes the “air-fuel ratio imbalance among cylinders determination routine” shown in FIG. 19 every elapse of a predetermined time. Accordingly, when the predetermined timing comes, the CPU starts the process from step 1900 and proceeds to step 1905 to determine whether or not the value of the hydrogen amount difference instruction parameter acquisition completion flag XAVE is “0”.

  The value of the hydrogen amount difference instruction parameter acquisition completion flag XAVE is set to “0” by an initial routine (not shown) that is executed when the ignition key switch is switched from the off position to the on position. Yes. When the value of the hydrogen amount difference indicating parameter acquisition completion flag XAVE is “0”, it indicates that the hydrogen amount difference indicating parameter Avesfbg has not yet been obtained after the current start of the engine 10, and the value is “1”. ”Indicates that the hydrogen amount difference indicating parameter Avesfbg is obtained after the engine 10 is started this time.

  Assume that the value of the hydrogen amount difference instruction parameter acquisition completion flag XAVE is “0”. According to this assumption, the CPU makes a “Yes” determination at step 1905 to proceed to step 1910 to determine whether or not the “precondition of abnormality determination (air-fuel ratio imbalance determination)” is satisfied. . The precondition for this abnormality determination is also the “hydrogen amount difference instruction parameter acquisition permission condition”. Further, when the precondition for the abnormality determination is not satisfied, the “determination prohibition condition” for the air-fuel ratio imbalance among cylinders is satisfied. When the “determination prohibition condition” for the air-fuel ratio imbalance among cylinders is satisfied, the “air-fuel ratio described below” using the “hydrogen amount difference indicating parameter (imbalance determination parameter) calculated based on the sub-FB learning value Vafsfbg” The determination of “imbalance between cylinders” is not executed.

  Preconditions for this abnormality determination can be, for example, the following (condition D1) to (condition D4). This precondition is satisfied when all of (Condition D1) to (Condition D4) are satisfied. In other words, the determination prohibition condition is satisfied when any one of (condition D3) to (condition D4) is not satisfied. Further, any one or more combinations of these conditions may be used as a precondition, and other conditions may be added.

  (Condition D1) The ability of the upstream catalyst 53 to oxidize hydrogen is not less than the first predetermined ability. That is, when the capacity of the upstream catalyst 53 to oxidize hydrogen is greater than the first predetermined capacity. In other words, this condition is “the state of the upstream catalyst 53 is in a state in which hydrogen flowing into the upstream catalyst 53 can be purified by a predetermined amount or more (that is, a hydrogen purifying state)”.

The reason for providing this (condition D1) is as follows.
If the ability of the upstream catalyst 53 to oxidize hydrogen is less than or equal to the first predetermined ability, hydrogen is not sufficiently purified in the upstream catalyst 53 and hydrogen may flow downstream of the upstream catalyst 53. As a result, the output value Voxs of the downstream air-fuel ratio sensor 56 may be affected by the selective diffusion of hydrogen, or the air-fuel ratio of the gas downstream of the upstream catalyst 53 is “supplied to the entire engine 10. It does not agree with the “true average value of the air-fuel ratio of the mixture”. Therefore, the output value Voxs of the downstream air-fuel ratio sensor 56 corresponds to “the true average value of the air-fuel ratio that has been excessively corrected by the air-fuel ratio feedback control using the output value Vabyfs of the upstream air-fuel ratio sensor 55”. It is likely that no value is shown. Therefore, if air-fuel ratio imbalance determination between cylinders is executed in such a state (or if a hydrogen amount difference instruction parameter is acquired), there is a high possibility of erroneous determination.

  The above (Condition D1) can be a condition that is established when, for example, the oxygen storage amount of the upstream catalyst 53 is equal to or greater than the first threshold oxygen storage amount. In this case, it can be determined that the ability of the upstream catalyst 53 to oxidize hydrogen is greater than the first predetermined ability.

  The CPU separately acquires the oxygen storage amount of the upstream catalyst 53 by a well-known method. For example, the oxygen storage amount OSA of the upstream catalyst 53 sequentially adds an amount corresponding to the amount of excess oxygen flowing into the upstream catalyst 53 and also adds to the amount of excess unburned components flowing into the upstream catalyst 53. It is obtained by sequentially subtracting the corresponding amount. That is, the oxygen excess / deficiency ΔO2 (ΔO2 = k · mfr · (abyfs−stoich)) is obtained at every elapse of a predetermined time based on the difference between the upstream air-fuel ratio abyfs and the stoichiometric air-fuel ratio stoich. The oxygen storage amount OSA is obtained by integrating the excess / deficiency amount ΔO2 of 0.23, mfr is the amount of fuel supplied during the predetermined time), and the excess / deficiency amount ΔO2 (for example, JP 2007-239700 A). JP, 2003-336535, A, JP, 2004-036475, etc.). Note that the oxygen storage amount OSA obtained in this way is regulated to a value between the maximum oxygen storage amount Cmax of the upstream catalyst 53 and “0”.

(Condition D2) The ability of the upstream catalyst 53 to oxidize hydrogen is less than the second predetermined ability. The second predetermined ability is an ability larger than the first predetermined ability.

The reason for providing this (condition D2) is as follows.
During a period in which the upstream catalyst 53 has the ability to oxidize hydrogen equal to or greater than the second predetermined capacity, the average value of the air-fuel ratio of the exhaust gas flowing out from the upstream catalyst 53 is “the true value corrected excessively by the air-fuel ratio feedback control”. May not show a value corresponding to the "air-fuel ratio". For example, since the oxygen storage amount of the upstream catalyst 53 is very large immediately after the fuel cut, the air-fuel ratio of the exhaust gas downstream of the upstream catalyst 53 is “the true air-fuel ratio that has been excessively corrected by the air-fuel ratio feedback control”. ”Is not shown. In other words, when the hydrogen oxidation capacity of the upstream catalyst 53 is “between the first predetermined capacity and the second predetermined capacity”, the hydrogen amount difference indicating parameter (imbalance determination parameter) A value that accurately represents the degree of balance.

  The above (Condition D2) can be a condition that is established when the oxygen storage amount of the upstream catalyst 53 is not equal to or greater than the second threshold oxygen storage amount, for example. When the oxygen storage amount of the upstream catalyst 53 is equal to or greater than the second threshold oxygen storage amount, it can be determined that the ability of the upstream catalyst 53 to oxidize hydrogen is equal to or greater than the second predetermined capability. Note that the second threshold oxygen storage amount is larger than the first threshold oxygen storage amount.

  (Condition D3) The flow rate of exhaust gas discharged from the engine 10 is not equal to or higher than the threshold exhaust gas flow rate. That is, the flow rate of the exhaust gas discharged from the engine 10 is less than the threshold exhaust gas flow rate.

The reason for providing this condition (condition D3) is as follows.
If the flow rate of exhaust gas discharged from the engine 10 is equal to or greater than the threshold exhaust gas flow rate, the amount of hydrogen flowing into the upstream catalyst 53 exceeds the hydrogen oxidation capability of the upstream catalyst 53, and hydrogen flows downstream from the upstream catalyst 53. There is a case. Therefore, there is a high possibility that the output value Voxs of the downstream air-fuel ratio sensor 56 is affected by the selective diffusion of hydrogen. Alternatively, the air-fuel ratio of the gas downstream of the catalyst does not match the “true average value of the air-fuel ratio of the air-fuel mixture supplied to the entire engine”. As a result, even when the air-fuel ratio imbalance among cylinders occurs, the output value Voxs of the downstream air-fuel ratio sensor 56 is “by the air-fuel ratio feedback control using the output value Vabyfs of the upstream air-fuel ratio sensor 55. There is a high possibility that a value corresponding to the “overcorrected true air-fuel ratio” is not exhibited. Therefore, if the determination of the air-fuel ratio imbalance among cylinders is performed in such a state (or if the hydrogen amount difference instruction parameter is acquired), there is a high possibility that the determination is erroneous.

  The condition (condition D3) can be a condition that is satisfied when the load of the engine 10 (load factor KL, throttle valve opening TA, accelerator pedal operation amount Accp, etc.) is equal to or less than a threshold load. Alternatively, the above (Condition D3) can be a condition that is satisfied when the intake air amount Ga per unit time of the engine 10 is equal to or less than the threshold intake air amount GAth.

  (Condition D4) The upstream target air-fuel ratio abyfr is the stoichiometric air-fuel ratio stoich.

  Now, it is assumed that the above-described preconditions for abnormality determination (all of (Condition D1) to (Condition D4)) are satisfied. In this case, the CPU makes a “Yes” determination at step 1910, and the CPU executes the processing after step 1915 described below. The processing after step 1915 is part of the processing for abnormality determination (air-fuel ratio imbalance determination between cylinders).

  Step 1915: The CPU determines whether or not the current time is “a time immediately after the sub FB learning value Vafsfbg is updated (a time immediately after the sub FB learning value is updated)”. If the current time is immediately after the sub FB learning value is updated, the CPU proceeds to step 1920 to step 1935. If the current time is not immediately after the sub FB learning value is updated, the CPU proceeds directly to step 1995 to end the present routine tentatively.

Step 1920: The CPU increases the value of the learning value integration counter Cexe by “1”.
Step 1925: The CPU reads the sub FB learning value Vafsfbg calculated at step 1640 of the routine of FIG.
Step 1930: The CPU updates the integrated value SVafsfbg of the sub FB learning value Vafsfbg. That is, the CPU adds the “sub-FB learning value Vafsfbg read in step 1925” to the “current integrated value SVafsfbg” to obtain a new integrated value SVafsfbg.

  This integrated value SVafsfbg is set to “0” by the above-described initial routine. Further, the integrated value SVafsfbg is also set to “0” by the processing in step 1980 described later. This step 1980 is executed when an abnormality determination (air-fuel ratio imbalance among cylinders determination, steps 1965 to 1975) is executed. Therefore, the integrated value SVafsfbg becomes an integrated value of the sub FB learning value Vafsfbg in “when the abnormality determination precondition is satisfied” after the start of the engine 10 or after execution of the immediately preceding abnormality determination.

  Step 1935: The CPU determines whether or not the value of the learning value integration counter Cexe is greater than or equal to the counter threshold value Cth. If the value of the learning value integration counter Cexe is smaller than the counter threshold Cth, the CPU makes a “No” determination at step 1935 to directly proceed to step 1995 to end the present routine tentatively. On the other hand, if the value of the learning value integration counter Cexe is equal to or greater than the counter threshold Cth, the CPU makes a “Yes” determination at step 1935 to proceed to step 1940 and step 1945.

  Step 1940: The CPU obtains the sub FB learning value average value Avesfbg by dividing “the integrated value SVafsfbg of the sub FB learning value Vafsfbg” by the “learning value integration counter Cexe”. As described above, the sub-FB learning value average value Avesfbg is the amount of hydrogen contained in the exhaust gas before passing through the upstream catalyst 43 and the amount of hydrogen contained in the exhaust gas after passing through the upstream catalyst 43. This is a “hydrogen amount difference indicating parameter (imbalance determination parameter)” that increases as the difference increases.

  Step 1945: The CPU sets the value of the hydrogen amount difference instruction parameter acquisition completion flag XAVE to “1”.

  Next, the CPU proceeds to step 1950 to determine whether or not the value of the rotation speed fluctuation parameter acquisition completion flag XKH is “1”. That is, the CPU determines whether or not the rotational speed variation parameter ΔT180AVE has been acquired after the current start of the engine 10. At this time, if the value of the rotation speed fluctuation parameter acquisition completion flag XKH is “0”, that is, if the rotation speed fluctuation parameter ΔT180AVE has not been acquired, the CPU makes a “No” determination at step 1950 to execute step 1995. Go directly to, and end this routine once.

  In this state, the value of the hydrogen amount difference instruction parameter acquisition completion flag XAVE is set to “1”. Therefore, when the CPU restarts the process from step 1900 and proceeds to step 1905, the CPU makes a “No” determination at step 1905 to directly proceed to step 1950. Therefore, the CPU monitors that the value of the rotation speed fluctuation parameter acquisition completion flag XKH is “1”.

  Then, the rotational speed fluctuation parameter ΔT180AVE is acquired in step 1850 of FIG. 18 by the routine of FIG. 17 and FIG. 18, so that the value of the rotational speed fluctuation parameter acquisition completion flag XKH is set to “1” in step 1855. Then, when the CPU proceeds to step 1950 in FIG. 19, the CPU makes a “Yes” determination at step 1950 and executes the processing of predetermined steps from step 1955 to step 1980 described below.

  Step 1955: The CPU obtains a threshold correction coefficient Khosei by applying the rotation speed fluctuation parameter ΔT180AVE to the table MapKhosei.

According to this table MapKhosei, as shown in the right part of the drawing in step 1955 of FIG.
(A) The threshold value correction coefficient Khosei is “1” when the absolute value of the rotational speed fluctuation parameter ΔT180AVE is between 0 and the first value f1,
(B) Between the first value f1 of the rotational speed fluctuation parameter ΔT180AVE and the second value f2 greater than the first value f1, the threshold correction coefficient Khosei becomes a first negative value as the rotational speed fluctuation parameter ΔT180AVE increases. Gradually decreasing to have a slope,
(C) When the absolute value of the rotational speed fluctuation parameter ΔT180AVE is equal to or greater than the second value f2, the threshold correction coefficient Khosei gradually decreases to have a second negative slope as the rotational speed fluctuation parameter ΔT180AVE increases. Note that the absolute value of the second negative slope is smaller than the absolute value of the first negative slope.

  That is, the threshold correction coefficient Khosei is maintained at “1” when the rotational speed variation parameter ΔT180AVE is within a predetermined range including 0 (that is, 0 to the first value f1). Further, the threshold correction coefficient Khosei is determined to be smaller in a range of “1” or less as the rotational speed fluctuation parameter ΔT180AVE increases from the first value f1.

  Step 1960: The CPU corrects the reference value Athbase with the threshold correction coefficient Khosei (multiplies the reference value Athbase by the threshold correction coefficient Khosei) to obtain the imbalance determination threshold Ath. As a result, the imbalance determination threshold (first imbalance determination threshold) Ath is changed so as to decrease as the acquired rotation speed fluctuation parameter ΔT180AVE increases.

  More specifically, the imbalance determination threshold (first imbalance determination threshold) Ath is when the rotational speed fluctuation parameter ΔT180AVE is within a predetermined range including 0 (when it is greater than or equal to 0 and less than the first value f1). , A constant reference value (Athbase) is maintained. In addition, the imbalance determination threshold Ath is continuously increased from the reference value Athbase as the rotational speed fluctuation parameter ΔT180AVE increases when the rotational speed fluctuation parameter ΔT180AVE is outside a predetermined range (that is, when the rotational speed fluctuation parameter ΔT180AVE is greater than or equal to the first value f1). (See FIG. 5).

  Step 1965: The CPU determines whether or not the hydrogen amount difference indicating parameter (sub FB learning value average value) Avesfbg is equal to or greater than “the imbalance determination threshold Ath obtained in step 1960”.

  As described above, when the air-fuel ratio non-uniformity among the cylinders is excessive and the “air-fuel ratio imbalance among cylinders” occurs, the sub feedback amount Vafsfb is the air-fuel ratio of the air-fuel mixture supplied to the engine 10. Since it is going to be a value that is largely corrected to the rich side, the hydrogen amount difference indicating parameter (sub FB learning value average value) Avesfbg, which is the average value of the sub FB learning value Vafsfbg, is also “the mixture supplied to the engine 10”. This value is a value that corrects the air / fuel ratio of the gas to the rich side (a value equal to or greater than the imbalance determination threshold Ath).

  Further, the imbalance determination threshold Ath is changed so as to decrease as the rotational speed fluctuation parameter ΔT180AVE increases. Therefore, as shown in FIG. 5, even when the upstream air-fuel ratio sensor 55 receives the “influence of exhaust gas from a specific cylinder” more strongly than “influence of exhaust gas from other cylinders”, If an air-fuel ratio imbalance among cylinders has occurred, the hydrogen amount difference indicating parameter (sub-FB learning value Vafsfbg) becomes equal to or greater than the imbalance determination threshold Ath (see curves C2 and C3 in FIG. 5).

  Therefore, the CPU determines that the air-fuel ratio imbalance among cylinders is occurring when the hydrogen amount difference indicating parameter Avesfbg is equal to or greater than the imbalance determination threshold Ath. That is, if the hydrogen amount difference instruction parameter Avesfbg is equal to or greater than the imbalance determination threshold Ath, the CPU makes a “Yes” determination at step 1965 to proceed to step 1970 to set the value of the abnormality occurrence flag XIJO to “1”. To do. That is, the value of the abnormality occurrence flag XIJO being “1” indicates that an air-fuel ratio imbalance among cylinders has occurred. Note that the value of the abnormality occurrence flag XIJO is stored in the backup RAM. Further, when the value of the abnormality occurrence flag XIJO is set to “1”, the CPU may turn on a warning lamp (not shown).

  On the other hand, if the hydrogen amount difference indicating parameter Avesfbg is smaller than the imbalance determination threshold Ath, the CPU makes a “No” determination at step 1965 to proceed to step 1975. In step 1975, the CPU sets the value of the abnormality occurrence flag XIJO to “0” so as to indicate that the “air-fuel ratio imbalance among cylinders” has not occurred.

  Step 1980: The CPU proceeds to Step 1980 from either Step 1970 or Step 1975, sets the value of the learning value integration counter Cexe to “0” (resets), and sets the integration value SVafsfbg of the sub FB learning value to “ Set to 0 (reset).

  If the precondition for abnormality determination is not satisfied when the processing of step 1910 is executed, the CPU proceeds to step 1995 after executing the processing of step 1980 and temporarily ends this routine. Furthermore, when the CPU executes the process of step 1915 and the time is not immediately after the update of the sub FB learning value Vafsfbg, the CPU directly proceeds from step 1915 to step 1995 to end the present routine tentatively.

As described above, the determination apparatus according to the first embodiment of the present invention is
Applied to a multi-cylinder internal combustion engine (10) having a plurality of cylinders;
From an exhaust gas collecting portion in which exhaust gas discharged from the combustion chamber (21) of at least two or more cylinders (all cylinders in the first embodiment) of the plurality of cylinders gathers in the exhaust passage of the engine And a catalyst (43) disposed at a downstream site,
A fuel injection valve (25) disposed corresponding to each of the at least two cylinders and injecting fuel contained in an air-fuel mixture supplied to each combustion chamber of the two or more cylinders;
A diffusion resistance layer disposed between the exhaust collection portion or the exhaust collection portion of the exhaust passage and the catalyst, and in contact with the exhaust gas before passing through the catalyst; and covered with the diffusion resistance layer; An upstream air-fuel ratio sensor (55) having an air-fuel ratio detection element that outputs an output value corresponding to the air-fuel ratio of the exhaust gas that has passed through the diffusion resistance layer;
A downstream air-fuel ratio sensor (56) for outputting an output value corresponding to the air-fuel ratio of the exhaust gas after passing through the catalyst;
Air-fuel ratio feedback control means for feedback-controlling a fuel injection amount that is the amount of fuel injected from the fuel injection valve so that the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor matches the stoichiometric air-fuel ratio ( (See the routines of FIGS. 14 and 15)
An imbalance determination means for executing an air-fuel ratio imbalance determination between cylinders to determine whether or not an imbalance has occurred between the air-fuel ratios of the cylinders, which is the air-fuel ratio of the air-fuel mixture supplied to each of the at least two cylinders; (See routines in FIGS. 17-19)
In an internal combustion engine air-fuel ratio imbalance determination apparatus for an internal combustion engine comprising:
The imbalance determination means
The amount of hydrogen contained in the exhaust gas before passing through the catalyst and the amount of hydrogen contained in the exhaust gas after passing through the catalyst based on the output value of the downstream air-fuel ratio sensor when the feedback control is being executed. A hydrogen amount difference indicating parameter acquisition means (see steps 1915 to 1940 in FIG. 19) for acquiring a hydrogen amount difference indicating parameter (Avesfbg) that increases as the difference from the amount increases.
Rotational speed fluctuation parameter acquisition means (see the routines of FIGS. 17 and 18) for acquiring a rotational speed fluctuation parameter (ΔT180AVE) that changes in accordance with the fluctuation of the rotational speed of the engine;
Imbalance determination execution means for executing the air-fuel ratio imbalance among cylinders determination using both the acquired hydrogen amount difference indicating parameter (Avesfbg) and the acquired rotation speed fluctuation parameter (ΔT180AVE) (step of FIG. 19) 1955 to step 1975.)
Is provided.

Further, the air / fuel ratio feedback control means of the determination device comprises:
A sub-feedback amount (Vafsfb) for making the output value of the downstream air-fuel ratio sensor coincide with a value corresponding to the theoretical air-fuel ratio each time a predetermined first update timing arrives (every execution timing of the routine of FIG. 16). Sub-feedback amount update means (see Steps 1605 to 1630 in FIG. 16) for updating the value based on the deviation (DVoxs) between the output value (Voxs) of the downstream air-fuel ratio sensor and the theoretical air-fuel ratio;
Each time a predetermined second update timing arrives (every execution timing of the routine of FIG. 14), the fuel injection amount (at least the output value (Vabyfs) of the upstream air-fuel ratio sensor and the sub feedback amount (Vafsfb)) A fuel injection amount determining means for determining Fi) (see the routines of FIGS. 15 and 14);
A means for updating the learning value (Vafsfbg) of the sub-feedback amount based on the sub-feedback amount (Vafsfb) every time a predetermined third update timing arrives (every execution timing of the routine of FIG. 16). Sub feedback amount learning means for updating the learning value (Vafsfbg) so that the learning value Vafsfbg approaches the steady component of the sub feedback amount (see steps 1635 to 1655 of the routine of FIG. 16);
Is configured to include
The hydrogen amount difference indicating parameter acquisition means includes
The hydrogen amount difference indicating parameter is calculated based on the learned value of the sub feedback amount (see Step 1915 to Step 1940 in FIG. 19).

Further, the imbalance determination executing means includes
When the acquired hydrogen amount difference indicating parameter (Avesfbg) is larger than a predetermined first imbalance determination threshold (Ath), it is determined that the air-fuel ratio imbalance among cylinders is occurring, and the acquired rotational speed fluctuation is determined. The first imbalance determination threshold value is decreased as the parameter increases (see step 1955 and step 1960 of the routine of FIG. 19).

In addition, the imbalance determination execution means
When the acquired rotation speed variation parameter is within a predetermined range (0 to first value f1) including 0, the first imbalance determination threshold is maintained at a constant reference value (Athbase), and the acquired rotation When the speed fluctuation parameter (ΔT180AVE) is outside the predetermined range, the first imbalance determination threshold (Ath) is reduced as the rotational speed fluctuation parameter (ΔT180AVE) increases (in the routine of FIG. 19). (See Step 1955 and Step 1960.)

  Therefore, even when the upstream air-fuel ratio sensor 55 is strongly influenced by the exhaust gas discharged from the specific cylinder, it can be reliably determined that the air-fuel ratio imbalance among cylinders has occurred.

(Second Embodiment)
Next, an air-fuel ratio imbalance among cylinders determination apparatus (hereinafter simply referred to as “second determination apparatus”) of a multi-cylinder internal combustion engine according to a second embodiment of the present invention will be described. This second determination device replaces the determination threshold for determining the air-fuel ratio imbalance among cylinders based on the rotational speed fluctuation parameter ΔT180AVE, and changes the hydrogen amount difference indicating parameter (Avesfbg) based on the rotational speed fluctuation parameter ΔT180AVE. Only the change (correction) is different from the determination apparatus of the first embodiment. Therefore, hereinafter, this difference will be mainly described.

  Among the routines executed by the CPU of the determination apparatus according to the first embodiment, the CPU of the second determination apparatus replaces “steps 1955 to 1965 shown in the routine of FIG. 19” with “step 2010 shown in the routine of FIG. The only difference from the first embodiment is that the routine replaced with “step 2030” is executed.

  That is, if the CPU of the second determination apparatus determines “Yes” in step 1950, the CPU proceeds to step 2010 in FIG. 20 to obtain the hydrogen amount difference instruction parameter correction coefficient Kh2. Specifically, the CPU obtains the hydrogen amount difference instruction parameter correction coefficient Kh2 by applying the rotation speed variation parameter ΔT180AVE to the table MapKh2.

According to this table MapKh2, as shown in step 2010 of FIG.
(A) When the absolute value of the rotational speed fluctuation parameter ΔT180AVE is between 0 and the first value f1, the hydrogen amount difference indicating parameter correction coefficient Kh2 is “1”.
(B) Between the first value f1 of the rotational speed fluctuation parameter ΔT180AVE and the second value f2 larger than the first value f1, the hydrogen amount difference indicating parameter correction coefficient Kh2 increases as the rotational speed fluctuation parameter ΔT180AVE increases. Gradually increasing to have a positive slope of 3,
(C) When the absolute value of the rotational speed fluctuation parameter ΔT180AVE is equal to or greater than the second value f2, the hydrogen amount difference indicating parameter correction coefficient Kh2 gradually increases so as to have a fourth positive slope as the rotational speed fluctuation parameter ΔT180AVE increases. Increase. Note that the fourth positive slope is smaller than the third positive slope.

  That is, the hydrogen amount difference indicating parameter correction coefficient Kh2 is maintained at “1” when the rotational speed variation parameter ΔT180AVE is within a predetermined range including 0 (that is, 0 to the first value f1). Further, the hydrogen amount difference indicating parameter correction coefficient Kh2 is determined so as to increase in the range of “1” or more as the rotational speed fluctuation parameter ΔT180AVE increases from the first value f1.

  Next, the CPU proceeds to step 2020 and corrects the hydrogen amount difference indicating parameter Avesfbg (sub-FB learning value average value Avesfbg) by the hydrogen amount difference indicating parameter correction coefficient Kh2 (the hydrogen amount difference indicating parameter Avesfbg is changed to the hydrogen amount difference indicating parameter). The corrected hydrogen amount difference indicating parameter Avesfbg is obtained by multiplying the correction coefficient Kh2. As a result, the hydrogen amount difference indicating parameter Avesfbg is changed so as to increase as the acquired rotation speed fluctuation parameter ΔT180AVE increases.

  Next, the CPU proceeds to step 2030 to determine whether or not the corrected hydrogen amount difference indicating parameter Avesfbg is equal to or greater than the second imbalance determination threshold value (the above-described constant reference value) Athbase. If the corrected hydrogen amount difference indicating parameter Avesfbg is equal to or greater than the second imbalance determination threshold Athbase, the CPU determines that an air-fuel ratio imbalance among cylinders has occurred, and proceeds to step 1970 in FIG. On the other hand, if the corrected hydrogen amount difference indicating parameter Avesfbg is smaller than the second imbalance determination threshold value Athbase, the CPU determines that no air-fuel ratio imbalance among cylinders has occurred, and proceeds to step 1975 in FIG. move on.

As described above, the imbalance determination execution means of the second determination device is
The acquired hydrogen amount difference indicating parameter (Avesfbg) is corrected so as to increase as the acquired rotation speed fluctuation parameter ΔT180AVE increases, and the corrected hydrogen amount difference indicating parameter (corrected hydrogen amount difference indicating parameter) Is greater than a predetermined second imbalance determination threshold value (Athbase), it is determined that an air-fuel ratio imbalance among cylinders has occurred (see the routine of FIG. 20).

  Therefore, when the upstream air-fuel ratio sensor 55 is strongly influenced by the exhaust gas discharged from the specific cylinder, the hydrogen amount difference indicating parameter before correction is “curve C3 in which the imbalance ratio is positive in FIG. 3”. Even when the imbalance ratio changes as shown by the curve C2 in the negative region in FIG. 3, when the imbalance ratio increases to a certain extent, the corrected hydrogen amount difference indicating parameter is The value exceeds the balance determination threshold (Athbase). Therefore, the second determination device can reliably determine that the air-fuel ratio imbalance among cylinders has occurred.

(Third embodiment)
Next, an air-fuel ratio imbalance among cylinders determination device (hereinafter simply referred to as “third determination device”) of a multi-cylinder internal combustion engine according to a third embodiment of the present invention will be described. In this third determination device, instead of changing the determination threshold value for determining the air-fuel ratio imbalance among cylinders based on the rotational speed fluctuation parameter ΔT180AVE, the hydrogen amount difference indicating parameter (Avesfbg) is a predetermined third imbalance. Even when it is determined that an air-fuel ratio imbalance among cylinders is occurring when the determination threshold value (Athbase) is larger, the hydrogen amount difference indicating parameter (Avesfbg) is smaller than a predetermined third imbalance determination threshold value (Athbase). When the “obtained rotational speed fluctuation parameter ΔT180AVE” is larger than the fourth imbalance determination threshold (threshold Bth in FIG. 4), it is determined that the air-fuel ratio imbalance among cylinders is occurring.

  More specifically, the CPU of the third determination apparatus executes “steps 1955 to 1965 shown in the routine of FIG. 19” among the routines executed by the CPU of the determination apparatus according to the first embodiment. The second embodiment is different from the first embodiment only in that the routine replaced with step 2110 and step 2120 "shown in the routine of FIG.

  That is, if the CPU of the third determination apparatus determines “Yes” in step 1950 of FIG. 19, the CPU proceeds to step 2110 of FIG. 21, and the hydrogen amount difference indicating parameter Avesfbg is set to It is determined whether or not the reference value is greater than Athbase. If the hydrogen amount difference indicating parameter Avesfbg is equal to or greater than the third imbalance determination threshold Athbase, the CPU determines that an air-fuel ratio imbalance among cylinders has occurred, and proceeds to step 1970 in FIG.

  On the other hand, if the hydrogen amount difference instruction parameter Avesfbg is smaller than the third imbalance determination threshold value Athbase, the CPU makes a “No” determination at step 2110 to proceed to step 2120, where the rotational speed variation parameter ΔT180AVE is “rotation”. It is determined whether or not it is equal to or greater than a fourth imbalance determination threshold value (threshold value Bth in FIG. 4) that is a “threshold value for speed fluctuation parameter”. If the rotational speed variation parameter ΔT180AVE is equal to or greater than the fourth imbalance determination threshold Bth, the CPU determines that an air-fuel ratio imbalance among cylinders has occurred, and proceeds from step 2120 to step 1970 in FIG.

  If the rotational speed variation parameter ΔT180AVE is smaller than the fourth imbalance determination threshold Bth, the CPU determines “No” in step 2120 (that is, determines that no air-fuel ratio imbalance among cylinders has occurred). ), The process proceeds to step 1975 in FIG.

As described above, the imbalance determination execution means of the third determination device is
When the hydrogen amount difference indicating parameter (Avesfbg) acquired in step 1940 of FIG. 19 is larger than a predetermined third imbalance determination threshold (Athbase), it is determined that an air-fuel ratio imbalance among cylinders has occurred, and the acquisition is performed. Even if the obtained hydrogen amount difference indicating parameter (Avesfbg) is smaller than the predetermined third imbalance determination threshold (Athbase), the rotational speed fluctuation parameter ΔT180AVE acquired in step 1850 of FIG. When larger than the determination threshold Bth, it is determined that an air-fuel ratio imbalance among cylinders is occurring (see the routine of FIG. 21).

  According to this, in the example shown in FIG. 5, when the imbalance ratio is in a predetermined range including 0 (range where the imbalance ratio is approximately −10% to + 40%), the hydrogen amount difference indicating parameter and the third By comparing the imbalance determination threshold value (Athbase), it is possible to reliably detect that the air-fuel ratio imbalance among cylinders has occurred. When the imbalance ratio is outside the predetermined range, the rotational speed fluctuation parameter ΔT180AVE that is not directly related to the output value Vabyfs of the upstream air-fuel ratio sensor 55 is compared with the fourth imbalance determination threshold Bth. Therefore, even if the upstream air-fuel ratio sensor reacts sensitively to hydrogen contained in the exhaust gas from the specific cylinder, it is determined whether or not the air-fuel ratio imbalance between cylinders has occurred. It is possible to reliably determine that the occurrence has occurred.

  As described above, the air-fuel ratio imbalance determining apparatus according to each embodiment of the present invention is an air-fuel ratio cylinder even when the upstream air-fuel ratio sensor is strongly influenced by exhaust gas from a specific cylinder. It can be reliably determined that an imbalance has occurred.

  Various modifications may be employed within the scope of the present invention. For example, in each of the determination devices, the sub FB learning value average value Avesfbg is acquired as the hydrogen amount difference indicating parameter. However, when the abnormality determination precondition is satisfied, the “sub FB learning value Vafsfbg itself or The average value of the sub feedback amount Vafsfb ”may be acquired as an imbalance determination parameter.

  Further, the sub feedback control of the determination device apparently corrects the air-fuel ratio abyfs detected by the upstream air-fuel ratio sensor 55 so that the output value Voxs of the downstream air-fuel ratio sensor 56 matches the downstream target value Voxsref. (Refer to the above formula (2).) On the other hand, as disclosed in Japanese Patent Application Laid-Open No. 6-010738, the sub-feedback control uses an air-fuel ratio correction coefficient created based on the output value of the upstream air-fuel ratio sensor 55 as “downstream air-fuel ratio sensor”. It may be changed based on the “sub-feedback amount obtained by integrating 56 output values Voxs”.

In addition, as described in JP 2007-77869 A, JP 2007-146661 A, JP 2007-162565 A, and the like, the above-described determination device is configured to output the output value Vabyfs of the upstream air-fuel ratio sensor 55. The main feedback amount KFmain is calculated by high-pass filtering the difference between the upstream air-fuel ratio abyfs obtained based on this and the upstream target air-fuel ratio abyfr, and the output value Voxs of the downstream air-fuel ratio sensor 56 and the downstream target value Voxsref The sub-feedback amount Fisub may be obtained by performing a proportional integration process on a value obtained by performing a low-pass filter process on the deviation. In this case, as shown in the following equation (14), the feedback amounts are used for correcting the basic fuel injection amount Fbase in a form independent of each other, thereby obtaining the final fuel injection amount Fi. May be.
Fi = KFmain · Fbase + Fisub (14)

  In addition, in the routine of FIG. 19, when the CPU determines “No” in step 1910, the CPU proceeds to step 1980, but when it determines “No” in step 1910, the CPU may directly proceed to step 1995.

In addition, the determination device may update the sub FB learning value Vafsfbg according to the following equation (15). In this case, as is apparent from the equation (15), the sub FB learning value Vafsfbg is a value obtained by performing “filter processing for noise removal” on the “sub feedback amount Vafsfb”. In other words, the sub FB learning value Vafsfbg may be a primary delay amount (smoothing value) of the sub feedback amount Vafsfb. In the equation (15), the value p is a constant larger than 0 and smaller than 1.
Vafsfbgnew = (1-p) · Vafsfbg + p · Vafsfb (19)

  Furthermore, this determination apparatus may store “a value SDVoxs based on an integral value of the output deviation amount DVoxs” obtained when calculating the sub feedback amount Vafsfb in the backup RAM as the sub FB learning value Vafsfbg. In this case, Ki · Vafsfbg may be used as the sub-feedback amount Vafsfb during the sub-feedback control period. At this time, Vafsfbg in the above equation (2) is set to “0”. Further, in this case, the sub FB learning value Vafsfbg may be adopted as the initial value of the integrated value SDVoxs of the output deviation amount at the start of the sub feedback control.

  In addition, this determination apparatus updates the sub FB learning value Vafsfbg immediately after the output value Voxs of the downstream air-fuel ratio sensor 56 crosses the theoretical air-fuel ratio equivalent value Vst (0.5 V) (during rich-lean reversal). Can be configured to do.

  Moreover, this determination apparatus is applicable also to a V-type engine, for example. In that case, the V-type engine has a right bank upstream side catalyst (an exhaust passage of the engine and at least two of the plurality of cylinders of the plurality of cylinders) downstream of the exhaust collecting portion of the two or more cylinders belonging to the right bank. Catalyst located in the downstream side of the exhaust collecting part where the exhaust gas discharged from the combustion chamber collects), and the left bank upstream side catalyst downstream of the exhaust collecting part of two or more cylinders belonging to the left bank (Exhaust passage of the engine in a portion downstream of the exhaust collecting portion where the exhaust gas discharged from the combustion chambers of the remaining two or more cylinders other than at least two of the plurality of cylinders collects Disposed catalyst). Furthermore, the V-type engine includes an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor for the right bank upstream and downstream of the right bank upstream catalyst, and an upstream for the left bank upstream and downstream of the left bank upstream catalyst. A side air-fuel ratio sensor and a downstream air-fuel ratio sensor can be provided. In this case, the main feedback control and the sub feedback control for the right bank are executed, and the main feedback control and the sub feedback control for the left bank are executed independently.

  Furthermore, the rotational speed variation parameter may not be acquired based on “the absolute value | T180new−T180old | of the difference between the current time T180new and the previous time T180old.” That is, for example, the rotational speed variation parameter is as follows. Can be acquired.

  The CPU acquires time T30 (0) and time T30 (90). Time T30 (0) is the time from the compression top dead center of a certain cylinder until the crank angle of that cylinder reaches the 30 degree crank angle after the compression top dead center. Time T30 (90) is the time for a certain cylinder to reach the crank angle from 90 ° crank angle after compression top dead center to 120 ° crank angle after compression top dead center.

  The CPU obtains the difference ΔT30 by subtracting T30 (0) from T30 (90). The CPU obtains an absolute value ΔΔT30 of a difference between the difference ΔT30 between the cylinders that have passed the expansion stroke this time and the ΔT30 (= ΔT30old) of the cylinder that has finished the expansion stroke immediately before the expansion stroke. Then, the CPU acquires a plurality of data for the absolute value ΔΔT30 of the difference, and obtains an average value thereof as a rotation speed variation parameter.

  DESCRIPTION OF SYMBOLS 10 ... Multi-cylinder internal combustion engine, 25 ... Fuel injection valve, 40 ... Exhaust system, 41 ... Exhaust manifold, 41b ... Exhaust collection part, 42 ... Exhaust pipe, 43 ... Upstream catalyst (catalyst), 55 ... Upstream air-fuel ratio sensor 55a ... solid electrolyte layer, 55b ... exhaust gas side electrode layer, 55c ... air side electrode layer, 55d ... diffusion resistance layer, 56 ... downstream air-fuel ratio sensor, 60 ... electric control device.

Claims (6)

Applied to a multi-cylinder internal combustion engine having a plurality of cylinders,
A catalyst disposed in a portion downstream of an exhaust collecting portion in which exhaust gas discharged from a combustion chamber of at least two of the plurality of cylinders is an exhaust passage of the engine;
A fuel injection valve that is disposed corresponding to each of the at least two cylinders and injects fuel contained in an air-fuel mixture supplied to each combustion chamber of the two or more cylinders;
A diffusion resistance layer disposed between the exhaust collection portion or the exhaust collection portion of the exhaust passage and the catalyst, and in contact with the exhaust gas before passing through the catalyst; and covered with the diffusion resistance layer; An upstream air-fuel ratio sensor having an air-fuel ratio detection element that outputs an output value corresponding to the air-fuel ratio of the exhaust gas that has passed through the diffusion resistance layer;
A downstream air-fuel ratio sensor that outputs an output value corresponding to the air-fuel ratio of the exhaust gas after passing through the catalyst;
Air-fuel ratio feedback control means for feedback-controlling a fuel injection amount which is the amount of fuel injected from the fuel injection valve so that the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor matches the stoichiometric air-fuel ratio; ,
An imbalance determination means for performing an air-fuel ratio imbalance determination between cylinders as to whether or not an imbalance has occurred between cylinder-by-cylinder air-fuel ratios, which is an air-fuel ratio of an air-fuel mixture supplied to each of the at least two cylinders; ,
In an internal combustion engine air-fuel ratio imbalance determination apparatus for an internal combustion engine comprising:
The imbalance determination means
The amount of hydrogen contained in the exhaust gas before passing through the catalyst and the amount of hydrogen contained in the exhaust gas after passing through the catalyst based on the output value of the downstream air-fuel ratio sensor when the feedback control is being executed. A hydrogen amount difference indicating parameter acquisition means for acquiring a hydrogen amount difference indicating parameter that increases as the difference from the amount increases;
Rotational speed fluctuation parameter acquisition means for acquiring a rotational speed fluctuation parameter that changes according to fluctuations in the rotational speed of the engine;
An imbalance determination execution means for executing the air-fuel ratio imbalance among cylinders determination using both of the acquired hydrogen amount difference indicating parameter and the acquired rotation speed fluctuation parameter;
An air-fuel ratio imbalance among cylinders determination device. The air-fuel ratio imbalance among cylinders determination apparatus for an internal combustion engine according to claim 1,
The air-fuel ratio feedback control means includes
Every time the predetermined first update timing arrives, the sub feedback amount for making the output value of the downstream air-fuel ratio sensor coincide with the value corresponding to the theoretical air-fuel ratio is updated based on the output value of the downstream air-fuel ratio sensor. Sub-feedback amount updating means for
Fuel injection amount determination means for determining the fuel injection amount based on at least the output value of the upstream air-fuel ratio sensor and the sub feedback amount each time a predetermined second update timing arrives;
A means for updating the learning value of the sub-feedback amount based on the sub-feedback amount every time a predetermined third update timing arrives, so that the learning value approaches the stationary component of the sub-feedback amount. A sub-feedback amount learning means for updating a value;
Is configured to include
The hydrogen amount difference indicating parameter acquisition means includes
The hydrogen amount difference indicating parameter is configured to be calculated based on the learned value of the sub feedback amount.
Air-fuel ratio imbalance among cylinders determination device. The air-fuel ratio imbalance among cylinders determination apparatus for an internal combustion engine according to claim 1 or 2,
The imbalance determination execution means
When the acquired hydrogen amount difference indicating parameter is larger than a predetermined first imbalance determination threshold, it is determined that the air-fuel ratio imbalance among cylinders is occurring, and the larger the acquired rotation speed variation parameter, the more the first 1 configured to reduce the imbalance threshold,
Air-fuel ratio imbalance among cylinders determination device. The air-fuel ratio imbalance among cylinders determination apparatus for an internal combustion engine according to claim 3,
The imbalance determination execution means
When the acquired rotational speed fluctuation parameter is within a predetermined range including 0, the first imbalance determination threshold is maintained at a constant reference value, and the acquired rotational speed fluctuation parameter is out of the predetermined range. The first imbalance determination threshold is configured to decrease as the rotation speed variation parameter increases.
Air-fuel ratio imbalance among cylinders determination device. The air-fuel ratio imbalance among cylinders determination apparatus for an internal combustion engine according to claim 1 or 2,
The imbalance determination execution means
The acquired hydrogen amount difference indicating parameter is corrected so as to increase as the acquired rotation speed variation parameter increases, and the corrected hydrogen amount difference indicating parameter is greater than a predetermined second imbalance determination threshold value. Configured to determine that an air-fuel ratio imbalance among cylinders has occurred,
Air-fuel ratio imbalance among cylinders determination device. The air-fuel ratio imbalance among cylinders determination apparatus for an internal combustion engine according to claim 1 or 2,
The imbalance determination execution means
When the acquired hydrogen amount difference indicating parameter is larger than a predetermined third imbalance determination threshold value, it is determined that the air-fuel ratio imbalance among cylinders is occurring, and the acquired hydrogen amount difference indicating parameter is a predetermined first value. Even if it is smaller than the 3 imbalance determination threshold, it is configured to determine that the air-fuel ratio imbalance among cylinders has occurred when the acquired rotational speed variation parameter is larger than the fourth imbalance determination threshold.
Air-fuel ratio imbalance among cylinders determination device.

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