JP2012017657A - Fuel injection amount control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel injection amount control device of an internal combustion engine, capable of avoiding the increase of an NOdischarge amount as much as possible when the nonuniformity of air/fuel ratios among cylinders occurs.SOLUTION: The control device performs main feedback control such that the air/fuel ratio represented by an output value of an upstream side air/fuel ratio sensor is conformed to the target air/fuel ratio. Further, the control device gains an air/fuel ratio imbalance index value which becomes larger as the differences among the cylinders of the air/fuel ratio of a mixed gas supplied to respective combustion chambers is large, and the target air/fuel ratio is corrected to the rich side as the gained air/fuel ratio imbalance index value is large. Then, the control device separately calculates a stoichiometric correction term (first correction amount) for conforming a real average air/fuel ratio to the reference air/fuel ratio in the range of the window of catalysts and an enriching correction term (second correction amount) for conforming the real average air/fuel ratio to an air/fuel ratio of the reference air/fuel ratio or less, based on the air/fuel ratio imbalance index value, and determines the target air/fuel ratio by using them.

Description

  The present invention relates to a fuel injection amount control device for a multi-cylinder internal combustion engine.

  Conventionally, as shown in FIG. 1, a three-way catalyst (53) disposed in an exhaust passage of an internal combustion engine, and an upstream air-fuel ratio sensor (67) disposed upstream of the three-way catalyst (53). Are widely known.

  This air-fuel ratio control device is based on the output value of the upstream air-fuel ratio sensor so that the air-fuel ratio of the air-fuel mixture supplied to the engine (the air-fuel ratio of the engine and hence the exhaust gas air-fuel ratio) matches the target air-fuel ratio. Thus, the air-fuel ratio feedback amount (main feedback amount) is calculated, and the air-fuel ratio of the engine is feedback controlled (main feedback control) based on the air-fuel ratio feedback amount. The air-fuel ratio feedback amount used in such an air-fuel ratio control device is a control amount common to all cylinders. The target air-fuel ratio is set to a predetermined reference air-fuel ratio (for example, theoretical air-fuel ratio) within the window of the three-way catalyst (53).

  In general, such an air-fuel ratio control device is applied to an internal combustion engine that employs an electronically controlled fuel injection device. The internal combustion engine includes at least one fuel injection valve (39) in each cylinder or an intake port communicating with each cylinder. Therefore, 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 (indicated fuel injection amount)”, the mixture supplied to the specific cylinder Only the air air-fuel ratio (the air-fuel ratio of the specific cylinder) largely 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 “cylinder air-fuel ratio” that is the air-fuel ratio of the air-fuel mixture supplied to each cylinder.

  In the following, a cylinder corresponding to a fuel injection valve having a characteristic of injecting an amount of fuel that is larger or smaller than the commanded fuel injection amount is also referred to as an imbalance cylinder, and the remaining cylinders (the fuel of the commanded fuel injection amount) The cylinder corresponding to the fuel injection valve to be injected) is also referred to as a non-imbalance cylinder (or normal cylinder).

  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 indicated fuel injection amount”, the average of the air-fuel ratio of the air-fuel mixture supplied to the entire engine becomes the reference air-fuel ratio. The air-fuel ratio becomes richer than the set target air-fuel ratio. Therefore, the air-fuel ratio of the specific cylinder is changed to the lean side so that the air-fuel ratio of the specific cylinder approaches the reference air-fuel ratio by the air-fuel ratio feedback amount common to all the cylinders. It is made to change to the lean side so that it may be kept away from. As a result, the average air-fuel ratio of the air-fuel mixture supplied to the entire engine (the temporal average of the air-fuel ratio of the exhaust gas) matches the air-fuel ratio in the vicinity of the reference air-fuel ratio. In the following, “the average of the air-fuel ratio of the air-fuel mixture supplied to the entire engine” is also simply referred to as “the true average air-fuel ratio of the engine”. The true average air-fuel ratio of the engine is synonymous with the true average air-fuel ratio of “the air-fuel mixture supplied to the combustion chambers of a plurality of cylinders”.

  However, the air-fuel ratio of the specific cylinder is still richer than the reference air-fuel ratio, and the air-fuel ratio of the remaining cylinders is leaner than the reference air-fuel ratio. As a result, the amount of emissions discharged from each cylinder (the amount of unburned matter and / or the amount of nitrogen oxides) is increased as compared with the case where the air-fuel ratio of each cylinder is the reference air-fuel ratio. For this reason, even if the true average air-fuel ratio of the engine is the reference air-fuel ratio, the three-way catalyst cannot completely purify the increased emission, and as a result, the emission may be deteriorated.

  Therefore, detecting that the air-fuel ratio non-uniformity among cylinders is excessive (the air-fuel ratio imbalance condition between cylinders) is detected, and taking some measures will worsen the emissions. It is important not to let it. The air-fuel ratio imbalance state between cylinders also occurs when the characteristic of the fuel injection valve of a specific cylinder becomes “a characteristic for injecting an amount of fuel that is smaller than the indicated fuel injection amount”.

  One of the conventional fuel injection amount control devices acquires the locus length of the output value (output signal) of the upstream air-fuel ratio sensor (67). Further, the control device compares the trajectory length with a “reference value that changes according to the engine speed” and determines whether or not an air-fuel ratio imbalance among cylinders has occurred based on the comparison result. (For example, see Patent Document 1).

  Another conventional fuel injection amount control device analyzes the output value of the upstream air-fuel ratio sensor (67) and detects the air-fuel ratio for each cylinder. Then, this control device determines whether or not an air-fuel ratio imbalance among cylinders has occurred based on the detected difference between the cylinder-by-cylinder air-fuel ratios (see, for example, Patent Document 2).

US Pat. No. 7,152,594 JP 2000-220489 A

  By the way, when the air-fuel ratio non-uniformity occurs between cylinders (when the air-fuel ratio by cylinder becomes non-uniform), the true average air-fuel ratio of the engine is represented by the air value expressed based on the output value of the upstream air-fuel ratio sensor. The main feedback control for matching the fuel ratio to the “target air-fuel ratio set to the reference air-fuel ratio such as the stoichiometric air-fuel ratio” is controlled to be “an air-fuel ratio larger than the reference air-fuel ratio (lean air-fuel ratio)”. In some cases, nitrogen oxide emissions may increase. The reason for this will be described below.

The fuel supplied to the engine is a compound of carbon and hydrogen. Therefore, if the air-fuel ratio of the air-fuel mixture used for combustion is an air-fuel ratio richer than the stoichiometric air-fuel ratio, unburned substances such as “hydrocarbon HC, carbon monoxide CO and hydrogen H ₂ ” are intermediate products. Is generated as 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, as shown in FIG. 2, the amount of unburned matter (HC, CO, and H ₂ ) increases as the air-fuel ratio of the air-fuel mixture supplied to the cylinder becomes richer (for example, two It increases in terms of a function.

  The horizontal axis of the graph shown in FIG. 2 is also an “imbalance ratio” representing the degree of non-uniformity of the air-fuel ratio between cylinders. The imbalance ratio is, for example, when the fuel injection amount injected from the fuel injection valve of the non-imbalance cylinder is 1, and when the fuel injection amount injected from the fuel injection valve of the imbalance cylinder is 1 + α, Is the value.

  On the other hand, the upstream air-fuel ratio sensor (67) generally includes a diffusion resistance layer. The upstream air-fuel ratio sensor (67) passes through the diffusion resistance layer and reaches the exhaust gas-side electrode layer (surface of the air-fuel ratio detection element) of the upstream air-fuel ratio sensor (67). A value corresponding to “oxygen partial pressure” or unburned substance amount (unburned substance concentration, unburned substance partial pressure) ”is output.

On the other hand, hydrogen H ₂ is a small molecule compared to hydrocarbon HC and carbon monoxide CO. Accordingly, hydrogen H ₂ diffuses more quickly in the diffusion resistance layer of the upstream air-fuel ratio sensor (67) than other unburned substances (HC, CO). That is, selective diffusion (preferential diffusion) of hydrogen H ₂ occurs in the diffusion resistance layer.

  When air-fuel ratio non-uniformity occurs between cylinders, the average of the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor (67) is the true average of the engine due to the selective diffusion of hydrogen. The air-fuel ratio is richer than the air-fuel ratio.

  More specifically, 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 Assume that / F0 is the stoichiometric air-fuel ratio (eg, 14.6). Further, for simplicity of explanation, it is assumed that the target air-fuel ratio is a stoichiometric air-fuel ratio.

  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 air-fuel ratio of the engine is 4 · A0 / (4.4 · F0) = A0 / (1.1 · F0).

  The control device stores in advance the relationship between the output value of the upstream air-fuel ratio sensor (67) and the air-fuel ratio when the air-fuel ratio non-uniformity does not occur between the cylinders, and the relationship between the upstream side and the actual upstream side. The air-fuel ratio is detected based on the output value of the side air-fuel ratio sensor (67). The air-fuel ratio detected in this way is referred to as a detected air-fuel ratio abyfs. Therefore, in the case of the above example, the detected air-fuel ratio abyfs is the air-fuel ratio A0 / (1.1 · F0) equal to the true average air-fuel ratio of the engine.

  As a result, 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 target air-fuel ratio” by the main feedback control. That is, the amount of fuel supplied to each cylinder is reduced by 10% by main feedback control, and 1 · F0 fuel is supplied to each cylinder. That is, the air-fuel ratio of each cylinder matches the theoretical air-fuel ratio A0 / F0 in any cylinder.

Next, 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 each of the remaining three cylinders It is assumed that the amount is an appropriate amount (the amount of fuel necessary for the air-fuel ratio of each cylinder to coincide with the stoichiometric air-fuel ratio, in this case F0). That is, it is assumed that “the non-uniformity of the air-fuel ratio among the cylinders” occurs in which only the air-fuel ratio of the specific cylinder is greatly shifted to the rich side. 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 side air-fuel ratio (small air-fuel ratio). At this time, an extremely large amount of unburned matter (HC, CO, H ₂ ) is discharged from the specific cylinder.

  More specifically, according to the above assumption, 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 air-fuel ratio of the engine is 4 · A0 / (4.4 · F0) = A0 / (1.1 · F0). In other words, the true average air-fuel ratio of the engine in this case is the same value as the above-mentioned “in the case where the amount of fuel supplied to each cylinder is equally excessive by 10%”.

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, “the amount of hydrogen H ₂ contained in the exhaust gas when only the amount of fuel supplied to a specific cylinder is 40% excessive” is “the amount of fuel supplied to each cylinder”. When the amount becomes an excessive amount by 10% evenly, the amount becomes significantly larger than the “amount of hydrogen H ₂ contained in the exhaust gas”.

As a result, due to the “selective diffusion of hydrogen” described above, the average of the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor (67) is “the true average air-fuel ratio of the engine (A0 / (1 .1 · F0)) ”on the rich side. That is, even if the true average air-fuel ratio of the exhaust gas is “predetermined rich air-fuel ratio”, the exhaust-gas-side electrode layer of the upstream air-fuel ratio sensor (67) when the air-fuel ratio imbalance among cylinders is occurring. The concentration of hydrogen H ₂ that reaches 1 is much higher than the concentration of hydrogen H ₂ that reaches the exhaust gas side electrode layer when the air-fuel ratio imbalance among cylinders does not occur. Therefore, the output value of the upstream air-fuel ratio sensor (67) is a value indicating the richer air-fuel ratio than the true average air-fuel ratio of the engine.

  As a result, the true average air-fuel ratio of the engine is controlled to be leaner than the stoichiometric air-fuel ratio by main feedback control based on the output value of the upstream air-fuel ratio sensor. The above is the reason why the true average air-fuel ratio of the engine is controlled to be leaner than the target air-fuel ratio when air-fuel ratio non-uniformity occurs between the cylinders. In the following, such “transition of the air-fuel ratio to the lean side caused by selective diffusion of hydrogen and main feedback control” is also simply referred to as “lean miscontrol”.

  The “lean erroneous control” occurs in the same manner even when the air-fuel ratio of the imbalance cylinder shifts to the lean side from the air-fuel ratio of the non-imbalance cylinder. The reason for this will be described later.

  When the lean miscontrol occurs, the true average air-fuel ratio of the engine (therefore, the true average air-fuel ratio of the exhaust gas flowing into the three-way catalyst) becomes “the air-fuel ratio leaner than the three-way catalyst window (large air-fuel ratio). May occur. In this case, the purification efficiency of the three-way catalyst decreases, and the amount of NOx (nitrogen oxide) emissions increases.

  In addition, even if the “true average air-fuel ratio of the engine” is matched with the reference air-fuel ratio, when the air-fuel ratio non-uniformity occurs between the cylinders, the spatial distribution of exhaust gas in the exhaust passage is uniform. It is not like. In other words, the three-way catalyst has the first air-fuel ratio exhaust gas from the imbalance cylinder that is different from the reference air-fuel ratio, and the exhaust gas from the non-imbalance cylinder that is different from the reference air-fuel ratio. The second air-fuel ratio exhaust gas alternately flows. Therefore, as the degree of non-uniformity of the air-fuel ratio by cylinder increases, among these exhaust gases, the exhaust gas of “the air-fuel ratio deviating to the lean side with respect to the window of the three-way catalyst” flows into the three-way catalyst. Cases arise. In this case, the NOx emission amount increases rapidly.

  Accordingly, one of the objects of the present invention is to “maximize NOx emission when the air-fuel ratio non-uniformity between cylinders occurs (when the lean lean control occurs)” as much as possible. An object of the present invention is to provide a fuel injection amount control device for an internal combustion engine that can be avoided. Hereinafter, the fuel injection amount control apparatus of the present invention is also simply referred to as “the present invention apparatus”.

  The device according to the present invention executes main feedback control for making the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor coincide with the target air-fuel ratio. When the non-uniformity of the cylinder-by-cylinder air-fuel ratio does not occur, the target air-fuel ratio is set to “a predetermined reference air-fuel ratio (for example, theoretical air-fuel ratio) within the three-way catalyst window”.

  Furthermore, the device of the present invention sets the target air-fuel ratio to “a smaller air-fuel ratio (corrected air-fuel ratio)” as the degree of non-uniformity of the cylinder-by-cylinder air-fuel ratio increases. That is, the target air-fuel ratio is corrected so as to decrease in a range smaller than the reference air-fuel ratio as the degree of non-uniformity of the cylinder-by-cylinder air-fuel ratio increases.

  Even when the target air-fuel ratio is set to "an air-fuel ratio smaller than the reference air-fuel ratio" as the degree of non-uniformity of the cylinder-by-cylinder air-fuel ratio becomes large, the upstream is attributed to the selective diffusion of hydrogen. The air-fuel ratio represented by the output value of the side air-fuel ratio sensor shifts to a rich air-fuel ratio with respect to the true air-fuel ratio of the exhaust gas. Therefore, by the main feedback control, the true average air-fuel ratio of the engine (that is, the true average air-fuel ratio of the air-fuel mixture supplied to the combustion chambers of the plurality of cylinders exhausting the exhaust gas reaching the air-fuel ratio sensor) is The air-fuel ratio is controlled to be larger than the target air-fuel ratio (lean air-fuel ratio). However, since the corrected air-fuel ratio is smaller than the reference air-fuel ratio (the air-fuel ratio on the rich side), the true average air-fuel ratio of the engine obtained as a result of the main feedback control can be obtained by appropriately setting the corrected air-fuel ratio. The fuel ratio can be made to coincide with the “air-fuel ratio below the reference air-fuel ratio”. Therefore, it is possible to avoid an increase in the NOx emission amount.

  More specifically, the device of the present invention corrects the target air-fuel ratio as described above using the first correction amount and the second correction amount. That is, the device of the present invention further modifies the “first corrected air-fuel ratio obtained by correcting the reference air-fuel ratio based on the first correction amount” by using the second correction amount. The obtained second corrected air-fuel ratio is obtained as “the corrected air-fuel ratio set as the target air-fuel ratio”.

  The first correction amount and the second correction amount are both calculated based on the air-fuel ratio imbalance index value. The air-fuel ratio imbalance index value is calculated based on various methods as will be described later. The air-fuel ratio imbalance index value is a predetermined reference value (for example, “0”) when the non-uniformity of the cylinder-by-cylinder air-fuel ratio does not occur, and is a value that increases as the degree of non-uniformity of the cylinder-by-cylinder air-fuel ratio increases. As required.

  The first correction amount is a value for obtaining the first correction air-fuel ratio by correcting the reference air-fuel ratio with the first correction amount. The first corrected air-fuel ratio is an air-fuel ratio equal to or lower than the reference air-fuel ratio, and when the target air-fuel ratio is the first corrected air-fuel ratio, the true average air-fuel ratio of the engine is determined by the main feedback control. An air-fuel ratio set to coincide with the "reference air-fuel ratio".

  The second correction amount is a value for obtaining the second corrected air-fuel ratio by correcting the first corrected air-fuel ratio with the second correction amount. The second corrected air-fuel ratio is an air-fuel ratio equal to or lower than the first corrected air-fuel ratio, and the true average air-fuel ratio of the engine is the main feedback when the target air-fuel ratio is the second corrected air-fuel ratio. The air-fuel ratio is set so as to coincide with “the air-fuel ratio equal to or lower than the reference air-fuel ratio” by the control. Thus, since the second corrected air-fuel ratio is an air-fuel ratio determined so that the true average air-fuel ratio of the engine matches the air-fuel ratio “below” the reference air-fuel ratio, It may be “an amount that does not correct the corrected air-fuel ratio”.

  According to the device of the present invention, when the non-uniformity of the cylinder-by-cylinder air-fuel ratio does not occur (that is, when the air-fuel ratio imbalance index value is the reference value), the target air-fuel ratio is “within the window of the three-way catalyst”. Is maintained at a reference air-fuel ratio which is a predetermined air-fuel ratio. In this case, the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor does not include a detection error due to the selective diffusion of hydrogen, and therefore depends on the true air-fuel ratio of the exhaust gas that has reached the upstream air-fuel ratio sensor. The air / fuel ratio becomes high. As a result, the true average air-fuel ratio of the engine is maintained near the reference air-fuel ratio by the main feedback control. Therefore, the NOx emission amount is small.

  On the other hand, when the non-uniformity of the air-fuel ratio by cylinder occurs (that is, when the air-fuel ratio imbalance index value is different from the reference value), the target air-fuel ratio is set as “the corrected air-fuel ratio. The second corrected air-fuel ratio is set.

  As described above, the second correction amount may be an amount that does not correct the first correction air-fuel ratio. Therefore, when the second correction amount is an amount that does not correct the first correction air-fuel ratio, the correction air-fuel ratio matches the first correction air-fuel ratio, and the target air-fuel ratio is made to match the first correction air-fuel ratio. In this case, the main feedback control is executed so that the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor matches the “first corrected air-fuel ratio”. As a result, since the true average air-fuel ratio of the engine matches the reference air-fuel ratio, it is possible to avoid an increase in NOx emission when the degree of non-uniformity of the cylinder-by-cylinder air-fuel ratio increases.

  For example, when the air-fuel ratio imbalance index value is a predetermined value, in order to make the “true average air-fuel ratio of the engine” obtained as a result of the main feedback control coincide with the reference air-fuel ratio, Whether the air-fuel ratio should be set (that is, what air-fuel ratio the first corrected air-fuel ratio should be) can be obtained in advance by experiments or the like. In other words, the target air-fuel ratio when the “true average air-fuel ratio of the engine” coincides with the reference air-fuel ratio by the main feedback control is obtained in advance for various air-fuel ratio imbalance index values. The relationship between the index value and the first corrected air-fuel ratio, and therefore the relationship between the air-fuel ratio imbalance index value and the first correction amount can be determined in advance. The first correction amount can be calculated by applying the “actually acquired air-fuel ratio imbalance index value” to the relationship.

  In addition, when the second correction amount is an amount for correcting the first correction air-fuel ratio, the corrected air-fuel ratio matches the second correction air-fuel ratio, and the target air-fuel ratio is made equal to the second correction air-fuel ratio. . In this case, the main feedback control is executed so that the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor matches the “second corrected air-fuel ratio”. As a result, the true average air-fuel ratio of the engine coincides with the “air-fuel ratio smaller than the reference air-fuel ratio”, so that the exhaust gas spatial distribution is uniform when, for example, the degree of non-uniformity of the air-fuel ratio by cylinder increases. It is possible to prevent the exhaust gas of the air-fuel ratio deviating to the lean side with respect to the three-way catalyst window from flowing into the three-way catalyst. Therefore, even when the degree of non-uniformity of the cylinder-by-cylinder air-fuel ratio increases, it is possible to avoid an increase in the NOx emission amount.

In one aspect of the device of the present invention,
The target air-fuel ratio determining means for determining the target air-fuel ratio is
A downstream air-fuel ratio sensor that is disposed at a position downstream of the three-way catalyst in the exhaust passage and outputs an output value corresponding to an air-fuel ratio of exhaust gas that passes through the disposed position;
Sub-feedback for correcting the target air-fuel ratio based on the output value of the downstream air-fuel ratio sensor so that the output value of the downstream air-fuel ratio sensor matches the downstream target value corresponding to the reference air-fuel ratio. Sub feedback control means for calculating the amount and correcting the target air-fuel ratio based on the calculated sub feedback amount;
Is further provided.

  The sub-feedback amount may be an amount that directly corrects the target air-fuel ratio in the main feedback control. The sub-feedback amount is used for main feedback control by correcting the output value of the upstream air-fuel ratio sensor. It may be an “amount that substantially corrects the fuel ratio”. That is, “correcting the output value of the upstream air-fuel ratio sensor” used for the main feedback control has substantially the same meaning as correcting the target air-fuel ratio in the main feedback control.

  The feedback control using the sub feedback amount is also referred to as sub feedback control. According to the sub-feedback control, the output value of the downstream air-fuel ratio sensor can be matched with the downstream target value corresponding to the reference air-fuel ratio. By the way, the output value itself of the upstream air-fuel ratio sensor changes due to the way the gas from each cylinder reaches the upstream air-fuel ratio sensor, the individual difference of the upstream air-fuel ratio sensor, and the like. Therefore, even when the “main feedback control for matching the target air-fuel ratio to the first corrected air-fuel ratio” is executed when the second correction amount is an amount that does not correct the first corrected air-fuel ratio, In some cases, the average air-fuel ratio cannot be made to completely match the reference air-fuel ratio. On the other hand, “exhaust gas in which unburned matter such as hydrogen has been purified in the three-way catalyst” reaches the downstream air-fuel ratio sensor. Further, the downstream air-fuel ratio sensor is not affected by the difference between the cylinders in the way the exhaust gas reaches from each cylinder. Therefore, the output value of the downstream air-fuel ratio sensor becomes a value corresponding to the true average air-fuel ratio of the engine. Therefore, if the sub-feedback control is executed, the true average air-fuel ratio of the engine can be controlled in the vicinity of the reference air-fuel ratio, so that the emission can be maintained well.

  By the way, the second correction amount is an amount for shifting the true average air-fuel ratio of the engine to “the air-fuel ratio richer than the reference air-fuel ratio”. On the other hand, the sub-feedback amount is an amount that changes so that the true average air-fuel ratio of the engine matches the “reference air-fuel ratio”. Therefore, when the second correction amount is “a value for correcting the true average air-fuel ratio of the engine to a specific air-fuel ratio within a range smaller than the reference air-fuel ratio by a positive threshold air-fuel ratio”, If the feedback control is continued, the effect of the second correction amount (the effect of setting the true average air-fuel ratio of the engine to an air-fuel ratio richer than the reference air-fuel ratio) is canceled by the sub-feedback control.

Therefore, one aspect of the device of the present invention further includes:
Whether the second corrected air-fuel ratio is a specific air-fuel ratio for making the true average air-fuel ratio of the engine coincide with an air-fuel ratio in a range smaller than the reference air-fuel ratio by a positive threshold air-fuel ratio or more. Sub-feedback for determining based on the second correction amount and stopping the calculation of the sub-feedback amount by the sub-feedback control means when it is determined that the second correction air-fuel ratio is the specific air-fuel ratio. Amount calculation stop means.

  According to this, it is possible to avoid that the effect of the second correction amount is canceled by the sub feedback control.

In another embodiment of the device of the present invention,
The target air-fuel ratio determining means includes
The first correction amount is correlated with the intake air amount so that the first correction amount becomes a value that corrects the first correction air-fuel ratio to a smaller air-fuel ratio as the intake air amount of the engine increases. It is configured to be determined based on parameters (for example, intake air amount, throttle valve opening, engine load factor, etc.).

  Even if the degree of “lean miscontrol” (that is, how much lean air-fuel ratio is controlled by the main feedback control) is a specific value with a degree of non-uniformity of air-fuel ratio between cylinders, for example, The larger the intake air amount of the engine, the larger it becomes. This is estimated to be because the absolute amount of hydrogen discharged from the imbalance cylinder increases as the intake air amount increases. Therefore, as in the above aspect, the true average air-fuel ratio of the engine can be more reliably controlled to be close to the reference air-fuel ratio by determining the first correction amount based on the parameter correlated with the intake air amount. it can.

Similarly, in another embodiment of the device of the present invention,
The target air-fuel ratio determining means includes
The second correction amount is correlated with the intake air amount so that the second correction amount becomes a value that corrects the second corrected air-fuel ratio to a smaller air-fuel ratio as the intake air amount of the engine increases. It is configured to be determined based on parameters (for example, intake air amount, throttle valve opening, engine load factor, etc.).

  As described above, the degree of “lean erroneous control” increases as the intake air amount of the engine increases, for example, even if the degree of non-uniformity of the air-fuel ratio between the cylinders is a specific value. Accordingly, by determining the second correction amount based on the parameter that correlates with the intake air amount as in the above-described aspect, the true average air-fuel ratio of the engine is determined as “the target air-fuel ratio richer than the reference air-fuel ratio. It is possible to control to “near” more surely.

The imbalance index value acquisition means for acquiring the air-fuel ratio imbalance index value can take the following various modes.
(A) The imbalance index value acquisition means sets a value that increases as the fluctuation of the air-fuel ratio of the exhaust gas that passes through the position where the upstream air-fuel ratio sensor is disposed increases as the air-fuel ratio imbalance index value. It may be configured to acquire based on an output value of the air-fuel ratio sensor.

In this case, more specifically, the imbalance index value acquisition unit may have the following mode.
(A-1)
The imbalance index value acquisition means includes
The differential value d (Vabyfs) / dt with respect to the time of the output value Vabyfs of the upstream air-fuel ratio sensor is acquired, and a value correlated with the acquired differential value d (Vabyfs) / dt is obtained as the air-fuel ratio imbalance index value. Can be configured to obtain as
(A-2)
The imbalance index value acquisition means includes
The differential value d (abyfs) / dt for the time of the detected air-fuel ratio abyfs represented by the output value Vabyfs of the upstream side air-fuel ratio sensor is acquired and correlated with the acquired differential value d (abyfs) / dt value A value may be obtained as the air-fuel ratio imbalance index value.
(A-3)
The imbalance index value acquisition means includes
The second-order differential value d ² (Vabyfs) / dt ² for the time of the output value Vabyfs of the upstream air-fuel ratio sensor is acquired, and a value correlated with the acquired second-order differential value d ² (Vabyfs) / dt ² is It may be configured to obtain as an air-fuel ratio imbalance index value.
(A-4)
The imbalance index value acquisition means includes
Obtaining the second derivative d ² (abyfs) / dt ² for the time of the detected air-fuel ratio abyfs represented by the output value Vabyfs of the upstream air-fuel ratio sensor, and obtaining the second derivative d ² (abyfs) / A value correlated with dt ² may be obtained as the air-fuel ratio imbalance index value.
(A-5)
The imbalance index value acquisition means includes
A value correlated with the difference between the maximum value and the minimum value of the output value Vabyfs of the upstream air-fuel ratio sensor in a predetermined period, or a predetermined period of the detected air-fuel ratio abyfs expressed by the output value Vabyfs of the upstream air-fuel ratio sensor A value that correlates with a difference between the maximum value and the minimum value at the time can be obtained as the air-fuel ratio imbalance index value.
(A-6)
The imbalance index value acquisition means includes
As the air-fuel ratio imbalance index value, a value that correlates with the locus length of the output value Vabyfs of the upstream air-fuel ratio sensor in a predetermined period, or a detected air-fuel ratio abyfs expressed by the output value Vabyfs of the upstream air-fuel ratio sensor A value that correlates to the trajectory length in the predetermined period of time may be acquired.

  Other objects, other features and attendant advantages of the inventive device will be readily understood from the description of each embodiment of the inventive device described with reference to the following drawings.

FIG. 1 is a schematic plan view of an internal combustion engine to which a fuel injection amount control device according to each embodiment of the present invention is applied. FIG. 2 is a graph showing the relationship between the air-fuel ratio of the air-fuel mixture supplied to the cylinder and the amount of unburned components discharged from the cylinder. 3 is a sectional view of the internal combustion engine shown in FIG. FIG. 4 is a partial schematic perspective view (perspective view) of the air-fuel ratio sensor (upstream air-fuel ratio sensor) shown in FIGS. 1 and 3. FIG. 5 is a partial cross-sectional view of the air-fuel ratio sensor shown in FIGS. 1 and 3. 6A to 6C are schematic cross-sectional views of an air-fuel ratio detection unit provided in the air-fuel ratio sensor (upstream air-fuel ratio sensor) shown in FIGS. FIG. 7 is a graph showing the relationship between the air-fuel ratio of exhaust gas and the limit current value of the air-fuel ratio sensor. FIG. 8 is a graph showing the relationship between the air-fuel ratio of exhaust gas and the output value of the air-fuel ratio sensor. FIG. 9 is a graph showing the relationship between the air-fuel ratio of exhaust gas and the output value of the downstream air-fuel ratio sensor shown in FIGS. FIG. 10 is a time chart showing the behavior of each value related to the air-fuel ratio imbalance index value when the air-fuel ratio imbalance state between cylinders occurs and when the same state does not occur. FIG. 11 is a graph showing the relationship between the actual imbalance ratio and the air-fuel ratio imbalance index value acquired based on the detected air-fuel ratio change rate. FIG. 12 is a flowchart showing a routine executed by the CPU of the fuel injection amount control apparatus (first control apparatus) according to the first embodiment of the present invention. FIG. 13 is a flowchart showing a routine executed by the CPU of the first control device. FIG. 14 is a flowchart showing a routine executed by the CPU of the first control device. FIG. 15 is a flowchart showing a routine executed by the CPU of the first control device. FIG. 16 is a flowchart showing a routine executed by the CPU of the first control device. FIG. 17 is a flowchart showing a routine executed by the CPU of the fuel injection amount control device (second control device) according to the second embodiment of the present invention. FIG. 18 is a flowchart showing a routine executed by the CPU of the fuel injection amount control device (third control device) according to the third embodiment of the present invention.

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

<First Embodiment>
(Constitution)
FIG. 3 shows a system in which the control device according to the first embodiment (hereinafter also referred to as “first control device”) is applied to a 4-cycle, spark ignition type, multi-cylinder (in-line 4-cylinder) internal combustion engine 10. The schematic structure of is shown. FIG. 3 shows only a cross section of a specific cylinder, but the other cylinders have the same configuration.

  The internal combustion engine 10 includes a cylinder block portion 20, a cylinder head portion 30, an intake system 40, and an exhaust system 50.

  The cylinder block unit 20 includes a cylinder block, a cylinder block lower case, an oil pan, and the like. The cylinder head part 30 is fixed on the cylinder block part 20. The intake system 40 includes various members that supply gasoline mixture to the cylinder block unit 20. The exhaust system 50 includes various members for releasing the exhaust gas discharged from the cylinder block unit 20 to the outside.

  The cylinder block unit 20 includes a cylinder 21, a piston 22, a connecting rod 23 and a crankshaft 24. The piston 22 reciprocates in the cylinder 21, and the reciprocating motion of the piston 22 is transmitted to the crankshaft 24 through the connecting rod 23, whereby the crankshaft 24 rotates. The wall surface of the cylinder 21 and the upper surface of the piston 22 form a combustion chamber 25 together with the lower surface of the cylinder head portion 30.

  The cylinder head unit 30 includes an intake port 31, an intake valve 32, a variable intake timing control device 33, an exhaust port 34, an exhaust valve 35, a variable exhaust timing control device 36, a spark plug 37, an igniter 38, and a fuel injection valve (fuel injection means). , Fuel supply means) 39.

  The intake port 31 communicates with the combustion chamber 25. The intake valve 32 opens and closes the intake port 31. The variable intake timing control device 33 includes an intake camshaft that drives the intake valve 32, and an actuator 33a that continuously changes the phase angle of the intake camshaft. The exhaust port 34 communicates with the combustion chamber 25. The exhaust valve 35 opens and closes the exhaust port 34. The variable exhaust timing control device 36 includes an exhaust camshaft that drives the exhaust valve 35, and an actuator 36a that continuously changes the phase angle of the exhaust camshaft. One spark plug 37 is disposed in each combustion chamber 25. One igniter 38 is provided for each spark plug 37. The igniter 38 includes an ignition coil.

  One fuel injection valve 39 is provided for each combustion chamber 25. The fuel injection valve 39 is provided in each intake port 31 that communicates with each combustion chamber 25. That is, each of the plurality of cylinders includes a fuel injection valve 39 that supplies fuel independently from the other cylinders. In response to the injection instruction signal, the fuel injection valve 39 injects “the fuel of the indicated fuel injection amount included in the injection instruction signal” into the corresponding intake port 31 when it is normal. More specifically, the fuel injection valve 39 is supplied with fuel maintained at a constant pressure. The fuel injection valve 39 opens for a time corresponding to the command fuel injection amount. Therefore, if the fuel injection valve 39 is normal, the fuel injection valve 39 injects the indicated fuel injection amount of fuel. However, when an abnormality occurs in the fuel injection valve 39, the fuel injection valve 39 injects an amount of fuel different from the command fuel injection amount.

  The intake system 40 includes an intake manifold 41, an intake pipe 42, an air filter 43, and a throttle valve 44.

  As shown in FIG. 1, the intake manifold 41 includes a plurality of branch portions 41a and a surge tank 41b. One end of each of the plurality of branch portions 41a is connected to each of the plurality of intake ports 31 as shown in FIG. The other ends of the plurality of branch portions 41a are connected to the surge tank 41b. One end of the intake pipe 42 is connected to the surge tank 41b. The air filter 43 is disposed at the other end of the intake pipe 42. The throttle valve 44 is provided in the intake pipe 42 so that the opening cross-sectional area of the intake passage is variable. The throttle valve 44 is rotationally driven in the intake pipe 42 by a throttle valve actuator 44a (a part of the throttle valve driving means) made of a DC motor.

  The exhaust system 50 includes an exhaust manifold 51, an exhaust pipe 52, an upstream catalyst 53 disposed in the exhaust pipe 52, and a downstream catalyst (not shown) disposed in the exhaust pipe 52 downstream of the upstream catalyst 53. I have.

  As shown in FIG. 1, the exhaust manifold 51 includes a plurality of branch portions 51a each having one end connected to the exhaust port, and the other ends of the plurality of branch portions 51a and all the branch portions 51a. Are gathering portions 51b. The collecting portion 51b is also referred to as an exhaust collecting portion HK because exhaust gas discharged from a plurality of (two or more, four in this example) cylinders gathers. The exhaust pipe 52 is connected to the collecting portion 51b. As shown in FIG. 3, the exhaust port 34, the exhaust manifold 51, and the exhaust pipe 52 constitute an exhaust passage.

Each of the upstream side catalyst 53 and the downstream side catalyst is a so-called three-way catalyst device (exhaust purification catalyst) carrying an active component made of a noble metal (catalyst substance) such as platinum, rhodium and palladium. Each catalyst has HC, CO, H _2, etc., when the air-fuel ratio of the gas flowing into each catalyst is “a predetermined air-fuel ratio within the window range (a reference air-fuel ratio in the vicinity of the theoretical air-fuel ratio)”. It has a function (oxidation / reduction function) of oxidizing unburned components and reducing nitrogen oxides (NOx). This function is also called a catalyst function. Further, each catalyst has an oxygen storage function for storing (storing) oxygen. Each catalyst can purify unburned components and nitrogen oxides even if the air-fuel ratio shifts from the stoichiometric air-fuel ratio due to the oxygen storage function. That is, the width of the window of the three-way catalyst (upstream catalyst 53 and downstream catalyst) is expanded by the oxygen storage function. The oxygen storage function is provided by an oxygen storage material such as ceria (CeO ₂ ) supported on the catalyst.

  This system includes a hot-wire air flow meter 61, a throttle position sensor 62, a water temperature sensor 63, a crank position sensor 64, an intake cam position sensor 65, an exhaust cam position sensor 66, an upstream air-fuel ratio sensor 67, a downstream air-fuel ratio sensor 68, An accelerator opening sensor 69 is provided.

  The air flow meter 61 outputs a signal corresponding to the mass flow rate (intake air flow rate) Ga of intake air flowing in the intake pipe 42. That is, the intake air flow rate Ga represents the intake air amount Ga that is taken into the engine 10 per unit time.

  The throttle position sensor 62 detects the opening degree of the throttle valve 44 (throttle valve opening degree) and outputs a signal representing the throttle valve opening degree TA.

  The water temperature sensor 63 detects the temperature of the cooling water of the internal combustion engine 10 and outputs a signal representing the cooling water temperature THW. The coolant temperature THW is a parameter that represents the warm-up state of the engine 10 (temperature of the engine 10).

  The crank position sensor 64 outputs a signal having a narrow pulse every time the crankshaft 24 rotates 10 °, and a wide pulse every time the crankshaft 24 rotates 360 °. This signal is converted into an engine speed NE by an electric control device 70 described later.

  The intake cam position sensor 65 outputs one pulse every time the intake cam shaft rotates 90 degrees, 90 degrees, and 180 degrees from a predetermined angle. The electric control device 70 described later acquires an absolute crank angle CA based on the compression top dead center of the reference cylinder (for example, the first cylinder) based on signals from the crank position sensor 64 and the intake cam position sensor 65. It has become. The absolute crank angle CA is set to “0 ° crank angle” at the compression top dead center of the reference cylinder, and increases to a 720 ° crank angle according to the rotation angle of the crankshaft 24. Set to an angle.

  The exhaust cam position sensor 66 outputs one pulse every time the exhaust cam shaft rotates 90 degrees from a predetermined angle, then 90 degrees, and then 180 degrees.

  As shown in FIG. 1, the upstream side air-fuel ratio sensor 67 is located at either the exhaust manifold 51 or the exhaust pipe 52 (that is, the exhaust pipe 52 (ie, exhaust gas) at a position between the collecting portion 51 b of the exhaust manifold 51 and the upstream catalyst 53. Channel) ”.

  The upstream side air-fuel ratio sensor 67 is disclosed in, for example, “Limit current type wide area air-fuel ratio including diffusion resistance layer” disclosed in JP-A-11-72473, JP-A-2000-65782, JP-A-2004-69547, and the like. Sensor ".

  As shown in FIGS. 4 and 5, the upstream air-fuel ratio sensor 67 includes an air-fuel ratio detector 67 a, an outer protective cover 67 b, and an inner protective cover 67 c.

  The outer protective cover 67b is a hollow cylindrical body made of metal. The outer protective cover 67b accommodates the inner protective cover 67c so as to cover the inner protective cover 67c. The outer protective cover 67b has a plurality of inflow holes 67b1 on its side surface. The inflow hole 67b1 is a through hole for allowing exhaust gas (exhaust gas outside the outer protective cover 67b) EX flowing in the exhaust passage to flow into the outer protective cover 67b. Further, the outer protective cover 67b has an outflow hole 67b2 on the bottom surface for allowing the exhaust gas inside the outer protective cover 67b to flow out (exhaust passage).

  The inner protective cover 67c is a hollow cylindrical body made of metal and having a diameter smaller than that of the outer protective cover 67b. The inner protective cover 67c accommodates the air-fuel ratio detection unit 67a so as to cover the air-fuel ratio detection unit 67a. The inner protective cover 67c has a plurality of inflow holes 67c1 on its side surface. The inflow hole 67c1 is a through-hole for allowing exhaust gas flowing into the “space between the outer protective cover 67b and the inner protective cover 67c” through the inflow hole 67b1 of the outer protective cover 67b to flow into the inner protective cover 67c. It is. Further, the inner protective cover 67c has an outflow hole 67c2 for allowing the exhaust gas inside the inner protective cover 67c to flow out to the outside.

  As shown in FIGS. 6A to 6C, the air-fuel ratio detection unit 67a includes a solid electrolyte layer 671, an exhaust gas side electrode layer 672, an atmosphere side electrode layer (reference gas side electrode layer) 673, A diffusion resistance layer (porous layer) 674, a first wall portion 675, a catalyst portion 676, a second wall portion 677, and a heater 678 are included.

The solid electrolyte layer 671 is an oxygen ion conductive oxide sintered body. In this example, the solid electrolyte layer 671 is a “stabilized zirconia element” in which CaO as a stabilizer is dissolved in ZrO ₂ (zirconia). The solid electrolyte layer 671 exhibits well-known “oxygen battery characteristics” and “oxygen pump characteristics” when its temperature is equal to or higher than the activation temperature.

  The exhaust gas side electrode layer 672 is made of a noble metal having high catalytic activity such as platinum (Pt). The exhaust gas side electrode layer 672 is formed on one surface of the solid electrolyte layer 671. The exhaust gas side electrode layer 672 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 673 is made of a noble metal having high catalytic activity such as platinum (Pt). The atmosphere-side electrode layer 673 is formed on the other surface of the solid electrolyte layer 671 so as to face the exhaust gas-side electrode layer 672 with the solid electrolyte layer 671 interposed therebetween. The atmosphere-side electrode layer 673 is formed to have sufficient permeability (that is, in a porous shape) by chemical plating or the like.

  The diffusion resistance layer (diffusion limiting layer) 674 is made of porous ceramic (heat-resistant inorganic substance). The diffusion resistance layer 674 is formed by, for example, a plasma spraying method or the like so as to cover the outer surface of the exhaust gas side electrode layer 672.

  The first wall portion 675 is made of alumina ceramic that is dense and does not allow gas to pass therethrough. The first wall portion 675 is formed so as to cover the diffusion resistance layer 674 except for a corner (a part) of the diffusion resistance layer 674. That is, the first wall portion 675 includes a penetration portion that exposes a part of the diffusion resistance layer 674 to the outside.

  The catalyst part 676 is formed in the penetration part so as to close the penetration part of the first wall part 675. Similar to the upstream catalyst 53, the catalyst unit 676 carries a catalyst material that promotes a redox reaction and an oxygen storage material that exhibits an oxygen storage function. The catalyst portion 676 is a porous body. Accordingly, as indicated by the white arrows in FIGS. 6B and 6C, the exhaust gas (the exhaust gas flowing into the inner protective cover 67c described above) passes through the catalyst portion 676. The exhaust gas reaches the diffusion resistance layer 674, and the exhaust gas further passes through the diffusion resistance layer 674 and reaches the exhaust gas side electrode layer 672.

  The second wall portion 677 is made of alumina ceramic that is dense and does not allow gas to pass therethrough. The second wall portion 677 is configured to form an “atmosphere chamber 67 </ b> A” that is a space for accommodating the atmosphere-side electrode layer 673. The atmosphere is introduced into the atmosphere chamber 67A.

  A power source 679 is connected to the upstream air-fuel ratio sensor 67. The power source 679 applies the voltage V (= Vp) so that the atmosphere side electrode layer 673 side has a high potential and the exhaust gas side electrode layer 672 has a low potential.

  The heater 678 is embedded in the second wall 677. The heater 678 generates heat when energized by an electric control device 70 described later, heats the solid electrolyte layer 671, the exhaust gas side electrode layer 672, and the atmosphere side electrode layer 673, and adjusts the temperatures thereof.

  As shown in FIG. 6B, the upstream side air-fuel ratio sensor 67 having such a structure has the diffusion resistance layer 674 when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio. The oxygen that passes through and reaches the exhaust gas side electrode layer 672 is ionized and passed through the atmosphere side electrode layer 673. As a result, a current I flows from the positive electrode to the negative electrode of the power source 679. As shown in FIG. 7, when the voltage V is set to a predetermined value Vp, the magnitude of the current I is the amount of oxygen that reaches the exhaust gas side electrode layer 672 (oxygen concentration, oxygen partial pressure, ie, exhaust gas empty space). It becomes a constant value proportional to (fuel ratio). The upstream air-fuel ratio sensor 67 outputs a value obtained by converting this current (that is, the limit current Ip) into a voltage as an output value Vabyfs.

On the other hand, as shown in FIG. 6C, when the air-fuel ratio of the exhaust gas is an air-fuel ratio richer than the stoichiometric air-fuel ratio, the upstream air-fuel ratio sensor 67 detects oxygen present in the atmospheric chamber 67A. Is ionized to be led to the exhaust gas side electrode layer 672, and unburned substances (HC, CO, H _{2 and the} like) reaching the exhaust gas side electrode layer 672 through the diffusion resistance layer 674 are oxidized. As a result, a current I flows from the negative electrode of the power source 679 to the positive electrode. As shown in FIG. 7, when the voltage V is set to a predetermined value Vp, the magnitude of the current I is also the amount of unburned matter that has reached the exhaust gas side electrode layer 672 (unburned matter concentration, unburned matter partial pressure). That is, it becomes a constant value proportional to the air-fuel ratio of the exhaust gas). The upstream air-fuel ratio sensor 67 outputs a value obtained by converting this current (that is, the limit current Ip) into a voltage as an output value Vabyfs.

  That is, as shown in FIG. 4, the air-fuel ratio detection unit 67a flows through the position where the upstream air-fuel ratio sensor 67 is disposed, and passes through the inlet hole 67b1 of the outer protective cover 67b and the inlet hole 67c1 of the inner protective cover 67c. An output value Vabyfs corresponding to the air-fuel ratio of the gas passing through and reaching the air-fuel ratio detector 67a is output as an “air-fuel ratio sensor output”. The output value Vabyfs increases as the air-fuel ratio of the gas reaching the air-fuel ratio detection unit 67a increases (lean). That is, as shown in FIG. 8, the output value Vabyfs is substantially proportional to the air-fuel ratio of the exhaust gas that has reached the air-fuel ratio detector 67a. FIG. 8 shows a “relationship between the output value Vabyfs and the air-fuel ratio of exhaust gas” when there is no non-uniformity in the air-fuel ratio for each cylinder. The output value Vabyfs in this case coincides with the stoichiometric air-fuel ratio equivalent value Vstoich when the air-fuel ratio of the gas reaching the air-fuel ratio detection unit 67a is the stoichiometric air-fuel ratio.

  As is clear from the above, “the upstream air-fuel ratio sensor 67 is the exhaust collecting portion HK of the exhaust passage of the engine in which exhaust gas discharged from a plurality of cylinders (at least two, preferably three or more cylinders) collects or the same. An air-fuel ratio sensor disposed in a portion of the exhaust passage downstream of the exhaust collecting portion HK, the solid electrolyte layer 671, the exhaust gas side electrode layer 672 formed on one surface of the solid electrolyte layer 671, and the exhaust gas A diffusion resistance layer 674 that covers the side electrode layer and reaches the exhaust gas, and an air-fuel ratio detection unit that is formed on the other surface of the solid electrolyte layer 671 and that is exposed to the atmosphere chamber 67A and exposed to the atmosphere chamber 67A And an output value Vabyfs (an output value Vabyfs corresponding to the air-fuel ratio of the exhaust gas) corresponding to the “amount of oxygen and unburned matter” contained in the exhaust gas reaching the exhaust gas-side electrode layer 672. Is the difference. Can be said.

  The electric control device 70 stores the air-fuel ratio conversion table (map) Mapabyfs shown in FIG. The electric control device 70 detects the actual upstream air-fuel ratio abyfs (ie, acquires the detected air-fuel ratio abyfs) by applying the output value Vabyfs of the air-fuel ratio sensor 67 to the air-fuel ratio conversion table Mapabyfs. The air-fuel ratio conversion table Mapabyfs is created in advance based on the “relation between the air-fuel ratio of exhaust gas and the output value Vabyfs” when there is no non-uniformity of air-fuel ratio between cylinders.

  By the way, the upstream air-fuel ratio sensor 67 is disposed at a position between the exhaust collecting portion HK and the upstream catalyst 53 as described above. Further, the upstream side air-fuel ratio sensor 67 is disposed so that the outer protective cover 67 b is exposed either in the exhaust manifold 51 or in the exhaust pipe 52.

  More specifically, in the air-fuel ratio sensor 67, as shown in FIGS. 4 and 5, the bottom surface of the protective cover (67b, 67c) is parallel to the flow of the exhaust gas EX, and the protective cover (67b, 67c) The central axis CC is disposed in the exhaust passage so as to be orthogonal to the flow of the exhaust gas EX. As a result, the exhaust gas EX in the exhaust passage that has reached the inflow hole 67b1 of the outer protective cover 67b is caused by the flow of the exhaust gas EX in the exhaust passage flowing in the vicinity of the outflow hole 67b2 of the outer protective cover 67b. It is sucked into the cover 67c.

  Therefore, the exhaust gas EX flowing through the exhaust passage passes through the inflow hole 67b1 of the outer protective cover 67b and is located between the outer protective cover 67b and the inner protective cover 67c as shown by the arrow Ar1 in FIGS. Inflow. Next, the exhaust gas passes through the “inflow hole 67c1 of the inner protective cover 67c” as shown by the arrow Ar2 and then flows into the “inside of the inner protective cover 67c”, and then reaches the air-fuel ratio detection unit 67a. Thereafter, the exhaust gas flows out into the exhaust passage through the “outflow hole 67c2 of the inner protective cover 67c and the outflow hole 67b2 of the outer protective cover 67b” as indicated by an arrow Ar3.

  Therefore, the flow rate of the exhaust gas inside the “outer protective cover 67b and the inner protective cover 67c” is the flow rate of the exhaust gas EX flowing in the vicinity of the outflow hole 67b2 of the outer protective cover 67b (hence, the intake air amount per unit time). It changes according to the air amount Ga). In other words, the time from “when the exhaust gas having a certain air-fuel ratio (first exhaust gas) reaches the inflow hole 67b1” to “when the first exhaust gas reaches the air-fuel ratio detection unit 67a” is equal to the intake air amount Ga. Depends on the engine speed NE. Therefore, the output responsiveness (responsiveness) of the air-fuel ratio sensor 67 to “the air-fuel ratio of the exhaust gas flowing through the exhaust passage” is larger as the flow rate (flow velocity) of the exhaust gas flowing near the outer protective cover 67 b of the air-fuel ratio sensor 67 is larger. That is, the larger the intake air amount Ga, the better. This is also true when the upstream air-fuel ratio sensor 67 has only the inner protective cover 67c.

  Referring to FIG. 3 again, the downstream air-fuel ratio sensor 68 is the exhaust pipe 52 that is downstream of the upstream catalyst 53 and upstream of the downstream catalyst (ie, the upstream catalyst 53 and the downstream side). (Exhaust passage between catalyst). The downstream air-fuel ratio sensor 68 is a known electromotive force type oxygen concentration sensor (a well-known concentration cell type oxygen concentration sensor using stabilized zirconia). The downstream air-fuel ratio sensor 68 generates an output value Voxs corresponding to the air-fuel ratio of the gas to be detected, which is a gas that passes through the exhaust passage and the portion where the downstream air-fuel ratio sensor 68 is disposed. ing. In other words, the output value Voxs is the air-fuel ratio of the gas flowing out from the upstream catalyst 53 and flowing into the downstream catalyst (thus, the temporal average value of the air-fuel ratio of the air-fuel mixture supplied to the engine 10, the true value of the engine 10). The average air-fuel ratio).

  As shown in FIG. 9, the output value Voxs 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. The output value Vabyfs becomes the minimum output value min (for example, about 0.1 V) when the air-fuel ratio of the detected gas is leaner than the stoichiometric air-fuel ratio. Further, the output value Voxs becomes a voltage Vst (intermediate voltage Vst, for example, about 0.5 V) approximately between the maximum output value max and the minimum output value min when the air-fuel ratio of the detected gas is the stoichiometric air-fuel ratio. The 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. Similarly, the output value Voxs suddenly changes from the minimum output value min to the maximum output value max when the air-fuel ratio of the detected gas changes from an air-fuel ratio leaner than the stoichiometric air-fuel ratio to a rich air-fuel ratio.

  The accelerator opening sensor 69 shown in FIG. 3 outputs a signal representing the operation amount Accp (accelerator pedal operation amount Accp) of the accelerator pedal 81 operated by the driver. The accelerator pedal operation amount Accp increases as the operation amount of the accelerator pedal 81 (the opening degree of the accelerator pedal 81) increases.

  The electric control device 70 includes a “CPU 71, a ROM 72 in which a program executed by the CPU 71, tables (maps, functions), constants, and the like are stored in advance, a RAM 73 in which the CPU 71 temporarily stores data as necessary, a backup RAM 74, and an AD converter. It is a well-known microcomputer composed of an interface 75 including

  The backup RAM 74 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 74 stores data (data is written) in accordance with an instruction from the CPU 71 and holds (stores) the data so that the data can be read. Therefore, the backup RAM 74 can hold data even when the operation of the engine 10 is stopped.

  The backup RAM 74 cannot retain data when 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 74 is resumed, the CPU 71 initializes (sets to a default value) data to be held in the backup RAM 74. Note that the backup RAM 74 may be a readable / writable nonvolatile memory such as an EEPROM.

  The interface 75 is connected to the sensors 61 to 69 and supplies signals from these sensors to the CPU 71. Further, the interface 75 is provided with an actuator 33a of the variable intake timing control device 33, an actuator 36a of the variable exhaust timing control device 36, an igniter 38 of each cylinder, and a fuel injection valve provided corresponding to each cylinder in response to an instruction from the CPU 71. 39, a drive signal (instruction signal) is sent to the throttle valve actuator 44a, the heater 678 of the air-fuel ratio sensor 67, and the like.

  The electric control device 70 is configured to send an instruction signal to the throttle valve actuator 44a so that the throttle valve opening degree TA increases as the acquired accelerator pedal operation amount Accp increases. That is, the electric control device 70 changes the opening degree of the “throttle valve 44 disposed in the intake passage of the engine 10” according to the acceleration operation amount (accelerator pedal operation amount Accp) of the engine 10 changed by the driver. Throttle valve drive means is provided.

(About lean miscontrol)
When the air-fuel ratio of the imbalance cylinder shifts to a richer side than the air-fuel ratio of the non-imbalance cylinder, the air-fuel ratio feedback control (main feedback control) based on the output value Vabyfs of the upstream air-fuel ratio sensor 67 The reason why the air-fuel ratio shifts to the lean side has been described above.

That is, 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 as shown in FIG. . Therefore, according to FIG. 2, the total amount SH1 of hydrogen H ₂ contained in the exhaust gas when “only the amount of fuel supplied to the specific cylinder becomes an excess amount by 40%” is SH1 = H3 + H0 + H0 + H0. = H3 + 3 · H0.

  Here, 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. According to this assumption, the air-fuel ratio of the engine when “only the amount of fuel supplied to the specific cylinder is excessive by 40%” is 4 · A0 / (4.4 · F0) = A0 / (1.1 · F0).

On the other hand, according to FIG. 8, 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. The air-fuel ratio of the engine in this case is also 4 · A0 / (4.4 · F0) = A0 / (1.1 · F0). The amount H1 is slightly larger than the amount H0, but both the amount H1 and the amount H0 are very 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).

  As described above, even if the true average air-fuel ratio of the 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 the same as the air-fuel ratio imbalance among cylinders. If not, the total amount SH2 of hydrogen contained in the exhaust gas is significantly larger.

Therefore, when only the amount of fuel supplied to the specific cylinder becomes an excessive amount by 40%, the upstream air-fuel ratio sensor is caused by “selective diffusion of hydrogen H ₂ ” in the diffusion resistance layer 674. The detected air-fuel ratio abyfs expressed by the output value Vabyfs is an air-fuel ratio (smaller air-fuel ratio) that is richer than the “true average air-fuel ratio (A0 / (1.1 · F0)) of the engine 10”.

In other words, even if the average value of the true air-fuel ratio of the exhaust gas is the same, when the air-fuel ratio imbalance among cylinders occurs (when the non-uniformity of the air-fuel ratio among the cylinders is large), the air-fuel ratio cylinder Since the concentration of hydrogen H ₂ in the exhaust gas side electrode layer 672 of the upstream air-fuel ratio sensor 67 is higher than when no imbalance occurs, the output value Vabyfs of the upstream air-fuel ratio sensor 67 is “true exhaust gas true Therefore, the air-fuel ratio is richer than the air-fuel ratio.

  As a result, when the main feedback control is executed to make the air-fuel ratio represented by the output value Vabyfs coincide with the “target air-fuel ratio abyfr set to the reference air-fuel ratio such as the stoichiometric air-fuel ratio”, the true average air-fuel ratio of the engine 10 becomes Therefore, the air-fuel ratio is controlled to be leaner than the reference air-fuel ratio. The first control apparatus and the control apparatus according to another embodiment of the present invention compensate for such a correction to the lean side (lean miscontrol) by correcting the target air-fuel ratio, and thus, nitrogen oxides Reduce the amount of emissions.

  Even when the air-fuel ratio of the imbalance cylinder shifts leaner than the air-fuel ratio of the non-imbalance cylinder, “lean miscontrol” occurs. Such a situation occurs, for example, when the injection characteristic of the fuel injection valve 39 provided for the specific cylinder becomes “a characteristic for injecting a fuel amount considerably smaller than the command fuel injection amount”.

  Now, the amount of fuel supplied to one specific cylinder (for convenience, the first cylinder) is an amount that is too small (ie, 0.6 · F0) by 40%, and the remaining three cylinders ( The amount of fuel supplied to the second, third, and fourth cylinders is assumed to be the amount of fuel (ie, F0) such that the air-fuel ratio of these cylinders matches the stoichiometric air-fuel ratio. . 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 to fourth cylinders 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 air-fuel ratio of the engine 10 is 4 · A0 / (4 · F0) = A0 / F0, that is, the stoichiometric air-fuel ratio.

However, in actuality, “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. H4 is the amount of hydrogen generated when the air-fuel ratio is A0 / (0.7 · F0), and is smaller than H1 and H0 (the amount of hydrogen generated when the air-fuel ratio is the stoichiometric air-fuel ratio) and H0. Is almost equal. Accordingly, the total amount SH3 is substantially (H0 + 3 · H1).

On the other hand, when the air-fuel ratio imbalance among cylinders does not occur and the true average air-fuel ratio of the engine 10 is the stoichiometric air-fuel ratio, “the total amount SH4 of hydrogen H ₂ contained in the exhaust gas” 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).

  Therefore, even when “the air-fuel ratio of the imbalance cylinder shifts leaner than the air-fuel ratio of the non-imbalance cylinder”, the influence of the selective hydrogen diffusion affects the output value Vabyfs of the upstream air-fuel ratio sensor 67. appear. That is, the detected air-fuel ratio abyfs obtained by applying the output value Vabyfs to the air-fuel ratio conversion table Mapabyfs is an air-fuel ratio richer than the true air-fuel ratio of the exhaust gas. As a result, when the main feedback control is further executed, the true average air-fuel ratio of the engine 10 is corrected to be leaner than the reference air-fuel ratio. The first control device and the control device according to another embodiment of the present invention compensate by correcting the target air-fuel ratio, thereby reducing the emission amount of nitrogen oxides.

(Overview of fuel injection amount control)
Next, an overview of fuel injection amount control executed by the first control device will be described.
The first control device increases or decreases the command fuel injection amount so that the detected air-fuel ratio abyfs represented by the output value Vabyfs of the upstream air-fuel ratio sensor 67 matches the target air-fuel ratio (upstream target air-fuel ratio) abyfr. Perform feedback correction (main feedback control).

That is, the first control device calculates a basic fuel injection amount for obtaining the target air-fuel ratio abyfr, calculates a main feedback amount for making the detected air-fuel ratio abyfs coincide with the target air-fuel ratio abyfr, and the main feedback amount Based on this, the basic fuel injection amount is feedback corrected. The target air-fuel ratio abyfr is a reference air-fuel ratio (a predetermined air-fuel ratio within the window of the three-way catalyst (upstream catalyst 53) (
Usually, the stoichiometric air-fuel ratio is set. The basic fuel injection amount becomes the command fuel injection amount in a state where no correction is made. Therefore, it can be said that the main feedback control is control for feedback correction of the commanded fuel injection amount by the main feedback amount.

  When the degree of non-uniformity of the cylinder-by-cylinder air-fuel ratio becomes large, the true average air-fuel ratio of the engine is controlled to be an air-fuel ratio leaner than the target air-fuel ratio abyfr by the lean erroneous control described above. Therefore, if the target air-fuel ratio abyfr is maintained at the reference air-fuel ratio abyfr0 when the degree of non-uniformity of the cylinder-by-cylinder air-fuel ratio becomes large, the true average air-fuel ratio of the engine becomes "the reference air-fuel ratio abyfr0 The lean air-fuel ratio is controlled. As a result, the NOx emission amount increases.

  Therefore, when there is no non-uniformity of the cylinder-by-cylinder air-fuel ratio (that is, when the value of the air-fuel ratio imbalance index value RIMB described later in detail is a reference value), the first control device uses the target air-fuel ratio abyfr as the reference air The fuel ratio is maintained at abyfr0 (for example, the stoichiometric air-fuel ratio stoich). Further, the first control device sets the target air-fuel ratio abyfr to “reference air as the degree of non-uniformity in the air-fuel ratio by cylinder increases (that is, as the air-fuel ratio imbalance index value RIMB increases away from the reference value). The air-fuel ratio is changed to a richer air-fuel ratio (that is, the corrected air-fuel ratio) than the fuel ratio abyfr0. As a result, the true average air-fuel ratio of the engine obtained as a result of the main feedback control is controlled to the reference air-fuel ratio abyfr0 or an air-fuel ratio richer than the reference air-fuel ratio abyfr0 (an air-fuel ratio smaller than the reference air-fuel ratio abyfr0) can do. As a result, it is possible to avoid an increase in NOx emission.

More specifically, the first control device determines “the stoichiometric correction term α as the first correction amount” based on the air-fuel ratio imbalance index value RIMB. The stoichiometric correction term α is 0 or more. The first control device determines “the enrichment correction term β as the second correction amount” based on the air-fuel ratio imbalance index value RIMB. The enrichment correction term β is a value of 0 or more. The first control device calculates the corrected air-fuel ratio γ by correcting (correcting) the reference air-fuel ratio abyfr0 using the stoichiometric correction term α and the enrichment correction term β. That is, the first control device obtains the corrected air-fuel ratio γ, for example, according to the following equation (1). The first control device sets the corrected air-fuel ratio γ as the target air-fuel ratio abyfr.

γ = (abyfr0−α) −β (1)

  The value (abyfr0−α) on the right side of the equation (1) is an air / fuel ratio obtained by correcting the reference air / fuel ratio abyfr0 by the stoichiometric correction term α, and is referred to as a first corrected air / fuel ratio. Therefore, the corrected air-fuel ratio γ is an air-fuel ratio obtained by correcting the first corrected air-fuel ratio (abyfr0−α) by the enrichment correction term β, and is also referred to as a second corrected air-fuel ratio. In other words, the stoichiometric correction term α is a value for obtaining the first corrected air-fuel ratio (abyfr0−α) by correcting the reference air-fuel ratio abyfr0 with the stoichiometric correction term α. Similarly, the enrichment correction term β is used to obtain the second modified air-fuel ratio {(abyfr0-α) -β} by modifying the first modified air-fuel ratio (abyfr0−α) with the enrichment correction term β. Value.

  As described above, the first control device performs the first operation with respect to the “first corrected air-fuel ratio (abyfr0−α) obtained by correcting the reference air-fuel ratio abyfr0 based on the first correction amount (stoichiometric correction term α)”. 2 The second corrected air-fuel ratio {(abyfr0−α) −β} obtained by further correcting using the correction amount (enrichment correction term β) is obtained as “the corrected air-fuel ratio γ”.

  Since the stoichiometric correction term α is a value equal to or greater than 0, the first corrected air-fuel ratio (abyfr0−α) is an air-fuel ratio equal to or lower than the reference air-fuel ratio abyfr0. When the target air-fuel ratio abyfr is the first corrected air-fuel ratio (abyfr0-α), the first average air-fuel ratio (abyfr0-α) is determined by the main feedback control as “the reference air-fuel ratio abyfr0 The air-fuel ratio is determined so as to coincide with "."

  Since the enrichment correction term β is a value equal to or greater than 0, the second corrected air-fuel ratio {(abyfr0−α) −β} is an air-fuel ratio equal to or lower than the first corrected air-fuel ratio (abyfr0−α). The second modified air-fuel ratio {(abyfr0-α) -β} is such that the true average air-fuel ratio of the engine 10 is the main air-fuel ratio when the target air-fuel ratio abyfr is the second modified air-fuel ratio {(abyfr0-α) -β}. The air-fuel ratio is determined to be equal to “the air-fuel ratio below the reference air-fuel ratio abyfr0 (reference air-fuel ratio abyfr0 or an air-fuel ratio richer than the reference air-fuel ratio abyfr0)” by feedback control.

  Thus, the second modified air-fuel ratio {(abyfr0−α) −β} is an air-fuel ratio that matches the true average air-fuel ratio of the engine 10 with the air-fuel ratio equal to or lower than the reference air-fuel ratio abyfr0 “below”. It may be equal to the corrected air-fuel ratio (abyfr0−α). In other words, the enrichment correction term β may be “amount that does not correct the first corrected air-fuel ratio (abyfr0−α)”, that is, “0”.

  For example, when the air-fuel ratio imbalance index value RIMB is “a predetermined value different from the reference value”, the “true average air-fuel ratio of the engine 10” obtained as a result of the main feedback control is made to coincide with the reference air-fuel ratio abyfr0. Therefore, what air-fuel ratio should be set as the target air-fuel ratio abyfr can be obtained by experiments in advance. The obtained target air-fuel ratio abyfr is the first corrected air-fuel ratio (abyfr0−α). Therefore, the relationship between the air-fuel ratio imbalance index value RIMB and the stoichiometric correction term α can be determined in advance.

  In addition, for example, when the air-fuel ratio imbalance index value RIMB is “a predetermined value different from the reference value”, the “true average air-fuel ratio of the engine 10” obtained as a result of the main feedback control is set to “the predetermined rich It is possible to determine in advance by experiment what kind of air-fuel ratio the target air-fuel ratio abyfr should be set to match the air-fuel ratio (for example, the air-fuel ratio that does not increase the NOx emission amount). The obtained target air-fuel ratio abyfr is the second modified air-fuel ratio {(abyfr0−α) −β}. Accordingly, the relationship between the air-fuel ratio imbalance index value RIMB and the enrichment correction term β can be determined in advance based on the target air-fuel ratio abyfr and the stoichiometric correction term α.

  As described above, the first control device sets the target air-fuel ratio abyfr to the corrected air-fuel ratio γ (= (abyfr0−α) −β). Therefore, when the enrichment correction term β is “0” (when the enrichment correction term β is a value that does not correct the first corrected air-fuel ratio (abyfr0−α)), the true average air-fuel ratio of the engine 10 is the reference. It is made to coincide with the air-fuel ratio abyfr0. When the enrichment correction term β is not “0” (when the enrichment correction term β is a value for correcting the first corrected air-fuel ratio (abyfr0−α)), the true average air-fuel ratio of the engine 10 is “reference air-fuel ratio. The air-fuel ratio is richer than abyfr0 and is made to coincide with the air-fuel ratio that is leaner than the corrected air-fuel ratio γ (= (abyfr0−α) −β) that is the second corrected air-fuel ratio. As a result, even when the degree of non-uniformity of the cylinder-by-cylinder air-fuel ratio increases, it is possible to avoid an increase in the NOx emission amount.

The first control device further determines the target air-fuel ratio abyfr (= corrected air-fuel ratio γ) based on the sub-feedback amount KSFB calculated based on the output value of the downstream air-fuel ratio sensor and the intake air amount Ga. And the corrected target air-fuel ratio is set to the final target air-fuel ratio abyfr of the main feedback control. That is, the first control device obtains the final target air-fuel ratio abyfr according to the following equation (2).

abyfr = γ / (1 + KSFB + KCAT) (2)

(Outline of acquisition of air-fuel ratio imbalance index value and air-fuel ratio imbalance determination between cylinders)
Next, acquisition of an air-fuel ratio imbalance index value and air-fuel ratio imbalance determination performed by the first control device will be described.

  The air-fuel ratio imbalance determination between cylinders is a determination for determining whether or not the degree of non-uniformity of the air-fuel ratio has exceeded a warning required value. When the magnitude of the difference between the air-fuel ratio of the imbalance cylinder and the air-fuel ratio of the non-imbalance cylinder (the air-fuel ratio difference for each cylinder) is greater than or equal to “an unacceptable emission level”, the first control device It is determined that an inter-cylinder imbalance state has occurred. The first control device determines whether or not the air-fuel ratio imbalance index value is equal to or greater than the imbalance determination threshold value, and when the air-fuel ratio imbalance index value is equal to or greater than the imbalance determination threshold value, It is determined that an imbalance condition has occurred.

The first control device acquires the air-fuel ratio imbalance index value as follows.
(1) When a predetermined parameter acquisition condition (air-fuel ratio imbalance index value acquisition condition) is satisfied, the first control device indicates “the air-fuel ratio (detected by the output value Vabyfs of the upstream air-fuel ratio sensor 67” “Amount of change per unit time (constant sampling time ts)” of “air-fuel ratio abyfs)” is acquired.

  The “change amount per unit time of the detected air-fuel ratio abyfs” is a differential value (time differential value d (abyfs)) with respect to the time of the detected air-fuel ratio abyfs when the unit time is an extremely short time, for example, about 4 milliseconds. / dt, first-order differential value d (abyfs) / dt). Therefore, “the amount of change in the detected air-fuel ratio abyfs per unit time” is also referred to as “the detected air-fuel ratio change rate ΔAF”. Further, the detected air-fuel ratio change rate ΔAF is also referred to as “basic index amount”.

(2) The first control device obtains an average value AveΔAF of the absolute values | ΔAF | of the plurality of detected air-fuel ratio change rates ΔAF acquired in one unit combustion cycle period. The unit combustion cycle period is a period in which the crank angle required to complete each one combustion stroke elapses in all of the cylinders that exhaust the exhaust gas that reaches one upstream air-fuel ratio sensor 67. The engine 10 of this example is an in-line four-cylinder four-cycle engine, and exhaust gas from the first to fourth cylinders reaches one upstream air-fuel ratio sensor 67. Therefore, the unit combustion cycle period is a period during which the 720 ° crank angle elapses.

(3) The first control device obtains an average value of the average values AveΔAF obtained for each of the plurality of unit combustion cycle periods, and adopts the value as an air-fuel ratio imbalance index value RIMB (parameter for imbalance determination). To do. The air-fuel ratio imbalance index value RIMB is also referred to as an air-fuel ratio imbalance ratio index value between cylinders or an imbalance ratio index value. The air-fuel ratio imbalance index value RIMB is not limited to the value obtained in this way, and can be obtained by various methods to be described later.

  The air-fuel ratio imbalance index value RIMB (a value correlated with the detected air-fuel ratio change rate ΔAF) obtained as described above is “the degree of air-fuel ratio non-uniformity (imbalance) between cylinders, that is, the air-fuel ratio for each cylinder. The value increases as “difference” increases. That is, the air-fuel ratio imbalance index value RIMB is a value that increases as the difference between the air-fuel ratios of the air-fuel mixture supplied to the combustion chambers of the plurality of cylinders (cylinder-specific air-fuel ratio difference) increases. Hereinafter, this reason will be described.

  The exhaust gas from each cylinder reaches the upstream air-fuel ratio sensor 67 in the ignition order (hence, the exhaust order). When there is no cylinder-by-cylinder air-fuel ratio difference (when there is no non-uniformity in the cylinder-by-cylinder air-fuel ratio), the air-fuel ratios of exhaust gases exhausted from each cylinder and reach the upstream air-fuel ratio sensor 67 are substantially the same. Therefore, the detected air-fuel ratio abyfs when there is no cylinder-by-cylinder air-fuel ratio difference changes, for example, as shown by the broken line C1 in FIG. That is, when there is no air-fuel ratio non-uniformity between cylinders, the waveform of the output value Vabyfs of the air-fuel ratio sensor 67 is substantially flat. Therefore, as indicated by the broken line C3 in FIG. 10C, when there is no cylinder-by-cylinder air-fuel ratio difference, the absolute value of the detected air-fuel ratio change rate ΔAF is small.

  On the other hand, if the characteristic of the “fuel injection valve 39 for injecting fuel to a specific cylinder (for example, the first cylinder)” becomes “characteristic for injecting fuel larger than the indicated fuel injection amount”, the air-fuel ratio difference for each cylinder growing. That is, the air-fuel ratio of the exhaust gas of the specific cylinder (the air-fuel ratio of the imbalance cylinder) is greatly different from the air-fuel ratio of the exhaust gas of the cylinders other than the specific cylinder (the air-fuel ratio of the non-imbalance cylinder).

  Therefore, the detected air-fuel ratio abyfs when the air-fuel ratio imbalance among cylinders is occurring varies greatly for each unit combustion cycle period, for example, as shown by the solid line C2 in FIG. Therefore, as indicated by the solid line C4 in FIG. 10C, when the air-fuel ratio imbalance among cylinders is occurring, the absolute value of the detected air-fuel ratio change rate ΔAF becomes large.

  In addition, the absolute value | ΔAF | of the detected air-fuel ratio change rate ΔAF varies greatly as the air-fuel ratio of the imbalance cylinder deviates from the air-fuel ratio of the non-imbalance cylinder. For example, it is assumed that the detected air-fuel ratio abyfs when the magnitude of the difference between the air-fuel ratio of the imbalance cylinder and the air-fuel ratio of the non-imbalance cylinder is the first value changes as indicated by a solid line C2 in FIG. For example, the detected air-fuel ratio abyfs when the difference between the air-fuel ratio of the imbalance cylinder and the air-fuel ratio of the non-imbalance cylinder is “a second value larger than the first value” is shown in FIG. B) It changes like the one-dot chain line C2a.

  Therefore, as shown in FIG. 11, the average value AveΔAF (air-fuel ratio imbalance index value RIMB) in the “plurality of unit combustion cycle periods” of the absolute value | ΔAF | of the detected air-fuel ratio change rate ΔAF is an imbalance cylinder. As the air-fuel ratio deviates from the air-fuel ratio of the non-imbalance cylinder (the degree of non-uniformity of the cylinder-by-cylinder air-fuel ratio increases), the air-fuel ratio increases.

  When acquiring the air-fuel ratio imbalance index value RIMB, the first control device compares the air-fuel ratio imbalance index value RIMB with the imbalance determination threshold value RIMBth. When the air-fuel ratio imbalance index value RIMB is larger than the imbalance determination threshold RIMBth, the first control device determines that an air-fuel ratio imbalance among cylinders has occurred. In contrast, when the air-fuel ratio imbalance index value RIMB is smaller than the imbalance determination threshold value RIMBth, the first control apparatus determines that the air-fuel ratio imbalance among cylinders has not occurred.

(Actual operation)
<Fuel injection amount control>
The CPU 71 of the first control device repeatedly executes the fuel injection control routine shown in FIG. 12 for each cylinder every time the crank angle of any cylinder reaches a predetermined crank angle before the intake top dead center. It has become. The predetermined crank angle is, for example, BTDC 90 ° CA (90 ° crank angle before intake top dead center). A cylinder whose crank angle coincides with the predetermined crank angle is also referred to as a “fuel injection cylinder”. The CPU 71 calculates the commanded fuel injection amount Fi and instructs fuel injection by this fuel injection control routine.

  When the crank angle of an arbitrary cylinder matches the predetermined crank angle before the intake top dead center, the CPU 71 starts processing from step 1200, and in step 1210, a fuel cut condition (hereinafter referred to as “FC condition”) is established. It is determined whether it is established.

  Assume that the FC condition is not satisfied. In this case, the CPU 71 makes a “No” determination at step 1210 to proceed to step 1220 to read the target air-fuel ratio (upstream target air-fuel ratio) abyfr. The target air-fuel ratio abyfr is calculated separately by a routine shown in FIG. Thereafter, the CPU 71 sequentially performs the processing from step 1230 to step 1260 described below, proceeds to step 1295, and once ends this routine.

  Step 1230: The CPU 71 determines that the “fuel injection cylinder” is based on “the intake air amount Ga measured by the air flow meter 61, the engine speed NE acquired based on the signal of the crank position sensor 64, and the lookup table MapMc”. In the one intake stroke, “in-cylinder intake air amount Mc (k)” which is “the amount of air sucked into the fuel injection cylinder” 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 1240: The CPU 71 obtains the basic fuel injection amount Fbase by dividing the in-cylinder intake air amount Mc (k) by the target air-fuel ratio abyfr. In other words, the basic fuel injection amount Fbase is a feedforward amount of the fuel injection amount necessary for calculation in order to obtain the target air-fuel ratio abyfr. That is, in step 1240, based on the intake air amount sucked into the engine 10, the basic fuel injection amount for making the air-fuel ratio (engine air-fuel ratio) of the air-fuel mixture supplied to the engine coincide with the target air-fuel ratio abyfr. Basic fuel injection amount calculating means for calculating Fbase is configured.

  Step 1250: The CPU 71 corrects the basic fuel injection amount Fbase with the main feedback amount DFi. More specifically, the CPU 71 calculates the final command fuel injection amount (final fuel injection amount) Fi by adding the main feedback amount DFi to the basic fuel injection amount Fbase. The main feedback amount DFi is an air-fuel ratio feedback amount that is required to make the detected air-fuel ratio abyfs represented by the output value Vabyfs of the upstream air-fuel ratio sensor 67 coincide with the target air-fuel ratio abyfr. The main feedback amount DFi is separately calculated by a routine shown in FIG.

  Step 1260: The CPU 71 generates an injection instruction signal for injecting the “fuel of the indicated fuel injection amount Fi” obtained in Step 1250 from the “fuel injection valve 39 provided corresponding to the fuel injection cylinder”. It is sent to the fuel injection valve 39. That is, the step 1260 is an injection instruction signal for sending an injection instruction signal to the plurality of fuel injection valves 39 so that an amount of fuel corresponding to the instruction fuel injection amount Fi is injected from the “multiple fuel injection valves 39”. It constitutes sending means.

  If the FC condition is satisfied when the CPU 71 executes the process of step 1210, the CPU 71 determines “Yes” in step 1210 and directly proceeds to step 1295 to end the present routine tentatively. In this case, fuel injection control (fuel supply stop control) is executed because fuel injection by the processing of step 1260 is not executed.

<Calculation of main feedback amount>
The CPU 71 repeatedly executes the “main feedback amount calculation routine” shown in the flowchart of FIG. 13 every elapse of a predetermined time. Therefore, when the predetermined timing comes, the CPU 71 starts processing from step 1300 and proceeds to step 1305 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.
(A1) The upstream air-fuel ratio sensor 67 is activated.
(A2) The engine load KL is equal to or less than the threshold KLth.
(A3) Fuel cut control is not being performed.

Here, the load KL is a load factor obtained by the following equation (3). Instead of the load KL, an accelerator pedal operation amount Accp may be used. 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% (3)

  The description will be continued assuming that the main feedback control condition is satisfied. In this case, the CPU 71 makes a “Yes” determination at step 1305 to sequentially perform the processing from step 1310 to step 1340 described below, and proceeds to step 1395 to end the present routine tentatively.

  Step 1310: The CPU 71 reads the target air-fuel ratio abyfr that is separately calculated by the routine shown in FIG.

Step 1315: The CPU 71 obtains the detected air-fuel ratio abyfs by applying the output value Vabyfs of the upstream air-fuel ratio sensor 67 to the table Mapabyfs shown in FIG. 8, as shown in the following equation (4).

abyfs = Mapabyfs (Vabyfs) (4)

Step 1320: The CPU 71, according to the following equation (5), “in-cylinder fuel supply amount Fc (k−N)” which is “the amount of fuel actually supplied to the combustion chamber 25 at the time N cycles before the current time”. " That is, the CPU 71 divides “the in-cylinder intake air amount Mc (k−N) at a time point N cycles before the current time (ie, N · 720 ° crank angle)” by “the detected air-fuel ratio abyfs”. The in-cylinder fuel supply amount Fc (k−N) is obtained.

Fc (k−N) = Mc (k−N) / abyfs (5)

  As described above, in order to obtain the in-cylinder fuel supply amount Fc (k−N), the in-cylinder intake air amount Mc (k−N) N cycles before the present time is divided by the detected air-fuel ratio abyfs. This is because it takes “a time corresponding to N cycles” until the “exhaust gas generated by the combustion of the air-fuel mixture in the chamber 25” reaches the air-fuel ratio sensor 67.

Step 1325: The CPU 71, according to the following equation (6), “target in-cylinder fuel supply amount Fcr (k) which is“ the amount of fuel that should have been supplied to the combustion chamber 25 at the time N cycles before the current time ”. -N) ". That is, the CPU 71 obtains the target in-cylinder fuel supply amount Fcr (k−N) by dividing the in-cylinder intake air amount Mc (k−N) N cycles before the current time by the target air-fuel ratio abyfr.

Fcr = Mc (k−N) / abyfr (6)

Step 1330: The CPU 71 acquires the in-cylinder fuel supply amount deviation DFc according to the following equation (7). That is, the CPU 71 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 (kN) -Fc (kN) (7)

Step 1335: The CPU 71 obtains the main feedback amount DFi according to the following equation (8). In this equation (6), Gp is a preset proportional gain, and Gi is a preset integral gain. Further, the “value SDFc” in the equation (6) is “an integral value of the in-cylinder fuel supply amount deviation DFc”. That is, the CPU 71 calculates the “main feedback amount DFi” by proportional-integral control for making the detected air-fuel ratio abyfs coincide with the target air-fuel ratio abyfr.

DFi = Gp · DFc + Gi · SDFc (8)

  Step 1340: The CPU 71 adds the in-cylinder fuel supply amount deviation DFc obtained in the above step 1330 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 is obtained. An integral value SDFc is obtained.

  As described above, the main feedback amount DFi is calculated by proportional integral control, and this main feedback amount DFi is reflected in the commanded fuel injection amount Fi by the processing of step 1250 of FIG.

  On the other hand, if the main feedback control condition is not satisfied at the time of determination in step 1305 of FIG. 13, the CPU 71 determines “No” in step 1305 and proceeds to step 1345 to set the value of the main feedback amount DFi to “0”. To "". Next, in step 1350, the CPU 71 stores “0” in the integral value SDFc of the in-cylinder fuel supply amount deviation. Thereafter, the CPU 71 proceeds to step 1395 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.

<Determination of target air-fuel ratio>
The CPU 71 repeatedly executes the “target air-fuel ratio determination (calculation) routine” shown in the flowchart of FIG. 14 every elapse of a predetermined time. Therefore, when the predetermined timing is reached, the CPU 71 starts processing from step 1400 and proceeds to step 1410 to determine whether or not the “main feedback control condition” described above is satisfied.

  The description will be continued assuming that the main feedback control condition is satisfied. In this case, the CPU 71 makes a “Yes” determination at step 1410 to sequentially perform the processing from step 1420 to step 1470 described below, and proceeds to step 1495 to end the present routine tentatively.

  Step 1420: The CPU 71 reads the air-fuel ratio imbalance index value RIMB (the learned value of the air-fuel ratio imbalance index value RIMB) from the backup RAM 74. The air-fuel ratio imbalance index value RIMB is separately calculated by a routine shown in FIG. 16 to be described later, and is stored in the backup RAM 74 each time it is calculated.

  Step 1430: The CPU 71 calculates a stoichiometric correction term α based on the air-fuel ratio imbalance index value RIMB. More specifically, the CPU 71 stores the first correction amount determination table (the relationship between the air-fuel ratio imbalance index value RIMB and the stoichiometric correction term α in the block B1 of FIG. The stoichiometric correction term α is calculated by applying the air-fuel ratio imbalance index value RIMB read in step 1420 to the relationship).

  According to the first correction amount determination table, the stoichiometric correction term α is determined so as to gradually increase from “0” as the air-fuel ratio imbalance index value RIMB increases. As described above, the stoichiometric correction term α is a correction term for changing the target air-fuel ratio abyfr in order to compensate for “lean erroneous control” by the main feedback control (see step 1470 described later). The stoichiometric correction term α is for correcting the target air-fuel ratio abyfr so that the “true average air-fuel ratio of the engine 10 obtained as a result of the main feedback control” matches the “reference air-fuel ratio abyfr0 which is the theoretical air-fuel ratio”. Amount.

  The stoichiometric correction term α is set to “0” (a value that does not correct the target air-fuel ratio) when the air-fuel ratio imbalance index value RIMB is “range from 0 to the first value R1”. It may be determined that the balance index value RIMB gradually increases from “0” as the value RMB becomes larger than the value R1.

  Step 1440: The CPU 71 calculates the enrichment correction term β based on the air-fuel ratio imbalance index value RIMB. More specifically, the CPU 71 reads in the second correction amount determination table (relationship between the air-fuel ratio imbalance index value RIMB and the enrichment correction term β) described in the block B2 of FIG. By applying the air-fuel ratio imbalance index value RIMB, the enrichment correction term β is calculated.

  According to the second correction amount determination table, the enrichment correction term β is determined so as to gradually increase from “0” as the air-fuel ratio imbalance index value RIMB increases. As described above, the enrichment correction term β is a value for calculating the corrected air-fuel ratio γ obtained by correcting the reference air-fuel ratio abyfr0 based on the stoichiometric correction term α and the enrichment correction term β. The true average air-fuel ratio of the engine 10 when the corrected air-fuel ratio γ is set to the target air-fuel ratio and the main feedback control is executed matches the “predetermined air-fuel ratio richer than the reference air-fuel ratio abyfr0”. The amount is as specified.

  The richening correction term β is set to “0” (a value that does not correct the target air-fuel ratio) when the air-fuel ratio imbalance index value RIMB is “range from 0 to the second value R2”. It may be determined that the imbalance index value RIMB gradually increases from “0” as the value RMB becomes larger than the value R2. At this time, the second value R2 is set to a value larger than “the first value R1”.

  Step 1450: The CPU 71 reads the sub feedback amount KSFB. The sub feedback amount KSFB is separately calculated by a routine shown in FIG. The sub feedback amount KSFB may be set to “0”. That is, the first control device does not have to execute the sub feedback control using the sub feedback amount KSFB.

  Step 1460: The CPU 71 calculates a window correction term KCAT based on the intake air amount Ga. The range of the air-fuel ratio in which the purification efficiency of the upstream catalyst 53 is not less than a predetermined value is referred to as a catalyst window. The window of the catalyst becomes gradually smaller from the stoichiometric air-fuel ratio (richer air-fuel ratio) as the intake air amount Ga becomes larger. The window correction term KCAT is a correction term for shifting the target air-fuel ratio abyfr to “an air-fuel ratio that is smaller than the theoretical air-fuel ratio as the intake air amount Ga increases” in consideration of the change in the window of the catalyst. The window correction term KCAT is described in detail in JP-A-2005-48711. The first control device may set the window correction term KCAT to “0”. That is, the first control device may not correct the target air-fuel ratio abyfr by the window correction term KCAT.

Step 1470: The CPU 71 calculates a target air-fuel ratio abyfr according to the following equation (9). In equation (9), abyfr0 is a predetermined air-fuel ratio in the window of the upstream catalyst 53, and is an air-fuel ratio referred to as a reference air-fuel ratio (base target air-fuel ratio). In this example, the reference air-fuel ratio abyfr0 is set to the stoichiometric air-fuel ratio stoich (for example, 14.6).

abyfr = {(abyfr0−α) −β} / (1 + KSFB + KCAT) (9)

  As understood from the equation (9), the target air-fuel ratio abyfr gradually decreases from the reference air-fuel ratio abyfr0 (here, stoichiometric air-fuel ratio stoich) as the stoichiometric correction term α increases. In other words, as the stoichiometric correction term α increases, the target air-fuel ratio abyfr gradually decreases so that the difference between the target air-fuel ratio abyfr0 and the reference air-fuel ratio abyfr0 increases in a range smaller than the reference air-fuel ratio abyfr0.

  As understood from the equation (9), the target air-fuel ratio abyfr gradually decreases as the enrichment correction term β increases. In other words, as the enrichment correction term β increases, the target air-fuel ratio abyfr gradually decreases so that the difference between the target air-fuel ratio abyfr0 and the reference air-fuel ratio abyfr0 increases in a range smaller than the reference air-fuel ratio abyfr0. More specifically, as the enrichment correction term β increases, the second corrected air-fuel ratio {(abyfr0−α) −β} is within a range smaller than the first corrected air / fuel ratio (abyfr0−α). The difference gradually decreases as the difference from the air-fuel ratio (abyfr0−α) increases.

  On the other hand, if the main feedback control condition is not satisfied at the time when the CPU performs the process of step 1410, the CPU 71 determines “No” in step 1410 and proceeds to step 1480. In step 1480, the CPU 71 sets the stoichiometric correction term α to “0” and sets the enrichment correction term β to “0”. Thereafter, the CPU 71 executes processing of step 1450 to step 1470. As a result, the target air-fuel ratio abyfr is corrected only by the sub feedback amount KSFB and the window correction term KCAT.

<Calculation of Sub Feedback Amount KSFB and Sub FB Learning Value KSFBg>
The CPU 71 repeatedly executes the “sub-feedback amount KSFB and sub-FB learning value KSFBg calculation routine” shown in the flowchart of FIG. 15 every elapse of a predetermined time. Therefore, when the predetermined timing comes, the CPU 71 starts processing from step 1500 and proceeds to step 1505 to determine whether or not the enrichment correction term β is equal to or greater than a predetermined threshold (predetermined value) βth. One of the sub-feedback control conditions is that the enrichment correction term β is equal to or greater than a predetermined threshold value βth. The threshold value βth may be “0” or a predetermined value larger than “0”.

  The sub feedback amount KSFB is a correction amount for making the output value Voxs of the downstream air-fuel ratio sensor 68 coincide with the downstream target value Voxsref. The downstream target value Voxsref is set to a value Vst (for example, 0.5 V) corresponding to the reference air-fuel ratio abyfr0 (the stoichiometric air-fuel ratio stoich in this example). On the other hand, the enrichment correction term β is an amount for shifting the true average air-fuel ratio of the engine 10 to an air-fuel ratio smaller (rich) than the reference air-fuel ratio abyfr0. Accordingly, when the enrichment correction term β is equal to or greater than the predetermined threshold value βth, the sub feedback amount KSFB is updated, and the correction of the air-fuel ratio (indicated fuel injection amount) of the air-fuel mixture supplied to the engine 10 by the sub feedback amount KSFB. (Fi correction), the effect of the richening correction term β does not appear. Therefore, when the enrichment correction term β is equal to or greater than the predetermined threshold βth, the update of the sub feedback amount KSFB is stopped (prohibited).

  Now, it is assumed that the enrichment correction term β is smaller than the threshold value βth. In this case, the CPU 71 determines “No” in step 1505 and proceeds to step 1510 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.
(B1) The main feedback control condition is satisfied.
(B2) The downstream air-fuel ratio sensor 68 is activated.

  The description will be continued assuming that the sub-feedback control condition is satisfied. In this case, the CPU 71 makes a “Yes” determination at step 1510 to execute processing (sub feedback amount calculation processing) from step 1515 to step 1535 described below, and then proceeds to step 1540.

Step 1515: The CPU 71 obtains an “output deviation amount DVoxs” that is a difference between the “downstream target value Voxsref” and the “output value Voxs of the downstream air-fuel ratio sensor 68” according to the following equation (10). That is, the CPU 71 obtains the “output deviation amount DVoxs” by subtracting the “current output value Voxs of the downstream air-fuel ratio sensor 68” from the “downstream target value Voxsref”.

DVoxs = Voxsref−Voxs (10)

Step 1520: The CPU 71 sets “the output deviation amount DVoxs obtained in step 1515 and the gain K” to “the integrated value SDVoxs (= SDVoxs (n−1)) of the output deviation amount at that time” according to the following equation (11). Is added to obtain a new output deviation integrated value SDVoxs (= SDVoxs (n)). The gain K is set to “1” here.

SDVoxs (n) = SDVoxs (n−1) + K · DVoxs (11)

  Step 1525: The CPU 71 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 1515”. Find the differential value DDVoxs of the output deviation amount.

Step 1530: The CPU 71 obtains the sub feedback amount KSFB according to the following equation (12). In this equation (12), 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).

KSFB = Kp · DVoxs + Ki · SDVoxs + Kd · DDVoxs (12)

  Step 1535: The CPU 71 stores “the output deviation amount DVoxs calculated in step 1515” as “the previous output deviation amount DVoxsold”.

  In this way, the CPU 71 calculates the “sub feedback amount KSFB” by proportional / integral / differential (PID) control for making the output value Voxs of the downstream air-fuel ratio sensor 68 coincide with the downstream target value Voxsref. This sub-feedback amount KSFB is used to calculate the target air-fuel ratio abyfr, as shown in the above equation (9).

  That is, when the output value Voxs is smaller than the downstream target value Voxsref (when lean), the sub feedback amount KSFB gradually increases. As the sub feedback amount KSFB increases, the target air-fuel ratio abyfr is corrected so as to decrease (becomes a rich-side air-fuel ratio). As a result, the true average air-fuel ratio of the engine 10 becomes small (becomes a rich-side air-fuel ratio), so that the output value Voxs increases so as to coincide with the downstream target value Voxsref.

  Conversely, when the output value Voxs is greater than the downstream target value Voxsref (when rich), the sub feedback amount KSFB gradually decreases. The target air-fuel ratio abyfr is corrected so as to increase (become the lean-side air-fuel ratio) as the sub-feedback amount KSFB decreases. As a result, the true average air-fuel ratio of the engine 10 increases (becomes a lean-side air-fuel ratio), so that the output value Voxs decreases so as to coincide with the downstream target value Voxsref.

  When the CPU 71 proceeds to step 1540, it determines whether or not the first time T1 has elapsed since the last update time of the sub feedback amount learning value (sub FB learning value) KSFBg. At this time, if the first time T1 has not elapsed since the last update of the sub FB learning value KSFBg, the CPU 71 makes a “No” determination at step 1540 to directly proceed to step 1595 to end the present routine tentatively. .

  On the other hand, when the CPU 71 executes the process of step 1540 and the first time T1 has elapsed since the last update of the sub FB learning value KSFBg, the CPU 71 determines “Yes” in step 1540. In step 1545, the product (Ki · SDVoxs) of the integration value SDVoxs and the integration gain Ki at that time is stored in the backup RAM 74 as the sub FB learning value KSFBg. Thereafter, the CPU 71 proceeds to step 1595 to end the present routine tentatively.

  As described above, the CPU 71 takes in the steady term Ki · SDVoxs of the sub feedback amount KSFB at the time when a period longer than the period in which the sub feedback amount KSFB is updated as the sub FB learning value KSFBg.

  On the other hand, when the value of the enrichment correction term β is equal to or greater than the threshold value βth at the time when the CPU 71 executes the process of step 1505, the CPU 71 determines “Yes” in step 1505 and proceeds to step 1550 to perform sub FB learning. The value KSFBg is set as the sub feedback amount KSFB. That is, the CPU 71 stops updating the sub feedback amount KSFB. Next, the CPU 71 proceeds to step 1555 and stores a value obtained by dividing the sub FB learning value KSFBg by the integral gain Ki (sub FB learning value KSFBg / integral gain Ki) in the backup RAM 74 as the integral value SDVoxs. Thereafter, the CPU 71 proceeds to step 1595 to end the present routine tentatively.

  Incidentally, the sub feedback amount KSFB and the sub FB learning value KSFBg should be set to values for making the true average air-fuel ratio of the engine 10 coincide with the reference air-fuel ratio abyfr0. Therefore, the target air-fuel ratio abyfr is corrected to the rich-side air-fuel ratio as the air-fuel ratio imbalance index value RIMB increases without distinguishing between the stoichiometric correction term α and the enrichment correction term β. Then, the updating (calculation) of the sub feedback amount KSFB and the sub FB learning value KSFBg must be stopped based on the target air-fuel ratio abyfr. However, since the target air-fuel ratio abyfr itself is corrected by the sub-feedback amount KSFB or the like, updating (calculation) of the sub-feedback amount KSFB and the sub-FB learning value KSFBg should be stopped when the target air-fuel ratio abyfr is any value. I cannot judge exactly. On the other hand, as described above, the first control device obtains the stoichiometric correction term α and the richening correction term β separately, so that the true average sky of the engine 10 is determined by the value of the richening correction term β. It is possible to easily determine whether or not the fuel ratio is about to be controlled to the reference air-fuel ratio abyfr0 (or an air-fuel ratio within a predetermined value from the reference air-fuel ratio abyfr0). As a result, it is possible to avoid setting the sub FB learning value KSFBg to an incorrect value.

  If the sub feedback control condition is not satisfied at the time when the CPU 71 executes the process of step 1510, the CPU 71 determines “No” in step 1510, and goes to step 1595 via steps 1550 and 1555. move on.

  The output value Voxs of the downstream air-fuel ratio sensor 68 is a value that reflects the true average air-fuel ratio of the engine 10 (thus, “the air-fuel ratio that has been excessively corrected to the lean side” by the main feedback control). This is because a large amount of hydrogen generated when non-uniformity of the air-fuel ratio between the cylinders is purified in the upstream catalyst 53. Therefore, by the sub-feedback control using the sub-feedback amount for making the output value Voxs coincide with the downstream target value Voxsref, the true average air-fuel ratio of the engine 10 becomes “the reference air-fuel ratio abyfr0 in the window of the three-way catalyst 53”. It is corrected to “corresponding value”. Therefore, if the sub-feedback amount has converged to an appropriate value, it is possible to avoid a large amount of NOx emission.

  However, the sub-feedback control is a control that gradually changes the “average of the air-fuel ratio of the engine”. Therefore, in general, the sub feedback amount KSFB is updated so that the target air-fuel ratio abyfr changes slowly. Therefore, for example, after the engine is started, a period in which the sub feedback amount is not an appropriate value occurs. In addition, the degree of “lean erroneous control” varies depending on the operating state of the engine 10 even if the degree of non-uniformity of the air-fuel ratios for each cylinder is “a specific value”. For example, the degree of lean miscontrol increases as the intake air flow rate Ga increases.

  Therefore, when there is non-uniformity of the air-fuel ratio among the cylinders, and during transient operation where the intake air amount changes suddenly (especially increases) after the engine is started, the sub-feedback is performed. The period during which the amount is an inappropriate value becomes longer, and the true average air-fuel ratio of the engine 10 may not be corrected to the reference air-fuel ratio abyfr0.

  On the other hand, the first controller corrects the target air-fuel ratio abyfr using the stoichiometric correction term α. Therefore, even if the sub feedback amount KSFB is not an appropriate value, the true average air-fuel ratio of the engine 10 can be maintained in the vicinity of the reference air-fuel ratio abyfr0. In other words, it can be said that the stoichiometric correction term α is an amount corresponding to the “feed forward amount” of the sub feedback amount KSFB.

<Acquisition of air-fuel ratio imbalance index value and air-fuel ratio imbalance determination between cylinders>
Next, processing for executing “acquisition of air-fuel ratio imbalance index value and determination of air-fuel ratio imbalance among cylinders” will be described. The CPU 71 executes the routine shown by the flowchart in FIG. 16 every time 4 ms (predetermined constant sampling time ts) elapses.

  Accordingly, when the predetermined timing comes, the CPU 71 starts processing from step 1600 and proceeds to step 1610 to determine whether or not the value of the parameter acquisition permission flag Xkyoka is “1”.

  The value of the parameter acquisition permission flag Xkyoka is “1” when a parameter acquisition condition (air-fuel ratio imbalance index acquisition permission condition) described later is satisfied when the absolute crank angle CA becomes 0 ° crank angle. And is immediately set to “0” when the parameter acquisition condition is not satisfied.

  The parameter acquisition condition is satisfied when all of the following conditions (conditions C1 to C5) are satisfied. Accordingly, the parameter acquisition condition is not satisfied when at least one of the following conditions (conditions C1 to C5) is not satisfied. Of course, the conditions constituting the parameter acquisition conditions are not limited to the following conditions C1 to C5.

(Condition C1) The intake air amount Ga acquired by the air flow meter 61 is within a predetermined range. That is, the intake air amount Ga is not less than the low threshold air flow rate GaLoth and not more than the high threshold air flow rate GaHith.
(Condition C2) The engine speed NE is within a predetermined range. That is, the engine rotational speed NE is equal to or higher than the low-side threshold rotational speed NELoth and equal to or lower than the high-side threshold rotational speed NEHith.
(Condition C3) Cooling water temperature THW is equal to or higher than threshold cooling water temperature THWth.
(Condition C4) The main feedback control condition is satisfied.
(Condition C5) Fuel cut control is not being performed.

  Assume that the value of the parameter acquisition permission flag Xkyoka is “1”. In this case, the CPU 71 makes a “Yes” determination at step 1610 to proceed to step 1615 to acquire “the output value Vabyfs of the upstream air-fuel ratio sensor 67 at that time” by AD conversion.

  Next, the CPU 71 proceeds to step 1620 and applies the output value Vabyfs acquired in step 1615 to the air-fuel ratio conversion table Mapabyfs shown in FIG. 8 to acquire the current detected air-fuel ratio abyfs. Note that the CPU 71 stores the detected air-fuel ratio abyfs acquired when this routine was previously executed as the previous detected air-fuel ratio abyfsold before the process of step 1620. That is, the previous detected air-fuel ratio abyfsold is the detected air-fuel ratio abyfs at a time point 4 ms (sampling time ts) before the current time. The initial value of the previous detected air-fuel ratio abyfsold is set to a value corresponding to the theoretical air-fuel ratio in the initial routine. The initial routine is a routine executed by the CPU 71 when the ignition key switch of the vehicle on which the engine 10 is mounted is changed from OFF to ON.

Next, the CPU 71 proceeds to step 1625, and
(A) Obtain the detected air-fuel ratio change rate ΔAF (differential value d (abyfs) / dt),
(B) updating the integrated value SAFD of the absolute value | ΔAF | of the detected air-fuel ratio change rate ΔAF;
(C) Update the integration number counter Cn of the absolute value | ΔAF | of the detected air-fuel ratio change rate ΔAF to the integrated value SAFD.
Hereinafter, these update methods will be described in detail.

(A) Acquisition of detected air-fuel ratio change rate ΔAF.
The detected air-fuel ratio change rate ΔAF (differential value d (abyfs) / dt) is data (basic index amount) that is the original data of the air-fuel ratio imbalance index value RIMB. The CPU 71 acquires the detected air-fuel ratio change rate ΔAF by subtracting the previous detected air-fuel ratio abyfsold from the current detected air-fuel ratio abyfs. In other words, when the detected air-fuel ratio abyfs this time is expressed as abyfs (n) and the previous detected air-fuel ratio abyfsold is expressed as abyfs (n-1), the CPU 71 determines in step 1625 “current detected air-fuel ratio change rate ΔAF (n)”. Is obtained according to the following equation (13).

ΔAF (n) = abyfs (n) −
abyfs (n-1) (13)

(B) Updating the integrated value SAFD of the absolute value | ΔAF | of the detected air-fuel ratio change rate ΔAF.
The CPU 71 obtains the current integrated value SAFD (n) according to the following equation (14). That is, the CPU 71 adds the absolute value | ΔAF (n) | of the detected air-fuel ratio change rate ΔAF (n) calculated this time to the previous integrated value SAFD (n−1) at the time of proceeding to Step 1625. Then, the integrated value SAFD is updated.

SAFD (n) = SAFD (n−1) + | ΔAF (n) | (14)

  The reason for adding “the absolute value of the detected air-fuel ratio change rate | ΔAF (n) |” to the integrated value SAFD is understood from FIGS. 10B and 10C. This is because the rate ΔAF (n) can be a positive value or a negative value. The integrated value SAFD is also set to “0” in the above-described initial routine.

(C) Update of the integration number counter Cn to the integrated value SAFD of the absolute value | ΔAF | of the detected air-fuel ratio change rate ΔAF.
The CPU 71 increases the value of the counter Cn by “1” according to the following equation (15). Cn (n) is the updated counter Cn, and Cn (n−1) is the updated counter Cn. The value of the counter Cn is set to “0” in the above-described initial routine, and is also set to “0” in step 1675 and step 1680 described later. Therefore, the value of the counter Cn indicates the number of data of the absolute value | ΔAF | of the detected air-fuel ratio change rate ΔAF integrated with the integrated value SAFD.

Cn (n) = Cn (n−1) +1 (15)

  Next, the CPU 71 proceeds to step 1630 to determine whether or not the crank angle CA (absolute crank angle CA) based on the compression top dead center of the reference cylinder (first cylinder in this example) is a 720 ° crank angle. judge. At this time, if the absolute crank angle CA is less than the 720 ° crank angle, the CPU 71 makes a “No” determination at step 1630 to directly proceed to step 1695 to end the present routine tentatively.

  Step 1630 is a step of determining a minimum unit period for obtaining an average value of the absolute values | ΔAF | of the detected air-fuel ratio change rate ΔAF. Here, “720 ° crank angle as a unit combustion cycle period” is set. This corresponds to the minimum period. Of course, this minimum period may be shorter than the 720 ° crank angle, but it is desirable that the minimum period be a period more than a multiple of the sampling time ts. Furthermore, it is desirable that the minimum period be a natural number times the unit combustion cycle period.

  On the other hand, if the absolute crank angle CA is 720 ° crank angle when the CPU 71 performs the process of step 1630, the CPU 71 determines “Yes” in step 1630 and proceeds to step 1635.

In step 1635, the CPU 71
(D) calculating an average value AveΔAF of the absolute value | ΔAF | of the detected air-fuel ratio change rate ΔAF;
(E) update the integrated value Save of the average value AveΔAF, and
(F) Update the cumulative number counter Cs.
Hereinafter, these update methods will be described in detail.

(D) Calculation of the average value AveΔAF of the absolute value | ΔAF | of the detected air-fuel ratio change rate ΔAF.
The CPU 71 calculates an average value AveΔAF of the absolute value | ΔAF | of the detected air-fuel ratio change rate ΔAF by dividing the integrated value SAFD by the value of the counter Cn as shown in the following equation (16). Thereafter, the CPU 71 sets the integrated value SAFD and the value of the counter Cn to “0”.

AveΔAF = SAFD / Cn (16)

(E) Update of the integrated value Save of the average value AveΔAF.
The CPU 71 calculates the current integrated value Save (n) according to the following equation (17). That is, the CPU 71 updates the integrated value Save by adding the calculated average value AveΔAF to the previous integrated value Save (n−1) at the time of proceeding to Step 1635. The value of the integrated value Save (n) is set to “0” in the above-described initial routine, and is also set to “0” in step 1675 described later.

Save (n) = Save (n−1) + AveΔAF (17)

(F) Update of the cumulative number counter Cs.
The CPU 71 increases the value of the counter Cs by “1” according to the following equation (18). Cs (n) is the updated counter Cs, and Cs (n−1) is the updated counter Cs. The value of the counter Cs is set to “0” in the above-described initial routine, and is also set to “0” in step 1675 described later. Therefore, the value of the counter Cs indicates the number of data of the average value AveΔAF integrated with the integrated value Save.

Cs (n) = Cs (n−1) +1 (18)

  Next, the CPU 71 proceeds to step 1640 to determine whether or not the value of the counter Cs is greater than or equal to the threshold value Csth. At this time, if the value of the counter Cs is less than the threshold value Csth, the CPU 71 makes a “No” determination at step 1640 to directly proceed to step 1695 to end the present routine tentatively. Note that the threshold Csth is a natural number and is desirably 2 or more.

On the other hand, if the value of the counter Cs is equal to or greater than the threshold value Csth at the time when the CPU 71 performs the process of step 1640, the CPU 71 determines “Yes” in step 1640 and proceeds to step 1645. In step 1645, the CPU 71 divides the integrated value Save by the value of the counter Cs (= Csth) according to the following equation (19), thereby obtaining the air-fuel ratio imbalance index value RIMB (= air-fuel ratio fluctuation index amount AFD). get. The air-fuel ratio imbalance index value RIMB is a value obtained by averaging the average value AveΔAF in each unit combustion cycle period of the absolute value | ΔAF | of the detected air-fuel ratio change rate ΔAF for a plurality of (Csth) unit combustion cycle periods. The air-fuel ratio imbalance index value RIMB is also referred to as an imbalance determination parameter.

RIMB = AFD = Save / Csth (19)

  The air-fuel ratio imbalance index value RIMB is stored (stored) in the backup RAM 74 as a learned value RIMBgaku of the air-fuel ratio imbalance index value RIMB.

  Next, the CPU 71 proceeds to step 1650 to determine whether or not the value of the index value acquisition flag XIMBget is “0”. The index value acquisition flag XIMBget is set to “0” in the above-described initial routine. Therefore, when the air-fuel ratio imbalance index value RIMB is not acquired after the engine 10 is started this time, the value of the index value acquisition flag XIMBget is “0”. In this case, the CPU makes a “Yes” determination at step 1650 to proceed to step 1655 to determine whether or not the air-fuel ratio imbalance index value RIMB is greater than the imbalance determination threshold value RIMBth. That is, the CPU 71 determines whether or not an air-fuel ratio imbalance among cylinders has occurred.

  At this time, if the air-fuel ratio imbalance index value RIMB is larger than the imbalance determination threshold value RIMBth, the CPU 71 determines “Yes” in step 1655 and proceeds to step 1660 to set the value of the imbalance occurrence flag XIMB to “1”. To "". That is, the CPU 71 determines that an air-fuel ratio imbalance among cylinders has occurred. Further, at this time, the CPU 71 may turn on a warning lamp (not shown). The value of the imbalance occurrence flag XIMB is stored in the backup RAM 74. Thereafter, the CPU 71 proceeds to step 1670.

  On the other hand, if the air-fuel ratio imbalance index value RIMB is less than the imbalance determination threshold RIMBth at the time when the CPU 71 performs the process of step 1655, the CPU 71 determines “No” in step 1655 and proceeds to step 1665. Then, the value of the imbalance occurrence flag XIMB is set to “2”. That is, “the air-fuel ratio imbalance among cylinders as a result of the imbalance determination between air-fuel ratios is determined to have been determined not to have occurred” is stored. Thereafter, the CPU 71 proceeds to step 1670.

  Note that the CPU 71 replaces the air-fuel ratio imbalance index value RIMB with the imbalance determination threshold value RIMBth in step 1655 instead of comparing the air-fuel ratio imbalance index value RIMBth with the learning value RIMBgaku and the imbalance determination threshold value RIMBth. May be compared to execute imbalance determination.

  In step 1670, the CPU 71 sets the value of the index value acquisition flag XIMBget to “1”. Next, the CPU proceeds to step 1675 to set “each value used for calculating the air-fuel ratio imbalance index value RIMB (ΔAF, SAFD, Cn, AveΔAF, Save, Cs, etc.)” to “0” ( clear. Thereafter, the CPU 71 proceeds to step 1695 to end the present routine tentatively.

  After this point, when the CPU 71 proceeds to step 1650, it determines “No” in step 1650 and proceeds directly to step 1675. Therefore, the CPU 71 determines whether or not the air-fuel ratio imbalance among cylinders has occurred. The operation of the engine 10 is temporarily stopped and then the engine 10 is started and a new air-fuel ratio imbalance index value RIMB is obtained. Do not run until acquired. However, the CPU 71 repeatedly updates the air-fuel ratio imbalance index value RIMB during one operation from when the engine 10 is started to when it is stopped. The CPU 71 executes step 1655 every time the air-fuel ratio imbalance index value RIMB is acquired, so that the air-fuel ratio imbalance among cylinders during one operation from when the engine 10 is started until it is stopped. It may be repeatedly determined whether or not a state has occurred.

  On the other hand, if the value of the parameter acquisition permission flag Xkyoka is not “1” when the CPU 71 proceeds to step 1610, the CPU 71 determines “No” in step 1610 and proceeds to step 1680. In step 1680, the CPU 71 sets (clears) “each value used to calculate the average value AveΔAF (ΔAF, SAFD, Cn, etc.)” to “0”. Next, the CPU 71 proceeds to step 1695 to end the present routine tentatively.

  The air-fuel ratio imbalance index value RIMB obtained in this way is the detected air-fuel ratio change rate when non-uniformity of the air-fuel ratio by cylinder does not occur (that is, when the air-fuel ratio of all the cylinders is the same). Since ΔAF is “0”, the reference value is “0”.

  As described above, the first control device is applied to the multi-cylinder internal combustion engine 10 having a plurality of cylinders. The engine 10 is arranged corresponding to each of a plurality of cylinders (at least two or more cylinders, preferably three or more cylinders, in this example, four cylinders of the first cylinder # 1 to the fourth cylinder # 4). In addition, there are provided a plurality of fuel injection valves 39 that respectively inject fuel in an amount corresponding to the indicated fuel injection amount Fi, which is fuel contained in the mixture supplied to the respective combustion chambers 25 of the plurality of cylinders.

Furthermore, the first control device
Main for feedback correction of the amount of fuel injected from the fuel injection valve 39 so that the air-fuel ratio (detected air-fuel ratio abyfs) represented by the output value Vabyfs of the upstream air-fuel ratio sensor 67 matches the target air-fuel ratio abyfr By calculating the feedback amount DFi based on the output value Vabyfs of the upstream air-fuel ratio sensor 67 (see the routine of FIG. 13), and executing the main feedback control using the calculated main feedback amount DFi. Commanded fuel injection amount determining means for determining “a commanded fuel injection amount Fi that is a commanded value of the amount of fuel injected from the fuel injection valve 39” (see step 1220 to step 1250 in FIG. 12);
An injection instruction signal sending means (step 1260 in FIG. 12) for sending an injection instruction signal to the plurality of fuel injection valves 39 so that an amount of fuel corresponding to the indicated fuel injection amount Fi is injected from the plurality of fuel injection valves 39. A fuel injection amount control device for an internal combustion engine.

Furthermore, the first control device
When there is no difference between the plurality of cylinders in “the air-fuel ratio of the air-fuel mixture supplied to the respective combustion chambers of the plurality of cylinders”, the predetermined reference value (“0”) is obtained. An imbalance index value acquisition means (see Steps 1610 to 1645 in FIG. 16) for acquiring an air-fuel ratio imbalance index value RIMB that increases as the degree of non-uniformity among the plurality of cylinders of different air-fuel ratios increases. ,
When the target air-fuel ratio abyfr has no difference between the air-fuel ratio imbalance index value RIMB and the reference value (that is, when the air-fuel ratio imbalance index value RIMB is “0”), the upstream side catalyst 53 (three-way) The reference air-fuel ratio abyfr0 in the catalyst) window and the difference between the air-fuel ratio imbalance index value RIMB and the reference value increases (the air-fuel ratio imbalance index value RIMB increases). Target air-fuel ratio determining means for determining the target air-fuel ratio abyfr so that the corrected air-fuel ratio γ decreases in a range smaller than the air-fuel ratio abyfr0 (see step 1420, step 1440 and step 1470 in FIG. 14). ,
Is provided.

Further, the target air-fuel ratio determining means includes
The first correction amount (stoichiometric correction term α) and the second correction amount (riching correction term β) are calculated based on the air-fuel ratio imbalance index value RIMB, and the reference air-fuel ratio abyfr0 is corrected based on the first correction amount. The second corrected air-fuel ratio {(abyfr0-α) -β} obtained by further correcting the first corrected air-fuel ratio (abyfr0-α) obtained by performing the second correction amount is corrected. The rear air-fuel ratio γ is calculated.

  As a result, the true average air-fuel ratio of the engine 10 is controlled to an air-fuel ratio below the reference air-fuel ratio abyfr0 even when the degree of non-uniformity of the cylinder-by-cylinder air-fuel ratio becomes large. An increase in the amount can be avoided.

<Second Embodiment>
Next, a control device (hereinafter simply referred to as “second control device”) according to a second embodiment of the present invention will be described. The second control device determines the first correction amount (stoichiometric correction term α) and the second correction amount (riching correction term β) depending on the air-fuel ratio imbalance index value RIMB, the engine load such as the intake air amount Ga, and the like. It differs from the first control device only in that it is determined based on the first value.

(Actual operation)
The CPU 71 of the second control device executes the routine shown in FIG. 17 when proceeding to step 1430 of FIG.

  More specifically, when the processing of step 1420 in FIG. 14 ends, the CPU 71 proceeds to step 1710 in FIG. 17 and calculates a base stoichiometric correction term α0 based on the intake air amount Ga. More specifically, the CPU 71 sets the first corrected base amount determination table (the relationship between the intake air amount Ga and the base stoichiometric correction term α0 described in step 1710 in FIG. The base stoichiometric correction term α0 is calculated by applying the actual intake air amount Ga to the above relationship. The base stoichiometric correction term α0 is calculated so as to increase as the intake air amount Ga increases.

  Next, the CPU 71 proceeds to step 1720 to calculate the first reflection rate Ka based on the air-fuel ratio imbalance index value RIMB. More specifically, the CPU 71 stores the first reflection rate determination table (the relationship between the air / fuel ratio imbalance index value RIMB and the first reflection rate Ka in the step 1720 of FIG. The first reflection rate Ka is calculated by applying the actual air-fuel ratio imbalance index value RIMB to the relationship). The first reflection rate Ka is calculated so as to increase as the air-fuel ratio imbalance index value RIMB increases.

  Next, the CPU 71 proceeds to step 1730 to calculate the stoichiometric correction term α by multiplying the base stoichiometric correction term α0 by the first reflection rate Ka. Thereafter, the CPU 71 proceeds to step 1440 in FIG.

  Further, when the CPU 71 proceeds to step 1440 in FIG. 14, the CPU 71 proceeds to step 1810 in FIG. 18 and calculates the base enrichment correction term β0 based on the intake air amount Ga. More specifically, the CPU 71 stores the second corrected base amount determination table (the relationship between the intake air amount Ga and the base enrichment correction term β0 described in step 1810 in FIG. The base enrichment correction term β0 is calculated by applying the actual intake air amount Ga to the above relationship. The base enrichment correction term β0 is calculated so as to increase as the intake air amount Ga increases.

  Next, the CPU 71 proceeds to step 1820 to calculate the second reflection rate Kb based on the air-fuel ratio imbalance index value RIMB. More specifically, the CPU 71 stores the second reflection rate determination table (the relationship between the air / fuel ratio imbalance index value RIMB and the second reflection rate Kb in the step 1820 in FIG. The second reflection rate Kb is calculated by applying the actual air-fuel ratio imbalance index value RIMB to the relationship). The second reflection rate Kb is calculated so as to increase as the air-fuel ratio imbalance index value RIMB increases.

  Next, the CPU 71 proceeds to step 1830 to calculate the stitch correction term β by multiplying the base enrichment correction term β0 by the second reflection rate Kb. Thereafter, the CPU 71 proceeds to step 1450 in FIG.

  As described above, according to the second control device, the stoichiometric correction term α is calculated so as to increase as the intake air amount Ga increases and the air-fuel ratio imbalance index value RIMB increases. In other words, the first corrected air-fuel ratio (abyfr0−α) is so small that the difference from the reference air-fuel ratio abyfr0 becomes larger as the intake air amount Ga is larger and the air-fuel ratio imbalance index value RIMB is larger. Become.

  The degree of “lean erroneous control” increases as the intake air flow rate Ga increases, for example, even if the degree of air-fuel ratio non-uniformity between cylinders is a specific value. Therefore, the target air-fuel ratio abyfr is set to the first corrected air-fuel ratio (abyfr0−α) by determining the stoichiometric correction term α as the first correction amount based on the intake air amount Ga as in the second control device. In this case, the true average air-fuel ratio of the engine 10 obtained by the main feedback control can be controlled more reliably to the vicinity of the reference air-fuel ratio abyfr0.

  In addition, according to the second control device, the enrichment correction term β is calculated so as to increase as the intake air amount Ga increases and the air-fuel ratio imbalance index value RIMB increases. In other words, the second corrected air-fuel ratio {(abyfr0−α) −β} becomes smaller as the intake air amount Ga is larger and the air-fuel ratio imbalance index value RIMB is larger.

  As described above, the degree of “lean erroneous control” increases as the intake air amount of the engine increases, for example, even if the degree of non-uniformity of the air-fuel ratio between the cylinders is a specific value. Therefore, as in the second control device, the enrichment correction term β as the second correction amount is determined based on the intake air amount Ga, so that the true average air-fuel ratio of the engine 10 becomes “richer than the reference air-fuel ratio. To the vicinity of the target air-fuel ratio on the side.

  The second control device uses the table that defines the relationship between the combination of the air-fuel ratio imbalance index value RIMB and the intake air amount Ga and the stoichiometric correction term α to the actual air-fuel ratio imbalance index value RIMB and the actual air-fuel ratio imbalance index value RIMB. The stoichiometric correction term α may be calculated by applying the intake air amount Ga.

  Further, the second control device may calculate the stoichiometric correction term α based also on the engine rotational speed NE. For example, the second control device uses the table that defines the relationship between the combination of the engine rotational speed NE and the intake air amount Ga and the base stoichiometric correction term α0 to the actual engine rotational speed NE and the actual intake air amount Ga. May be calculated, and the stoichiometric correction term α may be calculated by multiplying the base stoichiometric correction term α0 by the first reflection factor Ka.

  In addition, the second control device replaces the intake air amount Ga used when obtaining the stoichiometric correction term α with parameters (for example, throttle valve opening TA, accelerator pedal operation amount Accp, and engine Load factor KL, etc.) may be used.

  In addition, the second control device sets the actual air-fuel ratio imbalance index value RIMB and the table including the relationship between the combination of the air-fuel ratio imbalance index value RIMB and the intake air amount Ga and the enrichment correction term β. The enrichment correction term β may be calculated by applying the actual intake air amount Ga.

  Further, the second control device may calculate the enrichment correction term β based also on the engine speed NE. For example, the second control device sets the actual engine speed NE and the actual intake air amount in a table that defines the relationship between the combination of the engine speed NE and the intake air amount Ga and the base enrichment correction term β0. The base enrichment correction term β0 may be calculated by applying Ga, and the enrichment correction term β may be calculated by multiplying the base enrichment correction term β0 by the second reflection rate Kb.

  In addition, the second control device replaces the intake air amount Ga used when obtaining the enrichment correction term β with parameters (for example, throttle valve opening TA, accelerator pedal operation amount Accp, and engine) that correlate with the intake air amount Ga. Load factor KL, etc.) may be used.

  As described above, the embodiment according to the present invention can avoid an increase in the NOx emission amount even when the degree of non-uniformity of the cylinder-by-cylinder air-fuel ratio increases. In addition, this invention is not limited to the said embodiment, A various modification can be employ | adopted within the scope of the present invention.

For example, the sub feedback amount KSFB is an amount that directly corrects the target air-fuel ratio abyfr as shown in the equation (9) and step 1470 in FIG. 14, but is detected in step 1315 in FIG. It may be a value for correcting the output value Vabyfs (that is, a value applied to the table Mapabyfs shown in FIG. 8) of the upstream air-fuel ratio sensor 67 which is the basis of the air-fuel ratio abyfs. That is, if the “value applied to the table Mapabyfs to obtain the detected air-fuel ratio abyfs in step 1315” is expressed as a control output value Vabyfsc, the sub feedback amount KSFB corrects the output value Vabyfs as shown in the following equation (20). It may be a value. Such a sub feedback amount KSFB is an amount that substantially corrects the target air-fuel ratio abyfr in the main feedback control.

Vabyfsc = Vabyfs + KSFB (20)

Further, the method for calculating the target air-fuel ratio abyfr is not limited to the above equation (9). For example, the target air-fuel ratio abyfr may be obtained according to any one of the following formulas (21) to (27) or other formulas.

abyfr = (abyfr0−α) −β (21)

abyfr = abyfr / (1 + α + β) (22)

abyfr = abyfr / (1 + KSFB + KCAT + α + β) (23)

abyfr = (abyfr0 · α) / (1 + β) (24)

abyfr = (abyfr0 · α) / (1 + KSFB + KCAT · β) (25)

abyfr = (abyfr0−α) / (1 + β) (26)

abyfr = (abyfr0−α) / (1 + KSFB + KCAT + β) (27)

  Further, the main feedback control (and sub feedback control) may be executed by the methods described in, for example, Japanese Patent Application Laid-Open No. 2008-128022 and Japanese Patent Application Laid-Open No. 2008-215106. In this case, the sub feedback amount Fisub is preferably configured to correct the upstream target air-fuel ratio abyfr. In addition, the upstream air-fuel ratio sensor may be an “electromotive force type oxygen concentration sensor similar to the downstream air-fuel ratio sensor 68”.

  Further, the imbalance index value acquisition means for acquiring the air-fuel ratio imbalance index value RIMB may be as follows.

(A-1)
The imbalance index value acquisition means includes
The differential value d (Vabyfs) / dt with respect to the time of the output value Vabyfs of the upstream air-fuel ratio sensor 67 is acquired, and a value correlated with the acquired differential value d (Vabyfs) / dt is set as an air-fuel ratio imbalance index value RIMB. It can be configured to obtain.

  An example of a value correlated with the acquired differential value d (Vabyfs) / dt is the average of the absolute values of the differential value d (Vabyfs) / dt acquired in a unit combustion cycle or a period that is a natural number times the unit combustion cycle. Value. This average value can be obtained by a routine similar to the routine of FIG. Another example of a value that correlates with the acquired differential value d (Vabyfs) / dt is the maximum absolute value of the differential value d (Vabyfs) / dt acquired in the unit combustion cycle. Is an averaged value.

(A-2)
The imbalance index value acquisition means includes
The differential value d (abyfs) / dt with respect to the time of the detected air-fuel ratio abyfs represented by the output value Vabyfs of the upstream side air-fuel ratio sensor 67 is acquired and correlated with the acquired differential value d (abyfs) / dt value The value may be obtained as an air-fuel ratio imbalance index value RIMB. An example of this is shown in FIG.

  An example of a value correlated with the acquired differential value d (abyfs) / dt value is an absolute value of the differential value d (abyfs) / dt acquired in a unit combustion cycle or a period that is a natural number times the unit combustion cycle. Average value (see routine in FIG. 16). Another example of a value that correlates with the acquired differential value d (abyfs) / dt is the maximum absolute value of the differential value d (abyfs) / dt acquired in the unit combustion cycle. Is an averaged value.

(A-3)
The imbalance index value acquisition means includes
The second-order differential value d ² (Vabyfs) / dt ² for the time of the output value Vabyfs of the upstream air-fuel ratio sensor 67 is acquired, and a value correlated with the acquired second-order differential value d ² (Vabyfs) / dt ² is empty. It may be configured to obtain the fuel ratio imbalance index value RIMB. Since the output value Vabyfs and the detected air-fuel ratio abyfs are substantially proportional (see FIG. 8), the second-order differential value d ² (Vabyfs) / dt ² is the second-order differential value with respect to the time of the detected air-fuel ratio abyfs. It shows the same tendency as d ² (abyfs) / dt ² . Therefore, the second-order differential value d ² (Vabyfs) / dt ² becomes a relatively small value as shown by the broken line C5 in FIG. When the difference is large, the value is relatively large as shown by a solid line C6 in FIG.

The second-order differential value d ² (Vabyfs) / dt ² is obtained by subtracting the output value Vabyfs before a certain sampling time from the current output value Vabyfs to obtain the differential value d (Vabyfs) / dt at a certain sampling time. It can be obtained by subtracting the differential value d (Vabyfs) / dt before a certain sampling time from the newly obtained differential value d (Vabyfs) / dt.

An example of a value correlated with the acquired second order differential value d ² (Vabyfs) / dt ² value is a unit combustion cycle or a plurality of second order differential values d ² (Vabyfs) / is the mean value of the absolute value of dt ^2. Another example of the obtained second-order differential value d ² (Vabyfs) value correlated with / dt ² values, the maximum absolute value of the second-order differential value d ² (Vabyfs) / dt ² values plurality obtained in unit combustion cycle The value is an averaged value for a plurality of unit combustion cycles.

(A-4)
The imbalance index value acquisition means includes
The second-order differential value d ² (abyfs) / dt ² for the time of the detected air-fuel ratio abyfs expressed by the output value Vabyfs of the upstream air-fuel ratio sensor 67 is acquired, and the acquired second-order differential value d ² (abyfs) / A value correlated with dt ² may be obtained as the air-fuel ratio imbalance index value RIMB. The second-order differential value d ² (abyfs) / dt ² becomes a relatively small value as shown by the broken line C5 in FIG. 10D when the cylinder-by-cylinder air-fuel ratio difference is small, and the cylinder-by-cylinder air-fuel ratio difference becomes smaller. If it is large, it becomes a relatively large value as shown by the solid line C6 in FIG.

The second order differential value d ² (abyfs) / dt ² is obtained by subtracting the detected air-fuel ratio change rate ΔAF obtained before a certain sampling time from the detected air-fuel ratio change rate ΔAF obtained in step 1625 of FIG. It can ask for.

An example of a value correlated with the acquired second order differential value d ² (abyfs) / dt ² value is a unit combustion cycle or a plurality of second order differential values d ² (abyfs) / is the mean value of the absolute value of dt ^2. Another example of the obtained second-order differential value d ² (abyfs) value correlated with / dt ² is the maximum value of the absolute values of the plurality obtained second-order differential value d ² (abyfs) / dt ² in the unit combustion cycle These are average values for a plurality of unit combustion cycles.

For each of `` differential value d (Vabyfs) / dt, differential value d (abyfs) / dt, second order differential value d ² (Vabyfs) / dt ² , and second order differential value d ² (abyfs) / dt ² '' Although the correlated values are affected by the intake air amount Ga, they are hardly affected by the engine rotational speed NE. This is because the flow rate of the exhaust gas inside the “outer protective cover 67b and inner protective cover 67c of the upstream air-fuel ratio sensor 67” is the flow rate of the exhaust gas EX flowing in the vicinity of the outflow hole 67b2 of the outer protective cover 67b (accordingly, per unit time). This is because it changes according to the intake air amount Ga). Therefore, since the air-fuel ratio imbalance index value RIMB obtained based on these values accurately represents the cylinder-by-cylinder air-fuel ratio difference without being affected by the engine speed NE, the stoichiometric correction term α and the enrichment correction term β Is a preferable parameter for calculating.

(A-5)
The imbalance index value acquisition means includes
A value that correlates with the difference ΔX between the maximum value and the minimum value in a predetermined period (for example, a period that is a natural number multiple of the unit combustion cycle period) of the output value Vabyfs of the upstream air-fuel ratio sensor 67, or the upstream air-fuel ratio sensor 67 A value correlated with the difference ΔY between the maximum value and the minimum value of the detected air-fuel ratio abyfs expressed by the output value Vabyfs in a predetermined period may be acquired as the air-fuel ratio imbalance index value RIMB. As is apparent from the solid line C2 and the broken line C1 shown in FIG. 10B, the difference ΔY (the absolute value of ΔY) increases as the cylinder-by-cylinder air-fuel ratio difference increases. Therefore, the difference ΔX (the absolute value of ΔX) increases as the cylinder-by-cylinder air-fuel ratio difference increases. An example of a value correlated with the difference ΔX (or the difference ΔY) is an average value of absolute values of a plurality of differences ΔX (or ΔY) acquired in a unit combustion cycle or a period that is a natural number times the unit combustion cycle.

(A-6)
The imbalance index value acquisition means includes
As the air-fuel ratio imbalance index value RIMB, a value correlated with the locus length of the output value Vabyfs of the upstream air-fuel ratio sensor 67 in a predetermined period or the detected air-fuel ratio abyfs expressed by the output value Vabyfs of the upstream air-fuel ratio sensor 67 A value that correlates to the trajectory length in the predetermined period of time may be acquired. As is clear from FIG. 10B, these trajectory lengths increase as the air-fuel ratio difference for each cylinder increases. The value correlated with the trajectory length is, for example, an average value of absolute values of trajectory lengths acquired in a unit combustion cycle or a period that is a natural number times the unit combustion cycle.

  For example, the trajectory length of the detected air-fuel ratio abyfs acquires the output value Vabyfs every time the fixed sampling time ts elapses, converts the output value Vabyfs to the detected air-fuel ratio abyfs, and the detected air-fuel ratio abyfs The absolute value of the difference between the detected air-fuel ratio abyfs acquired before the certain sampling time ts can be obtained by integration.

(B) The imbalance index value acquisition means includes:
A value (rotational fluctuation correlation value) that increases as the rotational speed fluctuation of the engine 10 increases may be acquired as the air-fuel ratio imbalance index value. The rotational fluctuation correlation value may be, for example, an average value in a unit combustion cycle of a plurality of absolute values of the change amount ΔNE of the engine rotational speed NE for each constant sampling and the absolute value of the change amount ΔNE.

  In addition, each control device described above can also be applied to a V-type engine. 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. And a catalyst disposed at a site downstream of the exhaust collecting portion where the exhaust gas discharged from the combustion chamber collects. Further, the V-type engine has a left bank upstream side catalyst (at least two or more cylinders of the plurality of cylinders in the exhaust passage of the engine) downstream of an exhaust collecting portion of two or more cylinders belonging to the left bank. And a catalyst disposed at a site downstream of the exhaust collecting portion where exhaust gases discharged from the combustion chambers of the remaining two or more cylinders other than the above are gathered.

  In addition, 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, respectively, and a left bank upstream and downstream of the left bank upstream catalyst. The upstream air-fuel ratio sensor and the downstream air-fuel ratio sensor can be provided. Each upstream air-fuel ratio sensor, like the air-fuel ratio sensor 67, is disposed between the exhaust collection portion of each bank and the upstream catalyst of each bank. 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.

  In this case, the control device obtains the “right-bank air-fuel ratio imbalance index value RIMB” based on the output value of the upstream-side air-fuel ratio sensor for the right bank, and uses this to determine the main feedback for the cylinders belonging to the right bank. Correct the control target air-fuel ratio abyfr. Similarly, the control unit obtains the “left bank air-fuel ratio imbalance index value RIMB” based on the output value of the upstream bank air-fuel ratio sensor for the left bank, and uses it to perform main feedback to the cylinders belonging to the left bank. Correct the control target air-fuel ratio abyfr.

  In addition, in the control device according to the above-described embodiment, when the air-fuel ratio of the imbalance cylinder shifts to the rich side from the stoichiometric air-fuel ratio stoich, the air-fuel ratio of the imbalance cylinder to the lean side from the stoichiometric air-fuel ratio stoich. The “stoichiometric correction term α and the enrichment correction term β” were calculated according to the air-fuel ratio imbalance index value RIMB without distinguishing from the case of shifting. This is because, in any of the cases, if the air-fuel ratio imbalance index value RIMB is the same, the degree of lean miscontrol is the same.

  In contrast, when the air-fuel ratio of the imbalance cylinder shifts to a richer side than the stoichiometric air-fuel ratio stoich, the stoichiometric correction term α and the enrichment correction term β and the air-fuel ratio of the imbalance cylinder are the stoichiometric air-fuel ratio stoich. Alternatively, the “stoichiometric correction term α and the richening correction term β” when shifted to the lean side may be calculated independently of each other.

  Whether the air-fuel ratio of the imbalance cylinder is shifted to the rich side from the stoichiometric air-fuel ratio stoich or whether it is shifted to the lean side from the stoichiometric air-fuel ratio stoich can be determined based on the rotational fluctuation. Good (rotational fluctuation when the air-fuel ratio of the imbalance cylinder shifts leaner than the stoichiometric air-fuel ratio stoich, when the air-fuel ratio of the imbalance cylinder shifts richer than the stoichiometric air-fuel ratio stoich Or can be determined as follows.

The CPU obtains an average value PAF of “a differential value d (abyfsvir) / dt that is a positive value” among the differential values d (abyfsvir) / dt in a unit combustion cycle.
The CPU obtains an absolute value average value NAF of “negative differential value d (abyfsvir) / dt” in the unit combustion cycle among the differential values d (abyfsvir) / dt.
If the average value NAF is larger than the average value PAF, the CPU determines that the air-fuel ratio of the imbalance cylinder is shifted to the rich side with respect to the stoichiometric air-fuel ratio stoich.
If the average value NAF is smaller than the average value PAF, the CPU determines that the air-fuel ratio of the imbalance cylinder is shifted leaner than the stoichiometric air-fuel ratio stoich.

Explanation of sign

  DESCRIPTION OF SYMBOLS 10 ... Multi-cylinder internal combustion engine, 25 ... Combustion chamber (cylinder), 39 ... Fuel injection valve, 51 ... Exhaust manifold, 51b ... Collecting part (exhaust collecting part HK), 52 ... Exhaust pipe, 53 ... Upstream catalyst (three-way) Catalyst), 67 ... upstream air-fuel ratio sensor, 67a ... air-fuel ratio detector, 671 ... solid electrolyte layer, 672 ... exhaust gas side electrode layer, 673 ... air side electrode layer, 674 ... diffusion resistance layer (porous layer), 68 ... downstream air-fuel ratio sensor, 70 ... electric control device.

Claims (4)

A three-way catalyst disposed at a position downstream of the exhaust collecting portion of the exhaust passage of the engine in which exhaust gases discharged from a plurality of cylinders of the multi-cylinder internal combustion engine gather;
A plurality of fuel injection valves, each of which is configured to inject fuel contained in an air-fuel mixture supplied to each combustion chamber of the plurality of cylinders;
Exhaust gas disposed in the exhaust passage at a position between the exhaust collecting portion and the three-way catalyst, and disposed so as to face the air-fuel ratio detection element and the air-fuel ratio detection element therebetween An air-fuel ratio sensor having a side electrode layer, a reference gas side electrode layer, and a porous layer covering the exhaust gas side electrode layer, wherein the porous gas in the exhaust gas passing through a position where the air fuel ratio sensor is disposed An upstream air-fuel ratio sensor that outputs an output value corresponding to the amount of oxygen and the amount of unburned matter contained in the exhaust gas that has reached the exhaust gas side electrode layer through the catalyst layer;
A main feedback amount for feedback correcting 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 target air-fuel ratio is the upstream air-fuel ratio. An instruction fuel injection amount that is an instruction value of the amount of fuel injected from the fuel injection valve by performing main feedback control using the calculated main feedback amount while calculating based on the output value of the sensor. Instruction fuel injection amount determination means for determining;
Injection instruction signal sending means for sending an injection instruction signal to the plurality of fuel injection valves so that an amount of fuel corresponding to the instruction fuel injection amount is injected from the plurality of fuel injection valves;
A fuel injection amount control device for an internal combustion engine comprising:
When there is no difference between the plurality of cylinders in the air-fuel ratio of each cylinder, which is the air-fuel ratio of the air-fuel mixture supplied to the respective combustion chambers of the plurality of cylinders, the predetermined reference value is obtained. An imbalance index value acquisition means for acquiring an air-fuel ratio imbalance index value that increases as the degree of non-uniformity among the cylinders increases.
The target air-fuel ratio becomes a predetermined reference air-fuel ratio in the window of the three-way catalyst when there is no difference between the air-fuel ratio imbalance index value and the reference value, and the air-fuel ratio imbalance index value and the reference Target air-fuel ratio determining means for determining the target air-fuel ratio so that the corrected air-fuel ratio becomes smaller in the range smaller than the reference air-fuel ratio as the magnitude of the difference from the value increases;
With
The target air-fuel ratio determining means includes
A first correction amount and a second correction amount are calculated based on the air-fuel ratio imbalance index value, and the first correction air-fuel ratio obtained by correcting the reference air-fuel ratio based on the first correction amount is calculated. A second corrected air-fuel ratio obtained by further correcting using the second correction amount is calculated as the corrected air-fuel ratio,
The first correction amount is a combustion chamber of the plurality of cylinders when the first correction air-fuel ratio is an air-fuel ratio equal to or lower than the reference air-fuel ratio and the target air-fuel ratio is the first correction air-fuel ratio. A value that corrects the reference air-fuel ratio so that the true average air-fuel ratio of the engine that is the true average air-fuel ratio of the air-fuel mixture supplied to Yes,
The second correction amount is obtained when the second correction air-fuel ratio is an air-fuel ratio equal to or lower than the first correction air-fuel ratio and the target air-fuel ratio is the second correction air-fuel ratio. The first air-fuel ratio is a value that corrects the first air-fuel ratio so that the average air-fuel ratio becomes an air-fuel ratio that is equal to or lower than the reference air-fuel ratio by the main feedback control
A fuel injection amount control device for an internal combustion engine. The fuel injection amount control apparatus for an internal combustion engine according to claim 1,
The target air-fuel ratio determining means includes
A downstream air-fuel ratio sensor that is disposed at a position downstream of the three-way catalyst in the exhaust passage and outputs an output value corresponding to an air-fuel ratio of exhaust gas that passes through the disposed position;
Sub-feedback for correcting the target air-fuel ratio based on the output value of the downstream air-fuel ratio sensor so that the output value of the downstream air-fuel ratio sensor matches the downstream target value corresponding to the reference air-fuel ratio. Sub feedback control means for calculating the amount and correcting the target air-fuel ratio based on the calculated sub feedback amount;
Whether the second corrected air-fuel ratio is a specific air-fuel ratio for making the true average air-fuel ratio of the engine coincide with an air-fuel ratio in a range smaller than the reference air-fuel ratio by a positive threshold air-fuel ratio or more. Sub-feedback for determining based on the second correction amount and stopping the calculation of the sub-feedback amount by the sub-feedback control means when it is determined that the second correction air-fuel ratio is the specific air-fuel ratio. A quantity calculation stop means;
A fuel injection amount control device further comprising: The fuel injection amount control device for an internal combustion engine according to claim 1 or 2,
The target air-fuel ratio determining means includes
The first correction amount is correlated with the intake air amount so that the first correction amount becomes a value that corrects the first correction air-fuel ratio to a smaller air-fuel ratio as the intake air amount of the engine increases. A fuel injection amount control device configured to be determined based on a parameter. The fuel injection amount control device for an internal combustion engine according to claim 1 or 2,
The target air-fuel ratio determining means includes
The second correction amount is correlated with the intake air amount so that the second correction amount becomes a value that corrects the second corrected air-fuel ratio to a smaller air-fuel ratio as the intake air amount of the engine increases. A fuel injection amount control device configured to determine based on a parameter.

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