JP2016044576A - Cylinder air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cylinder air-fuel ratio control device for an internal combustion engine, which is enabled to suppress the increase in the air-fuel ratio dispersion between cylinders due to the fluctuation of a main F/B (feedback) correction value.SOLUTION: A main F/B correction value is so calculated to correct the air/fuel ratios of individual cylinders generally that the air-fuel ratio of exhaust gases may be identical to a target air/fuel ratio on the basis of the output of an air-fuel ratio sensor 36. On the basis of the detected value of the air/fuel ratios of the air/fuel ratio sensor 36, moreover, the air-fuel ratio of each cylinder is estimated on the basis of the detected value of the air/fuel ratio sensor so that the air/fuel ratio of each cylinder is controlled on the basis of the estimated air/fuel ratio of each cylinder. At this time, it is decided, on the basis of the magnitude of an initial estimation air/fuel ratio, whether or not an inter-cylinder imbalance trouble is possible in an engine 11 (or whether or not an air-fuel ratio dispersion between the cylinders is relatively high). If it is decided that the imbalance trouble between the cylinders is possible, and that the load on the engine 11 is at or higher than a predetermined value, the variation of the main F/B correction value is restricted by a predetermined guard value thereby to suppress the fluctuation of the main F/B correction value.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a cylinder-by-cylinder air-fuel ratio control apparatus for an internal combustion engine that estimates the air-fuel ratio of each cylinder of the internal combustion engine and controls the air-fuel ratio of each cylinder based on the estimated air-fuel ratio.

  Conventionally, in order to increase the exhaust gas purification rate of a catalyst for purifying exhaust gas of an internal combustion engine, the air-fuel ratio of the exhaust gas is made to coincide with the target air-fuel ratio based on the output of the air-fuel ratio sensor installed in the exhaust pipe. In addition, there is a type in which main feedback control is performed in which the main feedback correction value is calculated and the air-fuel ratio (for example, fuel injection amount) of each cylinder is uniformly corrected.

  Further, as a technique for reducing variation in air-fuel ratio between cylinders of an internal combustion engine, for example, as described in Patent Document 1 (Japanese Patent Laid-Open No. 2013-253593), each cylinder is based on a detection value of an air-fuel ratio sensor. The air-fuel ratio of each cylinder is estimated for each cylinder, and the cylinder-by-cylinder air-fuel ratio control is performed in which the air-fuel ratio of each cylinder is controlled for each cylinder based on the estimated air-fuel ratio of each cylinder.

JP 2013-253593 A

  By the way, when the air-fuel ratio variation between the cylinders of the internal combustion engine is relatively large, if the load on the internal combustion engine is large (that is, the exhaust flow rate is fast), the amplitude of the air-fuel ratio sensor output increases and the air-fuel ratio sensor output fluctuates greatly. To do. In such a case, the main feedback correction value by the main feedback control fluctuates greatly, which may increase the air-fuel ratio variation between the cylinders. For this reason, it becomes difficult to reduce the variation in air-fuel ratio among the cylinders by the cylinder-by-cylinder air-fuel ratio control (to converge the estimated air-fuel ratio of each cylinder), which may cause deterioration in exhaust emission.

  Accordingly, an object of the present invention is to provide a cylinder-by-cylinder air-fuel ratio control device for an internal combustion engine that can suppress an increase in variation in air-fuel ratio among cylinders due to fluctuations in a main feedback correction value.

  In order to solve the above-mentioned problem, the invention according to claim 1 is directed to an air-fuel ratio for detecting the air-fuel ratio of the exhaust gas in the exhaust collecting portion (34a) through which the exhaust gas of each cylinder of the internal combustion engine (11) flows. A sensor (36) is installed, and based on the output of the air-fuel ratio sensor (36), the main feedback correction value is calculated so that the air-fuel ratio of the exhaust gas matches the target air-fuel ratio, and the air-fuel ratio of each cylinder is uniformly corrected. The cylinder-by-cylinder air-fuel ratio estimation for estimating the air-fuel ratio of each cylinder for each cylinder based on the detected value of the main feedback control means (39) and the air-fuel ratio sensor (36) detected at each air-fuel ratio detection timing of each cylinder And a cylinder-by-cylinder air-fuel ratio control means (39) for performing cylinder-by-cylinder air-fuel ratio control for controlling the air-fuel ratio of each cylinder for each cylinder based on the estimated air-fuel ratio of each cylinder. ) In the cylinder-by-cylinder air-fuel ratio control apparatus, a determination means (39) for determining whether or not there is a possibility of an imbalance failure between cylinders of the internal combustion engine (11); When the load of the internal combustion engine (11) is equal to or greater than a predetermined value, a limiting means (39) is provided for limiting the amount of change in the main feedback correction value with a predetermined guard value.

  When it is determined that there is a possibility of imbalance failure between cylinders (that is, the air-fuel ratio variation between cylinders is relatively large) and the load of the internal combustion engine is equal to or higher than a predetermined value (that is, the exhaust flow rate is relatively high), the air-fuel ratio sensor There is a possibility that the main feedback correction value fluctuates greatly as the output amplitude increases.

  Therefore, when it is determined that there is a possibility of an imbalance failure between cylinders and the load of the internal combustion engine is equal to or greater than a predetermined value, the amount of change in the main feedback correction value is limited by a predetermined guard value. The fluctuation can be moderately suppressed, and an increase in the air-fuel ratio variation between the cylinders due to the fluctuation of the main feedback correction value can be suppressed. Thus, even when it is determined that there is a possibility of an imbalance failure between cylinders (the air-fuel ratio variation between cylinders is relatively large), the air-fuel ratio variation between cylinders is quickly reduced by the air-fuel ratio control for each cylinder (each The estimated air-fuel ratio of the cylinder can be converged), and the exhaust emission can be prevented from deteriorating.

FIG. 1 is a diagram showing a schematic configuration of an engine control system in one embodiment of the present invention. FIG. 2 is a block diagram illustrating the air-fuel ratio control function. FIG. 3 is a flowchart showing the flow of processing of the main F / B control routine. FIG. 4 is a flowchart showing the flow of the cylinder-by-cylinder air-fuel ratio estimation routine. FIG. 5 is a flowchart (No. 1) showing the flow of processing of the cylinder-by-cylinder air-fuel ratio control and convergence determination routine. FIG. 6 is a flowchart (No. 2) showing the flow of processing of the cylinder-by-cylinder air-fuel ratio control and convergence determination routine. FIG. 7 is a flowchart illustrating a process flow of an imbalance failure possibility determination routine according to the first embodiment. FIG. 8 is a time chart illustrating an execution example of the control according to the first embodiment. FIG. 9 is a time chart showing the effect of the first embodiment. FIG. 10 is a flowchart illustrating a process flow of an imbalance failure possibility determination routine according to the second embodiment. FIG. 11 is a flowchart illustrating a process flow of an imbalance failure possibility determination routine according to the third embodiment.

  Hereinafter, some embodiments embodying the mode for carrying out the present invention will be described.

A first embodiment of the present invention will be described with reference to FIGS.
First, a schematic configuration of the entire engine control system will be described with reference to FIG.
An in-line four-cylinder engine 11 that is an internal combustion engine, for example, has four cylinders, a first cylinder # 1 to a fourth cylinder # 4, and an air cleaner 13 is provided at the most upstream portion of the intake pipe 12 of the engine 11. An air flow meter 14 for detecting the intake air amount is provided on the downstream side of the air cleaner 13. A throttle valve 15 whose opening is adjusted by a motor or the like and a throttle opening sensor 16 for detecting the opening (throttle opening) of the throttle valve 15 are provided on the downstream side of the air flow meter 14.

  Further, a surge tank 17 is provided on the downstream side of the throttle valve 15, and an intake pipe pressure sensor 18 for detecting the intake pipe pressure is provided in the surge tank 17. The surge tank 17 is provided with an intake manifold 19 for introducing air into each cylinder of the engine 11, and fuel is injected into the intake port at or near the intake port connected to the intake manifold 19 of each cylinder. A fuel injection valve 20 is attached. Alternatively, a fuel injection valve 20 that directly injects fuel into the cylinder may be attached to each cylinder of the engine 11. During engine operation, the fuel in the fuel tank 21 is sent to the delivery pipe 23 by the fuel pump 22 and fuel is injected from the fuel injection valve 20 of each cylinder at each injection timing of each cylinder. A fuel pressure sensor 24 that detects fuel pressure (fuel pressure) is attached to the delivery pipe 23.

  Further, the engine 11 is provided with variable valve timing mechanisms 27 and 28 for changing the valve timing (opening / closing timing) of the intake valve 25 and the exhaust valve 26, respectively. Further, the engine 11 is provided with an intake cam angle sensor 31 and an exhaust cam angle sensor 32 that output a cam angle signal in synchronization with the rotation of the intake cam shaft 29 and the exhaust cam shaft 30, and the crank of the engine 11. A crank angle sensor 33 that outputs a pulse of a crank angle signal every predetermined crank angle (for example, every 30 CA) in synchronization with the rotation of the shaft is provided.

On the other hand, in the exhaust pipe 34 of the engine 11, exhaust gas in the exhaust collecting portion 34 a (the portion where the exhaust manifold 35 of each cylinder gathers or the downstream side thereof) flows. An air-fuel ratio sensor 36 for detecting the fuel ratio is provided. A catalyst 37 such as a three-way catalyst for purifying CO, HC, NO _{x and the} like in the exhaust gas is provided on the downstream side of the air-fuel ratio sensor 36. A cooling water temperature sensor 38 for detecting the cooling water temperature is attached to the cylinder block of the engine 11.

  Outputs of these various sensors are input to an electronic control unit (hereinafter referred to as “ECU”) 39. The ECU 39 is mainly composed of a microcomputer, and executes various engine control programs stored in a built-in ROM (storage medium), thereby depending on the engine operating state, the fuel injection amount, the ignition timing. The throttle opening (intake air amount) and the like are controlled.

  At that time, the ECU 39 executes a main F / B control routine of FIG. 3 to be described later, so that when a predetermined main F / B control execution condition is satisfied, the exhaust gas emission is determined based on the output of the air-fuel ratio sensor 36. A main F / B control is performed in which the main F / B correction value is calculated so that the air-fuel ratio matches the target air-fuel ratio and the air-fuel ratio (for example, fuel injection amount) of each cylinder is uniformly corrected. Here, “F / B” means “feedback” (hereinafter the same).

  Specifically, as shown in FIG. 2, first, the air-fuel ratio deviation calculating unit 40 calculates the deviation between the detected air-fuel ratio (the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensor 36) and the target air-fuel ratio, The main F / B control unit 41 calculates the main F / B correction value so that the deviation between the detected air-fuel ratio and the target air-fuel ratio becomes small. Then, the injection amount calculation unit 42 calculates the fuel injection amount based on the base injection amount, the main F / B correction value, etc. calculated based on the engine speed and load (intake pipe pressure, intake air amount, etc.). The fuel injection valve 20 of each cylinder is controlled based on the fuel injection amount.

  Further, the ECU 39 executes the routines of FIGS. 4 to 6 to be described later, thereby calculating the air-fuel ratio of each cylinder based on the detection value of the air-fuel ratio sensor 36 detected at each air-fuel ratio detection timing of each cylinder. The cylinder-by-cylinder air-fuel ratio estimation is performed for each cylinder, and the cylinder-by-cylinder air-fuel ratio control is performed to control the air-fuel ratio of each cylinder for each cylinder based on the estimated air-fuel ratio of each cylinder.

  Specifically, as shown in FIG. 2, first, the cylinder-by-cylinder air-fuel ratio estimation unit 43 uses a detection value of the air-fuel ratio sensor 36 (exhaust gas flowing through the exhaust collecting unit 34 a) using a cylinder-by-cylinder air-fuel ratio estimation model described later. Based on the actual air-fuel ratio), the air-fuel ratio of each cylinder is estimated for each cylinder, the reference air-fuel ratio calculating unit 44 calculates the average value of the estimated air-fuel ratios of all the cylinders, and the average value is used as the reference air-fuel ratio. Set. Thereafter, the cylinder-by-cylinder air-fuel ratio deviation calculating unit 45 calculates the deviation between the estimated air-fuel ratio of each cylinder and the reference air-fuel ratio for each cylinder, and the cylinder-by-cylinder air-fuel ratio control unit 46 calculates the estimated air-fuel ratio of each cylinder and the reference air-fuel ratio. For example, a fuel correction amount (a fuel injection amount correction amount) is calculated for each cylinder as a cylinder specific correction value so that the deviation from the air-fuel ratio becomes small. By correcting the fuel injection amount of each cylinder for each cylinder based on the calculation result, the air-fuel ratio of the air-fuel mixture supplied to each cylinder is corrected for each cylinder to reduce the air-fuel ratio variation among the cylinders.

  Here, a specific model for estimating the air-fuel ratio of each cylinder based on the detected value of the air-fuel ratio sensor 36 (actual air-fuel ratio of the exhaust gas flowing through the exhaust collecting portion 34a) (hereinafter referred to as "cylinder-specific air-fuel ratio estimation model"). An example will be described.

  Paying attention to the gas exchange in the exhaust collecting portion 34a, the detected value of the air-fuel ratio sensor 36 is given a predetermined weight to the estimated air-fuel ratio history of each cylinder and the detected value history of the air-fuel ratio sensor 36 in the exhaust collecting portion 34a. The model is obtained by multiplying and adding, and the air-fuel ratio of each cylinder is estimated using this model. At this time, a Kalman filter is used as an observer.

More specifically, a gas exchange model in the exhaust collecting portion 34a is approximated by the following equation (1).
ys (t) = k1 * u (t-1) + k2 * u (t-2) -k3 * ys (t-1) -k4 * ys (t-2)
...... (1)
Here, ys is a detected value of the air-fuel ratio sensor 36, u is an air-fuel ratio of the gas flowing into the exhaust collecting portion 34a, and k1 to k4 are constants.

  In the exhaust system, there are a first-order lag element for gas inflow and mixing in the exhaust collecting portion 34 a and a first-order lag element due to a response delay of the air-fuel ratio sensor 36. Therefore, in the above equation (1), the history for the past two times is referred to in consideration of these first order lag elements.

When the above equation (1) is converted into a state space model, the following equations (2a) and (2b) are derived.
X (t + 1) = A.X (t) + B.u (t) + W (t) (2a)
Y (t) = C · X (t) + D · u (t) (2b)
Here, A, B, C, and D are model parameters, Y is a detected value of the air-fuel ratio sensor 36, X is an estimated air-fuel ratio of each cylinder as a state variable, and W is noise.

Further, when the Kalman filter is designed by the above equations (2a) and (2b), the following equation (3) is obtained.
X ^ (k + 1 | k) = A.X ^ (k | k-1) + K {Y (k) -C.A.X ^ (k | k-1)} (3) where X ^ (X hat) is the estimated air-fuel ratio of each cylinder, and K is the Kalman gain. The meaning of X ^ (k + 1 | k) represents that the estimated value of the next time (k + 1) is obtained from the estimated value of time (k).

  As described above, the cylinder-by-cylinder air-fuel ratio estimation model is configured by the Kalman filter type observer, whereby the air-fuel ratio of each cylinder can be sequentially estimated as the combustion cycle proceeds.

  Next, a method for setting the air-fuel ratio detection timing of each cylinder (sample timing of the output of the air-fuel ratio sensor 36) will be described. The delay until the exhaust gas discharged from each cylinder reaches the vicinity of the air-fuel ratio sensor 36 and the air-fuel ratio is detected (hereinafter referred to as “exhaust system response delay”) varies depending on the engine operating state. In consideration of this, in this embodiment, the air-fuel ratio detection timing of each cylinder is set by a map according to the engine operating state (for example, engine speed and load), and the output of the air-fuel ratio sensor 36 is taken into the ECU 39. ing. In general, as the load on the engine 11 decreases, the response delay of the exhaust system increases, so the air-fuel ratio detection timing of each cylinder is set to shift to the retard side as the load on the engine 11 decreases. .

  However, the length of the exhaust manifold 35 from the exhaust port of each cylinder to the air-fuel ratio sensor 36 is different for each cylinder, and the flow of exhaust gas in each cylinder is in the engine operating state (engine rotation speed, in-cylinder charged air amount, etc.). ). In addition, since the response delay of the exhaust system also changes due to manufacturing variations and aging of the engine 11, the response delay of the exhaust system of each cylinder (air-fuel ratio detection timing of each cylinder) and the load in the engine design and manufacturing process. It is difficult to map the relationship accurately. For this reason, the air-fuel ratio detection timing of each cylinder may deviate from the proper air-fuel ratio detection timing.

  If the air-fuel ratio detection timing of each cylinder shifts, the estimation accuracy of the air-fuel ratio of each cylinder deteriorates, and the estimated air-fuel ratio of each cylinder does not converge even if the cylinder-by-cylinder air-fuel ratio control is continued (estimation between cylinders). The variation of the air-fuel ratio is not reduced).

  Therefore, the ECU 39 performs air-fuel ratio detection timing determination for determining whether there is a deviation in the air-fuel ratio detection timing based on the estimated air-fuel ratio during the cylinder-by-cylinder air-fuel ratio control, and when it is determined that there is a deviation in the air-fuel ratio detection timing Correct the air-fuel ratio detection timing.

  For example, the local learning for correcting the air-fuel ratio detection timing is executed so that the variation (fluctuation) of the detection value of the air-fuel ratio sensor 36 is maximized within one cycle (720 CA) of the engine 11. After the execution of the local learning, the air-fuel ratio detection timing is determined based on the relationship between the change in the estimated air-fuel ratio of at least one cylinder and the change in the cylinder-specific correction value (for example, fuel correction amount) of the cylinder during the cylinder-by-cylinder air-fuel ratio control. The global learning for correcting is performed. In this global learning, the change in the estimated air-fuel ratio of at least one cylinder and the change in the estimated air-fuel ratio in each of the cases where each cylinder number assumed for the estimated air-fuel ratio of each cylinder is virtually changed in plural ways. A correlation value with a change in the cylinder-specific correction value of the cylinder number is calculated, and the air-fuel ratio detection timing is corrected so that this correlation value becomes maximum.

  Alternatively, after the local learning is executed (that is, after the air-fuel ratio detection timing is corrected by the local learning), the air-fuel ratio detection timing is set to the combustion interval ( (In the case of four cylinders, 180 CA) or a global learning that corrects multiple times of each is performed to replace the air-fuel ratio detection timing of each cylinder with the air-fuel ratio detection timing of the other cylinders. May be corrected to the correct air-fuel ratio detection timing.

  Incidentally, when the air-fuel ratio variation between the cylinders of the engine 11 is relatively large, if the load on the engine 11 is large (that is, the exhaust flow rate is fast), the amplitude of the output of the air-fuel ratio sensor 36 increases as shown by the broken line in FIG. As a result, the output of the air-fuel ratio sensor 36 fluctuates greatly. In such a case, if the main F / B control is executed as usual, the main F / B correction value fluctuates greatly, which may increase the variation in air-fuel ratio among the cylinders. For this reason, it becomes difficult to reduce the variation in air-fuel ratio among the cylinders by the cylinder-by-cylinder air-fuel ratio control (to converge the estimated air-fuel ratio of each cylinder), which may cause deterioration in exhaust emission.

  As a countermeasure against this, the ECU 39 determines whether or not there is a possibility of an imbalance failure between cylinders of the engine 11 by executing routines of FIGS. 3 and 7 described later, and the possibility of an imbalance failure between cylinders. When it is determined that there is a load and the load on the engine 11 is greater than or equal to a predetermined value, the change amount of the main F / B correction value is limited by a predetermined guard value.

  Here, the inter-cylinder imbalance failure is, for example, a state in which the variation in air-fuel ratio between cylinders exceeds an allowable level. In addition, the possibility of an imbalance failure between cylinders is a state in which an imbalance failure between cylinders may occur. For example, there is a relatively large variation in air-fuel ratio between cylinders (a level that does not exceed an allowable level). Is a large state).

  When it is determined that there is a possibility of an imbalance failure between cylinders (that is, the air-fuel ratio variation between cylinders is relatively large) and the load of the engine 11 is equal to or greater than a predetermined value (that is, the exhaust flow rate is relatively fast), There is a possibility that the main F / B correction value fluctuates greatly as the amplitude of the 36 outputs increases.

  Therefore, when it is determined that there is a possibility of an imbalance failure between cylinders and the load of the engine 11 is equal to or greater than a predetermined value, the amount of change in the main F / B correction value is limited by a predetermined guard value, so that FIG. As indicated by the solid line, fluctuations in the main F / B correction value can be moderately suppressed, and an increase in air-fuel ratio variation between cylinders due to fluctuations in the main F / B correction value can be suppressed.

  Further, in the first embodiment, the possibility of an imbalance failure between cylinders based on the magnitude of the initial estimated air-fuel ratio by the cylinder-by-cylinder air-fuel ratio estimation (for example, the maximum value among the absolute values of the initial estimated air-fuel ratio of each cylinder). Whether or not there is is determined.

If the variation in air-fuel ratio between cylinders is small, the estimated air-fuel ratio of each cylinder is within a predetermined range even before or immediately after the start of cylinder-by-cylinder air-fuel ratio control. Before or immediately after the start of control, the estimated air-fuel ratio of each cylinder does not fall within the predetermined range (the estimated air-fuel ratio of at least one cylinder falls outside the predetermined range). Accordingly, if the magnitude of the initial estimated air-fuel ratio (the estimated air-fuel ratio before or after the start of the cylinder-by-cylinder air-fuel ratio control) is monitored, there is a possibility of an imbalance failure between cylinders (the variation in air-fuel ratio between cylinders is relatively small). It is possible to accurately determine whether or not it is large.
Hereinafter, the processing content of each routine of FIG. 3 thru | or FIG. 7 which ECU39 performs in the present Example 1 is demonstrated.

[Main F / B control routine]
The main F / B control routine shown in FIG. 3 is started at every predetermined crank angle (for example, every 30 CA) in synchronization with the output pulse of the crank angle sensor 33, and serves as a main feedback control means and a limiting means in the claims. To play a role.

When this routine is started, first, at step 101, it is determined whether or not an execution condition for main F / B control is satisfied. As execution conditions of the main F / B control, for example, there are the following conditions (1) and (2).
(1) The air-fuel ratio sensor 36 is in an active state
(2) The air-fuel ratio sensor 36 is not judged to be abnormal (failure)

  When both of these two conditions (1) and (2) are satisfied, the execution condition of the main F / B control is satisfied, and if there is a condition that does not satisfy any one of them, the execution condition is not satisfied. If the execution condition is not satisfied, this routine is terminated without performing the processing after step 102.

  On the other hand, if the execution condition is satisfied, the routine proceeds to step 102 where it is determined whether or not the load of the engine 11 (for example, intake air amount or intake pipe pressure) is equal to or greater than a predetermined value. This predetermined value is, for example, such a value that the amplitude of the air-fuel ratio sensor 36 output becomes relatively large when there is a possibility of an imbalance failure between cylinders.

  If it is determined in step 102 that the load of the engine 11 is equal to or greater than the predetermined value, the process proceeds to step 103, and an imbalance failure possibility flag described later indicates whether or not there is a possibility of an imbalance failure between cylinders. Judgment is made based on whether or not it is “1”.

  If it is determined in step 102 that the load of the engine 11 is equal to or greater than a predetermined value and it is determined in step 103 that there is a possibility of an imbalance failure between cylinders (imbalance failure possibility flag = 1), the process proceeds to step 104. Then, the change guard value for limiting the change amount of the main F / B correction value is set to a predetermined value.

  On the other hand, if it is determined in step 102 that the load of the engine 11 is smaller than a predetermined value, or it is determined in step 103 that there is no possibility of an imbalance failure between cylinders (imbalance failure possibility flag = 0). If YES in step 105, the flow advances to step 105 to make no change amount guard value (not set).

Thereafter, the routine proceeds to step 106, where the main F / B correction value is calculated so that the deviation between the detected air-fuel ratio (the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensor 36) and the target air-fuel ratio becomes small.
Thereafter, the process proceeds to step 107, where the difference between the current main F / B correction value and the main F / B correction value before a predetermined period (for example, one before combustion or one TDC) is determined as the change amount of the main F / B correction value. Calculate as

  Thereafter, the process proceeds to step 108, and when the change amount guard value is set in step 104, the change amount of the main F / B correction value is limited by the change amount guard value. Specifically, when the change amount of the main F / B correction value is larger than the change amount guard value, the main F / B correction value is guarded with the change amount guard value, and the main F / B correction value is guarded. The change amount of the correction value is set as a change amount guard value (main F / B correction value change amount = change amount guard value). On the other hand, when the change amount of the main F / B correction value is equal to or less than the change guard value, the change amount of the main F / B correction value is adopted as it is. If the change amount guard value is omitted in step 105, the main F / B correction value change amount is used as it is.

  Thereafter, the process proceeds to step 109, and the final main F / B correction value is obtained by adding the amount of change of the main F / B correction value to the main F / B correction value before a predetermined period. If the change amount guard value is omitted in step 105, the main F / B correction value calculated in step 106 may be directly used as the final main F / B correction value.

  Thereafter, the process proceeds to step 110, where the learning value of the main F / B correction value is calculated based on the final main F / B correction value. Specifically, the main F / B correction value is subjected to filter processing (for example, smoothing processing, averaging processing, and first-order lag processing) to obtain a learning value of the main F / B correction value. By storing the learning value of the main F / B correction value in a rewritable nonvolatile memory (a rewritable memory that retains stored data even when the ECU 39 is powered off) such as a backup RAM (not shown) of the ECU 39, The main F / B correction value is learned. At this time, while the change amount of the main F / B correction value is limited (that is, when the change amount guard value is set in step 104), the learning update speed of the main F / B correction value is reduced (main F / B). The interval for updating the learning value of the B correction value is made longer than usual) or the learning of the main F / B correction value is stopped (the calculation of the learning value of the main F / B correction value is stopped).

[Individual air-fuel ratio estimation routine]
The cylinder-by-cylinder air-fuel ratio estimation routine shown in FIG. 4 is started at predetermined crank angles (for example, every 30 CA) in synchronization with the output pulse of the crank angle sensor 33, and serves as cylinder-by-cylinder air-fuel ratio estimation means. Play a role.

When this routine is started, first, at step 201, it is determined whether or not an execution condition for the cylinder-by-cylinder air-fuel ratio control is satisfied. The execution conditions of the cylinder-by-cylinder air-fuel ratio control include, for example, the following conditions (1) to (3).
(1) The air-fuel ratio sensor 36 is in an active state
(2) The air-fuel ratio sensor 36 is not judged to be abnormal (failure)
(3) The engine operating range (for example, engine speed and intake pipe pressure) must be an operating range where air-fuel ratio estimation accuracy can be ensured.

  The execution condition of the cylinder-by-cylinder air-fuel ratio control is satisfied when all of these three conditions (1) to (3) are satisfied. If any one of the conditions is not satisfied, the execution condition is not satisfied. If the execution condition is not satisfied, this routine is terminated without performing the processing from step 202 onward.

  On the other hand, if the execution condition is satisfied, the routine proceeds to step 202, where the air-fuel ratio detection timing of each cylinder (sample timing of the output of the air-fuel ratio sensor 36) is set to the load of the engine 11 at that time (for example, intake pipe pressure). Set according to the map. Note that the air-fuel ratio detection timing of each cylinder may be set by a map according to the load of the engine 11 and the engine speed. The map for setting the air-fuel ratio detection timing is learned and corrected by an unillustrated routine for correcting the air-fuel ratio detection timing (for example, a local learning execution routine or a global learning execution routine).

  Thereafter, the routine proceeds to step 203, where it is determined whether or not the current crank angle is the air-fuel ratio detection timing set in step 202. If it is not the air-fuel ratio detection timing, this routine is executed without performing the subsequent processing. Exit.

  On the other hand, if the current crank angle is the air-fuel ratio detection timing set in step 202, the process proceeds to step 204, and the output of the air-fuel ratio sensor 36 (air-fuel ratio detection value) is read. Thereafter, the routine proceeds to step 205, where the air-fuel ratio of the cylinder that is the current air-fuel ratio estimation target is estimated based on the detected value of the air-fuel ratio sensor 36 using the cylinder-by-cylinder air-fuel ratio estimation model.

[Cylinder-specific air-fuel ratio control and convergence determination routine]
The cylinder-by-cylinder air-fuel ratio control and convergence determination routine shown in FIG. 5 and FIG. 6 is started at every predetermined crank angle (for example, every 30 CA) in synchronization with the output pulse of the crank angle sensor 33. Serves as air-fuel ratio control means. When this routine is started, first, in step 301, it is determined whether or not the execution condition of the cylinder-by-cylinder air-fuel ratio control (the same condition as in step 201 in FIG. 4) is satisfied. If the execution condition is not satisfied, this routine is terminated without performing the processing after step 302.

  On the other hand, if the execution condition is satisfied, the routine proceeds to step 302, where observation is performed based on the detected value φ of the air-fuel ratio sensor 36 (actual air-fuel ratio of exhaust gas flowing through the exhaust collecting portion 34a) and the estimated air-fuel ratio φ ^. The residual err is calculated by the following equation (4). At this time, the observation residual err is normalized using the amplitude of the detected value φ of the air-fuel ratio sensor 36 (difference from the target air-fuel ratio tφ). The detected value φ of the air-fuel ratio sensor 36, the estimated air-fuel ratio φ ^, and the target air-fuel ratio tφ are each calculated with an equivalence ratio (the reciprocal of the excess air ratio).

Here, τ is a time constant, and s is a Laplace operator.
Thereafter, the routine proceeds to step 303, where it is determined whether or not the per-cylinder air-fuel ratio control permission flag is “1” (cylinder-by-cylinder air-fuel ratio control is permitted). If it is determined in step 303 that the cylinder-by-cylinder air-fuel ratio control permission flag is “0” (cylinder-by-cylinder air-fuel ratio control is prohibited), the process proceeds to step 304 where the observation residual err is It is determined whether or not it is smaller than the permission threshold value K1on for the separate air-fuel ratio control.

  If it is determined in step 304 that the observation residual err is greater than or equal to the permission threshold value K1on, the process proceeds to step 306 where the cylinder-by-cylinder air-fuel ratio control permission flag is maintained at “0” and the convergence determination permission flag. Is maintained at “0”. Thereafter, if it is determined in step 304 that the observation residual err is smaller than the permission threshold K1on, the process proceeds to step 307, the permission flag for cylinder-by-cylinder air-fuel ratio control is set to “1”, and convergence determination is performed. The permission flag is set to “1”.

  On the other hand, if it is determined in step 303 that the cylinder-by-cylinder air-fuel ratio control permission flag is “1” (cylinder-by-cylinder air-fuel ratio control is permitted), the process proceeds to step 305 and the observation residual err Is smaller than the prohibition threshold K1off of the cylinder-by-cylinder air-fuel ratio control. This prohibition threshold value K1off is set to a value larger than the permission threshold value K1on.

  If it is determined in step 305 that the observation residual err is smaller than the prohibition threshold K1off, the process proceeds to step 307, where the permission flag for cylinder-by-cylinder air-fuel ratio control is maintained at “1” and the convergence determination permission flag. Is maintained at “1”. Thereafter, if it is determined in step 305 that the observation residual err is greater than or equal to the prohibition threshold value K1off, the process proceeds to step 306 where the per-cylinder air-fuel ratio control permission flag is reset to “0” and the convergence determination is performed. The permission flag is reset to “0”.

During the period when the cylinder-by-cylinder air-fuel ratio control permission flag is “1” (that is, during the period when the convergence determination permission flag is “1”), the routine proceeds to step 308 in FIG. 6 and the initial value calculation end flag is “1”. It is determined whether or not the calculation of the initial estimated air-fuel ratio has been completed. If it is determined in step 308 that the initial value calculation end flag is “0” (calculation of the initial estimated air-fuel ratio has not been completed), the process proceeds to step 309 and the initial estimation of each cylinder is performed by the following equation. Calculate the air-fuel ratio initφ ^ # i.
initφ ^ # i = {1 / (τ × 2 × s + 1)} × φ ^ # i
Here, φ ^ # i is the estimated air-fuel ratio of i-th cylinder #i, and init φ ^ # i is the initial estimated air-fuel ratio of i-th cylinder #i.

  Thereafter, the process proceeds to step 310, the count value of the initial value calculation counter is incremented, and then the process proceeds to step 311 to determine whether or not the count value of the initial value calculation counter is larger than a predetermined value. If it is determined in step 311 that the count value of the initial value calculation counter is equal to or smaller than the predetermined value, this routine is ended while the initial value calculation end flag is maintained at “0”.

  Thereafter, if it is determined in step 311 that the count value of the initial value calculation counter is larger than the predetermined value, the process proceeds to step 312 and the initial value calculation end flag is set to “1”, and this routine is executed. finish.

  By performing the processing in steps 308 to 312, the initial estimated air-fuel ratio init φ ^ # i of each cylinder is determined for each cylinder based on the estimated air-fuel ratio φ ^ # i of each cylinder in a predetermined period before the start of the cylinder-by-cylinder air-fuel ratio control. While calculating, the cylinder-by-cylinder air-fuel ratio control is prohibited until the calculation of the initial estimated air-fuel ratio initφ ^ # i is completed.

On the other hand, if it is determined in step 308 that the initial value calculation end flag is “1” (calculation of the initial estimated air-fuel ratio is ended), the air-fuel ratio control for each cylinder in steps 313 to 315 is performed. In addition to executing the processing, the processing related to the convergence determination of the estimated air-fuel ratio in steps 316 to 322 is executed.
First, in step 313, an average value of estimated air-fuel ratios of all cylinders is calculated, and the average value is set as a reference air-fuel ratio baseφ.

Thereafter, the process proceeds to step 314, where a deviation (baseφ−φ ^ # i) between the estimated air-fuel ratio φ ^ # i and the reference air-fuel ratio baseφ of each cylinder is calculated, and the deviation (baseφ−φ ^ # i) is calculated. The fuel correction amount Cmp # i is calculated by the following equation as a cylinder-specific correction value for each cylinder so as to decrease.
Cmp # i = ∫ (baseφ−φ ^ # i) dt
Here, Cmp # i is the fuel correction amount of the i-th cylinder #i.

  Thereafter, the process proceeds to step 315, and the air-fuel ratio of the air-fuel mixture supplied to each cylinder is corrected for each cylinder by correcting the fuel injection amount of each cylinder based on the fuel correction amount Cmp # i of each cylinder. Control is performed so as to reduce variations in the air-fuel ratio between cylinders.

  Thereafter, the process proceeds to step 316, and a convergence determination threshold value of each cylinder is set for each cylinder by a map or a mathematical formula or the like according to the initial estimated air-fuel ratio initφ ^ # i of each cylinder (initial estimated air-fuel ratio of i-th cylinder #i The convergence determination threshold value of the i-th cylinder #i is set according to initφ ^ # i). For example, the convergence determination threshold map or formula is set such that the deviation of the convergence determination threshold from the reference air-fuel ratio increases as the deviation of the initial estimated air-fuel ratio from the reference air-fuel ratio increases.

  Further, a convergence determination time common to all the cylinders is set by a map or a mathematical expression in accordance with the initial estimated air-fuel ratio initφ ^ # i (for example, the maximum value of the initial estimated air-fuel ratio initφ ^ # i of each cylinder). For example, the convergence determination time map or formula is set such that the convergence determination time becomes longer as the deviation of the maximum value of the initial estimated air-fuel ratio from the reference air-fuel ratio is smaller.

  Thereafter, the routine proceeds to step 317, where the estimated air-fuel ratio φ ^ # i of each cylinder is determined depending on whether or not the estimated air-fuel ratio φ ^ # i of each cylinder is closer to the target value (for example, the reference air-fuel ratio) than the convergence determination threshold value. Is within the convergence determination region.

  If it is determined in step 317 that the estimated air-fuel ratio φ ^ # i of at least one cylinder is outside the convergence determination region, the process proceeds to step 320, where the estimated air-fuel ratio convergence state (the estimated air-fuel ratio of each cylinder is This routine is terminated while the convergence determination flag is maintained at “0”.

  Thereafter, in step 317, it is determined that the estimated air-fuel ratio φ ^ # i of each cylinder is within the convergence determination region (the estimated air-fuel ratios φ ^ # i of all cylinders are on the target value side of the convergence determination threshold). If so, the process proceeds to step 318 to increment the count value of the convergence determination counter.

  Thereafter, the process proceeds to step 319, where it is determined whether or not the count value of the convergence determination counter is equal to or longer than the convergence determination time. If it is determined in step 319 that the count value of the convergence determination counter is smaller than the convergence determination time, this routine is terminated while the convergence determination flag is maintained at “0”.

  Thereafter, when it is determined in step 319 that the count value of the convergence determination counter is equal to or greater than the convergence determination time, the estimated air-fuel ratio φ ^ # i of each cylinder is closer to the target value than the convergence determination threshold value. When it is determined that the state has continued for the convergence determination time or longer, the process proceeds to step 321, where it is determined that the estimated air-fuel ratio convergence state is set, and the convergence determination flag is set to “1”.

  Thereafter, the process proceeds to step 322, and when it is determined that the estimated air-fuel ratio has converged, the amount of change of the estimated air-fuel ratio φ ^ # i (for example, the average value of the amount of change of the estimated air-fuel ratio φ ^ # i of each cylinder) The steady gain Kdc is calculated based on the ratio between the change amount of the fuel correction amount Cmp # i and the change amount of the fuel correction amount Cmp # i (for example, the average value of the change amount of the fuel correction amount Cmp # i of each cylinder). Reflect in the estimation model. Specifically, the parameter B in the above equation (2a) is multiplied by the steady gain Kdc.

  After determining that the estimated air-fuel ratio has converged, if it is determined in step 317 that the estimated air-fuel ratio φ ^ # i of at least one cylinder is outside the convergence determination region, that is, in the estimated air-fuel ratio converged state. If the estimated air-fuel ratio φ ^ # i diverges after it is determined that there is, the process proceeds to step 320 and the determination is reset. That is, it is determined that the estimated air-fuel ratio convergence state is not established, and the convergence determination flag is reset to “0”.

[Imbalance failure possibility determination routine]
The imbalance failure possibility determination routine shown in FIG. 7 is started at every predetermined crank angle (for example, every 30 CA) in synchronization with the output pulse of the crank angle sensor 33, and serves as a determination means in the claims. .

  When this routine is started, first, at step 401, it is determined whether or not an imbalance failure possibility flag described later is “0”. If it is determined in this step 401 that the imbalance failure possibility flag is “0”, the routine proceeds to step 402 where the maximum value max of the absolute values of the initial estimated air-fuel ratio initφ ^ # i of each cylinder is reached. It is determined whether [abs (initial estimated air-fuel ratio)] is equal to or greater than a predetermined value A1.

  If it is determined in step 402 that the maximum value max [abs (initial estimated air-fuel ratio)] is smaller than the predetermined value A1, the process proceeds to step 406, where it is determined that there is no possibility of an inter-cylinder imbalance failure. The routine is terminated while the imbalance failure possibility flag is maintained at “0”.

  On the other hand, if it is determined in step 402 that the maximum value max [abs (initial estimated air-fuel ratio)] is greater than or equal to the predetermined value A1, the process proceeds to step 403, where there is a possibility of an imbalance failure between cylinders (cylinders). After determining that the imbalance failure possibility flag is set to “1”, the routine is terminated.

  Thereafter, when it is determined in the above step 401 that the imbalance failure possibility flag is “1”, the process proceeds to step 404 where the convergence determination flag indicates whether or not the estimated air-fuel ratio of each cylinder has converged. Judgment is made based on whether or not it is “1”.

  If it is determined in step 404 that the estimated air-fuel ratio of each cylinder has not converged (convergence determination flag = 0), the process proceeds to step 405. In this step 405, a state where the maximum value max [abs (estimated air-fuel ratio)] of the absolute values of the estimated air-fuel ratio φ ^ # i of each cylinder is smaller than a predetermined value A2 continues for a predetermined time or more (that is, each cylinder It is determined whether or not the estimated air-fuel ratio of the engine has reached the target value side of the predetermined value A2 for a predetermined time or longer). Here, the predetermined value A2 is set to the same value as the predetermined value A1 or a value smaller than the predetermined value A1.

  If it is determined in step 405 that the state where the maximum value max [abs (estimated air-fuel ratio)] is smaller than the predetermined value A2 does not continue for the predetermined time or longer, the imbalance failure possibility flag is set to “1”. This routine is terminated while maintaining the above.

  Thereafter, when it is determined in step 404 that the estimated air-fuel ratio of each cylinder has converged (convergence determination flag = 1), or in step 405, the maximum value max [abs (estimated air-fuel ratio)] is greater than the predetermined value A2. If it is determined that the smaller state has continued for a predetermined time or longer, the process proceeds to step 406. In step 406, the determination that there is a possibility of an imbalance failure between cylinders is reset, it is determined that there is no possibility of an imbalance failure between cylinders, and the imbalance failure possibility flag is reset to “0”. This routine ends.

An execution example of the control of the first embodiment described above will be described with reference to FIG.
Cylinder air-fuel ratio estimation for estimating the air-fuel ratio of each cylinder based on the detection value of the air-fuel ratio sensor 36 detected at each air-fuel ratio detection timing of each cylinder at the time t1 when the execution condition of the cylinder-by-cylinder air-fuel ratio control is satisfied. To start. Furthermore, processing for calculating the observation residual err based on the detected value of the air-fuel ratio sensor 36 and the estimated air-fuel ratio is started.

  When the observation residual err is equal to or greater than the cylinder-by-cylinder air-fuel ratio control permission threshold K1on, the cylinder-by-cylinder air-fuel ratio control permission flag is maintained at “0” to prohibit the cylinder-by-cylinder air-fuel ratio control and set the convergence determination permission flag to “ “0” is maintained, and the convergence determination of the estimated air-fuel ratio is prohibited.

  Thereafter, at time t2 when the observation residual err becomes smaller than the cylinder-by-cylinder air-fuel ratio control permission threshold K1on, the cylinder-by-cylinder air-fuel ratio control permission flag is set to “1” and the cylinder-by-cylinder air-fuel ratio control is permitted. The convergence determination permission flag is set to “1” and the convergence determination of the estimated air-fuel ratio is permitted.

  When the cylinder-by-cylinder air-fuel ratio control is permitted, the initial estimated air-fuel ratio of each cylinder is calculated for each cylinder based on the estimated air-fuel ratio of each cylinder in a predetermined period A before the start of the cylinder-by-cylinder air-fuel ratio control. Further, a convergence determination threshold value for each cylinder is set for each cylinder according to the initial estimated air-fuel ratio of each cylinder, and a convergence determination time is set according to the initial estimated air-fuel ratio (for example, the maximum value of the initial estimated air-fuel ratio for each cylinder). Set.

  At the time t3 when the calculation of the initial estimated air-fuel ratio is completed, the cylinder-by-cylinder air-fuel ratio control is started. Further, if it is determined that the maximum value max [abs (initial estimated air-fuel ratio)] of the absolute values of the initial estimated air-fuel ratio of each cylinder is equal to or greater than the predetermined value A1, there is a possibility of an inter-cylinder imbalance failure. It is determined that there is (the air-fuel ratio variation between cylinders is relatively large), and the imbalance failure possibility flag is set to “1”.

  Furthermore, when it is determined that there is a possibility of an imbalance failure between cylinders (imbalance failure possibility flag = 1) and the load of the engine 11 is equal to or greater than a predetermined value, a change amount guard value is set. The amount of change in the main F / B correction value is limited. Thereby, when the change amount of the main F / B correction value is larger than the change amount guard value, the change amount of the main F / B correction value is guarded with the change amount guard value, and the main F / B correction value Is set as a change amount guard value (main F / B correction value change amount = change amount guard value). On the other hand, when the change amount of the main F / B correction value is equal to or less than the change guard value, the change amount of the main F / B correction value is adopted as it is.

  Thereafter, at a time point t4 when the estimated air-fuel ratio of each cylinder is closer to the target value than the convergence determination threshold value, a process of incrementing the count value of the convergence determination counter is started. Until the count value of the convergence determination counter reaches the convergence determination time, it is determined that the estimated air-fuel ratio convergence state is not established, and the convergence determination flag is maintained at “0”.

  Thereafter, at the time t5 when the count value of the convergence determination counter becomes equal to or longer than the convergence determination time, it is determined that the estimated air-fuel ratio convergence state is set, the convergence determination flag is set to “1”, and an inter-cylinder imbalance failure is possible. It is determined that there is no possibility of imbalance failure between cylinders, and the imbalance failure possibility flag is reset to “0”. Thereby, the restriction on the change amount of the main F / B correction value is ended (the restriction by the change guard value is released).

  In the first embodiment described above, it is determined whether or not there is a possibility of an imbalance failure between cylinders of the engine 11, it is determined that there is a possibility of an imbalance failure between cylinders, and the load of the engine 11 is greater than or equal to a predetermined value. Sometimes, the change amount of the main F / B correction value is limited by the change amount guard value. As a result, as shown by the solid line in FIG. 9, fluctuations in the main F / B correction value can be moderately suppressed, and increase in air-fuel ratio variation between cylinders due to fluctuations in the main F / B correction value can be suppressed. . Thus, even when it is determined that there is a possibility of an imbalance failure between cylinders (the air-fuel ratio variation between cylinders is relatively large), the air-fuel ratio variation between cylinders is quickly reduced by the air-fuel ratio control for each cylinder (each The estimated air-fuel ratio of the cylinder can be converged), and the exhaust emission can be prevented from deteriorating.

  Further, in the first embodiment, after determining that there is a possibility of an imbalance failure between cylinders, when it is determined that the estimated air-fuel ratio of each cylinder has converged, or the estimated air-fuel ratio of each cylinder is lower than a predetermined value. When the state of the target value side continues for a predetermined time or more, the determination that there is a possibility of an imbalance failure between cylinders is reset, and it is determined that there is no possibility of an imbalance failure between cylinders. In this way, the timing at which it is determined that the estimated air-fuel ratio of each cylinder has converged and the timing at which the estimated air-fuel ratio of each cylinder has reached the target value side from the predetermined value have continued for a predetermined time or longer. At the earlier timing, the restriction of the change amount of the main F / B correction value is finished (the restriction by the change amount guard value is released), and the normal main F / B control can be restored.

  In the first embodiment, while the change amount of the main F / B correction value is limited (that is, when the change guard value is set), the learning update speed of the main F / B correction value is reduced or the main F / B correction value is reduced. / B correction value learning is stopped. In this way, the learning update speed of the main F / B correction value is reduced in response to the fluctuation of the main F / B correction value becoming smaller while the change amount of the main F / B correction value is limited. Alternatively, the learning of the main F / B correction value can be stopped to reduce the calculation load on the ECU 39.

  Next, Embodiment 2 of the present invention will be described with reference to FIG. However, description of substantially the same parts as those in the first embodiment will be omitted or simplified, and different parts from the first embodiment will be mainly described.

  In the second embodiment, the ECU 39 executes an imbalance failure possibility determination routine shown in FIG. 10 to be described later, whereby the magnitude of the correction value for each cylinder by the air-fuel ratio control for each cylinder (for example, the absolute value of the correction value for each cylinder of each cylinder). It is determined whether or not there is a possibility of an imbalance failure between cylinders based on the maximum value).

  If the variation in air-fuel ratio between cylinders is small, the correction value for each cylinder (fuel correction amount Cmp # i) falls within a predetermined range. If the variation in air-fuel ratio between cylinders is large, the correction value for each cylinder in each cylinder. Does not fall within the predetermined range (the cylinder-specific correction value of at least one cylinder is outside the predetermined range). Accordingly, by monitoring the magnitude of the cylinder specific correction value, it is possible to accurately determine whether or not there is a possibility of an imbalance failure between cylinders (the air-fuel ratio variation between cylinders is relatively large).

  The routine of FIG. 10 executed in the second embodiment is obtained by changing the process of step 402 of the routine of FIG. 7 described in the first embodiment to the process of step 402a. It is the same as FIG.

  In the imbalance failure possibility determination routine of FIG. 10, when it is determined in step 401 that the imbalance failure possibility flag is “0”, the process proceeds to step 402a and the cylinder-specific correction value (fuel) for each cylinder is determined. It is determined whether or not the maximum value max [abs (correction value for each cylinder)] among the absolute values of the correction amount Cmp # i) is equal to or greater than a predetermined value.

  If it is determined in step 402a that the maximum value max [abs (correction value for each cylinder)] is smaller than a predetermined value, the process proceeds to step 406, where it is determined that there is no possibility of an imbalance failure between cylinders. The imbalance failure possibility flag is maintained at “0”.

  On the other hand, if it is determined in step 402a that the maximum value max [abs (correction value for each cylinder)] is equal to or greater than a predetermined value, the process proceeds to step 403, where there is a possibility of an imbalance failure between cylinders (inter-cylinder). And the imbalance failure possibility flag is set to “1”.

  Thereafter, if it is determined in step 401 that the imbalance failure possibility flag is “1”, whether or not the estimated air-fuel ratio of each cylinder has converged in step 404 (convergence determination flag = 1). In step 405, it is determined whether or not a state where the maximum value max [abs (estimated air-fuel ratio)] is smaller than a predetermined value A2 has continued for a predetermined time or more.

  If “Yes” is determined in Step 404 or “Yes” is determined in Step 405, the process proceeds to Step 406 to reset the determination that there is a possibility of an imbalance failure between cylinders. Then, it is determined that there is no possibility of an imbalance failure between cylinders, and the imbalance failure possibility flag is reset to “0”.

  In the second embodiment described above, substantially the same effects as those of the first embodiment can be obtained.

  Next, Embodiment 3 of the present invention will be described with reference to FIG. However, description of substantially the same parts as those in the first embodiment will be omitted or simplified, and different parts from the first embodiment will be mainly described.

  In the third embodiment, the possibility of an imbalance failure between cylinders is determined based on the amplitude value during one cycle of the output of the air-fuel ratio sensor 36 by executing an imbalance failure possibility determination routine of FIG. Whether or not there is is determined.

  If the variation in the air-fuel ratio between the cylinders is small, the amplitude value during one cycle of the output of the air-fuel ratio sensor 36 becomes small. If the variation in the air-fuel ratio between the cylinders is large, the amplitude during one cycle of the output of the air-fuel ratio sensor 36. The value increases. Therefore, by monitoring the amplitude value of the output of the air-fuel ratio sensor 36 for one cycle, it is possible to accurately determine whether there is a possibility of imbalance failure between cylinders (the air-fuel ratio variation between cylinders is relatively large). Can do.

  In the routine of FIG. 11 executed in the third embodiment, the process of step 402 in the routine of FIG. 7 described in the first embodiment is changed to the process of step 402b. It is the same as FIG.

  In the imbalance failure possibility determination routine of FIG. 11, when it is determined in step 401 that the imbalance failure possibility flag is “0”, the process proceeds to step 402 b and one cycle of the output of the air-fuel ratio sensor 36 is performed. It is determined whether or not the amplitude value between them is greater than or equal to a predetermined value. Here, the amplitude value during one cycle of the output of the air-fuel ratio sensor 36 is, for example, the difference between the maximum value and the minimum value during one cycle of the output of the air-fuel ratio sensor 36, or the maximum value and the target air-fuel ratio. Difference.

  If it is determined in step 402b that the amplitude value is smaller than the predetermined value, the process proceeds to step 406, where it is determined that there is no possibility of an imbalance failure between cylinders, and the imbalance failure possibility flag is set to “0”. To maintain.

  On the other hand, if it is determined in step 402b that the amplitude value is greater than or equal to the predetermined value, the process proceeds to step 403, where there is a possibility of an imbalance failure between cylinders (the air-fuel ratio variation between cylinders is relatively large). Determination is made and an imbalance failure possibility flag is set to “1”.

  Thereafter, if it is determined in step 401 that the imbalance failure possibility flag is “1”, whether or not the estimated air-fuel ratio of each cylinder has converged in step 404 (convergence determination flag = 1). In step 405, it is determined whether or not a state where the maximum value max [abs (estimated air-fuel ratio)] is smaller than a predetermined value A2 has continued for a predetermined time or more.

If “Yes” is determined in Step 404 or “Yes” is determined in Step 405, the process proceeds to Step 406 to reset the determination that there is a possibility of an imbalance failure between cylinders. Then, it is determined that there is no possibility of an imbalance failure between cylinders, and the imbalance failure possibility flag is reset to “0”.
In the third embodiment described above, substantially the same effect as in the first embodiment can be obtained.

  Note that two or three of the imbalance failure possibility determination routines (FIGS. 7, 10, and 11) of the first to third embodiments may be combined. Specifically, when two or three of the processes of steps 402, 402a, and 402b are performed, and “Yes” is determined in two or three of steps 402, 402a, and 402b. In step 403, it is determined that there is a possibility of an imbalance failure between cylinders, and an imbalance failure possibility flag is set to “1”.

  Moreover, in the imbalance failure possibility determination routines (FIGS. 7, 10, and 11) of the first to third embodiments, the processes of steps 404 and 405 are performed. Only one of 405 may be performed (that is, one of steps 404 and 405 is omitted).

  In the first to third embodiments, the initial estimated air-fuel ratio is calculated based on the estimated air-fuel ratio before the start of the cylinder-by-cylinder air-fuel ratio control. However, the present invention is not limited to this. The initial estimated air-fuel ratio may be calculated based on the estimated air-fuel ratio immediately after the start.

Further, the method of correcting the air-fuel ratio detection timing when it is determined that there is a deviation in the air-fuel ratio detection timing is not limited to the method described in the above embodiment, and may be changed as appropriate.
In the first to third embodiments, the present invention is applied to a four-cylinder engine. However, the present invention is not limited to this, and the present invention can be applied to a two-cylinder engine, a three-cylinder engine, or an engine having five or more cylinders. good.

  DESCRIPTION OF SYMBOLS 11 ... Engine (internal combustion engine), 12 ... Intake pipe, 34 ... Exhaust pipe, 34a ... Exhaust collecting part, 36 ... Air-fuel ratio sensor, 39 ... ECU (Main feedback control means, Cylinder air-fuel ratio estimation means, Cylinder air-fuel ratio Control means, judgment means, restriction means)

Claims (7)

An air-fuel ratio sensor (36) for detecting the air-fuel ratio of the exhaust gas is installed in the exhaust collecting part (34a) where the exhaust gas of each cylinder of the internal combustion engine (11) flows and flows, and the air-fuel ratio sensor (36) Main feedback control means (39) for uniformly correcting the air-fuel ratio of each cylinder by calculating a main feedback correction value so that the air-fuel ratio of the exhaust gas matches the target air-fuel ratio based on the output, and each cylinder Cylinder-by-cylinder air-fuel ratio estimation means for executing cylinder-by-cylinder air-fuel ratio estimation for estimating the air-fuel ratio of each cylinder for each cylinder based on the detected value of the air-fuel ratio sensor (36) detected at each air-fuel ratio detection timing of 39) and cylinder-by-cylinder air-fuel ratio control means (39) for performing cylinder-by-cylinder air-fuel ratio control for controlling the air-fuel ratio of each cylinder based on the estimated air-fuel ratio of each cylinder. Air-fuel ratio by cylinder In the control device,
Determination means (39) for determining whether or not there is a possibility of an imbalance failure between cylinders of the internal combustion engine (11);
Limiting means (39) for limiting the amount of change in the main feedback correction value with a predetermined guard value when it is determined that there is a possibility of an imbalance failure between cylinders and the load of the internal combustion engine (11) is a predetermined value or more. And an air-fuel ratio control apparatus for each cylinder of an internal combustion engine.   The determination means (39) determines whether or not there is a possibility of an inter-cylinder imbalance failure based on a magnitude of an initial estimated air-fuel ratio by the cylinder-by-cylinder air-fuel ratio estimation. The cylinder-by-cylinder air-fuel ratio control apparatus according to claim.   The determination means (39) determines whether or not there is a possibility of an imbalance failure between cylinders based on a magnitude of a cylinder specific correction value by the cylinder specific air-fuel ratio control. The cylinder-by-cylinder air-fuel ratio control apparatus according to claim 2.   The determination means (39) determines whether or not there is a possibility of an imbalance failure between cylinders based on an amplitude value during one cycle of an output of the air-fuel ratio sensor (36). The air-fuel ratio control apparatus for each cylinder of the internal combustion engine according to any one of 1 to 3.   After the determination means (39) determines that there is a possibility of an imbalance failure between cylinders and then determines that the estimated air-fuel ratio of each cylinder has converged, there is a possibility of an imbalance failure between cylinders. 5. The cylinder-by-cylinder air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the determination is reset.   When the determination means (39) determines that there is a possibility of an imbalance failure between the cylinders, the state where the estimated air-fuel ratio of each cylinder has become a target value side from a predetermined value continues for a predetermined time or more. 6. The cylinder-by-cylinder air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the determination that there is a possibility of an imbalance failure between cylinders is reset.   The limiting means (39) reduces the learning update speed of the main feedback correction value or stops learning of the main feedback correction value while limiting the change amount of the main feedback correction value. Item 7. The air-fuel ratio control device for each cylinder of the internal combustion engine according to any one of Items 1 to 6.

Images