Method and device for controlling air-fuel ratio of an internal combustion engine

Abstract

In the present invention, the oxygen storage capacity of a three-way catalyst (15) is estimated on the basis of an upstream-side exhaust air–fuel ratio (Fr A / F) (S1), and a target air–fuel ratio is controlled so that the estimated oxygen storage capacity matches a prescribed target oxygen storage capacity (S6). If a downstream-side exhaust air–fuel ratio (Rr A / F) detected by a downstream-side air–fuel ratio sensor (20) is equal to or less than a threshold value (RAF1) corresponding to a first oxygen storage capacity (OSA1), the estimated oxygen storage capacity is reset to the value of the first oxygen storage capacity (OSA1) (S2, S3). If the downstream-side exhaust air–fuel ratio (Rr A / F) is equal to or greater than a threshold value (RAF2) corresponding to a second oxygen storage capacity (OSA2), the estimated oxygen storage capacity is reset to the value of the second oxygen storage capacity (OSA2) (S4, S5). The target oxygen storage capacity is set to be closer to the first oxygen storage capacity (OSA1) than the median value of the first oxygen storage capacity (OSA1) and the second oxygen storage capacity (OSA2).

Description

[Technical Field]

[0001] The present invention relates to a control method and device for controlling the air-fuel ratio so as to maintain the oxygen storage amount of an exhaust purification catalyst provided in an exhaust passage of an internal combustion engine at a target oxygen storage amount. [Background technology]

[0002] For example, a three-way catalyst is an exhaust purification catalyst that can oxidize CO and HC in exhaust gas and reduce NOx. However, in order to achieve a high level of both oxidation and reduction through catalytic action, the catalyst's ability to store and release oxygen, or so-called oxygen storage capacity, is important. Therefore, a technique is known that estimates the oxygen storage capacity of a three-way catalyst and controls a target air-fuel ratio to maintain this oxygen storage capacity within an appropriate range.

[0003] Patent Document 1 discloses a technology in which a downstream air-fuel ratio sensor is provided downstream of an exhaust purification catalyst, and when the air-fuel ratio detected by this downstream air-fuel ratio sensor becomes a rich judged air-fuel ratio, the target air-fuel ratio is switched to a lean air-fuel ratio, and when the lean judged air-fuel ratio becomes a lean judged air-fuel ratio, the target air-fuel ratio is switched to a rich air-fuel ratio. Furthermore, as one-sided failure control, after switching to a lean air-fuel ratio, the target air-fuel ratio is switched to a rich air-fuel ratio when the estimated oxygen storage amount reaches a predetermined switching reference storage amount.

[0004] However, this patent document 1 basically relates to a technology for actively increasing or decreasing the oxygen storage capacity of the exhaust purification catalyst, and does not attempt to converge the oxygen storage capacity to a fixed target oxygen storage capacity. When the air-fuel ratio detected by the downstream air-fuel ratio sensor becomes a rich judged air-fuel ratio, CO and HC have already begun to flow out of the exhaust purification catalyst. Similarly, when the air-fuel ratio becomes a lean judged air-fuel ratio, NOx has already begun to flow out of the exhaust purification catalyst. In particular, NOx has a characteristic of rapidly increasing downstream of the exhaust purification catalyst when the oxygen storage capacity exceeds a level corresponding to the lean judged air-fuel ratio, so a delay in control is likely to result in NOx emissions exceeding an allowable level. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-71959 Summary of the Invention

[0006] The present invention provides an air-fuel ratio control method for an internal combustion engine, which is provided with an exhaust purification catalyst having an oxygen storage capacity in an exhaust passage, and controls the air-fuel ratio so that the oxygen storage amount of the exhaust purification catalyst becomes a target oxygen storage amount, comprising: an oxygen storage amount of the exhaust purification catalyst is estimated based on an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst; controlling the air-fuel ratio of the internal combustion engine so that the estimated oxygen storage amount coincides with the target oxygen storage amount; detecting an air-fuel ratio of exhaust gas flowing out from the exhaust purification catalyst downstream of the exhaust purification catalyst; resetting the estimated oxygen storage amount to the first oxygen storage amount when it is detected based on the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst that the oxygen storage amount of the exhaust purification catalyst is equal to or less than a predetermined first oxygen storage amount that is smaller than the target oxygen storage amount; resetting the estimated oxygen storage amount to the second oxygen storage amount when it is detected based on the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst that the oxygen storage amount of the exhaust purification catalyst is equal to or greater than a predetermined second oxygen storage amount that is greater than the target oxygen storage amount; Here, the target oxygen storage amount is set in a range in which the oxygen storage amount is smaller than the median value between the first oxygen storage amount and the second oxygen storage amount.

[0007] In the above configuration, the air-fuel ratio of the internal combustion engine is controlled so that the estimated oxygen storage amount matches the target oxygen storage amount. Ideally, the actual oxygen storage amount of the exhaust purification catalyst is maintained near the target oxygen storage amount. If the estimated oxygen storage amount deviates from the actual oxygen storage amount due to disturbances or some other factor, for example, the actual oxygen storage amount may be too small, causing the air-fuel ratio of the exhaust gas flowing out of the exhaust purification catalyst to be equal to or lower than the air-fuel ratio corresponding to the first oxygen storage amount. This will detect that the actual oxygen storage amount is equal to or lower than the first oxygen storage amount. Alternatively, the actual oxygen storage amount may be too large, causing the air-fuel ratio of the exhaust gas flowing out of the exhaust purification catalyst to be equal to or higher than the air-fuel ratio corresponding to the second oxygen storage amount. This will detect that the actual oxygen storage amount is equal to or higher than the second oxygen storage amount.

[0008] In such a case, the estimated oxygen storage amount is reset to the first oxygen storage amount or the second oxygen storage amount, respectively. As a result, a large difference appears between the estimated oxygen storage amount and the target oxygen storage amount, and the air-fuel ratio of the internal combustion engine is controlled in a manner corresponding to the difference.

[0009] In the present invention, the target oxygen storage capacity is not set to the median between the first and second oxygen storage capacities, but is set to a value closer to the first oxygen storage capacity. In other words, the difference between the target oxygen storage capacity and the second oxygen storage capacity is greater than the difference between the target oxygen storage capacity and the first oxygen storage capacity. Therefore, in the event of an estimation error due to a disturbance or some other factor, the actual oxygen storage capacity reaches the second oxygen storage capacity and the estimated oxygen storage capacity is reset less frequently than the actual oxygen storage capacity reaches the first oxygen storage capacity and the estimated oxygen storage capacity is reset less frequently. This suppresses the emission of NOx, which tends to increase suddenly when the oxygen storage capacity exceeds the second oxygen storage capacity. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram illustrating the configuration of an internal combustion engine equipped with a three-way catalyst according to an embodiment of the present invention; [Figure 2]A characteristic diagram showing the relationship between the oxygen storage capacity of a three-way catalyst and the CO and NOx that flow out of the three-way catalyst. [Figure 3] 3 is a flowchart of an air-fuel ratio control according to an embodiment. [Figure 4] FIG. 3 is a block diagram relating to the calculation of a target air-fuel ratio. [Figure 5] 5 is a time chart showing changes in the downstream exhaust air-fuel ratio, the oxygen storage amount, and the target air-fuel ratio. DETAILED DESCRIPTION OF THE INVENTION

[0011] An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is an explanatory diagram showing a schematic configuration of an internal combustion engine 1 of one embodiment to which the present invention is applied. The internal combustion engine 1 of the one embodiment is a four-stroke spark-ignition internal combustion engine (a so-called gasoline engine), and each cylinder is provided with an intake valve 2, an exhaust valve 3, and an ignition plug 4. The illustrated example is configured as a direct injection type engine, and a fuel injection valve 5 that injects fuel into the cylinder is disposed, for example, on the intake valve 2 side. Note that the engine may also be configured as a port injection type that injects fuel toward an intake port 6.

[0012] An electronically controlled throttle valve 10, the opening of which is controlled by a control signal from an engine controller 9, is installed upstream of a collector section 8 in an intake passage 7 connected to the intake port 6 of each cylinder. An air flow meter 11, which detects the amount of intake air, is disposed upstream of the throttle valve 10, and an air cleaner 12 is disposed further upstream.

[0013] The exhaust ports 13 of each cylinder are joined together to form a single exhaust passage 14, which is provided with an exhaust purification catalyst for purifying the exhaust, such as a three-way catalyst 15. The three-way catalyst 15 is, for example, a monolithic ceramic catalyst in which a catalyst layer containing catalytic metal is coated on the surface of a monolithic ceramic body having fine passages formed therein. The three-way catalyst 15 may also be configured to further include a downstream catalyst (a so-called underfloor catalyst) arranged in series.

[0014] An upstream air-fuel ratio sensor 19 is disposed in the exhaust passage 14 on the inlet side of the three-way catalyst 15, i.e., on the upstream side of the three-way catalyst 15, for detecting the air-fuel ratio of the exhaust gas emitted by the internal combustion engine 1 (in other words, the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 15). This upstream air-fuel ratio sensor 19 is a so-called wide-range air-fuel ratio sensor that obtains an output corresponding to the exhaust air-fuel ratio. In addition, a downstream air-fuel ratio sensor 20 is disposed on the outlet side or downstream side of the three-way catalyst 15, for detecting the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15. Like the upstream air-fuel ratio sensor 19, the downstream air-fuel ratio sensor 20 is a wide-range air-fuel ratio sensor that obtains an output corresponding to the exhaust air-fuel ratio.

[0015] Detection signals from the air-fuel ratio sensors 19, 20 and the air flow meter 11 are input to the engine controller 9. Further detection signals from a number of sensors, such as a crank angle sensor 21 for detecting the engine speed, a water temperature sensor 22 for detecting the coolant temperature, and an accelerator position sensor 23 for detecting the amount of depression of the accelerator pedal operated by the driver, are input to the engine controller 9. Based on these input signals, the engine controller 9 optimally controls the amount and timing of fuel injection by the fuel injection valve 5, the ignition timing by the spark plug 4, the opening of the throttle valve 10, etc.

[0016] As one of various controls of the internal combustion engine 1, the engine controller 9 performs air-fuel ratio control to maintain the oxygen storage capacity of the three-way catalyst 15 at a target oxygen storage capacity in order to optimize the exhaust purification performance of the three-way catalyst 15. In the air-fuel ratio control, the fuel injection amount is feedback controlled (for example, PID control) so that the exhaust air-fuel ratio detected by the upstream air-fuel ratio sensor 19 (hereinafter referred to as the upstream exhaust air-fuel ratio) is in line with the target air-fuel ratio. Here, the target air-fuel ratio is calculated so that the oxygen storage capacity of the three-way catalyst 15 estimated from the upstream exhaust air-fuel ratio matches the target oxygen storage capacity. Therefore, the oxygen storage capacity of the three-way catalyst 15 is basically maintained near the target oxygen storage capacity.

[0017] On the other hand, if the estimated oxygen storage capacity deviates from the actual oxygen storage capacity due to some disturbance or estimation error, the actual oxygen storage capacity of the three-way catalyst 15 becomes smaller or larger than the target oxygen storage capacity, and the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15, i.e., the exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor 20 (hereinafter referred to as the downstream exhaust air-fuel ratio), changes to the rich side or the lean side, respectively. Based on such a change in the downstream exhaust air-fuel ratio, the estimated oxygen storage capacity is reset to match the actual oxygen storage capacity.

[0018] That is, in the above embodiment, a first oxygen storage amount OSA1, which is an oxygen storage amount smaller than the target oxygen storage amount, and a second oxygen storage amount OSA2, which is an oxygen storage amount larger than the target oxygen storage amount, are set in advance, and corresponding downstream exhaust air-fuel ratio thresholds RAF1 and RAF2 are set. The threshold RAF1 is slightly richer than the air-fuel ratio equivalent to the stoichiometric air-fuel ratio, and the threshold RAF2 is slightly leaner than the air-fuel ratio equivalent to the stoichiometric air-fuel ratio. When the downstream exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor 20 becomes equal to or less than the threshold RAF1, the actual oxygen storage amount of the three-way catalyst 15 is considered to be equal to or less than the first oxygen storage amount OSA1, and the estimated oxygen storage amount is reset using the value of the first oxygen storage amount OSA1. Similarly, when the downstream exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor 20 becomes equal to or greater than the threshold value RAF2, the actual oxygen storage capacity of the three-way catalyst 15 is deemed to be equal to or greater than the second oxygen storage capacity OSA2, and the estimated oxygen storage capacity is reset using the value of the second oxygen storage capacity OSA2.

[0019] In this way, the accuracy of the estimated oxygen storage amount is ensured by resetting the estimated oxygen storage amount using the value of the first oxygen storage amount OSA1 or the second oxygen storage amount OSA2. At the same time, by resetting the estimated oxygen storage amount, a large difference appears between the estimated oxygen storage amount and the target oxygen storage amount, and the air-fuel ratio of the internal combustion engine 1 (in other words, the fuel injection amount) is controlled in a manner corresponding to each difference, so that the actual oxygen storage amount of the three-way catalyst 15 quickly approaches the target oxygen storage amount.

[0020] FIG. 3 is a flowchart showing the flow of this air-fuel ratio control based on the oxygen storage capacity. In step 1, the oxygen storage capacity of the three-way catalyst 15 is estimated based on the upstream exhaust air-fuel ratio (FrA / F) detected by the upstream air-fuel ratio sensor 19 and the intake air volume detected by the air flow meter 11, which corresponds to the gas flow rate flowing into the three-way catalyst 15. Note that the "intake air volume" does not refer to the air volume per cylinder cycle, but rather to the air flow rate per unit time taken into the internal combustion engine 1 (i.e., passing through the air flow meter 11). The estimated oxygen storage capacity is calculated by adding or subtracting the oxygen storage capacity based on the upstream exhaust air-fuel ratio at each calculation cycle of the engine controller 9. In other words, simply put, if the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 15 is lean, the oxygen storage capacity increases, and if it is rich, the oxygen storage capacity decreases. Therefore, the oxygen storage capacity at that time is estimated by integrating both positive and negative values.

[0021] In step 2, the downstream exhaust air-fuel ratio (RrA / F) detected by the downstream air-fuel ratio sensor 20 is compared with a threshold value RAF1 corresponding to the first oxygen storage amount OSA1 described above. If the downstream exhaust air-fuel ratio is equal to or less than the threshold value RAF1, the process proceeds from step 2 to step 3, where the estimated oxygen storage amount is reset to the value of the first oxygen storage amount OSA1. After the reset, the process proceeds to step 6. If the downstream exhaust air-fuel ratio is greater than the threshold value RAF1, the process proceeds to step 4, where the downstream exhaust air-fuel ratio is compared with a threshold value RAF2 corresponding to the second oxygen storage amount OSA2 described above. If the downstream exhaust air-fuel ratio is equal to or greater than the threshold value RAF2, the process proceeds from step 4 to step 5, where the estimated oxygen storage amount is reset to the value of the second oxygen storage amount OSA2. After the reset, the process proceeds to step 6.

[0022] If the downstream exhaust air-fuel ratio is between two threshold values ​​RAF1, RAF2 that sandwich the stoichiometric air-fuel ratio, the value of the estimated oxygen storage amount estimated in step 1 is maintained as is, and the process proceeds to step 6.

[0023] In step 6, a required target air-fuel ratio is calculated based on the estimated oxygen storage amount and a predetermined target oxygen storage amount so that the estimated oxygen storage amount coincides with the target oxygen storage amount.

[0024] 4 is a block diagram showing the processing of step 6. In the target air-fuel ratio calculation unit 31, the difference between the estimated oxygen storage amount and the target oxygen storage amount is found, and the target air-fuel ratio is calculated so that the oxygen storage amount changes at an appropriate speed. For example, if the estimated oxygen storage amount is greater than the target oxygen storage amount, the target air-fuel ratio is set richer than the stoichiometric air-fuel ratio. Conversely, if the estimated oxygen storage amount is smaller than the target oxygen storage amount, the target air-fuel ratio is set leaner than the stoichiometric air-fuel ratio. Note that the fuel injection amount is controlled to achieve this target air-fuel ratio, so this target air-fuel ratio can basically be considered to be equal to the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1, i.e., the upstream exhaust air-fuel ratio detected by the upstream air-fuel ratio sensor 19.

[0025] By this process, the oxygen storage capacity of the three-way catalyst 15 is maintained near the target oxygen storage capacity, and the downstream exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor 20 is ideally between the two threshold values ​​RAF1 and RAF2. Therefore, the oxidation of CO and HC in the exhaust gas and the reduction of NOx are effectively performed.

[0026] In the present invention, the target oxygen storage amount is not set to the median value between the first oxygen storage amount OSA1 and the second oxygen storage amount OSA2, but is set to a range in which the oxygen storage amount is smaller than the median value. The oxygen storage amount can be expressed in terms of the mass of oxygen (unit: g), but it can also be expressed as a percentage, with the maximum oxygen storage amount of the three-way catalyst 15 being 100(%).

[0027] For example, the first oxygen storage amount OSA1 is greater than 10% of the maximum oxygen storage amount of the three-way catalyst 15, and the second oxygen storage amount OSA2 is less than 90% of the maximum oxygen storage amount of the three-way catalyst 15. The target oxygen storage amount is less than 40% of the maximum oxygen storage amount of the three-way catalyst 15.

[0028] In a preferred embodiment, the first oxygen storage amount OSA1 is 20% of the maximum oxygen storage amount of the three-way catalyst 15, and the second oxygen storage amount OSA2 is 60% of the maximum oxygen storage amount of the three-way catalyst 15. The target oxygen storage amount is 35% of the maximum oxygen storage amount of the three-way catalyst 15.

[0029] In this way, the target oxygen storage amount is not set to the median value between the first oxygen storage amount OSA1 and the second oxygen storage amount OSA2, but is set to a value closer to the first oxygen storage amount OSA1, i.e., a value smaller than this. In other words, the difference in the oxygen storage amount from the target oxygen storage amount to the second oxygen storage amount OSA2 is larger than the difference in the oxygen storage amount from the target oxygen storage amount to the first oxygen storage amount OSA1. Therefore, when there is an estimation error due to a disturbance or some other factor, the actual oxygen storage amount reaches the second oxygen storage amount OSA2 and the estimated oxygen storage amount is reset less frequently than the actual oxygen storage amount reaches the first oxygen storage amount OSA1 and the estimated oxygen storage amount is reset.

[0030] FIG. 2 is a characteristic diagram that schematically shows the relationship between the oxygen storage capacity of the three-way catalyst 15 and the CO and NOx that flow out from the three-way catalyst 15. As shown in the figure, when the oxygen storage capacity of the three-way catalyst 15 is within a certain intermediate range, both CO emissions and NOx emissions are minimized. When the oxygen storage capacity falls below a certain level, CO flows out from the three-way catalyst 15. The amount of CO emissions increases proportionally as the oxygen storage capacity decreases. The same tendency applies to HC, which requires oxidation.

[0031] On the other hand, when the oxygen storage capacity exceeds a certain level, NOx begins to flow out from the three-way catalyst 15. The amount of NOx that flows out increases rapidly when the oxygen storage capacity exceeds a certain level. As the oxygen storage capacity approaches 100%, the rate at which NOx increases becomes gentler.

[0032] Basically, the first oxygen storage amount OSA1 is set to an oxygen storage amount at which the amount of CO flowing downstream of the three-way catalyst 15 becomes an allowable limit, and the second oxygen storage amount OSA2 is set to an oxygen storage amount at which the amount of NOx flowing downstream of the three-way catalyst 15 becomes an allowable limit. However, for example, when the estimated oxygen storage amount is reset to the first oxygen storage amount OSA1 based on the downstream exhaust air-fuel ratio assuming that the oxygen storage amount is equal to or less than the first oxygen storage amount OSA1, there is a delay before the air-fuel ratio of the internal combustion engine 1 becomes lean as the estimated oxygen storage amount is reset, the actual oxygen storage amount begins to increase, and the outflow of CO is suppressed. Similarly, when the estimated oxygen storage amount is reset to the second oxygen storage amount OSA2 based on the downstream exhaust air-fuel ratio assuming that the oxygen storage amount is equal to or greater than the second oxygen storage amount OSA2, there is a delay before the air-fuel ratio of the internal combustion engine 1 becomes rich as the estimated oxygen storage amount is reset, the actual oxygen storage amount begins to decrease, and the outflow of NOx is suppressed. As described above, CO tends to increase proportionally to the amount of oxygen stored, so the outflow of CO due to this delay is relatively small. In contrast, NOx tends to increase rapidly, so the outflow of NOx due to this delay is significant.

[0033] In the above embodiment, the target oxygen storage amount is set closer to the first oxygen storage amount OSA1, which is smaller than the median between the first oxygen storage amount OSA1 and the second oxygen storage amount OSA2, so the frequency with which the estimated oxygen storage amount is reset when the actual oxygen storage amount reaches the second oxygen storage amount OSA2 is relatively lower than the frequency with which the estimated oxygen storage amount is reset when the actual oxygen storage amount reaches the first oxygen storage amount OSA1, thereby suppressing the outflow of NOx as described above.

[0034] 5 is a time chart showing an example of changes in the oxygen storage amount and the like due to the control of the above embodiment. From top to bottom, the chart shows (a) the downstream exhaust air-fuel ratio (RrA / F), (b) the oxygen storage amount, and (c) the target air-fuel ratio. The target air-fuel ratio is also the upstream exhaust air-fuel ratio (FrA / F). In the (b) oxygen storage amount column, the estimated oxygen storage amount b1 and the actual oxygen storage amount b2 are shown overlapping each other.

[0035] In this example time chart, at time t1, the downstream exhaust air-fuel ratio becomes equal to or less than the threshold value RAF1 corresponding to the first oxygen storage amount OSA1, and accordingly, the estimated oxygen storage amount b1 is reset to the first oxygen storage amount OSA1, causing the target air-fuel ratio to change in a stepwise manner to the lean side.

[0036] At time t2, the downstream exhaust air-fuel ratio becomes equal to or greater than the threshold value RAF2 corresponding to the second oxygen storage amount OSA2, and the estimated oxygen storage amount b1 is reset to the second oxygen storage amount OSA2. As a result, the target air-fuel ratio changes in a stepwise manner to the rich side. At time t3, the downstream exhaust air-fuel ratio again becomes equal to or less than the threshold value RAF1, the estimated oxygen storage amount b1 is reset to the first oxygen storage amount OSA1, and the target air-fuel ratio changes in a stepwise manner to the lean side.

[0037] 5 is an exaggerated illustration for the purpose of explaining the reset operation, and as described above, ideally the downstream exhaust air-fuel ratio is maintained between the two threshold values ​​RAF1 and RAF2, and air-fuel ratio control based on the estimated oxygen storage amount b1 continues without resetting. Also, resetting by threshold value RAF1 and resetting by threshold value RAF2 do not necessarily occur alternately.

[0038] Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and various modifications are possible. For example, in the above embodiment, the three-way catalyst 15 is used as an example of an exhaust purification catalyst, but the present invention can be similarly applied to exhaust purification catalysts other than the three-way catalyst that have oxygen storage capacity.

[0039] Furthermore, although not shown in detail, when a temporary stop of the internal combustion engine 1 is required in an idle stop or series hybrid vehicle, it is desirable to operate the internal combustion engine 1 with a rich air-fuel ratio before the temporary stop so that the oxygen storage amount of the three-way catalyst 15 is smaller than the target oxygen storage amount. This suppresses NOx emissions at the initial stage of restarting the internal combustion engine 1.

Claims

1. An air-fuel ratio control method for an internal combustion engine, which is provided with an exhaust purification catalyst having an oxygen storage capacity in an exhaust passage, and controls the air-fuel ratio so that the oxygen storage amount of the exhaust purification catalyst becomes a target oxygen storage amount, an oxygen storage amount of the exhaust purification catalyst is estimated based on an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst; controlling the air-fuel ratio of the internal combustion engine so that the estimated oxygen storage amount coincides with the target oxygen storage amount; detecting an air-fuel ratio of exhaust gas flowing out from the exhaust purification catalyst downstream of the exhaust purification catalyst; when it is detected based on the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst that the oxygen storage amount of the exhaust purification catalyst is equal to or less than a predetermined first oxygen storage amount that is smaller than the target oxygen storage amount, resetting the estimated oxygen storage amount to the first oxygen storage amount, resetting the estimated oxygen storage amount to the second oxygen storage amount when it is detected based on the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst that the oxygen storage amount of the exhaust purification catalyst is equal to or greater than a predetermined second oxygen storage amount that is greater than the target oxygen storage amount; Here, the target oxygen storage amount is set to a range in which the oxygen storage amount is smaller than the median value between the first oxygen storage amount and the second oxygen storage amount. A method for controlling the air-fuel ratio of an internal combustion engine.

2. the first oxygen storage amount is greater than 10% of the maximum oxygen storage amount of the exhaust purification catalyst, the second oxygen storage amount is smaller than 90% of the maximum oxygen storage amount of the exhaust purification catalyst; 2. The method for controlling an air-fuel ratio of an internal combustion engine according to claim 1.

3. the target oxygen storage amount is smaller than 40% of the maximum oxygen storage amount of the exhaust purification catalyst; 3. The air-fuel ratio control method for an internal combustion engine according to claim 2.

4. the first oxygen storage amount is 20% of the maximum oxygen storage amount of the exhaust purification catalyst, the second oxygen storage amount is 60% of the maximum oxygen storage amount of the exhaust purification catalyst, 3. The air-fuel ratio control method for an internal combustion engine according to claim 2.

5. the target oxygen storage amount is 35% of the maximum oxygen storage amount of the exhaust purification catalyst, 5. The method for controlling an air-fuel ratio of an internal combustion engine according to claim 4.

6. when a temporary suspension of the internal combustion engine is requested, before the temporary suspension is executed, the internal combustion engine is operated with a rich air-fuel ratio so that the oxygen storage amount of the exhaust purification catalyst becomes smaller than the target oxygen storage amount; 2. The method for controlling an air-fuel ratio of an internal combustion engine according to claim 1.

7. An air-fuel ratio control device for an internal combustion engine, comprising: an exhaust purification catalyst having an oxygen storage capacity, which is provided in an exhaust passage of the internal combustion engine; an upstream air-fuel ratio sensor provided upstream of the exhaust purification catalyst; a downstream air-fuel ratio sensor provided downstream of the exhaust purification catalyst; and a controller for controlling the air-fuel ratio of the internal combustion engine, The above controller is an oxygen storage amount of the exhaust purification catalyst based on the air-fuel ratio detected by the upstream air-fuel ratio sensor, and controlling the air-fuel ratio of the internal combustion engine so that the estimated oxygen storage amount coincides with a target oxygen storage amount; When it is detected based on the air-fuel ratio detected by the downstream air-fuel ratio sensor that the oxygen storage amount of the exhaust purification catalyst is equal to or less than a predetermined first oxygen storage amount that is smaller than the target oxygen storage amount, the estimated oxygen storage amount is reset to the first oxygen storage amount, and when it is detected that the oxygen storage amount of the exhaust purification catalyst is equal to or greater than a predetermined second oxygen storage amount that is larger than the target oxygen storage amount, the estimated oxygen storage amount is reset to the second oxygen storage amount, Here, the target oxygen storage amount is set to a range in which the oxygen storage amount is smaller than the median value between the first oxygen storage amount and the second oxygen storage amount. Air-fuel ratio control device for internal combustion engines.

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