Air-fuel ratio control method and device for internal combustion engine

Abstract

The amount of oxygen reserve in the three-way catalyst (15) is estimated (S1) on the basis of the upstream-side exhaust air-fuel ratio (FrA / F), and the target air-fuel ratio is controlled (S6) such that the estimated amount of oxygen reserve matches the determined target amount of oxygen reserve. If a downstream-side exhaust air-fuel ratio (RrA / F) detected by a downstream-side air-fuel ratio sensor (20) is less than or equal to a threshold value (RAF1) corresponding to a first oxygen storage amount (OSA1), the estimated oxygen storage amount is reset to the value of the first oxygen storage amount (OSA1) (S2, S3). If the downstream-side exhaust air-fuel ratio (RrA / F) is greater than or equal to a threshold value (RAF2) corresponding to the second oxygen storage amount (OSA2), the estimated oxygen storage amount is reset to the value of the second oxygen storage amount (OSA2) (S4, S5). The target oxygen storage amount is set so as to be closer to the first oxygen storage amount (OSA1) than the central value between the first oxygen storage amount (OSA1) and the second oxygen storage amount (OSA2).

Description

Technical Field

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

[0002] As an exhaust gas purification catalyst, for example, a three-way catalyst can achieve oxidation of CO and HC in the exhaust gas and reduction of NOx, but in order to achieve both the above oxidation and reduction based on the catalytic action at a high level, the so-called oxygen storage capacity of the catalyst for storing / releasing oxygen is more important. Therefore, the following technology is known, that is, estimating the oxygen storage amount of the three-way catalyst and controlling the target air-fuel ratio in a manner that maintains the oxygen storage amount within an appropriate range.

[0003] Patent Document 1 discloses a technique in which a downstream air-fuel ratio sensor is provided on the downstream side of an exhaust purification catalyst, and when the air-fuel ratio detected by the downstream air-fuel ratio sensor reaches a rich determination air-fuel ratio, the target air-fuel ratio is switched to a lean air-fuel ratio, and when the lean determination air-fuel ratio is reached, the target air-fuel ratio is switched to a rich air-fuel ratio. In addition, as a one-side breakdown control, 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 after switching to a lean air-fuel ratio.

[0004] However, Patent Document 1 is basically a technique for actively increasing or decreasing the oxygen storage amount of the exhaust gas purification catalyst, and does not converge it to a constant target oxygen storage amount. When the detected air-fuel ratio obtained by the downstream air-fuel ratio sensor reaches the rich judgment air-fuel ratio, CO and HC have already begun to flow out of the exhaust gas purification catalyst. Similarly, when the lean judgment air-fuel ratio is reached, NOx has already begun to flow out of the exhaust gas purification catalyst. In particular, NOx has the following characteristics, that is, when the oxygen storage amount exceeds the level corresponding to the lean judgment air-fuel ratio, it increases sharply downstream of the exhaust gas purification catalyst, so it is easy to generate NOx emissions exceeding the permissible level due to the lag of control.

[0005] Patent Document 1: Japanese Patent Application Publication No. 2015-71959 Summary of the invention

[0006] The present invention is a method for controlling the air-fuel ratio of an internal combustion engine, wherein an exhaust gas purification catalyst is provided in an exhaust passage, the exhaust gas purification catalyst having an oxygen storage capacity, and the air-fuel ratio is controlled in such a way that the oxygen storage amount of the exhaust gas purification catalyst reaches a target oxygen storage amount, wherein:

[0007] estimating the oxygen storage amount of the exhaust gas purification catalyst based on the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst,

[0008] The air-fuel ratio of the internal combustion engine is controlled so that the estimated oxygen storage amount coincides with the target oxygen storage amount.

[0009] detecting the air-fuel ratio of the exhaust gas flowing out of the exhaust purification catalyst at the downstream side of the exhaust purification catalyst,

[0010] When it is detected based on the air-fuel ratio of the exhaust gas flowing out of the exhaust purification catalyst that the oxygen storage amount of the exhaust purification catalyst is less than or equal to a predetermined first oxygen storage amount, resetting the estimated oxygen storage amount to the first oxygen storage amount, the first oxygen storage amount being smaller than the target oxygen storage amount,

[0011] when it is detected based on the air-fuel ratio of the exhaust gas flowing out of the exhaust purification catalyst that the oxygen storage amount of the exhaust purification catalyst is greater than or equal to a predetermined second oxygen storage amount, resetting the estimated oxygen storage amount to the second oxygen storage amount, the second oxygen storage amount being greater than the target oxygen storage amount,

[0012] Here, the target oxygen storage amount is set within a range such that the oxygen storage amount is smaller than a median value between the first oxygen storage amount and the second oxygen storage amount.

[0013] Regarding the above structure, the air-fuel ratio of the internal combustion engine is controlled in such a way that the estimated oxygen storage amount is consistent with the target oxygen storage amount, so it is ideal to maintain the actual oxygen storage amount of the exhaust purification catalyst near the target oxygen storage amount. If the estimated oxygen storage amount deviates from the actual oxygen storage amount due to external interference or some other factors, for example, the actual oxygen storage amount is too small, so that the air-fuel ratio of the exhaust gas flowing out of the exhaust purification catalyst may be less than or equal to the air-fuel ratio equivalent to the first oxygen storage amount. Thus, it is detected that the actual oxygen storage amount is less than or equal to the first oxygen storage amount. Alternatively, because the actual oxygen storage amount is too large, it is possible that the air-fuel ratio of the exhaust gas flowing out of the exhaust purification catalyst is greater than or equal to the air-fuel ratio equivalent to the second oxygen storage amount. Thus, it is detected that the actual oxygen storage amount is greater than or equal to the second oxygen storage amount.

[0014] At this time, the estimated oxygen storage amount is reset to the first oxygen storage amount or the second oxygen storage amount. As a result, the difference between the estimated oxygen storage amount and the target oxygen storage amount becomes large, and the air-fuel ratio of the internal combustion engine is controlled accordingly.

[0015] In the present invention, the target oxygen storage amount is not set to the middle value between the first oxygen storage amount and the second oxygen storage amount, but is set to a value that makes the oxygen storage amount smaller than the middle value, that is, near the first oxygen storage amount. In other words, the difference in oxygen storage amount from the target oxygen storage amount to the second oxygen storage amount is larger than the difference in oxygen storage amount from the target oxygen storage amount to the first oxygen storage amount. Therefore, in the case where an estimation error occurs due to external disturbance or some other factor, the frequency of resetting the estimated oxygen storage amount when the actual oxygen storage amount reaches the second oxygen storage amount is relatively small compared to the frequency of resetting the estimated oxygen storage amount when the actual oxygen storage amount reaches the first oxygen storage amount. As a result, the emission of NOx, which has the characteristic of increasing sharply when the oxygen storage amount is equal to or greater than the second oxygen storage amount, is suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a diagram for explaining the structure of an internal combustion engine having an embodiment of a three-way catalyst.

[0017] Figure 2 This is a characteristic diagram showing the relationship between the oxygen storage amount of the three-way catalyst and CO and NOx flowing out of the three-way catalyst.

[0018] Figure 3 is a flow chart of air-fuel ratio control according to an embodiment.

[0019] Figure 4 This is a block diagram regarding the calculation of the target air-fuel ratio.

[0020] Figure 5 It 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

[0021] Hereinafter, one embodiment of the present invention will be described in detail based on the drawings. Figure 1 1 is an explanatory diagram showing a schematic structure of an internal combustion engine 1 to which an embodiment of the present invention is applied. The internal combustion engine 1 of one embodiment is a spark ignition type internal combustion engine (so-called gasoline internal combustion engine) of a four-stroke cycle, and has an intake valve 2, an exhaust valve 3, and a spark plug 4 in each cylinder. In addition, the illustrated example is configured as a direct-injection type internal combustion engine, and a fuel injection valve 5 that injects fuel into the cylinder is, for example, arranged on the side of the intake valve 2. In addition, a port injection type structure that injects fuel into the intake port 6 may be used.

[0022] An electronically controlled throttle valve 10 is installed on the upstream side of the manifold 8 of the intake passage 7 connected to the intake port 6 of each cylinder, and the opening degree of the throttle valve 10 is controlled according to a control signal from the engine controller 9. An air flow meter 11 for detecting the intake air amount is arranged on the upstream side of the throttle valve 10, and an air filter 12 is arranged further upstream.

[0023] The exhaust ports 13 of the cylinders merge into one exhaust passage 14, and an exhaust purification catalyst for exhaust purification, such as a three-way catalyst 15, is provided in the exhaust passage 14. The three-way catalyst 15 is, for example, a so-called single ceramic catalyst in which a catalyst layer including a catalyst metal is coated on the surface of a single ceramic body having a fine passage. In addition, the three-way catalyst 15 may be a structure that also includes a catalyst on the downstream side arranged in series (so-called underfloor catalyst).

[0024] An upstream air-fuel ratio sensor 19 for detecting the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1 (in other words, the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 15) is arranged at the inlet side of the three-way catalyst 15 of the exhaust passage 14, that is, at a position upstream of the three-way catalyst 15. The upstream air-fuel ratio sensor 19 is a so-called wide-range air-fuel ratio sensor that can obtain an output corresponding to the exhaust air-fuel ratio. In addition, a downstream air-fuel ratio sensor 20 for detecting the air-fuel ratio of the exhaust gas flowing out of the three-way catalyst 15 is arranged from the outlet side to the downstream side of the three-way catalyst 15. Similar to 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.

[0025] The detection signals of the air-fuel ratio sensors 19 and 20 and the air flow meter 11 are input to the engine controller 9. The detection signals of a plurality of sensors such as a crank angle sensor 21 for detecting the rotation speed of the internal combustion engine, a water temperature sensor 22 for detecting the temperature of the cooling water, and an accelerator opening sensor 23 for detecting the amount of depression of the accelerator pedal operated by the driver are also input to the engine controller 9. Based on the above input signals, the engine controller 9 controls the fuel injection amount and injection timing of the fuel injection valve 5, the ignition timing of the spark plug 4, the opening of the throttle valve 10, etc. to be optimal.

[0026] As one of the various controls of the internal combustion engine 1, the engine controller 9 performs air-fuel ratio control for maintaining the oxygen storage amount of the three-way catalyst 15 at a target oxygen storage amount in order to optimize the exhaust purification performance based on the three-way catalyst 15. In the air-fuel ratio control, the fuel injection amount is feedback-controlled (for example, PID control, etc.) in such a manner 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) follows the target air-fuel ratio. Here, the target air-fuel ratio is calculated in such a manner that the oxygen storage amount of the three-way catalyst 15 estimated based on the upstream exhaust air-fuel ratio is consistent with the target oxygen storage amount. Therefore, the oxygen storage amount of the three-way catalyst 15 is basically maintained near the target oxygen storage amount.

[0027] On the other hand, if the estimated oxygen storage amount deviates from the actual oxygen storage amount due to some external disturbance or estimation error, the actual oxygen storage amount of the three-way catalyst 15 deviates to a direction smaller than or larger than the target oxygen storage amount, and the air-fuel ratio of the exhaust gas flowing out of the three-way catalyst 15, that is, 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 the change in the downstream exhaust air-fuel ratio, the estimated oxygen storage amount is reset in such a manner that it follows the actual oxygen storage amount.

[0028] That is, in the above-mentioned embodiment, the first oxygen storage amount OSA1 in which the oxygen storage amount is smaller than the target oxygen storage amount and the second oxygen storage amount OSA2 in which the oxygen storage amount is larger than the target oxygen storage amount are preset, and the threshold values ​​RAF1 and RAF2 of the downstream exhaust air-fuel ratio are assigned correspondingly, respectively. The threshold value RAF1 is slightly on the rich side compared with the air-fuel ratio equivalent to the theoretical air-fuel ratio, and the threshold value RAF2 is slightly on the lean side compared with the air-fuel ratio equivalent to the theoretical air-fuel ratio. If the downstream exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor 20 is less than or equal to the threshold value RAF1, it is regarded that the actual oxygen storage amount of the three-way catalyst 15 is less than or equal to 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, if the downstream exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor 20 is greater than or equal to the threshold RAF2, the actual oxygen storage amount of the three-way catalyst 15 is deemed to be greater than or equal to the second oxygen storage amount OSA2, and the estimated oxygen storage amount is reset using the value of the second oxygen storage amount OSA2.

[0029] By resetting the estimated oxygen storage amount using the value of the first oxygen storage amount OSA1 or the second oxygen storage amount OSA2, the accuracy of the estimated oxygen storage amount is ensured. In addition, by resetting the estimated oxygen storage amount at the same time, the difference between the estimated oxygen storage amount and the target oxygen storage amount is made large, and the air-fuel ratio (in other words, the fuel injection amount) of the internal combustion engine 1 is controlled in a corresponding manner, so that the actual oxygen storage amount of the three-way catalyst 15 quickly approaches the target oxygen storage amount.

[0030] Figure 31 is a flowchart showing the flow of air-fuel ratio control based on the oxygen storage amount. In step 1, the oxygen storage amount 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 amount detected by the air flow meter 11, which is equivalent to the gas flow rate flowing into the three-way catalyst 15. In addition, the "intake air amount" does not mean the amount of air in each cycle of the cylinder, but means the flow rate of air sucked into the internal combustion engine 1 (i.e., passing through the air flow meter 11) per unit time. In each calculation cycle of the engine controller 9, the oxygen storage amount is added and subtracted based on the upstream exhaust air-fuel ratio at that time, thereby obtaining the estimated oxygen storage amount. That is, in short, if the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 15 is lean, the oxygen storage amount increases, and if it is rich, the oxygen storage amount decreases, so the oxygen storage amount at that time point is estimated by integrating in both positive and negative directions.

[0031] In step 2, the downstream exhaust air-fuel ratio (RrA / F) detected by the downstream air-fuel ratio sensor 20 is compared with the threshold RAF1 corresponding to the above-mentioned first oxygen storage amount OSA1. If the downstream exhaust air-fuel ratio is less than or equal to the threshold RAF1, the process proceeds from step 2 to step 3, and the estimated oxygen storage amount is reset to the value of the first oxygen storage amount OSA1. Moreover, after the reset, the process proceeds to step 6. If the downstream exhaust air-fuel ratio is greater than the threshold RAF1, the process proceeds to step 4, and the downstream exhaust air-fuel ratio is compared with the threshold RAF2 corresponding to the above-mentioned second oxygen storage amount OSA2. If the downstream exhaust air-fuel ratio is greater than or equal to the threshold RAF2, the process proceeds from step 4 to step 5, and the estimated oxygen storage amount is reset to the value of the second oxygen storage amount OSA2. Moreover, after the reset, the process proceeds to step 6.

[0032] If the downstream exhaust gas air-fuel ratio is between the two threshold values ​​RAF1 and RAF2 that sandwich the stoichiometric air-fuel ratio, the value of the estimated oxygen storage amount estimated in step 1 is maintained unchanged and the process proceeds to step 6 .

[0033] In step 6, based on the estimated oxygen storage amount and the predetermined target oxygen storage amount, a required target air-fuel ratio is calculated so that the estimated oxygen storage amount coincides with the target oxygen storage amount.

[0034] Figure 4The processing of step 6 is shown as a block diagram. The target air-fuel ratio calculation unit 31 calculates the difference between the estimated oxygen storage amount and the target oxygen storage amount, and calculates the target air-fuel ratio in such a manner 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 to the rich side compared with the theoretical air-fuel ratio. On the contrary, if the estimated oxygen storage amount is less than the target oxygen storage amount, the target air-fuel ratio is set to the lean side compared with the theoretical air-fuel ratio. In addition, the fuel injection amount is controlled in such a manner as to achieve the target air-fuel ratio, so that the target air-fuel ratio can be basically regarded as being equal to the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1, that is, the upstream exhaust air-fuel ratio detected by the upstream air-fuel ratio sensor 19.

[0035] By this process, the oxygen storage amount of the three-way catalyst 15 is maintained near the target oxygen storage amount, and the downstream exhaust air-fuel ratio detected by the downstream air-fuel ratio sensor 20 is ideally between the two thresholds RAF1 and RAF2. Therefore, the oxidation of CO and HC and the reduction of NOx in the exhaust gas are effective.

[0036] Here, in the present invention, the target oxygen storage amount is not set to the median value of the first oxygen storage amount OSA1 and the second oxygen storage amount OSA2, but is set to a range where the oxygen storage amount is smaller than the median value. In addition, the oxygen storage amount can be processed by the mass of oxygen (unit g), and as a general rule, the maximum oxygen storage amount of the three-way catalyst 15 can be set to 100 (%) and expressed as a percentage relative to this.

[0037] 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 . Furthermore, the target oxygen storage amount is less than 40% of the maximum oxygen storage amount of the three-way catalyst 15 .

[0038] 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 . Moreover, the target oxygen storage amount is 35% of the maximum oxygen storage amount of the three-way catalyst 15 .

[0039] In this way, the target oxygen storage amount is not set to the middle value between the first oxygen storage amount OSA1 and the second oxygen storage amount OSA2, but is set to the side of the oxygen storage amount smaller than the middle value, that is, near the first oxygen storage amount OSA1. In other words, the difference between the target oxygen storage amount and the second oxygen storage amount OSA2 is larger than the difference between the target oxygen storage amount and the first oxygen storage amount OSA1. Therefore, in the case where an estimation error is caused by external disturbance or some other factor, the frequency of resetting the estimated oxygen storage amount when the actual oxygen storage amount reaches the second oxygen storage amount OSA2 is relatively small compared to the frequency of resetting the estimated oxygen storage amount when the actual oxygen storage amount reaches the first oxygen storage amount OSA1.

[0040] Figure 2 This is a characteristic diagram schematically showing the relationship between the oxygen storage amount of the three-way catalyst 15 and the CO and NOx flowing out of the three-way catalyst 15. As shown in the figure, if the oxygen storage amount of the three-way catalyst 15 is in a constant range in the middle, both the emission of CO and the emission of NOx are minimum. If the oxygen storage amount is less than a certain level, CO flows out of the three-way catalyst 15. The emission of CO increases in proportion to the decrease in the oxygen storage amount. In addition, HC that needs to be oxidized also shows the same trend.

[0041] On the other hand, when the oxygen storage amount exceeds a certain level, NOx flows out from the three-way catalyst 15. The outflow of NOx has the characteristic of increasing rapidly when the oxygen storage amount exceeds a certain level. Furthermore, as the oxygen storage amount approaches 100%, the gradient of NOx increase becomes gentle.

[0042] Basically, the first oxygen storage amount OSA1 is set to an oxygen storage amount at which CO flowing out downstream of the three-way catalyst 15 reaches an allowable limit, and the second oxygen storage amount OSA2 is set to an oxygen storage amount at which NOx flowing out downstream of the three-way catalyst 15 reaches 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 so that the oxygen storage amount is less than or equal to the first oxygen storage amount OSA1, the air-fuel ratio of the internal combustion engine 1 becomes leaner as the estimated oxygen storage amount is reset, and there is a lag until the actual oxygen storage amount turns to increase and the CO outflow is suppressed. Similarly, when the estimated oxygen storage amount is reset to the second oxygen storage amount OSA2 based on the downstream exhaust gas air-fuel ratio so that the oxygen storage amount is greater than or equal to the second oxygen storage amount OSA2, there is a hysteresis until the actual oxygen storage amount turns to decrease and the NOx outflow is suppressed as the air-fuel ratio of the internal combustion engine 1 becomes richer with the reset of the estimated oxygen storage amount. Here, as described above, CO has a tendency to increase in proportion to the oxygen storage amount, so the outflow of CO with this hysteresis is small. On the other hand, NOx has a tendency to increase sharply, so the outflow of NOx with the hysteresis becomes significant.

[0043] In the above embodiment, the target oxygen storage amount is set to the first oxygen storage amount OSA1 which is close to the oxygen storage amount being less than the median value between the first oxygen storage amount OSA1 and the second oxygen storage amount OSA2. Therefore, the frequency of resetting the estimated oxygen storage amount when the actual oxygen storage amount reaches the second oxygen storage amount OSA2 is relatively small compared to the frequency of resetting the estimated oxygen storage amount when the actual oxygen storage amount reaches the first oxygen storage amount OSA1. Thus, the outflow of NOx is suppressed.

[0044] Figure 5 This is a timing chart showing an example of changes in the oxygen storage amount and the like obtained by the control of the above-mentioned embodiment. From the top of the figure, (a) the downstream exhaust air-fuel ratio (RrA / F), (b) the oxygen storage amount, and (c) the target air-fuel ratio are shown in order. The target air-fuel ratio is also the upstream exhaust air-fuel ratio (FrA / F). (b) In the oxygen storage amount column, the estimated oxygen storage amount b1 and the actual oxygen storage amount b2 are shown in an overlapping manner.

[0045] In the example of this time chart, at time t1, the downstream exhaust air-fuel ratio is less than or equal to the threshold 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. Therefore, the target air-fuel ratio changes stepwise toward the lean side.

[0046] In addition, at time t2, the downstream exhaust air-fuel ratio is greater than or equal to the threshold value RAF2 corresponding to the second oxygen storage amount OSA2, and accordingly, the estimated oxygen storage amount b1 is reset to the second oxygen storage amount OSA2. Therefore, the target air-fuel ratio changes stepwise to the rich side. At time t3, the downstream exhaust air-fuel ratio becomes less than or equal to the threshold value RAF1 again, and the estimated oxygen storage amount b1 is reset to the first oxygen storage amount OSA1, and the target air-fuel ratio changes stepwise to the lean side.

[0047] also, Figure 5 Although the illustration is exaggerated for the purpose of explaining the reset operation, as mentioned above, it is ideal to maintain the downstream exhaust air-fuel ratio between the two thresholds RAF1 and RAF2, and not to continue the air-fuel ratio control based on the estimated oxygen storage amount b1 with the reset. In addition, the reset based on the threshold RAF1 and the reset based on the threshold RAF2 do not necessarily need to be performed alternately.

[0048] One embodiment of the present invention has been described above, but 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 shown as an example of an exhaust gas purification catalyst, but the present invention can also be applied to an exhaust gas purification catalyst having oxygen storage capacity other than the three-way catalyst.

[0049] Although not shown in detail, it is preferable to drive such that, when a temporary stop of the internal combustion engine 1 is requested in an idling stop or a series hybrid vehicle, the air-fuel ratio of the internal combustion engine 1 is made richer before the temporary stop is performed so that the oxygen storage amount of the three-way catalyst 15 is less than the target oxygen storage amount. Thus, the emission of NOx at the initial stage of restarting the internal combustion engine 1 is suppressed.

Claims

1. A method for controlling an air-fuel ratio of an internal combustion engine, wherein an exhaust gas purification catalyst is provided in an exhaust passage, the exhaust gas purification catalyst having an oxygen storage capacity, and the air-fuel ratio is controlled in such a way that the oxygen storage amount of the exhaust gas purification catalyst reaches a target oxygen storage amount, wherein: estimating the oxygen storage amount of the exhaust gas purification catalyst based on the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst, The air-fuel ratio of the internal combustion engine is controlled so that the estimated oxygen storage amount coincides with the target oxygen storage amount. detecting, on the downstream side of the exhaust gas purification catalyst, the air-fuel ratio of the exhaust gas flowing out from the exhaust gas purification catalyst; When it is detected based on the air-fuel ratio of the exhaust gas flowing out of the exhaust purification catalyst that the oxygen storage amount of the exhaust purification catalyst is less than or equal to a predetermined first oxygen storage amount, resetting the estimated oxygen storage amount to the first oxygen storage amount, the first oxygen storage amount being smaller than the target oxygen storage amount, when it is detected based on the air-fuel ratio of the exhaust gas flowing out of the exhaust purification catalyst that the oxygen storage amount of the exhaust purification catalyst is greater than or equal to a predetermined second oxygen storage amount, resetting the estimated oxygen storage amount to the second oxygen storage amount, the second oxygen storage amount being greater than the target oxygen storage amount, Here, the target oxygen storage amount is set within a range such that the oxygen storage amount is smaller than a median value between the first oxygen storage amount and the second oxygen storage amount.

2. The air-fuel ratio control method of an internal combustion engine according to claim 1, wherein: 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.

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

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

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

6. The air-fuel ratio control method of an internal combustion engine according to claim 1, wherein: When temporary stop of the internal combustion engine is requested, before the temporary stop is executed, an operation is performed to make the air-fuel ratio of the internal combustion engine rich so that the oxygen storage amount of the exhaust purification catalyst becomes smaller than the target oxygen storage amount.

7. An air-fuel ratio control device for an internal combustion engine, comprising: an exhaust gas purification catalyst, which is arranged in an exhaust passage of the internal combustion engine and has an oxygen storage capacity; an upstream air-fuel ratio sensor, which is arranged on the upstream side of the exhaust gas purification catalyst; a downstream air-fuel ratio sensor, which is arranged on the downstream side of the exhaust gas purification catalyst; and a controller, which controls the air-fuel ratio of the internal combustion engine, wherein: The controller estimates the oxygen storage amount of the exhaust purification catalyst based on the detected air-fuel ratio of the upstream air-fuel ratio, and controls the air-fuel ratio of the internal combustion engine so that the estimated oxygen storage amount coincides with the target oxygen storage amount. When it is detected based on the detected air-fuel ratio of the downstream air-fuel ratio sensor that the oxygen storage amount of the exhaust purification catalyst is less than or equal to a prescribed first oxygen storage amount, the estimated oxygen storage amount is reset to the first oxygen storage amount, the first oxygen storage amount being less than the target oxygen storage amount, and when it is detected that the oxygen storage amount of the exhaust purification catalyst is greater than or equal to a prescribed second oxygen storage amount, the estimated oxygen storage amount is reset to the second oxygen storage amount, the second oxygen storage amount being greater than the target oxygen storage amount, Here, the target oxygen storage amount is set within a range such that the oxygen storage amount is smaller than a median value between the first oxygen storage amount and the second oxygen storage amount.

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