Exhaust gas purification system of an internal combustion engine

Abstract

Exhaust gas purification system of an internal combustion engine, which features: a catalyst (20) arranged in an exhaust duct and capable of storing oxygen; an upstream air-fuel ratio sensor (40) which is arranged in the direction of exhaust gas flow on an upstream side of the catalyst (20) and which detects an air-fuel ratio of the incoming exhaust gas flowing into the catalyst (20); a downstream air-fuel ratio sensor (41) arranged in the direction of exhaust gas flow on a downstream side of the catalyst (20) and which detects an air-fuel ratio of the outgoing exhaust gas flowing from the catalyst (20); and an air-fuel ratio control device (31) configured to control an air-fuel ratio of the incoming exhaust gas, wherein the air-fuel ratio control device (31) is configured to switch a target air-fuel ratio of the incoming gas between a rich air-fuel ratio (TAFrich), which is richer than a stoichiometric air-fuel ratio, and a lean air-fuel ratio (TAFlean), which is leaner than a stoichiometric air-fuel ratio; an oxygen storage quantity (OSA), which is an estimated value of the amount of oxygen stored in the catalyst (20) while the target air-fuel ratio is maintained at the lean air-fuel ratio (TAFlean); and an oxygen discharge quantity (ODA), which is an estimated value of the amount of oxygen discharged from the catalyst (20) while the target air-fuel ratio is maintained at the rich air-fuel ratio (TAFrich).to calculate, based on an air-fuel ratio detected by the upstream air-fuel ratio sensor (40), to update a learning value based on a difference between the oxygen storage quantity (OSA) and the oxygen discharge quantity (ODA), and to correct an air-fuel ratio-related parameter based on the learning value so that the difference between the oxygen storage quantity (OSA) and the oxygen discharge quantity (ODA) becomes smaller, and , where an operating state of the internal combustion engine changes between a first state and a second state and the air-fuel ratio control device (31) is configured to change a condition for switching the target air-fuel ratio between the first state and the second state, to store the learned value as a first state value at the time when the operating state of the internal combustion engine changes from the first state to the second state, and to update the learned value to the first state value when the operating state of the internal combustion engine returns from the second state to the first state.

Description

Area

[0001] The present invention relates to an exhaust gas purification system of an internal combustion engine. background

[0002] It has been known in the past to install a catalyst with oxygen storage capacity in the exhaust passage of an internal combustion engine and to use the catalyst to remove unburned gas (HC, CO, etc.) and NOx from the exhaust gas. The higher the oxygen storage capacity of the catalyst, the greater the amount of oxygen that can be stored in the catalyst, and the better the exhaust gas purification performance of the catalyst.

[0003] To maintain the oxygen storage capacity of the catalyst, the oxygen storage amount of the catalyst preferably fluctuates, so that the oxygen storage amount of the catalyst is not kept constant. In the internal combustion engine described in JP 2015-071963 A, the target air-fuel ratio of the exhaust gas flowing into the catalyst is alternately switched between a lean air-fuel ratio leaner than a stoichiometric air-fuel ratio and a rich air-fuel ratio richer than the stoichiometric air-fuel ratio to fluctuate the oxygen storage amount of the catalyst.Specifically, when the air-fuel ratio estimated on the downstream side of the air-fuel ratio sensor becomes a rich air-fuel ratio richer than the stoichiometric air-fuel ratio or less, the target air-fuel ratio is switched from the rich air-fuel ratio to the lean air-fuel ratio, while when the estimated value of the amount of oxygen stored in the catalyst becomes a switching reference value or more while the target air-fuel ratio is maintained at the lean air-fuel ratio, the target air-fuel ratio is switched from the lean air-fuel ratio to the rich air-fuel ratio.

[0004] Further, when such control is performed, an air-fuel ratio-related parameter is corrected by learning control to prevent the exhaust emission from deteriorating due to the deviation of the output value of the upstream air-fuel ratio sensor.Specifically, the oxygen storage value, which is the estimated value of the amount of oxygen stored on the catalyst while the target air-fuel ratio is maintained at the lean air-fuel ratio, and the oxygen discharge amount, which is the estimated value of the amount of oxygen discharged from the catalyst while the target air-fuel ratio is maintained at the rich air-fuel ratio, are calculated, the learning value is updated based on a difference between the oxygen storage amount and the oxygen discharge amount, and the air-fuel ratio-related parameter is corrected based on the learning value so that the difference between the oxygen storage amount and the oxygen discharge amount becomes smaller. SummaryTechnical problem

[0005] In this regard, the condition of the exhaust gas flowing into the catalyst fluctuates depending on the engine operating condition, even if the target air-fuel ratio is set. Therefore, it is sometimes desirable to change the target air-fuel ratio switching condition (rich estimated air-fuel ratio and switching reference value in JP 2015-071963 A) depending on the engine operating condition to prevent exhaust emission deterioration while maintaining the catalyst's oxygen storage capacity.

[0006] For example, as the richness of the rich-estimated air-fuel ratio increases, the timing for switching the target air-fuel ratio from the rich air-fuel ratio to the lean air-fuel ratio becomes delayed. As a result, the time period for maintaining the target air-fuel ratio at the rich air-fuel ratio becomes longer, and the oxygen discharge amount becomes larger. On the other hand, as the switching reference value increases, the timing for switching the target air-fuel ratio from the lean air-fuel ratio to the rich air-fuel ratio becomes delayed. As a result, the time period for maintaining the target air-fuel ratio at the lean air-fuel ratio becomes longer, and the oxygen storage amount becomes larger.

[0007] Therefore, when the target air-fuel ratio switching condition changes, even if the output of the upstream air-fuel ratio sensor is normal, the learning value calculated from the oxygen storage amount and the oxygen discharge amount will sometimes change. As a result, the appropriate learning value will fluctuate depending on the engine operating condition. Therefore, if the learning value is maintained when the engine operating condition changes, the air-fuel ratio of the exhaust gas flowing into the catalyst will become a value that does not match the changed operating condition, and exhaust emissions are likely to deteriorate.

[0008] Therefore, in view of the above problem, the object of the present invention is to prevent the exhaust emission from deteriorating when the condition for switching the target air-fuel ratio of the exhaust gas flowing into the catalyst changes in association with the operating state of the internal combustion engine. Solution to the problem

[0009] The summary of the present disclosure is as follows. (1) An exhaust gas purification system of an internal combustion engine comprises: a catalyst arranged in an exhaust passage and capable of storing oxygen; an upstream air-fuel ratio sensor arranged on an upstream side of the catalyst in the flow direction of the exhaust gas, which detects an air-fuel ratio of the inflowing exhaust gas flowing into the catalyst; a downstream air-fuel ratio sensor arranged on a downstream side of the catalyst in the flow direction of the exhaust gas, which detects an air-fuel ratio of the outflowing exhaust gas flowing out of the catalyst; and an air-fuel ratio control device configured to control an air-fuel ratio of the inflowing exhaust gas, wherein the air-fuel ratio control device is configuredalternatively switch a target air-fuel ratio of the inflowing exhaust gas between a rich air-fuel ratio that is richer than a stoichiometric air-fuel ratio and a lean air-fuel ratio that is leaner than a stoichiometric air-fuel ratio, calculate an oxygen storage amount, which is an estimated value of an amount of oxygen stored in the catalyst while the target air-fuel ratio is maintained at the lean air-fuel ratio, and calculate an oxygen discharge amount, which is an estimated value of an amount of oxygen discharged from the catalyst while the target air-fuel ratio is maintained at the rich air-fuel ratio, based on an air-fuel ratio,which is detected by the upstream-side air-fuel ratio sensor, update a learning value based on a difference in the oxygen storage amount and the oxygen discharge amount, and correct an air-fuel ratio-related parameter based on the learning value so that the difference in the oxygen storage amount and the oxygen discharge amount becomes smaller, and wherein an operating state of the internal combustion engine changes between a first state and a second state, and the air-fuel ratio control device is configured to change a condition for switching the target air-fuel ratio between the first state and the second state, store the learning value at the time when the operating state of the internal combustion engine changes from the first state to the second state as a first state value, and update the learning value to the first state value,when the operating state of the internal combustion engine returns from the second state to the first state., (2) The exhaust gas purification system of an internal combustion engine described in (1) above, wherein the air-fuel ratio control device is configured to store the learning value at the time when the operating state of the internal combustion engine changes from the second state to the first state as a second state value, and to update the learning value to the second state value when the operating state of the internal combustion engine returns from the first state to the second state. (3) The exhaust gas purification system of an internal combustion engine described in (1) or (2) above, wherein the air-fuel ratio control device is configured to switch the target air-fuel ratio from the rich-set air-fuel ratio to the lean-set air-fuel ratio when the air-fuel ratio detected by the downstream-side air-fuel ratio sensor reaches a rich-estimated air-fuel ratio, and to switch the target air-fuel ratio from the lean-set air-fuel ratio to the rich-set air-fuel ratio when the air-fuel ratio detected by the downstream-side air-fuel ratio sensor reaches a lean-estimated air-fuel ratio, wherein the rich-estimated air-fuel ratio is an air-fuel ratio,which is richer than a stoichiometric air-fuel ratio and leaner than the rich-set air-fuel ratio, and wherein the lean-estimated air-fuel ratio is an air-fuel ratio that is leaner than a stoichiometric air-fuel ratio and richer than the lean-set air-fuel ratio, and the air-fuel ratio control device is configured to change a value of the rich-estimated air-fuel ratio and / or the lean-estimated air-fuel ratio between the first state and the second state. (4) The exhaust gas purification system of an internal combustion engine described in (3) above, wherein, when the oxygen storage amount reaches a threshold before the air-fuel ratio detected by the downstream air-fuel ratio sensor reaches the lean-estimated air-fuel ratio, the air-fuel ratio control device is configured to switch the target air-fuel ratio from the lean-set air-fuel ratio to the rich-set air-fuel ratio when the oxygen storage amount reaches the threshold, and the air-fuel ratio control device is configured to update the threshold based on the oxygen storage amount and the oxygen discharge amount, the threshold at the time when the operating state of the internal combustion engine changes from the first state to the second state,as a first state threshold, and update the threshold to the first state threshold when the operating state of the internal combustion engine returns from the second state to the first state., (5) The exhaust gas purification system of an internal combustion engine described in (4) above, wherein the air-fuel ratio control device is configured to store the threshold value at the time when the operating state of the internal combustion engine changes from the second state to the first state as a second state threshold value, and to update the threshold value to the second state threshold value when the operating state of the internal combustion engine returns from the first state to the second state. (6) The exhaust gas purification system of an internal combustion engine described in (1) or (2) above, wherein the air-fuel ratio control device is configured to switch the target air-fuel ratio from the rich-set air-fuel ratio to the lean-set air-fuel ratio when the air-fuel ratio detected by the downstream-side air-fuel ratio sensor reaches a rich-estimated air-fuel ratio, and to switch the target air-fuel ratio from the lean-set air-fuel ratio to the rich-set air-fuel ratio when the oxygen storage amount reaches a switched storage amount that is smaller than a maximum oxygen storage amount, the rich-estimated air-fuel ratio being an air-fuel ratio,which is richer than a stoichiometric air-fuel ratio and leaner than the rich-set air-fuel ratio, and wherein the air-fuel ratio control device is configured to change a value of the rich-estimated air-fuel ratio and / or the switched storage amount between the first state and the second state. (7) The exhaust gas purification system of an internal combustion engine described in any one of (1) to (6) above, wherein the air-fuel ratio control device is configured to change a value of the rich-set air-fuel ratio and / or the lean-set air-fuel ratio between the first state and the second state. (8) The exhaust gas purification system of an internal combustion engine described in any one of (1) to (7) above, wherein the first state is a non-stationary state and the second state is a stationary state. (9) The exhaust gas purification system of an internal combustion engine described in any one of (1) to (7) above, wherein the first state is a steady state and the second state is a non-steady state. (10) The exhaust gas purification system of an internal combustion engine described in any one of (1) to (7) above, wherein an EGR passage provided to recirculate a part of exhaust gas flowing through the exhaust passage as an EGR gas to an intake passage is provided to the internal combustion engine, and the first state is a low EGR state in which an EGR gas flow rate is less than a first predetermined value, and the second state is a high EGR state in which the EGR gas flow rate is the first predetermined value or more, or the first state is a low EGR state in which the EGR rate is less than a second predetermined value, and the second state is a high EGR state in which the EGR rate is the second predetermined value or more. (11) The exhaust gas purification system of an internal combustion engine described in any one of (1) to (7) above, wherein an EGR passage provided to recirculate a part of the exhaust gas flowing through the exhaust passage to the intake passage as EGR gas is provided to the internal combustion engine, and the first state is a high EGR state in which an EGR gas flow rate is a first predetermined value or more, and the second state is a low EGR state in which the EGR gas flow rate is less than the first predetermined value, or the first state is a high EGR state in which the EGR rate is a second predetermined value or more, and the second state is a low EGR state in which the EGR rate is less than the second predetermined value. (12) The exhaust gas purification system of an internal combustion engine described in any one of (1) to (7) above, wherein the first state is a high-load state in which an engine load is a predetermined value or more, and the second state is a low-load state in which the engine load is less than the predetermined value. (13) The exhaust gas purification system of an internal combustion engine described in any one of (1) to (7) above, wherein the first state is a low-load state in which an engine load is lower than a predetermined value, and the second state is a high-load state in which the engine load is the predetermined value or more. Advantageous effect of the invention

[0010] According to the present invention, it is possible to prevent the exhaust emission from deteriorating when the condition for switching the target air-fuel ratio of the exhaust gas flowing into the catalyst changes in association with the operating state of the internal combustion engine. Short description of the drawings Fig. 1 is a view schematically showing an internal combustion engine in which an exhaust gas purification system of an internal combustion engine according to a first embodiment of the present invention is provided. Fig. 2 shows a cleaning characteristic of a three-way catalyst. Fig. 3 is a view showing a relationship between a sensor applied voltage and an output current at two different exhaust air-fuel ratios. Fig. 4 is a view showing a relationship between an exhaust air-fuel ratio and the output current when a sensor applied voltage is kept constant. Fig. 5 is a time chart showing an operating state of the internal combustion engine, etc., when the air-fuel ratio control is performed in the first embodiment. Fig. 6 is a control block diagram of the air-fuel ratio control. Fig. 7 is a flowchart showing a control flow for processing for setting a control condition in the first embodiment. Fig. 8 is a flowchart showing a control flow for processing for updating a learning value in the first embodiment. Fig. 9 is a flowchart showing a control procedure for processing the setting of a target air-fuel ratio in the first embodiment. Fig. 10 is a flowchart showing a control flow for processing for updating a threshold in a second embodiment. Fig. 11 is a flowchart showing a control flow for processing for setting a target air-fuel ratio in the second embodiment. Fig. 12 is a flowchart showing a control flow for processing for setting a control condition in a third embodiment. Fig. 13 is a flowchart showing a control flow for processing for setting a target air-fuel ratio in the third embodiment. Description of the embodiments

[0011] Embodiments of the present invention will be described in detail below with reference to the figures. Note that similar components are assigned the same reference numerals in the following explanation. First embodiment

[0012] First, with reference to Fig. 1 to Fig. 9 explains a first embodiment of the present invention. Explanation of the entire combustion engine

[0013] Fig. Fig. 1 is a view schematically showing an internal combustion engine provided with an exhaust gas purification system of an internal combustion engine according to a first embodiment of the present invention. The internal combustion engine shown in Fig. The engine shown in Figure 1 is a spark-ignition internal combustion engine. The engine is mounted in a vehicle.

[0014] With reference to Fig. 1 denotes an engine body, 2 a cylinder block, 3 a piston that reciprocates within the cylinder block 2, 4 a cylinder head that is attached to the cylinder block, 5 a combustion chamber formed between the piston 3 and the cylinder head 4, 6 an intake valve, 7 an intake port, 8 an exhaust valve, and 9 an exhaust port. The intake valve 6 opens and closes the intake port 7, while the exhaust valve 8 opens and closes the exhaust port 9.

[0015] As it is in Fig. As shown in Figure 1, a spark plug 10 is disposed in the central portion of the inner wall surface of the cylinder head 4. A fuel injector 11 is disposed around the inner wall surface of the cylinder head 4. The spark plug 10 is configured to cause the generation of a spark in association with an ignition signal. Furthermore, the fuel injector 11 injects a predetermined amount of fuel into the combustion chamber 5 in association with an ignition signal. In the present embodiment, gasoline with a stoichiometric air-fuel ratio of 14.6 is used as the fuel.

[0016] The intake port 7 in each cylinder is connected to a surge tank 14 through a corresponding intake port 13. The surge tank 14 is connected to an air cleaner 16 through an intake pipe 15. The intake port 7, the intake port 13, the surge tank 14, the intake pipe 15, etc., form an intake passage that supplies air to the combustion chamber 5. Furthermore, a throttle valve 18 is arranged within the intake pipe 15, which is driven by a throttle valve drive actuator 17. The throttle valve 18 can be rotated by the throttle valve drive actuator 17 to thereby change the opening area of ​​the intake passage.

[0017] On the other hand, the exhaust port 9 in each cylinder is connected to an exhaust manifold 19. The exhaust manifold 19 has a plurality of inlets connected to the exhaust ports 9 and has a head where these inlets are collected. The head of the exhaust manifold 19 is connected to an upstream-side casing 21 in which an upstream-side catalyst 20 is installed. The upstream-side casing 21 is connected to a downstream-side casing 23 in which a downstream catalyst 24 is installed via an exhaust pipe 22. The exhaust port 9, the exhaust manifold 19, the upstream-side casing 21, the exhaust pipe 22, the downstream-side casing 23, etc., form an exhaust passage that discharges exhaust gas produced by the combustion of the air-fuel mixture in the combustion chamber 5.

[0018] Various control operations of the internal combustion engine are performed by an electronic control unit (ECU) 31. The ECU 31 is composed of a digital computer provided with components connected to each other by a bidirectional bus 32, such as a RAM (random access memory) 33, ROM (read-only memory), CPU (microprocessor) 35, an input port 36, and an output port 37. An air flow meter 39 is disposed in the intake pipe 15 to detect the flow rate of air flowing through the intake pipe 15. The output of the air flow meter 39 is input to the input port 36 through a corresponding AD converter 38.

[0019] Furthermore, at the head of the exhaust manifold 19, that is, on the upstream side of the upstream catalyst 20 in the flow direction of the exhaust gas, an upstream air-fuel ratio sensor 40 is arranged, which detects the air-fuel ratio of the exhaust gas flowing through the interior of the exhaust manifold 19 (that is, the exhaust gas flowing into the upstream catalyst 20). The output of the upstream air-fuel ratio sensor 40 is input to the input port 36 through the corresponding AD converter 38.

[0020] Furthermore, a downstream air-fuel ratio sensor 41 for detecting an air-fuel ratio of the exhaust gas flowing through the interior of the exhaust pipe 22 (i.e., the exhaust gas flowing out of the upstream catalyst 20) is disposed within the exhaust pipe 22, that is, on the downstream side of the upstream catalyst 20 in the flow direction of the exhaust gas. The output of the downstream air-fuel ratio sensor 41 is input to the input port 36 through a corresponding AD converter 38.

[0021] Furthermore, an accelerator pedal 42 is connected to a load sensor 43, which generates an output voltage proportional to the amount of depression of the accelerator pedal 42. The output voltage of the load sensor 43 is input to the input port 36 through a corresponding AD converter 38. A crank angle sensor 44 generates an output pulse each time the crankshaft rotates, for example, by 15 degrees. This output pulse is input to the input port 36. In the CPU 35, the engine speed is calculated from the output pulse of the crank angle sensor 44. On the other hand, the output port 37 is connected to the spark plugs 10, the fuel injectors 11, and the throttle valve drive actuator 17 through corresponding drive circuits 45.

[0022] It should be noted that the above-mentioned internal combustion engine is a non-supercharged internal combustion engine fueled by gasoline, but the configuration of the internal combustion engine is not limited to the above configuration. Therefore, the cylinder arrangement, fuel injection mode, intake and exhaust system configuration, valve operating mechanism configuration, the presence of a supercharger, and other specific parts of the internal combustion engine configuration may differ from the configuration described in Fig. 1. For example, the fuel injectors 11 may be arranged to inject fuel into the intake ports 7. Explanation of the catalyst

[0023] The upstream catalyst 20 and the downstream catalyst 24, which are arranged in the exhaust passage, have the same configurations. The catalysts 20 and 24 are catalysts that have oxygen storage capabilities, such as three-way catalysts. Specifically, the catalysts 20 and 24 have supports made of ceramics on which a noble metal with a catalytic effect (e.g., platinum (PT)) and a co-catalyst with oxygen storage capability (e.g., cerium (CEO2)) are supported.

[0024] Fig. Figure 2 shows the cleaning properties of a three-way catalyst. As shown in Fig. 2, the purification rates of unburned gas (HC, CO) and nitrogen oxides (NOx) by the catalysts 20 and 24 become extremely high when the air-fuel ratio of the exhaust gas flowing into the catalysts 20 and 24 is in the range close to the stoichiometric air-fuel ratio (purification window A in Fig. 2). Therefore, the catalysts 20 and 24 can effectively remove unburned gas and NOx when the air-fuel ratio of the exhaust gas is maintained at the stoichiometric air-fuel ratio.

[0025] Furthermore, catalysts 20 and 24 store oxygen or release it through the co-catalyst in relation to the fuel ratio of the exhaust gas. Specifically, catalysts 20 and 24 store excess oxygen from the exhaust gas when the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio. Conversely, catalysts 20 and 24 release the amount of additional oxygen required to oxidize the unburned exhaust gas when the exhaust air-fuel ratio is richer than the stoichiometric air-fuel ratio. As a result, even if the air-fuel ratio of the exhaust gas deviates from the stoichiometric air-fuel ratio, the air-fuel ratio at the surface of the catalysts 20 and 24 remains close to the stoichiometric air-fuel ratio, and the unburned gas and NOx are effectively removed at the catalysts 20 and 24.

[0026] It should be noted that as long as the catalysts 20 and 24 have a catalytic effect and oxygen storage capabilities, these can be catalysts different from a three-way catalyst. Output characteristics of air-fuel ratio sensors

[0027] Next, with reference to Fig. 3 and Fig. 4 explains the output characteristics of the air-fuel ratio sensors 40, 41 in the present embodiment. Fig. Fig. 3 is a view showing the voltage-current (VI) characteristics of the air-fuel ratio sensors 40, 41 in the present embodiment, while Fig. 4 is a view showing the relationship between the air-fuel ratio of the exhaust gas circulating around the air-fuel ratio sensors 40, 41 (hereinafter referred to as the "exhaust air-fuel ratio") and the output current I when the applied voltage is kept constant. Note that, in the present embodiment, the same configurations of air-fuel ratio sensors are used as the air-fuel ratio sensors 40, 41.

[0028] As it turns out Fig. 3, in the air-fuel ratio sensors 40, 41 of the present embodiment, the higher the exhaust air-fuel ratio (the leaner the exhaust air-fuel ratio), the larger the output current I becomes. Furthermore, in the VI line of each exhaust air-fuel ratio, there is a region substantially parallel to the V axis, that is, a region in which the output current hardly changes when the applied voltage changes. This voltage range is called the "limit current range." The current at this time is referred to as the "limit current." Fig. 3, the limiting current range and the limiting current are each given as W 18 and I 18 displayed when the exhaust air-fuel ratio is 18. Therefore, the air-fuel ratio sensors 40, 41 are limit flow air-fuel ratio sensors.

[0029] Fig. Figure 4 is a view showing the relationship between the exhaust air-fuel ratio and the output current I when the applied voltage is 0.45 V or so. As can be seen from Fig. 4, in the air-fuel ratio sensors 40, 41, the larger the output current I of the air-fuel ratio sensors 40, 41, the higher the exhaust air-fuel ratio becomes (i.e., the leaner). In addition, the air-fuel ratio sensors 40, 41 are configured so that the output current I becomes zero when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio. Accordingly, the air-fuel ratio sensors 40, 41 can continuously (linearly) detect the exhaust air-fuel ratio. Note that when the exhaust air-fuel ratio increases by a certain amount or more, or when it decreases by a certain amount or less, the ratio of the change in the output current with respect to the change in the exhaust air-fuel ratio becomes smaller.

[0030] It should be noted that in the above example, limit-flow air-fuel ratio sensors are used as the air-fuel ratio sensors 40, 41. However, it is also possible to use any other air-fuel ratio sensor, such as non-limit-flow air-fuel ratio sensors, as long as the output flow changes linearly with respect to the exhaust air-fuel ratio. Furthermore, the air-fuel ratio sensors 40, 41 may also be air-fuel ratio sensors with structures different from each other. Exhaust gas purification system of the combustion engine

[0031] An exhaust gas purification system of an internal combustion engine according to a first embodiment of the present invention (hereinafter referred to simply as "exhaust gas purification system") will be explained below. The exhaust gas purification system includes an upstream catalyst 20, a downstream catalyst 24, an upstream air-fuel ratio sensor 40, a downstream air-fuel ratio sensor 41, and an air-fuel ratio control device. In the present embodiment, the ECU 31 functions as the air-fuel ratio control device.

[0032] The air-fuel ratio control device controls the air-fuel ratio of the exhaust gas flowing into the upstream-side catalyst 20 (hereinafter referred to as "inflowing exhaust gas"). Specifically, the air-fuel ratio control device sets the target air-fuel ratio of the inflowing exhaust gas and controls the amount of fuel supplied to the combustion chambers 5 so that the air-fuel ratio of the inflowing exhaust gas corresponds to the target air-fuel ratio. In the present embodiment, the air-fuel ratio control device controls the amount of fuel supplied to the combustion chambers 5 through feedback so that the output air-fuel ratio of the upstream-side air-fuel ratio sensor 40 corresponds to the target air-fuel ratio.It should be noted that “the output air-fuel ratio” means the air-fuel ratio corresponding to the output value of the air-fuel ratio sensor, that is, the air-fuel ratio detected by the air-fuel ratio sensor.

[0033] The air-fuel ratio control device switches the target air-fuel ratio of the inflowing exhaust gas alternatively between the rich air-fuel ratio and the lean air-fuel ratio to fluctuate the oxygen storage amount of the upstream-side catalyst 20.Specifically, the air-fuel ratio control device switches the target air-fuel ratio from the rich-set air-fuel ratio to the lean-set air-fuel ratio when the output air-fuel ratio of the downstream-side air-fuel ratio sensor 41 reaches the rich-estimated air-fuel ratio, and switches the target air-fuel ratio from the lean-set air-fuel ratio to the rich-set air-fuel ratio when the output air-fuel ratio of the downstream-side air-fuel ratio sensor 41 reaches the lean-estimated air-fuel ratio.

[0034] The rich air-fuel ratio is an air-fuel ratio richer than the stoichiometric air-fuel ratio (14.6 in the present embodiment), for example, 13 to 14.4. The rich estimated air-fuel ratio is an air-fuel ratio richer than the stoichiometric air-fuel ratio and leaner than the rich air-fuel ratio, for example, 14.55 to 14.4. The lean air-fuel ratio is an air-fuel ratio leaner than the stoichiometric air-fuel ratio, for example, 14.8 to 16.5. The lean estimated air-fuel ratio is an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio and richer than the lean set air-fuel ratio, and is, for example, 14.65 to 14.8.

[0035] When the output air-fuel ratio of the downstream-side air-fuel ratio sensor 41 becomes the rich-estimated air-fuel ratio or less, the oxygen storage amount of the downstream-side catalyst 20 may be zero. On the other hand, when the output air-fuel ratio of the downstream-side air-fuel ratio sensor 41 becomes the lean-estimated air-fuel ratio or more, the oxygen storage amount of the upstream-side catalyst 20 may be the maximum value. The air-fuel ratio control device may detect the oxygen storage amount of the upstream-side catalyst 20 being zero or the maximum value from the output of the downstream-side air-fuel ratio sensor 41, so that the oxygen storage amount of the upstream-side catalyst 20 may fluctuate between zero and the maximum value.This can prevent the oxygen storage capacity of the upstream catalyst 20 from decreasing.

[0036] In this regard, the usage characteristic of the air-fuel ratio sensor sometimes deteriorates gradually with use. For example, when the usage characteristic of the upstream-side air-fuel ratio sensor 40 changes, the output air-fuel ratio of the upstream-side air-fuel ratio sensor 40 and the actual air-fuel ratio of the inflowing exhaust gas sometimes deviate from each other. In this case, the output air-fuel ratio of the upstream-side air-fuel ratio sensor 40 deviates toward the rich side or the lean side from the actual air-fuel ratio of the inflowing exhaust gas.

[0037] Furthermore, hydrogen in the unburned gas has a high velocity at which it passes through the diffusion control layer of the air-fuel ratio sensor. For this reason, when the concentration of hydrogen in the exhaust gas is high, the output air-fuel ratio of the upstream-side air-fuel ratio sensor 40 deviates toward the lean side (i.e., toward the rich side) from the actual air-fuel ratio of the inflowing exhaust gas. When the deviation of the output air-fuel ratio of the upstream-side air-fuel ratio sensor 40 occurs in this way, the actual air-fuel ratio of the inflowing exhaust gas is likely to deviate from the target air-fuel ratio, and exhaust emission is likely to deteriorate.

[0038] For this reason, the air-fuel ratio control device performs the following learning control to compensate for any deviation in the output air-fuel ratio of the upstream-side air-fuel ratio sensor 40. The air-fuel ratio control device calculates the oxygen storage amount, which is the estimated value of the amount of oxygen stored in the upstream-side catalyst 20 while maintaining the target air-fuel ratio at the lean air-fuel ratio, and calculates the oxygen storage amount, which is the estimated value of the amount of oxygen discharged from the upstream-side catalyst 20 while maintaining the target air-fuel ratio at the rich air-fuel ratio.The air-fuel ratio control device gradually adds the oxygen excess / deficiency to the stoichiometric air-fuel ratio of the incoming exhaust gas to thereby calculate the oxygen storage amount and the oxygen discharge amount.

[0039] Note that the "oxygen excess / deficiency with respect to the stoichiometric air-fuel ratio of the inflowing exhaust gas" means the amount of oxygen that becomes excessive or the amount of oxygen that becomes deficient when attempting to adjust the air-fuel ratio of the inflowing exhaust gas to the stoichiometric air-fuel ratio. The oxygen excess / deficiency OED is calculated, for example, based on the output air-fuel ratio of the upstream air-fuel ratio sensor 40 and the fuel injection amount by the following formula (1). OED=0.23⋅(AFup−AFR)⋅Qi where 0.23 is the oxygen concentration in the air, Qi is the fuel injection amount, AFup is the output air-fuel ratio of the upstream air-fuel ratio sensor 40, and AFR is the control center air-fuel ratio. The initial value of the control center air-fuel ratio before performing the learning control explained later is the stoichiometric air-fuel ratio (14.6).

[0040] It should be noted that the oxygen excess / deficiency OED can be calculated based on the output of the upstream air-fuel ratio sensor 40 and the intake air amount by the following formula (2). OED=0.23⋅(AFup−AFR)⋅Ga÷AFup where 0.23 is the oxygen concentration in the air, Ga is the intake air quantity, AFup is the output air-fuel ratio of the upstream air-fuel ratio sensor 40, and AFR is the control center air-fuel ratio. The intake air quantity Ga is detected by an air flow meter 39. The initial value of the control center air-fuel ratio before performing the learning control explained later is the stoichiometric air-fuel ratio (14.6).

[0041] When the target air-fuel ratio is maintained at the lean air-fuel ratio, the upstream catalyst 20 stores oxygen, so the oxygen excess / deficiency value OED becomes positive. The oxygen storage amount is calculated as the cumulative oxygen excess / deficiency value calculated when the target air-fuel ratio is maintained at the lean air-fuel ratio. On the other hand, when the target air-fuel ratio is maintained at the rich air-fuel ratio, the upstream catalyst 20 discharges oxygen, so the oxygen excess / deficiency value OED becomes negative. The oxygen discharge amount is calculated as the absolute value of the cumulative oxygen excess / deficiency value calculated when the target air-fuel ratio is maintained at the rich air-fuel ratio.

[0042] The oxygen storage amount of the upstream-side catalyst 20 changes from the maximum value to zero in the period from when the target air-fuel ratio is set to the rich air-fuel ratio to when it is switched to the lean air-fuel ratio, that is, in the period when the target air-fuel ratio is maintained at the rich air-fuel ratio. On the other hand, the oxygen storage amount of the upstream-side catalyst 20 changes from zero to the maximum value in the period from when the target air-fuel ratio is set to the lean air-fuel ratio to when it is switched to the rich air-fuel ratio, that is, in the period when the target air-fuel ratio is maintained at the lean air-fuel ratio.For this reason, when the precise air-fuel ratio control is performed, the oxygen storage amount and the oxygen discharge amount assume the same values.

[0043] However, the oxygen storage amount and the oxygen discharge amount are calculated based on the output air-fuel ratio of the upstream-side air-fuel ratio sensor 40, so when the deviation occurs in the output air-fuel ratio of the upstream-side air-fuel ratio sensor 40, the oxygen storage amount and the oxygen discharge amount change in association with this deviation. When the output air-fuel ratio of the upstream-side air-fuel ratio sensor 40 deviates toward the rich side, the oxygen storage amount is calculated to be smaller than the actual oxygen storage amount, and the oxygen discharge amount is calculated to be larger than the actual oxygen discharge amount. For this reason, the oxygen discharge amount becomes larger than the oxygen storage amount.On the other hand, when the output air-fuel ratio of the upstream air-fuel ratio sensor 40 deviates toward the lean side, the oxygen storage amount is calculated to be larger than the actual oxygen storage amount, and the oxygen discharge amount is calculated to be smaller than the actual oxygen discharge amount. Therefore, the oxygen storage amount becomes larger than the oxygen discharge amount.

[0044] In the present embodiment, the control central air-fuel ratio is calculated based on the deviation DOA between the oxygen storage amount OSA and the oxygen discharge amount ODA (= ODA - OSA, hereinafter referred to as the "oxygen amount deviation"). The air-fuel ratio control device calculates the learning value based on the oxygen amount deviation and corrects the control main air-fuel ratio based on the learning value so that the oxygen amount deviation becomes smaller.

[0045] Specifically, the air-fuel ratio control device updates the learning value sfbg by the following formula (3) and corrects the control central air-fuel ratio AFR by the following formula (4): sfbg(n)=sfbg(n−1)+k1⋅DOA AFR=AFRbase−sfbg(m)

[0046] It should be noted that in the above formula (3), "n" indicates the number of calculations or time. Therefore, sfbg(n) indicates the current learning value after a change, while sfbg(n-1) indicates the previous learning value before a change. Furthermore, k1 in the above formula (3) is an increment that shows the amount of update of the learning value with respect to the deviation of the oxygen storage amount DOA. The larger the value of the increment k1, the greater the amount of change of the learning value with respect to the deviation of the oxygen storage amount DOA. Furthermore, in the above formula (4), the basic control center air-fuel ratio AFRbase is the initial value of the control center air-fuel ratio AFR. In the present embodiment, it is the stoichiometric air-fuel ratio. Furthermore, the initial value sfbg(0) of the learning value is zero.

[0047] As can be seen from the above formula (3), the learning value is updated to decrease when the deviation of the oxygen amount DOA is positive, that is, when the oxygen discharge amount ODA is greater than the oxygen storage amount OSA. On the other hand, when the deviation of the oxygen storage amount DOA is negative, that is, when the oxygen storage amount OSA is greater than the oxygen discharge amount ODA, the learning value is updated to increase.

[0048] Furthermore, the target air-fuel ratio of the inflowing exhaust gas is calculated by adding a predetermined air-fuel ratio correction amount to the control center air-fuel ratio (AFR). The air-fuel ratio correction amount corresponding to the rich air-fuel ratio is a negative value, while the air-fuel ratio correction amount corresponding to the lean air-fuel ratio is a positive value. As understood from the above formula (4), when the learning value is positive, the control center air-fuel ratio (AFR) is made smaller, and as a result, the target air-fuel ratio is corrected to the rich side. On the other hand, when the learning value is negative, the control center air-fuel ratio (AFR) is made larger, and as a result, the target air-fuel ratio is corrected to the lean side.

[0049] However, it is sometimes desirable to change the condition for switching the target air-fuel ratio (rich estimated air-fuel ratio and lean estimated air-fuel ratio in the present embodiment) in order to prevent exhaust emission from deteriorating while maintaining the oxygen storage capacity of the upstream catalyst 20. In the present embodiment, the air-fuel ratio control device changes the condition for switching the target air-fuel ratio between the first state and the second state when the operating state of the internal combustion engine changes between a first state and a second state.

[0050] As the richness of the rich-estimated air-fuel ratio increases as the operating state of the internal combustion engine changes from the first state to the second state, the timing for switching the target air-fuel ratio from the rich-set air-fuel ratio to the lean-set air-fuel ratio becomes delayed in the second state. As a result, in the second state, the time period during which the target air-fuel ratio is maintained at the rich-set air-fuel ratio becomes longer, and the oxygen storage amount increases. Note that "richness" means the difference between an air-fuel ratio richer than the stoichiometric air-fuel ratio and the stoichiometric air-fuel ratio.

[0051] On the other hand, as the leanness of the lean-estimated air-fuel ratio increases as the operating state of the internal combustion engine changes from the first state to the second state, the timing of switching the target air-fuel ratio from the lean-set air-fuel ratio to the rich-set air-fuel ratio becomes delayed in the second state. As a result, in the second state, the time period during which the target air-fuel ratio is maintained at the lean-set air-fuel ratio becomes longer, and the oxygen storage amount increases. Note that "leanness" means the difference between an air-fuel ratio leaner than the stoichiometric air-fuel ratio and the stoichiometric air-fuel ratio.

[0052] Therefore, when the condition for switching the target air-fuel ratio is changed, even if the output of the upstream air-fuel ratio sensor 40 is normal, the learning value calculated from the oxygen storage amount and the oxygen discharge amount will sometimes change. As a result, the appropriate learning value fluctuates according to the operating state of the internal combustion engine. For this reason, if the learning value is maintained when the operating state of the internal combustion engine changes, the air-fuel ratio of the incoming exhaust gas is likely to become a value that does not match the changed operating state, and exhaust emission is likely to deteriorate.

[0053] Therefore, the air-fuel ratio control device in the present embodiment stores the learning value as the first state value at the time when the operating state of the internal combustion engine changes from the first state to the second state, and updates the learning value to the first state value when the operating state of the internal combustion engine returns from the second state to the first state. As a result, the inappropriate learning value updated in the second state is not used in the first state, so it is possible to prevent the exhaust emission from deteriorating after the operating state of the internal combustion engine returns from the second state to the first state.Therefore, when the condition for switching the target air-fuel ratio of the inflowing exhaust gas changes in accordance with the operating state of the internal combustion engine, it is possible to prevent the exhaust emission from deteriorating.

[0054] Note that, in addition to the above control, the air-fuel ratio control device may store the learning value when the operating state of the internal combustion engine changes from the second state to the first state as the second state value, and update the learning value to the second state value when the operating state of the internal combustion engine returns from the first state to the second state. Thereby, the inappropriate learning value updated in the first state is not used in the second state, so that it is possible to prevent the exhaust emission from deteriorating after the operating state of the internal combustion engine returns from the first state to the second state.

[0055] The operating state of the internal combustion engine changes between the steady state and the non-stationary state. The following explains the case where the first state is the non-stationary state, while the second state is the steady state.

[0056] In order to maintain the oxygen storage capacity of the upstream catalyst 20 when the oxygen storage amount of the upstream catalyst 20 fluctuates, it is desirable to completely discharge the oxygen from the upstream catalyst 20 and cause the entire upstream catalyst 20 to store oxygen. To discharge the oxygen stored in a deep part of the upstream catalyst 20, it is necessary to increase the richness of the rich air-fuel ratio. Furthermore, as the richness of the rich-estimated air-fuel ratio is increased, the time period for maintaining the target air-fuel ratio at the rich air-fuel ratio becomes longer, so it is possible to reduce the remaining amount of oxygen stored in the upstream catalyst 20.

[0057] On the other hand, in order to cause the upstream catalyst 20 to store oxygen in its deep part, it is necessary to increase the leanness of the lean-set air-fuel ratio. Furthermore, as the leanness of the lean-estimated air-fuel ratio is increased, the time period during which the target air-fuel ratio is maintained at the lean-set air-fuel ratio becomes longer, so it is possible to increase the amount of oxygen stored in the upstream catalyst.

[0058] Furthermore, it is possible to increase the richness degree of the rich-set air-fuel ratio and / or the rich-estimated air-fuel ratio, thereby periodically supplying a predetermined amount of unburned gas to the downstream catalyst 24. On the other hand, it is possible to increase the leanness degree of the lean-set air-fuel ratio and / or the lean-estimated air-fuel ratio, thereby periodically supplying a predetermined amount of oxygen to the downstream catalyst 24. As a result, it is possible to periodically change the oxygen storage amount of the downstream catalyst 24, and thereby prevent the oxygen storage capacity of the downstream catalyst from decreasing.

[0059] However, if the richness degree of the rich-set air-fuel ratio and / or the rich-estimated air-fuel ratio is increased when the air-fuel ratio of the inflowing exhaust gas temporarily deviates from the target air-fuel ratio due to external disturbances, a large amount of unburned gas is likely to flow out of the upstream-side catalyst 20. On the other hand, if the leanness degree of the lean-set air-fuel ratio and / or the lean-estimated air-fuel ratio is increased when the air-fuel ratio of the inflowing exhaust gas temporarily deviates from the target air-fuel ratio due to external disturbances, a large amount of NOx is likely to flow out of the upstream-side catalyst 20.

[0060] The operating state of an internal combustion engine fluctuates between a non-stationary state, where the engine load fluctuation is large, and a steady state, where the engine load fluctuation is small. During acceleration, deceleration, etc., of the vehicle in which the internal combustion engine is installed, the operating state of the internal combustion engine becomes a non-stationary state. External disturbances are likely to occur when the operating state of the internal combustion engine is a non-stationary state.

[0061] For this reason, the air-fuel ratio control device in the present embodiment changes the condition for switching the target air-fuel ratio between the rich set air-fuel ratio and the lean set air-fuel ratio, that is, it changes the values ​​of the rich estimated air-fuel ratio and the lean estimated air-fuel ratio between the non-steady state and the steady state.Specifically, the air-fuel ratio control device sets the rich-estimated air-fuel ratio and the lean-estimated air-fuel ratio to a first rich-estimated air-fuel ratio and a first lean-estimated air-fuel ratio when the operating state of the internal combustion engine is a non-steady state, and sets the rich-estimated air-fuel ratio and the lean-estimated air-fuel ratio to a second rich-estimated air-fuel ratio and a second lean-estimated air-fuel ratio when the operating state of the internal combustion engine is a steady state. The second rich-estimated air-fuel ratio is richer than the first rich-estimated air-fuel ratio, while the second lean-estimated air-fuel ratio is leaner than the first lean-estimated air-fuel ratio.

[0062] Furthermore, the air-fuel ratio control device changes the values ​​of the rich air-fuel ratio and the lean air-fuel ratio between the non-steady state and the steady state. Specifically, the air-fuel ratio control device sets the rich air-fuel ratio and the lean air-fuel ratio to a first rich air-fuel ratio and a first lean air-fuel ratio when the operating state of the internal combustion engine is a non-steady state, and sets the rich air-fuel ratio and the lean air-fuel ratio to a second lean air-fuel ratio and a second lean air-fuel ratio when the operating state of the internal combustion engine is the steady state.The second rich air-fuel ratio is richer than the first rich air-fuel ratio, while the second lean air-fuel ratio is leaner than the first lean air-fuel ratio.

[0063] Due to the above-described control, in the steady state, the richness degrees of the rich-set air-fuel ratio and the rich-estimated air-fuel ratio become larger, and the leanness degrees of the lean-set air-fuel ratio and the lean-estimated air-fuel ratio become larger, compared to the non-steady state. In the steady state, the air-fuel ratio of the inflowing exhaust gas is stable compared to the non-steady state. For this reason, by performing such control, it is possible to prevent the exhaust emission from deteriorating while maintaining the oxygen storage capacity of the upstream-side catalyst 20 and preventing the oxygen storage capacity of the downstream-side catalyst 24 from decreasing. Explanation of air-fuel ratio control using a timing chart

[0064] With reference to Fig. 5, the air-fuel ratio control in the present embodiment is explained in particular. Fig. 5 is a time chart showing parameters when the air-fuel ratio control is performed in the first embodiment, such as the operating state of the internal combustion engine, the control center air-fuel ratio, the air-fuel ratio correction amount, the learning value, the cumulative value of the oxygen excess / deficiency with respect to the stoichiometric air-fuel ratio of the inflowing exhaust gas (cumulative oxygen excess / deficiency), and the output air-fuel ratio of the downstream air-fuel ratio sensor 41. The cumulative oxygen excess / deficiency is calculated by cumulatively adding the oxygen excess / deficiency calculated by the above formula (1) or (2). Furthermore, the control center air-fuel ratio changes in association with the learning value based on the above formula (4).The target air-fuel ratio of the inflowing exhaust gas is calculated by adding the air-fuel ratio correction amount to the control center air-fuel ratio.

[0065] In the illustrated example, the operating state of the internal combustion engine at time t0 is the non-steady state. In the non-steady state, the rich correction amount is set to the first rich correction amount AFCrich1, and the lean correction amount is set to the first lean correction amount AFClean1. Furthermore, the rich-estimated air-fuel ratio is set to the first rich-estimated air-fuel ratio AFrich1, while the lean-estimated air-fuel ratio is set to the first lean-estimated air-fuel ratio AFlean1. The first rich correction amount AFCrich1 corresponds to the first rich-set air-fuel ratio, while the first lean correction amount AFClean1 corresponds to the first lean-set air-fuel ratio.

[0066] Further, the air-fuel ratio correction amount is set to the first rich correction amount AFCrich1 at time t0. The air-fuel ratio of the inflowing exhaust gas becomes richer than the stoichiometric air-fuel ratio. For this reason, the upstream-side catalyst 20 discharges an amount of oxygen equal to the amount insufficient for oxidizing the unburned gas. The cumulative oxygen excess / deficiency gradually decreases. The outflowing exhaust gas contains no unburned gas and no NOx due to the purification of the upstream-side catalyst 20, so the output air-fuel ratio of the downstream-side air-fuel ratio sensor 41 becomes substantially the stoichiometric air-fuel ratio.

[0067] When the oxygen storage amount of the upstream-side catalyst 20 becomes zero, a part of the unburned gas flowing into the upstream-side catalyst starts to flow out of the upstream-side catalyst 20. As a result, the output air-fuel ratio of the upstream-side air-fuel ratio sensor 41 gradually decreases and reaches the first rich estimated air-fuel ratio AFrich1 at time t1.

[0068] To increase the oxygen storage amount of the upstream catalyst 20, the air-fuel ratio correction amount is switched from the first rich correction amount AFCrich1 to the first lean correction amount AFClean1 at time t1. That is, the target air-fuel ratio is switched from the first rich air-fuel ratio to the first lean air-fuel ratio. Furthermore, the learning value is updated at time t1, and the cumulative value of the oxygen excess / deficiency is reset to zero. In this example, the oxygen discharge amount ODA is greater than the oxygen storage amount OSA (not shown), so the learning value becomes larger.

[0069] When the air-fuel ratio of the inflowing exhaust gas becomes leaner than the stoichiometric air-fuel ratio, the upstream-side catalyst 20 stores the excess oxygen from the inflowing exhaust gas, and the cumulative oxygen excess / deficiency gradually increases. Therefore, at time t1, along with the increase in the oxygen storage amount of the upstream-side catalyst 20, the air-fuel ratio of the outflowing exhaust gas changes from an air-fuel ratio richer than the stoichiometric air-fuel ratio to the stoichiometric air-fuel ratio, and the output air-fuel ratio of the downstream-side air-fuel ratio sensor 41 converges to the stoichiometric air-fuel ratio.

[0070] Thereafter, when the oxygen storage amount of the upstream catalyst 20 reaches the maximum oxygen storage amount, a portion of the oxygen and NOx flowing into the upstream catalyst 20 begins to flow out of the upstream catalyst 20. As a result, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 gradually increases. At time t2, it reaches the first lean-estimated air-fuel ratio AFlean1.

[0071] To cause the oxygen storage amount of the upstream catalyst 20 to decrease, at time t2, the air-fuel ratio correction amount is switched from the first lean correction amount AFClean1 to the first rich correction amount AFCrich1. That is, the target air-fuel ratio is switched from the first lean air-fuel ratio to the first rich air-fuel ratio. Furthermore, at this time, the cumulative value of the oxygen excess / deficiency is reset to zero.

[0072] In the same manner as at time t1, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 reaches the first rich estimated air-fuel ratio AFrich1 at time t3. For this reason, at time t3, the air-fuel ratio correction amount is switched from the first rich correction amount AFCrich1 to the first lean correction amount AFClean1. That is, the target air-fuel ratio is switched from the first rich set air-fuel ratio to the first lean set air-fuel ratio. Furthermore, the learning value is updated at time t3, and the cumulative value of the oxygen excess / deficiency is reset to zero.In this example, the oxygen storage amount OSA from time t1 to time t2 and the oxygen discharge amount ODA from time t2 to time t3 are almost the same, so the learning value does not change overall.

[0073] Thereafter, at time t4, the operating state of the internal combustion engine changes from the non-steady state to the steady state. In the steady state, the rich correction amount is set to the second rich correction amount AFCrich2, while the lean correction amount is set to the second lean correction amount AFClean2. The second rich correction amount AFCrich2 is smaller than the first rich correction amount AFCrich1, while the second lean correction amount AFClean2 is larger than the first lean correction amount AFClean1. The second rich correction amount AFCrich2 corresponds to the second rich-set air-fuel ratio, while the second lean correction amount AFClean2 corresponds to the second lean-set air-fuel ratio.

[0074] Furthermore, the rich-estimated air-fuel ratio in the steady state is set to the second rich-estimated air-fuel ratio AFrich2, while the lean-estimated air-fuel ratio is set to the second lean-estimated air-fuel ratio AFlean2. The second lean-estimated air-fuel ratio AFrich2 is richer than the first rich-estimated air-fuel ratio AFrich1, while the second lean-estimated air-fuel ratio AFlean2 is leaner than the first lean-estimated air-fuel ratio AFlean1.

[0075] For this reason, at time t4, the air-fuel ratio correction amount is switched from the first lean correction amount AFClean1 to the second lean correction amount AFlean2. That is, the target air-fuel ratio is switched from the first lean air-fuel ratio to the second lean air-fuel ratio. Furthermore, the learning value at the time when the operating state of the internal combustion engine changes from the non-steady state to the steady state is stored at time t4.

[0076] Thereafter, at time t5, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 reaches the second lean-estimated air-fuel ratio AFlean2. For this reason, the air-fuel ratio correction amount is switched from the second lean correction amount AFClean2 to the second rich correction amount AFCrich2. That is, the target air-fuel ratio is switched from the second lean-set air-fuel ratio to the second rich-set air-fuel ratio. Furthermore, the cumulative value of the oxygen excess / deficiency at this time is reset to zero.

[0077] At time t6, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 reaches the second rich estimated air-fuel ratio AFrich2. For this reason, at time t6, the air-fuel ratio correction amount is switched from the second rich correction amount AFCrich2 to the second lean correction amount AFClean2. That is, the target air-fuel ratio is switched from the second rich set air-fuel ratio to the second lean set air-fuel ratio. Furthermore, the learning value is updated at time t6, and the cumulative value of the oxygen excess / deficiency is reset to zero.

[0078] In this example, the leanness degree of the second lean-estimated air-fuel ratio AFlean2 becomes greater than the richness degree of the second rich-estimated air-fuel ratio AFrich2 in the steady state to reliably supply oxygen to the downstream catalyst 24. For this reason, the oxygen storage amount OSA from time t3 to time t5 becomes greater than the oxygen discharge amount ODA from time t5 to time t6, and the learning value becomes smaller.

[0079] At time t7, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 reaches the second lean-estimated air-fuel ratio AFlean2. For this reason, the air-fuel ratio correction amount is switched from the second lean correction amount AFClean2 to the second rich correction amount AFCrich2. That is, the target air-fuel ratio is switched from the second lean-set air-fuel ratio to the second rich-set air-fuel ratio. Furthermore, at this time, the cumulative value of the oxygen excess / deficiency is reset to zero.

[0080] At time t8, the output air-fuel ratio of the upstream-side air-fuel ratio sensor 41 reaches the second rich-estimated air-fuel ratio AFrich2. For this reason, at time t8, the air-fuel ratio correction amount is switched from the second rich correction amount AFCrich2 to the second lean correction amount AFClean2. That is, the target air-fuel ratio is switched from the second rich-set air-fuel ratio to the second lean-set air-fuel ratio. Furthermore, the learning value is updated at time t8, and the cumulative value of the oxygen excess / deficiency is reset to zero.

[0081] Due to the updating of the learning value at time t6, the difference between the oxygen storage amount OSA from time t6 to time t7 and the oxygen discharge amount ODA from time t7 to time t8 becomes smaller. However, the oxygen storage amount OSA from time t6 to time t7 is slightly larger than the oxygen discharge amount ODA from time t7 to time t8, so the learning value at time t8 becomes slightly smaller.

[0082] After that, the operating state of the internal combustion engine changes from steady state to non-steady state. Therefore, the air-fuel ratio correction amount is switched from the second lean correction amount AFClean2 to the first lean correction amount AFClean1. That is, the target air-fuel ratio is switched from the second lean air-fuel ratio to the first lean air-fuel ratio. Furthermore, the learning value at time t9 is updated to the learning value stored at time t4.

[0083] At time t10, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 reaches the first lean-estimated air-fuel ratio AFlean1. Therefore, the air-fuel ratio correction amount is switched from the first lean correction amount AFClean1 to the first rich correction amount AFCrich1. That is, the target air-fuel ratio is switched from the first lean-set air-fuel ratio to the first rich-set air-fuel ratio. Furthermore, at this time, the cumulative value of the oxygen excess / deficiency is reset to zero.

[0084] At time t11, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 reaches the first rich estimated air-fuel ratio AFrich1. For this reason, at time t11, the air-fuel ratio correction amount is switched from the first rich correction amount AFCrich1 to the first lean correction amount AFClean1. That is, the target air-fuel ratio is switched from the first rich set air-fuel ratio to the first lean set air-fuel ratio. Furthermore, the learning value is updated at time t11, and the cumulative value of the oxygen excess / deficiency is reset to zero. In this example, the oxygen discharge amount ODA from time t10 to time t11 is larger than the oxygen storage amount OSA from time t8 to time t10, so the learning value becomes larger. Block diagram of the control

[0085] The following is based on Fig. 6 to Fig. 9 explains the air-fuel ratio control in the present embodiment in detail. Fig. Figure 6 is a block diagram for the air-fuel ratio control system. The air-fuel ratio control system includes functional blocks A1 to A10. The functional blocks are explained below.

[0086] First, the calculation of the fuel injection amount will be explained. To calculate the fuel injection amount, a cylinder intake air calculation device A1, a basic fuel injection calculation device A2, and a fuel injection calculation device A3 are used.

[0087] The cylinder intake air calculation device A1 calculates the intake air quantity Mc for the cylinders based on the intake air quantity Ga, the engine speed NE, and the map or calculation formula stored in the ROM 34 of the ECU 31. The intake air quantity Ga is detected by the air flow meter 39, while the engine speed NE is calculated based on the output of the crank angle sensor 44.

[0088] The base fuel injection calculation device A2 divides the cylinder intake air amount Mc calculated by the cylinder intake air calculation device A1 by the target air-fuel ratio TAF to calculate the base fuel injection amount Qbase (Qbase = Mc / TAF). The target air-fuel ratio TAF is calculated by the target air-fuel ratio setting device A8, explained later.

[0089] The fuel injection calculation device A3 adds the F / D correction amount DQi, explained later, to the basic fuel injection amount Qbase calculated by the basic fuel injection calculation device A2 to calculate the fuel injection amount Qi (Qi = Qbase + DQi). An injection instruction is issued to the fuel injectors 11, so that the fuel in the thus calculated fuel injection amount Qi is injected from the fuel injectors 11.

[0090] Next, the calculation of the target air-fuel ratio is explained. To calculate the target air-fuel ratio, the oxygen excess / deficiency calculation device A4, the air-fuel ratio correction calculation device A5, the learning value calculation device A6, the control center air-fuel ratio calculation device A7, and the target air-fuel ratio setting device A8 are used.

[0091] The oxygen excess / deficiency calculation device A4 calculates the oxygen excess / deficiency by the above formulas (1) or (2) based on the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40, the fuel injection amount Qi calculated by the fuel injection calculation device A3, or the intake air amount Ga. Further, the oxygen excess / deficiency calculation device A4 cumulatively adds the oxygen excess / deficiency to calculate the cumulative oxygen excess / deficiency ΣOED.

[0092] In the air-fuel ratio correction calculation device A5, the air-fuel ratio correction amount AFC of the target air-fuel ratio is calculated based on the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41. Specifically, the air-fuel ratio correction amount AFC is calculated based on the flowchart shown in Fig. 9 is shown.

[0093] In the learning value calculation device A6, the learning value sfbg is calculated based on the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41, the cumulative oxygen excess / deficiency ΣOED calculated by the oxygen excess / deficiency calculation device A4, etc. Specifically, the learning value sfbg is calculated based on the flowchart shown in Fig. 8 is shown.

[0094] In the control center air-fuel ratio calculation device A7, the control center air-fuel ratio AFR is calculated based on the basic control center air-fuel ratio AFRbase (in the present embodiment, the stoichiometric air-fuel ratio), and the learning value sfbg is calculated by the learning value calculation device A6. Specifically, the control center air-fuel ratio AFR is calculated by subtracting the learning value sfbg from the basic control center air-fuel ratio AFbase, as shown by the above formula (4).

[0095] The air-fuel ratio adjusting device A8 adds the air-fuel ratio correction amount AFC calculated by the air-fuel ratio correction calculating device A5 to the control center air-fuel ratio AFR calculated by the control center air-fuel ratio calculating device A7 to calculate the target air-fuel ratio TAF. The thus calculated target air-fuel ratio TAF is input to the basic fuel injection calculating device A2 and to the air-fuel ratio deviation calculating device A9 explained later.

[0096] Next, the calculation of the F / B correction amount based on the output air-fuel ratio AFup of the upstream catalyst 40 will be explained. To calculate the F / B correction amount, the air-fuel ratio deviation calculation device A9 and the F / B correction calculation device A10 are used.

[0097] The air-fuel ratio deviation calculation device A9 subtracts the target air-fuel ratio TAF calculated by the target air-fuel ratio setting device A8 from the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 to calculate the air-fuel ratio deviation DAF (DAF = AFup - TAF). This air-fuel ratio deviation DAF is a value indicating the excess or deficiency of the fuel supply amount with respect to the target air-fuel ratio TAF.

[0098] The F / B correction calculation device A10 processes the air-fuel ratio deviation DAF calculated by the air-fuel ratio deviation calculation device A9 through proportional-integral-derivative (PID) processing to calculate the F / B correction amount DQi for compensating for the excess or deficiency of the amount of supplied fuel based on the following formula (5). The thus calculated F / B correction amount DQi is input to the fuel injection calculation device A3. DQI=Kp⋅DAF+Ki⋅SDAF+Kd⋅DDAF

[0099] It should be noted that in the above formula 5, Kp is a preset proportional increase (proportional constant), Ki is the preset integral increase (integral constant), and Kd is the preset differential increase (differential constant). Furthermore, DDAF is the time differential of the air-fuel ratio deviation DAF and is calculated by dividing the difference between the currently updated air-fuel ratio deviation DAF and the previous air-fuel ratio deviation DAF by the time corresponding to the update interval. Furthermore, SDAF is the time integral of the air-fuel ratio deviation DAF and is calculated by adding the currently updated air-fuel ratio deviation DAF to the previous time integral SDAF. Processing for setting the control condition

[0100] Fig. Fig. 7 is a flowchart showing a control flow for processing for setting a control condition in the first embodiment. The control flow is repeatedly executed at predetermined time intervals by the ECU 31 after the engine is started.

[0101] First, at step S101, it is judged whether the operating state of the internal combustion engine is the steady state. For example, if the amount of change in the engine load per unit time is a predetermined value or less, it is judged that the internal combustion engine is in the steady state, while if the amount of change in the engine load per unit time is greater than the predetermined value, it is judged that the internal combustion engine is in the non-steady state. The engine load is detected by the load sensor 43. Further, if the amount of change in the intake air amount of the internal combustion engine per unit time is a predetermined value or less, it can be judged that the internal combustion engine is in the steady state, while if the amount of change in the intake air amount of the internal combustion engine per unit time is greater than the predetermined value, it can be judged that the internal combustion engine is in the non-steady state.The intake air quantity is detected by the air flow meter 39.

[0102] If it is judged at step S101 that the operating state of the internal combustion engine is the non-steady state, the present control flow proceeds to step S102. At step S102, the rich estimated air-fuel ratio AFrich is set to the first rich estimated air-fuel ratio AFrich1, while the lean estimated air-fuel ratio AFlean is set to the first lean estimated air-fuel ratio AFlean1. Next, at step S103, the rich correction amount AFCrich is set to the first rich correction amount AFCrich1, while the lean correction amount AFClean is set to the first lean correction amount AFClean1.That is, the rich air-fuel ratio is set to the first rich air-fuel ratio, while the lean air-fuel ratio is set to the first lean air-fuel ratio. After step S103, the present control flow ends.

[0103] On the other hand, if it is judged at step S101 that the operating state of the internal combustion engine is in the steady state, the present control flow proceeds to step S104. At step S104, the rich estimated air-fuel ratio AFrich is set to the second rich estimated air-fuel ratio AFrich2, while the lean estimated air-fuel ratio AFlean is set to the second lean estimated air-fuel ratio AFlean2. Next, at step S105, the rich correction amount AFCrich is set to the second rich correction amount AFCrich2, while the lean correction amount AFClean is set to the second lean correction amount AFClean2.That is, the rich air-fuel ratio is set to the second rich air-fuel ratio, while the lean air-fuel ratio is set to the second lean air-fuel ratio. After step S105, the present control flow ends.

[0104] It should be noted that the value of both the rich-estimated air-fuel ratio AFrich and the lean-estimated air-fuel ratio AFlean can be changed between the steady state and the non-steady state. Furthermore, the value of both the rich correction amount AFCrich and the lean correction amount AFClean can be changed between the steady state and the non-steady state. Furthermore, the rich correction amount AFCrich and the lean correction amount AFClean do not need to be changed between the steady state and the non-steady state. In this case, steps S103 and S105 are omitted.

[0105] Furthermore, it is not necessary for the rich-estimated air-fuel ratio AFrich, the lean-estimated air-fuel ratio AFlean, the rich correction amount AFCrich, and the lean correction amount AFClean to be switched at the time when the engine operating state changes between the steady state and the non-steady state. For example, these switching operations can be performed at the time when the target air-fuel ratio is switched after the engine operating state changes between the steady state and the non-steady state. Processing to update the learning value

[0106] Fig. 8 is a flowchart showing a control flow for processing for updating the learning value in the first embodiment. The control flow is repeatedly executed at predetermined time intervals by the ECU 31 after the engine is started.

[0107] First, at step S201, it is judged whether the operating state of the internal combustion engine has changed between the steady state and the non-steady state in the period from when step S201 is performed in the predetermined control flow to when step S201 is performed in the current control flow. If it is judged that the operating state of the internal combustion engine has not changed, the present control flow proceeds to step S205.

[0108] At step S205, the cumulative oxygen excess / deficiency ΣOED is calculated. The cumulative oxygen excess / deficiency ΣOED is calculated by cumulatively adding the oxygen excess / deficiency calculated by the above formulas (1) or (2). Next, at step S206, it is judged whether the target air-fuel ratio has been switched in the period from when step S206 is performed to the predetermined control flow until when step S206 is performed to the current control flow. If it is judged that the target air-fuel ratio has not been switched, the present control flow ends. On the other hand, if it is judged that the target air-fuel ratio has been switched, the present control flow proceeds to step S207.

[0109] At step S207, it is judged whether the target air-fuel ratio has been switched from the rich air-fuel ratio TAFrich to the lean air-fuel ratio TAFlean. If it is judged that the target air-fuel ratio has been switched from the lean air-fuel ratio TAFlean to the rich air-fuel ratio TAFrich, the present control flow proceeds to step S208. At step S208, the oxygen storage amount OSA is updated to the value of the cumulative oxygen excess / deficiency ΣOED. Thereafter, the cumulative oxygen excess / deficiency ΣOED is reset to zero. After step S208, the present control flow ends.

[0110] On the other hand, if it is judged at step S207 that the target air-fuel ratio has been switched from the rich air-fuel ratio TAFrich to the lean air-fuel ratio TAFlean, the present control flow proceeds to step S209. At step S209, the oxygen discharge amount ODA is updated to the absolute value of the cumulative oxygen excess / deficiency ΣOED. Thereafter, the cumulative oxygen excess / deficiency ΣOED is reset to zero.

[0111] Next, at step S210, the oxygen amount DOA deviation is calculated by subtracting the oxygen storage amount OSA from the oxygen discharge amount ODA. Next, at step S211, the learning value sfbg is updated based on the oxygen amount DOA deviation by the above formula (3). After step S211, the present control flow ends.

[0112] Furthermore, if it is judged at step S201 that the operating state of the internal combustion engine has changed, the present control flow proceeds to step S202. At step S202, it is judged whether the operating state of the internal combustion engine has changed from the non-stationary state to the steady state. If it is judged that the operating state of the internal combustion engine has changed from the non-stationary state to the steady state, the present control flow proceeds to step S203. At step S203, the learning value sfbg(sw) at the time the operating state of the internal combustion engine changes from the non-stationary state to the steady state is stored.

[0113] On the other hand, if it is judged at step S202 that the operating state of the internal combustion engine has changed from the steady state to the non-steady state, the present control flow proceeds to step S204. At step S204, the learned value sfbg is updated to the learned value sfbg(sw) stored at step S203.

[0114] It should be noted that step S210 and step S211 are performed after step S208. Furthermore, at step S203, the learning value sfbg(sw1) may be stored at the time when the operating state of the internal combustion engine has changed from the non-stationary state to the steady state; at step S204, the learning value sfbg may be updated to the learning value sfbg(sw1); at step S204, the learning value sfbg(sw2) may be stored at the time when the operating state of the internal combustion engine changes from the steady state to the non-stationary state; and at step S203, the learning value sfbg may be changed to the learning value sfbg(sw2). In this example, the first state is still the non-stationary state while the second state is the stationary state, but the first state can also be the stationary state and the second state can be the non-stationary state.In this case, it is judged at step S202 whether the operating state of the internal combustion engine changes from the steady state to the non-steady state. Processing for setting the target air-fuel ratio

[0115] Fig. 9 is a flowchart showing a control flow for processing for setting a target air-fuel ratio in the first embodiment. The control flow is repeatedly executed at a predetermined time interval by the ECU 31 after starting the engine.

[0116] First, in step S301, it is judged whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is the rich estimated air-fuel ratio AFrich or less. The rich estimated air-fuel ratio AFrich is determined in step S102 or step S104 from Fig. 7 set.

[0117] If it is judged at step 301 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is the rich estimated air-fuel ratio AFrich or less, the present control flow proceeds to step 302. At step S302, the air-fuel ratio correction amount AFC is set to the lean correction amount AFClean. That is, the target air-fuel ratio is set to the lean set air-fuel ratio. The lean correction amount AFClean is calculated at steps S103 and S105 from Fig. 7 set.

[0118] On the other hand, if it is judged at step 301 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is higher than the rich air-fuel ratio AFrich, the present control flow proceeds to step 303. At step S303, it is judged whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is the lean estimated air-fuel ratio AFlean or more. The lean estimated air-fuel ratio AFlean is determined at step S102 or step S104 from Fig. 7 set.

[0119] If it is judged at step S303 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is the lean-judged air-fuel ratio AFlean or more, the present control flow proceeds to step S304. At step S304, the air-fuel ratio correction amount AFC is set to the rich correction amount AFCrich. That is, the target air-fuel ratio is set to the rich-set air-fuel ratio. The rich correction amount AFCrich is determined at step S103 or at step S105 from Fig. 7. After step S304, the present control flow ends.

[0120] On the other hand, if it is judged at step S303 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is lower than the lean-estimated air-fuel ratio AFlean, the present control flow ends. In this case, the air-fuel ratio correction amount AFC is maintained at the currently set value. Second embodiment

[0121] The structure and control of the exhaust gas purification system of an internal combustion engine in a second embodiment are substantially similar to the exhaust gas purification system of an internal combustion engine in the first embodiment, except for the points explained below. Therefore, the second embodiment of the present invention will be explained below, focusing on the parts that are different from the first embodiment.

[0122] The air-fuel ratio control device can detect the oxygen storage amount of the upstream catalyst 20, which is zero or the maximum value, from the output of the downstream air-fuel ratio sensor 41, so that the oxygen storage amount of the upstream catalyst 20 can be caused to fluctuate between zero and the maximum value. However, due to the effect of hydrogen or nitrogen discharged from the upstream catalyst 20, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 sometimes becomes richer than the actual air-fuel ratio.In this case, the time until the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes the lean-estimated air-fuel ratio or more becomes longer, and the timing for switching the target air-fuel ratio from the lean-set air-fuel ratio to the rich-set air-fuel ratio is delayed. As a result, a large amount of NOx is likely to be exhausted, and exhaust emission is likely to deteriorate while the target air-fuel ratio is set to the lean-set air-fuel ratio.

[0123] Therefore, the air-fuel ratio control device in the second embodiment switches the target air-fuel ratio from the lean-set air-fuel ratio to the rich-set air-fuel ratio when the oxygen storage amount reaches the threshold value, if the oxygen storage amount reaches the threshold value before the output air-fuel ratio of the downstream-side air-fuel ratio sensor 41 reaches the lean-estimated air-fuel ratio. Thereby, due to the effect of the hydrogen or nitrogen discharged from the upstream-side catalyst 20, it is possible to prevent a large amount of NOx from flowing out of the upstream-side catalyst 20 during the period during which the target air-fuel ratio is set to the lean-set air-fuel ratio.

[0124] This air-fuel ratio control device updates the threshold based on the oxygen storage amount and the oxygen discharge amount. For example, the air-fuel ratio control device calculates the maximum oxygen storage amount Cmax based on the oxygen storage amount OSA and the oxygen discharge amount ODA using the following formula (6), and calculates the threshold OEDth based on the maximum oxygen storage amount Cmax using the following formula (7): CMAX=(OSA+ODA) / 2 OEDth=Cmax⋅A

[0125] The coefficient A is a value greater than 1, for example, 1.1 to 1.5, preferably 1.2. The threshold OEDth is a value greater than the maximum oxygen storage amount Cmax, so that when the oxygen storage amount OSA reaches the threshold OEDth, it can be considered that the actual oxygen storage amount of the upstream-side catalyst 20 reaches the maximum value.

[0126] As explained above, when the condition for switching the target air-fuel ratio changes between the first state and the second state of the internal combustion engine's operating state, the oxygen storage amount and / or oxygen discharge amount fluctuates. As a result, the threshold value will fluctuate according to the operating state of the internal combustion engine, as shown in the above formulas (6) and (7). For this reason, if the threshold value remains the same when the operating state of the internal combustion engine changes, the threshold value is likely to become a value that does not match the changed operating state, and the exhaust emission is likely to deteriorate.

[0127] Therefore, the air-fuel ratio control device in the second embodiment stores the threshold value at the time the operating state of the internal combustion engine changes from the first state to the second state as the first state threshold value, and updates the threshold value to the first state threshold value when the operating state of the internal combustion engine returns from the second state to the first state. As a result, the inappropriate threshold value updated in the second state is not used in the first state, so it is possible to prevent exhaust emission from deteriorating after the operating state of the internal combustion engine returns from the second state to the first state.

[0128] Note that, in addition to the above control, the air-fuel ratio control device stores the threshold value at the time the operating state of the internal combustion engine changes from the second state to the first state as the second state threshold value, and updates the threshold value to the second state threshold value when the operating state of the internal combustion engine returns from the first state to the second state. Therefore, the inappropriate threshold value updated in the first state is not used in the second state, so it is possible to prevent the exhaust emission from deteriorating after the operating state of the internal combustion engine returns from the first state to the second state. Processing to update the threshold

[0129] The air-fuel ratio control in the second embodiment will be described in detail below. In the following example, the first state is the non-stationary state, while the second state is the stationary state. In the second embodiment, in addition to the control flows for the processing for setting the control condition, Fig. 7 and the processing for updating the learning value of Fig. 8 the control flow for processing to update the threshold is performed.

[0130] Fig. 10 is a flowchart showing a control flow for processing for updating the threshold value in the second embodiment. The control flow is repeatedly executed at predetermined time intervals by the ECU 31 after the engine is started.

[0131] First, at step S401, it is judged whether the operating state of the internal combustion engine has changed between the steady state and the non-steady state in the period from when step S401 was performed in the previous control flow until when step S401 is performed for the current control flow. If it is judged that the operating state of the internal combustion engine has not changed, the present control flow proceeds to step S405.

[0132] At step S405, the cumulative oxygen excess / deficiency ΣOED is calculated. The cumulative oxygen excess / deficiency ΣOED is calculated by the cumulative addition of the oxygen excess / deficiency calculated by the above formulas (1) or (2). Next, at step S406, it is judged whether the target air-fuel ratio has been switched in the period from when step S406 was performed in the previous control flow until when step S406 is performed for the current control flow. If it is judged that the target air-fuel ratio has not been switched, the present control flow ends. On the other hand, if it is judged that the target air-fuel ratio has been switched, the present control flow proceeds to step S407.

[0133] At step S407, it is judged whether the target air-fuel ratio has switched from the rich air-fuel ratio TAFrich to the lean air-fuel ratio TAFlean. If it is judged that the target air-fuel ratio has switched from the lean air-fuel ratio TAFlean to the rich air-fuel ratio TAFrich, the control flow proceeds to step S408. At step S408, the oxygen storage amount OSA is updated to the value of the cumulative oxygen excess / deficiency ΣOED. Thereafter, the cumulative oxygen excess / deficiency ΣOED is reset to zero.

[0134] On the other hand, if it is judged at step S407 that the target air-fuel ratio has been switched from the rich air-fuel ratio TAFrich to the lean air-fuel ratio TAFlean, the present control flow proceeds to step S409. At step S409, the oxygen discharge amount ODA is updated to the absolute value of the cumulative oxygen excess / deficiency value ΣOED. Thereafter, the cumulative oxygen excess / deficiency ΣOED is reset to zero.

[0135] After step S408 or step S409, the maximum oxygen storage amount Cmax of the upstream catalyst 20 is calculated by the above formula (6) at step S410. Note that the maximum oxygen storage amount Cmax may be calculated as the oxygen discharge amount ODA or the oxygen storage amount OSA.

[0136] Next, at step S411, the threshold value OEDth is updated based on the maximum oxygen storage amount Cmax by the above formula (7). After step S411, the present control flow ends.

[0137] Furthermore, if it is judged at step S401 that the operating state of the internal combustion engine has changed, the present control flow proceeds to step S402. At step S402, it is judged whether the operating state of the internal combustion engine has changed from the non-stationary state to the steady state. If it is judged that the operating state of the internal combustion engine has changed from the non-stationary state to the steady state, the present control flow proceeds to step S403. At step S403, the threshold value OEDth(sw) at the time when the operating state of the internal combustion engine changes from the non-stationary state to the steady state is stored.

[0138] On the other hand, if it is judged at step S402 that the operating state of the internal combustion engine has changed from the steady state to the non-steady state, the present control flow proceeds to step S404. At step S404, the threshold value OEDth is updated to the threshold value OEDth(sw), which is stored at step S403.

[0139] It should be noted that at step S403, the threshold value OEDth(sw1) at the time when the operating state of the internal combustion engine changes from the non-stationary state to the steady state may be stored, at step S404, the threshold value OEDth may be updated to the threshold value OEDth(sw1), at step S404, the threshold value OEDth(sw2) at the time when the operating state of the internal combustion engine changes from the steady state to the non-stationary state may be stored, and at step S403, the threshold value OEDth may be updated to the threshold value OEDth(sw2). Furthermore, in this example, the first state is the non-stationary state, while the second state is the steady state, but the first state may be the steady state and the second state may be the non-stationary state.In this case, it is judged at step S402 whether the operating state of the internal combustion engine has changed from the steady state to the non-steady state. Processing for setting the target air-fuel ratio

[0140] Fig. 11 is a flowchart showing a control flow for processing for setting the target air-fuel ratio in the second embodiment. The control flow is repeatedly executed at predetermined time intervals by the ECU 31 after the engine is started.

[0141] First, in step S501, it is judged whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is the rich estimated air-fuel ratio AFrich or less. The rich estimated air-fuel ratio AFrich is calculated from the Fig. 7. When it is judged at step S501 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is the rich estimated air-fuel ratio AFrich or less, the present control flow proceeds to step S502.

[0142] At step S502, the air-fuel ratio correction amount AFC is set to the lean correction amount AFClean. That is, the target air-fuel ratio is set to the lean air-fuel ratio. The lean correction amount AFClean is determined at step S103 or at step S105 from Fig. 7. Further, at step S502, the lean flag Flean is set to "1." The lean flag Flean is a flag that is set to "1" when the target air-fuel ratio is set to the lean air-fuel ratio, and is set to zero when the target air-fuel ratio is set to the rich air-fuel ratio.

[0143] On the other hand, if it is judged at step S501 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is higher than the rich estimated air-fuel ratio AFrich, the present control flow proceeds to step S503. At step S503, it is judged whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is the lean estimated air-fuel ratio AFlean or more. The lean estimated air-fuel ratio AFlean is determined at step S102 or at step S104 from Fig. 7 set.

[0144] If it is judged at step S503 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is the lean-judged air-fuel ratio AFlean or more, the present control flow proceeds to step S504. At step S504, the air-fuel ratio correction amount AFC is set to the rich correction amount AFCrich. That is, the target air-fuel ratio is set to the rich-set air-fuel ratio. The rich correction value AFCrich is determined at step S103 or at step S105 from Fig. 7. Furthermore, in step S504, the lean flag Flean is set to zero. In step S504, the present control flow ends.

[0145] On the other hand, if it is judged at step S503 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is less than the lean-judged air-fuel ratio AFlean, the present process flow proceeds to step S505. At step S505, it is judged whether the lean flag Flean is "1." If it is judged that the lean flag Flean is zero, the present control flow ends. In this case, the air-fuel ratio correction amount AFC is maintained at the currently set value.

[0146] On the other hand, if it is judged at step S505 that the lean flag Flean is "1", the present control flow proceeds to step S506. At step S506, it is judged whether the cumulative oxygen excess / deficiency ΣOED is the threshold value OEDth or more. The threshold value OEDth is determined in the control flow from Fig. 10. The cumulative oxygen excess / deficiency ΣOED is calculated by the cumulative addition of the oxygen excess / deficiency calculated by the above formula (1) or (2). Note that the cumulative oxygen excess / deficiency ΣOED calculated when the target air-fuel ratio is set to the lean air-fuel ratio corresponds to the oxygen storage amount. Furthermore, the cumulative oxygen excess / deficiency ΣOED is calculated at step S408 or step S409 from Fig. 10 reset to zero.

[0147] If it is judged at S506 that the cumulative oxygen excess / deficiency ΣOED is less than the threshold value OEDth, the current control flow ends. In this case, the air-fuel ratio correction amount AFC is maintained at the currently set value.

[0148] On the other hand, if it is judged at step S506 that the cumulative oxygen excess / deficiency ΣOED is the threshold value OEDth or more, the present control flow proceeds to step S504. At step S504, the air-fuel ratio correction amount AFC is set to the rich correction amount AFCrich, and the lean flag Flean is set to zero. After step S504, the present control flow ends. Third embodiment

[0149] The structure and control of the exhaust gas purification system of an internal combustion engine in the third embodiment are substantially similar to the exhaust gas purification system of an internal combustion engine in the first embodiment, except for the points explained below. Therefore, the third embodiment of the present invention will be explained below, focusing on the parts that differ from the first embodiment.

[0150] In the third embodiment, the air-fuel control device switches the target air-fuel ratio from the rich air-fuel ratio to the lean air-fuel ratio when the output air-fuel ratio of the downstream-side air-fuel ratio sensor 41 reaches the rich estimated air-fuel ratio, and switches the target air-fuel ratio from the lean air-fuel ratio to the rich air-fuel ratio when the oxygen storage amount reaches a switched storage amount smaller than the maximum oxygen storage amount. Due to this control, it is possible to prevent NOx from flowing out of the upstream-side catalyst 20 because the oxygen storage amount of the upstream-side catalyst 20 will basically not reach the maximum oxygen storage amount.

[0151] Furthermore, the air-fuel ratio control device changes the condition for switching the target air-fuel ratio, ie, the value of the rich estimated air-fuel ratio and / or the switched storage amount between the first state and the second state.

[0152] For example, the air-fuel ratio control device sets the rich-estimated air-fuel ratio and the switched storage amount to a first rich-estimated air-fuel ratio and a first switched storage amount when the operating state of the internal combustion engine is a non-steady state, and sets the rich-estimated air-fuel ratio and the switched storage amount to a second rich-estimated air-fuel ratio and a second switched storage amount when the operating state of the internal combustion engine is the steady state. The second rich-estimated air-fuel ratio is richer than the first rich-estimated air-fuel ratio, while the second switched storage amount is greater than the first switched storage amount.

[0153] Furthermore, the air-fuel ratio control device changes the values ​​of the rich air-fuel ratio and the lean air-fuel ratio between the non-steady state and the steady state. For example, the air-fuel ratio control device sets the rich air-fuel ratio and the lean air-fuel ratio to a first rich air-fuel ratio and a first lean air-fuel ratio when the operating state of the internal combustion engine is a non-steady state, and sets the rich air-fuel ratio and the lean air-fuel ratio to a second rich air-fuel ratio and a second lean air-fuel ratio when the operating state of the internal combustion engine is the steady state.The second rich air-fuel ratio is richer than the first rich air-fuel ratio, while the second lean air-fuel ratio is leaner than the first lean air-fuel ratio. Processing for setting the tax condition

[0154] Fig. 12 is a flowchart showing a control flow for the control condition setting processing in the third embodiment. The control flow is repeatedly executed at different time intervals by the ECU 31 after the engine is started.

[0155] First, at step S601, in the same manner as at step S101 of Fig. 7, it is judged whether the operating state of the internal combustion engine is the steady state. If the operating state of the internal combustion engine is judged to be a non-steady state, the present control flow proceeds to step S602. At step S602, the rich estimated air-fuel ratio AFrich is set to the first rich estimated air-fuel ratio AFrich1, and the switched storage amount Csw is set to the first switched storage amount Csw1. Next, at step S603, the rich correction amount AFCrich is set to the first rich correction amount AFCrich1, while the lean correction amount AFClean is set to the first lean correction amount AFClean1.That is, the rich air-fuel ratio is set to the first rich air-fuel ratio, while the lean air-fuel ratio is set to the first lean air-fuel ratio. After step S603, the present control flow ends.

[0156] On the other hand, if it is judged at step S601 that the operating state of the internal combustion engine is the steady state, the present control flow proceeds to step S604. At step S604, the rich estimated air-fuel ratio AFrich is set to the second rich estimated air-fuel ratio AFrich2, while the switched storage amount Csw is set to the second switched storage amount Csw2. Next, at step S605, the rich correction amount AFCrich is set to the second rich correction amount AFCrich2, while the lean correction amount AFClean is set to the second lean correction amount AFClean2. That is, the rich-set air-fuel ratio is set to the second rich-set air-fuel ratio, while the lean-set air-fuel ratio is set to the second lean-set air-fuel ratio.After step S605, the present control flow ends.

[0157] It should be noted that only the value of the rich estimated air-fuel ratio AFrich may change between the steady state and the non-steady state. Furthermore, only the value of either the rich correction amount AFCrich or the lean correction amount AFClean may change between the steady state and the non-steady state. Furthermore, the rich correction amount AFCrich and the lean correction amount AFClean may not change between the steady state and the non-steady state. In this case, step S603 and step S605 are omitted.

[0158] Furthermore, the rich estimated air-fuel ratio AFrich, the switched storage amount Cref, the rich correction amount AFCrich, and the lean correction amount AFClean do not need to be switched at the time when the engine operating state changes between the steady state and the non-steady state. For example, these switching operations can be performed at the time when the target air-fuel ratio is switched after the engine operating state has changed between the steady state and the non-steady state. Processing for setting the target air-fuel ratio

[0159] Fig. 13 is a flowchart showing a control flow for processing for setting the target air-fuel ratio in the third embodiment. The control flow is repeatedly executed by the ECU 31 at predetermined time intervals after the engine is started.

[0160] First, in step S701, it is judged whether the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is the rich estimated air-fuel ratio AFrich or less. The rich estimated air-fuel ratio AFrich is determined from Fig. 12. When it is judged at step 701 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is the rich estimated air-fuel ratio AFrich or less, the present control flow proceeds to step S702.

[0161] At step S702, the air-fuel ratio correction amount AFC is set to the lean correction amount AFClean. That is, the target air-fuel ratio is set to the lean air-fuel ratio. The lean correction amount AFClean is determined at step S603 or step S605 from Fig. 12. Furthermore, the lean flag Flean is set to "1." The lean flag Flean is a flag that is set to "1" when the target air-fuel ratio is set to the lean air-fuel ratio, and that is set to zero when the target air-fuel ratio is set to the rich air-fuel ratio. Furthermore, at step S702, the cumulative oxygen excess / deficiency ΣOED is reset to zero.

[0162] On the other hand, if it is judged at step S701 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is higher than the rich-estimated air-fuel ratio AFrich, the present control flow proceeds to step S703. At step S703, it is judged whether the lean flag Flean is "1." If it is judged that the lean flag Flean is zero, the present control flow ends. In this case, the air-fuel ratio correction amount AFC is maintained at the currently set value.

[0163] On the other hand, if it is judged at step S702 that the lean flag Flean is "1", the present control flow proceeds to step S704. At step S704, it is judged whether the cumulative oxygen excess / deficiency ΣOED is the switched storage amount Csw or more. The switched storage amount Csw is determined at step S602 or at step S604 from Fig. 12. The cumulative oxygen excess / deficiency ΣOED is calculated cumulatively by adding the oxygen excess / deficiency calculated by the above formulas (1) or (2). Note that the cumulative oxygen excess / deficiency ΣOED calculated when the target air-fuel ratio is set to the lean air-fuel ratio corresponds to the oxygen storage amount.

[0164] If it is judged at S704 that the cumulative oxygen excess / deficiency ΣOED is less than the switched storage amount Csw, the current control flow ends. In this case, the air-fuel ratio correction amount AFC is maintained at the currently set value.

[0165] On the other hand, if it is judged at step S704 that the cumulative oxygen excess / deficiency ΣOED is the switched storage amount Csw or more, the present control flow proceeds to step S705. At step S705, the air-fuel ratio correction amount AFC is set to the rich correction amount AFCrich. That is, the target air-fuel ratio is set to the rich air-fuel ratio. The rich correction amount AFCrich is determined at step S603 or at step S605 from Fig. 12. Furthermore, the lean flag Flean is set to zero at step S705, and then the cumulative oxygen excess / deficiency ΣOED is set to zero. After step S705, the present control flow ends.

[0166] It should be noted that in the third embodiment, in the same manner as in the first embodiment, the control flow for the learning value update processing of Fig.8 is executed. Other embodiments

[0167] Preferred embodiments according to the present invention have been explained above, but the present invention is not limited to these embodiments. Various revisions and changes can be made within the scope of the claims. For example, as the parameter corrected based on the learning values, other air-fuel ratio-related parameters other than the control center air-fuel ratio can be used. Examples of other air-fuel ratio-related parameters include the amount of fuel supplied to the interior of the combustion chamber 5, the output air-fuel ratio of the upstream air-fuel ratio sensor 40, the air-fuel ratio correction amount, etc.

[0168] Furthermore, hazardous substances in the exhaust gas are substantially removed at the upstream catalyst 20. For this reason, the downstream catalyst 24 can be eliminated from the exhaust gas purification system.

[0169] Furthermore, when an EGR passage is provided for recirculating a portion of the exhaust gas flowing through the exhaust passage as EGR gas to the intake passage in the internal combustion engine, a low EGR state in which the EGR gas flow rate or the EGR rate is less than a predetermined value may be the first state, while a high EGR state in which the EGR gas flow rate or the EGR rate is the predetermined value or more may be the second state. The EGR gas flow rate is detected, for example, by a flow rate sensor provided in the EGR passage. The EGR rate is estimated, for example, by a known device based on the output of the air flow meter 39, the opening degree of the EGR valve provided in the EGR passage, etc.It should be noted that the "EGR rate" is the ratio of the amount of EGR gas to the total amount of gas supplied to the interior of the cylinders (total intake air amount and EGR gas amount). The larger the EGR gas flow rate and the EGR rate, the more the NOx concentration in the exhaust gas decreases. For this reason, for example, in the first embodiment or the second embodiment, the lean estimated air-fuel ratio in the high EGR state is made leaner than the lean estimated air-fuel ratio in the low EGR state. Furthermore, for example, in the third embodiment, the switched storage amount in the high EGR state is made larger than the switched storage amount in the low EGR state. It should be noted that the high EGR state may be the first state, and the low EGR state may be the second state.

[0170] Furthermore, the high-load condition in which the engine load is a predetermined value or more may be the first condition, while the second load condition in which the engine load is less than a predetermined value may be the second condition. The engine load is detected by the load sensor 43. In the low-load condition, the fluctuation in the air-fuel ratio of the inflowing exhaust gas due to external disturbance is small even if an external disturbance occurs. For this reason, for example, in the first embodiment or the second embodiment, the rich-estimated air-fuel ratio in the low-load condition is made richer than the rich-estimated air-fuel ratio in the high-load condition, while the lean-estimated air-fuel ratio in the low-load condition is made leaner than the lean-estimated air-fuel ratio in the high-load condition.Furthermore, for example, in the third embodiment, the rich-estimated air-fuel ratio in the low-load state is made richer than the rich-estimated air-fuel ratio in the high-load state, while the switched storage amount in the low-load state is made larger than the switched storage amount in the high-load state. Note that the low-load state may be the first state, while the high-load state may be the second state. List of reference symbols 20 Upstream catalyst 31 ECU 40 Upstream air-fuel ratio sensor 41 Downstream air-fuel ratio sensor

Claims

[1] Exhaust gas purification system of an internal combustion engine, comprising: a catalyst (20) arranged in an exhaust gas duct and capable of storing oxygen; an upstream air-fuel ratio sensor (40) arranged on an upstream side of the catalyst (20) in the flow direction of the exhaust gas and detecting an air-fuel ratio of the inflowing exhaust gas flowing into the catalyst (20); a downstream air-fuel ratio sensor (41) arranged on a downstream side of the catalyst (20) in the flow direction of the exhaust gas and detecting an air-fuel ratio of the exhaust gas flowing out of the catalyst (20); and an air-fuel ratio control device (31) configured to control an air-fuel ratio of the incoming exhaust gas, wherein the air-fuel ratio control device (31) is configured to alternatively switch a target air-fuel ratio of the inflowing gas between a rich air-fuel ratio (TAFrich) that is richer than a stoichiometric air-fuel ratio and a lean air-fuel ratio (TAFlean) that is leaner than a stoichiometric air-fuel ratio, an oxygen storage amount (OSA), which is an estimated value of an amount of oxygen stored in the catalyst (20) while the target air-fuel ratio is maintained at the lean air-fuel ratio (TAFlean), and an oxygen discharge amount (ODA), which is an estimated value of an amount of oxygen discharged from the catalyst (20) while the target air-fuel ratio is maintained at the rich air-fuel ratio (TAFrich),based on an air-fuel ratio detected by the upstream air-fuel ratio sensor (40), update a learning value based on a difference in the oxygen storage amount (OSA) and the oxygen discharge amount (ODA), and correct an air-fuel ratio-related parameter based on the learning value so that the difference in the oxygen storage amount (OSA) and the oxygen discharge amount (ODA) becomes smaller, and, an operating state of the internal combustion engine changes between a first state and a second state, and the air-fuel ratio control device (31) is configured to change a condition for switching the target air-fuel ratio between the first state and the second state, store the learning value at the time the operating state of the internal combustion engine changes from the first state to the second state as a first state value, and update the learning value to the first state value when the operating state of the internal combustion engine returns from the second state to the first state. [2] An exhaust gas purification system of an internal combustion engine according to claim 1, wherein the air-fuel ratio control device (31) is configured to store the learning value at the time when the operating state of the internal combustion engine changes from the second state to the first state as a second state value, and to update the learning value to the second state value when the operating state of the internal combustion engine returns from the first state to the second state. [3] Exhaust gas purification system of an internal combustion engine according to claim 1 or 2, wherein the air-fuel ratio control device (31) is configured to switch the target air-fuel ratio from the rich air-fuel ratio (TAFrich) to the lean air-fuel ratio (TAFlean) when the air-fuel ratio detected by the downstream air-fuel ratio sensor (41) reaches a rich estimated air-fuel ratio, and to switch the target air-fuel ratio from the lean air-fuel ratio (TAFlean) to the rich air-fuel ratio (TAFrich) when the air-fuel ratio detected by the downstream air-fuel ratio sensor (41) reaches a lean estimated air-fuel ratio, wherein the rich estimated air-fuel ratio is an air-fuel ratio,which is richer than a stoichiometric air-fuel ratio and leaner than the rich air-fuel ratio (TAFrich), and wherein the lean estimated air-fuel ratio is an air-fuel ratio that is leaner than a stoichiometric air-fuel ratio and richer than the lean air-fuel ratio (TAFlean), and, the air-fuel ratio control device (31) is configured to change a value of the rich estimated air-fuel ratio (TAFrich) and / or the lean estimated air-fuel ratio between the first state and the second state. [4] Exhaust gas purification system of an internal combustion engine according to claim 3, wherein when the oxygen storage amount (OSA) reaches a threshold value before the air-fuel ratio detected by the downstream air-fuel ratio sensor (41) reaches the lean estimated air-fuel ratio, the air-fuel ratio control device (31) is configured to switch the target air-fuel ratio from the lean set air-fuel ratio (TAFlean) to the rich set air-fuel ratio (TAFrich) when the oxygen storage amount (OSA) reaches the threshold value, and the air-fuel ratio control device (31) is configured to update the threshold value based on the oxygen storage amount (OSA) and the oxygen discharge amount (ODA), to store the threshold value as a first state threshold value at the time when the operating state of the internal combustion engine changes from the first state to the second state, and to update the threshold value to the first state threshold value when the operating state of the internal combustion engine returns from the second state to the first state. [5] An exhaust gas purification system of an internal combustion engine according to claim 4, wherein the air-fuel ratio control device (31) is configured to store the threshold value at the time when the operating state of the internal combustion engine changes from the second state to the first state as a second state threshold value, and to update the threshold value to the second state threshold value when the operating state of the internal combustion engine returns from the first state to the second state. [6] An exhaust gas purification system of an internal combustion engine according to claim 1 or 2, wherein the air-fuel ratio control device (31) is configured to switch the target air-fuel ratio from the rich air-fuel ratio (TAFrich) to the lean air-fuel ratio (TAFlean) when the air-fuel ratio detected by the downstream air-fuel ratio sensor (41) reaches a rich estimated air-fuel ratio, and to switch the target air-fuel ratio from the lean air-fuel ratio (TAFlean) to the rich air-fuel ratio (TAFrich) when the oxygen storage amount (OSA) reaches a switched storage amount that is smaller than a maximum oxygen storage amount (OSA), wherein the rich estimated air-fuel ratio is an air-fuel ratio that is richer than a stoichiometric air-fuel ratio and leaner than the rich Air-fuel ratio is (TAFrich),and, the air-fuel ratio control device (31) is configured to change a value of the rich estimated air-fuel ratio and / or the switched storage amount between the first state and the second state. [7] An exhaust gas purification system of an internal combustion engine according to any one of claims 1 to 6, wherein the air-fuel ratio control device (31) is configured to change a value of the rich set air-fuel ratio (TAFrich) and / or the lean set air-fuel ratio (TAFlean) between the first state and the second state. [8] An exhaust gas purification system of an internal combustion engine according to any one of claims 1 to 7, wherein the first state is a non-stationary state and the second state is a stationary state. [9] An exhaust gas purification system of an internal combustion engine according to any one of claims 1 to 7, wherein the first state is a steady state and the second state is a non-steady state. [10] Exhaust gas purification system of an internal combustion engine according to one of claims 1 to 7, wherein an EGR passage provided to return a part of the exhaust gas flowing through the exhaust passage as an EGR gas to an intake passage provided on the internal combustion engine, and the first state is a low EGR state in which an EGR gas flow rate is less than a first predetermined value, and the second state is a high EGR state in which the EGR gas flow rate is the first predetermined value or more, or the first state is a low EGR state in which the EGR rate is less than a second predetermined value, and the second state is a high EGR state in which the EGR rate is the second predetermined value or more. [11] Exhaust gas purification system of an internal combustion engine according to one of claims 1 to 7, wherein an EGR passage provided to return a portion of the exhaust gas flowing through the exhaust passage to the intake passage as EGR gas, provided on the internal combustion engine, and the first state is a high EGR state where an EGR gas flow rate is a first predetermined value or more, and the second state is a low EGR state where the EGR gas flow rate is less than the first predetermined value, or the first state is a high EGR state where the EGR rate is a second predetermined value or more, and the second state is a low EGR state where the EGR rate is less than the second predetermined value. [12] An exhaust gas purification system of an internal combustion engine according to any one of claims 1 to 7, wherein the first state is a high load state in which an engine load is a predetermined value or more, and the second state is a low load state in which the engine load is less than the predetermined value. [13] An exhaust gas purification system of an internal combustion engine according to any one of claims 1 to 7, wherein the first state is a low load state in which an engine load is lower than a predetermined value, and the second state is a high load state in which the engine load is the predetermined value or more.

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