JP2006274910A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device of an internal combustion engine capable of uniformizing the distribution of the stored amount of NOx in the termination of each of lean operation and rich operation. <P>SOLUTION: This exhaust emission control device comprises an NOx storage catalyst installed in the exhaust passage of the internal combustion engine, having a plurality of flow passages arranged parallel with each other therein, combining the exhaust flows in these flow passages with each other in a series direction, and storing NOx in exhaust gases in lean operation and releasing and reducing NOx stored in rich operation, selector valves disposed in the NOx storage catalyst and capable of reversing the flow direction of the exhaust gases in the flow passages combined with each other in the series direction, a rich operation discriminating means S303 discriminating the time of the termination of rich operation, and a selector valve control means S304 operating the selector valves so that these flow of the exhaust gases in the flow passages combined with each other in the series direction can be reversed a the time of the termination of rich operation. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to an exhaust gas purification apparatus suitable for an internal combustion engine that releases and reduces NOx stored in a NOx storage catalyst by a rich spike.

This type of exhaust purification device solves a problem that is difficult to deal with only by improving the combustion performed in the cylinder. As this exhaust purification device, a NOx storage catalyst is known that stores NOx in exhaust during lean operation and releases and reduces NOx stored during rich operation.
And the technique provided with the switching valve which can reverse the flow direction of the exhaust_gas | exhaustion discharged | emitted from the engine within this catalyst is disclosed (for example, refer patent document 1).
Japanese Patent Laid-Open No. 2004-28004

By the way, in the exhaust emission control device described in Patent Document 1, the direction of the exhaust gas is not reversed and the operation of the switching valve is prohibited until a predetermined period elapses after the end of the rich operation. This is to prevent the fuel from flowing out as it is through the catalyst.
However, if the exhaust flow direction is not reversed at the end of the rich operation, the amount of occlusion on the downstream side during the next lean operation continues to increase each time the rich operation is performed. That is, if the period until switching becomes long, NOx that is occluded on the upstream side during the lean operation and released in the subsequent rich operation is occluded again on the downstream side. Since the reducing agent is difficult to be supplied to the downstream side, the amount of occlusion on the downstream side continues to increase. As a result, the NOx occlusion amount at the end of the lean operation was initially distributed only on the upstream side, whereas it is distributed from the upstream side to the downstream side from the end point of the next lean operation. It becomes difficult to make uniform. Furthermore, the NOx occlusion amount on the downstream side at each end point of the rich operation continues to increase greatly and exceeds the allowable amount of the catalyst at an early stage.

Here, it is conceivable to determine the period from the required load of the engine, the rotational speed and the amount of fuel added to the switching, but in this case, the state in the catalyst cannot be grasped, and the operation of the switching valve is rejected and the above distribution of stored amount There is a concern that it will prevent the homogenization.
The present invention has been made in view of such a problem, and provides an exhaust purification device for an internal combustion engine that can achieve uniform distribution of the stored amount of NOx at each end point of lean operation and rich operation. Objective.

  In order to achieve the above object, the exhaust gas purification apparatus for an internal combustion engine according to claim 1 is interposed in an exhaust passage of the internal combustion engine, and has a plurality of flow paths arranged in parallel therein, and in each flow path. The NOx storage catalyst that stores NOx in the exhaust during lean operation and releases and reduces the NOx stored during rich operation and the NOx storage catalyst are combined in series. In addition, a switching valve that switches the direction of the exhaust flow in each flow path so that it can be reversed, rich operation determination means that determines when the rich operation ends, and each flow path combined in series at the end of rich operation And a switching valve control means for operating the switching valve so that the direction of the exhaust flow in the interior is reversed.

  The invention according to claim 2 further comprises NOx occlusion amount estimating means for estimating the occlusion amount of NOx with respect to both ends of the exhaust flow in each flow passage combined in the series direction. Based on the signals from the NOx occlusion amount estimating means and the rich operation determining means, the direction of the exhaust flow in each flow path is reversed.

Therefore, according to the exhaust gas purification apparatus for an internal combustion engine of the first aspect of the present invention, when the rich operation determining means determines the end point of the rich operation, the switching valve control means immediately passes each flow without waiting for the elapse of the predetermined period. Since the direction of the exhaust flow in the passage is reversed, the NOx occlusion amount distribution is surely made uniform at each end point of the lean operation and the rich operation.
Specifically, at the end of the lean operation, that is, at the start of the rich operation, a distribution that the NOx storage amount is always increased on the upstream side as much as possible and the storage amount on the downstream side is decreased is obtained. . Further, at the end of the rich operation, that is, at the start of the lean operation, the NOx storage amount is always reduced on the upstream side as much as possible, and the downstream storage amount is larger than the upstream storage amount at this time. A distribution is obtained.

  In other words, if the direction of the exhaust flow in each flow path is reversed immediately before the start of the lean operation, in this lean operation, the downstream side at the end of the previous rich operation switches to the upstream side, and a large amount of NOx Is always acceptable, and in the subsequent rich operation, a high concentration reducing agent can always be supplied to the upstream side. Further, in the lean operation, the upstream side at the end of the previous rich operation can be switched to the downstream side to reduce the storage amount, and NOx released and reduced from the upstream side in the subsequent rich operation can be stored again. Is less affected.

As a result, it is possible to release and reduce NOx more efficiently than before, and the NOx purification performance is maintained for a long period of time. Therefore, the frequency of rich spikes is reduced and fuel consumption can be reduced.
According to the second aspect of the invention, the direction of the exhaust flow in each flow path is reversed based on the NOx occlusion amount at both ends of the exhaust flow in addition to the end point of the rich operation. Therefore, the switching in which the state in the NOx storage catalyst is accurately grasped is possible, and the fail safe is achieved.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a diesel engine (hereinafter referred to as an engine) 2 to which an exhaust emission control device according to the present invention is applied. As shown in the figure, each cylinder 4 of the engine 2 is provided with a fuel supply system 6 having a fuel injection device. An intake passage 12 for introducing fresh air into the combustion chamber 10 by opening the intake valve 8 and an exhaust passage 16 for deriving exhaust gas from the combustion chamber 10 by opening the exhaust valve 14 are connected to the cylinder 4. ing.

A supercharger 18 is interposed on the upstream side of the intake passage 12, and an air cleaner (not shown) is connected to the tip of the intake passage 12. Further, an intercooler 20 is interposed at an appropriate position of the intake passage 12, and an air supply throttle 22 that adjusts the flow passage area of the intake passage 12 is disposed.
On the other hand, a later-described NOx storage catalyst 50 is interposed at an appropriate position on the downstream side of the exhaust passage 16. The NOx storage catalyst 50 stores NOx in the exhaust when the exhaust air-fuel ratio is leaner than the stoichiometric state, whereas the NOx storage catalyst 50 has a reducing agent in the exhaust when the exhaust air-fuel ratio is rich. When unburned fuel (HC) or carbon monoxide (CO) is present, the stored NOx is released and reduced. The function of the NOx storage catalyst 50 is known.

Further, an EGR passage 24 branches and extends from the exhaust passage 16, and the tip of the EGR passage 24 is connected to the intake passage 12. The EGR passage 24 is electrically connected to an EGR cooler 26 and an electronic control unit (ECU) 60. An EGR valve 28 connected to is provided.
Fresh air from the air cleaner enters the intake passage 12 via the supercharger 18, reaches the intercooler 20, is adjusted by the air supply throttle 22, and is then introduced into the combustion chamber 10 of each cylinder 4. Then, the crankshaft 30 and the flywheel 32 are operated by combustion of fuel supplied from the fuel supply system 6. When combustion ends, the exhaust is discharged into the exhaust passage 16 and introduced into the NOx storage catalyst 50.

The NOx storage catalyst 50 of the present embodiment is configured to be able to reverse the direction of the flow of exhaust gas introduced from the exhaust passage 16.
Specifically, as shown in FIG. 2, the NOx storage catalyst 50 includes a cylindrical catalyst body 500, and three flow paths 501, 502, and 503 are partitioned inside the catalyst body 500. More specifically, the first flow path 501 and the third flow path 503 extend in a cylindrical shape along the longitudinal axis direction of the catalyst body 500 (FIG. 5A), and the flow paths 501 and 503 are the catalyst. It is formed inside the edge along the outer peripheral edge of the main body 500. A flow path 501 is formed on the upper side of the catalyst main body 500 with a diameter portion (FIG. 5B), and a flow path 503 is formed on the lower side. (FIG. 3C) are arranged.

The second flow path 502 is disposed inside the flow path 501 and the flow path 503 and extends in a cylindrical shape along the longitudinal axis direction of the catalyst main body 500. The flow path 501, the flow path 502, and the flow path 503 are provided. Are arranged side by side ((a) in the figure). Further, the flow path 501, the flow path 502, and the flow path 503 have the same cross-sectional area, and have a catalyst equally.
A cylindrical rod support portion 504 is disposed along the longitudinal axis direction in the central axis portion of the catalyst body 500, that is, in the central portion of the flow path 502, and the rod 506 is provided in the rod support portion 504. It is inserted. Both end portions of the rod 506 protrude from both end surfaces of the catalyst body 500, and an inflow direction switching valve (switching valve) 508 is fixed to one end side of the rod 506 via a locking portion 514, and the other end side of the rod 506. In addition, an inflow direction switching valve (switching valve) 518 is fixed via a locking portion 516. Further, the other end of the rod 506 is connected to a drive shaft 526 via a joint 524, and the drive shaft 526 is rotated in response to an instruction signal from the ECU 60. That is, when an ON signal is output from the ECU 60, the rotation of the drive shaft 526 is transmitted to the rod 506, and the inflow direction switching valves 508 and 518 are also rotated in the same direction as the rod 506.

  The inflow direction switching valve 508 has a semicircular lid portion 510 based on the diameter of the catalyst body 500 and a semicircular shape based on the diameter of the flow path 502, as shown in FIG. It is comprised from the cover part 511, and the cover part 510 and the cover part 511 are integrally formed. The inflow direction switching valve 508 covers either the flow path 502 and the flow path 503 (FIG. 5B) or the flow path 501 and the flow path 502 according to the rotation of the rod 506. Further, a passage 512 is formed between the lid portion 510 and the lid portion 511 and one end face side of the catalyst body 500 (FIG. 5A).

  On the other hand, the inflow direction switching valve 518 is also based on the diameter of the semicircular lid portion 520 based on the diameter of the catalyst body 500 and the flow path 502, as shown in FIG. The lid portion 521 is formed in a semicircular shape, and the lid portion 520 and the lid portion 521 are integrally formed. The inflow direction switching valve 518 covers either the flow path 501 and the flow path 502 (FIG. 5C) or the flow path 502 and the flow path 503 according to the rotation of the rod 506. Further, a passage 522 is also formed between the lid portion 520 and the lid portion 521 and the other end surface side of the catalyst main body 500 (FIG. 5A).

  By the way, the inflow direction switching valve 508 and the inflow direction switching valve 518 are arranged in opposite phases. Specifically, as shown in FIG. 6A, the inflow direction switching valve 518 is in a position where the inflow direction switching valve 508 covers the flow path 502 and the flow path 503 (FIG. 5B). It is provided at a position covering the path 502 ((c) in the figure). As a result, when the inflow direction switching valve 508 covers the upstream side of the flow path 502 and the flow path 503, the inflow direction switching valve 518 covers the downstream side of the flow path 501 and the flow path 502, and the flow path 501 is the flow path. The channel 502 is positioned upstream of the channel 503 and is positioned upstream of the channel 502.

On the other hand, when the inflow direction switching valve 508 covers the upstream side of the flow path 501 and the flow path 502, the inflow direction switching valve 518 covers the downstream side of the flow path 502 and the flow path 503, and the flow path 503 becomes the flow path 502. The flow path 502 is located upstream of the flow path 501.
Returning to FIG. 1 again, in the present embodiment, an addition injector 44 that directly supplies HC to the NOx storage catalyst 50 is disposed at an appropriate position upstream of the NOx storage catalyst 50, and the addition injector 44 is configured to add fuel. It is connected to pump 48 via line 46.

  Further, at an appropriate position upstream of the NOx storage catalyst 50 in the exhaust passage 16, a NOx sensor 36 that detects the NOx concentration, that is, the NOx amount based on the output voltage, and an exhaust temperature sensor 38 that detects the temperature in the exhaust passage 16. Are arranged respectively. Furthermore, a NOx sensor 40 that detects the amount of NOx and a catalyst temperature sensor 42 that detects the temperature of the NOx storage catalyst 50 are disposed at appropriate positions downstream of the NOx storage catalyst 50, respectively. , 40 and 42 are electrically connected to the ECU 60.

  On the input side of the ECU 60, various sensors for detecting the operating state of the engine 2, such as the crank angle sensor 34, in addition to the NOx sensor 36, the exhaust temperature sensor 38, the NOx sensor 40, and the catalyst temperature sensor 42 described above are also electrically connected. It is connected to the. On the other hand, on the output side of the ECU 60, the above-described fuel supply system 6, the air supply throttle 22, the addition injector 44, the actuator for rotating the drive shaft 526, the pump 48, and the like are electrically connected.

Further, the ECU 60 is provided with various maps, for example, various maps relating to NOx occlusion amount estimation such as a NOx release amount map.
Here, the above-described NOx storage catalyst 50 stores NOx in the exhaust in an oxidizing atmosphere, while the rich operation is performed before the NOx storage amount reaches saturation in order to suppress a decrease in the performance of the catalyst accompanying an increase in the NOx storage amount. The NOx storage catalyst 50 is regenerated by performing a rich spike that is intermittently switched to. As a result, the exhaust gas is well purified.

  Specifically, the rich spike of the present embodiment is performed in the out-cylinder rich. That is, when a rich spike is instructed in accordance with signals from various sensors 36, 38, 40, 42, etc., the HC pumped from the pump 48 is exhausted into the exhaust gas using the addition injector 44 provided in the exhaust passage 16. Direct injection is performed to create a condition for rich operation. If this condition is satisfied, NOx is released and reduced. The inflow direction switching valves 508 and 518 are operated after the NOx release reduction is completed.

More specifically, the ECU 60 includes a rich operation determination unit (rich operation determination unit) 62, a NOx storage amount estimation unit (NOx storage amount estimation unit) 64, and a switching valve control unit (switching valve control unit) 66. .
The rich operation determination unit 62 determines the presence or absence of a rich spike instruction according to the signals from the various sensors 36, 38, 40, 42, etc., and also determines the end point of the rich operation, and the result is the NOx storage. It is output to the quantity estimation unit 64.

  The NOx occlusion amount estimation unit 64 estimates the NOx occlusion amount for the flow path 501 and the flow path 503. More specifically, during lean operation, the NOx occlusion amount is calculated based on the exhaust flow rate of the exhaust passage 16 obtained from the intake air amount and the NOx concentration from the NOx sensors 36 and 40. On the other hand, during rich outside the cylinder, the NOx release amount is calculated by the map based on the exhaust flow rate, the exhaust temperature from the exhaust temperature sensor 38, and the catalyst temperature from the catalyst temperature sensor 42. Then, the calculated NOx release amount is subtracted from the calculated NOx occlusion amount to estimate the current NOx occlusion amounts of the respective channels 501 and 503. The result is output to the switching valve control unit 66.

The switching valve controller 66 rotates the drive shaft 526 so that the estimated NOx occlusion amount of the flow path 501 or the flow path 503 is the most upstream side at the end of the rich operation. The inflow direction switching valves 508 and 518 are operated.
FIG. 3 shows a flowchart of the exhaust inflow direction switching control by the rich operation determination unit 62, the NOx occlusion amount estimation unit 64, and the switching valve control unit 66. Hereinafter, the exhaust gas purification configured as described above will be described. The operation of the apparatus according to the present invention will be described.

In step S301 in the figure, the lean operation is performed, the NOx occlusion amount estimation unit 64 calculates the NOx occlusion amount, and the process proceeds to step S302.
In step S302, the lean operation is finished and the rich operation is started. This rich operation is monitored by the rich operation determination unit 62. Further, the NOx occlusion amount estimation unit 64 calculates the NOx release amount, and estimates the current NOx occlusion amounts of the flow paths 501 and 503, respectively.

  Next, in step S303, the rich operation determination unit 62 determines whether the rich operation has ended. If it is determined that the rich operation is finished and the lean operation is started, that is, if YES, the process proceeds to step S304, and the switching valve control unit 66 estimates the flow path 501 or the flow path 503. Further, the position where the amount of stored NOx is larger is switched to the upstream side.

  That is, when the inflow direction switching valves 508 and 518 are at positions where the flow path 501 is located upstream during the previous lean and rich operations, the NOx in the flow path 501 is almost released and reduced at the end of the rich operation. Since it can be occluded again in the downstream channel 503, the NOx occlusion amount in the channel 503 is larger than that in the channel 501. Therefore, in this case, the switching valve control unit 66 reverses the flow path 503 to a position on the upstream side.

  Thus, the inflow direction switching valve 508 covers the upstream side of the flow path 501 and the flow path 502, and the inflow direction switching valve 518 is operated to a position that covers the downstream side of the flow path 502 and the flow path 503. Exit. In this case, the exhaust flow in the flow path 503, the flow path 502, and the flow path 501 are combined in this order in series, and the exhaust from the exhaust passage 16 is introduced into the flow path 503 as the most upstream side, and then Then, after being introduced into the flow path 502 through the passage 522 and introduced into the flow path 501 through the passage 512, it goes to the outside.

  On the other hand, when the inflow direction switching valves 508 and 518 are at positions where the flow path 503 is located upstream during the previous lean and rich operations, the direction of the exhaust flow is reversed from the above. In other words, the inflow direction switching valve 508 covers the upstream side of the flow path 502 and the flow path 503, and the inflow direction switching valve 518 is operated to a position covering the downstream side of the flow path 501 and the flow path 502, respectively. Exit the routine. In this case, the exhaust flows in the flow path 501, the flow path 502, and the flow path 503 are combined in this order in the series direction, introduced into the flow path 501 as the most upstream side, and introduced into the flow path 502 through the passage 522. Is done. Next, after being introduced into the flow path 503 through the passage 512, it goes to the outside.

As described above, according to the present invention, the direction of the exhaust flow in the flow paths 501, 502, and 503 immediately after the switching valve control unit 66 finishes a predetermined period of time each time a pair of lean / rich operations ends. Reverse. Therefore, the NOx occlusion distribution can be made uniform evenly at the end of each of the lean operation and the rich operation.
More specifically, as shown in FIG. 4 (a), the characteristics of the NOx occlusion catalyst are as follows. When the NOx occlusion amount at the end of lean operation, that is, at the start of rich operation, the inlet side is large and the outlet side is When it decreases, the distribution decreases to the right (indicated by hatching in the figure). This is because NOx is easily occluded on the inlet side and becomes difficult to occlude as it is located on the outlet side. On the other hand, as shown in FIG. 4 (b), when the NOx occlusion amount at the end of the rich operation, that is, the start of the lean operation is seen, the NOx occlusion amount is hardly recognized on the inlet side, and rises to the right when the outlet side increases. It becomes a distribution (indicated by diagonal lines in the figure). This is because a high concentration reducing agent is easily supplied to the inlet side, and NOx occluded on this inlet side (indicated by a dotted line in the figure) is almost released and reduced, but the reducing agent is located on the outlet side. This is because part of the NOx released and reduced from the inlet side is occluded again on the outlet side because it becomes difficult to be supplied.

  If the exhaust flow direction is not reversed at the end of the rich operation as in the conventional case, and the exhaust flow direction is not reversed at the end of the rich operation, at the end of the next lean operation, not only at the inlet side but also at the outlet side. It will be occluded and it will become difficult to obtain the distribution of the said right downward. Further, at the end of the subsequent rich operation, the NOx on the inlet side is released and reduced, and the above-mentioned upward distribution is obtained, but a large amount is occluded on the outlet side and the allowable amount of the catalyst is easily reached.

  However, according to the present invention, the exhaust flow direction in the flow paths 501, 502, and 503 is reversed at the end of the rich operation. Therefore, in the lean operation, the end of the previous rich operation (see FIG. )) Is set on the upstream side, and a large amount of NOx can be received on the upstream side. In addition, the inlet side at the end of the previous rich operation ((b) in the figure) is set to the downstream side, and the amount of occlusion on the downstream side can always be reduced. That is, the above-mentioned right-down distribution is always obtained and uniformization is achieved. In the subsequent rich operation, it is always possible to supply a high concentration reducing agent to the NOx occluded in a large amount on the upstream side for release reduction, and the above-mentioned upward distribution is obtained to achieve uniformization. At the same time, a part of the released and reduced NOx is occluded on the inlet side at the end of the previous rich operation ((b) in the same figure), so that the occlusion amount is difficult to reach the allowable amount of the catalyst.

As a result, it is possible to release and reduce NOx more efficiently than before, and the NOx purification performance is maintained for a long period of time. Therefore, the frequency of rich spikes is reduced and fuel consumption can be reduced.
Further, when the switching valve control unit 66 reverses the direction of the exhaust flow in the flow paths 501, 502, and 503 based on the NOx occlusion amount in the flow paths 501 and 503 in addition to the end of the rich operation, the NOx occlusion is performed. Switching which accurately grasps the state in the catalyst 50 is possible, which is fail-safe, and the NOx occlusion distribution is more evenly distributed at each end of the lean operation and the rich operation.

Furthermore, when the switching valve control unit 66 reverses the direction of the exhaust flow in the flow paths 501, 502, 503, the temperature distribution in the NOx storage catalyst is made uniform.
Specifically, as shown in FIG. 5, in the temperature distribution during the rich operation in the NOx storage catalyst, immediately after the start of the rich operation, the temperature on the inlet side (indicated by the solid line in the figure) is the center temperature. (It is indicated by a one-dot chain line in the figure) and the outlet side temperature (indicated by a two-dot chain line in the figure) is always high, but according to the present invention, the switching of the exhaust flow direction described above is possible. Therefore, the same flow path is unlikely to be thermally deteriorated by S purge or the like. This contributes to improvement in durability of the NOx storage catalyst and maintenance of performance after deterioration. Note that the switching timing in this case is not necessarily performed every time a rich operation is performed.

The description of one embodiment of the present invention is finished above, but the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.
For example, in the above embodiment, the switching valve control unit 66 immediately turns the drive shaft 526 on the basis of the signal at the end of the rich operation from the rich operation determination unit 62 without considering the current estimated NOx storage amount. The inflow direction switching valves 508 and 518 may be operated by rotating. Also in this case, similarly to the above, the NOx occlusion amount distribution at the end points of the lean operation and the rich operation can be made uniform.

  In the above embodiment, the NOx storage catalyst 50 is partitioned into the three flow paths 501, 502, and 503, but the present invention is not necessarily limited to this form. That is, it can be partitioned into a plurality of arbitrary flow paths.

1 is an engine system configuration diagram to which an exhaust gas purification apparatus for an internal combustion engine according to an embodiment of the present invention is applied. It is sectional drawing of a NOx storage catalyst. 3 is a flowchart showing an exhaust inflow direction switching control routine executed by an ECU. It is a figure explaining NOx occlusion amount distribution in a catalyst. It is a figure explaining the temperature distribution in a catalyst.

Explanation of symbols

2 Internal combustion engine 16 Exhaust passage 50 NOx storage catalyst 60 ECU (electronic control unit)
62 Rich operation determination unit (rich operation determination means)
64 NOx occlusion amount estimation unit (NOx occlusion amount estimation means)
66 Switching valve control section (switching valve control means)
501 First channel (channel)
502 Second channel (channel)
503 Third channel (channel)
508 Inflow direction switching valve (switching valve)
518 Inflow direction switching valve (switching valve)

Claims (2)

It is interposed in the exhaust passage of the internal combustion engine, and has a plurality of flow paths arranged in parallel, and combines the exhaust flow in each flow path in series, and stores NOx in the exhaust during lean operation. A NOx storage catalyst for releasing and reducing the stored NOx during rich operation;
A switching valve that is disposed in the NOx storage catalyst and switches the direction of the exhaust flow in each flow path combined in the series direction so as to be able to reversely rotate;
Rich operation determining means for determining the end point of the rich operation;
An internal combustion engine characterized by comprising switching valve control means for operating the switching valve so that the direction of the exhaust flow in each flow path combined in the series direction is reversed at the end of the rich operation. Engine exhaust purification system. Further comprising NOx occlusion amount estimation means for estimating the occlusion amount of NOx with respect to both ends of the exhaust flow in each flow path combined in the series direction;
The internal combustion engine according to claim 1, wherein the switching valve control means reverses the direction of the exhaust flow in each flow path based on signals from the NOx occlusion amount estimation means and the rich operation determination means. Engine exhaust purification system.

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