JP5482222B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust gas purification device for an internal combustion engine, and more particularly to an exhaust gas purification device for an internal combustion engine that includes a NOx storage reduction catalyst and a NOx selective reduction catalyst.

  Conventionally, as disclosed in, for example, Japanese Patent Application Laid-Open No. 2009-111489, an SOx trap catalyst, a NOx occlusion reduction catalyst (hereinafter referred to as “NSR catalyst”), and NOx selective reduction from the upstream side to the exhaust passage of the internal combustion engine An exhaust purification device provided with a catalyst (hereinafter referred to as “SCR”) is known. The NSR catalyst has an occlusion function that occludes nitrogen oxide (NOx) contained in combustion gas discharged from the internal combustion engine inside the catalyst, and a catalytic function that purifies NOx and hydrocarbons (HC) and the like. It is a catalyst. When the internal combustion engine is operated at a lean air-fuel ratio, exhaust gas containing a large amount of NOx is discharged. For this reason, the above-mentioned NSR catalyst occludes this NOx therein, and suppresses the situation where the NOx is released downstream of the catalyst. The SOx trap catalyst is a catalyst that can capture SOx contained in the exhaust gas, and suppresses SOx occlusion in the NSR catalyst.

  The SCR is a catalyst capable of selectively reducing NOx with ammonia. When the engine operation is temporarily controlled to a rich air-fuel ratio, ammonia is generated by the reduction reaction of NOx in the NSR catalyst. In SCR, NOx is selectively reduced using this ammonia.

JP 2009-111487 A JP 2004-525299 A

  When the internal combustion engine is operated at a lean air-fuel ratio, the exhaust gas can be effectively purified by the exhaust gas purification device including the NSR catalyst and the SCR as in the conventional device described above. However, the lean operation is not always performed depending on the operating conditions of the internal combustion engine. That is, the operating air-fuel ratio of the internal combustion engine varies depending on the operating conditions. For example, when a high load request is issued to the internal combustion engine, stoichiometric operation with the operating air-fuel ratio as stoichiometric may be performed. In this case, there is a possibility that the exhaust gas during the stoichiometric operation cannot be effectively purified with an NSR catalyst having a low ternary activity. For this reason, in an internal combustion engine capable of lean burn operation, an exhaust purification device in which an NSR catalyst and an SCR are simply arranged cannot maintain high exhaust purification performance, and an improvement has been desired.

  The present invention has been made to solve the above-described problems, and in an internal combustion engine capable of lean burn operation, exhaust purification of the internal combustion engine that can maintain high purification performance regardless of the engine operating air-fuel ratio. An object is to provide an apparatus.

In order to achieve the above object, a first invention is an exhaust purification device for an internal combustion engine capable of lean burn operation,
A NOx occlusion reduction catalyst (hereinafter referred to as NSR catalyst) disposed in the exhaust passage of the internal combustion engine;
A NOx selective reduction catalyst (hereinafter referred to as SCR) disposed downstream of the NSR catalyst;
A branch passage branched from the upstream side of the NSR catalyst in the exhaust passage;
A three-way catalyst disposed in the branch passage;
A flow rate control valve that is disposed at a branch portion between the exhaust passage and the branch passage, and variably sets a flow rate ratio of exhaust gas flowing in the exhaust passage downstream of the branch portion and the branch passage;
Control means for controlling the flow rate control valve based on the operating air-fuel ratio of the internal combustion engine,
The control means controls the flow rate control valve so that at least a part of the exhaust gas flows to the branch passage during the stoichiometric operation of the internal combustion engine, and a load during the stoichiometric operation of the internal combustion engine is predetermined. The flow rate control valve is controlled so that the exhaust gas flows to both the exhaust passage and the branch passage downstream of the branch portion when the load value is higher than the high load value .

According to a second invention, in the first invention,
The control means controls the flow rate control valve so that the entire amount of exhaust gas flows through the exhaust passage downstream of the branch portion during the lean burn operation of the internal combustion engine.

According to a third invention, in the first or second invention,
The control means controls the flow rate control valve so that the entire amount of exhaust gas flows into the branch passage when the load during the stoichiometric operation of the internal combustion engine is lower than the predetermined high load value. And

A fourth invention is any one of the first to third inventions,
The downstream end of the branch passage is joined to a position downstream of the NSR catalyst and upstream of the SCR in the exhaust passage.

According to the first invention, during the stoichiometric operation of the internal combustion engine, at least a part of the exhaust gas is circulated to the branch passage in which the three-way catalyst is arranged. For this reason, according to the present invention, even during a stoichiometric operation in which a high purification rate cannot be maintained by an NSR catalyst having a low three-way activity, a high three-way catalyst is used to maintain a high purification performance. can do.
Further, according to the present invention, when the load during the stoichiometric operation of the internal combustion engine is higher than a predetermined high load value, the branch passage in which the three-way catalyst is arranged, the exhaust passage in which the NSR catalyst and the SCR are arranged, Exhaust gas is circulated through both. During high-load operation, the exhaust gas flow rate is large, so that unpurified exhaust gas may slip through the branch passage downstream of the three-way catalyst. On the other hand, the NSR catalyst has not only a NOx storage function but also a ternary function. For this reason, according to the present invention, in addition to the three-way function of the three-way catalyst, the three-way function of the NSR catalyst can be used effectively, so that high purification performance can be maintained even during high-load stoichiometric operation. Can do.

  According to the second invention, during the lean burn operation of the internal combustion engine, the entire amount of the exhaust gas is circulated to the exhaust passage provided with the NSR catalyst and the SCR. Therefore, according to the present invention, since the NSR catalyst and the SCR can be used effectively during the lean burn operation in which the high purification rate cannot be maintained by the three-way catalyst, high purification performance even during the lean burn operation. Can be maintained.

According to the third invention, when the load during the stoichiometric operation of the internal combustion engine is lower than a predetermined high load value , the entire amount of the exhaust gas is circulated to the branch passage having the three-way catalyst. For this reason, according to the present invention, since the three-way catalyst can be utilized to the maximum extent during the stoichiometric operation in which the high purification rate cannot be maintained by the NSR catalyst and the SCR, high purification performance can be obtained even during the stoichiometric operation. Can be maintained.

According to the fourth invention, non-methane hydrocarbon (NMHC) components and the like that are discharged downstream of the three-way catalyst disposed in the branch passage can be purified by the SCR.

It is a figure for demonstrating the structure of Embodiment 1 of this invention. It is a figure for demonstrating the NOx purification rate at the time of lean operation in an exhaust system provided with an NSR catalyst and SCR. It is a figure which shows the ammonia generation amount in a NSR catalyst and TWC. It is a figure for demonstrating control of the flow regulating valve during lean operation. It is a figure for demonstrating control of the flow regulating valve during stoichi operation. It is a flowchart of the routine performed in Embodiment 1 of the present invention. It is a figure for demonstrating the modification of the system of this Embodiment 1. FIG. It is a figure for demonstrating control of the flow regulating valve during high load stoichiometric operation. It is a flowchart of the routine performed in Embodiment 2 of this invention.

  Several embodiments of the present invention will be described below with reference to the drawings. In addition, the same code | symbol is attached | subjected to the element which is common in each figure, and the overlapping description is abbreviate | omitted. The present invention is not limited to the following embodiments.

Embodiment 1 FIG.
[Configuration of Embodiment 1]
FIG. 1 is a diagram for explaining the configuration of the first embodiment of the present invention. As shown in FIG. 1, the system of the present embodiment includes an internal combustion engine (engine) 10. An exhaust passage 12 communicates with the exhaust side of the internal combustion engine 10. A start catalyst (hereinafter referred to as “SC”) 14 that is a three-way catalyst is disposed in the exhaust passage 12.

  The exhaust passage 12 branches into a first exhaust passage 121 and a second exhaust passage 122 on the downstream side of the SC 14. An NSR catalyst (NOx storage reduction catalyst) 16 is disposed in the middle of the first exhaust passage 121, and an SCR (NOx selective reduction catalyst) 18 is disposed downstream thereof. A three-way catalyst (hereinafter referred to as “TWC”) 20 is disposed in the middle of the second exhaust passage 122.

The internal combustion engine 10 easily discharges HC and CO when the air-fuel ratio is rich. Further, it is easy to exhaust NOx when the air-fuel ratio is lean. The SC 14 and the TWC 20 reduce NOx (purify to N ₂ ) while adsorbing oxygen (O ₂ ) in a lean atmosphere. On the other hand, in a rich atmosphere, HC and CO are oxidized (purified to H ₂ O, CO ₂ ) while releasing oxygen. Further, in a rich atmosphere, ammonia (NH ₃ ) is generated by the reaction of nitrogen contained in the exhaust gas with hydrogen.

The NSR catalyst 16 occludes NOx contained in the exhaust gas under a lean atmosphere. Further, the NSR catalyst 16 releases the stored NOx in a rich atmosphere. NOx released in a rich atmosphere is reduced by HC and CO. At this time, NH ₃ is also generated in the NSR catalyst 16 as in the case of SC14.

The SCR 18 has a function in which the SC 14 and the NSR catalyst 16 occlude NH ₃ generated under a rich atmosphere, and under a lean atmosphere, the NO ₃ in the exhaust gas is selectively reduced using NH ₃ as a reducing agent. Yes. According to the SCR 18, it is possible to effectively prevent a situation in which NH ₃ and NOx blown downstream of the NSR catalyst 16 are released into the atmosphere.

  The system shown in FIG. 1 includes a flow rate adjustment valve 22 at a branch point of the exhaust passage 12, that is, a communication portion between the upstream ends of the first exhaust passage 121 and the second exhaust passage 122 and the exhaust passage 12. The flow rate adjusting valve 22 can switch the flow destination of the exhaust gas exhausted from the internal combustion engine 10 between the first exhaust passage 121 and the second exhaust passage 122, or can adjust the flow rate to a predetermined flow rate ratio.

In addition, the system shown in FIG. 1 includes an air-fuel ratio (A / F) sensor 24 on the upstream side of the SC 14 in the exhaust passage 12. The A / F sensor 24 can detect the exhaust air / fuel ratio of the internal combustion engine 10. Further, an oxygen (O ₂ ) sensor 26 is provided at a position upstream of the flow rate adjustment valve 22 and downstream of the SC 14 in the exhaust passage 12 and downstream of the junction of the first exhaust passage 121 and the second exhaust passage 122. , 28. The O ₂ sensors 26 and 28 are sensors that generate a signal corresponding to the oxygen concentration in the exhaust gas.

The system according to the present embodiment includes an ECU (Electronic Control Unit) 30 as shown in FIG. Various actuators such as the flow rate adjusting valve 22 described above are connected to the output portion of the ECU 30. In addition to the A / F sensor 24 and the O ₂ sensors 26 and 28 described above, various sensors for detecting the operating condition and operating state of the internal combustion engine 10 are connected to the input unit of the ECU 30. The ECU 30 can control the state of the system shown in FIG. 1 based on various types of input information.

[Operation of Embodiment 1]
First, the function and operation of the NSR catalyst 16 will be described. The ECU 30 normally operates the internal combustion engine 10 at a lean air-fuel ratio (lean operation). During lean operation, oxidizers such as NOx are discharged in a larger amount than reducing agents such as HC and CO. For this reason, even if it is going to purify the said exhaust gas using TWC20, all NOx cannot be purified by lack of a reducing agent. Therefore, in the system of the first embodiment, during the lean operation, the flow rate adjustment valve 22 is controlled so that the entire amount of exhaust gas flows from the exhaust passage 12 to the first exhaust passage 121. The NSR catalyst 16 is disposed in the first exhaust passage 121. The NSR catalyst 16 has a function of storing NOx as nitrates such as Ba (NO ₃ ) ₂ . For this reason, according to the system of the first embodiment, it is possible to effectively suppress the situation where the NOx is released into the atmosphere even during the lean operation.

However, the NOx storage performance of the NSR catalyst 16 decreases as the storage amount increases. For this reason, if the lean operation is continued for a long time, NOx that has not been occluded will blow through downstream of the catalyst. Therefore, in the system of the first embodiment, rich spike control is performed in which NOx occluded in the NSR catalyst 16 is periodically desorbed and processed. More specifically, the exhaust air-fuel ratio of the internal combustion engine 10 is temporarily made rich (for example, A / F = 12) at a predetermined timing when the storage performance of the NSR catalyst 16 is lowered. The exhaust gas during execution of rich spike contains a large amount of reducing agent such as HC, CO, H _{2 and the} like. For this reason, when these reducing agents are introduced into the NSR catalyst 16, NOx stored as nitrate is reduced to NO and desorbed from the base. The desorbed NOx is purified to N ₂ or the like on the catalyst in the NSR catalyst 16 and processed. Thus, by executing the rich spike during the lean operation, the NOx occluded in the NSR catalyst 16 can be desorbed, so that the NOx occlusion performance can be effectively recovered.

  Next, the function of the SCR 18 will be described. As described above, the NOx storage performance of the NSR catalyst 16 can be effectively recovered by executing the rich spike. However, when the rich spike is executed, part of the NOx desorbed from the NSR catalyst 16 is blown downstream without being purified. Further, as described above, there is also NOx that is blown downstream without being stored in the NSR catalyst 16 during the lean operation. If these blow-by NOx are released into the atmosphere as they are, emissions will be deteriorated.

Therefore, the system according to the first embodiment includes an SCR 18 for processing NOx blown through the downstream side of the NSR catalyst 16. As described above, the SCR 18 occludes therein NH ₃ produced by the SC 14 and the NSR catalyst 16 in a rich atmosphere. For this reason, according to the SCR 18, NOx blown downstream of the NSR catalyst 16 can be selectively reduced and purified by NH ₃ . Thereby, the situation where NOx is released into the atmosphere and the emission deteriorates can be effectively prevented.

  The purification performance of the exhaust system having the NSR catalyst and the SCR will be discussed in detail below. FIG. 2 is a diagram for explaining the NOx purification rate during the lean operation in the exhaust system including the NSR catalyst and the SCR. In this figure, (a) has the same configuration as the exhaust system on the first exhaust passage 121 side in the system of the first embodiment, and SC, NSR catalyst, and SCR are arranged from the upstream side of the exhaust passage, respectively. (B) shows, as a comparative example, an exhaust system in which SC, TWC, and double SCR are respectively arranged from the upstream side of the exhaust passage. Further, the exhaust gas samples are those collected at the position on the outlet side of the NSR catalyst or TWC and the position on the outlet side of the SCR.

As shown in this figure, in the exhaust system configuration of (a), the NOx purification rate is three times or more than that of (b) in any of the NSR catalyst outlet and SCR outlet samples. From this, it can be seen that during lean operation, the NSR catalyst can purify NOx more effectively than the TWC. FIG. 3 is a graph showing the amount of ammonia generated in the NSR catalyst and TWC. As shown in this figure, the NSR catalyst produces a large amount of ammonia (NH ₃ ), whereas the TWC cannot achieve this because of its nature. From this result, it can be seen that an exhaust system configuration in which an NSR catalyst is arranged upstream of the SCR is suitable for increasing the NOx purification rate in the SCR.

[Characteristic operation of the first embodiment]
As described above, the ECU 30 normally operates the internal combustion engine 10 at a lean air-fuel ratio (lean operation). FIG. 4 is a diagram for explaining the control of the flow rate adjusting valve during the lean operation. As shown in this figure, during the lean operation, the flow rate adjustment valve 22 is controlled so that the entire amount of exhaust gas flows from the exhaust passage 12 to the first exhaust passage 121. In this case, the entire amount of the exhaust gas is introduced into the NSR catalyst 16 and the SCR 18 disposed in the first exhaust passage 121, so that the lean gas as the exhaust gas can be effectively purified.

  However, when an operation request such as an acceleration request or a high load request is issued to the internal combustion engine 10, it may be preferable to perform a stoichiometric operation in which the internal combustion engine 10 is operated at a stoichiometric air-fuel ratio. In this case, since the ECU 30 causes the internal combustion engine 10 to perform a stoichiometric operation, the NSR catalyst 16 having a poor three-way function may not be able to purify all of the stoichiometric gas.

  Therefore, in the system according to the first embodiment, during the stoichiometric operation, the flow rate adjustment valve 22 is controlled so that the entire amount of exhaust gas flows from the exhaust passage 12 to the second exhaust passage 122. FIG. 5 is a diagram for explaining the control of the flow rate adjustment valve during the stoichiometric operation. As shown in this figure, the TWC 20 is disposed in the second exhaust passage 122. The TWC 20 has a higher stoichiometric activity than the NSR catalyst 16. For this reason, according to the system of the first embodiment, it is possible to effectively purify the stoichiometric gas as the exhaust gas even during the stoichiometric operation.

  Thus, according to the system of the present embodiment, the flow rate adjustment valve 22 is controlled according to the operating air-fuel ratio of the internal combustion engine 10. As a result, the exhaust gas can be effectively purified regardless of whether the internal combustion engine 10 is in a lean operation or a stoichiometric operation.

[Specific Processing in Embodiment 1]
Next, with reference to FIG. 6, the specific content of the process performed in this Embodiment is demonstrated. FIG. 6 is a flowchart of a routine that the ECU 30 executes. Note that the routine shown in FIG. 6 is repeatedly executed during operation of the internal combustion engine 10.

  In the routine shown in FIG. 6, first, the base position of the flow rate adjustment valve 22 is set (step 100). Here, specifically, the valve position of the flow rate adjustment valve 22 is set to a position where the entire amount of exhaust gas flows to the first exhaust passage 121. Next, it is determined whether or not a stoichiometric operation request for the internal combustion engine 10 has been issued (step 102). As a result, when the stoichiometric operation request is not issued, it is determined that the lean operation is being performed, and the process of step 100 is repeatedly executed. On the other hand, if the stoichiometric operation request is issued in step 102, it is determined that the entire amount of stoichiometric gas cannot be purified using the NSR catalyst 16 and the SCR 18, and the process proceeds to the next step, where the exhaust gas is exhausted. The flow rate adjustment valve 22 is set so that the total amount of the refrigerant flows through the second exhaust passage 122 (step 104). As a result, the entire amount of exhaust gas is distributed to the TWC 20.

  In the routine shown in FIG. 6, it is next determined whether or not the stoichiometric operation request is still continuing (step 106). As a result, when it is determined that the stoichiometric operation request is continuing, it is determined that the stoichiometric gas is still exhausted, and the process of step 104 is repeatedly executed. On the other hand, if it is determined in step 106 that the stoichiometric operation request is not continued, it is determined that the operation is switched to the lean operation, and the valve position of the flow rate adjusting valve 22 is returned to the base position again.

  As described above, according to the system of the present embodiment, the communication destination of the exhaust passage 12 is set on the first exhaust passage 121 side during the lean operation, and is set on the second exhaust passage 122 side during the stoichiometric operation. . As a result, the lean gas can be effectively purified using the NSR catalyst 16 and the SCR 18 during the lean operation, and the stoichiometric gas that cannot be completely purified by the NSR catalyst 16 and the SCR 18 is effective using the TWC 20 during the stoichiometric operation. Can be purified. Thereby, it is possible to effectively purify the exhaust gas even during the lean operation or the stoichiometric operation.

  Incidentally, in the first embodiment described above, the SCR 18 is arranged on the downstream side of the NSR catalyst 16 in the first exhaust passage 121, but the arrangement of the SCR 18 is not limited to this. FIG. 7 is a diagram for explaining a modification of the system according to the first embodiment. As shown in this figure, the SCR 18 may be arranged in the exhaust passage 12 after the first exhaust passage 121 and the second exhaust passage 122 merge. Thereby, the non-methane hydrocarbon (NMHC) component etc. which are discharged | emitted downstream of TWC20 can also be purified in the said SCR18.

  In the first embodiment described above, the valve position of the flow rate adjustment valve 22 is controlled based on whether or not the stoichiometric operation request has been issued, but the exhaust gas detected by the A / F sensor 24 is controlled. The valve control may be executed based on whether the air-fuel ratio of the gas is a predetermined stoichiometric air-fuel ratio.

  In the first embodiment described above, the NSR catalyst 16 is the “NSR catalyst” in the first invention, the SCR 18 is the “SCR” in the first invention, and the TWC 20 is the “three” in the first invention. In the original catalyst, the first exhaust passage 121 corresponds to the “exhaust passage downstream of the branch portion” in the first invention, and the second exhaust passage 122 corresponds to the “branch passage” in the first invention. . In the first embodiment described above, the “control means” according to the first aspect of the present invention is implemented when the ECU 30 executes the process of step 104 described above.

  In the first embodiment described above, the “control means” according to the second aspect of the present invention is implemented when the ECU 30 executes the process of step 100 or 108.

  In the first embodiment described above, the “control means” according to the third aspect of the present invention is implemented when the ECU 30 executes the process of step 104 described above.

Embodiment 2. FIG.
[Features of Embodiment 2]
Next, a second embodiment of the present invention will be described with reference to FIGS. The system of the present embodiment can be realized by causing the ECU 30 to execute a routine shown in FIG. 9 described later using the hardware configuration shown in FIG.

  In the system of the first embodiment described above, the entire amount of exhaust gas is circulated to the first exhaust passage 121 during the lean operation of the internal combustion engine 10, and the entire amount of exhaust gas is circulated to the second exhaust passage 122 during the stoichiometric operation. It is said. Here, when a high load request is issued during the stoichiometric operation of the internal combustion engine 10, it may be assumed that the stoichiometric gas passes through the downstream side of the TWC 20 and the entire amount of exhaust gas cannot be purified.

  Therefore, in the system according to the second embodiment, when a predetermined high load request is issued during the stoichiometric operation, a part of the exhaust gas is also circulated to the first exhaust passage 121 side. FIG. 8 is a diagram for explaining the control of the flow rate adjustment valve during the high load stoichiometric operation. As shown in this figure, during the high load stoichiometric operation, the flow rate ratio of the exhaust gas flowing through the first exhaust passage 121 side and the second exhaust passage 122 side is set to a predetermined ratio (for example, 2: 7). The valve position of the flow rate adjustment valve 22 is controlled. The NSR catalyst 16 has not only a NOx storage function but also a ternary function. As a result, the stoichiometric gas at the time of high load is distributed to the first exhaust passage 121 side and the second exhaust passage 122 side, so that the exhaust gas is effectively purified using the three-way function of the TWC 20 and the NSR catalyst 16. be able to. Incidentally, the ternary activity of the TWC 20 is higher than that of the NSR catalyst 16. For this reason, when setting the flow rate ratio during the stoichiometric operation, it is preferable that the second exhaust passage 122 side is the main and the first exhaust passage 121 is the subordinate.

[Specific Processing in Second Embodiment]
Next, with reference to FIG. 9, the specific content of the process performed in this Embodiment is demonstrated. FIG. 9 is a flowchart of a routine that the ECU 30 executes. Note that the routine shown in FIG. 9 is repeatedly executed during operation of the internal combustion engine 10.

  In the routine shown in FIG. 9, first, the base position of the flow rate adjustment valve 22 is set (step 200). Next, it is determined whether or not a stoichiometric operation request for the internal combustion engine 10 has been issued (step 202). Here, specifically, the same processing as in steps 100 to 102 is executed. As a result, when the stoichiometric operation request is not issued, it is determined that the lean operation is being performed, and the process of step 200 is repeatedly executed.

  On the other hand, when the stoichiometric operation request is issued in step 202, it is determined that the entire amount of stoichiometric gas cannot be purified using the NSR catalyst 16 and the SCR 18, and the process proceeds to the next step. It is determined whether or not the operation is a high load operation, for example, whether or not the load KL> 90 is satisfied (step 204). As a result, if establishment of KL> 90 is not recognized, it is determined that the entire amount of exhaust gas can be purified by the TWC 20, and the process proceeds to the next step, where the entire amount of exhaust gas flows to the second exhaust passage 122. Thus, the flow rate adjustment valve 22 is set (step 206).

  On the other hand, if the establishment of KL> 90 is recognized in step 204, it is determined that unpurified exhaust gas passes through the downstream side of the TWC 20, and the process proceeds to the next step, where the first exhaust passage. The flow rate adjusting valve 22 is set so that the flow rate ratio of the exhaust gas flowing through 121 and the second exhaust passage 122 becomes a predetermined ratio (step 208).

  In the routine shown in FIG. 9, it is next determined whether or not the stoichiometric operation request is still continuing (step 210). As a result, when it is determined that the stoichiometric operation request is continuing, it is determined that the stoichiometric gas is still exhausted, and the process of step 204 is repeatedly executed. On the other hand, when it is determined in step 210 that the stoichiometric operation request is not continued, it is determined that the operation is switched to the lean operation, and the valve position of the flow rate adjusting valve 22 is returned to the base position again.

  As described above, according to the system of the second embodiment, during the high load stoichiometric operation, the distribution destination of the exhaust gas is distributed to the first exhaust passage 121 side and the second exhaust passage 122 side at a predetermined flow rate ratio. Is done. As a result, part of the exhaust gas is circulated to the first exhaust passage 121 side, so that these gases can be effectively purified using the three-way function of the NSR catalyst 16. Further, when the distribution destination of the exhaust gas is dispersed, the flow rate of the exhaust gas flowing to the second exhaust passage 122 side is reduced, so that the situation in which unpurified exhaust gas passes through the downstream side of the TWC 20 is effectively suppressed. be able to.

  In the second embodiment described above, the SCR 18 is arranged on the downstream side of the NSR catalyst 16 in the first exhaust passage 121, but the arrangement of the SCR 18 is not limited to this. That is, as shown in FIG. 7 described above, the SCR 18 may be disposed in the exhaust passage 12 after the first exhaust passage 121 and the second exhaust passage 122 merge. Thereby, the non-methane hydrocarbon (NMHC) component etc. which are discharged | emitted downstream of TWC20 can also be purified in the said SCR18.

  In the second embodiment described above, the valve position of the flow rate adjustment valve 22 is controlled based on whether or not the stoichiometric operation request is issued. However, the exhaust gas detected by the A / F sensor 24 is controlled. The valve control may be executed based on whether the air-fuel ratio of the gas is a predetermined stoichiometric air-fuel ratio.

  In the second embodiment described above, the NSR catalyst 16 is the “NSR catalyst” in the first invention, the SCR 18 is the “SCR” in the first invention, and the TWC 20 is the “three” in the first invention. In the original catalyst, the first exhaust passage 121 corresponds to the “exhaust passage downstream of the branch portion” in the first invention, and the second exhaust passage 122 corresponds to the “branch passage” in the first invention. . In the second embodiment described above, the “control means” according to the first aspect of the present invention is implemented when the ECU 30 executes the processing of step 106 or 208 described above.

  In the second embodiment described above, the “control means” according to the second aspect of the present invention is implemented when the ECU 30 executes the process of step 200 or 212.

  In the second embodiment described above, the “control means” according to the third aspect of the present invention is implemented when the ECU 30 executes the process of step 206.

In the second embodiment described above, the “control means” according to the first aspect of the present invention is implemented when the ECU 30 executes the process of step 208 described above.

10 Internal combustion engine
12 Exhaust passage 121 First exhaust passage 122 Second exhaust passage 14 Start catalyst (SC)
16 NOx storage reduction catalyst (NSR catalyst)
18 NOx selective reduction catalyst (SCR)
20 Three-way catalyst (TWC)
22 Flow control valve 24 A / F sensor 26 O ₂ sensor 28 O ₂ sensor 30 ECU (Electronic Control Unit)

Claims (4)

An exhaust purification device for an internal combustion engine capable of lean burn operation,
A NOx occlusion reduction catalyst (hereinafter referred to as NSR catalyst) disposed in the exhaust passage of the internal combustion engine;
A NOx selective reduction catalyst (hereinafter referred to as SCR) disposed downstream of the NSR catalyst;
A branch passage branched from the upstream side of the NSR catalyst in the exhaust passage;
A three-way catalyst disposed in the branch passage;
A flow rate control valve that is disposed at a branch portion between the exhaust passage and the branch passage, and variably sets a flow rate ratio of exhaust gas flowing in the exhaust passage downstream of the branch portion and the branch passage;
Control means for controlling the flow rate control valve based on the operating air-fuel ratio of the internal combustion engine,
The control means controls the flow rate control valve so that at least a part of the exhaust gas flows to the branch passage during the stoichiometric operation of the internal combustion engine, and a load during the stoichiometric operation of the internal combustion engine is predetermined. An exhaust emission control device for an internal combustion engine , wherein the flow control valve is controlled so that exhaust gas flows through both the exhaust passage downstream of the branch section and the branch passage when the load is higher than a high load value .   2. The internal combustion engine according to claim 1, wherein the control unit controls the flow rate control valve so that the entire amount of exhaust gas flows to the exhaust passage downstream of the branch portion during the lean burn operation of the internal combustion engine. Engine exhaust purification system. The control means controls the flow rate control valve so that the entire amount of exhaust gas flows into the branch passage when the load during the stoichiometric operation of the internal combustion engine is lower than the predetermined high load value. The exhaust emission control device for an internal combustion engine according to claim 1 or 2. The exhaust of the internal combustion engine according to any one of claims 1 to 3 , wherein a downstream end of the branch passage joins a position downstream of the NSR catalyst and upstream of the SCR in the exhaust passage. Purification equipment.

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