JP2012127295A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine including an NSR catalyst and an SCR that suppresses production reaction of N2O and achieves a high NOx purification rate.SOLUTION: The exhaust emission control device for an internal combustion engine 10 having a lean operation mode includes: an NOstorage-reduction catalyst (NSR catalyst) 16 arranged in an exhaust passage 12 of the internal combustion engine 10; an NOselective-reduction catalyst (SCR) 18 arranged downstream from the NSR catalyst 16; and a rich spike device for performing rich spike operation at predetermined timing during the lean operation. The rich spike device is configured to perform two-stage rich spike for switching an exhaust air-fuel ratio during rich spike from a first rich air-fuel ratio (A/F=12) to a second rich air-fuel ratio (A/F=13) after a predetermined lean period.

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. 2008-303759, a three-way catalyst, a NOx storage reduction catalyst (hereinafter referred to as “NSR catalyst”), and a NOx selective reduction catalyst (hereinafter referred to as “SCR”). Are known in this order from the upstream side of the exhaust passage of the internal combustion engine. 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 (lean operation), exhaust gas containing a large amount of NOx is discharged. The NSR catalyst occludes and holds this NOx therein, and suppresses the situation where the NOx is released downstream of the catalyst.

  During lean operation, the NOx occlusion retention amount in the NSR catalyst increases with time. Therefore, in the above-described conventional system, rich spike control for temporarily enriching the exhaust air-fuel ratio of the internal combustion engine is executed at a point where the NOx occlusion holding amount in the NSR catalyst approaches a predetermined occlusion capacity. Thereby, NOx occluded and held in the NSR catalyst is purified.

When rich spike control is executed, ammonia (NH ₃ ) is generated in the three-way catalyst and the NSR catalyst. SCR has a function of adsorbing ammonia (NH _3), absorbing the NH ₃ generated by the three-way catalyst and the NSR catalyst therein. The stored NH ₃ is used when NOx flowing into the SCR is selectively reduced. Thus, in the conventional system, NOx discharged from the internal combustion engine is purified by a combination of the NSR catalyst and the SCR.

JP 2008-303759 A Japanese Patent Laid-Open No. 2004-211676

By the way, when the rich spike is executed during the lean operation, the NSR catalyst and the SCR cause a nitrous oxide (N ₂ O) generation reaction in addition to the NOx purification reaction. N ₂ O is said to be a greenhouse gas about 300 times that of carbon dioxide, and tends to be generated particularly in a low activity environment where the catalyst bed temperature is low. For this reason, if the above-described rich spike is executed when the internal combustion engine is under a low load, a large amount of N ₂ O may be generated in these catalysts.

The present invention has been made to solve the above-described problems. In an internal combustion engine including an NSR catalyst and an SCR, the present invention achieves a high NOx purification rate while suppressing N ₂ O production reaction. An object of the present invention is to provide an exhaust purification device for an internal combustion engine.

In order to achieve the above object, a first invention is an exhaust purification device for an internal combustion engine capable of lean 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;
Rich spike means for executing a rich spike at a predetermined timing during lean operation,
The rich spike means executes a two-stage rich spike that switches the exhaust air / fuel ratio in the rich spike from a first rich air / fuel ratio to a second rich air / fuel ratio with a predetermined lean period interposed therebetween. Yes.

According to a second invention, in the first invention,
The second rich air-fuel ratio is a lean rich air-fuel ratio that is leaner than the first air-fuel ratio.

According to a third invention, in the first or second invention,
The NSR catalyst includes an OSC material therein,
The predetermined lean period is a period until the exhaust gas inside the NSR catalyst is replaced from rich to lean.

A fourth invention is any one of the first to third inventions,
A bed temperature acquisition means for acquiring a bed temperature of the NSR catalyst and / or the SCR;
When the bed temperature is higher than a predetermined reference value, limiting means for restricting execution of the two-stage rich spike in the rich spike means;
Is further provided.

When the rich spike is executed, NOx stored in the NSR catalyst (NOx storage reduction catalyst) is desorbed and purified, and NH ₃ is generated in the NSR catalyst. The produced NH ₃ is stored in an SCR (NOx selective reduction catalyst) disposed on the downstream side through the exhaust passage. In the SCR, the stored NH ₃ is used to selectively reduce NOx blown downstream of the NSR catalyst.

According to the first invention, in the two-stage rich spike, a predetermined lean period is provided between the rich spike caused by the first rich air-fuel ratio and the rich spike caused by the second rich air-fuel ratio. For this reason, according to the present invention, the bed temperature of the NSR catalyst and the SCR can be effectively increased, so that the situation in which a large amount of N ₂ O is generated by these catalysts during a rich spike can be effectively suppressed. Can do.

According to the second aspect of the invention, the rich spike due to the second rich air-fuel ratio is executed at a lean rich air-fuel ratio that is leaner than that of the first rich air-fuel ratio. Therefore, according to the present invention, the NOx desorption / reduction reaction in the NSR catalyst is effectively promoted by the rich spike due to the first rich air-fuel ratio, and at the NSR catalyst due to the rich spike due to the second rich air-fuel ratio. NH ₃ production reaction can be activated.

  According to the third invention, the NSR catalyst contains an OSC material having an oxygen absorption / release function. A predetermined lean period between the rich spike due to the first rich air-fuel ratio and the rich spike due to the second rich air-fuel ratio is set to a period until the exhaust gas inside the NSR catalyst is replaced with the lean gas. ing. For this reason, according to the present invention, it is possible to shift to the rich spike by the second rich air-fuel ratio without oxygen being stored in the OSC material of the NSR catalyst. Immediately after the start, the situation where the rich component of the rich spike is consumed for the release of oxygen in the OSC material can be effectively suppressed.

  According to the fourth invention, when the bed temperature of the NSR catalyst and / or the SCR is higher than the predetermined reference temperature, the execution of the two-stage rich spike provided with the predetermined lean period is restricted. For this reason, according to this invention, the fall of the NOx purification rate by raising a catalyst bed temperature more than necessary can be suppressed effectively.

It is a figure for demonstrating the structure of embodiment of this invention. It is a figure for demonstrating operation | movement at the time of changing the air fuel ratio in a rich spike in two steps. It is a figure which shows the relationship between a rich spike and the bed temperature of an NSR catalyst. It is a figure which shows the relationship between a rich spike and the bed temperature of an NSR catalyst. It is a figure which shows typically the mode of the change of the actual air fuel ratio at the time of a rich spike. It is a figure which shows the NOx purification rate with respect to SCR bed temperature. It is a flowchart of the routine performed in Embodiment 1 of the present invention.

  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 embodiment of the present invention. As shown in FIG. 1, the system of the present embodiment includes an internal combustion engine 10. The internal combustion engine 10 is configured as a V-type gasoline engine including a right bank 101 and a left bank 102. The cylinder group belonging to the right bank 101 communicates with the exhaust passage 121. The cylinder group belonging to the left bank 102 communicates with the exhaust passage 122. The exhaust passages 121 and 122 communicate with one end of the exhaust passage 123 after joining downstream. Hereinafter, when the exhaust passages 121, 122, and 123 are not particularly distinguished, they are simply referred to as “exhaust passage 12”.

  In the exhaust passages 121 and 122, start catalysts (hereinafter referred to as “SC”) 141 and 142, which are three-way catalysts, are arranged, respectively. Further, a NOx storage reduction catalyst (NSR catalyst) 16 is disposed in the exhaust passage 123. Further, a NOx selective reduction catalyst (SCR) 18 is disposed downstream of the NSR catalyst 16 in the exhaust passage 123. Hereinafter, when the SCs 141 and 142 are not particularly distinguished, these are simply referred to as “SC 14”.

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 reduces NOx (purifies 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. In a rich atmosphere, ammonia (NH ₃ ) is generated by the reaction of nitrogen and hydrogen or HC and NOx contained in the exhaust gas.

The NSR catalyst 16 occludes NOx contained in the exhaust gas under a lean atmosphere. Further, the NSR catalyst 16 releases NOx stored 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 16 as in the case of the SC 14.

The SCR 18 is configured as an Fe-based zeolite catalyst, and the SC 14 and the NSR catalyst 16 occlude NH ₃ produced in a rich atmosphere. In the lean atmosphere, the NO ₃ in the exhaust gas is selectively used with NH ₃ as a reducing agent. It has the function of reducing to 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.

  In the vicinity of the downstream side of the NSR catalyst 16 and the SCR 18, temperature sensors 20 and 22 are arranged, respectively. These temperature sensors 20 and 22 can detect the temperature of the exhaust gas discharged from each catalyst.

  The system according to the present embodiment includes an ECU (Electronic Control Unit) 30 as shown in FIG. Various actuators such as a fuel injection device (not shown) are connected to the output portion of the ECU 30. In addition to the temperature sensors 20 and 22 described above, the input unit of the ECU 30 includes a crank angle sensor for detecting the engine speed, a throttle opening sensor (not shown) for detecting the throttle opening, and the like. Various sensors are connected. 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, rich spike control executed in the system of the present embodiment 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 exhaust gas using a three-way catalyst, it is not possible to purify all NOx due to a shortage of reducing agent. Therefore, the system according to the first embodiment is provided with the NSR catalyst 16 in the exhaust passage 123. 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 increased at a timing when the NOx amount stored in the NSR catalyst 16 reaches a predetermined storage amount (for example, an amount corresponding to 80% of the maximum storage amount). To rich (for example, target A / F = 12). 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.

  The target A / F at the time of rich spike can be set as appropriate, but the rich air-fuel ratio is more preferable from the viewpoint of improving the NOx purification performance of the NSR catalyst 16. However, since fuel efficiency deteriorates as a contradiction, it is preferable to set the exhaust air-fuel ratio as lean as possible when NOx purification in the SCR 18 described later can be expected.

  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 using NH ₃ . Thereby, the situation where NOx is released into the atmosphere and the emission deteriorates can be effectively prevented.

According to the opinion of the inventor of the present application, the amount of NH ₃ produced in the SC 14 and the NSR catalyst 16 (particularly the NSR catalyst 16) is a slite in which the exhaust air-fuel ratio at the time of rich spike is around A / F = 13 The amount becomes large when the air-fuel ratio is rich. For this reason, the NOx purification performance of the SCR 18 can be effectively enhanced by controlling the target A / F during the rich spike to the light rich air-fuel ratio.

[Characteristic operation of the first embodiment]
Next, characteristic operations of the present embodiment will be described with reference to FIGS. As described above, from the viewpoint of enhancing the NOx purification performance of the NSR catalyst 16, it is preferable to control the target A / F during the rich spike to a rich air-fuel ratio (for example, A / F = 12). On the other hand, however, from the viewpoint of increasing the amount of NH ₃ produced in the SC 14 and the NSR catalyst 16 and promoting NOx purification in the SCR 18, the target A / F at the time of the rich spike is set to a slightly rich air-fuel ratio (for example, A / F = 13) is preferable.

  As a technique for achieving both of these two viewpoints, it is conceivable to change the exhaust air-fuel ratio during the rich spike stepwise. FIG. 2 is a diagram for explaining the operation when the air-fuel ratio during the rich spike is changed in two stages. In the example shown in this figure, the target A / F at the front stage (first stage) of the rich spike is set to a predetermined rich air-fuel ratio (A / F = 12), and the target A / F at the rear stage (second stage) is set. It is supposed to be set to a predetermined slight rich air-fuel ratio (A / F = 13). Thereby, a high NOx purification rate can be exhibited by the combination of the NSR catalyst 16 and the SCR 18.

  However, the above-described two-stage rich spike control has a problem that the temperature rise performance of the catalyst is poor. FIG. 3 is a graph showing the relationship between the rich spike and the bed temperature of the NSR catalyst. As shown in this figure, when the above-described two-stage rich spike is performed during the lean operation (for example, A / F = 25), the bed temperature of the NSR catalyst 16 gradually increases, but the increase width is relatively small. It is a thing (around Δ30 ° C.).

Here, in the NSR catalyst 16 and the SCR 18, a nitrous oxide (N ₂ O) generation reaction occurs in addition to the NOx purification reaction. N ₂ O is said to be a greenhouse gas about 300 times as much as carbon dioxide, and it is preferable to suppress emissions as much as possible. This N ₂ O production reaction tends to occur particularly in a low activity environment where the catalyst bed temperature is low. For this reason, if the above-described two-stage rich spike control is performed, a large amount of N ₂ O may be generated at the rich spike timing.

Therefore, in the system of the first embodiment, a lean period for temporarily controlling the exhaust air-fuel ratio to the lean air-fuel ratio is provided after the end of the first-stage rich spike and before the second rich spike is started. I will do it. FIG. 4 is a graph showing the relationship between the rich spike and the bed temperature of the NSR catalyst. As shown in this figure, when the above-described two-stage rich spike is performed during the lean operation (for example, A / F = 25), the bed temperature of the NSR catalyst 16 is greatly increased (Δ50). Around ℃). Therefore, by executing such a rich spike, the generation of N ₂ O can be drastically reduced.

  For the SC14 and NSR catalyst 16 having a three-way function, an OSC material such as ceria is used as one component of the carrier for the purpose of enhancing robustness. This OSC material has an oxygen absorption / release capability, absorbs oxygen in a lean atmosphere, and releases oxygen in a rich atmosphere. For this reason, a part of the reducing agent discharged in the early stage of the rich spike tends to be used for the oxygen release reaction in the OSC material.

  FIG. 5 is a diagram schematically showing how the actual air-fuel ratio changes during a rich spike. As shown in (A) of this figure, if the lean period between the rich spikes described above is too long, oxygen is occluded again in the OSC material inside the NSR catalyst 16, and therefore the execution of the second rich spike is performed. Even in the initial stage, a part of the reducing agent is consumed, which is not preferable for the purpose of reducing and purifying NOx.

  Therefore, in the rich spike control of the first embodiment, as shown in (B) in the figure, oxygen is not occluded in the OSC material during the lean period between the first stage rich spike and the second stage rich spike. It is preferable to set a period of about. More specifically, the lean period is preferably set to a period until the inside of the NSR catalyst 16 is replaced from rich gas to lean gas. Thereby, the situation where the reducing agent in the second rich spike is consumed in the oxygen releasing reaction in the OSC material can be effectively suppressed.

Further, during high load operation where the catalyst bed temperature is relatively high (for example, the bed temperature of the NSR catalyst 16> 450 ° C., the bed temperature of the SCR 18> 350 ° C.), the above-described rich spike control is not executed. It is assumed that the generation of N ₂ O is suppressed. Conversely, if the catalyst temperature rises more than necessary, the NOx purification rate will decrease. FIG. 6 is a diagram showing the NOx purification rate with respect to the SCR bed temperature. As shown in this figure, it can be seen that the NOx purification rate tends to gradually decrease from the vicinity where the bed temperature of the SCR 18 exceeds 350 ° C.

Therefore, in the system of the first embodiment, the bed temperatures of the NSR catalyst 16 and the SCR 18 are estimated, and when the bed temperature is higher than a predetermined temperature, the normal two-stage rich spike control that does not sandwich the lean period is performed. Execute it. Thereby, it is possible to effectively suppress a decrease in the NOx purification rate in the NSR catalyst 16 and the SCR 18 while suppressing the generation of N ₂ O.

[Specific Processing in Embodiment 1]
Next, with reference to FIG. 7, the specific content of the process performed in this Embodiment is demonstrated. FIG. 7 is a flowchart of a routine in which the ECU 30 executes rich spike control. Note that the routine shown in FIG. 7 is repeatedly executed during operation of the internal combustion engine 10.

  In the routine shown in FIG. 7, it is first determined whether or not the rich spike execution condition is satisfied (step 100). Specifically, it is determined whether or not the lean operation is being performed, and whether or not the current NOx storage amount stored in the NSR catalyst 16 exceeds a predetermined storage amount. As a result, when the lean operation is not being performed, or when establishment of NOx occlusion amount> predetermined occlusion amount is not recognized, it is determined that the rich spike need not be executed, and this routine is immediately terminated.

  On the other hand, when the lean operation is being performed and the establishment of NOx storage amount> storage limit amount is recognized in step 100, it is determined that the rich spike needs to be executed, and the process proceeds to the next step. The bed temperatures Tn and Ts of the NSR catalyst 16 and the SCR 18 are estimated (step 102). Specifically, the temperatures of the exhaust gases detected by the temperature sensors 20 and 22 are used as the bed temperatures Tn and Ts of the NSR catalyst 16 and SCR 18.

Next, it is determined whether or not the bed temperatures Tn and Ts of the NSR catalyst 16 and SCR 18 are higher than the predetermined temperatures Kn and Ks, respectively (step 104). As the predetermined temperatures Kn and Ks, preset temperatures (for example, Kn = 450 ° C., Ks = 350 ° C.) are used as temperatures at which the generation of N ₂ O in each catalyst is suppressed. As a result, if the establishment of Tn> Kn and / or Ts> Ks is not recognized, it is determined that a large amount of N ₂ O is generated at the time of rich spike, the process proceeds to the next step, and the lean period is set. Two-stage rich spike control is performed between them (step 106). Specifically, the rich spike at the first stage is a predetermined rich air-fuel ratio (for example, A / F = 12), and the rich spike at the second stage is a predetermined rich-rich air-fuel ratio (for example, A / F = 13). ) Is controlled. Further, the lean period between the first stage rich spike and the second stage rich spike is controlled to a period until the exhaust gas in the NSR catalyst 16 is replaced with the rich gas from the rich gas.

On the other hand, if the establishment of Tn> Kn and / or Ts> Ks is recognized in step 106, it is determined that N ₂ O is not generated even if the catalyst bed temperature is not further raised, The process proceeds to step, and normal two-stage rich spike control is executed (step 108). Specifically, the normal two-stage rich spike switching from the first-stage rich spike to the second-stage rich spike is executed without interposing the lean period.

As described above, according to the system of the present embodiment, when the catalyst bed temperature is lower than the predetermined temperature, the two-stage rich spike with the lean period in between is executed. Thus, it is possible to increase the catalyst bed temperature, can be effectively suppress the occurrence of N _{2 O} during the rich spike.

  In addition, according to the system of the present embodiment, when the catalyst bed temperature is higher than the predetermined temperature, the catalyst temperature raising operation is not performed. For this reason, the situation where the catalyst bed temperature rises more than necessary and the NOx purification rate falls can be effectively suppressed.

  By the way, in Embodiment 1 mentioned above, based on the exhaust gas temperature detected by the temperature sensors 20 and 22, the bed temperature of the NSR catalyst 16 and SCR 18 is estimated. However, the method for estimating the catalyst bed temperature is not limited to this. For example, the catalyst bed temperature may be estimated from the load state of the internal combustion engine 10.

  In the first embodiment described above, the NSR catalyst 16 corresponds to the “NSR catalyst” in the first invention, and the SCR 18 corresponds to the “SCR” in the first invention. In the first embodiment described above, the ECU 30 executes the process of step 106, so that the “rich spike means” in the first aspect of the invention executes the process of step 102. The “restricting means” in the fourth invention is realized by the “floor temperature acquisition means” in the fourth invention executing the processing of step 104 described above.

10 Internal combustion engine
12 Exhaust passage 14 Start catalyst (SC)
16 NOx storage reduction catalyst (NSR catalyst)
18 NOx selective reduction catalyst (SCR)
20, 22 Temperature sensor 30 ECU (Electronic Control Unit)

Claims (4)

An exhaust purification device for an internal combustion engine capable of lean 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;
Rich spike means for executing a rich spike at a predetermined timing during lean operation,
The rich spike means executes a two-stage rich spike for switching the exhaust air / fuel ratio in the rich spike from a first rich air / fuel ratio to a second rich air / fuel ratio with a predetermined lean period interposed therebetween. An exhaust purification device for an internal combustion engine.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the second rich air-fuel ratio is a lean rich air-fuel ratio leaner than the first air-fuel ratio. The NSR catalyst includes an OSC material therein,
The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2, wherein the predetermined lean period is a period until the exhaust gas inside the NSR catalyst is replaced from rich to lean. A bed temperature acquisition means for acquiring a bed temperature of the NSR catalyst and / or the SCR;
When the bed temperature is higher than a predetermined reference value, limiting means for restricting execution of the two-stage rich spike in the rich spike means;
The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 3, further comprising:

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