JP2018035766A - Exhaust gas purification device for internal combustion engine - Google Patents

Abstract

A selective reduction NOx catalyst (SCR catalyst) is prevented from being poisoned with ammonium nitrate by reducing the amount of ammonium nitrate produced. The maximum value of the NOx purification rate in the SCR catalyst when NO2 is contained in the exhaust gas flowing into the SCR catalyst and estimated when ammonium nitrate is not present in the SCR catalyst. Based on the NOx purification rate, a production amount of ammonium nitrate in a predetermined period is calculated, and based on this production amount and the amount of ammonia supplied when the exhaust gas temperature is in a predetermined temperature region in the predetermined period, Correct the amount of ammonia to be supplied. [Selection] Figure 4

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

When ammonia is supplied to a selective reduction NOx catalyst (hereinafter also referred to as SCR catalyst) when the temperature of the exhaust gas of the internal combustion engine is low, ammonium nitrate (NH ₄ NO ₃ ) produced by reaction of ammonia and NO ₂ May poison the SCR catalyst. For this reason, when it is judged that the adsorption amount of ammonium nitrate is large, a technique for removing ammonium nitrate from the SCR catalyst by increasing NO in exhaust gas and promoting the reaction between ammonium nitrate and NO is known (for example, , See Patent Document 1).

JP 2011-102573 A

  However, even if ammonium nitrate is removed, the SCR catalyst may be poisoned again by ammonium nitrate depending on the subsequent supply amount of ammonia, and the NOx purification rate may be reduced.

  The present invention has been made in view of the above-described problems, and an object thereof is to suppress poisoning of the selective reduction type NOx catalyst with ammonium nitrate by reducing the amount of ammonium nitrate produced. is there.

In order to solve the above problems, a selective reduction type NOx catalyst that is provided in an exhaust passage of an internal combustion engine and reduces NOx using ammonia as a reducing agent, a supply device that supplies ammonia to the selective reduction type NOx catalyst, and the selective reduction in the exhaust purification system of an internal combustion engine comprising a processing unit that performs processing to prevent contains nO ₂ in the exhaust gas flowing into the mold NOx catalyst, wherein the time to have performed said processing by said processing means The maximum value of the NOx purification rate in the selective reduction type NOx catalyst and the NOx purification estimated when ammonium nitrate is not present in the selective reduction type NOx catalyst when the processing is performed by the processing means. The amount of ammonium nitrate produced in a predetermined period is calculated based on the rate, and the amount of ammonium nitrate produced in the predetermined period And the amount of ammonia supplied by the supply device when the exhaust temperature is in a predetermined temperature range where ammonium nitrate is generated, and the amount of ammonia supplied when the exhaust temperature is in the predetermined temperature range. Control means for correcting the amount is provided.

  According to the present invention, it is possible to suppress the selective reduction NOx catalyst from being poisoned with ammonium nitrate by reducing the amount of ammonium nitrate produced.

It is a figure which shows schematic structure of the internal combustion engine which concerns on an Example. It is a time chart which showed transition of NOx concentration downstream from an SCR catalyst. It is the figure which showed the relationship between exhaust temperature and NOx purification rate. It is the flowchart which showed the flow of ammonia addition control which concerns on an Example. It is the figure which showed the relationship between NOx purification rate difference and the production amount of ammonium nitrate.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be exemplarily described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified.

<Example>
FIG. 1 is a diagram illustrating a schematic configuration of an internal combustion engine according to the present embodiment. An internal combustion engine 1 shown in FIG. 1 is a diesel engine. An exhaust passage 2 is connected to the internal combustion engine 1. In the middle of the exhaust passage 2, an oxidation catalyst 3, a filter 4, a reducing agent addition valve 5, and a selective reduction type NOx catalyst 6 (hereinafter referred to as an SCR catalyst 6) are provided in this order from the upstream side.

  The oxidation catalyst 3 may be any catalyst having an oxidation function, and may be, for example, a three-way catalyst or an occlusion reduction type NOx catalyst. The filter 4 collects particulate matter (PM) in the exhaust gas. The filter 4 may carry a catalyst. In this case, the oxidation catalyst 3 is not always necessary.

The reducing agent addition valve 5 injects a reducing agent into the exhaust gas in the exhaust passage 2. Ammonia (NH ₃ ) is used as the reducing agent. The reducing agent addition valve 5 may inject urea, which is a precursor of ammonia, instead of injecting ammonia. The urea injected from the reducing agent addition valve 5 is hydrolyzed by the heat of the exhaust or the heat from the SCR catalyst 6 to become ammonia, and is adsorbed on the SCR catalyst 6. This ammonia is used as a reducing agent in the SCR catalyst 6. In this embodiment, ammonia is injected from the reducing agent addition valve 5. In this embodiment, the reducing agent addition valve 5 corresponds to the supply device in the present invention.

  The SCR catalyst 6 adsorbs a reducing agent, and when NOx passes through the SCR catalyst 6, it selectively reduces NOx with the adsorbed reducing agent. Therefore, if ammonia is adsorbed in advance as a reducing agent on the SCR catalyst 6, NOx can be reduced with ammonia in the SCR catalyst 6.

  Further, in the exhaust passage 2 downstream from the filter 4 and upstream from the SCR catalyst 6, an upstream temperature sensor 11 for detecting the temperature of the exhaust, an upstream NOx sensor 12 for detecting the NOx concentration in the exhaust, Is attached. Note that the temperature of the filter 4 or the temperature of the SCR catalyst 6 can be detected by the upstream temperature sensor 11. Further, the upstream NOx sensor 12 can detect the NOx concentration in the exhaust gas flowing into the SCR catalyst 6. Further, a downstream temperature sensor 13 for detecting the temperature of the exhaust and a downstream NOx sensor 14 for detecting the NOx concentration in the exhaust are attached to the exhaust passage 2 downstream of the SCR catalyst 6. The temperature of the SCR catalyst 6 can be detected by the downstream temperature sensor 13. Further, the downstream NOx sensor 14 can detect the NOx concentration in the exhaust gas flowing out from the SCR catalyst 6. The ECU 10 described later can also estimate the temperature of the SCR catalyst 6 based on the operating state of the internal combustion engine 1. For example, since the engine speed, the fuel injection amount, the intake air amount, and the temperature of the SCR catalyst 6 have a correlation, these relationships may be obtained in advance through experiments or the like and mapped.

The internal combustion engine 1 is provided with a fuel injection valve 7 for injecting fuel into the cylinder. An intake passage 8 is connected to the internal combustion engine 1. In the middle of the intake passage 8, a throttle 9 for adjusting the intake air amount of the internal combustion engine 1 is provided. An air flow meter 15 that detects the intake air amount of the internal combustion engine 1 is attached to the intake passage 8 upstream of the throttle 9.

  The internal combustion engine 1 configured as described above is provided with an ECU 10 that is an electronic control unit for controlling the internal combustion engine 1. The ECU 10 controls the internal combustion engine 1 in accordance with the operating conditions of the internal combustion engine 1 and the driver's request. In addition to the above sensors, the ECU 10 outputs an electric signal corresponding to the amount of depression of the accelerator pedal 16 by the driver to detect the engine load, and an accelerator position sensor 17 for detecting the engine speed. 18 are connected via electric wiring, and output signals of these various sensors are input to the ECU 10. On the other hand, the reducing agent addition valve 5, the fuel injection valve 7, and the throttle 9 are connected to the ECU 10 through electric wiring, and these devices are controlled by the ECU 10.

The ECU 10 supplies ammonia to the SCR catalyst 6 by controlling the reducing agent addition valve 5. However, when ammonia is added from the reducing agent addition valve 5 when the exhaust temperature is low, ammonium nitrate (NH ₄ NO ₃ ) is generated from the ammonia added from the reducing agent addition valve 5 and NO ₂ contained in the exhaust. Is done. When the SCR catalyst 6 is poisoned by the ammonium nitrate, the NOx purification performance is lowered. Further, ammonium nitrate is thermally decomposed when the temperature is increased, and N ₂ O and H ₂ O are generated. Since this N ₂ O is known as a greenhouse gas, it is preferable to reduce the emission amount to the atmosphere. Therefore, it is preferable to reduce the amount of ammonium nitrate produced.

Therefore, in this embodiment, when the exhaust temperature is within a predetermined temperature region where ammonium nitrate is generated (hereinafter also referred to as a predetermined region), the ammonia addition amount is corrected so that the amount of ammonium nitrate generated is reduced. . In order to correct the amount of added ammonia, first, the amount of ammonium nitrate generated at the time of adding ammonia before correcting the amount of added ammonia is detected. When detecting the production amount of ammonium nitrate, the combustion mode is set so that NO ₂ is not included in the NOx discharged from the internal combustion engine 1. During this combustion mode, only NO is included in NOx. Then, a correction coefficient for the ammonia addition amount is calculated based on the production amount of ammonium nitrate, and the subsequent ammonia addition amount is corrected according to this correction coefficient.

Here, FIG. 2 is a time chart showing the transition of the NOx concentration downstream of the SCR catalyst 6. FIG. 2 shows the transition of the NOx concentration when the exhaust temperature is within the predetermined region, and shows the transition of the NOx concentration when the exhaust temperature is, for example, 180 ° C. In FIG. 2, ammonia is added from the reducing agent addition valve 5 over the entire period. The ammonia addition amount is set so as not to be excessive or insufficient with respect to the NOx amount flowing into the SCR catalyst 6. In FIG. 2, since the NOx concentration in the exhaust gas flowing into the SCR catalyst 6 is constant, the ammonia addition amount is also constant. In FIG. 2, a period from 0 to T2 is a period in which NO and NO ₂ are discharged from the internal combustion engine 1. That is, the period from 0 to T2 is set to the combustion mode in which NO and NO ₂ are discharged from the internal combustion engine 1. Further, the period from T2 to T4 is a period during which it is discharged NO from the internal combustion engine 1 does not discharge the NO _2. That is, the period from the T2 to T4 is to discharge the NO from the internal combustion engine 1 is set to the combustion mode that does not discharge the NO _2. T2 is the time when the travel distance of the vehicle on which the internal combustion engine 1 is mounted becomes longer than a predetermined distance, for example. The predetermined distance is a distance suitable for correcting the ammonia addition amount. C1 is the NOx concentrations without ammonia supply, C2 is the NOx concentration when it is discharged NO from the internal combustion engine 1 does not exist ammonium nitrate when not discharged NO _2, C3 is , the minimum value of the NOx concentration in the period from T2 to T4, C4 is the NOx concentration at which ammonium nitrate does not exist in the case of discharging the nO and nO ₂ from the internal combustion engine 1. C4 is the NOx concentration when the NOx purification rate is, for example, 80%.

In the period from 0 to T1, NOx is purified in the SCR catalyst 6 by addition of ammonia, so the NOx concentration decreases from C1. During this period from 0 to T1, NO and NO ₂ exist in the exhaust gas, and ammonia is adsorbed on the SCR catalyst 6, so that the Fast reaction shown in the following (formula 1) is performed in the SCR catalyst 6. Occur. This reaction is faster than the standard reaction described later.
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (Formula 1)

This reaction reduces the NOx concentration. However, the NOx concentration starts to increase at T1. The NOx concentration increases in this way because NO ₂ and ammonia in the exhaust gas react to produce ammonium nitrate, and the SCR catalyst 6 is poisoned by this ammonium nitrate. As the poisoning amount of the SCR catalyst 6 increases, the NOx purification rate decreases, so the NOx concentration downstream of the SCR catalyst 6 increases. In order to suppress the production of ammonium nitrate, the amount of ammonia added may be reduced. However, if the amount of ammonia added is excessively reduced, a sufficient amount of ammonia for purifying NOx cannot be supplied to the SCR catalyst 6. In this embodiment, ammonia is produced according to the amount of ammonium nitrate produced. Reduce the amount added. In order to detect the production of ammonium nitrate, it shifts to combustion mode no NO ₂ is generated in the internal combustion engine 1 from T2.

In the combustion mode in which NO ₂ is not generated, for example, the fuel injection timing of the fuel injection valve 7 is advanced, or when an EGR device is provided, the amount of EGR gas is decreased or the supply of EGR gas is stopped. Thus, rapid combustion occurs in the combustion chamber of the internal combustion engine 1. By shifting to such a combustion mode, NO can be generated and NO ₂ can be prevented from being generated. In addition, for example, when an NSR catalyst and a bypass passage that bypasses the NSR catalyst are provided upstream of the SCR catalyst 6, by adjusting the amount of exhaust gas flowing through the bypass passage, It is possible to prevent NO from flowing in and NO ₂ from flowing in. Since easily occluded better of NO ₂ than NO when the occluding NOx in the NSR catalyst, the consequently pass through the SCR catalyst 6 is only to NO. Therefore, if the capacity of the NSR catalyst is adjusted so that NOx passes through the NSR catalyst, it is possible to supply NO to the SCR catalyst 6 and not supply NO ₂ . When supplying NO and NO ₂ to the SCR catalyst 6, the exhaust gas may be circulated through the bypass passage.

During the period from T2 to T4, the reaction of NO and ammonia causes the Standard reaction shown in the following (Formula 2) to occur.
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (Formula 2)

Since NO is purified by this reaction, the NOx concentration decreases. However, the NOx purification amount per unit time is smaller in the standard reaction of (Formula 2) than in the case of the Fast reaction of (Formula 1). For this reason, the NOx concentration does not decrease to C4 as before T2. Here, when ammonium nitrate is present in the SCR catalyst 6, the reaction shown by the following (formula 3) also occurs.
NH ₄ NO ₃ + NO → N2 + H ₂ O (Formula 3)

  That is, NO is also purified by ammonium nitrate, and since this reaction is close to the Fast reaction and faster than the Standard reaction, NO is purified by ammonium nitrate during the period from T2 to T4. As a result, there is a period in which the NOx concentration is lower than C2. The difference between C2 and C3 is caused by the purification of NO by ammonium nitrate. The difference between C2 and C3 correlates with the amount of ammonium nitrate produced. For this reason, in this embodiment, the actual NOx purification rate at T3 (that is, the maximum value of the NOx purification rate) and the NOx purification rate when the NOx concentration is C2 (that is, the NOx purification rate when there is no ammonium nitrate). Based on the difference, the amount of ammonium nitrate is calculated.

  FIG. 3 is a graph showing the relationship between the exhaust temperature and the NOx purification rate. In FIG. 3, T1 to T4 correspond to the time points T1 to T4 in FIG. According to FIG. 3, it can be seen that the difference in the NOx purification rate between the Fast reaction and the Standard reaction becomes relatively large when the temperature is, for example, 200 ° C. or less. In such a temperature range, the NOx purification rate by the reaction close to the Fast reaction shown in (Equation 3) also becomes high. That is, since there is a large difference between the NOx purification rate by the reaction shown in (Formula 2) and the NOx purification rate by the reaction shown in (Formula 3), it can be understood by looking at the NOx purification rate which reaction is dominant. . Therefore, by detecting the NOx purification rate, it is possible to determine whether the NOx is purified by the reaction of (Equation 2) or (Equation 3).

  Here, the difference between the NOx purification rate corresponding to C3 in FIG. 2 and the NOx purification rate corresponding to C2 is caused by NO being purified by ammonium nitrate. That is, the difference between the NOx purification rate corresponding to C3 and the NOx purification rate corresponding to C2 correlates with the production amount of ammonium nitrate. Therefore, if the relationship between the difference between the NOx purification rate corresponding to C3 and the NOx purification rate corresponding to C2 and the amount of ammonium nitrate produced is obtained in advance through experiments or simulations, the NOx purification corresponding to C3. Based on the difference between the rate and the NOx purification rate corresponding to C2, the amount of ammonium nitrate produced can be calculated. Note that the NOx purification rate corresponding to C2 is obtained in advance by experiments or simulations. Further, the NOx purification rate corresponding to C2 can be obtained by the upstream NOx sensor 12 and the downstream NOx sensor 14.

  As shown in FIG. 3, even when the temperature is higher than when the NOx purification rate is at its peak, the difference in the NOx purification rate between the Fast reaction and the Standard reaction becomes large. However, in this case, the amount of NOx emitted from the internal combustion engine 1 increases due to the high combustion temperature. In this embodiment, the amount of ammonium nitrate produced is detected on the low temperature side in order to reduce the NOx emission.

  And based on the production amount of ammonium nitrate, the ammonia addition amount when the exhaust temperature is within the predetermined region is corrected. Since ammonium nitrate is generated when the exhaust temperature is within a predetermined region, it is calculated how much ammonium nitrate is generated relative to the amount of ammonia added when the exhaust temperature is within the predetermined region. Then, the ratio of the production amount of ammonium nitrate to the added ammonia amount is calculated as a correction coefficient.

  FIG. 4 is a flowchart showing a flow of ammonia addition control according to the present embodiment. This flowchart is repeatedly executed by the ECU 10 every predetermined time.

  In step S101, it is determined whether there is an ammonia addition request. For example, when the temperature of the SCR catalyst 6 is equal to or higher than a temperature capable of purifying NOx (for example, 100 ° C.), ammonia is consumed by purifying NOx in the SCR catalyst 6, and therefore there is a request for addition of ammonia. Determined. If an affirmative determination is made in step S101, the process proceeds to step S102. On the other hand, if a negative determination is made, this flowchart is terminated.

  In step S102, it is determined whether or not the exhaust temperature is within a predetermined region that is a temperature region in which ammonium nitrate is generated. If the exhaust temperature is lower than 200 ° C., for example, the exhaust temperature is 100 ° C. or higher and lower than 200 ° C., and it is determined that the exhaust temperature is within the predetermined region. If an affirmative determination is made in step S102, the process proceeds to step S103, whereas if a negative determination is made, the process proceeds to step S104.

  In step S103, the ammonia addition amount is integrated. That is, the amount of ammonia added when the exhaust temperature is within the predetermined range is integrated.

  In step S104, it is determined whether the travel distance of the vehicle on which the internal combustion engine 1 is mounted is longer than a predetermined distance. The predetermined distance is a distance suitable for correcting the ammonia addition amount, and is, for example, 10,000 km. In step S104, it is determined whether a predetermined period has elapsed. If an affirmative determination is made in step S104, the process proceeds to step S105. On the other hand, if a negative determination is made, this flowchart is terminated.

  In step S105, it is determined whether the exhaust temperature is equal to or lower than a predetermined temperature. As described with reference to FIG. 3, the predetermined temperature is a temperature that becomes a threshold value at which the difference in the NOx purification rate between the Fast reaction and the Standard reaction becomes large, for example, 200 ° C. That is, in this step S105, it is determined whether or not the exhaust temperature is a temperature suitable for calculating the production amount of ammonium nitrate. If an affirmative determination is made in step S105, the process proceeds to step S106. On the other hand, if a negative determination is made, this flowchart is ended.

In step S106, the combustion mode is switched to a combustion mode in which NO is discharged from the internal combustion engine 1 and NO ₂ is not discharged. That is, the NOx component is only NO. In this embodiment, the ECU 10 processes step S106 to function as processing means in the present invention.

  In step S107, ammonia addition is performed. At this time, the ammonia amount is determined and added so as not to be excessive or insufficient with respect to the NOx amount. The NOx amount is calculated from the NOx concentration detected by the upstream NOx sensor 12, the intake air amount detected by the air flow meter 15, and the fuel injection amount.

  In step S108, the NOx purification rate is calculated. In this flowchart, the NOx purification rate calculated this time is the NOx purification rate (n), and the previously calculated NOx purification rate is the NOx purification rate (n−1). The NOx purification rate is calculated from the NOx concentration detected by the upstream NOx sensor 12 and the NOx concentration detected by the downstream NOx sensor 14.

  In step S109, it is determined whether or not the currently calculated NOx purification rate (n) is equal to or less than the previously calculated NOx purification rate (n-1). In this step S109, it is determined whether or not the increase in the NOx purification rate has stopped or the NOx purification rate has started to decline. That is, it is determined whether or not the NOx purification rate has reached the maximum value. If an affirmative determination is made in step S109, the process proceeds to step S110. On the other hand, if a negative determination is made, the process returns to step S108. Before returning to step S108, the NOx purification rate (n) calculated this time is substituted into the NOx purification rate (n-1) calculated last time.

  In step S110, it is determined whether or not the previously calculated NOx purification rate (n-1) is greater than the reference NOx purification rate. The previously calculated NOx purification rate (n-1) is the maximum value of the NOx purification rate. If the maximum value of the NOx purification rate is larger than the reference NOx purification rate, it is considered that NO is purified by ammonium nitrate. Therefore, in this step S109, it is determined whether or not ammonium nitrate is present. The reference NOx purification rate is the NOx purification rate when ammonium nitrate is not present, and is obtained in advance by experiments or simulations and stored in the ECU 10. If an affirmative determination is made in step S110, the process proceeds to step S111. On the other hand, if a negative determination is made, the process proceeds to step S117.

  In step S111, a NOx purification rate difference that is a difference between the previously calculated NOx purification rate (n-1) and the reference NOx purification rate is calculated.

  In step S112, the production amount of ammonium nitrate is calculated based on the NOx purification rate difference. The relationship between the NOx purification rate difference and the amount of ammonium nitrate produced is obtained in advance through experiments or simulations and stored in the ECU 10. Here, FIG. 5 is a diagram showing the relationship between the NOx purification rate difference and the amount of ammonium nitrate produced. It can be said that the larger the NOx purification rate difference is, the more ammonium nitrate is produced.

  In step S113, the ammonia addition amount is corrected. The ammonia addition amount correction coefficient is calculated by dividing the ammonium nitrate generation amount by the integrated value of the ammonia addition amount calculated in step S103. In this embodiment, the ECU 10 processes step S112 and step S113, thereby functioning as control means in the present invention.

  In step S114, ammonia addition is performed. At this time, the ammonia amount corresponding to the correction coefficient calculated in step S113 is reduced and added to the ammonia amount so that the NOx amount is not excessive or insufficient.

  In step S115, the current NOx purification rate is calculated. In step S116, it is determined whether or not the NOx purification rate is equal to or less than a reference NOx purification rate. If an affirmative determination is made in step S116, it can be determined that ammonium nitrate has been removed, and the process proceeds to step S117. On the other hand, if a negative determination is made, the process returns to step S115.

In step S117, the combustion mode is changed to the normal combustion mode. The normal change mode is a combustion mode before the combustion mode is changed in step S106, and is a combustion mode in which NO and NO ₂ are discharged from the internal combustion engine 1. In step S118, the travel distance used in step S104 and the integrated value of the ammonia addition amount calculated in step S103 are reset.

  As described above, according to the present embodiment, the amount of ammonium nitrate produced can be calculated, and the amount of ammonia added can be corrected according to the amount of ammonium nitrate produced. Thereby, since excessive addition of ammonia can be suppressed, it can suppress that the SCR catalyst 6 is poisoned with ammonium nitrate and a NOx purification rate falls.

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 2 Exhaust passage 3 Oxidation catalyst 4 Filter 5 Reducing agent addition valve 6 Selective reduction type NOx catalyst (SCR catalyst)
7 Fuel injection valve 8 Intake passage 9 Throttle 10 ECU
11 upstream temperature sensor 12 upstream NOx sensor 13 downstream temperature sensor 14 downstream NOx sensor 15 air flow meter 16 accelerator pedal 17 accelerator opening sensor 18 crank position sensor

Claims (1)

A selective reduction type NOx catalyst provided in an exhaust passage of the internal combustion engine for reducing NOx using ammonia as a reducing agent;
A supply device for supplying ammonia to the selective reduction type NOx catalyst;
Processing means for performing processing for preventing NO _{2 from} being contained in the exhaust gas flowing into the selective reduction type NOx catalyst;
An exhaust gas purification apparatus for an internal combustion engine comprising:
The maximum value of the NOx purification rate in the selective reduction type NOx catalyst when the processing is being performed by the processing means, and ammonium nitrate is temporarily added to the selective reduction type NOx catalyst when the processing is being performed by the processing means. The amount of ammonium nitrate produced in a predetermined period is calculated based on the NOx purification rate estimated when NO is present, and the ammonium nitrate is produced in the predetermined period and the exhaust temperature in the predetermined period. An internal combustion engine comprising control means for correcting the amount of ammonia to be supplied when the exhaust gas temperature is in the predetermined temperature range based on the amount of ammonia supplied by the supply device when in the predetermined temperature range Exhaust purification equipment.

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