DE102017204300A1 - Method for diagnosing an SCR system - Google Patents

Abstract

The invention relates to a method for the diagnosis of an SCR system, which has a feed pump and a metering valve. The method comprises the following steps: Initially, a print error is detected in the SCR system, whereupon a setting of a pressure-controlled mode of the SCR system is performed. Subsequently, a mass deviation (Δm) of the reducing agent solution is detected. Finally, an error is output to the feed pump when the mass deviation (Δm) is above a first threshold (S ₁ ) and an error is output to the metering valve when the mass deviation (Δm) is below or equal to the first threshold (S ₁ ) is.

Description

The present invention relates to a method for diagnosing an SCR system when a printing error has been detected in the SCR system. Furthermore, the invention relates to a computer program that performs each step of the method when it runs on a computing device, and a machine-readable storage medium that stores the computer program. Finally, the invention relates to an electronic control device which is set up to carry out the method according to the invention.

State of the art

Today, SCR catalysts (Selective Catalytic Reduction) are used to reduce nitrogen oxides (NOx) in the exhaust gas of internal combustion engines in motor vehicles. Here are nitrogen oxide molecules, which are located on an SCR catalyst surface, in the presence of ammonia (NH3) as a reducing agent, reduced to elemental nitrogen. The reducing agent is provided in the form of a urea-water solution from which ammonia is split off, also known commercially as AdBlue®. A feed pump delivers the reductant solution from a reductant tank to a metering module which then injects the reductant solution into an exhaust line upstream of the SCR catalyst. The control of the dosing takes place in an electronic control unit, in which strategies for operation and monitoring of the SCR system are stored.

In a so-called "volumetric mode" one makes use of the generally high accuracy of the feed pump and the property that in the stationary state, the very well-known, promoted by the feed pump mass of the reducing agent solution also leaves the system as a metered mass again. In combination with the comparatively small tolerances of the conveyed mass of the reducing agent solution by the feed pump on average a high mass accuracy. In terms of pressure, however, there is generally no closed loop.

A "pressure-controlled mode" is generally based on the principle that the feed pump provides a desired system pressure and regulates to the narrowest possible pressure range around the specified target pressure. The dosing module then doses the desired mass of the reducing agent solution starting from this system pressure by setting a suitable valve opening time.

If the mass balance is disturbed in volumetric mode, for example due to a defect in the feed pump or in the dosing module, the metered mass can no longer be checked directly. It is known in this case to monitor the pressure, for example by means of a pressure sensor. For this purpose, an overpressure threshold and a vacuum threshold are established, which form an allowable pressure range around a target pressure. If the pressure leaves this permitted pressure range during the dosing, a (pressure) error is output and an erroneous operation is set in the SCR system. As a result, it can no longer be guaranteed that emission regulations will be complied with.

In motor vehicles, in faulty mode, usually an engine control light is activated, which signals the driver to visit a workshop. Optionally, in addition, an emergency operation can be set, in which the SCR system is operated with different operating parameters and beyond, for example, the performance of the internal combustion engine is throttled. In the workshop, the delivery pump and / or the dosing module are repaired or changed on the basis of the assessment or the knowledge and experience of the responsible personnel, therefore "on suspicion".

The DE 10 2008 005 988 A1 describes a method and apparatus for diagnosing an exhaust aftertreatment device that meters a reagent into the exhaust region. The reagent is brought from a pump to a metering pressure and then metered. The diagnosis is based on an evaluation of the pressure drop of the reagent after switching off the pump. In this case, leakage losses of the pump are taken into account.

From the DE 10 2010 002 620 A1 is a so-called adaptation factor known. Based on a model, a required mass of the reducing agent solution, which is metered in upstream of an SCR catalyst, and a modeled nitrogen oxide value downstream of the SCR catalyst is calculated. In addition, a nitrogen oxide sensor arranged downstream of the SCR catalytic converter measures a measured nitrogen oxide value. The modeled nitrogen oxide value and the measured nitrogen oxide value are compared with one another and, in the event of a deviation, an underdosing or an overdose of the reducing agent solution is detected. Depending on this, an adaptation is then carried out and the dosage is increased or reduced by adjusting the adaptation factor.

Disclosure of the invention

A method is proposed for the diagnosis of an SCR system, which has a feed pump and a metering valve, for an SCR catalytic converter for exhaust aftertreatment, in particular of an internal combustion engine in a motor vehicle. At the beginning, a printing error in the SCR Detected system, for example, when a system pressure between the pump and metering valve during metering leaves an allowable pressure range, which is formed by an overpressure threshold and a vacuum threshold. If the printing error has been detected, a desired or required mass of the reducing agent solution and an actually metered mass of the reducing agent solution may diverge, so that it is no longer possible to ensure that emission regulations are adhered to.

In a further step, a pressure-controlled mode of the SCR system is set. In the usual case that the SCR system operates in a volumetric mode prior to detecting the printing error in order to achieve the highest possible mass accuracy of the metered reducing agent solution, the SCR system is switched to the pressure-controlled mode after the error has been detected. In pressure-controlled mode, the feed pump provides a desired system pressure and regulates it to a very narrow pressure range around a specified target pressure. As a result, a mass deviation of the reducing agent solution may occur in the system. In a further step, this mass deviation of the reducing agent solution is detected. If the detected mass deviation is above a first threshold, it is possible to conclude that there is a defect in the feed pump and an error is output for the feed pump. The basis for this is that the defect in the feed pump due to the pressure control too much or too little mass of the reducing agent based on the metered mass is promoted, so that increases the mass deviation. Otherwise, if the mass deviation is below or equal to the first threshold, an error is output to the metering valve. The basis for this is that due to the defect in the dosing module, the mass deviation due to the pressure control during dosing is compensated. The first threshold may preferably have a value of up to 40%, although it may also assume negative values, therefore down to -40%. This method enables the component - delivery module or metering valve - from which the printing error indicates to diagnose during operation. As a result, the repair or replacement of the defective components is simplified, for example, in a workshop, since it is already known in advance which component is defective.

Preferably, after the printing error has been detected in the SCR system, a setting of a faulty mode of the SCR system is performed in which error-specific measures are performed. As one of the measures in faulty mode, it is possible to activate an engine control lamp, which for example signals a driver of the motor vehicle to the faulty mode of the SCR system, whereupon the latter should drive to the workshop. Thus, the pertinent statutory requirements are met. In addition, a further measure in faulty mode may provide for performing emergency operation, at least for the SCR system, where the SCR system is provided with other operating parameters, such as those shown in FIG. Metering and / or Dosiermenge operated, and beyond the performance of the engine can be throttled. To prevent excess ammonia, which is toxic to humans and the environment, from escaping from the exhaust aftertreatment system, provision may be made to completely shut down the SCR system. In the course of this usually the reductant solution is recycled from the SCR system. If this is the case can be provided in this method, before setting the pressure-controlled mode to refill the SCR system with reducing agent solution.

According to a preferred aspect, the setting of the pressure-controlled mode is realized by changing the opening duration of the metering valve. The opening time of the metering valve is increased when the pressure above the target pressure continues to increase, so that more reducing agent solution is metered. On the other hand, the opening duration of the metering valve is reduced when the pressure below the target pressure continues to drop, so that less reducing agent is metered.

According to a further aspect, the setting of the pressure-controlled mode is realized by changing the control of the pump. Depending on the pressure, the rotational speed of the pump or, if appropriate, the mass conveyed at the same time can be changed, so that the total mass conveyed changes.

For detecting the mass deviation of the reducing agent solution, it may be provided to carry out the following steps. Initially, the SCR catalyst is emptied. If several SCR catalysts are present in the SCR system, it is meant that all SCR catalysts are emptied. Then, at least one desired mass flow of the reducing agent solution is metered in a substoichiometric ratio upstream of the SCR catalyst. Preferably, several such desired mass flows are metered with different substoichiometric ratios in order to cover the largest possible coverage. Subsequently, a nitrogen oxide concentration is detected downstream of the SCR catalyst. For this purpose, a nitrogen oxide sensor, which is arranged downstream of the SCR catalytic converter, can be used. In addition, a nitrogen oxide concentration upstream of the SCR catalyst is detected. Analogous to this, a nitrogen oxide sensor which is located upstream of the SCR Catalyst is arranged to be used. If no such nitrogen oxide sensor is present upstream of the SCR catalyst, a nitrogen oxide concentration, which was measured with the SCR catalyst deflated before metering in the desired mass flow of the reducing agent solution from the nitrogen oxide sensor downstream, can also be the nitrogen oxide concentration upstream of the SCR Catalyst can be used, since in this case no nitrogen oxide was reduced by the SCR catalyst. A difference in nitrogen oxide concentration downstream of the SCR catalyst and nitrogen oxide concentration upstream of the SCR catalyst is used to calculate an actual mass flow of the reductant solution through the SCR catalyst, taking into account the substoichiometric ratio and a total mass exhaust gas mass. From the actual mass flow, the actually metered mass of the reducing agent solution can be determined, while the desired mass flow is related to the desired mass of the reducing agent solution. Finally, a comparison of the desired mass flow and the actual mass flow is carried out in order to determine the mass deviation.

Alternatively, for detecting the mass deviation of the reducing agent solution, the following steps may be performed. Before the pressure-controlled mode is set, an adaptation factor, for example according to the DE 10 2010 002 620 A1 , detected in a known manner and then stored as a nominal adaptation factor. After the pressure-controlled mode has been set, the adaptation factor is reset and a new adaptation is started. During the pressure-controlled mode, a new adaptation factor is learned in the same way depending on the new adaptation over a learning time. Finally, a comparison of the nominal adaptation factor and the new adaptation factor is carried out in order to determine the mass deviation.

The computer program is set up to perform each step of the method, in particular when it is performed on a computing device or controller. It allows the implementation of the method in a conventional electronic control unit without having to make any structural changes. For this purpose it is stored on the machine-readable storage medium.

By loading the computer program onto a conventional electronic control unit, the electronic control unit is set up, which is set up to carry out the diagnosis of the SCR system.

list of figures

• 1 zeigt eine schematische Darstellung eines SCR-Systems für einen SCR-Katalysator in einem Abgasstrang einer Verbrennungsmaschine, bei dem das erfindungsgemäße Verfahren angewendet werden kann.
• 2 zeigt ein Diagramm eines Drucks des SCR-Systems aus Figur 1 für einen volumetrischen Modus über der Zeit.
• 3 zeigt ein Ablaufdiagramm eines ersten Ausführungsbeispiels des erfindungsgemäßen Verfahrens.
• 4 zeigt ein Ablaufdiagramm eines zweiten Ausführungsbeispiels des erfindungsgemäßen Verfahrens.
• 5 zeigt ein Diagramm des Drucks des SCR-Systems aus 1 und der Massenabweichung der Reduktionsmittellösung für einen druckgeregelten Modus über der Zeit, gemäß einem Ausführungsbeispiel des erfindungsgemäßen Verfahrens.

Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.

• 1 shows a schematic representation of an SCR system for an SCR catalyst in an exhaust line of an internal combustion engine, in which the inventive method can be applied.
• 2 FIG. 12 is a graph of pressure of the SCR system of FIG. 1 for a volumetric mode over time.
• 3 shows a flowchart of a first embodiment of the method according to the invention.
• 4 shows a flowchart of a second embodiment of the method according to the invention.
• 5 shows a diagram of the pressure of the SCR system 1 and the mass deviation of the reducing agent solution for a pressure-controlled mode over time, according to an embodiment of the method according to the invention.

Embodiment of the invention

1 shows a schematic representation of an SCR system 1 for an SCR catalyst 2 in an exhaust line 3 an internal combustion engine 4 , The SCR system 1 has a delivery module 10 with a feed pump 11 containing a reducing agent solution from a reducing agent tank 12 via a pressure line 13 to a metering module 14 promotes. Via a dosing valve 15 in Dosiermodul 14, the reducing agent solution is then in the exhaust line 3 upstream of the SCR catalyst 2 metered. Non-metered reducing agent solution flows over one with the pressure line 13 connected return 16 in the reducing agent tank 12 back. In the return 16 a return throttle 17 is arranged, which by the return 16 flowing mass of the reducing agent solution limited. Furthermore, there is a pressure sensor 18 in the pressure line 13 arranged there, which measures the pressure p of the therein located reducing agent solution. In the exhaust system 3 is a first nitric oxide sensor 31 downstream of the SCR catalyst 2 arranged there, there is a nitrogen oxide concentration NOxN downstream of the SCR catalyst 2 measures, and a second nitric oxide sensor 32 upstream of the SCR catalyst 2 arranged there, a nitrogen oxide concentration NOxV upstream of the SCR catalyst 2 measures. Furthermore, an electronic control unit 5 provided, which at least with the delivery module 10 or the feed pump 11 and the metering module 14 and the metering valve 15 is connected and can control these. In addition, the pressure sensor 18 and the two nitric oxide sensors 31 . 32 with the electronic control unit 5 connected and send this their measured values.

In general, such SCR systems are operated in volumetric mode, in which the by the feed pump 11 delivered mass of the reducing agent solution as metered mass through the metering valve 15 is metered. On average, a high mass accuracy sets in, but there is no closed loop for the pressure p. In 2 Two pressure curves p ₁ and p _{2 are shown} in a graph of the pressure p over the time t for the volumetric mode, both of which lead to a printing error. In a first area 100 in which the metering valve 15 is closed, the feed pump 11 promotes reducing agent, so that the pressure p for both pressure curves p ₁ and p ₂ from the ambient pressure p _a increases until a desired pressure p _{s is} reached. From a "Begin of Injection Point" BIP opens the metering valve 15 and the reducing agent solution enters the exhaust line 3 metered. In a second region 101, in which metering takes place, the two pressure profiles p ₁ and p ₂ differ. The first pressure curve p ₁ represents a defect either of the feed pump 11, is promoted by the more reducing agent solution than intended, or the metering valve 15 , is metered through the less reducing agent solution than intended. Accordingly, the first pressure curve p ₁ continues to rise and finally exceeds an overpressure threshold p _o . Conversely, the second pressure curve p ₂ represents a defect either of the feed pump 11 , is promoted by the less reducing agent solution than intended, or the metering valve 15 , is dosed by the more reducing agent solution than intended. Accordingly, the second pressure curve p ₂ decreases and finally falls below a vacuum threshold p _u . If the pressure p leaves after the "Begin of Injection Point" BIP a permitted pressure range between the overpressure threshold p _o and the negative pressure threshold p _u , a printing error is detected 200.

3 shows a flowchart of a first embodiment of the method according to the invention. As described above, a printing error in the SCR system 1 If 200 is detected, a setting takes place 201 a faulty mode FM. On the one hand, in this case an engine control light is activated 202. For an embodiment in which the SCR system 1 is used in a motor vehicle, the engine control lamp on a dashboard (both not shown) is arranged so that it signals a faulty mode FM to a driver of the motor vehicle. Furthermore, in faulty mode FM is an emergency operation 203 set, in which the SCR system 1 is operated with other operating parameters. In some embodiments, moreover, during emergency operation 203 , the performance of the internal combustion engine 5 be throttled. In the present embodiment, in emergency mode 203 in addition, the SCR system 1 is emptied, ie the reducing agent solution is conveyed back into the reducing agent tank 12, in order to prevent free ammonia from the exhaust gas line due to overdosing 3 escapes. In this embodiment, therefore, a refilling takes place 204 of the SCR system 1 with the reducing agent solution.

According to the invention, a pressure-controlled mode DM for the SCR system 1 set. In the exemplary embodiments described here, this is achieved by a change 205 an opening period of the metering valve 15 realized. In further embodiments, the setting of the pressure-controlled mode DM by changing the control of the feed pump 11 will be realized.

In the in 3 illustrated first embodiment, a detection of a mass deviation Δm is as follows. An emptying 210 of the SCR catalyst 2 is performed, for example by less reducing agent solution is metered into the exhaust line 3, as would be necessary for a complete SCR of the nitrogen oxides, so that no more ammonia in the SCR catalyst 2 is included. Subsequently, a predetermined desired mass flow ṁ _{s of} the reducing agent solution with a substoichiometric ratio α of, for example, α = 0.5 upstream of the SCR catalyst 2 dosed 211.

Then, the nitrogen oxide concentration NOxN becomes downstream of the SCR catalyst 2 via the first nitrogen oxide sensor 31 determines 212 and the nitrogen oxide concentration NOxV upstream of the SCR catalyst 2 via the second nitrogen oxide sensor 32 determines 213. In further embodiments, in particular when no second nitrogen oxide sensor 32 upstream of the SCR catalyst 2, the nitrogen oxide concentration NOxV may be upstream of the SCR catalyst 2 also be modeled. About a difference in nitrogen oxide concentration NOxN downstream of the SCR catalyst 2 and the nitrogen oxide concentration NOxV upstream of the SCR catalyst 2 becomes a calculation 214 the actual metered actual mass flow ṁ _l performed. At the calculation 214 the stoichiometric ratio α and the total exhaust mass flow ṁ _{A are} taken into account. The nitrogen oxides which have been reduced at the SCR catalyst 2 depend directly on the SCR catalyst 2 existing mass of the reducing agent from ammonia, which was actually metered. By means of a comparison 215 of the desired mass flow ṁ _s and the actual mass flow ṁ _l , the mass deviation Δm is determined.

Finally, the mass deviation Δm of the reducing agent is compared 220 with a first threshold S ₁ , which should be 25% in this example. If the mass deviation .DELTA.m above the first threshold S ₁ , an error 221 of the feed pump 11 output. This is due to the fact that the change 205 the opening duration of the metering valve 15 to the printing error of the defective feed pump 11 in the pressure-controlled mode DM, the mass deviation Δm of the metered reducing agent solution increases. If, on the other hand, the mass deviation Δm is below the first threshold S ₁ or is equal to this, then an error occurs 222 of the metering valve 15 is output. Here is the change 205 the opening duration of the metering valve 15 to that the defect of the metering valve 15 is balanced so that the mass deviation Δm decreases.

In 4 a second embodiment of the method according to the invention is shown. Same reference numerals as in 3 mean that the steps correspond, therefore, for their description on the description of 3 directed. In this second embodiment, an adaptation of the dosage by means of an adaptation factor a is performed in a known manner. In short, a modeled nitrogen oxide value and a downstream of the SCR catalyst are shown 2 arranged measured first nitric oxide sensor 31 measured nitrogen oxide value with each other and executed depending on the adaptation. After the printing error has been detected 200, the adaptation factor a is detected 230 and stored as a nominal adaptation factor a _A 231. Since a defect either in the feed pump 11 or the metering valve 15, the nominal adaptation factor a _A assumes an increased or a decreased value, for example at a mass deviation of 50%, a value of 1.5 based on the stoichiometric ratio. In further embodiments, it may be provided to detect the printing error by means of a comparison of the adaptation factor a or of the nominal adaptation factor a _A with a threshold 200. This is done in the same way as with reference to FIG 3 already described, the setting 201 the faulty mode FM and the change 205 the opening duration of the metering valve 15 , After the pressure-controlled mode DM has been set as a result, the adaptation factor a is reset to 1 and a new adaptation is started 233. During this new adaptation, the teaching-in takes place 234 a new adaptation factor a _N over a learning time t _L. Due to the pressure regulation taking place, the new adaptation factor a _N assumes a new value deviating from the nominal adaptation factor. About a comparison 235 of the new adaptation factor a _N with the nominal adaptation factor a _A , an adaptation adaptation Δa is determined, which is proportional to the mass deviation Δm.

According to one embodiment, the mass deviation Δm can be determined first and then, as already in 3 shown (steps 220 to 222 ), 220 are compared with the first threshold S ₁ . In the in 4 1, an adaptation adaptation Δa, which is proportional to the mass deviation Δm, is compared 240 with a second threshold S _2. As a concrete example, the second threshold S _{2 should be} 30%. If the adaptation adaptation Δa is above the second threshold S ₂ , therefore above 25%, it follows that the mass deviation Δm is greater than the first threshold S ₁ 241 and it is analogous to 3 a mistake 242 the feed pump 11 issued. If this is not the case, it follows that the mass deviation Δm is less than the first threshold S ₁ or equal to this 243 and it is analogous to 3 a mistake 244 of the metering valve 15 output.

5 shows a diagram of the pressure p and the mass deviation Δm of the reducing agent solution over the time t for a metering, in which the SCR system 1 is already operated in the pressure-controlled mode DM. In a first area 110 in which the metering valve 15 is closed, the feed pump 11 promotes reducing agent solution, so that a third pressure curve p ₃ from the ambient pressure p _a increases until the target pressure p _{s is} reached. Since the metering valve 15 is closed, no reducing agent in the exhaust line 3 metered in so that the mass deviation Δm is zero. From the "Begin of Injection Point" BIP opens the metering valve 15 and the reducing agent solution enters the exhaust line 3 metered. In a second area 111 , in which is metered, the pressure p is permanently adjusted to the target pressure p _s , whereby the third pressure curve p ₃ oscillates to the target pressure p _s . As already described above, this has an effect on the mass deviation Δm. The first threshold S ₁ is 25% in this example. The mass deviation Δm increases in this embodiment and exceeds the first threshold S ₁ . Accordingly, according to the method, the error 221 . 242 the feed pump 11 issued. In addition, the learning time t _L is shown here by way of example. The learning time t _L begins with the pressure control at the "Begin of Injection Point" BIP and can be any length in the second range 111 be executed. In addition, the learning time can be continued in other embodiments over several doses.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102008005988 A1 [0007]
• DE 102010002620 A1 [0008, 0015]

Claims (10)

A method of diagnosing an SCR system (1) comprising a delivery pump (11) and a metering valve (15), comprising the steps of: - detecting (200) a printing error in the SCR system (1); - Setting a pressure-controlled mode (DM) of the SCR system (1); - detecting a mass deviation (Δm) of the reducing agent solution; - outputting a fault (221; 242) for the feed pump (11) if the mass deviation (Δm) is above a first threshold (S ₁ ); and - outputting a fault (222; 244) for the metering valve (15) if the mass deviation (Δm) is less than or equal to the first threshold (S ₁ ). Method according to Claim 1 , characterized in that after the printing error has been detected (200) in the SCR system, an adjustment (201) of a faulty mode (FM) of the SCR system (1) is made. Method according to Claim 2 characterized in that, upon setting (200) of the faulty mode (FM), an engine control lamp (202) is activated which signals the faulty mode (FM) of the SCR system (1). Method according to one of Claims 1 to 3 , characterized in that the setting of the pressure-controlled mode (DM) by changing (205) an opening duration of the metering valve (15) is realized. Method according to one of Claims 1 to 3 , characterized in that the setting of the pressure-controlled mode is realized by changing the control of the feed pump. Method according to one of Claims 1 to 5 characterized in that for detecting the mass deviation (Δm) of the reducing agent solution, the following steps are carried out: - emptying (210) an SCR catalyst (2) of the SCR system (1); - dosing (211) at least one desired mass flow (ṁ _s ) of the reducing agent solution in a substoichiometric ratio (a) upstream of the SCR catalyst (2); - determining (212, 213) a nitrogen oxide concentration (NOxN) downstream of the SCR catalyst (2) and a nitrogen oxide concentration (NOxV) upstream of the SCR catalyst (2); Calculating (214) an actual mass flow (ṁ _l ) of the reducing agent solution via a difference of the nitrogen oxide concentration (NOxN) downstream of the SCR catalytic converter (2) and the nitrogen oxide concentration (NOxV) upstream of the SCR catalytic converter (2), taking into account the substoichiometric ratio (a) and a total exhaust mass flow (ṁ _A ); - Comparison (215) of the desired mass flow (ṁ _s ) and the actual mass flow (ṁ _l ) to determine the mass deviation (Δm). Method according to one of Claims 1 to 5 , characterized in that for detecting the mass deviation (Δm) of the reducing agent solution, the following steps are performed: - detecting (230) an adaptation factor (a) before the pressure-controlled mode (DM) is set; - storing (231) the adaptation factor (a) as a nominal adaptation factor (a _A ); - resetting (232) the adaptation factor (a) after the pressure-controlled mode (DM) has been set; - Teaching (234) a new adaptation factor (a _N ) during the pressure-controlled mode (DM) over a learning time (t _L ); and - comparing (235) the nominal adaptation factor (a _A ) and the new adaptation factor (a _N ) to determine the quantity deviation (Δm). Computer program, which is set up, each step of the procedure according to one of Claims 1 to 7 perform. Machine-readable storage medium on which a computer program is based Claim 8 is stored. Electronic control unit (5), which is arranged to operate by means of a method according to one of Claims 1 to 7 to carry out a diagnosis of the SCR system (1).

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