JP5206558B2 - Exhaust gas reforming system - Google Patents

Description

  The present invention relates to an exhaust gas reforming system for reforming exhaust gas.

Conventionally, a system that can improve the fuel efficiency of an internal combustion engine by reforming exhaust gas is known (see, for example, Patent Document 1).
This system adds fuel to a part of the exhaust gas taken out from the exhaust passage of the internal combustion engine to generate a mixed gas, introduces the generated mixed gas into the reforming catalyst, and discards the exhaust gas passing through the exhaust passage. Heat the reforming catalyst using heat. Thereby, this system reforms the mixed gas to generate a reformed gas containing hydrogen, and the reformed gas generated through the intake passage is recirculated to the internal combustion engine.

JP 2004-92520 A

  However, in the system described in Patent Document 1, since the amount of exhaust gas actually introduced into the combustion chamber of the internal combustion engine cannot be detected with high accuracy, a large amount of exhaust gas is introduced into the combustion chamber, causing problems such as misfire. There was a risk that the fuel consumption would decrease.

  Accordingly, an object of the present invention is to provide an exhaust gas reforming system that can improve fuel efficiency.

An exhaust gas reforming system according to the present invention includes an external circulation path that externally circulates exhaust gas discharged from an engine to an exhaust pipe to an intake pipe that the engine uses for intake, and an external circulation path that is externally circulated by the external circulation path. A reforming catalyst that reforms a mixed gas in which EGR and fuel are mixed to generate hydrogen, an opening / closing timing of an exhaust valve that opens and closes an exhaust pipe, and an opening / closing timing of an intake valve that opens and closes an intake pipe Based on the first calculation means for calculating the amount of internal EGR that is the exhaust gas that is circulated internally by the engine, and the total EGR amount obtained by adding the amount of external EGR and the amount of internal EGR calculated by the first calculation means. a second calculating means for calculating the hydrogen concentration is the amount of hydrogen quality catalyst caused, concentrated hydrogen pumping gas recirculation rate is a circulation rate of the external EGR and internal EGR, was calculated in the second calculating means So that the total exhaust gas recirculation rate to optimize the fuel consumption of the engine which is determined by, is characterized in that it comprises an adjusting valve for adjusting the amount of external EGR external circulation path to the external circulation.
According to this configuration, not only can the internal EGR amount be calculated based on the opening / closing timing of the exhaust valve and the opening / closing timing of the intake valve, but also the total exhaust gas recirculation rate that provides the best fuel efficiency based on the calculated internal EGR amount. Thus, the external EGR amount can be adjusted. For this reason, it is possible not only to prevent a combustion failure due to an increase in exhaust gas, which is an incombustible gas, but also to improve thermal efficiency and fuel consumption.

  According to the exhaust gas reforming system disclosed in this specification, fuel efficiency can be improved.

It is a block diagram which shows one Embodiment of the exhaust-gas reforming system of this invention. It is a block diagram showing the example of 1 structure of a control apparatus. It is a figure showing an example of the information which a storage part memorizes. It is a flowchart showing an example of the control processing which a control apparatus performs.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Embodiments of the invention will be described below with reference to the accompanying drawings.

FIG. 1 is a configuration diagram showing an embodiment of an exhaust gas reforming system 1 of the present invention.
An exhaust gas reforming system 1 shown in FIG. 1 is mounted on a vehicle. The exhaust gas reforming system 1 includes an external circulation path 10, a fuel supply device 12, a cooler 14, a flow rate adjustment valve 15, a heat exchanger 20, a throttle 30, a fuel injection device 40, an intake pipe 51, an exhaust pipe 52, an engine 60, And a control device 70. The exhaust gas reforming system 1 reforms a mixed gas obtained by mixing exhaust gas and fuel using a reforming catalyst. Here, the reforming catalyst generates a reformed gas containing, for example, hydrogen, carbon monoxide, and nitrogen from a mixed gas of exhaust gas and fuel by a reforming reaction that is an endothermic reaction. Although the reforming catalyst is described as being a rhodium-based catalyst, for example, it is not limited to this.

  The external circulation path 10 circulates a part of the exhaust gas discharged from the engine 60 to the exhaust pipe 52, and causes the engine 60 to intake air again (hereinafter simply referred to as external circulation). Specifically, the external circulation path 10 supplies the engine 60 with a reformed gas obtained by reforming a mixed gas of exhaust gas and fuel circulated by the reforming catalyst. The recirculated exhaust gas that is externally circulated by the external circulation path 10 is referred to as external EGR (Exhaust Gas Recirculation).

Next, the configuration of the external circulation path 10 will be described.
The external circulation path 10 includes an exhaust gas introduction path 11 and a reformed gas conduit 13, and a fuel supply device 12, a cooler 14, and a flow rate adjustment valve 15 are installed.
The exhaust gas introduction path 11 is connected to the heat exchanger 20 and the exhaust pipe 52, and introduces a part of the exhaust gas that passes through the exhaust pipe 52 to the heat exchanger 20. Further, the exhaust gas introduction path 11 includes a fuel supply device 12 downstream of a connection point with the exhaust pipe 52.

  The fuel supply device 12 is constituted by, for example, an injector. The fuel supply device 12 injects an amount of fuel according to the control of the control device 70 into the exhaust gas passing through the exhaust gas introduction path 11 to generate a mixed gas in which the exhaust gas and the fuel are mixed.

Here, the heat exchanger 20 will be described once.
The heat exchanger 20 includes a detection device 20a, an exhaust passage 21, and a reforming chamber 22. The heat exchanger 20 moves the heat of the exhaust gas passing through the exhaust passage 21 to the reforming catalyst supported in the reforming chamber 22. The detection device 20a is constituted by, for example, a temperature sensor, detects the bed temperature of the reforming catalyst, and outputs a signal representing the detected temperature to the control device 70.
A part of the exhaust gas discharged from the engine 60 is supplied to the reforming chamber 22 from the exhaust gas introduction path 11. Specifically, since a mixed gas of exhaust gas and fuel is supplied to the reforming chamber 22, the reforming catalyst carried in the reforming chamber 22 reforms the mixed gas using the heat of the exhaust gas. . A reformed gas conduit 13 is connected downstream of the reforming chamber 22.

Next, the configuration of the external circulation path 10 will be continuously described.
The reformed gas conduit 13 is connected to the reforming chamber 22 upstream, the cooler 14 and the flow rate adjusting valve 15 are sequentially installed in the middle flow, and merge with the intake pipe 32 downstream. The reformed gas conduit 13 supplies the reformed gas generated in the reforming chamber 22 to the engine 60 via the intake pipe 32. The cooler 14 cools the reformed gas passing through the reformed gas conduit 13 to increase the volume flow rate of the gas.

  The flow rate adjustment valve 15 adjusts the flow rate (that is, the external EGR amount) of exhaust gas that is circulated externally by the external circulation path 10. That is, the flow rate adjustment valve 15 controls not only the external EGR amount supplied to the reforming chamber 22 but also the external EGR amount to be returned to the engine 60. Here, the flow rate adjusting valve 15 controls the total amount of recirculation gas (hereinafter simply referred to as the total EGR rate) by controlling the amount of reformed gas passing through the reformed gas conduit 13 according to the control of the control device 70. To do. The total EGR rate is a ratio obtained by dividing the total EGR amount by the sum of the air amount taken in by the engine 60 and the total EGR amount. The total EGR amount refers to the sum of the external EGR amount and the exhaust gas amount (hereinafter referred to as the internal EGR amount) that the engine 60 circulates internally.

The throttle 30 is connected to an accelerator sensor (not shown). The throttle 30 adjusts the opening / closing amount of the intake pipe 51 according to the accelerator operation detected by the accelerator sensor. That is, the throttle 30 controls the amount of air taken in by the engine 60 by opening and closing the intake pipe 51. The throttle 30 can also be connected to the control device 70 and adopt a configuration in which the total EGR rate is controlled by controlling the amount of air taken in by the engine 60 according to the control of the control device 70.
The fuel injection device 40 is composed of, for example, an injector. The fuel injection device 40 injects an amount of fuel controlled by the control device 70 into the intake pipe 51.

The intake pipe 51 is connected to the throttle 30 upstream and is connected to the intake port of the engine 60 downstream. Further, the intake pipe 51 joins the reformed gas conduit 13 in the middle flow, and the fuel injection device 40 is installed at a point between a point where the reformed gas conduit 13 joins and a point connected to the engine 60. The intake pipe 51 includes an amount of air controlled by the throttle 30, an amount of reformed gas controlled by the flow rate adjusting valve 15 provided in the reformed gas conduit 13, and an amount of fuel injected by the fuel injection device 40. Is used by the engine 60 to take in the air-fuel mixture.
The exhaust pipe 52 is connected upstream with the exhaust port of the engine 60 and discharges the exhaust gas discharged by the engine 60 to the atmosphere outside the vehicle. Further, the exhaust pipe 52 branches off from the external circulation path 10 downstream, and the heat exchanger 20 is installed downstream of the branch point. Further, the exhaust pipe 52 includes a detection device 52 a that detects the temperature of the exhaust gas passing through the inside and outputs a signal representing the detected temperature to the control device 70. In addition, the detection apparatus 52a is comprised with a temperature sensor, for example.

The engine 60 is provided with a detection device 60a. The detection device 60a is composed of, for example, an NE sensor (Number of Engine speed), detects the rotational speed of the engine 60, and outputs an NE signal representing the detected rotational speed to the control device 70.
The engine 60 is composed of, for example, a gasoline engine. The engine 60 generates power for running the vehicle by burning the mixed gas sucked from the intake pipe 51. The engine 60 includes a variable valve timing (VVT), and the exhaust gas is internally circulated by changing the opening / closing timings of the intake valve that opens and closes the intake pipe 51 and the exhaust valve that opens and closes the exhaust pipe 52. The gas amount (that is, the internal EGR amount) is changed.

  Here, the total amount of non-combustible gas calculated from the amount of air (oxygen gas amount) supplied to the engine 60 and the total amount of EGR recirculated to the engine 60 is used to prevent misfire of combustion in the engine 60. Supply limit amount (hereinafter referred to as EGR limit). However, since hydrogen contained in the mixed gas enables rapid combustion in the engine 60, this EGR limit is expanded. Thereby, not only the pump loss is reduced in the engine 60 but also the fuel efficiency is improved by the improvement of the thermal efficiency due to the increase of the specific heat ratio. Therefore, in order to maximize the fuel efficiency effect of the reformed gas, it is necessary to accurately control the increase amount of the total EGR based on the EGR limit that expands according to the amount of reformed gas generated. This is because an excessive amount of exhaust gas that is non-combustible gas recirculated to the engine 60 may cause combustion failure of the engine 60, failing to improve thermal efficiency, and improving fuel efficiency. Therefore, the exhaust gas reforming system 1 of the present invention that can calculate the amount of internal EGR that changes due to the operation of the VVT will be described in order to reliably improve the fuel consumption.

The control apparatus 70 is comprised by ECU (Electronic Control Unit), for example. The control device 70 controls each device and equipment to be connected so as to improve fuel efficiency by executing control processing that is software processing.
Here, with reference to FIG. 2A, the hardware configuration used by the control device 70 to execute software processing will be described. FIG. 2A is a hardware configuration diagram illustrating a configuration example of the control device 70.
The control device 70 shown in FIG. 2A includes an execution unit 70a configured by a CPU (Central Processing Unit), a storage unit 70b configured by a RAM (Random Access Memory), and an AD converter (Analog-to-Digital). The input / output unit 70c is configured. The software processing is realized by the execution unit 70a reading the program stored in the storage unit 70b and performing calculations according to the execution procedure of the software processing represented by the read program. In addition, the calculation result which the execution part 70a performed is written in the memory | storage part 70b. Further, as necessary, the input / output unit 70c inputs a signal input by the detection device 20a, 52a, 60a, or the like as a calculation target, and outputs a calculation result to the fuel supply device 12, the flow rate adjustment valve 15, or the like.

Next, with reference to FIG. 2B, the configuration of the control device 70 will be described focusing on functions. FIG. 2B is a functional block diagram illustrating a configuration example of the control device 70.
The control device 70 includes an acquisition unit 71, an optimal injection rate storage unit 72, a hydrogen concentration calculation unit 73, an internal EGR amount calculation unit 74, a hydrogen concentration recalculation unit 75, an optimal total EGR rate storage unit 76, and an optimal external EGR amount selection unit. 77, an optimal injection amount selection unit 78, an injection control unit 79a, and an EGR rate control unit 79b.

The acquisition unit 71 is realized by the execution unit 70a executing the acquisition process. The acquisition unit 71 acquires signals output from the detection devices 20a, 52a, and 60a.
The optimal injection rate storage unit 72 includes, for example, a storage unit 70b. As shown in FIG. 3A, the optimum injection rate storage unit 72 stores the bed temperature of the reforming catalyst and the optimum injection rate of the reforming catalyst at that temperature in association with each other. The optimal injection rate is the ratio of the fuel injection amount that can be reformed by the reforming catalyst in the external EGR amount supplied to the reforming catalyst.

  The hydrogen concentration calculation unit 73 is realized by the execution unit 70a executing the hydrogen concentration calculation process. The hydrogen concentration calculation unit 73 searches for the optimal injection rate that the optimal injection rate storage unit 72 stores in association with the bed temperature of the reforming catalyst acquired by the acquisition unit 71 from the detection device 20a. Next, the hydrogen concentration calculation unit 73 calculates the amount of hydrogen generated by the reforming catalyst based on the searched optimum injection rate and the external EGR amount controlled by the EGR rate control unit 79b. Thereafter, the hydrogen concentration calculation unit 73 calculates the hydrogen concentration by dividing the calculated hydrogen amount by the external EGR amount.

  The internal EGR amount calculation unit 74 is realized by the execution unit 70a executing an internal EGR amount calculation process. The internal EGR amount calculation unit 74 calculates the amount of exhaust gas that the engine 60 circulates internally (that is, the internal EGR amount) based on the advance value of the VVT included in the engine 60. Specifically, the internal EGR amount calculation unit 74 calculates the internal EGR amount of the engine 60 based on the opening / closing timing of the exhaust valve provided in the engine 60 and the opening / closing timing of the intake valve. More specifically, the internal EGR amount calculation unit 74 calculates the volume of the combustion chamber when the exhaust valve is closed and the temperature of the exhaust gas acquired by the acquisition unit 71 from the detection device 52a based on the advance value of the VVT. Based on the above, the internal EGR amount is calculated.

  The hydrogen concentration recalculation unit 75 is realized by the execution unit 70a executing the hydrogen concentration recalculation process. The hydrogen concentration recalculation unit 75 divides the hydrogen amount calculated by the hydrogen concentration calculation unit 73 by the total EGR amount obtained by adding the internal EGR amount calculated by the internal EGR amount calculation unit 74 to the external EGR amount, thereby regenerating the hydrogen concentration. calculate.

  The optimum total EGR rate storage unit 76 includes, for example, a storage unit 70b. The optimal total EGR rate storage unit 76 is the optimal total EGR when the total EGR of the hydrogen concentration is returned to the engine 60 mounted on the vehicle in the traveling state, the hydrogen concentration of the total EGR, and the vehicle in the traveling state. The rate is stored in association with the rate. Note that the optimal total EGR rate refers to the total EGR rate at which the fuel efficiency is optimal.

Here, with reference to FIG.3 (b), the relationship between a fuel consumption, hydrogen concentration, and a total EGR rate is demonstrated.
In FIG. 3B, the vertical axis represents the fuel consumption of the engine 60, and the horizontal axis represents the total EGR rate of the engine 60. A curve C0 represents the relationship between the fuel efficiency and the total EGR rate when the hydrogen concentration is 0% (that is, when exhaust gas reforming is not performed). A curve Cmax represents the relationship between the fuel efficiency and the total EGR rate when the hydrogen concentration generated by the reforming of the exhaust gas is the maximum concentration. Furthermore, the curves C2 and C1 between the curves Cmax and C0 have a lower hydrogen concentration than the lower curve (that is, when the internal EGR amount not including the reformed gas in the total EGR amount increases). Represents the relationship between the fuel efficiency and the total EGR rate. The total EGR rate corresponding to the lowest points of the curves C0, C1, C2, and Cmax is the optimal total EGR rate at each hydrogen concentration. Here, as shown in FIG. 3 (b), the optimum total EGR rate of the curve C0 without exhaust gas reforming is greater than the optimum total EGR rate of the curve Cmax that maximizes the hydrogen concentration by reforming the exhaust gas. Is also small. This is because hydrogen gas expands the EGR limit. The optimum total EGR rate varies depending on the design of the exhaust gas reforming system 1 and the vehicle on which the exhaust gas reforming system 1 is mounted, but can be determined by a person skilled in the art through experiments.

  The optimum external EGR amount selection unit 77 is realized by the execution unit 70a executing the optimum external amount EGR selection process. The optimal external EGR amount selection unit 77 selects an external EGR amount (hereinafter simply referred to as an optimal external EGR amount) that provides the best fuel efficiency. Specifically, first, the optimum external EGR amount selection unit 77 specifies the traveling state of the vehicle on which the engine 60 is mounted. More specifically, the optimum external EGR amount selection unit 77 specifies the traveling state of the vehicle based on the rotation speed of the engine 60 acquired by the acquisition unit 71 from the detection device 60a and the load (torque) of the engine 60. . Next, the optimum external EGR amount selection unit 77 searches for the optimum total EGR rate that the optimum total EGR rate storage unit 76 stores in association with the identified running state and the hydrogen concentration recalculated by the hydrogen concentration recalculation unit 75. To do. Thereafter, the optimum external EGR amount selection unit 77 calculates a total EGR amount (hereinafter simply referred to as an optimum total EGR amount) that provides the best fuel efficiency based on the searched optimum total EGR rate and the air amount introduced by the throttle 30. get. Next, the optimum external EGR amount selection unit 77 obtains the optimum external amount EGR by subtracting the internal EGR amount calculated by the internal EGR amount calculation unit 74 from the acquired optimum total EGR amount.

  The optimal injection amount selection unit 78 is realized by the execution unit 70a executing the optimal injection amount selection process. The optimal injection amount selection unit 78 selects the fuel injection amount (hereinafter simply referred to as the optimal injection amount) of the fuel supply device 12 that provides the best fuel efficiency. Specifically, the optimum injection amount selection unit 78 selects the optimum injection amount based on the optimum injection rate searched by the hydrogen concentration calculation unit 73 and the optimum external EGR amount selected by the optimum external EGR amount selection unit 77. .

The injection control unit 79a is realized by the execution unit 70a executing the injection control process. The injection control unit 79a controls the fuel supply device 12 to inject the fuel of the optimal injection amount selected by the optimal injection amount selection unit 78.
The EGR rate control unit 79b is realized by the execution unit 70a executing the EGR rate control process. The EGR rate control unit 79b causes the flow rate adjustment valve 15 to adjust the external EGR amount so that the difference between the optimal total EGR rate searched by the optimal external EGR amount selection unit 77 and the total EGR rate of the engine 60 decreases. .
According to this configuration, not only can the internal circulation amount of the exhaust gas be calculated based on the advance angle of the engine, but also the difference from the total exhaust gas recirculation rate at which the fuel efficiency is optimal based on the calculated internal circulation amount is reduced. As described above, the amount of exhaust gas to be externally circulated can be adjusted. For this reason, it is possible not only to prevent a combustion failure due to an increase in exhaust gas, which is an incombustible gas, but also to improve thermal efficiency and fuel consumption.

Specifically, the EGR rate control unit 79b controls the flow rate adjustment valve 15 so that the total EGR rate of the engine 60 becomes the optimum total EGR rate. More specifically, the EGR rate control unit 79b controls the flow rate adjustment valve 15 so that the external EGR of the optimal external EGR amount selected by the optimal external EGR amount selection unit 77 flows.
According to this configuration, it is possible not only to reliably prevent a combustion failure but also to reliably improve thermal efficiency and fuel consumption.

Next, control processing executed by the control device 70 will be described with reference to FIG. FIG. 4 is a flowchart illustrating an example of a control process executed by the control device 70.
First, the control device 70 determines whether or not the bed temperature of the reforming catalyst satisfies a condition for reforming the exhaust gas (hereinafter referred to as a reformable condition) (step S01). The reformable condition is, for example, a condition that the bed temperature exceeds the temperature at which the exhaust gas can be reformed. When it is determined that the reformable condition is satisfied, the control device 70 executes the process of step S02, and otherwise ends the control process.

  In step S01, when it is determined that the reformable condition is satisfied, the control device 70 calculates the hydrogen concentration from the bed temperature of the reforming catalyst (step S02). Next, control device 70 calculates the internal EGR amount from the VVT advance angle and the exhaust temperature (step S03). Thereafter, the control device 70 recalculates the hydrogen concentration obtained by adding the internal EGR amount (step S04). Next, the control device 70 selects the optimum external EGR amount associated with the recalculated hydrogen concentration (step S05). Thereafter, the control device 70 selects an optimal injection amount that provides the hydrogen concentration calculated in the selected optimal external EGR amount (step S06). Next, the control device 70 controls the flow rate adjustment valve 15 so that the external EGR amount becomes the selected optimal external EGR amount (step S07). Further, the control device 70 controls the fuel supply device 12 so as to inject the fuel of the selected optimal injection amount (step S08). Then, the control apparatus 70 returns to step S01, and repeats the said process.

In FIG. 4, step S02 corresponds to an example of a hydrogen concentration calculation process for realizing the hydrogen concentration calculation unit 73, and step S03 is an example of an internal EGR amount calculation process for realizing the internal EGR amount calculation unit 74. It corresponds to. Step S04 corresponds to an example of a hydrogen concentration recalculation process for realizing the hydrogen concentration recalculation unit 75, and step S05 is an example of an optimal external EGR amount selection process for realizing the optimal external EGR amount selection unit 77. It corresponds to. Further, step S06 corresponds to an example of an optimal injection amount selection process for realizing the optimal injection amount selection unit 78, and step S07 corresponds to an example of an injection control process for realizing the injection control unit 79a, and step S08. Corresponds to an example of an EGR rate control process for realizing the EGR rate control unit 79b.
In this embodiment, the internal EGR amount calculation unit 74 corresponds to an example of a first calculation unit, and the hydrogen concentration recalculation unit 75 corresponds to an example of a second calculation unit.

  Part or all of the functions realized by executing software processing by the control device 70 can be realized by using a hardware circuit.

  A program describing a processing procedure executed by the control device 70 can be provided by being stored and distributed in a magnetic disk, optical disk, semiconductor memory, or other recording medium, or distributed via a network.

  The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the specific embodiments, and various modifications, within the scope of the gist of the present invention described in the claims, It can be changed.

1 ... Exhaust gas reforming system 10 ... External circulation path
11 ... Exhaust gas introduction path 12 ... Fuel supply device
13 ... reformed gas conduit 14 ... cooler
15 ... Flow control valve 20 ... Heat exchanger
20a ... Detection device 21 ... Exhaust passage
22 ... reforming chamber 30 ... throttle
40 ... Fuel injection device 51 ... Intake pipe
52 ... Exhaust pipe 52a ... Detection device
60 ... Engine 60a ... Detection device
70 ... Control device 70a ... Execution unit
70b: Storage unit 70c: Input / output unit
70f ... Bus 71 ... Acquisition part
72 ... Optimal injection amount storage unit 73 ... Hydrogen concentration calculation unit
74: Internal EGR amount calculation unit (first calculation means)
75 ... Hydrogen concentration recalculation unit (second calculation means)
76 ... Optimal EGR amount storage unit 77 ... Optimal external EGR amount selection unit
78 ... Optimal injection amount selection unit 79a ... Injection control unit
79b ... EGR rate control unit

Claims (1)

An external circulation path for externally circulating the exhaust gas discharged to the exhaust pipe by the engine to the intake pipe used by the engine for intake;
A reforming catalyst that generates hydrogen by reforming a mixed gas obtained by mixing an external EGR that is an exhaust gas that is externally circulated by the external circulation path and fuel;
Based on the opening / closing timing of the exhaust valve that opens and closes the exhaust pipe and the opening / closing timing of the intake valve that opens and closes the intake pipe, the amount of internal EGR that is the exhaust gas circulated internally by the engine is calculated. 1 calculating means;
Second calculation means for calculating a hydrogen concentration that is the amount of the hydrogen generated by the reforming catalyst with respect to a total EGR amount obtained by adding the amount of the external EGR and the amount of the internal EGR calculated by the first calculation means. When,
The total exhaust gas recirculation rate, which is the circulation rate between the external EGR and the internal EGR, is the total exhaust gas recirculation rate that provides the best fuel efficiency of the engine determined by the hydrogen concentration calculated by the second calculating means. the exhaust gas reforming system, comprising an adjustment valve that adjusts the amount of the external EGR in which the external circulation path is external circulation.

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