JP6705294B2 - Exhaust gas purification system for internal combustion engine and exhaust gas purification method for internal combustion engine - Google Patents

Description

The present invention relates to an exhaust gas purification system for an internal combustion engine and an exhaust gas purification method for an internal combustion engine.

It is necessary to branch the exhaust pipe between SCRF (filter coated with SCR) and SCR (catalyst of selective catalytic reduction system), and to provide an exhaust control valve and a cooling element in this branch pipe to cool the exhaust gas. In some cases, an exhaust gas purification system for an internal combustion engine has been proposed which controls the temperature of the exhaust gas flowing into the SCR by increasing the amount of the exhaust gas passing through the branch pipe according to the target catalyst temperature (for example, patents). Reference 1).

JP, 2005-86848, A

By the way, in the above exhaust gas purification system, the amount of urea water supplied from the reducing agent injection valve to the SCRF and SCR is set in consideration of the amount of ammonia that flows out (slips) to the exhaust pipe downstream of the SCR. Even under the conditions, the ammonia slips when the SCR temperature rises rapidly or when the amount of ammonia stored in the SCR exceeds the ammonia storage capacity of the SCR even if the temperature does not rise suddenly. I will end up. Therefore, it is necessary to provide an ammonia slip catalyst device on the downstream side of the SCR and purify the ammonia slipped from the SCR to nitrogen by this ammonia slip catalyst device. However, nitrogen oxide (NOx) is a by-product of this purification reaction. However, there is a problem in that the NOx purification performance of the entire exhaust gas purification system deteriorates.

An object of the present invention is to suppress the ammonia slip from the selective reduction catalyst device, while maintaining the NOx purification performance of the entire exhaust gas purification system, the exhaust gas purification system of the internal combustion engine and the exhaust gas purification of the internal combustion engine. To provide a method.

An exhaust gas purifying system for an internal combustion engine according to the present invention to achieve the above object, an exhaust gas of an internal combustion engine configured to include a urea water supply device and a selective reduction catalyst device in the exhaust passage of the internal combustion engine in this order from the upstream side. In the gas purification system, a branch passage having an exhaust gas cooling device is provided in the exhaust passage upstream of the urea water supply device in parallel with the exhaust passage, and a passage switching device is provided at a branch point of the branch passage. In addition, a temperature detecting device is provided inside the selective reduction catalyst device or in the exhaust passage upstream or downstream of the selective reduction catalyst device, and the exhaust passage downstream of the selective reduction catalyst device. When the control device for controlling the exhaust gas purification system is equipped with an ammonia concentration detection device, the detected value of the temperature detection device becomes equal to or higher than a preset cooling start temperature threshold value, or the ammonia concentration detection device When the detection value of the device is equal to or higher than a preset cooling start concentration threshold value, the flow passage switching device is controlled to control the flow of exhaust gas from the exhaust passage to the branch passage. To be done.

Further, an exhaust gas purifying method for an internal combustion engine according to the present invention for achieving the above object, comprises a urea water supply device and a selective reduction type catalyst device in the exhaust passage of the internal combustion engine in this order from the upstream side. Exhaust of an internal combustion engine configured such that a branch passage having an exhaust gas cooling device is provided in the exhaust passage upstream of the device in parallel with the exhaust passage, and a flow path switching device is provided at a branch point of the branch passage. In the gas purification method, when the temperature of the selective reduction catalyst device is equal to or higher than a preset cooling start temperature threshold value, or of the exhaust gas passing through the exhaust passage on the downstream side of the selective reduction catalyst device. When the ammonia concentration exceeds a preset cooling start concentration threshold value, the flow passage switching device is controlled to switch the flow of exhaust gas from the exhaust passage to the branch passage. ..

According to the exhaust gas purification system for an internal combustion engine and the exhaust gas purification method for an internal combustion engine of the present invention, NOx purification performance of the entire exhaust gas purification system is maintained while suppressing ammonia slip from the selective reduction catalyst device. You can

It is a figure which shows the structure of the exhaust gas purification system of the internal combustion engine of this invention. It is a figure which shows the temperature of a selective catalytic reduction apparatus, and the relationship of ammonia storage capacity and NOx purification rate. It is a figure which shows the control flow of the exhaust gas purification method of the internal combustion engine of this invention. It is a figure which shows the structure of the exhaust gas purification system of the internal combustion engine of a prior art.

Hereinafter, an exhaust gas purification system for an internal combustion engine and an exhaust gas purification method for an internal combustion engine according to embodiments of the present invention will be described with reference to the drawings.

As shown in FIG. 1, an exhaust gas purification system 1 of the present invention includes an oxidation catalyst device (DOC) 11, a fine particle collection device 12, and a urea water supply device in an exhaust passage (exhaust pipe) 10 of an engine 2 in order from an upstream side. This is a system that is equipped with a selective reduction catalyst device (SCR) 13.

The oxidation catalyst device 11 is configured such that a base material forming a honeycomb structure carries a noble metal catalyst (oxidation catalyst) that oxidizes hydrocarbons (HC) and carbon monoxide (CO) of the exhaust gas G. As the noble metal catalyst, a platinum (Pt)-based catalyst that oxidizes hydrocarbon into water and carbon dioxide and carbon monoxide into carbon dioxide is preferable.

Since the oxidation reaction of hydrocarbons and carbon monoxide by this noble metal catalyst is an exothermic reaction, the exhaust gas G rises in temperature due to this heat generation. Utilizing this, when a high temperature exhaust gas G is required, such as during forced PM regeneration control of the particulate matter collection device 12, it is included in the exhaust gas G passing through the exhaust passage 10 on the upstream side of the oxidation catalyst device 11. The exhaust gas G is heated to a high temperature by temporarily increasing the amount of hydrocarbons and oxidizing the increased amount of hydrocarbons by the oxidation catalyst device 11.

As a method of temporarily increasing the amount of hydrocarbons, for example, a method of performing post-injection of fuel in the cylinder (cylinder) 2a of the engine 2 or a method of supplying fuel to the exhaust passage 10 on the upstream side of the oxidation catalyst device 11 is performed. There is a method of injecting fuel from this fuel injection device by providing an injection device (not shown).

The particulate matter collecting device 12 is configured to include a filter therein to collect the particulate matter (PM) in the exhaust gas G. This filter is a monolith honeycomb type wall flow type filter in which the inlets and outlets of cells (channels) of a porous ceramic honeycomb are alternately plugged.

Exhaust gas G flows in from the inlet of the unplugged cell of the particulate collection device 12, passes through the PM trapping wall formed at the boundary with the adjacent unplugged cell, and It flows out from the outlet of an unsealed cell. The PM contained in the exhaust gas G is collected by the PM collection wall, but the collection amount is limited. Therefore, before the amount of collected PM reaches the limit value, the high temperature exhaust gas G is passed through the inside of the particulate collection device 12, and the heat of the exhaust gas G collects inside the particulate collection device 12. The forced PM regeneration control for burning and removing the generated PM is regularly performed.

The selective reduction catalyst device 13 reduces ammonia (NH ₃ ) generated by hydrolyzing the urea water U injected from the urea water supply device 15 provided in the exhaust passage 10 at the preceding stage with the heat of the exhaust gas G. The agent is a device that purifies nitrogen oxides (NOx) contained in the exhaust gas G into nitrogen (N ₂ ).

It should be noted that ammonia that is not used for purifying NOx contained in the exhaust gas G is stored inside the selective reduction catalyst device 13 or flows out (slip) to the exhaust passage 10 on the downstream side of the selective reduction catalyst device 13. ) Do. Further, as shown in FIG. 2, the ammonia storage capacity A (upper limit amount of ammonia that can be stored) A of the selective reduction catalyst device 13 decreases as the temperature of the selective reduction catalyst device 13 increases.

Further, a control device 40 that controls the exhaust gas purification system 1 of the present invention is provided. The control device 40 is a device that controls the supply amount and the like of the urea water U from the urea water supply device 15 based on the detection values of various sensors that represent the operating state of the engine 2.

The supply amount of the urea water U from the urea water supply device 15 is controlled by the control device 40 according to the operating state of the engine 2, but precise control of this supply amount is difficult. Therefore, for example, when the operating state of the engine 2 is drastically changed and the urea water supply device 15 excessively supplies the urea water U to the exhaust passage 10, a large amount of generated ammonia is contained in the exhaust gas G. There is a risk that ammonia is discharged to the exhaust passage 10 on the downstream side of the selective reduction catalyst device 13, that is, so-called ammonia slip, which is not used for purifying the exhaust gas and for storing ammonia in the selective reduction catalyst device 13. Further, for example, when the selective reduction catalyst device 13 rapidly rises in temperature as the exhaust gas G rapidly rises in temperature, the ammonia storage capacity of the selective reduction catalyst device 13 sharply decreases. There is a possibility that part of the ammonia stored in the exhaust gas may flow out (slip) to the exhaust passage 10 on the downstream side.

In an exhaust gas purification system 1X for an internal combustion engine according to the related art, as shown in FIG. 4, an ammonia slip catalyst device (having the same structure as the oxidation catalyst device 11) 14 is provided on the downstream side of the selective reduction catalyst device 13. The ammonia flowing out of the selective reduction catalyst device 13 was purified to nitrogen by the ammonia slip catalyst device 14. However, there is a problem that NOx may be produced as a by-product of this purification reaction, and the NOx purification performance of the entire exhaust gas purification system may deteriorate.

Therefore, in the exhaust gas purification system 1 of the present invention, as shown in FIG. 1, a branch passage 20 having an exhaust gas cooling device 21 is arranged in parallel with the exhaust passage 10 on the upstream side of the urea water supply device 15. And a three-way valve (flow path switching device) 22 at the branch point of the branch passage 20. Further, the temperature detection sensor 30 for detecting the temperature of the selective reduction catalyst device 13 is disposed inside the selective reduction catalyst device 13 (at this position in FIG. 1) or exhaust gas on the upstream side or the downstream side of the selective reduction catalyst device 13. In addition to being provided in the passage 10, an ammonia concentration detection sensor 31 is provided in the exhaust passage 10 on the downstream side of the selective reduction catalyst device 13.

In the exhaust gas cooling device 21, a cooling medium such as air, engine cooling water, radiator cooling water, etc. circulates in a cooling medium passage (not shown) therein, and is adjacent to the cooling medium passage. The exhaust gas Gb passing through the exhaust gas passage (not shown) is cooled by a cooling medium.

Regarding the installation position of the temperature detection sensor 30, it is preferable that the temperature detection sensor 30 is provided inside the selective reduction catalyst device 13 as shown in FIG. 1 to directly detect the temperature of the selective reduction catalyst device 13. The temperature detection sensor 30 may be provided in the exhaust passage 10 on the upstream side or the downstream side of the selective reduction catalyst device 13, and the detection value of the temperature detection sensor 30 may be used as the temperature of the selective reduction catalyst device 13.

Then, the control device 40 starts cooling when the detection value T of the temperature detection sensor 30 becomes equal to or higher than the preset cooling start temperature threshold T1 or when the detection value D of the ammonia concentration detection sensor 31 is preset. When the concentration becomes equal to or higher than the threshold value D1, the three-way valve 22 is controlled so that the flow of the exhaust gas G is switched from the exhaust passage 10 to the branch passage 20. This cooling start temperature threshold value T1 is a value at which the ammonia adsorption capacity A to the selective reduction catalyst device 13 is reduced and the ammonia adsorption source performance is reduced when the temperature of the exhaust gas G is further increased. It is a value preset by the above. Further, the cooling start concentration threshold value D1 is a value that is desired to prevent the ammonia slip amount from increasing above this concentration, and is a value set in advance by experiments or the like.

That is, when ammonia slip from the selective reduction catalyst device 13 is detected, or when the temperature T of the selective reduction catalyst device 13 is high and the ammonia storage capacity A is small, the ammonia flows into the selective reduction catalyst device 13. The exhaust gas cooling device 21 cools the entire amount of the exhaust gas G before the operation, and the exhaust gas G whose temperature has been lowered is caused to flow into the selective reduction catalyst device 13, whereby the ammonia storage capacity A is rapidly increased and selected. The ammonia slip from the reduction catalyst device 13 is prevented or prevented.

Therefore, according to this configuration, ammonia slip from the selective reduction catalyst device 13 can be suppressed to an amount that does not require the ammonia slip catalyst device 14 in the exhaust passage 10, and the ammonia slip catalyst device 14 can be omitted. At the same time, since the amount of NOx produced by the ammonia purification reaction in the ammonia slip catalyst device 14 can be reduced, the NOx purification performance of the entire exhaust gas purification system 1 can be maintained.

In the exhaust gas purification system 1 for the internal combustion engine, the control device 40 causes the flow path switching device 22 to operate when the detected value T of the temperature detection sensor 30 becomes equal to or lower than the preset cooling end temperature threshold T2. The flow of the exhaust gas G is controlled to switch from the branch passage 20 to the exhaust passage 10. This cooling end temperature threshold value T2 is a value lower than the cooling start temperature threshold value T1, and when the temperature of the exhaust gas G is further reduced, the temperature of the catalyst carried by the selective reduction catalytic converter 13 becomes too low. , A value at which the NOx reduction performance deteriorates, and is a value set in advance by experiments or the like.

According to this configuration, it is possible to prevent the temperature of the catalyst carried by the selective reduction catalyst device 13 from dropping to a temperature outside the activation temperature range, so that ammonia slip from the selective reduction catalyst device 13 is prevented. The NOx purification performance can be maintained.

As shown in FIG. 2, the cooling start temperature threshold value T1 and the cooling end temperature threshold value T2 are in a temperature range in which the NOx purification rate P of the selective reduction catalytic converter 13 is equal to or higher than the preset NOx purification rate P1. It is more preferable to set within a certain required NOx purification rate range (T2≦T≦T3 in FIG. 2). The NOx purification rate P1 is a value preset by experiments or the like according to the target of NOx purification.

Although the three-way valve 22 is provided as the flow path switching device in FIG. 1, a flow rate adjusting valve (flow rate adjusting device) 22 having not only the flow path switching function but also the flow rate adjusting function may be provided instead. In this case, the exhaust gas G is not passed through either the exhaust passage 10 or the branch passage 20 like the three-way valve 22, but the exhaust gas G is passed through either the exhaust passage 10 or the branch passage 20 or both. It can be distributed.

When the flow of the exhaust gas G between the exhaust passage 10 and the branch passage 20 is instantly switched like the three-way valve 22, the control of the flow passage switching device becomes simple and flows into the selective reduction catalyst device 13. Since the temperature of the exhaust gas G can be rapidly reduced and the ammonia storage capacity A can be immediately increased by the temperature reduction of the selective reduction catalyst device 13, the ammonia slip from the selective reduction catalyst device 13 can be more reliably and reliably performed. It can be prevented quickly.

On the other hand, when the control device 40 switches the flow of the exhaust gas G between the exhaust passage 10 and the branch passage 20, the control device 40 does not switch it immediately like the three-way valve 22, but switches gradually or stepwise. Therefore, it is possible to prevent a sudden change in the temperature of the selective reduction catalyst device 13 and prevent a sudden change in the NOx purification rate. Further, the distribution amount of the exhaust gas G flowing in the exhaust passage 10 and the branch passage 20 can be adjusted and controlled, and the temperature of the exhaust gas G can be finely adjusted.

Further, at the time of this switching, the control device 40 sets the variation amount ΔV of the flow rate of the exhaust gas G passing through the branch passage 20 in accordance with the variation amount ΔD of the detection value of the ammonia concentration detection sensor 31, and the flow rate adjusting valve 22. The exhaust gas cooling device 21 immediately increases the amount of the exhaust gas G cooled by the exhaust gas cooling device 21 even when the ammonia slip amount from the selective reduction catalyst device 13 suddenly increases. By promoting the cooling of G, the ammonia storage capacity A of the selective reduction catalyst device 13 can be increased, so that ammonia slip from the selective reduction catalyst device 13 can be prevented more quickly.

At this time, it is preferable that the amount of the cooling medium passing through the exhaust gas cooling device 21 is larger than the amount of the normal cooling medium, because the cooling of the exhaust gas G can be further promoted.

Next, a control flow of the exhaust gas purifying method for an internal combustion engine of the present invention using the above exhaust gas purifying system 1 for an internal combustion engine will be described with reference to FIG. The control flow of FIG. 3 is a control flow that is called by a higher-level control flow and starts at every control time set in advance while the engine 2 is operating.

When the control flow of FIG. 3 starts, in step S10, the exhaust gas G flowing into the selective reduction catalyst device 13 is cooled by the exhaust gas cooling device 21 to increase the ammonia storage capacity A of the selective reduction catalyst device 13. It is determined whether it is necessary to do. This determination is made based on whether or not the temperature T of the selective reduction catalyst device 13 (detection value of the temperature detection sensor 30) becomes equal to or higher than the cooling start temperature threshold T1, or the exhaust passage 10 on the downstream side of the selective reduction catalyst device 13. This is performed depending on whether or not the ammonia concentration (detection value of the ammonia concentration detection sensor 31) D of the exhaust gas Gc passing through is equal to or higher than the cooling start concentration threshold D1.

In step S10, if the temperature T of the selective reduction catalytic converter 13 is lower than the cooling start temperature threshold T1 and the ammonia concentration D of the exhaust gas Gc is lower than the cooling start concentration threshold D1 (NO), the exhaust gas is exhausted. Assuming that the cooling of the gas G is unnecessary, the process proceeds to the return and the control flow ends.

On the other hand, in step S10, if the temperature T of the selective reduction catalyst device 13 is the cooling start temperature T1 or higher, or if the ammonia concentration D of the exhaust gas Gc is the cooling start concentration threshold D1 or higher (YES). Assuming that the exhaust gas Gc needs to be cooled, the process proceeds to step S20, and in step S20, the flow path switching device 22 is controlled to switch the flow of the exhaust gas G from the exhaust passage 10 to the branch passage 20. .. When the three-way valve 22 is used as the flow passage switching device, the exhaust gas G is switched from the exhaust passage 10 to the branch passage 20 all at once. On the other hand, when the flow rate control valve 22 is used as the flow path switching device, the flow rate of the exhaust gas Ga flowing into the exhaust passage 10 is reduced (while the flow rate of the exhaust gas Gb flowing into the branch passage 22 is increased), The flow of G is gradually switched from the exhaust passage 10 to the branch passage 20. Alternatively, the flow rate of the exhaust gas Gb flowing into the branch passage 20 is increased. After performing the control in step S20, the process proceeds to step S30.

At the time of this switching, if the flow rate adjustment valve 22 is controlled by setting the flow rate variation amount ΔV of the exhaust gas Gb passing through the branch passage 20 in accordance with the variation amount ΔD of the ammonia concentration of the exhaust gas Gc, the selective reduction type Even when the amount of ammonia slip from the catalyst device 13 suddenly increases, the amount of the exhaust gas Gb cooled by the exhaust gas cooling device 21 is immediately increased to accelerate the cooling of the exhaust gas Gb, thereby performing the selective reduction. Since the temperature of the catalytic converter 13 can be lowered and the ammonia storage capacity A can be increased, ammonia slip from the selective reduction catalytic converter 13 can be prevented more quickly.

In step S30, it is determined whether cooling of the exhaust gas G flowing into the selective catalytic reduction device 13 has become unnecessary. This determination is made based on whether or not the temperature T of the selective reduction catalyst device 13 (detection value of the temperature detection sensor 30) becomes equal to or lower than the cooling end temperature threshold T2 which is a value lower than the cooling start temperature threshold T1.

In step S30, when the temperature of the selective reduction catalyst device 13 is higher than the cooling end temperature threshold value T2 (NO), it is determined that the exhaust gas G still needs to be cooled, and the preset control is performed through experiments or the like. After the lapse of time, the determination in step S30 is performed again.

On the other hand, if the temperature of the selective reduction catalyst device 13 becomes equal to or lower than the cooling end temperature threshold T2 in step S30 (YES), the process proceeds to step S40, and the flow path switching device 22 is controlled in step S40. Then, control is performed to switch the flow of the exhaust gas G from the branch passage 20 to the exhaust passage 10. When the three-way valve 22 is used as the flow path switching device, the exhaust gas G is switched from the total amount branch passage 20 to the exhaust passage 10 at once. On the other hand, when the flow rate adjusting valve 22 is used as the flow path switching device, the flow rate of the exhaust gas Gb flowing into the branch passage 20 is decreased (while the flow rate of the exhaust gas Ga flowing into the exhaust passage 10 is increased), and the exhaust gas is reduced. The flow of G is gradually switched from the branch passage 20 to the exhaust passage 10. Alternatively, the flow rate of the exhaust gas Gb flowing into the branch passage 20 is reduced. After performing the control of step S40, the process proceeds to return and the present control flow ends.

As described above, the exhaust gas purification method for an internal combustion engine according to the present invention based on the exhaust gas purification system 1 for an internal combustion engine according to the present invention is applied to the exhaust passage 10 of the engine 2 in order from the upstream side to the urea water supply device 15 and the selective reduction. Type catalyst device 13 is provided, and a branch passage 20 having an exhaust gas cooling device 21 is provided in the exhaust passage 13 upstream of the urea water supply device 15 in parallel with the exhaust passage 10 at a branch point of the branch passage 20. In an exhaust gas purification method for an internal combustion engine configured to include a flow path switching device 22, when the temperature T of the selective reduction catalyst device 13 becomes equal to or higher than a cooling start temperature threshold T1 preset by experiments or the like, or When the ammonia concentration D of the exhaust gas Gc passing through the exhaust passage 10 on the downstream side of the selective reduction catalyst device 13 becomes equal to or higher than the cooling start concentration threshold D1 preset by experiments or the like, the flow path switching device 22 is turned on. The method is characterized by controlling and switching the flow of the exhaust gas G from the exhaust passage 10 to the branch passage 20.

According to the exhaust gas purification system 1 for an internal combustion engine and the exhaust gas purification method for an internal combustion engine of the present invention, NOx purification performance of the entire exhaust gas purification system 1 can be improved while suppressing ammonia slip from the selective reduction catalyst device 13. Can be maintained.

1, 1X Exhaust Gas Purification System for Internal Combustion Engine 2 Engine 10 Exhaust Passage 13 Selective Reduction Catalyst (SCR)
15 Urea water supply device 20 Branch passage 21 Exhaust gas cooling device 22 Three-way valve (flow path switching device), flow rate adjusting valve (flow rate adjusting device)
30 Temperature detection sensor (temperature detection device)
31 Ammonia concentration detection sensor (ammonia concentration detection device)
40 Control Device G Exhaust Gas Ga Exhaust Gas Gb Passing Through Exhaust Passage Exhaust Gas T Passing Through Branch Passage T Temperature of Selective Reduction Type Catalyst Device (Detection Value of Temperature Detection Sensor)
T1 cooling start temperature threshold value T2 cooling end temperature threshold value D detection value of ammonia concentration detection sensor D1 cooling start concentration threshold value ΔD fluctuation amount of detection value of ammonia concentration detection sensor ΔV fluctuation amount of exhaust gas flow rate passing through branch passage

Claims (4)

In an exhaust gas purifying system for an internal combustion engine, which comprises an urea water supply device and a selective reduction type catalyst device in order from the upstream side in an exhaust passage of the internal combustion engine,
In the exhaust passage upstream of the urea water supply device, a branch passage having an exhaust gas cooling device is provided in parallel with the exhaust passage, and a flow path switching device is provided at a branch point of the branch passage,
A temperature detecting device is provided inside the selective reduction catalyst device or in the exhaust passage upstream or downstream of the selective reduction catalyst device, and an ammonia concentration is provided in the exhaust passage downstream of the selective reduction catalyst device. Equipped with a detector,
A control device for controlling the exhaust gas purification system,
When the detection value of the temperature detection device is equal to or higher than a preset cooling start temperature threshold value, or when the detection value of the ammonia concentration detection device is equal to or higher than a preset cooling start concentration threshold value, An exhaust gas purification system for an internal combustion engine configured to control a flow passage switching device to perform control to switch a flow of exhaust gas from the exhaust passage to the branch passage. The control device is
When the detected value of the temperature detection device becomes equal to or lower than the cooling end temperature threshold value preset as a value lower than the cooling start temperature threshold value, the flow path switching device is controlled to change the flow of exhaust gas to the branch passage. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the exhaust gas purification system is configured to perform control to switch from the exhaust passage to the exhaust passage. While configuring the flow path switching device as a flow rate adjusting device capable of flowing exhaust gas in both the exhaust passage or the branch passage,
The control device is
When switching the flow of exhaust gas between the exhaust passage and the branch passage,
The amount of fluctuation of the flow rate of the exhaust gas passing through the branch passage is set according to the amount of fluctuation of the detection value of the ammonia concentration detecting device, and the flow rate adjusting device is controlled. An exhaust gas purification system for an internal combustion engine as described. The exhaust passage of the internal combustion engine is provided with a urea water supply device and a selective reduction type catalyst device in order from the upstream side, and a branch passage having an exhaust gas cooling device is provided in the exhaust passage upstream of the urea water supply device. In parallel with, in the exhaust gas purification method of an internal combustion engine configured to include a flow path switching device at the branch point of the branch passage,
When the temperature of the selective reduction catalyst device becomes equal to or higher than a preset cooling start temperature threshold value, or the ammonia concentration of the exhaust gas passing through the exhaust passage downstream of the selective reduction catalyst device is preset. An exhaust gas purifying method for an internal combustion engine, comprising controlling the flow path switching device to switch the flow of exhaust gas from the exhaust passage to the branch passage when the cooling start concentration threshold is exceeded.

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