JP5716687B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

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

  2. Description of the Related Art An exhaust gas purification device for an internal combustion engine that includes a NOx purification catalyst that purifies nitrogen oxide (NOx) in exhaust gas is known. In such an exhaust purification device, for example, a reducing agent such as urea water is injected toward the exhaust passage. When the injected reducing agent reaches the NOx purification catalyst, it is adsorbed on the NOx purification catalyst as ammonia. The NOx is reduced and purified by ammonia adsorbed on the catalyst.

  Further, in the apparatus described in Patent Document 1, when the storage capacity of the reducing agent in the NOx purification catalyst is determined to be larger than the allowable upper limit capacity, the supply of the reducing agent is prohibited, thereby reducing the reducing agent adsorption amount of the NOx purification catalyst. Is set to an appropriate amount.

JP 2008-255937 A

  By the way, as the temperature of the NOx purification catalyst increases, the amount of adsorbed ammonia decreases and the amount of ammonia desorbed from the NOx purification catalyst increases. Therefore, when the exhaust temperature is likely to be high, such as when the vehicle is running, a sufficient amount of ammonia cannot be adsorbed even if a reducing agent is added, causing a reduction in the NOx purification rate of the NOx purification catalyst, for example. There is a fear.

  The present invention has been made in view of such circumstances, and an object thereof is to provide an exhaust purification device for an internal combustion engine that can suitably perform ammonia adsorption on a NOx purification catalyst.

In the following, means for achieving the above object and its effects are described.
The invention according to claim 1 is an exhaust gas purification apparatus for an internal combustion engine including a NOx purification catalyst that purifies NOx by adding a reducing agent, and the reducing agent is used during a fuel cut in which fuel injection from a fuel injection valve is stopped. When the temperature of the NOx purification catalyst is lower than a predetermined value, the reducing agent is added, and when the temperature of the NOx purification catalyst during fuel cut is equal to or higher than the predetermined value, the exhaust flow rate in the exhaust passage is increased. The gist is to increase it .

  When fuel injection from the fuel injection valve is being performed, NOx is generated in the combustion chamber. When the reducing agent is added in a state where NOx is generated in this way, a part of the reducing agent reacts with NOx in the exhaust gas before reaching the NOx purification catalyst. Therefore, the amount of reducing agent that reaches the NOx purification catalyst is reduced by the amount of reaction with NOx, and the ammonia adsorption amount is reduced by the amount of reduction of the reducing agent.

  On the other hand, NOx is not generated in the combustion chamber during fuel cut in which fuel injection from the fuel injection valve is stopped. Therefore, in this configuration, a reducing agent is added during fuel cut. Thus, the reducing agent added during the fuel cut reaches the NOx purification catalyst as it is without reacting with the NOx in the exhaust gas. Therefore, when the reducing agent is added during the fuel cut, the amount of the reducing agent reaching the NOx purification catalyst is increased compared to the case where the same amount of reducing agent is added during the fuel injection, and the ammonia adsorption amount is also increased. Become more.

Further, during fuel cut, the temperature in the exhaust passage is lower than during fuel injection, so the temperature of the NOx purification catalyst is also lowered, and more ammonia can be adsorbed.
In this way, according to the same configuration, the added reducing agent can be supplied to the NOx purification catalyst more reliably, and the reducing agent is added when more ammonia can be adsorbed. Ammonia adsorption of the catalyst can be performed more suitably.
Further, as described above, as the temperature of the NOx purification catalyst increases, the amount of adsorbed ammonia decreases and the amount of ammonia desorbed from the NOx purification catalyst increases. Therefore, in order to appropriately perform ammonia adsorption on the NOx purification catalyst, it is desirable to add a reducing agent when the temperature of the NOx purification catalyst is lower than a predetermined value, that is, when ammonia can be adsorbed to some extent.
However, for example, when the fuel is cut immediately after a high load operation, the temperature of the NOx purification catalyst may become equal to or higher than the predetermined value. Cannot be added. Therefore, in this configuration, when the temperature of the NOx purification catalyst during the fuel cut is equal to or higher than the predetermined value and the reducing agent cannot be added, the exhaust flow rate in the exhaust passage is increased. When the exhaust gas flow rate is increased during the fuel cut in this way, the amount of fresh air that flows into the exhaust passage substantially increases, so that cooling of the NOx purification catalyst by fresh air is promoted. Therefore, according to the configuration, even when the NOx purification catalyst during fuel cut is at a high temperature, the reducing agent can be added earlier.

  According to a second aspect of the present invention, in the exhaust gas purification apparatus for an internal combustion engine according to the first aspect, the ammonia adsorption amount of the NOx purification catalyst reaches a target adsorption amount set based on the temperature of the NOx purification catalyst. The gist of the invention is that the reducing agent is added and the target adsorption amount set during fuel cut is made larger than the target adsorption amount set during fuel injection.

  As described above, the amount of ammonia adsorbed on the NOx purification catalyst changes according to the temperature of the NOx purification catalyst. Therefore, as in the same configuration, the target adsorption amount of ammonia is set based on the temperature of the NOx purification catalyst, and the reducing agent is added until the ammonia adsorption amount of the NOx purification catalyst reaches the target adsorption amount. A target amount of ammonia can be adsorbed on the catalyst. Here, as described above, during the fuel cut, the ammonia adsorption amount of the NOx purification catalyst increases compared to during the fuel injection. Therefore, in the same configuration, the target adsorption amount set during the fuel cut is set to the fuel adsorption amount. The target adsorption amount set during the injection is increased. Therefore, as compared with the case where the target adsorption amount is not changed in this way, more ammonia can be adsorbed to the NOx purification catalyst during the fuel cut.

According to a third aspect of the present invention, in the exhaust gas purification apparatus for an internal combustion engine according to the first or second aspect , a variable capacity supercharger having a nozzle vane is provided in the exhaust passage of the internal combustion engine. The gist of the invention is that when the temperature of the NOx purification catalyst during fuel cut is equal to or higher than the predetermined value, the opening degree of the nozzle vane is corrected to increase.

  According to this configuration, when the opening degree of the nozzle vane is corrected to be increased, the pressure loss on the exhaust inlet side of the supercharger is reduced, thereby increasing the exhaust flow rate in the exhaust passage.

According to a fourth aspect of the present invention, in the exhaust gas purification apparatus for an internal combustion engine according to the first or second aspect , a throttle valve for adjusting an intake air amount is provided in the intake passage of the internal combustion engine, The gist is that when the temperature of the NOx purification catalyst being cut is equal to or higher than the predetermined value, the opening of the throttle valve is corrected to increase.

  According to this configuration, the opening amount of the throttle valve provided in the intake passage is corrected to increase, so that the amount of fresh air flowing into the exhaust passage during fuel cut increases, thereby The exhaust gas flow rate increases.

As for the temperature of the NOx purification catalyst, as in the fifth aspect of the invention, a configuration is adopted in which the temperature of the exhaust gas flowing into the NOx purification catalyst is used as an approximate value of the temperature of the NOx purification catalyst. It is also possible.

BRIEF DESCRIPTION OF THE DRAWINGS Schematic which shows the internal combustion engine to which this is applied, and its periphery structure about 1st Embodiment of the exhaust gas purification device of the internal combustion engine concerning this invention. The graph which shows the relationship between catalyst temperature and ammonia adsorption amount. The flowchart which shows the procedure about the adsorption process of ammonia in the embodiment. The flowchart which shows the one part procedure about the adsorption process of ammonia in 2nd Embodiment.

(First embodiment)
A first embodiment of an internal combustion engine exhaust gas purification apparatus according to the present invention will be described below with reference to FIGS. Note that “upstream” and “downstream” described in this specification are based on the flow direction of the exhaust gas in the exhaust system.

FIG. 1 is a schematic configuration diagram showing a diesel engine (hereinafter simply referred to as “engine”) to which the exhaust emission control device according to the present embodiment is applied, and a peripheral configuration thereof.
The engine 1 is provided with a plurality of cylinders # 1 to # 4. A plurality of fuel injection valves 4 a to 4 d are attached to the cylinder head 2. These fuel injection valves 4a to 4d inject fuel into the combustion chambers of the cylinders # 1 to # 4. Also, the cylinder head 2 is provided with intake ports for introducing fresh air into the cylinders and exhaust ports 6a to 6d for discharging combustion gas to the outside of the cylinders corresponding to the respective cylinders # 1 to # 4. It has been.

  The fuel injection valves 4a to 4d are connected to a common rail 9 that accumulates high-pressure fuel. The common rail 9 is connected to the supply pump 10. The supply pump 10 sucks fuel in the fuel tank and supplies high-pressure fuel to the common rail 9. The high-pressure fuel supplied to the common rail 9 is injected into the cylinder from the fuel injection valves 4a to 4d when the fuel injection valves 4a to 4d are opened.

  An intake manifold 7 is connected to the intake port. The intake manifold 7 is connected to the intake passage 3. An intake throttle valve 16 for adjusting the intake air amount is provided in the intake passage 3.

An exhaust manifold 8 is connected to the exhaust ports 6a to 6d. The exhaust manifold 8 is connected to the exhaust passage 26.
In the middle of the exhaust passage 26, there is provided a variable capacity turbocharger 11 that supercharges intake air introduced into the cylinders using exhaust pressure. In the turbocharger 11, a nozzle vane 11v for adjusting the exhaust flow rate is provided on the inlet side of the exhaust side turbine, and the opening degree of the nozzle vane 11v is changed according to the engine operating state. For example, by reducing the opening of the nozzle vane 11v in the low load region, the dynamic pressure of the exhaust is increased even when the exhaust flow rate is small. Further, by increasing the opening degree of the nozzle vane 11v in the high load region, the pressure loss of the exhaust gas when the exhaust gas flow rate is large is reduced.

  An intercooler 18 is provided in the intake passage 3 between the intake side compressor of the turbocharger 11 and the intake throttle valve 16. The intercooler 18 cools the intake air whose temperature has risen due to supercharging of the turbocharger 11.

  A first purification member 30 that purifies the exhaust gas is provided in the middle of the exhaust passage 26 and downstream of the exhaust side turbine of the turbocharger 11. Inside the first purification member 30, an oxidation catalyst 31 and a DPF catalyst 32 are arranged in series with respect to the flow direction of the exhaust gas.

  The oxidation catalyst 31 carries a catalyst for oxidizing HC in the exhaust. The DPF catalyst 32 is a filter that collects PM (particulate matter) in the exhaust gas and is composed of a porous ceramic, and further supports a catalyst for promoting oxidation of PM. . The PM in the exhaust gas is collected when it passes through the porous wall of the DPF catalyst 32. The DPF catalyst 32 constitutes the exhaust purification member.

  Further, a fuel addition valve 5 for supplying fuel as an additive to the oxidation catalyst 31 and the DPF catalyst 32 is provided in the vicinity of the collecting portion of the exhaust manifold 8. The fuel addition valve 5 is connected to the supply pump 10 through a fuel supply pipe 27. The position of the fuel addition valve 5 can be changed as appropriate as long as it is in the exhaust system and upstream of the first purification member 30.

  When the amount of PM collected by the DPF catalyst 32 exceeds a predetermined value, regeneration processing of the DPF catalyst 32 is started, and fuel is injected from the fuel addition valve 5 into the exhaust manifold 8. The fuel injected from the fuel addition valve 5 is combusted when it reaches the oxidation catalyst 31, thereby increasing the exhaust temperature. The exhaust gas heated by the oxidation catalyst 31 flows into the DPF catalyst 32, whereby the DPF catalyst 32 is heated, whereby the PM deposited on the DPF catalyst 32 is oxidized to regenerate the DPF catalyst 32. Is planned.

  A second purification member 40 that purifies the exhaust gas is provided in the middle of the exhaust passage 26 and downstream of the first purification member 30. Inside the second purification member 40, a selective reduction type NOx catalyst (hereinafter referred to as an SCR catalyst) 41 is disposed as a NOx purification catalyst that reduces and purifies NOx in the exhaust using a reducing agent.

  Further, a third purification member 50 for purifying exhaust gas is provided in the middle of the exhaust passage 26 and downstream of the second purification member 40. Inside the third purification member 50, an ammonia oxidation catalyst 51 for purifying ammonia in the exhaust is disposed.

  The engine 1 is provided with a urea water supply mechanism 200 as a reducing agent supply mechanism that supplies a reducing agent to the SCR catalyst 41. The urea water supply mechanism 200 includes a tank 210 that stores urea water, a urea addition valve 230 that injects urea water into the exhaust passage 26, a supply passage 240 that connects the urea addition valve 230 and the tank 210, and a supply passage 240. The pump 220 is provided in the middle.

  The urea addition valve 230 is provided in the exhaust passage 26 between the first purification member 30 and the second purification member 40, and the injection hole is opened toward the SCR catalyst 41. When the urea addition valve 230 is opened, urea water is injected and supplied into the exhaust passage 26 via the supply passage 240. The urea addition valve 230 constitutes the reducing agent injection valve.

  The pump 220 is an electric pump, and at the time of forward rotation, the urea water is fed from the tank 210 toward the urea addition valve 230. On the other hand, during reverse rotation, urea water is sent from the urea addition valve 230 toward the tank 210. In other words, during the reverse rotation of the pump 220, urea water is recovered from the urea addition valve 230 and the supply passage 240 and returned to the tank 210.

  A dispersion plate 60 is provided in the exhaust passage 26 between the urea addition valve 230 and the SCR catalyst 41 to promote atomization of the urea water by dispersing the urea water injected from the urea addition valve 230. It has been.

  When urea water injected from the urea addition valve 230 reaches the SCR catalyst 41, it is adsorbed as ammonia. Then, NOx is reduced and purified by the ammonia adsorbed on the SCR catalyst 41.

  In addition, the engine 1 is provided with an exhaust gas recirculation device (hereinafter referred to as an EGR device). This EGR device is a device that reduces the combustion temperature in the cylinder by introducing a part of the exhaust gas into the intake air, thereby reducing the amount of NOx generated. This exhaust gas recirculation device includes an EGR passage 13 that communicates the intake passage 3 and the exhaust manifold 8, an EGR valve 15 provided in the EGR passage 13, an EGR cooler 14, and the like. By adjusting the opening degree of the EGR valve 15, the exhaust gas recirculation amount introduced into the intake passage 3 from the exhaust passage 26, that is, the so-called external EGR amount is adjusted. Further, the temperature of the exhaust gas flowing through the EGR passage 13 is lowered by the EGR cooler 14.

  Various sensors for detecting the engine operation state are attached to the engine 1. For example, the air flow meter 19 detects the intake air amount GA in the intake passage 3. The throttle valve opening sensor 20 detects the opening of the intake throttle valve 16. The engine rotation speed sensor 21 detects the rotation speed of the crankshaft, that is, the engine rotation speed NE. The accelerator sensor 22 detects an accelerator pedal depression amount, that is, an accelerator operation amount ACCP. The outside air temperature sensor 23 detects the outside air temperature THout. The vehicle speed sensor 24 detects the vehicle speed SPD of the vehicle on which the engine 1 is mounted. The ignition switch 25 detects a start operation and a stop operation of the engine 1 by a vehicle driver.

  The first exhaust temperature sensor 100 provided upstream of the oxidation catalyst 31 detects the first exhaust temperature TH1 that is the exhaust temperature before flowing into the oxidation catalyst 31. The differential pressure sensor 110 detects the pressure difference ΔP between the exhaust pressure upstream and downstream of the DPF catalyst 32.

  A second exhaust temperature sensor 120 and a first NOx sensor 130 are provided in the exhaust passage 26 between the first purification member 30 and the second purification member 40 and upstream of the urea addition valve 230. The second exhaust temperature sensor 120 detects a second exhaust temperature TH2, which is the exhaust temperature before flowing into the SCR catalyst 41. The first NOx sensor 130 detects a first NOx concentration N1, which is the NOx concentration in the exhaust before flowing into the SCR catalyst 41.

  The exhaust passage 26 downstream of the third purification member 50 is provided with a second NOx sensor 140 that detects a second NOx concentration N2 that is the NOx concentration of the exhaust purified by the SCR catalyst 41.

  Outputs from these various sensors are input to the control device 80. The control device 80 includes a central processing control device (CPU), a read-only memory (ROM) that stores various programs and maps in advance, a random access memory (RAM) that temporarily stores CPU calculation results, a timer counter, an input The microcomputer is mainly configured with an interface, an output interface, and the like.

  Then, the controller 80 controls, for example, the fuel injection amount control / fuel injection timing control of the fuel injection valves 4a to 4d and the fuel addition valve 5, the discharge pressure control of the supply pump 10, and the drive amount of the actuator 17 that opens and closes the intake throttle valve 16. Various controls of the engine 1 such as control, opening control of the EGR valve 15 and opening control of the nozzle vane 11v are performed.

The exhaust gas purification control such as the regeneration process for burning the PM collected by the DPF catalyst 32 is also performed by the controller 80.
The control device 80 also performs urea water addition control by the urea addition valve 230 as one of such exhaust gas purification controls. In this addition control, the urea addition amount QE necessary for reducing the NOx discharged from the engine 1 is calculated based on the engine operating state and the like, and the calculated urea addition amount QE is injected from the urea addition valve 230. Thus, the valve opening state of the urea addition valve 230 is controlled.

By the way, the ammonia adsorption amount of the SCR catalyst 41 varies depending on the temperature of the SCR catalyst 41 (approximately, the second exhaust temperature TH2) and the state of fuel injection.
FIG. 2 shows the relationship between the temperature of the SCR catalyst 41 and the ammonia adsorption amount. Note that a line L1 shown in FIG. 2 indicates the amount of ammonia adsorbed when fuel injection from the fuel injection valves 4a to 4d is being performed, and a line L2 indicates that fuel injection from the fuel injection valves 4a to 4d is stopped. The amount of adsorbed ammonia when the fuel is cut, that is, when the fuel cut is performed.

  As shown in FIG. 2, the higher the second exhaust temperature TH2, that is, the higher the temperature of the SCR catalyst 41, the smaller the amount of ammonia adsorbed to the SCR catalyst 41, and adsorbs ammonia when the adsorption limit temperature UG is exceeded. Can not do.

  Further, when fuel injection from the fuel injection valves 4a to 4d is being executed, NOx is generated in the combustion chamber of each cylinder. When urea water is added in a state where NOx is generated in this way, a part of the urea water reacts with NOx in the exhaust gas before reaching the SCR catalyst 41 to purify the NOx. Therefore, the amount of urea water reaching the SCR catalyst 41 is reduced by the amount of reaction with NOx, and the amount of ammonia adsorption is reduced by the amount of reduction of the urea water.

  On the other hand, NOx is not generated in the combustion chamber during the fuel cut in which the fuel injection from the fuel injection valves 4a to 4d is stopped. Therefore, when urea water is added during fuel cut, the added urea water reaches the SCR catalyst 41 as it is without reacting with NOx in the exhaust gas. Therefore, when urea water is added during fuel cut, the amount of urea water reaching the SCR catalyst 41 is larger than when urea water of the same amount is added during fuel injection. Therefore, even when the temperature of the SCR catalyst 41 is the same, when the urea water is added during the fuel cut, the ammonia adsorption amount is larger than when the same amount of urea water is added during the fuel injection. (The amount of adsorbed ammonia increases by the amount of increased adsorption KZ shown in FIG. 2).

  Therefore, in the present embodiment, the ammonia adsorption process shown in FIG. 3 is performed in order to perform the urea water addition control in consideration of the ammonia adsorption amount that varies depending on the temperature of the SCR catalyst 41 and the fuel injection state. Yes.

  Hereinafter, the ammonia adsorption process will be described with reference to FIG. This process is repeatedly executed by the control device 80 at predetermined intervals. In this process, the second exhaust temperature TH2 is used as an approximate value of the temperature of the SCR catalyst 41. However, the temperature of the SCR catalyst 41 may be directly detected by a temperature sensor.

  When this process is started, first, the urea addition amount QE is calculated based on the second exhaust temperature TH2 and the intake air amount GA (S100). This urea addition amount is an addition amount of urea water that is sufficient for reducing NOx, and is an addition amount necessary per unit time. As the engine load is higher and the second exhaust temperature TH2 is higher, the amount of NOx discharged from the combustion chamber per unit time tends to increase. In addition, as the intake air amount GA increases, the amount of NOx discharged from the combustion chamber per unit time tends to increase. Therefore, the urea addition amount QE is variably set so that the urea addition amount QE increases as the second exhaust temperature TH2 increases or the intake air amount GA increases.

  Next, it is determined whether or not the fuel injection from the fuel injection valves 4a to 4d is stopped, that is, whether or not the fuel is being cut (S110). As is well known, such fuel cut is performed, for example, when the engine 1 is decelerated.

  When the fuel injection from the fuel injection valves 4a to 4d is being executed and the fuel is not being cut (S110: NO), the first target adsorption amount NHp1 is set based on the second exhaust temperature TH2 (S120). ). This first target adsorption amount NHp1 is a target value of the ammonia adsorption amount of the SCR catalyst 41 during fuel injection, and as the second exhaust temperature TH2 increases, the first target adsorption amount NHp1 increases as the second exhaust temperature TH2 increases. The target adsorption amount NHp1 is reduced.

  Next, it is determined whether the ammonia adsorption amount NHr of the SCR catalyst 41 is smaller than the first target adsorption amount NHp1 (S130). This ammonia adsorption amount NHr is estimated by an appropriate method. For example, the ammonia adsorption amount NHr is estimated based on parameters correlated with the ammonia adsorption amount, such as urea addition amount, exhaust temperature, exhaust flow rate, and the like.

  When the ammonia adsorption amount NHr is smaller than the first target adsorption amount NHp1 (S130: YES), urea addition is executed to increase the ammonia adsorption amount NHr of the SCR catalyst 41 (S140). Is terminated.

On the other hand, when the ammonia adsorption amount NHr is equal to or greater than the first target adsorption amount NHp1 (S130: YES), urea addition is stopped (S150), and this process is temporarily terminated.
On the other hand, when it is determined in step S110 that the fuel is being cut (S110: YES), the second target adsorption amount NHp2 is set based on the second exhaust temperature TH2 (S160). The second target adsorption amount NHp2 is a target value of the ammonia adsorption amount of the SCR catalyst 41 during fuel cut, and the second target adsorption amount NHp2 increases as the second exhaust temperature TH2 increases as in the case of the line L2 shown in FIG. The target adsorption amount NHp2 is reduced. Even at the same second exhaust temperature TH2, the second target adsorption amount NHp2 is made larger than the first target adsorption amount NHp1.

  Next, it is determined whether or not the ammonia adsorption amount NHr of the SCR catalyst 41 is smaller than the second target adsorption amount NHp2 (S170). This ammonia adsorption amount NHr is the same as the ammonia adsorption amount NHr described in step S130.

  When the ammonia adsorption amount NHr is smaller than the second target adsorption amount NHp2 (S170: YES), urea addition is executed to increase the ammonia adsorption amount NHr of the SCR catalyst 41 (S180). Is terminated.

On the other hand, when the ammonia adsorption amount NHr is equal to or larger than the second target adsorption amount NHp2 (S170: YES), urea addition is stopped (S190), and this process is temporarily terminated.
Next, the operation of this embodiment will be described.

  In the present embodiment, urea water is added during the fuel cut in which the fuel injection from the fuel injection valves 4a to 4d is stopped. Thus, the urea water added during the fuel cut reaches the SCR catalyst 41 as it is without reacting with NOx in the exhaust gas. Therefore, the amount of urea water that reaches the SCR catalyst 41 increases, and thereby the amount of ammonia adsorbed on the SCR catalyst 41 increases.

  Further, since the temperature in the exhaust passage 26 is lower during fuel cut than during fuel injection, the temperature of the SCR catalyst 41 is also lower. Therefore, more ammonia can be adsorbed.

  Further, as described above, the amount of ammonia adsorbed on the SCR catalyst 41 varies according to the temperature of the SCR catalyst 41. Therefore, in the present embodiment, the target adsorption amount of ammonia is set based on the second exhaust temperature TH2, which is an approximate value of the temperature of the SCR catalyst 41, and urea is used until the ammonia adsorption amount NHr of the SCR catalyst 41 reaches the target adsorption amount. Water is added so that ammonia corresponding to the target adsorption amount is adsorbed on the SCR catalyst 41. Here, as described above, during the fuel cut, the amount of ammonia adsorbed by the SCR catalyst 41 increases as compared to during fuel injection. Therefore, in the present embodiment, the target adsorption amount (second target adsorption amount NHr2) set during fuel cut is made larger than the target adsorption amount (first target adsorption amount NHp1) set during fuel injection. . Therefore, as compared with the case where the target adsorption amount is not changed in this way, more ammonia is adsorbed to the SCR catalyst 41 during the fuel cut.

As described above, according to the present embodiment, the following effects can be obtained.
(1) The urea water is added during the fuel cut in which the fuel injection from the fuel injection valves 4a to 4d is stopped. Therefore, when urea water is added during fuel cut, the amount of urea water reaching the SCR catalyst 41 is larger than when adding the same amount of urea water during fuel injection. Ammonia adsorption amount also increases. Further, during the fuel cut, the temperature in the exhaust passage 26 is lower than during the fuel injection, so the temperature of the SCR catalyst 41 is also lower, and more ammonia can be adsorbed. As described above, according to the present embodiment, the added urea water can be supplied to the SCR catalyst 41 more reliably and the urea water is added when more ammonia can be adsorbed. The ammonia adsorption of the catalyst 41 can be performed more suitably.

(2) The urea water is added until the ammonia adsorption amount NHr of the SCR catalyst 41 reaches the target adsorption amount set based on the second exhaust temperature TH2, which is an approximate value of the temperature of the SCR catalyst 41. . The target adsorption amount (second target adsorption amount NHr2) set during the fuel cut is made larger than the target adsorption amount (first target adsorption amount NHr1) set during fuel injection. Therefore, as compared with the case where the target adsorption amount is not changed in this way, more ammonia can be adsorbed to the SCR catalyst 41 during the fuel cut.
(Second Embodiment)
Next, a second embodiment in which the exhaust gas purification apparatus for an internal combustion engine according to the present invention is embodied will be described with reference to FIG.

  As described above, the amount of ammonia adsorbed decreases as the temperature of the SCR catalyst 41 increases, and when the temperature exceeds a certain threshold value α (for example, the adsorption limit temperature UG), ammonia cannot be adsorbed. Therefore, in order to appropriately adsorb ammonia on the SCR catalyst 41, it is desirable to add urea water when the temperature of the SCR catalyst 41 is lower than the threshold value α, that is, when ammonia can be adsorbed to some extent.

  However, for example, at the time of fuel cut immediately after high-load operation, the temperature of the SCR catalyst 41 may become equal to or higher than the threshold value α. In this case, urea water is added until the temperature of the SCR catalyst 41 becomes lower than the threshold value α. I can't do it.

  Therefore, in the present embodiment, urea water is added when the temperature of the SCR catalyst 41 is lower than the threshold value α. When the temperature of the SCR catalyst 41 during the fuel cut is equal to or higher than the threshold value α, the exhaust flow rate in the exhaust passage 26 is increased, so that even if the SCR catalyst 41 during the fuel cut is hot, it is earlier. In this way, urea water can be added.

  This embodiment can be realized by changing a part of the ammonia adsorption process shown in FIG. 3, more specifically, by changing the process after step S170. Therefore, in the following, the ammonia adsorption process in the present embodiment will be described focusing on the changes from the adsorption process described in the first embodiment.

FIG. 4 shows a part of the procedure for the adsorption process of ammonia in the present embodiment.
As shown in FIG. 4, when it is determined in step S170 in FIG. 3 that the ammonia adsorption amount NHr of the SCR catalyst 41 is smaller than the second target adsorption amount NHp2 (S170: YES), the second It is determined whether or not the exhaust gas temperature TH2 is higher than the threshold value α (S200). As the threshold value α, for example, the adsorption limit temperature UG can be set.

  When the second exhaust temperature TH2 is higher than the threshold value α (S200: YES), it is determined that the temperature of the SCR catalyst 41 is excessively high and ammonia cannot be adsorbed appropriately. Next, an exhaust flow rate increasing process is performed (S210), and this process is temporarily terminated. In the exhaust gas flow rate increasing process, the opening degree of the nozzle vane 11v is corrected to increase. When the opening degree of the nozzle vane 11v is corrected to be increased in this way, the pressure loss on the exhaust inlet side of the turbocharger 11 is reduced, and thereby the exhaust flow rate in the exhaust passage 26 is increased.

  On the other hand, when the second exhaust temperature TH2 is lower than the threshold value α (S200: NO), it is determined that the temperature of the SCR catalyst 41 is a temperature at which ammonia can be adsorbed appropriately. Next, urea addition is executed (S180), and this process is temporarily terminated.

  On the other hand, when it is determined in step S170 in FIG. 3 that the ammonia adsorption amount NHr of the SCR catalyst 41 is equal to or larger than the second target adsorption amount NHp2 (S170: NO), urea addition is stopped (S190). ), This process is temporarily terminated.

Next, the operation of this embodiment will be described.
In the present embodiment, urea addition is performed when the second exhaust temperature TH2 is low below the threshold value α. When it is determined that the second exhaust temperature TH2 during the fuel cut is equal to or higher than the threshold value α and the temperature of the SCR catalyst 41 is excessively high, a process for increasing the exhaust flow rate in the exhaust passage 26 is performed. When the exhaust gas flow rate is increased during the fuel cut in this way, the amount of fresh air flowing into the exhaust passage 26 substantially increases, so that the SCR catalyst 41 is cooled by the fresh air and the second exhaust temperature TH2 is increased. Decrease is promoted. Therefore, the time until the second exhaust gas temperature TH2 becomes equal to or lower than the threshold value α is shortened, so that urea water is added early.

As described above, according to the present embodiment, the following effects can be obtained in addition to the effects described in (1) and (2) above.
(3) The urea water is added when the second exhaust temperature TH2, which is an approximate value of the temperature of the SCR catalyst 41, is lower than the threshold value α. When the second exhaust temperature TH2 during fuel cut is equal to or higher than the threshold value α, an increase process for increasing the exhaust flow rate in the exhaust passage 26 is performed. Therefore, even when the SCR catalyst 41 during fuel cut is at a high temperature, the urea water can be added earlier.

  (4) When the second exhaust temperature TH2 during fuel cut is equal to or higher than the threshold value α, the opening degree of the nozzle vane 11v is corrected to be increased. Therefore, the exhaust flow rate in the exhaust passage 26 can be actually increased.

In addition, each said embodiment can also be changed and implemented as follows.
In the second embodiment, the opening degree of the nozzle vane 11v is corrected to be increased during the exhaust gas flow rate increasing process, but the exhaust gas flow rate may be increased in another manner. For example, when the opening degree of the intake throttle valve 16 can be increased, the opening degree may be corrected to increase. In this case, since the amount of fresh air flowing into the exhaust passage 26 during the fuel cut increases, the exhaust flow rate in the exhaust passage 26 also increases.

  Although urea water is used as the reducing agent, other reducing agents may be used.

  DESCRIPTION OF SYMBOLS 1 ... Engine, 2 ... Cylinder head, 3 ... Intake passage, 4a-4d ... Fuel injection valve, 5 ... Fuel addition valve, 6a-6d ... Exhaust port, 7 ... Intake manifold, 8 ... Exhaust manifold, 9 ... Common rail, DESCRIPTION OF SYMBOLS 10 ... Supply pump, 11 ... Turbocharger, 11v ... Nozzle vane, 13 ... EGR passage, 14 ... EGR cooler, 15 ... EGR valve, 16 ... Intake throttle valve, 17 ... Actuator, 18 ... Intercooler, 19 ... Air flow meter, 20 ... throttle valve opening sensor, 21 ... engine rotation speed sensor, 22 ... accelerator sensor, 23 ... outside air temperature sensor, 24 ... vehicle speed sensor, 25 ... ignition switch, 26 ... exhaust passage, 27 ... fuel supply pipe, 30 ... first Purification member, 31 ... oxidation catalyst, 32 ... filter, 40 ... second purification member, 41 ... NOx purification catalyst (selective return) Type NOx catalyst: SCR catalyst), 50 ... third purification member, 51 ... ammonia oxidation catalyst, 60 ... dispersion plate, 80 ... control device, 100 ... first exhaust temperature sensor, 110 ... differential pressure sensor, 120 ... second exhaust Temperature sensor, 130 ... first NOx sensor, 140 ... second NOx sensor, 200 ... urea water supply mechanism, 210 ... tank, 220 ... pump, 230 ... urea addition valve, 240 ... supply passage.

Claims (5)

In an exhaust gas purification apparatus for an internal combustion engine including a NOx purification catalyst that purifies NOx by adding a reducing agent,
Adding the reducing agent during fuel cut when fuel injection from the fuel injection valve is stopped ,
When the temperature of the NOx purification catalyst is lower than a predetermined value, the reducing agent is added, and when the temperature of the NOx purification catalyst during fuel cut is equal to or higher than the predetermined value, the exhaust flow rate in the exhaust passage is increased. An exhaust gas purification apparatus for an internal combustion engine characterized by the above. The reducing agent is added until the ammonia adsorption amount of the NOx purification catalyst reaches a target adsorption amount set based on the temperature of the NOx purification catalyst, and the target adsorption amount set during the fuel cut is The exhaust emission control device for an internal combustion engine according to claim 1, wherein the exhaust gas purification device is set to be larger than the target adsorption amount set during injection. The exhaust passage of the internal combustion engine is provided with a variable capacity supercharger provided with a nozzle vane. When the temperature of the NOx purification catalyst during fuel cut is equal to or higher than the predetermined value, the opening degree of the nozzle vane is The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2 , wherein the exhaust gas purification apparatus is corrected for increase. The intake passage of the internal combustion engine is provided with a throttle valve that regulates the amount of intake air. When the temperature of the NOx purification catalyst during fuel cut is equal to or higher than the predetermined value, the opening of the throttle valve increases. The exhaust emission control device for an internal combustion engine according to claim 1 or 2 , which is corrected. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 2 to 4 , wherein the temperature of the exhaust gas flowing into the NOx purification catalyst is used as an approximate value of the temperature of the NOx purification catalyst.