JP2010053703A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device capable of realizing excellent exhaust emission control efficiency by adsorbing a proper quantity of ammonia to an SCR catalyst, while restraining sedimentation of solid matter such as urea generated from urea water. <P>SOLUTION: An ECU 56 executes adsorption control for controlling a urea water injector 46 so that a required quantity of ammonia for selective reduction in NOx is supplied to an ammonia selective reduction type NOx catalyst 42, while adsorbing the ammonia to the ammonia selective reduction type NOx catalyst 42 so that an actual adsorption quantity Qa in the ammonia selective reduction type NOx catalyst 42 becomes a target adsorption quantity Qt. In the adsorption control, the urea water injector 46 is controlled so as to reduce a urea water supply quantity for increasing the actual adsorption quantity Qa toward the target adsorption quantity Qt in response to an increase in the actual adsorption quantity Qa toward the target adsorption quantity Qt. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an exhaust purification device for purifying engine exhaust, and more particularly, to an ammonia selective reduction type NOx catalyst that selectively reduces NOx in exhaust using ammonia generated from urea water supplied in the exhaust as a reducing agent. The present invention relates to an exhaust emission control device.

  The exhaust of the engine contains NOx (nitrogen oxide) as one of the pollutants. As an exhaust purification device for purifying the exhaust by removing this NOx from the exhaust, it is provided in the exhaust passage of the engine. 2. Description of the Related Art An exhaust gas purification apparatus is known in which an ammonia selective reduction type NOx catalyst (hereinafter referred to as an SCR catalyst) is disposed, and ammonia is supplied to the SCR catalyst as a reducing agent to reduce NOx and purify exhaust gas. .

  In such an exhaust purification device, in order to supply ammonia to the SCR catalyst, urea water, which is easier to handle than ammonia, is generally supplied into the exhaust gas, and the exhaust gas is exhausted using a urea water injector or the like. The urea water is injected into the tank. The atomized urea water supplied into the exhaust gas from the urea water injector is hydrolyzed by the heat of the exhaust gas, and the resulting ammonia is supplied to the SCR catalyst. Then, the ammonia supplied to the SCR catalyst is used as a reducing agent, and NOx in the exhaust gas is selectively reduced by the SCR catalyst, thereby purifying the exhaust gas.

  When the exhaust gas temperature is relatively low in the temperature range where the SCR catalyst is activated, most of the ammonia supplied to the SCR catalyst is once adsorbed by the SCR catalyst, and NOx is selectively reduced mainly using the adsorbed ammonia. . While there is a limit to the amount of ammonia adsorbed on the SCR catalyst, the purification rate of the SCR catalyst changes depending on the amount of ammonia adsorbed, and as the ammonia adsorption amount increases, the purification rate of the SCR catalyst tends to improve. If the supply amount is not adjusted appropriately, there arises a problem that ammonia flows out from the SCR catalyst due to excessive supply of ammonia, that is, ammonia slip occurs, or the exhaust purification rate of the SCR catalyst cannot be maintained satisfactorily. Therefore, in order to solve such a problem, it is necessary to appropriately control the amount of ammonia adsorbed on the SCR catalyst. For example, Patent Document 1 proposes an exhaust purification device that performs such control. .

In the exhaust purification device of this Patent Document 1, the ammonia adsorption amount in the SCR catalyst is accurately estimated, and the ammonia adsorption amount in the SCR catalyst is appropriately controlled, thereby preventing the occurrence of ammonia slip and the SCR catalyst. The exhaust gas purification rate is maintained at a good level.
JP 2003-314256 A

  However, if urea water is supplied into the exhaust when the exhaust temperature is relatively low, the urea water is not easily vaporized in the exhaust, and the urea water atomized in the exhaust is applied to the inner wall surface of the exhaust passage. Since the temperature drops further when touched, the urea water atomized in the exhaust gas becomes liquid again and adheres to the inner wall surface of the exhaust passage, and solid matter such as urea generated from the urea water by evaporation of water ( (Hereinafter collectively referred to as urea solids).

  Therefore, when the exhaust gas temperature is relatively low, in order to make the SCR catalyst adsorb ammonia to the urea water supply amount necessary for the selective reduction of NOx in the SCR catalyst in order to make the ammonia adsorption amount to the SCR catalyst appropriate. When the urea water supply amount is added and the urea water supply amount is increased, the urea water supply amount increases, so that there is a problem that the urea solids as described above are likely to be deposited. When urea solid matter accumulates in the exhaust passage or the like, the engine operating efficiency is reduced with an increase in exhaust flow resistance, and the amount of ammonia supplied to the SCR catalyst is reduced, resulting in a reduction in the exhaust purification rate of the SCR catalyst. Occurs.

Further, in order to solve such problems, it is not possible to adsorb a sufficient amount of ammonia to the SCR catalyst by simply reducing the urea water supply amount, and the exhaust of the SCR catalyst in the selective reduction of NOx. There arises a problem that the purification efficiency is lowered.
The present invention has been made in view of such problems, and the object of the present invention is to adsorb an appropriate amount of ammonia to the SCR catalyst while suppressing deposition of urea solids generated from urea water. Another object of the present invention is to provide an exhaust emission control device capable of realizing good exhaust emission purification efficiency.

  In order to achieve the above object, an exhaust emission control device of the present invention includes an ammonia selective reduction type NOx catalyst that selectively reduces NOx in engine exhaust using ammonia as a reducing agent, and an upstream side of the ammonia selective reduction type NOx catalyst. Urea water supply means for supplying urea water into the exhaust, and the ammonia selective reduction type NOx so that the actual adsorption amount, which is the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst, becomes a predetermined target adsorption amount. Control means for performing adsorption control for controlling the urea water supply means so that ammonia necessary for the selective reduction of NOx is supplied to the ammonia selective reduction type NOx catalyst while adsorbing ammonia to the catalyst; And the control means is configured to increase the actual adsorption amount toward the target adsorption amount when executing the adsorption control. The urea water supply means is controlled so that the supply amount of urea water into the exhaust gas for increasing the actual adsorption amount toward the target adsorption amount is decreased (Claim 1). .

  According to the exhaust gas purification apparatus configured as described above, the control unit performs adsorption control to control the urea water supply unit, so that urea is introduced into the exhaust gas from the urea water supply unit upstream of the ammonia selective reduction type NOx catalyst. Water is supplied. The urea water in the exhaust gas supplied in this way is hydrolyzed by the heat of the exhaust gas to generate ammonia. The produced ammonia flows into the ammonia selective reduction type NOx catalyst together with the exhaust gas. In the ammonia selective reduction type NOx catalyst, a part of the ammonia in the exhaust gas that has flowed in is temporarily adsorbed on the ammonia selective reduction type NOx catalyst, and the remaining ammonia and ammonia adsorbed on the ammonia selective reduction type NOx catalyst are used as a reducing agent. NOx in the exhaust is selectively reduced, and the exhaust is purified. At this time, the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst increases toward a predetermined target adsorption amount.

  In such adsorption control, the control means is an exhaust for increasing the actual adsorption amount toward the target adsorption amount as the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst increases toward the target adsorption amount. The urea water supply means is controlled so that the amount of urea water supplied therein decreases. Thereby, when the amount of ammonia required for the selective reduction of NOx in the ammonia selective reduction type NOx catalyst is the same, the case where the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst is relatively small is relatively large. A larger amount of urea water is supplied into the exhaust than in the case.

  As a result, when the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst is relatively small, the urea water is supplied in preference to the increase in the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst. A reduction in exhaust purification efficiency of the reduced NOx catalyst is suppressed. On the other hand, when the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst is relatively large, there is a low possibility that the exhaust gas purification efficiency of the ammonia selective reduction type NOx catalyst is lowered. Priority is given to preventing the accumulation of substances, and the supply amount of urea water is reduced, so that the deposition of solid substances such as urea is suppressed.

  More specifically, in the exhaust purification apparatus, the control means sets the target adsorption amount based on an upper limit adsorption amount of ammonia that can be adsorbed on the ammonia selective reduction type NOx catalyst when the adsorption control is executed. Then, based on the relationship between the target adsorption amount and the actual adsorption amount, the urea water supply amount from the urea water supply means is decreased stepwise in accordance with the increase in the actual adsorption amount ( Claim 2).

  When the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst reaches the upper limit adsorption amount, the ammonia further supplied to the ammonia selective reduction type NOx catalyst flows out of the ammonia selective reduction type NOx catalyst as it is, and so-called ammonia slip occurs. To do. Therefore, the target adsorption amount is set based on the upper limit adsorption amount in order to adsorb as much ammonia as possible to the ammonia selective reduction type NOx catalyst while preventing such ammonia slip. Since the upper limit adsorption amount changes depending on the operating state of the engine or the like, the target adsorption amount also changes as the upper limit adsorption amount changes. In the adsorption control, the control means reduces the supply amount of urea water from the urea water supply means step by step as the actual adsorption amount of ammonia increases and approaches the target adsorption amount. The urea water supply amount giving priority to the exhaust gas purification efficiency of the NOx catalyst is changed to the urea water supply amount giving priority to the prevention of solid substances such as urea. By such adsorption control, even if the upper limit adsorption amount changes due to a change in the operating state of the engine, etc., the target adsorption amount that changes with the change in the upper limit adsorption amount can be used to reduce the possibility of solid substances such as urea being deposited. While decreasing, the actual adsorption amount of ammonia can be increased to the target adsorption amount as quickly as possible, and as much ammonia as possible can be adsorbed on the ammonia selective reduction type NOx catalyst.

In the exhaust emission control device, the control means supplies urea water into the exhaust gas when the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst exceeds the target adsorption amount during execution of the adsorption control. The urea water supply means is controlled so as to stop the operation (Claim 3).
If the supply of urea water is continued even after the actual adsorption amount of ammonia reaches the target adsorption amount, ammonia slip may occur from the ammonia selective reduction type NOx catalyst due to excessive supply of ammonia generated from the urea water. . However, according to the exhaust gas purification apparatus configured as described above, if the actual adsorption amount of ammonia exceeds the target adsorption amount in the adsorption control, the control unit stops the urea water supply from the urea water supply unit. Such ammonia slip is prevented from occurring.

  More specifically, in the exhaust purification apparatus, the control means performs a first reduction of urea water necessary for selectively reducing NOx in the exhaust in the ammonia selective reduction type NOx catalyst when the adsorption control is executed. The urea water supply means is controlled based on the sum of the target supply amount and the second target supply amount of urea water necessary for increasing the actual adsorption amount toward the target adsorption amount, and the actual adsorption amount The amount of urea water supplied into the exhaust gas is reduced by decreasing the second target supply amount as the value increases (claim 4).

  According to the exhaust gas purification apparatus configured as described above, the urea water supply from the urea water supply means in the adsorption control is the urea water necessary for selectively reducing NOx in the exhaust gas in the ammonia selective reduction type NOx catalyst. This is performed based on the sum of the first target supply amount and the second target supply amount of urea water necessary for increasing the actual adsorption amount of ammonia toward the target adsorption amount. As the actual adsorption amount of ammonia increases, the second target supply amount is decreased to reduce the urea water supply amount, thereby reducing the urea water supply amount and suppressing the deposition of solid substances such as urea. In this case as well, it is possible to ensure the supply amount of ammonia necessary for selectively reducing NOx in the exhaust gas in the ammonia selective reduction type NOx catalyst.

  Specifically, in the exhaust emission control device, the control means causes the engine to enter a predetermined operating state preset as an operating state in which the upper limit adsorption amount of ammonia that can be adsorbed to the ammonia selective reduction type NOx catalyst decreases. When the engine is in the predetermined operating state, an ammonia having an equivalent ratio of 1 to NOx selectively reduced in the ammonia selective reduction type NOx catalyst is selected by the ammonia selection. Equivalence ratio control for controlling the urea water supply means so as to be supplied to the reduced NOx catalyst is performed, while the adsorption control is performed when the engine is not in the predetermined operation state. (Claim 5).

  As described above, the upper limit adsorption amount of ammonia that can be adsorbed on the ammonia selective reduction type NOx catalyst varies depending on the operating state of the engine, etc., and the upper limit adsorption amount decreases, so that ammonia is not adsorbed so much on the ammonia selective reduction type NOx catalyst. There is a case. In such a case, if the adsorption control based on ammonia adsorption is performed, the control is not suitable for the actual situation, and the amount of ammonia supplied to the ammonia selective reduction type NOx catalyst becomes inadequate, causing the occurrence of ammonia slip and ammonia. There is a possibility that the exhaust purification efficiency of the selective reduction type NOx catalyst is lowered.

Therefore, when it is determined that the engine is operating in a predetermined operation state set in advance as an operation state in which the upper limit adsorption amount is significantly reduced, the control means performs the NOx selectively reduced in the ammonia selective reduction type NOx catalyst. The urea water supply means is controlled by performing equivalent ratio control so that an amount of ammonia with an equivalent ratio of 1 is supplied to the ammonia selective reduction type NOx catalyst.
When the upper limit adsorption amount is significantly reduced, most of the ammonia supplied to the ammonia selective reduction type NOx catalyst is not adsorbed to the ammonia selective reduction type NOx catalyst, but directly flows into the ammonia selective reduction type NOx catalyst. Used for. At this time, ammonia equivalent to the NOx selectively reduced in the ammonia selective reduction type NOx catalyst by the equivalent ratio control described above is supplied to the ammonia selective reduction type NOx catalyst, so that ammonia slip occurs. Therefore, it is possible to supply an appropriate amount of ammonia to the ammonia selective reduction type NOx catalyst while preventing the exhaust gas purification efficiency of the ammonia selective reduction type NOx catalyst.

  On the other hand, when the engine is operated in an operation state in which the upper limit adsorption amount does not decrease significantly and it is determined that the engine is not in a predetermined operation state, the control unit controls the urea water supply unit by adsorption control. In this case, a part of the ammonia supplied to the ammonia selective reduction type NOx catalyst is once adsorbed to the ammonia selective reduction type NOx catalyst, and the remaining ammonia and the ammonia adsorbed to the ammonia selective reduction type NOx catalyst are NOx. Used for selective reduction of At this time, since the control means performs adsorption control, the possibility of depositing solid substances such as urea is suppressed as low as possible, and the ammonia selective reduction type while appropriately adjusting the ammonia adsorption amount in the ammonia selective reduction type NOx catalyst. It is possible to ensure good exhaust purification efficiency of the NOx catalyst.

More specifically, in the exhaust purification apparatus, when the control means determines that the temperature of the ammonia selective reduction type NOx catalyst is equal to or higher than a predetermined reference catalyst temperature, the engine is in the predetermined operation state. It is characterized by determining (Claim 6).
The upper limit adsorption amount varies according to the temperature of the ammonia selective reduction type NOx catalyst, and tends to decrease as the temperature of the ammonia selective reduction type NOx catalyst increases. Accordingly, when the control means determines that the temperature of the ammonia selective reduction type NOx catalyst is equal to or higher than the predetermined reference catalyst temperature, the control means determines that the engine is in the predetermined operation state, thereby actually reducing the upper limit adsorption amount. It is possible to accurately determine the operation state to be performed. As a result, it is possible to appropriately switch between the equivalence ratio control and the adsorption control.

Alternatively, more specifically, in the exhaust purification apparatus, the control means is configured such that the temperature of the ammonia selective reduction NOx catalyst is equal to or higher than a predetermined reference catalyst temperature, and the exhaust flow rate of the engine is equal to or higher than a predetermined reference exhaust flow rate. When it is determined that the engine is in the predetermined operating state, it is determined that the engine is in the predetermined operating state (claim 7).
The upper limit adsorption amount is also affected by the exhaust gas flow rate of the engine, and tends to decrease as the exhaust gas flow rate increases in the state where the temperature of the ammonia selective reduction type NOx catalyst does not change. Therefore, when the control means determines that the temperature of the ammonia selective reduction type NOx catalyst is equal to or higher than the reference catalyst temperature and the exhaust flow rate is equal to or higher than the predetermined reference exhaust flow rate, it determines that the engine is in the predetermined operating state. By doing so, it is possible to accurately determine the operation state in which the upper limit adsorption amount actually decreases. As a result, it is possible to appropriately switch between the equivalence ratio control and the adsorption control.

Or more specifically, in the exhaust emission control device, the control means determines that the engine speed is equal to or higher than a predetermined reference speed and the engine load is equal to or higher than a predetermined reference load. In addition, it is determined that the engine is in the predetermined operating state (claim 8).
When the engine speed becomes equal to or higher than the predetermined reference speed and the engine load becomes equal to or higher than the predetermined reference load, the exhaust temperature of the engine becomes high and the exhaust flow rate increases. As the exhaust temperature of the engine becomes high, the temperature of the ammonia selective reduction type NOx catalyst also becomes high, and the upper limit adsorption amount decreases as the temperature of the ammonia selective reduction type NOx catalyst increases or the exhaust flow rate increases. Therefore, the control means determines that the engine is in the predetermined operating state when it is determined that the engine speed is equal to or higher than the predetermined reference speed and the engine load is equal to or higher than the predetermined reference load. Thus, it is possible to accurately determine the operation state in which the upper limit adsorption amount actually decreases. As a result, it is possible to appropriately switch between the equivalence ratio control and the adsorption control.

Alternatively, more specifically, in the exhaust gas purification apparatus, the engine includes a particulate filter that collects particulates contained in the exhaust gas, and the control means performs forced regeneration of the particulate filter. It is determined that the engine is in the predetermined operation state when the vehicle is on.
When the forced regeneration of the particulate filter is executed, the exhaust gas is heated to a temperature high enough to burn the particulates in order to remove the particulates accumulated on the particulate filter. Therefore, even when the particulate filter is forcibly regenerated, the upper limit adsorption amount is significantly reduced by increasing the temperature of the ammonia selective reduction type NOx catalyst. Therefore, the control means can accurately determine the operation state in which the upper limit adsorption amount actually decreases by determining that the engine is in the predetermined operation state when forced regeneration of the particulate filter is being performed. Can do. As a result, it is possible to appropriately switch between the equivalence ratio control and the adsorption control.

In the exhaust emission control device, the control means determines that the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst has fallen below a predetermined lower limit adsorption amount when switching from the adsorption control to the equivalent ratio control. Until then, the urea water supply by the urea water supply means is stopped (claim 10).
If quantitative ratio control is started in a state where a certain amount of ammonia is adsorbed on the ammonia selective reduction type NOx catalyst, an excess will be caused when the adsorbed ammonia is released from the ammonia selective reduction type NOx catalyst. May cause ammonia slip. Therefore, when the control means switches from adsorption control to equivalent ratio control, the supply of urea water by the urea water supply means is temporarily stopped, and it is determined that the ammonia adsorption amount in the ammonia selective reduction type NOx catalyst has fallen below a predetermined lower limit adsorption amount. Then, the equivalence ratio control is performed.

  Therefore, the supply of urea water by the equivalence ratio control is started after the possibility that a large amount of ammonia slip is generated due to the release of ammonia adsorbed on the ammonia selective reduction type NOx catalyst is reduced. Further, while the supply of urea water is stopped, ammonia adsorbed on the ammonia selective reduction type NOx catalyst is used for selective reduction of NOx, so that the exhaust gas purification efficiency of the ammonia selective reduction type NOx catalyst is good. Maintained.

Specifically, in the exhaust purification apparatus, the lower limit adsorption amount is set as an ammonia adsorption amount that does not cause ammonia slip from the ammonia selective reduction type NOx catalyst even when the equivalence ratio control is executed. (Claim 11).
According to the exhaust purification apparatus configured as described above, when the control unit switches from the adsorption control to the equivalence ratio control, the supply of the urea water by the urea water supply unit is temporarily stopped, and the ammonia in the ammonia selective reduction type NOx catalyst is temporarily stopped. The equivalence ratio control is started after it is determined that the actual adsorption amount falls below the ammonia adsorption amount at which ammonia slip from the ammonia selective reduction type NOx catalyst does not occur even when the equivalence ratio control is executed.

More specifically, the lower limit adsorption amount is an adsorption amount that can be considered that ammonia is not substantially adsorbed in the ammonia selective reduction type NOx catalyst (claim 12).
According to the exhaust purification apparatus configured as described above, when the control unit switches from the adsorption control to the equivalence ratio control, the urea water supply unit temporarily stops supplying the urea water, and the ammonia selective reduction type NOx catalyst is supplied with ammonia. The equivalence ratio control is started after the state of substantially no adsorption.

  According to the exhaust emission control device of the present invention, when the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst is relatively small when the control means executes the adsorption control, the actual amount of ammonia in the ammonia selective reduction type NOx catalyst is determined. Since a relatively large amount of urea water is supplied into the exhaust gas in preference to the increase in the adsorption amount, the actual adsorption amount of ammonia can be increased rapidly, and good exhaust purification efficiency of the ammonia selective reduction type NOx catalyst can be ensured. Is possible. On the other hand, when the ammonia adsorption amount in the ammonia selective reduction type NOx catalyst is relatively large, a relatively small amount of urea water is supplied into the exhaust gas in order to prevent the solid matter such as urea generated from the urea water from being deposited. Therefore, deposition of solid substances such as urea generated from urea water can be suppressed. That is, when the control means executes such adsorption control, the actual adsorption amount is increased as quickly as possible toward the target adsorption amount while suppressing the accumulation of solid substances such as urea generated from urea water, It is possible to ensure good exhaust purification efficiency of the ammonia selective reduction type NOx catalyst.

  Further, according to the exhaust purification device of the second aspect, as the actual adsorption amount of ammonia increases and approaches the target adsorption amount, the supply amount of urea water decreases step by step. Even if the upper limit adsorption amount changes, the target adsorption amount that changes as the upper limit adsorption amount changes is used, and the actual adsorption amount of ammonia is increased to the target adsorption amount as quickly as possible while suppressing the accumulation of solid substances such as urea. Thus, it is possible to ensure good exhaust purification efficiency of the ammonia selective reduction type NOx catalyst.

According to the exhaust emission control device of claim 3, since the supply of urea water is stopped when the actual adsorption amount of ammonia exceeds the target adsorption amount in the adsorption control, after the actual adsorption amount of ammonia reaches the target adsorption amount. In addition, it is possible to reliably prevent the occurrence of ammonia slip due to the continued supply of urea water.
According to the exhaust emission control device of claim 4, when the supply amount of urea water is decreased in accordance with the increase in the actual adsorption amount of ammonia in the adsorption control, the actual adsorption amount is increased toward the target adsorption amount. Therefore, the second target supply amount of urea water necessary for the reduction is reduced, so that it is possible to reliably ensure the amount of ammonia necessary for the selective reduction of NOx in the exhaust gas in the ammonia selective reduction type NOx catalyst. As a result, the exhaust gas purification efficiency of the ammonia selective reduction type NOx catalyst can be satisfactorily maintained while suppressing the accumulation of solid substances such as urea.

  According to the exhaust purification device of claim 5, when the engine is operated in a predetermined operation state in which the upper limit adsorption amount of ammonia that can be adsorbed to the ammonia selective reduction type NOx catalyst is greatly reduced, the control means controls the equivalence ratio control. I do. As a result, while preventing the occurrence of ammonia slip due to excessive supply of ammonia, an appropriate amount of ammonia is secured for the selective reduction of NOx in the ammonia selective reduction type NOx catalyst, and exhaust purification of the ammonia selective reduction type NOx catalyst is ensured. Efficiency can be maintained well. On the other hand, when the engine is operated in an operation state in which the upper limit adsorption amount does not decrease significantly, the control means determines that the engine is not in the predetermined operation state and performs adsorption control, so that solids such as urea are removed. It is possible to ensure good exhaust purification efficiency of the ammonia selective reduction type NOx catalyst while suppressing the possibility of depositing as low as possible and appropriately adjusting the ammonia adsorption amount in the ammonia selective reduction type NOx catalyst.

According to the exhaust purification device of claim 6, when it is determined that the temperature of the ammonia selective reduction type NOx catalyst is equal to or higher than the predetermined reference catalyst temperature, it is determined that the engine is in the predetermined operation state. It is possible to accurately determine the operation state in which the upper limit adsorption amount actually decreases. As a result, it is possible to appropriately switch between the equivalence ratio control and the adsorption control.
According to the exhaust purification device of claim 7, when it is determined that the temperature of the ammonia selective reduction type NOx catalyst is equal to or higher than the reference catalyst temperature and the exhaust flow rate is equal to or higher than the predetermined reference exhaust flow rate, the engine By determining that it is in the predetermined operation state, it is possible to accurately determine the operation state in which the upper limit adsorption amount actually decreases. As a result, it is possible to appropriately switch between the equivalence ratio control and the adsorption control.

  According to the exhaust emission control device of claim 8, when it is determined that the engine speed is equal to or higher than the predetermined reference speed and the engine load is equal to or higher than the predetermined reference load, the engine performs the predetermined operation. By determining that it is in the state, it is possible to accurately determine the operation state in which the upper limit adsorption amount actually decreases. As a result, it is possible to appropriately switch between the equivalence ratio control and the adsorption control.

According to the exhaust emission control device of claim 9, when the forced regeneration of the particulate filter is being performed, it is determined that the engine is in the predetermined operation state, so that the operation for actually reducing the upper limit adsorption amount is performed. The state can be accurately determined. As a result, it is possible to appropriately switch between the equivalence ratio control and the adsorption control.
According to the exhaust emission control device of claim 10, when the control means switches from the adsorption control to the equivalence ratio control, the supply of urea water is temporarily stopped, and the ammonia adsorption amount in the ammonia selective reduction type NOx catalyst is a predetermined lower limit. After determining that the amount is below the adsorption amount, supply of urea water by equivalent ratio control is started. Therefore, after switching from the adsorption control to the equivalence ratio control, it is possible to prevent the occurrence of ammonia slip due to the ammonia adsorbed on the ammonia selective reduction type NOx catalyst being released as the upper limit adsorption amount decreases. It becomes possible. Since the ammonia adsorbed on the ammonia selective reduction type NOx catalyst is used for selective reduction of NOx while the supply of urea water is stopped, the exhaust gas purification efficiency of the ammonia selective reduction type NOx catalyst is good. Maintained.

  According to the exhaust emission control device of claim 11, when switching from the adsorption control to the equivalence ratio control, the supply of urea water is temporarily stopped, and the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst is controlled by the equivalence ratio control. After it is determined that the ammonia adsorption amount from which ammonia slip from the ammonia selective reduction type NOx catalyst does not occur is less than the ammonia adsorption amount, the supply of urea water by the equivalence ratio control is started. Therefore, after switching from the adsorption control to the equivalence ratio control, it is possible to reliably prevent the occurrence of ammonia slip caused by the release of the ammonia adsorbed on the ammonia selective reduction type NOx catalyst as the upper limit adsorption amount decreases. It becomes possible.

  According to the exhaust purification device of claim 12, when switching from the adsorption control to the equivalence ratio control, the urea water supply is temporarily stopped, and the ammonia is not substantially adsorbed on the ammonia selective reduction type NOx catalyst. Then, the equivalence ratio control is started. Therefore, after switching from adsorption control to equivalent ratio control, it is possible to reliably prevent the occurrence of ammonia slip due to the release of ammonia adsorbed on the ammonia selective reduction type NOx catalyst as the upper limit adsorption amount decreases. Is possible.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows an overall configuration diagram of a four-cylinder diesel engine (hereinafter referred to as an engine) to which an exhaust emission control device according to an embodiment of the present invention is applied, and the exhaust emission control device according to the present invention is based on FIG. Will be described.
The engine 1 includes a high-pressure accumulator chamber (hereinafter referred to as a common rail) 2 common to each cylinder, and a fuel injection valve 4 in which high-pressure fuel supplied from a fuel injection pump (not shown) and stored in the common rail 2 is provided in each cylinder. The fuel is injected into each cylinder from each fuel injection valve 4.

  The intake passage 6 is equipped with a turbocharger 8. The intake air drawn from an air cleaner (not shown) flows into the compressor 8a of the turbocharger 8 from the intake passage 6, and the intake air supercharged by the compressor 8a is intercooler. 10 and the intake control valve 12 are introduced into the intake manifold 14. Further, an intake air amount sensor 16 that detects an intake air flow rate to the engine 1 is provided upstream of the compressor 8 a in the intake passage 6.

  On the other hand, an exhaust port (not shown) through which exhaust is discharged from each cylinder of the engine 1 is connected to an exhaust pipe 20 via an exhaust manifold 18. An EGR passage between the exhaust manifold 18 and the intake manifold 14 is connected to the exhaust manifold 18 and the intake manifold 14 via an EGR valve 22 to recirculate a part of the exhaust gas of the engine 1 to the intake side. 24 is provided.

The exhaust pipe 20 is connected to the exhaust aftertreatment device 28 via the turbine 8 b of the turbocharger 8 and the exhaust throttle valve 26. The rotating shaft of the turbine 8b is mechanically connected to the rotating shaft of the compressor 8a, and the turbine 8b receives the exhaust flowing in the exhaust pipe 20 and drives the compressor 8a.
The exhaust aftertreatment device 28 includes a cylindrical upstream casing 30 connected to the exhaust pipe 20 and a cylindrical downstream casing 34 communicated with the downstream side of the upstream casing 30 through a communication path 32. . The exhaust gas flowing into the upstream casing 30 from the exhaust pipe 20 flows into the downstream casing 34 through the communication path 32 after passing through the upstream casing 30, and the exhaust gas that has passed through the downstream casing 34 is It is discharged from the tail pipe 36 into the atmosphere.

A pre-stage oxidation catalyst 38 is accommodated in the upstream casing 30, and a particulate filter (hereinafter referred to as a filter) 40 is accommodated on the downstream side of the pre-stage oxidation catalyst 38. The filter 40 is made of a honeycomb-type ceramic body, and traps particulates in the exhaust as the exhaust of the engine 1 circulates inside. Since the pre-stage oxidation catalyst 38 oxidizes NO (nitrogen monoxide) in the exhaust gas to generate NO ₂ (nitrogen dioxide), the pre-stage oxidation catalyst 38 and the filter 40 are arranged in this manner, so that the filter 40 captures them. The collected particulates react with NO ₂ supplied from the pre-stage oxidation catalyst 38 and oxidize, whereby the filter 40 is continuously regenerated.

On the other hand, in the downstream casing 34, an ammonia selective reduction type NOx catalyst (hereinafter referred to as an SCR catalyst) 42 that purifies exhaust by selectively reducing NOx (nitrogen oxide) in the exhaust using ammonia in the exhaust as a reducing agent. Is housed, and a downstream oxidation catalyst 44 for removing ammonia flowing out of the SCR catalyst 42 from the exhaust gas is housed downstream of the SCR catalyst 42. The post-stage oxidation catalyst 44 also has a function of oxidizing CO (carbon monoxide) generated when particulates are incinerated by forced regeneration of the filter 40, which will be described later, and discharging it to the atmosphere as CO ₂ (carbon dioxide). is doing.

A part of the NO ₂ generated by the front-stage oxidation catalyst 38 is used for continuous regeneration of the filter 40 as described above, but the remaining NO ₂ is supplied to the SCR catalyst 42 and the NO ₂ with respect to the NO in the exhaust gas is supplied. The exhaust purification efficiency of the SCR catalyst 42 is increased by increasing the ratio.
A urea water injector (urea water supply means) 46 for supplying urea water into the exhaust gas flowing out from the filter 40 and flowing into the communication passage 32 is provided on the downstream side of the filter 40 of the upstream casing 30. The urea water is supplied to the urea water injector 46 from a urea water tank (not shown).

The urea water injected from the urea water injector 46 is hydrolyzed by the heat of the exhaust to become ammonia, and is supplied to the SCR catalyst 42 as a reducing agent. The SCR catalyst 42 promotes a denitration reaction between the supplied ammonia and NOx in the exhaust, thereby reducing NOx and converting it into harmless N ₂ or the like. At this time, when ammonia flows out of the SCR catalyst 42 without reacting with NOx, the ammonia is excluded from the exhaust gas by the post-stage oxidation catalyst 44.

  The upstream casing 30 is provided with an upstream pressure sensor 48 for detecting the exhaust pressure upstream of the filter 40 on the upstream side of the filter 40, and a downstream pressure sensor 50 for detecting the exhaust pressure downstream of the filter 40 is provided for the filter 40. It is provided on the downstream side. A first exhaust temperature sensor 52 that detects the temperature of the exhaust gas that flows out of the filter 40 and flows into the communication passage 32 is provided in the vicinity of the urea water injector 46.

  As shown in FIG. 1, the urea water injector 46 and the first exhaust temperature sensor 52 are attached to a step portion formed in a direction approaching the central axis from the circumferential surface of the cylindrical upstream casing 30. By mounting the urea water injector 46 and the first exhaust temperature sensor 52 at such a position, the exhaust temperature detection point of the first exhaust temperature sensor 52 is brought close to the central portion of the exhaust gas flowing out from the filter 40 and the exhaust temperature is adjusted. The detection accuracy is improved, and the urea water injected from the urea water injector 46 is diffused as evenly as possible into the exhaust gas flowing out from the filter 40.

  Further, the urea water injector 46 is mounted at a position facing the connecting portion between the upstream casing 30 and the communication path 32, and the urea water injection direction is changed to the communication path 32 as shown by a one-dot chain line in FIG. 1. Is directed. In this way, by injecting urea water from the urea water injector 46 into the communication path 32, it is possible to avoid the injected urea water from colliding with the inner wall surface of the communication path 32 as much as possible. The accumulation of solid matter such as urea (hereinafter collectively referred to as urea solid matter) on the inner wall surface of the steel is suppressed.

The downstream casing 34 is provided with a second exhaust temperature sensor 54 that detects the temperature of the exhaust gas that passes through the SCR catalyst 42 and then flows into the post-stage oxidation catalyst 44.
The ECU (control means) 56 is a control device for performing comprehensive control including the operation control of the engine 1, and includes a CPU, a memory, a timer counter, and the like, and performs various control amount calculations. Various devices are controlled based on the control amount.

  In addition to the intake air amount sensor 16, the upstream pressure sensor 48, the downstream pressure sensor 50, the first exhaust temperature sensor 52, and the second exhaust temperature sensor 54, the ECU 56 collects information necessary for various controls on the input side. Various sensors such as a rotational speed sensor 58 for detecting the rotational speed of the engine 1 and an accelerator opening sensor 60 for detecting a depression amount of an accelerator pedal (not shown) are connected. On the output side of the ECU 56, various devices such as the fuel injection valve 4, the intake control valve 12, the EGR valve 22, the exhaust throttle valve 26, and the urea water injector 46 of each cylinder that are controlled based on the calculated control amount. Is connected.

  The ECU 56 also performs calculation of the fuel supply amount to each cylinder of the engine 1 and control of fuel supply from the fuel injection valve 4 based on the calculated fuel supply amount. The fuel supply amount (main injection amount) necessary for the operation of the engine 1 is preliminarily determined based on the engine speed detected by the engine speed sensor 58 and the accelerator pedal depression amount detected by the accelerator opening sensor 60. It is determined by reading from the stored map. The amount of fuel supplied to each cylinder is adjusted by the opening time of the fuel injection valve 4, and each fuel injection valve 4 is driven to open during a driving time corresponding to the determined fuel amount, and main injection to each cylinder. As a result, the amount of fuel necessary for the operation of the engine 1 is supplied.

In addition to the operation control of the engine 1, the ECU 56 performs various controls for maintaining a good exhaust purification function by the exhaust aftertreatment device 28. As one of them, the filter 56 is forcibly regenerated and functions. Forced regeneration control for recovery is also performed.
Particulates deposited on the filter 40 are oxidized and removed by continuous regeneration by reaction with NO ₂ flowing into the filter 40 from the pre-stage oxidation catalyst 38 as described above, but when the operation state where the exhaust temperature is low continues for a long time. In such a case, the deposited particulates may not be sufficiently oxidized and removed only by such continuous regeneration. If such a state continues, particulates may accumulate excessively in the filter 40 and the filter 40 may be clogged. Therefore, the ECU 56 appropriately forcibly regenerates the filter 40 according to the accumulation state of particulates in the filter 40.

The accumulation state of the particulates is estimated based on the detection values of the upstream pressure sensor 48, the downstream pressure sensor 50, and the intake air amount sensor 16 provided on the upstream side and the downstream side of the filter 40, respectively. When it is determined that the curated deposition amount has reached a predetermined amount, the forced regeneration control is started.
In forced regeneration control, based on the exhaust temperature detected by the first exhaust temperature sensor 52, the intake control valve 12 and the exhaust throttle valve 26 are controlled in the closing direction as necessary to raise the exhaust temperature, and the engine 1 expands. In the stroke or exhaust stroke, fuel is supplied from the fuel injection valve 4 into the exhaust gas by post-injection, and the exhaust gas flowing into the filter 40 is heated to a temperature at which particulates accumulated on the filter 40 can be incinerated (for example, 600 ° C.). Increase temperature.

That is, the HC supplied into the exhaust gas by the post injection from the fuel injection valve 4 reaches the front-stage oxidation catalyst 38, and the high-temperature exhaust gas whose temperature has risen due to the HC oxidation reaction in the front-stage oxidation catalyst 38 flows into the filter 40. To do. The particulates deposited on the filter 40 are incinerated by the exhaust gas that has become high in this way, and the filter 40 is forcibly regenerated.
As another control for maintaining the exhaust purification function of the exhaust after-treatment device 28 satisfactorily, the ECU 56 selectively reduces the amount of ammonia required for purifying exhaust by selectively reducing NOx in the exhaust gas. The urea water supply control for supplying to is also performed.

FIG. 2 shows a flowchart of urea water supply control executed by the ECU 56. The ECU 56 performs urea water supply control at a predetermined control cycle in accordance with the flowchart of FIG. 2 until the engine 1 is started and stopped. Execute.
When the urea water supply control is started, the ECU 56 determines in step S1 whether a condition for enabling the urea water supply is satisfied. Specifically, based on the exhaust temperature detected by the first exhaust temperature sensor 52, it is confirmed whether or not the urea water supplied into the exhaust gas has a temperature at which vaporization is possible, and the first exhaust temperature sensor 52 and the first exhaust temperature sensor 52 2. Based on the exhaust temperatures detected by the two exhaust temperature sensors 54, it is confirmed whether or not the SCR catalyst 42 is activated. When the exhaust temperature detected by the first exhaust temperature sensor 52 is a temperature at which urea water can be vaporized and it is determined that the SCR catalyst 42 is activated, the ECU 56 satisfies a condition for enabling urea water supply. It is determined that

  If it is determined in step S1 that urea water cannot be supplied, the ECU 56 advances the process to step S5 to stop the supply of urea water from the urea water injector 46, and then ends the control cycle. Then, the ECU 56 starts the process again from step S1 in the next control cycle, and determines whether or not a condition for enabling urea water supply is satisfied. Accordingly, until it is determined in step S1 that the condition for enabling the supply of urea water is satisfied, the processes in steps S1 and S5 are repeated every control cycle, and the supply of urea water from the urea water injector 46 is stopped. The state continues. If it is determined in step S1 that the condition for enabling the supply of urea water is satisfied, the ECU 56 advances the process to step S2. In the following description, it is assumed that the conditions for enabling urea water supply are satisfied.

In step S2, the ECU 56 determines a predetermined high temperature operation state (predetermined operation) that is set in advance as an operation state in which the upper limit adsorption amount, which is the limit value of the ammonia adsorption amount that can be adsorbed to the SCR catalyst 42, greatly decreases. State).
The upper limit adsorption amount of ammonia in the SCR catalyst 42 is affected by the catalyst temperature Tc of the SCR catalyst 42. FIG. 3 is a graph showing an example of the relationship between the catalyst temperature Tc and the upper limit adsorption amount when the exhaust gas flow rate is assumed to be constant. As shown in FIG. 3, the upper limit adsorption amount increases the catalyst temperature Tc. It tends to decrease according to.

  Therefore, in this embodiment, it is determined whether or not the engine 1 is in a high temperature operation state by determining whether or not the catalyst temperature Tc is equal to or higher than a predetermined reference catalyst temperature Tr. The reference catalyst temperature Tr used here is a temperature set in advance so that the upper limit adsorption amount of ammonia in the SCR catalyst 42 greatly decreases when the catalyst temperature Tc further increases, and is set to 300 ° C., for example. Is done. The catalyst temperature Tc is an average value of the exhaust temperature detected by the first exhaust temperature sensor 52 and the exhaust temperature detected by the second exhaust temperature sensor 54, that is, on the upstream side and the downstream side of the SCR catalyst 42, respectively. The average value of the exhaust temperature is used.

When it is determined that the catalyst temperature Tc has not reached the reference catalyst temperature Tr and the engine 1 is not in the high temperature operation state, the ECU 56 proceeds to step S3 to select the adsorption control mode, and urea water by the urea water injector 46 As the supply control, adsorption control, which is control based on the assumption of ammonia adsorption in the SCR catalyst 42, is performed.
When the suction control mode is selected in step S3, the ECU 56 ends the control cycle, and starts the process from step S1 again in the next control cycle. Then, in subsequent step S2, it is determined whether or not the engine 1 is in a high temperature operation state. Therefore, while the state in which the catalyst temperature Tc has not reached the reference catalyst temperature Tr continues, the ECU 56 repeats the determination that the engine 1 is not in the high temperature operation state in step S2 in each control cycle, thereby in step S3. The suction control mode is selected, and the suction control is continuously executed.

  As described above, the adsorption control is based on the assumption that ammonia is adsorbed on the SCR catalyst 42. When the adsorption control mode is selected in step S3, the adsorption control is executed in a routine different from the flowchart of FIG. When the engine 1 is not in a high temperature operation state, the upper limit adsorption amount is not reduced so much and it is possible to adsorb a certain amount of ammonia to the SCR catalyst 42, so that the ECU 56 does not absorb ammonia in the SCR catalyst 42. The urea water supply amount from the urea water injector 46 is controlled so that the actual adsorption amount that is the actual adsorption amount becomes a predetermined target adsorption amount. Details of the adsorption control will be described later.

As described above, in this embodiment, it is determined whether or not the engine 1 is in a high temperature operation state by determining whether or not the catalyst temperature Tc is equal to or higher than a predetermined reference catalyst temperature Tr. However, the method for determining whether or not the engine 1 is in the high temperature operation state is not limited to this, and various methods can be adopted.
For example, as described above, the catalyst temperature Tc of the SCR catalyst 42 is obtained from the exhaust temperatures on the upstream side and the downstream side of the SCR catalyst 42. Therefore, when the exhaust temperature of the engine 1 is equal to or higher than a predetermined reference exhaust temperature. In addition, it may be determined that the engine 1 is in a high temperature operation state.

  Further, the upper limit adsorption amount of ammonia in the SCR catalyst 42 is influenced not only by the catalyst temperature Tc of the SCR catalyst 42 but also by the flow rate of exhaust gas discharged from the engine 1. FIG. 4 is a graph showing an example of the relationship between the exhaust flow rate of the engine 1 and the upper limit adsorption amount when the catalyst temperature Tc is assumed to be constant. Therefore, as shown in FIG. 3 and FIG. 4, the upper limit adsorption amount tends to decrease as the catalyst temperature Tc of the SCR catalyst 42 increases, and to decrease as the exhaust gas flow rate in the SCR catalyst 42 increases. Therefore, when the catalyst temperature Tc of the SCR catalyst 42 is equal to or higher than the predetermined reference catalyst temperature and the exhaust flow rate of the engine 1 is equal to or higher than the predetermined reference exhaust flow rate, it is determined that the engine 1 is in a high temperature operation state. Also good. In this case, the reference catalyst temperature may be the same as the reference catalyst temperature Tr used in the determination of step S2 in the present embodiment, or a different value may be used in consideration of the combined use of the exhaust flow rate. Further, in this case, the determination may be made based on the exhaust temperature of the engine 1 instead of the catalyst temperature Tc as described above. The exhaust flow rate used here is obtained by performing a calculation based on the intake air flow rate detected by the intake air amount sensor 16, the engine speed detected by the rotation speed sensor 62, and the fuel injection amount calculated in the ECU 56. be able to.

  Further, as the rotational speed of the engine 1 increases and the load on the engine 1 increases, the exhaust temperature of the engine 1 becomes high and the exhaust flow rate increases. As described above, when the exhaust temperature of the engine 1 rises, the catalyst temperature of the SCR catalyst 42 rises and the upper limit adsorption amount decreases as the catalyst temperature rises or the exhaust flow rate increases. May be determined to be in the high temperature operation state when it is determined that the engine 1 is equal to or higher than the predetermined reference speed and the load of the engine 1 is equal to or higher than the predetermined reference load. In this case, from the relationship shown in FIG. 3 and FIG. 4, the engine speed and the engine load that become the catalyst temperature and the exhaust flow rate at which the upper limit adsorption amount greatly decreases are obtained in advance, and based on these engine speed and engine load, The number and the reference load may be set. As the engine load, for example, the fuel injection amount calculated by the ECU 56 or the accelerator pedal depression amount detected by the accelerator opening sensor 60 may be used. That is, when the fuel injection amount is used as the engine load, a predetermined reference fuel injection amount is used as the reference load, and when the accelerator pedal depression amount is used, the predetermined reference depression amount is used as the reference load. Further, the value detected by the rotation speed sensor 58 can be used as the rotation speed of the engine 1.

  Further, when the forced regeneration of the filter 38 is executed, the exhaust gas is heated to a temperature high enough to burn the particulates in order to remove the particulates accumulated on the filter 38 as described above. Therefore, even when the filter 38 is forcibly regenerated, the catalyst temperature of the SCR catalyst 40 greatly increases, so that the upper limit adsorption amount of ammonia in the SCR catalyst 40 is greatly reduced. Therefore, it may be determined that the engine 1 is in a high temperature operation state when the forced regeneration of the filter 38 is being performed.

As described above, there are various methods for determining whether or not the engine 1 is in the high temperature operation state. In any method, the operation state in which the upper limit adsorption amount of ammonia in the SCR catalyst 40 is actually greatly reduced. Can be accurately determined.
When the adsorption control mode is selected in the urea water supply control and the adsorption control is performed in this way, if it is determined in step S2 of FIG. 2 that the engine 1 is in a high temperature operation state, the ECU 56 performs the process in step S2. To step S4.

  In step S4, the ECU 56 determines whether the actual adsorption amount Qa, which is the actual adsorption amount of ammonia in the SCR catalyst 42 at that time, has fallen below a predetermined lower limit adsorption amount Qo. This lower limit adsorption amount Qo is set in advance as an ammonia adsorption amount that does not cause ammonia slip from the SCR catalyst 42 even when the equivalence ratio control described later is executed. In this embodiment, the lower limit adsorption amount Qo A very small amount of adsorption that can be considered that ammonia is not substantially adsorbed is set as the lower limit adsorption amount Qo.

  The actual adsorption amount Qa of ammonia in the SCR catalyst 42 is also necessary for the adsorption control, and the ECU 56 continuously calculates the actual adsorption amount Qa from the time when the adsorption control is being executed. Specifically, the ECU 56 obtains the actual adsorption amount Qa from the ammonia consumption amount for selective reduction of NOx in the SCR catalyst 42 and the ammonia supply amount to the SCR catalyst 42 by urea water supply control.

  Here, the ammonia consumption amount in the SCR catalyst 42 is obtained as follows. That is, the amount of NOx selectively reduced in the SCR catalyst 42 is obtained from the NOx emission amount of the engine 1 and the purification rate of the SCR catalyst 42. Then, the amount of ammonia necessary to selectively reduce this NOx corresponding to the amount of NOx that is selectively reduced is obtained as the ammonia consumption amount in the SCR catalyst 42.

  More specifically, the ECU 56 first obtains the exhaust gas purification rate by the NOx reduction of the SCR catalyst 42 using the purification rate map stored in advance. The exhaust gas purification rate by the NOx reduction of the SCR catalyst 42 varies depending on the actual adsorption amount Qa at that time, and also varies depending on the catalyst temperature Tc of the SCR catalyst 42. The relationship between the actual adsorption amount Qa and the purification rate is as shown in the graph of FIG. 5 when the catalyst temperature Tc is constant, and the purification rate tends to increase as the actual adsorption amount Qa increases. The relationship between the catalyst temperature Tc of the SCR catalyst 42 and the purification rate is as shown in the graph of FIG. 6 when the actual adsorption amount Qa of ammonia is constant, and the purification rate increases as the catalyst temperature Tc increases. There is a tendency. The purification rate map stored in the ECU 56 is a map obtained by obtaining these relationships in advance by changing the operating state of the engine 1 through experiments or the like. The ECU 56 uses the purification rate map set in this way, and based on the actual adsorption amount Qa of ammonia in the SCR catalyst 42 determined so far and the catalyst temperature Tc of the SCR catalyst 42, the purification of the SCR catalyst 42 at that time. Find the rate.

  Next, the ECU 56 obtains the NOx emission amount per unit time from the engine 1 based on the fuel injection amount per unit time of the engine 1 calculated in the fuel supply control and the engine speed detected by the rotational speed sensor 58. . Further, the ECU 56 obtains the amount of NOx selectively reduced per unit time by the SCR catalyst 42 from the purification rate of the SCR catalyst 42 and the NOx emission amount per unit time of the engine 1, and this amount of NOx selectively reduced. The amount of ammonia per unit time required for the selective reduction of NOx is determined as the ammonia consumption amount per unit time in the SCR catalyst 42.

  Since the ammonia supply amount per unit time to the SCR catalyst 42 can be obtained from the supply amount per unit time of urea water supplied into the exhaust gas via the urea water injector 46 by the urea water supply control, the ECU 56 The actual adsorption amount per unit time in the SCR catalyst 42 can be obtained by subtracting the ammonia consumption amount obtained as described above from the ammonia supply amount. The ECU 56 grasps the actual adsorption amount per unit time in the SCR catalyst 42 by such calculation from the state in which no ammonia is adsorbed on the SCR catalyst 42. Therefore, the ECU 56 determines the actual adsorption amount per unit time in the SCR catalyst 42 by the integration. The actual adsorption amount Qa of ammonia can be obtained.

  If it is determined in step S4 of the urea water supply control that the actual adsorption amount Qa of ammonia is not lower than the lower limit adsorption amount Qo, the ECU 56 advances the process to step S5 and stops supplying the urea water from the urea water injector 46. Thereafter, the control cycle ends. Therefore, if the adsorption control mode has been selected until then, the selection of the adsorption control mode is canceled, and the control mode in the urea water supply control is not selected.

  In the next control cycle, the process starts again from step S1, and if it is determined in step S2 that the engine 1 is in a high temperature operation state, the ECU 56 again determines whether the actual ammonia adsorption amount Qa has fallen below the lower limit adsorption amount Qo in step S4. Determine whether or not. Therefore, when the engine 1 is in a high temperature operation state, as long as it is determined in step S4 that the actual adsorption amount Qa of ammonia is not lower than the lower limit adsorption amount Qo, the urea water from the urea water injector 46 is processed by the process in step S5. Supply remains stopped.

By stopping the supply of the urea water from the urea water injector 46 in this way, the ammonia adsorbed on the SCR catalyst 42 is consumed for the selective reduction of NOx in the SCR catalyst 42, and the ammonia adsorption on the SCR catalyst 42. The amount will decrease.
If the ammonia adsorption amount in the SCR catalyst 42 decreases due to the stop of the supply of urea water, and it is determined in step S4 that the actual adsorption amount Qa of ammonia is below the lower limit adsorption amount Qo, the ECU 56 advances the process from step S4 to step S6. In step S6, the ECU 56 selects the equivalence ratio control mode as the urea water supply control by the urea water injector 46, and executes the equivalence ratio control. This equivalence ratio control is based on the premise that ammonia is hardly adsorbed on the SCR catalyst 42, and the supplied ammonia is used as it is for selective reduction of NOx, so that the ECU 56 supplies an appropriate amount of urea water. Controls the urea water injector 46.

  When the ECU 56 selects the equivalence ratio control mode in step S6, the control cycle ends. Then, the ECU 56 starts the process again from step S1 in the next control cycle. When the engine 1 is in a high temperature operation state, whether or not the actual adsorption amount Qa of ammonia has fallen below the lower limit adsorption amount Qo in step S4. Make a decision. As described above, since the actual ammonia adsorption amount Qa is already less than the lower limit adsorption amount Qo, the ECU 56 further proceeds to step S6 to select the equivalence ratio control mode. Therefore, after determining that the actual adsorption amount Qa of ammonia has fallen below the lower limit adsorption amount Qo, as long as the engine 1 is in a high temperature operation state, the equivalence ratio control mode is repeatedly selected at each control cycle in step S6, and the equivalence ratio control is performed. Will continue to be executed.

  As described above, the equivalence ratio control is a control based on the premise that the supplied ammonia is hardly adsorbed on the SCR catalyst 42 and is used for selective reduction of NOx as it is. Is selected, it is executed in a routine different from the flowchart of FIG. In the equivalence ratio control, the ECU 56 controls the urea water injector 46 based on the target supply amount Qinj of the urea water obtained from the ammonia supply amount at which the equivalent ratio becomes 1 with respect to NOx selectively reduced in the SCR catalyst 42. .

  As described above, when the engine 1 is in a high temperature operation state, the upper limit adsorption amount of ammonia in the SCR catalyst 42 is greatly reduced, and ammonia is hardly adsorbed on the SCR catalyst 42. Therefore, most of the ammonia supplied to the SCR catalyst 42 is not adsorbed by the SCR catalyst 42 but is used for selective reduction of NOx in the SCR catalyst 42 as it is. Therefore, when the engine 1 is in a high temperature operation state, urea water is supplied so that an amount of ammonia necessary for selective reduction of NOx in the SCR catalyst 42 is supplied by performing equivalence ratio control on the assumption of such a situation. The injector 46 is controlled. Details of the equivalence ratio control performed by the ECU 56 are as follows.

  The ECU 56 obtains the purification rate of the SCR catalyst 42 at that time based on the actual adsorption amount Qa of ammonia in the SCR catalyst 42 and the catalyst temperature Tc using the purification rate map as described above. Next, the ECU 56 determines in the SCR catalyst 42 from the NOx emission amount per unit time obtained from the engine 1 based on the fuel injection amount per unit time of the engine 1 and the engine speed, and the purification rate of the SCR catalyst 42. The amount of NOx selectively reduced per unit time is obtained. The ECU 56 obtains the amount of ammonia whose equivalence ratio is 1 with respect to the amount of NOx thus selectively reduced as the target supply amount of ammonia, and unit time of urea water for obtaining this target supply amount of ammonia Per unit supply amount is set as a target supply amount Qinj of urea water. Then, the ECU 56 controls the urea water injector 46 so that the urea water of the target supply amount Qinj determined in this way is injected from the urea water injector 46.

As described above, the mist-like urea water injected from the urea water injector 46 is hydrolyzed to ammonia by the heat of the exhaust, and this ammonia is supplied to the SCR catalyst 42. Most of the ammonia supplied to the SCR catalyst 42 is not adsorbed by the SCR catalyst 42, but is used for selective reduction of NOx in the exhaust gas by the catalytic action of the SCR catalyst 42, and NOx is reduced to generate harmless N _{2 and the} like. Is done.

  When the engine 1 is in a high temperature operation state, the equivalence ratio control is performed in this way, so that the amount of ammonia used for the selective reduction of NOx with almost no adsorption to the SCR catalyst 42 can be selected. It can be controlled to an appropriate amount necessary for reduction. As a result, it is possible to satisfactorily prevent a decrease in exhaust gas purification efficiency of the SCR catalyst 42 due to a shortage of the ammonia supply amount and an ammonia slip due to an excessive supply of ammonia.

  Further, if the equivalence ratio control is started immediately after the operating state of the engine 1 becomes the high temperature operating state, the ammonia released when the ammonia adsorbed on the SCR catalyst 42 is released from the SCR catalyst so far. Becomes surplus in the selective reduction of NOx. Therefore, in the present embodiment, as described above, when the operation state of the engine 1 shifts to the high temperature operation state, the urea water injector 46 starts until the actual adsorption amount Qa of ammonia in the SCR catalyst 42 falls below the lower limit adsorption amount Qo. Since the urea water supply is stopped, when the equivalence ratio control is started, ammonia is hardly adsorbed on the SCR catalyst 42. Accordingly, it is possible to prevent excess ammonia in the SCR catalyst 42 when the above-described equivalence ratio control is started. As a result, it is possible to satisfactorily prevent the occurrence of ammonia slip from the SCR catalyst 42 when switching from adsorption control to equivalent ratio control.

  Further, in the present embodiment, as the lower limit adsorption amount Qo, a minute adsorption amount that can be considered that ammonia is not substantially adsorbed in the SCR catalyst 42 is used, so that the equivalence ratio control is performed from the adsorption control as described above. It is possible to reliably prevent the occurrence of ammonia slip when switching to. The lower limit adsorption amount Qo may be an ammonia adsorption amount that can suppress the occurrence of ammonia slip from the SCR catalyst 42 when the control is shifted to the equivalence ratio control. That is, for example, when a lower limit adsorption amount larger than the lower limit adsorption amount Qo used in the present embodiment is used, there is a possibility that ammonia slip slightly occurs when shifting to the equivalence ratio control. However, it is possible to suppress the occurrence of ammonia slip as compared with the case where the control immediately shifts to the equivalence ratio control.

In this way, when the equivalence ratio control mode is selected in step S6 and the equivalence ratio control is being performed, if the operation state of the engine 1 is not the high temperature operation state, the ECU 56 performs the process in step S3 according to the determination in step S2. Therefore, the supply of urea water in each subsequent control cycle is performed by the adsorption control described above.
Next, details of the suction control executed by the ECU 56 when the suction control mode is selected in step S3 will be described. As described above, the adsorption control is based on the assumption that ammonia is adsorbed on the SCR catalyst 42, and is performed in a state where ammonia is relatively easily adsorbed on the SCR catalyst 42.

  Therefore, most of the ammonia supplied to the SCR catalyst 42 is once adsorbed on the SCR catalyst 42, and the remaining ammonia and the ammonia adsorbed on the SCR catalyst 42 are used for selective reduction of NOx in the SCR catalyst 42 and consumed. The As described above, the exhaust purification rate by selective reduction of NOx of the SCR catalyst 42 is compared with the actual adsorption of ammonia in the SCR catalyst 42 as shown in FIG. The amount Qa tends to increase as the amount Qa increases. That is, a higher exhaust gas purification rate can be achieved by adsorbing as much ammonia as possible within a range not exceeding the upper limit adsorption amount to the SCR catalyst 42. Therefore, in the adsorption control, the urea water injector 46 is controlled so that the actual adsorption amount Qa of ammonia becomes the predetermined target adsorption amount Qt set based on the upper limit adsorption amount.

  While the adsorption control mode is selected in step S3 of the flowchart of FIG. 2 in the urea water supply control, the ECU 56 performs the predetermined period according to the flowchart of FIG. 7 based on the target adsorption amount Qt and the actual adsorption amount Qa. Repeated adsorption control is executed. When the suction control is started, first, the ECU 56 calculates the actual suction amount Qa and calculates and sets the target suction amount Qt in step S11. Here, the calculation method of the target adsorption amount Qt and the actual adsorption amount Qa is as follows.

  As described above, the upper limit adsorption amount of ammonia in the SCR catalyst 42 varies depending on the catalyst temperature Tc of the SCR catalyst 42 and the exhaust flow rate of the engine 1, and is exemplified in FIGS. 3 and 4 with respect to the catalyst temperature Tc and the exhaust flow rate. There is a relationship. The ECU 56 stores an upper limit adsorption amount map set based on the relationship between the catalyst temperature Tc and the upper limit adsorption amount with respect to the exhaust gas flow rate when the operating state of the engine 1 is variously changed in advance through experiments or the like. Then, the ECU 56 uses the upper limit adsorption amount map to calculate the intake air by calculating the catalyst temperature Tc, which is the average value of the exhaust temperatures detected by the first exhaust temperature sensor 52 and the second exhaust temperature sensor 54, and the calculation as described above. An upper limit adsorption amount corresponding to the exhaust flow rate of the engine 1 obtained from the flow rate, the engine speed and the fuel injection amount is obtained.

  The ECU 56 sets the target adsorption amount Qt of ammonia for the SCR catalyst 42 by multiplying the upper limit adsorption amount thus obtained by a safety factor (for example, 0.9). Here, the safety factor is multiplied when ammonia is adsorbed up to the upper limit adsorption amount due to a sudden change in the operating state of the engine 1 due to some factor or the exhaust purification characteristics of the SCR catalyst 42 or variations in the operating performance of the engine 1. This is to avoid excess ammonia that may be supplied relative to the upper limit adsorption amount. The value of the safety factor can be changed to an appropriate value according to the characteristics of the SCR catalyst 42 and the engine 1.

  On the other hand, the ECU 56 determines the actual adsorption amount Qa of ammonia in the SCR catalyst 42 as described above. That is, the amount of NOx selectively reduced per unit time by the SCR catalyst 42 is obtained from the purification rate of the SCR catalyst 42 and the NOx emission amount per unit time from the engine 1, and the amount of NOx selectively reduced is obtained. Find the ammonia consumption per unit time. Then, from the ammonia consumption amount per unit time and the ammonia supply amount per unit time to the SCR catalyst 42, the actual adsorption amount of ammonia per unit time in the SCR catalyst 42 is obtained and integrated. The actual adsorption amount Qa of ammonia in the SCR catalyst 42 is obtained.

  When the actual adsorption amount Qa and the target adsorption amount Qt are obtained in step S11, the ECU 56 advances the processing to step S12, and determines whether Qa / Qt, which is the ratio of the actual adsorption amount Qa to the target adsorption amount Qt, is 1 or more. judge. Since the target adsorption amount Qt is obtained by multiplying the upper limit adsorption amount by the safety factor as described above, when Qa / Qt is 1 or more, the actual adsorption amount Qa of ammonia in the SCR catalyst 42 is the upper limit adsorption amount. If the supply of ammonia continues, the surplus ammonia may flow out of the SCR catalyst 42 and ammonia slip may occur.

  Therefore, when it is determined in step S12 that Qa / Qt is 1 or more, the ECU 56 advances the process to step S13, controls the urea water injector 46 to stop the supply of urea water, and ends the control cycle. Also in the next control cycle, the ECU 56 determines the actual adsorption amount Qa and the target adsorption amount Qt as described above at step S11, and then determines whether Qa / Qt is 1 or more at step S12. Therefore, as long as it is determined in step S12 that Qa / Qt is 1 or more in the adsorption control, the urea water supply from the urea water injector 46 is stopped. Therefore, a situation in which ammonia is further supplied to the SCR catalyst 42 from the state where the actual adsorption amount Qa of ammonia is close to the upper limit adsorption amount is avoided, and the occurrence of ammonia slip from the SCR catalyst 42 due to excessive supply of ammonia. Can be reliably prevented.

  When it is determined in step S12 that Qa / Qt is 1 or more and the supply of urea water is stopped, the ammonia adsorbed on the SCR catalyst 42 is consumed for the selective reduction of NOx, and the actual adsorption amount Qa of ammonia Gradually decreases. If the actual adsorption amount Qa of ammonia decreases in this way and it is determined in step S12 that Qa / Qt is less than 1, the ECU 56 advances the process from step S12 to step S14.

  The target adsorption amount Qt is obtained for each control cycle in step S11, and the upper limit adsorption amount serving as the base of the target adsorption amount Qt varies depending on the catalyst temperature Tc, the exhaust flow rate, etc., as described above. It is determined that / Qt is less than 1 not only due to fluctuations in the actual adsorption amount Qa, but fluctuations in the target adsorption amount Qt also affect the determination in step S12.

  In consideration of such fluctuations in the target adsorption amount Qt, the process proceeds from step S12 to step S13 in order to avoid repeated reversal of determination in step S12 when the actual adsorption amount Qa is in the vicinity of the target adsorption amount Qt. The Qa / Qt determination value may be changed depending on whether the process proceeds from step S12 to step S14. That is, for example, when Qa / Qt is 1 or more, the process proceeds from step S12 to step S13. On the other hand, once Qa / Qt is determined to be 1 or more, Qa / Qt is less than 0.8. Then, the process may continue from step S12 to step S13, and the process may proceed from step S12 to step S14 when Qa / Qt falls below 0.8. In this case, once it is determined that Qa / Qt is less than 0.8, the process continues from step S12 to step S14 until Qa / Qt becomes 1 or more. In this example, the determination values are 1 and 0.8. However, these values can be changed to appropriate values as necessary.

  If it is determined in step S12 in the flowchart of FIG. 7 that Qa / Qt is less than 1, the ECU 56 advances the process to step S14, and increases the amount used when calculating the target supply amount Qinj of urea water in the next step S15. Set the value of the coefficient K. The value of the increase coefficient K is stored in the ECU 56 as a preset increase coefficient map. In this increase coefficient map, the increase coefficient K is determined using Qa / Qt as a parameter, and the relationship between Qa / Qt and the increase coefficient K is as shown in FIG. 8, for example. That is, when Qa / Qt is 0.5 or less, the increase coefficient K is set to K1, and when Qa / Qt exceeds 0.5 and is 0.75 or less, the increase coefficient K is set to K2. When Qa / Qt exceeds 0.75 and is 1 or less, the increase coefficient K is set to K3. Here, the magnitude relationship between K1, K2, and K3 is K1> K2> K3, for example, K1 = 1, K2 = 0.5, and K3 = 0.25.

Thus, when the increase coefficient K is set based on Qa / Qt in step S14, the ECU 56 advances the process to step S15, and uses the set increase coefficient K and the reference supply amount Qb of urea water, the following equation (1): To calculate the target supply amount Qinj.
Qinj = (1 + K) · Qb (1)
The reference supply amount Qb used in the formula (1) is a unit of urea water necessary for obtaining an amount of ammonia having an equivalent ratio of 1 with respect to the amount of NOx selectively reduced per unit time in the SCR catalyst 42. Supply amount per hour. The amount of NOx selectively reduced in the SCR catalyst 42 is determined based on the NOx emission amount from the engine 1 obtained based on the fuel injection amount of the engine 1 and the engine speed as described above, and the purification rate of the SCR catalyst 42. Desired. The ECU 56 obtains the supply amount of urea water for obtaining the amount of ammonia having an equivalence ratio of 1 with respect to the amount of NOx to be selectively reduced thus obtained as the reference supply amount Qb for each control cycle in step S15. Used in the above formula (1).

  As shown in the above equation (1), the target supply amount Qinj of urea water is obtained as the sum of the reference supply amount Qb and the supply amount obtained by multiplying the reference supply amount Qb by the increase coefficient K. The reference supply amount Qb is the supply amount of urea water (first target supply amount) necessary to obtain the amount of ammonia with an equivalent ratio of 1 with respect to the amount of NOx selectively reduced in the SCR catalyst 42. The supply amount obtained by multiplying the reference supply amount Qb by the increase coefficient K is the urea water supply amount (second target supply amount) for increasing the actual ammonia adsorption amount Qa in the SCR catalyst 42.

  As described above, the increase coefficient K is changed in size according to Qa / Qt, and the increase coefficient K is set to be larger as Qa / Qt is smaller. Therefore, the smaller the Qa / Qt is, the larger the urea water supply amount for increasing the actual adsorption amount Qa of ammonia in the SCR catalyst 42 is set. In other words, the urea water supply amount for increasing the actual adsorption amount Qa of ammonia is set smaller as the actual adsorption amount Qa of ammonia in the SCR catalyst 42 approaches the target adsorption amount Qt.

  Specifically, when Qa / Qt is 0.5 or less, the increase coefficient K is set to the largest K1 in the present embodiment, and urea water supply for increasing the actual adsorption amount Qa of ammonia in the SCR catalyst 42 is performed. The amount is calculated relatively large. Here, when Qa / Qt is 0.5 or less, it means that the actual adsorption amount Qa is smaller than the target adsorption amount Qt. Therefore, a sufficient ammonia adsorption amount is not secured in the SCR catalyst 42, and the purification efficiency of the SCR catalyst 42 is relatively low from the relationship shown in FIG. Therefore, when Qa / Qt is 0.5 or less, the actual amount of adsorption Qa in the SCR catalyst 42 is rapidly increased to realize a good purification rate of the SCR catalyst 42, so that the increase coefficient K is set to this embodiment. The target supply amount Qinj is set by setting the largest K1 in FIG.

  When Qa / Qt exceeds 0.75, the increase coefficient K is set to the smallest K3 in this embodiment, and the urea water supply amount for increasing the actual adsorption amount Qa of ammonia in the SCR catalyst 42 is relatively high. Calculated slightly. Here, when Qa / Qt exceeds 0.75, it means that the actual adsorption amount Qa is close to the target adsorption amount Qt. Therefore, a certain amount of ammonia adsorption is secured in the SCR catalyst 42, and the purification efficiency of the SCR catalyst 42 is maintained relatively well from the relationship shown in FIG. Therefore, when Qa / Qt exceeds 0.75, urea solids from urea water by supplying a large amount of urea water into the exhaust gas rather than rapidly increasing the actual adsorption amount Qa in the SCR catalyst 42. In order to give priority to the suppression of the accumulation of materials, the increase coefficient K is set to the smallest K3 in the present embodiment, and the target supply amount Qinj is set while suppressing the increase relative to the reference supply amount Qb.

  Furthermore, when Qa / Qt exceeds 0.5 and is 0.75 or less, the purification efficiency of the SCR catalyst 42 is Qa / Qt of 0.5 or less because it is in the middle of the above two cases. Although it is improved compared with the case, it is lower than the case where Qa / Qt exceeds 0.75. Therefore, the increase coefficient K is also set to an intermediate size K2, and the increase relative to the reference supply amount Qb is suppressed as compared with the case where Qa / Qt is 0.5 or less, while the urea solids are prevented from being deposited as soon as possible. The actual adsorption amount Qa of ammonia is increased toward the target adsorption amount Qt.

Thus, when the target supply amount Qinj of urea water is obtained in step S16, the ECU 56 advances the process to step S16, and controls the urea water injector 46 so that the urea water of the target supply amount obtained in step S15 is supplied. The control cycle ends.
Also in the next control cycle, the ECU 56 starts processing from step S11. After obtaining the actual adsorption amount Qa and the target adsorption amount Qt of ammonia in step S11, the ratio of the actual adsorption amount Qa to the target adsorption amount Qt in step S12. It is determined whether Qa / Qt is 1 or more. If Qa / Qt has not reached 1, the ECU 56 advances the process to step S14, and obtains an increase coefficient corresponding to Qa / Qt at that time from the increase coefficient map as described above. Further, the ECU 56 calculates the target supply amount Qinj in step S15 and controls the urea water injector 46 according to the obtained target supply amount Qinj. Therefore, until the adsorption control is performed and Qa / Qt reaches 1, from the urea water injector 46 according to the target supply amount Qinj calculated in step S15 using the increase coefficient obtained in step S14 for each control cycle. Urea water is supplied into the exhaust.

  As described above, the target supply amount Qinj in the adsorption control is to increase the actual ammonia adsorption amount Qa to the reference supply amount Qb that is the supply amount of urea water necessary for the selective reduction of NOx in the SCR catalyst 42. Thus, by repeating the processes of steps S11, S12, S14 to S16 in this way, the actual adsorption amount Qa of ammonia in the SCR catalyst 42 is directed toward the target adsorption amount Qt. Gradually increase. Then, as Qa / Qt increases as the actual adsorption amount Qa approaches the target adsorption amount Qt, the increase coefficient K is switched to a small value in steps, and the actual adsorption amount Qa for ammonia is increased. As the urea water supply amount decreases according to the change in the increase coefficient, the target supply amount Qinj also decreases stepwise.

Therefore, when the actual adsorption amount Qa of ammonia in the SCR catalyst 42 is relatively small with respect to the target adsorption amount Qt, priority is given to the increase in the actual adsorption amount Qa of ammonia and a relatively large amount of urea water is supplied into the exhaust gas. Therefore, the actual adsorption amount Qa of ammonia increases rapidly, and it becomes possible to ensure good exhaust purification efficiency of the SCR catalyst 42.
On the other hand, when the actual adsorption amount Qa of ammonia is relatively large and close to the target adsorption amount Qt, the exhaust purification efficiency of the SCR catalyst 42 is ensured to some extent, so that it is possible to prevent the accumulation of urea solids generated from urea water. Preferentially, a relatively small amount of urea water is supplied into the exhaust gas, so that the accumulation of urea solids can be satisfactorily suppressed. That is, by performing such adsorption control, the actual adsorption amount Qa is increased as quickly as possible toward the target adsorption amount Qt while suppressing the accumulation of urea solids, and the good exhaust purification efficiency of the SCR catalyst 42 is achieved. Can be secured.

  Further, since the target adsorption amount Qt is set based on the upper limit adsorption amount, the target adsorption amount Qt also changes as the upper limit adsorption amount changes. Therefore, the ECU 56 controls the adsorption so that the actual adsorption amount Qa of the SCR catalyst 42 approaches the target adsorption amount Qt that changes in accordance with the fluctuation of the upper limit adsorption amount, thereby suppressing urea solid matter accumulation. At the same time, it is possible to adsorb as much ammonia as possible to the SCR catalyst 42 while preventing the occurrence of ammonia slip, and to maintain the exhaust purification efficiency of the SCR catalyst 42 in a satisfactory manner.

  When the actual adsorption amount Qa of ammonia in the SCR catalyst 42 increases and reaches the target adsorption amount Qt by the processing of steps S14 to S16, the ECU 56 determines that Qa / Qt is 1 or more in step S12 and performs processing. In step S13, the urea water injector 46 is controlled so as to stop the supply of urea water as described above. Therefore, generation of ammonia slip from the SCR catalyst 46 due to excessive supply of urea water can be reliably prevented.

Although the description of the exhaust emission control device according to one embodiment of the present invention is finished above, the present invention is not limited to the above embodiment.
For example, in the adsorption control of the above embodiment, as the actual adsorption amount Qa of ammonia in the SCR catalyst 42 approaches the target adsorption amount Qt, the increase coefficient K is gradually switched from K1 to K2, K3, and the urea water target. Although the supply amount Qinj is decreased stepwise, the method of decreasing the target supply amount Qinj is not limited to this. That is, for example, the increase coefficient K may be continuously decreased, and the target supply amount Qinj of urea water may be continuously decreased. In this case, for example, the relationship between the ratio Qa / Qt of the actual adsorption amount Qa to the target adsorption amount Qt and the increase coefficient K shown in FIG. Or you may make it change in a curve.

Alternatively, the target supply amount Qinj of urea water may be reduced by another method without using the increase coefficient K. Also in this case, the supply amount equal to or higher than the reference supply amount Qb is set as the target supply amount Qinj, and the increase from the reference supply amount Qb is reduced as the actual adsorption amount Qa of ammonia approaches the target adsorption amount Qt. That's fine.
Further, even when the target supply amount Qinj of urea water is changed stepwise as in the above embodiment, the increase coefficient K is not limited to three of K1 to K3 as in the above embodiment. Further, more increase coefficients may be determined and changed in multiple stages, or only two increase coefficients may be used and changed in two stages. Furthermore, the value of the increase coefficient K used in the above embodiment is an example, and can be variously changed as necessary.

  Further, in the above embodiment, the average values of the upstream and downstream exhaust temperatures of the SCR catalyst 42 detected by the first exhaust temperature sensor 52 and the second exhaust temperature sensor 54, respectively, are used as the catalyst temperature Tc of the SCR catalyst 42. Although used, the detection method of the catalyst temperature Tc is not limited to this. For example, the catalyst temperature Tc may be obtained using only one of both detection values of the first exhaust temperature sensor 52 and the second exhaust temperature sensor 54, or the catalyst temperature Tc may be calculated by weighting both detection values. You may make it ask. Further, the catalyst temperature Tc may be estimated from a parameter representing the operating state of the engine 1 or the like.

Further, the configuration of the exhaust aftertreatment device 28 is not limited to that of the above-described embodiment, and the exhaust aftertreatment device 28 includes an SCR catalyst 42 that uses ammonia generated from urea water supplied in the exhaust as a reducing agent. The present invention can be applied to any processing apparatus.
Furthermore, in the above embodiment, the engine 1 is a four-cylinder diesel engine, but the number of cylinders and the type of the engine 1 are not limited to this.

1 is an overall configuration diagram of an engine system to which an exhaust emission control device according to an embodiment of the present invention is applied. It is a flowchart of the urea water supply control which ECU performs. It is a graph which shows an example of the relationship between catalyst temperature and the upper limit adsorption amount of ammonia in an SCR catalyst. It is a graph which shows an example of the relationship between an exhaust gas flow rate and the upper limit adsorption amount of ammonia in an SCR catalyst. It is a graph which shows an example of the relationship between the actual adsorption amount of ammonia in an SCR catalyst, and the purification rate of an SCR catalyst. It is a graph which shows an example of the relationship between a catalyst temperature and the exhaust gas purification rate of an SCR catalyst. It is a flowchart of the adsorption | suction control which ECU performs. It is a graph which shows the relationship between ratio Qa / Qt of the actual adsorption amount with respect to the target adsorption amount of ammonia, and the increase coefficient used by adsorption control.

Explanation of symbols

1 Engine 40 Particulate filter 42 SCR catalyst (ammonia selective reduction type NOx catalyst)
46 Urea water injector (urea water supply means)
56 ECU (control means)

Claims (12)

An ammonia selective reduction type NOx catalyst that selectively reduces NOx in engine exhaust using ammonia as a reducing agent;
Urea water supply means for supplying urea water into the exhaust gas upstream of the ammonia selective reduction type NOx catalyst;
Necessary for selective reduction of NOx while adsorbing ammonia on the ammonia selective reduction type NOx catalyst so that the actual adsorption amount, which is the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst, becomes a predetermined target adsorption amount. Control means for performing adsorption control for controlling the urea water supply means so that fresh ammonia is supplied to the ammonia selective reduction type NOx catalyst,
The control means, when executing the adsorption control, in the exhaust for increasing the actual adsorption amount toward the target adsorption amount as the actual adsorption amount increases toward the target adsorption amount. An exhaust emission control device, wherein the urea water supply means is controlled so that the amount of urea water supplied to the water decreases.   The control means sets the target adsorption amount based on the upper limit adsorption amount of ammonia that can be adsorbed to the ammonia selective reduction type NOx catalyst when executing the adsorption control, and the target adsorption amount and the actual adsorption amount are set. 2. The exhaust gas purification apparatus according to claim 1, wherein the urea water supply amount from the urea water supply means is decreased stepwise in accordance with an increase in the actual adsorption amount.   When the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst exceeds the target adsorption amount during execution of the adsorption control, the control means stops the urea water supply into the exhaust gas. The exhaust gas purification device according to claim 1, wherein the water supply means is controlled.   When the control means executes the adsorption control, the first selective supply amount of urea water required to selectively reduce NOx in the exhaust gas in the ammonia selective reduction type NOx catalyst and the actual adsorption amount are set as described above. The urea water supply means is controlled based on the sum of the second target supply amount of urea water necessary for increasing toward the target adsorption amount, and the second target supply is increased as the actual adsorption amount increases. The exhaust emission control device according to claim 1, wherein the amount of urea water supplied into the exhaust gas is reduced by decreasing the amount.   The control means determines whether or not the engine is in a predetermined operation state set in advance as an operation state in which the upper limit adsorption amount of ammonia that can be adsorbed to the ammonia selective reduction type NOx catalyst is reduced. When in the predetermined operating state, the urea is so supplied that an amount of ammonia having an equivalent ratio of 1 to NOx selectively reduced in the ammonia selective reduction NOx catalyst is supplied to the ammonia selective reduction NOx catalyst. 2. The exhaust emission control device according to claim 1, wherein when the engine is not in the predetermined operation state, the adsorption control is performed while performing the equivalence ratio control for controlling the water supply unit.   6. The control unit according to claim 5, wherein when the temperature of the ammonia selective reduction type NOx catalyst is determined to be equal to or higher than a predetermined reference catalyst temperature, the control unit determines that the engine is in the predetermined operation state. The exhaust emission control device described.   When the control means determines that the temperature of the ammonia selective reduction type NOx catalyst is equal to or higher than a predetermined reference catalyst temperature and the exhaust flow rate of the engine is equal to or higher than a predetermined exhaust flow rate, the engine is in the predetermined operating state. The exhaust emission control device according to claim 5, wherein the exhaust gas purification device is determined to be present.   The control means determines that the engine is in the predetermined operation state when it is determined that the engine speed is equal to or higher than a predetermined reference speed and the engine load is equal to or higher than a predetermined reference load. The exhaust emission control device according to claim 5, wherein: The engine includes a particulate filter that collects particulates contained in the exhaust,
6. The exhaust emission control device according to claim 5, wherein the control means determines that the engine is in the predetermined operation state when forced regeneration of the particulate filter is being executed.   When the control means switches from the adsorption control to the equivalence ratio control, the urea water supply means until it is determined that the actual adsorption amount of ammonia in the ammonia selective reduction type NOx catalyst has fallen below a predetermined lower limit adsorption amount. The exhaust purification device according to claim 5, wherein the supply of urea water is stopped.   11. The lower limit adsorption amount is set as an ammonia adsorption amount that does not cause ammonia slip from the ammonia selective reduction type NOx catalyst even when the equivalence ratio control is executed. Exhaust purification device.   The exhaust purification device according to claim 10, wherein the lower limit adsorption amount is an adsorption amount that can be considered that ammonia is not substantially adsorbed in the ammonia selective reduction type NOx catalyst.

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