JP2018071490A - Deterioration diagnosis device of selective reduction catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the lowering of diagnosis accuracy resulting from the lamination adsorption of NH, in a deterioration diagnosis device which performs a deterioration diagnosis of an SCR catalyst on the basis of an NHconcentration of exhaust emission flowing out of the SCR catalyst when an additive is added from an adding device.SOLUTION: In a deterioration diagnosis device which performs a deterioration diagnosis of an SCR catalyst on the basis of an NHconcentration of exhaust emission flowing out of the SCR catalyst in a state that diagnosis addition processing for adding a large amount of additives larger than those at a normal time into the exhaust emission from an adding device is performed, in a period in which the SCR catalyst is brought into a lamination adsorptive state out of an execution period of the diagnosis addition processing, an atmosphere of the SCR catalyst is made to be an atmosphere in which lamination adsorption hardly occurs by raising a water component concentration of the exhaust emission flowing into the SCR catalyst.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a technique for diagnosing deterioration of a selective reduction catalyst (SCR (Selective Catalytic Reduction) catalyst) disposed in an exhaust passage of an internal combustion engine.

Conventionally, as a system for purifying NO _X contained in exhaust gas, an SCR catalyst and an addition device for adding an additive that is an NH ₃ or a precursor of NH ₃ to the exhaust gas upstream of the SCR catalyst are used as exhaust gas for an internal combustion engine. An apparatus is known that is arranged in a passage and supplies an additive in accordance with the amount of NO _x flowing into the SCR catalyst.

In the system as described above, it is also important to diagnose the deterioration of the SCR catalyst. As a technique for diagnosing the deterioration of the SCR catalyst, there are an SCR catalyst, an adding device for adding an aqueous urea solution into the exhaust upstream of the SCR catalyst, an NH ₃ sensor for detecting the concentration of NH _{3 in} the exhaust flowing out of the SCR catalyst, Is known to diagnose that the SCR catalyst has deteriorated if the NH ₃ concentration detected by the NH ₃ sensor is larger than a threshold value when adding the urea aqueous solution from the addition device. (For example, refer to Patent Document 1).

International Publication No. 2006/046339 JP2013-124642A JP 2010-043585 A

In the above prior art, when the deterioration of the SCR catalyst proceeds, the adsorption sites that can be activated are reduced among the adsorption sites (for example, acid sites such as Cu and Fe supported on the catalyst carrier) of the SCR catalyst. the amount of NH ₃ slip through the SCR catalyst when the aqueous urea solution from the addition device is added (NH ₃ slip amount) is based on the finding that increases. Therefore, in order to accurately diagnose the deterioration of the SCR catalyst using the above prior art method, the NH ₃ adsorption capacity of the deteriorated SCR catalyst (the amount of NH ₃ that can be adsorbed by the active adsorption site) is determined. It is necessary to supply a large amount of NH ₃ exceeding the SCR catalyst. That is, when supplying NH ₃ for the purpose of degradation diagnosis of the SCR catalyst supply when supplying NH ₃ for the purpose of NO _X purification by SCR catalyst compared to (normal), more the NH ₃ in the SCR catalyst There is a need to.

Incidentally, the form in which the SCR catalyst adsorbs NH _3, forms NH ₃ adsorption sites of the SCR catalyst as described above is adsorbed (hereinafter referred to as "chemical adsorption") and, NH _3, which is chemically adsorbed on the SCR catalyst There is a mode in which NH ₃ in the exhaust is adsorbed and stacked (hereinafter referred to as “stacked adsorption”). Here, if the NH ₃ adsorption capacity of the SCR catalyst is not saturated, the chemical adsorption is superior to the laminated adsorption, so that the laminated adsorption is hardly exhibited. However, if the NH ₃ adsorbing ability of the SCR catalyst is saturated, NH ₃ cannot be adsorbed by chemical adsorption, so that adsorption of NH ₃ by stack adsorption is likely to occur. Therefore, by supplying a relatively large amount of NH ₃ to the deteriorated SCR catalyst, even if the NH ₃ adsorption ability of the SCR catalyst is saturated, stacked adsorption may occur. When stacked adsorption occurs, the amount of NH ₃ that passes through the deteriorated SCR catalyst is less than the amount commensurate with the degree of deterioration, which may make it difficult to accurately diagnose the deterioration of the SCR catalyst. .

The present invention has been made in view of the above circumstances, and its purpose is based on the NH ₃ concentration of the exhaust gas flowing out from the SCR catalyst in a state where the additive is added to the exhaust gas from the addition device. An object of the present invention is to suppress degradation in diagnostic accuracy caused by NH ₃ stack adsorption in a degradation diagnostic apparatus that diagnoses degradation of an SCR catalyst.

In order to solve the above-described problems, the present invention is directed to the NH of exhaust gas flowing out from the SCR catalyst in a state where diagnostic addition processing for adding more additive into the exhaust gas from the addition device than usual is performed. ₃ is a deterioration diagnosis device for diagnosing deterioration of an SCR catalyst based on the concentration, and during the period of execution of the addition process for diagnosis, the atmosphere of the SCR catalyst is stacked and adsorbed during a period in which the SCR catalyst can be stacked and adsorbed. By making the atmosphere less likely to occur, the degradation of diagnostic accuracy was suppressed.

Specifically, the present invention relates to an SCR catalyst disposed in an exhaust passage of an internal combustion engine, an addition device for adding an additive that is NH ₃ or a precursor of NH ₃ into the exhaust upstream of the SCR catalyst, detecting means for detecting the NH ₃ concentration in exhaust gas flowing out of the SCR catalyst, is added a number of additives amounts than additive amount to be added from the addition device for the purpose of NO _X purification by the SCR catalyst from said adding device Control means for executing a diagnostic addition process, and diagnostic means for diagnosing deterioration of the SCR catalyst based on the NH ₃ concentration detected by the detection means during execution of the diagnostic addition process. It is a deterioration diagnosis device. Then, the deterioration diagnosis device is configured to acquire the temperature of the SCR catalyst and the acquisition in a period in which the temperature acquired by the acquisition unit is equal to or lower than a predetermined temperature in the execution period of the diagnostic addition process. And a processing means for performing a process of increasing the moisture concentration of the exhaust gas flowing into the SCR catalyst as compared with a period in which the temperature acquired by the means is higher than the predetermined temperature.

The “predetermined temperature” here is an upper limit value of the temperature at which the SCR catalyst can stack and adsorb NH ₃ in the exhaust gas. Note that as the amount of additive added per unit time from the addition device into the exhaust gas increases, NH _{3 in} the exhaust gas is more easily stacked and adsorbed on the SCR catalyst, so that it is added from the addition device into the exhaust gas per unit time. The predetermined temperature may be set to a higher temperature as the amount of the additive increases.

According to the present invention, based on the added device in NH ₃ concentration of the exhaust gas additive to the exhaust gas flows out from the SCR catalyst in a state of being added, in degradation diagnostic apparatus for performing the deterioration diagnosis of the SCR catalyst, NH ₃ It is possible to suppress a decrease in diagnostic accuracy due to the stacked adsorption.

It is a figure which shows schematic structure of the exhaust system of the internal combustion engine to which this invention is applied. Is determined from the temperature and the adsorbed NH ₃ amount of the SCR catalyst, it shows a degradation diagnosis execution region. It is a figure which shows the lamination | stacking adsorption | suction expression area | region specified from the temperature of an SCR catalyst, and the quantity of the urea aqueous solution added per unit time. It is a figure which shows the correlation with an EGR rate and the water concentration in exhaust_gas | exhaustion. It is a flowchart which shows the process routine performed by ECU when performing the deterioration diagnosis process of an SCR catalyst.

  Hereinafter, specific embodiments of the present invention will be described with reference to the drawings. The dimensions, materials, shapes, relative arrangements, and the like of the components described in the present embodiment are not intended to limit the technical scope of the invention to those unless otherwise specified.

  FIG. 1 is a diagram showing a schematic configuration of an exhaust system of an internal combustion engine to which the present invention is applied. An internal combustion engine 1 shown in FIG. 1 is a compression ignition type internal combustion engine (diesel engine) operated in a lean combustion mode. The internal combustion engine 1 may be a spark ignition type internal combustion engine (gasoline engine) capable of lean burn operation.

  Connected to the internal combustion engine 1 is an exhaust passage 2 for circulating burned gas (exhaust gas) discharged from the cylinder. A first catalyst casing 3 is disposed in the middle of the exhaust passage 2. A second catalyst casing 4 is disposed in the exhaust passage 2 downstream from the first catalyst casing 3.

The first catalyst casing 3 accommodates a catalyst carrier in which an NSR catalyst is supported in a cylindrical casing, and a particulate filter. The NSR catalyst stores NO _X in the exhaust when the air-fuel ratio of the exhaust is a lean air-fuel ratio, and releases the stored NO _X when the air-fuel ratio of the exhaust is a rich air-fuel ratio. It is reduced to N ₂ by reacting with a reducing component (such as HC or CO). The particulate filter collects PM (Particulate Matter) contained in the exhaust gas.

The second catalyst casing 4 accommodates a catalyst carrier carrying an SCR catalyst in a cylindrical casing. The catalyst carrier is, for example, a monolith type base material having a honeycomb-shaped cross section formed of cordierite, Fe-Cr-Al heat-resistant steel, or the like, coated with an alumina-based or zeolite-based catalyst carrier. It is. The catalyst carrier carries a transition metal element such as Cu or Fe after ion exchange. The SCR catalyst configured in this manner chemically adsorbs NH ₃ contained in exhaust gas at acid sites (adsorption sites) such as Cu and Fe, and in the exhaust gas using NH ₃ adsorbed on the adsorption site as a reducing agent. NO _X is reduced to N ₂ .

In the exhaust passage 2 between the first catalyst casing 3 and the second catalyst casing 4, an addition valve 5 for adding (injecting) NH ₃ or an additive which is a precursor of NH ₃ into the exhaust is disposed. ing. The addition valve 5 is connected to an additive tank 51 via a pump 50. The pump 50 sucks the additive stored in the additive tank 51 and pumps the sucked additive to the addition valve 5. The addition valve 5 injects the additive pumped from the pump 50 into the exhaust passage 2. The combination of the addition valve 5, the pump 50, and the additive tank 51 corresponds to the addition apparatus according to the present invention. As the additive stored in the additive tank 51, NH ₃ gas or an aqueous solution such as urea or ammonium carbamate can be used. In this embodiment, an aqueous urea solution is used.

When the urea aqueous solution is injected from the addition valve 5, the urea aqueous solution flows into the second catalyst casing 4 together with the exhaust gas. At that time, the urea aqueous solution is thermally decomposed by the heat of the exhaust or is hydrolyzed by the SCR catalyst. When the urea aqueous solution is thermally decomposed or hydrolyzed, NH ₃ is generated. The NH ₃ thus generated is adsorbed or occluded by the SCR catalyst. NH ₃ adsorbed or occluded by the SCR catalyst reacts with NO _X contained in the exhaust to generate N ₂ and H ₂ O. That is, NH ₃ functions as a NO _X reducing agent.

  In addition, an EGR device is attached to the internal combustion engine 1. The EGR device changes the passage cross-sectional area of the EGR passage 15 and the EGR passage 15 for guiding a part of the exhaust gas (EGR gas) from the exhaust passage 2 upstream of the first catalyst casing 3 to the intake passage 14. And an EGR valve 16 for adjusting the amount of EGR gas flowing through the EGR passage 15.

The internal combustion engine 1 configured as described above is provided with an ECU 10. The ECU 10
An electronic control unit including a CPU, ROM, RAM, backup RAM, and the like. The ECU 10 includes various sensors such as a first NO _X sensor 6, a second NO _X sensor 7, an exhaust temperature sensor 8, an NH ₃ sensor 9, a crank position sensor 11, an accelerator position sensor 12, and an air flow meter 13. It is connected.

The first NO _X sensor 6 is disposed in the exhaust passage 2 between the first catalyst casing 3 and the second catalyst casing 4, and outputs an electrical signal correlated with the NO _X concentration of the exhaust flowing into the second catalyst casing 4. . The second NO _X sensor 7 is disposed in the exhaust passage 2 downstream from the second catalyst casing 4, and outputs an electrical signal that correlates with the NO _X concentration of the exhaust gas flowing out from the second catalyst casing 4. The exhaust temperature sensor 8 is disposed in the exhaust passage 2 downstream from the second catalyst casing 4 and outputs an electrical signal correlated with the temperature of the exhaust gas flowing out from the second catalyst casing 4. The NH ₃ sensor 9 is disposed in the exhaust passage 2 downstream of the second catalyst casing 4 and outputs an electrical signal correlated with the NH ₃ concentration of the exhaust gas flowing out from the second catalyst casing 4. It corresponds to “detection means”. Since the second NO _X sensor 7 has a characteristic of reacting to NH ₃ in the exhaust in addition to NO _X in the exhaust, the second NO _X sensor 7 is also used as the NH ₃ sensor. Also good. In that case, the second NO _X sensor 7 corresponds to the “detecting means” according to the present invention.

  The crank position sensor 11 outputs an electrical signal correlated with the rotational position of the output shaft (crankshaft) of the internal combustion engine 1. The accelerator position sensor 12 outputs an electrical signal that correlates with the amount of operation of the accelerator pedal (accelerator opening). The air flow meter 13 outputs an electrical signal correlated with the amount (mass) of air taken into the internal combustion engine 1.

  The ECU 10 is electrically connected to the above-described addition valve 5, EGR valve 16, pump 50 and the like in addition to various devices (for example, a fuel injection valve) attached to the internal combustion engine 1. The ECU 10 electrically controls various devices of the internal combustion engine 1, the addition valve 5, the EGR valve 16, the pump 50, and the like based on the output signals of the various sensors described above. For example, the ECU 10 performs the SCR catalyst accommodated in the second catalyst casing 4 in addition to known controls such as fuel injection control of the internal combustion engine 1, EGR control, and addition control for intermittently injecting the additive from the addition valve 5. A process (deterioration diagnosis process) for diagnosing the deterioration of is executed.

Hereinafter, a method for executing the deterioration diagnosis process in the present embodiment will be described. As a method of diagnosing the deterioration of the SCR catalyst, NO _X purification rate of the SCR catalyst based methods (with respect to _the amount of NO _X flowing into the SCR catalyst, _the amount of NO _X ratio of being purified by the SCR catalyst) is considered . However, as shown in FIG. 1, in a configuration in which the NSR catalyst is placed from the SCR catalyst in the upstream, since the absolute amount of the NO _X flowing into the SCR catalyst is small, can be difficult to accurately perform the deterioration diagnosis There is sex.

To the above-described problems (for the purpose of purification of the NO _X by the SCR catalyst, when is added the aqueous urea solution from the addition valve 5) normal from the addition valve 5 and the diagnostic addition process to add more of the aqueous urea solution from A method of diagnosing the deterioration of the SCR catalyst based on the measured value (NH ₃ concentration) of the NH ₃ sensor 9 during the execution of the diagnostic addition process is conceivable. Specifically, a predetermined amount of urea aqueous solution is added from the addition valve 5. The predetermined amount here is a larger amount than usual as described above. More specifically, if the SCR catalyst is in a normal state in which the SCR catalyst is not deteriorated, the NH ₃ adsorption capacity of the SCR catalyst is not saturated, and if the SCR catalyst is in a deteriorated state, the predetermined amount is An amount of NH ₃ that saturates the NH ₃ adsorption capacity is determined to be supplied to the SCR catalyst. Then, if the NH ₃ concentration measured by the NH ₃ sensor 9 during the execution period of the diagnostic addition process is equal to or lower than the predetermined threshold concentration, it is diagnosed that the SCR catalyst is normal, and during the execution period of the diagnostic addition process If the NH ₃ concentration measured by the NH ₃ sensor 9 exceeds a predetermined threshold concentration, it is diagnosed that the SCR catalyst is deteriorated. The threshold concentration here is a value that is assumed that the NH ₃ concentration measured by the NH ₃ sensor 9 during the execution period of the diagnostic addition process exceeds the threshold concentration if the SCR catalyst is deteriorated, Assume that it is obtained in advance based on the results of experiments and simulations.

Incidentally, the form in which NH ₃ is adsorbed on the SCR catalyst, and forms NH ₃ adsorption site as described above are chemically adsorbed, is NH ₃ in the exhaust gas NH _3, which is chemically adsorbed to the adsorption sites laminated adsorption And forms. When the SCR catalyst is deteriorated, among the adsorption sites of the SCR catalyst, the active adsorption sites are reduced, and the NH ₃ adsorption ability of the SCR catalyst is reduced. For this reason, it has been considered that the concentration of the exhaust gas flowing out from the SCR catalyst during the execution of the diagnostic addition process is larger when the SCR catalyst is deteriorated than when the SCR catalyst is not deteriorated. However, in the state where the NH ₃ adsorption capacity of the SCR catalyst is not saturated, the stacked adsorption is difficult to develop, but in the state where the NH ₃ adsorption capacity of the SCR catalyst is saturated, the stacked adsorption is likely to occur. . Therefore, by supplying a relatively large amount of NH ₃ to the deteriorated SCR catalyst during the execution period of the diagnostic addition process, even if the NH ₃ adsorption capacity of the SCR catalyst is saturated, it is possible to develop stacked adsorption. There is sex. Accordingly, there is a possibility that a decrease in NH ₃ adsorption capacity due to deterioration is compensated by stack adsorption. In that case, the NH ₃ concentration of the exhaust gas flowing out from the SCR catalyst during the execution period of the diagnostic addition process may be smaller than the concentration commensurate with the deterioration state of the SCR catalyst. As a result, although the SCR catalyst is deteriorated, there is a possibility that the SCR catalyst is erroneously diagnosed as being normal.

Here, based on the above-described adsorption form of NH ₃ by the SCR catalyst, it is conceivable to perform the deterioration diagnosis by the above-described method when the SCR catalyst is in a state in which the NH ₃ is not stacked and adsorbed. NH _{3 that} is stacked and adsorbed on the SCR catalyst is more easily desorbed from the SCR catalyst in a lower temperature range than NH ₃ that is chemisorbed on the SCR catalyst. Therefore, the high temperature range NH ₃ stacked adsorbed on the SCR catalyst is eliminated, in other words, in the high temperature region where the SCR catalyst is not laminated adsorb NH _3, it is conceivable to perform the deterioration diagnosis of the SCR catalyst. However, the maximum amount of NH ₃ that can be chemically adsorbed by the SCR catalyst (the amount of adsorption when the NH ₃ adsorption capacity of the SCR catalyst is saturated (hereinafter referred to as “saturated adsorption amount”)) is the temperature of the SCR catalyst. There is a tendency to get smaller as it gets higher. Therefore, as the temperature of the SCR catalyst increases, the difference between the saturated adsorption amount when the SCR catalyst is normal and the saturated adsorption amount when the SCR catalyst is deteriorated may be reduced. Therefore, it may be difficult to accurately diagnose the deterioration of the SCR catalyst.

Therefore, in this embodiment, the deterioration diagnosis is performed in a low temperature range where the difference between the saturated adsorption amount when the SCR catalyst is normal and the saturated adsorption amount when the SCR catalyst is deteriorated is relatively large. In the period during which the diagnostic addition process is performed, in the period in which the SCR catalyst is in a stackable state, the atmosphere of the SCR catalyst is set to an atmosphere in which stacking adsorption is unlikely to occur, so that the NH measured by the NH ₃ sensor 9 is measured. The correlation between the ₃ concentration and the degree of deterioration of the SCR catalyst was increased.

Here, FIG. 2 is a diagram showing a deterioration diagnosis execution region determined from the temperature of the SCR catalyst and the NH ₃ adsorption amount (chemically adsorbed NH ₃ amount) of the SCR catalyst. The solid line in FIG. 2 shows the amount of saturated adsorption when the SCR catalyst is normal. A one-dot chain line in FIG. 2 indicates a saturated adsorption amount when the SCR catalyst is deteriorated. Temp 1 in FIG. 2 is a lower limit value of the temperature at which the aqueous urea solution added from the addition valve 5 can be thermally decomposed and hydrolyzed to produce NH ₃ . Temp2 in FIG. 2 is an upper limit value of the temperature at which a difference that can perform a highly accurate deterioration diagnosis between the saturated adsorption amount when the SCR catalyst is normal and the saturated adsorption amount when the SCR catalyst is deteriorated. Further, the NH ₃ adsorption amount indicated by A2 in FIG. 2 is a predetermined margin from the saturated adsorption amount (A1 in FIG. 2) when the SCR catalyst is normal and the temperature of the SCR catalyst is Temp2. Is the amount after subtracting. As shown in FIG. 2, the degradation diagnosis execution region (shaded portion in FIG. 2) of the present embodiment is such that the temperature of the SCR catalyst belongs to the range of Temp1 to Temp2, and the NH ₃ adsorption amount of the SCR catalyst is A2 or less. It is determined to be an area.

FIG. 3 is a diagram showing a region (stacked adsorption developing region) where the SCR catalyst can stack and adsorb NH ₃ . It is the same as Temp1 and Temp2 in FIG. 3, and Temp1 and Temp2 in FIG. A hatched portion in FIG. 3 indicates a stacked adsorption expression region. As shown in FIG. 3, in the temperature range of Temp1 to Temp2, stacked adsorption does not occur in a region where the temperature of the SCR catalyst is higher than the predetermined temperature Temp3. On the other hand, in the region where the temperature of the SCR catalyst is equal to or lower than the predetermined temperature Temp3, stacked adsorption can occur. Specifically, in the region where the temperature of the SCR catalyst is equal to or lower than the predetermined temperature Temp3, in the region where the amount of the aqueous urea solution added per unit time from the addition valve 5 is relatively small, stacked adsorption does not occur. However, in the region where the temperature of the SCR catalyst is equal to or lower than the predetermined temperature Temp3, in the region where the amount of the urea aqueous solution added per unit time from the addition valve 5 is increased to some extent, stacked adsorption occurs. In addition, as described above, in the diagnostic addition process, it is necessary to add a larger amount of urea aqueous solution than usual, but it is also necessary to quickly terminate the diagnostic addition process. It is necessary to relatively increase the amount of the aqueous urea solution added. Therefore, when the temperature of the SCR catalyst is equal to or lower than the predetermined temperature Temp3 during the execution period of the diagnostic addition process, it is assumed that the SCR catalyst is in a state where it can be stacked and adsorbed.

Next, a method for setting the atmosphere of the SCR catalyst to an atmosphere in which stacking adsorption hardly occurs in the stacking adsorption development region will be described. NH ₃ laminated and adsorbed on the SCR catalyst is desorbed from the SCR catalyst by being exposed to an atmosphere having a high moisture concentration (H ₂ O concentration). Therefore, during the period in which the deterioration diagnosis process for the SCR catalyst is performed, in the period in which the state of the SCR catalyst is in the stacked adsorption development region, the moisture concentration of the exhaust gas flowing into the SCR catalyst is increased, thereby stacking NH _{3 on} the SCR catalyst. (Hereinafter referred to as “lamination adsorption suppression process”). As a method for increasing the moisture concentration of the exhaust gas flowing into the SCR catalyst, a method for increasing the EGR rate by the EGR device can be used. FIG. 4 is a diagram showing a correlation between the EGR rate and the moisture concentration of the exhaust gas flowing into the SCR catalyst. As shown in FIG. 4, the greater the EGR rate, the higher the moisture concentration of the exhaust flowing into the SCR catalyst. Therefore, in the present embodiment, a target water concentration Ctrg, which is a water concentration that can suppress the stacked adsorption of NH _{3 on} the SCR catalyst, is obtained from the results of experiments and simulations, and the target EGR rate at which the target water concentration Ctrg can be realized. Assume that R1 is also obtained. The target water concentration Ctrg here is a concentration obtained by adding a predetermined margin to the minimum water concentration that can suppress the stacking adsorption of NH _{3 in} the SCR catalyst.

When the stack adsorption suppression process is executed in the period in which the state of the SCR catalyst is in the stack adsorption development region in the deterioration diagnosis execution period of the SCR catalyst, the stack adsorption of NH _{3 on} the SCR catalyst is suppressed. It is possible to perform a high deterioration diagnosis.

  Hereinafter, a specific execution procedure of the deterioration diagnosis process in the present embodiment will be described with reference to FIG. FIG. 5 is a flowchart showing a processing routine executed by the ECU 10 when the deterioration diagnosis process for the SCR catalyst is performed. This processing routine is a processing routine that is repeatedly executed by the ECU 10 during operation of the internal combustion engine 1. In the present embodiment, it is assumed that the deterioration diagnosis process is executed once per trip.

In the processing routine of FIG. 5, the ECU 10 first determines whether or not the value of the diagnostic flag is “0” in the processing of S101. The diagnosis flag is set to “1” when the deterioration diagnosis of the SCR catalyst is finished (for example, when the processing of S111 or S112 described later is finished), or when the operation of the internal combustion engine 1 is stopped, or the internal combustion engine This flag is reset to “0” when 1 is started. When the value of the diagnosis flag is “1”, the deterioration diagnosis process in the trip has already been completed. Therefore, when a negative determination is made in the process of S101, the ECU 10 ends the process of this process routine. On the other hand, when the value of the diagnosis flag is “0”, the deterioration diagnosis process in the trip has not been completed yet. Therefore, if an affirmative determination is made in the process of S101, the ECU 10 proceeds to the process of S102.

In the process of S102, the ECU 10 determines whether or not an execution condition for the deterioration diagnosis process is satisfied, that is, whether or not the state of the SCR catalyst belongs to the deterioration diagnosis execution region shown in FIG. At this time, NH ₃ adsorption amount of the SCR catalyst, and those using NH ₃ adsorption amount estimated in separate processing routine. In a separate processing routine, the NH ₃ amount supplied to the SCR catalyst per unit time, the amount of NH ₃ is consumed in the SCR catalyst per unit time (the amount of NH ₃ to be consumed in the reduction of NO _X) units It shall be estimated by integrating the subtracted value and the NH ₃ slip amount per time. The amount of NH ₃ supplied to the SCR catalyst is calculated using the amount of urea aqueous solution added from the addition valve 5 as a parameter. The amount of NH ₃ consumed in the SCR catalyst is calculated using the NO _X inflow amount and the NO _X purification rate as parameters. The NO _X purification rate here is calculated using the flow rate of the exhaust gas flowing into the SCR catalyst and the temperature of the SCR catalyst as parameters under the assumption that the SCR catalyst is normal. The NH ₃ slip amount is calculated using the previous estimated value of the NH ₃ adsorption amount, the temperature of the SCR catalyst, and the flow rate of exhaust gas passing through the SCR catalyst per unit time as parameters. On the other hand, the temperature of the SCR catalyst is calculated from the measured value of the exhaust temperature sensor 8. The ECU 10 calculates the temperature of the SCR catalyst from the measured value of the exhaust gas temperature sensor 8, thereby realizing “acquiring means” according to the present invention. If a negative determination is made in step S102, the ECU 10 ends the processing routine. On the other hand, if an affirmative determination is made in the process of S102, the ECU 10 proceeds to the process of S103.

In the process of S103, the ECU 10 starts a diagnostic addition process. That is, the ECU 10 controls the addition valve 5 so as to add a predetermined amount of urea aqueous solution. As described above, the predetermined amount here is that the NH ₃ adsorption capacity of the SCR catalyst is not saturated if the SCR catalyst is in a normal state where the SCR catalyst is not deteriorated, and the SCR catalyst is deteriorated if the SCR catalyst is in a deteriorated state. The amount of the urea aqueous solution is determined so that NH _{3 in} an amount that saturates the NH ₃ adsorption capacity of the catalyst is supplied to the SCR catalyst, and is larger than usual. It should be noted that the “control means” according to the present invention is realized when the ECU 10 executes the process of S103. Subsequently, the ECU 10 proceeds to the process of S104 and increments the value of the timer t by a predetermined time Δt. Specifically, the ECU 10 updates the value of the timer t by adding a predetermined time Δt to the previous value told (initial value is zero). This timer t measures the execution time of the diagnostic addition process. The predetermined time Δt corresponds to the elapsed time from the previous execution of S104.

In the process of S105, the ECU 10 determines whether or not the SCR catalyst is in a state capable of stacking and adsorbing NH ₃ , that is, whether or not the state of the SCR catalyst belongs to the above-described stacked adsorption expression region shown in FIG. Determine. Specifically, it is determined whether or not the temperature of the SCR catalyst is equal to or lower than a predetermined temperature Temp3. At that time, the temperature of the SCR catalyst is acquired by the same method as the process of S102. If an affirmative determination is made in the process of S105, the ECU 10 executes the stacked adsorption suppression process in the process of S106 because the SCR catalyst may stack and adsorb NH ₃ . Specifically, the EGR valve 16 is controlled so that the EGR rate becomes the aforementioned target EGR rate R1. Thus, the “processing means” according to the present invention is realized by the ECU 10 executing the processing of S106. If a negative determination is made in the process of S105, the ECU 10 proceeds to the process of S107 because there is no possibility that the SCR catalyst stacks and adsorbs NH ₃ . If the stacked adsorption suppression process has already been performed at the time when the process of S107 is executed, the ECU 10 stops the stacked adsorption suppression process by returning the opening of the EGR valve 16 to the normal opening. Further, if the stacked adsorption suppression process is not performed at the time when the process of S107 is performed, the ECU 10 maintains the opening degree of the EGR valve 16 at the normal opening degree, thereby stopping the stacked adsorption suppression process. To maintain.

  When the ECU 10 finishes executing the process of S106 or S107, the ECU 10 proceeds to the process of S108. In the process of S108, the ECU 10 determines again whether or not the state of the SCR catalyst belongs to the deterioration diagnosis execution region. This is because the temperature of the SCR catalyst or the like may change due to the execution of the stacked adsorption suppression process. If a negative determination is made in step S108, the ECU 10 sequentially executes steps S114 and S115 described later. On the other hand, when an affirmative determination is made in the process of S108, the ECU 10 proceeds to the process of S109.

In the process of S109, the ECU 10 reads the measured value (NH ₃ concentration) CNH ₃ of the NH ₃ sensor 9, and determines whether or not the NH ₃ concentration CNH ₃ is less than or equal to the threshold concentration Cthre. As described above, the threshold concentration Cthre here is a value that is assumed that the measured value of the NH ₃ sensor 9 exceeds the threshold concentration Cthre during the execution of the diagnostic addition process if the SCR catalyst is deteriorated. It is.

If an affirmative determination is made in the process of S109, the ECU 10 proceeds to the process of S110, and determines whether or not the value of the timer t is equal to or greater than a predetermined value t0. The predetermined value t0 is an amount that is assumed that the NH ₃ adsorption capacity of the SCR catalyst is not saturated if the SCR catalyst is normal, and that the NH ₃ adsorption capacity of the SCR catalyst is saturated if the SCR catalyst is deteriorated. This is the time required for the urea aqueous solution to be added from the addition valve 5. If a negative determination is made in the process of S110, the ECU 10 returns to the process of S104. On the other hand, when an affirmative determination is made in the process of S110, the ECU 10 proceeds to the process of S111 and diagnoses that the SCR catalyst is normal. When a negative determination is made in the process of S109, the ECU 10 proceeds to the process of S112 and diagnoses that the SCR catalyst has deteriorated. The ECU 10 executes the processes of S109, S111, and S112, thereby realizing the “diagnostic unit” according to the present invention.

  When the ECU 10 finishes executing the process of S111 or S112, the ECU 10 proceeds to the process of S113, and changes the value of the diagnostic flag from “0” to “1”. Next, the ECU 10 proceeds to the process of S114 and stops the diagnostic addition process. At that time, if the stacked adsorption suppression process is being executed, the stacked adsorption suppression process is also stopped. Then, the ECU 10 resets the value of the timer t to zero in the process of S115, and ends the execution of this process routine.

When the deterioration diagnosis process of the SCR catalyst is executed according to the procedure described above, even if the state of the SCR catalyst enters the stacked adsorption expression region during the execution period of the deterioration diagnosis process, the NH ₃ stack adsorption on the SCR catalyst is suppressed. be able to. Therefore, the amount of NH ₃ that passes through the SCR catalyst during the execution period of the deterioration diagnosis process is an amount that matches the deterioration state of the SCR catalyst. As a result, it is possible to accurately diagnose the deterioration of the SCR catalyst.

<Other embodiments>
In the above-described embodiment, the method of increasing the EGR rate is exemplified as the execution method of the stacked adsorption suppression process. However, the water injection valve for injecting water into the exhaust passage 2 in the cylinder or upstream from the first catalyst casing 3 is exemplified. Is attached to the internal combustion engine 1, a method of injecting water from the water injection valve or increasing the amount of water injection from the water injection valve when the state of the SCR catalyst belongs to the stacked adsorption expression region Thus, the stacked adsorption suppression process may be executed. In this case, the amount of water injection per unit time may be increased as the temperature of the SCR catalyst is increased and the amount of urea aqueous solution added per unit time from the addition valve 5 is increased.

1 Internal combustion engine 2 Exhaust passage 3 First catalyst casing 4 Second catalyst casing 5 Addition valve 6 First NO _X sensor 7 Second NO _X sensor 8 Exhaust temperature sensor 9 NH ₃ sensor 10 ECU
14 Intake passage 15 EGR passage 16 EGR valve

Claims (1)

A selective reduction catalyst disposed in an exhaust passage of the internal combustion engine;
An addition device for adding an additive which is NH ₃ or a precursor of NH ₃ into the exhaust gas upstream of the selective catalytic reduction catalyst;
Detecting means for detecting the NH ₃ concentration in the exhaust gas flowing out from the selective catalytic reduction catalyst;
And control means for performing the selective reduction catalyst by NO _X purifying the added amounts of additive than additive to be added from the device is a process of adding from the addition device diagnostic addition processing purposes,
Diagnostic means for diagnosing deterioration of the selective catalytic reduction catalyst based on the NH ₃ concentration detected by the detection means when the diagnostic addition process is performed;
A degradation diagnosis apparatus for a selective catalytic reduction catalyst, comprising:
Obtaining means for obtaining the temperature of the selective catalytic reduction catalyst;
Of the execution period of the diagnostic addition process, the selective reduction is performed in a period in which the temperature acquired by the acquisition unit is equal to or lower than a predetermined temperature compared to a period in which the temperature acquired by the acquisition unit is higher than the predetermined temperature. Processing means for performing processing for increasing the moisture concentration of the exhaust gas flowing into the catalyst,
A deterioration reduction apparatus for a selective catalytic reduction catalyst, further comprising:

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