JP6278002B2 - Failure diagnosis device for exhaust purification system - Google Patents

Description

  The present invention relates to a failure diagnosis device for an exhaust gas purification device that purifies exhaust gas from an internal combustion engine.

  A technique is known in which an exhaust passage of an internal combustion engine is provided with a filter that collects particulate matter (hereinafter sometimes referred to as “PM”) in exhaust gas. In addition to PM, ash derived from oil is also collected in this filter. The PM deposited on the filter can be removed from the filter by executing a so-called filter regeneration process that is a process of oxidizing the PM by raising the temperature of the filter. However, since the ash deposited on the filter contains a metal component, it is not removed from the filter even if the filter regeneration process is executed. Therefore, the ash deposited on the filter remains continuously.

  Patent Document 1 discloses a technique relating to a method for estimating an ash accumulation amount in a filter. Further, in Patent Document 2, when clogging of the exhaust purification catalyst due to adhesion of fuel or soot in the exhaust occurs, the exhaust gas is exhausted because the reducing agent used for the reduction reaction in the exhaust purification catalyst is insufficient. It is described that the exhaust gas purification rate at the purification catalyst decreases.

JP 2007-303334 A JP 2005-248760 A

  In some cases, an SCR filter having a configuration in which an SCR catalyst (selective reduction type NOx catalyst) for reducing NOx in the exhaust is carried by the filter is provided in the exhaust passage of the internal combustion engine. In some cases, a downstream SCR catalyst for reducing NOx in the exhaust gas is further provided in the exhaust passage downstream of the SCR filter. In the present invention, whether or not the exhaust gas purification apparatus having such an SCR filter and a rear SCR catalyst has failed is detected by a NOx sensor provided in an exhaust passage between the SCR filter and the rear SCR catalyst. It aims at improving the diagnostic accuracy when using and diagnosing.

  In the first invention, the NOx purification rate in the SCR filter is calculated using the detected value of the NOx sensor provided in the exhaust passage between the SCR filter and the subsequent SCR catalyst. When the NOx purification rate in the SCR filter is equal to or less than a predetermined determination purification rate, it is determined that the exhaust purification device having the SCR filter and the subsequent SCR catalyst has failed. At this time, the value of the determination purification rate is changed according to the ash accumulation amount in the SCR filter.

More specifically, the failure diagnosis device for an exhaust gas purification device according to the first invention is an SCR filter having a configuration in which an SCR catalyst for reducing NOx in exhaust gas is supported on a filter that collects particulate matter in exhaust gas. An SCR filter provided in the exhaust passage of the internal combustion engine, and a post-stage SCR catalyst for reducing NOx in the exhaust gas, the post-stage SCR catalyst provided in the exhaust passage downstream of the SCR filter, In a failure diagnosis device for an exhaust gas purification device for diagnosing whether or not the exhaust gas purification device has a failure, a NOx sensor provided in an exhaust passage between the SCR filter and the rear SCR catalyst, and detection of the NOx sensor A NOx purification rate calculating unit for calculating a NOx purification rate in the SCR filter using the value, and the SCR filter calculated by the NOx purification rate calculating unit. When the NOx purification rate in the filter is equal to or less than a predetermined judgment purification rate, a determination unit that determines that the exhaust purification device has failed, and an ash accumulation amount estimation unit that estimates an ash accumulation amount in the SCR filter, In addition, when the ash accumulation amount in the SCR filter estimated by the ash accumulation amount estimation unit is large, the determination purification rate is smaller than when the ash accumulation amount is small.

  In an exhaust purification device having an SCR filter and a rear SCR catalyst, it is necessary to ensure a desired NOx purification rate by both the SCR filter and the rear SCR catalyst. Here, normally, if the deterioration of the SCR catalyst carried on the SCR filter progresses, the deterioration of the post-stage SCR catalyst also progresses (Note that “deterioration” here refers to deterioration over time or thermal deterioration. This is a deterioration in which the NOx purification function of the SCR catalyst itself decreases.) Therefore, there is a correlation between the degree of deterioration of the SCR catalyst carried on the SCR filter (that is, the degree of deterioration of the NOx purification function) and the degree of deterioration of the subsequent SCR catalyst. Therefore, in a state where the NOx purification rate in the SCR filter has decreased to a certain extent, there is a high possibility that the NOx purification rate in the rear-stage SCR catalyst is also reduced by an amount corresponding to the degree of the reduction.

  Therefore, in the present invention, the NOx purification rate in the SCR filter is calculated using the detected value of the NOx sensor provided in the exhaust passage between the SCR filter and the subsequent SCR catalyst. When the NOx purification rate in the SCR filter is equal to or less than a predetermined determination purification rate, it is determined that the exhaust purification device having the SCR filter and the subsequent SCR catalyst has failed. Here, when the NOx purification rate in the SCR filter is reduced below the judgment purification rate, the NOx purification rate in the rear-stage SCR catalyst is also lowered to the extent that it should be determined that the exhaust purification device has failed. It is a value that is assumed to be.

  On the other hand, the SCR filter collects not only PM in exhaust but also ash in exhaust. Therefore, ash is deposited on the SCR filter in addition to PM. The ash once deposited on the SCR filter is not removed. When ash is deposited on the SCR filter, adsorption of ammonia on the SCR catalyst supported on the SCR filter is inhibited by the ash. As a result, even if the NOx purification function itself of the SCR catalyst carried by the SCR filter is not lowered, the NOx purification rate in the SCR filter is lowered. That is, even if the degree of deterioration of the SCR catalyst carried on the SCR filter is the same, the NOx purification rate in the SCR filter becomes lower as the ash accumulation amount in the SCR filter increases. However, even if the ash accumulation amount in the SCR filter increases, the degree of deterioration of the post-stage SCR catalyst does not advance. Therefore, even if the NOx purification rate in the SCR filter decreases due to an increase in the amount of ash accumulated in the SCR filter, the NOx purification rate in the subsequent SCR catalyst also decreases by an amount corresponding to the degree of the decrease. is not.

  Here, it is assumed that the determination purification rate is a constant value regardless of the ash accumulation amount in the SCR filter. In this case, even if the NOx purification rate in the SCR filter becomes equal to or lower than the determination purification rate due to an increase in the amount of ash accumulation in the SCR filter, the NOx purification rate in the subsequent SCR catalyst is maintained at a high value to some extent. It is highly possible that Therefore, even if the NOx purification rate in the SCR filter is equal to or less than the determination purification rate, the NOx purification rate as a whole (purification rate within an allowable range) may be secured as a whole exhaust purification device. Therefore, if it is determined at this time that the exhaust emission control device is out of order, a fault diagnosis is made as a failure diagnosis of the exhaust emission purification device.

Therefore, in the present invention, when the ash accumulation amount in the SCR filter is large, the determination purification rate is set to a smaller value than when the ash accumulation amount is small. According to this, N in the SCR filter due to an increase in the amount of ash deposition in the SCR filter.
When the Ox purification rate decreases, the value of the determination purification rate decreases according to the degree of decrease. Therefore, it is suppressed that the exhaust gas purification device is determined to be malfunctioning even though a certain amount of NOx purification rate (purification rate within an allowable range) is secured as a whole. Accordingly, it is possible to improve the diagnosis accuracy of the failure diagnosis of the exhaust purification device.

  In the second invention as well, as in the first invention, the NOx purification rate in the SCR filter is calculated using the detected value of the NOx sensor provided in the exhaust passage between the SCR filter and the subsequent SCR catalyst. Then, the calculated NOx purification rate in the SCR filter is corrected according to the ash accumulation amount in the SCR filter. When the value corrected in this way is equal to or less than a predetermined determination purification rate, it is determined that the exhaust purification device having the SCR filter and the post-stage SCR catalyst has failed.

  More specifically, the failure diagnosis device for an exhaust gas purification apparatus according to the second invention is an SCR filter having a configuration in which an SCR catalyst for reducing NOx in exhaust gas is supported on a filter that collects particulate matter in exhaust gas. An SCR filter provided in the exhaust passage of the internal combustion engine, and a post-stage SCR catalyst for reducing NOx in the exhaust gas, the post-stage SCR catalyst provided in the exhaust passage downstream of the SCR filter, In the failure diagnosis device of the exhaust gas purification device for diagnosing whether or not the exhaust gas purification device has a failure, a NOx sensor provided in an exhaust passage between the SCR filter and the post-stage SCR catalyst, and detection of the NOx sensor A NOx purification rate calculating unit that calculates a NOx purification rate in the SCR filter using a value, and an ash accumulation amount in the SCR filter. And correcting the NOx purification rate in the SCR filter calculated by the NOx purification rate estimation unit and the NOx purification rate calculation unit based on the ash deposition amount in the SCR filter estimated by the ash deposition amount estimation unit. When the corrected purification rate calculated by the corrected purification rate calculation unit and the corrected purification rate calculation unit is equal to or less than a predetermined determination purification rate, it is determined that the exhaust purification device has failed. And when the ash accumulation amount in the SCR filter estimated by the ash accumulation amount estimation unit is large, the corrected purification rate calculation unit includes the SCR compared to when the ash accumulation amount is small. The corrected purification rate is calculated by correcting the NOx purification rate in the filter to a larger value.

  In the present invention, the corrected purification rate is calculated by correcting the NOx purification rate in the SCR filter based on the ash accumulation amount in the SCR catalyst. Then, by comparing the corrected purification rate with the determined purification rate, it is determined whether or not the exhaust purification device has failed. At this time, when the ash accumulation amount in the SCR filter is large, the correction width (increase width) from the NOx purification rate to the correction purification rate in the SCR filter is increased compared to when the ash accumulation amount is small. According to this, even if the value of the NOx purification rate in the SCR filter is the same, if the NOx purification rate is reduced due to an increase in the amount of ash accumulation in the SCR filter, depending on the degree of reduction. As a result, the corrected purification rate increases. Therefore, according to the present invention, as in the first aspect of the invention, the exhaust purification device fails as a whole even though a certain degree of NOx purification rate (purification rate within an allowable range) is ensured. It is suppressed that it determines with being. Accordingly, it is possible to improve the diagnosis accuracy of the failure diagnosis of the exhaust purification device.

  According to the present invention, a failure of an exhaust purification device having an SCR filter and a subsequent SCR catalyst is diagnosed using a detection value of a NOx sensor provided in an exhaust passage between the SCR filter and the subsequent SCR catalyst. , Diagnostic accuracy can be improved.

1 is a diagram illustrating a schematic configuration of an internal combustion engine and an intake / exhaust system thereof according to Embodiment 1. FIG. Is a diagram showing an image of the adsorption state of ammonia (NH ₃₎ in the SCR filter. It is a figure which shows the correlation with the ash deposition amount Aash in an SCR filter, and the NOx purification rate Rf in this SCR filter. It is the figure which showed the NOx purification rate in an exhaust gas purification apparatus with the bar graph. FIG. 4A shows the NOx purification rate when the NOx purification function of the exhaust purification device is normal and ash is not deposited on the SCR filter. FIG. 4 (b) shows the NOx purification rate when the NOx purification function of the exhaust purification device is out of order. FIG. 4C shows the NOx purification rate when the NOx purification function of the exhaust purification device is normal and ash is deposited on the SCR filter. 3 is a flowchart showing a failure diagnosis flow of the exhaust gas purification apparatus according to the first embodiment. It is a figure which shows the correlation of the ash deposition amount Aash in an SCR filter, and the coefficient k1 used for calculation of the determination purification | cleaning rate Rfth. 6 is a flowchart showing a failure diagnosis flow of the exhaust gas purification apparatus according to the second embodiment. It is a figure which shows the correlation with the ash deposition amount Aash in a SCR filter, and the coefficient k2 used for calculation of correction | amendment purification rate Rfm.

  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.

<Example 1>
[Schematic configuration]
FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine and its intake / exhaust system according to the present embodiment. An internal combustion engine 1 shown in FIG. 1 is a compression ignition type internal combustion engine (diesel engine) using light oil as fuel. However, the present invention can also be applied to a spark ignition type internal combustion engine using gasoline or the like as fuel.

  The internal combustion engine 1 includes a fuel injection valve 3 that injects fuel into the cylinder 2. The internal combustion engine 1 is connected to the intake passage 4. An air flow meter 40 and a throttle valve 41 are provided in the intake passage 4. The air flow meter 40 outputs an electrical signal corresponding to the amount (mass) of intake air (air) flowing through the intake passage 4. The throttle valve 41 is disposed downstream of the air flow meter 40 in the intake passage 4. The throttle valve 41 adjusts the intake air amount of the internal combustion engine 1 by changing the passage cross-sectional area in the intake passage 4.

  The internal combustion engine 1 is connected to the exhaust passage 5. The exhaust passage 5 is provided with an oxidation catalyst 50, an SCR filter 51, and a post-stage SCR catalyst 52. The SCR filter 51 is configured such that an SCR catalyst is supported on a wall flow type filter formed of a porous base material. The SCR filter 51 configured as described above has a PM collection function for collecting PM in exhaust gas and a NOx purification function for reducing NOx in exhaust gas using ammonia as a reducing agent. The oxidation catalyst 50 is provided in the exhaust passage 5 upstream of the SCR filter 51. The rear stage SCR catalyst 52 is provided in the exhaust passage 5 on the downstream side of the SCR filter 51. The post-stage SCR catalyst 52 is a catalyst having a NOx purification function for reducing NOx in the exhaust gas using ammonia as a reducing agent, similarly to the SCR catalyst carried by the SCR filter 51.

A fuel addition valve 53 is provided in the exhaust passage 5 upstream of the oxidation catalyst 50. The fuel addition valve 53 adds fuel into the exhaust flowing through the exhaust passage 5. A urea water addition valve 54 is provided in the exhaust passage 5 downstream of the oxidation catalyst 50 and upstream of the SCR filter 51. The urea water addition valve 54 adds urea water into the exhaust flowing through the exhaust passage 5. When urea water is added into the exhaust gas from the urea water addition valve 54, the urea water is supplied to the SCR filter 51. In the SCR filter 51, ammonia generated by hydrolysis of the supplied urea is adsorbed on the SCR catalyst. Then, NOx in the exhaust is reduced using ammonia adsorbed on the SCR catalyst as a reducing agent. A part of the urea water added to the exhaust gas from the urea water addition valve 54 passes through the SCR filter 51 and is supplied to the post-stage SCR catalyst 52. Also in the post-stage SCR catalyst 52, the ammonia produced by the hydrolysis of the supplied urea is adsorbed, and NOx in the exhaust is reduced using the adsorbed ammonia as a reducing agent.

  In addition to the urea water addition valve 54, another urea water addition valve may be provided between the SCR filter 51 and the post-stage SCR catalyst 52. Further, instead of the urea water addition valve, an ammonia addition valve for adding ammonia gas into the exhaust gas may be provided. Instead of the urea water addition valve, an ammonia generation catalyst such as an NSR catalyst (storage reduction type NOx catalyst) may be provided. The ammonia generation catalyst is a catalyst having a function of generating ammonia when HC (fuel) is supplied.

  A temperature sensor 55 is provided in the exhaust passage 5 downstream of the oxidation catalyst 50 and upstream of the urea water addition valve 54. A NOx sensor 56 is provided in the exhaust passage 5 downstream of the SCR filter 51 and upstream of the rear-stage SCR catalyst 52 (that is, the exhaust passage 5 between the SCR filter 51 and the rear-stage SCR catalyst 52). The temperature sensor 55 is a sensor that outputs an electrical signal corresponding to the temperature of the exhaust gas. The NOx sensor 56 is a sensor that outputs an electrical signal corresponding to the NOx concentration of the exhaust gas.

  The internal combustion engine 1 is provided with an electronic control unit (ECU) 10. The ECU 10 is a unit that controls the operating state and the like of the internal combustion engine 1. An air flow meter 40, a temperature sensor 55, and a NOx sensor 56 are electrically connected to the ECU 10. Further, various sensors such as an accelerator position sensor 7 and a crank position sensor 8 are electrically connected to the ECU 10. The accelerator position sensor 7 is a sensor that outputs an electrical signal correlated with an operation amount (accelerator opening) of an accelerator pedal (not shown). The crank position sensor 8 is a sensor that outputs an electrical signal correlated with the rotational position of the engine output shaft (crankshaft) of the internal combustion engine 1. Then, the output signals of these sensors are input to the ECU 10.

  Various devices such as the fuel injection valve 3, the throttle valve 41, the fuel addition valve 53, and the urea water addition valve 54 are electrically connected to the ECU 10. These devices are controlled by the ECU 10. For example, the ECU 10 adds urea water in order to maintain the ammonia adsorption amount in the SCR filter 51 (that is, the ammonia adsorption amount in the SCR catalyst supported by the SCR filter 51) and the ammonia adsorption amount in the subsequent SCR catalyst 52 at the target adsorption amount. The urea water addition by the valve 54 is controlled.

  Further, when the estimated value of the PM accumulation amount in the SCR filter 51 reaches a predetermined PM accumulation amount, the ECU 10 performs filter regeneration processing by adding fuel from the fuel addition valve 53. In the filter regeneration process, the temperature of the SCR filter 51 is raised by the oxidation heat generated when the fuel added from the fuel addition valve 53 is oxidized in the oxidation catalyst 50. At this time, the ECU 10 estimates the temperature of the SCR filter 51 based on the output value of the temperature sensor 55. Then, the ECU 10 controls the amount of fuel added from the fuel addition valve 53 so that the estimated temperature of the SCR filter 51 becomes the target temperature. As a result, PM deposited on the SCR filter 51 is oxidized and removed from the SCR filter 51.

  The SCR filter 51 collects ash in the exhaust in addition to PM in the exhaust. Ash is a substance derived from oil. Ash collected by the SCR filter 51 is deposited on the SCR filter 51. Since this ash contains a metal component, it is not removed from the SCR filter 51 even if the filter regeneration process is executed. Therefore, the ash deposited on the SCR filter 51 remains continuously. In the present embodiment, the ash accumulation amount in the SCR filter 51 is estimated by the ECU 10. Specifically, first, oil consumption in the internal combustion engine 1 is calculated based on the history of the engine load of the internal combustion engine 1. The amount of ash outflow from the internal combustion engine 1 has a correlation with this oil consumption. Therefore, the ash accumulation amount in the SCR filter 51 is estimated based on the oil consumption amount. The estimated ash accumulation amount is stored in the ECU 10.

  Note that the ash accumulation amount in the SCR filter 51 can be estimated by a method different from the above method. For example, the ash accumulation amount in the SCR filter 51 may be estimated based on the travel distance of the vehicle on which the internal combustion engine 1 is mounted. Further, when a differential pressure sensor for detecting an exhaust pressure difference between the upstream side and the downstream side of the SCR filter 51 is provided, the ash accumulation amount in the SCR filter 51 may be estimated based on the exhaust pressure difference. . As described above, the ash deposited on the SCR filter 51 is not removed from the SCR filter 51 even when the filter regeneration process is executed. For this reason, at the time immediately after the end of the execution of the filter regeneration process, almost only ash is deposited on the SCR filter 51. Therefore, the exhaust differential pressure detected by the differential pressure sensor at this time has a high correlation with the ash accumulation amount in the SCR filter 51. Therefore, the ash accumulation amount in the SCR filter 51 can be estimated based on the exhaust pressure difference detected by the differential pressure sensor immediately after the completion of the filter regeneration process.

[Fault diagnosis]
As described above, in this embodiment, the SCR filter 51 and the post-stage SCR catalyst 52 have a NOx purification function. Therefore, a desired NOx purification rate (a ratio of the amount of NOx reduced in the SCR filter 51 and the rear SCR catalyst 52 to the amount of NOx flowing into the SCR filter 51) is secured by both the SCR filter 51 and the rear SCR catalyst 52. Yes. Therefore, in this embodiment, the SCR filter 51 and the post-stage SCR catalyst 52 are regarded as one exhaust purification device 60, and whether or not the exhaust purification device 60 is out of order (that is, the NOx purification function of the exhaust purification device 60 is effective). Failure diagnosis is performed to diagnose whether or not a failure has occurred. In the SCR filter 51, the PM collection function may be broken. In the case of distinguishing and detecting the failure of the NOx purification function of the exhaust purification device 60 and the failure of the PM collection function of the exhaust purification device 60 (that is, the failure of the PM collection function of the SCR filter 51), The failure diagnosis according to the present embodiment is executed as a failure diagnosis for detecting a failure of the NOx purification function of the exhaust purification device 60.

  Here, in this embodiment, the SCR filter 51 and the rear stage SCR catalyst 52 are arranged in series in the exhaust passage 5. Therefore, normally, if the deterioration of the SCR catalyst carried on the SCR filter 51 progresses, the deterioration of the post-stage SCR catalyst 52 also progresses. That is, there is a correlation between the degree of deterioration of the SCR catalyst carried on the SCR filter 51 and the degree of deterioration of the subsequent SCR catalyst 52. Therefore, in a state where the NOx purification rate in the SCR filter 51 (the ratio of the NOx amount reduced in the SCR filter 51 to the NOx amount flowing into the SCR filter 51) has decreased to some extent, the amount corresponding to the degree of the decrease. The NOx purification rate in the rear SCR catalyst 52 (the ratio of the amount of NOx reduced in the rear SCR catalyst 52 to the amount of NOx flowing into the rear SCR catalyst 52) can also be regarded as decreasing.

  Therefore, in this embodiment, it is determined whether or not the exhaust purification device 60 has failed based on the NOx purification rate in the SCR filter 51. More specifically, the NOx purification rate in the SCR filter 51 is calculated using the detected value of the NOx sensor 56 provided in the exhaust passage 5 between the SCR filter 51 and the subsequent SCR catalyst 52. The NOx concentration of the exhaust gas flowing into the SCR filter 51 (hereinafter also referred to as “inflow exhaust gas”) can be estimated based on the operating state of the internal combustion engine 1. Then, the NOx concentration of the exhaust gas flowing out from the SCR filter 51 (hereinafter sometimes referred to as “outflow exhaust gas”) is detected by the NOx sensor 56. The NOx purification rate in the SCR filter 51 is calculated based on the estimated value of the NOx concentration of the inflowing exhaust gas and the detected value of the NOx concentration of the outflowing exhaust gas. When the NOx purification rate in the SCR filter 51 is equal to or less than a predetermined determination purification rate, it is determined that the exhaust purification device 60 has failed. Here, when the NOx purification rate in the SCR filter 51 falls below the judgment purification rate, it is assumed that the NOx purification rate in the entire exhaust purification device 60 is below the lower limit value of the allowable range. Value. This determination purification rate is a value determined in consideration of the correlation between the degree of deterioration of the SCR catalyst carried on the SCR filter 51 and the degree of deterioration of the rear-stage SCR catalyst 52. That is, when the NOx purification rate in the SCR filter 51 falls below the judgment purification rate, the judgment purification rate is such that the NOx purification rate in the rear-stage SCR catalyst 52 is such that the exhaust purification device 60 should be judged as malfunctioning. It is a value that is assumed to be lower.

On the other hand, as described above, not only PM but also ash is deposited on the SCR filter 51. The ash deposited on the SCR filter 51 remains continuously. Here, FIG. 2 is a diagram showing an image of the adsorption state of ammonia (NH ₃ ) in the SCR filter 51. FIG. 2 (a) shows the adsorption state of ammonia when ash is not deposited on the SCR filter, and FIG. 2 (b) shows the adsorption state of ammonia when ash is deposited on the SCR filter. Show. In FIGS. 2A and 2B, arrows indicate the flow of exhaust.

  As shown in FIGS. 2A and 2B, in the SCR filter 51, an SCR catalyst is supported on the partition walls of the filter carrier. And the ammonia produced | generated by the urea supplied to the SCR filter 51 hydrolyzing adsorb | sucks to this SCR catalyst. Here, as shown in FIG. 2B, when ash is deposited on the SCR filter 51, the ash is continuously deposited on the SCR catalyst. Therefore, when ash is accumulated on the SCR filter 51, the adsorption of ammonia to the SCR catalyst supported on the SCR filter 51 is inhibited by the ash. That is, the amount of ammonia that serves as a reducing agent when NOx is reduced in the SCR filter 51 is reduced. As a result, even if the NOx purification function of the SCR catalyst carried by the SCR filter 51 is not lowered, the NOx purification rate in the SCR filter 51 is lowered. The degree of decrease in the NOx purification rate in the SCR filter 51 at this time increases as the ash accumulation amount in the SCR filter 51 increases.

  FIG. 3 is a diagram showing a correlation between the ash accumulation amount Aash in the SCR filter 51 and the NOx purification rate Rf in the SCR filter 51. In FIG. 3, the horizontal axis represents the ash accumulation amount Aash in the SCR filter 51, and the vertical axis represents the NOx purification rate Rf in the SCR filter 51. FIG. 3 shows the correlation between the two when the degree of deterioration of the SCR catalyst carried by the SCR filter 51 is constant. As shown in FIG. 3, even if the degree of deterioration of the SCR catalyst carried on the SCR filter 51 is the same, the NOx purification rate in the SCR filter 51 decreases as the ash accumulation amount in the SCR filter 51 increases. Become.

However, ash is difficult to deposit on the post-stage SCR catalyst 52. Therefore, the latter stage SCR catalyst 5
The NOx purification rate of 2 is hardly affected by ash. Further, even if the ash accumulation amount in the SCR filter 51 increases, the degree of deterioration of the post-stage SCR catalyst 52 does not advance. Therefore, even if the NOx purification rate in the SCR filter 51 is reduced due to an increase in the ash accumulation amount in the SCR filter 51, the NOx purification rate in the post-stage SCR catalyst 52 is also increased according to the degree of the reduction. It does not decrease. This is different from the case where the NOx purification rate in the SCR filter 51 is lowered due to the deterioration of the SCR catalyst carried on the SCR filter 51.

  FIG. 4 is a bar graph showing the NOx purification rate in the exhaust purification device 60. FIGS. 4A and 4C show the NOx purification rate when the NOx purification function of the exhaust purification device 60 is normal. However, FIG. 4A shows the NOx purification rate when ash is not deposited on the SCR filter 51. On the other hand, FIG. 4C shows the NOx purification rate when ash is accumulated on the SCR filter 51. FIG. 4B shows the NOx purification rate when the NOx purification function of the exhaust purification device 60 is malfunctioning. 4A, 4B, and 4C, the lattice portion Rf indicates the NOx purification rate in the SCR filter 51, and the hatched portion Rp indicates the NOx purification rate in the rear-stage SCR catalyst 52. In FIG. 4, Rfth is the determination purification rate, and Rath is the lower limit value of the allowable range of the NOx purification rate in the entire exhaust purification device 60.

  If the exhaust purification device 60 is normal, the degree of deterioration of both the SCR catalyst carried by the SCR filter 51 and the rear-stage SCR catalyst 52 is small. In this case, as shown in FIG. 4 (a), the NOx purification rate Rf in the SCR filter 51 is larger than the determination purification rate Rfth, and the NOx purification rate in the entire exhaust purification device 60 is also larger than the lower limit value Rath of the allowable range. ing. On the other hand, when the SCR catalyst carried by the SCR filter 51 and the rear-stage SCR catalyst 52 deteriorate and the exhaust purification device 60 fails, as shown in FIG. 4B, the NOx purification rate Rf in the SCR filter 51 becomes the judgment purification rate Rfth. Smaller. Further, the NOx purification rate Rp in the rear-stage SCR catalyst 52 is also reduced compared to the normal time by the amount corresponding to the degree of reduction in the NOx purification rate Rf in the SCR filter 51. Therefore, the NOx purification rate in the entire exhaust purification device 60 is smaller than the lower limit value Rath of the allowable range. When ash is accumulated on the SCR filter 51, as shown in FIG. 4C, even if the degree of deterioration of the SCR catalyst carried on the SCR filter 51 is small, the NOx purification rate Rf in the SCR filter 51 is reduced to the exhaust gas. In some cases, the purification device 60 may be reduced to the same level as when the purification device 60 breaks down (same as the NOx purification rate in the SCR filter 51 shown in FIG. 4B). However, even in this case, since the NOx purification function in the post-stage SCR catalyst 52 is normal, the NOx purification rate Rp of the post-stage SCR catalyst 52 is the same as the NOx purification rate Rp of the post-stage SCR catalyst 52 shown in FIG. Maintained to a certain extent. In this case, the NOx purification rate in the entire exhaust purification device 60 is larger than the lower limit value Rath of the allowable range.

  In other words, if the determination purification rate is constant at a value such as Rfth in FIG. 4 regardless of the ash accumulation amount in the SCR filter 51, the SCR filter 51 is caused by an increase in the ash accumulation amount in the SCR filter 51. Even if the NOx purification rate Rf at NO becomes equal to or less than the determination purification rate Rfth, there is a high possibility that the NOx purification rate Rp in the rear-stage SCR catalyst 52 is maintained at a high value to some extent. Therefore, as shown in FIG. 4C, even if the NOx purification rate in the SCR filter 51 is equal to or less than the determination purification rate Rfth, the exhaust purification device 60 may be able to ensure a NOx purification rate within an allowable range. . Therefore, if it is determined at this time that the exhaust purification device 60 has failed, a fault diagnosis has been made as a failure diagnosis of the exhaust purification device 60.

Therefore, in this embodiment, the determination purification rate is changed based on the ash accumulation amount in the SCR filter 51 estimated by the ECU 10. Specifically, when the ash accumulation amount in the SCR filter 51 is large, the determination purification rate is set to a smaller value than when the ash accumulation amount is small. According to this, when the NOx purification rate in the SCR filter 51 is reduced due to an increase in the ash accumulation amount in the SCR filter 51, the value of the determination purification rate is reduced according to the degree of the reduction. That is, the influence of the ash accumulation amount in the SCR filter 51 on the determination as to whether or not the exhaust purification device 60 has failed can be reduced. Therefore, the exhaust purification device 60 as a whole can be prevented from being determined to be out of order even though the NOx purification rate within the allowable range is secured.

[Failure diagnosis flow]
FIG. 5 is a flowchart showing a failure diagnosis flow of the exhaust gas purification apparatus according to the present embodiment. According to this flow, the ECU 10 performs a failure diagnosis of the exhaust purification device 60 during the operation of the internal combustion engine 1.

  In this flow, first, in S101, it is determined whether or not an execution condition for failure diagnosis of the exhaust purification device is satisfied. Execution conditions for failure diagnosis of the exhaust gas purification device are predetermined. As the execution conditions, the operating state of the internal combustion engine 1 is a steady state, the NOx sensor 56 is normal, the flow rate of the exhaust gas flowing through the exhaust passage 5 is within a predetermined range, and the SCR filter 51 For example, the temperature is within a predetermined range. The diagnosis as to whether or not the NOx sensor 56 is normal is performed by the ECU 10 according to a flow different from this flow, and the diagnosis result is stored in the ECU 10. Further, the NOx purification rate in the SCR filter 51 changes according to the ammonia adsorption amount in the SCR filter 51. The ammonia adsorption amount in the SCR filter 51 is affected by the flow rate of the exhaust gas flowing through the exhaust passage 5 and the temperature of the SCR filter 51. Therefore, when performing a failure diagnosis of the exhaust purification device based on the NOx purification rate in the SCR filter 51, it is preferable that both the flow rate of the exhaust gas flowing through the exhaust passage 5 and the temperature of the SCR filter 51 are within a predetermined range.

  If a negative determination is made in S101, the execution of this flow is temporarily terminated. On the other hand, when a positive determination is made in S101, the process of S102 is executed next. In S102, the NOx purification rate Rf in the SCR filter 51 is calculated. Here, as described above, the NOx purification rate Rf in the SCR filter 51 is based on the NOx concentration of the inflowing exhaust estimated based on the operating state of the internal combustion engine 1 and the NOx concentration of the outflowing exhaust detected by the NOx sensor 56. Is calculated.

  Next, in S103, the ash accumulation amount Aash in the SCR filter 51 calculated by the above-described method and stored in the ECU 10 is read. Next, in S104, a coefficient k1 used to calculate a later-described determination purification rate Rfth is calculated based on the ash accumulation amount Aash in the SCR filter 51 read in S103. FIG. 6 is a diagram showing the correlation between the ash deposition amount Aash and the coefficient k1 in the SCR filter 51. As shown in FIG. As shown in FIG. 6, the coefficient k1 is 1 when the ash deposition amount Aash is zero. Then, the value of the coefficient k1 decreases as the value of the ash deposition amount Aash increases. In the ECU 10, the correlation between the ash deposition amount Aash and the coefficient k1 in the SCR filter 51 as shown in FIG. 6 is stored as a map. In S104, the coefficient k1 is calculated using this map.

Next, a determination purification rate Rfth is set in S105. The determination purification rate Rfth set here is calculated by the following equation (1).
Rfth = Rfth0 × k1 (1)
Rfth0: reference determination purification rate k1: coefficient calculated in S104 Here, the reference determination purification rate Rfth0 is a determination purification rate when it is assumed that no ash is accumulated in the SCR filter 51. This reference determination purification rate Rfth0 is determined in advance based on experiments or the like. The determination purification rate Rfth calculated by the above equation (1) becomes smaller as the ash accumulation amount Aash in the SCR filter 51 is larger.

  Note that, as described above, the exhaust gas purifier failure diagnosis execution conditions include that the flow rate of the exhaust gas flowing through the exhaust passage 5 is within a predetermined range, and that the temperature of the SCR filter 51 is within a predetermined range. Although included, the determination purification rate Rfth may be further corrected based on the flow rate of the exhaust gas flowing through the exhaust passage 5 and the temperature of the SCR filter 51 in order to further improve the diagnostic accuracy. Further, the determination purification rate Rfth may be changed stepwise in accordance with an increase in the ash accumulation amount Aash in the SCR filter 51.

  Next, in S106, it is determined whether or not the NOx purification rate Rf in the SCR filter 51 calculated in S102 is greater than the determination purification rate Rfth set in S105. If an affirmative determination is made in S106, it is then determined in S107 that the exhaust purification device 60 is normal. On the other hand, if a negative determination is made in S106, that is, if the NOx purification rate Rf in the SCR filter 51 is equal to or less than the determination purification rate Rfth, then in S108, it is determined that the exhaust purification device 60 has failed. After it is determined in S107 that the exhaust purification device 60 is normal, or after it is determined in S108 that the exhaust purification device 60 has failed, the execution of this flow is terminated.

  According to the present embodiment, the exhaust purification device 60 as a whole can be prevented from being determined to have failed even though the NOx purification rate within the allowable range is secured. it can. Therefore, the diagnostic accuracy of the fault diagnosis of the exhaust purification device 60 can be improved.

<Example 2>
The schematic configuration of the internal combustion engine and its intake / exhaust system according to this embodiment is the same as that of the first embodiment. Hereinafter, differences from the first embodiment in the failure diagnosis of the exhaust gas purification apparatus according to the present embodiment will be described.

  In the failure diagnosis of the exhaust gas purification apparatus according to the first embodiment, the determination purification rate that is a threshold for comparison with the NOx purification rate in the SCR filter 51 is changed based on the ash accumulation amount in the SCR filter 51. In the failure diagnosis of the exhaust purification apparatus according to the present embodiment, instead of this, the corrected purification rate is calculated by correcting the NOx purification rate in the SCR filter 51 based on the ash accumulation amount in the SCR filter 51. And it is discriminate | determined whether the exhaust gas purification device 60 is out of order by comparing a correction | amendment purification rate with a determination purification rate.

  Specifically, when the ash accumulation amount in the SCR filter 51 is large, the corrected purification rate is calculated by correcting the NOx purification rate in the SCR filter 51 to a larger value than when the ash accumulation amount is small. That is, when the ash accumulation amount in the SCR filter 51 is large, the correction width (increase) from the NOx purification rate to the correction purification rate in the SCR filter 51 is increased compared to when the ash accumulation amount is small. According to this, even when the value of the NOx purification rate in the SCR filter 51 is the same, if the NOx purification rate is reduced due to an increase in the ash deposition amount in the SCR filter 51, the degree of reduction is reduced. Accordingly, the value of the correction purification rate increases.

By performing failure diagnosis using such a corrected purification rate, the ash accumulation amount in the SCR filter 51 is changed as in the case of changing the determination purification rate based on the ash accumulation amount in the SCR filter 51 as in the first embodiment. The influence on the determination as to whether or not the exhaust gas purification device 60 has failed can be reduced. Therefore, the exhaust purification device 60 as a whole can be prevented from being determined to be out of order even though the NOx purification rate within the allowable range is secured. Therefore, the diagnostic accuracy of the fault diagnosis of the exhaust purification device 60 can be improved.

[Failure diagnosis flow]
FIG. 7 is a flowchart showing a failure diagnosis flow of the exhaust gas purification apparatus according to the present embodiment. According to this flow, the ECU 10 performs a failure diagnosis of the exhaust purification device 60 during the operation of the internal combustion engine 1. Note that, in this flow, steps in which the same processing as the processing in each step of the flow shown in FIG.

  In this flow, after the process of S103 is executed, the process of S204 is executed. In S204, a coefficient k2 used to calculate a corrected purification rate Rfm, which will be described later, is calculated based on the ash accumulation amount Aash in the SCR filter 51 read in S103. FIG. 8 is a diagram showing the correlation between the ash deposition amount Aash and the coefficient k2 in the SCR filter 51. As shown in FIG. As shown in FIG. 8, when the ash deposition amount Aash is zero, the coefficient k2 is 1. The value of the coefficient k2 increases as the value of the ash deposition amount Aash increases. In the ECU 10, the correlation between the ash deposition amount Aash and the coefficient k2 in the SCR filter 51 as shown in FIG. 8 is stored as a map. In S204, the coefficient k2 is calculated using this map.

Next, in S205, a corrected purification rate Rfm is calculated. Here, the corrected purification rate Rfm is calculated by the following equation (2).
Rfm = Rf × k2 (2)
Rf: NOx purification rate in the SCR filter 51 calculated in S102 k2: Coefficient calculated in the S104 The modified purification rate Rfm calculated by the above equation (2) is the same when the NOx purification rate Rf in the SCR filter 51 is the same. The larger the ash deposition amount Aash in the SCR filter 51, the larger the amount. The corrected purification rate Rfm may be calculated by increasing the NOx purification rate Rf in the SCR filter 51 stepwise in accordance with the increase in the ash accumulation amount Aash in the SCR filter 51.

  Next, in S206, it is determined whether or not the corrected purification rate Rfm calculated in S205 is greater than the determined purification rate Rfth. Here, the determination purification rate Rfth is a constant value determined in advance based on experiments or the like, and the determination purification rate when it is assumed that no ash is accumulated on the SCR filter 51 (reference determination purification according to the first embodiment). Rate Rfth0). If an affirmative determination is made in S206, it is then determined in S107 that the exhaust purification device 60 is normal. On the other hand, if a negative determination is made in S206, it is then determined in S108 that the exhaust purification device 60 has failed.

  It should be noted that the exhaust gas purification apparatus failure diagnosis methods according to the first embodiment and the second embodiment may be combined. That is, when performing failure diagnosis of the exhaust purification device 60, the ECU 10 sets the determination purification rate to a smaller value when the ash accumulation amount in the SCR filter 51 is large than when the ash accumulation amount is small. At the same time, the corrected purification rate may be calculated by correcting the NOx purification rate in the SCR filter 51 to a larger value. And ECU10 may discriminate | determine whether the exhaust gas purification apparatus 60 is out of order by comparing these determination purification rates and correction purification rates.

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 4 ... Intake passage 5 ... Exhaust passage 50 ... Oxidation catalyst 51 ... SCR filter 52 ... Rear stage SCR catalyst 53 ... Fuel addition valve 54 ... Urea water addition valve 55 ... Temperature sensor 56 .. NOx sensor 60 .. Exhaust gas purification device 10 .. ECU

Claims (2)

An SCR filter configured to carry a SCR catalyst for reducing NOx in exhaust gas on a filter that collects particulate matter in exhaust gas, the SCR filter provided in an exhaust passage of an internal combustion engine, Failure of the exhaust gas purification device for diagnosing whether or not the exhaust gas purification device having a rear-stage SCR catalyst for reducing NOx and having a rear-stage SCR catalyst provided in an exhaust passage downstream of the SCR filter has failed In the diagnostic device,
A NOx sensor provided in an exhaust passage between the SCR filter and the rear SCR catalyst;
A NOx purification rate calculating unit for calculating a NOx purification rate in the SCR filter using a detection value of the NOx sensor;
A determination unit that determines that the exhaust purification device has failed when the NOx purification rate in the SCR filter calculated by the NOx purification rate calculation unit is equal to or less than a predetermined determination purification rate;
An ash accumulation amount estimation unit for estimating an ash accumulation amount in the SCR filter,
A fault diagnosis device for an exhaust gas purification apparatus, wherein when the ash accumulation amount in the SCR filter estimated by the ash accumulation amount estimation unit is large, the determination purification rate is smaller than when the ash accumulation amount is small. An SCR filter configured to carry a SCR catalyst for reducing NOx in exhaust gas on a filter that collects particulate matter in exhaust gas, the SCR filter provided in an exhaust passage of an internal combustion engine, Failure of the exhaust gas purification device for diagnosing whether or not the exhaust gas purification device having a rear-stage SCR catalyst for reducing NOx and having a rear-stage SCR catalyst provided in an exhaust passage downstream of the SCR filter has failed In the diagnostic device,
A NOx sensor provided in an exhaust passage between the SCR filter and the rear SCR catalyst;
A NOx purification rate calculating unit for calculating a NOx purification rate in the SCR filter using a detection value of the NOx sensor;
An ash accumulation amount estimation unit for estimating an ash accumulation amount in the SCR filter;
Correction for calculating a corrected purification rate by correcting the NOx purification rate in the SCR filter calculated by the NOx purification rate calculation unit based on the ash deposition amount in the SCR filter estimated by the ash deposition amount estimation unit A purification rate calculator,
A determination unit that determines that the exhaust purification device has failed when the corrected purification rate calculated by the corrected purification rate calculation unit is equal to or less than a predetermined determination purification rate;
When the ash accumulation amount in the SCR filter estimated by the ash accumulation amount estimation unit is large, the modified purification rate calculation unit increases the NOx purification rate in the SCR filter more than when the ash accumulation amount is small. A fault diagnosis device for an exhaust gas purification device that calculates the corrected purification rate by correcting to a large value.

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