JP2017115805A - Failure diagnosis device for exhaust emission control system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve diagnostic accuracy when diagnosing an exhaust emission control system having an SCR filter to determine whether the system is broken.SOLUTION: When an NOx elimination rate of an SCR filter calculated by using a detection value obtained by an NOx sensor provided downstream of the SCR filter is equal to or lower than a prescribed determination elimination rate, a determination that an exhaust emission control system is broken is made. At this time, an increment of a differential pressure conversion value per unit increment of filter PM accumulation amount is defined as a differential pressure change rate. If the differential pressure change rate at time when the detection value obtained by the NOx sensor and used for calculation of the NOx elimination rate is detected is a prescribed threshold value or higher, the abnormality diagnosis is not made.SELECTED DRAWING: Figure 7

Description

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

  A technique is known in which an SCR filter having a configuration in which an SCR catalyst (selective reduction type NOx catalyst) is supported on a filter is provided in an exhaust passage of an internal combustion engine. Here, the SCR catalyst has a function of reducing NOx in the exhaust gas using ammonia as a reducing agent. Further, the filter has a function of collecting particulate matter (Particulate Matter: hereinafter also referred to as “PM”) in the exhaust gas. In an exhaust gas purification system for an internal combustion engine having such an SCR filter, ammonia as a reducing agent is supplied to the SCR filter by an ammonia supply device provided in the exhaust passage.

  Patent Document 1 discloses a technique related to a failure diagnosis device for an exhaust purification system including an SCR catalyst provided in an exhaust passage. In the technique described in Patent Document 1, a predetermined threshold for failure diagnosis is based on an intermediate characteristic between a purification characteristic when the exhaust purification system is normal and a purification characteristic when the exhaust purification system is out of order. And a correction coefficient is set. The NOx purification rate calculated using the NOx outflow amount from the SCR catalyst as a parameter is corrected by the set correction coefficient. By comparing the corrected NOx purification rate with a predetermined threshold value, it is diagnosed whether or not the exhaust purification system has failed.

  Further, in Non-Patent Document 1, when the PM accumulation amount in the SCR filter increases, the ammonia adsorption amount that is the amount of ammonia adsorbed on the SCR catalyst supported on the SCR filter tends to increase. It is disclosed.

Japanese Patent Laying-Open No. 2015-010589

"Physico-Chemical Modeling of an Integrated SCR on DPF (SCR / DPF) System," SAE International Journal of Engines, August 2012 vol. 5 no. 3, 958-974

  A technique for diagnosing whether or not an exhaust purification system including an SCR filter has failed based on a NOx purification rate in the SCR filter (a ratio of a NOx amount reduced in the SCR filter to a NOx amount flowing into the SCR filter) It has been known. Here, as disclosed in the above-described prior art documents, in the SCR filter, the ammonia adsorption amount on the SCR catalyst supported on the SCR filter may fluctuate due to the influence of the PM deposition state. When the ammonia adsorption amount at the SCR catalyst varies, the NOx purification rate in the SCR filter varies accordingly. Therefore, in order to perform a failure diagnosis of the exhaust purification system with high accuracy using the NOx purification rate in the SCR filter, it is necessary to consider the influence of the PM accumulation state in the SCR filter.

  The present invention has been made in view of the above problems, and an object of the present invention is to improve diagnosis accuracy when diagnosing whether or not an exhaust purification system including an SCR filter has failed.

  PM in the exhaust gas is collected on the SCR filter, and the collected PM gradually accumulates. At this time, in the SCR filter, first, PM is deposited in the partition walls (that is, in the pores formed in the partition walls). And PM accumulates on the surface of a partition, after the deposition amount of PM in a partition reaches an upper limit. Hereinafter, the deposition of PM in the partition wall of the SCR filter is sometimes referred to as “in-wall PM deposition”, and the period during which PM deposition in the wall is progressing may be referred to as “in-wall PM deposition period”. Further, the amount of PM deposited in the partition wall of the SCR filter may be referred to as “in-wall PM deposition amount”. Further, the deposition of PM on the surface of the partition wall of the SCR filter is sometimes referred to as “surface layer PM deposition”, and the period during which surface layer PM deposition is in progress may be referred to as “surface layer PM deposition period”. In addition, the amount of PM deposited on the surface of the partition wall of the SCR filter may be referred to as “surface layer PM deposition amount”.

  As described above, conventionally, it has been considered that when the amount of PM deposited on the SCR filter increases, the ammonia adsorption amount on the SCR catalyst supported on the SCR filter tends to increase. However, the detailed correlation between the PM accumulation state on the SCR filter and the increasing tendency of the ammonia adsorption amount on the SCR catalyst has been unknown so far. However, the inventor of the present invention tends to increase the ammonia adsorption amount on the SCR catalyst when the PM deposition amount in the wall in the SCR filter is large compared to when the PM deposition amount in the wall is small. It was newly found that the increase or decrease in the surface PM deposition amount in the SCR filter has a tendency to hardly affect the increase or decrease in the ammonia adsorption amount in the SCR catalyst. Here, when the amount of PM deposition in the wall is large, the amount of ammonia adsorbed on the SCR catalyst tends to increase more easily than when the amount of PM deposition in the wall is small. This is thought to be because the amount of ammonia adsorbed on the SCR catalyst increases and the amount of ammonia desorbed from the SCR catalyst decreases accordingly. On the other hand, even if the surface PM deposition amount changes, the saturated adsorption amount of ammonia in the SCR catalyst hardly changes, and therefore the ammonia amount desorbed from the SCR catalyst hardly changes. For this reason, the increase or decrease in the surface PM deposition amount is considered to have little effect on the increase or decrease in the ammonia adsorption amount on the SCR catalyst. The present invention reflects the above new knowledge in failure diagnosis of an exhaust purification system using the NOx purification rate in the SCR filter.

  More specifically, the failure diagnosis device for an exhaust gas purification system according to the present invention is an SCR filter that is provided in an exhaust passage of an internal combustion engine and has a SCR catalyst supported on a filter, An SCR filter having a function of reducing NOx in exhaust gas using ammonia as a reducing agent, the filter having a function of collecting particulate matter in the exhaust gas, and an ammonia supply device for supplying ammonia to the SCR filter; , A NOx sensor provided in an exhaust passage downstream of the SCR filter, and a detected value of the NOx sensor. And a NOx purification rate calculation unit that calculates a NOx purification rate in the SCR filter, and a NOx purification rate calculation unit. A diagnosis unit that performs a failure diagnosis to determine whether or not the exhaust purification system has failed by comparing the issued NOx purification rate with a predetermined determination purification rate, and upstream of the SCR filter Based on parameters other than the differential pressure conversion value, a conversion value obtained by converting the difference in the exhaust pressure with the downstream into a value when the flow rate of the exhaust gas flowing into the SCR filter is assumed to be constant is defined as a differential pressure conversion value. The increase amount of the differential pressure conversion value per unit increase amount of the filter PM deposition amount when the deposition amount of the particulate matter in the estimated SCR filter is the filter PM deposition amount is defined as the differential pressure change rate, The time when the detected value of the NOx sensor used for the calculation of the NOx purification rate by the NOx purification rate calculation unit is detected as a sensor detection time, and the diagnosis unit performs the sensor detection time at the sensor detection time. The pressure change rate does not execute the failure diagnosis when the predetermined threshold value or more, the difference pressure change rate to execute the failure diagnosis when less than the predetermined threshold.

  In the exhaust purification system according to the present invention, ammonia as a reducing agent is supplied to the SCR filter by the ammonia supply device. Then, the supplied ammonia is adsorbed on the SCR catalyst supported on the SCR filter. The ammonia supply device may supply ammonia as a gas or liquid, or may supply an ammonia precursor.

  Further, when the NOx purification function of the SCR filter is lowered due to deterioration of the SCR catalyst carried on the SCR filter, the NOx purification rate in the SCR filter is lowered. Further, when the ammonia amount supplied to the SCR filter becomes smaller than a desired amount due to an abnormality in the ammonia supply device, the NOx purification rate in the SCR filter is lowered. Therefore, the failure of the exhaust purification system according to the present invention includes not only the deterioration of the NOx purification function of the SCR filter but also the abnormality of the ammonia supply device.

  In the present invention, the NOx purification rate calculation unit calculates the NOx purification rate in the SCR filter using the detected value of the NOx sensor provided in the exhaust passage downstream of the SCR filter. Then, the diagnosis unit executes a failure diagnosis for determining whether or not the exhaust purification system has failed by comparing the calculated NOx purification rate with a predetermined determination purification rate. Here, the determination purification rate is a value set as a threshold value to determine that the exhaust purification system is out of order when the NOx purification rate in the SCR filter falls below the determination purification rate.

  According to the new knowledge described above, the NOx purification function itself of the SCR filter is in the same state, and even if the ammonia amount supplied to the SCR filter is the same, the amount of PM deposition in the wall in the SCR filter is different. In some cases, the amount of ammonia adsorbed on the SCR catalyst is different. Specifically, the amount of ammonia adsorbed on the SCR catalyst tends to increase as the amount of PM deposition in the wall in the SCR filter increases. If the parameters correlated with the NOx purification rate other than the ammonia adsorption amount at the SCR catalyst are the same, the NOx purification rate at the SCR filter increases as the ammonia adsorption amount at the SCR catalyst increases.

  Therefore, the NOx purification function itself of the SCR filter is in the same state, and even if the amount of ammonia supplied to the SCR filter is the same, that is, the state of the exhaust purification system is the same, During the in-wall PM deposition period in which the in-wall PM deposition amount varies, the NOx purification rate calculated by the NOx purification rate calculation unit may be different due to the difference in the in-wall PM deposition amount. .

  On the other hand, during the surface PM deposition period, the PM deposition amount in the wall is constant at the upper limit value. Moreover, according to the new knowledge mentioned above, the increase / decrease in the surface PM deposition amount in the SCR filter has little influence on the increase / decrease in the ammonia adsorption amount in the SCR catalyst. Therefore, the increase / decrease in the surface PM deposition amount in the SCR filter has little influence on the NOx purification rate of the SCR filter. Therefore, during the surface PM deposition period, if the state of the exhaust purification system is the same and other parameters correlated with the NOx purification rate have the same value, regardless of the PM deposition amount in the SCR filter (surface PM) Regardless of the accumulation amount, the NOx purification rate calculated by the NOx purification rate calculation unit is a constant value.

  Therefore, in the present invention, whether or not to perform failure diagnosis of the exhaust purification system using the NOx purification rate is determined according to whether the sensor detection time is during the PM deposition period in the wall or during the surface PM deposition period. Is done. Here, the sensor detection time is the time when the detection value of the NOx sensor used for calculating the NOx purification rate in the SCR filter is detected.

  Specifically, when the differential pressure change rate at the sensor detection time is equal to or greater than a predetermined threshold, the diagnosis unit does not execute the fault diagnosis and performs the fault diagnosis when the differential pressure change rate is smaller than the predetermined threshold. Run. Here, the differential pressure change rate is an increase amount of the differential pressure conversion value per unit increase amount of the filter PM accumulation amount. The filter PM accumulation amount is a value estimated based on parameters other than the differential pressure conversion value. The differential pressure change rate defined in this way is a smaller value during the surface PM deposition period than during the in-wall PM deposition period. Therefore, the predetermined threshold value according to the present invention is set to a value that can distinguish whether it is during the in-wall PM deposition period or the surface layer PM deposition period.

  The PM deposition in the SCR filter changes to the surface layer PM deposition after the PM deposition in the wall reaches the upper limit value. On the other hand, the oxidation of PM in the SCR filter occurs both in the partition and on the surface of the partition. Can happen. Therefore, even after PM deposition on the SCR filter has once shifted to surface PM deposition, the amount of PM deposition in the wall may decrease due to oxidation of PM in the partition wall. In this case, when PM deposition is resumed, PM is deposited again in the partition walls (that is, transition from surface PM deposition to in-wall PM deposition). Therefore, whether the PM deposition period in the wall or the surface layer PM deposition period is based only on the elapsed time from the point when PM starts to deposit on the SCR filter, or only on the filter PM deposition amount (PM deposition amount on the entire SCR filter). It is difficult to distinguish exactly what is inside. Therefore, in the present invention, the differential pressure change rate is used as a parameter for distinguishing between the in-wall PM deposition period and the surface layer PM deposition period.

  By determining whether or not the failure diagnosis of the exhaust purification system can be performed as described above, the failure diagnosis is not performed when the sensor detection time is in the in-wall PM deposition period, and the sensor detection time is not in the surface PM deposition period. It will be executed when inside. Therefore, according to the present invention, it is possible to improve the diagnostic accuracy of the fault diagnosis of the exhaust purification system provided with the SCR filter.

  ADVANTAGE OF THE INVENTION According to this invention, the diagnostic accuracy at the time of diagnosing whether the exhaust gas purification system provided with the SCR filter has failed can be improved.

It is a figure which shows schematic structure of the internal combustion engine which concerns on the Example of this invention, and its intake / exhaust system. It is a block diagram which shows the function of PM deposition amount calculation part in ECU which concerns on the Example of this invention. It is a block diagram which shows the function of the adsorption amount calculation part in ECU which concerns on the Example of this invention. It is a figure for demonstrating the influence which the accumulation condition of PM in an SCR filter has on the saturated adsorption amount of ammonia of the SCR catalyst carry | supported by this SCR filter. It is a figure which shows the correlation with the accumulation condition of PM in an SCR filter, and the saturated adsorption amount of ammonia of an SCR catalyst. It is a figure which shows transition of the differential pressure | voltage conversion value according to the increase in filter PM deposition amount. It is a flowchart which shows the flow of a failure diagnosis of the exhaust gas purification system which concerns on the Example of this invention.

  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>
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. When the internal combustion engine 1 is a spark ignition type internal combustion engine, the fuel injection valve 3 may be configured to inject fuel into the intake port.

  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, a fuel addition valve 52, and a urea water addition valve 53. The SCR filter 51 is configured by supporting a SCR catalyst 51a on a wall flow type filter formed of a porous base material. The filter has a function of collecting PM in the exhaust. The SCR catalyst 51a has a function of reducing NOx in the exhaust gas using ammonia as a reducing agent. Therefore, the SCR filter 51 has a PM collection function and a NOx purification function. The oxidation catalyst 50 is provided in the exhaust passage 5 upstream of the SCR filter 51. The fuel addition valve 52 is provided in the exhaust passage 5 further upstream than the oxidation catalyst 50. The fuel addition valve 52 adds fuel to the exhaust flowing in the exhaust passage 5. The urea water addition valve 53 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 53 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 53, the urea water is supplied to the SCR filter 51. That is, urea, which is a precursor of ammonia, is supplied to the SCR filter 51. In the SCR filter 51, ammonia produced by hydrolysis of the supplied urea is adsorbed to the SCR catalyst 51a. Then, NOx in the exhaust is reduced using ammonia adsorbed on the SCR catalyst 51a as a reducing agent. Instead of the urea water addition valve 53, an ammonia addition valve for adding ammonia gas into the exhaust gas may be provided.

In the exhaust passage 5 downstream of the oxidation catalyst 50 and upstream of the urea water addition valve 53, an O ₂ sensor 54, an upstream temperature sensor 55, and an upstream NOx sensor 57 are provided. A downstream temperature sensor 56 and a downstream NOx sensor 58 are provided in the exhaust passage 5 downstream of the SCR filter 51. The O ₂ sensor 54 outputs an electrical signal corresponding to the O ₂ concentration of the exhaust. The upstream temperature sensor 55 and the downstream temperature sensor 56 output an electrical signal corresponding to the exhaust temperature. The upstream NOx sensor 57 and the downstream NOx sensor 58 output an electrical signal corresponding to the NOx concentration of the exhaust. A differential pressure sensor 59 is provided in the exhaust passage 5. The differential pressure sensor 59 outputs an electrical signal corresponding to a difference in exhaust pressure between the upstream and downstream of the SCR filter 51 (hereinafter also referred to as “filter differential pressure”).

The internal combustion engine 1 is also 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. The ECU 10 includes an accelerator position sensor in addition to the air flow meter 40, the O ₂ sensor 54, the upstream temperature sensor 55, the upstream NOx sensor 57, the downstream temperature sensor 56, the downstream NOx sensor 58, and the differential pressure sensor 59. 7 and various sensors such as a crank position sensor 8 are electrically connected. The accelerator position sensor 7 is a sensor that outputs an electrical signal corresponding to an operation amount (accelerator opening) of an accelerator pedal (not shown). The crank position sensor 8 is a sensor that outputs an electrical signal corresponding to 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. The ECU 10 estimates the temperature of the SCR filter 51 (hereinafter also referred to as “filter temperature”) based on the output value of the downstream temperature sensor 56. Further, the ECU 10 estimates the flow rate of exhaust gas flowing into the SCR filter 51 (hereinafter sometimes simply referred to as “exhaust gas flow rate”) based on the output value of the air flow meter 40.

  The ECU 10 is electrically connected to various devices such as the fuel injection valve 3, the throttle valve 41, the fuel addition valve 52, and the urea water addition valve 53. The ECU 10 controls the various devices described above based on the output signals of the sensors as described above. For example, the ECU 10 controls the urea water addition amount from the urea water addition valve 53 in order to maintain or adjust the ammonia adsorption amount at the SCR catalyst 51a at a predetermined target adsorption amount. The predetermined target adsorption amount is a value that can secure a desired NOx purification rate in the SCR filter 51 and that can suppress the outflow amount of ammonia from the SCR filter 51 within an allowable range in advance based on experiments and the like. It is a defined value.

  Further, the ECU 10 starts from the fuel addition valve 52 when the PM accumulation amount (hereinafter also referred to as “filter PM accumulation amount”) in the SCR filter 51 estimated by a method described later reaches a predetermined accumulation amount. Filter regeneration processing is executed by adding fuel. 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 52 is oxidized in the oxidation catalyst 50. As a result, PM deposited on the SCR filter 51 is burned and removed.

(Estimation of filter PM accumulation)
In this embodiment, the ECU 10 repeatedly calculates the filter PM accumulation amount at a predetermined calculation cycle. FIG. 2 is a block diagram illustrating the function of the PM accumulation amount calculation unit in the ECU 10. The PM accumulation amount calculation unit 110 is a functional unit for calculating the filter PM accumulation amount, and is realized by executing a predetermined program in the ECU 10. The PM accumulation amount calculation unit 110 according to the present embodiment is a differential pressure conversion that is a conversion value obtained by converting a filter differential pressure detected by the differential pressure sensor 59, which will be described later, to a value when the exhaust flow rate is assumed to be constant. The amount of filter PM deposition is calculated without using a value. Further, in the PM accumulation amount calculation unit 110 according to the present embodiment, the filter PM accumulation amount is calculated on the assumption that the PM collection function of the SCR filter 51 is in a normal state.

  The PM accumulation amount calculation unit 110 integrates the PM collection amount, which is the PM amount collected by the SCR filter 51, and the PM oxidation amount, which is the amount of PM oxidized by the SCR filter 51. The amount of accumulated filter PM is calculated. Specifically, the PM accumulation amount calculation unit 110 includes a PM collection amount calculation unit 111 and a PM oxidation amount calculation unit 112. The PM collection amount calculation unit 111 calculates the PM amount collected by the SCR filter 51 during the first predetermined period corresponding to the calculation period of the filter PM accumulation amount as the PM collection amount. The PM oxidation amount calculation unit 112 calculates the amount of PM oxidized in the SCR filter 51 during the first predetermined period as the PM oxidation amount.

  The PM collection amount calculation unit 111 receives a PM amount discharged from the internal combustion engine 1 during the first predetermined period (hereinafter may be simply referred to as “PM discharge amount”). The PM emission amount can be estimated based on the operating state of the internal combustion engine 1. The PM collection amount calculation unit 111 multiplies the input PM discharge amount by a predetermined PM collection rate (the ratio of the PM amount collected by the SCR filter 51 to the PM amount flowing into the SCR filter 51). Thus, the amount of PM trapped is calculated. Note that the predetermined PM collection rate may be a value estimated based on the exhaust gas flow rate.

On the other hand, the PM oxidation amount calculation unit 112 flows into the filter temperature, the O ₂ concentration of exhaust gas flowing into the SCR filter 51 (hereinafter also referred to as “inflow O ₂ concentration”), and the SCR filter 51. The NO ₂ concentration of exhaust gas (hereinafter sometimes referred to as “inflow NO ₂ concentration”) is input. The filter temperature can be estimated based on the output value of the downstream temperature sensor 56. The inflow O ₂ concentration is detected by the O ₂ sensor 54. The inflow O ₂ concentration can also be estimated based on the air-fuel ratio of the exhaust, the operating state of the internal combustion engine 1, and the like. The inflow NO ₂ concentration can be estimated based on the output value of the air flow meter 40, the output value of the upstream temperature sensor 55, the output value of the upstream NOx sensor 57, and the like. More specifically, the NOx amount in the exhaust gas can be estimated based on the output value of the upstream NOx sensor 57 and the exhaust gas flow rate. Further, based on the temperature of the oxidation catalyst 50 estimated based on the output value of the upstream temperature sensor 55 and the exhaust gas flow rate, the ratio of the NO ₂ amount in the NOx amount in the exhaust gas can be estimated. The inflow NO ₂ concentration can be estimated based on the NOx amount in the exhaust gas and the estimated value of the ratio of the NO ₂ amount in the NOx amount in the exhaust gas. Further, the PM oxidation amount calculation unit 112 receives the filter PM accumulation amount calculated in the previous calculation (hereinafter also referred to as “deposition amount previous value”). Then, the PM oxidation amount calculation unit 112 calculates the PM oxidation amount based on the input filter temperature, inflow O ₂ concentration, inflow NO ₂ concentration, and the previous deposition amount value.

  Then, the PM accumulation amount calculation unit 110 adds the PM collection amount that is an increase to the previous value of the accumulation amount, and subtracts the PM oxidation amount that is a decrease, thereby obtaining the current filter PM accumulation amount. (Current filter PM accumulation amount) is calculated. The calculated current filter PM accumulation amount is used as the previous accumulation amount value in the next calculation.

  Note that the method for calculating the filter PM accumulation amount according to the present invention is not limited to the above method. As the filter PM accumulation amount according to the present invention, any known method may be adopted as long as it is a calculation method using parameters other than the differential pressure conversion value described later.

(Estimation of ammonia adsorption amount)
In the present embodiment, the ECU 10 repeatedly calculates the ammonia adsorption amount, which is the ammonia amount adsorbed on the SCR catalyst 51a, at a predetermined calculation cycle. FIG. 3 is a block diagram illustrating the function of the adsorption amount calculation unit in the ECU 10. The adsorption amount calculation unit 120 is a functional unit for calculating the ammonia adsorption amount in the SCR catalyst 51a, and is realized by executing a predetermined program in the ECU 10. In the adsorption amount calculation unit 120 according to the present embodiment, the ammonia adsorption amount is calculated on the assumption that the NOx purification function of the SCR filter 51 is in a normal state. Further, in the adsorption amount calculation unit 120 according to the present embodiment, the ammonia adsorption amount is calculated on the assumption that PM is not deposited on the SCR filter 51 (that is, the ammonia adsorption amount calculated by the adsorption amount calculation unit 120). The value of the amount is a value that does not consider the influence on the ammonia adsorption amount in the SCR catalyst 51a due to the PM accumulation state in the SCR filter 51 as will be described later.

In the adsorption amount calculation unit 120, an ammonia supply amount which is an ammonia amount supplied to the SCR filter 51, an ammonia consumption amount which is an ammonia amount consumed for NOx reduction in the SCR catalyst 51a, and a desorption from the SCR catalyst 51a. The current ammonia adsorption amount is calculated by integrating the ammonia desorption amount, which is the amount of ammonia to be produced. Specifically, the adsorption amount calculation unit 120 includes a consumption amount calculation unit 121 and a desorption amount calculation unit 122. The consumption amount calculation unit 121 calculates, as the ammonia consumption amount, the ammonia amount consumed for the reduction of NOx in the SCR catalyst 51a during the second predetermined period according to the calculation period of the ammonia adsorption amount. The desorption amount calculation unit 122 calculates the ammonia amount desorbed from the SCR catalyst during the second predetermined period as the ammonia desorption amount. Further, the adsorption amount calculation unit 120 estimates the ammonia amount supplied to the SCR filter 51 during the second predetermined period as the ammonia supply amount. As mentioned above, SC
Ammonia supplied to the R filter 51 is generated by hydrolysis of urea contained in the urea water added from the urea water addition valve 53. Therefore, the ammonia supply amount can be estimated based on the urea water amount added from the urea water addition valve 53 during the second predetermined period.

  The consumption amount calculation unit 121 includes the NOx concentration of exhaust gas flowing into the SCR filter 51 (hereinafter also referred to as “inflow NOx concentration”), the exhaust flow rate, the filter temperature, and the SCR calculated in the previous calculation. An ammonia adsorption amount at the catalyst 51a (hereinafter, also referred to as “adsorption amount previous value”) may be input. The inflow NOx concentration is detected by the upstream NOx sensor 57. Here, the NOx purification rate at the SCR catalyst 51a has a correlation with the exhaust gas flow rate, the filter temperature, and the ammonia adsorption amount at the SCR catalyst 51a. Therefore, the consumption amount calculation unit 121 is based on the input exhaust gas flow rate, filter temperature, and previous adsorption amount value, and the NOx purification rate (hereinafter, “estimated”) that is estimated to be exhibited in the SCR catalyst 51a at the present time. May be referred to as “NOx purification rate”). Further, in the consumption amount calculation unit 121, the NOx amount flowing into the SCR filter 51 during the second predetermined period (hereinafter referred to as “inflow NOx amount”) based on the input inflow NOx concentration and the exhaust gas flow rate. In some cases). Then, the ammonia consumption amount is calculated based on the calculated estimated NOx purification rate and inflow NOx amount.

  On the other hand, the desorption amount calculation unit 122 receives the filter temperature and the previous adsorption amount value. If the ammonia adsorption amount at the SCR catalyst 51a is the same, the ammonia desorption amount increases as the filter temperature increases. If the filter temperatures are the same, the ammonia desorption amount increases as the ammonia adsorption amount on the SCR catalyst 51a increases. Based on these correlations, the desorption amount calculation unit 122 calculates the ammonia desorption amount based on the input filter temperature and the previous adsorption amount value.

  Then, the adsorption amount calculation unit 120 adds the ammonia supply amount that is an increase to the previous value of the adsorption amount, and subtracts the ammonia consumption amount and the ammonia desorption amount that are a decrease, thereby obtaining the current SCR. The ammonia adsorption amount at the catalyst 51a is calculated.

(Relationship between PM deposition status and ammonia adsorption amount)
Here, the relationship between the PM accumulation state in the SCR filter 51 and the ammonia adsorption amount in the SCR catalyst 51a will be described. As described above, the inventor of the present invention has found new knowledge about the correlation between the PM accumulation state in the SCR filter and the increasing tendency of the ammonia adsorption amount in the SCR catalyst. According to this knowledge, even when the filter temperature and the ammonia adsorption amount at the SCR catalyst 51a are the same, when the PM deposition amount (PM deposition amount in the wall) in the partition wall of the SCR filter 51 is large, the PM in the wall The ammonia desorption amount is smaller than when the deposition amount is small. As a result, even if the value of the other parameter related to the amount of increase in the amount of ammonia adsorbed by the SCR catalyst 51a is the same, the SCR catalyst is greater when the PM deposition amount in the wall is large than when the PM deposition amount is small. The ammonia adsorption amount at 51a is likely to increase. Further, the amount of PM deposition in the wall in the SCR filter 51 has reached the upper limit, and after the PM deposition in the SCR filter 51 has shifted from the PM deposition in the wall to the surface PM deposition, the filter temperature and the SCR catalyst 51a If the ammonia adsorption amount is the same, the ammonia desorption amount hardly changes even if the filter PM deposition amount (that is, the surface PM deposition amount) changes. Therefore, the increase / decrease in the surface PM deposition amount has little influence on the increase / decrease in the ammonia adsorption amount in the SCR catalyst 51a.

The fluctuation tendency of the ammonia adsorption amount in the SCR catalyst 51a with respect to the PM accumulation state in the SCR filter 51 is as follows. The PM accumulation state in the SCR filter 51 and the saturated adsorption amount of ammonia in the SCR catalyst 51a This is considered to be due to the correlation with the upper limit of the amount of ammonia that can be adsorbed (hereinafter sometimes simply referred to as “saturated adsorption amount”). FIG. 4 is a diagram for explaining the influence of the PM accumulation state in the SCR filter 51 on the saturated adsorption amount of the SCR catalyst 51a. In FIG. 4, the horizontal axis represents the filter temperature, and the vertical axis represents the saturated adsorption amount of the SCR catalyst 51a. In FIG. 4, a line L <b> 1 indicates the correlation between the filter temperature and the saturated adsorption amount when PM is not deposited on the SCR filter 51. On the other hand, in FIG. 4, a line L <b> 2 indicates the correlation between the filter temperature and the saturated adsorption amount when PM is deposited on the SCR filter 51. The higher the filter temperature (that is, the higher the temperature of the SCR catalyst 51a), the easier it is for ammonia to desorb from the SCR catalyst 51a. Therefore, the saturated adsorption amount of the SCR catalyst 51a decreases as the filter temperature increases. In other words, the saturation adsorption amount of the SCR catalyst 51a increases as the filter temperature decreases. At this time, as shown in FIG. 4, if the filter temperature is the same, when PM is accumulated in the SCR filter 51, compared to when PM is not accumulated in the SCR filter 51, The saturated adsorption amount of the SCR catalyst 51a increases.

  Here, a more detailed correlation between the PM accumulation state in the SCR filter 51 and the saturated adsorption amount of the SCR catalyst 51a will be described with reference to FIG. FIG. 5 is a diagram showing a correlation between an assumed PM accumulation state in the SCR filter 51 and a saturated adsorption amount of the SCR catalyst 51a. In FIG. 5, the horizontal axis represents the filter PM accumulation amount, and the vertical axis represents the saturated adsorption amount of the SCR catalyst 51a. FIG. 5 shows the transition of the saturated adsorption amount of the SCR catalyst 51a under a constant filter temperature.

  As shown in FIG. 5, when PM is deposited on the SCR filter 51, first, PM is deposited in the partition walls (that is, in the pores formed in the partition walls). And PM accumulates on the surface of a partition, after PM deposition amount in a wall reaches an upper limit. That is, after the amount of PM deposition in the wall reaches the upper limit, the PM deposition in the SCR filter 51 shifts from PM deposition in the wall to surface PM deposition. At this time, as shown in FIG. 5, during the in-wall PM accumulation period, the saturated adsorption amount of the SCR catalyst 51a increases in accordance with the increase in the filter PM accumulation amount (that is, the in-wall PM accumulation amount). On the other hand, during the surface PM deposition period, the saturated adsorption amount of the SCR catalyst 51a does not increase even if the filter PM deposition amount (that is, the surface layer PM deposition amount) increases. However, during the surface PM deposition period, the PM deposition amount in the wall is an upper limit value. Therefore, during the surface PM deposition period, the saturated adsorption amount of the SCR catalyst 51a is constant at the amount when the in-wall PM deposition amount reaches the upper limit value. That is, as shown in FIG. 4, the difference in the saturated adsorption amount of the SCR catalyst 51a between the state where PM is accumulated on the SCR filter 51 and the state where PM is not accumulated on the SCR filter 51 is as follows. This is considered to be caused by PM deposition in the wall.

  When the saturated adsorption amount of the SCR catalyst 51a increases, ammonia is difficult to desorb from the SCR catalyst 51a. Therefore, when the filter temperature, which is another parameter correlated with the amount of ammonia desorption, and the amount of ammonia adsorbed on the SCR catalyst 51a are the same, when the amount of PM deposition in the wall is large, the amount of PM deposition in the wall is small. The amount of ammonia desorbed is smaller than that. Therefore, during the in-wall PM accumulation period in which the in-wall PM accumulation amount fluctuates, even if the filter temperature and the ammonia adsorption amount in the SCR catalyst 51a are the same, the ammonia desorption amount depends on the in-wall PM accumulation amount. Is a different amount. Therefore, during the PM deposition period in the wall, even if the values of other parameters related to the increase in the amount of adsorption of ammonia in the SCR catalyst 51a are the same, the ammonia in the SCR catalyst 51a depends on the PM deposition amount in the wall. The amount of adsorption is different.

On the other hand, as described above, since the PM deposition amount in the wall is constant at the upper limit value during the surface PM deposition period, even if the filter PM deposition amount increases (that is, the surface PM deposition amount increases), the SCR catalyst 51a. The saturated adsorption amount of does not increase. Therefore, during the surface layer PM deposition period, if the filter temperature and the ammonia adsorption amount on the SCR catalyst 51a are in the same state, the ammonia desorption amount does not change even if the surface layer PM deposition amount changes. Therefore, during the surface PM deposition period, the increase or decrease in the filter PM deposition amount is considered to have little effect on the increase or decrease in the ammonia adsorption amount on the SCR catalyst.

(Failure diagnosis)
In this embodiment, when the NOx purification function of the SCR filter 51 is lowered due to deterioration of the SCR catalyst 51a or the like, the NOx purification rate in the SCR filter 51 is lowered. Further, when the urea water addition amount is reduced due to an abnormality in the urea water addition valve 53 and the amount of ammonia supplied to the SCR filter 51 is smaller than a desired amount, the NOx in the SCR filter 51 is also reduced. The purification rate decreases. Therefore, in this embodiment, both the decrease in the NOx purification function of the SCR filter 51 and the abnormality of the urea water addition valve 53 are detected as a failure of the exhaust purification system 60 including the SCR filter 51 and the urea water addition valve 53. Hereinafter, failure diagnosis for detecting a failure of the exhaust purification system 60 according to the present embodiment will be described.

  In this embodiment, it is determined whether or not the exhaust purification system 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 based on the detection value of the upstream NOx sensor 57 (that is, the inflow NOx concentration) and the detection value of the downstream NOx sensor 58 (that is, the outflow NOx concentration). The The inflow NOx concentration can be estimated based on the operating state of the internal combustion engine 1. 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 system 60 has failed. Here, the determination purification rate is a value set as a threshold value to determine that the exhaust purification system 60 is out of order when the NOx purification rate in the SCR filter 51 is lowered to the determination purification rate or less.

  Here, the NOx purification rate in the SCR filter 51 calculated based on the detection values of the upstream NOx sensor 57 and the downstream NOx sensor 58 is the SCR catalyst 51a at the sensor detection timing, which is the timing at which these detection values are detected. It becomes a different value depending on the ammonia adsorption amount at. That is, the greater the amount of ammonia adsorbed on the SCR catalyst 51a, the greater the amount of NOx reduced in the SCR catalyst 51a. Therefore, if the parameters correlated with the NOx purification rate other than the ammonia adsorption amount are the same, the NOx purification rate increases as the ammonia adsorption amount increases. Then, as described above, during the PM deposition period in the wall, even if the values of other parameters related to the increase in the amount of adsorption of ammonia in the SCR catalyst 51a are the same, the SCR catalyst depends on the PM deposition amount in the wall. The amount of ammonia adsorption at 51a is different. Therefore, even if the NOx purification function itself of the SCR filter 51 is in the same state and the amount of ammonia supplied to the SCR filter 51 is the same, that is, the state of the exhaust purification system 60 is the same. During the in-wall PM accumulation period, the calculated value of the NOx purification rate in the SCR filter 51 may be different due to the difference in the amount of PM accumulation in the wall at the sensor detection time. That is, even when the NOx purification function of the SCR filter 51 is reduced to the same extent, or even when the ammonia supply amount to the SCR filter 51 is reduced to the same extent due to the abnormality of the urea water addition valve 53. The calculated value of the NOx purification rate in the SCR filter 51 may be a different value. Therefore, if a failure diagnosis of the exhaust purification system 60 is executed based on the NOx purification rate when the sensor detection time is during the PM deposition period in the wall, there is a risk of causing a false diagnosis.

On the other hand, as described above, during the surface PM deposition period, the increase or decrease in the filter PM deposition amount has little effect on the increase or decrease in the ammonia adsorption amount in the SCR catalyst 51a. Therefore, the increase or decrease in the surface PM deposition amount in the SCR filter 51 hardly affects the NOx purification rate in the SCR filter 51. Therefore, during the surface PM deposition period, if the state of the exhaust purification system 60 is the same and the values of other parameters correlated with the NOx purification rate are the same, regardless of the PM deposition amount in the SCR filter 51 ( Regardless of the surface PM deposition amount), the calculated value of the NOx purification rate in the SCR filter 51 is a constant value. Therefore, a stable diagnosis result can be obtained by executing a failure diagnosis of the exhaust purification system 60 based on the NOx purification rate when the sensor detection time is during the surface PM deposition period.

  Therefore, in this embodiment, in order to suppress misdiagnosis in the failure diagnosis of the exhaust purification system 60, depending on whether the sensor detection time is in the wall PM deposition period or the surface layer PM deposition period, Whether to perform failure diagnosis of the exhaust purification system using the NOx purification rate is determined. That is, the failure diagnosis of the exhaust purification system 60 is not executed when the sensor detection time is during the in-wall PM deposition period, but the failure diagnosis of the exhaust purification system 60 is executed when the sensor detection time is during the surface PM deposition period. .

  Next, a method for distinguishing between the in-wall PM deposition period and the surface layer PM deposition period according to the present embodiment will be described with reference to FIG. FIG. 6 is a diagram illustrating the transition of the differential pressure conversion value according to the increase in the amount of filter PM accumulation. In FIG. 6, the horizontal axis represents the filter PM accumulation amount, and the vertical axis represents the differential pressure conversion value.

Here, the differential pressure conversion value is a conversion value obtained by converting the filter differential pressure detected by the differential pressure sensor 59 into a value when the exhaust flow rate is assumed to be constant. More specifically, the differential pressure conversion value according to the present embodiment is expressed by the following formula 1.
Ap = dP / Qg Formula 1
Ap: differential pressure conversion value dP: filter differential pressure (detected value of differential pressure sensor 59)
Qg: Exhaust flow rate

Further, the increase amount of the differential pressure conversion value per unit increase amount of the filter PM accumulation amount (that is, the slope of the line in FIG. 6) is defined as the differential pressure change rate. This differential pressure change rate is expressed by the following equation 2.
Rp = dAp / dQpm Equation 2
Rp: differential pressure change rate dAp: increase amount of differential pressure conversion value during third predetermined period dQpm: increase amount of filter PM deposition amount during third predetermined period Here, the length of the third predetermined period is the differential pressure In order to calculate the rate of change, it is predetermined based on the calculation cycle. Further, dAp and dQpm are the increase amount of the differential pressure conversion value and the increase amount of the filter PM accumulation amount during the third predetermined period at the same time.

  As shown in FIG. 6, the differential pressure conversion value increases as the filter PM deposition amount increases. Here, in the SCR filter 51, when PM is deposited in the partition wall, the influence on the filter differential pressure is larger than when PM is deposited on the surface of the partition wall. Therefore, if the increase amount of the PM deposition amount is the same, the increase amount of the differential pressure conversion value is larger when the PM deposition amount in the wall is increased than when the surface layer PM deposition amount is increased. Therefore, as shown in FIG. 6, the differential pressure change rate is larger during the in-wall PM deposition period than during the surface layer PM deposition period. In other words, when the PM deposition on the SCR filter 51 shifts from in-wall PM deposition to surface PM deposition, the differential pressure change rate decreases. That is, based on the differential pressure change rate, it is possible to distinguish between the PM deposition period in the wall and the surface PM deposition period. Specifically, if the differential pressure change rate is equal to or greater than a predetermined threshold, it can be determined that the in-wall PM deposition period is in progress. Moreover, if the differential pressure change rate is smaller than a predetermined threshold value, it can be determined that the surface PM deposition period is in progress.

As described above, the PM deposition in the SCR filter 51 changes in the order from the in-wall PM deposition to the surface layer PM deposition. However, the oxidation of PM in the SCR filter 51 can occur both inside the partition and on the surface of the partition. For this reason, even after the transition to surface PM deposition, the amount of PM deposition in the wall may decrease due to oxidation. When PM deposition in the SCR filter 51 is resumed, PM is first deposited in the partition wall. At this time, in-wall PM deposition may proceed while PM remains on the surface of the partition wall. Therefore, only the elapsed time from the time when PM starts to accumulate on the SCR filter 51 (for example, the elapsed time from the time when the filter regeneration process is completed) and the filter PM accumulation amount (the PM accumulation amount in the entire SCR filter 51). Based on the above, it is difficult to accurately distinguish whether it is during the in-wall PM deposition period or the surface PM deposition period. Therefore, by using the differential pressure change rate as a parameter for discriminating between the in-wall PM deposition period and the surface layer PM deposition period, these can be distinguished with higher accuracy.

(Failure diagnosis flow)
Hereinafter, the failure diagnosis flow of the exhaust purification system according to the present embodiment will be described with reference to FIG. FIG. 7 is a flowchart showing a flow of failure diagnosis of the exhaust purification system according to the present embodiment. In accordance with this flow, the ECU 10 performs a failure diagnosis of the exhaust purification system 60 during 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 system 60 is satisfied. Execution conditions for failure diagnosis of the exhaust purification system 60 are determined in advance. This execution condition is that the difference between the NOx purification rate in the SCR filter 51 when the exhaust purification system 60 is normal and the NOx purification rate in the SCR filter 51 when the exhaust purification system 60 is out of order is clear to some extent. It is set as a condition that appears. Specifically, as the execution conditions, the operating state of the internal combustion engine 1 is a steady state, the upstream side NOx sensor 57 and the downstream side NOx sensor 58 are normal, and the exhaust gas flow rate is within a predetermined range. The inflow NOx concentration is within a predetermined range, the filter temperature is within a predetermined range, and the ammonia adsorption amount at the SCR catalyst 51a calculated by the adsorption amount calculation unit 120 is within a predetermined range. It can be illustrated.

  The diagnosis as to whether or not the upstream NOx sensor 57 and the downstream NOx sensor 58 are 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, as described above, the ammonia adsorption amount in the SCR catalyst 51 a calculated by the adsorption amount calculation unit 120 (hereinafter, this calculated value may be referred to as “reference adsorption amount”) is the PM in the SCR filter 51. This is a value that does not consider the influence on the ammonia adsorption amount in the SCR catalyst 51a due to the deposition state. Therefore, there is a possibility that this reference adsorption amount is different from the actual ammonia adsorption amount in the SCR catalyst 51a. However, there is a certain degree of correlation between the reference adsorption amount and the actual ammonia adsorption amount. When determining whether or not the condition for executing the failure diagnosis of the exhaust purification system 60 is satisfied, it is determined whether or not the SCR catalyst 51a has adsorbed an amount of ammonia that can diagnose the failure of the exhaust purification system 60. It only has to be discriminated. That is, it is not always necessary to use the correct ammonia adsorption amount in the SCR catalyst 51a to determine whether or not the execution condition of the failure diagnosis of the exhaust purification system 60 is satisfied. Therefore, in S101, the reference adsorption amount can be used as one of parameters for determining whether or not failure diagnosis of the exhaust purification system 60 can be performed. As described above, the ammonia adsorption amount in the SCR filter 51 is affected by the exhaust flow rate and the filter temperature. Therefore, when performing failure diagnosis of the exhaust purification system 60 based on the NOx purification rate in the SCR filter 51, it is preferable that both the exhaust flow rate and the filter temperature 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, a determination purification rate Rnoxth is calculated. Here, the determination purification rate Rnoxth is calculated as a determination purification rate when it is assumed that the amount of PM deposition in the wall in the SCR filter 51 has reached the upper limit value. That is, when the NOx purification rate in the SCR filter 51 in a state where the amount of PM accumulated in the wall has reached the upper limit value is reduced below the judgment purification rate Rnoxth, the exhaust purification system 60 fails. It is a threshold value to be determined to be.

  The determination purification rate Rnoxth is calculated based on the reference adsorption amount calculated by the adsorption amount calculation unit 120. However, the reference adsorption amount is a value calculated on the assumption that PM does not accumulate on the SCR filter 51. Therefore, the ammonia adsorption amount in the SCR catalyst 51a when the PM accumulation amount in the wall in the SCR filter 51 reaches the upper limit value is larger than the reference adsorption amount. Therefore, the NOx purification rate in the SCR filter 51 when the PM accumulation amount in the wall reaches the upper limit is higher than the NOx purification rate in the SCR filter 51 when the ammonia adsorption amount in the SCR catalyst 51a is the reference adsorption amount. Become. Therefore, in the present embodiment, as described above, the correlation between the NOx purification rate and the reference adsorption amount in the SCR filter 51 when the in-wall PM accumulation amount reaches the upper limit value (specifically, in-wall PM accumulation). The correlation between the predetermined reference adsorption amount and the determined purification rate Rnoxth based on the difference between the NOx purification rate when the amount reaches the upper limit and the NOx purification rate when PM does not accumulate on the SCR filter 51) Is stored in the ECU 10 as a map or function. In S102, the determination purification rate Rnoxth is calculated using this map or function.

  Next, in S103, the NOx purification rate Rnox in the SCR filter 51 is calculated. Here, as described above, the NOx purification rate Rnox in the SCR filter 51 is calculated based on the inflow NOx concentration that is the detection value of the upstream NOx sensor 57 and the outflow NOx concentration that is the detection value of the downstream NOx sensor 58. The

  Next, in S104, the differential pressure change rate Rp at the sensor detection time is calculated using the above equation 2. The sensor detection time here is the time when the detected values of the upstream NOx sensor 57 and the downstream NOx sensor 58 used for calculating the NOx purification rate Rnox in S103 are detected. Next, in S105, it is determined whether or not the differential pressure change rate Rp calculated in S104 is smaller than a predetermined threshold value Rpth. Here, the predetermined threshold value Rpth is a threshold value for distinguishing whether the sensor detection time is in the in-wall PM deposition period or the surface layer PM deposition period. The predetermined threshold value Rpth is determined in advance based on experiments or the like and is stored in the ECU 10.

  When an affirmative determination is made in S105, it can be determined that the sensor detection time is during the surface PM deposition period. Therefore, in this case, failure diagnosis of the exhaust purification system 60 is executed. Therefore, if an affirmative determination is made in S105, it is then determined in S106 whether the NOx purification rate Rnox in the SCR filter 51 calculated in S103 is greater than the determination purification rate Rnoxth calculated in S102. If an affirmative determination is made in S106, it is then determined in S107 that the exhaust purification system 60 is normal. On the other hand, if a negative determination is made in S106, that is, if the NOx purification rate Rnox in the SCR filter 51 is equal to or less than the determination purification rate Rnoxth, then in S108, it is determined that the exhaust purification system 60 has failed. After it is determined that the exhaust purification system 60 is normal in S107, or after it is determined that the exhaust purification system 60 has failed in S108, the execution of this flow is terminated.

  On the other hand, if a negative determination is made in S105, it can be determined that the sensor detection time is during the in-wall PM accumulation period. Therefore, in this case, the execution of this flow is temporarily terminated. That is, if a negative determination is made in S105, failure diagnosis of the exhaust purification system 60 is not executed.

  According to the above fault diagnosis flow, the calculated value of the NOx purification rate may be different due to the difference in the PM accumulation amount in the wall at the sensor detection time. When it is in the middle, failure diagnosis of the exhaust purification system 60 is not executed. Then, when the sensor detection time is during the surface PM deposition period and a stable diagnosis result can be obtained, failure diagnosis of the exhaust purification system 60 is executed. Therefore, the diagnostic accuracy of the fault diagnosis of the exhaust purification system 60 can be improved.

  In this embodiment, the SCR filter 51 corresponds to the “SCR filter” according to the present invention, and the urea water addition valve 53 corresponds to the “ammonia supply device” according to the present invention. In this embodiment, the downstream NOx sensor 58 corresponds to the “NOx sensor” according to the present invention. In the present embodiment, the ECU 10 executes the processing from S105 to S108 in the failure diagnosis flow shown in FIG.

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 4 ... Intake passage 5 ... Exhaust passage 50 ... Oxidation catalyst 51 ... SCR filter 51a ... SCR catalyst 53 ... Urea water addition valve 57 ... Upstream NOx sensor 58 ... Downstream NOx sensor 59 ・ ・ Differential pressure sensor 10 ・ ・ ECU

Claims (1)

An SCR filter provided in an exhaust passage of an internal combustion engine and having a SCR catalyst supported on a filter, the SCR catalyst having a function of reducing NOx in exhaust using ammonia as a reducing agent, Exhaust gas purification that diagnoses whether or not an exhaust gas purification system having an SCR filter having a function of collecting particulate matter in exhaust gas and an ammonia supply device that supplies ammonia to the SCR filter has failed In the system fault diagnosis device,
A NOx sensor provided in an exhaust passage downstream of the SCR filter;
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 diagnosis unit that executes a failure diagnosis for determining whether or not the exhaust purification system has failed by comparing the NOx purification rate calculated by the NOx purification rate calculation unit with a predetermined determination purification rate. ,
A differential pressure conversion value obtained by converting a difference in exhaust pressure between upstream and downstream of the SCR filter into a value when the flow rate of exhaust gas flowing into the SCR filter is assumed to be constant is used as the differential pressure conversion value. The amount of increase in the differential pressure conversion value per unit increase amount of the filter PM deposition amount when the accumulation amount of the particulate matter in the SCR filter estimated based on parameters other than the filter PM deposition amount is set as a difference. The rate of pressure change,
The time when the detected value of the NOx sensor used for the calculation of the NOx purification rate by the NOx purification rate calculation unit is detected as the sensor detection time,
When the differential pressure change rate at the sensor detection timing is equal to or greater than a predetermined threshold, the diagnosis unit does not execute the fault diagnosis and performs the fault diagnosis when the differential pressure change rate is smaller than the predetermined threshold. Fault diagnosis device for exhaust purification system to be executed.

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