JP2012145052A - Degradation detecting device of scr system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a degradation detecting device of an SCR system capable of precisely detecting the degradation of an SCR catalyst even in the case NOratio transiently changes.SOLUTION: An ECU 50 calculates a real cleaning ratio (step 100), estimates an NOratio from the calculated real cleaning ratio (step 110), and determines whether or not the difference of the estimated NOratio and the value thereof at preceding time is less than a certain value (step 120). The ECU 50 determines whether or not the adsorbing quantity of an SCR catalyst is unsaturated (step 140) in the case it is determined that the difference is not less than the certain value in the step 120. The ECU 50 calculates an adsorption change quantity of components derived from the NO(step 150) and a correction coefficient from the calculated adsorption change quantity (step 160) in the case it is determined that the adsorption quantity of the SCR catalyst is unsaturated in the step 140.

Description

  The present invention relates to a degradation detection device for an SCR (Selective Catalytic Reduction) system, and more particularly to a degradation detection device for an SCR system that detects degradation of an SCR catalyst constituting the SCR system.

Conventionally, for example, Patent Document 1 compares the NOx purification rate calculated from the concentration of NOx (refer to NO and NO _2. Hereinafter the same.) At upstream and downstream of the SCR catalyst, and a NOx purification rate at the time of the catalyst non-degraded In the apparatus for diagnosing the deterioration of the SCR catalyst, it is disclosed that a necessary correction relating to the temperature of the SCR catalyst is performed on the NOx purification rate when the catalyst is not deteriorated. The SCR catalyst changes its NOx purification capacity depending on its temperature. Therefore, as the required correction, a process of multiplying the NOx purification rate when the catalyst is not deteriorated by a correction coefficient corresponding to the temperature of the SCR catalyst is performed. Thereby, the deterioration of the SCR catalyst can be diagnosed after eliminating the influence of the change in the purification capacity due to the temperature of the SCR catalyst.

Further, for example, in Patent Document 2, in an SCR system in which two NOx sensors for detecting upstream and downstream NOx concentrations of an SCR catalyst and an ammonia adding means as a NOx reducing agent are provided in the SCR catalyst, these two NOx sensors are described. It is disclosed that when the NOx purification rate is calculated from the sensor value, a necessary correction is performed on the sensor value. Even if the NOx concentration in the exhaust gas flowing into the SCR catalyst is constant, if the NO ₂ ratio in the NOx is different, the output value of the NOx sensor is different. In addition, the reaction of NOx and ammonia proceeds on the SCR catalyst, but the reaction rate of this reaction varies depending on the temperature range of the SCR catalyst. That is, if the NO ₂ ratio and the temperature range of the SCR catalyst are different, the NOx purification rate is different. From this point of view, in Patent Document 2, as the required correction, the sensor value is multiplied by a correction coefficient corresponding to the change in the NO ₂ ratio or the temperature of the SCR catalyst.

JP-A-11-117726 JP 2010-133375 A

In Patent Document 2, the required correction is performed on the assumption that the NO ₂ ratio is steady. However, the NO ₂ ratio may change transiently depending on the operating conditions of the internal combustion engine, etc., and it is highly possible that the correction of the sensor value will be insufficient with the correction based on only the steady state. Therefore, when such a sensor correction value is used, there is a problem that the degradation diagnosis accuracy of the SCR catalyst is lowered.

The present invention has been made to solve the above-described problems. That is, an object of the present invention is to provide a deterioration detection device for an SCR system that can accurately detect deterioration of an SCR catalyst even when the NO ₂ ratio changes transiently.

In order to achieve the above object, a first invention is a deterioration detection apparatus for an SCR system,
An SCR catalyst provided in an exhaust passage of the internal combustion engine and adsorbing a NOx-derived component in the exhaust;
NOx purification rate calculating means for calculating a NOx purification rate in the SCR catalyst from NOx concentrations upstream and downstream of the SCR catalyst;
And the NOx purification rate calculated by the NOx purification ratio calculating means, and a temperature of the SCR catalyst, and NO ₂ ratio estimating means for estimating the NO ₂ proportion in the exhaust gas flowing into the SCR catalyst,
A purification rate correction unit that corrects the NOx purification rate calculated by the NOx purification rate calculation unit according to the adsorption amount of the NOx-derived component that increases or decreases with a change in the NO ₂ ratio estimated by the NO ₂ rate estimation unit. When,
SCR catalyst deterioration determination means for determining deterioration of the SCR catalyst by comparing the corrected NOx purification rate after correction by the purification rate correction means with a predetermined deterioration determination value;
It is characterized by providing.

According to a second invention, in the first invention,
An oxidation catalyst provided in the exhaust passage upstream of the SCR catalyst;
NOx reducing agent addition means for adding a NOx reducing agent to the SCR catalyst;
Transient determination means for determining whether a deviation between the estimated values of the NO ₂ ratio estimated by the NO ₂ ratio estimation means is equal to or greater than a predetermined transient determination value;
When the deviation between the estimated values is equal to or greater than the predetermined transient determination value, the degree of change between the purification rates of the corrected NOx purification rate and the degree of change between the addition amounts of the NOx reducing agent added to the SCR catalyst. A failure identifying means for identifying which of the oxidation catalyst and the NOx reducing agent addition means is malfunctioning;
It is characterized by providing.

According to a third invention, in the first or second invention,
The failure identifying means identifies a failure of the oxidation catalyst when the degree of change between the purification rates is greater than the degree of change between the addition amounts, and the degree of change between the purification rates is a change between the addition amounts. When it is smaller than the degree, it is specified that the NOx reducing agent adding means is out of order.

If the NO ₂ ratio changes, the adsorption amount of the NOx-derived component on the SCR catalyst changes. In particular, if the NO ₂ ratio changes transiently, this change in adsorption amount is large. According to the first aspect of the invention, the NOx purification rate can be corrected according to the amount of adsorption of the NOx-derived component that increases or decreases with the change in the NO ₂ ratio estimated by the NO ₂ ratio estimating means. Therefore, the deterioration determination of the SCR catalyst can be performed with high accuracy regardless of whether the NO ₂ ratio is steady or transient.

According to the first invention, the NOx purification rate can be corrected to a constant value regardless of whether the NO ₂ ratio is steady or transient. However, when a failure occurs in the oxidation catalyst or the NOx reducing agent addition means, a deviation occurs in the corrected NOx purification rate. According to the second and third inventions, when such a divergence occurs, the degree of change between the purification rates of the NOx purification rate after correction by the purification rate correction means and the NOx reducing agent added to the SCR catalyst It is possible to specify which of the oxidation catalyst and the NOx reducing agent addition means is malfunctioning using the degree of change between the addition amounts of NO. Therefore, it is possible to prevent in advance that the degradation determination accuracy of the SCR catalyst is lowered due to the occurrence of a failure of the oxidation catalyst or the NOx reducing agent addition means.

2 is a diagram for describing a system configuration according to Embodiment 1. FIG. It is an example of the conversion map used for the deterioration determination method of an SCR catalyst. NO ₂ ratio in the case of using the conversion map in FIG. 2 is a diagram showing the time variation of the correction coefficient and the NOx purification rate. It is the figure which showed typically the mechanism of the purification reaction on an SCR catalyst. 4 is a timing chart showing the SCR catalyst inflow NOx concentration (ppm), the actual purification rate (%), the adsorption rate (%) in the SCR catalyst, the transient correction coefficient, and the corrected NOx purification rate in the first embodiment. 4 is a flowchart showing transient correction control executed by an ECU 50 in the first embodiment. NO ₂ ratio flowing into the SCR catalyst is a diagram showing a time change of the NOx amount and the NOx purification ratio after the correction flowing into the SCR catalyst. 6 is a flowchart showing transient correction control executed by an ECU 50 in the second embodiment. It is a characteristic diagram showing the relationship between the steady / transient change degree of the NO ₂ ratio and the steady / transient change degree of the urea addition amount.

Embodiment 1 FIG.
[Description of system configuration]
Hereinafter, Embodiment 1 of the present invention will be described with reference to FIGS. FIG. 1 is a diagram for explaining a system configuration according to the first embodiment of the present invention. The system according to the present embodiment includes an engine 10 as an internal combustion engine. The number of cylinders and the cylinder arrangement of the engine 10 are not particularly limited.

  In the exhaust passage 12 of the engine 10, in order from the upstream side, a CCO (catalytic converter oxidation) 14 that oxidizes HC, CO, and NO contained in the exhaust gas and particulate matter in the exhaust gas are captured. A DPR (Diesel Particulate Reduction) 16 that collects and removes combustion and an SCR catalyst 18 as a catalyst for purifying NOx in the exhaust gas are provided in series.

  Further, a urea addition valve 20 for supplying urea as a NOx reducing agent is provided in the exhaust passage 12 upstream of the SCR catalyst 18. The urea addition valve 20 is connected to a urea tank 22 that stores urea water therein via a supply pump (not shown). It is assumed that the urea water in the urea tank 22 is injected and supplied from the urea addition valve 20 toward the SCR catalyst 18 by driving the supply pump. The urea addition valve 20 and the urea tank 22 constitute a urea addition system 30 together with the supply pump and an ECU 50 described later.

  Further, the system of the present embodiment includes an ECU (Electronic Control Unit) 50. On the input side of the ECU 50, a NOx sensor 24 provided on the downstream side of the SCR catalyst 18 and other various sensors necessary for controlling the vehicle and the engine 10 (for example, a crank angle sensor for detecting the engine speed and the crank angle, An air-fuel ratio sensor for detecting the exhaust air-fuel ratio, a throttle sensor for detecting the opening of the throttle valve, etc.) are connected. On the other hand, various actuators including the urea addition valve 20 are connected to the output side of the ECU 50. The ECU 50 detects the operation information of the engine 10 by the various sensors described above, and performs various controls by driving the actuators based on the detection results.

[Features of Embodiment 1]
By the way, if the SCR catalyst 18 is deteriorated, the purification capability is reduced accordingly. Therefore, it is desirable that the deterioration of the SCR catalyst 18 can be detected with high accuracy. Here, as one of the SCR catalyst deterioration determination methods, a NOx sensor or the like is used to detect or estimate the NOx concentration in the upstream and downstream of the SCR catalyst, and an actual NOx purification rate (hereinafter referred to as “actual purification”) based on these. In some cases, the actual purification rate is converted into a NOx purification rate in a specified state (reference state) and compared with a threshold value for deterioration determination.

FIG. 2 shows an example of a conversion map used for such an SCR catalyst deterioration determination method. The conversion map of FIG. 2 defines the NOx purification rate in the SCR catalyst by the NO ₂ ratio in NOx and the temperature (bed temperature) of the SCR catalyst. This is because the NOx purification rate in the SCR catalyst changes depending on the NO ₂ ratio in NOx and the temperature of the SCR catalyst. Specifically, as shown in FIG. 2, if the NO ₂ ratio is constant, the NOx purification rate increases as the temperature of the SCR catalyst increases, and if the SCR catalyst temperature is constant, the NO ₂ ratio is around 50%. The NOx purification rate becomes the highest.

Specifically, the conversion of the actual purification rate is performed by correcting the difference between the current state (the state at the time of calculating the actual purification rate) and the specified state. For example, it is assumed that the temperature of the SCR catalyst is 200 ° C. and the NO ₂ ratio is 30%. In this case, with reference to the conversion map of FIG. 2, the correction coefficient corresponding to the deviation from the specified state (SCR catalyst temperature 280 ° C., NO ₂ ratio 50%) is multiplied to convert to the NOx purification rate in the specified state. .

FIG. 3 is a diagram showing temporal changes in the NO ₂ ratio, correction coefficient, and NOx purification rate (actual purification rate and corrected NOx purification rate) when the conversion map of FIG. 2 is used. It is assumed that the NO ₂ ratio gradually changes from about 40% to about 60% from time t ₀ to time t ₁ (the temperature of the SCR catalyst is constant) (FIG. 3A). In this case, the correction coefficient is changed so as to be the minimum in the vicinity of about 50% with reference to the conversion map of FIG. 2 (FIG. 3B). By multiplying the actual purification rate by such a correction coefficient, the corrected NOx purification rate is always a constant value (FIG. 3 (c)). Accordingly, it is possible to determine the deterioration while eliminating the influence due to the change in the NO ₂ ratio. When the temperature of the SCR catalyst changes simultaneously with the NO ₂ ratio, the correction coefficient shown in FIG. 3B may be further multiplied by the correction coefficient corresponding to the temperature change.

However, the conversion map of FIG. 2 is established when the NO ₂ ratio is steady, and is not necessarily established when the NO ₂ ratio changes transiently depending on the operating conditions of the engine 10. This is because, when the NO ₂ ratio is changed transitionally _is, ^NO 2 ^_-, NO 3 -, because adsorption amount onto SCR catalyst NO ₂ derived components such as _NH 4 NO ₃ changes transitionally is there. These transient changes in the amount of adsorption will be described with reference to FIG. FIG. 4 is a diagram schematically showing the mechanism of the purification reaction on the SCR catalyst.

As shown in FIG. 4, it is considered that the reactions of the following formulas (i) to (vii) proceed competitively on the SCR catalyst.
2NO + O ₂ → 2NO ₂ (i)
2NO ₂ + O ²⁻ → 2NO ₃ ⁻ (ii)
NO ₃ ⁻ + NH ₄ ⁺ → NH ₄ NO ₃ (iii)
NH ₄ NO ₃ → NH ₃ + HNO ₃ (iv)
NH ₄ NO ₃ → N ₂ O + 2H ₂ O (v)
NO ₃ ⁻ + NO → NO ₂ ⁻ + NO ₂ (vi)
^{_{NO 2 - + NH 4 + →}} N 2 + 2H 2 O ··· (vii)

For example, if the ratio of NO ₂ flowing into the SCR catalyst increases, the reaction of the above formula (ii) proceeds to the right and NO ₃ ⁻ is generated. If NO ₃ ⁻ is generated, the reaction of the above formula (iii) proceeds in the right direction to generate NH ₄ NO ₃ . Here, the produced NO ₃ ^— and NH ₄ NO ₃ continue to be adsorbed by the SCR catalyst until the adsorption amount is saturated. If NO ₃ ^— or NH ₄ NO ₃ continues to be adsorbed, the NOx concentration on the downstream side of the SCR catalyst does not change despite the increase in the NO ₂ ratio, and the actual purification rate apparently increases.

On the other hand, if the ratio of NO ₂ flowing into the SCR catalyst decreases (if the NO ratio increases), the reaction of the above formula (vi) proceeds in the right direction to generate NO ₂ ⁻ or NO ₂ . Here, the produced NO ₂ ⁻ is adsorbed until the adsorbed amount of the SCR catalyst is saturated, like NO ₃ ⁻ and NH ₄ NO ₃ . However, since the generated NO ₂ flows out downstream of the SCR catalyst, the NOx concentration on the downstream side of the SCR catalyst increases and apparently the actual purification rate decreases.

Incidentally, if the range increases or decreases slightly in NO ₂ ratio flowing into the SCR catalyst, the above formula (ii), since hardly proceeds reaction or (iii) (vi), the SCR catalyst of the NO ₂ derived components The amount of adsorption up can be considered constant.

Thus, if the NO ₂ ratio changes, the amount of adsorption of the NO ₂ -derived component on the SCR catalyst changes. Therefore, if the NO ₂ ratio changes transiently, the adsorption amount of the NO ₂ -derived component also changes transiently. That is, the NOx concentration on the downstream side of the SCR catalyst is temporarily changed by the total adsorption change amount of the NO ₂ -derived component, and the actual purification rate is temporarily changed. Therefore, in the present embodiment, when the NO ₂ ratio changes transiently, the transient correction control is performed in consideration of the adsorption change amount of the NO ₂ -derived component. As a result, even when the NO ₂ ratio changes transiently, the deterioration of the SCR catalyst can be diagnosed in the same manner as in the steady state.

FIG. 5 is a timing chart showing the SCR catalyst inflow NOx concentration (ppm), the actual purification rate (%), the adsorption rate (%) in the SCR catalyst, the transient correction coefficient, and the corrected NOx purification rate in this embodiment. In Figure 5, it shows that the transient correction control is started at time t _2. As shown in FIG. 5 (a), NO and NO ₂ concentration flowing into the SCR catalyst, changes in the time _{t 2, the} then together takes a constant value.

For example, assume that the NO ₂ ratio changes from 50% to 80% at time t ₂ . Then, since the NO ₂ ratio has changed transiently, as shown in FIG. 5B, the actual purification rate rises at a stroke at time t ₂ and then converges to a constant value. Further, as described in the description of FIG. 4, when the NO ₂ ratio increases, NO ₃ ^— or NH ₄ NO ₃ is generated and adsorbed on the SCR catalyst. Therefore, as shown in FIG. 5C, the adsorption amount of NO ₃ ⁻ or NH ₄ NO ₃ continues to increase from time t ₂ and becomes a constant value after time t ₃ . The time t ₃ adsorption amount of these later becomes a constant value, because the adsorption amount with respect to the SCR catalyst is saturated.

Transient correction control, as shown in FIG. 5 (d), between the time t ₂ to time t _3, is carried out by multiplying the transient correction factor the actual purification rate of FIG. 5 (b). The transient correction coefficient can be determined from the adsorption change amount of the NO ₂ -derived component. Here, the adsorption amount of change in the NO ₂ derived component calculation, the previously adsorbed amount of adsorption already said NO ₂ derived components in the SCR catalyst, the model represented by the change of the NO ₂ ratio entering the SCR catalyst it can. Thus, as shown in FIG. 5 (e), NOx purifying ratio after correction, so takes a certain value before and after the time t _2, the as with steady diagnose the deterioration of the SCR catalyst.

[Specific Processing in Embodiment 1]
Next, specific processing for realizing the above-described transient correction control will be described with reference to FIG. FIG. 6 is a flowchart showing transient correction control executed by the ECU 50 in the present embodiment. Note that the routine shown in FIG. 6 is periodically and repeatedly executed when the engine 10 is in operation.

  In the routine shown in FIG. 6, first, the ECU 50 calculates the actual purification rate (step 100). The actual purification rate is calculated from the estimated value of the NOx concentration upstream of the SCR catalyst 18 and the detected value of the NOx sensor 24. The upstream NOx concentration can be estimated by a known method that is estimated based on the rotational speed of the engine 10, the amount of injected fuel, the temperature of the CCO 14, and the like.

Subsequently, the ECU 50 estimates the NO ₂ ratio from the actual purification rate calculated in step 100 (step 110). In this step, the NO ₂ ratio is estimated by referring to the conversion map used in the description of FIG. 2 as data. Subsequently, the ECU 50 determines whether or not the deviation between the NO ₂ ratio estimated in step 110 and the previous value is a certain value or more (step 120). In this step, as the constant value, a value stored in advance in the ECU 50 as a threshold value at which the NO ₂ ratio is recognized to change transiently is used.

If it is determined in step 120 that the deviation is less than a certain value, it can be determined that the NO ₂ ratio is steady and the amount of adsorption of the NO ₂ -derived component onto the SCR catalyst 18 does not change. Therefore, the ECU 50 calculates a correction coefficient from the conversion map of FIG. 2 (step 130). On the other hand, when it is determined that the deviation is equal to or greater than a certain value, the ECU 50 determines whether the adsorption amount of the SCR catalyst 18 is not saturated (step 140). Whether or not the adsorption amount is unsaturated can be determined, for example, by whether or not the integrated value of the NO ₂ -derived components adsorbed on the SCR catalyst 18 exceeds a threshold value.

If it is determined in step 140 that the adsorption amount of the SCR catalyst 18 is saturated, it can be determined that the adsorption amount of the NO ₂ -derived component is constant regardless of a transient change in the NO ₂ ratio. Therefore, the ECU 50 estimates the NO ₂ ratio with reference to the conversion map used in the description of FIG. 2 as data, as in the normal state (step 130). On the other hand, if it is determined in step 140 that the adsorption amount of the SCR catalyst 18 is not saturated, the ECU 50 calculates the adsorption change amount of the NO ₂ -derived component (step 150), and the calculated adsorption change amount. A correction coefficient is calculated from (step 160).

  Subsequent to step 130 or step 160, the ECU 50 corrects the actual purification rate calculated in step 100 using the correction coefficients calculated in these steps (step 170). Thereby, the NOx purification rate after correction and the threshold value for deterioration determination are compared, and deterioration determination of the SCR catalyst can be performed (step 180).

As described above, according to the routine shown in FIG. 6, the correction coefficient can be calculated from the conversion map of FIG. 2 when the NO ₂ ratio is steady or when the adsorption amount of the SCR catalyst is saturated. Further, when the NO ₂ ratio changes transiently, the correction coefficient can be calculated by calculating the adsorption change amount of the NO ₂ -derived component on the assumption that the adsorption amount of the SCR catalyst is not saturated. Therefore, the deterioration determination of the SCR catalyst can be performed with high accuracy regardless of whether the NO ₂ ratio is steady or transient.

By the way, in Embodiment 1 mentioned above, in Step 120 of FIG. 6, it was determined whether or not the deviation between the NO ₂ ratio estimated in Step 110 and its previous value is greater than or equal to a certain value. It may be omitted. Since the adsorption change amount of the NO ₂ -derived component can be calculated by the deviation between the NO ₂ ratio estimated in step 110 and its previous value, it is not always necessary to determine whether or not these deviations are equal to or greater than a certain value. That is, even if the processing of step 140 for determining whether or not the adsorption amount of the SCR catalyst is unsaturated after estimating the NO ₂ ratio in step 110, the same effect as in the present embodiment can be obtained. .

In the first embodiment described above, the “NO ₂ ratio estimating means” in the first aspect of the present invention is executed by the ECU 50 executing the process of step 110 of FIG. By executing the process, the “purification rate correction means” in the first invention and the “SCR catalyst deterioration determining means” in the first invention are realized by executing the process of step 180 in FIG. ing.

Embodiment 2. FIG.
[Features of Embodiment 2]
Next, a second embodiment of the present invention will be described with reference to FIGS. The present embodiment is characterized in that the fault identification control shown in FIG. 8 is executed using the system of FIG. Therefore, in this embodiment, the difference from the first embodiment will be mainly described, and the description of the same matters will be simplified or omitted.

According to the transient correction control of the first embodiment, since the correction coefficient can be calculated separately and multiplied by the actual purification rate for the steady state and the transient state of the NO ₂ ratio, the corrected NOx purification rate is always set to a constant value. (See FIG. 3C). By the way, this transient correction control is based on the assumption that the CCO 14 and the urea addition system 30 are functioning normally. Therefore, if a failure occurs in these, a difference may occur in the corrected NOx purification rate.

The occurrence of such divergence will be described with reference to FIG. FIG. 7 is a graph showing the change over time in the ratio of NO ₂ flowing into the SCR catalyst, the amount of NOx flowing into the SCR catalyst (amount of urea added to the SCR catalyst), and the corrected NOx purification rate. FIG. 7 shows that the NO ₂ ratio changes transiently at time t _{4 and the} urea addition amount changes transiently at time t ₆ .

As described above, if the transient correction control is executed, the corrected NOx purification rate can be made constant, so the corrected NOx purification rate should be maintained at a constant value, although there is some increase or decrease. However, when a failure occurs in the CCO 14, the oxidation of NO in the exhaust gas (ie, conversion to NO ₂ ) is insufficient. Therefore, reduced NO ₂ ratio abruptly flowing into the SCR catalyst, as shown in FIG. 7 (a), as shown in FIG. (C), the NOx purification ratio after the correction from time _{t 4} to time _{t 5} It fluctuates greatly temporarily. Similarly, urea addition when a failure in the system 30 occurs, 7 urea addition amount decreases abruptly (b), the as shown in FIG. (C), the time from time t ₆ t The NOx purification rate after the correction up to ₇ temporarily fluctuates greatly.

Therefore, in the present embodiment, when the corrected NOx purification rate temporarily increases, a failure occurs in either the CCO 14 or the urea addition system 30 from the degree of change in the NO ₂ ratio and the urea addition amount. It was decided to execute failure identification control to identify whether or not

[Specific Processing in Second Embodiment]
Next, specific processing for realizing the above-described failure identification control will be described with reference to FIG. FIG. 8 is a flowchart showing the failure specifying control executed by the ECU 50 in the present embodiment. It is assumed that the routine shown in FIG. 8 is periodically and repeatedly executed when the engine 10 is operating, like the routine shown in FIG.

  In the routine shown in FIG. 8, first, the ECU 50 estimates the degree of deterioration of the SCR catalyst 18 (step 200), and changes the correction coefficient based on the estimated degree of deterioration (step 210). The degree of deterioration of the SCR catalyst 18 is a value of the corrected NOx purification rate with respect to the threshold value for deterioration determination, and can be estimated by executing the processing of step 180 in FIG.

Subsequently, the ECU 50 obtains the corrected NOx purification rate at the steady state of the NO ₂ ratio or urea addition amount and the corrected NOx purification rate at the corresponding transient time (step 220). The steady / transient state of the NO ₂ ratio can be distinguished, for example, by executing the processing of steps 100 to 120 in FIG. Further, the steady / transient of the urea addition amount can be distinguished by, for example, an addition command value from the ECU 50 to the urea addition valve 20. Therefore, the ECU 50 acquires the corrected NOx purification rate in the steady state and in the transient state, respectively.

Subsequently, the ECU 50 determines whether or not the deviation between the corrected NOx purification rates acquired in step 220 is a certain value or more (step 230). In this step, as the constant value, a value stored in advance in the ECU 50 is used as a threshold value that is recognized to be an obstacle to the deterioration determination of the SCR catalyst. If it is determined in step 230 that the divergence is less than a certain value, it can be determined that the change is in the NO ₂ ratio or the urea addition amount due to the change in the operating condition of the engine 10. Therefore, the ECU 50 determines that there is no failure (step 240) and ends this routine.

On the other hand, if it is determined in step 230 that the divergence is greater than or equal to a certain value, the ECU 50 determines the steady / transient change degree of the NO ₂ ratio used in the process of step 220 and the urea addition amount. From the steady / transient change degree, it is specified whether the difference between the corrected NOx purification rates is due to the CCO 14 or the urea addition system 30 (step 250). FIG. 9 is a characteristic diagram showing the relationship between the steady / transient change degree of the NO ₂ ratio and the steady / transient change degree of the urea addition amount.

As shown in FIG. 9, in the region A where the change degree of the urea addition amount is larger than the change degree of the NO ₂ ratio, the possibility of failure of the urea addition system 30 is high, and the change degree of the NO ₂ ratio is the urea addition amount. In the region B that is larger than the degree of change, it is determined that the possibility of CCO 14 failure is high. In this step 250, with reference to the map of the characteristic diagram shown in FIG. 9, it is specified whether it is caused by the CCO 14 or the urea addition system 30. If it is determined in step 250 that the region is in the area A, it is determined that the urea addition system 30 is in failure (step 260). If it is determined that the region is in the region B, it is determined that the CCO 14 is in failure. Specify (step 270).

  As described above, according to the routine shown in FIG. 8, when the deviation between the corrected NOx purification rates is a certain value or more, this deviation is caused by any failure of the CCO 14 and the urea addition system 30. Can be identified. Therefore, it can be prevented in advance that the deterioration determination accuracy of the SCR catalyst of the first embodiment is lowered due to the occurrence of these failures.

  In the first embodiment described above, the ECU 50 executes the process of step 230 in FIG. 8 so that the “transient determination means” in the second invention executes the processes of steps 250, 260, and 270 in FIG. As a result, the “failure identifying means” in the second aspect of the present invention is realized.

10 Engine 12 Exhaust passage 14 CCO
16 DPR
18 SCR catalyst 20 Urea addition valve 22 Urea tank 24 NOx sensor 30 Urea addition system 50 ECU

Claims (3)

An SCR catalyst provided in an exhaust passage of the internal combustion engine and adsorbing a NOx-derived component in the exhaust;
NOx purification rate calculating means for calculating a NOx purification rate in the SCR catalyst from NOx concentrations upstream and downstream of the SCR catalyst;
And the NOx purification rate calculated by the NOx purification ratio calculating means, and a temperature of the SCR catalyst, and NO ₂ ratio estimating means for estimating the NO ₂ proportion in the exhaust gas flowing into the SCR catalyst,
A purification rate correction unit that corrects the NOx purification rate calculated by the NOx purification rate calculation unit according to the adsorption amount of the NOx-derived component that increases or decreases with a change in the NO ₂ ratio estimated by the NO ₂ rate estimation unit. When,
SCR catalyst deterioration determination means for determining deterioration of the SCR catalyst by comparing the corrected NOx purification rate after correction by the purification rate correction means with a predetermined deterioration determination value;
An apparatus for detecting deterioration of an SCR system, comprising: An oxidation catalyst provided in the exhaust passage upstream of the SCR catalyst;
NOx reducing agent addition means for adding a NOx reducing agent to the SCR catalyst;
Transient determination means for determining whether a deviation between the estimated values of the NO ₂ ratio estimated by the NO ₂ ratio estimation means is equal to or greater than a predetermined transient determination value;
When the deviation between the estimated values is equal to or greater than the predetermined transient determination value, the degree of change between the purification rates of the corrected NOx purification rate and the degree of change between the addition amounts of the NOx reducing agent added to the SCR catalyst. A failure identifying means for identifying which of the oxidation catalyst and the NOx reducing agent addition means is malfunctioning;
The deterioration detection apparatus for an SCR system according to claim 1, comprising:   The failure identifying means identifies a failure of the oxidation catalyst when the degree of change between the purification rates is greater than the degree of change between the addition amounts, and the degree of change between the purification rates is a change between the addition amounts. 3. The deterioration detection apparatus for an SCR system according to claim 2, wherein when the degree is smaller than the degree, the failure is identified as a failure of the NOx reducing agent addition means.

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