JP2015014213A - Deterioration detection device for selective reduction type catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a deterioration detection device for an SCR (Selective Catalytic Reduction)catalyst which is arranged in an exhaust passage downstream of an NSR (NOx Storage Reduction) catalyst, for accurately detecting the deteriorating condition of the SCR catalyst.SOLUTION: The deterioration detection device for the SCR catalyst diagnoses the deterioration of a selective reduction type catalyst under circumstances in which the ammonia adsorption amount of the SCR catalyst is greater than the amount of ammonia which the SCR catalyst in an abnormal condition can absorb, using an integrated value for a NOinflow amount and an integrated value for a NOoutflow amount as parameters when an integrated value for a NOamount flowing into the SCR catalyst reaches a reference NOamount. In this configuration, the deterioration diagnosis of the selective reduction type catalyst is performed under conditions that a difference between the NOeliminating performance of the selective reduction type catalyst in a normal condition and the NOeliminating performance of the selective reduction type catalyst in the abnormal condition is greater, so that the abnormality of the selective reduction type catalyst can be more accurately detected.

Description

  The present invention relates to a technique for detecting deterioration of a selective catalytic reduction catalyst disposed in an exhaust passage of an internal combustion engine, and more particularly to a technique for detecting degradation of a selective catalytic reduction catalyst disposed downstream of an occlusion reduction type catalyst.

As a technology for purifying exhaust gas of an internal combustion engine that is operated with lean burn (lean burn operation), an exhaust purification device including a selective reduction catalyst (SCR (Selective Catalytic Reduction) catalyst),
A technique is known in which an addition valve for adding ammonia (NH ₃ ) or an additive that is a precursor of ammonia to an exhaust gas flowing into an exhaust gas purification device is disposed in an exhaust passage of an internal combustion engine.

In the above configuration, by calculating the NO _X purification rate of the SCR catalyst from the NO _X runoff flows out NO _X flowing amount flowing into the exhaust purification apparatus from the exhaust purification apparatus, the deterioration of the SCR catalyst based on the NO _X purification rate Techniques for detecting or determining the state are also known.

Incidentally, NO _X purification rate of the SCR catalyst, the ammonia adsorbed on the SCR catalyst amount and NO ₂ ratio of the exhaust gas flowing into the SCR catalyst (the amount of NO ₂ with respect to the amount of NO _X contained in the exhaust For example, the ratio, or the ratio of the amount of NO _{2 to} the total amount of NO and NO ₂ contained in the exhaust gas).

Therefore, by correcting the NO _X purification rate of NO ₂ ratio of the ammonia adsorption amount and the exhaust of the SCR catalyst as a parameter, a technique for detecting deterioration of the SCR catalyst based the NO _X purification rate after the correction has been proposed (e.g. , See Patent Document 1).

JP 2011-220142 A JP 2012-237296 A

By the way, in contrast to the configuration in which ammonia or an additive that is a precursor of ammonia is supplied to the SCR catalyst from the addition valve, a storage reduction type catalyst (NSR (NO _X Storage Reduction) catalyst) arranged upstream of the SCR catalyst is arranged. A configuration in which ammonia is generated by the NSR catalyst has been proposed.

  An object of the present invention is to accurately detect a deterioration state of an SCR catalyst in an SCR catalyst deterioration detection device disposed in an exhaust passage downstream of the NSR catalyst.

The present invention employs the following means in order to solve the above-described problems.
That is, the degradation detection apparatus for a selective catalytic reduction catalyst according to the present invention is:
A selective reduction catalyst that is disposed downstream of the storage reduction catalyst in the exhaust passage of the internal combustion engine, adsorbs ammonia generated by the storage reduction catalyst, and reduces NO _X in the exhaust gas using the adsorbed ammonia as a reducing agent;
Acquisition means for acquiring the ammonia adsorption amount of the selective catalytic reduction catalyst;
When the ammonia adsorption amount acquired by the acquisition means exceeds the lower limit, which is the amount of ammonia that can be adsorbed by the selective reduction catalyst in the abnormal state, the temperature of the selective reduction catalyst and the ammonia adsorption acquired by the acquisition means Calculating means for calculating a reference NO _X amount, which is the amount of NO _X that can be purified by the selective catalytic reduction catalyst in a normal state, using the amount as a parameter;
NO ammonia adsorption amount obtained by the obtaining unit is the amount of the NO _X flowing out from the NO _X flow rate and the selective reduction catalyst is the amount of the NO _X flowing into the selective reduction catalyst after becoming larger than the lower limit value An integration means for integrating each _X outflow amount;
Diagnosis for diagnosing NO _X flow rate integrated value and degradation of the selective reduction catalyst based on the integrated value of the NO _X runoff when NO _X flow rate, which is integrated by the integration means becomes equal to the reference amount of NO _X Means,
I was prepared to.

When the selective reduction catalyst (SCR catalyst) is in an abnormal state, the amount of ammonia that can be adsorbed by the SCR catalyst is smaller than when it is in a normal state. Therefore, when the SCR catalyst is in an abnormal state than when in the normal state, the SCR catalyst is an amount of capable of purifying NO _X is reduced. However, when the ammonia adsorption amount of the SCR catalyst is smaller than the amount of ammonia that can be adsorbed by the abnormal SCR catalyst (hereinafter referred to as the “lower limit value”), the NO _X that can be purified by the normal SCR catalyst is reduced. The difference between the amount and the amount of NO _x that can be purified by the abnormal SCR catalyst is reduced. Therefore, on the basis of the NO _X purification performance when the ammonia adsorption amount of SCR catalyst is above the lower limit, it is preferred that the SCR catalyst to determine whether it is abnormal.

Further, even when the ammonia adsorption amount of SCR catalyst is above the lower limit value, the less the amount of the NO _X flowing into the SCR catalyst (NO _X flowing amount), NO _X purification performance and abnormality of the normal sometimes the difference between the NO _X purification performance decreases. In particular, when the amount of NO _X inflow is smaller than the amount of NO _X that can be purified by the ammonia adsorbed on the SCR catalyst (reference NO _X amount), the NO _X purification performance at normal time and the NO NO at abnormal time The difference with _X purification performance becomes small.

To problems as described above, the present invention is, in a situation where the ammonia adsorption amount of the SCR catalyst is above the reference value, NO _X when the integrated value of the NO _X amount flowing into the SCR catalyst has reached the reference amount of NO _X diagnosing the deterioration of the SCR catalyst the integrated value of the integrated value and the NO _X outflow of inflow as a parameter. According to this method, so that the deterioration diagnosis of the SCR catalyst is performed under conditions where the difference between the NO _X purification performance of the SCR catalyst of the NO _X purification performance and the abnormal state of the SCR catalyst in the normal state becomes large. Therefore, even in a configuration in which an occlusion reduction type catalyst (NSR catalyst) is arranged upstream of the SCR catalyst, it is possible to detect an abnormality of the SCR catalyst more accurately.

Note that in the period from when the ammonia adsorption amount exceeds the lower limit value to the point where the integrated value of the NO _X flow rate reaches the reference amount of NO _X when the air-fuel ratio of the exhaust gas flowing into the NSR catalyst is made rich, There is a possibility that the ammonia adsorption amount and the lower limit of the SCR catalyst will change.

Therefore, according to the present invention, the period from the time when the ammonia adsorption amount acquired by the acquiring means exceeds the reference value to the time when the NO _X inflow amount integrated by the integrating means becomes equal to the reference NO _X amount is NSR. You may make it further provide the maintenance means which maintains the air fuel ratio of the exhaust gas which flows into a catalyst lean.

  According to such a configuration, it is possible to prevent the ammonia adsorption amount and the lower limit value of the SCR catalyst from changing during the period, and in particular, the ammonia adsorption amount from being lower than the lower limit value. As a result, it is possible to suppress a decrease in the degradation detection accuracy of the SCR catalyst.

  According to the present invention, the deterioration state of the SCR catalyst can be accurately detected in the deterioration detection device for the SCR catalyst disposed in the exhaust passage downstream of the NSR catalyst.

It is a figure which shows schematic structure of the internal combustion engine to which this invention is applied, and its exhaust system. It is a flowchart which shows the processing routine which ECU performs when detecting degradation of an SCR catalyst. It is a figure which shows the relationship between the temperature of a SCR catalyst, and the quantity of ammonia which a SCR catalyst can adsorb | suck. Is a graph showing the relationship between the temperature and the NO _X purification rate of the SCR catalyst of the SCR catalyst.

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

  FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine to which the present invention is applied and its exhaust system. An internal combustion engine 1 shown in FIG. 1 is a spark ignition type internal combustion engine capable of a lean combustion operation. The internal combustion engine 1 may be a compression ignition type internal combustion engine that is operated with lean combustion.

  The internal combustion engine 1 includes a fuel injection valve 2. The fuel injection valve 2 may be a valve device that injects fuel into an intake passage (for example, an intake port), or may be a valve device that injects fuel into a cylinder.

  The internal combustion engine 1 is connected to the exhaust passage 3. The exhaust passage 3 is a passage through which gas (exhaust gas) combusted in the cylinder of the internal combustion engine 1 flows. A first catalyst casing 4 is disposed in the middle of the exhaust passage 3. The first catalyst casing 4 contains a three-way catalyst composed of a honeycomb structure covered with a coat layer such as alumina and a noble metal (for example, platinum, palladium, rhodium, etc.) supported on the coat layer. To do.

A second catalyst casing 5 is disposed in the exhaust passage 3 downstream of the first catalyst casing 4. The second catalyst casing 5 includes a honeycomb structure covered with a coat layer such as alumina, a noble metal (platinum, palladium, rhodium, etc.) supported on the coat layer, and an NO _X storage agent (alkaline) supported on the coat layer. Occlusion, alkaline earth, etc.).

  A third catalyst casing 6 is disposed in the exhaust passage 3 downstream of the second catalyst casing 5. The third catalyst casing 6 includes a honeycomb structure made of cordierite or Fe-Cr-Al heat-resistant steel, an alumina-based or zeolite-based coat layer that covers the honeycomb structure, and a noble metal supported on the coat layer ( A selective reduction catalyst (SCR catalyst) composed of platinum, palladium, or the like) is accommodated.

The internal combustion engine 1 configured as described above is provided with an ECU 7. The ECU 7 is an electronic control unit that includes a CPU, a ROM, a RAM, a backup RAM, and the like. The ECU 7 includes an air-fuel ratio sensor (A / F sensor) 8, an oxygen concentration sensor (O ₂ sensor) 9, a first temperature sensor 10, a first NO _X sensor 11, a second NO _X sensor 12, a second temperature sensor 13, It is electrically connected to various sensors such as an accelerator position sensor 14, a crank position sensor 15, and an air flow meter 16.

The air-fuel ratio sensor 8 is attached to the exhaust passage 3 upstream from the first catalyst casing 4 and outputs an electrical signal correlated with the air-fuel ratio of the exhaust gas flowing into the first catalyst casing 4. The oxygen concentration sensor 9 is attached to the exhaust passage 3 between the first catalyst casing 4 and the second catalyst casing 5 and outputs an electrical signal correlated with the concentration of oxygen contained in the exhaust gas flowing out from the first catalyst casing 4. To do. The first temperature sensor 10 is attached to the exhaust passage 3 between the second catalyst casing 5 and the third catalyst casing 6, and outputs an electrical signal correlated with the temperature of the exhaust gas flowing out from the second catalyst casing 5. The first NO _X sensor 11 is attached to the exhaust passage 3 between the second catalyst casing 5 and the third catalyst casing 6, and exhaust gas flowing out from the second catalyst casing 5 (exhaust gas flowing into the third catalyst casing 6). and it outputs an electrical signal correlated to the amount of NO _X (NO _X flow rate) included in the. Second NO _X sensor 12 is mounted from the third catalyst casing 6 downstream of the exhaust passage 3, an electrical signal correlated to the amount of the NO _X contained in the exhaust gas flowing out of the third catalyst casing 6 (NO _X outflow) Is output. The second temperature sensor 13 is attached to the exhaust passage 3 downstream from the third catalyst casing 6 and outputs an electrical signal correlated with the temperature of the exhaust gas flowing out from the third catalyst casing 6. The accelerator position sensor 14 outputs an electrical signal that correlates with the amount of operation of the accelerator pedal (accelerator opening). The crank position sensor 15 outputs an electrical signal correlated with the rotational position of the output shaft (crankshaft) of the internal combustion engine 1. The air flow meter 16 outputs an electrical signal correlated with the amount of air taken into the cylinder of the internal combustion engine 1 (intake air amount).

  The ECU 7 controls the operating state of the internal combustion engine 1 based on the output signals of the various sensors described above. For example, the ECU 7 calculates the target air-fuel ratio of the air-fuel mixture based on the engine speed calculated based on the output signal of the crank position sensor 15 and the output signal (accelerator opening) of the accelerator position sensor 14. The ECU 7 calculates the target fuel injection amount (fuel injection period) of the fuel injection valve 2 based on the target air-fuel ratio and the output signal (intake air amount) of the air flow meter 16, and operates the fuel injection valve 2 according to the target fuel injection amount. Let The ECU 7 sets the target air-fuel ratio to a lean air-fuel ratio that is higher than the stoichiometric air-fuel ratio when the operating state of the internal combustion engine 1 is in the low rotation / low load region or the medium rotation / medium load region. The ECU 7 sets the target air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio lower than the stoichiometric air-fuel ratio when the operating state of the internal combustion engine 1 is in a high load region or a high rotation region. As described above, when the operation state of the internal combustion engine 1 belongs to the low rotation / low load region or the medium rotation / medium load region (hereinafter, these operation regions are referred to as “lean operation regions”), the internal combustion engine 1 is lean. By performing the combustion operation, the fuel consumption can be reduced. Further, the ECU 7 performs air-fuel ratio feedback control for correcting the target fuel injection amount so that the output signal of the air-fuel ratio sensor 8 matches the target air-fuel ratio, and air-fuel ratio feedback control based on the output signal of the oxygen concentration sensor 9. Learning control of the correction coefficient used is performed.

Meanwhile, if the target air-fuel ratio is set to a lean air-fuel ratio (when the internal combustion engine 1 is lean-burn operation) is, NO _X purifying performance of the three-way catalyst contained in the first catalyst casing 4 is lowered. Therefore, if the target air-fuel ratio is set to a lean air-fuel ratio, it is necessary to purify NO _X in the exhaust gas by the NSR catalyst of the second catalyst casing 5 and the SCR catalyst of the third catalyst casing 6.

NSR catalyst when the oxygen concentration of the exhaust gas flowing into the second catalyst casing 5 is high (when the air-fuel ratio of the exhaust gas is lean), the the _the NO _X storage or adsorbed in the exhaust gas. The NSR catalyst has a low oxygen concentration in the exhaust gas flowing into the second catalyst casing 5 and contains a reducing component such as hydrocarbon (HC) or carbon monoxide (CO) (the exhaust air-fuel ratio is rich). When there is, the NO _X stored in the NSR catalyst is released, and the released NO _X is reduced to nitrogen (N ₂ ).

Therefore, the ECU 8 periodically executes rich spike processing in the lean operation region. The rich spike process is a process for adjusting the fuel injection amount and the intake air amount so that the oxygen concentration in the exhaust gas is low and the HC and CO concentrations are high. Rich spike treatment, when _the NO _X storage amount of the NSR catalyst becomes more than a certain amount, previous rich spike action operating time from the end (preferably, operating time the target air-fuel ratio is set to a lean air-fuel ratio) Is executed when the travel distance from the end of the previous rich spike processing (preferably, the travel distance in which the target air-fuel ratio is set to the lean air-fuel ratio) exceeds the predetermined distance. That's fine. As a specific execution method of the rich spike processing, a method of executing at least one of processing for increasing the fuel injection amount of the fuel injection valve 2 or processing for reducing the opening of the intake throttle valve (throttle valve) is performed. Can be used. In the configuration in which the fuel injection valve 2 directly injects fuel into the cylinder, the rich spike processing may be executed by a method of injecting fuel from the fuel injection valve 2 during the exhaust stroke of the cylinder.

The SCR catalyst adsorbs ammonia (NH ₃ ) contained in the exhaust gas. The SCR catalyst reduces NO _X to nitrogen (N ₂ ) by reacting NH ₃ adsorbed on the SCR catalyst with NO _X in the exhaust. Note that NH ₃ supplied to the SCR catalyst is generated in a three-way catalyst or an NSR catalyst. For example, if the rich-spike treatment is executed, part of the NO _X in the three-way catalyst is reduced to NH _3, part of the NO _X released from the NSR catalyst is reduced to NH ₃ in the NSR catalyst . At that time, the amount of NH ₃ produced in the NSR catalyst varies depending on the interval at which the rich spike process is executed, the air-fuel ratio at the time when the rich spike process is executed, and the like. Therefore, when NH ₃ is supplied to the SCR catalyst, the execution interval of the rich spike process is set to an interval suitable for the generation of NH ₃ , or the air-fuel ratio at the execution of the rich spike process is an empty space suitable for the generation of NH ₃ What is necessary is just to set to an air-fuel ratio (for example, about 14.1).

By the rich spike action as described above is performed, it is possible even when the internal combustion engine 1 is lean-burn operation to purify NO _X in the exhaust gas. By the way, when the purification performance of the SCR catalyst housed in the third catalyst casing 6 deteriorates, NO _X that is not purified by the NSR catalyst may be discharged into the atmosphere. Therefore, if the NO _X purification performance of the SCR catalyst is deteriorated, and quickly detect a deterioration of the SCR catalyst, or encourage repair of the SCR catalyst to the driver of the vehicle, or prohibit lean-burn operation of the internal combustion engine 1 There is a need to. Hereinafter, a method for detecting the deterioration of the SCR catalyst in the present embodiment will be described.

When the SCR catalyst is in an abnormal state, the amount of ammonia that can be adsorbed by the SCR catalyst is smaller than when the SCR catalyst is in a normal state. Therefore, when the SCR catalyst is in an abnormal state than when in the normal state, the amount of the NO _X capable SCR catalyst to purify decreases. However, when the amount of ammonia adsorbed by the SCR catalyst is smaller than the amount of ammonia that can be adsorbed by the abnormal SCR catalyst (lower limit), the amount of NO _X that can be purified by the normal SCR catalyst and the abnormal SCR the difference between the amount of the NO _X capable catalyst for purifying decreases.

Further, even when the ammonia adsorption amount of SCR catalyst is above the lower limit value, the less the NO _X flow rate, the difference between the NO _X purification performance decreases in the NO _X purification performance and abnormality of the normal . In particular, when the SCR catalyst is normal, of the NO _X which can be purified by ammonia adsorbed on the SCR catalyst amount when NO _X flow rate with respect to (a reference amount of NO _X) is small, the normal the difference between the NO _X purification performance and abnormal of the NO _X purification performance decreases.

Accordingly, in this embodiment, under conditions of ammonia adsorption amount of SCR catalyst is above the lower limit value, NO _X flow rate of the integrated value when the integrated value of the NO _X flow rate has reached the reference amount of NO _X and the cumulative value of the NO _X outflow to diagnose the deterioration of the SCR catalyst as a parameter.

  Here, the procedure for detecting the deterioration of the SCR catalyst in this embodiment will be described with reference to FIG. FIG. 2 is a flowchart showing a processing routine executed when the ECU 7 detects the deterioration of the SCR catalyst. This processing routine is stored in advance in the ROM or the like of the ECU 7, and is periodically executed by the ECU 7 (CPU) when the internal combustion engine 1 is in a lean combustion operation.

In the processing routine of FIG. 2, the ECU 7 first acquires the ammonia adsorption amount of the SCR catalyst in the processing of S101. The ammonia adsorption amount of the SCR catalyst includes the integrated amount of NO _X purified by the SCR catalyst (in other words, the integrated amount of ammonia consumed for NO _X purification by the SCR catalyst), and the integrated amount of ammonia flowing into the SCR catalyst. Can be calculated as parameters.

The accumulated amount of NO _X purified by the SCR catalyst can be obtained by integrating the difference between the NO _X inflow amount and the NO _X outflow amount. The integrated amount of ammonia flowing into the SCR catalyst can be obtained by integrating the amount of ammonia flowing into the SCR catalyst when the rich spike process is executed. The amount of ammonia flowing into the SCR catalyst during the execution of the rich spike control can calculate _the NO _X storage amount of the NSR catalyst, the air-fuel ratio in the rich spike control execution, the execution time of the rich spike processing as a parameter.

Further, since the NO _X sensor reacts to ammonia in addition to NO _X in the exhaust gas, the amount of ammonia flowing into the SCR catalyst when the rich spike process is executed may be measured by the first NO _X sensor 11. Here, immediately after the rich spike processing is started, oxygen is released from the three-way catalyst, so the air-fuel ratio of the exhaust gas flowing into the NSR catalyst is maintained at an air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio that is higher than the desired rich air-fuel ratio. Is done. Thereafter, when all the oxygen stored in the three-way catalyst is released, the air-fuel ratio of the exhaust gas flowing into the NSR catalyst is lowered to a desired rich air-fuel ratio. When the air-fuel ratio of the exhaust gas flowing into the NSR catalyst is maintained at an air-fuel ratio in the vicinity of the theoretical air-fuel ratio, a part of the NO _X stored in the NSR catalyst is discharged from the NSR catalyst without being reduced. After the air-fuel ratio of the exhaust gas flowing into the NSR catalyst is lowered to a desired rich air-fuel ratio, a part of NO _X released from the NSR catalyst is reduced to ammonia. In view of the above-described characteristics, the timing at which NO _X is discharged from the NSR catalyst and the timing at which ammonia is discharged are different, so that the amount of NO _{X and} the amount of ammonia discharged from the NSR catalyst are each determined as the first NO. It can be measured by the _X sensor 11. If integrating the amount of ammonia to be measured by such a method, it is possible to obtain the integrated amount of the NO _X flowing into the SCR catalyst.

  The acquisition means according to the present invention is realized by the ECU 7 acquiring the ammonia adsorption amount of the SCR catalyst by the method described above.

  In the process of S102, the ECU 7 determines whether or not the ammonia adsorption amount acquired in the process of S101 is larger than a lower limit value. The lower limit value corresponds to the amount of ammonia that can be adsorbed by the abnormal SCR catalyst. Here, the relationship between the temperature of the SCR catalyst and the amount of ammonia that can be adsorbed by the SCR catalyst is shown in FIG. The solid line in FIG. 3 indicates the amount (lower limit) of ammonia that can be adsorbed by the SCR catalyst when the SCR catalyst is abnormal, and the alternate long and short dash line in FIG. 3 indicates that the SCR catalyst has a normal state when the SCR catalyst is normal. Indicates the amount of ammonia that can be adsorbed. As shown in FIG. 3, the amount of ammonia that can be adsorbed by the SCR catalyst varies according to the temperature of the SCR catalyst. Therefore, it is desirable that the lower limit value be changed using the temperature of the SCR catalyst as a parameter. Although the temperature of the SCR catalyst may be estimated from the operating state of the internal combustion engine 1, the output signal of the second temperature sensor 13 may be used as a correlation value of the temperature of the SCR catalyst.

Here, returning to the processing routine of FIG. 2, when the ECU 7 makes a negative determination in the processing of S102, the execution of this routine is temporarily terminated. On the other hand, when an affirmative determination is made in the process of S102, the ECU 7 proceeds to the process of S103 and prohibits the execution of the rich spike process. As described above, the ECU 7 prohibits the rich spike processing, so that the air-fuel ratio of the exhaust gas flowing into the NSR catalyst is maintained lean. Thus, the maintenance means according to the present invention is realized by the ECU 7 executing the process of S103.

In the process of S104 is, ECU 7 is, NO _X storage capability of the NSR catalyst is determined whether or not saturated. Specifically, the ECU 7 saturates the NO _X storage capacity of the NSR catalyst when the output signal (NO _X inflow amount) of the first NO _X sensor 11 becomes zero or more than an amount obtained by adding a predetermined margin to zero. It is determined that When a negative determination is made in the process of S104, the ECU 7 returns to the process of S103 and maintains the rich spike process prohibited state. On the other hand, when a positive determination is made in the process of S104, the ECU 7 proceeds to the process of S105.

  In the process of S105, the ECU 7 acquires the ammonia adsorption amount of the SCR catalyst again. Subsequently, the ECU 7 proceeds to the process of S106, and again determines whether or not the ammonia adsorption amount acquired in the process of S105 is larger than the lower limit value. When the ammonia adsorption amount of the SCR catalyst becomes equal to or lower than the lower limit due to the temperature change of the SCR catalyst, the ECU 7 once ends the execution of this routine. On the other hand, if an affirmative determination is made in the process of S106, the ECU 7 proceeds to the process of S107.

In the process of S107, the ECU 7 uses the output signals of the first NO _X sensor 11 and the second NO _X sensor 12 to start integration of the NO _X inflow amount and the NO _X outflow amount. Preferably, the ECU 7 may start the accumulation of the NO _X inflow amount and the NO _X outflow amount from the time when the NO _X storage capacity of the NSR catalyst is saturated. In this way, the ECU 7 calculates the integrated value of the NO _X inflow amount and the NO _X outflow amount, thereby realizing the integrating means according to the present invention.

In the process of S108, it is determined whether or not the integrated value of the NO _X inflow amount is greater than or equal to the reference NO _X amount. The reference NO _X amount corresponds to the amount of NO _X that can be purified by the normal SCR catalyst. The amount of NO _{X that} can be purified by the SCR catalyst in the normal state is calculated using the ammonia adsorption amount of the SCR catalyst and the temperature of the SCR catalyst as parameters. Specifically, ECU 7, the amount of the NO _X ammonia adsorption amount and the equivalent ratio of the SCR catalyst is 1, in other words, if all of the ammonia adsorbed on the SCR catalyst is assumed to have been consumed in the reduction of the NO _X of the NO _X reduction amount (hereinafter, referred to as "reference reduction amount") is calculated. Subsequently, ECU 7 has a temperature of the SCR catalyst as a parameter, and calculates the maximum of the NO _X purification rate by the SCR catalyst in a normal state. Here, the relationship between NO _X purification rate temperature and SCR catalyst of the SCR catalyst when the SCR catalyst is normal in Fig. As shown in FIG. 4, the maximum NO _X purification rate when the SCR catalyst is normal (hereinafter referred to as “maximum NO _X purification rate”) varies depending on the temperature of the SCR catalyst. Therefore, the current temperature of the SCR catalyst, the amount of the NO _X capable SCR catalyst normal state is reduced corresponds to the product of the reference amount of reduction and the maximum NO _X purification rate. Accordingly, ECU 7 calculates a reference amount of NO _X by multiplying the reference amount of reduction and the maximum NO _X purification rate. By such a method, by ECU7 to calculating the reference amount of NO _X, calculating means according to the present invention is implemented.

When a negative determination is made in the processing of the S108, the ECU 7, until the integrated value of the NO _X flow rate becomes equal to or higher than the reference amount of NO _X, repeatedly executes the processes in S107 and S108. On the other hand, if an affirmative determination is made in the process of S108, the ECU 7 proceeds to the process of S109. In the process of S109, the ECU 7 has an integrating value of the integrated value of the NO _X flow rate at the time when the integration value of the NO _X flow rate is equal to or greater than the amount of the reference NO _X and NO _X outflow as a parameter, the SCR catalyst NO _X purification rate Enox is calculated. Specifically, ECU 7 calculates the NO _X purification rate Enox based on the following equation (1).
Enox = (ΣAnoxin−ΣAnoxout) / ΣAnoxin (1)
In formula (1), ΣAnoxin is an integrated value of the NO _X inflow amount, and ΣAnoxout is an integrated value of the NO _X outflow amount.

In the process of S110 is, ECU 7 is, NO _X purification rate Enox calculated in the processing of the step S109 wherein it is determined whether or not the threshold value or more. Threshold value obtained by adding a predetermined margin to the maximum of the NO _X purification rate of the SCR catalyst abnormal condition, or corresponds to the minimum of the NO _X purification rate of the SCR catalyst in a normal state, in advance experimentally determined value is there.

  If an affirmative determination is made in step S110, the ECU 7 proceeds to step S111 and determines that the SCR catalyst is normal. On the other hand, if a negative determination is made in the process of S110, the ECU 7 proceeds to the process of S112 and determines that the SCR catalyst is abnormal. In this way, the diagnostic means according to the present invention is realized by the ECU 7 executing the processing of S108 to S112.

When the abnormality detection process of the SCR catalyst is executed according to the procedure described above, the integrated value of the NO _X amount flowing into the SCR catalyst reaches the reference NO _X amount under the situation where the ammonia adsorption amount of the SCR catalyst exceeds the reference value. The deterioration diagnosis of the SCR catalyst is performed using the integrated value of the NO _X inflow amount and the integrated value of the NO _X outflow amount as parameters. As a result, the deterioration diagnosis of the SCR catalyst is performed under the condition that the difference between the NO _X purification performance of the normal SCR catalyst and the NO _X purification performance of the abnormal SCR catalyst becomes large. Therefore, even in the configuration in which the NSR catalyst is arranged upstream of the SCR catalyst, it is possible to detect the abnormality of the SCR catalyst more accurately.

1 Internal combustion engine 2 Fuel injection valve 3 Exhaust passage 4 First catalyst casing 5 Second catalyst casing 6 Third catalyst casing 7 ECU
8 Air-fuel ratio sensor 9 Oxygen concentration sensor 10 First temperature sensor 11 First NO _X sensor 12 Second NO _X sensor 13 Second temperature sensor 14 Accelerator position sensor

Claims (2)

A selective reduction catalyst that is disposed in an exhaust passage downstream of the storage reduction catalyst, adsorbs ammonia generated by the storage reduction catalyst, and reduces NO _X in the exhaust gas using the adsorbed ammonia as a reducing agent;
Acquisition means for acquiring the ammonia adsorption amount of the selective catalytic reduction catalyst;
When the ammonia adsorption amount acquired by the acquisition means exceeds the lower limit, which is the amount of ammonia that can be adsorbed by the selective reduction catalyst in the abnormal state, the temperature of the selective reduction catalyst and the ammonia adsorption acquired by the acquisition means Calculating means for calculating a reference NO _X amount, which is the amount of NO _X that can be purified by the selective catalytic reduction catalyst in a normal state, using the amount as a parameter;
NO ammonia adsorption amount obtained by the obtaining unit is the amount of the NO _X flowing out from the NO _X flow rate and the selective reduction catalyst is the amount of the NO _X flowing into the selective reduction catalyst after becoming larger than the lower limit value An integration means for integrating each _X outflow amount;
Diagnosis for diagnosing NO _X flow rate integrated value and degradation of the selective reduction catalyst based on the integrated value of the NO _X runoff when NO _X flow rate, which is integrated by the integration means becomes equal to the reference amount of NO _X Means,
A deterioration detection apparatus for a selective catalytic reduction catalyst. 2. The period from the time when the ammonia adsorption amount acquired by the acquiring means exceeds the lower limit to the time when the NO _X inflow amount integrated by the integrating means becomes equal to the reference NO _X amount in claim 1 A selective reduction type catalyst deterioration detection device further comprising a maintaining means for maintaining the air-fuel ratio of the exhaust gas flowing into the type catalyst lean.

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