JP5252217B2 - Engine exhaust purification system - Google Patents

Description

  The present invention relates to an engine exhaust purification device, and more particularly to an engine exhaust purification device including a selective reduction NOx catalyst.

  Conventionally, engine exhaust purification that reduces NOx emission concentration by supplying urea water from an injection valve toward a selective reduction NOx catalyst provided in an exhaust passage and reducing nitrogen oxide (NOx) in exhaust gas to nitrogen An apparatus is known (see, for example, Patent Documents 1 and 2).

  As described in Patent Document 1, such an engine exhaust gas purification device has a heat damage problem of the urea water injection valve. That is, since the heat resistance temperature of the urea water injection valve is lower than the exhaust temperature, there is a problem with the heat resistance of the urea water injection valve. For this reason, in the apparatus described in Patent Document 1, a cooling chamber for circulating a cooling medium around the injection valve is provided to cool the injection valve. Thereby, in this apparatus, the injection valve can be thermally protected.

  Further, as described in Patent Document 2, in the engine exhaust gas purification device, the selective reduction NOx catalyst deteriorates with use, or the NOx concentration sensor or the like generates a detection abnormality. For this reason, the apparatus described in Patent Document 2 is configured to determine the deterioration abnormality of the selective reduction NOx catalyst and the sensors and notify the driver of this. By this abnormality notification, the driver can know the performance deterioration of the purification system.

Japanese Patent Laid-Open No. 2007-321647 Japanese Patent Laid-Open No. 2006-37770

  However, when the cooling chamber for circulating the cooling medium is provided in the injection valve as in Patent Document 1, the entire apparatus becomes complicated and the manufacturing cost increases. For this reason, instead of providing such an additional structure, it is conceivable to obtain the cooling action of the injection valve by the internal flow itself of the urea water to be injected. In this case, it is possible to protect the injection valve thermally without an additional structure by securing the injection amount of the urea water at a predetermined amount or more.

  However, when the injection valve self-cools with the internal flow of urea water, the amount of urea water injection required is increased according to the temperature of the injection valve (or the exhaust temperature associated with the temperature of the injection valve). Thus, when the injection amount of urea water increases, there is a problem in that ammonia slip occurs in which excess ammonia is discharged into the atmosphere.

  On the other hand, if ammonia slip has occurred, the NOx concentration sensor provided downstream of the catalyst detects ammonia in addition to the exhausted NOx, so the NOx concentration is erroneously detected higher. For this reason, even in a case where the purification system is not abnormal, there is a possibility that the purification system is erroneously determined to be abnormal due to erroneous detection of the NOx concentration caused by ammonia slip even in a device such as Patent Document 2. It was.

  The present invention has been made to solve such a problem, and it is possible to suppress thermal damage to the urea water injection valve by injection of urea water, and to prevent erroneous determination of abnormality of the purification system. An object of the present invention is to provide an engine exhaust purification device that can be used.

  In order to solve the above-described problems, the present invention provides a selective reduction NOx catalyst provided in an exhaust passage of an engine and an aqueous urea solution provided upstream of the selective reduction NOx catalyst toward the selective reduction NOx catalyst. In an engine exhaust gas purification apparatus that includes a urea water injection valve that injects water and a control unit that controls the urea water injection valve, the control unit controls the urea water injection valve to adjust the injection amount of urea water An injection amount control means, a heat damage area determination means for determining whether or not the heat damage to the urea water injection valve should be suppressed based on the temperature related to the urea water injection valve, and reduction of NOx in the exhaust gas The first setting means for setting the injection amount according to the engine operating state for the purpose, the second setting means for setting the injection amount so as to suppress the heat damage, and the purification for acquiring the NOx purification rate by the selective reduction NOx catalyst Rate acquisition means, Abnormality determining means for determining an abnormality of the engine exhaust purification device based on the NOx purification rate acquired by the rate acquiring means, and the injection amount control means is configured to operate in the first state when it is in the operating region where heat damage should be suppressed. The urea water injection valve is controlled so that urea water is injected with a larger injection amount among the injection amounts set by the setting means and the second setting means, and the injection amount set by the second setting means is the first setting. When the injection amount set by the means is larger, the NOx purification abnormality determination by the abnormality determination means is prohibited.

  In the present invention configured as described above, in an engine operating state in which thermal damage to the urea water injection valve is to be suppressed, an optimal injection amount according to the engine operating state and an injection set to suppress thermal damage The larger one of the amounts is employed, and urea water is injected from the injection valve at this amount. Therefore, in the operation region where thermal damage should be suppressed, at least an injection amount set to suppress thermal damage or an injection amount larger than this is ensured, so that the thermal damage of the injection valve can be reliably suppressed. it can.

  Furthermore, in the present invention, in the engine operating state in which thermal damage should be suppressed, when the injection amount set to suppress thermal damage is larger than the optimal injection amount according to the engine operating state, Since urea water is injected with an injection amount set so as to suppress thermal damage, ammonia slip is likely to occur. Under such circumstances where ammonia slip is likely to occur, the NOx concentration cannot be accurately detected due to ammonia in the exhaust gas, and an accurate NOx purification rate may not be obtained. For this reason, in the present invention, it is possible to prevent erroneous determination by prohibiting abnormality determination of the exhaust purification system under this situation.

  In the present invention, preferably, the abnormality determination means determines at least one of deterioration of the selective reduction NOx catalyst or abnormality of the purification rate acquisition means. In the present invention configured as described above, it is possible to accurately determine the deterioration of the selective reduction NOx catalyst or the abnormality of the purification rate acquisition means based on the NOx purification rate.

  In the present invention, preferably, the purification rate acquisition means includes NOx concentration estimating means for estimating the NOx concentration on the exhaust upstream side of the selective reduction NOx catalyst according to the engine operating state, and NOx on the exhaust downstream side of the selective reduction NOx catalyst. NOx concentration detecting means for detecting the concentration. In the present invention configured as described above, the NOx concentration upstream and downstream of the selective reduction NOx catalyst can be accurately detected by the NOx concentration estimating means and the NOx concentration detecting means.

  Preferably, in the present invention, the injection amount control means stops the urea water injection for a predetermined period when the abnormality determination means determines abnormality, and after the predetermined period has elapsed after the stop, The concentration difference of the downstream NOx concentration is calculated. When the concentration difference is equal to or less than the predetermined concentration difference, it is determined that the selective reduction NOx catalyst has deteriorated. When the concentration difference exceeds the predetermined concentration difference, the NOx concentration detecting means Judge as abnormal. In the present invention configured as described above, the deterioration of the selective reduction NOx catalyst and the abnormality of the NOx concentration detection means downstream of the catalyst can be identified and determined.

  According to the engine exhaust gas purification apparatus of the present invention, the thermal damage to the urea water injection valve can be suppressed by the injection of urea water, and erroneous determination of abnormality of the purification system can be prevented.

1 is an overall view of an engine exhaust purification device according to an embodiment of the present invention. It is an exhaust purification treatment flow in the embodiment of the present invention. It is a graph for the injection amount setting in the embodiment of the present invention.

Next, embodiments of the present invention will be described with reference to the accompanying drawings.
An engine exhaust gas purification apparatus according to an embodiment of the present invention will be described with reference to FIGS.
As shown in FIG. 1, an engine exhaust gas purification apparatus 1 according to an embodiment of the present invention reduces the exhaust concentration of nitrogen oxides (NOx) in vehicle engine exhaust by an SCR (selective reduction catalyst) method. Selective reduction NOx catalyst 10 (hereinafter referred to as “catalyst 10”) provided in the exhaust passage 3 of the engine 2 and urea water for injecting the urea water 4 into the exhaust passage 3 between the catalyst 10 and the engine 2 An injection device 20 and a control unit 30 for controlling the injection amount of the urea water 4 from the urea water injection device 20 are provided.

In the engine exhaust gas purification device 1 of this embodiment, when the urea water 4 is sprayed into the exhaust passage 3 by the urea water injection device 20, the urea water 4 is hydrolyzed and thermally decomposed by the exhaust heat from the exhaust in the exhaust passage 3. Ammonia is produced. This ammonia is adsorbed by the catalyst 10, and a denitration reaction is promoted between the adsorbed ammonia and NOx in the exhaust. Thereby, even in the presence of oxygen, NOx is selectively reduced and decomposed into nitrogen gas N ₂ and water H ₂ O, which are discharged into the atmosphere.

The urea water injection device 20 includes a urea water injection valve 21 (hereinafter referred to as “injection valve 21”) for injecting urea water attached to the exhaust passage 3 between the engine 2 and the catalyst 10, and a urea water 4 Is provided with a urea water tank 22, a pump 23, and a pressure control valve 24. The urea water 4 is supplied to the pump 23 from the urea water tank 22 through the filter 25 and the pipe 26a. The pump 23 pumps the urea water 4 toward the pressure control valve 24 through the pipe 26b. The pressure control valve 24 supplies the urea water 4 at a predetermined pressure to the injection valve 21 through the pipe 26c. The pressure control valve 24 is provided with a pipe 26 d that is a return path to the urea water tank 22.
The injection valve 21 includes, for example, a solenoid valve and a drive mechanism for the solenoid valve, and the opening degree of the solenoid valve is adjusted based on a drive signal from the control unit 30.

  The control unit 30 is an ECU provided in the vehicle, receives signals from various sensors, and calculates the injection amount of the urea water 4 supplied to the catalyst 10 based on these signals. And the control part 30 outputs a drive signal to the injection valve 21, and adjusts the valve opening so that the urea water 4 may be injected with the calculated injection quantity. The urea water 4 may be continuously injected from the injection valve 21 or may be intermittently injected.

  Further, the control unit 30 outputs an abnormality detection signal to the notification unit 5 when an abnormality of the purification system including the catalyst 10 and the NOx concentration sensor 12 described later is detected. The notification means 5 is, for example, a monitor, a lamp, a speaker or the like, and notifies the driver of the abnormality based on the abnormality detection signal.

The engine speed sensor 2 a of the engine 2 outputs an engine speed signal representing the engine speed Ne to the control unit 30.
The accelerator opening sensor 6 outputs an accelerator opening signal representing the accelerator opening θ to the control unit 30.

  Further, the catalyst 10 is provided with a temperature sensor 11 for measuring the temperature of the catalyst 10 in its housing. The temperature sensor 11 outputs a catalyst temperature signal indicating the temperature Ts of the catalyst 10 to the control unit 30. In the present embodiment, the catalyst temperature Ts is related to the temperature of the injection valve 21, and the temperature of the injection valve 21 is estimated or substantially regarded as the temperature of the injection valve 21. Not only this but you may provide the temperature sensor which measures the temperature of the injection valve 21 directly.

  The exhaust passage 3 downstream of the catalyst 10 is provided with a NOx concentration sensor 12 as NOx concentration detection means. The NOx concentration sensor 12 detects the NOx concentration Dr in the exhaust gas purified by the catalyst 10, and outputs a NOx concentration detection signal representing the NOx concentration Dr to the control unit 30.

Next, the operation of the engine exhaust gas purification apparatus 1 according to the embodiment of the present invention will be described based on the exhaust gas purification process flow of FIG. The control unit 30 repeatedly performs the process shown in FIG. 2 at predetermined time intervals.
First, the control unit 30 reads signals representing the engine speed Ne, the accelerator opening θ, the NOx concentration Dr, and the catalyst temperature Ts received from each sensor (step S1).
The control unit 30 calculates the required engine torque Te based on the read engine rotational speed Ne and accelerator opening degree θ (step S2).

  Further, the control unit 30 that is a NOx concentration estimating means is based on the engine rotational speed Ne and the required engine torque Te, and the exhausted NOx concentration is calculated from the amount of NOx that is estimated to be exhausted from the engine 2 according to the current operating state. Df is calculated (step S3). In the present embodiment, the exhaust NOx concentration Df is calculated according to the operating state of the engine 2, but the present invention is not limited to this, and a NOx concentration sensor is provided on the exhaust upstream side of the catalyst 10, and this NOx concentration sensor The exhaust NOx concentration Df may be detected.

  Then, the control unit 30 as the purification rate acquisition means calculates the NOx purification rate α based on the acquired NOx concentration, that is, the exhaust NOx concentration Df and the NOx concentration Dr at the exhaust upstream / downstream position of the catalyst 10 (step S4). .

  Further, the control unit 30 as the first setting means, based on the exhausted NOx concentration Df, the NOx concentration Dr, and the catalyst temperature Ts, optimal injection amount of the urea water 4 to be supplied according to the current operating state of the engine 2. Q1 is calculated (step S5). In this process, the control unit 30 sets the target ammonia adsorption amount from the maximum ammonia adsorption amount that can be adsorbed by the catalyst 10 at the catalyst temperature Ts, and changes the NOx concentration Df, Dr upstream and downstream of the catalyst 10 to the NOx purification rate α. Calculate the corresponding ammonia consumption. Then, the control unit 30 calculates the injection amount so that the target ammonia adsorption amount is achieved according to the calculated ammonia consumption amount. Therefore, if the urea water 4 is injected with the injection amount Q1 calculated at this time, ammonia slip does not occur. The actual ammonia adsorption amount can be calculated from the integrated value of the injected urea water and the ammonia consumption amount.

Next, the control unit 30 as the heat-damaged area determination unit determines whether or not the catalyst temperature Ts is equal to or higher than a predetermined threshold temperature Ts ₀ (step S6). The threshold temperature Ts ₀ is a temperature threshold that requires countermeasures against heat damage of the injection valve 21, and the threshold temperature Ts ₀ or higher is a heat damage region. Accordingly, the temperature Ts ₀ is set so that the injection valve 21 can be thermally protected regardless of the amount of injection when used below this temperature.

If the catalyst temperature Ts is the threshold temperature Ts ₀ or more (Step S6; Yes), the case of the injection valve 21 should be protected from thermal damage, the control unit 30 as a second setting means resets the counter to be described later After returning to the initial value (step S7), the injection amount Q2 corresponding to the catalyst temperature Ts is calculated (step S8). In the present embodiment, the injection amount Q2 is set based on the catalyst temperature Ts and a table (see FIG. 3) stored in the internal memory.

As shown in FIG. 3, this table is set at the catalyst temperature Ts equal to or higher than the threshold temperature Ts ₀ , and the injection amount Q2 is larger as the catalyst temperature Ts is higher. That is, the thermal damage to the injection valve 21 is suppressed by increasing the injection amount Q2 of the urea water 4 and increasing the self-cooling action of the injection valve 21 as the catalyst temperature Ts increases. By injecting urea water with the injection amount Q2, the thermal protection of the injection valve 21 can be achieved.

  When the injection amount Q2 is set, the control unit 30 determines whether or not the injection amount Q2 is larger than the injection amount Q1 (step S9). In an operation state in which the concentration of NOx discharged from the engine 2 is high, the injection amount Q1 may be calculated to be a larger value than the injection amount Q2 required for heat damage countermeasures. For this reason, in the following process, it is comprised so that the heat damage countermeasure of the injection valve 21 can be taken at least by selecting the larger one among the injection quantities Q1 and Q2.

  When the injection amount Q2 is larger than the injection amount Q1 (step S9; Yes), the control unit 30 as the injection amount control means adopts the injection amount Q2, and the urea water 4 is injected from the injection valve 21 at the injection amount Q2. A drive signal is output so as to be injected (step S10). Thereby, the injection valve 21 can be protected thermally. On the other hand, since the injection amount Q2 is larger than the injection amount Q1, ammonia slip easily occurs due to an excessive injection amount (ammonia slip region).

  Therefore, when the injection amount Q2 is set, the control unit 30 prohibits a purification system abnormality determination described later (step S11). That is, when ammonia slip occurs, the NOx concentration sensor 12 may detect the NOx concentration higher due to ammonia contained in the exhaust gas downstream of the catalyst 10, and thus the calculated NOx purification rate may be inaccurate. . For this reason, if the NOx purification rate is inaccurate, the control unit 30 erroneously determines that the purification system is abnormal. In the present embodiment, in order to prevent such erroneous determination, in the ammonia slip region, abnormality determination of the purification system is prohibited and abnormality determination processing is not executed. The control unit 30 outputs this abnormality determination prohibition to the notification unit 5, and the notification unit 5 notifies the driver that the abnormality determination is being prohibited.

On the other hand, when the catalyst temperature Ts is equal to or higher than the threshold temperature Ts ₀ but the injection amount Q2 is not greater than the injection amount Q1 (step S9; No), the NOx concentration discharged from the engine 2 is in an operating state, and the injection is performed. The control unit 30 as the amount control means adopts the injection amount Q1 and outputs a drive signal so that the urea water 4 is injected from the injection valve 21 at the injection amount Q1 (step S12). Thereby, the injection valve 21 can be thermally protected without causing ammonia slip.

When the injection amount Q1 is set, the control unit 30 serving as the abnormality determination means executes the purification system abnormality determination process (steps S13 to S16). First, the control unit 30 determines whether or not the NOx purification rate α calculated in step S4 is equal to or less than a predetermined threshold (step S13).
When the NOx purification rate α is equal to or less than the predetermined threshold (step S13; Yes), the predetermined NOx purification rate is not obtained, and therefore the control unit 30 sets the deterioration abnormality flag to 1 (step S14). A deterioration abnormality flag of 0 means that the purification system is operating normally, and a deterioration abnormality flag of 1 means that some abnormality has occurred in the purification system.

  That is, the decrease in the NOx purification rate α is caused by the deterioration of the catalyst 10 of the purification system and the abnormality of the detection sensor (NOx concentration sensor 12 in this embodiment). When the catalyst 10 deteriorates, the NOx purification rate actually decreases. On the other hand, when the NOx concentration sensor 12 is abnormal, the NOx purification rate is calculated to be low although NOx is actually purified appropriately. However, it cannot be specified which of the catalyst 10 and the NOx concentration sensor 12 is the cause only from the NOx purification rate α. For this reason, the control unit 30 stores the abnormality detection of the purification system without distinguishing between catalyst deterioration or NOx concentration sensor abnormality, and outputs an abnormality detection signal to the notification means 5 (step S15). The notification means 5 receives this abnormality detection signal and notifies the driver that the abnormality is being detected.

  On the other hand, when the NOx purification rate α is not less than or equal to the predetermined threshold (step S13; No), since the predetermined NOx purification rate is obtained, the control unit 30 sets the deterioration abnormality flag to 0 (step S16).

Further, in step S6, when the catalyst temperature Ts is not the threshold temperature Ts ₀ or more (Step S6; No), the case where the thermal damage suppression of the injection valve 21 may not be performed, the control unit 30 first degradation It is determined whether or not the abnormality flag is 1 (step S17).
When the deterioration abnormality flag is not 1 (step S17; No), it is a state in which no abnormality of the purification system is detected, and the control unit 30 performs the processing of the above-described steps S12 to S16.

  On the other hand, when the deterioration abnormality flag is 1 (step S17; Yes), it is a state in which an abnormality of the purification system is detected, and it is not necessary to take measures for suppressing heat damage of the injection valve 21. . In this case, the control unit 30 sets the injection amount of the urea water 4 from the injection valve 21 to 0, stops the injection (step S18), and performs a countdown process of the internal counter (step S19). This counter is provided inside the control unit 30 and is provided to stop the injection for a predetermined period. In the process of step S19, the current counter value is decremented by a predetermined number. Specifically, a process of subtracting 1 from the current counter value is performed.

Next, the control unit 30 determines whether or not the counter value has reached 0 (step S20).
When the counter value is not 0 (step S20; No), the control unit 30 ends the process and repeats the process of step S1 again.
On the other hand, when the counter value is 0 (step S20; Yes), since the predetermined time has elapsed from the stop of injection, the control unit 30 resets the counter (step S21). Since a predetermined time has elapsed since the stop of the injection, the ammonia adsorbed on the catalyst 10 is consumed to purify NOx in the exhaust gas that has passed during the stop period, and the ammonia adsorption amount is zero. Therefore, at this time, the catalyst 10 does not purify NOx.

Next, the control unit 30 determines whether or not the concentration difference between the exhaust NOx concentration Df and the NOx concentration Dr when the catalyst 10 does not purify NOx is equal to or less than a predetermined concentration (step S22).
At this time, since the catalyst 10 is not purifying NOx, the concentration difference becomes 0, and thus the concentration difference becomes a predetermined concentration or less.

  However, if the concentration difference is not less than or equal to the predetermined concentration (step S22; No), the reason why the concentration difference does not become the predetermined concentration or less is considered to be an abnormal detection of the NOx concentration sensor 12. Therefore, the control unit 30 identifies the cause of the deterioration abnormality flag being 1 as the abnormality of the NOx concentration sensor 12, and outputs an abnormality detection signal representing the sensor abnormality to the notification unit 5 (step S23). The notification means 5 receives this abnormality detection signal and notifies the driver that the sensor is abnormal.

  On the other hand, when the concentration difference is equal to or less than the predetermined concentration (step S22; Yes), the NOx concentration sensor 12 is regarded as normal, and the control unit 30 identifies the cause of the deterioration abnormality flag being 1 as deterioration of the catalyst 10. Then, an abnormality detection signal indicating catalyst deterioration is output to the notification means 5 (step S24). The notification means 5 receives this abnormality detection signal and notifies the driver that the catalyst is deteriorated.

  As described above, in the engine exhaust gas purification apparatus 1 of the present embodiment, in the engine operation state in which the heat damage to the injection valve 21 is to be suppressed, the optimal injection amount Q1 corresponding to the engine operation state and the heat damage are suppressed. The larger one of the injection amounts Q2 set in the above is adopted, and the urea water 4 is injected from the injection valve 21 with this injection amount. Therefore, in the operation region in which the heat damage should be suppressed, at least the injection amount Q2 set to suppress the heat damage or an injection amount larger than this is secured, so the heat damage of the injection valve 21 is surely suppressed. be able to.

  Furthermore, in the engine exhaust purification apparatus 1 of the present embodiment, when the injection amount Q2 for suppressing thermal damage is larger than the optimal injection amount Q1 in the engine operating state where thermal damage should be suppressed, injection is performed. Since the urea water 4 is injected with the amount Q2, ammonia slip easily occurs. Under such circumstances where ammonia slip is likely to occur, the NOx concentration sensor 12 cannot accurately detect the NOx concentration due to ammonia in the exhaust gas, and there is a possibility that an accurate NOx purification rate α cannot be obtained. For this reason, in this embodiment, an erroneous determination can be prevented by prohibiting the abnormality determination of the exhaust purification system (catalyst 10, NOx concentration sensor 21) in a situation where ammonia slip is likely to occur.

DESCRIPTION OF SYMBOLS 1 Engine exhaust gas purification device 2 Engine 2a Engine rotational speed sensor 3 Exhaust passage 4 Urea water 5 Notification means 6 Accelerator opening degree sensor 10 Selective reduction NOx catalyst 11 Temperature sensor 12 NOx concentration sensor 20 Urea water injection device 21 Urea water injection valve 22 Urea Water tank 23 Pump 30 Controller

Claims (4)

A selective reduction NOx catalyst provided in the exhaust passage of the engine, a urea water injection valve provided on the exhaust upstream side of the selective reduction NOx catalyst and injecting urea water toward the selective reduction NOx catalyst, and the urea water In an engine exhaust purification device comprising a control unit that controls an injection valve,
The controller is
Injection amount control means for adjusting the urea water injection amount by controlling the urea water injection valve;
A heat damage area determination means for determining whether or not the operation area is to suppress a heat damage to the urea water injection valve based on a temperature related to the urea water injection valve;
First setting means for setting the injection amount according to the engine operating state for the reduction of NOx in the exhaust;
Second setting means for setting the injection amount so as to suppress the heat damage;
A purification rate acquisition means for acquiring a NOx purification rate by the selective reduction NOx catalyst;
An abnormality determination means for determining an abnormality of the engine exhaust purification device based on the NOx purification rate acquired by the purification rate acquisition means,
When the injection amount control means is in an operation region in which the heat damage should be suppressed, urea water is injected with a large injection amount among the injection amounts set by the first setting means and the second setting means. The urea water injection valve is controlled, and when the injection amount set by the second setting means is larger than the injection amount set by the first setting means, NOx purification abnormality determination by the abnormality determination means is prohibited. An engine exhaust purification device characterized by:   2. The engine exhaust gas purification apparatus according to claim 1, wherein the abnormality determination unit determines at least one of deterioration of the selective reduction NOx catalyst and abnormality of the purification rate acquisition unit. .   The purification rate acquisition means includes NOx concentration estimation means for estimating the NOx concentration on the exhaust upstream side of the selective reduction NOx catalyst according to an engine operating state, and NOx for detecting the NOx concentration on the exhaust downstream side of the selective reduction NOx catalyst. The engine exhaust purification device according to claim 1, further comprising a concentration detection unit.   The injection amount control means stops the urea water injection for a predetermined period when the abnormality determination means makes an abnormality determination, and the NOx on the exhaust upstream side and downstream side of the selective reduction NOx catalyst after a predetermined period has elapsed after the stop. A concentration difference between the concentrations is calculated. If the concentration difference is less than or equal to a predetermined concentration difference, it is determined that the selective reduction NOx catalyst has deteriorated. If the concentration difference exceeds the predetermined concentration difference, an abnormality in the NOx concentration detection means is detected. The engine exhaust gas purification apparatus according to claim 3, wherein the engine exhaust gas purification apparatus is determined.

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