JP6638331B2 - Selective catalyst reduction system and dosing valve sticking detection method - Google Patents

Description

  The present invention relates to a selective catalytic reduction system for detecting an abnormality of a dosing valve and a dozing valve sticking detection method in an exhaust gas purification system for selectively reducing NOx in exhaust gas of a diesel engine using urea water.

  As an exhaust gas purification system for purifying NOx in exhaust gas of diesel engines mounted on vehicles such as trucks and buses, selective catalytic reduction that reduces NOx to nitrogen and water using urea water or the like as a reducing agent (SCR: Selective Catalytic Reduction) system has been developed.

  The SCR system supplies the urea water stored in the urea water tank to an exhaust pipe upstream of the SCR device, hydrolyzes urea with the heat of exhaust gas to produce ammonia, and uses the ammonia to generate NOx at a catalyst in the SCR device. Is to reduce. An appropriate amount of urea water is supplied by, for example, a dosing valve provided in an exhaust pipe.

  Supply of urea water to the dosing valve is performed by a supply module including a supply pump and a urea water pressure sensor. The supply module is connected to the urea water tank via the suction line, and supplies the urea water sucked up from the urea water tank through the suction line to the dosing valve through a pressure feed line connecting the supply module and the dosing valve. The opening and closing of the dosing valve is controlled by a DCU (dosing control unit) according to the detection value of a NOx sensor provided upstream and / or downstream of the SCR device.

JP 2006-132442 A

  As described above, the dosing valve is provided in the exhaust pipe through which high-temperature exhaust gas flows. When the temperature of the dosing valve becomes high, the temperature of the urea water also becomes high, and components in the urea water may crystallize and adhere to the dosing valve. Specifically, for example, the crystallized object is fixed to the valve body of the dosing valve, and the urea water cannot be normally supplied from the dosing valve to the exhaust pipe, or the crystallized object is a valve of the dosing valve. It may be caught between the body and the cylinder, making it impossible to stop the injection of urea water. In this specification, the former is referred to as closed fixing and the latter is referred to as open fixing.

  Since the behavior of the exhaust gas purifying system differs from the action required for eliminating the fixed fixation and the fixed fixation, there has been a demand to accurately distinguish between the fixed fixation and the fixed fixation. Conventionally, there has been a method of determining whether the fixing is closed or open based on the output of a supply pump that sends urea water to the dosing valve. However, in this method, the output of the supply pump changes for reasons other than the fixing of the dosing valve. As a result, it was not possible to accurately judge whether the liquid was closed or fixed.

  The present invention has been made to solve such a problem, and an object thereof is to provide a selective catalyst capable of accurately determining whether a dosing valve is stuck open due to a pump output regardless of a change in a supply pump output. It is to provide a reduction system and a dosing valve sticking detection method.

A selective catalytic reduction system according to the present invention includes a dosing valve that supplies urea water into an exhaust pipe, a supply pump that supplies urea water in a urea tank to the dosing valve, and a dosing valve control unit that controls the operation of the dosing valve. And an atmospheric pressure sensor for acquiring the atmospheric pressure in the urea tank , wherein the dosing valve control unit performs the urea water injection at an injection amount set based on a detection value of a NOx sensor in the exhaust pipe. A drive current control unit that supplies a drive current for driving the dosing valve so as to supply the exhaust gas into the exhaust pipe, and that determines that the dosing valve is stuck when the drive current is outside the normal operation range. the anchoring thresholds fixing generated is used to determine whether open sticking or stuck closed, the atmospheric pressure is obtained from the atmospheric pressure sensor Is higher than the first atmospheric air bubbles appeared in serial urea in water, set to a predetermined initial value, when the atmospheric pressure is less than or equal to the first atmospheric pressure, set to a value obtained by correcting the initial value A sticking threshold setting unit that determines whether the sticking generated in the dosing valve is open sticking or closed sticking by comparing the output of the supply pump with the sticking threshold. And a part.

The dosing valve sticking detection method according to the present invention is directed to a selective catalyst reduction system comprising: a dosing valve for supplying urea water into an exhaust pipe; and a supply pump for supplying urea water in a urea tank to the dosing valve. A dozing valve sticking detection method for detecting the occurrence of sticking in the exhaust pipe, wherein the dosing valve is driven to supply urea water into the exhaust pipe at an injection amount set based on a detection value of a NOx sensor in the exhaust pipe. Determining whether the drive current to be performed is outside the normal operation range; and, when the drive current is determined to be outside the normal operation range, determining that the dosing valve is stuck, The sticking threshold used to determine whether the dosing valve is open or closed There is set to a predetermined initial value is higher than the first atmospheric air bubbles generated in the urea in water, when the atmospheric pressure is less than the first atmosphere is set to a value obtained by correcting the initial value And determining whether the dosing valve is open or closed using the sticking threshold value.

  According to the present disclosure, it is possible to accurately determine whether the dosing valve is stuck open due to the pump output regardless of the change in the supply pump output.

Diagram showing an example of the configuration of an SCR system Diagram for explaining an example of the configuration of a DCU Diagram showing an example of the relationship between the sticking threshold value and the atmospheric pressure Flowchart for explaining an operation example of the SCR system

<Background leading to the invention>
The opening and closing of the dosing valve is controlled by the DCU by increasing and decreasing the drive current for controlling the opening and closing of the valve. If the dosing valve is stuck, this coil current indicates an abnormal value, so that the DCU can detect the stuck or not by monitoring the driving current of the dosing valve. However, even if the presence of the fixation can be detected, it is not known from the drive current of the dosing valve whether the fixation is a closed fixation or an open fixation, so that it was necessary to determine this separately.

  Conventionally, as a method of determining whether the dosing valve is stuck closed or open, a method in which the dosing valve is stuck based on the drive current and the pump output of the supply pump is equal to or higher than a predetermined threshold. If there is, there is a method of determining that the sticking is open.

  By the way, the sticking of the dosing valve may be naturally resolved by the pressure of the urea water in the pressure feed line. If the dosing valve is not fixed, the pressure in the pressure feed line immediately drops to a predetermined pressure, and the output of the supply pump is adjusted according to the pressure in the pressure feed line.

  Here, since the determination of the presence or absence of the sticking based on the driving current of the dosing valve is performed based on the driving current at the time of driving a plurality of times, it may take time until the determination result is obtained. In the case where it takes a long time to determine the sticking, the sticking is determined based on the driving current of the dosing valve, and the pressure in the pressure feed line due to the sticking is eliminated, although the sticking has already been eliminated. In order to respond to the decrease, the output of the supply pump is increased. Here, if the output of the supply pump is equal to or greater than the predetermined threshold value, it is determined that the sticking is open even though the sticking has actually been eliminated.

  The cause of the increase in the output of the supply pump includes, for example, the following situations. For example, when the vehicle is located at a high altitude, the air dissolved in the urea water precipitates as bubbles due to a low atmospheric pressure, and this may enter the supply pump to lower the pump efficiency. Also, if the temperature of the urea water in the urea water tank rises for some reason, air dissolved in the urea water precipitates as air bubbles, which may drop into the supply pump and reduce pump efficiency. is there.

  To cope with such a decrease in the efficiency of the supply pump, the DCU increases the pump output. However, since the predetermined threshold value for determining the above-described open sticking is a duty ratio with respect to the pump output, if the pump output is increased, the pump output will exceed the above threshold value even if it is not open sticking, There is a possibility that an erroneous detection of open fixation may occur.

  To cope with this, the threshold may be set to a higher pump output. However, if the threshold value is set to a higher value, a case may occur in which the pump output cannot be determined to be open and fixed unless the pump output is increased.

  From such circumstances, in the present invention, the pump output decreases due to, for example, a change in the atmospheric pressure or a change in the urea water temperature, and even when the DCU performs control to increase the pump output, it is erroneously determined to be the open fixation. In order to avoid such a situation, a selective catalyst reduction system has been devised which can accurately judge whether the fuel is closed or open.

  In addition, if the closed fixation and the open fixation are erroneously determined, the following situation may occur. Ammonia generated from urea water is usually stored in the SCR device. However, when open sticking is detected, the amount of ammonia stored in the SCR device and the actual amount of ammonia stored are determined. In order to prevent the difference width from increasing, the storage amount of ammonia is calculated on the assumption that the urea water is injected at the maximum injection amount. If the open sticking is erroneously detected, the stored amount is calculated as being injected at the maximum injection amount even though it is not actually injected at the maximum injection amount. There is a large divergence between the above storage amount. As a result, it takes time until the injection amount becomes appropriate after the sticking is eliminated, and the risk of ammonia slip (flow of ammonia to the downstream side of the SCR device) increases. In order to avoid such a situation, it is an object of the present invention to accurately determine whether it is closed or locked.

<Embodiment>
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing an example of the configuration of an SCR system 100 corresponding to the selective catalytic reduction system of the present invention. An SCR device 11 is connected to an exhaust pipe 10 of a diesel engine mounted on a vehicle such as a truck or a bus. A dosing valve 12 for injecting urea water U is provided upstream of the SCR device 11, and a NOx sensor 13 is provided upstream and downstream of the SCR device 11.

  The SCR device 11 reduces the NOx to nitrogen gas and water vapor by a catalyst in the SCR device 11 using ammonia generated by hydrolysis of urea water injected from the dosing valve 12 due to high temperature of the exhaust gas. Purify gas. The NOx sensor upstream of the SCR device 11 and the NOx sensor 13 downstream detect NOx in the exhaust gas after the processing of the SCR device 11, and confirm that the NOx value in the exhaust gas downstream of the SCR device 11 is steady. Monitor and transmit the measured value to DCU14. Although the NOx sensor 13 has been described as being arranged upstream and downstream of the SCR device 11, it is not always necessary to arrange the NOx sensor 13 on both the upstream side and the downstream side, and is arranged upstream and / or downstream of the SCR device 11. Just do it.

  The detection value of the NOx sensor 13 is input to a DCU (Dosing Control Unit) 14. The DCU 14 controls the opening and closing of the dosing valve 12 according to the detection value of the NOx sensor 13 and the output value from an ECM 26 described later. The DCU 14 corresponds to the dosing valve control unit of the present invention.

  Although not shown, the DCU 14 performs the following operation by, for example, reading and executing a predetermined program from a memory by a CPU.

  The DCU 14 controls the dosing valve 12 according to a value detected by the NOx sensor 13 and the like. Specifically, the DCU 14 calculates the amount and timing of injecting the urea water U into the SCR device 11 and drives the supply pump 18 to increase the urea water U to a specified pressure, and opens and closes the dosing valve 12. The calculated amount of urea water U is injected at the calculated timing.

  Urea water U injected from dosing valve 12 is stored in urea water tank 15. When the dosing valve 12 is in the open state, the urea water U is sucked from the suction line 16 to the supply pump 18 of the supply module 17 and is pressure-fed to the dosing valve 12 via the pressure feeding line 20. At this time, foreign matter in the urea water is removed through the filter 19. Excess urea water U is returned to the urea water tank 15 from the pressure feed line 20 on the discharge side of the filter 19 by the return line 21.

  A urea water sensor 22 is provided in the urea water tank 15. The urea water sensor 22 measures the level of the urea water U in the urea water tank 15 and transmits the same to the DCU 14.

  The DCU 14 is connected to an ECM (engine control module) 26 that mainly performs fuel injection control. Further, the ECM 26 obtains information on the atmospheric pressure from the connected atmospheric pressure sensor 27. Information about the atmospheric pressure, a vehicle speed signal, various input information, control information of the ECM 26, and the like are transmitted from the ECM 26 to the DCU 14. Then, a battery 24 is connected to the DCU 14, and power is supplied. Further, the ON / OFF signal of the ignition key 25 is input to the DCU 14. The atmospheric pressure sensor 27 does not need to be connected to the ECM 26, and may be connected to another configuration, for example, the DCU 14.

  In such an SCR system 100, when a component in the urea water is stuck to the dosing valve 12, the DCU 14 detects the stuck and accurately determines whether the stuck is an open stuck or a closed stuck. Is determined. Hereinafter, the configuration of the DCU 14 and the operation of each configuration will be described in detail.

  FIG. 2 is a diagram for describing an example of the configuration of the DCU 14. As shown in FIG. 2, the DCU 14 includes an injection amount instruction unit 141, a drive current control unit 142, a pump output control unit 143, a sticking threshold value setting unit 144, a sticking determination unit 145, and a return detection unit 146.

  The injection amount instructing unit 141 determines the amount of the urea water U injected into the exhaust pipe 10 by the dosing valve 12 based on the detection value of the NOx sensor 13, the flow rate of the exhaust gas flowing through the exhaust pipe 10, the temperature of the exhaust gas, and the like. Instruct. The flow rate and temperature of the exhaust gas are acquired by an exhaust gas flow rate sensor (not shown) or an estimated exhaust gas flow rate or an exhaust gas temperature sensor calculated by the ECM 26 installed in the exhaust pipe 10.

  The drive current control unit 142 energizes the dosing valve 12 for the time t1 at predetermined time intervals t0 according to the injection amount of the urea water U set by the injection amount instruction unit 141. Inject. The time t1 is a time determined by the injection amount of the urea water U set by the injection amount instruction unit 141. The method of determining the time t1 according to the injection amount of the urea water U set by the injection amount instruction unit 141 is not particularly limited in the present invention. Hereinafter, the current supplied to the dosing valve 12 by the drive current control unit 142 is referred to as the drive current of the dosing valve 12.

  In addition, when the dozing valve 12 is stuck, the driving current control unit 142 detects the abnormality of the driving current to the dosing valve 12 by detecting the abnormality. The determination as to whether the drive current is abnormal may be made by detecting whether or not the current value of the drive current is outside the normal operation range of the drive current obtained experimentally or empirically. The normal operating range of the drive current is a range of the drive current generated when the drive current control unit 142 performs the opening / closing control of the dosing valve in a state where the dosing valve 12 is not stuck.

  The pump output control unit 143 controls the pump output of the supply pump 18 that pumps the urea water U from the urea water tank 15 to the dosing valve 12 via the suction line 16 and the pressure feed line 20. The pump output control unit 143 is based on, for example, a deviation between the pressure in the pumping line 20 measured by a pressure sensor (not shown) and the target value so that the pressure in the pumping line 20 is maintained at a predetermined target value. Thus, the output of the supply pump 18 is feedback-controlled.

  The sticking threshold value setting unit 144 sets a sticking threshold value of the pump output for the sticking determination unit 145 described later to determine the type of sticking. The setting method of the sticking threshold value by the sticking threshold value setting unit 144 may be, for example, as follows.

  First, the sticking threshold setting unit 144 sets an initial value of the sticking threshold based on the pump output of the supply pump 18. The initial value of the sticking threshold is not particularly limited in the present invention, and may be, for example, a value set experimentally or empirically. Specifically, the initial value of the sticking threshold may be, for example, 1% of the pump output (duty output).

  Next, the sticking threshold setting unit 144 corrects the initial value of the set sticking threshold according to the atmospheric pressure, and determines the final sticking threshold. Information on the atmospheric pressure is acquired from the atmospheric pressure sensor 27. The sticking threshold value setting unit 144 corrects the sticking threshold value so that the sticking threshold value is high when the atmospheric pressure is low and is low when the atmospheric pressure is high.

  FIG. 3 shows an example of the relationship between the sticking threshold value and the atmospheric pressure. As shown in FIG. 3, although the sticking threshold value and the atmospheric pressure do not have a linear relationship, the sticking threshold value increases as the atmospheric pressure decreases, and the sticking threshold value increases as the atmospheric pressure increases. May be set so as to lower the threshold value.

  In addition, as the correction amount of the sticking threshold value according to the atmospheric pressure, for example, a value obtained experimentally or empirically may be used. As an example, for example, the sticking threshold may be set to be higher than the atmospheric pressure at which bubbles are generated in the urea water, and the sticking threshold may be kept at the initial value at a higher atmospheric pressure. .

  Further, the sticking threshold value setting section 144 may further correct the sticking threshold value according to the temperature of the urea water U in the urea water tank 15. The temperature of the urea water U in the urea water tank 15 is detected by, for example, a urea water temperature sensor or the like (not shown), and when the urea water temperature is higher than a predetermined temperature, the fixing threshold may be set to be higher. The predetermined temperature is, for example, a temperature at which bubbles are generated in the urea water.

  The final sticking threshold value set by the sticking threshold value setting unit 144 is obtained by correcting the initial value of the predetermined sticking threshold value with the atmospheric pressure and / or the temperature of the urea water, as described above. . This correction of the sticking threshold is based on the assumption that when the atmospheric pressure is low or when the temperature of the urea water is high, bubbles are generated in the urea water and the bubbles enter the supply pump 18 to reduce the pump output. ing. That is, even when the atmospheric pressure is low or the urea water temperature is high, the sticking threshold is set according to the situation, so that the sticking determination unit 145 described later can accurately determine the type of sticking. become.

  When the drive current control unit 142 detects the occurrence of the sticking, the sticking determination unit 145 determines the type of the sticking, that is, determines whether the sticking is the open sticking or the closed sticking. The sticking determination unit 145 determines that the pump is open when the current pump output is higher than the sticking threshold set by the sticking threshold setting unit 144, and otherwise determines that the pump is closed. The reason for this determination is that the pump output is larger in the case of open fixation than in the case of close fixation.

  The return detection unit 146 monitors the pump output of the supply pump 18 after the drive current control unit 142 detects the fixation, and detects when the dosing valve 12 returns from the fixation. The return detection by the return detection unit 146 is performed when the abnormality of the drive current to the dosing valve 12 by the drive current control unit 142 is eliminated and the pump output of the supply pump 18 is equal to or less than a predetermined return threshold. It is determined that the dosing valve 12 has returned from the fixation.

  This return threshold is a value independent of the above-described sticking threshold. The return threshold is set, for example, by the return detection unit 146 according to the pump output of the supply pump 18. As the return threshold, a value obtained experimentally or empirically based on the fluctuation range of the value of the supply pump 18 when the SCR system 100 is operating normally may be used. As an example, the return threshold is set, for example, according to the normal output value of the supply pump 18 according to the atmospheric pressure acquired from the atmospheric pressure sensor 27 and the injection amount of the dosing valve 12 set by the injection amount instruction unit 141. Is done. That is, for example, the output of the supply pump 18 decreases below the atmospheric pressure at which bubbles are generated in the urea water, so that the return threshold may be set low. Alternatively, for example, when the injection amount of the dosing valve 12 set by the injection amount instruction unit 141 is small, the pump output decreases, so the return threshold value is low. When the injection amount is large, the pump output increases. Therefore, the return threshold may be set high.

  By appropriately setting the return threshold in this way, the return detection unit 146 can accurately detect whether or not the dosing valve 12 has returned from being stuck.

  The configuration of the SCR system 100 according to the present embodiment has been described above. Hereinafter, an operation example of the SCR system 100 will be described. FIG. 4 is a flowchart for explaining an operation example of the SCR system 100.

  In step S1, the drive current control unit 142 determines whether the drive current of the dosing valve 12 is an abnormal value. That is, when the current value of the drive current is out of the normal operation range, it is determined that the current value is abnormal, and otherwise, it is determined that the current value is normal. If it is determined in step S1 that the drive current is an abnormal value, the dozing valve 12 is stuck. Therefore, the process proceeds to step S2; otherwise, the stiction has occurred. Therefore, the process of step S1 is repeated.

  When it is determined in step S1 that sticking has occurred, in step S2, the sticking threshold value setting unit 144 sets a sticking threshold value. As described above, the sticking threshold is set based on, for example, the pump output of the supply pump 18, the atmospheric pressure, the temperature of the urea water U in the urea water tank 15, and the like.

  In step S3, the fixation determination unit 145 determines the type of fixation, that is, whether it is open fixation or closed fixation. As described above, the sticking determination unit 145 determines that the pump output is open when the current pump output is higher than the sticking threshold value set by the sticking threshold value setting unit 144, and otherwise determines that the pump output is closed. I do.

  In step S4, the return detection unit 146 sets a return threshold value for return detection. As described above, the return threshold is set based on, for example, the pump output when the supply pump 18 is normal, the atmospheric pressure, and the like.

  In step S5, the return detection unit 146 determines whether the drive current is within the normal operation range and whether the pump output of the supply pump 18 is within the normal operation range. It is determined whether or not has returned from the fixation. When the drive current is within the normal operation range and the pump output of the supply pump 18 is smaller than the return threshold, the return detection unit 146 determines that the dosing valve 12 has returned from being stuck. In this step S5, if it is determined that the dosing valve 12 has returned from the fixation, the process ends, otherwise, step S5 is repeated.

  As described above, the SCR system 100 according to the embodiment of the present invention includes a dosing valve that supplies urea water into the exhaust pipe, a supply pump that supplies urea water in the urea tank to the dosing valve, and a dosing valve. A dosing valve control unit that controls the operation, the dosing valve control unit controls the dosing valve to supply the urea water into the exhaust pipe at an injection amount set based on a detection value of the NOx sensor in the exhaust pipe. A drive current control unit that supplies a drive current to be driven and determines that the dozing valve is stuck when the drive current is outside the normal operation range, and determines whether the stuck is open or closed. Threshold setting unit that sets the sticking threshold value used for air pressure based on the atmospheric pressure, and the sticking generated in the dosing valve is stuck open Whether a certain or stuck closed, having and determining freeze determining section by comparing the fixed threshold and the output of the supply pump.

  With such a configuration, the SCR system 100 according to the embodiment of the present invention is configured such that when the driving current detects the fixation of the dosing valve 12, the fixation set based on the pump output of the supply pump 18 and the atmospheric pressure. Using the threshold value, it is determined whether it is open or closed. For this reason, even if the atmospheric pressure fluctuates, it is possible to determine whether it is open or closed using the fixation threshold value corresponding to the change in the atmospheric pressure, so that the determination can be made accurately.

  Further, the SCR system 100 according to the embodiment of the present invention, after the drive current control unit determines that the fixation has occurred, the drive current supplied by the drive current control unit returns to within the normal operation range, When the pump output of the supply pump is smaller than a predetermined return threshold, the supply pump further includes a return detection unit that determines that the dosing valve is not fixed. The predetermined return threshold is an output value of the supply pump, It is set based on the atmospheric pressure and the injection amount set based on the detection value of the NOx sensor.

  With such a configuration, when the SCR system 100 according to the embodiment of the present invention determines whether or not the dosing valve 12 has returned from being stuck, the return threshold corresponding to the pump output and the atmospheric pressure at that time is determined. Since the determination is performed using the value, the determination can be accurately performed.

  The present invention is useful for a selective catalytic reduction system that purifies NOx in exhaust gas.

Reference Signs List 100 SCR system 10 Exhaust pipe 11 SCR device 12 Dosing valve 13 NOx sensor 14 DCU
141 Injection amount instruction unit 142 Drive current control unit 143 Pump output control unit 144 Sticking threshold value setting unit 145 Sticking determination unit 146 Return detection unit 15 Urea water tank 16 Suction line 17 Supply module 18 Supply pump 19 Filter 20 Pressure feed line 21 Return Line 22 Urea water sensor 24 Battery 25 Ignition key 26 ECM
27 Atmospheric pressure sensor

Claims (6)

A dosing valve for supplying urea water into the exhaust pipe,
A supply pump for supplying urea water in the urea tank to the dosing valve,
A dosing valve control unit for controlling the operation of the dosing valve,
An atmospheric pressure sensor for obtaining the atmospheric pressure in the urea tank,
Has,
The dosing valve control unit,
A drive current for driving the dosing valve to supply urea water into the exhaust pipe at an injection amount set based on a detection value of the NOx sensor in the exhaust pipe, and the drive current is out of a normal operating range. A drive current control unit that determines that the dosing valve is stuck if
The sticking threshold value used to determine whether the sticking occurred is open sticking or closed sticking, when the atmospheric pressure obtained from the atmospheric pressure sensor is higher than the first atmospheric pressure at which bubbles are generated in the urea water. Setting a predetermined initial value, and when the atmospheric pressure is equal to or lower than the first atmospheric pressure, a sticking threshold value setting unit that sets the initial value to a corrected value ;
Whether the fixation generated in the dosing valve is open fixation or closed fixation, a fixation determination unit that determines by comparing the output of the supply pump and the fixation threshold,
A selective catalytic reduction system having: The fixed threshold setting unit, the initial value is set based on the pump output of the supply pump,
The selective catalyst reduction system according to claim 1. The sticking threshold value setting unit sets the sticking threshold value based on the temperature of the urea water before being supplied to the exhaust pipe.
The selective catalytic reduction system according to claim 1 or 2. When the output of the supply pump is higher than the sticking threshold, the sticking determination unit determines that the sticking is open.
The selective catalyst reduction system according to any one of claims 1 to 3. After the drive current control unit determines that the sticking has occurred, the drive current supplied by the drive current control unit returns to within the normal operation range, and the pump output of the supply pump returns to a predetermined return threshold. If the value is smaller than the value, a return detection unit that determines that the dosing valve has been fixed is removed,
Further having
The predetermined return threshold is set to a first return threshold when atmospheric pressure is higher than the first atmospheric pressure, and when the atmospheric pressure is equal to or lower than the first atmospheric pressure, the first return threshold is set. Set to a lower value than the value
The selective catalytic reduction system according to any one of claims 1 to 4. In a selective catalytic reduction system having a dosing valve for supplying urea water into an exhaust pipe and a supply pump for supplying urea water in a urea tank to the dosing valve, dozing valve sticking for detecting occurrence of sticking in the dosing valve. A detection method,
It is determined whether or not a drive current for driving the dosing valve to supply urea water into the exhaust pipe at an injection amount set based on a detection value of the NOx sensor in the exhaust pipe is outside a normal operation range. Steps to perform;
When it is determined that the drive current is out of the normal operation range, it is determined that the dosing valve is stuck, and is used to determine whether the dozing valve is stuck open or stuck closed. The fixing threshold is set to a predetermined initial value when the atmospheric pressure is higher than a first atmospheric pressure at which air bubbles occur in the urea water, and when the atmospheric pressure is equal to or lower than the first atmospheric pressure, Setting the initial value to a corrected value ;
Using the sticking threshold, determining whether the dosing valve sticking is open sticking or closed sticking;
A dosing valve sticking detection method comprising:

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