JP2018062891A - Urea water detection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an urea water detection device capable of more accurately detecting urea water amount injected by an urea water injection device.SOLUTION: An urea water detection device 30 is applied to an SCR system 1 including: an SCR device 11 disposed in an exhaust passage 10 of an internal combustion engine; and an urea water injection device 12 for injecting urea water to the exhaust passage 10 upstream of the SCR device 11. The urea water detection device includes: a sensor section 31 disposed in the exhaust passage 10 downstream of an injection position of the urea water injection device 12 and having at least one pair of electrode members 31a opposing to each other while sandwiching a hollow region of the exhaust passage 10 in which exhaust gas flows; and a calculation section 32 for calculated urea water amount in exhaust gas on the basis of electrostatic capacitance detected by the sensor section 31.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a urea water detection device.

  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 (reducing NOx to nitrogen and water using ammonia as a reducing agent) Selective Catalytic Reduction (hereinafter referred to as “SCR”) system is known.

  In general, an SCR system supplies urea water stored in a urea water tank to an exhaust passage upstream of the SCR device, and hydrolyzes urea with the heat of exhaust gas to generate ammonia, which is used in the SCR device. NOx is reduced by the SCR catalyst. The supply of urea water to the exhaust passage is generally performed by a urea water injection device having a dosing valve inside.

  By the way, it is known that components in urea water are crystallized and fixed inside the urea water injection device. For example, when the urea water injection device reaches a high temperature, the components in the urea water are crystallized. Then, the crystallized object adheres to the valve body of the dosing valve and the urea water cannot be normally supplied to the exhaust passage, or the crystallized object does not contact the valve body and the cylinder of the dosing valve. It may be sandwiched between them, and the urea water injection cannot be stopped. For this reason, a sensor for detecting the state of the urea water injection device is disposed inside the urea water injection device (see, for example, Patent Document 1).

JP 2006-132442 A

  However, in the conventional technology such as Patent Document 1, since the internal state of the urea water injection device is partially monitored by a pressure sensor or the like, the urea water injection device actually detects the amount of urea water injected. There was a problem that could not.

  If the urea water amount actually injected by the urea water injection device can be accurately detected, calibration according to the state of the urea water injection device may be performed in addition to the diagnosis of the urea water injection device. Is possible.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a urea water detection device capable of detecting the amount of urea water injected by the urea water injection device with higher accuracy.

  The main present invention that solves the above-described problems is an SCR system having an SCR device disposed in an exhaust passage of an internal combustion engine and a urea water injection device that injects urea water into the exhaust passage upstream of the SCR device. A urea water detection device applicable to the above, and is disposed in an exhaust passage downstream of the injection position of the urea water injection device, and at least facing a hollow region through which exhaust gas flows in the exhaust passage. A urea water detection device comprising: a sensor unit having a pair of electrode members; and a calculation unit for deriving an amount of urea water in the exhaust gas based on a capacitance detected by the sensor unit.

  The urea water detection device according to the present invention can detect the amount of urea water injected by the urea water injection device with higher accuracy.

The figure which shows an example of a structure of the SCR system which concerns on embodiment The figure which shows an example of a structure of the urea water detection apparatus which concerns on embodiment The exploded view which shows the structure of the sensor part of the urea water detection apparatus which concerns on embodiment The figure which shows an example of the operation | movement flow of the urea water detection apparatus which concerns on embodiment The figure which shows typically the operation | movement which detects a moisture occupation rate in a sensor part. The figure which shows an example of a structure of the sensor part of the urea water detection apparatus which concerns on the modification 1.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram illustrating an example of the overall configuration of an SCR system 1 according to the present embodiment.

  The SCR system 1 according to the present embodiment includes an SCR device 11, a urea water injection device 12, a pumping line 13, a supply pump 14, a urea water tank 15, a DCU (Dosing Control Unit) 20, and a urea water detection device 30. Composed.

  The SCR system 1 according to the present embodiment is applied to, for example, a diesel engine, and is provided on the downstream side of the oxidation catalyst and the PM filter of the exhaust passage 10 of the diesel engine. Of course, it can be applied to a gasoline engine or the like instead of the diesel engine.

  The SCR device 11 reduces NOx to nitrogen and water using urea water injected into the urea water injection device 12 on the upstream side of the exhaust passage 10. The SCR device 11 incorporates, for example, Fe zeolite, Cu zeolite, vanadium, or the like as the SCR catalyst. As the SCR catalyst, a urea water adsorption type catalyst that converts urea water to ammonia and reduces NOx on the catalyst may be used.

  The urea water injected from the urea water injection device 12 is hydrolyzed by the high temperature of the exhaust gas, converted into ammonia, and supplied to the SCR device 11. Ammonia is adsorbed on the SCR catalyst and reacts with NOx by the action of the SCR catalyst to reduce and purify NOx.

The chemical reaction in which urea water reduces and purifies NOx is typically represented by the following chemical reaction formulas (1) to (4).
CO (NH ₂ ) ₂ + H ₂ O → 2NH ₃ + CO ₂ Formula (1)
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O Formula (2)
6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O Formula (3)
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O Formula (4)

  The urea water injection device 12 injects a predetermined amount of urea water into the exhaust passage 10 by adjusting the opening of a built-in dosing valve. The opening degree of the dosing valve is controlled by a control signal output from the DCU 20, for example.

  The urea water injected from the urea water injection device 12 is stored in the urea water tank 15. The urea water stored in the urea water tank 15 is sucked into the supply pump 14 and pumped to the urea water injector 12 through the pumping line 13. The urea water is stored in the urea water tank 15 at a predetermined concentration of about 30%, for example.

  The DCU 20 performs overall control of the entire SCR system 1. The DCU 20 performs data communication with a vehicle ECU (not shown) that performs fuel injection control and the urea water detection device 30. The DCU 20 also receives detection signals from various sensors such as a NOx sensor, a flow rate sensor, and a temperature sensor disposed in the exhaust passage 10.

  For example, the DCU 20 calculates the amount and timing of urea water to be injected from the urea water injection device 12 based on detection values indicated by a NOx sensor, a flow sensor, a temperature sensor, and the like. Then, the DCU 20 drives the supply pump 14 to increase the urea water in the pumping line 13 to the specified pressure, and also drives the dosing valve of the urea water injection device 12 to inject the amount of urea water calculated at the calculated timing. .

  The urea water detection device 30 is a capacitance type sensor arranged to detect the amount of urea water injected by the urea water injection device 12.

  FIG. 2 is a diagram illustrating an example of the configuration of the urea water detection device 30. FIG. 3 is an exploded view showing the configuration of the sensor unit 31 of the urea water detection device 30. In addition, the arrow in FIG. 2, FIG. 3 represents the distribution direction of exhaust gas.

  The urea water detection device 30 includes a sensor unit 31 and a calculation unit 32 (corresponding to a calculation unit).

  The sensor unit 31 includes at least a pair of electrode members 31a that face each other across a hollow region, and a case 31b that houses the electrode members 31a. The sensor unit 31 constitutes a capacitor having a dielectric in the hollow region of the pair of electrode members 31a. The exhaust gas of the exhaust passage 10 flows through the hollow regions of the pair of electrode members 31a, and the sensor unit 31 thereby detects a change in capacitance according to the amount of urea water contained in the exhaust gas.

  The sensor unit 31 is disposed in the exhaust passage 10 on the downstream side of the position where the urea water injection device 12 injects urea water (hereinafter abbreviated as “injection position”) and on the upstream side of the SCR device 11. The exhaust passage 10 is attached to the exhaust pipe wall by bolts or the like.

  Each electrode member 31a according to the present embodiment has a flat plate shape, and is housed in the case 31b with the other electrode member 31a adjacent to the plate surface parallel to the plate surface. Further, the electrode member 31a is arranged so as to face another adjacent electrode member 31a along the exhaust gas flow direction in order to avoid an increase in engine back pressure.

  Adjacent electrode members 31a are arranged close to each other in order to improve capacitance detection accuracy. Then, three or more electrode members 31a (four electrode members 31a are shown in FIG. 3) are arranged in parallel, thereby ensuring a wide detection range in the exhaust passage 10. As described above, each electrode member 31a forms a capacitor so as to face the other adjacent electrode member 31a across the hollow region, and the exhaust gas flows through each hollow region. Thus, the sensor part 31 comprises a plurality of capacitors in the case 31b.

  The case 31 b has openings 31 c and 31 d on the upstream side and the downstream side of the exhaust passage 10. The exhaust gas flows into the case 31b through the upstream opening 31c, passes through the region where the electrode members 31a face each other, and flows out of the case 31b through the downstream opening 31d.

  Each electrode member 31a is connected to a capacitance detection circuit (not shown) via a conductive wire, and thereby a capacitance of a capacitor formed by the adjacent electrode member 31a and a hollow region (hereinafter referred to as “sensor portion”). Is detected). Then, the arithmetic unit 32 derives the amount of water contained in the exhaust gas from the capacitance, and converts it into the amount of urea water actually injected by the urea water injection device 12 (details will be described later).

  The arithmetic unit 32 is, for example, a microcomputer including a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), an input port, an output port, and the like. In addition, the arithmetic unit 32 according to this embodiment also includes a capacitance detection circuit that detects the capacitance of the sensor unit 31.

  The arithmetic unit 32 includes a urea water amount deriving unit 32a and a diagnosis unit 32b.

  The urea water amount deriving unit 32 a derives the urea water amount in the exhaust gas based on the capacitance detected by the sensor unit 31. Then, the urea water amount deriving unit 32 a is based on the capacitance detected by the sensor unit 31 and the volume flow rate of the exhaust gas flowing through the exhaust passage 10, and the urea water actually injected from the urea water injection device 12. The quantity is derived (details will be described later).

  The diagnosis unit 32b diagnoses whether or not the urea water injection device 12 has failed based on the urea water amount derived by the urea water amount deriving unit 32a. The diagnosis unit 32b, for example, includes a urea water amount (hereinafter referred to as “injection amount command value”) instructed by the DCU 20 to the urea water injector 12 and a urea water amount actually injected by the urea water injector 12 (hereinafter referred to as “injection amount command value”). , Referred to as “actual value of injection amount”), a failure diagnosis of the urea water injection device 12 is performed.

  The urea water amount deriving unit 32a and the diagnosis unit 32b are realized, for example, when the CPU refers to a control program or various data stored in a ROM, a RAM, or the like. However, the function is not limited to processing by software, and can of course be realized by a dedicated hardware circuit.

[Operation flow of urea water detector]
Next, with reference to FIGS. 4-5, the operation | movement flow for the urea water detection apparatus 30 which concerns on this embodiment to diagnose the failure of the urea water injection apparatus 12 is demonstrated.

  FIG. 4 is a diagram illustrating an example of an operation flow of the urea water detection device 30 according to the present embodiment. Note that the operation flow shown in FIG. 4 is executed by the arithmetic unit 32 according to a computer program, for example. This operation flow is repeatedly executed at a predetermined interval (for example, every one second) when the urea water injection device 12 is operating, for example.

First, the urea water amount deriving unit 32a of the arithmetic unit 32 actually injects from the urea water injection device 12 based on the capacitance detected by the sensor unit 31 and the volume flow rate (m ³ / s) of the exhaust gas, for example. The amount of the urea aqueous solution (actual value of the injection amount) is derived (step S1).

  FIG. 5 is a diagram schematically illustrating the operation of detecting the moisture occupancy rate by the sensor unit 31. As described above, the exhaust gas containing moisture (indicated by W in FIG. 5) flows between the electrode members 31a of the sensor unit 31, and the sensor unit 31 corresponds to the moisture occupancy in the exhaust gas. The capacitance detected by changes. At this time, since the water discharged from the combustion chamber of the internal combustion engine has already been steamed, it can be assumed that the water detected by the sensor unit 31 is substantially due to urea water injected by the urea water injection device 12. it can. Further, the PM discharged from the combustion chamber of the internal combustion engine is also removed by the PM filter on the upstream side of the SCR system 1, and therefore does not affect the capacitance.

Therefore, the capacitance detected by the sensor unit 31 is based on the urea water component injected by the approximately urea water injection device 12 and the exhaust gas component of substantially air. From this point of view, the occupation ratio of urea water in the exhaust gas can be derived, for example, by calculating the equation (5) below. Here, it is assumed that the relative dielectric constant of the exhaust gas is equivalent to that of air, and the relative dielectric constant ε _r is approximately 1. Further, since the relative dielectric constant of water (ε _r = approximately 80) and the relative dielectric constant of urea (ε _r = approximately 5) are significantly different, the relative dielectric constant of urea water is assumed to be equivalent to that of water. The rate ε _r is approximately 80.
80 × A + 1 × (1-A) = B (5)
(However, A: urea water occupancy (%), B: capacitance detected by sensor unit 31)

On the other hand, the urea water actually injected from the urea water injection device 12 is dispersed according to the volume flow rate (m ³ / s) of the exhaust gas in the exhaust passage 10.

Therefore, the urea water amount deriving unit 32a can obtain the urea water amount from the relationship between the occupation ratio (%) of urea water obtained by the equation (5) and the volume flow rate (m ³ / s) of the exhaust gas. The urea water amount deriving unit 32a obtains the volume flow rate (m ³ / s) of the exhaust gas by adding the fuel flow rate to the detection value of a flow rate sensor (not shown) disposed in the intake passage, for example. At this time, the urea water amount deriving unit 32a may obtain the volume flow rate (m ³ / s) of the exhaust gas from the detection value of a flow rate sensor (not shown) disposed in the exhaust passage 10. Note that the urea water amount deriving unit 32a actually derives the urea water amount with reference to a calculation map using a preset capacitance and volume flow rate (m ³ / s) as variables and the urea water amount as a derived value. .

  The urea water amount deriving unit 32a may derive the urea water amount injected by the urea water injection device 12 by a more detailed calculation method. For example, in the above formula (5), the dielectric constant of the amount of urea water may be changed according to the concentration of urea water. When three or more electrode members 31a are used, an average value of capacitance detected between them may be used, or an added value may be used. On the other hand, in addition to the urea water amount derived above, the urea water amount evaporated from the injection position of the urea water injection device 12 until it is detected by the sensor unit 31 may be considered (described later in Modification 2).

  The subsequent steps S <b> 2 to S <b> 10 are processes of the diagnosis unit 32 b of the arithmetic unit 32.

  The diagnosis unit 32b first acquires a command value for the injection amount from the DCU 20 (step S2).

  Next, the diagnosis unit 32b compares the command value of the injection amount with the actual value of the injection amount, and determines whether or not the actual value of the injection amount of the urea water injector 12 is considerably less than the command value. (Step S3). In other words, it is determined whether or not the dosing valve inside the urea water injection device 12 is clogged.

  In step S3, the diagnosis unit 32b sets a value obtained by integrating a predetermined subtraction coefficient α (for example, 0.8) less than 1 with respect to the command value of the injection amount, and the actual value of the injection amount is the value. It is determined whether or not the number is exceeded. When the actual value of the injection amount does not exceed the value (step S3: NO), the diagnosis unit 32b accumulates the number of errors as an error because the actual value of the injection amount is too small (step S5). . On the other hand, when the actual value of the injection amount exceeds the value (step S3: YES), the subsequent determination process of step S4 is performed.

  The diagnosis unit 32b compares the command value of the injection amount with the actual value of the injection amount, and determines whether or not the actual value of the injection amount of the urea water injector 12 is considerably higher than the command value (step S4). ). In other words, it is determined whether a leak or the like has occurred in the dosing valve inside the urea water injector 12.

  In step S4, the diagnosis unit 32b sets a value obtained by integrating a predetermined addition coefficient β (for example, 1.2) of 1 or more with respect to the command value of the injection amount, and the actual value of the injection amount is the value. It is determined whether or not the number is exceeded. When the actual value of the injection amount exceeds the value (step S4: NO), the diagnosis unit 32b accumulates the number of errors as an error because the actual value of the injection amount is excessive (step S8). ). On the other hand, when the actual value of the injection amount does not exceed the value (step S4: YES), it can be determined that there is no failure in the urea water injection device 12, and thus the series of processes is terminated.

  In the above, when the error determination is made because the actual value of the injection amount is too small (step S3: NO, step S5), the diagnosis unit 32b determines whether the integrated value of the number of errors exceeds the set value ( Step S6). Then, when the integrated value of the number of errors does not exceed the set value (step S6: NO), the diagnosis unit 32b repeatedly executes the determination process with the number of errors held. Similarly, in the above, when the error determination is made because the actual value of the injection amount is too large (step S4: NO, step S8), the diagnosis unit 32b determines whether the integrated value of the number of errors exceeds the set value. If the accumulated value of the number of errors does not exceed the set value (step S9: NO), the determination process is repeatedly executed with the number of errors held. On the other hand, if no error is detected continuously, the series of processing ends as described above. Here, the reason why the diagnosis unit 32b performs the final failure determination by accumulating the number of errors is to prevent erroneous determination due to noise components such as foreign matters flowing through the exhaust passage 10.

  Then, when the integrated value of the number of errors exceeds the set value (step S6: YES, step S9: YES), the diagnosis unit 32b determines that the actual injection amount of the urea water injection device 12 is too small. Is determined (step S7), and the determination is made that the failure state is that the actual injection amount of the urea water injection device 12 is too large (step S10). At this time, for example, the diagnosis unit 32b may output a stop command for the urea water injection device 12 to the DCU 20, or may output a notification sound to notify the driver of the failure state.

  As described above, according to the urea water detection device 30 according to this embodiment, the amount of urea water contained in the exhaust gas can be detected in real time. In particular, since the urea water detection device 30 derives the urea water amount based on the capacitance detected by the sensor unit 31, it can detect the urea water after injection dispersed in a relatively wide range with high accuracy. it can.

  As a result, the amount of urea water actually injected from the urea water injection device 12 can be monitored, and it is possible to make a failure determination such as sticking of urea components or leakage of urea water in the urea water injection device 12. . This also makes it possible to calibrate the urea water injection device 12.

(Modification 1)
The urea water detection device 30 according to Modification 1 is different from the above embodiment in the configuration of the sensor unit 31.

  FIG. 6 is a diagram illustrating an example of the configuration of the sensor unit 31 of the urea water detection device 30 according to the first modification. In addition, the arrow in FIG. 6 represents the distribution route of exhaust gas.

  The case 31b of the urea water detection device 30 according to the modified example 1 has the opening 31c for taking in the exhaust gas in the exhaust gas flow direction in the exhaust passage 10 and the opening 31d for discharging the exhaust gas orthogonal to the flow direction. Configured to face in the direction of

  Further, the case 31b of the urea water detection device 30 according to the modified example 1 has a guide portion 31e in the opening 31c for taking in the exhaust gas.

  The case 31b changes the direction of the exhaust gas flowing through the exhaust passage 10 in a direction perpendicular to the flow direction by the guide portion 31e, and takes in the exhaust gas decelerated. Each electrode member 31a is disposed so as to extend along the flow direction of the exhaust gas taken into the case 31b. In other words, each electrode member 31 a extends along a direction orthogonal to the flow direction of the exhaust gas flowing through the exhaust passage 10.

  By adopting such a configuration, the sensor unit 31 can detect the capacitance in a state where the exhaust gas is decelerated, so that the detection accuracy of the capacitance can be improved. Thereby, the urea water detection device 30 can detect the amount of urea water injected from the urea water injection device 12 with higher accuracy.

(Modification 2)
The urea water detection device 30 according to the modified example 2 is different from the above-described embodiment in the method of deriving the urea water amount by the urea water amount deriving unit 32a.

  The urea water amount deriving unit 32a according to the modification 2 further adds the amount of urea water evaporated from the injection position of the urea water injection device 12 until it is detected by the sensor unit 31, and the urea water injection device 12 injects the urea water amount. The amount of urea water obtained is derived.

  The amount of urea water evaporated between the injection position of the urea water injection device 12 and the detection position of the urea water detection device 30 is lost between the injection position of the urea water injection device 12 and the detection position of the urea water detection device 30. It can be determined by the amount of heat.

  More specifically, the urea water amount deriving unit 32 a detects the detected value of a temperature sensor (not shown) that detects the temperature of the exhaust gas at the injection position of the urea water injection device 12 and the exhaust gas at the detection position of the urea water detection device 30. The amount of urea water evaporated by calculating the lost heat amount from the detection value of a temperature sensor (not shown) that detects the temperature of the gas and dividing by the heat energy required for 1 g of urea water to evaporate. Is derived.

The urea water amount deriving unit 32a derives the evaporation amount of urea water using, for example, the following equation (6).
Y = (amount of heat A (J) at the injection position of the urea water injection device 12−amount of heat B (J) at the position of the sensor unit 31)
/ Energy required to evaporate 1g of urea water X (J) ... Formula (6)
(However, Y (g): Amount of urea water evaporation
A (J): exhaust gas flow rate (kg / s) x exhaust gas specific heat (J / Kg / K) x exhaust gas temperature Ta (K),
B (J): exhaust gas flow rate (kg / s) x exhaust gas specific heat (J / Kg / K) x exhaust gas temperature Tb (K),
X (J / g): Sensible heat (373-T1 (K)) x 4.19 J / g / K + latent heat of vaporization 3257 (J / g))

  Thereby, the urea water detection device 30 can detect the amount of urea water injected from the urea water injection device 12 with higher accuracy.

  In the above description, when the urea water amount deriving unit 32 a derives the evaporated urea water amount, the detection value of the temperature sensor that detects the temperature of the exhaust gas at the injection position of the urea water injection device 12 and the detection of the urea water detection device 30. The aspect using the detected value of the temperature sensor which detects the temperature of the exhaust gas in the position was shown. However, if the temperature at these positions can be detected, the temperature sensor is unnecessary. For example, the temperature of the exhaust gas at the injection position of the urea water injection device 12 and the temperature of the exhaust gas at the detection position of the urea water detection device 30 are closely related to the engine speed and the engine load. It is also possible to predict from the load. Therefore, these relationships may be obtained in advance, and the urea water amount deriving unit 32a may identify these temperatures based on the engine speed and the engine load.

(Other embodiments)
The present invention is not limited to the above embodiment, and various modifications can be considered.

  In the above embodiment, as an example of the arithmetic unit 32 of the urea water detection device 30, the functions of the urea water amount deriving unit 32 a and the diagnosis unit 32 b are described as being realized by a single computer, but are realized by a plurality of computers. Of course, it is good. For example, the function of the diagnosis unit 32 may be realized as a function of the DCU 20. On the other hand, all of the functions of the urea water amount deriving unit 32a and the diagnosis unit 32b may be incorporated in the DCU 20.

  In the above embodiment, the urea water amount deriving unit 32a and the diagnosis unit 32b are shown as being executed in a series of flows as an example of the configuration of the urea water detection device 30. The units may be executed in parallel.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

  The urea water detection device according to the present invention is useful for an SCR system.

DESCRIPTION OF SYMBOLS 1 SCR system 10 Exhaust passage 11 SCR apparatus 12 Urea water injection apparatus 13 Pumping line 14 Supply pump 15 Urea water tank 20 DCU
30 Urea water detector 31 Sensor unit 32 Arithmetic unit

Claims (6)

A urea water detection device applicable to an SCR system comprising: an SCR device disposed in an exhaust passage of an internal combustion engine; and a urea water injection device that injects urea water into the exhaust passage upstream of the SCR device. ,
A sensor unit that is disposed in an exhaust passage downstream of the injection position of the urea water injection device and has at least a pair of electrode members facing each other across a hollow region through which exhaust gas flows in the exhaust passage;
Based on the capacitance detected by the sensor unit, a calculation unit for deriving the amount of urea water in the exhaust gas,
A urea water detection device comprising: The calculation unit derives the urea water amount injected into the exhaust passage by the urea water injection device based on the capacitance detected by the sensor unit and the volume flow rate of the exhaust gas flowing through the exhaust passage. ,
The urea water detection device according to claim 1. The arithmetic unit diagnoses the normality or abnormality of the urea water injection device based on the urea water amount injected by the urea water injection device and the urea water amount commanded to be injected to the urea water injection device.
The urea water detection device according to claim 2. The sensor unit is disposed in the exhaust passage on the downstream side of the injection position of the urea water injection device and on the upstream side of the SCR device,
The urea water detection device according to any one of claims 1 to 3. The sensor unit has three or more plate-like electrode members arranged in parallel.
The urea water detection apparatus as described in any one of Claims 1 thru | or 4. The sensor unit further includes a case that houses the electrode member, decelerates the exhaust gas flowing through the exhaust passage, and guides it to a region where the electrode members face each other.
The urea water detection device according to any one of claims 1 to 5.

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