EP2547988A1

EP2547988A1 - Method for determining the fill level in a reducing agent tank

Info

Publication number: EP2547988A1
Application number: EP11706843A
Authority: EP
Inventors: Peter Bauer; Jan Hodgson
Original assignee: Emitec Gesellschaft fuer Emissionstechnologie mbH
Current assignee: Continental Automotive GmbH
Priority date: 2010-03-11
Filing date: 2011-03-08
Publication date: 2013-01-23
Also published as: JP2013522515A; US20130255234A1; US8955308B2; WO2011110574A1; US9074510B2; EP2547877B1; DE102010011151A1; JP2013522584A; EP2547877A1; WO2011110573A1; US20140096512A1

Abstract

The invention relates to a method for operating a tank (1) for reducing agent, in particular aqueous urea solution, comprising a sensor (5) having a first electrical contact (6) and a second electrical contact (7). According to the method, a conductance value for liquid reducing agent (14), a conductance value for frozen reducing agent (15), and a conductance value for air (16) are first determined (steps a.1) to a.3)). A voltage is then applied between the first electrical contact (6) and the second electrical contact (7) (step b)). A conductance value between the first electrical contact (6) and the second electrical contact (7) is then determined (step c)). The conductance value determined in step c) is then compared to the conductance values determined in steps a.1) to a.3) and a determination is made as to whether liquid reducing agent (14), frozen reducing agent (15), or air (16) is present.

Description

The invention relates to a method for determining the fill level in a tank in which (liquid) reducing agent, such as a urea water solution, is stored, in particular for a mobile application in the motor vehicle sector.

In particular, for mobile internal combustion engines in motor vehicles exhaust gas purification devices are known, in which a reducing agent for reducing certain exhaust gas components is supplied. For example, nitrogen oxide compounds (NOx) in the exhaust gas can be removed particularly effectively if ammonia is supplied to the exhaust gas as a reducing agent. Typical reducing agents, such as ammonia, are hazardous substances and therefore should not be stored directly in motor vehicles. Therefore, reducing agent is kept regularly as reducing agent precursor in a separate tank as additional fuel in the motor vehicle. A typical reducing agent precursor is, for example, urea. This is z. B. in the form of a 32.5-urea-water solution stored in the vehicle. Such a urea-water solution is available, for example, under the trade name "AdBIue." Such a urea-water solution typically freezes at temperatures of -11 ^° C. A device for conveying or metering liquid reducing agent is then Such low temperatures can occur in motor vehicles, in particular as a consequence of long downtimes. It is desirable that it can be reliably determined whether liquid or frozen reducing agent is present in a tank for reducing agent Only for the sake of completeness, it should be noted here In addition, the consumption of reducing agent is regularly low, and typically the consumption of reducing agent is about 0.5% to 10% of the fuel consumption of one reducing agent Therefore, a simple and cost-effective sensor for filling level determination is desired, but a complex continuous level determination method is not normally required, but at the same time high demands are made on the determination of a reserve level in order to always ensure the desired cleaning effect of the exhaust gas treatment system It is an object of the present invention at least partially to solve the technical problems described in connection with the prior art, in particular a method for operating a reduction sanksanks are described with a level determination.

These objects are achieved by a method having the features of patent claim 1. Further advantageous embodiments and applications of the method are specified in the dependent formulated patent claims. It should be pointed out that the features listed individually in the patent claims can be combined with one another in any technologically meaningful manner, and thus further embodiments of the invention are shown. The description, in particular in conjunction with the figures, leads to further particularly preferred embodiments.

The invention relates to a method for operating a tank, which has a sensor with a first electrical contact and a second electrical contact. have contact, which comprises at least the following steps:

a.l) determining a conductance for liquid reducing agent;

a.2) determining a conductance for frozen reducing agent;

a.3) setting a conductance for air;

b) applying a voltage between the first electrical contact and the second electrical contact;

c) determining a conductance between the first electrical contact and the second electrical contact;

d) comparing the conductance determined in step c) with the guide values determined in steps al) to a.3) and determining whether liquid reducing agent, frozen reducing agent or air is present. The method according to the invention is also partially illustrated in connection with various embodiments of a tank for a reducing agent. Accordingly, this tank is particularly suitable for carrying out the method according to the invention and is therefore described in advance for the purpose of illustration. It has a tank wall and an at least partially bounded by the tank wall interior, wherein on the tank wall, a sensor having a first electrical contact and a second electrical contact is arranged, wherein the first electrical contact and the second electrical contact with the interior in electrically conductive Connection, the tank wall penetrate from the inner space to an outer side of the tank wall, and at a first distance of less than 5 cm [centimeters] are arranged to each other. The first electrical contact and the second electrical contact are preferably arranged at a first distance of less than 3 cm, more preferably even less than 2 cm. So that the first electrical contact and the second electrical contact penetrate the tank wall, it is essentially meant that the first electrical contact and the second electrical contact makes electrical connection from the interior of the tank to an outside of the tank.

At least the first electrical contact and / or the second electrical contact may be formed by a metallic pot which is arranged in the tank. Such a pot can be, for example, a housing for a metering device, which serves to convey reducing agent out of the tank. At least the first electrical contact and / or the second electrical contact can also be formed by a withdrawal line, a return line, a withdrawal or a discharge for the metering device. An extraction line, a return line, a withdrawal and a discharge are different lines that connect the metering device for promoting reducing agent with the interior of the tank s. For the mode of operation of the tank, it is necessary for the first electrical contact to be electrically insulated from the second electrical contact so that electrical properties of the reducing agent in the tank can be determined. The tank wall is preferably made of plastic. The electrical contacts, which together form the sensor, are preferably cast in the tank wall. Optionally, in addition at least one sealing element may be cast into the tank wall, which seals the electrical contacts against the tank wall. The electrical contacts are preferably designed as metallic pins. Optionally, these metallic pins may have a surface structure that favors the sealing of the tank wall to the metallic pins. Optionally, a groove may also be formed in the metallic pins into which a sealing element, such as an O-ring seal, engages. Optionally, the metallic pins may also have a recess through which improved sealing of the pins in the tank wall is achieved. It is also possible that the electrical contacts penetrate the tank wall individually. But it is also possible that the metallic pins are arranged in a common sealing element and this sealing element is inserted as a whole in the tank wall or penetrates the tank wall.

In principle, a plurality of such sensors may be provided, but it is preferred to provide only a single sensor on such a tank. Several such sensors may for example be provided in a tank in order to be able to reliably measure at least one of the sensors at low filling levels and / or an inclined position of the tank. At low levels, it may happen due to an inclined position of the tank, that no reducing agent is present at a sensor, although at a certain amount of reducing agent stored in the tank with horizontal alignment of the tank to the sensor would actually be present reducing agent. Consequently, the error rate of the level detection system can be reduced.

As far as "electrical contacts" is mentioned here, the first electrical contact and the second electrical contact is meant, which is not to be expressed by this designation that the first electrical contact and the second electrical contact must then always be made the same Rather, it should be expressed that at least one of the contacts can be designed in this way, the same applies here to other generalizations, such as, for example, to pins, seals, etc. A particularly preferred embodiment of the tank is when the sensor With the sensor described above, a fill level in the tank can be determined possible to determine a reserve level, because with the help of a measurement between the two electrical contacts a discrete level determination is possible. For this purpose, a voltage is applied to the electrical contacts and an electrical resistance (or a conductance = reciprocal of the resistance) between the electrical contacts determined. Depending on this measured value, a conclusion can be drawn as to whether and / or how much reducing agent is present in the first distance between the two electrical contacts of the sensor and / or which aggregate state (eg liquid or frozen) the reducing agent Has.

The reserve level in the tank is preferably determined near the bottom of the tank because near the bottom both electrical contacts may be located at an equal height. In addition, an arrangement in the vicinity of the tank bottom allows a particularly advantageous determination of a residual volume. In fact, a residual volume regularly only represents a surface covering of the tank bottom. A sensor arranged in the tank bottom can also be arranged in the middle of the tank bottom. As a result, the sensor is less susceptible to sloshing in the tank and / or to a possible tilt of the tank, because sloshing and / or an inclined position (particularly pronounced) change the level just away from the center - at the edge near the side tank walls - cause. In addition, tanks in motor vehicles may be installed in such a way that access to the tank is only possible from below. Therefore, the sensor z. B. particularly well accessible for maintenance when it is located in the tank bottom. It is particularly preferred that the electrical contacts for a first length of at most 5 cm [centimeters], preferably between 0.2 cm and 2 cm, particularly preferably between 0.5 cm and 1 cm, protrude from the tank bottom in the tank interior. In the region of this first length, the electrical contacts are preferably bare, ie in particular not electrically insulated. Consequently, an electric current can pass from the electrical contacts into the reducing agent over the entire first length.

In a further advantageous embodiment, the sensor can also be arranged on a tank wall. In particular, it is also possible to distribute a plurality of sensors, for example between two sensors and ten sensors, at a specific height over a circumference of the tank. Thus, monitoring of the tank can take place by means of this plurality of sensors, whereby at least one of the sensors is suitable for a representative measurement even in the case of inclined position and in the event of sloshing movements in the tank.

The level signals determined by the individual sensors can be evaluated in a suitable controller in order to arrive at a corrected fill level signal. For example, an average value can be formed from the individual fill level signals in order to determine whether a reserve level to be monitored has been undershot or not. In a further embodiment variant, a distinction can also be made as to whether a reserve level has fallen below or not by comparing the number of sensors in which reducing agent is present with the number of all available sensors. If z. B. more than half of the sensors reports that the reserve level is below, the reserve height can be set as below. A statistical evaluation of the level signals of the plurality of sensors, for example by means of a principal component analysis, is also possible.

In a further advantageous embodiment, a shoulder with a reserve height is arranged in the region of the electrical contacts. This paragraph isolates the electrical contacts up to the reserve height. In this way, a reserve level amount is precisely defined when arranged in the tank bottom electrical contacts. Further advantageous is the tank when the tank has a heater, and the heater is disposed at a distance of less than 50 cm [centimeter] to the sensor. Preferably, the second distance is less than 20 cm, and more preferably less than 10 cm. The heater is arranged for this purpose, in particular in the vicinity of the tank bottom. This heater is preferably a controllable, electrical heater (eg having at least one element from the group heating wire, heating foil, PTC element, cooling water heating). In a cooling water heater, a heating coil is preferably passed through the tank through which the cooling water heated by the internal combustion engine flows and gives off heat energy to the reducing agent in the tank.

When the reducing agent in the tank is completely frozen, the operation of a heater in the vicinity of the tank bottom forms a cavity in the frozen reducing agent (so-called "ice cave") in which (partially) liquid reducing agent is present The sensor can be used to determine the size of this ice cave, which can be determined by means of a conductivity measurement to determine whether liquid reducing agent or frozen reducing agent is present on the sensor In order to determine a temperature distribution in the tank, the energy introduced into the tank by the heater can also be taken into account.

In addition, it is proposed that at least at an electrical contact on the outside of the tank wall, a temperature sensor is mounted. Due to their own electrical conductivity, electrically conductive contacts also regularly have a good thermal conductivity. The tank wall preferably made of plastic, however, has a poor thermal conductivity. The electrical contacts This can be used to attach a temperature sensor on the outside of a tank wall and to determine with this temperature sensor via one of the two electrical contacts a temperature on the inside of the tank wall or in the interior of the tank. Such a sensor further improves the possibilities for determining a temperature distribution in the tank.

With regard to the method according to the invention, it should first be pointed out that electrical variables are always addressed here (conductance, voltage, contact, resistance, etc.). It should also be noted that the steps a1), a.2) and / or a.3) do not have to be carried out each time a determination of the fill level and / or the state of matter has taken place, but possibly only once. The corresponding guide values can then be stored as a guide value or tolerance range (permanently) and used as a reference for the guide values currently measured in step c) in step d). Consequently, the conductance values from the steps al), a.2) and a.3) can also be referred to as reference conductance values. The guide values of liquid reducing agent and frozen reducing agent are regularly so different that it can be concluded by determining the conductance whether liquid reducing agent or frozen reducing agent is present. Air is compared to reducing agent is a very good electrical insulator, so that air can be detected by a conductance determination between the two electrical contacts and air. The conductance of frozen reducing agent and the conductance of air are similar. In particular, it should be noted that the difference between the conductance of the frozen reductant and the conductance of air is significantly smaller than the differences between the conductance of air and the conductance of liquid reductant, and the conductance of frozen reductant and the conductance of liquid reductant. The method is particularly advantageous if a temperature sensor is provided on the tank and in step d) a temperature measured with this temperature sensor is taken into account. As already explained, the conductance values of frozen reducing agent and of air do not differ so much from each other as the liquid reducing agent conductance. For this reason, it may be advantageous if, in order to distinguish whether air or frozen reducing agent is present, a measured temperature is taken into account in step d). If the temperature is above a threshold temperature, for example, more than -10 ^° C or preferably more than -5 ^° C and more preferably more than 0 ^° C, no frozen reducing agent may be present, so that based on the conductance only between air and liquid Reducing agent must be distinguished. In addition, it can be taken into account in step d) which values were measured in previous process iterations. In addition, in step d), it can be taken into account whether and / or to what extent a heater was operated in a time interval before the procedure was carried out. If, for example, frozen reducing agent was detected in a preceding process iteration and, in addition, a heater was operated in step d) for thawing the reducing agent, it can be expected that liquid reducing agent will first be detected before air is detected. A conductance, which is normally characteristic of air, can thus be evaluated in step d) in such a way that frozen reducing agent is present if no liquid reducing agent could be detected in the meantime.

The method according to the invention is furthermore particularly advantageous when the method steps a1) to a.3) are carried out in advance, the guide values of liquid reducing agent, frozen reducing agent and air are stored in a first store and then for step d) the guide values of liquid reducing agent, frozen reductant Onsmittel and air are read out of this first memory. This first memory can be stored in a control unit such as the engine control of a motor vehicle. As a reference for the liquid reducing agent conductance, a conductance measured at an earlier time point can be used. At this time, there should certainly have been liquid reductant in the tank. Whether this is the case can be determined with a temperature sensor. When a temperature above a specified threshold temperature has been measured with a temperature sensor positioned on or in the tank or on or in a reductant dosing unit, it can be safely assumed that there is liquid reductant in the tank. In addition, it is also possible for steps a) to a.3) to be carried out in advance in a test setup. These guide values can also be stored in a first memory, from which they are read out for the execution of step d) in later operation. The memory may in this case be a read-only memory which is not rewritable.

The quality and composition of reducing agent are not always exactly constant. As already stated, reducing agent is regularly a 32.5 percent urea-water solution. In such a solution, on the one hand, the urea content may vary somewhat. On the other hand, impurities can also occur in the solution. Thus, it is possible that the conductance values (in particular the conductivities of liquid reducing agent and frozen reducing agent) shift slightly. Therefore, the method steps a1) to a.3) should at least be carried out if the properties of the reducing agent (detectable) could have changed. This is especially to be done after filling the tank with reducing agent, because the filled reducing agent may have different properties. In an advantageous embodiment of the method according to the invention, an alternating voltage is applied in step b) to the first electrical contact and to the second electrical contact, which changes between a positive voltage value and a negative voltage value. Preferably, the AC voltage is rectangular. Further preferably, the AC voltage is symmetrical. By this is meant that the negative voltage proportion and the positive voltage proportion correspond in form and amount. Thus it can be prevented that form deposits on one of the two contacts as a result of electrolysis.

Furthermore, the method is advantageous if the tank has a heater and the method is extended by the following steps:

e.l) activating the heater if it has been determined in step d) that frozen reducing agent is present;

e.2) Disabling the heater if it has been determined in step d) that air is present.

If there is an ice cave in the frozen reducer around a heater in the tank, liquid reductant should be present in that ice cave to allow the heat to be transferred from the heater to the remaining frozen reductant. By contrast, air in the ice cave is a thermal insulator. If there is no compound of liquid reductant between the heater and the remaining frozen reductant in the ice cave, then further operation of the heater is usually not practical because the heat energy delivered by the heater is not can get more efficiently to the frozen reducing agent. In the case of a PTC heating (PTC: positive temperature coefficient), such a situation can be detected by monitoring the current consumption of the heating system. If the heater already has a reduced power consumption shortly after switching on, there is an ice cave. With resistance heating, one of these be noted similar situation in which the current consumption is set in relation to the heating voltage. If this ratio is greatly reduced, there is an ice cave. For this reason it makes sense to deactivate the heating if an insulating air layer has been detected in an ice cave. In a further process, the heating can also be operated with a reduced power, if there is air in the tank.

If there is still frozen reducing agent on the sensor, it is immaterial here in a corresponding configuration of the tank, whether in the immediate vicinity of the heating an ice cave exists, which is filled with reducing agent or with air, because the second distance between the heater and the sensor chosen is that an optionally present between the sensor and the heater ice cave is so small that despite the ice cave sufficient heat transfer from the (planar) heating can be done in the frozen reducing agent. Especially in the case of very small ice cavities, the heat transportability of the air from the heater to the frozen reducing agent is sufficient, so that the heating can remain activated.

The particular advantages and refinements described with the tank are applicable and transferable to the method according to the invention. Particularly preferably, the invention finds application in a motor vehicle, comprising an internal combustion engine with an exhaust gas treatment device having a metering device for reducing agent, wherein the metering device has a tank described herein and a controller, and the controller is adapted to carry out the method according to the invention. The invention and the technical environment will be explained in more detail with reference to FIGS. The figures show particularly preferred embodiments, to which the invention is not limited. In particular, it should be noted that the figures and in particular the illustrated proportions are only schematic. Show it:

1: a tank for reducing agent,

2 shows a first embodiment variant for electrical contacts,

3 shows a second embodiment for electrical contacts,

4 shows a third embodiment variant for electrical contacts,

5 shows a fourth embodiment variant for electrical contacts,

6 shows a tank having a temperature sensor,

7: a motor vehicle with a tank,

8 shows a flow chart of the method according to the invention,

9 shows a tank with a fifth embodiment for electrical contacts, and

10 shows a tank with a sixth embodiment for electrical contacts.

In Fig. 1, a tank 1 is shown. This tank 1 has a tank wall 3 which limits an interior 4. In the tank 1 is frozen reducing agent 15, in which an ice cave 33 is formed. The ice cave 33 is partially filled with air 16 and partly with liquid reductant. onsmittel 14 (here in particular a urea-water solution) filled. The ice cave 33 is formed around a (electrically controllable, areal) heater 11. The heater 11 is arranged in the region of the tank bottom 10 on the tank wall 3. In a second distance 12 to the heater 11 is a sensor 5. The sensor 5 is also there in the tank wall 3, namely the tank bottom 10, respectively. The sensor 5 has a first electrical contact 6 and a second electrical contact 7. The first electrical contact 6 and the second electrical contact 7 are arranged at a first distance 9 from each other and guided with a seal 20 through the tank wall 3 of the tank 1 therethrough. At the first electrical contact 6, a temperature sensor 13 is attached, with which from an outside 8 of the tank 1, the temperature in the interior 4 of the tank 1 and the temperature of the reducing agent can be detected.

FIG. 2 shows an example of how electrical contact can penetrate a tank wall 3. The tank wall 3 has an indentation 23 into which a threaded sleeve 34 is inserted. In the threaded sleeve 34, the first electrical contact 6 with a seal 20 is arranged. The interior 4 of a tank, which is filled, for example, with frozen reducing agent 15, is sealed with the seal 20 against an outside 8.

FIG. 3 shows a further example of how a first electrical contact 6 and a second electrical contact 7 can penetrate a tank wall 3. The tank wall 3 is penetrated here in the region of the tank bottom 10. The first electrical contact 6 and the second electrical contact 7 are embedded by means of seals 20 in the tank wall 3. The filled with reducing agent 15 interior 4 of a tank 1 is sealed against an outer side 8. On the outer side 8, a protective frame 28 is attached to the tank wall 3, through which the first electrical contact 6 and the second electrical Kon- 7 are protected. This protective frame 28 may be provided directly in the manufacture of the tank 1. The protective frame 28 may for example be injection-molded or cast on the tank 1. The protective frame 28 may also form a plug. A cable with a corresponding connector can then be connected directly to the first electrical contact 6 and to the second electrical contact 7. The protective frame 28 then gives mechanical stability to the connection between the tank 1 and the connector. In the interior 4, a shoulder 21 is provided in each case in the region of the first electrical contact 6 and of the second electrical contact 7 on the tank wall 3 for the first electrical contact 6 and the second electrical contact 7. By paragraphs 21, a reserve level 22 is defined in the tank (the reserve level represents the level in the tank, if only the reserve volume of liquid reducing agent is present). The first electrical contact 6 and the second electrical contact 7 each protrude out of the shoulders 21 for a first length 32. Thus, a secure electrically conductive contact between the reducing agent present in the interior 4 and the first electrical contact 6 and the second electrical contact 7 is ensured. The arrangement according to FIG. 3 is particularly suitable for an arrangement of the sensor in the tank bottom 10.

4 shows another example of how an electrical contact can penetrate a tank wall 3. Here, the first electrical contact 6 is designed here in the form of a rivet 25, wherein the rivet 25 braces a rubber sleeve 29 as a seal 20 with the tank wall 3.

FIG. 5 shows a first electrical contact 6 inserted in a lateral region of a tank wall 3 (tank side wall). Here, too, only the first electrical contact 6 is represented by way of example. This first electrical contact 6 is also designed as a rivet 25, which is inserted by means of a seal 20 in a recess 23 of the tank wall 3. A reserve height 22 is defined here by the arrangement of the first electrical contact 6 in the tank wall 3 and not by the height of a shoulder 21. The further the first electrical contact 6 or the electrical contacts on the tank wall 3 are positioned away from the tank bottom, the larger the reserve height 22.

In addition, particularly preferred forms of a first electrical contact 6 are shown in FIGS. 4 and 5. These shapes are chosen so that no deposits and / or accumulations of reducing agent and / or reducing agent residues may occur in the first electrical contact 6, or that such deposits and / or accumulations are avoided as far as possible. Such deposits may cause a short circuit between the first electrical contact 6 and the tank wall 3 and / or a short circuit between the first electrical contact 6 and the second electrical contact 7. In particular, the end of the first electrical contact 6 can be designed appropriately here. FIG. 4 shows, for example, a lens mold 37 for the end of the first electrical contact 6. Fig. 5 shows a first electrical contact 6, which has a preferably circumferential chamfer 38 at its end. In addition, the thickness 40 of the first electrical contact 6 can be suitably selected. Preferably, the thickness 40 is at least 0.5 mm [millimeter], preferably at least 1 mm, and more preferably at least 2 mm. In addition, it is important for avoiding a short circuit due to deposits and / or accumulations that the insulation of the first electrical contact 6 to the tank wall 3 and to a second electrical contact 7 has a sufficient width 40. By a width 40 is preferably meant a shortest path on the surface of the insulation from the tank wall 3 to the first electrical contact 6. The insulation is formed according to FIGS. 4 and 5 by the seal 20. Preferably, the width 39 is at least 0.5 mm [millimeters] and more preferably at least 1 mm.

These special embodiments presented for the first electrical contact 6 can be transferred in an analogous manner to a second electrical contact 7, which however is not shown separately in FIGS. 4 and 5 for the sake of simplicity.

FIG. 6 shows a further tank 1 with a sensor 5, which is designed with a first electrical contact 6 and a second electrical contact 7. The tank 1 has a metallic pot 27, in which a conveying unit 26 for transporting or for metering the reducing agent is arranged. By means of the delivery unit 26, the liquid reducing agent 14 can be removed via a removal 35 from the interior of the tank 1 out. Part of the conveyor unit 26 may, for. Example, a filter, a pump, a valve, transport lines, etc., which are integrated with in the metallic pot 27. Starting from this delivery unit 26, the liquid reducing agent (possibly under elevated pressure) via a discharge line 36, for example, fed to an (not shown here) addition point or Zudosierstelle an exhaust system.

7 shows a motor vehicle 17 having an internal combustion engine 18 and an exhaust gas treatment device 19. In the exhaust gas treatment device 19, a metering device 2 is provided, which has a tank 1. Liquid reducing agent stored in the tank 1 can be added by means of a (preferably integrated in the tank) conveying unit of the exhaust gas treatment device 19 via an injector 30 with predetermined amounts. Fig. 8 shows a flow diagram of the method according to the invention. It can be seen the process steps al) to a.3), b), c) and d) and el) and e.2). It can also be seen that the process takes the form of a fe iteratively (several times) can be carried out repeatedly, the method steps al) to a.3) need not be carried out at each iteration of the process with. In addition, shown is a first memory 41, in which in the process steps al) to. a.3) certain conductivity values can be temporarily stored. The guide values stored in the first memory 41 can be taken into account in step d). This is indicated by corresponding signal arrows in FIG. 8. In addition, in step d), guide values can be taken into account that were measured in previous iterations of the method in step c) and stored in a second memory 42. The deposit of the guide values is indicated by a signal arrow from step c) to the second memory 42. The consideration of the stored conductance in step d) is indicated by the signal arrow from the second memory 42 to the step d). Also, in step d) temperature signals 43 and information stored in the second memory 42 about the operation of a heater during a preceding time interval can be taken into account. This is also indicated in each case by corresponding signal arrows. The first memory and / or the second memory may be provided in a control unit of a motor vehicle. Information about the operation of a heater can for example be obtained from the process steps el) and e.2). On the basis of the switch-on or the switch-off of a heater can be detected when the heater was operated. The information about the operation of a heater can also be obtained from the control of the heater itself.

9 shows a fifth embodiment variant of a first electrical contact 6 and of a second electrical contact 7 for a tank 1 with a sensor 5. A metallic pot 27 is inserted in the tank 1. In this metallic pot 27, a conveyor unit 26 is arranged for the promotion of reducing agent. The metallic pot 27 forms a first electrical contact 6 of the sensor 5. In addition, a second ter electrical contact 7 is provided. The second electrical contact 7 penetrates in the embodiment of FIG. 9, the metallic pot 27, wherein it is sealed with a sealing element against the metallic pot 27. The sealing element is designed in the present case as a rubber sleeve 29. However, other embodiments of the sealing element are also conceivable. In a modified embodiment, the second electrical contact 7, the tank wall 3 also separated from the metallic pot 27 penetrate. For example, the second electrical contact 7 can be arranged next to the metallic pot 27 in the tank wall. The designed as a metallic pot 27 first electrical contact 6 and the second electrical contact 7 preferably have a first distance 9 of less than 5 cm [centimeters] to each other, as is in the sense of the tank 1. FIG. 10 shows a sixth embodiment variant for a first electrical contact 6 and a second electrical contact 7 for a tank 1 with a sensor 5. A metallic pot 27 with a delivery unit 26 is likewise inserted into the tank 1. Again, the metallic pot 27 forms the first electrical contact 6. Around the metallic pot 27 around here is a filter 44 is arranged. On the metallic pot 27, a removal 35 for reducing agent is arranged by which reducing agent is transported from the tank 1 to the conveyor unit 26. Reducing agent, which passes from the tank 1 to the removal 35, is filtered by the filter 44. The second electrical contact 7 is arranged next to the metallic pot 27 with the filter 44. Between the first electrical contact 6 and the second electrical contact 7 there is also a distance of less than 5 cm [centimeters]. The filter 44 is disposed between the first electrical contact 6 and the second electrical contact 7. However, this is not disadvantageous for the measurement of the electrical properties of the reducing agent between the first electrical contact 6 and the second electrical contact 7. Thus, a particularly advantageous method for operating ei ^¬ nes reducing agent tank has been specified with a level determination here.

LIST OF REFERENCE NUMBERS

1 tank

2 metering device

3 tank wall

4 interior

5 sensor

6 first electrical contact

7 second electrical contact

8 outside

9 first distance

10 tank bottom

11 heating

12 second distance

13 temperature sensor

14 liquid reducing agent

15 frozen reducing agent

16 air

17 motor vehicle

18 internal combustion engine

19 exhaust treatment device

20 seal

21 paragraph

22 reserve height

23 invagination

24 threaded bush

25 rivet

26 conveyor unit

27 metallic pot

28 protective frame 29 rubber sleeve

30 injector

31 control

32 first length

33 Ice Cave

34 threaded sleeve

35 removal

36 derivative

37 lens shape

38 chamfer

39 width

40 thickness

41 first memory

42 second memory

43 temperature signal

Claims

claims

A method of operating a tank (1), comprising a sensor (5) having a first electrical contact (6) and a second electrical contact (7), which comprises at least the following steps:

a.l) determining a conductance for liquid reducing agent (14); a.2) establishing a guide value for frozen reducing agent (15);

a.3) setting a conductance for air (16);

b) applying a voltage between the first electrical contact (6) and the second electrical contact (7);

c) determining a conductance between the first electrical contact (6) and the second electrical contact (7);

d) comparing the guide value determined in step c) with the guide values determined in steps a.l) to a.3) and determining whether liquid reducing agent (14), frozen reducing agent (15) or air (16) is present.

Method according to claim 1, wherein a temperature sensor (13) is provided on the tank (1) and in step d) a temperature measured with this temperature sensor (13) is taken into account.

Method according to one of the preceding claims, wherein the method steps al) to a.3) are carried out in advance and the conductance of liquid reducing agent (14), frozen reducing agent (15) and air (16) are stored in a memory (41) and for step d) the guide values of liquid reduction medium (14), frozen reducing agent (15) and air (16) are read from this memory (41).

Method according to one of the preceding claims, wherein in step b) to the first electrical contact (6) and to the second electrical contact (7) an alternating voltage is applied, which alternates between a positive voltage value and a negative voltage value.

Method according to one of the preceding claims, wherein the tank (1) has a heater (11) and the method is extended by the following steps:

e.l) activating the heater (11) if it has been determined in step d) that frozen reducing agent (15) is present;

e.2) deactivating the heater (11) if it was determined in step d) that air (16) is present.

Motor vehicle (17) comprising an internal combustion engine (18) with an exhaust gas treatment device (19) having a metering device (2) for reducing agent, wherein the metering device (2) comprises a controller (31) and the controller (31) for performing a method according to of the preceding claims.