Method for operating a device for conveying a liquid

Abstract

Method for operating a device (1) for conveying a liquid with a conveying line (2) which can be divided into a suction line section (4) from a filter chamber (6) in a tank (7) to at least one pump (3) and into a pressure line section (5) from the at least one pump (3) to at least one injector (8), wherein the filter chamber (6) is separated from an interior space (10) of the tank (7) by at least one filter layer (9), and wherein the method comprises at least the following steps: a) sucking back a liquid located in the pressure line section (5) with the at least one pump (3) against a regular conveying direction (11); b) monitoring at least one operating parameter of the at least one pump (3) during the back suction, wherein the at least one operating parameter is representative of a back pressure against which the at least one pump (3) operates during the back suction; and c) Detect an increase in back pressure and stop the backflow.

Description

The invention relates to a method for operating a device for conveying a liquid, in particular for emptying the same. The device can be used, for example, in a motor vehicle in order to convey a liquid additive to an exhaust gas treatment device for cleaning the exhaust gases of an internal combustion engine of the motor vehicle and to supply it in metered fashion to the exhaust gas treatment device.In exhaust gas treatment devices to which a liquid additive for exhaust gas purification is supplied, the selective catalytic reduction (SCR) method is carried out, for example. In this method, nitrogen oxide compounds in the exhaust gas of an internal combustion engine are reduced with the aid of a reducing agent. Ammonia is usually used as the reducing agent. Ammonia is normally not stored directly in motor vehicles, but rather in the form of a reducing agent precursor solution. A reducing agent precursor solution which is particularly frequently used is liquid urea-water solution. A 32.5 percent urea-water liquid solution is available under the trade name AdBlue®. The method described herein is suitable for the operation of an apparatus by which a reducing agent precursor solution can be supplied to an exhaust gas treatment apparatus.DE 10 2008 010 105 A1 discloses a dosing system for dosing a pollutant-reducing medium into an exhaust gas, in particular for introducing a reducing agent and / or a reducing agent precursor into the exhaust gas. The dosing system comprises a storage tank for providing the pollutant-reducing medium and an intake system for intake of the pollutant-reducing medium from the storage tank. The intake system comprises at least two intake points. The dosing system further comprises at least one switch configured to switch between the at least two intake points.DE 10 2011 112 325 A1 discloses a device for providing liquid reducing agent for an exhaust gas treatment device. The device has a tank and a delivery unit, with an intake point in the tank, at which reducing agent can be sucked out of the tank. The intake point is covered by a separating layer, so that a closed intermediate space is present between the intake point and the separating layer, wherein the separating layer has a higher flow resistance for reducing agent in an outflow direction from the intermediate space into the tank than in an inflow direction from the tank into the intermediate space.In the development, production and operation of devices for providing such liquids for exhaust gas purification, it must regularly be taken into account that the (aqueous) liquids can freeze at low temperatures. The 32.5% strength urea-water solution freezes at -11° C., for example. Such low temperatures can occur in the motor vehicle sector, in particular during long idle times in winter. In the case of the Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Z-Freezing of the liquid typically occurs by volume expansion. This volume expansion can damage the device for conveying the liquid.For this reason, it is known to empty such a device at the stop of operation. During emptying, the liquid is removed from the device and replaced by air from the environment. Air is typically drawn into the device via an injector on an exhaust line, while at the same time the liquid is conveyed back into a tank. Then, during the standstill phase after an operation stop, no liquid is present within the device. Accordingly, no volume expansion of the liquid can occur within the device. However, it is problematic that a emptied device must be filled again before it is put back into operation. When refilling the device, there is the problem, in particular, that an undesired leakage of liquid through the injector can occur, in particular if the quantity of liquid that has to be supplied to the device for complete refilling is not known exactly. Moreover, an unnecessarily long operation of a pump should be avoided both during emptying of the device and during filling of the device. A shorter operating time of the pump reduces, on the one hand, the energy consumption during filling or emptying. On the other hand, damage to the pump can thus also be avoided or increased wear as a result occurs.Proceeding from this, it is an object of the present invention to solve or at least alleviate the technical problems described. In particular, a particularly advantageous method for operating a device for conveying a liquid is to be presented, in which the device is filled during startup and emptied during deactivation.These objects are achieved by a method according to the features of claim 1. The features individually listed in the patent claims can be combined with one another in any desired, technologically meaningful manner and can be supplemented by explanatory facts from the description, in particular from the description of the figures, wherein further embodiment variants of the invention are shown.The invention relates to a method for operating a device for conveying a liquid having a conveying line which can be divided in a suction line section from a filter space in a tank to at least one pump and into a pressure line section from the at least one pump to at least one injector, wherein the filter space is separated from an interior space of the tank by at least one filter layer and wherein the method comprises at least the following steps: a) back-sucking a liquid located in the pressure line section with the at least one pump counter to a regular conveying direction; b) monitoring at least one operating parameter of the at least one pump during the back-sucking, wherein the at least one operating parameter is representative of a counter-pressure against which the at least one pump operates during the back-sucking; and c) determining an increase in the counter-pressure and stopping the back-sucking.A device for conveying a liquid, for which device it is possible to drain using the method described here, is preferably provided to convey a liquid exhaust gas purification additive into an exhaust gas treatment device of an internal combustion engine. The device is preferably designed without a return line. This means that there is no branch from the pressure line section back into the tank downstream of the pump in the conveying direction from the tank to the injector. Thus, a circulation of liquid through the suction line section, the pump and back into the tank with the device is not possible. The emptying of the device must accordingly take place by a return (suction) of the liquid by the at least one pump, wherein the liquid is moved in the opposite direction to the regular / normal conveying direction. Here, the regular conveying direction is regularly defined as the direction along the conveying line from the tank toward the injector. The conveying direction opposite to the regular conveying direction, which occurs during the suck back, can also be referred to as the suck back direction.The at least one pump is preferably a pump with a reversible delivery direction. This pump can be, for example, an orbital pump or a peristaltic pump, in which the conveying direction can be reversed by the drive of the pump operating in the opposite direction. In the case of an orbital pump or in the case of a peristaltic pump, at least one seal of a conveying path is displaced along the conveying path for conveying within the pump. A seal means a closed region of the conveying path that cannot be passively flown through by a fluid. By displacing the seal in the conveying direction along a conveying path, a closed pump volume is moved along the conveying path and the liquid contained in the closed pump volume is conveyed along the conveying path. In a tube pump, the delivery path is formed by a tube and the seal is a compressed portion of the tube. In an orbital pump, the delivery path is formed by a gap between a housing and a deformable membrane, and the seal is formed by a region in which the deformable membrane is pressed against the housing and bears against it. By reversing the operating direction of the drive of a peristaltic pump or an orbital pump, the direction of movement of the seal along the conveying path can be reversed, so that the conveying direction of the pump is also reversed.According to one embodiment variant, the at least one pump is a reciprocating piston pump or a diaphragm pump, the delivery direction of which can be reversed with the aid of a valve arrangement. According to yet another variant, the pump is a flow pump with a reversible delivery direction.The method is based in particular on the operating situation that the delivery line (here in particular the pressure line section) is (completely) filled with liquid and previously, if applicable, a metering of liquid has taken place via the injector. It is now desired to deactivate the device and thus to drain it.During the back-suction of the liquid in step a), air (possibly also residual constituents of exhaust gas) is sucked in by the injector and the liquid is pressed (back) into the filter space at a suction point at which the suction line section of the conveying line opens into the filter space. Starting from the filter chamber, the liquid is forced further through the filter layer into the interior of the tank. In this case, the liquid now flows counter to the normal conveying direction, i.e. away from the injector towards the pump or also towards the tank.The operating parameter monitored in step b) is representative of the pressure which the pump builds up in the suction line section during the suck-back. During normal metering operation of the pump, no overpressure is built up in the suction line section, but only in the pressure line section. Under these conditions, a negative pressure is even present in the suction line section, so that the liquid is sucked from the filter space into the suction line section of the conveying line. In the case of a reverse conveying direction during the suck-back, these pressure conditions in the suction line section and the pressure line section are virtually reversed. Then, a negative pressure exists in the pressure line section, while a positive pressure exists in the suction line section. The operating parameter monitored by the pump is representative of the pressure in the suction line section. The pressure in the suction line section depends on the flow resistance of the liquid when it flows out of the suction line section into the filter space or into the interior of the tank. Among other things, the flow resistance of the filter layer starting from the filter chamber back into the interior of the tank is also decisive for the pressure.If an increase in the counter pressure is detected in step c), the air has entered the filter space. The increase in pressure is triggered by the air being forced by the pump against the filter layer. The filter layer is selectively permeable to the liquid. The permeability to the liquid is considerably higher than the permeability to air. Therefore, an increased counterpressure arises as soon as the air has reached the filter space or the at least one filter layer. Then, the suck back is stopped. The stopping of the suck back in step b) can take place, for example, if the counterpressure corresponding to the at least one operating parameter is greater than a predefined threshold value. Thus, an increase in the back pressure is detected and the suck back is stopped depending on the back pressure. This can prevent the filter chamber from being emptied (predominantly or even completely). In particular, it can be ensured that liquid still remains in the filter space and the liquid is not replaced there (completely or in the region of the intake point) by air. This makes it possible, on the one hand, to shorten the time of the suck-back, because the filter space usually has a relatively large volume in relation to the conveying line. At the same time, there is protection of the delivery line, the pump and further components (such as pressure sensors) adjoining the delivery line from freezing liquid additive.Selective permeability of the filter layer can be achieved, for example, by the filter layer being a non-woven material. A nonwoven material may be constructed from fibers (e.g., wires) that are at least partially air repellent, thus preventing air from entering the filter layer. When such a non-woven material is wetted by the liquid, it is very easy for the liquid to pass. When air is applied to this material, penetration of air into the material by capillary forces is prevented. A flow through the material for air is thus also prevented. If the pressure difference between the filter space and the interior of the tank present at the filter layer exceeds a limit value, however, a passage of air can occur despite such air-repellent properties of the material of the filter layer. Typically, suitable materials for the filter layer can maintain this selective permeability to the liquid only in the range of the usual operating pressures.The method is particularly advantageous if at least the following further step is carried out after step c):d) conveying liquid in a regular conveying direction until the filter layer is again completely filled with liquid.If in step c) an increase in the counter pressure is detected, air is already present in some regions at the at least one filter layer in the filter chamber. This air can permanently block the filter layer. It is therefore advantageous if after step c) a conveying takes place in a regular conveying direction. By this conveying, the conveying line or the filter chamber is at least partially filled again. It can thus be ensured that the filter layer is charged with liquid during deactivation of the device. Step d) can be carried out, for example, in a volume-controlled and / or time-controlled manner. In a volume-controlled implementation of step d), the pump is operated in step d) such that a predefined quantity of liquid (for example between 2 ml [milliliters] and 50 ml) is conveyed. This quantity is selected in particular such that the filter space is again completely filled with liquid and the filter is thus again completely wetted with liquid. In a time-controlled execution of step d), the pump is operated in such a way that the pump is operated for a predefined time period (for example between 2 seconds and 5 seconds), wherein this time period is selected in such a way that the filter space is again completely filled with liquid and the filter is thus again completely wetted with liquid. Step d) is preferably carried out in such a way that it is ensured that the liquid does not penetrate as far as the components adjoining the conveying path, which could be damaged by freezing liquid. It is also possible that the execution of step d) is (actively) monitored in order to stop the conveying during step d) as soon as the filter space or the conveying line is sufficiently filled again. This monitoring can be carried out, for example, with the aid of a pressure sensor.The method is particularly advantageous if the at least one filter layer extends at least over a section of a height of the tank, so that the liquid can flow from the interior into the filter space at different heights.The filter space is preferably a partial volume at the bottom of the tank delimited by the filter from the interior of the tank. The filter space preferably has a height extending from the tank bottom in the direction of the top side of the tank. In other words, this means, for example, that the filter space is separated from the tank interior by a vertically arranged filter layer, wherein the liquid can therefore also flow through the filter layer (depending on the fill level) over the entire vertical extension of the filter layer. Alternatively, it is possible for the filter chamber to be arranged in the manner of a sump below the tank. However, additional installation space below the tank is then required for the filter space.Furthermore, the method is advantageous if the suction line section of the conveying line starts from an upper region of the filter chamber.Preferably, at least one suction point exists in the (vertically) upper region of the filter chamber, at which the conveying line opens into the filter chamber. This at least one suction point is preferably arranged at a distance of less than 10 cm [centimeters] from a highest position of the filter layer. As already described above, the filter layer preferably has a selective permeability for the liquid and is in particular less permeable to air than to the liquid. If the interior of the tank is only partially filled with liquid, it may happen that liquid is present in a lower region on the surface of the filter layer oriented toward the interior of the tank, while air is present in an upper region. Under these conditions, the suction point in the upper region of the filter chamber ensures that the filter chamber is always completely filled with liquid during suction. If air is present in the upper region on the surface of the filter layer, this cannot be sucked into the filter space. Instead, the liquid is sucked through the filter space (along the filter layer) towards the suction point in the upper region.Furthermore, the method is advantageous if the filter chamber forms a jacket which delimits a housing with the at least one pump from the interior of the tank, wherein at least one heater for heating the liquid is additionally arranged in the housing.The housing preferably forms a (dry) chamber on the tank base of the tank, which chamber is separated from the interior of the tank and from the filter chamber and is surrounded by the filter chamber. If the filter chamber were to be completely emptied during emptying, the filter chamber would therefore form an air jacket around the housing. This air jacket would have an insulating effect. A heater arranged in the housing would then no longer be able to heat the liquid stored in the interior of the tank, or would only be able to heat it to a limited extent, because the air jacket would act as an insulation layer between the housing and the interior of the tank. For this reason, it is advantageous to stop the suck back as soon as air is present within the filter space. It is thus possible to prevent a closed air jacket from being formed around the housing.In particular in the case that the filter space forms such a shortcoming, it is advantageous if step c) is followed by step d explained above). If an increase in the counter pressure is detected in step c), air is already present at the filter layer in the filter space. Although the filter chamber has not been completely emptied due to the execution of steps a) to c), nevertheless a (slight) partial emptying of the filter chamber has already been carried out. This partial emptying is reversed again by step d). Thus, a regionally (heat) insulation effect of the air within the filter space can also be prevented.The still desired filling degree of the filter chamber with liquid at the end of the suck-back process can be adjusted in particular on the basis of the predetermined or adapted threshold value for the detected operating parameter.The heater is preferably self-regulating. The heating can comprise, for example, at least one PTC heating element (PTC=positive temperature coefficient). In the case of such a heating element, a reduction in the heating power automatically occurs when the temperature of the heating element reaches a limit value.Furthermore, the method is advantageous if operating parameters of the pump monitored in step b) are at least one of the following parameters:an electric power absorbed by a drive of the pump;an electric current taken up by a drive of the pump;an operating voltage received by a drive of the pump;a moving speed of a drive of the pump;a pressure or a pressure gradient which the pump builds up in the suction line section during the suck back.The electric power consumed by the pump, the electric current consumed and the operating voltage consumed are representative of the electric energy consumed by the pump during the suck-back, wherein it may depend on the type of pump, which of the said parameters is particularly suitable for detecting the electric energy consumed. The higher this electric energy, the more back pressure occurs in the suction pipe portion during suck back. For this reason, these parameters can be used (alone or in combination) to measure the counterpressure in the suction line section and to carry out step c) of the method described. In this case, it is possible to monitor these parameters without additional components for monitoring the counterpressure being necessary at the suction line section. Only suitable electronics are necessary for evaluating the supply voltage or the supply current of the pump.The drive of the pump, the movement speed of which can be evaluated, comprises, for example, a movable pump element which is moved translationally for delivery, or a rotational drive which rotates for delivery. A movable pump element can be, in particular, a piston or a membrane. A movement speed of a drive is understood here to mean a movement speed of such a movable pump element or of a rotary drive. Preferably, a drive of the pump has at least one coil which is configured to build up a magnetic field which exerts a force on the movable pump element or on the rotary drive, such that the movable pump element or the rotary drive moves.When the pump drive moves rapidly, there is a low back pressure. The higher the counter pressure, the greater is the resistance which opposes the movement of the drive. Therefore, the moving speed of the pump is also a parameter which may be representative of the back pressure in the suction line portion during the suck back depending on the type of the pump. The speed of movement of the drive of the pump can be monitored, for example, by detecting an induced voltage in a drive coil of the pump.It is also possible for a pressure or a pressure gradient in the suction line section to be monitored directly. It is possible, for example, for a pressure sensor to be arranged on the suction line section, with which pressure in the suction line section can be monitored.Furthermore, a motor vehicle is proposed, having at least one internal combustion engine, an exhaust gas treatment device for cleaning the exhaust gases of the internal combustion engine and a device for conveying a liquid to the exhaust gas treatment device, wherein the device is configured to be operated according to the described method.The device is preferably also configured to convey the liquid into the exhaust gas treatment device. The liquid is preferably a urea-water solution which can be used in the exhaust gas treatment device for exhaust gas cleaning. In the exhaust gas treatment device, there is preferably an SCR catalyst on which nitrogen oxide compounds in the exhaust gas of the internal combustion engine can be reduced with the aid of the liquid. The device can have all the device features explained in connection with the described method.The invention and the technical field are explained in more detail below with reference to the figures. The figures show particularly preferred exemplary embodiments, to which the invention is, however, not limited. In particular, it should be pointed out that the figures and in particular the size relationships illustrated are only schematic. The following are shown: FIG. 1 : a first variant embodiment of a device for the described method, FIG. 2 : shows a motor vehicle having a device for the method described, FIG. 3 shows a second variant of a device for the method described, and FIG. 4 shows a curve of the pressure in the suction line section during the execution of the described method.FIG. 1 shows a first variant embodiment of a device 1 which can be emptied according to the described method. The device 1 comprises a tank 7, in the interior 10 of which the liquid (in particular an aqueous urea-water solution) is stored. A housing 16 is arranged in the tank 7, which extends from the tank bottom 33 into the interior 10 of the tank 7. The tank 7 has a height 13, and the housing 16 extends over a section 12 of the height 13 of the tank 7. The filter space 6 is separated from the interior 10 of the tank 7 by a filter layer 9. A heater 17 is disposed in the housing 16. The heater 17 is configured to heat liquid stored in the interior 10 of the tank 7 through the filter space 6 and the filter layer 9. For this purpose, it is provided that the filter chamber 6 is filled with liquid so that a good thermal conductivity exists through the filter chamber 6.The housing 16 contains functional components of the device 1, which take over the delivery of the liquid from the tank 7 to an injector 8. These functional components comprise in particular a pump 3 which performs the delivery of the liquid and which has a drive 18 which is preferably an electric motor.Proceeding from the filter chamber 6, a delivery line 2 extends from the filter chamber 6 to the injector 8, and the pump 3 is also arranged on this delivery line 2. The delivery line 2 is divided by the pump 3 into a suction line section 4 from the filter chamber 6 to the pump 3 and a pressure line section 5 from the pump to the injector 8. Starting from the filter chamber 6 toward the injector 8, the pump 3 delivers the liquid with a regular delivery direction 11 The delivery line 2 or the suction line section 4 of the delivery line 2 opens at a suction point 34 into an upper region 14 of the filter chamber 6.The motor vehicle 19 shown in FIG. 2 has an internal combustion engine 20 and an exhaust gas treatment device 21 for purifying the exhaust gases of the internal combustion engine 20. A liquid for exhaust gas cleaning can be supplied to the exhaust gas treatment device 21 by means of a device 1. This liquid is preferably a reducing agent precursor solution. An SCR catalytic converter 23 is provided in the exhaust gas treatment device 21, with which nitrogen oxide compounds in the exhaust gas of the internal combustion engine 20 can be reduced. The device 1 conveys the liquid from a tank 7 in which the liquid is stored. To provide the liquid at the exhaust gas treatment device 21, an injector 8 is provided, which enables a metered dispensing of the liquid. The motor vehicle 19 preferably has a control unit 22, which is connected at least to the pump, not shown here, and to the injector 8 of the device 1, in order to control the operation of the device 1. In particular, routines for carrying out the described method are also stored in the control device.FIG. 3 shows a modification of the device 1 according to FIG. 1, wherein a pressure sensor 24 is arranged in the suction line section 4, with which pressure sensor the pressure in the suction line section 4 can be actively monitored during the suck-back.In FIG. 4, an exemplary pressure profile 27 in the suction line section is shown on the time axis 26 via the pressure axis 25, which pressure profile occurs immediately before the suck back and during the suck back. Along the time axis 26, a suction phase 28 is initially shown, during which regular operation (metering) of the device still takes place and during which a negative pressure 31 occurs in the suction line section. At the time when the pressure curve 27 intersects the time axis 26, the emptying phase 29 begins, during which the liquid is sucked back. The pressure in the suction line section is now positive because the liquid from the pressure line section is now forced through the suction line section back into the filter space or into the interior space of the tank. At the end of the emptying phase 29, a significant increase in pressure 30 occurs, which is detected (method step c)). Thus, upon reaching a threshold value 35, the emptying phase 29 can be stopped.The method described is particularly advantageous in order to drain the device for supplying liquid in a targeted manner and in the process to use as little energy as possible for suck back and at the same time to ensure that the device is only drained as far as is necessary in order to avoid freezing liquid in the conveying line of the device damaging components of the device.List of reference characters1 Device 2 Delivery line 3 Pump 4 Suction line section 5 Pressure line section 6 Filter chamber 7 Tank 8 Injector 9 Filter layer 10 Interior 11 Regular delivery direction 12 Section 13 Height 14 Upper region 15 Jacket 16 Housing 17 Heater 18 Drive 19 Motor vehicle 20 Internal combustion engine 21 Exhaust gas treatment device 22 Control device 23 SCR catalyst 24 Pressure sensor 25 Pressure axis 26 Time axis 27 Pressure profile 28 Suction phase 29 Emptying phase 30 Abrupt pressure increase 31 Negative pressure 32 Positive pressure 33 Tank base 34 Suction point 35 Threshold value

Claims

Method for operating a device (1) for conveying a liquid, having a conveying line (2) which can be divided into a suction line section (4) from a filter space (6) in a tank (7) to at least one pump (3) and into a pressure line section (5) from the at least one pump (3) to at least one injector (8), wherein the filter space (6) is separated from an interior space (10) of the tank (7) by at least one filter layer (9) and wherein the method comprises at least the following steps: a) sucking back a liquid located in the pressure line section (5) by means of the at least one pump (3) counter to a regular conveying direction (11); b) monitoring at least one operating parameter of the at least one pump (3) during suck back, wherein the at least one operating parameter is representative of a counter pressure against which the at least one pump (3) operates during suck back; and c) detecting an increase in the counter pressure and stopping the suck back.Method according to claim 1, wherein after step c) at least the following further step is carried out: d) conveying liquid in regular conveying direction (11) until the filter space (6) is again completely filled with liquid.Method according to claim 1 or 2, wherein the at least one filter layer (9) extends at least over a section (12) of a height (13) of the tank (7) so that the liquid can flow at different heights from the interior (10) into the filter space (6).Method according to one of the preceding patent claims, wherein the suction line section (4) of the conveying line starts from an upper region (14) of the filter chamber (6).Method according to one of the preceding patent claims, wherein the filter space (6) forms a jacket (15) which delimits a housing (16) with the at least one pump (3) from the interior space (10) of the tank (7), wherein at least one heater (17) for heating the liquid is additionally arranged in the housing (16).Method according to one of the preceding patent claims, wherein the operating parameter of the pump (3) monitored in step b) is at least one of the following parameters: - an electrical power consumed by a drive (18) of the pump (3); - an electrical current consumed by a drive (18) of the pump (3); - an operating voltage consumed by a drive (18) of the pump (3); - a movement speed of a drive (18) of the pump (3); - a pressure or a pressure gradient which the pump (3) builds up in the suction line section (4) during the suck-back.Motor vehicle (19), comprising at least one internal combustion engine (20), an exhaust gas treatment device (21) for cleaning the exhaust gases of the internal combustion engine (20) and a device for conveying a liquid to the exhaust gas treatment device (21), wherein the device (1) is configured to be operated according to a method according to one of the preceding patent claims.

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