Abstract:
The invention relates to a device ( 40 ) for reducing pollutants, in particular nitrogen oxides, in an exhaust gas flow of an internal combustion engine, comprising a reducing agent ( 46 ), in particular a urea-water solution, which is stored in a storage tank ( 42 ), a metering module, in particular a nozzle ( 68 ), for injecting the reducing agent ( 46 ) into the exhaust gas flow, and a pump ( 56, 100 ) for delivering the reducing agent ( 46 ) from a suction point ( 62 ) in the storage tank ( 42 ) to the metering module via lines ( 58, 66 ). According to the invention, the pump ( 56, 100 ) lies beneath a reducing agent level ( 44 ) in the storage tank ( 42 ), and a non-return valve is provided. As a result of the non-return valve, the device ( 40 ) is prevented from refilling with the reducing agent ( 46 ) after the reducing agent ( 46 ) is completely sucked back from the device ( 40 ) after shutting off the internal combustion engine, in particular after extended idle times of the internal combustion engine. In a first embodiment, the non-return valve is a filter ( 64 ) that is formed with a plurality of screens ( 80, 82, 84, 86 ) which are connected one behind the other, which have a special microstructure, and which are formed with a plastic material. In the case of a second embodiment, the non-return valve is formed with a hermetically sealing valve ( 158 ) which is heavily preloaded by a spring force in particular and which is directly integrated into the pump ( 100 ) that is necessary for delivering the reducing agent ( 46 ) in the device ( 40 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    In motor vehicles with internal combustion engines, increasingly stringent exhaust-gas limit values require the reduction, inter alia, of atmospheric pollutants such as for example nitrogen oxides (NOx) in the exhaust gas stream. One widespread method which is used for such purposes is catalytic reduction, i.e. the “SCR” (“Selective Catalytic Reduction”) method. In this method, a liquid reducing agent is delivered by means of a pump from a storage container to a feed module in the region of a catalyst on the exhaust gas pipe during operation of the internal combustion engine. The reducing agent used generally takes the form of a urea/water solution with the brand name “AdBlue®”. Account needs to be taken of the fact that this urea/water solution freezes at a temperature of −11° C. and decomposes thermally above 60° C., such that heating devices have to be provided in particular for low-temperature winter operation. To ensure the necessary frost resistance of the exhaust-gas purification system at low operating temperatures in the region of −11° C. and below, the reducing agent is aspirated completely back out of the feed module, the lines and the delivery device into the storage tank once the internal combustion engine has been turned off. In the process, air and/or residual exhaust gas flows into the cavities left empty by the reducing agent when it is pumped out. 
         [0002]    The principle problem of this return procedure is the spatial arrangement of the pump below the liquid level in the storage tank, which may however be necessary due to specific structural requirements. This is because on the one hand the inlet and outlet valves of the delivery module do not close absolutely tightly and on the other hand the feed module on the exhaust gas line is not hermetically gas-tight, such that the complete system, in particular with prolonged down times, can slowly fill back up with the reducing liquid after completion of the return process. This is all the more the case given that the nozzle for injecting the reducing agent is generally positioned in the region of the lowest point of the system, i.e. directly on the exhaust gas pipe or on the catalytic converter. If the pump and the nozzle including the connecting lines are not ice-pressure resistant, i.e. burst-resistant in the event of cooling-related volumetric expansion of the reducing agent, they may be damaged when the volume increases during the phase transition of the reducing agent from liquid to solid, so making the exhaust-gas purification system no longer functional. 
         [0003]    Solutions known from the prior art have hitherto dealt with this problem by always positioning the pump above the liquid level in the storage tank, so ensuring that the reducing agent cannot flow back under the effect of gravity. However, this structural situation significantly restricts the installation space available for incorporating the exhaust-gas aftertreatment system into the motor vehicle. Alternatively, a siphon may be provided between the delivery pump and the intake point, the elbow of which lies above the liquid level of the reducing agent. With regard to the spatial arrangement of its components, the second variant solution allows greater flexibility, but has the disadvantage that, due to the siphon and the consequently longer line arrangements, a greater line length has to be heated to ensure winter operation, so increasing the energy input for the heating and at the same time also the space required for the lines. 
         [0004]    A system is known from DE 10 2006 044 246 A1 for aftertreatment of an exhaust gas from an internal combustion engine with a reducing agent, wherein the reducing agent is delivered from a storage container to a nozzle arranged on a catalytic converter with the assistance of a hydraulically driven diaphragm pump. In the process, the diaphragm pump builds up sufficient operating pressure for the process of injecting the reducing agent. However, the system does not disclose any means of aspirating the reducing agent back out of the line arrangement once the internal combustion engine has been turned off and moreover does not disclose any precautions for ensuring a hermetic seal against the reducing agent flowing back once it has been aspirated out. 
         [0005]    It is therefore an object of the invention to provide a device for a reducing agent for cutting levels of atmospheric pollutants, in particular nitrogen oxides (NOx), in the exhaust gas of an internal combustion engine, in which the reducing agent is reliably prevented from flowing back out of the storage tank into the system once the engine has been turned off and the reducing liquid return process is complete. 
       SUMMARY OF THE INVENTION 
       [0006]    A device for reducing nitrogen oxides in the exhaust gas stream of an internal combustion engine is proposed which comprises, inter alia, a storage tank containing the reducing agent, a feed module, in particular a nozzle, for injecting the reducing agent into the exhaust gas stream and a pump for delivering the reducing agent from an intake point in the storage tank via lines to the feed module. 
         [0007]    According to the invention, the pump is arranged below the level of the reducing agent in the storage tank and a backflow barrier prevents the reducing agent from flowing back undesirably to the nozzle, in particular due to the effect of gravity and incompletely closing valves in the pump, once it has been aspirated back out of the exhaust gas aftertreatment system after the internal combustion engine has been turned off. 
         [0008]    In a first variant, the backflow barrier is embodied by a filter disposed upstream of the pump intake line, said filter taking the form of at least two close-meshed plastics screens arranged in series. Due to the microstructure of the screens, fine exhaust gas and air bubbles settle in the screen mesh during return of the reducing agent. In this way, the gas-filled screen achieves hydrophobic characteristics, while the reducing agent is hydrophilic due to its water content. The hydrophobicity of the screen thus reliably prevents backflow after the engine has been turned off, since the relatively low static pressure of the reducing agent is insufficient to overcome the water-repellent effect of the air- and exhaust gas-filled screens. When the internal combustion engine is started up again, however, the pump used for delivery produces a vacuum in the region of the filter, which is sufficient to overcome the repulsive forces brought about by the filter screens and the reducing agent is again able to flow unimpeded through the filter. 
         [0009]    In the case of a second variant, the backflow barrier is implemented technically with a sealing edge which, when idle, i.e. when the pump used to deliver the reducing agent is in a de-energized state, is pressed with a defined level of mechanical contact pressure against a rubberized limit stop. This brings about a reliable, hermetic seal between the storage tank with the reducing agent and the inlet of the pump, which prevents uncontrolled backflow of the reducing agent when the internal combustion engine is turned off. A particular advantage of this configuration is that the backflow barrier is incorporated directly into the pump, such that no further components are necessary. An external shut-off valve, optionally requiring heating, or the filter in accordance with the first variant embodiment may be dispensed with, thereby significantly reducing the cost and effort required to produce the device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention will now be explained in greater detail with reference to the drawings, in which: 
           [0011]      FIG. 1  shows a device for treating exhaust gas, in accordance with the prior art; 
           [0012]      FIG. 2  shows a first variant embodiment of the device according to the invention; 
           [0013]      FIG. 3  is a detail view of the filter used in the first variant embodiment according to  FIG. 2 , and 
           [0014]      FIG. 4  is a schematic sectional representation of the pump with integral backflow barrier used in the second variant embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  shows a device known from the prior art for treating the exhaust gas of an internal combustion engine, in particular of a diesel engine. The device  10  comprises inter alia a storage tank  12 , which is filled with a reducing agent  14 , for example a urea/water solution, for chemically reducing the nitrogen oxides contained in the exhaust gas stream from an internal combustion engine. 
         [0016]    The storage tank  12  is conventionally filled up to a reducing agent level  16  and then contains between 10 l and 20 l of the reducing agent  14 . At the bottom, a functional module  18  is welded into the storage tank  12  or attached in some other way. This comprises inter alia a pump  20  and an electrical heating unit  22 . The pump  20  may deliver the reducing agent  14  both from the storage tank  12  to the nozzle  28  and in the opposite direction from the nozzle  28  back into the storage tank  12 . Furthermore, the functional module  18  comprises a plurality of measuring sensors, such as for example temperature sensors, pressure sensors, sensors for measuring flow, level gauges, position sensors and an open- and/or closed-loop control unit. By means of the pump  20 , the reducing agent  14  is delivered during operation of the internal combustion engine from an intake point  24 , which is preferably arranged in the region of the lowest point of the storage tank  12 , the “pump sump”, via a line  26  to the nozzle  28 . The nozzle  28  allows finely divided, disperse injection of the reducing agent  14  into the catalytic converter of the exhaust gas line of the internal combustion engine, which is not shown in  FIG. 1  for the sake of greater drawing clarity. In general, the nozzle  28  simultaneously has a valve function, i.e. injection of the reducing agent  14  proceeds only once a defined minimum pressure is reached. Once the internal combustion engine has been turned off, the reducing agent  14  is initially aspirated completely back out of the nozzle  28 , the line  26  and a siphon  30  to the storage tank  12  by means of the pump  20 , which is then operating in reverse mode. In the process, the stated components of the system fill up with residual quantities of the gas from the exhaust gas system of the internal combustion engine. 
         [0017]    Uncontrolled backflow of the reducing agent  14 , in particular in the case of prolonged internal combustion engine shutdown, is prevented by the siphon  30 . Although the above-explained device optionally also allows arrangement of the pump  20  of the exhaust gas aftertreatment system below the reducing agent level  16  in the storage tank  12 , the need for the siphon  30  increases the line length which has to be provided and which has to be heated to ensure functionality of the device  10  at low temperatures. 
         [0018]      FIG. 2  illustrates a first variant of the device according to the invention. A device  40  inter alia comprises a storage tank  42  which is filled up to a reducing agent level  44  with a reducing agent  46 . The reducing agent  46  is preferably a urea/water solution, by means of which efficient reduction of nitrogen oxides (NOx) in the exhaust gas of a internal combustion engine is achieved, the reducing agent being injected in a finely divided manner by a nozzle into a catalytic converter through which the exhaust gas stream flows. An electrical (resistance) heating unit  50  with a passage  52  for the reducing agent  46  is located on the bottom  48  of the storage tank  42 , in the region of a raised portion, not shown. The heating unit  50  is joined to the bottom  48  of the storage tank  42  by a weld seam  54 . A pump  56  is additionally provided, which is connected via a line  58  to the passage  52 . The line  58  may for example take the form of a flexible hose line and be fastened to the passage  52  with a clip  60 . In this variant, a backflow barrier in the form of a filter  64  is located in the region of an intake point  62 . The pump  56  is connected via a further line  66  to the nozzle  68 , by means of which the reducing agent  46  is injected in a finely atomized manner into an exhaust gas line, not shown in  FIG. 2 , of an internal combustion engine. The nozzle  68  also has a valve function in addition to its primary atomizing function, i.e. injection of the reducing agent  46  only proceeds once a predetermined minimum pressure has been exceeded. As is clear from the illustration in  FIG. 2 , both the storage tank  42  and the line  58  leading to the pump  56  may be electrically heated by means of the heating unit  50 . 
         [0019]    As is indicated by the small black double-headed arrow in the pump symbol, the pump  56  is configured such that it is able to deliver the reducing agent  46  both in a (main) delivery direction from the storage tank  42  to the nozzle  68  and in a reverse return direction from the nozzle  68  back into the storage tank  42  again. 
         [0020]    In addition, the device has a plurality of sensors, not shown for the sake of greater drawing clarity, for example at least one pressure sensor, a temperature sensor, a filling level sensor and optionally a sensor for precise flow measurement, in order to identify the physical parameters relevant to functioning of the device  40  to ensure the reducing agent  46  is injected in a precise, metered amount, which parameters are additionally optimally adapted to the current operating state of the internal combustion engine, so as in particular to optimize consumption of the reducing agent  46 . In addition, an open- and/or closed-loop control unit, likewise not shown, is provided for controlling all the operating sequences within the device  40 , the sensors, the pump  56  and the heating unit  50 , inter alia, being connected to said control unit. For example, the heating unit  50  may be switched on automatically by means of the open- and/or closed-loop control unit, if a temperature sensor signals that the temperature of the reducing agent  46  has fallen below or reached its freezing temperature of −11° C., the internal combustion engine has been turned off and the return of the reducing agent has not yet taken place. 
         [0021]    Due to its special microstructure, the filter  64  provided according to the invention prevents backflow of the reducing agent  46  out of the storage tank  4  when the internal combustion engine is turned off, but allows unhindered flow of the reducing agent  46  through the passage  52  in the direction of the black arrow  70  when the internal combustion engine is in operation. Reference is made to the description of  FIG. 3  for the further structural details of the structure of the filter  64  and its mode of operation. A (coarse) purification filter  72  is connected upstream of the filter  64 , to prevent the device  40  from becoming clogged or contaminated with foreign bodies, which may be present in the reducing agent  46 . The storage tank  42  additionally has a vent valve  74  arranged at the top. The electrical heating unit  50  is supplied with power via an electrical (plug-in) connection  76 . 
         [0022]      FIG. 3  illustrates a possible embodiment of the filter  64  in accordance with  FIG. 2 . 
         [0023]    In the exemplary embodiment shown, the filter  64  comprises four series-connected screens  80  to  86 , which are each accommodated with a spacing  88  of between 1 mm and 5 mm in a housing  90 . The mesh size  92  of the opening  94 , which is provided with a reference numeral representatively for all the others, in the screens  80  to  86  preferably amounts in each case to between 10 μm and 50 μm. As shown in  FIG. 3 , it is not necessary for the spacing  88  between the screens  80 ,  82  to be constant for all further screens. It is also possible to select different mesh sizes for each of the screens  80  to  86 . The screens  80  to  86  are preferably made from a thermoplastic material, for example of polyamide (PA). Others plastics materials, such as for example polyethylene (PE) or polytetrafluoroethylene (PTFE), may likewise be used to produce the screens. 
         [0024]    When the reducing agent is aspirated back through the filter  64  in the direction of the large white arrow  96  by means of the pump which is then operating in the return direction (cf.  FIG. 2 ), the residual exhaust gas taken in therewith or the air from the exhaust gas line, not shown, arrives between the screens  80  to  86  and thus also enters the openings  94 . The entrained air bubbles which form close up the openings in the screens  80  to  86  due to their inherent surface tension, such that the filter  64  becomes water-repellent, i.e. hydrophobic. Once the internal combustion engine has been turned off, the water-containing reducing agent  46  can therefore no longer pass through the completely gas-filled, hydrophobic filter  64  contrary to the direction of arrow  96  due to its only low inherent static pressure. This reliably prevents the slow “trickle” of reducing agent  46  into the pump  56 , including the lines  58 ,  66  and the nozzle  68 , and the associated undesired consequences, in particular during prolonged internal combustion engine shutdown (cf.  FIG. 2 ). When the internal combustion engine is started up again, the pump  56  aspirates the reducing agent  46  contrary to the direction of the arrow  96  through the filter  64 , simultaneously expelling the gas or quantities of residual exhaust gas out of the filter  64 . Aspirating the gas back off renders the filter  64  hydrophilic again, so that it allows reducing agent to pass through unimpeded. 
         [0025]    A slow “trickle” of reducing agent  46  into the exhaust gas treatment system is problematic in particular because on the one hand at low temperatures it leads to freezing of the reducing agent and associated consequences if the system is not 100% ice-pressure resistant and on the other hand reducing agent  46  dripping out slowly but constantly in the region of the nozzle  68  results in crystallization. Said crystallization, which increases maintenance work, at the very least inhibits outlet of the reducing agent  46  and reduces atomization, i.e. the dispersion action of the nozzle  68 , which at the same time impairs the catalytic reduction action in the exhaust gas stream. In any event, uncontrolled flow of the reducing agent  46  back in after it has been aspirated out of the exhaust gas treatment system results in undesirable functional impairment. 
         [0026]      FIG. 4  shows an exemplary embodiment of a second embodiment of the device  40  according to the invention, not shown in full here however for the sake of greater drawing clarity, in which a pump is used which has an automatically closing valve as backflow barrier. 
         [0027]    A pump  100  comprises inter alia a housing top  102  with an inlet  104  and a housing bottom  106  with an outlet  108  for the reducing agent  46 . The housing parts  102 ,  106  are joined together for example by a flange and bolt joint, not shown. In the housing bottom  106  there is located an electromagnet  110 , which comprises a magnetic core  112  and a cylindrical coil  114 . The electromagnet  110  is covered with an isolating disc  116 , which is preferably formed of a non-magnetic material (air gap effect), to prevent it from sticking magnetically to the armature plate. The magnetic core  112 , on the other hand, is made of a magnetic material preferably with a laminated structure so as better to direct and intensify the magnetic flux of the electromagnet  110 . In the region of the housing top  102  there is located a disc-shaped armature plate  118 , which is displaceable substantially parallel to an axis of symmetry or longitudinal axis  120  of the pump  100  due to the magnetic force action of the electromagnet  110 . Between the armature plate  118  and the isolating disc  116 , which is firmly connected to the magnetic core  112  of the electromagnet  110 , a peripheral resilient sealing membrane  122  is provided to define at least in part a delivery chamber  124  and to guide the armature plate  118 . The sealing membrane  122  may be formed of any desired resilient material, which has however to display sufficient resistance relative to the reducing agent  46 . A suitable example is an elastomer, such as for example rubber or a resilient membrane (bellows), which is made of a stainless steel alloy. 
         [0028]    A continuous isolating disc and magnetic core bore  126 ,  128  is introduced into the isolating disc  116  and magnetic core  112 , wherein the diameter, not shown, of the isolating disc bore  126  is preferably less than the diameter of the magnetic core bore  128 . A pretensioned compression spring  132  is located between a magnetic core recess  130  and the armature plate  118 , wherein the magnetic core  112  is sealed in the region of this recess relative to the housing bottom  106  by means of a resilient seal, in particular an O-ring  134 . 
         [0029]    A conical raised part  136  of the armature plate  118 , arranged centrally thereon and into which a continuous armature plate bore  138  is introduced to create a sealing edge  140 , is pressed under the force of the compression spring  132 , when the electromagnet  110  is in the de-energized state, with a defined contact pressure of 1 N to 10 N, preferably however more than 10 N, against a limit stop  142  in the region of the housing top  102 , in order to achieve hermetically tight closure of the inlet  104  when the internal combustion engine is turned off and the pump  100  is in the de-energized state. The limit stop  142  is surrounded by an annular groove  144 , in order in particular to counter the occurrence of stress cracking in the housing top  102 . To improve the sealing action, the limit stop  142  is provided with a preferably circular rubber coating  146 . This may for example be vulcanized onto the limit stop  142 . Instead of the rubber coating  146 , it is also possible to use any desired thermoplastic or thermosetting plastics material as the sealant. To achieve pulsing pump operation through periodic energization of the electromagnet  110  by means of the open- and/or closed-loop control unit, an inlet valve  148  is provided in the region of the armature plate bore  138  and an outlet valve  150  is provided in the region of the magnetic core bore  128 . 
         [0030]    The situation illustrated in  FIG. 4  almost corresponds (since in the position shown of the armature plate  118  there is still a narrow gap (not shown) between the conical raised portion  136  and the limit stop  142 ) to a idle state of the pump  100 , in which the armature plate  118  is pressed firmly against the housing top  102  by the compression spring  132  when the electromagnet  110  is de-energized. 
         [0031]    Both the inlet valve  148  and the outlet valve  150  are constructed, by way of example, as flap valves and are not passively opened and closed by the reducing agent  46  flowing through them or as a function of the flow direction, but rather may be opened and closed under the control of the above-mentioned open- and/or closed-loop control unit by means of actuators, not shown. Instead of the embodiment shown of the inlet and outlet valves  148 ,  150  in the form of flap valves, ball valves may for example also be used. 
         [0032]    The direction of flow, i.e. the normal (main) delivery direction of the reducing agent  46  to be delivered by means of the pump  100 , is indicated by the black arrow line  152 . The armature plate  118  travels a travel distance  154  in each case when the electromagnet  110  is energized or, as a result of the restoring action of the compression spring  132 , when the electromagnet  110  is de-energized, which is parallel to the longitudinal axis  120  and which is defined at the top by the limit stop  142  and at the bottom by the isolating disc  116 . The magnetic core  112  or the magnetic core bore  128  is continued downward into a delivery pipe  156 , which is if necessary connected directly to the catalytic converter used for exhaust gas purification in the exhaust gas line of the internal combustion engine. The outlet valve  150  is connected to the isolating disc  116  for example by a peripheral weld seam. In cooperation with the rubberized limit stop  142  and the contact pressure built up by the compression spring  132 , the sealing edge  140  constitutes a hermetically sealing valve  158 , which, when the internal combustion engine is turned off, reliably prevents the reducing agent  46  from flowing out of the storage tank (cf.  FIG. 2 ), not shown here, via the inlet  104  or the not completely tightly closing inlet and outlet valves  148 ,  150  of the pump  100 . 
         [0033]    To prepare for delivery of the reducing agent  46 , i.e. in particular after a prolonged internal combustion engine shutdown and with the device completely emptied of any residue, the electromagnet  110  is energized in a controlled manner by the open- and/or closed-loop control unit, whereby the armature plate  118  moves downward parallel to the longitudinal axis  120  over the travel distance  154  and strikes against the isolating disc  116 . At the same time, the open-and/or closed-loop control unit opens the inlet valve  148  and closes the outlet valve  150 , such that the delivery chamber  124  arising above the armature plate  118  may fill up at least partially with the reducing agent  46  as a result of the static pressure. 
         [0034]    Once the electromagnet  110  has been switched off, the armature plate  118  is forced back upwards by the compression spring  132  as far as the limit stop  142 , wherein the inlet valve  148  is at the same time held open, in order to allow the reducing agent  46  to flow into the delivery chamber  124  volume enlarging in the process. Once the armature plate  118  reaches its upper end position against the limit stop  142 , the valve  158  consisting of the sealing edge  140  and the limit stop  142  closes automatically while the open- and/or closed-loop control unit closes the inlet valve  148 , in order to prevent further reducing agent  46  from being aspirated in via the inlet  152  on a renewed downward movement of the armature plate  118  caused by renewed energization of the electromagnet  110 . To control the inlet and outlet valves  148 ,  150 , position sensors may be provided, not shown in  FIG. 4 , which output signals to the open- and/or closed-loop control unit as to whether the armature plate  118  is located against the limit stop  142 , is briefly occupying an intermediate position or is resting firmly against the isolating disc  116 . 
         [0035]    In order now to be able to discharge reducing agent  46  located in the delivery chamber  124  in the delivery direction of the pump  100 , i.e. parallel to the arrow line  152 , the electromagnet  110  is again energized, wherein the open- and/or closed-loop control unit opens the outlet valve  150  and keeps the inlet valve  148  closed. In this way, the quantity of reducing agent  46  located in the delivery chamber  124  is delivered out of the pump  100  through the delivery pipe  156  or the outlet  108  at a pressure of up to  10  bar. This delivery process is repeated over the entire service life of the internal combustion engine by periodic energization or de-energization of the electromagnet  110  and corresponding activation of the inlet and outlet valves  148 ,  150 , in order to deliver the reducing agent  46  in the (main) delivery direction of the pump  100  in each case in defined volume units in a pulsed manner from the inlet  104  to the outlet  108  of the pump  100 , and thus to bring about the desired, continuous catalytic reduction of the nitrogen oxides in the exhaust gas of the internal combustion engine while the latter is in operation. 
         [0036]    In order to be able to aspirate the reducing agent  46  contained in the device completely back into the storage tank when the internal combustion engine is turned off, i.e. to achieve complete emptying of the device, it is necessary to operate the pump  100  in the “return” direction. This means that the reducing agent  46  flows through the pump  100  contrary to the direction of the arrow line  152 . To explain what takes place during the return process, it is assumed in the remainder of the description that the delivery chamber  124  is completely full of reducing agent  46  and the armature plate  118  is in the position indicated in  FIG. 4 . 
         [0037]    First of all the electromagnet  110  is energized, such that the armature plate  118  moves from its idle position downward toward the isolating disc  116 . At the same time, both the inlet valve  148  and the outlet valve  150  are opened. As a result of the open valves  148 ,  150 , the armature plate  118  may move downward virtually without resistance, irrespective of the fact that the delivery chamber  124  is at this point still completely full of reducing agent  46 , wherein, during this process, due to the relatively low static pressure in conjunction with the inertia of said reducing agent  46  in the storage tank, only a negligible quantity continues flowing from the storage tank into the pump  100 . 
         [0038]    When the electromagnet  110  is de-energized again, the compression spring  132  forces the armature plate  118  back into its upper idle position, wherein the volume of the delivery chamber  124  increases and the resultant vacuum generated thereby in the delivery chamber  124  causes the reducing liquid  46  to be aspirated from the outlet  108  via the delivery pipe  156  into the delivery chamber  124 . 
         [0039]    During this process the outlet valve  150  remains open, while the inlet valve  148  is closed. Once the armature plate  118  has reached the end position, the valve  158  closes automatically. When the electromagnet  110  is re-energized, the reducing liquid  46  accommodated in the delivery chamber  124  is delivered out of the delivery chamber  124  in the direction of the inlet  104  due to the resultant reduction in volume of said chamber. During this process, the outlet valve  150  is conversely closed, while the inlet valve  148  remains open. 
         [0040]    The stated procedures are repeated by periodic energization of the electromagnet  110  and corresponding activation of the inlet and outlet valves  148 ,  150  by means of the open- and/or closed-loop control unit until the reducing agent  46  has been completely aspirated out of the device and returned to the storage tank. Finally, the electromagnet  110  and the inlet and outlet valves  148 ,  150  are finally switched off by the open- and/or closed-loop control unit, wherein according to the invention the valve  158  ensures hermetic sealing of the inlet  104  even when the pump  100  is in the de-energized state. 
         [0041]    Of crucial significance in achieving this reversal of delivery direction of the pump  100  (conveying direction           return direction) is the individual activation of the inlet valve  148  and the outlet valve  150  in conjunction with energization of the electromagnet  110  to move the armature plate  118  by means of corresponding signals from the open- and/or closed-loop control unit. Actuation of the valve  158 , which in this variant embodiment functions as a backflow barrier according to the invention, proceeds purely passively through the upward and downward movement of the armature plate  118  with its conical raised portion  136  relative to the sealing edge  140 . 
         [0042]    All the components of the pump  100  which come into contact with the reducing agent  46  to be delivered (“AdBlue®” or urea/water (H 2 O) solution), have to be made of sufficiently corrosion-resistant materials, such as for example thermoplastics, thermosetting plastics, titanium or stainless steel alloys, due to the high chemical activity of the reducing agent  46 . In addition, the freezing point of the reducing agent  46  needs to be taken into account, to ensure the necessary ice-pressure resistance. Since thermal decomposition of the reducing agent  46  sets in above a temperature of 60° C. due to intensifying crystallization effects, precautions have to be additionally taken which counteract an excessive reducing agent  46  temperature. 
         [0043]    As a result of the hermetically sealing valve  158  incorporated into the pump  100  in accordance with the invention, in particular during prolonged internal combustion engine shutdown, slow filling of the device with the reducing agent  46 , in particular through not fully closing inlet and outlet valves  148 ,  150 , after prior return of the reducing agent  46  into the storage tank  42 , is reliably prevented in this second embodiment too. The valve  158  incorporated directly into the pump  100  as an active backflow barrier allows compact and at the same time inexpensive production of the device with simultaneously high variability of installation in the motor vehicle.