Abstract:
The invention relates to a pumping device ( 20 ) for supplying an exhaust gas aftertreament system ( 10 ) of an internal combustion engine with a reductant ( 14 ), in particular with a urea-water solution, in order to reduce nitrogen oxides (NOx) in the exhaust gas flow of the internal combustion engine, comprising a motor ( 32 ) for driving two pumps ( 22, 24 ). According to the invention, the first pump ( 22 ) is connected to the motor ( 32 ) by means of a first coupling, and the second pump ( 24 ) is connected to the motor by means of a second coupling. In a preferred embodiment, the couplings are designed as freewheel couplings ( 26, 28 ) acting in opposite directions, so that a switch between a “pumping state” and a “suck-back state” can be made by simply reversing the direction of rotation of the motor ( 32 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    In the case of motor vehicles with internal combustion engines, ever stricter exhaust emissions limits mean that air pollutants, such as nitrogen oxides (NOx), in the exhaust gas flow must be reduced to a greater and greater extent. One known method which is used in this context is catalytic reduction (known as “Selective Catalytic Reduction” or SCR). In this case, a reductant is pumped out of a tank as far as a metering module in the region of the exhaust line by means of a pump. The metering module required to inject the reductant is arranged within the exhaust line, generally ahead of the catalyst in which the reduction of nitrogen oxides takes place. A 32.5% aqueous urea solution (known as “AdBlue®”) is generally used as a reductant. In many cases, diaphragm pumps are used to deliver the reductant, and these generally have a preferred direction of delivery. 
         [0002]    In many cases, the diaphragm pumps are driven by means of an electric motor having an eccentric connected by a connecting rod to the pump diaphragm. If the eccentric is set in rotation with the aid of the electric motor, the pump diaphragm is periodically raised and lowered by the connecting rod, with the result that the reductant is drawn in from the storage tank and pumped as far as the metering module. An orifice or restrictor arranged downstream of the diaphragm pump prevents a pressure rise in the system when the metering module is closed or is delivering only a very small quantity of the reductant into the exhaust line. For this purpose, the delivered quantity that is not required is directed back into the storage tank by the orifice via an additional return line. The exhaust gas aftertreatment system furthermore has a complex open-loop and/or closed-loop control device for controlling all the system processes and a large number of sensors and actuators, which are interconnected via a bidirectional bus system. 
         [0003]    The aqueous urea solution freezes below −11° C. In order to ensure the required ice pressure resistance after the internal combustion engine is switched off, the reductant must as far as possible be sucked back completely out of all regions which are exposed over a prolonged period to a temperature of −11° C. or less. In order to be able to implement the suck-back process by means of the diaphragm pumps which are conventionally employed, a separate 4/2-way valve is generally used. 
         [0004]    In normal delivery mode, in particular during the normal operation of the internal combustion engine, the 4/2-way valve is in the deenergized idle state, allowing the reductant to be pumped out of the tank with the aid of the diaphragm pump and reach the metering module via the 4/2-way valve. While the reductant is being sucked back out of the exhaust gas aftertreatment system, the delivery direction of the diaphragm pump can be maintained unaltered. Only the 4/2-way valve is activated. In normal delivery mode, the reductant flows in opposite directions through two parallel ducts in the 4/2-way valve while, in suck-back mode, the reductant flows in opposite directions through two further, intersecting ducts in the 4/2-way valve. 
         [0005]    However, multi-way valves of this kind involve a construction of complex design and are therefore also expensive to produce. Moreover, such valves are prone to leaks, are susceptible to wear and require a lot of maintenance. 
       SUMMARY OF THE INVENTION 
       [0006]    It is therefore the object of the invention to provide a pumping device for exhaust gas aftertreatment systems, in particular those operating by the “SCR” method, which has little tendency to wear, has a high degree of fail safety and furthermore requires little maintenance. 
         [0007]    A pumping device for supplying an exhaust gas aftertreatment system of an internal combustion engine with a reductant, in particular with an aqueous urea solution, in order to reduce nitrogen oxides in the exhaust gas flow of the internal combustion engine is disclosed, having a motor for driving two pumps. 
         [0008]    According to the invention, the first pump is connected to the motor by means of a first coupling, and the second pump is connected to the motor by means of a second coupling. 
         [0009]    The couplings are preferably designed as freewheel couplings but, as an alternative, can also be embodied as switchable (releasable) couplings. It is thereby possible in a simple way selectively to connect the first pump (suck-back pump) or the second pump (delivery pump) to the drive motor and thus to switch from what is referred to as a normal “delivery state” while the internal combustion engine is running to what is known as a “suck-back state”, in particular after a relatively long stoppage time of the internal combustion engine. The required ice pressure resistance at ambient temperatures of −11° C. or below is achieved by means of the, ideally complete, sucking back of the reductant out of the exhaust gas aftertreatment system in order to prepare for a relatively long stoppage time of the internal combustion engine. 
         [0010]    In the preferred embodiment of the pumping device, the couplings are freewheel couplings acting in opposite directions. 
         [0011]    It is thereby possible to switch from the “delivery state” to the “suck-back state” and vice versa by simply reversing the direction of rotation of the motor. When the motor is stationary, neither of the two pumps is driven—irrespective of the switching state of the couplings—and therefore the pumping device is in what is known as the “idle state”. Each of the pumps connected for operation in opposite directions is therefore only ever operated in the delivery direction thereof, irrespective of the system state (“delivery state”/“suck-back state”). 
         [0012]    Another advantageous embodiment of the pumping device envisages that a metering module of the exhaust gas aftertreatment system is connected to a discharge line of the second pump and to a suction line of the first pump by means of a shuttle valve. The metering module is used to inject a precisely determined quantity of the reductant into an exhaust pipe containing a catalyst for reducing the nitrogen oxides. It is in the catalyst that the actual selective chemical reduction of nitrogen oxides in the exhaust gas flow of the internal combustion engine to water (H 2 O) and nitrogen (N 2 ) takes place. The shuttle valve allows an effective hydraulic separation between the two pumps, with the shuttle valve being switched over automatically simply by the pressure conditions prevailing at a pressure port and a suction port of the shuttle valve. The filter unit and the metering module arranged downstream thereof are connected to the shuttle valve via a third port of the shuttle valve, through which the reductant flows in both directions (bidirectional port). 
         [0013]    In a preferred embodiment, a suction line of the second pump and a discharge line of the first pump are connected to a storage tank for the reductant. 
         [0014]    Owing to this arrangement of the lines, the reductant can be drawn in from the storage tank by means of the second pump during the normal operation of the internal combustion engine and pumped onward at excess pressure as far as the metering module via the shuttle valve. In addition, this line routing enables the reductant to be sucked back out of the metering module via the shuttle valve as far as the storage tank by means of the first pump in order to initiate a relatively long stoppage time of the internal combustion engine. 
         [0015]    Another advantageous embodiment of the pumping device envisages that both pumps are designed as diaphragm pumps. 
         [0016]    The diaphragm pumps make possible a pumping device construction of simple design. Moreover, diaphragm pumps have good corrosion resistance since the pumping space is separated completely from the drive zone by the diaphragm. The use of diaphragm pumps increases the operational reliability of the pumping device for the reductant, which is generally chemically aggressive, and, at the same time, considerably reduces the outlay on maintenance. In general, diaphragm pumps allow virtually maintenance-free operation for the entire life of a motor vehicle. 
         [0017]    According to another advantageous embodiment, it is envisaged that an electric motor, in particular an external rotor motor, is used to drive the pumps. 
         [0018]    First of all, the embodiment of the motor as an electric motor has the advantage of ease of closed-loop and/or open-loop control. Moreover, the use of an external rotor motor has the advantage that the delivery flow can be made more uniform through the high moment of inertia of the rotor rotating around the stator. 
         [0019]    According to another advantageous development of the pumping device, at least one return flow restrictor is provided. 
         [0020]    Particularly where use is made of an external rotor motor, the rotating rotor of which generally has a high moment of inertia, the motor remains continuously switched on in the “delivery state” in order to ensure as a uniform a delivery flow is possible and to minimize the run-up times of the motor. Moreover, a motor which is permanently switched on allows a uniform delivery flow, thereby ensuring a reliable and continuous supply of the reductant to the metering module. However, continuous operation of the motor and of the associated second pump in the “delivery state” can lead to an unwanted pressure increase in the region of the discharge line between the second pump, the shuttle valve and the metering module if too little or no reductant is discharged by the metering module. In order to avoid such a pressure rise, a return flow restrictor or a return flow orifice can preferably be provided in the discharge line between the second pump and the shuttle valve, directing the excess reductant that is not required by the metering module back to the storage tank via an additional return flow line. 
         [0021]    In an alternative embodiment, the return flow restrictor can be an integral part of the shuttle valve. In general, a restrictor or an orifice with a cylindrical bore and a small cross-sectional area is implemented technically. 
         [0022]    In addition, a method, in particular for operating a pumping device of this kind, for supplying an exhaust gas aftertreatment system of an internal combustion engine with a reductant, in particular with an aqueous urea solution, in order to reduce nitrogen oxides in the exhaust gas flow of the internal combustion engine, is disclosed, having a motor for driving two pumps. 
         [0023]    According to the method, a switch is made between a “delivery state” and a “suck-back state” by reversing the direction of rotation of the motor, wherein, in the delivery state, the reductant is pumped out of a storage tank for the reductant, via a shuttle valve, as far as a metering module by means of the second pump and, in the “suck-back state”, the reductant is sucked back out of the metering module, via the shuttle valve, into the storage tank by means of the first pump. 
         [0024]    Simply by changing the direction of rotation of the motor, the method allows a quick change between the “delivery state”, in which the metering module of the exhaust gas aftertreatment system is supplied with the reductant, and the “suck-back state”, in which the reductant contained in the exhaust gas aftertreatment system is returned almost completely to the storage tank in order to achieve the required ice pressure resistance in the case of relatively long stoppage times. After the motor has been switched off, the system is in an “idle state”, in which there is no delivery of the reductant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention will be described in greater detail below by means of the drawings, in which: 
           [0026]      FIG. 1  shows a schematic illustration intended to elucidate the fundamental principle of operation of the pumping device; 
           [0027]      FIG. 2  shows a diagrammatic illustration of the drive motor with two freewheel couplings and the associated two pumps; 
           [0028]      FIG. 3  shows a cross section through the shuttle valve, and 
           [0029]      FIG. 4  shows a perspective view of the closing member of the shuttle valve. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]      FIG. 1  shows a schematic illustration intended to elucidate the fundamental principle of operation of the pumping device for an exhaust gas aftertreatment system operating by what is known as the “SCR” method. 
         [0031]    Among the components of the exhaust gas aftertreatment system  10  are a storage tank  12  for the reductant  14 , at least one filter unit  16  and a metering module  18 . 
         [0032]    For greater clarity, the exhaust line of the internal combustion engine, the catalyst required within the exhaust line for catalytic reduction, an open-loop and/or closed-loop control device required to control all the processes within the exhaust gas aftertreatment system  10 , and a large number of sensors and actuators which communicate with the open-loop and/or closed-loop control device via a bidirectional bus system are not shown in  FIG. 1 . 
         [0033]    The fundamental directions of flow of the reductant  14  within the exhaust gas aftertreatment system  10  are illustrated by white arrows, which are not provided with reference numerals. Among the components of a pumping device  20  designed in accordance with the invention for supplying the exhaust gas aftertreatment system  10  with the reductant  14  are a first pump  22  (suck-back pump) and a second pump  24  (delivery pump). The pumps  22 ,  24  are each connected mechanically, via the freewheel couplings  26 ,  28 , to a motor shaft  30  of a (drive) motor  32 . In a preferred embodiment, the motor  32  is designed as an electric external rotor motor. The (continuous) motor shaft  30  allows simultaneous driving of both freewheel couplings  26 ,  28 , which are designed to act in opposite directions according to the invention. 
         [0034]    This means that only the second pump  24  is driven when the motor  32  rotates clockwise (“R”), for example, while the first freewheel coupling  26  freewheels in this direction of rotation and consequently the first pump  22  is stationary. If the direction of rotation of the motor  32  is reversed, so that it then rotates counterclockwise (“L”), the second freewheel coupling  28  freewheels instead. As a result, the second pump  24  is stationary, and the first pump  22  is then driven via the first freewheel coupling  26 , which is in engagement in the direction of rotation (“L”). The two oppositely acting freewheel couplings  26 ,  28  thus allow alternating operation of the pumps  22 ,  24 , depending on the direction of rotation of the motor  32  and of the motor shaft  30 . 
         [0035]    A discharge line  34  of the second pump  24  is connected to a shuttle valve  36 , which is in the no-load idle position in the illustration in  FIG. 1 . In a corresponding manner, a suction line  38  of the first pump  22  is likewise connected to the shuttle valve  36 . The shuttle valve  36  is connected hydraulically, via lines that are not designated, to the filter unit  16  and to the metering module  18  of the exhaust gas aftertreatment system  10 . In the context of this description, the term “lines” is taken to mean both pipes and flexible hoses, including armored hoses. The directions of delivery of the two pumps  22 ,  24 , which are preferably designed as diaphragm pumps, are designed to be opposite one another. Consequently, the second pump  24  always draws in the reductant  14  from the storage tank via a suction line  40 , while the first delivery pump  22  always pumps the reductant  14  back into the storage tank  14  via a discharge line  42 . 
         [0036]    An illustrative sequence of the method during the operation of the pumping device  20  and, in association therewith, further details of the construction of the pumping device  20  will be explained in detail below: 
         [0037]    When the internal combustion engine is running or has just been started, the reductant  14  is drawn in from the storage tank  12  by the second pump  24  and passes via the discharge line  34  into the shuttle valve  36  (“delivery state”). Owing to the delivery pressure built up in this way, a closing member  44 , illustrated here schematically as a ball, rises from the right-hand valve seat  46  of the shuttle valve  36  counter to the spring force built up by the undesignated spring, and—counter to the spring force thereof—is pressed against the left-hand valve seat  48 . As a result, the reductant  14  can flow through the shuttle valve  36  and passes via the filter unit to the metering module  18 . During this process, the motor  32 , which is rotating clockwise (“R”), drives the second pump  24  via the motor shaft  30  and the second freewheel coupling  28 . Control of the speed and direction of rotation of the motor  32  is performed by means of the open-loop or closed-loop control device mentioned at the outset. 
         [0038]    An electric motor, designed, in particular, as an “external rotor”, is preferably used as a motor  32 . Using an electric motor makes it easy to perform open-loop and/or closed-loop control. Since, by virtue of its design, the rotor rotates around the stator in the case of an external rotor, the higher moment of inertia of the motor  32  due to this fact can be used in an advantageous manner, in particular to make the delivery flows of the pumps  22 ,  24  more uniform. Owing to the high moment of inertia of the motor  32 , however, it is advantageous to make the motor  32  run as continuously as possible, both in the “delivery state” and in the “suck-back state” of the reductant  14 , in order to avoid relatively long run-up times before a target motor speed is reached. After the suck-back process is fully ended, the motor  32  can be switched off. Owing to the fact that the motor  32  usually runs continuously, a particularly uniform supply of reductant  14  to the metering module  18  is furthermore obtained. 
         [0039]    Particularly in the “delivery state”, however, there can be an unwanted pressure increase in the region of the discharge line  34 , the shuttle valve  36 , the downstream filter unit  16  and/or the metering module  18  in this case. In order to prevent this, the pumping device  20  in the embodiment shown is fitted with a return flow restrictor  50  or a return flow orifice. Here, the return flow restrictor  50  is connected to the discharge line  34 . Excess reductant  14  which is not required in the metering module  18  can then flow back into the storage tank  12  via the return flow restrictor  50  and a return flow line  52  arranged downstream thereof. It is thereby possible to prevent the unwanted pressure increase. As an alternative, it is possible (cf, especially,  FIGS. 3 and 4 ) to design the return flow restrictor  50  or the return flow orifice as an integral part of the shuttle valve  36 , thereby enabling the number of line connections, sealing locations and system components to be reduced. To prepare for a relatively long stoppage time of the internal combustion engine, the reductant  14  is sucked back into the storage tank  12 . For this suck-back process, the direction of rotation of the motor  32  is reversed from clockwise (“R”) to counterclockwise (“L”), thereby initiating what is known as the “suck-back state”. Owing to the reversal in the direction of rotation, the second freewheel coupling  28  is in the freewheeling state, with the result that the second pump  24  is stationary. In contrast, the first pump  22  is set in motion by means of the first freewheel coupling  26 , which is in engagement in this direction of rotation. As a result, the reductant  14  is drawn in by the first pump  22  from the metering module  18 , via the filter unit  16  and the shuttle valve  36 , via the suction line  38 , and is pumped back into the storage tank  12  by means of the discharge line  42 . This “suck-back state” is maintained until, in the ideal case, all the reductant  14  has been sucked back out of the exhaust gas aftertreatment system  10 , thus establishing the required ice pressure resistance. Once the suck-back process is complete, the motor  32  can be switched off, with the result that both pumps  22 ,  24  stop and the pumping device  20  is in the “idle state”. 
         [0040]    In the “suck-back state”, the closing member  44  of the shuttle valve  36  is pressed firmly against the right-hand valve seat  46  owing to the action of the undesignated compression spring, and, at the same time, the left-hand valve seat  48  is exposed, allowing the reductant  14  to be drawn in by the first pump  22  against only a slight resistance. The shuttle valve  36  thus ensures effective hydraulic separation between the two pumps  22 ,  24  and the delivery branches connected thereto in both main states of the pumping device  20  in the form of the “delivery state” and the “suck-back state”. 
         [0041]    The shuttle valve  36  operates automatically since a pressure of up to 5.0 bar built up in the discharge line  34  by the second pump  24  in the “delivery state” is significantly higher in the discharge line  34  than a suction vacuum of about 0.5 bar brought about by the first pump  22  in the region of the suction line  38 . Consequently, the shuttle valve  36  responds solely on the basis of the respective pressure conditions in the region of the discharge line  34  and of the suction line  38 . 
         [0042]    If the internal combustion engine is to be restarted, the “delivery state” is initiated again, starting from the “idle state”, in that the motor  32  runs up in the clockwise direction of rotation “R”, with the result that the second delivery pump  24  pumps the reductant  14  out of the storage tank  12  as far as the metering module  18 . The cyclical change between the “delivery state”, the “suck-back state” and the “idle state” can be performed as often as required. 
         [0043]    Instead of the two freewheel couplings  26 ,  28  in the preferred embodiment, it is also possible to use couplings (not shown) that can be switched electromagnetically, for example, or in some other way, these being addressed by the open-loop and/or closed-loop control device in a controlled manner. In such a configuration, it is also possible to make both pumps  22 ,  24  deliver simultaneously if the metering module  18  is taking off too little reductant  14 , such that the excess reductant  14  is pumped back immediately into the storage tank  12  by means of the first pump  22  (“recirculation”). It is thereby possible to avoid a pressure rise while the motor  32  is running. In certain circumstances, this may make the return flow restrictor  50  or the return flow line  52  superfluous. In such an embodiment, the shuttle valve  36  may also be unnecessary if the two pumps  22 ,  24  are sufficiently pressure-resistant or secure against throughflow on both sides when stationary, such that they themselves act as closed “valves” when stationary. Consequently, it may be necessary to form the pumps  22 ,  24  with some other type of pump than the diaphragm pump that is used for preference here. 
         [0044]      FIG. 2  illustrates a sectional view of a possible illustrative embodiment of the motor having two freewheel couplings and associated (diaphragm) pumps. 
         [0045]    The motor  32 , which is preferably designed as an external rotor, is flanged to a housing  60 . The power supply to the motor  32  is via a plug connection  62  or a plug connector. The first and second freewheel couplings  26 ,  28  are firmly connected to the motor shaft  32  and are driven by the latter. The freewheel couplings  26 ,  28  are connected to two eccentrics  64 ,  66 , on each of which a ball bearing  68 ,  70  is mounted. Two connecting rods  72 ,  74 , to each of which a pump diaphragm  76 ,  78  is pivotally attached, are rotatably mounted on the ball bearings  68 ,  70 . 
         [0046]    By means of the two eccentrics  64 ,  66  and the connecting rods  72 ,  74 , the rotary motion of the motor shaft  30  is transformed into a linear motion, which is transmitted to the diaphragms  76 ,  78  of the pumps  22 ,  24  by the connecting rods  72 ,  74 . As a result, the diaphragms  76 ,  78  perform a periodic upward and downward motion, parallel to the two undesignated white arrows, and pump the reductant through the pumping device  20 —as explained in greater detail as part of the description of  FIG. 1 . Owing to the opposite action of the two freewheel couplings  26 ,  28 , only that pump of the two pumps  22 ,  24  is in delivery mode, of which the freewheel coupling  26 ,  28  is in engagement—depending on the direction of rotation of the motor  32 . Any check valves that are still required for the delivery mode of the pumps  22 ,  24  are not shown in  FIG. 2 . 
         [0047]      FIG. 3  illustrates a more detailed cross section through one embodiment of the shuttle valve  36  in a no-load “idle position”. 
         [0048]    The fundamental flow conditions of the reductant through the shuttle valve  36  are once again illustrated by the three undesignated white arrows. Among the components of the shuttle valve  36  is a housing  80 , in which an approximately cup-shaped closing member  82  is accommodated in a sprung manner in such a way that it can be moved parallel to a vertical longitudinal axis  84 . The closing member  82  has an encircling projection  86  with a first seal  88  (sealing lip), which is pressed against a right-hand housing wall  92  as a sealing surface owing to the action of a compression spring  90 . A right-hand pressure port  94  of the shuttle valve  36  is thereby sealed off, said port normally being connected to the discharge line  34  of the second pump  24  (cf  FIG. 1 ). 
         [0049]    If the pressure of the reductant in the region of the pressure port  94  rises due to a pumping action of the second pump  24  to such an extent that the spring force of the compression spring  90  is overcome, the closing member  82  moves to the left, parallel to the valve longitudinal axis  84 , until a second seal  96  (sealing lip) rests against a left-hand housing wall  98  as a sealing surface. The first seal  88  is situated on an undesignated front side of the projection  86  or of the closing member  82 , while the second seal  96  is situated on an undesignated rear side of the projection  86 . 
         [0050]    In this “open position” of the closing member  82 , the reductant can flow from the pressure port  94 , through a large, approximately cylindrical chamber  100  within the housing  80 , as far as a bidirectional port  102 , which is generally connected to the filter unit  16  or the downstream metering module  18 . In order to ensure proper functioning of the shuttle valve  36 , the compression spring  90  should have a spring force sufficient to ensure that the movement of the closing member  82  to the left takes place at the earliest from a pressure of 1.5 bar in the region of the pressure port  94 . 
         [0051]    If the pump  24  ceases delivery, then, after a sufficient pressure drop, the closing member  82  is once again pushed to the right by the spring force of the compression spring  90  until seal  88  is resting against the right-hand housing wall  92  as a sealing seat, and the “idle position” shown in  FIG. 3  has been reached again. 
         [0052]    In the “suck-back state”, the closing member  82  is in the “idle position” shown in  FIG. 3 , allowing the reductant to flow into the large chamber  100  from above through the (bidirectional) port  102 . From there, the reductant flows via a multiplicity of suck-back bores—of which just one suck-back bore  104  is designated—into a hollow-cylindrical stem  106  of the closing member  82 . From there, the reductant flows into a smaller, cylindrical chamber  108  of the housing  80 , which opens into a suction port  110 . The suction port  110  is generally connected to the first pump  22 , which is used to pump the reductant back into the storage tank (cf  FIG. 1 ). 
         [0053]    In the embodiment shown in  FIG. 3 , the shuttle valve  36  furthermore also performs the function of a return flow restrictor or a return flow line (cf  FIG. 1 ) in the pumping device, which can be eliminated as a result. 
         [0054]    For this purpose, an orifice opening  114  with a small cross-sectional area is introduced into a base  112  of the closing member  82 , forming a return flow restrictor or return flow orifice in terms of hydraulics. By means of the orifice opening  114 , which is formed by a conically countersunk but otherwise cylindrical bore, excess reducing fluid which is not discharged in the metering module owing, for example, to special operating states of the internal combustion engine, can return from the pressure port  94 , via the suction port  110 , to the storage tank  12  through the first pump. As a result, an excessive pressure rise in the system is avoided. The two seals  88 ,  96  are produced from an elastomer which is sufficiently resistant to chemicals, especially to the reductant (“AdBlue®”), such as an EPDM (ethylene-propylene-diene monomer). In principle, the closing member  82  or the projection  86  can be produced from a metal alloy or from a plastic material as long as the required resistance to the reductant is ensured. In a preferred embodiment, however, the closing member  82  is produced from a thermoplastic or a thermosetting plastic (“TP”/“TS”), the thermosetting plastic being preferred in the case of the integrated throttling function in  FIG. 3  since this is more resistant to “flow abrasion”, which occurs to a greater extent as compared with that in the orifice opening  114 . The seals  88 ,  96  can be formed integrally with the closing member  82  by what is known as the “2-C” injection molding method (“two-component” injection molding method), for example, or by way of what is known as the “compression molding method”. 
         [0055]      FIG. 4  shows an enlarged perspective view of the closing member  82  from  FIG. 3 . 
         [0056]    A multiplicity of suck-back bores are introduced at uniform spacings around the circumference in the sleeve-type stem  106  of the closing member  82 , one of said suck-back bores bearing the reference numeral  104 . The compression spring  90  is inserted into the stem  106  and is guided radially by the latter. The first and second seals  88 ,  96  are arranged above and below the encircling projection  86 . In the event that the closing member  82  is formed by a plastic material, the two seals  88 ,  96  can be produced integrally (in one piece) with the body of the closing member  82  by the two-component injection molding method (known as the “2-C” injection molding method), for example, or by what is known as the “compression molding method”. 
         [0057]    The pumping device according to the invention for an exhaust gas aftertreatment system in a motor vehicle, having two pumps and a drive motor, which are each alternately driven by the drive motor via oppositely acting freewheel couplings in the preferred embodiment, allows a reliable and uniform supply of the reductant required for catalytic exhaust gas purification to a metering module of the exhaust gas aftertreatment system. 
         [0058]    Moreover, the pumping device allows reliable and yet low-maintenance operation of the exhaust gas aftertreatment system in comparison with previously known solutions having a 4/2-way valve, and the optimized suck-back process furthermore ensures the required ice pressure resistance at low motor vehicle operating temperatures.