Abstract:
The invention relates to a dosing pump unit for mixing a liquid reducing agent with an exhaust gas flow by means of a dosing pump ( 2 ) which is connected, on the input side thereof, to a connection connectable to a reducing agent tank ( 4 ) and, on the output side thereof, to a pressure pipe ( 16 ) and a pipe ( 10 ) for returning flow back to said reducing agent tank ( 4 ), wherein a non-return valve ( 20 ) which is prestressed against a hydraulic pressure produced by the dosing pump ( 2 ) is arranged in the pressure line ( 16 ), the return pipe ( 10 ) is connected to the pressure pipe ( 16 ) upstream of the non-return valve ( 20 ) in the reducing agent flow direction and a first stop valve ( 12 ) for selectively closing the return pipe ( 10 ) is placed therein.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Section 371 of International Application No. PCT/EP2005/014111, filed Dec. 29, 2005, which was published in the German language on Jul. 13, 2006, under International Publication No. WO 2006/072443 A1, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a metering pump assembly for admixing a fluid reduction agent into an exhaust gas flow 
         [0003]    With self-ignition internal combustion engines, nitrogen oxides arise on operation with excess oxygen, which is the case in the majority of operating conditions, and specifically in particular with direct injection into the main combustion space, as is typically the case with diesel motors. In order to reduce these nitrogen oxide emissions, it is known to lead the exhaust gas flow to a reduction catalyser. Thereby, as a reduction agent, usually an aqueous urea solution is led in a finely distributed manner to the exhaust gas before entry into the catalyser. Thereby, the supplied quantity of urea is to be matched as precisely as possible to the combustion process, in order on the one hand to ensure an as complete as possible reduction within the catalyser, and on the other hand to prevent an excess of urea. 
         [0004]    A special metering pump assembly for metering and feeding reduction agent to the exhaust gas flow is known from EP 1 435 458 A1. With this metering pump assembly, the supplied reduction agent, before feeding to the exhaust gas flow, is mixed or impinged with pressurized air in the metering pump assembly. In order to be able to firstly bleed the system on starting operation, a 3/2-way valve as a preflushing valve is arranged in front of a premixing device, in which the mixing of the reduction agent with pressurized air is effected. On starting operation of the assembly, this preflushing valve is firstly switched such that the metering pump conveys the aspirated reduction agent back into the reduction agent tank, until the conduits are completely filled with reduction agent. The premixing valve is then switched over, so that the reduction agent is supplied to the premixing device. 
         [0005]    The object of the present invention is to simplify the construction of the metering pump assembly known from EP 1 435 458 A1. 
         [0006]    This object is achieved by a metering pump assembly with the features specified in claim  1 . Preferred embodiments are to be deduced from the dependent claims. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    According to the invention, the desired simplification of the construction of the metering pump assembly results on account of the following construction. The metering pump assembly for admixing a fluid reduction agent into an exhaust gas flow comprises a metering pump which on the entry side is connected to a connection for connecting to a reduction agent tank, and at the exit side is connected to a pressure conduit as well as a return conduit to the reduction agent tank. Connections may be provided on the metering pump assembly, for the suction conduit as well as the return conduit to the reduction agent tank, to which conduits or flexible tubing which lead to the reduction agent tank are connected. The pressure conduit which extends behind the metering pump in the flow direction, is part of the metering pump assembly, and for example leads to a premixing device in which the delivered reduction agent is impinged with pressurized gas. The return conduit branches away from the pressure conduit and is in connection with this. According to the invention, a return valve which is biased against the fluid pressure produced by the metering pump, is arranged in the pressure conduit. This means that the biasing retains the return valve closed against the fluid pressure produced by the metering pump, as long as the fluid pressure does not exceed a certain pressure value, at which the force acting on the return valve on account of the fluid pressure corresponds to the biasing force of the return valve. If the fluid pressure exceeds this limit value, the return valve is opened against its biasing by way of the delivered reduction agent. The return conduit is connected to the pressure conduit in front of the return valve in the flow direction of the reduction agent. Furthermore, a first shut-off valve for the selective closure of the return conduit is arranged in the return conduit. One may make do without a complicated 3/2-way valve for bleeding and filling the system by way of this arrangement, and instead of this, provide a simple switching- or shut-off valve in the return conduit. 
         [0008]    On starting operation of the metering pump assembly, the metering pump, for example a membrane pump, firstly delivers reduction agent out of the reduction agent tank and pumps this through the return conduit back into the tank, until the metering pump and the pressure conduit are completely filled with reduction agent. Since the provided return valve is biased, as long as the fluid pressure of the reduction agent does not reach the pressure value for opening the return valve, one may ensure that the reduction agent does not flow further for example to a premixing device and in particular into the exhaust gas system of the combustion motor, but firstly through the opened shut-off valve and the return conduit back into the reduction agent tank. After the complete filling of the system with reduction agent, the return conduit may be closed by the shut-off valve, so that the reduction agent may no longer flow back into the reduction agent tank. If then the fluid pressure in the pressure conduit is increased, the return valve opens, and the reduction agent may, as the case may be, via a premixing device, be delivered into the exhaust gas system of the combustion engine. 
         [0009]    In order to ensure the opening of the return valve at the correct point in time, the biasing force which keeps the return valve closed, is preferably larger than the force acting on the return valve on account of the fluid pressure with an opened return conduit. By way of this, it is ensured that no reduction agent may flow through the return valve during the preflushing or filling. Further preferably, the biasing force is selected such that with a closed return conduit, i.e. when the shut-off valve blocks the return conduit, the force acting on the return valve due to the fluid pressure exceeds the biasing force of the return valve, so that this is opened and the reduction agent may flow through the return valve. If after closure of the return conduit, the metering pump continues to deliver reduction agent, the pressure in the pressure conduit automatically increases downstream behind the metering pump, so that when the biasing force of the return valve is selected in a suitable manner, this is automatically opened. Since reduction agent may flow away out of the pressure conduit with an opened return conduit, a reduction of the pressure in the pressure conduit automatically occurs, so that the return valve is automatically closed in this condition with a suitably selected biasing force. The return valve may for example be biased by a compression spring, for example a helical spring, whose spring force is matched to the fluid pressure occurring in the pressure conduit, such that the automatic closure and opening of the return valve, as previously described, is ensured. 
         [0010]    It is further preferable to arrange the return valve directly on a mixing chamber of a premixing device, in which the reduction agent delivered by the metering pump is impinged or mixed with a pressurized gas. With this arrangement, the return valve may assume a double function. On the one hand the return valve, as previously described, on bleeding or preflushing the metering pump, ensures that the reduction agent firstly does not yet flow further into the premixing device and the exhaust gas system of the combustion engine. On the other hand, the return valve directly on the premixing device ensures that no pressurized gas, in particular pressurized air, with the reduction agent, may flow back into the pressure conduit. A return flow of reduction agent, and in particular a reduction agent/pressurized gas mixture is undesirable, since for example urea tends to crystallize in contact with air. For this reason, the return valve is preferably arranged directly on the mixing region, in which the pressurized gas is mixed with the reduction agent, so that the region located downstream of the return valve may be freed from reduction agent residues by way of the pressurized gas, and the conduit region located upstream of the return valve is securely kept held free of reduction agent and pressurized gas flowing back. The construction of the whole metering pump assembly may be considerably simplified, and simultaneously a large operational reliability is ensured, by way of this double function of the return valve. 
         [0011]    It is further preferable to arrange a pressure sensor detecting the fluid pressure, in the pressure conduit or return conduit, in front of the return valve and the shut-off valve in the flow direction. Various operating conditions of the metering pump assembly, and in particular possible faults on operation of the metering pump assembly, may be detected via this pressure sensor. Firstly, on preflushing on starting operation of the metering pump assembly, the pressure sensor may be used to determine when the pressure conduit is completely filled with reduction agent, and thus when the metering pump assembly is ready for operation. When the pressure sensor detects the pressure threshold value indicating this operation condition, the shut-off valve in the return conduit may then be closed, so that the metering pump then delivers the reduction agent via the return valve, as the case may be, via a premixing device, into the exhaust gas flow. Furthermore, the pressure sensor may be used to detect a blockage of the pressure conduit situated further downstream, and in particular of the injection nozzle in the exhaust gas flow. Then specifically the pressure in the pressure conduit increases above a pressure value occurring on normal operation, which is detected by the pressure sensor. A corresponding warning notice may be outputted e.g. by a control device, and the metering pump assembly may be switched off as the case may be. Furthermore, by way of the pressure sensor, one may also determine when the reduction agent supply in the reduction agent tank has been consumed. Specifically, the pressure in the pressure conduit on operation then sinks to below a pressure value occurring during normal operation. If accordingly, the pressure sensor detects a lower pressure limit value, then the lacking reduction agent supply may be detected and be suitably displayed e.g. via a control device. In this condition too, the metering pump assembly may be firstly automatically switched off, until the filling of the reduction agent by way of a control device, in order to avoid damage on operation without reduction agent. 
         [0012]    In order to ensure the starting operation of the metering pump assembly after the preflushing or bleeding, the first shut-off valve is advantageously coupled to the pressure sensor in a manner such that the first shut-off valve is switched over into its position closing the return conduit on reaching a predefined fluid pressure detected by the pressure sensor. The predefined limit value for the fluid pressure at which the return conduit is closed, is selected such that the complete filling of the pressure conduit with reduction agent on starting operation of the metering pump assembly may always be ensured. This means that the metering pump is firstly bled, before the metering pump assembly is started in operation, and reduction agent is injected into the exhaust gas flow. This is particularly important when the metering pump assembly is started in operation for the first time after filling the reduction agent tank. 
         [0013]    The first shut-off valve is preferably biased in a manner such that in the idle position, it is held in its position opening the return conduit. In this manner, it is ensured that the return conduit is open to the reduction agent tank when the metering pump assembly is not in operation, so that the entire system and in particular the pressure conduit are not under pressure. In this manner, one may ensure that the biased return valve securely keeps the pressure conduit closed in the switched-off condition of the metering pump assembly, so that reduction agent may not inadvertently exit from the metering pump assembly. The biasing may be effected for example by way of spring force. An actuation force which is larger than the biasing force must be mustered for the closure of the first shut-off valve. 
         [0014]    It is particularly preferable to provide a premixing device, in which the reduction agent delivered by the metering pump is impinged or mixed with a pressurized gas, and to arrange a second shut-off valve for the selective interruption of the pressurized gas supply, at the entry side in a pressurized gas conduit leading to the premixing device. The pressurized gas supply may for example be effected from a central pressurized air supply of a motor vehicle, in particular lorry. The second shut-off valve then serves for being able to switch the pressurized gas or pressurized air supply into the metering pump assembly on and off in a targeted manner. On starting operation of the metering pump assembly, the second shut-off valve is opened in order to let pressurized gas flow into the metering pump assembly, and to mix the delivered reduction agent with pressurized gas in the premixing device. Thereby, a fine mixing between the reduction agent and the pressurized gas does not need to be effected in the premixing device, but rather it is sufficient to impinge the reduction agent only with pressurized gas, so that larger reduction agent droplets are conveyed further to an injection nozzle into the exhaust gas flow by way of the pressurized gas. A finer atomization of the reduction agent in the exhaust gas flow is effected on exit from the injection nozzle. 
         [0015]    It is particularly preferable for the first shut-off valve in the return conduit to be coupled to the second shut-off valve in the pressurized gas conduit, in a manner such that the return conduit is closed when the pressurized gas conduit is open. This permits a very simple and secure starting operation of the metering pump assembly, since both shut-off valves do not need to be switched independently of one another. On account of the coupling, it is indeed possible to actuate only one of the valves, for example the second shut-off valve, and simultaneously to automatically also switch over the other one, for example the first shut-off valve. Thus for example it is possible, by way of opening the second shut-off valve, to firstly switch on the pressurized gas supply to the metering pump assembly, wherein then the return conduit is automatically closed by way of the coupling, so that the reduction agent delivered by the metering pump amid the opening of the return valve, is then also conveyed into the premixing device, where it is mixed with the pressurized gas. Conversely, on account of the coupling on switching off the metering pump assembly, it is ensured that when the pressurized gas supply is interrupted by the closure of the second shut-off valve, the first shut-off valve in the return conduit is also automatically opened, so that the reduction agent which is delivered further by the metering pump may flow back into the reduction agent tank, or the pressure conduit is switched in a pressureless manner. In this manner, it is ensured that no reduction agent gets into the premixing device and further downstream to the injection nozzle into the exhaust gas flow, if no pressurized gas flows into the metering pump assembly. An undesirable blocking or clogging of the conduits downstream of the return valve in the pressure conduit is thus prevented. 
         [0016]    The first shut-off valve is particularly preferably actuatable by pressurized gas for coupling the two shut-off valves, wherein the pressurized gas conduit leading to the premixing device is connected to an actuation connection of the first shut-off valve, e.g. via an actuation conduit, behind the second shut-off valve in the flow direction. This permits a purely pneumatic coupling between the two shut-off valves, so that the number of the electrically actuated components in the metering pump assembly may be reduced, which leads to a further simplified construction. The arrangement permits the first shut-off valve to be closed by way of the pressure of the pressurized gas led to the premixing device, preferably against a spring bias. Thus no additional actuation elements whatsoever are required for the closure of the first shut-off valve. 
         [0017]    The actuation connection of the first shut-off valve is preferably connected to the pressurized gas conduit leading to the premixing device, in a region which in the flow direction lies on front of a throttle location in the pressurized gas conduit. In this manner, an adequately high gas pressure at the actuation connection of the first shut-off valve is ensured for its actuation, whilst the premixing device is supplied with a lower gas- or air pressure via the throttle location. 
         [0018]    It is further preferable for the second shut-off valve to be coupled to a pressure sensor arranged in the flow direction in front of the return valve and the first shut-off valve, in the pressure conduit or the return conduit, in a manner such that the second shut-off valve is switched over into its position opening the pressurized air conduit on reaching a defined fluid pressure detected by the pressure sensor. This means that on starting operation of the metering pump assembly, one may firstly switch on the metering pump, wherein the pressurized gas supply is firstly still closed by the second shut-off valve. Firstly, a preflushing or bleeding of the metering pump is effected via the return conduit which is still open. If then it is ascertained by way of the pressure sensor, that a certain pressure value has been reached in the pressure conduit, which indicates that the pressure conduit is completely filled with reduction agent, the second shut-off valve is then opened, so that the pressurized gas may flow into the metering pump assembly and to the premixing device. If then, as described previously, the first shut-off valve is also coupled to the second shut-off valve, then simultaneously the return to the reduction agent tank may be closed, so that the reduction agent is delivered into the premixing device via the return valve which then opens. The coupling of the pressure sensor and the first shut-off valve may be realized electronically via a control device but also mechanically, pneumatically or hydraulically. 
         [0019]    The second shut-off valve in the pressurized gas conduit is particularly preferably biased in a manner such that in the idle position, it is held in its position interrupting the pressurized gas conduit. In this manner, on switching off the metering pump assembly, it is always ensured that the pressurized gas supply to the metering pump assembly is interrupted. The second shut-off valve may for example be biased by way of spring force, so that it must be opened by an actuation force to be mustered externally. Thereby, the actuation force is preferably produced electrically e.g. electromagnetically. If then the current supply, e.g. the flow of current in a coil in the electromagnet, is switched off, the actuation force stops and the bias automatically closes the shut-off valve. 
         [0020]    Furthermore, a further return valve may be arranged in the premixing device on the exit side of the pressurized gas supply, and this further return valve prevents reduction agent from being able to flow from the premixing device back into the pressurized gas conduit on switching off the pressurized gas supply to the premixing device. It may thus be ensured that the pressurized gas conduit is kept free from contamination. This second return valve is preferably likewise biased, so that it opens at a certain pressure in the pressurized gas conduit, and automatically closes on falling short of this pressure value or on switching off the pressurized gas supply, for example by way of closure of the second shut-off valve. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0021]    The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
           [0022]    In the drawings: 
           [0023]      FIG. 1  is a circuit diagram of the hydraulic components of a metering pump assembly according to the invention; 
           [0024]      FIG. 2  is a sectioned cut-out, the premixing device of a metering pump assembly according to  FIG. 1 ; 
           [0025]      FIG. 3  is a separate sectional view, the premixing device according to  FIG. 2 , with closed return valves; 
           [0026]      FIG. 4  is the premixing device according to  FIG. 3 , with an opened return valve for the supply of pressurized gas; 
           [0027]      FIG. 5  is the premixing device according to  FIGS. 3 and 4  with an opened return valve for the supply of reduction agent, and an opened return valve for the supply of pressurized gas; 
           [0028]      FIG. 6  is a sectioned view of the shut-off valve in the return conduit, in the closed condition; and 
           [0029]      FIG. 7  is a sectioned view of the shut-off valve according to  FIG. 6 , in the opened condition. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    The basic function of one embodiment example of a metering pump assembly according to the invention is firstly described by way of the circuit diagram in  FIG. 1 . 
         [0031]    The core piece of the metering pump assembly is a metering pump  2  which is designed as a membrane pump, with an associated drive. By way of the control of the drive, and in particular the control of the number of pump strokes and/or the stroke speed, one may set the quantity of the reduction agent delivered by the metering pump  2 , in order to be able to adapt the reduction agent quantity exactly to the requirements with regard to the currently occurring combustion process in the motor. An aqueous urea solution is preferably applied as a reduction agent. The reduction agent is kept ready in a reduction agent tank  4  from which it is suctioned by the metering pump  2  via a suction conduit  5 . In the shown example, in each case a return valve  6  which is an essential constituent of the membrane pump, is arranged in the known manner in front of and behind the metering pump  2 . Thus the return valve  6  which in the flow direction is situated in front of the pump, ensures that the reduction agent is not delivered back into the reduction agent tank  4  with a pump stroke. The return valve  6  lying behind the metering pump  2  in the flow direction conversely ensures that on suctioning, reduction agent is suctioned only from the reduction agent tank  4 , and not suctioned back out of the pressure conduit. 
         [0032]    A return conduit  10  which leads back to the reduction agent tank  4  and serves for bleeding the system on starting operation of the metering pump  2 , branches behind the metering pump  2  and the second return valve  6  at a branching point  8 . 
         [0033]    An interruption- or shut-off valve  12 , by way of which the return conduit  10  may be closed, so that no reduction agent may flow back through the return conduit  10  to the reduction agent tank  4 , is arranged in the return conduit  10 . In the shown idle condition, the shut-off valve  12  is situated in the open position, in which the return conduit  10  is released. A pressure sensor  14  which detects the fluid pressure in front of the shut-off valve  12  and thus in the pressure conduit  16  behind the metering pump  2 , is arranged in the return conduit  10  and in the flow direction in front of the shut-off valve  12  and behind the branching point  8 . 
         [0034]    The pressure conduit  16  leads from the metering pump  2  via the branching point  8  to the mixing region or the mixing chamber  18  of a device, in which the reduction agent is impinged or mixed with pressurized gas, in this case pressurized air. A return valve  20  is arranged directly in front of the mixing region  18  in the pressure conduit  16 , i.e. at the end of the pressure conduit  16 . This return valve is held in the shown idle position in a closed position by way of biasing, for example a spring, and a backflow of reduction agent and in particular of pressurized gas from the mixing region  18  into the pressure conduit  16  is prevented. A conduit  22  which leads to the injection nozzle in the exhaust gas system of the motor vehicle connects behind the mixing region  18  in the flow direction. 
         [0035]    The pressurized air used as a pressurized gas in this shown example is made available by a pressurized air supply  24  of the motor vehicle. Such pressurized air supply systems are usually present in lorries, in particular for actuating the brakes. The shown metering pump assembly is connected to this central pressurized air supply  24 , wherein the metering pump assembly on the entry side comprises a solenoid or magnetic valve  26  which selectively connects the pressurized air conduit  28  which leads to the mixing region  18 , to the pressurized air supply  24  or to the atmosphere  30 . The electrically actuated magnetic valve  26  is biased, such that in its idle position, it is kept in the shown position in which the pressurized air conduit  28  is opened to the atmosphere  30 . 
         [0036]    A pressure regulator  32  and behind this, a throttle  34  are arranged in the pressurized air conduit  28  behind the magnetic valve  26  in the flow direction. 
         [0037]    The pressurized air conduit at the mixing chamber or the mixing region  18  ends in a return valve  36  which is biased such that it is closed in the shown idle position, and may be opened against its biasing by way of the pressure acting in the pressurized air conduit  28 . It is thus ensured, that when no pressurized air flows out of the pressurized air conduit  28  into the mixing chamber  18 , the return valve  36  is always closed, so that no reduction agent may penetrate from the mixing region  18  into the pressurized air conduit  28 . 
         [0038]    The shut-off valve  12  in the return conduit  10  is actuated by pressurized air and is connected to the pressurized air conduit  28  via an actuation conduit  38 , wherein the actuation conduit  38  is in connection with the pressurized air conduit  28  between the pressure regulator  32  and the throttle  34 . The actuation conduit  38  ensures that when the magnetic valve  26  is switched over so that the pressurized air conduit  28  is in connection with the pressurized air supply  24 , the actuation conduit  38  is also subjected to pressure. The air pressure prevailing in the actuation conduit  38  then effects a switching-over of the shut-off valve  12  against its biasing, so that the return conduit  10  is closed. 
         [0039]    The previously described elements which lie within the border indicated by the dashed line D in  FIG. 1 , i.e. in particular the metering pump  2 , the shut-off valve  12 , the magnetic valve  26 , the mixing point  18 , the return valves and the conduits connecting these components, are all integrated into the metering pump assembly, so that the metering pump assembly apart from electrical connections to the outside, only has four fluid connections, specifically for the connection to the pressurized air supply  24 , for the connection of the conduit  22  leading to the injection nozzle, for the connection of the suction conduit  5  leading to the reduction agent tank  4 , and for the connection of the return conduit  10  to the reduction agent tank  4 . 
         [0040]    On starting operation of the system, the magnetic valve  26  firstly remains in its closed idle position (shown position) in which the pressurized air conduit  28  is separated from the pressurized air supply  24 . Firstly the metering pump  2  is set into operation, which via the suction conduit  5  suctions reduction agent out of the reduction agent tank  4 . The biasing of the return valve  20  in the shown closed position is selected such that with the return conduit  10  opened, the pressure in the pressure conduit  16  is not sufficient to open the return valve against the biasing. 
         [0041]    Since firstly no pressure is present in the pressurized air conduit  28 , the actuation conduit  38  is at first also without pressure, so that the shut-off valve  12  remains in its opened idle position, and the return conduit  10  is opened. In this manner, the metering pump  2  firstly delivers reduction agent from the reduction agent tank  4  and via the branching point  8  through the return conduit  10  back into the reduction agent tank  4 . This serves for bleeding the system on starting operation, i.e. firstly of ensuring that the pressure conduit  15  is completely filled with reduction agent. 
         [0042]    If the pressure conduit  16  and the return conduit  10  are completely filled with reduction agent, the fluid pressure in the pressure conduit  16  and in the return conduit  10  in front of the shut-off valve  12  reaches a certain limit value, which is detected by the pressure sensor  14 . When this limit value is detected by the pressure sensor  14 , a control switches over the magnetic valve  26 , so that the pressurized air conduit  28  is supplied with pressurized air via the pressurized air supply  24  of the lorry. The actuation conduit  38  is also impinged with pressure by way of this, by which means the shut-off valve  12  is switched over against the spring biasing, and the return conduit  10  is closed in this manner. Since now the pressure conduit  16  is no longer open to the reduction agent tank  4  via the return conduit  10 , on further operation of the metering pump  2 , the fluid pressure in the pressure conduit  16  increases to such an extent, that the pressure is sufficient to open the return valve  20  against its spring biasing, so that the reduction agent may flow into the mixing region  18  and is impinged there with pressurized air from the pressurized air conduit  28 . The pressurized air and the reduction agent then together flow through the conduit  22  to an injection nozzle in the exhaust gas conduit of the lorry. 
         [0043]    On operation, the quantity of the supplied reduction agent may be set by way of the number of pump strokes. The pressurized air flow through the pressurized air conduit  28  into the mixing region  18  is thereby constant. 
         [0044]    If the installation is taken out of operation, in particular on switching off the vehicle, firstly the metering pump  2  is switched off, so that reduction agent may no longer be delivered from the reduction agent tank  4 . The pressure in the pressure conduit  16 , by way of this reduces to such an extent, that the return valve  20  closes on account of its biasing, and prevents further reduction agent from penetrating into the mixing region  18 . Since the magnetic valve  26  at first continues to be open, pressurized air continues to flow through the return valve  36  into the mixing region  18 , and flushes out the reduction agent residues which are still present, via the conduit  22 . 
         [0045]    If then the magnetic valve  26  is closed by way of switching off the current supply, the pressurized air flow through the pressurized air conduit  28  and the return valve  36  is also switched off, so that the whole system is taken out of operation. In this condition, the shut-off valve  12  again switches back into its idle position, i.e. the return conduit  10  is opened. 
         [0046]    On account of the arrangement of the return valve  20 , it is ensured that no air may penetrate from the mixing chamber or the mixing region  18  into the pressure conduit  16 . Thus one may prevent a crystallization of the reduction agent in the pressure conduit  16 . Since furthermore after switching off the metering pump  2 , the mixing region  18  is automatically flushed out by the constant flow of pressurized air in this, one may also prevent a crystallization of reduction agent in the mixing region  18  and in the connecting conduit  22 . 
         [0047]    The pressure sensor  14  which preferably emits an electrical signal, apart from detecting the complete bleeding of the pressure conduit  16 , also serves for the recognition of further undesired operating conditions. Thus one may recognize a blocked return conduit  10  by way of the pressure sensor  14 , specifically when, with an opened shut-off valve, the pressure exceeds a predefined value which may not normally occur with an opened return conduit  10 . The pressure sensor  14  may also detect that the injection nozzle in the exhaust gas conduit of the vehicle is blocked. Then, specifically the pressure in the pressure conduit  16  given an opened magnetic valve  26  likewise increases above a predefined limit value, which may not normally occur with a correctly functioning injection nozzle. Furthermore, one may also detect whether the reduction agent tank  4  is empty by way of the pressure sensor  14 . Then specifically, on operation, the pressure in the pressure conduit  16  sinks below a predefined limit value, which may not normally occur in normal operation with a closed return conduit  10 . 
         [0048]    An exemplary construction of the premixing device essentially consisting of the mixing region  18  and the return valves  20  and  36 , is described hereinafter by way of  FIGS. 2 to 5 . 
         [0049]      FIG. 2  shows a sectioned view of the premixing device  39  in a condition installed into a pump head. The pump head is formed essentially by a central plate  40  and an end-plate  42  bearing on this, wherein flow channels are designed and the premixing device  39  arranged between the plates  40  and  42 . 
         [0050]    The conduit  22  in the end plate  42  is designed as a connection, to which a fluid conduit which leads to an injection nozzle in the exhaust gas system of the vehicle may be connected. The pressure conduit  16  as well as the pressurized air conduit  28  in the form of channels in the surface and through-holes connecting thereto, are formed in the central plate  40 . 
         [0051]    The premixing device as a central component comprises a cylindrical bush  44  with a cylindrical inner wall  46 . A necking  48  which divides the inner space of the bush  46  into two parts, is formed in the inside of the bush  44 . The first part of the inner space, proceeding from the necking  48 , widens towards a first end-side  50  of the bush  44  in a funnel-like manner. This region is the actual mixing region  18  or the mixing chamber  18  of the premixing device  39 . Recesses or openings  52  which serve as entry openings for the pressurized air are formed in the peripheral wall of the mixing region  18  distributed uniformly over the periphery. The opening surrounded by the necking  48 , in the inside of the bush  44 , serves as an entry opening for the reduction agent into the mixing region  18 . This region is closed by way of a piston  54  with an O-ring  56  inserted into a peripheral groove. Thereby, the O-ring  56 , as shown in the  FIGS. 3 and 4 , may come to bear on the funnel-like inner wall of the mixing region  18  in the inside of the bush  44  in a sealing manner. 
         [0052]    The piston  54  extends with the piston rod  55  through the necking  48  into the second region in the inside of the bush  44  to the second end-side  58  which is distant to the first end-side  50 . A compression spring which with its first end is supported on the necking  48 , is arranged in the second part of the inner space of the bush  44 . The opposite end of the compression spring  60  which is designed as a helical spring, on a guide bush  62  surrounding the piston rod  55 , bears on a shoulder which faces the necking  48 . The guide bush  62  leads the piston rod and thus the piston  54  in the inside of the bush  44 , in which it bears on the inner wall of the bush  44 . The guide bush  62  is supported via a spring ring  64  on the longitudinal end of the piston rod  55  which is distant to the piston  54 . Thus the compression spring  60  presses the piston rod  55  in the direction of the second end-side of the bush  40 , so that the piston  54  is pressed with the O-ring  56  against the funnel-like or conical inner wall of the mixing region  18 . In this manner, the piston  54  which forms the return valve  20  in  FIG. 1 , is kept in its idle position in the closed condition, as is shown in  FIGS. 3 and 4 . 
         [0053]    The guide bush on its outer periphery comprises longitudinal grooves  66  (not shown in the  FIGS. 3 to 5 ), through which reduction agent may flow through the pressure conduit  16  into the inside of the bush  44  to the necking  48 . One prevents reduction agent from flowing past the bush  44  to the outside, by way of the O-ring  58  surrounding the bush  44  at the outside. In the inserted condition, the O-ring  68  seals the outer wall of the bush  44  with respect to the inner wall of the recess, in which the bush  44  is arranged in the central plate  40 . 
         [0054]    The fluid pressure in the pressure conduit  16 , in the inside of the bush  44  acts on the piston  54  in the direction of the longitudinal axis of the piston rod  55 . With an adequately high fluid pressure in the pressure conduit  16 , the force acting on the piston  54  by way of the pressure, exceeds the spring force of the compression spring  60 , so that the piston  54  with the piston rod  55  is displaced in the direction of the first end-side  50  of the bush  44 , and the piston  54  with the O-ring  56  lifts from the conical inner wall of the mixing region  18 , as is shown in the  FIGS. 2 and 5 . Thus, an annular gap arises between the piston  54  or the O-ring  56  and the surrounding inner wall of the bush  44  or the mixing region  18  respectively, through which the reduction agent may flow into the mixing region  18 . 
         [0055]    The second return valve  36  of the premixing device  39  is formed by an annular, elastic sleeve  70  which is clamped between the central plate  40  and the end-plate  42 . Thereby, in particular, a thickened region at the outer periphery of the sleeve  70  comes to bear on the central plate  40  as well as on the end-plate  42 , so that the pressurized air from the pressurized air conduit  28  may not flow past the outer periphery of the collar  70 . 
         [0056]    The sleeve  70  at its outer periphery is extended in a sleeve-like manner towards the end-side  50  of the bush  44  in the axial direction, so that a collar  72  is formed. This collar  72  extends inclined in a slightly conical manner to the outer wall  46  of the bush  44 , and comes to bear on this with its free end-side. Thereby, the sleeve  70  or the collar  72  are designed in an elastic manner, such that the sleeve in its idle position is sealingly held on the outer wall  46  of the bush  44 , as is shown in the  FIGS. 2 and 3 . 
         [0057]    If pressurized air is introduced into the pressurized air conduit  28 , the pressurized air in the central plate  40 , firstly on the outer periphery  46  flows around the whole bush  44 , since the recess accommodating the bush  44 , in the central plate  40 , in the region distant to the pressure conduit  16 , is larger than the outer diameter of the bush  44 . The pressurized air then flows into the region between the collar  72  of the sleeve  70 , and the outer wall  46  of the bush  44 , wherein by way of the air pressure, the collar  72  is pressed away from the outer wall of the bush  44 , so that an annular gap  74  arises between the outer wall  46  and the inner periphery of the sleeve  70  or the collar  72 , through which the pressurized air may flow into the recess  76  in which the bush  44  is arranged in the end-plate  42 . The pressurized air then flows from the recess  46  through the recesses or openings  52  into the mixing region  18 , and from there, together with the supplied reduction agent, flows through the conduit or connection  22  further to the injection nozzle in the exhaust gas system of the vehicle. 
         [0058]    If the supply of pressurized air in the pressurized air conduit  28  is switched off, the sleeve  70  with its collar  72  again is sealingly applied onto the outer wall  46  of the bush  44  on account of its elasticity. On account of the collar  72  of the sleeve  70  projecting into the recess  76 , a higher pressure in the recess  76  succeeds in pressing the sleeve-like extension or the collar  72  of the sleeve  70  against the outer wall  46  to an even greater extent, and thus in securely closing the return valve  36 . 
         [0059]    The recesses or openings  52  are designed such that they extend in the longitudinal direction of the bush  44  up to the outer side of the piston  54 . Furthermore, the recesses  52  are shaped such that they widen towards the inside of the bush  44 , i.e. towards the mixing region  18 . By way of this, one succeeds in pressurized air which flows through the recesses  52  into the mixing region  18 , completely flowing over the whole mixing region  18  at its inner wall and in particular also the outer side of the piston  54 , so that reduction agent residues may be completely flushed out of the mixing region  18 . 
         [0060]    The construction of the shut-off valve  12  is hereinafter described in more detail by way of the  FIGS. 6 and 7 . The shut-off valve  12  is arranged in the central plate  40  in a recess  78 . The recess  78  is formed in the surface of the central plate  40  which is distant to the end-plate  42 , and is closed by the front plate  80  of a drive housing of the metering pump assembly, to which the central plate  40  is attached in a flat manner. 
         [0061]    The recess  78  on its base is formed in a cylindrical manner, and opens towards the front plate  80  in a funnel-like manner. An inlet connection piece  82  extends from the base of the recess  78  into the recess  78 , centrally in the cylindrical section. The return conduit  10  branching from the pressure conduit  8  runs in the inside of the inlet connection piece  82 , i.e. concentrically to this, such that it is open to the end-side of the inlet connection piece  82 . The end-side of the inlet connection piece  82  which is distant to the base of the recess  78  thus forms a valve seat  84 , on which a valve element  86  designed in a membrane-like manner sealingly bears in the closed condition, which is shown in  FIG. 6 . The valve element  86  is designed as a circular membrane, which at its outer periphery  88  is held between the surfaces of the central plate  40  and of the front plate  80 , which are adjacent to one another. The central region of the valve membrane  86  is movable with respect to the periphery  88  in the direction of extension of the inlet connection piece  82 , which is ensured by the elasticity of the membrane. 
         [0062]    The valve membrane or the valve element  86  comprises a carrier  90  which is enclosed or peripherally injected by an elastic material  92 , which also defines the sealing surface  94  coming to bear on the valve seat  84 . 
         [0063]    A guide bush  96  which comprises openings  92  in its peripheral wall, surrounding the sealing surface  94  and proceeding from the valve element  86 , extends concentrically to the inlet connection piece  92 . The guide bush  96  is integrally connected to the elastic material  92 , and via this, to the carrier  90  of the valve element  86 . Preferably, the carrier  90  and the guide bush  96  are peripherally injected with the elastic material  92  and thus connected to one another with a positive fit. 
         [0064]    A compression spring  100  in the form of a helical spring is arranged or guided in the inside of the guide bush  96 , so that the compression spring  100  extends parallel to the longitudinal axis of the inlet connection piece  82  between its outer periphery and the inner periphery of the guide bush  96 . The compression spring  100  with a longitudinal end is supported on the base of the recess  78 , and with the opposite longitudinal end is supported on the valve element  86  at the periphery of the sealing surface  94 . The compression spring  100  is dimensioned such that it presses the valve element  96  into its opened position, i.e. its position distanced to the valve seat  84 , which is shown in  FIG. 7 . In this position of the shut-off valve  12  shown in  FIG. 7 , the reduction agent which is delivered by the metering pump into the return conduit  10 , may flow through the inlet connection piece  82  and through the annular gap between the sealing surface  94  and the valve seat  84 , into the inside of the guide bush  96 . The reduction agent may flow through the opened end-side distant to the valve element  86 , as well as the openings  98  of the guide bush  96 , into the recess  78 . The reduction agent from the recess  78 , flows through a channel  102  opening at the periphery of the recess  78 , to a connection piece of the metering pump assembly, and from there further through the return conduit to the reduction agent tank  4 . 
         [0065]    In order to close the shut-off valve  12 , the membrane-like valve element  86  is impinged with the pressurized air from the pressurized air conduit  28 , from its side distant to the inlet connection piece  82 , via the actuation conduit  38 . The air pressure acting on the surface  104  of the valve element  86  moves the valve element  86  against the spring force of the compression spring  100  in the direction of the longitudinal axis of the inlet connection piece  82  to this, so that the valve element  86  with its sealing surface  84  comes to bear on the valve seat  84  in a sealing manner. In this condition shown in  FIG. 6 , no reduction agent may flow out of the inlet connection piece  82  into the inside of the recess  78 , so that the return conduit  10  is closed or interrupted by the shut-off valve  12 . 
         [0066]    The actuation conduit  38  through which the pressurized air flows for impinging the surface  104 , is designed as a channel in the inside of the central plate  40 , which opens out between the central plate  40  and the front plate  80 , in the region of the surface  104  of the valve element  86 . For this, in the shown example, an open annular channel  106  which faces the valve element  86  and in which the pressurized air may distribute, is formed in the front plate  80 , so that the pressurized air acts uniformly on the whole surface  104 . Furthermore, the surface  104  is designed in a curved manner, such that in an annular region bordering the peripheral region  88  in a radially inner-lying manner, it is formed distanced to the plane of the surface of the central plate  40 . 
         [0067]    The opposite central region  108  on the surface  104  of the sealing surface  94 , is designed as an abutment surface, which in the opened condition (see  FIG. 7 ) of the shut-off valve  12  comes to bear on the surface of the front plate  80 , and thus limits the path of the valve element  86  in the opened position. The central region  108  centrically comprises a projection, which for guiding the valve element, engages into a hole in the surface of the front plate  80 . 
         [0068]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims. 
         [0069]    The invention relates to a metering pump assembly for admixing a fluid reduction agent into an exhaust gas flow. 
         [0070]    With self-ignition internal combustion engines, nitrogen oxides arise on operation with excess oxygen, which is the case in the majority of operating conditions, and specifically in particular with direct injection into the main combustion space, as is typically the case with diesel motors. In order to reduce these nitrogen oxide emissions, it is known to lead the exhaust gas flow to a reduction catalyser. Thereby, as a reduction agent, usually an aqueous urea solution is led in a finely distributed manner to the exhaust gas before entry into the catalyser. Thereby, the supplied quantity of urea is to be matched as precisely as possible to the combustion process, in order on the one hand to ensure an as complete as possible reduction within the catalyser, and on the other hand to prevent an excess of urea. 
         [0071]    A special metering pump assembly for metering and feeding reduction agent to the exhaust gas flow is known from EP 1 435 458 A1. With this metering pump assembly, the supplied reduction agent, before feeding to the exhaust gas flow, is mixed or impinged with pressurised air in the metering pump assembly. In order to be able to firstly bleed the system on starting operation, a 3/2-way valve as a preflushing valve is arranged in front of a premixing device, in which the mixing of the reduction agent with pressurised air is effected. On starting operation of the assembly, this preflushing valve is firstly switched such that the metering pump conveys the aspirated reduction agent back into the reduction agent tank, until the conduits are completely filled with reduction agent. The premixing valve is then switched over, so that the reduction agent is supplied to the premixing device. 
         [0072]    The object of the present invention is to simplify the construction of the metering pump assembly known from EP 1 435 458 A1. 
         [0073]    This object is achieved by a metering pump assembly with the features specified in claim  1 . Preferred embodiments are to be deduced from the dependent claims. 
         [0074]    According to the invention, the desired simplification of the construction of the metering pump assembly results on account of the following construction. The metering pump assembly for admixing a fluid reduction agent into an exhaust gas flow comprises a metering pump which on the entry side is connected to a connection for connecting to a reduction agent tank, and at the exit side is connected to a pressure conduit as well as a return conduit to the reduction agent tank. Connections may be provided on the metering pump assembly, for the suction conduit as well as the return conduit to the reduction agent tank, to which conduits or flexible tubing which lead to the reduction agent tank are connected. The pressure conduit which extends behind the metering pump in the flow direction, is part of the metering pump assembly, and for example leads to a premixing device in which the delivered reduction agent is impinged with pressurised gas. The return conduit branches away from the pressure conduit and is in connection with this. According to the invention, a return valve which is biased against the fluid pressure produced by the metering pump, is arranged in the pressure conduit. This means that the biasing retains the return valve closed against the fluid pressure produced by the metering pump, as long as the fluid pressure does not exceed a certain pressure value, at which the force acting on the return valve on account of the fluid pressure corresponds to the biasing force of the return valve. If the fluid pressure exceeds this limit value, the return valve is opened against its biasing by way of the delivered reduction agent. The return conduit is connected to the pressure conduit in front of the return valve in the flow direction of the reduction agent. Furthermore, a first shut-off valve for the selective closure of the return conduit is arranged in the return conduit. One may make do without a complicated 3/2-way valve for bleeding and filling the system by way of this arrangement, and instead of this, provide a simple switching- or shut-off valve in the return conduit. 
         [0075]    On starting operation of the metering pump assembly, the metering pump, for example a membrane pump, firstly delivers reduction agent out of the reduction agent tank and pumps this through the return conduit back into the tank, until the metering pump and the pressure conduit are completely filled with reduction agent. Since the provided return valve is biased, as long as the fluid pressure of the reduction agent does not reach the pressure value for opening the return valve, one may ensure that the reduction agent does not flow further for example to a premixing device and in particular into the exhaust gas system of the combustion motor, but firstly through the opened shut-off valve and the return conduit back into the reduction agent tank. After the complete filling of the system with reduction agent, the return conduit may be closed by the shut-off valve, so that the reduction agent may no longer flow back into the reduction agent tank. If then the fluid pressure in the pressure conduit is increased, the return valve opens and the reduction agent may, as the case may be, via a premixing device, be delivered into the exhaust gas system of the combustion engine. 
         [0076]    In order to ensure the opening of the return valve at the correct point in time, the biasing force which keeps the return valve closed, is preferably larger than the force acting on the return valve on account of the fluid pressure with an opened return conduit. By way of this, it is ensured that no reduction agent may flow through the return valve during the preflushing or filling. Further preferably, the biasing force is selected such that with a closed return conduit, i.e. when the shut-off valve blocks the return conduit, the force acting on the return valve due to the fluid pressure exceeds the biasing force of the return valve, so that this is opened and the reduction agent may flow through the return valve. If after closure of the return conduit, the metering pump continues to deliver reduction agent, the pressure in the pressure conduit automatically increases downstream behind the metering pump, so that when the biasing force of the return valve is selected in a suitable manner, this is automatically opened. Since reduction agent may flow away out of the pressure conduit with an opened return conduit, a reduction of the pressure in the pressure conduit automatically occurs, so that the return valve is automatically closed in this condition with a suitably selected biasing force. The return valve may for example be biased by a compression spring, for example a helical spring, whose spring force is matched to the fluid pressure occurring in the pressure conduit, such that the automatic closure and opening of the return valve, as previously described, is ensured. 
         [0077]    It is further preferable to arrange the return valve directly on a mixing chamber of a premixing device, in which the reduction agent delivered by the metering pump is impinged or mixed with a pressurised gas. With this arrangement, the return valve may assume a double function. On the one hand the return valve, as previously described, on bleeding or preflushing the metering pump, ensures that the reduction agent firstly does not yet flow further into the premixing device and the exhaust gas system of the combustion engine. On the other hand, the return valve directly on the premixing device ensures that no pressurised gas, in particular pressurised air, with the reduction agent, may flow back into the pressure conduit. A return flow of reduction agent, and in particular a reduction agent/pressurised gas mixture is undesirable, since for example urea tends to crystallize in contact with air. For this reason, the return valve is preferably arranged directly on the mixing region, in which the pressurised gas is mixed with the reduction agent, so that the region located downstream of the return valve may be freed from reduction agent residues by way of the pressurised gas, and the conduit region located upstream of the return valve is securely kept held free of reduction agent and pressurised gas flowing back. The construction of the whole metering pump assembly may be considerably simplified, and simultaneously a large operational reliability are ensured, by way of this double function of the return valve. 
         [0078]    It is further preferable to arrange a pressure sensor detecting the fluid pressure, in the pressure conduit or return conduit, in front of the return valve and the shut-off valve in the flow direction. Various operating conditions of the metering pump assembly, and in particular possible faults on operation of the metering pump assembly, may be detected via this pressure sensor. Firstly, on preflushing on starting operation of the metering pump assembly, the pressure sensor may be used to determine when the pressure conduit is completely filled with reduction agent, and thus when the metering pump assembly is ready for operation. When the pressure sensor detects the pressure threshold value indicating this operation condition, the shut-off valve in the return conduit may then be closed, so that the metering pump then delivers the reduction agent via the return valve, as the case may be, via a premixing device, into the exhaust gas flow. Furthermore, the pressure sensor may be used to detect a blockage of the pressure conduit situated further downstream, and in particular of the injection nozzle in the exhaust gas flow. Then specifically the pressure in the pressure conduit increases above a pressure value occurring on normal operation, which is detected by the pressure sensor. A corresponding warning notice may be outputted e.g. by a control device, and the metering pump assembly may be switched off as the case may be. Furthermore, by way of the pressure sensor, one may also determine when the reduction agent supply in the reduction agent tank has been consumed. Specifically, the pressure in the pressure conduit on operation then sinks to below a pressure value occurring during normal operation. If accordingly, the pressure sensor detects a lower pressure limit value, then the lacking reduction agent supply may be detected and be suitably displayed e.g. via a control device. In this condition too, the metering pump assembly may be firstly automatically switched off, until the filling of the reduction agent by way of a control device, in order to avoid damage on operation without reduction agent. 
         [0079]    In order to ensure the starting operation of the metering pump assembly after the preflushing or bleeding, the first shut-off valve is advantageously coupled to the pressure sensor in a manner such that the first shut-off valve is switched over into its position closing the return conduit on reaching a predefined fluid pressure detected by the pressure sensor. The predefined limit value for the fluid pressure at which the return conduit is closed, is selected such that the complete filling of the pressure conduit with reduction agent on starting operation of the metering pump assembly may always be ensured. This means that the metering pump is firstly bled, before the metering pump assembly is started in operation, and reduction agent is injected into the exhaust gas flow. This is particularly important when the metering pump assembly is started in operation for the first time after filling the reduction agent tank. 
         [0080]    The first shut-off valve is preferably biased in a manner such that in the idle position, it is held in its position opening the return conduit. In this manner, it is ensured that the return conduit is open to the reduction agent tank when the metering pump assembly is not in operation, so that the entire system and in particular the pressure conduit are not under pressure. In this manner, one may ensure that the biased return valve securely keeps the pressure conduit closed in the switched-off condition of the metering pump assembly, so that reduction agent may not inadvertently exit from the metering pump assembly. The biasing may be effected for example by way of spring force. An actuation force which is larger than the biasing force must be mustered for the closure of the first shut-off valve. 
         [0081]    It is particularly preferable to provide a premixing device, in which the reduction agent delivered by the metering pump is impinged or mixed with a pressurised gas, and to arrange a second shut-off valve for the selective interruption of the pressurised gas supply, at the entry side in a pressurised gas conduit leading to the premixing device. The pressurised gas supply may for example be effected from a central pressurised air supply of a motor vehicle, in particular lorry. The second shut-off valve then serves for being able to switch the pressurised gas or pressurised air supply into the metering pump assembly on and off in a targeted manner. On starting operation of the metering pump assembly, the second shut-off valve is opened in order to let pressurised gas flow into the metering pump assembly, and to mix the delivered reduction agent with pressurised gas in the premixing device. Thereby, a fine mixing between the reduction agent and the pressurised gas does not need to be effected in the premixing device, but rather it is sufficient to impinge the reduction agent only with pressurised gas, so that larger reduction agent droplets are conveyed further to an injection nozzle into the exhaust gas flow by way of the pressurised gas. A finer atomisation of the reduction agent in the exhaust gas flow is effected on exit from the injection nozzle. 
         [0082]    It is particularly preferable for the first shut-off valve in the return conduit to be coupled to the second shut-off valve in the pressurised gas conduit, in a manner such that the return conduit is closed when the pressurised gas conduit is open. This permits a very simple and secure starting operation of the metering pump assembly, since both shut-off valves do not need to be switched independently of one another. On account of the coupling, it is indeed possible to actuate only one of the valves, for example the second shut-off valve, and simultaneously to automatically also switch over the other one, for example the first shut-off valve. Thus for example it is possible, by way of opening the second shut-off valve, to firstly switch on the pressurised gas supply to the metering pump assembly, wherein then the return conduit is automatically closed by way of the coupling, so that the reduction agent delivered by the metering pump amid the opening of the return valve, is then also conveyed into the premixing device, where it is mixed with the pressurised gas. Conversely, on account of the coupling on switching off the metering pump assembly, it is ensured that when the pressurised gas supply is interrupted by the closure of the second shut-off valve, the first shut-off valve in the return conduit is also automatically opened, so that the reduction agent which is delivered further by the metering pump may flow back into the reduction agent tank, or the pressure conduit is switched in a pressureless manner. In this manner, it is ensured that no reduction agent gets into the premixing device and further downstream to the injection nozzle into the exhaust gas flow, if no pressurised gas flows into the metering pump assembly. An undesirable blocking or clogging of the conduits downstream of the return valve in the pressure conduit is thus prevented. 
         [0083]    The first shut-off valve is particularly preferably actuatable by pressurised gas for coupling the two shut-off valves, wherein the pressurised gas conduit leading to the premixing device is connected to an actuation connection of the first shut-off valve, e.g. via an actuation conduit, behind the second shut-off valve in the flow direction. This permits a purely pneumatic coupling between the two shut-off valves, so that the number of the electrically actuated components in the metering pump assembly may be reduced, which leads to a further simplified construction. The arrangement permits the first shut-off valve to be closed by way of the pressure of the pressurised gas led to the premixing device, preferably against a spring bias. Thus no additional actuation elements whatsoever are required for the closure of the first shut-off valve. 
         [0084]    The actuation connection of the first shut-off valve is preferably connected to the pressurised gas conduit leading to the premixing device, in a region which in the flow direction lies on front of a throttle location in the pressurised gas conduit. In this manner, an adequately high gas pressure at the actuation connection of the first shut-off valve is ensured for its actuation, whilst the premixing device is supplied with a lower gas- or air pressure via the throttle location. 
         [0085]    It is further preferable for the second shut-off valve to be coupled to a pressure sensor arranged in the flow direction in front of the return valve and the first shut-off valve, in the pressure conduit or the return conduit, in a manner such that the second shut-off valve is switched over into its position opening the pressurised air conduit on reaching a defined fluid pressure detected by the pressure sensor. This means that on starting operation of the metering pump assembly, one may firstly switch on the metering pump, wherein the pressurised gas supply is firstly still closed by the second shut-off valve. Firstly, a preflushing or bleeding of the metering pump is effected via the return conduit which is still open. If then it is ascertained by way of the pressure sensor, that a certain pressure value has been reached in the pressure conduit, which indicates that the pressure conduit is completely filled with reduction agent, the second shut-off valve is then opened, so that the pressurised gas may flow into the metering pump assembly and to the premixing device. If then, as described previously, the first shut-off valve is also coupled to the second shut-off valve, then simultaneously the return to the reduction agent tank may be closed, so that the reduction agent is delivered into the premixing device via the return valve which then opens. The coupling of the pressure sensor and the first shut-off valve may be realized electronically via a control device but also mechanically, pneumatically or hydraulically. 
         [0086]    The second shut-off valve in the pressurised gas conduit is particularly preferably biased in a manner such that in the idle position, it is held in its position interrupting the pressurised gas conduit. In this manner, on switching off the metering pump assembly, it is always ensured that the pressurised gas supply to the metering pump assembly is interrupted. The second shut-off valve may for example be biased by way of spring force, so that it must be opened by an actuation force to be mustered externally. Thereby, the actuation force is preferably produced electrically e.g. electromagnetically. If then the current supply, e.g. the flow of current in a coil in the electromagnet, is switched off, the actuation force stops and the bias automatically closes the shut-off valve. 
         [0087]    Furthermore, a further return valve may be arranged in the premixing device on the exit side of the pressurised gas supply, and this further return valve prevents reduction agent from being able to flow from the premixing device back into the pressurised gas conduit on switching off the pressurised gas supply to the premixing device. It may thus be ensured that the pressurised gas conduit is kept free from contamination. This second return valve is preferably likewise biased, so that it opens at a certain pressure in the pressurised gas conduit, and automatically closes on falling short of this pressure value or on switching off the pressurised gas supply, for example by way of closure of the second shut-off valve. 
         [0088]    The invention is hereinafter described by way of example and the attached figures. In these there are shown in: 
         [0089]      FIG. 1  a circuit diagram of the hydraulic components of a metering pump assembly according to the invention, 
         [0090]      FIG. 2  in a sectioned cut-out, the premixing device of a metering pump assembly according to  FIG. 1 , 
         [0091]      FIG. 3  in a separate sectional view, the premixing device according to  FIG. 2 , with closed return valves, 
         [0092]      FIG. 4  the premixing device according to  FIG. 3 , with an opened return valve for the supply of pressurised gas, 
         [0093]      FIG. 5  the premixing device according to  FIGS. 3 and 4  with an opened return valve for the supply of reduction agent, and an opened return valve for the supply of pressurised gas, 
         [0094]      FIG. 6  a sectioned view of the shut-off valve in the return conduit, in the closed condition and 
         [0095]      FIG. 7  a sectioned view of the shut-off valve according to  FIG. 6 , in the opened condition. 
         [0096]    The basic function of one embodiment example of a metering pump assembly according to the invention is firstly described by way of the circuit diagram in  FIG. 1 . 
         [0097]    The core piece of the metering pump assembly is a metering pump  2  which is designed as a membrane pump, with an associated drive. By way of the control of the drive, and in particular the control of the number of pump strokes and/or the stroke speed, one may set the quantity of the reduction agent delivered by the metering pump  2 , in order to be able to adapt the reduction agent quantity exactly to the requirements with regard to the currently occurring combustion process in the motor. An aqueous urea solution is preferably applied as a reduction agent. The reduction agent is kept ready in a reduction agent tank  4  from which it is suctioned by the metering pump  2  via a suction conduit  5 . In the shown example, in each case a return valve  6  which is an essential constituent of the membrane pump, is arranged in the known manner in front of and behind the metering pump  2 . Thus the return valve  6  which in the flow direction is situated in front of the pump, ensures that the reduction agent is not delivered back into the reduction agent tank  4  with a pump stroke. The return valve  6  lying behind the metering pump  2  in the flow direction conversely ensures that on suctioning, reduction agent is suctioned only from the reduction agent tank  4 , and not suctioned back out of the pressure conduit. 
         [0098]    A return conduit  10  which leads back to the reduction agent tank  4  and serves for bleeding the system on starting operation of the metering pump  2 , branches behind the metering pump  2  and the second return valve  6  at a branching point  8 . 
         [0099]    An interruption- or shut-off valve  12 , by way of which the return conduit  10  may be closed, so that no reduction agent may flow back through the return conduit  10  to the reduction agent tank  4 , is arranged in the return conduit  10 . In the shown idle condition, the shut-off valve  12  is situated in the open position, in which the return conduit  10  is released. A pressure sensor  14  which detects the fluid pressure in front of the shut-off valve  12  and thus in the pressure conduit  16  behind the metering pump  2 , is arranged in the return conduit  10  and in the flow direction in front of the shut-off valve  12  and behind the branching point  8 . 
         [0100]    The pressure conduit  16  leads from the metering pump  2  via the branching point  8  to the mixing region or the mixing chamber  18  of a device, in which the reduction agent is impinged or mixed with pressurised gas, in this case pressurised air. A return valve  20  is arranged directly in front of the mixing region  18  in the pressure conduit  16 , i.e. at the end of the pressure conduit  16 . This return valve is held in the shown idle position in a closed position by way of biasing, for example a spring, and a backflow of reduction agent and in particular of pressurised gas from the mixing region  18  into the pressure conduit  16  is prevented. A conduit  22  which leads to the injection nozzle in the exhaust gas system of the motor vehicle connects behind the mixing region  18  in the flow direction. 
         [0101]    The pressurised air used as a pressurised gas in this shown example is made available by a pressurised air supply  24  of the motor vehicle. Such pressurised air supply systems are usually present in lorries, in particular for actuating the brakes. The shown metering pump assembly is connected to this central pressurised air supply  24 , wherein the metering pump assembly on the entry side comprises a solenoid or magnetic valve  26  which selectively connects the pressurised air conduit  28  which leads to the mixing region  18 , to the pressurised air supply  24  or to the atmosphere  30 . The electrically actuated magnetic valve  26  is biased, such that in its idle position, it is kept in the shown position in which the pressurised air conduit  28  is opened to the atmosphere  30 . 
         [0102]    A pressure regulator  32  and behind this, a throttle  34  are arranged in the pressurised air conduit  28  behind the magnetic valve  26  in the flow direction. 
         [0103]    The pressurised air conduit at the mixing chamber or the mixing region  18  ends in a return valve  36  which is biased such that it is closed in the shown idle position, and may be opened against its biasing by way of the pressure acting in the pressurised air conduit  28 . It is thus ensured, that when no pressurised air flows out of the pressurised air conduit  28  into the mixing chamber  18 , the return valve  36  is always closed, so that no reduction agent may penetrate from the mixing region  18  into the pressurised air conduit  28 . 
         [0104]    The shut-off valve  12  in the return conduit  10  is actuated by pressurised air and is connected to the pressurised air conduit  28  via an actuation conduit  38 , wherein the actuation conduit  38  is in connection with the pressurised air conduit  28  between the pressure regulator  32  and the throttle  34 . The actuation conduit  38  ensures that when the magnetic valve  26  is switched over so that the pressurised air conduit  28  is in connection with the pressurised air supply  24 , the actuation conduit  38  is also subjected to pressure. The air pressure prevailing in the actuation conduit  38  then effects a switching-over of the shut-off valve  12  against its biasing, so that the return conduit  10  is closed. 
         [0105]    The previously described elements which lie within the border indicated by the dashed line D in  FIG. 1 , i.e. in particular the metering pump  2 , the shut-off valve  12 , the magnetic valve  26 , the mixing point  18 , the return valves and the conduits connecting these components, are all integrated into the metering pump assembly, so that the metering pump assembly apart from electrical connections to the outside, only has four fluid connections, specifically for the connection to the pressurised air supply  24 , for the connection of the conduit  22  leading to the injection nozzle, for the connection of the suction conduit  5  leading to the reduction agent tank  4 , and for the connection of the return conduit  10  to the reduction agent tank  4 . 
         [0106]    On starting operation of the system, the magnetic valve  26  firstly remains in its closed idle position (shown position) in which the pressurised air conduit  28  is separated from the pressurised air supply  24 . Firstly the metering pump  2  is set into operation, which via the suction conduit  5  suctions reduction agent out of the reduction agent tank  4 . The biasing of the return valve  20  in the shown closed position is selected such that with the return conduit  10  opened, the pressure in the pressure conduit  16  is not sufficient to open the return valve against the biasing. 
         [0107]    Since firstly no pressure is present in the pressurised air conduit  28 , the actuation conduit  38  is at first also without pressure, so that the shut-off valve  12  remains in its opened idle position, and the return conduit  10  is opened. In this manner, the metering pump  2  firstly delivers reduction agent from the reduction agent tank  4  and via the branching point  8  through the return conduit  10  back into the reduction agent tank  4 . This serves for bleeding the system on starting operation, i.e. firstly of ensuring that the pressure conduit  15  is completely filled with reduction agent. 
         [0108]    If the pressure conduit  16  and the return conduit  10  are completely filled with reduction agent, the fluid pressure in the pressure conduit  16  and in the return conduit  10  in front of the shut-off valve  12  reaches a certain limit value, which is detected by the pressure sensor  14 . When this limit value is detected by the pressure sensor  14 , a control switches over the magnetic valve  26 , so that the pressurised air conduit  28  is supplied with pressurised air via the pressurised air supply  24  of the lorry. The actuation conduit  38  is also impinged with pressure by way of this, by which means the shut-off valve  12  is switched over against the spring biasing, and the return conduit  10  is closed in this manner. Since now the pressure conduit  16  is no longer open to the reduction agent tank  4  via the return conduit  10 , on further operation of the metering pump  2 , the fluid pressure in the pressure conduit  16  increases to such an extent, that the pressure is sufficient to open the return valve  20  against its spring biasing, so that the reduction agent may flow into the mixing region  18  and is impinged there with pressurised air from the pressurised air conduit  28 . The pressurised air and the reduction agent then together flow through the conduit  22  to an injection nozzle in the exhaust gas conduit of the lorry. 
         [0109]    On operation, the quantity of the supplied reduction agent may be set by way of the number of pump strokes. The pressurised air flow through the pressurised air conduit  28  into the mixing region  18  is thereby constant. 
         [0110]    If the installation is taken out of operation, in particular on switching off the vehicle, firstly the metering pump  2  is switched off, so that reduction agent may no longer be delivered from the reduction agent tank  4 . The pressure in the pressure conduit  16 , by way of this reduces to such an extent, that the return valve  20  closes on account of its biasing, and prevents further reduction agent from penetrating into the mixing region  18 . Since the magnetic valve  26  at first continues to be open, pressurised air continues to flow through the return valve  36  into the mixing region  18 , and flushes out the reduction agent residues which are still present, via the conduit  22 . 
         [0111]    If then the magnetic valve  26  is closed by way of switching off the current supply, the pressurised air flow through the pressurised air conduit  28  and the return valve  36  is also switched off, so that the whole system is taken out of operation. In this condition, the shut-off valve  12  again switches back into its idle position, i.e. the return conduit  10  is opened. 
         [0112]    On account of the arrangement of the return valve  20 , it is ensured that no air may penetrate from the mixing chamber or the mixing region  18  into the pressure conduit  16 . Thus one may prevent a crystallization of the reduction agent in the pressure conduit  16 . Since furthermore after switching off the metering pump  2 , the mixing region  18  is automatically flushed out by the constant flow of pressurised air in this, one may also prevent a crystallization of reduction agent in the mixing region  18  and in the connecting conduit  22 . 
         [0113]    The pressure sensor  14  which preferably emits an electrical signal, apart from detecting the complete bleeding of the pressure conduit  16 , also serves for the recognition of further undesired operating conditions. Thus one may recognize a blocked return conduit  10  by way of the pressure sensor  14 , specifically when, with an opened shut-off valve, the pressure exceeds a predefined value which may not normally occur with an opened return conduit  10 . The pressure sensor  14  may also detect that the injection nozzle in the exhaust gas conduit of the vehicle is blocked. Then, specifically the pressure in the pressure conduit  16  given an opened magnetic valve  26  likewise increases above a predefined limit value, which may not normally occur with a correctly functioning injection nozzle. Furthermore, one may also detect whether the reduction agent tank  4  is empty by way of the pressure sensor  14 . Then specifically, on operation, the pressure in the pressure conduit  16  sinks below a predefined limit value, which may not normally occur in normal operation with a closed return conduit  10 . 
         [0114]    An exemplary construction of the premixing device essentially consisting of the mixing region  18  and the return valves  20  and  36 , is described hereinafter by way of  FIGS. 2 to 5 . 
         [0115]      FIG. 2  shows a sectioned view of the premixing device  39  in a condition installed into a pump head. The pump head is formed essentially by a central plate  40  and an end-plate  42  bearing on this, wherein flow channels are designed and the premixing device  39  arranged between the plates  40  and  42 . 
         [0116]    The conduit  22  in the end plate  42  is designed as a connection, to which a fluid conduit which leads to an injection nozzle in the exhaust gas system of the vehicle may be connected. The pressure conduit  16  as well as the pressurised air conduit  28  in the form of channels in the surface and through-holes connecting thereto, are formed in the central plate  40 . 
         [0117]    The premixing device as a central component comprises a cylindrical bush  44  with a cylindrical inner wall  46 . A necking  48  which divides the inner space of the bush  46  into two parts, is formed in the inside of the bush  44 . The first part of the inner space, proceeding from the necking  48 , widens towards a first end-side  50  of the bush  44  in a funnel-like manner. This region is the actual mixing region  18  or the mixing chamber  18  of the premixing device  39 . Recesses or openings  52  which serve as entry openings for the pressurised air are formed in the peripheral wall of the mixing region  18  distributed uniformly over the periphery. The opening surrounded by the necking  48 , in the inside of the bush  44 , serves as an entry opening for the reduction agent into the mixing region  18 . This region is closed by way of a piston  54  with an O-ring  56  inserted into a peripheral groove. Thereby, the O-ring  56 , as shown in the  FIGS. 3 and 4 , may come to bear on the funnel-like inner wall of the mixing region  18  in the inside of the bush  44  in a sealing manner. 
         [0118]    The piston  54  extends with the piston rod  55  through the necking  48  into the second region in the inside of the bush  44  to the second end-side  58  which is distant to the first end-side  50 . A compression spring which with its first end is supported on the necking  48 , is arranged in the second part of the inner space of the bush  44 . The opposite end of the compression spring  60  which is designed as a helical spring, on a guide bush  62  surrounding the piston rod  55 , bears on a shoulder which faces the necking  48 . The guide bush  62  leads the piston rod and thus the piston  54  in the inside of the bush  44 , in which it bears on the inner wall of the bush  44 . The guide bush  62  is supported via a spring ring  64  on the longitudinal end of the piston rod  55  which is distant to the piston  54 . Thus the compression spring  60  presses the piston rod  55  in the direction of the second end-side of the bush  40 , so that the piston  54  is pressed with the O-ring  56  against the funnel-like or conical inner wall of the mixing region  18 . In this manner, the piston  54  which forms the return valve  20  in  FIG. 1 , is kept in its idle position in the closed condition, as is shown in  FIGS. 3 and 4 . 
         [0119]    The guide bush on its outer periphery comprises longitudinal grooves  66  (not shown in the  FIGS. 3 to 5 ), through which reduction agent may flow through the pressure conduit  16  into the inside of the bush  44  to the necking  48 . One prevents reduction agent from flowing past the bush  44  to the outside, by way of the O-ring  58  surrounding the bush  44  at the outside. In the inserted condition, the O-ring  68  seals the outer wall of the bush  44  with respect to the inner wall of the recess, in which the bush  44  is arranged in the central plate  40 . 
         [0120]    The fluid pressure in the pressure conduit  16 , in the inside of the bush  44  acts on the piston  54  in the direction of the longitudinal axis of the piston rod  55 . With an adequately high fluid pressure in the pressure conduit  16 , the force acting on the piston  54  by way of the pressure, exceeds the spring force of the compression spring  60 , so that the piston  54  with the piston rod  55  is displaced in the direction of the first end-side  50  of the bush  44 , and the piston  54  with the O-ring  56  lifts from the conical inner wall of the mixing region  18 , as is shown in the  FIGS. 2 and 5 . Thus, an annular gap arises between the piston  54  or the O-ring  56  and the surrounding inner wall of the bush  44  or the mixing region  18  respectively, through which the reduction agent may flow into the mixing region  18 . 
         [0121]    The second return valve  36  of the premixing device  39  is formed by an annular, elastic sleeve  70  which is clamped between the central plate  40  and the end-plate  42 . Thereby, in particular, a thickened region at the outer periphery of the sleeve  70  comes to bear on the central plate  40  as well as on the end-plate  42 , so that the pressurised air from the pressurised air conduit  28  may not flow past the outer periphery of the collar  70 . 
         [0122]    The sleeve  70  at its outer periphery is extended in a sleeve-like manner towards the end-side  50  of the bush  44  in the axial direction, so that a collar  72  is formed. This collar  72  extends inclined in a slightly conical manner to the outer wall  46  of the bush  44 , and comes to bear on this with its free end-side. Thereby, the sleeve  70  or the collar  72  are designed in an elastic manner, such that the sleeve in its idle position is sealingly held on the outer wall  46  of the bush  44 , as is shown in the  FIGS. 2 and 3 . 
         [0123]    If pressurised air is introduced into the pressurised air conduit  28 , the pressurised air in the central plate  40 , firstly on the outer periphery  46  flows around the whole bush  44 , since the recess accommodating the bush  44 , in the central plate  40 , in the region distant to the pressure conduit  16 , is larger than the outer diameter of the bush  44 . The pressurised air then flows into the region between the collar  72  of the sleeve  70 , and the outer wall  46  of the bush  44 , wherein by way of the air pressure, the collar  72  is pressed away from the outer wall of the bush  44 , so that an annular gap  74  arises between the outer wall  46  and the inner periphery of the sleeve  70  or the collar  72 , through which the pressurised air may flow into the recess  76  in which the bush  44  is arranged in the end-plate  42 . The pressurised air then flows from the recess  46  through the recesses or openings  52  into the mixing region  18 , and from there, together with the supplied reduction agent, flows through the conduit or connection  22  further to the injection nozzle in the exhaust gas system of the vehicle. 
         [0124]    If the supply of pressurised air in the pressurised air conduit  28  is switched off, the sleeve  70  with its collar  72  again is sealingly applied onto the outer wall  46  of the bush  44  on account of its elasticity. On account of the collar  72  of the sleeve  70  projecting into the recess  76 , a higher pressure in the recess  76  succeeds in pressing the sleeve-like extension or the collar  72  of the sleeve  70  against the outer wall  46  to an even greater extent, and thus in securely closing the return valve  36 . 
         [0125]    The recesses or openings  52  are designed such that they extend in the longitudinal direction of the bush  44  up to the outer side of the piston  54 . Furthermore, the recesses  52  are shaped such that they widen towards the inside of the bush  44 , i.e. towards the mixing region  18 . By way of this, one succeeds in pressurised air which flows through the recesses  52  into the mixing region  18 , completely flowing over the whole mixing region  18  at its inner wall and in particular also the outer side of the piston  54 , so that reduction agent residues may be completely flushed out of the mixing region  18 . 
         [0126]    The construction of the shut-off valve  12  is hereinafter described in more detail by way of the  FIGS. 6 and 7 . The shut-off valve  12  is arranged in the central plate  40  in a recess  78 . The recess  78  is formed in the surface of the central plate  40  which is distant to the end-plate  42 , and is closed by the front plate  80  of a drive housing of the metering pump assembly, to which the central plate  40  is attached in a flat manner. 
         [0127]    The recess  78  on its base is formed in a cylindrical manner, and opens towards the front plate  80  in a funnel-like manner. An inlet connection piece  82  extends from the base of the recess  78  into the recess  78 , centrally in the cylindrical section. The return conduit  10  branching from the pressure conduit  8  runs in the inside of the inlet connection piece  82 , i.e. concentrically to this, such that it is open to the end-side of the inlet connection piece  82 . The end-side of the inlet connection piece  82  which is distant to the base of the recess  78  thus forms a valve seat  84 , on which a valve element  86  designed in a membrane-like manner sealingly bears in the closed condition, which is shown in  FIG. 6 . The valve element  86  is designed as a circular membrane, which at its outer periphery  88  is held between the surfaces of the central plate  40  and of the front plate  80 , which are adjacent to one another. The central region of the valve membrane  86  is movable with respect to the periphery  88  in the direction of extension of the inlet connection piece  82 , which is ensured by the elasticity of the membrane. 
         [0128]    The valve membrane or the valve element  86  comprises a carrier  90  which is enclosed or peripherally injected by an elastic material  92 , which also defines the sealing surface  94  coming to bear on the valve seat  84 . 
         [0129]    A guide bush  96  which comprises openings  92  in its peripheral wall, surrounding the sealing surface  94  and proceeding from the valve element  86 , extends concentrically to the inlet connection piece  92 . The guide bush  96  is integrally connected to the elastic material  92 , and via this, to the carrier  90  of the valve element  86 . Preferably, the carrier  90  and the guide bush  96  are peripherally injected with the elastic material  92  and thus connected to one another with a positive fit. 
         [0130]    A compression spring  100  in the form of a helical spring is arranged or guided in the inside of the guide bush  96 , so that the compression spring  100  extends parallel to the longitudinal axis of the inlet connection piece  82  between its outer periphery and the inner periphery of the guide bush  96 . The compression spring  100  with a longitudinal end is supported on the base of the recess  78 , and with the opposite longitudinal end is supported on the valve element  86  at the periphery of the sealing surface  94 . The compression spring  100  is dimensioned such that it presses the valve element  96  into its opened position, i.e. its position distanced to the valve seat  84 , which is shown in  FIG. 7 . In this position of the shut-off valve  12  shown in  FIG. 7 , the reduction agent which is delivered by the metering pump into the return conduit  10 , may flow through the inlet connection piece  82  and through the annular gap between the sealing surface  94  and the valve seat  84 , into the inside of the guide bush  96 . The reduction agent may flow through the opened end-side distant to the valve element  86 , as well as the openings  98  of the guide bush  96 , into the recess  78 . The reduction agent from the recess  78 , flows through a channel  102  opening at the periphery of the recess  78 , to a connection piece of the metering pump assembly, and from there further through the return conduit to the reduction agent tank  4 . 
         [0131]    In order to close the shut-off valve  12 , the membrane-like valve element  86  is impinged with the pressurised air from the pressurised air conduit  28 , from its side distant to the inlet connection piece  82 , via the actuation conduit  38 . The air pressure acting on the surface  104  of the valve element  86  moves the valve element  86  against the spring force of the compression spring  100  in the direction of the longitudinal axis of the inlet connection piece  82  to this, so that the valve element  86  with its sealing surface  84  comes to bear on the valve seat  84  in a sealing manner. In this condition shown in  FIG. 6 , no reduction agent may flow out of the inlet connection piece  82  into the inside of the recess  78 , so that the return conduit  10  is closed or interrupted by the shut-off valve  12 . 
         [0132]    The actuation conduit  38  through which the pressurised air flows for impinging the surface  104 , is designed as a channel in the inside of the central plate  40 , which opens out between the central plate  40  and the front plate  80 , in the region of the surface  104  of the valve element  86 . For this, in the shown example, an open annular channel  106  which faces the valve element  86  and in which the pressurised air may distribute, is formed in the front plate  80 , so that the pressurised air acts uniformly on the whole surface  104 . Furthermore, the surface  104  is designed in a curved manner, such that in an annular region bordering the peripheral region  88  in a radially inner-lying manner, it is formed distanced to the plane of the surface of the central plate  40 . 
         [0133]    The opposite central region  108  on the surface  104  of the sealing surface  94 , is designed as an abutment surface, which in the opened condition (see  FIG. 7 ) of the shut-off valve  12  comes to bear on the surface of the front plate  80 , and thus limits the path of the valve element  86  in the opened position. The central region  108  centrically comprises a projection, which for guiding the valve element, engages into a hole in the surface of the front plate  80 . 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           2 —metering pump 
           4 —reduction agent tank 
           5 —suction conduit 
           6 —return valve 
           8 —branching point 
           10 —return conduit 
           12 —shut-off valve 
           14 —pressure sensor 
           16 —pressure conduit 
           18 —mixing region 
           20 —return valve 
           22 —conduit 
           24 —pressurised air conduit 
           26 —magnetic valve 
           28 —pressurised air conduit 
           30 —atmosphere 
           32 —pressure regulator 
           34 —throttle 
           36 —return valve 
           38 —actuation conduit 
           39 —premixing device 
           40 —central plate 
           42 —end-plate 
           44 —bush 
           46 —outer wall 
           48 —narrowing 
           50 —end-side 
           52 —recesses 
           54 —piston 
           55 —piston rod 
           56 —O-ring 
           58 —end-side 
           60 —compression spring 
           62 —guide bush 
           64 —spring ring 
           66 —longitudinal grooves 
           68 —O-ring 
           70 —sleeve 
           72 —collar 
           74 —gap 
           76 —recess 
           78 —recess 
           80 —front plate 
           82 —inlet connection piece 
           84 —valve seat 
           86 —valve element 
           88 —outer periphery of the valve element 
           90 —carrier 
           92 —elastic material 
           94 —sealing surface 
           96 —guide bush 
           98 —openings 
           100 —compression spring 
           102 —channel 
           104 —surface 
           106 —annular channel 
           108 —central region 
         D—boundary of the metering pump assembly