Abstract:
Proposed is a dosing module for injecting liquid urea-water solution into the exhaust tract of an internal combustion engine, which dosing module is composed of two pumps, specifically a delivery pump ( 5 ) and an aeration pump ( 15 ). This permits firstly the injection of urea-water solutions and secondly safe and reliable ventilation of the system when the internal combustion engine is to be shut down.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    In internal combustion engines which operate according to the diesel process, an SCR catalyst is often provided in the exhaust gas system in order to meet environmental regulations. In order that the SCR catalyst can convert the NOx compounds contained in the exhaust gas to water and atmospheric nitrogen, a liquid urea-water solution (reducing agent) has to be injected into the exhaust tract upstream of the SCR catalyst. To this end, a dosing system comprising a tank, a pump and a dosing module, which operates similarly to the injector of a fuel injection system, is used. The pump is also referred to as the delivery module. 
         [0002]    It is the task of the delivery module or the pump to draw urea-water solution from a tank and to build up a sufficient pressure on the pressure side so that the liquid urea-water solution is finely atomized as soon as the dosing module opens in a demand-controlled manner. Just as the delivery module, the injector is connected to a control device of the internal combustion engine and is opened by said device in accordance with the demand and closed again by the same. The same process also applies to the operation of the delivery pump. Because urea-water solution has the property of freezing at low temperatures and thereby increasing the volume thereof by approximately 11%, measures must be taken to prevent damage to the dosing system due to freezing urea-water solution. 
         [0003]    For this reason, it is known from the German patent application DE 10 2004 054 238 to aerate the lines carrying the urea-water solution. To this end, the pump is designed having a reversible delivery direction, or a valve for reversing the delivery direction of the pump is provided. 
         [0004]    It is known from the German patent application DE 10 2009 029 408 to integrate a 4/2-way valve into the dosing system. In a first switching position of the 4/2-way valve, the pump delivers reducing agent from the tank to the dosing module. When the internal combustion engine is to be shut down, the 4/2-way valve is brought into the second switching position; thus enabling the pump of the delivery module to deliver liquid reducing agent from the dosing module to the tank and thereby ventilate parts of the dosing system. This assumes that the dosing module is open and air or exhaust gas can flow out of the exhaust tract into the dosing system. 
         [0005]    By the partial aeration of the dosing system, a compressible air bubble forms so that when the remaining residues of the reducing agent freeze in the dosing system, the resulting ice pressure is so small that no damage occurs to the dosing system. Such a 4/2-way valve is however subject to malfunction and expensive. 
       SUMMARY OF THE INVENTION 
       [0006]    The inventive dosing system is characterized in that said system is very cost effective and ensures a reliable evacuation or aeration of the dosing system after shutting down the internal combustion engine. Because the inventive aeration pump only serves to aerate or evacuate the dosing system, a very small delivery rate is sufficient. Moreover, only small demands are placed on the delivery pressure of the aeration pump. This leads to the aeration pump according to the invention being more cost effective than a 4/2-way valve. In addition, such a pump is less subject to malfunction than a switchable 4/2-way valve. 
         [0007]    The inventive delivery pump and/or the inventive aeration pump are preferably designed as diaphragm pumps. The invention is however not limited to diaphragm pumps. Other designs known from the prior art can also be used. 
         [0008]    It has proven to be particularly advantageous if the inventive delivery pump and/or the inventive ventilation pump are driven by an electromagnetic (linear) actuator, which is also referred to as a solenoid. The system can then namely forego a conversion of the rotational movement of an electric motor into, for example, an oscillating conveying movement of the pump. 
         [0009]    In a very simple and cost effective manner, the direct drive of the diaphragm pump via an electromagnetic actuator allows the injected quantity of the reducing agent to be ascertained very precisely via the stroke of the actuator. 
         [0010]    For example, the stroke of the actuator can be inferred from the profile of the armature current through the electromagnetic actuator. The stroke of the actuator is a direct measurement for the conveyed quantity of the reducing agent. It is therefore possible to dispense with a separate pressure sensor without compromising the dosing accuracy of the dosing system according to the invention. 
         [0011]    In order to optimize the operation of the delivery pump and/or the ventilation pump, a check valve is provided in each case on the suction side and/or the delivery side of both pumps. It is also alternatively possible for a throttle or a diaphragm to be provided either on the suction side or the delivery side and a check valve to be provided either on the pressure side or the suction side. 
         [0012]    In an advantageous embodiment of the dosing system according to the invention, a second check valve is provided on the suction side of the aeration pump parallel to the first check valve, wherein the blocking direction of the second check valve is opposite to the blocking direction of the first check valve. 
         [0013]    It is thereby possible to use the aeration pump according to the invention as a pressure equalization element. If an inadmissibly high pressure namely occurs in the pressure line during the operation of the delivery pump, damage can thereby occur to the dosing module or the pressure line. 
         [0014]    In the dosing system according to the invention, the aeration pump is used as a pressure equalization element during the operation of the delivery pump. If too high a pressure prevails specifically in the pressure line so that the first check valve on the suction side of the aeration pump opens, the high pressure from the pressure line then acts on the diaphragm of the aeration pump. The diaphragm can yield to this high pressure by said diaphragm expanding in the direction of the electrical actuator. As a result, the volume on the pressure side of the inventive dosing system increases and the pressure spike is dissipated. 
         [0015]    It is also alternatively possible to configure the pressure-side check valve in the aeration line in such a manner that said check valve opens when an inadmissibly high pressure occurs in the pressure line and therefore a portion of the urea-water solution conveyed by the delivery pump flows out of the pressure line back into the suction line. In so doing, an effective pressure limitation is likewise achieved. In addition, no additional costs are necessary to achieve this end. 
         [0016]    Of course, a combination of the two variants, namely of the elastic deformation of the diaphragm of the aeration pump and the opening of the aeration line, can be implemented. 
         [0017]    Provision is made in a further advantageous embodiment of the invention for a throttle or a diaphragm to be provided on the pressure side of the aeration pump parallel to the check valve. By means of said throttle or diaphragm, the electrical actuator can be of smaller design. As a result, the electrical power consumption is reduced. In addition, weight requirements as well as installation space requirements are reduced. 
         [0018]    In the case of diaphragm pump, provision is made in a particularly advantageous embodiment of the invention for the diaphragm to seal off the aeration line on the pressure side or the suction side of the aeration pump when the actuator is not being supplied with current. In so doing, the aeration pump according to the invention assumes the function of a switchable directional valve without additional use of and expenditure for components. This is possible because the delivery operation, i.e. when the diaphragm presses reducing agent out of the delivery chamber into the aeration line, is performed by a spring acting on the diaphragm. 
         [0019]    By means of a suitable constructive embodiment, it is therefore readily possible for the diaphragm to be pressed by the spring against the connection of the aeration line in the pump housing and therefore closing the same. 
         [0020]    In order to increase the sealing effect or the maximum pressure in the delivery chamber against which the diaphragm of the aeration pump can seal off the aeration line, a cross-sectional constriction in the housing can be provided. Said cross-sectional constriction can concurrently be configured as a throttle or a diaphragm. 
         [0021]    It is furthermore possible to increase the impermeability or the maximum holding pressure/closing pressure of the diaphragm by an annular bead being configured which surrounds the end of the pressure line or the suction line. As a result, an increased surface pressure occurs between the bead and the diaphragm; thus enabling the impermeability of the diaphragm pump used as a controllable directional valve to be increased. The costs for the additional bead are also thereby insignificant because the housing of the pumps is, generally speaking, manufactured as a plastic injection molded part or as a cast metal part and thus no additional manufacturing costs for the bead are incurred. 
         [0022]    It is also alternatively possible for the membrane to directly or indirectly exert a closing force on a valve member of the check valves when the actuator of the delivery pump and/or the aeration pump is deenergized. As a result, the impermeability of the check valves is increased. This too can be achieved without additional manufacturing costs. This improved impermeability makes it possible to simultaneously reduce the pre-load on the closing springs in the check valves. The delivery work to be provided by the electromagnetic actuator is thereby reduced; and the electromagnetic actuator can therefore be of smaller, more energy efficient and more cost effective design. This is an aspect which relates to the aeration pump as well as to the delivery pump. 
         [0023]    In order to achieve a particularly compact design, provision is furthermore made for the aeration pump to be integrated into the delivery pump. This not only has advantages with regard to the hydraulics of the dosing system but additionally has the advantage that the signal lines for actuating both pumps can be conjointly guided into the housing. 
         [0024]    In the instance that the reducing agent freezes in the delivery pump, there is furthermore the advantage that the aerated delivery chamber of the aeration pump, which chamber does indeed serve as a compensation volume for the reducing agent situated in the delivery pump, is located in direct proximity to the delivery pump and thus the pressure equalization between the two pumps is very well possible. 
         [0025]    According to an advantageous embodiment of the inventive dosing system, at least one capacitor is provided so that the electrical charge stored in the capacitor can be used to energize the electrical actuator. Because a capacitor can very quickly release the electrical charge stored therein, it is possible, should necessity require it, to very quickly apply large currents to the actuator of the aeration pump; thus enabling the diaphragm to be abruptly raised which results in a very rapid suction of liquid reducing agent by the aeration pump. Due to this dynamic suction process, a so-called pulse back suction of liquid reducing agent takes place. Said pulse back suction is ultimately nothing other than the utilization of the elasticity of the pressure line and of the liquid reducing agent that is subjected to pressure therein. In the case of an abrupt drop in pressure, the pressure line compresses to some extent and thereby delivers a small amount of liquid reducing agent in the direction of the aeration pump. This leads to at least a part of the pressure line as well as the dosing module being no longer filled with liquid reducing agent but instead with air or exhaust gases. As a result, the risk of damage due to ice pressure is reduced. 
         [0026]    Provision is made in a further advantageous embodiment of the dosing system according to the invention for the delivery pump and/or the aeration pump to include an electrical actuator comprising a magnet and an armature, a diaphragm, a valve diaphragm plate and a valve plate and for a rubber plate serving as a valve element and sealing element to be provided between the valve diaphragm plate and the valve plate. 
         [0027]    The check valves according to the invention and/or throttles can be manufactured in a simple and cost effective manner by means of this sandwich-like design of the delivery pump and/or the aeration pump. Thus, an additional aperture in the valve plate has, for example, only to be provided for an additional check valve, and corresponding recesses are to be provided in the rubber plate acting as a valve element. 
         [0028]    In a similar manner, it is possible for the valve diaphragm plate and the diaphragm of the aeration pump together with the electrical actuator to form a controllable shut-off valve. This too does not require any significant additional manufacturing costs. 
         [0029]    In a further advantageous embodiment of the invention, a valve disk is formed on the armature, said disk working together with a sealing bead of the valve diaphragm as a switchable directional or check valve. Provision is furthermore made for the diaphragm to be disposed on the armature so as to be offset with respect to the valve disk. As a result, it is possible that, on the one hand, the pressure prevailing in the delivery chamber acts to some extent on the back side of the valve disk and consequently presses the same against the sealing seat in the valve diaphragm plate. As a result, the impermeability is increased. At the same time, it is possible for the diaphragm to give way in the stroke direction and thus dissipate a pressure spike. The diaphragm can thus operate as a pressure equalization element. In order to be able to constructively define the elasticity of the diaphragm within narrow limits, it is advantageous to design the diaphragm wavelike in cross section. At the same time, it is advantageous if the armature of the electrical actuator delimits the travel of the diaphragm in the stroke direction so that there is no risk of bursting or tearing of the diaphragm when inadmissible high pressures are applied to said diaphragm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Further advantages and advantageous embodiments of the invention can be extracted from the following drawings, the description of said drawings and the patent claims. In the drawings: 
           [0031]      FIG. 1  shows a block diagram of a first exemplary embodiment of a dosing system according to the invention; 
           [0032]      FIG. 2  shows the exemplary embodiment pursuant to  FIG. 1  when the system is being aerated; 
           [0033]      FIG. 3  shows the block diagram of a second exemplary embodiment, in which the aeration pump embodied as a diaphragm pump simultaneously operates as controlled check valve during the normal operation of the dosing system; 
           [0034]      FIG. 4  shows a third exemplary embodiment of an inventive dosing system comprising a throttle instead of a check valve on the suction side of the aeration pump; 
           [0035]      FIG. 5  shows a further exemplary embodiment of an inventive dosing system comprising a throttle on the pressure side/delivery side of the aeration pump according to the invention; 
           [0036]      FIG. 6  shows a further exemplary embodiment of an inventive dosing system in which the diaphragm of the delivery pump is used as a controlled check valve; 
           [0037]      FIGS. 7 &amp; 8  show further exemplary embodiments of dosing systems according to the invention and 
           [0038]      FIGS. 9-16  show constructive details of different exemplary embodiments of aeration pumps according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    A first exemplary embodiment of a dosing system according to the invention is depicted as a block diagram in  FIG. 1 . Liquid reducing agent (urea-water solution) is situated in a tank  1 . A delivery pump  5  draws liquid reducing agent as required out of the tank via a suction line  3  and delivers the same via a pressure line  7  to a dosing module  9 . The terms suction line  3  and pressure delivery line  7  relate to the normal operation of the dosing system when reducing agent is namely being delivered from the tank to the dosing module  9 . 
         [0040]    The dosing module  9  is depicted in the block diagram as a combination of a throttle  11  and a switchable 2/2-way valve  13 . The directional valve  13  is closed in the deenergized state. No liquid reducing agent is then injected into the exhaust tract of the internal combustion engine (not depicted). If reducing agent is being delivered by the delivery pump  5  and therefore the reducing agent is subjected to an increased pressure in the pressure line  7 , the directional valve  13  can be opened by the engine control device (not depicted); thus enabling liquid reducing agent from the throttle  11  to be atomized in the dosing module  9  and injected into the exhaust pipe of the internal combustion engine in a finely dispersed manner. 
         [0041]    The quantity of liquid reducing agent injected into the exhaust tract can be controlled by the delivery pressure of the delivery pump  5  and the opening time of the directional valve  13 . In the dosing system according to the invention, an inventive aeration pump  15  is provided in parallel with the delivery pump, however having an opposite delivery direction. 
         [0042]    If the delivery pump  5  is in operation, the aeration pump  15  is not in operation and vice versa. There are however operating states of the dosing system according to the invention in which neither of the two pumps  5 ,  15  is in operation. 
         [0043]    A check valve  17 ,  19  is provided in each case on the suction side and the delivery side  5  of the delivery pump  5 . In a corresponding manner, check valves  21  and  23  are likewise provided on the suction side and the pressure side of the aeration pump  15 . Because the delivery directions of the delivery pump  15  and the aeration pump  15  are opposite to one another, the blocking directions of the check valves  17 ,  19  and  21 ,  23  are also oppositely oriented. 
         [0044]    The aeration pump  15  is hydraulically integrated into the suction line  3  and the pressure line  7  of the delivery pump  5  via an aeration line  25 . The suction-side (in relation to the aeration pump  15 ) section of the aeration line  25  has the reference numeral  25 . 1 . The pressure-side (in relation to the aeration pump  15 ) section of the aeration line  25  has the reference numeral  25 . 2 . 
         [0045]    In the normal operation of the dosing system depicted in  FIG. 1 , the check valves  21  and  23  block the aeration line  25  as long as the pressure in the pressure line  7  lies below the opening pressure of said check valves. 
         [0046]    The same exemplary embodiment of the dosing system according to the invention is depicted in  FIG. 2  in the operating mode: aeration. In this case, the delivery pump  5  is not in operation and the aeration pump  15  delivers liquid reducing agent from the dosing module  9  back into the tank  1 . In order that the aeration pump  15  can aerate the dosing module  9  as well as a portion of the pressure line  7 , the 2/2-way valve  13  of the dosing module  9  is open. This switching position is depicted in  FIG. 2 . 
         [0047]    During the aeration of the dosing system depicted in  FIG. 2 , the check valves  17  and  17  block sections of the suction line  3  and the pressure line  7  as long as the delivery pressure of the aeration pump  15 ,  7  lies below the opening pressure of said check valves. 
         [0048]    As soon as the aeration process has concluded, the directional valve  13  of the dosing module  9  is closed again and the aeration pump  15  is switched off. 
         [0049]    After the aeration process, the dosing module  9  as well as portions of the pressure line  7 , the aeration line  25  and the aeration pump  15  are filled with air or exhaust gas. The aforementioned regions that are filled with air are therefore available as a compensation volume to the regions of the dosing system that are still filled with liquid reducing agent, specifically above all the delivery pump  5 , the suction line  3  and a portion of the pressure line  7 , in the event that the reducing agent freezes. As a result, the forces occurring upon freezing of the reducing agent are reduced to the point where there is no longer risk of damage to the delivery pump  5  or the lines  3 ,  7 . This applies particularly to the case where the delivery pump  5  and the aeration pump  15  are disposed in a common housing. 
         [0050]    A second exemplary embodiment of the dosing system according to the invention is depicted in  FIG. 3 . A significant difference to the first exemplary embodiment is that the aeration pump  15  that is configured as a diaphragm pump is designed in such a manner that the diaphragm of the aeration pump  15  seals off the aeration line  15  whenever said aeration pump is deenergized. This is demonstrated by means of a switchable directional valve  26 . The section  25 . 2  of the aeration line  25  is thereby preferably closed although the directional valve  26  is illustrated in the section  25 . 1 . 
         [0051]    As soon as the actuator of the aeration pump  15  is energized, the diaphragm again unblocks the aeration line  25 ; thus enabling the operating mode described with the aid of  FIGS. 1 and 2  to again take place. The aeration pump  15  pursuant to the second exemplary embodiment has then additionally the function of a controlled shut-off valve  26 . Because no additional components are required to meet this end, this additional functionality is achieved without extra costs. 
         [0052]    The use of the delivery pump as a controlled shut-off valve  26  has the advantage that the aeration line  25  can be sealed off using very little spring pressure of the spring acting on the diaphragm by means of an appropriate design of the cross section of said aeration line. As a result, it is no longer necessary for one of the two check valves  21 ,  23  in the aeration line to be designed such that said valve is still leak tight with respect to the operating pressure of the delivery pump  5 . 
         [0053]    The opening pressure of the check valves  21  and  23  should be as low as possible because the electromagnetic actuator of the aeration pump  15  has to overcome the opening pressure with each stroke. The actuator can be designed smaller and lighter proportionally to how low the opening pressure is. By using the diaphragm of the aeration pump  15  as an additional shut-off valve, not only the opening pressure of the check valves  21 ,  23  can thus be reduced but the electromagnetic actuator of the aeration pump  15  can be of smaller design which saves on costs and installation space. In so doing, the power requirement for driving the aeration pump  15  is furthermore reduced. 
         [0054]    In the exemplary embodiment depicted in  FIG. 4 , a suction throttle  27  is provided on the suction side of the aeration pump  15  instead of a check valve  21  (see  FIGS. 1 to 3 ). Because the suction throttle  27  ultimately substantially consists only of a cross-sectional constriction in the aeration line  25 , the number of required components is thereby again reduced, which has a positive effect on the manufacturing costs and the robustness of the dosing system according to the invention. 
         [0055]    As can be seen in  FIG. 5 , the check valve  23  can also be replaced on the pressure side of the aeration pump  15  by a feed throttle  29 . It is important however that at least one check valve is present in the aeration line  25 . 
         [0056]    It goes without saying that the diaphragms of the delivery pump  5  as well as of the aeration pump  15  can be driven not only via an electromagnetic actuator but also by an electric motor. Another pump principle, such as, e.g., a piston pump, a gear pump or a vane pump among other things, can also be used. 
         [0057]    Depending on need and design, the check valves  17 ,  19 ,  21  and/or  23  can be loaded by spring elements so that the opening pressure of said check valves can be adjusted within wide limits by means of the preload force of the springs. Said check valves can also be partially replaced by throttles as explained using the exemplary embodiments pursuant to  FIGS. 4 and 5 . 
         [0058]    Filters which are possibly necessary on the suction side  3 , in the pressure line and/or in the aeration line  25  are partially required in practical applications but are not depicted for reasons of clarity. The same also applies to a pressure sensor or a flow rate sensor. If possible, the installation of such sensors should be avoided because they drive costs up. If necessary, an additional electrical heater can be installed. This is, however, not required in many cases because the waste heat of the pump drive is usually sufficient in preventing the dosing system from freezing. This, of course, does not apply to the liquid reducing agent situated in the tank  1 . In many cases, a heater is required here at least to thaw the frozen reducing agent (not depicted). 
         [0059]    A further exemplary embodiment of a dosing system according to the invention is depicted in  FIG. 6 . In this exemplary embodiment, the delivery pump is designed as a diaphragm pump and can also be used in a similar manner, as explained using  FIG. 3 , as a switchable shut-off valve  28 . Reference is therefore made in this case to that which has been described in connection with the aeration pump  15  in  FIG. 3 . 
         [0060]      FIG. 7  depicts a block diagram of a further exemplary embodiment of the dosing system according to the invention. In this exemplary embodiment, a second check valve  31  is provided in parallel with the first check valve  21  on the suction side of the aeration pump  15 . The blocking directions or the passage directions of the check valves  21  and  31  are thereby opposite to one another. 
         [0061]    If, for example, an inadmissibly high pressure now occurs in the pressure line  7  during the operation of the delivery pump  5 , the first check valve  21  then opens. As a result, the diaphragm (not depicted in  FIG. 7 ) of the aeration pump  15  is impinged with the higher pressure and said diaphragm is displaced due to the higher pressure. The volume of the delivery chamber in the aeration pump  15  is thereby enlarged, and the pressure spike is thus partially reduced. As soon as the pressure in the pressure line  7  returns again to normal values, the elastic diaphragm of the aeration pump  15  can push the quantity of liquid urea-water solution previously received in the delivery chamber back into the pressure line until an equalization of pressure occurs. 
         [0062]    If the excess pressure in the pressure line  7  is very high, it is also possible that the check valve  23  on the pressure side of the aeration pump  15  opens and a portion of the liquid delivered by the delivery pump  5  is led out of the pressure line  7  and back into the suction line  3 . As a result, a drop in pressure to admissible values or a pressure limitation also takes place. The system according to the invention is therefore very robust and does not incur damage even when inadmissibly high pressures occur. 
         [0063]    In the exemplary embodiment pursuant to  FIG. 8 , a throttle  33  is provided in parallel with the check valve  23  on the pressure side of the aeration pump  15 . By means of said throttle, it is possible for the actuator to be of smaller design. It has actually been found that a strong negative pressure can develop in the delivery chamber of the aeration pump  15  during the suction phase of the delivery pump  5  particularly if the diaphragm of the aeration pump  15  is designed as an additional shut-off valve  26  or pressure retention valve  26 . This results from the fact that the delivery chamber is connected via the aeration line  25  and the check valve  23  to the suction line  3 . The blocking action of the check valve  23  prevents an equalization in pressure between the delivery chamber of the aeration pump  15  and the suction line  3  if a negative pressure prevails in the delivery chamber. 
         [0064]    Said negative pressure in the delivery chamber can only be overcome by a very strong electrical actuator. By means of the throttle according to the invention, it is ensured that an equalization in pressure between the delivery chamber of the aeration pump  15  and the suction line  3  can take place if negative pressure prevails in the delivery chamber. As a result, the drive capacity of the electrical actuator can be reduced, which has a positive effect on the installation space requirements and the weight of the electrical actuator. Further details in this regard ensue from  FIGS. 14-16  and the descriptions thereof. 
         [0065]    A longitudinal section through an exemplary embodiment of an aeration pump  15  according to the invention is depicted in  FIG. 9 . 
         [0066]    The electrical actuator  35  essentially comprises an electromagnet  37  and an armature  39 . A spring  41  is located between the magnet  37  and the armature  39 , said spring pressing the armature  39  in  FIG. 9  to the left against a diaphragm  43 . By means of a bead  44 , the diaphragm  43  is externally clamped in a sealing manner in the housing  47  of the aeration pump  15 . Thus, no liquid is situated to the right of the diaphragm  43  in  FIG. 9 . On the other side of the diaphragm  43 , a delivery chamber  45  of the aeration pump  15  is configured in the housing  47 . Besides the delivery chamber  45 , the connections of the sections  25 . 1  and  25 . 2  are indicated in the housing  47  of the aeration pump  15 . The suction-side connection of the aeration pump  15  to the aeration line  25  is thereby denoted with the reference numeral  25 . 1 ; whereas the connection  25 . 2  denotes the pressure-side connection of the aeration pump  15  to the aeration line  25 . The check valves  21  and  23  are not depicted in  FIG. 9 . An annular sealing seat  49  is formed in the housing in the region of the pressure-side connection  25 . 2 . 
         [0067]    If the electrical actuator is deenergized, the spring  41  then pushes the armature  39  and with it the diaphragm  43  against the sealing seat  49  so that the connection  25 . 2  of the aeration line  25  is closed. As soon as the electrical actuator  35  is energized, the magnet  37  moves the armature in  FIG. 9  to the right; thus enabling the diaphragm  43  to lift off the sealing seat  49  and consequently a hydraulic connection is produced between the connection  25 . 1  and the delivery chamber  45 . The inventive aeration pump  15  pursuant to the exemplary embodiment of  FIG. 9  is thus simultaneously a controllable directional valve which closes the connection  25 . 2  of the aeration line  25  when power is switched off to the actuator  35 . This functionality does not require any additional components. It is achieved by an artful constructive design and tuning of the diaphragm  43 , the pump housing or the sealing seat  49  as well as electrical actuator  35 . As a result, no additional costs are incurred during manufacture. 
         [0068]    If the electrical actuator  35  is abruptly energized due to the discharge of one or a plurality of capacitors (not depicted), the armature is then pulled very quickly and with a large force so that a substantial and sudden drop in pressure occurs in the region of the pressure line  7  and a section  25 . 1  of the aeration line  25 . Due to the elasticity of the pressure line  7  or the aeration line  25  and the liquid which is situated therein and is subjected to pressure, the abrupt pressure release leads to a portion of the liquid situated in the pressure line  7  being pressed by the aeration pump  15  in the direction of the tank. As a result, a partial aeration of the dosing module  9  and the pressure line  7  is ensured even by means of a delivery stroke of the aeration pump  15  which admittedly occurs very quickly, so that no damage results from ice pressure even in the event that the system subsequently freezes. This highly dynamic process is referred to as pulse back suction in the context of the invention and can be employed in all inventive exemplary embodiments of dosing systems or aeration pumps  15 . 
         [0069]    In  FIG. 10 , a further exemplary embodiment of an aeration according to the invention is likewise depicted partially in cross section. In this exemplary embodiment, a sandwich-like design of the aeration pump  15  can be easily recognized. From top to bottom, the armature  39  is adjoined by the diaphragm  43  comprising the bead  44  thereof and a valve diaphragm plate  51 . 
         [0070]    In this exemplary embodiment, it can also be easily recognized that a valve disk  53 , which is overmolded with rubber or a similar elastic material, is formed at the end of the armature  39  which is the lower end in  FIG. 10 . The diaphragm  43  is produced from the same rubber material and is connected to the armature  39  in a positive-locking manner. 
         [0071]    There is however a certain distance between the valve disk  53  and the diaphragm  43  in the axial stroke direction of the armature  39 ; thus enabling the pressure prevailing in the delivery chamber  45  to also act on the valve disk  53  “from above” in  FIG. 10 . As a result, the pressure prevailing in the delivery chamber  45  simultaneously acts as a hydraulic closing force which presses the valve disk  53  against the sealing seat  49  in the valve diaphragm plate  51 . 
         [0072]    In the exemplary embodiment depicted in  FIG. 10 , the diaphragm  43  is designed wavelike in cross section. In so doing, the diaphragm  43  is more elastic and can therefore be displaced more easily if the pressure increases in the delivery chamber  45 . The diaphragm  43  in  FIG. 10  is then displaced upwards in the direction of the armature  39  until said diaphragm rests against the armature  39 . It is thereby ensured that the diaphragm  43  does not tear even when extremely large excess pressures occur in the delivery chamber  45 . 
         [0073]    Further connections can be seen in the valve diaphragm plate  51 , namely the connection  25 . 1  and a connection  25 . 3 . The pressure-side outlet  25 . 2  of the aeration pump  15  is covered in  FIG. 10  by the valve disk  53 . 
         [0074]    The connection  25 . 3  establishes the hydraulic connection to the second check valve  31  (see  FIG. 7 ) if the inventive aeration pump  15  is simultaneously used as a pressure equalization element. 
         [0075]      FIG. 11  shows a detail from  FIG. 10  that is even further enlarged and is supplemented by a valve disk  57  as well as a rubber plate  55 . A rubber plate  55  and a valve disk  57  are disposed below the valve diaphragm plate  51 . The valve diaphragm plate  51 , the rubber plate and the valve disk  57  form the check valve  21  below the connection  25 . 1 , the blocking direction of said check valve running from top to bottom. The passage direction is indicated by an arrow  59 . In order to illustrate which regions of the components  51 ,  55  and  57  form the check valve  21 , said regions are enclosed by a dashed line. 
         [0076]    A circumferential web  61  is formed in the valve disk  57 . The circumferential web interacts with a corresponding web  63  of the valve diaphragm plate  51  such that said web  61  clamps the rubber plate  55  in a sealing manner. A sealing seat  65  is configured coaxially to the web  61  in the valve plate  57 , on which seat the rubber plate  55  rests when the check valve  21  is closed. The sealing seat  65  and the web  61  together with the rubber plate  55  delimit an annular channel  67 . Above the annular channel  67 , a plurality of curvilinear apertures  69  are cut out in the rubber plate  55 . 
         [0077]    If the check valve  21  of the pressure line  7  (not depicted in  FIG. 11 ) is now impinged via the aeration line  25  with the pressure prevailing in the pressure line  7  and this pressure is larger than the opening pressure of said check valve  21 , the rubber plate  55  than lifts off the sealing seat  65  and a hydraulic connection thereby develops to the annular channel  67  in the valve plate  57 . From the annular channel  67 , the reducing agent enters by means of apertures  69  in the rubber plate  55  into the delivery chamber  45  of the aeration pump. 
         [0078]    This means that reducing agent can flow in the direction of arrow  59  through the bore  69  in the valve plate  57  if the difference between the pressure in the bore  71  and that in the delivery chamber  45  is large enough. 
         [0079]    As soon as the pressure of the reducing agent in the section  25 . 1  of the aeration line  25 , which is connected to the pressure line  7 , drops below the opening pressure of the check valve  21 , the rubber plate  55  lowers again onto the sealing seat  65  due to the elasticity of said plate and consequently seals off the delivery chamber  45 . 
         [0080]    The second check valve  31  has the same design but the opposite passage direction. For that reason, the annular channel  73  and the sealing seat  75  are disposed in the valve diaphragm plate  51 . 
         [0081]    In  FIG. 11 , the apertures  77  in the rubber plate  55  associated with the second check valve  31  can only be seen to a minor extent. 
         [0082]    When comparing the two check valves  21  and  31 , it is apparent that the sealing seat  75  of the second check valve  31  is smaller than the diameter of the sealing seat  65  of the first check valve  21 . As a result, the opening pressure of the two check valves  21  and  31  can be adjusted when the thickness of the rubber plate  55  is the same. As was already described in connection with  FIG. 7 , it is advantageous if the opening pressure of the second check valve  31  is higher than that of the first check valve  21 , which can be constructively implemented by means of the smaller diameter of the sealing seat  75 . 
         [0083]    It is already clear from  FIG. 11  that it is possible with minimal costs to integrate one or a plurality of check valves  21 ,  23 ,  31  into the aeration pump  15  according to the invention. Different variants of the inventive aeration pump  15  can thereby be produced by exchanging the valve diaphragm plate  51  or the valve disk  57 . 
         [0084]    A side view of the exemplary embodiment pursuant to  FIG. 11  is depicted in  FIG. 12 . In this embodiment, the check valve  23 , which connects the delivery chamber  45  to the pressure-side section  25 . 2  of the aeration line  25 , can be easily recognized. The passage direction of the check valve  23  is indicated by an arrow  79 . Also in this case, the same design is again recognizable. 
         [0085]    In the exemplary embodiment depicted in  FIG. 12 , an outer sealing seat  49 . 2  and an inner sealing seat  49 . 1  are formed in the valve diaphragm plate  51 . The valve disk  53  rests on said sealing seats when the actuator is deenergized; thus enabling a particularly good sealing of the delivery chamber  45  towards the pressure side of the aeration pump  15  to take place. The inner sealing bead  49 . 1  results in a leak free sealing being possible by means of the closing forces brought to bear by the spring  41 . This is primarily of importance when the motor vehicle has been shut down and the pressure line  7  and/or the dosing module and/or the exhaust gas system are to be reliably prevented from filling up with reducing agent without the spring  41  and thus the magnet  37  having to be larger than absolutely necessary. 
         [0086]    A sealing seat  81  and an annular channel  83 , which together with the rubber plate  55  constitutes the check valve  23 , are formed in the valve diaphragm plate  51 . It can be easily seen in this depiction how the valve disk  53  interacts with the sealing seat  49  and thereby relieves the second check valve  23 . 
         [0087]    It can also be easily recognized in  FIG. 12  that the magnet  37  has a toroidal recess which delimits the stroke- or the elastic deformation of the diaphragm  43 . As a result, damage to the diaphragm  43  can be prevented when inadmissibly high pressures occur in the delivery chamber  45 . 
         [0088]    A ledge  85  on the armature  39  allows on the one hand for the pressure spring  41  to be supported at the armature and on the other hand said ledge  85  can serve to guide the armature  39  in the magnet  37 . 
         [0089]    In  FIG. 13 , the rubber plate  55  is depicted transparently and “from below” so that the sealing seats in the valve diaphragm plate  51  and parts of the diaphragm  43  are also visible. The different diameters of the check valves  21 ,  23 ,  31  are also obvious in this depiction. 
         [0090]    The check valve  23  has the largest bore so that said valve already opens when there is a small excess pressure in the delivery chamber if the valve disk  53  does not close said valve. The energy requirements are thereby minimized when the aeration pump  15  is operating. In contrast, the second check valve  31  has the smallest diameter of the sealing seat  75  on the suction side of the aeration pump  15  so that this check valve opens only at a relatively high pressure. 
         [0091]    A further exemplary embodiment of an aeration pump  15  according to the invention is depicted in the  FIGS. 14 to 16 . 
         [0092]    The check valves  21  and  23  are somewhat different in design than the ones previously described. The function thereof is however unchanged. It can easily be seen in  FIGS. 14 and 15  how the diaphragm  43  rests on the sealing seat  49  which surrounds the connection  25 . 1  in the valve plate  51 . 
         [0093]    Particularly in  FIG. 15  which represents an enlarged detailed depiction of  FIG. 14 , it can likewise be easily seen that the diaphragm  43  abuts against a further bead  87 . The delivery chamber  45  thus has a circular ring-shaped geometry and is delimited radially on the outside by the bead  87  and on the inside by the sealing seat  49 . 
         [0094]    If liquid reducing agent is now drawn from the tank during the operation of the delivery pump  5  (see, e.g.,  FIG. 1 ), the pressure drops for a short time in the suction line  3 . As a result, the check valve  23  opens in the pressure-side portion of the aeration line  25  and consequently the pressure also drops in the delivery chamber  45 . This low pressure in the delivery chamber  45  also then remains intact on account of the blocking action of the check valve  23  if ambient pressure again prevails in the suction line  3 . 
         [0095]    Said low pressure in the delivery chamber  45  leads to the diaphragm  43  being pulled to some extent against the valve plate  51  or against the sealing seat  49  and the bead  87 . This means that a very large force has to be produced by the armature  39  or the magnet  37  in order to lift the armature  39  and with it the diaphragm  43  from the sealing seat  49  and the bead  87 . To meet this end, a larger, more expensive electrical actuator  35  would be required. 
         [0096]    According to the invention, a throttle  33  is therefore formed in the valve plate  57  which connects the delivery chamber  45  to the aeration line  25 . 2  or indirectly to the suction line  3  (see the block diagram in  FIG. 8  and also  FIG. 16 ). The throttle  33  provides for an equalization in pressure between the suction line  3  and the delivery chamber  45  so that the forces which are required to lift the diaphragm  43  from the sealing seat  49  or the bead  87  are drastically reduced. 
         [0097]    As a result, a smaller electrical actuator  35  can also be used, which saves on costs and installation space. In addition, the power requirements of the aeration pump  15  according to the invention are reduced.