Abstract:
The invention relates to an injection system ( 2 ) for metered injection of a fluid, particularly for an exhaust gas treatment device ( 1 ) in a motor vehicle, said system having: a tank ( 3 ) for storing the fluid; a metering valve ( 19 ); an actuatable venting valve ( 25 ) for admitting gas into or releasing gas from the injection system ( 2 ); and a metering pump ( 7 ) which is connected to the tank ( 3 ) and the metering valve ( 19 ) in order to convey the fluid from the tank ( 3 ) to the metering valve ( 19 ). The metering pump ( 19 ) is designed as a piston pump ( 8 ) and has a piston ( 10 ) axially movable in a cylinder ( 9 ) between an upper and a lower dead centre and forming, together with the cylinder ( 9 ), a pump chamber ( 12 ). Associated with the pump chamber ( 12 ), the cylinder ( 9 ) has a first connection ( 14 ) which is connected to the tank ( 3 ) via a tank line ( 6 ), a second connection ( 17 ) which is connected to the metering valve ( 19 ) via a conveying line ( 18 ), and a third connection ( 23 ) which is connected to the venting valve ( 25 ) via a venting line ( 24 ). The invention further relates to a metering pump and an exhaust gas treatment system. The invention additionally relates to a method for operating a corresponding injection system.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to an injection system for metered injection of a liquid, in particular for an exhaust gas treatment device in a motor vehicle, said system having a tank for storing the liquid, a metering valve, an actuable ventilation valve for admitting gas into or releasing gas from the injection system, and a metering pump, which is connected to the tank and the metering valve in order to deliver the liquid from the tank to the metering valve. 
         [0002]    The invention furthermore relates to a corresponding metering pump and to an exhaust gas treatment system. Moreover, the invention relates to a method for operating a corresponding injection system. 
         [0003]    Injection systems, metering pumps, exhaust gas treatment systems and methods for operating these are known from the prior art. In the case of motor vehicles having an internal combustion engine, the pollutant NOx among others must be reduced owing to the stricter exhaust gas laws expected in the coming years. A frequently used method is the SCR method (SCR=Selective Catalytic Reduction), in which the pollutant NOx is reduced to nitrogen and water with the aid of a liquid reducing agent. In this case, the liquid is delivered by means of a metering pump from a tank to a metering valve, which is often designed as a passive or pressure-controlled injection valve. The metering pump meters in the injection quantity and builds up a pressure in the connection to the injection valve. If this pressure exceeds a critical pressure, the injection valve opens. The known injection systems are faced with the problem that a liquid exhaust gas treatment medium, in particular exhaust gas treatment medium containing urea, exhibits a volume increase of about 10% during the phase transition from a liquid to a solid state, and therefore all injection systems operated with an exhaust gas treatment medium of this kind must be optimized in respect of the ice pressure resistance thereof. In the case of injection systems having an actuable metering valve, the direction of action of the metering pump could be switched over and the metering valve opened at the same time, with the result that a gas or ambient air or exhaust gas would be introduced into the injection system through the metering valve and ice pressure resistance could thereby be ensured. In the case of pressure-controlled injection valves, which open and close automatically, this is not possible however. 
         [0004]    EP 2 151 559 B1 discloses an injection system of the type stated in which the intention is to circumvent this problem by providing a ventilation valve for admitting gas into or releasing gas from the injection system between the metering pump and the metering valve. Depending on the delivery direction of the metering pump and in accordance with the switching state of the ventilation valve, gas can be introduced into the injection system here in order to ensure ice pressure resistance, and can subsequently be discharged in order to refill the injection system with the liquid exhaust gas treatment medium so as to reestablish the ability to function of the injection system within a short time. 
       SUMMARY OF THE INVENTION 
       [0005]    The injection system according to the invention has the advantage that the reversal of an operating direction of the metering pump is not necessary. Thus braking and restarting in the opposite direction of a drive of the metering pump is eliminated, for example. Instead, the metering pump is designed and incorporated into the injection system in such a way that the liquid is injected or gas introduced into the injection system or expelled therefrom by very simple means. According to the invention, provision is made, for this purpose, for the metering pump to be designed as a piston pump and to have a piston axially movable in a cylinder between an upper and a lower dead center and forming, together with the cylinder, a pump chamber, wherein, assigned to the pump chamber, the cylinder has a first connection, which is connected to the tank via a tank line, a second connection, which is connected to the injection valve via a delivery line, and a third connection, which is connected to the ventilation valve via a ventilation line. The ventilation line, the metering module and the tank are thus each connected directly to the metering pump. Since the metering pump is designed as a piston pump, it can set an excess pressure or a reduced pressure in the respective branch of the injection system, depending on the switching state of the different valves. This makes it possible to fill the injection system with gas without interrupting the operation of the piston pump, simply by setting or actuating the valves, in particular the ventilation valve, in order to ensure ice pressure resistance, or to expel the gas in order to put it back into operation. 
         [0006]    Provision is preferably made for the second connection to be arranged in an end region of the cylinder adjacent to the upper dead center. As a result, the second connection is situated in a region which uses the maximum delivery volume of the metering pump. As a particularly preferred option, the second connection is arranged in the end region on an end face of the cylinder. This end face then lies opposite a piston surface of the piston facing the pump chamber. This ensures that the maximum possible quantity of liquid is fed to the metering valve by means of the piston pump during a stroke of the piston. 
         [0007]    According to an advantageous development of the invention, provision is made for the third connection to be arranged in the end region of the cylinder adjacent to the upper dead center. Arranging the third connection in the end region likewise ensures that the piston pump draws in the maximum possible quantity of air during a movement of the piston from the upper dead center to the lower dead center and then introduces this into the injection system. In this case, the third connection can likewise be arranged on the end face of the cylinder, which is preferably at least substantially of pot-shaped design. However, the third connection is preferably formed in a circumferential wall of the cylinder. 
         [0008]    According to an advantageous development of the invention, provision is made for the first connection—as seen in the direction of motion of the piston—to be formed between the third connection and the lower dead center of the piston on the circumferential wall of the cylinder. The first connection is thus at a level between the third connection and the lower dead center of the piston. The result is that the piston will pass the first connection on the way from the lower dead center to the upper dead center, causing said first connection to lose its link to the pump chamber. Together with the piston, the first connection thus forms a spool valve, which is closed by the piston from a certain position between the upper and lower dead center. If the piston reaches the lower dead center while the ventilation valve is closed, it draws in liquid via the first connection as soon as it has passed said connection. On the return path, the first connection is closed again, and the liquid of the piston is delivered from the pump chamber to the metering module if the ventilation valve is closed. For the admission of air or to ensure ice pressure resistance, the ventilation valve is opened in the next cycle. As a result, a gas, e.g. ambient air or exhaust gas, flows into the piston chamber as soon as the piston moves from the upper dead center in the direction of the lower dead center. Owing to the geometry, the first connection is opened later, with the result that the pump chamber initially contains predominantly air. When the piston reaches the lower dead center, the valve is closed again, with the result that the gas in the pump chamber is compressed and forced into the first connection during a subsequent movement of the piston in the direction of the upper dead center, as a result of which the injection system is supplied at least locally with gas and thus ventilated on the tank side. As soon as the piston has crossed the first connection, the pressure in the pump chamber rises further until the metering valve, which is preferably designed as a pressure-controlled injection valve, opens owing to the high pressure. As a result, that part of the injection system which is on the metering module side is also supplied with the gas or freed from liquid, and the ice pressure resistance of the entire injection system is ensured. 
         [0009]    According to an alternative embodiment of the invention, provision is made for the first connection likewise to be arranged on the end face of the cylinder. This has the advantage that a larger quantity of gas for ventilating the injection system can be delivered into the tank-side part of the injection system or into the tank line since the entire piston stroke can be used. 
         [0010]    As a particularly preferred option, the first connection is then assigned an automatic suction valve which opens during a movement of the piston in the direction of the lower dead center and closes at least substantially during a movement of the piston in the direction of the upper dead center. Through the automatic opening and closing of the suction valve, the same functionality as with the spool valve described above is thus ensured, with the entire stroke of the piston and hence a larger delivery volume being used. 
         [0011]    As a particularly preferred option, the suction valve is designed as a spring-loaded check valve. Check valves of this kind are mass-produced and therefore inexpensive. Moreover, with this configuration, the piston stroke of the piston pump can be shortened and a corresponding drive, e.g. an electric drive, of the piston pump can be made smaller. The suction valve is preferably designed in such a way that it does not close completely in the compression stroke, i.e. when the piston moves from the lower dead center to the upper dead center, if there is air in the pump chamber or a ventilation process is taking place. The metering pump can carry out such frequent ventilation strokes until the line and the metering pump up to the tank have been emptied. If, on the other hand, there is liquid in the pump chamber, i.e. in the normal mode of the injection system, the suction valve remains closed. 
         [0012]    According to an alternative embodiment, the suction valve is designed as a diaphragm valve instead of as a spring-loaded check valve. The diaphragm thereof is expediently manufactured from an elastomeric material. The diaphragm preferably has a sealing frame which is connected by radially extending webs to a centrally arranged valve plate, in particular being formed integrally. The diaphragm is preferably designed in such a way that it completely closes the first connection during a liquid pumping operation and does not do so during a gas pumping operation. 
         [0013]    As an alternative to a design as a diaphragm valve, provision is preferably made for the suction valve to be designed as a piston valve with an integrated restrictor. The piston valve with an integrated restrictor operates in a manner similar to the diaphragm valve, depending on which medium is being delivered. If a gas is being pumped, the piston valve does not close the first connection or at least does not close it completely. If, on the other hand, liquid is being delivered, the piston valve closes the first connection completely, with the result that the liquid is prevented from being delivered back to the tank and a pressure which may open the metering valve builds up in the pump chamber. 
         [0014]    According to an advantageous development of the invention, provision is made for the ventilation valve to be connected to a gas connection, which is designed, in particular, as an ambient air connection, as an exhaust pipe connection, being connected to an exhaust pipe of the motor vehicle, or as a ventilation connection, being connected to the tank. In the case of an air admission process in the injection system, either ambient air, exhaust gas or gas from the tank storing the liquid is therefore introduced into the injection system. It is thereby possible to ensure the ice pressure resistance of the injection system in a simple manner. 
         [0015]    As a particularly preferred option, at least one hydrophobic diaphragm is arranged between the ventilation valve and the gas connection. On the one hand, the hydrophobic diaphragm forms a simple restrictor and thus prevents the air from escaping unhindered to the outside from the system. On the other hand, the hydrophobic diaphragm prevents small quantities of liquid from escaping outward into the environment, especially during a venting process. 
         [0016]    As a particularly preferred option, the hydrophobic diaphragm is assigned at least one reservoir for liquid or gaseous medium. Since, on the one hand, the hydrophobic diaphragm has a low restricting effect, it is possible, on the one hand, for the reservoir to hold excess air or gas when designed as an air reservoir. On the other hand, the reservoir can hold small quantities of liquid filtered out of the air stream by the hydrophobic diaphragm, thereby ensuring that the air permeability of the hydrophobic diaphragm is not impaired. It is particularly advantageous if a reservoir is provided on each side of the hydrophobic diaphragm. The reservoir can be designed as an elastically deformable hose section, for example, allowing its volume to adapt to the quantity of excess air or liquid. 
         [0017]    The metering pump according to the invention has the advantage that it does not have to be driven in different directions in order to switch over from a normal mode of an injection system to a ventilation mode. For this purpose, the metering pump is designed as a piston pump and has a piston axially movable in a cylinder between an upper and a lower dead center and forming, together with the cylinder, a pump chamber, wherein, assigned to the pump chamber, the cylinder has a first connection for a tank line, which can be connected to a tank of the injection system, a second connection for a delivery line, which can be connected to the injection valve of the injection system, and a third connection for a ventilation line, which can be connected to a ventilation valve, of the injection system. Further advantages and features can be obtained from what has been already been described. 
         [0018]    The exhaust gas treatment system according to the invention is characterized by an injection system of the kind described above. The advantages already mentioned are thereby obtained. 
         [0019]    The method according to the invention for operating an injection system is characterized in that, to switch over from a liquid-delivering normal mode to an ice-pressure-resistant state, the piston continues to be operated at least substantially as in the normal mode, and the ventilation valve is opened and closed in such a way, in accordance with the position of the piston, that a gas is introduced into the injection system. The working direction of a drive, e.g. an electric drive, driving the piston is thus not affected, and this leads to advantages in respect of the dimensioning and also in respect of the speed of the switchover, as already described above. Simply by actuating the ventilation valve, air can be admitted to and released from the injection system. Further advantages and features are obtained from the embodiments described previously. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention will be described below in greater detail with reference to the drawings, in which: 
           [0021]      FIG. 1  shows a first illustrative embodiment of an exhaust gas treatment device for a motor vehicle having an injection system, 
           [0022]      FIG. 2  shows a second illustrative embodiment of the exhaust gas treatment device, 
           [0023]      FIG. 3  shows an advantageous development of the exhaust gas treatment device, 
           [0024]      FIGS. 4A and 4B  show a first illustrative embodiment of a suction valve of the injection system, and 
           [0025]      FIG. 5  shows a second alternative illustrative embodiment of the suction valve. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  shows, in a simplified representation, an exhaust gas treatment device  1  for a motor vehicle which is not shown specifically here. To introduce an ammonia-containing exhaust gas treatment medium into the exhaust gas stream of an internal combustion engine of the motor vehicle, the exhaust gas treatment device has an injection system  2 . 
         [0027]    The injection system  2  has a tank  3 , which is used to store the liquid exhaust gas treatment medium  4 . The tank  3  is very largely filled, although an air buffer  5  remains therein. The tank  3  is connected to a metering pump  7  by a tank line  6 . The metering pump  7  is designed as a piston pump  8  and, for this purpose, has a substantially pot-shaped cylinder  9 , in which a piston  10  is arranged in such a way that it can be moved axially between an upper and a lower dead center. By way of example, the piston  10  is connected to a connecting rod, which imparts an oscillating backward and forward motion to the piston  10 , as indicated by a double arrow  11 , by means of a crankshaft driven, for example, by electric motor. Together with the cylinder  9 , the piston  10  forms a pump chamber  12 , the volume of which is changed by the motion of the piston  10 . In this case, the piston  10  is designed as a solid-body piston and rests sealingly by means of its outer circumferential surface against the inner surface of the circumferential wall  13  of the cylinder  9 . If appropriate, it is also possible for one or more sealing rings to be provided between the piston  10  and the cylinder  9 . On its circumferential wall  13 , the cylinder  9  has a first connection  14 , which is connected to the tank line  6 . Connection  14  opens into the pump chamber  12  of the piston pump  8  above or at a distance from the lower dead center of the piston  10 , as illustrated in  FIG. 1 . A filter  15  and other components  16  are optionally arranged in the tank line  6  between the tank  3  and the metering pump  7 . At least one of the components  16  can be designed as a heating device, for example. 
         [0028]    The metering pump  7  has a second connection  17 , which is connected to a metering valve  19  by a delivery line  18 . The metering valve  19  is assigned to an exhaust pipe  20  of the abovementioned internal combustion engine and is designed as a passive injection valve  21 , which opens and closes under pressure control. In the present case, the injection valve  21  is designed as a spring-loaded injection valve  21 . An additional check valve  22  is furthermore arranged between the metering valve  19  and the second connection  17 . 
         [0029]    The metering pump  7  furthermore has a third connection  23 , which is connected to a ventilation valve  25  by a ventilation line  24 . The second connection  17  and the third connection  23  are both arranged or formed in an end region  26  of the cylinder  9  remote from the piston  10 . However, while the third connection  23  likewise opens into the pump chamber  12  through the circumferential wall  13 , connection  17  is arranged on the closed or substantially closed end face  27  of the pot-shaped cylinder  9 . The first connection  14  thus lies axially between the lower dead center of the piston  10  and the third connection  23 . 
         [0030]    In the normal mode, i.e. when the liquid exhaust gas treatment medium  4  is to be mixed in with the exhaust gas flowing through the exhaust pipe  20 , the piston  10 , which is traveling in the direction of the lower dead center, draws the exhaust gas treatment medium  4  into the pump chamber  12  via connection  14 . On its path in the direction of the upper dead center, the piston  10  then drives the liquid through connection  17  into the delivery line  18  to the metering valve  19 , which opens when the pressure is sufficient and injects the exhaust gas treatment medium  4  into the exhaust pipe  20 . In this case, the ventilation valve  25  is continuously closed. 
         [0031]    If the injection system  2  is switched off, the ice pressure resistance thereof must be ensured. This means that, when the injection system  2  is stationary, it is necessary to prevent the injection system  2  from being damaged by an increase in the volume of the exhaust gas treatment medium  4  if the latter freezes. In order to ensure ice pressure resistance, provision is made to free the injection system  2  at least substantially from the liquid exhaust gas treatment medium  4  and to admit air thereto. In this case, the procedure is as follows: 
         [0032]    As soon as the piston  10  leaves the upper dead center, the ventilation valve  25  is opened. The ventilation valve  25  is connected to a gas connection  27 , which is designed as an ambient air connection, for example. If the ventilation valve  25  is opened, it thus connects the pressure chamber  12  to the ambient air. If the piston  10  is moving in the direction of the lower dead center, ambient air is thus drawn into the pump chamber  12  by opening the ventilation valve  25 . Owing to the low flow resistance of the gas or ambient air, no vacuum is built up in the piston chamber during this process. As an alternative, the gas connection  27  can also be designed as an exhaust pipe connection and, to this extent, can be connected to the exhaust pipe. The gas connection  27  can also be designed as a tank connection, which is arranged on the tank  3  in the region of the air buffer  5 . Designing the gas connection  27  as a tank connection has the advantage that the air taken from the tank is, where appropriate, enriched with ammonia, with the result that, when air is drawn in or during air admission to the injection system  2 , crystal formation in the system, which could otherwise prejudice the operation of the ventilation valve  25  for example, is prevented. 
         [0033]    Since connection  14  opens into the pump chamber  12  in the circumferential wall of the cylinder  9  below connection  23 , connection  14  acts as a spool valve together with the piston  10 . As soon as the piston  10  has been pushed past connection  14  in the direction of the lower dead center, connection  14  is exposed. Since, by virtue of its geometry, connection  14  is therefore opened only late, it is at first predominantly the gas or the ambient air which is present in the pump chamber  12 . This process is further assisted by the fact that, in comparison with the ambient air, the liquid exhaust gas treatment medium is more severely restricted when the piston  10  is moving in the direction of the lower dead center and has a greater inertia. During this process, the check valve  22  remains in the closed position thereof and thus has no effect on the events in the pump chamber  12 . 
         [0034]    When the piston  10  reaches the lower dead center, the ventilation valve  25  is closed. The air is thereby forced into connection  14  as soon as the piston  10  moves back in the direction of the upper dead center. This has the effect that the liquid in the tank line  6  is forced back in the direction of the tank  3 . If the filter  15  is arranged close enough to the metering pump  7 , the tank line  6  can be supplied with air or freed from the liquid up to the filter  15  through a single stroke process of the piston  10 . Otherwise, this process is carried out until air has been admitted to the desired extent to the tank line  6 . 
         [0035]    As soon as the piston  10  crosses connection  14  or the opening thereof on the way to the upper dead center and thereby closes it, the pressure in the pump chamber  12  rises. Depending on the chosen relationship between the volumes and the position of connection  14 , the metering valve  19  is opened earlier or later by the ambient air compressed in the pump chamber  12 . The liquid in the delivery line  18  is thereby forced into the exhaust pipe  20 , and air is admitted to the delivery line  18  up to the metering valve  19  and, as a result, also to the metering valve  19  itself. 
         [0036]    The piston  10  advantageously has a chamfer  29  on its piston surface  28  facing the end face  17 , with the result that, during a stroke motion of the piston  10 , the gas can flow or is forced for longer into the tank line  6  through connection  14 . 
         [0037]    If the injection system  2  is to be put back into operation, it must be vented or freed from the gas and filled with the exhaust gas treatment medium  4 . This can likewise be accomplished in a simple manner by means of the injection system  2  under consideration. As soon as the piston  10  leaves the upper dead center, the ventilation valve  25  is closed. A vacuum thus arises in the pump chamber  12  and then draws in the air in the injection system via connection  14 . When the piston moves back in the direction of the upper dead center from the lower dead center, the ventilation valve  25  is opened again, and the air in the pump chamber  12  is pumped back into the environment through connection  23 . 
         [0038]      FIG. 2  shows another illustrative embodiment of the exhaust treatment device  1  or injection system  2 . Elements that are already known from  FIG. 1  are provided with the same reference signs and therefore attention is drawn to this extent to the description above. It is essentially only the differences which will be explored below. 
         [0039]    Whereas, in the illustrative embodiment in  FIG. 1 , connection  14  opens into the pump chamber  12  or cylinder  9  through the circumferential wall  13 , it is arranged on the end face of the cylinder  9  according to the illustrative embodiment under consideration ( FIG. 2 ). Since connection  14  can then no longer interact with the piston  10  as a spool valve, connection  14  is assigned a suction valve  30 . In the illustrative embodiment under consideration, the suction valve  30  is designed as a spring-loaded check valve  31 . If the piston  10  moves downward, i.e. in the direction of the lower dead center, the suction valve  30  opens automatically as soon as a certain vacuum is present in the pump chamber  12 . In the normal mode, the ventilation valve  25  is closed. As the piston  10  moves downward, the check valve  22  closes due to the opening pressure set at the metering valve  19 . At the same time, the suction valve  30  opens owing to the vacuum which now arises in the pump chamber  12 . The suction valve  30  can have a very soft spring. The piston  10  reaches the lower dead center, and compression of the liquid drawn in by the suction valve  30  then begins. The suction valve  30  closes, and check valve  22  opens, with the result that the liquid is forced in the direction of the passive injection valve  21 , which opens when the trigger pressure thereof is exceeded and injects the exhaust treatment medium into the exhaust pipe  20 . 
         [0040]    In the air admission mode, the ventilation valve  25  is open during the downward movement of the piston  10 , allowing gas to flow into the pump chamber  12 . Between the suction valve  25  and the tank  3  there is a liquid column and, on the other side of the ventilation valve  25 , an air column by virtue of the principle involved. Owing to the inertia of the liquid and, in particular, the friction thereof on the tube wall of the respective line, the air, which, in contrast to the liquid, does not have a friction on the tubular wall, is drawn into the cylinder  10  or into the pump chamber  12  through the ventilation valve  25  during the movement of the piston  10  in the direction of the lower dead center. This behavior can be further promoted through appropriate restrictor matching. 
         [0041]    Owing to the dead volumes and wall friction of the liquid or of the liquid exhaust gas treatment medium  4 , it is advantageous if the ventilation valve  25  is arranged as close as possible to the cylinder  9 . Once the piston  10  has reached the lower dead center thereof, the ventilation valve  25  closes until the piston  10  has reached the upper dead center thereof again. The air which has collected in the pump chamber  12  is thus forced through the suction valve  30  into the injection system  12  in the direction of the tank  3  or into the tank line  6 . At the same time, the suction valve  30  is embodied in such a way that it does not close or does not close completely in the air compression stroke while, in the normal mode, when there is only liquid in the pump chamber  12 , it closes completely. In this case, check valve  22  and the injection valve  21  will also remain closed. Depending on the piston stroke depth, the metering pump  7  must carry out a corresponding number of air strokes until the tank line  6  and the metering pump  7  up to the tank  3  have been emptied. In this case, it is advantageous if the piston  10  is designed without the chamfer  29  in order to increase the efficiency of the metering pump  7 . In contrast to the previous illustrative embodiment, another filter  32  is furthermore arranged in the tank  3 , said filter being arranged ahead of the tank line  6 . The injection system  2  under consideration has the advantage that the full stroke of the piston  10  can be used to draw in and inject the liquid exhaust gas treatment medium  4  and to drive gas into the tank line  6  and/or into the delivery line  18 . 
         [0042]    Whereas the piston in the first illustrative embodiment according to  FIG. 1  has to be designed as a solid-body piston, it is also possible, in the case of the illustrative embodiment according to  FIG. 2 , for it to be of elastically deformable design in the form of a diaphragm, the metering pump  7  thus being designed as a piston pump  8  with a deformable piston or as a diaphragm pump. The mode of action corresponds to that of the piston pump  8  described. Construction as a spool valve is not possible by means of the diaphragm pump, and therefore it would be difficult, if not impossible, to implement an embodiment of this kind in the case of the first illustrative embodiment. 
         [0043]      FIG. 3  shows an advantageous development of the injection system  2  according to  FIG. 2 . However, this development can of course also be provided in the case of the illustrative embodiment according to  FIG. 1 . Elements that are already known from the previous figures are provided with the same reference signs, and therefore attention is drawn to this extent to the above description. 
         [0044]    The advantageous development in  FIG. 3  relates to the gas connection  27 . If gas is drawn in via the ventilation valve  25  from the exhaust gas or the environment in the air admission mode, there can be pronounced crystallization at the seat of the ventilation valve  25  owing to contact between ammonia from the exhaust gas treatment medium and fresh air. Moreover, air enriched with ammonia can be expelled into the environment when the injection system  2  is vented, and this could lead to unpleasant smells. 
         [0045]    In order to avoid this, a hydrophobic diaphragm  32  is inserted between the gas connection  27  and the ventilation valve  25  in the present case. On the one hand, the hydrophobic diaphragm  32  represents a slight restriction and thus prevents air or gas from escaping unhindered to the outside from the injection system  2 . On the other hand, the hydrophobic diaphragm  32  prevents small quantities of liquid from escaping outward into the environment, especially during the venting of the injection system  2 . 
         [0046]    Respective reservoirs  33  and  34 , each forming a reservoir volume, are furthermore provided on each side of the hydrophobic diaphragm  32 . The reservoirs  33  and  34  can be designed as elastically deformable hose sections, for example. Since the hydrophobic diaphragm  32  has a slight restricting effect, the reservoirs  33 ,  34  on the one hand act as air reservoirs, and, on the other hand, small quantities of liquid can be accepted, with the result that these quantities do not affect or hinder the air permeability of the hydrophobic diaphragm  32 . 
         [0047]    In the present case, the hydrophobic diaphragm  32  is arranged directly on the bottom of the reservoir  33 . A line  35  leads from the hydrophobic diaphragm  32  to the reservoir  34 . From the latter, a further line  36  then leads to the ventilation valve  25 , which is connected to the metering pump  7  via the ventilation line  24 , as described above. 
         [0048]    The line  36  on the valve side is advantageously connected to reservoir  34  at a level which is as low as possible, and line  35  is connected in the upper region of reservoir  34 . Reservoir  33 , which simultaneously also forms the gas connection  27 , has a screen  37  at its open or free end, said screen having particularly small holes, which represent large restrictors. Thus, only slight air exchange can take place between the environment and reservoir  33 . In the normal mode, when the ventilation valve  25  is closed, the section between the gas connection  27  and the ventilation valve  25  thus forms a kind of dead end in which the air is essentially held in the reservoirs  33  and  34 . 
         [0049]    The reservoirs  33 ,  34  have the advantage that air with a high ammonia content cannot suddenly enter the environment. The screen  37  ensures that fresh air is drawn into reservoir  33 . If the release of air from the injection system  2  is performed in a garage, for example, reservoir  33  prevents ammonia-containing air from being expelled into the environment. In the diagrammatically represented arrangement of reservoir  33 , there would always be air with only a low ammonia concentration in reservoir  33  in the normal mode, owing to the small holes of the screen  37 . Reservoir  34 , in contrast, contains air with a high ammonia concentration, but this will not escape into reservoir  33  in the normal mode, owing to the low throttling action of the hydrophobic diaphragm  32 . During an air admission process, the ammonia-rich air in reservoir  34  is drawn into the injection system  2 , and the low-ammonia air in reservoir  33  will enter reservoir  34 . As a counterbalance, fresh air is drawn into reservoir  33  from the outside. 
         [0050]    Since the air drawn into the injection system  2  from reservoirs  33  and  34  already contains ammonia, there will be no crystallization within the injection system  2 , in particular at the seat of the ventilation valve  25 . If the injection system  2  is put back into operation, the fresh air drawn into reservoir  33  during shutdown, which has become enriched with ammonia only to a very slight extent in the intervening period owing to the hydrophobic diaphragm, is released again outward into the environment. 
         [0051]    This reservoir arrangement with the hydrophobic diaphragm  32  is optionally also implemented on the tank side, with the result that the tank  3  has a hydrophobic ventilation diaphragm  38  in the region of the tank line  6 . In this case, the air buffer  5  forms a corresponding reservoir  39 . Adjoining the hydrophobic ventilation diaphragm  38  on the other side is a line  40 , in particular a long line  40 , having a screen  41  at the end, wherein line  40  forms a reservoir  42 . The reservoir device on the tank side thus corresponds substantially to the reservoir device on the ventilation-valve side. Via the metering pump  7 , the two reservoir devices communicate with one another. This ensures that the ammonia-laden air can be transferred or is transferred backward and forward between the two reservoir devices in a controlled manner during air admission to and release of air from the injection system  2 , and that only the gas with a low ammonia content in reservoirs  33  and  42  can enter the environment in small quantities. 
         [0052]    In the case where the gas connection  27  is designed as an exhaust pipe connection, reservoir  33  could also be omitted since, in this case, the exhaust pipe  20  acts as a reservoir. 
         [0053]    The reservoir devices described can each also be provided in an injection system  2  having a metering pump in accordance with the illustrative embodiment in  FIG. 1 . In the illustrative embodiment under consideration, the filter  15  and further components  16 , as described in the previous illustrative embodiments, can advantageously likewise be provided. 
         [0054]      FIGS. 4 and 5  show alternative embodiments of the suction valve  14 . 
         [0055]      FIG. 4  shows the suction valve  14  as a diaphragm valve  43 . In this regard,  FIG. 4A  shows a sheetlike diaphragm  44  of the diaphragm valve  43  in a plan view, wherein the diaphragm  44  has a continuous sealing frame  45 , which is connected to a centrally arranged valve plate  47  by webs  46 . In this case, the valve plate  47 , the webs  46  and the sealing frame  45  are formed integrally with one another. Here, the diaphragm  44  is manufactured from an elastically deformable elastomer material. 
         [0056]      FIG. 4B  shows a longitudinal section through the diaphragm valve  43 . The diaphragm valve  43  has a stub  48  adjacent to connection  14  and a stub  49  associated with the tank line  6 . The stubs  48  and  49  are arranged in alignment with one another and spaced apart. The diaphragm  44  extends transversely between the stubs  48  and  49 , resting sealingly on the free end of stub  48  in the initial state. For this purpose, the sealing frame  45  is held sealingly in a housing forming or holding the stubs  48  and  49 . Air or liquid can thus pass from one stub to the other past the webs  46  and the valve plate  47  only through the diaphragm  44 . If the piston  10  is moved in the direction of the upper dead center, then, in the normal mode, the liquid forms a pressure wave which presses the valve plate  47  sealingly against the free end of stub  49  owing to the pressure pulse thereof, with the result that the pump chamber is sealed off in the direction of the tank  3 , and the injection valve  21  opens owing to the excess pressure. If, on the other hand, air is compressed, there is no pronounced pressure wave. The valve plate  47  rises only to the extent that air can flow past it and enter the other stub  49 . In this case, the suction valve  14  is thus not closed, and the air can force the liquid exhaust gas treatment medium  4  in the tank line  6  back in the direction of the tank  3 . 
         [0057]      FIG. 5  shows an alternative embodiment of the suction valve  14 , which in this case is designed as a piston valve  50  with an integrated restrictor. In this case, the piston valve  50  has an axially movable piston  51 , through which there extends a channel  52 , forming a restrictor  53 . The piston  51  is guided in a socket  54 , the diameter of which corresponds substantially to the diameter of the piston  41 , ensuring that it rests in the socket  54  with sealing at the sides. The socket  54  is made axially somewhat longer than the piston  51 , allowing the latter to move axially in the socket  54 . Connection  14  opens into the socket  54  on one side, and the tank line  6  opens into it on the other side. Here, the opening of connection  14  is aligned relative to the piston  51  in such a way that connection  14  is connected to the channel  52 . The tank line  6 , on the other hand, opens into the socket  54  in such a way that it is at a lateral distance from the channel  51 . If the piston  51  has moved up to the upper stop, as illustrated, the channel  52  is closed by the housing  54  of the piston valve  50 . 
         [0058]    The mode of operation is similar to that of the diaphragm valve  43 . If a pressure is built up in the pump chamber  12  in the normal mode by a movement of the piston  10 , the pressure wave through the fluid ensures that the piston  51  is forced against the upper stop, and the channel  52  is thereby closed. If, on the other hand, air is compressed in the pump chamber  12 , this can flow through the channel  52  without the piston  51  being forced into the upper stop. As a result, the compressed air can pass through the channel  52  into the tank line  6  and can there force back the liquid exhaust gas treatment medium  4 . 
         [0059]    If liquid is drawn in in the normal mode, the piston  51  is forced against the lower stop, in which the channel  52  is in continuous communication with connection  14 , thus enabling the exhaust gas treatment medium  4  to flow continuously in the direction of the pump chamber  12 . In this version of the suction valve, it is advantageous if the ventilation valve  25  is opened shortly after the piston  10  reaches the upper dead center. This ensures that the piston  51  has reliably left the upper stop. If appropriate, the opening or movement of the piston into the lower stop could also be assisted by a compression spring, which is arranged in the socket  54  between the upper stop and the piston  51 , for example. 
         [0060]    If air is to be released from the injection system  2  again, the ventilation valve  25  is closed during a downward movement of the piston  10  in the direction of the lower dead center. As a result, the air in the injection system  2  enters the pump chamber  12 . During the subsequent upward movement of the piston  10 , the valve  25  is opened and the air in the pump chamber  12  leaves the injection system  2  again via the ventilation valve  25  and passes into the environment, into the tank  3  or into the exhaust pipe  20 . The restricting effect and the hydrostatic pressure in the direction of the tank  3  is such that the air will not escape in the direction of the tank  3  through connection  14 . If a higher pressure loss is to be expected beyond the ventilation valve  25 , it would be advantageous if the previously described filter  32  in the tank bottom were designed as a hydrophobic filter.