BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a dosing valve and to a dosing device for introducing a liquid medium into an exhaust gas stream of an internal combustion engine of a motor vehicle. 
         [0002]    For internal combustion engines, compliance with pollutant emissions limits in the exhaust gas is legally required. Particularly in the case of a diesel vehicle, nitrogen oxide reduction is absolutely essential. One possibility for nitrogen oxide reduction is the known method of selective catalytic reduction (SCR), for example. In this method, a liquid reducing agent, e.g. an aqueous urea solution (“AdBlue”), is introduced into the exhaust gas stream in the exhaust pipe. With the hot exhaust gas, the aqueous urea solution is converted into gaseous ammonia, by means of which the toxic nitrogen oxide is reduced to form harmless water and nitrogen. 
         [0003]    Pressure-wave-controlled dosing systems are known. In these systems, the injection valve consists exclusively of mechanical components. A valve needle is held shut by means of a spring holder and opens automatically above a certain hydraulic pressure. Injection is thus controlled by means of a pressure wave, which is produced with the aid of an external pump. One disadvantage is the low accuracy of metering since the injection quantity is heavily dependent on the shape of the pressure wave, which can easily be affected by external influences. 
         [0004]    Patent Application DE 10 2011 078 852 A1, which is a post-publication, discloses another embodiment of a dosing valve, in which a pump and a nozzle are integrated in one unit. This integrated dosing module (IDM) does not have a return. The pump is designed in such a way that it also assumes the function of metering an accurate quantity of the liquid medium. This is achieved by virtue of the fact that the pump delivers in a purely volumetric way, with the result that a defined quantity of the medium is delivered with each delivery stroke. Very good metering accuracy is thereby achieved. One disadvantage in the system is the complex production thereof. Currently known embodiments contain a control edge, which must be set precisely, and a plurality of guides, which must be manufactured with an accurate fit relative to one another. 
         [0005]    DE 10 2008 001 789 A1 shows a dosing device in which a dosing valve is controlled by an electromagnet. In this case, a flow rate of the aqueous urea solution when the valve is completely open is determined in accordance with a pressure of the aqueous urea solution. For this purpose, use is made of a characteristic which represents a relationship of the steady state flow rate against the pressure. This characteristic shows a relationship between the activation duration of the electromagnet and the injection quantity. The dosing of the aqueous urea solution is controlled by suitable activation of the valve. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention differs from the prior art in that a pump sleeve is guided sealingly in one section of the stepped bore, in that a hollow-bored piston is guided sealingly in a central bore of the pump sleeve, in that a magnet sleeve is fastened to the housing in the low-pressure space, in that a stop sleeve is arranged on an opposite side of the low-pressure space from the magnet sleeve, and in that a stroke of the piston is limited by the magnet sleeve and/or by the stop sleeve. 
         [0007]    A delivery volume of the dosing valve can be accurately determined by the defined limitation of the stroke of the piston. This results in a considerably improved metering accuracy of the dosing valve combined with simplified control. The dosing device according to the invention is of simple construction and inexpensive to produce and nevertheless operates very reliably. 
         [0008]    In a preferred embodiment, it is envisaged that the stop sleeve is pressed into a magnet pot and that the magnet pot is connected to the housing. Rigid and accurate seating of the stop sleeve is thereby ensured; the magnet pot is preferably connected materially to the housing, e.g. by welding. This means that the axial position of the stop sleeve relative to the housing can be set during installation. 
         [0009]    A length of the stop sleeve or the depth to which it is pressed into the magnet pot preferably determines the stroke of the piston and hence also the delivery volume of the dosing valve. Through an appropriately designed or press-fitted stop sleeve, it is thus possible to structurally define the delivery volume of the dosing valve and adapt it to different internal combustion engines and/or exhaust apparatus. The variation between different items within a series can also be greatly reduced. The construction of the dosing device can be standardized and is therefore suitable for large-scale manufacture. This is also the case especially because at least largely automated assembly is possible. 
         [0010]    It is furthermore envisaged that the dosing valve has a pump working space in addition to the low-pressure space, wherein the low-pressure space and the pump working space are separated by a mechanical check valve. The particular effect of the check valve is that the pump working space can be automatically refilled in a simple manner by means of an excess pressure in the low-pressure space if there is an operational pressure drop in the pump working space. The first pressure space of the dosing valve after the pump in the direction of flow is preferably the low-pressure space. 
         [0011]    Owing to the fact that an armature of the electromagnet is arranged in the low-pressure space and that the armature is connected to the piston, it is possible, on the one hand, to actuate the piston in a simple manner and, on the other hand, also to limit the stroke thereof in a simple manner. 
         [0012]    The dosing device according to the invention is also distinguished by the fact that the pump sleeve projects with a pressure plate into a nozzle space and that a nozzle needle is arranged in the nozzle space, wherein the nozzle space is connected hydraulically to the pump working space by means of a transverse bore when the pressure plate is raised from a sealing edge of the housing. The pump sleeve—and hence also the pressure plate—is subjected to pressure at least indirectly by the movable piston when the electromagnet is activated. During this process, the medium situated in the pump working space is also subjected to pressure by the piston. 
         [0013]    The nozzle space can be filled iteratively from the pump working space. Thus, when the electromagnet is activated, the nozzle space is filled with the liquid medium from the completely filled pump working space, and pressure is built up accordingly; when the energization of the electromagnet ends, the pump working space, which was previously at least partially emptied, is automatically fully refilled with liquid medium. 
         [0014]    By virtue of its construction and of its design, the dosing device according to the invention, in particular the dosing valve, does not make any special demands on production technology. Thus, the guides, for example, do not have to be aligned with an accurate fit relative to one another and can thus be manufactured with a relatively large tolerance. All the guides can be ground in a single operation. The design does not contain any control edge or other functional elements which must be measured in the assembled state and then adjusted in the disassembled state. For this reason too, the dosing device according to the invention is inexpensive to produce. 
         [0015]    It is also advantageous that the low-pressure space is connected hydraulically to the pump. Thus, a delivery pressure of the pump determines the pressure in the low-pressure space. In this case, the pump is preferably a pre-feed pump. The demands made on the pump, especially as regards a pressure buildup, are not particularly high. This also makes the dosing device inexpensive. 
         [0016]    In one embodiment, it is envisaged that the dosing device according to the invention injects a liquid reducing agent, e.g. “AdBlue”, for nitrogen oxide reduction into the exhaust gas stream. The dosing device according to the invention is thus part of a known system for selective catalytic reduction (SCR). In this case, the dosing valve is arranged ahead of an SCR catalyst in the direction of exhaust gas flow. 
         [0017]    In another embodiment, provision is made as an alternative or in addition for the dosing device according to the invention to inject diesel fuel for particulate filter regeneration into the exhaust gas stream when required. The dosing device according to the invention is thus part of a known system for removing soot particles from the particulate filter of a diesel engine. For this purpose, the dosing valve is arranged upstream of an oxidation catalyst in the exhaust pipe. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Further features, possible uses and advantages of the invention will become apparent from the following description of an illustrative embodiment of the invention, which is shown in the drawing. In this case, all the features described or shown, per se or in any combination, form the subject matter of the invention. In the drawings: 
           [0019]      FIG. 1  shows the environment of the invention, and 
           [0020]      FIG. 2  shows an illustrative dosing device according to the invention in detail. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    An internal combustion engine  1  having an exhaust gas aftertreatment device  3  is illustrated in a greatly simplified and schematic form in  FIG. 1  and shows the environment to the invention. The exhaust gas aftertreatment device  3  comprises an exhaust pipe  5 , an oxidation catalyst  7  and an SCR catalyst  11  for selective catalytic reduction of toxic nitrogen oxide. A particulate filter, which is usually arranged downstream of the oxidation catalyst  7 , is not illustrated. The direction of flow of the exhaust gas through the exhaust pipe  5  is indicated by arrows (without reference signs). 
         [0022]    In order to supply the SCR catalyst  11  with a liquid reducing agent, e.g. an aqueous urea solution (“AdBlue”) or some other liquid reducing agent, a dosing valve  13  for introducing the aqueous urea solution is arranged on the exhaust pipe  5  upstream of the SCR catalyst  11 . The dosing valve  13  injects the aqueous urea solution into the exhaust pipe  5  upstream of the SCR catalyst  11  when required, e.g. when a high concentration of nitrogen oxides is detected in the exhaust gas. With the hot exhaust gas, the aqueous urea solution is converted into gaseous ammonia, by means of which the toxic nitrogen oxide is reduced to form harmless water and nitrogen in the SCR catalyst  11 . 
         [0023]    The dosing valve  13  is part of a dosing device  15 . The dosing device  15  furthermore comprises a pump  23 , preferably a pre-feed pump, which is arranged in a delivery line  19  between a storage tank  21  and the dosing valve  13 . The delivery line  19  supplies the dosing valve  13  with aqueous urea solution from the storage tank  21 . 
         [0024]    For the sake of completeness, attention is also drawn to sensors arranged in the exhaust gas aftertreatment device  3 , namely a nitrogen oxide sensor  25  and temperature sensors  27  and  29 . However, the sensors  25 ,  27  and  29  shown here represent only an illustrative selection since, in real exhaust gas aftertreatment devices  3 , even more sensors can be arranged in the region of the exhaust pipe  5 . 
         [0025]    The sensors  25 ,  27  and  29  and the pre-feed pump  23  and the dosing valve  13  are connected to a control unit  31  by signal lines (without reference signs). The control unit  31  can also comprise a plurality of control units and control units arranged in a distributed manner. 
         [0026]      FIG. 2  shows the dosing device  15  according to the invention, in particular the dosing valve  13 , in detail. The dosing valve  13  is surrounded by a housing  33 , from which a nozzle body  35  projects. 
         [0027]    In the housing  33  there is a stepped bore  34  having a plurality of sections  34 . 1  to  34 . 4 . The nozzle body  35  is press fitted in the first section  34 . 1  of the stepped bore  34 . 
         [0028]    The second section  34 . 2  of the stepped bore  34  delimits a nozzle space  41 , in which there are, inter alia, a pressure plate  45  and a first compression spring  51 . 
         [0029]    A pump sleeve  47  is guided sealingly and yet with the ability for axial movement in the fourth section  34 . 4  of the stepped bore  34 . 
         [0030]    A magnet sleeve  67  is press fitted at the top end of the fourth section  34 . 4  in  FIG. 2 . Together with an armature  69 , the magnet sleeve  67  also serves as an end stop for a piston  49 . The armature  69  is firmly connected to the piston  49 . Grooves (without reference signs) or longitudinal bores, which establish a hydraulic connection between those parts of the low-pressure space  61  which are “separated” from the magnet sleeve  67 , are machined into the magnet sleeve  67 . 
         [0031]    An outward-opening nozzle needle  37  is guided in the nozzle body  35 . Through the spring force of a nozzle closing spring  39 , the nozzle needle  37  closes the nozzle body  35  in a valve seat  36 . The nozzle closing spring  39  is supported at one end against the nozzle body  35  and at the other end against stop  38  on the nozzle needle  37 . 
         [0032]    The nozzle needle  37  opens when the pressure in a nozzle space  41  is so great that the hydraulic forces acting on the nozzle needle  37  are greater than the forces of the nozzle closing spring  39  acting on the nozzle needle  37  in the closing direction. 
         [0033]    To set the stroke of the nozzle needle  37 , a stroke setting washer  43  can be arranged between the stop  38  and the nozzle body  35 . 
         [0034]    In the closed position illustrated in  FIG. 2 , the nozzle needle  41  is closed by the pressure plate  45  at an opposite end of the nozzle space  41  from the stop  38 . 
         [0035]    The pressure plate  45  is part of a pump sleeve  47 , which is guided in an axially movable manner in the fourth section  34 . 4  of the stepped bore  34  of the housing  33 . The pump sleeve  47  has a central bore  48 , in which the piston  49  is guided. The piston  49  is bored hollow. Moreover, a check valve  58  comprising a valve member  57 , which is designed as a ball, and a second compression spring  59 , as well as a return spring  66  are arranged in the central bore  48 . In this case, the valve member  57  is pressed against the opening of the hollow-bored piston  49  by the second compression spring  59 . 
         [0036]    The central bore  48 , the valve member  57  and the pressure plate  45  delimit a pump working space  55 . A transverse bore  50 , which connects the pump working space  55  to the third section  34 . 3  of the stepped bore  34 , is arranged in the pump sleeve  47 . 
         [0037]    The cavity in the piston  49  forms a low-pressure space  61  of the dosing valve  13 , wherein the low-pressure space  61  is connected to the delivery line  19  by a hydraulic connection  63 . It is therefore the delivery pressure produced by the pump  23  which prevails in the low-pressure space  61 . 
         [0038]    The hydraulic connection  63  is a separate component, which is inserted sealingly into a central bore (without a reference sign) of the magnet pot  71  and is fixed there. This can be accomplished by means of a press fit or a welded joint or some other joining method, for example. The hydraulic connection  63  is connected hydraulically to the delivery line  19 . 
         [0039]    The piston  49  is moved out of the closed position illustrated (downward in  FIG. 2 ) by energizing an electromagnet  65  and thus serves as a pump piston. The energization of the electromagnet  65  is controlled by the control unit  31 . The electromagnet  65  operates against a force of the return spring  66 , which is arranged coaxially with the piston  49 . At one end, the return spring  66  rests against an upper edge of the pump sleeve  47  and, at the other end, rests against a snap ring  68  arranged on the piston  49 . 
         [0040]    The electromagnet  65  is recessed in the housing  33  of the dosing valve  13  at an end opposite the nozzle body  35 . For electromagnetic actuation, the armature  69  is arranged on the piston  49 , being guided in the fourth section  34 . 4  of the stepped bore  34  of the housing  33 . The fourth section  34 . 4  has a diameter larger than the diameter of the piston  49 . 
         [0041]    The electromagnet  65  is covered by a magnet pot  71 , wherein the magnet pot  71  simultaneously also closes the dosing valve  13  with respect to the delivery line  19  with the hydraulic connection  63 . An electric terminal  72  for the electromagnet  65  is provided in the magnet pot  71 . The magnet pot  71  is connected to the housing  33  by weld seams  73 . 
         [0042]    In the closed position illustrated in  FIG. 2 , a stop sleeve  75  inserted into the central bore (without a reference sign) of the magnet pot  71  serves as a stop for the armature  69 . In this case, the stop sleeve  75  and the position of the hydraulic connection  63  in the central bore of the magnet pot limit the stroke motion of the piston  49 . By means of the length of the stop sleeve  75  but also the installation depth of the hydraulic connection  63  in the magnet pot  75 , it is possible during assembly to set a volume of the low-pressure space  61  and a stroke of the piston  49  and hence the desired delivery volume of the dosing valve  13 . It is thereby possible to compensate for manufacturing tolerances in the other components, ensuring that all items in a series have the same delivery volume for each piston stroke. 
         [0043]    The dosing device  15  according to the invention has the special feature that, with the exception of the electromagnet  65  and of the magnet pot  72  with the hydraulic connection  63 , all the components can be inserted from “below” into the housing  33 . Here, “below” refers to the orientation of the dosing device  15  illustrated in  FIG. 2 . 
         [0044]    This capacity for assembly from one side is an important production advantage because it facilitates automation of assembly or makes it possible for the first time. 
         [0045]    The dosing valve  13  functions as follows: 
         [0046]    The dosing valve  13  is supplied with aqueous urea solution from the storage tank  21  via the delivery line  19 . In the process, the aqueous urea solution flows at the delivery pressure of the pump  23  into the low-pressure space  61  of the dosing valve  13 . 
         [0047]    In the initial state illustrated in  FIG. 2 , the piston  49  is in an upper end position and rests against the stop sleeve  75 . If the electromagnet  65  is energized, a magnetic force acts on the armature  69  via the magnet sleeve  67 . As a consequence thereof, the piston  49  moves in the direction of the nozzle body  35 . As a result, the pressure in the pump working space  55  rises until the pump sleeve  47  and, with it, the pressure plate  45  likewise move in the direction of the nozzle body  35 . During this process, a sealing edge  53  between the housing  33  and the pump sleeve  47  is opened. 
         [0048]    The aqueous urea solution is forced into the nozzle space  41  by the piston  49  and the valve member  57 , with the result that the pressure likewise rises there. As soon as the hydraulic forces resulting from the pressure in the nozzle space  41  and acting on the nozzle needle  37  exceed the spring force of the nozzle closing spring  39 , the nozzle needle  37  opens. 
         [0049]    After the end of energization, the piston  49  moves upward in the direction of its initial position through the spring force of the return spring  66 . During this process, the pressure in the pump working space  55  falls. The nozzle needle  37  is closed due to the spring force of the nozzle closing spring  39 , and the sealing edge  53  closes due to the spring force of the first compression spring  51 . 
         [0050]    If the pressure level of the low-pressure space  61  is undershot in the pump working space  55 , the ball  57  of the check valve  58  opens the low-pressure space  61 . In this way, the pump working space  55  can be refilled via the central bore in the piston  49 , the transverse bore  50  in the pump sleeve  47  and section  34 . 3  of the stepped bore  34  until the ball  57  of the check valve  58  closes the low-pressure space  61 . This occurs as soon as the pressure in the pump working space  55  is equal to the pressure in the low-pressure space  61 . The filling of the pump working space  55  is thus complete. 
         [0051]    The spring force of the nozzle closing spring  39  is advantageously chosen in such a way that the pressure level in the nozzle space  41  as the nozzle needle  37  opens is significantly above the pressure level in the low-pressure space  61 . The pressure level prevailing there is based on the delivery pressure of the pump  23 . In the unactivated state, the nozzle needle  37  is thus always held shut with a sufficiently high closing force. 
         [0052]    The delivery volume of the dosing valve  13  results, on the one hand, from the upper end position of the body  49  against the stop on the stop sleeve  75  and, on the other hand, from the lower end position against the stop on the magnet sleeve  67 . Through variation of its length or of the depth to which it is pressed in, the stop sleeve  75  is suitable for setting the stroke of the piston  49  and hence for setting the delivery volume (injection quantity) of the dosing valve  13 . 
         [0053]    In an embodiment which is not illustrated, the storage tank  21  can be filled with diesel fuel. The dosing device  15  can thus be used to regenerate a particulate filter in the exhaust pipe  5 . 
         [0054]    Particularly in the case of larger diesel engines, a burner is provided upstream of the particulate filter and of the oxidation catalyst  7 , for example, producing a flame when required that serves to evaporate the fuel additionally injected into the exhaust gas. This exhaust gas/fuel mixture reacts in a highly exothermic way in the oxidation catalyst  7 . As a consequence thereof, the exhaust gas reaches the high temperature required to regenerate the particulate filter. For this purpose, the dosing valve  13  is arranged upstream of the oxidation catalyst  7  in the exhaust pipe  5 . This embodiment can be installed as an addition or as an alternative for the purpose of injecting an aqueous urea solution upstream of the SCR catalyst  11 .
The present invention relates to a dosing valve and to a dosing device ( 15) for introducing a liquid medium into an exhaust-gas stream of an internal combustion engine ( 1) of a motor vehicle. The dosing device ( 15) has a pump ( 23) for delivering the liquid medium. The medium is injected into the exhaust-gas stream by means of a dosing valve ( 13) which can be controlled by way of an electromagnet ( 65). The dosing device ( 13) comprises a housing ( 33) with a stepped bore ( 34, 34.1, 34.2, 34.3, 34.4), wherein a pump sleeve ( 47) is guided sealingly in one portion ( 34.4) of the stepped bore ( 34). A hollow-bored piston ( 49) is guided sealingly in a central bore ( 48) of the pump sleeve ( 47). In a low-pressure chamber ( 61), an armature ( 69) is fastened to the piston ( 49), and furthermore a magnet sleeve ( 67) is fastened to the housing ( 33). A stop sleeve ( 75) is arranged on a side of the low-pressure chamber ( 61) situated opposite the magnet sleeve ( 67), wherein a stroke of the piston ( 49) is limited by the magnet sleeve ( 67) and/or by the stop sleeve ( 75).