Abstract:
The present invention relates to a dosing device ( 15 ) for introducing a liquid medium into an exhaust gas flow of an internal combustion engine ( 1 ) of a motor vehicle. The dosing device ( 15 ) has a pump ( 23 ) to deliver the liquid medium and injects said medium into the exhaust gas flow via a controllable dosing valve ( 13 ). The dosing device ( 15 ) has a  2/2  directional valve ( 17 ) which is arranged in a delivery line ( 19 ) between the pump ( 23 ) and a first low pressure chamber ( 47 ) of the dosing valve ( 13 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates 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, adherence to limit values for pollutant emissions in the exhaust gas is a legal requirement. In the case of a diesel vehicle in particular, nitrogen oxide reduction is imperatively necessary. One possibility for nitrogen oxide reduction is, for example, the known method of selective catalytic reduction (SCR). In said system, a liquid reducing agent, for example a urea-water solution, is introduced into the exhaust-gas stream in the exhaust pipe. With the hot exhaust gas, ammonia gas is generated from the urea-water solution, by means of which ammonia gas the nitrogen oxide, which is harmful to health, is reduced to form non-hazardous water and nitrogen. 
         [0003]    DE 10 2010 28 866 A1 has disclosed a device for introducing the urea-water solution into the exhaust-gas stream. The urea-water solution is pressurized by means of an electrically driven pump. The injection itself is controlled by way of the electric actuation of a dosing valve. Owing to the electric actuator means, the dosing valve must generally be cooled. In the case of said system, a gas mixer is generally necessary in order to ensure a homogenous distribution of the urea-water solution. 
         [0004]    A dosing device known from DE 10 2011 078 850 A1, which constitutes a subsequent publication, is controlled by pressure waves. A valve needle of a dosing valve is held closed by means of a spring holder and automatically opens above a certain hydraulic pressure. The injection is thus controlled by way of the pressure wave that is generated by means of a pump. The metering accuracy of said system is not particularly high because the injection quantity is dependent on a profile of the pressure wave with respect to time, and said pressure wave form is influenced by external influences. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention differs from the prior art in that the dosing device has a 2/2 directional valve which is arranged in a delivery line between the pump and a first low-pressure chamber of the dosing valve. The dosing device according to the invention can be switched by means of the 2/2 directional valve. The 2/2 directional valve exhibits only two states: either it is open or it is closed. 
         [0006]    The required pressure for the liquid medium is generated by the pump, preferably by a predelivery pump, wherein no particularly great requirements have to be imposed on the pump, in particular with regard to a build-up of pressure. However, the pump should be capable of building up a delivery pressure of approximately 9 bar. Since the 2/2 directional valve and the pump are components of simple construction, the dosing device can be produced at low cost. Here, the 2/2 directional valve may be an integral constituent part of the dosing valve; it may however also be connected to the dosing valve via a line. 
         [0007]    In a preferred embodiment, it is provided that the dosing valve has at least one low-pressure chamber and at least one high-pressure chamber, and that the low-pressure chamber and the high-pressure chamber are delimited by a piston. Here, the pistons of the at least one low-pressure chamber and at least one high-pressure chamber have different piston diameters in each case. That pressure chamber of the dosing valve which is the first pressure chamber downstream of the pump or downstream of the 2/2 directional valve as viewed in the flow direction is preferably the low-pressure chamber. 
         [0008]    A (stepped) piston is arranged in displaceable fashion between the low-pressure chamber and the high-pressure chamber. That part of the piston which delimits the low-pressure chamber has a larger diameter than that part of the piston which delimits the high-pressure chamber. The (stepped) piston is preferably of unipartite form. 
         [0009]    Owing to the different piston diameters, a hydraulic pressure intensification action is realized which can advantageously generate very high injection pressures with simultaneously high delivery quantities. The high pressure that can thus be generated serves for the injection of the liquid medium into the exhaust pipe. Here, the liquid medium can be atomized to form very small droplets, and rapid mixture formation with high quality can be achieved even over extremely short mixing paths. A check valve is arranged between the low-pressure chamber and the high-pressure chamber. 
         [0010]    The dosing device according to the invention, in particular the dosing valve, is of relatively simple construction, which imposes no special requirements on the manufacturing technology used. Accordingly, it is for example possible for all of the guides to be ground in one working step without rechucking The dosing device according to the invention is thus inexpensive to produce and operates reliably. 
         [0011]    It is also advantageous for a delivery volume of the dosing device according to the invention to be constant. This is realized primarily by means of defined upper and lower stops for the (stepped) piston in the low-pressure chamber and/or in the high-pressure chamber. The end stops structurally define the delivery volume. Adherence to predefined exhaust-gas limit values in the exhaust pipe is thus ensured. Furthermore, the system is OBD-2 compliant. This is to be understood to mean the capability for the SCR system to be monitored during the operation of the internal combustion engine, and thus for functionality to be ensured over the entire service life of the internal combustion engine. 
         [0012]    It is furthermore advantageous for the dosing device to have at least one exchangeable spacer device for defining a piston stroke in at least one pressure chamber. Here, it is for example possible for at least one stop of the piston to be altered by the exchangeable spacer device, in order that the piston stroke and thus the delivery quantity of liquid medium can be adapted to different internal combustion engines or exhaust-gas devices, and accurately defined. By means of spacer devices of different thickness, it is also possible for tolerance-induced scatter in a mass production context to be compensated. The spacer device may in this case be designed, for example, as an annular disk (for example a so-called residual air gap disk) which is positioned relative to a fixedly arranged stop in the interior of the dosing valve. 
         [0013]    The disk may also bear against the housing of the dosing valve such that the stroke movement of the piston is limited by the disk. Through the selection of the thickness of the disk, the piston stroke can be correspondingly defined, wherein an assortment of disks of different thickness offers a selection with which the delivery volume of dosing valves of the same type of construction can be easily modified and adapted to different internal combustion engines or exhaust-gas devices. In this way, the construction of the dosing device can be standardized and used in substantially all exhaust-gas devices. 
         [0014]    In one embodiment, it is provided that the dosing device according to the invention injects a liquid reducing agent, for example urea-water solution, for nitrogen oxide reduction into the exhaust-gas stream. In this way, the dosing device according to the invention is part of a known method for selective catalytic reduction (SCR). Here, the dosing valve is arranged upstream of an SCR catalytic converter as viewed in the exhaust-gas flow direction. 
         [0015]    In a further embodiment, it is alternatively or additionally provided that the dosing device according to the invention, if required, injects diesel fuel for particle filter regeneration into the exhaust-gas stream. In this way, the dosing device according to the invention is part of a known method for the removal of soot particles at the particle filter of a diesel engine. For this purpose, the dosing valve is arranged in the exhaust pipe upstream of an oxidation catalytic converter. 
         [0016]    It is furthermore advantageous for a housing of the dosing valve to have cooling ducts. Here, the cooling ducts are preferably connected to the tank return line for the liquid medium. In this way, the quantity flowing back into a storage tank can be used for cooling purposes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Further features, possible uses and advantages of the invention will emerge from the following description of an exemplary embodiment of the invention, which is illustrated in the drawing. Here, all of the features described or illustrated form the subject matter of the invention individually or in any desired combination. In the drawing: 
           [0018]      FIG. 1  shows the context of the invention; 
           [0019]      FIG. 2  shows a dosing device according to the invention in detail. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates an internal combustion engine  1  with an exhaust-gas aftertreatment device  3  in highly simplified and schematic form, and shows the context of the invention. The exhaust-gas aftertreatment device  3  comprises an exhaust pipe  5 , an oxidation catalytic converter  7  and an SCR catalytic converter  11  for the selective catalytic reduction of nitrogen oxide, which is harmful to health. The illustration does not show a particle filter, which is normally arranged downstream of the oxidation catalytic converter  7 . The flow direction of the exhaust gas through the exhaust pipe  5  is indicated by arrows (without reference sign). 
         [0021]    To supply a liquid reducing agent, for example a urea-water solution or some other liquid reducing agent, to the SCR catalytic converter  11 , a dosing valve  13  for the urea-water solution is arranged on the exhaust pipe  5  upstream of the SCR catalytic converter  11 . The dosing valve  13  injects the urea-water solution into the exhaust pipe  5  upstream of the SCR catalytic converter  11  when required, for example when a high concentration of nitrogen oxides in the exhaust gas is detected. With the hot exhaust gas, ammonia gas is generated from the urea-water solution, by means of which ammonia gas the nitrogen oxide, which is harmful to health, is reduced in the SCR catalytic converter  11  to form non-hazardous water and nitrogen. 
         [0022]    The dosing valve  13  is part of a dosing device  15 . The dosing device  15  furthermore comprises a 2/2 directional valve  17  which is arranged in a delivery line  19  between a pump  23  and the dosing valve  13 . The delivery line  19  supplies urea-water solution to the dosing valve  13  from a storage tank  21 . For the delivery of the urea-water solution, the delivery line  19  has a pump  23 , preferably a predelivery pump, between the 2/2 directional valve  17  and the storage tank  21 . The predelivery pump  23  should preferably be capable of generating a delivery pressure of approximately  9  bar. Furthermore, a return line  24  into the storage tank  21  for excess urea-water solution is connected to the dosing valve  13 . The 2/2 directional valve  17  may—as illustrated in FIG.  1 —be arranged in the delivery line  19 , though may also be integrated in the dosing valve  13  (integrated dosing module IDM). 
         [0023]    For the sake of completeness, reference is also made to sensors arranged in the exhaust-gas aftertreatment device  3 , specifically a nitrogen oxide sensor  25  and temperature sensors  27  and  29 . The sensors shown here, however, constitute merely a certain exemplary selection, wherein yet further sensors may be arranged in the region of the exhaust pipe  5  in the real operating situation. 
         [0024]    The sensors  25 ,  27  and  29  and the predelivery pump  23  and the 2/2 directional valve  17  are connected by way of signal lines (without reference sign) to a control unit  31 . The control unit  31  may also comprise multiple control units in a distributed arrangement. 
         [0025]      FIG. 2  shows the dosing device  15  according to the invention, in particular the dosing valve  13 , in detail. The dosing device  13  is enclosed by a housing  33 , out of which a nozzle body  35  projects. 
         [0026]    An outwardly opening nozzle needle  37  is guided in the nozzle body  35 . The nozzle needle  37  closes the nozzle body  35  under the action of the spring force of a nozzle closing spring  39 . The nozzle needle  37  opens when the pressure in a high-pressure chamber  41  of the dosing valve  13  is of such a magnitude that the hydraulic forces acting on the nozzle needle  37  are greater than the forces of the nozzle closing spring  39  that act on the nozzle needle  37  in the closing direction. 
         [0027]    Between the nozzle body  35  and a hydraulic port  43  for the delivery line  19 , the housing  33  encloses a cylinder  45 . The delivery line  19  issues, directly downstream of the port  43  in the cylinder  45 , into a low-pressure chamber  47 , which in turn is delimited by a hollow-bored piston  49 . The hollow chamber of the piston  49  is part of the low-pressure chamber  47 . 
         [0028]    The piston  49  can be moved counter to the spring force of a restoring spring  51  under the action of the delivery pressure in the delivery line  19 . The restoring spring  51  is arranged coaxially with respect to a reduced-diameter section  49 . 2  of the piston  49 . A step in the cylinder  45  serves as a stroke stop  52  for the piston  49 . 
         [0029]    A residual air gap disk  53  is arranged as a spacer between the port  43  and the piston  49 , said residual air gap disk being arranged fixedly in the interior of the housing  33 . Here, the residual air gap disk  53  limits the stroke movement of the piston  49 . By way of the thickness of the residual air gap disk  53 , a volume of the low-pressure chamber  47 , and a possible stroke of the piston  49 , can be set at the assembly stage. 
         [0030]    In the section  49 . 2  of the piston  49 , the low-pressure chamber  47  is closed with respect to the high-pressure chamber  41  by a ball  55  of a check valve. Here, the ball  55  is pressed against an opening of the low-pressure chamber  47  by a compression spring  57 , counter to the action of the pressure in the low-pressure chamber  47 . The ball  55  is thus the valve element of a check valve between low-pressure chamber  47  and high-pressure chamber  41 . 
         [0031]    A diameter D of the piston  49  in the low-pressure chamber  47  in the region of the port  43  of the delivery line  19  is greater than the diameter d of the piston  49  that is subjected to pressure by the high-pressure chamber  41 . In this way, in the dosing valve  13 , by means of the delivery pressure of the pump  23  acting in the region of the port  43 , hydraulic pressure intensification from the low-pressure chamber  47  to the high-pressure chamber  41  can be realized. 
         [0032]    By means of a throttle  59  arranged in the section  49 . 2  of the piston  49 , the urea-water solution can be returned into the storage tank  21  via a return chamber  61 . 
         [0033]    The dosing valve  13  functions as follows: 
         [0034]    The dosing valve  13  is supplied with urea-water solution from the storage tank  21  via the delivery line  19 . The 2/2 directional valve  17  arranged in the delivery line  19  is, during operation, either open or closed. The corresponding position is specified by the control unit  31 . 
         [0035]    When the 2/2 directional valve  17  is opened, the pressure in the low-pressure chamber  47  is raised to the predelivery pressure (approximately  9  bar) generated by the pump  23 . Because the return chamber  61  is permanently connected to the return line into the storage tank  21  and is at ambient pressure, a resultant force acts on the piston  49  in the low-pressure chamber  47 . Said resultant force pushes the piston  49  downward, which ultimately results in an increase of pressure of the urea-water solution in the high-pressure chamber  41 . 
         [0036]    If the pressure built up in the high-pressure chamber  41  exceeds the spring force of the nozzle closing spring  39 , the nozzle needle  37  opens counter to the force of the nozzle closing spring  39 , and the urea-water solution is injected until the piston  49  has reached its lower stroke stop  52  on the housing  33 . When the end position is reached, the delivery into the high-pressure chamber  41  is ended. 
         [0037]    The 2/2 directional valve  17  is subsequently closed. As a result, the pressure in the low-pressure chamber  47  and in the high-pressure chamber  41  falls, and the nozzle needle  37  closes. 
         [0038]    The force of the restoring spring  51  pushes the piston  49  upward until the latter has reached its end position. Owing to the upward movement of the piston  49 , the pressure in the high-pressure chamber  41  falls to a value below the pressure level in the low-pressure chamber  47 , and the ball  55  of the check valve opens. In this phase, the high-pressure chamber  41  is refilled with the displaced volume from the low-pressure chamber  47 . Owing to the pressure intensification, a greater volume is displaced in the low-pressure chamber  47  than can be received in the high-pressure chamber  41 . The excess urea-water solution is discharged via the throttle  59  into the return line  24 . 
         [0039]    With every stroke of the piston  49 , the dosing valve  13  according to the invention always delivers an exactly defined volume. This is ensured by virtue of the fact that the 2/2 directional valve  17  is open for longer than the piston  49  requires to perform its maximum stroke. In this way, the dosing valve  13  can be operated volumetrically. In this way, for example in the context of on-board diagnosis, a time for the injection of the known delivery quantity can be determined in the control unit  31 , and it can thus be ensured that predefined exhaust-gas limit values are adhered to in the exhaust pipe  5 . The stroke of the pressure intensifier can be set exactly by way of the thickness of the residual air gap disk  53 , wherein the residual air gap disk  53  simultaneously defines the upper end position of the piston stroke. 
         [0040]    In an embodiment that is not illustrated, the housing  33  may comprise cooling ducts which are connected to the return line  24 . In this way, the quantity of urea-water solution flowing back into the storage tank  21  can be used for cooling purposes. 
         [0041]    In a further embodiment that is not illustrated, the storage tank  21  may be filled with diesel fuel. In this way, the dosing device  15  can be used for the regeneration of the particle filter in the exhaust pipe  5 . In particular in the case of large diesel engines, it is for example the case that a burner is provided upstream of the particle filter and of the oxidation catalytic converter  7 , which burner, when required, generates a flame which serves to evaporate the fuel additionally injected into the exhaust gas. This exhaust-gas/fuel mixture reacts in intensely exothermic fashion in the oxidation catalytic converter  7 . As a result, the exhaust gas reaches the high temperature required for the regeneration of the particle filter. For this purpose, the dosing valve  13  is arranged in the exhaust pipe  5  upstream of the oxidation catalytic converter  7 . This embodiment may be implemented in addition to or alternatively to the injection of urea-water solution upstream of the SCR catalytic converter  11 .