Abstract:
The invention relates to a device and method for metering a liquid, in particular a fuel, into an exhaust gas tract of an internal combustion engine, wherein the device comprises at least one injection valve that can be closed by a closing member and that has cooling circuit for regulating the temperature in the injection valve, and wherein a throttle element is connected upstream of the injection valve to control, by closed loop or open loop, the quantity of a volume flow of the liquid through the injection valve, in particular through the cooling circuit of the injection valve, wherein the injection valve opens when the throttle element de-throttles the volume flow of the liquid to the injection valve.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention proceeds from a device and a method for metering a liquid into the exhaust tract of an internal combustion engine according to the preamble of the independent claims. 
         [0002]    For the aftertreatment of exhaust gases from an internal combustion engine, DE 44 36 397 B4 discloses the delivery by a delivery element of a liquid, for example liquid urea solution or fuel, from a reservoir to an injection valve, which meters a required quantity of the liquid into the exhaust tract. Here the injection valve is arranged on the exhaust tract in such a way that its injection orifice is directed into the exhaust tract. 
         [0003]    In operation very high temperatures can occur on the injection valve, particularly at its injection orifice, due to hot exhaust gases in the exhaust tract. This negative effect may be further exacerbated by the need to arrange the injection valve in proximity to hot components, such as the internal combustion engine, for example, or an exhaust gas turbocharger. There is the risk here of an unwanted thermal decomposition of the liquid in the injection valve or a formation of deposits in the injection orifice of the injection valve. The metering accuracy of the injection valve may thereby be impaired, which in extreme cases can lead to failure of the injection valve. 
         [0004]    In order to avoid these very high temperatures in the injection valve, DE 10 2007 011 686 A1 discloses an injection valve with cooling in the area of the injection orifice. In addition, DE 10 2006 019 973 A1 discloses a metering system for providing reducing agents in the exhaust tract, in which a metering valve of the metering system can be cooled by a cooling circuit. 
       SUMMARY OF THE INVENTION 
       [0005]    The device according to the invention and the method according to the invention for metering a liquid into the exhaust tract of an internal combustion engine by contrast has the advantage that the injection valve opens when the restrictor element releases the volumetric flow of the liquid to the injection valve. This means that it is possible to adjust to any flow, however small, through the cooling circuit of the device, so that the cooling of the injection valve can be controlled as a function of the demand and does not always ensue at the maximum delivery of the pump. Less power is therefore needed in order to deliver the liquid through the injection valve, which leads to an increased efficiency of the internal combustion engine. In addition, a restrictor in the inlet to the injection valve gives the injection valve greater robustness to withstand pressure fluctuations in the liquid circuit upstream of the restrictor element, since such pressure fluctuations are damped by the restrictor element. Reducing such pressure fluctuations avoids injection control defects and leads to a higher metering accuracy of the injection valve. 
         [0006]    In an advantageous development the injection valve comprises a further restrictor, arranged in the closing member, for example, which has a greater restricting effect than the restrictor element when the flow through the first restrictor element is fully opened. The further restrictor serves to limit the the maximum cooling quantity through the cooling circuit of the injection valve, in particular the return quantity from the cooling circuit, so that, for example, a pressure can be built up in the injection valve. 
         [0007]    In a further advantageous development a first operating state, in which the restrictor element determines the volumetric flow through the cooling circuit of the injection valve, is succeeded by a second operating state, in which a pressure of the liquid in the injection valve is increased when the volumetric flow to the injection valve is released and is restricted by the further restrictor in the injection valve, and the injection valve opens when a defined pressure level is reached. This development allows the injection valve to be designed as a pressure-controlled valve, which opens in excess of a defined pressure threshold. Here the pressure in the injection valve may be adjusted solely via the restrictor element, if the further restrictor has a fixed cross section and therefore from a certain release of the restrictor element onwards becomes the flow-determining restrictor of the cooling circuit. 
         [0008]    In an advantageous embodiment of the device the closing member of the injection valve is embodied as a valve needle, in particular as a hollow needle. Designing the closing member as a valve needle allows a liquid return of the injection valve to be effected along the valve needle in the closing member, which return, in the case of a hollow needle, may be formed inside the hollow needle, so that the injection valve can be of very compact design. 
         [0009]    In a further advantageous embodiment of the device the restrictor element comprises a metering element, for example a metering pump, in particular an electrically controlled metering pump, a variable restrictor and/or a metering valve. A metering pump is advantageous since it is capable of boosting the pressure acting on the liquid, so that the liquid can be injected at a higher injection pressure and can be atomized more finely. 
         [0010]    In a particularly advantageous embodiment of the device the metering pump does not fully prevent the supply of liquid to the injection valve when the metering pump is in a deactivated operating state. This advantageously ensures that even with the metering pump deactivated liquid can be delivered through the injection valve for cooling purposes. Furthermore in the activated operating state the metering pump is capable of generating a pressure which exceeds the inlet pressure to the restrictor element and therefore leads to an excess pressure. The injection valve can therefore also be adjusted to opening pressures which exceed the inlet pressure to the restrictor element. The liquid supply to the restrictor element can thereby be of a particularly simple and cost-effective design, and this allows a connection to liquid circuits already existing, for example the low-pressure circuit of a fuel injection system. 
         [0011]    In a further advantageous development the injection valve comprises a cooling circuit with a liquid inlet and a liquid return, the liquid return being arranged in an inner area of the injection valve, in particular inside the closing member. Arranging the liquid return in an inner area of the injection valve means that the heated liquid can be discharged from the cooling circuit of the injection valve over a relatively short distance, so that further heating of the liquid and possible thermal decomposition or ageing are minimized. 
         [0012]    In a further advantageous development a filter element, for example a disk filter, is arranged in a housing of the injection valve, particularly in a liquid feed line to the cooling circuit. Incorporating a filter element in the injection valve serves to increase the robustness of the injection valve towards particles. With particles there is a risk of these particles being deposited in areas relevant to the functioning of the injection valve, in particular on the valve seat and in the further restrictor, and impairing the working of the valve or causing increased wear, and in extreme case particles may lead to a complete failure of the injection valve. The filter element reduces this risk, which leads to a longer service life and increased working accuracy over the operating life of the injection valve. 
         [0013]    In a further advantageous embodiment of the device the cooling circuit of the injection valve cools an area of the injection valve facing the exhaust tract, in particular a valve seat. The cooling of the valve seat by the cooling circuit affords the advantage that it specifically cools precisely that point of the injection valve subjected to the greatest thermal load, in particular the area of the valve seat and the injection orifice, and therefore minimizes the risk of deposits on the valve seat and coking of the injection orifices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Exemplary embodiments of the invention are represented in the drawings and are explained in more detail in the following description. 
           [0015]      FIG. 1  shows a schematic representation of the device according to the invention for metering a liquid into the exhaust tract of an internal combustion engine. 
           [0016]      FIG. 2  shows a sectional representation of a first exemplary embodiment of the device according to the invention. 
           [0017]      FIG. 3  shows a sectional representation of a further exemplary embodiment of the device according to the invention. 
           [0018]      FIG. 4  shows a sectional representation of a further exemplary embodiment of the device according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  represents the device  100  according to the invention for metering a liquid  12  into an exhaust tract  20  of an internal combustion engine  10 . The device  100  here comprises the components represented inside the dashed line. 
         [0020]    A reservoir  13  for storing the liquid  12  is connected to a suction-side inlet  21  of a pump  14  via a first connecting line  16 . Via its delivery-side outlet  22  the pump  14  is connected by a further connecting line  17  to a restrictor element  34 . From the restrictor element a further connecting line  18  leads to an injection valve  50 , which is arranged on the exhaust tract  20  of the internal combustion engine  10 . The injection valve  50  comprises a housing  51 , which is of trough-shaped design, and can be closed by a closing member  61  on its side facing the exhaust tract  20 . On its side remote from the exhaust tract  20  the housing  51  is closed by a cover  59 , which guides the closing member  61 . A cooling circuit  75 , which in an outer area of the injection valve  50  leads from the connecting line  18  to a valve seat  53 , which with the outwardly opening closing member  61  is closed by a valve disk  54 , is formed in the injection valve  50 . From the valve seat  53  the cooling circuit  75  in an inner area of the injection valve  50  leads along the closing member to the cover  59 . The cover  59  is connected via a return line  19  to the reservoir  13 . 
         [0021]    The pump  14  draws a volumetric flow  15  of the liquid  12  from the reservoir  13  via the connecting line  16  and delivers it via the connecting line  17  to the restrictor element  34 . In the initial state the restrictor element  34  has a restricting effect, so that the volumetric flow  15  flows from the restrictor element  34  at reduced pressure via the connecting line  18  to the injection valve  50 , the connecting line  18  being connected to the cooling circuit  75  of the injection valve  50 . The main direction of flow of the liquid  12  represented by arrows in the drawing. 
         [0022]    As it flows through the cooling circuit  75 , the volumetric flow  15  of the liquid  12  cools the area of the valve seat  53  of the injection valve  50  particularly subjected to thermal load and flows back to the reservoir  13  via the return line  19 , which is connected to the cover  59  of the injection valve  50 . If the restrictor element  34  is activated in such a way that the volumetric flow  15  to the injection valve  50  is released by the restrictor element  34 , a pressure of the liquid  12  acting on the closing member  61  increases in the injection valve  50 . If the pressure in the injection valve  50  reaches or exceeds a threshold, which is needed in order to overcome a closing force acting on the closing member  61 , the closing member  61  opens and allows a metering of the liquid  12  into the exhaust tract  20  of the internal combustion engine  10 . An aqueous urea solution or a fuel, in particular diesel fuel, is suitable as liquid  12  for use in this device  100 . 
         [0023]    Alternatively the liquid  12  may also be stored in and fed to the device  100  from a pressurized circuit, for example a low-pressure circuit of a fuel injection system. In this case it is possible to dispense with the pump  14 , if the pressurized circuit provides the liquid  12  with a pressure which exceeds the threshold, which is needed in order to overcome the closing force acting on the closing member  61  in the injection valve  50 . 
         [0024]    As a further alternative the injection valve  50  may also be embodied as an inwardly opening valve, in which the valve seat  53  may alternatively also be closed by a valve ball  55 . In the device claimed the cooling of the injection valve  50  by the cooling circuit  75  is not limited to the area of the valve seat  53  but may also dissipate the heat from elsewhere, in particular from the housing  51  of the injection valve  50 , so that a direct incident flow against the valve seat  53  is not absolutely essential. Here the cooling circuit  75  may also be arranged in the portion in the outer area of the injection valve  50  leading from the valve seat  53  to the return line  19 , in particular in the housing  51 . In this case the return line  19  may alternatively also be connected directly to the housing  51 . 
         [0025]      FIG. 2  shows a sectional representation of a further exemplary embodiment of the device  100  according to the invention. Here the reservoir  13  for storing the liquid  12  is connected via the first connecting line  16  to the suction-side inlet  21  of a pre-supply pump  23  of a fuel injection system  30 . The pre-supply pump  23  is connected, via the further connecting line  17  to a metering valve  32  acting as restrictor element  34 , and via a further connecting line  25  in a known manner to the fuel injection system  30  supplying the internal combustion engine  10 . The metering valve  32  is connected via the connecting line  18  to the injection valve  50 , which is arranged on the exhaust tract  20  of the internal combustion engine  10 . The injection valve  50  comprises a housing  51 , which is of trough-shaped design, and on its side facing the exhaust tract  20  can be closed by a closing member  61  situated on a central axis  60  of the injection valve  50 . Here a valve seat  53 , which can be closed by a valve disk  54  formed on the closing member  61 , is formed on an end face of the housing  51  facing the exhaust tract  20 . The closing member  61  is embodied as a hollow needle  63 , which is guided by an insert  52 , which is pressed in the cover  59 . A valve spring  64  is arranged between a bearing surface  68 , formed on the end face of the housing  51  facing the exhaust tract  20 , and a further bearing surface  69 , embodied as a spring plate  65  of the closing member  61 . The connecting line  18  opens via an angled port  81  and a connecting port  82  in the cover  59  into an annular orifice  57 , which is defined by the cover  59  and the housing  51 . Here a sealing element  72  in the form of an O-ring  73 , which seals off the injection valve  50  externally to prevent an unwanted escape of the liquid  12 , is arranged between the cover  59  and the housing  51 . The annular orifice  57  is hydraulically connected via a disk filter  58 , which is formed by the insert  52  and the housing  51 , to the cooling circuit  75 , which comprises a liquid inlet  76  and a liquid return  77 . In order to obtain an optimum cooling effect, the liquid inlet  76  arranged in an outer area of the injection valve  50  leads to the area of the valve seat  53  subjected to a high thermal load, whilst the liquid return  77  is arranged inside the closing member  61  embodied as a hollow needle  63 . The liquid inlet  76  and the liquid return  77  are connected to one another via a further restrictor  62 , which is embodied as a port  66  in the closing member  61  embodied as a hollow needle  63 . The liquid return  77  is connected via an orifice  83  in the insert  52  and an orifice  56  in the cover  59  to the return line  19 , which connects the injection valve  50  to the reservoir  13 . 
         [0026]    The pre-supply pump  23  of the fuel injection system  30  connected to the reservoir  13  via the connecting line  16  delivers a volumetric flow  15  of the liquid  12  to the metering valve  32  via the connecting line  17 . In the initial state the metering valve  32  restricts the volumetric flow  15  to the injection valve  50 , in such a way that the volumetric flow  15  of the liquid  12  is delivered to the injection valve  50  at reduced pressure via the connecting line  18 , the liquid  12  flowing via the angled port  81  and the connecting port  82  out of the connecting line  18  into the annular orifice  57  of the injection valve  50 . The liquid  12  passes via the disk filter  58  into the cooling circuit  75  of the injection valve  50 . As it flows through the liquid inlet  76 , the heat of the injection valve  50 , subjected to a thermal load, is absorbed and is dissipated via the liquid return  77 . In passing from the liquid inlet  76  into the liquid return  77  the liquid  12  of the port  66  flows through in the closing member  61 , which is embodied as a hollow needle  63  and which acts as a further restrictor  62 . Here the restriction effect of the port  66  in the initial state does not determine the rate of flow, so that only a slight pressure increase, if any, occurs in the liquid inlet  76  and the pressure in the liquid inlet is below the threshold, so that the pressure is not sufficient to overcome the spring force of the valve spring  64  and to lift the valve disk  64  of the closing member  61  off from the valve seat  53  of the injection valve  50 . In the initial state, therefore, no liquid  12  is metered into the exhaust tract  20  of the internal combustion engine  10 . The liquid  12  flows via the liquid return  77  through the orifice  83  in the insert  52  and the orifice  56  in the cover  59  and via the adjoining return line  19  back to the reservoir  13 . 
         [0027]    If the metering valve  32 , proceeding from the initial state described, is opened by electrical activation, the volumetric flow  15  of the liquid  12  to the injection valve  50  is released, the actuation of the metering valve  32  causing the volumetric flow  15  through the injection valve  50  to be limited by the port  66 , acting as further restrictor  62 , as it passes between the liquid inlet  76  and the liquid return  77  of the cooling circuit  75 . The pressure in the injection valve  50  thereby increases in the liquid inlet  76  or, at least briefly, exceeds the threshold  28 , which is sufficient to overcome the spring force of the valve spring  64 . Overcoming of the spring force of the valve spring  64  causes the valve disk  54  to lift from the valve seat  53  and allows metering of the liquid  12  into the exhaust tract  20  of the internal combustion engine  10 . Due to the metering or the restriction of the volumetric flow  15  by the metering valve  32 , the pressure in the liquid inlet  76  dips below the threshold  28  again, so that the closing member  61  is returned into the initial position again by the valve spring  64  and the injection valve  50  closes. 
         [0028]    Alternatively the metering valve  32  may also be actuated mechanically, pneumatically or hydraulically. The cover  59  of the injection valve  50  may alternatively also be integrally formed with the insert  52 , the closing member  61  also being alternatively guided in the housing  51  and/or in the cover  59 . The disk filter  58  can also possibly be dispensed with, particularly if a filter element is arranged in the connecting line  18  or in the annular orifice  57 . 
         [0029]    The sealing between the housing  51  and the cover  59  of the injection valve  50  is not limited to a sealing element  72 , for example an O-ring  73 , other alternatives here, for example, being to connect the cover  59  to the housing  51  by a cohesive material joint, for example by welding the cover  59  and the housing  51  together, or to connect them by positive interlock, for example by way of a sealing cone. 
         [0030]    Alternatively, as shown in  FIG. 3 , the pre-supply pump  23  may be connected by its delivery-side outlet  22  via the connecting line  17  to a variable restrictor  35 , which as restrictor element  34  is capable of limiting the volumetric flow  15  of the liquid  12  to the injection valve  50  through the connecting line  18 . Here the injection valve  50  is embodied as an inwardly opening injection valve  50 , the valve seat  53  in the housing  51  being closed by a valve ball  55  arranged between the closing member  61  and the valve seat  53 . In the case of the inwardly opening injection valve  50 , the valve spring  64  is positioned between the spring plate  65  of the closing member  61  and the insert  52  situated in the cover  59 , it being possible in all exemplary embodiments for the cover  59  to be integrally formed with the insert  52 . The embodiment of an inwardly opening injection valve  50  is not confined to the exemplary embodiment represented in  FIG. 3 , having a variable restrictor  35  between the pre-supply pump  23  and the injection valve  50 , but may also be transferred to the other exemplary embodiments outlined. A cardanic action of the valve ball  55  obviates the need for a highly precise alignment of the closing member  61  in the insert  52 . A further advantage accrues from the fact that an inwardly opening injection valve  50  can be configured so that opening of the injection valve gives rise to a hydraulically acting closing force, which presses the valve ball  55  back into the valve seat  53 . It is thereby possible to keep a valve lift of the closing member  61  small, improving the facility for metering minute quantities of the liquid  12 . Alternatively the valve ball  55  may also be integrally connected to the closing member  61 . 
         [0031]    In the exemplary embodiment sketched in  FIG. 3 , the volumetric flow  15  to the injection valve  50  is limited in the initial state by the variable restrictor  35 , in such a way that a pressure below the threshold  28  is set in the liquid inlet  76 . Here, as it flows through the cooling circuit  75 , the liquid  12  does not increase the pressure, or increases it only to such a degree that the threshold is not reached and the valve ball  55  of the closing member  61 , which is pressed into the valve seat  53  by the spring force of the valve spring  64 , is not lifted from the valve seat  53 . In the initial state the liquid  12  therefore flows via the liquid return  77  and the return line  19  back to the reservoir  13 . If, from the initial state, the variable restrictor  35  is activated to open it, the volumetric flow  15  of the liquid  12  in the cooling circuit  75  of the injection valve is released in such a way that the further restrictor  66  in the closing member  61  limits the flow. As a result the pressure in the liquid inlet  76  rises above the threshold  28 , so that the spring force of the valve spring  64  is overcome and the injection valve  50  allows metering of the liquid  12  into the exhaust tract  20 . A narrowing of the restrictor  35  causes the pressure in the liquid inlet  76  to drop below the threshold  28  again, so that the injection valve  50  closes again. Alternatively the pressure in the liquid inlet  76  may dip below the threshold due to the metering of the liquid  12 , that the injection valve  50  closes. 
         [0032]    In a further exemplary embodiment represented in  FIG. 4  the restrictor element  34  between the pre-supply pump  23  and the injection valve  50  is embodied as an electrically controlled metering pump  41 . 
         [0033]    The electrically controlled metering pump  41  comprises a pump housing  42 , in which a pressure chamber  49  is formed, in which a pressure can be built up by a pump piston  45 . The pump piston  45  is held in its initial position by a spring  46 , which is arranged between the pump housing  42  and a spring plate  47  formed on the pump piston  45 . The pump piston  45  additionally comprises an armature  44  integrally connected to the pump piston  45 , the armature  44  being capable of actuation by a solenoid assembly  43 , likewise arranged in the pump housing  42 . Branching off from the connecting line  17  between the pre-supply pump  23  and the electrically controlled metering pump  41  is a connecting line  37 , which is connected via a port  39  to a hydraulic working chamber  48  of the electrically controlled metering pump  41 . Here the hydraulic working chamber  48  comprises a spring chamber  85  and an armature chamber  84 , which are hydraulically connected to one another via a guide area  85  of the pump piston  45  formed in the pump housing  42 . An inlet restrictor  38  is arranged between the connecting line  17  and the pressure chamber  49  of the electrically controlled metering pump  41 . A non-return valve  36  arranged between the connecting line  17  and the pressure chamber  49  serves to prevent the liquid  12  flowing back out of the pressure chamber  49  to the pre-supply pump  23 . Here the non-return valve  36  is preferably arranged upstream of the inlet restrictor  38  in the direction of flow indicated by arrows in the figures. 
         [0034]    In the unactivated state of the metering pump  41 , the pre-delivery pump  23  delivers the liquid  12  into the pressure chamber  49  of the electrically controlled metering pump  41  via the non-return valve  36  and the inlet restrictor  38 . Here the pressure of the pre-supply pump  23  is sufficient to overcome the spring force of the non-return valve  36  and to open the non-return valve  36 . When the metering pump  41  is not activated, the pump piston  45  is pressure-balanced, since the hydraulic working chamber  48  of the electrically controlled metering pump  41  is hydraulically connected to the pre-supply pump  23  via the connecting line  37  and the port  39  in the housing  42 . The pump piston  45  is positioned in its initial position by the spring  46 . In the unactuated initial state of the electrically controlled metering pump  41  the liquid  12  circulates, as described in the preceding exemplary embodiments, through the cooling circuit  75  of the injection valve  50 , thereby cooling the injection valve  50 . If the solenoid assembly  43  of the electrically controlled metering pump  41  is activated, the armature  44  is pulled up by the magnetic force of the solenoid assembly  43 , overcoming the spring force of the spring  46  and causing the pump piston  45  to move in the direction of the pressure chamber  49 . As a result the pressure in the pressure chamber  49  increases. The non-return valve  36  closes due to the pressure increase in the pressure chamber  49 , preventing the liquid  12  from flowing back into the reservoir  13  counter to the direction of flow. Due to the pressure increase in the pressure chamber  49 , the pressure in the liquid inlet  76  of the cooling circuit  75  also increases. If the pressure in the liquid inlet  76  reaches or exceeds the threshold, the injection valve  50  opens and allows metering of the liquid  12  into the exhaust tract  20  of the internal combustion engine  10 . Due to the injection, the pressure in the liquid inlet  76  is reduced, so that the pressure again falls below the threshold when the pump piston  45  of the electrically controlled injection valve  41  has reached its limit position and no further pressure is being built up in the pressure chamber  49 . When the energizing of the solenoid assembly  43  ceases, the pump piston  45  is returned to its initial position again by the spring  46  and the non-return valve  36  opens again, so that the pressure chamber  49  is again filled with liquid  12 . 
         [0035]    Alternatively, instead of a magnetic circuit, which comprises the solenoid assembly  43  and the armature  44 , the electrically controlled metering pump  41  may also be controlled by a piezo-actuator. The invention is not limited to piston pumps; alternatively it is also possible to use other metering pumps  40 , for example diaphragm pumps or centrifugal pumps, which in the unactivated operating state restrict the volumetric flow  15  of the liquid  12  to the injection valve  50  and therefore allow a flow through the cooling circuit  75  in the injection valve  50 , and which in the activated state release the volumetric flow  15  of the liquid  12  to the injection valve, at least to a degree sufficient for the pressure in the liquid inlet  76  of the injection valve  50  to build up, at least until it reaches the threshold. 
         [0036]    The device according to the invention is likewise not limited to electrically controlled metering pumps  41  but also encompasses metering pumps  40 , which are activated pneumatically, hydraulically or mechanically, for example. 
         [0037]    As an alternative to pressure-balanced metering pumps, it is also possible to use metering pumps  40  in which, in the unactivated initial state, different pressures prevail in the hydraulic working chamber  48  and in the pressure chamber  49 , making it possible to dispense with the connecting line  37  and the port  39  in the pump housing  42 . As a further embodiment the pump piston  45  and the armature  44  may also be of two-part design.