Abstract:
A device for introducing a liquid reducing agent into the exhaust gas of an internal combustion engine includes an injection device and a retaining device. By means of the latter, the injection device is retained at least indirectly in the vicinity of an exhaust gas conduit and a cooling device cools the retaining device. An intermediate portion is disposed between the retaining device and the injection device, and this portion at least regionally influences the heat flow between the injection device and the retaining device.

Description:
[0001]    The invention relates to a device for introducing a liquid substance into the exhaust gas of an internal combustion engine as generically defined by the preamble to claim  1 . The subject of the invention is also a method for producing such a device. 
         [0002]    German Patent Disclosure DE 103 24 482 A1 describes a device for injecting a liquid reducing agent into the exhaust gas of an internal combustion engine. To that end, the reducing agent is pumped from a storage container into a hollow retaining device, and from there back to the fuel tank. Via a branch fluidically downstream of the retaining device, reducing agent can be delivered to an injection device, which is retained by the retaining device. Because of the flow of reducing agent through the retaining device, the retaining device is cooled; that is, the retaining device includes a cooling device. 
       DISCLOSURE OF THE INVENTION 
       [0003]    The object of the present invention is to refine a device for introducing a liquid substance into the exhaust gas of an internal combustion engine further in such a way that the liquid substance can be metered with high precision, and at the same time the service life of the device is improved. 
         [0004]    This object is attained by a device having the characteristics of claim  1 . Advantageous refinements are defined by dependent claims. A further means of attaining the object of the invention is disclosed by the coordinate claim, which pertains to a production method. Characteristics important to the invention are furthermore recited in the ensuing description and shown in the drawings, and the characteristics may be essential to the invention in quite various combinations. 
         [0005]    By means of the device and method of the invention, a targeted influence on the heat transfer between the cooled retaining device and the injection device is made possible, specifically with the goal of both an improved heat transfer for cooling the retaining device and a reduced heat transfer for at least regionally avoiding heating of the injection device. The targeted and possibly even regional variation of the temperature of the injection device relieves the injection device, which leads to a longer service life. Particularly if the liquid substance is a reducing agent, then it does not age as much in the injection device, because of the lesser temperature stress. Since typical reducing agents in the injection device change to the vapor phase at a temperature of approximately 160° C., it is possible by the targeted variation of the temperature of the injection device to avoid boiling of the reducing agent in the injection device and the incorrect metering that would be associated with it. 
         [0006]    This can be attained for instance by providing that between the relatively rigid injection device and the comparatively rigid retaining device, a less-rigid or even quite soft contact element is placed, which is deformed upon the mounting of the injection device on the retaining device. Since the contact element has a markedly lesser rigidity than the injection device and the retaining device, it is essentially only the contact element that is deformed, but not the injection device or the retaining device. Their function accordingly remains unimpaired. As a result of the deformation of the contact element, the contact element can press itself against, or in other words “conform” to, the injection device and the retaining element two-dimensionally and without play, creating an especially good thermal contact between the injection device and the contact element and between the contact element and the retaining device. Thus the contact element, because of its high thermal conductivity, can dissipate the heat, introduced into the injection device from the exhaust gas and the exhaust gas conduit, into the retaining device with good efficiency. 
         [0007]    One material which can be deformed easily and plastically and which at the same time has excellent thermal conductivity is graphite. 
         [0008]    Play-free deformation of the contact element with simultaneously only slight radial contact force can be attained if the contact element is annular, and if either the contact element or a contact region of the retaining element, oriented toward the contact element, has a slight conicity. 
         [0009]    The cooling device preferably includes a cooling conduit, which is disposed in the retaining device and through which a coolant flows. As the coolant, reducing agent, coolant, or even fuel can be used. A cooling device of this kind is very effective and robust and at the same can be produced economically. 
         [0010]    To minimize the heat input into the contact element and the retaining device, a thermal insulation means, such as a ceramic disk, should be disposed between the retaining device and the contact element, on the one hand, and the exhaust gas conduit, on the other. 
         [0011]    A further possibility of influencing the heat transfer between the retaining device and the injection device is that the intermediate portion has a slight air gap. Air is a poor thermal conductor and makes a targeted regional reduction in the heat transfer possible. Furthermore, the air gap can be designed and dimensioned in such a way that the air located in it is comparatively cool, and already by this means alone, an unwanted heating of the injection device is reduced. 
         [0012]    It is especially preferred if the slight air gap between the injection device and the retaining device is present in a region adjacent to the exhaust gas conduit. There, good insulation is especially important, to prevent a heat input from the retaining device, which in this region is heated especially strongly by the exhaust gas, into the injection device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    An especially preferred exemplary embodiment of the present invention is described in further detail below, in conjunction with the accompanying drawings. In the drawings: 
           [0014]      FIG. 1  is a schematic illustration of an internal combustion engine with a device for introducing a liquid reducing agent into the exhaust gas; 
           [0015]      FIG. 2  is a more-detailed view of the device of  FIG. 1 , in a plane perpendicular to it; and 
           [0016]      FIG. 3  is an enlarged view of an injection device and a first embodiment of a retaining device, with a contact element of the device of  FIG. 1 ; and 
           [0017]      FIG. 4  is a view similar to  FIG. 3  of a second embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    In  FIG. 1 , an internal combustion engine is identified overall by reference numeral  10 . It includes combustion chambers  12 , in which a fuel-air mixture is combusted. Hot combustion gases are removed from the combustion chambers  12  through an exhaust gas conduit  14 . In it, there is an exhaust gas posttreatment system, in the form of an SCR catalytic converter  16 . SCR stands for “Selective Catalytic Reduction”. In the SCR catalytic converter  16 , the pollutant NOx is reduced, with the aid of liquid reducing agent, to N2 and H2O. An oxidation catalytic converter is typically located upstream of the SCR catalytic converter but is not shown in  FIG. 1  for the sake of simplicity. Ammonia may be used as the reducing agent. For the sake of ease of handling, the ammonia is not metered into the exhaust gas in pure form but rather in the form of a precursor product. A urea-water solution can be considered in particular as the precursor product. 
         [0019]    The engine  10  also includes a device  18 , with which the reducing agent can be introduced into the exhaust gas flowing in the exhaust gas conduit  14 . The device  18  described here is not, however, limited to the delivery of a urea-water solution but instead can be used generally in conjunction with other reducing agents as well. For instance, even fuel can be metered as a reducing agent into the exhaust gas. The invention can furthermore be used in combination with other exhaust gas posttreatment provisions and systems that have storage-type catalytic converters and/or particle filters. 
         [0020]    The device  18  includes an injection device  20 , such as an injector, which in a manner to be described in further detail hereinafter is retained directly on the exhaust gas conduit  14  by a retaining device  22 . Through the injector  20 , the reducing agent, which is identified in the drawings overall by reference numeral  24 , finally reaches the exhaust gas conduit  14 . As can be seen particularly from  FIGS. 2 and 3 , the retaining device  22  is annular, with a bottom plate  26  oriented toward the exhaust gas conduit  14 , in which plate there is a through opening  28  through which an injection end  30  of the injector  20 , pointing toward the exhaust gas conduit  14 , is passed. A thermal insulation means  32  in the form of an annular disk, which is made for instance from plastic, is disposed between the bottom plate  26  and the exhaust gas conduit  14 . 
         [0021]    In a radially outer region, the bottom plate  26  is joined integrally to a hollow annular body  34  of overall rectangular cross section. The annular body  34  accordingly forms a radially outer region of the retaining device  22 . The annular body  34  and bottom plate  26  are made from a comparatively rigid steel, as is the injector  20 . A hollow chamber  36  in the annular body  34  forms an annular conduit with an inlet  38  and an outlet  40 . The function of the annular conduit  36  will be addressed in further detail hereinafter. 
         [0022]    Between the annular body  34  and the injector  20 , there is an annular contact element  42 . It is made from graphite and is retained in a slight press fit between the annular body  34  and the injector  20 . The retaining device  22  also includes a cover plate  43 , which represents the upper boundary, in  FIGS. 2 and 3 , of the retaining device  22 . In the installed position shown in  FIGS. 2 and 3 , an inner jacket face  43  of the contact element  42  rests two-dimensionally on an outer jacket face  45  of the injector  20 . An outer jacket face  46  of the contact element  42  rests two-dimensionally in the same way against an inner jacket face  48  of the annular body  34  of the retaining device  22 . Because of the press fit, the contact element  42  is thus received without play between the injector  20  and the annular body  34  of the contact element  42 . The two-dimensional and thus thermally optimal contact between the inner jacket face  44  and the outer jacket face  45 , and between the outer jacket face  46  and the inner jacket face  48 , is attained as a result of the comparatively low rigidity, compared to the injector  20  and the retaining device  22 , of the contact element  42  made from graphite and as a result of the resultant good deformability. 
         [0023]    As can be seen from  FIG. 3 , the outer jacket face  46  of the contact element  42  and the inner jacket face  48  of the annular body  34  are both embodied slightly conically, complementary to one another. As a result, the deformation of the contact element  42  and thus the attainment of a thermally optimal two-dimensional contact is promoted, while at the same time having an only slightly radial contact force of the contact element  42  against the outer jacket face  45  of the injector  20 . 
         [0024]    As can be seen from  FIGS. 1 and 2 , the reducing agent  24  is stored in a storage container  50 . From this container, it is pumped to the inlet  38  of the annular conduit  36  via a pump  52 . The outlet  40  of the annular conduit  36  communicates in turn with the storage container  50 , via a return  54  and a heat exchanger  56 . From the return  54 , a feed line  58  branches off; it leads to the injector  20 , and a metering valve  60  is disposed in it. 
         [0025]    The operation of the engine  10  and of the device  18  for introducing the reducing agent  24  into the exhaust gas of the engine  10  is controlled and regulated by a control and regulating unit  62 . To that end, the control and regulating unit  62  receives signals from various sensors, of which in  FIGS. 1 and 2  only one is shown as an example, referred to by reference numeral  64 . Among other things, the power of a drive motor  66 , which drives the pump  52 ; the electromagnetic metering valve  60 ; and various control devices of the engine  10 , such as injectors, with which the fuel is injected directly into the combustion chambers  12 , are varied by the control and regulating unit  62 . 
         [0026]    The device  18  functions as follows: The reducing agent  24  is pumped by the pump  52  into the annular conduit  36  via the inlet  38 . Since the reducing agent  24  coming from the storage container  50  is comparatively cold, it thus cools the annular body  34 . Thus to this extent, the annular conduit  36  and the annular body  34  form a cooling device  68 . Via the outlet  40  and the return  54 , at least some of the reducing agent  24  that is heated in the cooling device  68  reaches the heat exchanger  56 , where it is cooled down again before it returns to the storage container  50 . As a function of the triggering of the electromagnetic metering valve  60 , however, some of the reducing agent  24  flowing in the return  54  is carried via the feed line  58  to the injector  20  and is injected into the exhaust gas conduit  14 . 
         [0027]    Because of the heat of the exhaust gas flowing in the exhaust gas conduit  14 , the exhaust gas conduit  14  itself also heats up. A transfer of this heat to the injector  20 , however, is effectively reduced by the thermal insulation means  32 . To that end, heat from the injector  20  is diverted by the contact element  42  into the annular body  34  and from there into the reducing agent  24 , flowing into the annular conduit  36 , that to this extent acts as a coolant. 
         [0028]    The device  18  is produced such that upon the mounting of the injector  20  on the retaining device  22 , the contact element  42  is deformed more markedly than the injector  20  and in such a way that it comes into contact two-dimensionally and without play with the injector  20  and the retaining device  22 . It can also be seen from  FIG. 3  that the through opening  28  in the bottom plate  26  of the retaining device  22  has a somewhat greater diameter than the injection end  30  of the injector  20 . Between the bottom plate  26  and the injector  20  there is accordingly a gap  70 , by which a heat input into the bottom plate  26  is reduced. 
         [0029]    In the exemplary embodiment described above, the jacket face  46  of the contact element  42  and the jacket face  48  of the annular body  34  are embodied conically, complementary to one another. However, it is also possible for only of the two jacket faces to be conical, either that of the contact element  42  or that of the annular body  34 . 
         [0030]    A further embodiment of a retaining device is shown in  FIG. 4 . Here as below, those elements and regions which have equivalent functions to elements and regions described above have the same reference numerals and will not be described again in detail. 
         [0031]    The embodiment of  FIG. 4  does not have a contact element for improving the heat dissipation from the injector  20  into the retaining device  22 ; instead, it has a pronounced gap  70 , which reduces a heat input from the retaining device  22  into the injector  20 . The gap extends over the length of the entire injection end  30 . Comparatively cool air is present in the gap  70 . A thermal insulating means between the exhaust gas conduit  14  and the retaining device  22  can then optionally be dispensed with.