Abstract:
An apparatus for introducing a reducing agent into the exhaust of an internal combustion engine, having a reservoir, a delivery unit, and a flow path for the reducing agent. The apparatus also has a ventilation device for ventilating the flow path and is situated at a geodetic high point of the flow path and even in the closed state, permits a return of a minimal fluid quantity to the reservoir.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC 371 application of PCT/DE 2004/001984 filed on Sep. 7, 2004. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for introducing a reducing agent, which in particular contains urea, into the exhaust of an internal combustion engine, having a reservoir, a delivery unit, and a flow path for the reducing agent, and having a ventilation device for ventilating at least one region of the flow path. 
     2. Description of the Prior Art 
     An apparatus for aftertreating the exhaust of an internal combustion engine, known from DE 101 16 214 A1, stores a urea/water solution in a reservoir and delivers it to a mixing chamber by a delivery pump. Compressed air is blown into the mixing chamber. The resulting aerosol of compressed air and urea/water solution is blown into an exhaust line upstream of a catalytic converter. In this case, the urea serves to reduce NOx in the catalytic converter. 
     The previously known systems operate with a diaphragm pump to supply the urea/water solution but their delivery capacity is reduced when air is present in the flow path on the suction side and in particular in a pump head, and the system must therefore be bled. In the known apparatus, bleeding occurs by means of a solenoid valve that must be triggered by a control unit. In the operation of this solenoid valve, however, it has turned out that even when the solenoid valve is open, it is not always possible to assure the ventilation of the flow path for the reducing agent. 
     The object of the present invention, therefore, is to modify an apparatus of the type described above so that it functions in the most reliable way possible. 
     This object is attained in that the ventilation device is situated at a geodetic high point of the flow path and in that the ventilation device has an opening, which permits a constant return of a minimal fluid quantity to the reservoir. 
     SUMMARY AND ADVANTAGES OF THE INVENTION 
     The placement of the ventilation device at a geodetic high point of the flow path allows the ventilation to be executed at the point at which the air collects. This achieves a particularly effective ventilation, which results in a particularly high efficiency of the delivery unit, for example a diaphragm pump. The primary result of significantly reducing or completely eliminating air in the flow path is the significant reduction in the time that elapses from the moment the delivery unit is switched on to the moment it actually starts delivering the reducing agent. 
     A clogging of the ventilation device is prevented by the fact that it has an opening that always permits a return of fluid. Urea dissolved in water has a particular tendency to crystallize when exposed to air. Since the ventilation device is never completely closed, urea crystals possibly adhering to the opening are entrained by the returning fluid and dissolved again. The opening is thus “cleaned” and kept open by the constant flow of fluid. This measure according to the present invention assures the ventilation function of the ventilation device even if an actual ventilation has not been executed for an extended period of time. This significantly increases the operational reliability of the apparatus. 
     According to a first embodiment of the present invention, the ventilation device includes a float valve. In the presence of air bubbles or aspirated air, a float valve of this kind unblocks an enlarged opening, allowing the air to escape quickly. Then the float reduces the opening cross section back to its minimal dimension again in which a small, constant flow of fluid escapes. A float valve of this kind is relatively simple in design and prevents unnecessary flow losses when the fluid path is sufficiently ventilated. 
     As an alternative, it is also possible for the ventilation device to include a solenoid valve. A valve of this kind functions in a very precise manner. 
     The ventilation device is particularly simple in design when it is embodied in the form of a flow throttle. In order to prevent excessive flow losses during normal operation of the apparatus, however, the cross section of the flow throttle must be relatively small. 
     It is advantageous if the ventilation device is contained in a filter or in close proximity to a filter. This is based on the concept that urea/water solutions freeze at temperatures below −11° C., and in order to be able to carry out the NOx reduction in the catalytic converter even at such low temperatures, it is necessary to be able to heat the apparatus. Particularly after parking for long periods at low temperatures (less than −11° C.), the urea/water solution must first be thawed before it is possible to start dispensing the reducing agent. Air inclusions in the frozen urea/water solution and in particular in the region of a filter, however, reduce the transmission of heat from the heater into the urea/water solution and thus prolong the time required to thaw the frozen urea/water solution. The placement of the ventilation device in a filter or in close proximity to a filter reliably prevents the presence of such air inclusions in the urea/water solution in the vicinity of the filter, which accelerates the thawing of the frozen urea/water solution. 
     In a modification of this, the filter can operate in two installation positions that differ by approximately 90° and the ventilation device is situated at an angle of approximately 45° between the two installation positions. This makes it possible to install the apparatus, for example in motor vehicles, in the two main installation positions as a function of the installation requirements, without having to change the integration of the ventilation device. 
     It is also advantageous if the ventilation device is situated upstream of the delivery unit. This assures that no air is present in the flow path on the suction side of the delivery unit so that the delivery capacity of the delivery unit is available the moment it is switched on. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Particularly preferred exemplary embodiments of the present invention will be explained in greater detail below, in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic depiction of a first exemplary embodiment of an apparatus for introducing a reducing agent into the exhaust of an internal combustion engine; 
         FIG. 2  is a partial section through a ventilation device of the apparatus from  FIG. 1 ; and 
         FIG. 3  is a depiction similar to  FIG. 1  of a second exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a catalytic converter of an exhaust system of an internal combustion engine, which is labeled as a whole with the reference numeral  10 . The exhaust is supplied to the catalytic converter  10  from combustion chambers  12  of the internal combustion engine that are only depicted schematically in the drawing. The exhaust flows through the catalytic converter  10  and exits it in the direction of the arrow  14 . The catalytic converter  10 , in connection with the vaporized components of a urea-containing solution, acts to reduce NO x  components in the exhaust. To this end, a nozzle  16  injects an aerosol into the exhaust flow. This aerosol contains urea as a reducing agent for NO x  conversion. The aerosol is produced by an apparatus labeled as a whole with the reference numeral  18  in  FIG. 1 . 
     The apparatus  18  has a reservoir  20  in which a urea/water solution is stored. A delivery unit  26 , which is embodied in the form of a diaphragm pump, delivers the urea/water mixture from this reservoir  20  via a check valve  22  and a filter  24 , by means of a line  28  to mixing chamber  30  (the use of certain diaphragm pumps also permits elimination of the check valve  22 ). A pressure control valve  32  sets the pressure in the line  28 . The outlet of this pressure control valve  32  is connected via a return line  34  to the reservoir  20  in this instance (alternatively, it would also be possible to connect it to the suction side of the delivery unit  26 ). A compressed air supply  36  feeds compressed air to the mixing chamber  30  via a compressed air line  38 . From the mixing chamber  30 , the water/urea/air mixture travels to the nozzle  16 , where it is atomized, thus producing the aerosol. 
     The line  28  of the apparatus  18  is routed so that the filter  24  is situated at the geodetically highest point of the apparatus  18 . At this geodetically highest point, a ventilation device  40  is positioned in the line  28  directly upstream of the filter  24 . A ventilation line  42  leads from the ventilation device  40  back to the reservoir  20 . The entire assembly comprised of the reservoir  20 , valves  22  and  32 , filter  24 , and pump  26  can be heated. The corresponding heating device is indicated with a dot-and-dash line in  FIG. 1  and is labeled with the reference numeral  43 . 
     The ventilation device  40  in the current exemplary embodiment is a float valve. Its basic design is shown in  FIG. 2 : 
     The float valve  40  has a housing  44  with a circular, cylindrical, cup-shaped base section  46 . There is an opening  52  in a circumferential wall  48  of the base section  46 , in the region of a bottom  50  of the base section  46 . The line  28  coming from the reservoir  20  and/or from the check valve  22  feeds into this opening  52 . Above the base section  46 , there is a transition section  54  that tapers in a funnel shape, onto which a cylindrical valve section  56  is formed. A cylindrical valve element  58  is guided in a sliding, fluid-tight manner in this valve section  56 . A connecting rod  60  on the valve element  58  toward the base section  46  of the housing  44  is connected to a float  62 . 
     The valve section  56  of the housing  44  is closed toward the top by a cover  64 . In the middle of the cover  64 , a pin-like spacer  66  extends toward the valve element  58 . A compression spring  67  around this spacer  66  is clamped between the cover  64  and the valve element  58 . A circumferential wall  68  of the valve section  56  of the housing  44  is provided with a ventilation opening  70 , which is connected to the ventilation line  42 . 
     Diametrically opposite from the inlet opening  52  in the transition section  54 , just before it transitions into the valve section  56 , an outlet opening  72  is provided, which is connected to the line  28  leading to the filter  24 . The spacer  66  is long enough that when the valve element  58  contacts the spacer  66 , the ventilation opening  70  is not completely covered. A duct  74  passes through the valve element  58  in its longitudinal direction. The chamber enclosed by the housing  46  is referred to as the ventilation chamber  76 . 
     The float valve  40  functions as follows: 
     If air is present in the section of the line  28  upstream of the float valve  40 , then it collects at the geodetically highest point of the apparatus  18 , namely in the ventilation chamber  76  of the float valve  40 . But if the ventilation chamber  76  is filled with air, then the buoyancy of the float  62  does not come into play and the compression spring  67  presses the valve element  58  downward in  FIG. 2  so that it completely unblocks the ventilation opening  70 . This allows the air to escape from the ventilation chamber  76  through the duct  74  and the full cross section of the ventilation opening  70 , into the ventilation line  42  and on into the reservoir  20 . 
     During the ventilation process, the ventilation chamber  76  fills with the urea/water mixture. This buoys up the float  62 , causing it to press the valve element  58  upward counter to the force of the compression spring  67  until it comes into contact with the spacer  66 . This reduces the cross section of ventilation opening  70  and ends up closing it almost completely. The urea/water mixture now escapes from the ventilation chamber  76  through the outlet opening  72 , which has a comparatively large diameter, into the section of the line  28  downstream of the float valve  40  toward the filter  24 . 
     At the same time, a small flow of urea/water mixture travels through the duct  74  and the region of the ventilation opening  70  not completely covered by the valve element  58  and on through the ventilation line  42  back to the reservoir  20 . As a result, urea that has crystallized in the region of the ventilation opening  70  is entrained, dissolved, and washed back into the reservoir  20 . 
     The float valve  40  is designed so that it can function not only in the vertical position depicted in  FIG. 2 , but also in a position inclined by +/−45° in relation to this. It is consequently possible to install the apparatus  18  as a whole in a position range from 0 to 90° without requiring changes in the design of the float valve  40 . 
     In an exemplary embodiment not shown here, the ventilation device  40  is embodied as a solenoid valve that is opened or closed by a control unit. The solenoid valve is designed so that even in its “closed” position, a small quantity of the urea/water mixture can flow back to the reservoir via the ventilation line, thus allowing crystallized urea in the region of the solenoid valve to be entrained, dissolved, and washed back into the reservoir. 
     Another alternative embodiment form is depicted in  FIG. 3 . Elements and regions that have functions equivalent to elements and regions in the exemplary embodiment shown in  FIG. 1  have been provided with the same reference numerals. They are not explained again in detail. 
     In the exemplary embodiment depicted in  FIG. 3 , the ventilation device  40  is comprised of a flow throttle situated in the ventilation line  42 . The ventilation line  42  branches off directly from the filter  24  at the geodetically highest point of the apparatus  18 . During operation of the apparatus  18 , the ventilation line  42  with the flow throttle  40  continuously conveys air and/or urea/water mixture from the line  28  back to the reservoir  20 . The flow throttle  40  is dimensioned to allow the pressure in the line  28  required to generate the aerosol in the mixing chamber  30  to be maintained under all circumstances. 
     The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.