Abstract:
The invention relates to a device for supplying a reducing agent into an exhaust system of an internal combustion engine. The device includes a delivery pump for delivering the reducing agent from a storage tank into an exhaust tube of the exhaust system. A metering device is provided between the delivery pump and the exhaust tube, which metering device supplies reducing agent, which is delivered continuously by the delivery pump, in an intermittent fashion into the exhaust tube.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2007/052108 filed on Mar. 7, 2007. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a device for delivering a reducing agent, in particular a mixture of urea and water, to an exhaust system of an internal combustion engine. 
     In selective catalytic reduction or SCR, to reduce the proportion of nitrogen oxide in the exhaust gas from diesel engines, a mixture of urea and water, commonly also called AdBlue, is delivered to the exhaust system. The urea in the injected solution is converted in the exhaust system into ammonia (NH 3 ), which in a downstream SCR catalytic converter converts the nitrogen oxides (NO x ) contained in the exhaust gas, forming molecular nitrogen (N 2 ) and water (H 2 O). Since in this way the nitrogen oxides (NO x ) can be removed almost completely from the exhaust gas, diesel engines can be operated with a relatively lean mixture, which in turn makes fuel-saving operation of the engines possible. The use of other reducing agents, however, is also conceivable. 
     2. Prior Art 
     In known devices of the type defined at the outset, with which a mixture of urea and water is delivered as a reducing agent to the exhaust system, the deliver is effected either by means of a feed pump without a quantity-metering function, or by means of a metering pump. The feed pumps mentioned first have the advantage over metering pumps that they are not only chemically resistant to the urea or free ammonia in the mixture of urea and water, but if the mixture of urea and water freezes, they can also withstand the ice pressure resulting from its increase in volume without being damaged, which is not assured in the metering pumps mentioned second. 
     OBJECT AND SUMMARY OF THE INVENTION 
     With this as the point of departure, it is the object of the invention to improve a device of the type defined at the outset in such a way that it is not only resistant to reducing agent or its ingredients and proof against pressure in the case of reducing agents that contain water, but also allows quantity metering. 
     For attaining this object, it is proposed according to the invention that between the feed pump and the exhaust system, an additional metering device be provided, which delivers reducing agent, pumped continuously by the feed pump, to the exhaust system intermittently and thus takes on the quantity metering function. This embodiment makes it possible on the one hand to use time-tested conventional feed pumps without a quantity-metering function, which not only can be procured economically but also are resistant to reducing agent, such as a mixture of urea and water, and are proof against ice pressure. On the other, the feed pump can operate during the entire period of operation of the internal combustion engine, so that control of the pump can be dispensed with, which not only leads to cost savings but also, because of the sturdier construction of such feed pumps, also assures less vulnerability to malfunction. Moreover, the feed pump can optionally also be driven directly by the engine. 
     To provide for the most homogeneous possible distribution of the reducing agent in the exhaust gas, the reducing agent is injected into the exhaust system preferably by means of an injection nozzle. To avoid the necessity of an additional controller for the nozzle, this nozzle is expediently controlled by the pressure of the reducing agent upstream of the nozzle, and it opens when this pressure exceeds a defined opening pressure and closes when the pressure drops below the opening pressure. For controlling the pressure of the reducing agent upstream of the nozzle, the metering device is used, which in a preferred embodiment of the invention connects the compression side of the pump to a suction side of the pump when no reducing agent is to be delivered to the exhaust system, and which disconnects the compression side of the pump from the suction side when a pressure upstream of the nozzle is to be built up for the sake of delivering reducing agent to the exhaust system. 
     When the compression side of the pump is in communication with the suction side of the pump, this pump circulates the reducing agent. This means that a sufficient pressure that exceeds the opening pressure of the nozzle cannot build up upstream of the nozzle, and thus the nozzle remains closed. Conversely, when the compression side of the pump is disconnected from its suction side, a pressure that exceeds the opening pressure of the nozzle automatically builds up upstream of the nozzle as a result of the continuous pumping of the pump, so that the nozzle opens and assures a delivery of reducing agent into the exhaust system. 
     The metering device is preferably disposed along a pressure line, connecting the pump to the nozzle, as well as along an intake line, connecting the pump to the tank, and includes a return-flow conduit, which connects the pressure line and the intake line and can be opened or closed as needed by a controllable valve. 
     The valve is advantageously embodied as a magnet valve and has a valve member which is pressed by the force of a spring against a valve seat in the return-flow conduit, in order to close the return-flow conduit. By the deliver of current to the magnet valve, the valve member is lifted from the valve seat, in order to open the return-flow conduit. 
     Since at least some of the components of magnet valves, such as the coil and the current leads to the coil, are not resistant to typical reducing agents, such as a mixture of urea and water or ammonia outgassing from a mixture of urea and water, these components should not come into contact with the reducing agent, such as the mixture of urea and water, or its ingredients or products of decomposition, such as ammonia, so as to avoid corrosion and attendant problems in operation. For this reason, an opening penetrated by the valve member, between a coil chamber of the magnet valve and the return-flow conduit, around the valve member is hermetically sealed by an elastically deformable diaphragm. 
     To make the metering device resistant to ice pressure, so that it will not be damaged as a consequence of an expansion in volume of freezing reducing agent, such as a mixture of urea and water, in the metering device, the pressure line and/or the intake line inside the metering device are surrounded at least in part by elastically deformable compression bodies, which are compressed in the expansion in volume, caused by freezing, of the reducing agent and in this way assure a marked drop in the freezing pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in further detail below in conjunction with the drawings, in which: 
         FIG. 1  is a schematic illustration of a device according to the invention for delivering a reducing agent, in the form of a mixture of urea and water, to an exhaust system of an internal combustion engine; 
         FIG. 2  is an enlarged view, partly in section, of the detail marked II in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The device  2  shown in its entirety in  FIG. 1  serves to deliver a reducing agent to an exhaust tube  4 , through which exhaust gas from a diesel engine (not shown) flows in the direction of the arrow A. The delivered reducing agent is preferably a mixture of urea and water (AdBlue), but other reducing agents may also be used, in particular liquid reducing agents. The device  2  substantially comprises a tank  6  for the reducing agent, a conventional continuous-operation feed pump  8  for a liquid reducing agent, such as a mixture of urea and water, whose suction side communicates with the tank  6  through an intake line  10  and whose compression side communicates through a pressure line  12  with an injection nozzle  14  mounted on the exhaust tube  4 , as well as a metering unit  16  for intermittently metering the reducing agent, pumped by the feed pump  8 , through the injection nozzle  14  into the exhaust tube  4 . 
     The injection nozzle  14 , which for better mixing with the exhaust gas injects the reducing agent as a spray cone into the exhaust tube  4 , as represented schematically by arrows B in  FIG. 1 , is a pressure-controlled nozzle, which opens when the pressure in the pressure line  12  upstream of the nozzle  14  exceeds a preset opening pressure of the nozzle  14 . However, as long as the pressure in the pressure line  12  upstream of the nozzle  14  is below the opening pressure of the nozzle  14 , the nozzle stays closed. 
     As best shown in  FIG. 2 , both the pressure line  12 , leading from the feed pump  8  to the injection nozzle  14 , and the intake line  10  leading from the tank  6  to the feed pump  8  extend through the metering unit  16 , and line segments  18 ,  20  of the pressure line  12  and intake line  10  that are located inside the metering unit  16  are expediently oriented parallel to one another. These line segments  18 ,  20  communicate inside the metering unit  16  through a return-flow conduit  22 , which with the aid of a magnet valve  24  of the metering unit  16  can be selectively opened or closed. 
     The magnet valve  24  includes a housing  26 ; a coil  30 , accommodated in a coil chamber  28  of the housing  26 , that can be subjected to current through current leads (not shown); a magnet armature  32 , surrounded by the coil  30  and movable along a longitudinal axis L of the coil  30 ; a valve member  34 , in the form of a valve needle, that is rigidly connected to the magnet armature  32 ; and a helical compression spring  36 , which is disposed between the housing  26  and the face end, remote from the valve member  34 , of the magnet armature  32 . 
     In  FIG. 2 , to simplify the drawing, the pressure line  12  is shown above the intake line  10 , and the magnet valve  24  is shown above the pressure line, while in practice, the pressure line  12 , intake line  10 , and return-flow conduit  22  are preferably all located side by side in the same plane, and the magnet valve  24  is disposed outside this plane, above or below the return-flow conduit  22 . 
     In the currentless state of the coil  30 , the compression spring  36  presses the valve member  34  against a valve seat  38  in the return-flow conduit  22 , in order to close the latter, while a delivery of current to the coil  30  causes the magnet armature  32 , counter to the force of the compression spring  36 , to be displaced in the direction of the longitudinal axis L of the coil  30 , and as a result, the valve member  34  is lifted from the valve seat  38 , opening the return-flow conduit  22 . 
     With the return-flow conduit  22  open, the feed pump  8  circulates reducing agent, aspirated from the tank, through a line segment  40  of the pressure line  12 , which connects the compression side of the feed pump  8  with the return-flow conduit  22  in the metering unit  16 , through the return-flow conduit  22  and through a line segment  42  of the intake line  10 , which connects the return-flow conduit  22  to the suction side of the feed pump. As a result, in the line segment  44  of the pressure line  12  adjoining the metering unit  16  and leading to the injection nozzle  14 , no pressure can build up. Thus the injection nozzle  14  remains closed, and no reducing agent is delivered to the exhaust tube  4 . As soon as a delivery of reducing agent to the exhaust tube  4  is requested by a control unit (not shown) of the engine, the delivery of current to the coil  30  of the magnet valve  24  is discontinued, whereupon the compression spring  36  presses the valve member  34  against the valve seat  38  in the return-flow conduit  22  and closes this conduit, as shown in  FIG. 2 . When the feed pump  8 , with the return-flow conduit  22  closed, pumps reducing agent from the tank  6  into the pressure line  12 , as indicated by the arrows C and D in  FIG. 2 , a pressure builds up in the pressure line  12  that, if it exceeds the opening pressure of the injection nozzle  14 , leads to a delivery of reducing agent to the exhaust tube  4 . 
     The quantity of reducing agent delivered to the exhaust tube  4  depends on the capacity of the feed pump  8 , the characteristic curve of the injection nozzle  14 , and the duration of closure of the return-flow conduit  22  and can therefore be controlled by varying this last parameter. 
     If an additional pressure regulator (not shown) is provided between the feed pump  8  and the injection nozzle  14 , then the quantity of reducing agent delivered to the exhaust system  4  depends on the pressure set at the pressure regulator and on the duration of closure of the return-flow conduit  22 , so that once again, for controlling the quantity of reducing agent delivered to the exhaust tube  4 , the duration of closure of the return-flow conduit  22  can be varied. 
     Since the magnet valve  24 , or components of the magnet valve  24 , such as the coil  30 , the current supply lines to the coil  30  inside the housing  26 , and optionally also the magnet armature  32  are as a rule not chemically resistant to reducing agents, or their ingredients or products of decomposition, a stepped bore  46 , between the magnet valve  24  and the valve seat  38  of the return-flow conduit  22 , that is penetrated by the valve member  34  is closed in fluidtight and gastight fashion by an elastically deformable diaphragm  48 , so that reducing agent or its ingredients or products of decomposition from the return-flow conduit  22  is prevented from penetrating the coil chamber  28  of the magnet valve  24 . 
     To that end, the annular diaphragm  48  is embedded with its inner circumferential edge in fluidtight and gastight fashion in an encompassing groove in the circumference of the valve member  34 , while its outer circumferential edge is firmly clamped in fluidtight and gastight fashion between an annular shoulder of the stepped bore  46  and a coil holder protrudes partway into the stepped bore  46 . The diaphragm  48  that is undeformed when the return-flow conduit  22  is closed becomes elastically deformed when the magnet armature  32 , by delivery of current to the coil  30 , is shifted in the direction of the longitudinal axis L of the coil  30 , counter to the force of the spring  36 , and in the process, the valve member  34  is lifted from the valve seat  38 . However, in the process the annular gap between the valve member  34  and the wall of the stepped bore  46  is hermetically sealed by the diaphragm  48  in every valve position of the valve member  34 . 
     While the feed pump  8 , the pressure line  12 , the intake line  10 , the tank  6 , and a filter (not shown) that may optionally be disposed between the tank  6  and the feed pump  8  are embodied in a known manner such that they are resistant to the resultant ice pressure if the liquid reducing agent contained in them freezes, the resistance of the metering unit  16  shown in  FIG. 2  to ice pressure is attained by providing that the line segments  18  and  20 , of the pressure line  12  and intake line  10 , respectively, that extend through the metering unit are surrounded over a portion of their length by tubular compression bodies  52 . The compression bodies  52  are elastically deformable, so that if the volume of the liquid reducing agent contained in the metering unit  16  increases as a consequence of freezing of the reducing agent, the compression bodies are compressed somewhat, and upon thawing of the reducing agent, they resume their original shape. 
     Since with a metering unit  16  of this kind all the components of the device  2  can be embodied as resistant to ice pressure, the complicated evacuation of the lines  10 ,  12  can be dispensed with. Incorrect metering, which can ensue from incomplete evacuation of the lines  10 ,  12 , since the residual quantities of liquid reducing agent in the lines  10 ,  12  can lead to unwanted incorrect quantities during venting of the lines  10 ,  12  and thus can lead to deviations in the balance of the reducing agent delivered to the exhaust tube  4  and consequently deviations in the desired metering strategy as well, is also thus prevented. 
     The principle described for the metering unit  16  is applied to the injection nozzle  14  as well in order to make it proof against ice pressure, in that inside the injection nozzle  14 , at least a portion of the pressure line  12  is surrounded by a tubular compression body (not shown). 
     The foregoing relates to the preferred exemplary embodiment 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.