Abstract:
A method and device for treating exhaust gases of an internal combustion engine with a reducing agent introduced into the exhaust gases. The reducing agent is pumped out of a reactant reservoir and to a dosing valve so that after the dosing valve is opened, the reducing agent can be introduced into the engine&#39;s exhaust gas line in a dosed manner. Reducing agent can be conveyed back from the delivery device to the reservoir via a return line. The reducing agent can circulate between the delivery device and the reactant reservoir thereby, particularly when the dosing valve is closed, preventing the reactant from freezing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a 35 USC 371 application of PCT/EP 2005/056050 filed on Nov. 18, 2005. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention is based on a method and apparatus for aftertreatment of exhaust gases of an internal combustion engine. 
   2. Description of the Prior Art 
   DE 199 46 902 A1 has disclosed the introduction of a urea/water solution into the exhaust train downstream of an engine to remove nitrogen from the exhaust and the provision of a return line from a supply pump back to a urea tank, which line serves to reduce an excess pressure that may potentially be present in the urea line and, for example, to bleed air from the urea line; but especially in freeze-critical situations, particularly at times other than when the reducing agent is being metered into the exhaust train, no continuous heat exchange is provided between the urea/water solution and freeze-endangered regions of the exhaust aftertreatment apparatus. 
   In addition, DE 102 54 981 A1 has disclosed using compressed air to remove reducing agent still contained in reducing agent-conveying lines after the metering. 
   SUMMARY AND ADVANTAGES OF THE INVENTION 
   The method and apparatus according to the invention have the advantage over the prior art of offering a simple, space-saving, and effective alternative for preventing freezing of an exhaust gas aftertreatment apparatus, which alternative, on the mechanical side, can be largely implemented with already existing installations and can be easily implemented in a software-based way. In particular, a circulation of reactant provides good protection against freezing, even with low metering quantities and when there is a cold headwind. 
   Advantageous modifications and improvements of the method and apparatus for exhaust aftertreatment are disclosed. It is particularly advantageous to provide a circulation in a supply module that is connected to a metering device by means of a supply line. While taking up a small amount of space, this assures an effective freeze prevention, particularly in connecting regions or at connector plugs that produce connections with the reactant reservoir. 
   It is also advantageous if a reactant circulation is provided, particularly during times at which the metering operation is not occurring and/or when the internal combustion engine is switched off, so that pressure fluctuations in the lines that may be caused by the circulating operation do not interfere with the actual metering operation and a freeze prevention is assured even when the vehicle is parked. 
   It is also advantageous to provide a protection against drainage of the vehicle battery in order to assure that it is possible to restart the internal combustion engine at any time. 
   It also saves energy to provide a circulating operation only when the temperature falls below a certain threshold below which there is a danger of the on-board fluid reactant freezing in freeze-endangered regions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are explained in detail in the subsequent description, taken with the drawings, in which: 
       FIG. 1  shows an exhaust aftertreatment apparatus known from DE 199 46 902, 
       FIG. 2   a  shows an apparatus for reactant-assisted exhaust aftertreatment without compressed air assistance, equipped with a supply module, 
       FIG. 2   b  is a schematic depiction of an exhaust aftertreatment apparatus, 
       FIG. 3  shows another apparatus for exhaust aftertreatment, 
       FIG. 4  shows an apparatus with an alternative supply module, 
       FIG. 5  is a flowchart, and 
       FIG. 6  shows another apparatus for metered reactant introduction into the exhaust train without compressed air, equipped with means for blowing out the reactant. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a known apparatus for aftertreatment of exhaust gases. The reference numeral  1  indicates a urea tank from which a urea/water solution is drawn by a supply pump  4  via a line  1   a , for example a flexible tube, with a check valve  2  and a filter  3  embodied in the form of a filter screen and conveyed via an additional check valve  6  to a metering valve  7 . The metering valve  7  meters the required quantity of urea/water solution into a mixing chamber  8 . A potential overflow quantity of the urea/water solution can be returned to the urea tank  1  through a return line  12  passing through a pressure regulator  5  and an additional check valve  11 . A potentially required bleeding of air from the line  1   a  can be executed via a bleed circuit equipped with a bleed valve  10 . The reference numeral  20  indicates a compressed air reservoir from which compressed air can be introduced into the mixing chamber via a pressure limiter  21 , a 2/2-way valve  22 , and a check valve  23 . The provision of the check valve  23 —which can, for example, be embodied in the form of a ball valve or flat seat valve—can prevent a reducing agent/air mixture from flowing out of the mixing chamber and back into the compressed airline  24 . By comparison with the conventional systems, this sharply reduces the risk of a contamination of an on-board compressed air circuit communicating with the compressed airline  24 . 
   In the mixing chamber  8 , an aerosol is generated through the action of the compressed air on the urea/water solution and is introduced via an aerosol line  25  into a catalytic converter  30 . In this case, a control unit  40  detects signals that are received from an overriding motor control unit via a CAN data line  41  as well as the signals that are received from pressure sensors, temperature sensors, and fill-level sensors  50  through  55  whose functions are known in and of themselves and will not be explained in detail here. Based on these data, the control unit  40  calculates a urea metering quantity that should be added to an exhaust flowing through the catalytic converter  30 . 
   With the aid of the above-described solenoid valves, the control unit  40  controls the pressure in the compressed airline  24  and also monitors the pressure of the urea/water solution. The control unit  40  detects deviations and errors, stores them, and displays them, for example, on a PC by means of a diagnostic device that is not shown. 
     FIG. 2   a  shows an apparatus for exhaust aftertreatment in which components that are the same as those in the arrangement according to  FIG. 1  have been labeled with the same reference numerals and will not be described further. The electrical connections to a control unit  40  are the same as in the arrangement according to  FIG. 1 , but are not shown here. The check valve  2 , the filter  3 , the supply pump  4 , the pressure regulator  5 , the pressure sensor  50 , the line  1   a , the bleed valve  10 , the return line  12 , and the additional check valve  11  are structurally integrated into a supply module  61  that is enclosed by a housing  60 . The urea tank in this case is connected to the check valve  2  by means of a connector plug  72  that connects the line leading away from the tank to the line  1   a  that is situated inside the supply module and leads to the check valve  2 . Similarly, the additional check valve  11  in the return line  12  is connected by means of an additional connector plug  71  to an extension  75  of the return line, for example in the form of a flexible tube, leading to the urea tank. Downstream of the branch point  65  at which the return line  12  branches off, the line  1   a  is connected by means of an additional connector plug  70  to a supply line  76  that leads to a metering valve  80 . 
   By contrast with the apparatus according to  FIG. 1  the supply of the urea/water solution into the exhaust line occurs without the assistance of compressed air. If the valve  80  is open, the urea/water solution is injected directly into the exhaust train without being mixed with compressed air. To this end, the metering valve can be situated close to the exhaust line and its opening can protrude directly into the exhaust line. 
   Also in a schematic depiction,  FIG. 2   b  shows an apparatus equipped with a supply module  61  with a housing  60 , which is connected by means of flexible tube connections and connector plugs  70 ,  71 , and  72  to a tank  1  for a urea/water solution and to a metering valve  80 . 
   By means of a supply pump  4  integrated into it (see  FIG. 2   a ), for example, the supply module causes a suction of urea/water solution from the tank  1  via the line  1   a  and the connector plug  72 . As a result, urea/water solution remains available to be conveyed further via the connector plug  70  and the supply line  76  to the metering valve  80 , which, when it is open, assures a supply of urea/water solution into the exhaust train. Particularly when the metering valve  80  is closed, the supply module  61  continues to transport urea/water solution. The reactant continues to be drawn via the line  1   a , but is returned to the urea tank  1  via the connector plug  71  and the return tube  75  so that a fluid circulation can be maintained. 
     FIG. 3  shows an arrangement, which is equipped with components that have already been described in conjunction with  FIG. 1  and in which a urea/water solution is supplied to the exhaust train in a compressed air-assisted fashion through the production of an aerosol. In this case, the check valve  2 , the filter  3 , the supply pump  4 , the pressure regulator  5 , the pressure sensor  50 , the line  1   a , the bleed valve  10 , the return line  12 , and the additional check valve  11  are structurally integrated into a supply module  661  that is enclosed by a housing  660 . In a manner similar to the design shown in  FIG. 2   b , the line leading to the pump, the return line, and the line leading from the pump toward the metering valve  7  are connected by means of respective connector plugs  672 ,  671 , and  670  to lines that are connected to the urea tank or the metering valve. 
     FIG. 4  shows an arrangement that is equipped with the same components as in  FIG. 3 , but in addition to the check valve  2 , the filter  3 , the supply pump  4 , the pressure regulator  5 , the pressure sensor  50 , the line  1   a , the bleed valve  10 , the return line  12 , and the additional check valve  11 , the components  21  through  24  and  53  are integrated into a supply module  761  that is encompassed by a housing  760 . 
   With the apparatuses described in conjunction with  FIGS. 2   a  through  4 , i.e. both with compressed air-assisted reactant injections and with injections unassisted by compressed air, it is possible to execute a method of the kind schematically depicted in the form of a flowchart in  FIG. 5 . This method can be implemented in the control unit  40  in either a hardware-based or software-based fashion. The method step  810 , i.e. the normal metering operation, is followed by a query  820  to establish whether there is still a need for metering. The query  830  makes a continuation of the process contingent on the currently prevailing outside temperature. Method step  840  includes steps for freeze prevention and the query  850  determines whether reactant should continue being metered into the exhaust train. 
   During normal metering operation (method step  810 ), the metering valve  7  or  80  supplies reactant into the exhaust train downstream of the engine. In the course of this, a query is continuously repeated (query  820 ) as to whether there is still a need for metering and whether metering is still permitted. For example, metering is no longer permitted when the urea tank  1  is no longer fall enough—if more is withdrawn for metering purposes—to be able to assure a circulating operation, which will be described in detail below. Also in the event of a malfunction, metering is no longer permitted, for example, if the measurement of the catalytic converter temperature yields implausible values. It is also possible before the actual start of the metering operation to monitor the catalytic converter temperature and to only start the normal metering operation if the catalytic converter is sufficiently warm. If there is no longer a need for metering or if metering is no longer permitted, then in another step in query  830 , a query is made as to the temperature of the urea/water solution in the urea tank or alternatively a query is made as to the outside temperature. If the temperature is not below a certain threshold (temperature of the urea/water solution below −5 degrees Celsius or outside temperature below −11 degrees Celsius), then the process returns to the query  820 . But if the temperature has fallen below the relevant threshold, then in method step  840 , freeze-prevention steps that will be explained in detail below are initiated. In the course of this, a query is continuously repeated (query  850 ) as to whether further metering should occur. If not, then the process jumps back to the query  830 , otherwise, it returns to method step  810 . 
   In the arrangement according to  FIGS. 2   a  and  3 , when the metering valve  7  is closed and the bleed valve  10  is closed, the pump  4  is triggered so that it exceeds the threshold value of 3 bar of the pressure regulator  5  so that a circulation circuit of the reactant is produced, which continuously flushes the connector plugs  71  and  72  (or  671  and  672 ) with reactant. 
   Alternatively, when the metering valve is closed, the bleed valve  10  can be opened and the pump  4  can be switched on or remain switched on so that even at a pressure of less than 3 bar, a circulation circuit can be produced. If these connector plugs are flushed, it is possible to prevent a freezing of the reactant, particularly at times other than when metering operation is occurring, even in regions of the apparatus that are at risk of freezing. If special steps for freeze-prevention are not taken, then these regions are especially prone to the freezing of the reactant because the reactant has a particularly effective cooling contact with the environment due to the connection to the housing of the supply module. Alternatively, the pressure regulating valve  5  can also be replaced by an electrically triggerable valve that assumes the function of a pressure regulating valve during the metering operation and can be opened to establish a circulation circuit when the metering valve is closed. Alternatively, the above-mentioned electrically triggerable valve can also be provided in addition to the pressure regulator  5  and connected in parallel with it. In addition to the establishment of a circulation circuit, it is also possible for additional steps to be taken in order in particular to protect the region of the connector plug  70  ( FIG. 2   a ) or  670  ( FIG. 3 ) from a freezing of the reactant. These optional additional steps relating to the connector plug  70  or  670  will be described below in conjunction with  FIG. 6 . 
   The freeze-prevention method using a circulation circuit can also be continued after the internal combustion engine is switched off. In another alternative embodiment, it is also possible to eliminate the query of the temperature of the outside air or of the reactant contained in the reactant reservoir in order to establish a circulation circuit that does not include the metering device and/or the metering valve at all times except during metering operation—possibly even after the internal combustion engine is switched off. It is also possible for the circulation to occur only if or as long as the on-board electrical system of the vehicle of the internal combustion engine has sufficient energy reserves to start the engine again. 
     FIG. 6  illustrates a further modification of the system according to  FIG. 2   a , which was for an injection of reactant, in particular a urea/water solution, into the exhaust train without the use of compressed air. By contrast with the supply module  60  according to  FIG. 2   a , the supply module  960  also has a reversing valve  962  and a compressed air line  973  that connects the reversing valve to a compressed air connector plug  971 . The reversing valve (which can be triggered by the electric control unit that is not shown in detail) connects the connector plug  70  and metering valve  80  either to the line  1   a  connected to the pump  4  or to the connector plug  971 . At this connector plug  971 , the supply module  960  is connected by means of another compressed air line  972  to an electrically triggerable compressed air supply device  970 , which can likewise be controlled by means of the above-mentioned electric control unit. For example, this compressed air supply device includes a compressed air cartridge and a means for metering the compressed air, in particular a compressed air valve that can be electrically triggered. Alternatively, the means for metering the compressed air can also be supplied with compressed air coming from a compressed air source that is already provided on board the motor vehicle—or example a compressed air reservoir—or coming directly from a compressor in a pressure-controlled fashion. 
   In addition to the establishment of a circulation circuit for preventing the connector plugs  71  and  72  from freezing, as explained in the description of  FIG. 5 , an arrangement according to  FIG. 6  also provides a measure for preventing the connector plug  70  from freezing. To this end, the reversing valve  962  disconnects the connection between the line  1   a  and the supply line  76  and, as shown in  FIG. 6 , the metering valve is connected to the compressed air supply device  970  by means of the compressed air lines  972  and  973 . The means for metering the compressed air is triggered and reactant still present in the supply line  76  or in the region of the connector plug  70  is blown out into the exhaust train. After a certain time, the compressed air supply is switched off, but the reversing valve remains in the above-described position for the flushing of the metering valve in order to prevent reactant from flowing out of the line  1   a  into the region of the connector plug  70 . Only when metering operation is resumed is the reversing valve reversed again so that the metering valve can once again be acted on with reactant. As a result, the region of the connector plug  70  also remains protected from freezing. 
   The foregoing relates to a 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.