Abstract:
The invention relates to a method for the operation of an exhaust gas aftertreatment system in an exhaust tract of an internal combustion engine. The exhaust gas aftertreatment system includes an SCR particle filter, a first reducing agent feed device for introducing the reducing agent into the exhaust tract upstream of the SCR particle filter, continuous regeneration of the SCR particle filter being possible using nitrogen dioxide as oxidizing agent, an SCR catalytic converter element arranged downstream of the SCR particle filter, and a second reducing agent feed device for introducing the reducing agent into the exhaust tract downstream of the SCR particle filter and upstream of the SCR catalytic converter element. A control unit regulates a quantity of reducing agent introduced into the exhaust tract by the first reducing agent feed device and/or by the second reducing agent feed device as a function of the temperature (TscR-PF) of the SCR particle filter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of DE 10 2015 012 736.5 filed on Oct. 1, 2015, which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a method for the operation of an exhaust gas aftertreatment system for an internal combustion engine, in particular for a diesel engine, a device for performing the method, and to a vehicle, in particular a commercial vehicle, for performing the method and/or having the device. 
         [0003]    The provision of a particle filter in an exhaust tract of an internal combustion engine which serves to filter out from the exhaust gas and to store particles, in particular carbon particles, contained in the exhaust gas from the internal combustion engine, is known. The regeneration of such a particle filter can be accomplished, for example, through combustion of the particles stored in the particle filter. For this combustion, high temperatures (approximately 600° C.) are necessary, which cannot routinely be attained in conventional operation of the internal combustion engine, in particular of a diesel engine. In order to be able nevertheless to burn the particles accumulated in the particle filter, measures are usually taken to briefly increase the temperature of the particle filter. Such measures may include an increase in the quantity of fuel injected, for example, or heating of the particle filter by a heating device or the backflow of exhaust gas. Another known way of regenerating a particle filter is the continuous regeneration trap (CRT). In this regeneration of the particle filter an oxidation catalytic converter, which serves to convert the nitrogen monoxide contained in the exhaust gas from the diesel engine into nitrogen dioxide, is usually arranged in the exhaust tract upstream of the particle filter, viewed in the direction of flow of the exhaust gas. Even at low exhaust gas temperatures (approximately 250° C.), the nitrogen dioxide formed reacts with the carbon particles stored in the particle filter, thereby regenerating the particle filter. This usually ensues according to the following equation for the reaction: 
         [0000]      2 NO 2 +C→2 NO+CO 2  
 
         [0004]    In addition to the CRT, it is also known to provide an SCR-active catalytic material, which reduces the nitrogen oxides contained in the exhaust gas from the internal combustion engine using ammonia as reducing agent. The ammonia is usually introduced into the exhaust tract upstream of the particle filter, viewed in the direction of flow of the exhaust gas, in the form of an aqueous urea solution (“AdBlue”). The reduction of the nitrogen oxides usually ensues according to the following equations for the reaction: 
         [0000]      4 NH 3 +4 NO+O 2 →4 N 2 +6 H 2 O
 
         [0000]      2 NH 3 +NO+NO 2 →2 N 2 +3 H 2 O
 
         [0000]      4 NH 3 +3 NO 2 →3.5 N 2 +6 H 2 O
 
         [0005]    As is apparent from the stated equations for the reaction, in a particle filter which is to be continuously regenerated, i.e., using a CRT, and which also comprises a SCR catalytic material, a situation arises in which there is competition vis-a-vis the nitrogen dioxide contained in the exhaust gas. In order to counteract such a competitive situation, DE 10 2014 001 880 A1 discloses the arrangement of a further SCR catalytic converter downstream of the particle filter, viewed in the direction of flow of the exhaust gas, and the similar introduction of an aqueous urea solution into the exhaust tract between the particle filter and the further SCR catalytic converter, viewed in the direction of flow of the exhaust gas. This allows the quantity of aqueous urea solution introduced into the exhaust tract upstream of the particle filter to be reduced in such a way as to ensure the continuous regeneration of the particle filter. The desired reduction of the nitrogen oxides contained in the exhaust gas is ensured by the aqueous urea solution introduced into the exhaust tract downstream of the particle filter and upstream of the further SCR catalytic converter. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the invention is to provide a method for the operation of an exhaust gas aftertreatment system for an internal combustion engine, in particular for a diesel engine, together with a device, which effectively serve to increase the efficiency of the exhaust gas aftertreatment. 
         [0007]    According to an embodiment of the invention a method is proposed for the operation of an exhaust gas aftertreatment system for an internal combustion engine, in particular for a diesel engine, an exhaust tract being provided with at least one SCR particle filter, the SCR particle filter serving to filter out from the exhaust gas and to store particles, in particular carbon particles, contained in an exhaust gas of the internal combustion engine, the SCR particle filter serving to reduce nitrogen oxides contained in the exhaust gas from the internal combustion engine using ammonia as reducing agent, a first reducing agent feed device provided, which serves for introducing the reducing agent, particularly in the form of an aqueous urea solution, into the exhaust tract upstream of the SCR particle filter, viewed in the direction of flow of the exhaust gas, continuous regeneration of the SCR particle filter being possible using nitrogen dioxide as oxidizing agent, at least one SCR catalytic converter element being arranged in or on the exhaust tract downstream of the SCR particle filter and serving to reduce nitrogen oxides likewise contained in the exhaust gas from the internal combustion engine using ammonia as reducing agent, and a second reducing agent feed device being provided, which serves for introducing the reducing agent, particularly also in the form of an aqueous urea solution, into the exhaust tract downstream of the SCR particle filter and upstream of the SCR catalytic converter element. According to the invention a control unit is provided, which regulates and/or controls the quantity of reducing agent introduced into the exhaust tract by the first reducing agent feed device and/or by the second reducing agent feed device as a function of the temperature of the SCR particle filter. 
         [0008]    In this way the efficiency of the exhaust gas aftertreatment is effectively improved, since the quantity of reducing agent introduced into the exhaust tract by the first reducing agent feed device and/or the quantity of reducing agent introduced into the exhaust tract by the second reducing agent feed device is now adjusted or varied as a function of the temperature of the SCR particle filter. The temperature of the SCR particle filter has a great influence on the SCR catalytic effect and the continuous regeneration of the SCR particle filter and represents an important parameter in optimizing the system and the delivery of the reducing agent into the exhaust tract. Using the temperature of the SCR particle filter makes it possible to achieve the desired continuous regeneration of the SCR particle filter and the desired reduction of the nitrogen oxides contained in the exhaust gas from the internal combustion engine whilst at the same time minimizing the quantity of reducing agent introduced into the exhaust tract. A low reducing agent consumption reduces the operating costs of the exhaust gas aftertreatment system and, where the exhaust gas aftertreatment system is provided on a vehicle, increases the range of the vehicle. 
         [0009]    Here the temperature of the SCR particle filter can be obtained, for example, by providing temperature sensors, in particular thermocouples, in or on the exhaust tract. In one embodiment a first temperature sensor is disposed upstream of the SCR particle filter and a second temperature sensor is disposed downstream of the particle filter, in order to measure the exhaust gas temperature, the measured values from the sensors being used by the control unit to determine the temperature of the SCR particle filter. 
         [0010]    Furthermore, the term “exhaust tract” is here expressly to be interpreted in a wider sense. Thus the exhaust tract here encompasses all areas and components through which the exhaust gas from the internal combustion flows. 
         [0011]    In a preferred embodiment of the method no reducing agent is introduced into the exhaust tract by the first reducing agent feed device if the actual temperature of the SCR particle filter is lower than a defined particle filter temperature value. A defined quantity of reducing agent may then be introduced into the exhaust tract by the first reducing agent feed device, if the actual temperature of the SCR particle filter exceeds the defined particle filter temperature value. Thus reducing agent can be introduced into the exhaust tract by the first reducing agent feed device only when the SCR particle filter has a sufficiently high SCR catalytic effect. 
         [0012]    The defined particle filter temperature value is preferably lower than a SCR response temperature of the SCR particle filter, at which nitrogen oxides contained in the exhaust gas can be effectively reduced by the SCR particle filter. This reliably ensures that on reaching the 
         [0013]    SCR response temperature of the SCR particle filter a sufficient quantity of reducing agent is already present on the SCR particle filter to reduce the nitrogen oxides flowing through the SCR particle filter. Here the SCR response temperature of the SCR particle filter preferably lies in a temperature range from 220° C. to 250° C. Alternatively, however, the defined particle filter temperature value could naturally also be equal or identical to the SCR response temperature of the SCR particle filter. 
         [0014]    In a further preferred embodiment of the method, the control unit regulates and/or controls the quantity of reducing agent introduced into the exhaust tract by the first reducing agent feed device and/or the quantity of reducing agent introduced into the exhaust tract by the second reducing agent feed device also as a function of the temperature of the SCR catalytic converter element. In this way the efficiency of the exhaust gas aftertreatment can be further increased. The temperature of the SCR catalytic converter element can likewise be obtained by providing temperature sensors, in particular thermocouples, in or on the exhaust tract, including a first temperature sensor upstream of the SCR particle filter and a second temperature sensor downstream of the SCR catalytic converter element in order to measure the exhaust gas temperature, the measured values from which sensors are used by the control unit to determine the temperature of the SCR catalytic converter element. 
         [0015]    Here, preferably no reducing agent is introduced into the exhaust tract by the second reducing agent feed device if the actual temperature of the SCR catalytic converter element is lower than a defined catalytic converter element temperature value. A defined quantity of reducing agent may then be introduced into the exhaust tract by the second reducing agent feed device, if the actual temperature of the SCR particle filter exceeds the defined catalytic converter element temperature value. Reducing agent can thereby be introduced into the exhaust tract only when the SCR catalytic converter element has a sufficiently high SCR catalytic effect. This saves reducing agent and serves to counteract a flow of unused ammonia from the exhaust tract into the surroundings. 
         [0016]    In principle the defined catalytic converter temperature value may also be equal or identical to the SCR response temperature of the SCR catalytic converter element. It is preferable, however, if the defined catalytic converter element temperature value is likewise lower than the SCR response temperature, at which nitrogen oxides contained in the exhaust gas can be reduced by the SCR catalytic converter element. A reduction of the nitrogen oxides by the SCR catalytic converter element is thereby also ensured when the SCR catalytic converter element reaches its SCR response temperature. Here the SCR response temperature of the SCR catalytic converter element preferably lies in a temperature range from 220° C. to 250° C. 
         [0017]    The defined particle filter temperature value and/or the defined catalytic converter element temperature value more preferably lie in a temperature range from 150° C. to 190° C. Such a temperature range ensures an especially efficient use of the reducing agent. 
         [0018]    Furthermore, if the actual temperature of the SCR particle filter exceeds the defined particle filter temperature value and the actual temperature of the SCR catalytic converter element is lower than the defined catalytic converter temperature value, such a quantity of reducing agent is preferably introduced into the exhaust tract by the first reducing agent feed device that a substantially stoichiometric reducing agent-nitrogen oxide ratio prevails in the area of the first reducing agent feed device. A large quantity of nitrogen oxides contained in the exhaust gas can thereby be reduced by the SCR particle filter, if the SCR catalytic converter element has not yet attained its operating temperature. Moreover, such a quantity of reducing agent counteracts any deposit of aqueous urea solution in the exhaust tract. As an alternative to this stoichiometric introduction of reducing agent, such a quantity of reducing agent may also be introduced into the exhaust tract by the first reducing agent feed device that a greater quantity of reducing agent is present in the area of the first reducing agent feed device than is required for a stoichiometric reducing agent-nitrogen oxide ratio. A particularly large quantity of nitrogen oxides can thereby be reduced by the SCR particle filter. 
         [0019]    If the actual temperature of the SCR particle filter exceeds the defined particle filter temperature value and the actual temperature of the SCR catalytic converter element exceeds the defined catalytic converter temperature value, such a quantity of reducing agent is preferably introduced into the exhaust tract by the first reducing agent feed device, that a quantity of reducing agent smaller than is required for a stoichiometric reducing agent-nitrogen oxide ratio is present in the area of the first reducing agent feed device. This ensures that the SCR particle filter is continuously regenerated. Here such a quantity of reducing agent is preferably introduced into the exhaust tract by the first reducing agent feed device, that 50% to 80% of the quantity of reducing agent required for a stoichiometric reducing agent-nitrogen oxide ratio is present in the area of the first reducing agent feed device. An effective continuous regeneration of the SCR particle filter together with an effective reduction of the nitrogen oxides contained in the exhaust gas oxides are thereby achieved simultaneously by the SCR particle filter. 
         [0020]    Furthermore, if the actual temperature of the SCR particle filter exceeds the defined particle filter temperature value and the actual temperature of the SCR catalytic converter element exceeds the defined catalytic converter temperature value, such a quantity of reducing agent is preferably introduced into the exhaust tract by the second reducing agent feed device that a substantially stoichiometric reducing agent-nitrogen oxide ratio prevails in the area of the second reducing agent feed device. A large quantity of nitrogen oxides contained in the exhaust gas from the internal combustion engine can thereby be reduced by the SCR catalytic converter element. Moreover, such a quantity of reducing agent counteracts any deposit of aqueous urea solution in the exhaust tract. As an alternative, however, such a quantity of reducing agent could also be introduced into the exhaust tract by the second reducing agent feed device that a greater quantity of reducing agent is present in the area of the second reducing agent feed device than is required for a stoichiometric reducing agent-nitrogen oxide ratio. An especially large quantity of nitrogen oxides can thereby be reduced by the SCR catalytic converter element. 
         [0021]    In a further embodiment at least one oxidation catalytic converter, which oxidizes nitrogen monoxide contained in the exhaust gas from the internal combustion engine to form nitrogen dioxide, is arranged in or on the exhaust tract upstream of the SCR particle filter. Such an oxidation catalytic converter reliably serves to ensure the continuous regeneration of the SCR particle filter. 
         [0022]    At least one nitrogen oxide storage element that stores and releases nitrogen oxides contained in the exhaust gas as a function of the exhaust gas temperature, is furthermore preferably arranged in or on the exhaust tract. Here the nitrogen oxide storage element preferably stores the nitrogen oxides contained in the exhaust gas when the temperature of the nitrogen oxide storage element is lower than a defined temperature limit. If the nitrogen oxide storage element exceeds the defined temperature limit, the nitrogen oxide storage element then releases the stored nitrogen oxides. The temperature limit here preferably lies in a range from 100° C. to 200° C., more preferably in a range from 180° C. to 200° C. Such a nitrogen oxide storage element ensures that nitrogen oxides contained in the exhaust gas from the internal combustion engine are at first stored, if the SCR particle filter and/or at least the one SCR catalytic converter element has not yet reached its operating temperature. Once the SCR particle filter and/or the SCR catalytic converter element reach their operating temperature, the stored nitrogen oxides are released again and reduced by the SCR particle filter and/or of the SCR catalytic converter element. Here the nitrogen oxide storage element preferably contains barium nitrate and/or cerium oxide as storage material. The nitrogen oxide storage element may be arranged in or on the exhaust tract, for example, upstream of the SCR particle filter and/or an oxidation catalytic converter. Alternatively, and/or in addition the nitrogen oxide storage element may also be provided directly on the SCR particle filter and/or on the oxidation catalytic converter. 
         [0023]    The SCR particle filter and/or the SCR catalytic converter element preferably contains copper and/or vanadium and/or at least one zeolite material as SCR-effective catalytic material. Such a catalytic material ensures an especially effective reduction of the nitrogen oxides contained in the exhaust gas. 
         [0024]    The object of the present invention is also met by a device having an internal combustion engine, in particular a diesel engine, and an exhaust tract, the exhaust tract comprising at least one SCR particle filter, which serves to filter out from the exhaust gas and to store particles, in particular carbon particles, contained in an exhaust gas of the internal combustion engine, the SCR particle filter serving to reduce nitrogen oxides contained in the exhaust gas from the internal combustion engine using ammonia as reducing agent, a first reducing agent feed device, which serves for introducing the reducing agent, particularly in the form of an aqueous urea solution, in the exhaust tract upstream of the SCR particle filter, viewed in the direction of flow of the exhaust gas, continuous regeneration of the SCR particle filter being possible using nitrogen dioxide as oxidizing agent, at least one SCR catalytic converter element arranged in or on the exhaust tract downstream of the SCR particle filter and serving to reduce nitrogen oxides likewise contained in the exhaust gas from the internal combustion engine using ammonia as reducing agent, and a second reducing agent feed device, which serves for introducing the reducing agent, particularly also in the form of an aqueous urea solution, into the exhaust tract downstream of the SCR particle filter and upstream of the SCR catalytic converter element. According to the invention a control unit is provided, which regulates and/or controls the quantity of reducing agent introduced into the exhaust tract by the first reducing agent feed device and/or by the second reducing agent feed device as a function of the temperature of the SCR particle filter. 
         [0025]    The advantages accruing from the device according to the invention are identical to the advantages of the method according to the invention already disclosed, so that these will not be repeated at this juncture. 
         [0026]    The invention also relates to a vehicle, in particular a commercial vehicle, for implementing the method according to the invention and/or comprising the device according to the invention. The advantages accruing from this are likewise identical to the advantages of the method according to the invention already disclosed and similarly will not be repeated at this juncture. Alternatively to the vehicle, however, the device may naturally also be stationary or fixed. 
         [0027]    Except in cases of clear dependencies or irreconcilable alternatives, for example, the advantageous embodiments and/or developments of the invention explained above may be applied individually or also in any combination with one another. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The invention and its advantageous embodiments and/or developments together with the advantages thereof are explained in more detail below, merely by way of example, referring to drawings, of which: 
           [0029]      FIG. 1  is a side view of a truck having the device according to the invention; 
           [0030]      FIG. 2  is a schematic block drawing of the device according to the invention; 
           [0031]      FIG. 3  is a diagram explaining the method according to the invention; and 
           [0032]      FIG. 4  is a flow diagram explaining the sequence of the method according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]      FIG. 1  shows a vehicle  1 , here embodied as a truck, for example, having the device  3  ( FIG. 2 ) according to the invention. 
         [0034]    As  FIG. 2  shows, the device  3  comprises an internal combustion engine  5  as drive unit for the vehicle  1  and an exhaust tract  7 , which is connected to the internal combustion engine  5  and through which an exhaust gas  9  from the internal combustion engine  5  flows. Viewed in the direction of flow of the exhaust gas, the exhaust tract  7  here comprises an oxidation catalytic converter  11 , a first reducing agent introduction area  13 , a SCR particle filter  15 , a second reducing agent introduction area  17 , and a SCR catalytic converter element  19 . 
         [0035]    Starting from the internal combustion engine  5 , the exhaust gas  9  first flows through the oxidation catalytic converter  11 , which serves to partially oxidize nitrogen monoxide contained in the exhaust gas  9  to form nitrogen dioxide. Then the exhaust gas  9  flows past the first reducing agent introduction area  13 , where a reducing agent for reducing the nitrogen oxides contained in the exhaust gas  9 , here in the form of an aqueous urea solution, for example, can be introduced into the exhaust tract  7 . The aqueous urea solution here is stored in a urea tank  21  of the device  3  and is introduced into the exhaust tract  7  by a first reducing agent feed device  22 . The reducing agent feed device  22  here comprises a pump  23  and a straight-way valve  24 , for example. The straight-way valve  24  here is connected and sends signals to a control unit  25 , which serves to regulate and control the straight-way valve  24  and therefore the quantity of aqueous urea solution introduced into the exhaust tract  7  at the second reducing agent introduction area  17 . 
         [0036]    The exhaust gas  9  then flows through the SCR particle filter  15 , which serves to filter out from the exhaust gas  9  and to store particles, in particular carbon particles, contained in the exhaust gas  9  from the internal combustion engine  5 . The SCR particle filter  15  here is continuously regenerated by the nitrogen dioxide formed by the oxidation catalytic converter  11 . As an additional measure for regenerating the SCR particle filter  15 , the particles accumulated in the SCR particle filter  15  could also be burned at defined times. Here the times may be fixed at defined time intervals, for example, or determined by calculating the actual storage capacity of the SCR particle filter  15 . The particles may be burned, for example, by injecting an increased quantity of fuel, heating the particle filter or causing exhaust gas to back up. In addition, the SCR particle filter  15  also comprises a SCR catalytic material, which serves to reduce nitrogen oxides contained in the exhaust gas  9  from the internal combustion engine  5  using ammonia as reducing agent. The ammonia is introduced into the exhaust tract by the reducing agent feed device  22  in the form of the aqueous urea solution. The SCR particle filter  15  preferably comprises copper and/or vanadium and/or at least zeolite as SCR catalytic material. 
         [0037]    From the SCR particle filter  15  the exhaust gas  9  from the internal combustion engine  5  flows past the second reducing agent introduction area  17 , where the aqueous urea solution stored in the urea tank  21  can likewise be introduced into the exhaust tract  7 . The aqueous urea solution is here introduced into the exhaust tract  7  by a second reducing agent feed device  26 . The second reducing agent feed device  26  here likewise comprises a pump  27  and a straight-way valve  28 , for example. The straight-way valve  28  here is likewise connected and sends signals to a control unit  25 , which serves to regulate and control the straight-way valve  28  and therefore the quantity of aqueous urea solution introduced into the exhaust tract  7  at the second reducing agent introduction area  17 . The control device  25  controls the straight-way valves  24  and  28  based on temperatures of the SCR particle filter  15  and the SCR catalytic converter element  19 , e.g., using temperature sensors  14 ,  16 ,  18 . 
         [0038]    Finally, the exhaust gas  9  from the internal combustion engine  5  flows through the SCR catalytic converter element  19 , which serves to reduce nitrogen oxides likewise contained in the exhaust gas  9  from the internal combustion engine using ammonia as reducing agent. The ammonia needed for this purpose is introduced into the exhaust tract  7  by the reducing agent feed device  22  and/or by the reducing agent feed device  26 . The SCR catalytic converter element  19  likewise preferably comprises copper and/or vanadium and/or at least one zeolite as SCR catalytic material. 
         [0039]    In an optional embodiment, a nitrogen oxide storage element  24  (shown in dashed lines) that stores and releases nitrogen oxide as a function of the exhaust gas temperature is arranged on the SCR particle filter  15 . Alternatively, the nitrogen oxide storage element  24  could be arranged in the exhaust tract  7  upstream of the SCR particle filter  15  and/or upstream of the SCR catalytic converter element  19 , and/or on the SCR catalytic converter element  19 . 
         [0040]      FIG. 3  shows a diagram  29  representing the loading of the SCR particle filter  15  as a function of the quantity of aqueous urea solution introduced into the exhaust tract  7  by the reducing agent feed device  22 . From a time to up to a time t 1  such a quantity of aqueous urea solution is introduced into the exhaust tract  7  by the reducing agent feed device  22  that a greater quantity of reducing agent is present on the first reducing agent introduction area  13  than is required for a stoichiometric reducing agent-nitrogen oxide ratio. From the time t 1  to a time t 3  the reducing agent feed device  22  introduces such a quantity of aqueous urea solution into the exhaust tract  7 , that a quantity of reducing agent smaller than is required for a stoichiometric reducing agent-nitrogen oxide ratio is present on the first reducing agent introduction area  13 . Between the time t 1  and a time t 2  the reducing agent feed device  22  here introduces such a quantity of aqueous urea solution into the exhaust tract  7 , that 80% of the quantity of reducing agent which is required for a stoichiometric reducing agent-nitrogen oxide ratio is present on the first reducing agent introduction area  13 . Between the time t 2  and the time t 3  the reducing agent feed device  22  introduces such a quantity of aqueous urea solution into the exhaust tract  7  that 50% of the quantity of reducing agent which is required for a stoichiometric reducing agent-nitrogen oxide ratio is present on the first reducing agent introduction area  13 . 
         [0041]    It can be seen from the diagram  29  that the particle loading of the SCR particle filter  15  increases, when the reducing agent feed device  22  introduces a quantity of aqueous urea solution into the exhaust tract  7  in excess of the stoichiometric quantity. The reason for this is the reduced continuous regeneration of the SCR particle filter  15 , due to the competing reaction of the ammonia introduced with nitrogen dioxide. If a quantity of aqueous urea solution less than the stoichiometric quantity is introduced into the exhaust tract  7 , the particle loading of the SCR particle filter  15  diminishes again. The diagram  29  therefore clearly shows that the continuous regeneration of the SCR particle filter  15  is disturbed by the introduction of excessive quantities of ammonia into the exhaust tract  7 . 
         [0042]    In addition, the quantities of reducing agent introduced into the exhaust tract  7  by the reducing agent feed devices  22 ,  26  are regulated and controlled here by the control unit  25  as a function of the temperature of the SCR particle filter  15  and as a function of the temperature of the SCR catalytic converter element  19 . This regulation and control is explained in more detail below with reference to  FIG. 4 . 
         [0043]    In an initial state the internal combustion engine  5  here is switched off, for example. Here no aqueous urea solution is introduced into the exhaust tract  7 . After starting the internal combustion engine  5  at step  30 , it is examined, in a step  31 , whether the temperature of the SCR particle filter T SCR-PF  is greater than or equal to a first release temperature T release,1 . The release temperature T release,1  here, for example, is lower than a SCR response temperature of the SCR particle filter  15 , at which the nitrogen oxides contained in the exhaust gas  9  can be reduced by the SCR particle filter  15  or at which the SCR particle filter  15  reaches its operating temperature for reducing the nitrogen oxides. 
         [0044]    If the temperature of the SCR particle filter  15  T SCR-PF  is greater than or equal to the release temperature T release,1 , the first reducing agent feed device  22  introduces a first defined mass flow m 1  of aqueous urea solution into the exhaust tract  7 . The first defined mass flow m 1  here is designed in such a way that a substantially stoichiometric reducing agent-nitrogen oxide ratio prevails in the area of the first reducing agent feed device  22 . The second reducing agent feed device  26  here still does not introduce any aqueous urea solution into the exhaust tract  7 . 
         [0045]    In a step  33  it is then examined whether the temperature of the SCR catalytic converter element  19  T SCR-Kat , is greater than or equal to a second release temperature T release,2 . The release temperature T release,2  here, for example is likewise lower than a SCR response temperature of the SCR catalytic converter element  19 , at which the nitrogen oxides contained in the exhaust gas  9  can be reduced by the SCR catalytic converter element  19  or at which the SCR particle filter  15  reaches its operating temperature for reducing the nitrogen oxides. 
         [0046]    If the temperature of the SCR catalytic converter element  19  T SCR-Kat  is greater than or equal to the release temperature T release,2 , the first reducing agent feed device  22  introduces a second defined mass flow m 2  of aqueous urea solution into the exhaust tract  7 . The second defined mass flow m 2  here is designed in such away that a quantity of reducing agent smaller than is required for a stoichiometric reducing agent-nitrogen oxide ratio is present in the area of the first reducing agent feed device  22 . The second reducing agent feed device  26  then introduces a third defined mass flow m 3  of aqueous urea solution into the exhaust tract  7 . The third defined mass flow m 3  is here is designed in such a way that a substantially stoichiometric reducing agent-nitrogen oxide ratio prevails in the area of the second reducing agent feed device  26 . 
         [0047]    This regulation and control of the reducing agent feed devices  22 ,  26  serves for particularly efficient cleaning of the exhaust gas  9  from the internal combustion engine  5 . 
       LIST OF REFERENCE NUMERALS 
       [0048]      1  vehicle 
         [0049]      3  device 
         [0050]      5  internal combustion engine 
         [0051]      7  exhaust tract 
         [0052]      9  exhaust gas 
         [0053]      11  oxidation catalytic converter 
         [0054]      13  reducing agent introduction area 
         [0055]      15  SCR particle filter 
         [0056]      17  reducing agent introduction area 
         [0057]      19  SCR catalytic converter element 
         [0058]      21  urea tank 
         [0059]      22  reducing agent feed device 
         [0060]      23  pump 
         [0061]      24  straight-way valve 
         [0062]      25  control unit 
         [0063]      26  reducing agent feed device 
         [0064]      27  pump 
         [0065]      28  straight-way valve 
         [0066]      29  diagram 
         [0067]      31  step 
         [0068]      33  step 
         [0069]    m 1  first mass flow 
         [0070]    m 2  second mass flow 
         [0071]    m 3  third mass flow 
         [0072]    t time 
         [0073]    t 0  start time 
         [0074]    t 1  first time 
         [0075]    t 2  second time 
         [0076]    t 3  third time 
         [0077]    T release1  first release temperature 
         [0078]    T release2  second release temperature 
         [0079]    T SCR-PF  temperature of SCR particle filter 
         [0080]    T SCR-Kat  temperature of SCR catalytic converter element