Method for operating an exhaust gas purification system connected to a motor vehicle internal combustion engine, comprising an SCR catalyst

Abstract

Method for operating an exhaust gas purification system connected to a motor vehicle internal combustion engine (1) comprising an SCR catalyst (5) for the catalyzed conversion of nitrogen oxides contained in the exhaust gas of the internal combustion engine with ammonia, in which - an ammonia-containing reducing agent is added to the exhaust gas upstream of the SCR catalyst (5) at a predeterminable dosing rate, - a pressure value corresponding to an absolute pressure in the exhaust gas purification system (1) on the inlet side of the SCR catalyst (5) is determined and - the dosing rate is specified at least as a function of the pressure value, characterized in that a nitrogen oxide conversion of the SCR catalyst (5) is determined and, if a predefinable limit value for the determined nitrogen oxide conversion is undershot, the absolute pressure in the exhaust gas purification system (1) on the inlet side of the SCR catalyst (5) is increased by increasing a flow resistance for exhaust gas flowing out of the SCR catalyst (5), wherein, in order to increase the absolute pressure, a switching of an exhaust gas flow path is carried out from a first flow direction, in which exhaust gas of the motor vehicle internal combustion engine flows through a particle reduction unit (4) before flowing through the SCR catalyst (5), to a second flow direction, in which exhaust gas of the motor vehicle internal combustion engine flows through the SCR catalyst (5) before flowing through the particle reduction unit (4).

Description

[0001] The invention relates to a method for operating an exhaust gas purification system connected to a motor vehicle internal combustion engine, comprising an SCR catalyst for the catalyzed conversion of nitrogen oxides contained in the exhaust gas of the internal combustion engine with ammonia, in which an ammonia-containing reducing agent is added to the exhaust gas upstream of the SCR catalyst at a predeterminable dosing rate.

[0002] German patent DE 10 2008 036 885 A1 describes a process in which aqueous urea solution containing ammonia is added to an exhaust system with an SCR catalyst at a controlled and adjustable dosing rate. Depending on various parameters, the dosing rate is adjusted so that either a target fill level of ammonia stored in the SCR catalyst, predetermined by a computational model, or a predetermined target efficiency for nitrogen oxide conversion with ammonia stored in and / or added to the SCR catalyst is at least approximately achieved. In this way, an effective reduction of nitrogen oxides from the exhaust gas of the corresponding internal combustion engine in a motor vehicle can be achieved.

[0003] Furthermore, US Patent 2010 / 0281855A1 discloses a method for operating an exhaust gas purification system. This system includes an SCR catalyst in which a reducing agent is fed upstream of the catalyst. A pressure value is determined on the inlet side of the catalyst, and the dosing rate of the reducing agent is specified as a function of this pressure value.

[0004] A similar procedure is also known from DE 43 15 278 A1.

[0005] US Patent 2011 / 0239628A1 describes a method in which a throttle valve is installed in an exhaust gas purification system downstream of an SCR catalyst in an exhaust gas line to increase the exhaust gas pressure as needed.

[0006] The object of the invention is to provide a method which enables a further improved removal of nitrogen oxides from the exhaust gas of motor vehicles and internal combustion engines.

[0007] This problem is solved by a method having the features of claim 1.

[0008] In the inventive method for operating an exhaust gas purification system connected to a motor vehicle internal combustion engine, comprising an SCR catalyst for the catalyzed conversion of nitrogen oxides contained in the exhaust gas of the internal combustion engine with ammonia, an ammonia-containing reducing agent is added to the exhaust gas upstream of the SCR catalyst at a predefinable dosing rate. A pressure value corresponding to an absolute pressure in the exhaust gas purification system at the inlet side of the SCR catalyst is determined, and the dosing rate is predetermined, at least as a function of the pressure value. The dosing rate is non-zero and is preferably set so that a predefinable target value for nitrogen oxide conversion or a reduction of nitrogen oxides contained in the exhaust gas is at least approximately achieved. The dosing rate is preferably set by means of a closed-loop control system with feedback.However, forward control with an open-loop control system is also possible. With the absolute pressure-dependent and, in particular, regulated dosing rate according to the invention, a further improved utilization of the conversion potential of the SCR catalyst and thus a further improved reduction of nitrogen oxides from the exhaust gas is enabled. The inventive method takes into account the inventors' understanding that absolute pressure particularly influences mass transport processes, which in turn significantly determine the catalyzed nitrogen oxide conversion. Nitrogen oxides, hereinafter referred to simply as NOx, primarily include nitric oxide (NO) and nitrogen dioxide (NO2).

[0009] Furthermore, according to the invention, the NOx conversion of the SCR catalyst is determined, and if a predefinable limit value for the determined NOx conversion is undershot, the absolute pressure in the exhaust gas purification system on the inlet side of the SCR catalyst is increased by increasing the flow resistance for exhaust gas flowing out of the SCR catalyst. The increase of the absolute pressure to a predefinable value on the inlet side of the SCR catalyst can be achieved, for example, by actuating an exhaust gas flap located downstream of the SCR catalyst in the exhaust gas purification system. By increasing the absolute pressure on the inlet side of the SCR catalyst, the pressure in the catalyst element itself is also raised. As has been determined, this can positively influence the conversion of nitrogen oxides supplied to the SCR catalyst with ammonia (NH3) via the exhaust gas. The dosing rate can thus be increased, if necessary, and an increased nitrogen oxide conversion achieved.Although the mechanisms of action are not fully understood, an increased NH3 storage capacity resulting from the pressure increase, as well as a shift in the thermodynamic equilibrium of the conversion reaction in a desired direction or an improvement in the reaction kinetics, are considered crucial for improved NOx conversion by the SCR catalyst. Preferably, the absolute pressure at the inlet of the SCR catalyst is increased in conjunction with a cold start or warm-up of the internal combustion engine. The heating of the SCR catalyst to temperatures at which it exhibits good NOx reduction activity occurs more quickly because, due to the backing up of exhaust gas and the associated throttling effect, hotter exhaust gas is emitted from the internal combustion engine from the outset. Once the SCR catalyst has reached a predetermined temperature of approximately 250 °C, the pressure increase is increased.Once a predetermined activity of approximately 70% NOx conversion is achieved, the increase in absolute pressure can be reduced or completely reversed. However, an increase in absolute pressure has also proven advantageous after the internal combustion engine has warmed up. With the SCR catalyst at operating temperature above approximately 250 °C, a comparatively high NOx load on the SCR catalyst can occur, particularly under increased engine load (e.g., more than 70% of the rated load), due to elevated raw NOx emissions. At such operating points, reducing NOx emissions to the required limits is often difficult. Increasing the absolute pressure at the inlet of the SCR catalyst allows for an increase in its effectiveness.This allows high NOx reduction values ​​and compliance with strict limits to be achieved even at operating points with high NOx concentrations in the exhaust gas flowing into the SCR catalyst.

[0010] To increase the absolute pressure, the exhaust gas flow path is switched from a first flow direction, in which the exhaust gas from the vehicle's internal combustion engine flows through a particulate reduction unit before passing through the SCR catalyst, to a second flow direction, in which the exhaust gas from the vehicle's internal combustion engine flows through the SCR catalyst before passing through the particulate reduction unit. After switching the exhaust gas flow path, the particulate reduction unit is thus located downstream of the SCR catalyst, whereas before the switch it was located upstream. Therefore, after the switch, the exhaust gas exiting the SCR catalyst must overcome the flow resistance of the particulate reduction unit before being released into the environment.The flow resistance for exhaust gas exiting the SCR catalyst and the absolute pressure upstream of the SCR catalyst are therefore increased, as is its NOx conversion capacity. The switching of the exhaust gas flow path according to the invention proves particularly advantageous at low temperatures, at which the SCR catalyst can achieve a NOx conversion of less than approximately 50%, especially without an additional pressure increase, since the particulate reduction unit, acting as a heat sink for the hot exhaust gas from the internal combustion engine, is eliminated, and the SCR catalyst thus receives hotter exhaust gas. This is particularly advantageous during the warm-up phase of the internal combustion engine.

[0011] In a further embodiment of the invention, the increase in absolute pressure is adjusted depending on the operating parameters of the internal combustion engine and / or the SCR catalyst. This addresses the understanding that an increase in absolute pressure at the inlet of the SCR catalyst or within the SCR catalyst itself, via the exhaust gas purification system, affects the internal combustion engine and its operation. Undesirable consequences may occur in this regard. Furthermore, while positive effects on NOx conversion may be achieved, counterproductive effects on catalyst operating parameters can also occur. By increasing the absolute pressure depending on the operating parameters of the internal combustion engine and / or the SCR catalyst, these cross-influences can be taken into account, negative effects minimized, and an optimal compromise can be reached with respect to opposing operating parameters.

[0012] In a further embodiment of the invention, the increase in absolute pressure is adjusted such that the NOx conversion of the SCR catalyst increases at least approximately by a predefinable amount. For this purpose, previously determined and stored characteristic curves or maps are preferably used, which describe the pressure dependence of the NOx conversion of the SCR catalyst as a function of various operating parameters. The operating parameters can include one or more of the following: exhaust gas flow rate, catalyst temperature, NO2 or NOx inlet concentration, NH3 slip, and optionally other parameters.

[0013] In a further embodiment of the invention, a measure to influence the exhaust gas temperature on the inlet side of the SCR catalyst is implemented in parallel with increasing the absolute pressure. In this way, at least approximately optimal operating conditions for the SCR catalyst with regard to its NOx conversion capacity can be established. In particular, at low exhaust gas temperatures of, for example, 200 °C to 250 °C, measures to increase the exhaust gas temperature can be implemented in parallel with increasing the absolute pressure. Conversely, at high exhaust gas temperatures of, for example, more than 450 °C, measures to reduce the exhaust gas temperature can be implemented in parallel with increasing the absolute pressure.Preferably, one or more operating parameters of the internal combustion engine are changed to influence the exhaust gas temperature, such as a change in the timing and / or quantity of fuel pre-, main, and / or post-injection, exhaust gas recirculation rate, opening and / or closing times of internal combustion engine intake and / or exhaust valves.

[0014] In a further embodiment of the invention, when switching the exhaust gas flow path, it is particularly advantageous if, in the second flow direction, the SCR catalyst and the particle reduction unit are subjected to exhaust gas flow in the opposite direction compared to the first flow direction. Reversing the flow direction through the particle reduction unit allows for the removal of ash that accumulated during flow in the first direction. If, as is preferred, a so-called ammonia blocking catalyst is provided downstream of the SCR catalyst, it is preferably subjected to exhaust gas flow upstream of the SCR catalyst after switching the exhaust gas flow path to the second flow direction. Due to the oxidation-catalytic properties of the blocking catalyst, it increases the NO₂ to NO ratio in the exhaust gas.This in turn allows for an improvement in the catalytic effectiveness of the SCR catalyst, which is downstream of the blocking catalyst in the second flow direction.

[0015] Advantageous embodiments of the invention are illustrated in the drawings and described below. The features mentioned above and those to be explained below can be used not only in the combinations specified, but also in other combinations or individually, without departing from the scope of the present invention.

[0016] This shows: Fig. 1 a schematic representation of an advantageous embodiment of an exhaust gas purification system in which the method according to the invention can be used, Fig. 2 a schematic representation of a control unit for determining a dosing rate for the reducing agent to be added to the exhaust gas, Fig. 3 a diagram showing a schematic representation of the temperature and pressure dependence of the NOx conversion of an SCR catalyst, Fig. 4. A flowchart to explain the operation of the control unit according to Fig. 2 and Fig. 5 a schematic representation of a further advantageous embodiment of an exhaust gas purification system in which the method according to the invention can be used.

[0017] In Fig. Figure 1 is merely an exemplary and schematic representation of an advantageous embodiment of an exhaust gas purification system 1 in which the method according to the invention can be applied. The exhaust gas purification system 1 is associated with a motor vehicle internal combustion engine (not shown), hereinafter referred to simply as the engine. The engine is preferably designed as a direct-injection diesel engine, in particular of a commercial vehicle.

[0018] The exhaust gas emitted by the engine is taken up by an exhaust stream 2 of the exhaust aftertreatment system 1 and flows successively through a particle reduction unit 4 and an SCR catalyst 5 in a flow direction indicated by an arrow 3. Downstream of the SCR catalyst 5, an exhaust gas pressure relief valve 6 is arranged in the exhaust stream 2. This valve allows the exhaust gas to be dammed up, thereby variably increasing or adjusting the exhaust gas pressure, particularly upstream of the SCR catalyst 5. The exhaust gas pressure relief valve 6 is preferably continuously adjustable between an open and a closed position. It may be provided that, in the closed position, a predefinable amount of exhaust gas leakage can flow through the exhaust gas pressure relief valve 6.

[0019] Various sensors for pressure, temperature, and various exhaust gas components are provided on the inlet and outlet sides of the particle reduction unit 4 and the SCR catalyst 5. The following are shown as examples only: Fig. Figure 1 shows a first absolute pressure sensor 7 on the inlet side and a second absolute pressure sensor 8 on the outlet side of the particle reduction unit 4. The absolute pressure sensors 7 and 8 can be used to determine the differential pressure across the particle reduction unit 4 and thus the particle load filtered from the exhaust gas. The absolute pressure sensor 8 preferably also determines the absolute pressure at the inlet side of the SCR catalyst 5. Furthermore, a first temperature sensor 9 is provided upstream of the particle reduction unit 4 and a second temperature sensor 11 is provided upstream of the SCR catalyst 5. The temperature of the particle reduction unit 4 and the SCR catalyst 5 can be determined using the temperature sensors 9 and 11.Furthermore, a first exhaust gas sensor 10 sensitive to NOx and / or NH3 is provided behind the particle reduction unit 4 and a second, similar exhaust gas sensor 12 is provided behind the SCR catalyst 5, which serve to determine the NOx and / or NH3 content in the exhaust gas.

[0020] Furthermore, an injector 13 for releasing a NOx reducing agent into the exhaust gas is arranged in the exhaust gas stream 2 between the first exhaust gas sensor 10 and the second temperature sensor 11. The injector 13 is supplied with the reducing agent from a container (not shown), from which the reducing agent is pumped to the injector 13 by means of a reducing agent pump. Without loss of generality, it is assumed below that the reducing agent is an aqueous urea solution. In the hot exhaust gas, the actual active reducing agent NH3 is released from the urea by thermolysis and / or hydrolysis. This NH3 acts selectively with respect to the catalytic reduction of the NOx contained in the exhaust gas in the SCR catalyst 5. Accordingly, the SCR catalyst 5 is preferably designed as a complete catalyst based on V2O5 / WO3 / TiO2 or as a zeolite-coated SCR supported catalyst with a storage capacity for NH3.The SCR catalyst 5 has a honeycomb structure with a multitude of parallel flow channels and can comprise two SCR catalyst elements connected in series.

[0021] The particle reduction unit 4 preferably consists of an oxidation catalyst and a directly downstream particle filter, which is preferably designed as a wall-flow honeycomb structure based on silicon carbide, aluminum titanate, or cordierite, wherein the filter-effective walls are preferably at least partially provided with an oxidation-catalytically active coating. The oxidation catalyst and the particle filter are preferably arranged in close proximity within a common housing.

[0022] It goes without saying that this is in Fig. 1. The exhaust gas purification system shown in Figure 1 may include, or have included, further sensors, exhaust gas purification components, and other components not shown here for clarity. For example, additional temperature and pressure sensors, as well as exhaust gas sensors sensitive to NOx, oxygen, or other exhaust gas components, may be provided upstream or downstream of the particle reduction unit 4 and / or the SCR catalyst 5, or between the oxidation catalyst and the particulate filter of the particle reduction unit 4. Furthermore, a mixing unit for preparing the added urea solution may be arranged between the injector 13 and the SCR catalyst 5, or an NH3 blocking catalyst may be arranged downstream of the SCR catalyst 5. Preferably, a fuel injection unit is also provided upstream of the particle reduction unit 4. The engine preferably includes turbocharging units, exhaust gas recirculation units, and fuel injection devices.In exhaust system 2, further cleaning-effective components, addition devices for further auxiliary substances and further sensors and the like may be provided.

[0023] Sensors and actuators of the exhaust gas purification system and the engine are connected to an electronic control unit, which is capable of evaluating and processing recorded operating parameters and deriving control signals from them to control the exhaust gas purification system 1 and the engine. The following refers to a Fig. Figure 2 merely presents a schematically illustrated advantageous embodiment of such a control unit. It is understood that differently designed control unit architectures can also be used to control the engine and exhaust gas purification system 1.

[0024] The in Fig. The exemplary embodiment of an electronic control unit 20, as outlined in Figure 2, performs, among other things, a model-based determination of a dosing rate D for the reducing agent to be supplied to the exhaust gas via the injector 13. For this purpose, the control unit 20 comprises an engine control unit (ECU) and a processing unit (R) with a computational dosing model stored therein. The ECU receives input variables (ME) that essentially relate to engine operation. These include, in this case, current values ​​for the engine speed (n), the exhaust gas recirculation rate (EGR), and the intake air volume (m). L as well as other engine operating parameters not listed separately here. On the other hand, the engine control unit (ECU) outputs parameters MA for controlling engine operation. In this case, the MA output parameters include a settable engine torque M and the start of the control cycle t. ASB and approaching t ASEfor the engine's fuel injection injectors, fuel injection quantities, a control signal KL for controlling the exhaust gas flap 6 and other control variables not listed separately here.

[0025] The engine control unit (ECU) communicates with the processing unit (R), transmitting values ​​MAD to it and receiving values ​​DAM from the processing unit (R). The values ​​MAD transmitted to the processing unit (R) include, in this case, received or calculated values ​​for the current absolute pressure p at the inlet of the SCR catalyst (5), and a maximum permissible absolute pressure p, which is adjustable, in particular, by means of the exhaust gas flap (6). max on the inlet side of the SCR catalyst 5, an exhaust gas mass flow m A and other operating parameters not listed separately here, in particular those of the exhaust gas purification system. 1. When determining the maximum permissible absolute pressure p maxPreferably, compliance with specified boundary conditions, especially for engine operation, such as fuel consumption, soot emissions, torque dynamics and, if applicable, other parameters, is taken into account.

[0026] The values ​​DAM received by the engine control unit (ECU) of the processing unit R include a value p. soll for an absolute pressure p on the inlet side of the SCR catalyst 5, which can be set in particular by means of the exhaust gas flap 6, and a temperature increase ΔT that may need to be set for one of the exhaust aftertreatment components of the exhaust gas purification system 1.

[0027] The processing unit R receives as further input variables DE measured or calculated values ​​of operating variables, in particular of the exhaust gas purification system 1, such as values ​​for concentrations of nitrogen oxides in the exhaust gas c. NO , c NO2 inlet side of the SCR catalyst 5, c NOxinlet and outlet side of the SCR catalyst 5, an NH3 concentration c NH3 The output side of the SCR catalyst 5 and a temperature T of the SCR catalyst 5. From the received input variables MAD, DE, the dosing model of the computing unit R determines a dosing rate D to be set for the amount of reducing agent added and outputs this for the corresponding control of the reducing agent injector 13.

[0028] To determine the dosing rate D, the dosing model uses various stored characteristic curves and maps that describe the behavior of the SCR catalyst 5 as a function of various operating parameters, particularly those influencing NOx conversion. The characteristic curves and maps can be pre-determined and stored as such, or they can be dynamically generated or adapted during operation.

[0029] The following refers to Fig. 3. A characteristic curve of particular interest here, concerning the temperature and pressure dependence of the NOx conversion behavior of the SCR catalyst 5, is discussed in more detail. In the diagram of Fig. Figure 3 schematically illustrates a typical NOx conversion η of an SCR catalyst as a function of the catalyst temperature T, represented by a first solid curve 30. This assumes that NH3 is supplied to the SCR catalyst in a sufficient, or at least stoichiometric, quantity. As shown, the NOx conversion η increases steadily from low values ​​with increasing temperature T. At higher temperatures of approximately 300 °C or more, typically high conversions η of at least nearly 100% are achievable. Below a so-called start-up temperature T A The achievable sales η are low to negligible. The start-up temperature T Acan be defined, for example, by the intersection of a tangent to the sales curve 30 with the temperature axis.

[0030] As the inventors were able to determine, an increase in NOx conversion η is possible over a wide temperature range if the absolute pressure p of the gas contained in the SCR catalyst is increased. This is illustrated in the diagram of Fig. Figure 3 shows a second, dashed conversion curve 31 obtained by increasing the absolute pressure p while otherwise keeping conditions essentially unchanged. The second conversion curve 31 obtained by increasing the pressure typically represents approximately a shift of the first conversion curve 30 obtained at the correspondingly lower pressure p. As shown in the diagram of Fig. Figure 3 further illustrates that pressure increase can be achieved both at a comparatively high temperature T2, at which a comparatively high NOx conversion η can be achieved, and at a comparatively low temperature T1 near the start-up temperature T. A A noticeable increase Δη in the NOx conversion η is achieved. As can be seen, an increase in pressure also leads to a decrease in the start-up temperature T. A made possible.

[0031] It is therefore provided according to the invention to increase the absolute pressure p in the exhaust gas purification system 1 upstream of the SCR catalyst 5 as required, in particular by adjusting or closing the exhaust gas flap 6 to a greater or lesser extent, and thus to increase the NOx conversion η of the SCR catalyst 5 or to lower its start-up temperature T. A to achieve.

[0032] The determination of whether, and if so, to what extent, a pressure increase should be set, or whether and to what extent a pressure increase already implemented should be reversed, is carried out in the dosing model of the computing unit R. In parallel, a dosing rate D, which is to be set depending on the absolute pressure upstream of the SCR catalyst 5, is determined.

[0033] A preferred operating mode of the dosing model is in Fig. 4. This is roughly schematically represented in the form of a flowchart. The flowchart is executed periodically at a predefined frequency. After reading the input variables DE and MAD in block 40, the dosing model determines a target value Z and an actual, current value η. istfor the NOx conversion η of the SCR catalyst in block 41. The target value Z is preferably determined taking into account predefined maximum permissible values ​​for NOx tailpipe emissions. In addition, predefined boundary conditions that must be adhered to, such as NH3 slip, reducing agent consumption, and optionally other parameters, can preferably be considered. In order to obtain the most realistic and achievable target value Z possible, the dosing model preferably also uses stored characteristic curves concerning the temperature, exhaust gas flow rate, pressure, and NOx concentration dependence of the NOx conversion capacity of the SCR catalyst 5, and optionally other parameters influencing the conversion behavior of the SCR catalyst 5. The current NOx conversion η ist The result is then compared to the target value Z in blocks 42 and 43.

[0034] If it is determined in block 42 that the current NOx turnover η istIf the target value Z is exceeded by more than a predefinable measure Δ, the system jumps to block 44, in which one or more measures are selected from a plurality of measures M, with which the current NOx turnover η ist so that it can be brought as close as possible to the target value Z again.

[0035] Does the current NOx turnover exceed η ist If the target value Z does not change by more than a predefined measure Δ, the dosing model continues with query block 43 and determines whether the current NOx conversion η ist The value is smaller than the target value Z. If this is the case, the process continues by jumping to the aforementioned block 44 and also determines which of the planned measures M can best achieve the target value Z.

[0036] If block 44 is reached starting from query block 42, measures M may include, for example, a reduction in the dosing rate D, an increase in the raw NOx emissions of the engine with a corresponding reduction in fuel consumption, opening of the exhaust gas flap 6, or a reduction in the absolute pressure p on the inlet side of the SCR catalyst 5. If, on the other hand, block 44 is reached starting from block 43, it is determined whether the target value Z can be achieved by increasing the absolute pressure p on the inlet side of the SCR catalyst 5 or by another, potentially preferable, measure M, such as an increase in the dosing rate D, an increase in the exhaust gas temperature, a reduction in raw NOx emissions, or another measure M. To determine the pressure influence on the NOx conversion η, the dosing model uses the data in Fig. 3. A characteristic curve is shown schematically, or corresponding table values ​​are used. Preferably, a prioritization is provided for the available possible measures M. The prioritization preferably focuses primarily on achieving the lowest possible fuel consumption.

[0037] It is intended that, as shown, selection block 44 will be skipped if the current NOx turnover η ist by less than the intended tolerance dimension Δ is greater than the target value Z.

[0038] In any case, the dosing model progresses to block 45, in which a setpoint p is defined. soll The absolute pressure p is determined. If the setpoint value p is... soll above the maximum permissible absolute pressure p max , the latter is then used as the setpoint p to be adjusted soll chosen.

[0039] In the following block 46, the dosing model determines the previously determined setpoint p as a function of the input variables DE, MAD. soll for the absolute pressure p and, if applicable, further measures selected in block 44 that influence the NOx conversion η, M assigns a NOx conversion η to these data, which ideally corresponds to the target value Z, and ultimately an assigned dosing rate D.

[0040] In the final output block 47, the determined output sizes p are displayed. soll , D, as well as control signals designated as S(M), generated depending on the selected measures M, are output and transmitted to the intended units for processing.

[0041] In addition to or as an alternative to increasing the absolute pressure on the inlet side of the SCR catalyst 5 by actuating the flap 6 as in Fig. As shown in Figure 1, according to the invention it can be provided to increase the absolute pressure by switching the exhaust gas flow path in the exhaust gas purification system 1 from a first flow direction, in which exhaust gas from the engine flows through the particle reduction unit 4 before flowing through the SCR catalyst 5, to a second flow direction, in which exhaust gas from the engine flows through the SCR catalyst 5 before flowing through the particle reduction unit 4.

[0042] A preferred embodiment of the exhaust gas purification system 1 for this purpose is shown schematically in Fig. 5 is shown. This represents the following in relation to Fig. 1. Identical components use the same reference symbols. For the sake of clarity, a depiction of the preferably intended sensors has been omitted. However, it is understood that, analogous to the representation in Fig. 1 corresponding sensors for recording the relevant operating parameters of the exhaust gas purification system 1 in the same or similar manner also in the in Fig. The exhaust gas purification system 1 shown in section 5 is provided.

[0043] Compared to the one in Fig. In the embodiment shown in Figure 1, the routing of the exhaust stream 2 is modified, and an exhaust flow path switching device 50 is additionally provided. The exhaust flow path switching device 50 can be designed in the form of one or more suitably designed flaps or valve assemblies. In any case, the exhaust flow path switching device 50 allows the exhaust flow path in the exhaust aftertreatment system 1 to be switched from a first flow direction, in which exhaust gas from the engine flows through the particle reduction unit 4 before passing through the SCR catalyst 5, to a second flow direction, in which exhaust gas from the engine flows through the SCR catalyst 5 before passing through the particle reduction unit 4. The first flow direction is indicated by solid arrows 51, and the second flow direction by dashed arrows 52.As can be seen, in the second flow direction, the particle reduction unit 4 and the SCR catalyst 5 are subjected to flow in the opposite direction compared to the first exhaust gas flow direction.

[0044] Since the exhaust gas flow resistance of the particle reduction unit 4 is typically significantly higher than that of the SCR catalyst 5, switching the exhaust gas flow path from the first flow direction to the second flow direction results in an increase in the absolute pressure on the exhaust gas inlet side of the SCR catalyst 5 or in the catalyst itself.

[0045] Regarding the implementation of a switch from the first flow direction to the second flow direction, or vice versa, the metering model of the processing unit R determines whether such a switch is appropriate or should be carried out. The measure of switching the exhaust gas flow path is expediently included in the action catalog M in selection block 44 of the [document / section]. Fig. The dosing model outlined in section 4 applies. When deciding whether to switch from the first exhaust gas flow direction to the second, the particle loading of the particle reduction unit 4, or its effect on the flow resistance of the particle reduction unit 4, is preferably also taken into account. In other words, the exhaust gas flow path is switched depending on the differential pressure across the particle reduction unit 4. Reference symbol list: 1 Exhaust gas purification system 2 Exhaust system 3 Arrow 4 particle reduction units 5 SCR catalyst 6 Exhaust gas flap 7 First absolute pressure sensor 8 Second absolute pressure sensor 9 First temperature sensor 10 First exhaust gas sensor 11 Second temperature sensor 12 Second exhaust gas sensor 13 Injector 20 Control unit 30 Sales curve 31 Second sales curve 40 blocks Block 41 42 Block Block 43 44 Selection block 45 Block 46 Block 47 output block 50 Exhaust gas flow switching device 51 solid arrows 52 dashed arrows D Dosage rate DAM Received Values DE Input variables MA Output variables MAD values ME Input variables MSG engine control unit R processing unit

Claims

[1] Method for operating an exhaust gas purification system connected to a motor vehicle internal combustion engine (1) comprising an SCR catalyst (5) for the catalyzed conversion of nitrogen oxides contained in the exhaust gas of the internal combustion engine with ammonia, in which - an ammonia-containing reducing agent is added to the exhaust gas upstream of the SCR catalyst (5) at a predeterminable dosing rate, - a pressure value corresponding to an absolute pressure in the exhaust gas purification system (1) on the inlet side of the SCR catalyst (5) is determined and - the dosing rate is specified at least as a function of the pressure value, characterized by, that a nitrogen oxide conversion of the SCR catalyst (5) is determined and if a predefinable limit value for the determined nitrogen oxide conversion is undershot, the absolute pressure in the exhaust gas purification system (1) on the inlet side of the SCR catalyst (5) is increased by increasing a flow resistance for exhaust gas flowing out of the SCR catalyst (5), wherein to increase the absolute pressure a switching of an exhaust gas flow path from a first flow direction, in which exhaust gas of the motor vehicle internal combustion engine flows through a particle reduction unit (4) before flowing through the SCR catalyst (5), to a second flow direction, in which exhaust gas of the motor vehicle internal combustion engine flows through the SCR catalyst (5) before flowing through the particle reduction unit (4). [2] Method according to claim 1, characterized by, that the increase in absolute pressure is adjusted depending on the operating parameters of the internal combustion engine and / or the SCR catalyst (5). [3] Method according to claim 1 or 2, characterized by , that the increase in absolute pressure is adjusted such that the nitrogen oxide conversion of the SCR catalyst (5) increases at least approximately by a predeterminable amount. [4] Method according to any one of claims 1 to 3, characterized by , that in parallel with the increase in absolute pressure a measure is taken to influence the exhaust gas temperature on the inlet side of the SCR catalyst (5). [5] Method according to any one of claims 1 to 4, characterized by , that in the second flow direction the SCR catalyst (5) and the particle reduction unit (4) are subjected to exhaust gas flow in the opposite direction compared to the first flow direction.

Images