DE102015203425A1 - Method for operating an SCR catalyst system of an internal combustion engine, in particular of a motor vehicle - Google Patents

Abstract

The present invention relates to a method for operating an SCR catalyst system of an internal combustion engine, in particular of a motor vehicle, for exhaust gas after treatment of at least one exhaust gas component by means of at least one catalyst, wherein the exhaust aftertreatment system is disposed close to the internal combustion engine, wherein it is provided in particular that the at least one catalyst for a Model calculation both in the flow direction of the exhaust gas and transverse to the flow direction of the exhaust gas is divided into at least three discrete balance elements (300-310) and on the basis of the balance elements thus formed a calculation of the output of at least one catalyst resulting concentration of the at least one exhaust gas component is performed (430).

Description

The invention relates to a method for operating an SCR catalytic converter system of an internal combustion engine, in particular of a motor vehicle, according to the preamble of claim 1.

The present invention also relates to a computer program, a machine-readable data carrier for storing the computer program and an electronic control unit, by means of which the method according to the invention can be carried out.

State of the art

To meet the increasingly stringent emissions legislation (Euro6, Tier2Bin5 and further emissions regulations), it is necessary to reduce nitrogen oxides or nitrogen oxides (NOx) in the exhaust gas of internal combustion engines, in particular of diesel engines. For this purpose, it is known to arrange an SCR catalytic converter (selective catalytic reduction) in an exhaust gas line of an internal combustion engine, which reduces nitrogen oxides contained in the exhaust gas to nitrogen by means of a reducing agent. In the case of emission-relevant components, an on-board diagnosis (OBD) performed during operation of the motor vehicle is also required.

For the expiry of a said reduction of such nitrogen oxides ammonia (NH ₃ ) is required, which is added to the exhaust gas. Therefore, NH ₃ or NH ₃ -sabspendende reagents are metered into the exhaust line. As a rule, an aqueous urea solution (HWL) is used for this, which is injected downstream of the SCR catalytic converter. From this solution forms by hydrolysis ammonia, which acts as a reducing agent. A suitable 32.5% aqueous urea solution is commercially available under the trade name AdBlue ^®.

In order to achieve high conversion or conversion rates of the nitrogen oxides to be reduced in an SCR catalyst system, the SCR catalyst must be operated so that it is constantly filled to a certain level with the reducing agent ammonia. The DE 10 2004 031 624 A1 describes how such a process for an SCR catalyst system based on the ammonia level can be built. The FR 2 872 544 A1 describes a temperature-dependent desired level specification. In addition, the occurrence of ammonia slip downstream of the SCR catalyst must be avoided as much as possible, since ammonia in high concentration has a harmful effect.

Currently known and already used in series monitoring functions determine the efficiency of the nitrogen oxide reduction (NOx conversion rate) using a arranged downstream of the SCR catalyst first nitrogen oxide (NOx) sensor) and a second upstream of the SCR catalyst arranged nitrogen oxide sensor, optionally by a model-based replacement value is replaced. Due to the aging of the SCR catalyst, the achievable conversion rate decreases with increasing use time and the nitrogen oxide emission downstream of the SCR catalyst increases. From the maximum permissible nitrogen oxide emissions, a threshold for the efficiency can be determined, below which a system error is reported.

Out DE 10 2012 220 151 A1 Furthermore, a method for checking the functionality of components of an exhaust aftertreatment system affected here, in which a first SCR catalyst exhaust gas upstream of a second SCR catalyst is arranged. This makes it possible to check the ammonia storage capacity of the first and the second SCR catalytic converter, the functionality of an NH ₃ sensor arranged between the two SCR catalytic converters, and a NOx sensor arranged downstream of the second SCR catalytic converter. For this purpose, a change in an operating variable of the internal combustion engine, which influences the nitrogen oxide concentration of the exhaust gas, is detected.

In addition, methods for determining the temperature distribution of a NOx catalyst have become known in which the catalyst is divided into n disks with n> 1 in its axial direction and the temperature of each disk as a function of the temperature of the respective disk Exhaust gas is determined. The radial temperature distribution is assumed to be constant and an adiabatic heat transfer between an exhaust gas and a catalyst disk n is calculated.

Disclosure of the invention

According to the invention, it is proposed to divide the catalyst system or the catalyst not only in the flow direction into individual balance elements or finite elements in an SCR catalyst model, but also to calculate regions of different HWL or ammonia wetting arranged transversely to the flow direction by means of discrete or finite balance elements or in the model calculation or simulation. Here, a corresponding cross-sectional area of the respective catalyst is divided into the smallest possible number of more than two discrete areas, which are considered separately. This number is preferably less than five and more preferably three.

The method according to the invention is preferably applicable to an SCR catalyst system which is arranged in a region of an exhaust tract of the internal combustion engine close to the engine or in the vicinity of the combustion chamber in which an exhaust gas temperature in the range of 150-500 ° C. prevails during operation of the internal combustion engine. Because with such a combustion chamber near arrangement relatively high conversion rates are possible due to the naturally higher exhaust gas temperatures. However, in a close-to-motor arrangement, in contrast to a mostly existing underfloor arrangement, only relatively short mixing sections of the exhaust gas with the reducing agent are available.

The method according to the invention makes it possible to model, in comparison to the prior art, in particular real-time modeling of an unequal distribution of ammonia or HWL transversely to the exhaust gas flow direction during operation of a catalyst system affected here by means of a corresponding possibly already existing control device. As a result, a higher NH ₃ level can be set in the SCR catalytic converter, without increasing the stated risks of misdiagnosis and / or of the mentioned ammonia slip.

In the method according to the invention, in order to improve the real-time capability in the model calculation or simulation, it can further be provided that the at least one catalyst is subdivided into an odd number of balance elements transversely to the flow direction of the exhaust gas, around a symmetrical distribution of a balance element arranged around a middle balance element To provide balance elements. The number of balance elements formed transversely to the flow direction of the exhaust gas is again in particular less than five and preferably three.

In the method according to the invention, in order to improve the real-time capability in the calculation or simulation, it can be provided that the balance elements formed transversely to the flow direction of the exhaust gas are calculated independently of one another.

In the method according to the invention, in order to improve the real-time capability as well as the calculation quality in the calculation or simulation, it can further be provided that in the calculation of the concentration of the at least one exhaust gas component resulting from the exit of the at least one catalytic converter, i. Preferably, the concentration of said nitrogen oxides (NOx), transverse to the flow direction of the exhaust gas, a symmetrical unequal distribution u of the at least one exhaust gas component and in the flow direction of the exhaust gas, an exhaust gas temperature T1-Tx are taken as a basis.

In the method according to the invention, in order to improve the real-time capability as well as the calculation quality in the calculation or simulation, it can further be provided that the concentration of the at least one exhaust gas component resulting at the outlet of the at least one catalytic converter is calculated as an average of the concentration of the at least one exhaust gas component based on the equation NOx_means = v% · NOx_last_range_overdosing + (100 - 2 · v%) · NOx_last_range_nominal + v% · NOx_last_set_dosing where v is the area fraction of the respective balance element on a total cross-sectional area of the at least one catalyst in% and where the indices "Last-Reihe_überdosierend" and "Last-Reihe_ unterdosierend" at the output of at least one catalyst arranged balance elements denote, in which a relative to a nominal value with the index "last-Reihe_nominal" designated exceeding or falling below amount of reducing agent (HWL or ammonia) corresponding concentrations of at least one exhaust gas component result.

In the method according to the invention, in order to improve the real-time capability in the calculation or simulation it can further be provided that, for modeling the at least one catalyst, individual ones of the channels are sorted according to their concentration of the at least one exhaust gas component and, based on this sorting, a concentration distribution of the at least one exhaust gas component transversely to the flow direction of the exhaust gas is approximated by discretization.

In the method according to the invention, in order to improve the real-time capability in the calculation or simulation it can further be provided that an odd number of at least three balance spaces are formed by means of said discretization, each of which has an area fraction v of an overall cross-sectional area of the at least one catalyst of x%. , y% and 100% - x% - y% occupy, wherein an average range of the concentration distribution is applied to an average of the concentration of the at least one exhaust gas component and the at least two remaining areas with a b% increased and a% reduced concentration be charged.

The invention can in particular be used in an SCR catalytic converter system of an internal combustion engine, preferably a self-igniting internal combustion engine (diesel engine) of a motor vehicle. In addition, there is also one Application to other exhaust aftertreatment systems using similar catalysts as described herein is possible.

The computer program according to the invention is set up to carry out each step of the method, in particular if it runs on a computing device or a control device. It allows the implementation of the method according to the invention on an electronic control unit, without having to make structural changes to this. For this purpose, the machine-readable data carrier is provided on which the computer program according to the invention is stored. By loading the computer program according to the invention on an electronic control unit, the electronic control unit according to the invention is obtained, which is set up to control an exhaust aftertreatment system affected here by means of the method according to the invention.

Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination indicated, but also in other combinations or in isolation, without departing from the scope of the present invention.

Brief description of the drawings

1 shows an associated with an internal combustion engine SCR catalyst system according to the prior art, which can be operated by a method according to the invention.

2a , b show an exemplary modeling according to the invention of an unequal distribution of the ammonia concentration in an SCR catalytic converter transverse to the exhaust gas flow direction.

3 shows an exemplary embodiment of a modeling according to the invention with a discretization of the ammonia distribution transverse to the flow direction of the exhaust gas.

4 shows an embodiment of the method according to the invention with reference to a flowchart.

Description of exemplary embodiments

1 shows a per se known SCR catalyst system, which is provided for arrangement in the exhaust system of a diesel-powered motor vehicle. The direction of the exhaust gas flow is in 1 marked with an arrow.

In the exhaust line are three catalyst housing in the flow direction one behind the other 1 . 2 . 3 arranged. The first catalyst housing 1 has a Diesel Oxidation Catalyst (DOC) at its entrance 11 on. Downstream of the DOC 11 is in the case 1 a dosing module 12 arranged, which is adapted to meter urea water solution (HWL) into the exhaust gas. Downstream of this metering point is a first catalyst 13 arranged, which consists of a particulate filter with SCR coating (SCRoF). The second catalyst housing contains a second catalyst 21 which is an SCR catalyst 21 is. The third catalyst housing contains a clean-up catalyst 31 ,

Upstream of the first catalyst housing 1 is a first NH ₃ cross-sensitive nitric oxide sensor 41 arranged. Other NH ₃ -sensitive nitrogen oxide sensors 44 and 46 are between the SCRoF catalyst 13 and the SCR catalyst 21 and downstream of the clean-up catalyst 31 arranged. Between the SCRoF catalyst 13 and the SCR catalyst 21 is still an ammonia sensor 43 arranged. Between the DOC 11 and the SCRoF catalyst 13 , between the SCR catalyst 21 and the clean-up catalyst 31 and downstream of the clean-up catalyst 31 are three temperature sensors 42 . 45 . 47 arranged.

A control unit 5 is with the catalyst housings 1 . 2 . 3 and with the sensors 41 . 42 . 43 . 44 . 45 . 46 . 47 connected (compounds not shown). An internal combustion engine 6 is upstream of the first catalyst housing 1 arranged and introduces exhaust gas into this.

The review of the SCRoF catalyst 13 of the SCR catalyst 21 by assessing the degree of damping of a change in an operating variable exploits the fact that only in one of the catalysts 13 . 21 bound ammonia can contribute to nitrogen oxide conversion. In addition, the storage capacity of the catalysts decreases 13 . 21 during their aging. For the SCRoF catalyst 13 , which is directly downstream of the dosing module 12 is arranged in the exhaust line, positive nitrogen oxide sensor signals are observed in the operating state shown here, since a complete adsorption of ammonia is no longer possible and the nitrogen oxide conversion can not completely eliminate this effect.

The present invention relates in particular to motor vehicles in which exhaust gas aftertreatment systems, eg SCR catalyst systems described below, are provided to achieve the highest possible conversion rates of exhaust gas components such as nitrogen oxides as close as possible to the internal combustion engine. The higher conversion rates are made possible in particular by the higher exhaust gas temperatures in such an arrangement.

However, a disadvantage of such a close-coupled arrangement, in contrast to a mostly existing underfloor arrangement, is the relatively large lack of space in the engine compartment of the motor vehicle and the resulting relatively short mixing distances of the exhaust gas. Although the resulting deterioration of the exhaust gas mixture treatment per se is tolerable, such an exhaust aftertreatment system is difficult to control, since exhaust aftertreatment models implemented in a control unit or corresponding model calculations for the aforementioned conversion of exhaust gas components such as, for example, Nitrogen oxides can not or insufficiently take into account a poor mixture formation mentioned.

In the prior art, therefore, for example, close-coupled SCR catalyst systems are operated with a reduced amount of NH ₃ compared with the technically possible filling quantity, in order to prevent or minimize an abovementioned NH ₃ slip. However, there is a considerable risk of misdiagnosis, for example, of a named SCR on-board diagnosis, since a modeled and a measured efficiency can differ significantly due to the poor mixing of the exhaust gas.

With regard to said SCR modeling, a large number of approaches have become known, for example from the publication "Unsteady Analysis of NO Reduction on Selective Catalyst Reduction - De-NOx Monolith Catalysts", E. Tronconi et al., Ind. Eng. Chem. Res. 1998, 37, pages 2341-2349 , Such models implemented in modern vehicle control units map both the NOx conversion of the SCR catalyst and the NH ₃ slip. The models either operate on the basis of an average temperature or replicate the temperature and NH ₃ distribution in the SCR catalyst along the flow direction of the exhaust gas by dividing the catalyst into corresponding segments or finite elements.

In the 2a and 2 B is shown schematically how a division of a catalyst system according to the invention can be made transversely to the flow direction of the exhaust gas of the internal combustion engine into individual balance elements or finite elements in order to calculate areas of different HWL or ammonia wetting means such discrete balance elements.

Here, the cross-sectional area of the per se elongated catalyst system is divided in the present embodiment into three discrete areas or finite elements, which are considered separately with respect to a given model calculation. It should be noted that the number of regions thus formed may also be greater than three, but preferably less than five, and also preferably odd, to allow for symmetrical distribution about a central region.

In 2a is the frontal area 200 an SCR catalyst, which has a plurality of channels arranged in the flow direction of the exhaust gas 205 having. The face 200 is unevenly applied or wetted with a reducing agent involved here, ie in the case of an SCR system ammonia or a precursor substance such as called HWL. In the dashed line 210 demarcated inner area arrives at relatively more ammonia and in the outside of this inner area 215 arranged outer area 220 in terms of the inner area 215 relatively less ammonia or precursor substance.

To model the individual channels 205 of the catalyst according to its ammonia concentration, as shown in 2 B to see. This sorting allows the distribution of ammonia across the flow direction of the exhaust gas, ie in 2a at the paper level, by approximating a rough discretization to make the distribution accessible to a real-time numerical simulation. A modeling with a particularly coarse discretization with only a few areas can advantageously be carried out in real time so that it can be implemented in a control unit of the internal combustion engine or the SCR system and thus used during operation of the internal combustion engine or an underlying motor vehicle or can be applied.

In the 2 B shown discretization takes place in this embodiment in three balance areas, the area ratio v of the total in 2a shown end face 200 from x%, y% and 100% - x% - y% occupy. The middle area 225 the NH ₃ concentration distribution is averaged in the present model 230 the ammonia concentration or a said precursor substance acted upon. The left area 235 is charged with b% more 240 and the right area 245 but with a% less 250 , in each case measured on the average ammonia concentration 230 ,

In the 2 B drawn trace 255 - 265 NH ₃ concentrations measured at a test bench include three in 2a shown curve segments 255 . 260 . 265 , where the left curve segment 255 the inner area 215 the NH ₃ - Wetting, the middle curve segment 260 in the right part of the inner area 215 lightly displayed wetting 270 and the right curve segment 265 the NH ₃ addition in the outer area 220 correspond.

In the 2 B The advantage of the presented discretization is that by calculating the same SCR model three times, it is possible to simulate a correct consideration of the unequal distribution of ammonia and thus to bring the model and the reality into much better agreement.

3 shows a concrete embodiment of a discretization according to the invention, wherein the catalyst according to the 2a and 2 B transverse to the exhaust gas flow direction in each case three balance elements 300 . 305 . 310 and additionally along the exhaust gas flow direction in ten balance elements 315 . 320 . 325 , etc. is divided. As already mentioned, a symmetrical unequal distribution of ammonia is assumed for the modeling or simulation, ie in the upper part 300 of balance sheet elements 300 - 310 will be u% more ammonia and in the lower part 310 correspondingly u% less ammonia assumed. In addition, in the exemplary embodiment shown, an exhaust gas temperature profile of ten different exhaust gas temperatures is determined in the flow direction of the exhaust gas 315 . 320 . 325 , etc. (T1-T10).

For each of the thirty in 3 As described below, an SCR model is calculated which calculates the NOx conversion behavior, the NH ₃ level and the NH ₃ slip of each of the thirty balance elements. The inventively different admission of ammonia is initially only for the first disc formed from three balance elements 330 performed and then pulls from left to right through the entire model of the catalyst.

At the in 3 For the sake of simplification, it is assumed that between the in 2a shown individual channels 205 , and thus between the three balance sheet elements 300 - 310 a respective disc 330 . 335 . 340 , etc., no mass transfer to ammonia takes place. According to the aforementioned symmetrical unequal distribution are also the area proportions 345 of the three balance sheet items 300 - 310 a respective disc 330 . 335 . 340 , etc. with v% at the two outer balance elements 300 . 310 or 100 - 2 · v% at the middle balance element 305 formed symmetrically.

On the basis of in 3 Modeling shown is for each of the three balance element rows 300 - 310 the mean value behind the catalyst corresponding to the relative area proportions (v) 345 resulting individual concentrations of the substances involved (in this case preferably NOx and ammonia) calculated. For example, the mean value of the NOx concentration is calculated according to the following equation (1): NOx_means = v% · NOx_tube_10_overdosing + (100 - 2 · v%) · NOx_tube10_nominal + v% · NOx_tube10_substituting (1)

Accordingly, the substance concentration downstream of the tenth slice is taken and weighted according to its area fraction. The calculation shown in equation (1) is carried out in the same way for the ammonia concentration.

It should be noted that with the described modeling approach, it is also possible to model or simulate an SCR catalytic converter or system in which a NOx sensor is arranged off-center downstream of the catalytic converter. An NOx sensor arranged in this way is usually also flowed through in a non-uniform manner by the exhaust gas. In this arrangement, the three rear discs 350 . 355 . 360 converted into a concentration with a different weighting than the other discs in order to emulate the measurement signal of the NOx sensor in the model as well as possible. The same principle can be used to simulate the flow of a second downstream SCR catalyst.

4 shows an embodiment of the method according to the invention. After the start 400 The routine shown there is initially the creation or calculation 405 said balance elements, ie, assumed in the respective balance element wetting with ammonia or HWL solution. As input variables for this calculation 405 serve in the present embodiment, the entire, assumed in the catalyst adsorbed NH ₃ mass 410 , the area percentage (v) 415 of the respective balance element and the temperature profile 420 about the various, in the present embodiment, ten slices of the model.

• • Die NOx-Konzentration ist die NOx-Konzentration des stromauf gelegenen NOx-Sensors (oder eines geeigneten Ersatzmodells).
• • Die Anzahl an NH₃-Molekülen entspricht 50% der eingespritzten Harnstoff-Moleküle, und zwar multipliziert mit dem Flächenanteil der mittleren Scheibe. In diesem Fall geht das Modell von einer perfekten Thermolysierung des Harnstoffs zu Ammoniak und Isocyansäure aus. Mit Hilfe des Abgasmassenstroms kann hieraus natürlich auch eine Konzentration an NH₃ berechnet werden.
• • Die anderen 50% der Harnstoffmoleküle zerfallen zu Isocyansäure.
• • Die NOx-Konzentration ausgangs des Bilanzelements ergibt sich in diesem Ausführungsbeispiel folgendermaßen: – Es sei rNOx die Reaktionsgeschwindigkeit der genannten Thermolysierungsreaktion gemäß der Gleichung r_NOx = k_NOxc_NOxθexp(–E_NOx/RT), mit kNOx als applizierbarem Parameter, cNOx als NOx-Konzentration im Bilanzelement, theta als Bedeckungsgrad, d.h. als NH₃-Füllstand des Katalysators geteilt durch den maximalen Füllstand, ENOx als Aktivierungsenergie, R als universelle Gaskonstante und T als in dem Bilanzelement vorherrschender Temperatur. – Für die Konzentration ausgangs des Bilanzelements wird die folgende Gleichung zugrunde gelegt NOxDs = NOxUs – Int(rNOx), in der NOxDs die Konzentration ausgangs des Bilanzelements, NOxUs die Konzentration eingangs des Bilanzelements und Int() das zeitliche Integral über die Verweilzeit des Gases im entsprechenden Bilanzelement bezeichnen.
• • Die Bilanzierung für die anderen Spezies (NH₃, CHNO) erfolgt analog. Die Ansätze für die Reaktionsgeschwindigkeiten können der Literatur entnommen werden.

Based on the in step 405 certain balance sheet items takes place in step 430 for each of the thirty balancing items, a (balance) calculation of the values given on the right-hand side of equation (1), resulting in a NOx average value behind the catalytic converter. The following balance sheet can be used for NOx balancing, for example, of the first, middle balance element:

• • The NOx concentration is the NOx concentration of the upstream NOx sensor (or a suitable replacement model).
• The number of NH ₃ molecules corresponds to 50% of the injected urea molecules, multiplied by the area fraction of the middle slice. In this case, the model assumes a perfect thermolysis of the urea to ammonia and isocyanic acid. Of course, with the aid of the exhaust gas mass flow, a concentration of NH ₃ can also be calculated therefrom.
• • The other 50% of the urea molecules decompose to isocyanic acid.
• The NO x concentration output of the balance element in this exemplary embodiment results as follows: Let rNOx be the reaction rate of the said thermolysing reaction according to the equation r _NOx = k _NOx c _NOx θexp (-E _NOx / RT), with kNOx as the applicable parameter, cNOx as the NOx concentration in the balance element, theta as the coverage, ie as the NH ₃ level of the catalyst divided by the maximum fill level, ENOx as the activation energy, R as the universal gas constant and T as the prevailing temperature in the balance element. - For the concentration output of the balance element, the following equation is used NOxDs = NOxUs - Int (rNOx), in which NOxDs denote the concentration output of the balance element, NOxUs the concentration at the beginning of the balance element and Int () the time integral over the residence time of the gas in the corresponding balance element.
• • The balancing for the other species (NH ₃ , CHNO) is analogous. The approaches for the reaction rates can be found in the literature.

Thus one obtains the concentrations output of the first middle balance element, which in turn form the input values for the second middle balance element etc. For the other balance elements the calculation is done analogously.

In the present SCR catalyst model, the parameter v is assumed to be freely adjustable in this calculation, and the parameter u is assumed to have a first characteristic 435 determined or calculated, this first characteristic 435 of the total exhaust gas volume flow I _vol 445 in the catalyst depends. The mentioned flow of the NOx sensor, which as described depends on the exact lateral position of the NOx sensor, is determined via a further parameter v2 by means of a second characteristic curve 440 represented, which also depends on the exhaust gas volume flow I _vol .

For example, u is assumed to be a characteristic with two interpolation points 100m ³ / h and 200m ³ / h, which are respectively 0.5 and 0.2. This example assumes that the unequal distribution is lower at higher volume flows. In this case, the parameter v2 could be fed with the same nodes and the values 0.2 and 0.15 (again, the effect will be lower for larger volume flows).

The thus determined NOx average is in a test step 450 is compared with an NOx value measured by the NOx sensor. If the comparison shows that the deviation between these two values is greater than an empirically specified threshold value, the following step will be used 455 a corresponding countermeasure taken, for example, changed the catalyst supplied amount of HWL or ammonia. Alternatively, in the event of such a deviation, an error message can be output to a named on-board diagnostic system (OBD).

It is worth noting that the steps 430 to 455 as shown by the dashed line 425 is to be indicated, for each of the above (in the present case three) balance element rows are executed, ie these steps are executed either in a corresponding program loop or to improve the real-time behavior even in parallel.

The total adsorbed by the SCR catalyst NH ₃ mass is stored in a non-volatile memory, such as an EEPROM of a controller. Because it is particularly advantageous to store only a single value of the total mass and the total mass according to a common distribution between the in 3 split thirty balance sheet items shown. This is an even distribution along the ten slices 315 - 360 accepted. In the direction of the aforementioned unequal distribution transversely to the exhaust gas flow direction, the total mass is divided according to the root or the square of the coverage (corresponding to said parameter u), which is defined herein as a ratio of the thus divided NH ₃ mass to the maximum NH ₃ mass ,

Alternatively it can be provided that, instead of storing the total NH ₃ mass, which in all of the in 3 shown NH ₃ - or HWL wetting is stored in an EEPROM of said control device, whereby the modeling or simulation results can be much more precise.

Again alternatively, it can be provided that the parameter u depends on the exhaust gas flow, on the exhaust gas or catalyst temperature, on the metered quantity of ammonia or HWL, on the NOx mass flow or on a combination of these parameters. It can also be provided that the said discretization is chosen to be coarser or finer and / or that instead of ten temperature disks, a different number, for example five temperature disks, are used in the calculation or simulation. Also, the said unequal distribution can be represented with only two balance elements per slice or with significantly more balance elements per slice.

The method described can be realized in the form of a control program for an electronic control unit for controlling an exhaust aftertreatment system of an internal combustion engine, in particular an SCR catalyst system, or in the form of one or more corresponding electronic control units (ECUs).

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102004031624 A1 [0005]
• FR 2872544 A1 [0005]
• DE 102012220151 A1 [0007]

Cited non-patent literature

• "Unsteady Analysis of NO Reduction on Selective Catalyst Reduction - De-NOx Monolith Catalysts", E. Tronconi et al., Ind. Eng. Chem. Res. 1998, 37, pages 2341-2349 [0034]

Claims (11)

Method for operating an SCR catalyst system of an internal combustion engine, in particular of a motor vehicle, for the exhaust gas after treatment of at least one exhaust gas component by means of at least one catalyst, characterized in that the at least one catalyst for a model calculation both in the flow direction of the exhaust gas and transverse to the flow direction of the exhaust gas in at least two discrete balance sheet elements ( 300 - 310 ) and that, on the basis of the balance elements thus formed, a calculation of the concentration of the at least one exhaust gas component resulting at the outlet of the at least one catalytic converter is carried out ( 430 ). A method according to claim 1, characterized in that the SCR catalyst system is disposed in a region close to the combustion chamber of an exhaust tract of the internal combustion engine, in which an exhaust gas temperature in the range of 150-500 ° C is present during operation of the internal combustion engine. A method according to claim 1 or 2, characterized in that the at least one catalyst transversely to the flow direction of the exhaust gas in an odd number of at least three balance elements ( 300 - 310 ) to provide a symmetrical distribution of balance elements around a central balance element. Method according to one of the preceding claims, characterized in that the balance elements formed transversely to the flow direction of the exhaust gas ( 300 - 310 ) are calculated independently. Method according to one of the preceding claims, characterized in that in the calculation of the output of at least one catalyst resulting concentration of the at least one exhaust gas component transverse to the flow direction of the exhaust gas, a symmetrical unequal distribution u of at least one exhaust gas component and in the flow direction of the exhaust gas, an exhaust gas temperature T1- Tx. Method according to one of the preceding claims, characterized in that the at the output of the at least one catalyst resulting concentration of the at least one exhaust gas component as an average of the concentration of the at least one exhaust gas component based on the equation NOx_means = v% · NOx_last_range_overdosing + (100 - 2 · v%) · NOx_last_range_nominal + v% · NOx_last_set_dosing where v denotes the area fraction of the respective balance element on a total cross-sectional area of the at least one catalyst in% and wherein the indices "last-row overdosing" and "last-row under-dosing" at the outlet of the at least one catalyst arranged balance elements ( 360 ), in which corresponding concentrations of the at least one exhaust gas component result at a quantity of reducing agent which exceeds or falls below a nominal value designated by the index "last row_nominal". Method according to one of the preceding claims, wherein the at least one catalyst comprises a plurality of channels arranged in the flow direction of the exhaust gas ( 205 ), characterized in that for modeling the at least one catalyst, individual ones of the channels ( 205 ) are sorted by their concentration of at least one exhaust gas component and on the basis of this sorting a concentration distribution of at least one exhaust gas component is approximated transversely to the flow direction of the exhaust gas by means of discretization. A method according to claim 7, characterized in that by means of the discretization an odd number of at least three balance spaces are formed, each having an area fraction v on an entire cross-sectional area ( 200 ) of the at least one catalyst of x%, y% and 100% - x% - y%, wherein a middle range ( 225 ) of the concentration distribution with an average value ( 230 ) the concentration of the at least one exhaust gas component is applied and the at least two remaining areas ( 235 . 245 ) with a b% increase ( 240 ) and one with a% reduced ( 250 ) Concentration ( 230 ). A computer program configured to perform each step of a method according to any one of claims 1 to 8. Machine-readable data carrier on which a computer program according to claim 9 is stored. Electronic control unit, which is set up, an exhaust aftertreatment system of an internal combustion engine, in particular of a motor vehicle, which is provided for exhaust aftertreatment of at least one exhaust gas component by means of at least one catalyst, wherein the exhaust aftertreatment system is arranged close to the internal combustion engine to control by means of a method according to one of claims 1 to 8 ,

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