DE102008039687B4 - Process for the aftertreatment of an exhaust gas stream of an internal combustion engine - Google Patents

Abstract

Method for the after-treatment of an exhaust gas flow (7) of an internal combustion engine (3) of a motor vehicle (1) by means of a selective catalytic reduction, comprising: - determining a composition of the exhaust gas stream which is dependent on a first part size (47) and a second part size (49) Guide variable (21), wherein the first part size indicates a presence of pollutants in the exhaust gas flow and the second part size indicates the presence of a reducing agent (13), - adding the reducing agent (13) into the exhaust gas flow (7) as a function of the control variable (21), characterized by determining the second part size (cNH3) (49) by means of a catalytic exothermic oxidation of NH3 of the reducing agent (13) contained in the exhaust stream (7) on a catalytically active Pt surface (57) and determining a heat of reaction of the exothermic oxidation.

Description

The invention relates to a method for the after-treatment of an exhaust gas stream of an internal combustion engine of a motor vehicle by means of a selective catalytic reduction.

It is known for the aftertreatment of an exhaust gas stream, in particular for denitrification, to use so-called SCR catalysts (selective catalytic reduction). These can be used, for example, for diesel engines or any other engines with a lean of stoichiometric engine control, for example, engines with homogeneous compression ignition and / or lean-burn engines with spark ignition. By means of the SCR catalyst, pollutants entrained in the exhaust gas stream, for example NO, NO 2, can be converted into harmless components in the presence of an additionally attributable reducing agent, for example NH 3. For controlling and / or controlling a conversion rate of the pollutants, an addition amount of the reducing agent may be controlled. The DE 10 2007 044 193 A1 relates to a method and apparatus for controlling the injection of a reducing agent into an exhaust stream. From the DE 10 2006 041 676 A1 a method for dosing a reagent in the exhaust region of an internal combustion engine and apparatus for performing the method is known. The DE 60 2005 001 922 T2 discloses a method for controlling the addition of a reducing agent in the exhaust gas of an internal combustion engine.

From the DE 103 46 220 A1 as well as the DE 197 36 384 A1 In each case, a method for the aftertreatment of an exhaust gas stream of an internal combustion engine of a motor vehicle by means of a selective catalytic reaction is known, wherein determining a characteristic of the composition of the exhaust gas flow leading variable and an attributing reducing agent in the exhaust gas flow in dependence on the guide size.

The object of the invention is to enable an improved aftertreatment of an exhaust gas stream of an internal combustion engine of a motor vehicle by means of a selective catalytic reduction, in particular an improved control and / or control of an addition of a reducing agent in the exhaust gas stream.

The object is achieved with the features of claim 1.

• – Ermitteln einer von einer ersten Teilgröße (47) und einer zweiten Teilgröße (49) abhängigen, die Zusammensetzung des Abgasstromes kennzeichnenden Leitgröße (21), wobei die erste Teilgröße eine Anwesenheit von Schadstoffen im Abgasstrom und die zweite Teilgröße eine Anwesenheit eines Reduktionsmittels (13) kennzeichnet,
• – Zugeben des Reduktionsmittels (13) in den Abgasstrom (7) in Abhängigkeit von der Leitgröße (21), und zeichnet sich aus durch Ermitteln der zweiten Teilgröße (cNH3) (49) mittels einer katalytischen exothermen Oxidation von NH3 des im Abgasstrom (7) enthaltenen Reduktionsmittels (13) an einer katalytisch wirksamen Pt-Oberfläche (57) und Ermitteln einer Reaktionswärme der exothermen Oxidation.

In the method according to the invention for the aftertreatment of an exhaust gas stream of an internal combustion engine of a motor vehicle by means of a selective catalytic reduction is provided

• - Determining one of a first partial size ( 47 ) and a second partial size ( 49 ) dependent, the composition of the exhaust gas flow characterizing ( 21 ), wherein the first part size, a presence of pollutants in the exhaust stream and the second part size, a presence of a reducing agent ( 13 ),
• Adding the reducing agent ( 13 ) in the exhaust stream ( 7 ) as a function of the control variable ( 21 ) and is characterized by the determination of the second part size (cNH3) ( 49 ) by means of a catalytic exothermic oxidation of NH 3 in the exhaust gas stream ( 7 ) reducing agent ( 13 ) on a catalytically active Pt surface ( 57 ) and determining a heat of reaction of the exothermic oxidation.

According to the invention, therefore, a determination of the second part size (cNH3) by means of a catalytic exothermic oxidation of NH3 of the reducing agent contained in the exhaust stream to a catalytically active Pt surface and determining a heat of reaction of the exothermic oxidation is provided. Advantageously, at least a small part of the exhaust gas flow can be guided past the catalytically active Pt surface. Advantageously, NH 3 reacts in combination with oxygen and in the presence of platinum even at low temperatures, for example from about 200 ° C fast and highly exothermic. The reaction heat occurring can advantageously be a measure of a concentration of the entrained in the exhaust stream NH3.

For determining the control variable, a first partial variable (cNOx) characterizing a presence of pollutants in the exhaust gas flow and a second partial variable (cNH3) characterizing a presence of a reducing agent in the exhaust gas flow can be calculated by means of a calculation rule and an inverse operator applied to one of the partial quantities Calculation rule for determining a characteristic of the composition of the exhaust gas stream Leitgröße and adding the reducing agent may be provided in the exhaust stream in dependence on the Leitgröße. The calculation rule may have, for example, an addition or a multiplication. An inverse operator can be understood to mean an arithmetic operation which, as a result, supplies the neutral element of the calculation rule with regard to the associated arithmetic rule and applied to identical operands. In the case of an addition, then the calculation rule and the inverse operator in the form of a multiplication of one of the operands with (-1) result in the neutral element zero and in the case of a multiplication in a sweep formation the neutral element 1. Advantageous is by means of the comparison in comparison to usual summation signals, the leading variable clearly assigned to operating conditions of the internal combustion engine. Can be advantageous Disadvantages of otherwise resulting underdetermined regulation can be avoided.

In one embodiment of the method, determining the guide variable by means of subtracting the second part size from the first part size (cNOx-cNH3) is provided. Advantageously, the control variable can serve as an input variable of a regulator for the addition of the reducing agent, wherein advantageously both ammonia breakthroughs and operating states with too high NOx emissions can be detected.

In a further embodiment of the method, a determination of the guide variable by means of dividing the first part size by the second part size (cNOx / cNH3) is provided. A so determined control variable can also be clearly assigned to the operating conditions of the internal combustion engine and associated emissions of nitrogen oxides or ammonia and serve as an input variable of the controller.

In a further embodiment of the method, determining the first part size is provided as addition of a first nitrogen oxide size (cNO) and a second nitrogen oxide size (cNO2). The first part size can be formed as a sum signal of the nitrogen oxide sizes.

In a further embodiment of the method, a determination of the guide variable is provided by means of a NH3 / NOx ratio probe which can be assigned to the exhaust gas flow. Advantageously, basic redox chemical properties of ammonia can be exploited for detection. Advantageously, by means of only one probe, a ratio between the first and second part sizes can be formed for determining the guide variable.

In a further embodiment of the method, determining the first part size by means of a NOx sensor and / or determining the second part size by means of an NH3 sensor are provided. Advantageously, separate signals can be determined for the sub-quantities, which can be offset by means of a computing unit to the leading variable.

In a further embodiment of the method, a calculation of the first and the second part size is provided by means of a sum signal (cNO + cNO2 + CNH3) which can be determined by means of an electrochemical sensor and a correction signal for the second part size (cNH3). Advantageously, the sum signal can be determined by means of a conventional sensor for the quantitative detection of the first and second part sizes. The sum signal can be correspondingly corrected by means of the correction signal, so that an operating parameter of the internal combustion engine uniquely characterizing guide variable can be determined.

In a further embodiment of the method, taking into account a first time behavior of the NOx sensor and a second time behavior of the NH3 sensor and / or taking into account the first and second time behavior by measuring the first part size by means of the NOx sensor upstream of the second part size measuring NH3 Sensors, or vice versa. Advantageously, by means of taking into account the time behavior of the sensors, the billing to the control variable can be as little as possible time-skewed. A compensation or consideration of the different timing of the sensors can be done for example by means of a downstream processing unit, such as a motor control. Advantageously, this can also be done alternatively or additionally only by means of a suitable positioning of the sensors in the exhaust stream. These can be arranged at a certain distance from each other in the exhaust stream, so that due to the flow velocity, so the distance traveled between the sensors path of the entrained substances of the exhaust stream, a time delay or running time occurs. Advantageously, the distance or the resulting delay time can be selected so that the different time behavior of the sensors are compensated as best as possible. The distance between the sensors can be selected so that an optimal compensation of the time behavior of the sensors is possible at an operating point and / or a mean velocity of the exhaust gas flow.

The object is also designed with a motor vehicle having an exhaust aftertreatment device by means of a selective catalytic reduction for purifying an exhaust gas flow of an internal combustion engine of the motor vehicle, designed and / or set up to carry out a method described above. This results in the advantages described above.

Further advantages, features and details emerge from the following description in which an exemplary embodiment is described with reference to the drawing. The same, similar and / or functionally identical parts are provided with the same reference numerals. Show it:

1 a schematic representation of an internal combustion engine of a motor vehicle with a downstream emission control device;

2 another schematic representation of the in 1 shown internal combustion engine with the downstream exhaust gas purification device, in difference, two spaced-apart sensors are provided;

3 a sensor device assignable to an exhaust gas flow for determining a second partial variable (cNH3) for characterizing a presence of a reducing agent entrained in the exhaust gas flow;

4 a diagram of a transmission behavior with different offset values of a sensor assignable to the exhaust gas flow for determining a quantitative sum signal;

5 this in 4 shown graph, but in addition with a marked waveform of a determinable from a first part size and second part size guide.

1 shows a schematic view of a part of a motor vehicle 1 with an internal combustion engine 3 and a downstream exhaust system 5 , The exhaust system 5 is for cleaning and guiding an exhaust gas flow 7 of the internal combustion engine 3 designed. For cleaning the exhaust gas flow 7 can the exhaust system 5 more, not in 1 have shown components for exhaust aftertreatment, such as catalysts and sensors. In the internal combustion engine 3 it may be, for example, a diesel engine. However, it is also possible that it is any other lean-stoichiometric engine, such as an engine with a homogeneous compression ignition and / or a lean-burn engine, is.

The exhaust system 5 is for after treatment of the exhaust gas flow 7 designed by means of a selective catalytic reduction and has an SCR catalyst for this purpose 9 on. The SCR catalyst 9 can in the presence of a reducing agent 13 For example, NH3 in the exhaust stream entrained pollutants, such as NOx, convert into harmless components. Reducing agent may also be understood to mean a chemical precursor of such, in particular urea. The reducing agent 13 can by means of further, not shown components to the exhaust stream 7 be metered, for example in the form of an aqueous urea solution, the first in the exhaust gas stream 7 the NH3 releases. For dosing the reducing agent 13 , For example, this releasing aqueous urea solution, the exhaust system 5 a metering device 11 be assigned.

For controlling the dosing device 11 for metering in the reducing agent 13 in the exhaust stream 7 is the metering device 11 a control device 15 upstream. By means of the control device 15 can be a dosing 17 for controlling the metering device 11 be determined. By means of the control device 15 is also a mixture formation 19 of the internal combustion engine 3 controlled. By means of mixture formation 19 and the dosage 17 the control device 15 can be based on a composition of the exhaust gas flow 7 Be influenced. By means of one, for example the SCR catalyst 9 associated measuring device or sensor device, in particular a NH3 / NOx ratio probe 31 , can be a characteristic of the composition of the exhaust stream characteristic size 21 be determined. This benchmark 21 can be a regulator 23 be fed, which by means of a comparison with a predetermined setpoint 25 and control algorithms, not shown, a control variable 27 for controlling the control device 15 can generate. The regulator 23 and the control device 15 can in an engine control unit 29 of the internal combustion engine 3 be implemented.

For determining the control variable 21 can be the NH3 / NOx ratio probe 31 serve. The ratio probe 31 can the SCR catalyst 9 be assigned. Alternatively, however, it is also possible to use the ratio probe 31 the SCR catalyst 9 nachzuschalten. Advantageously, by means of the ratio probe 31 one in the exhaust stream 7 existing concentration of ammonia (cNH3) directly in relation to one in the exhaust stream 7 existing concentration of nitrogen oxides (NOx) are used. Advantageously, based on the guide size 21 by means of the regulator 23 a regulation of the metered addition of the reducing agent 13 be realized to achieve the best possible denitrification of the exhaust stream.

Alternatively and / or additionally, it is possible for the controller 23 to supply further input variables, in particular by means of a co-calculating model 33 the benchmark 21 to investigate.

2 shows a schematic representation of the in 1 shown motor vehicle 1 but with a modified measuring device 35 , The following is merely to the differences too 1 received. In contrast to the representation according to 1 is the measuring device 35 the SCR catalyst 9 downstream and has a first sensor 37 and one the first sensor 37 downstream second sensor 39 on. The first sensor 37 For example, as a NOx sensor and the second sensor 39 be designed as NH3 sensor. The sensors 37 and 39 are in one by means of a double arrow 41 indicated distance from each other. Due to the distance, entrained pollutant concentrations or concentrations of the reducing agent are carried in the exhaust gas flow 13 delayed by the sensors 37 . 39 measured. A mean velocity of the exhaust stream 7 is by means of an arrow 43 indicated. In the case of a change in the exhaust gas flow over time 7 or a composition of the exhaust stream 7 the first sensor registers 37 this change earlier than the second sensor 39 , Advantageously, the means of double arrow 41 symbolized distance between the sensors 37 . 39 be chosen so that there may be different timing between the sensors 37 . 39 can be compensated.

In the present case, the second sensor 39 a faster time response than the first sensor 37 exhibit. Nevertheless, despite this different timing, due to the means of the arrow 43 symbolized transit time of the exhaust stream 7 , the sensors are essentially equally fast on changing a composition of the exhaust gas flow 7 react. Advantageously, by means of a computing unit 45 For example, in the controller 23 can be implemented, one by means of the first sensor 37 ascertainable first part size 47 (cNOx) with a second part size 49 of the second sensor 39 (cNH3) to the benchmark 21 will be charged. The first part size 47 indicates a presence of pollutants, for example nitrogen oxides, of the exhaust gas flow 7 , The second part size 49 indicates a presence of the reducing agent 13 in the exhaust stream 7 , For example, a concentration of ammonia. The arithmetic unit 45 In particular, a division or a subtraction of the part sizes 47 and 49 carry out. In addition, it is possible to specify the part sizes 47 . 49 with different factors and / or offset values and / or alternatively and / or additionally the time behavior of the sensors 37 . 39 to compensate mathematically.

3 shows another measuring device 35 that have a presence of the second part size 49 can determine. For this purpose, the measuring device 35 the exhaust gas flow 7 a partial flow 51 and divide this on an empty path 53 and a catalytic path 55 , The catalytic path 55 has a catalytically active surface 57 on, on which the partial flow 51 in half, or is passed in a known ratio. To determine a heat of reaction are both the empty path 53 as well as the catalytic path 55 Temperature measurement points 59 assigned. The temperature measuring point 59 of the catalytic path 55 is the catalytic surface 57 downstream. Advantageously, by means of the temperature measuring point 59 a differential temperature can be formed, which depends on a temperature increase of the catalytic path 55 suggesting. Because at the catalytic surface 57 , which may comprise, for example, platinum, the reducing agent ammonia, provided in the exhaust gas stream 7 contained oxygen can be exothermally oxidized, the temperature increase is a measure of a heat of reaction of the oxidation and thus for a conversion rate and thus for a concentration of the in the exhaust stream 7 entrained reducing agent ammonia. Advantageously, thereby the first part size 47 be determined. Alternatively and / or additionally, it is conceivable that the catalytic surface 57 another temperature measuring point in 3 dotted is shown, to advance. The temperature difference before and after the catalytic surface 57 may also be a measure of conversion of the reducing agent 13 be. Alternatively and / or additionally, it is possible to use the empty path 53 and the catalytic path 55 sensory and with respect to a heat capacity to design identical and provide two temperature sensors.

4 shows a graph 61 with an x-axis 63 and a Y-axis 65 , On the X axis 63 is a ratio between the first part size 47 and the second part size 49 applied, for example between 0 and 1.5. On the Y axis 65 a sensor signal between -20 and +80 is plotted. In 4 are a total of three characteristics 67 a known from the prior art sensor for determining a quantitative sum signal of the first part size 47 and the second part size 49 applied. The three characteristics 67 differ only by different offset values, which in 4 by means of arrows 69 are indicated. It can be seen that the characteristics 67 at a ratio of 1 of the part sizes 47 and 49 each become minimal. The disadvantage is that the means of the curves 67 labeled sensor signals respectively for two completely different compositions or ratios of the part sizes 47 and 49 and thus operating conditions of the internal combustion engine 3 of the motor vehicle 1 to assume the same values. A distinction from, for example, a state of the internal combustion engine 3 and the downstream exhaust system 5 with a very high concentration of nitrogen oxides would therefore provide the same signal as in an ammonia breakthrough by the SCR catalyst 9 , Thus, without knowing further parameters, no functioning control strategy based on such sensor signals can be found.

In contrast, according to the 5 , which is also the graph 61 together with the characteristics 67 shows a course 71 the benchmark 21 located. The history 71 the benchmark 21 for example, directly by means of the ratio probe 31 or indirectly by means of the arithmetic unit 45 to be generated.

It can be seen that the course 71 the relationships between the first part size 49 and thus the operating conditions of the internal combustion engine 3 and the exhaust system 5 clearly identifies. It is advantageous on the basis of the Leitgröße 21 a regulation of the dosage of the reducing agent 13 by means of the metering device 11 possible.

Advantageously, a favorable for a complete implementation as possible ratio between NO, NO 2 and the reducing agent 13 (NH3) can be adjusted. For this purpose, the exhaust system 5 additionally have an oxidation catalyst. The reducing agent 13 can by means of the metering device 11 be added in the form of an aqueous urea solution. For this purpose, an unillustrated reservoir and a metering pump may be provided which is a dosage of the reducing agent 13 above the SCR catalyst 9 can effect. Advantageously, the metering device 11 by means of the guide size 21 that set off the first part size 47 and the second part size 49 represents done. In comparison to conventional NOx probes, for example, according to an amperometric principle, no disturbing, approximately quantitative cross-sensitivity to NH3 occurs. Advantageously, the course allows 71 the benchmark 21 a resolution of signal contributions of the reducing agent 13 and the pollutants characterized by the first part size. This in particular by means of the controller 23 realized control system is determined, with no engine operating conditions of the internal combustion engine 3 exist, the same sensor signals, that is, a same guide 21 would cause. Advantageously, by combustion parameters modified NOx emissions of the internal combustion engine 3 and a NH3 dosing amount are distinguished from each other, so that by means of the regulator 23 or the engine control unit 29 a readjustment or adaptation of the NH3 dosing amount, the amount of fuel, the amount of air and / or other parameters is possible without errors.

Advantageously, in addition to undesirable NH3 breakthroughs, underdoses or excessively high dosages in the exhaust gas flow 7 entrained NOx emissions are detected.

Advantageously, a control or control of the internal combustion engine 3 , in particular designed as a diesel engine, and / or an adaptation of an ammonia metering by means of the metering device 11 with the help of the benchmark 21 as (cNO2 + cNO) - cNH3 done. Alternatively and / or additionally, for example by a simple adaptation of the controller 23 it is also possible, with the same objective, to use an equivalent guide as (cNO2 + cNO) divided by cNH3.

The determination of the benchmark 21 can, for example, as in 2 represented by solitary sensors, by means of the first sensor 37 and the second sensor 39 respectively. The sensors 37 and 39 can, as in 2 represented, the SCR catalyst 9 be downstream or alternatively and / or additionally also within the SCR catalyst 9 be provided.

Alternatively, it is conceivable, as in 1 shown, a combined NH3 / NOx probe, the ratio probe 31 provided. Particularly advantageous may be the ratio probe 31 basic and / or redox chemical properties of the reducing agent 13 or the ammonia used for detection.

It is advantageous compared to the prior art at any time an unambiguous assignment of the sensor signal or the course 71 the benchmark 21 with the operating state of the internal combustion engine 3 or the NH3 metering amount of the metering device 11 possible and both nitrogen oxide and ammonia breakthroughs can be detected and corrected (comparison 5 ).

The control system with the controller 23 can, as shown in 2 with separate NOx and NH3 sensors 37 . 39 that are in or behind the SCR catalyst 9 are positioned to be built. Advantageously, the sensors 37 . 39 also be selected and dimensioned so that the first sensor 37 has a nearly quantitative cross-sensitivity to ammonia, while the second sensor 39 does not respond or at least only to a minimal extent to nitrogen oxides.

The separately obtained sensor signals, for example, the first part size 47 and the second part size 49 can by means of an ECU, for example with the arithmetic unit 45 , to the common benchmark 21 cNOx-cNH3 or cNOx / cNH3 for urea metering control and / or engine control by means of the engine control unit 29 are processed.

An installation of the sensors 37 . 39 in the SCR catalyst 9 can advantageously enable a faster control or adaptation behavior, wherein a positioning of the sensors 37 . 39 , as in 2 shown, a fundamentally simpler structure allows. Advantageously, as in 2 shown, a different retention behavior (storage behavior) of ammonia or nitrogen oxides in the SCR catalyst 9 and / or a different temporal response of the sensors 37 . 39 so by the positioning of the sensors 37 . 39 be compensated that when merging the sensor signals or the partial sizes 47 . 49 in the arithmetic unit 45 , the benchmark 21 cNOx-cNh3 or cNOx divided by cNH3 is obtained as precisely as possible and without temporal distortion.

As in 1 shown, the controller system with the controller 23 even with only one sensor type, the NH3 / NOx ratio probe 31 , being constructed. The NH3 / NOx ratio probe is advantageous 31 even in a position that in the exhaust or in the exhaust stream 7 present NOx / NH3 ratio to detect. The sensor signal or the reference variable 21 can in this embodiment then directly as input of the controller 23 and thus be used for dosing. It is advantageous to position the NH3 / NOx ratio probe 31 independent of different time behavior and can therefore be optimized in terms of installation costs, control speed and a minimization of stress due to temperature changes and possibly occurring poisoning advantageous.

As a functional principle for the NH3 / NOx ratio probe 31 For example, the very different acid / base properties of the nitrogen oxides and of the ammonia can be used with preference. Since ammonia is a strong Brönstedt base with a pK _B value of 4.75 and the higher nitrogen oxides have moderately acidic properties, assuming a sufficiently rapid neutralization reaction on an absorbent surface, the predominant species can be detected over a pH analogous size become. To determine such a pH-analogous size, temperature-resistant proton conductors, such as, for example, β-aluminates, Sr-, Ba-cerates or La-zirconates can advantageously be used. To a certain extent occurring cross-sensitivities to SO2 and / or H2O may occur in a design of the regulator 23 be taken into account.

Alternatively, it is conceivable to use thermally stable basic and / or acidic oxides and / or zeolites for the selective chemisorption of NH 3 and NO / NO 2. For detection, changes in the impedance, the electricity properties and / or a mass of the loaded material come into question.

In addition, it is conceivable, in particular for an even more selective and sensitive transmission behavior of the NH3 / NOx ratio probe 31 to provide semiconductor field effect transistors with a selectively sensitive gate for NO2 and / or for NH3.

Furthermore, to determine the guide size 21 Any other probes based on other chemical and / or physical properties of the ammonia or the nitrogen oxides are possible, for example a redox chemical behavior of these species.

It is also conceivable to use the NH3 / NOx ratio probe 31 be interpreted that it can be used as an electrochemical sensor, the partial concentrations of the nitrogen oxides and ammonia in the exhaust gas or exhaust stream 7 quantified, but has an additional function that allows the sum signal (cNO + cNO2 + cNH3) to be corrected for the present ammonia concentration (cNH3).

Such a correction function can be done using the very different acid-base properties of ammonia compared to nitrogen oxides. As already stated, ammonia is a strong Brönstedt base with a PK _B value of 4.75, whereas the higher nitrogen oxides have moderate to highly acidic properties, whereby a high proportion of ammonia can be detected by means of a solid proton conductor placed in the sensor.

Different solids with perovskite structure have a good proton conductivity in the application temperature range of diesel exhaust gases, for example for the charge carriers H3O + or NH4 +. An example of material or material group which can be used particularly advantageously at temperatures up to 650 ° C. as a "solid pH meter" are doped Li lanthanum titanates (for example Sr-doped LLT). However, a selection of suitable materials with cubic or tetragonal perovskite structure is by no means limited to the above-mentioned example. A measure of the H + ion current or the impedance at known temperature and voltage correlates with a NH3 concentration, ie the second part size 49 , where an occurring H2O cross sensitivity in the controller 23 is considered.

Alternatively, it is conceivable, a catalytic oxidation of NH3 on a Pt surface, for example, the in 3 represented catalytic surface 57 possible, wherein the measure of the metabolic rate of a catalyst of the catalytic surface 57 with the NH3 concentration, ie the second part size 49 correlated, with occurring cross-sensitivities to other oxidizable exhaust gas components, such as HCx in the design of the regulator 23 can be considered.

Ammonia is already catalytically oxidized rapidly and strongly exothermically at low temperatures, for example from 200 ° C., on a Pt surface. Such oxidation of ammonia is an undesirable side reaction in the SCR process. Advantageously, the SCR catalyst 9 have the highest possible SCR activity at the lowest possible NH3 oxidation activity and may advantageously contain as little as possible PGM (platinum group metals).

In the Pt-catalyzed oxidation of ammonia with O2, the following reactions occur: 4NH3 + 3O2 → 2N2 + 6H2O (main reaction up to about 300 ° C), heat of reaction +1266 kJ 2NH3 + 2O2 → 2N2O + 3H2O (main reaction from 300 to 400 ° C) reaction heat 550 kJ 4NH3 + 5O2 → 4NO + 6H2O (main reaction from 400 degrees C) heat of reaction +905 kJ.

As a side reaction, an oxidation with NO, which proceeds as follows: 4NH3 + 6NO → 5N2 + 6H2O + heat of reaction 1.808 kJ

Diemerization or disproportionation reactions and / or further oxidation to NO 2 can advantageously be disregarded because of their essentially low heat of reaction and / or their equilibrium position. A detection of a NH3 conversion corresponding to the above-mentioned 4 oxidation reactions on a finely divided catalytically highly active Pt surface, ie the catalytic surface 57 , Therefore, can be advantageous as a measure for determining the second part size 49 serve.

Advantageously, a measurement of the heat of reaction or heat of reaction of the oxidation reaction, as in 3 represented by a comparatively resistive measurement between a catalytically active and a passivated electron pad, for example as in 3 represented by a comparison of the empty path 53 with the catalytic path 55 ,

LIST OF REFERENCE NUMBERS

1

automotive

3

internal combustion engine

5

exhaust system

7

exhaust gas flow

9

SCR catalyst

11

metering

13

reducing

15

control device

17

dosage

19

mixture formation

21

command variable

23

regulator

25

setpoint

27

control variable

29

Engine control unit

31

NH3 / NOx ratio probe

33

model

35

measuring device

37

1st sensor

39

2nd sensor

41

double arrow

43

arrow

45

computer unit

47

1st part size

49

2nd part size

51

partial flow

53

empty path

55

catalytic path

57

catalytic surface

59

Temperature measuring point

61

graph

63

X axis

65

Y-axis

67

characteristics

69

arrow

71

course

Claims (9)

Process for the aftertreatment of an exhaust gas stream ( 7 ) of an internal combustion engine ( 3 ) of a motor vehicle ( 1 ) by means of a selective catalytic reduction, comprising: - determining a first partial size ( 47 ) and a second partial size ( 49 ) dependent, the composition of the exhaust gas flow characterizing ( 21 ), wherein the first part size, a presence of pollutants in the exhaust stream and the second part size, a presence of a reducing agent ( 13 ), - adding the reducing agent ( 13 ) in the exhaust stream ( 7 ) as a function of the control variable ( 21 ), characterized by determining the second part size (cNH3) ( 49 ) by means of a catalytic exothermic oxidation of NH 3 in the exhaust gas stream ( 7 ) reducing agent ( 13 ) on a catalytically active Pt surface ( 57 ) and determining a heat of reaction of the exothermic oxidation. Method according to claim 1, comprising: - determining the reference variable ( 21 ) by subtracting the second part size ( 49 ) of the first part size ( 47 ) (cNOx-cNH3). Method according to one of the preceding claims, comprising: - determining the reference variable ( 21 ) by dividing the first part size ( 47 ) by the second part size ( 49 ) (cNOx / cNH3) Method according to one of the preceding claims, comprising: determining the first part size ( 47 ) as the addition of a first nitrogen oxide (cNO) and a second nitrogen oxide (cNO2). Method according to one of the preceding claims, comprising: - determining the reference variable ( 21 ) by means of a flow of exhaust gas ( 7 ) assignable NH3 / NOx ratio probe ( 31 ). Method according to one of the preceding claims, with at least one of the following: determining the first part size ( 47 ) by means of a NOx sensor ( 37 ), - determining the second part size ( 49 ) by means of an NH3 sensor ( 39 ). A method according to any one of the preceding claims, comprising: - determining the first and second part sizes ( 47 . 49 ) by means of an electrochemical sensor ascertainable sum signal (cNO + cNO2 + cNH3) and a correction signal for the second part size (cNH3) ( 49 ). Method according to one of the preceding claims 6 or 7, with at least one of the following: - taking into account a first time behavior of the NOx sensor ( 37 ) and a second time behavior of the NH3 sensor ( 39 ), - taking into account the first and second time behavior by measuring the first part size ( 47 ) by means of the NOx sensor ( 37 ) upstream of the second part size ( 49 ) measuring NH3 sensor ( 39 ), or the other way around. Motor vehicle ( 1 ) with an exhaust aftertreatment device by means of a selective catalytic reduction for purifying an exhaust gas stream ( 7 ) of an internal combustion engine ( 3 ) of the motor vehicle ( 1 ), designed, constructed and / or arranged for carrying out a method according to one of the preceding claims.

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