DE102010028846A1 - Method for operating exhaust system of diesel engine, involves evaluating temporally increase of nitrogen oxide flow supplied from selective catalytic reduction catalyzer for closing aging state of catalyzer - Google Patents

Abstract

The method involves reducing nitrogen oxide by a selective catalytic reduction (SCR) catalyzer (32). An aging state of the SCR catalyzer is provided for monitoring amount of reducing agent i.e. ammonia, stored in the SCR catalyzer. Efficiency break-down of the SCR catalyzer is detected during a phase change of the amount of the reducing agent stored in the SCR catalyzer. Temporally increase of a nitrogen oxide flow supplied from the SCR catalyzer is evaluated for closing the aging state of the SCR catalyzer. Independent claims are also included for the following: (1) a computer program for operating an exhaust system of an internal combustion engine (2) a control and/or regulating device for operating an exhaust system of an internal combustion engine.

Description

State of the art

The invention relates to a method according to the preamble of claim 1, and a computer program and a control and / or regulating device according to the independent claims.

In the prior art, exhaust systems are known in motor vehicles, which are equipped with various devices for exhaust aftertreatment to meet existing emission regulations. The functionality of such devices must be monitored during on-board operation of the motor vehicle. As part of a so-called "on-board diagnosis" (OBD), it may be necessary, for example, to monitor an SCR catalytic converter (SCR = "Selective Catalytic Reduction") and, if necessary, detect it as defective.

The basic principle of the SCR catalyst then is that nitrogen oxide molecules (NOx) are reduced to elemental nitrogen on a catalyst surface in the presence of ammonia (NH3) as a reducing agent. The reducing agent is provided by a metering device upstream of the SCR catalyst. The determination of the desired metering rate takes place in an electronic control and / or regulating device in which strategies for the operation and monitoring of the SCR catalyst are stored.

The monitoring of an SCR catalyst can be done using at least one NOx sensor. Currently available on the market NOx sensors show a cross sensitivity to ammonia (NH3), that is, a sensor signal of the NOx sensor displays not only the respective NOx concentration, but a sum signal from the NOx and the NH3 concentration. For example, in the case of a NOx sensor located downstream of the SCR catalyst, an increase in the sensor signal may be due both to a decreasing NOx conversion rate (increase in NOx concentration) and breakthrough of pure ammonia (increase in NH3 concentration ) clues. A direct distinction between NOx and NH3 is therefore not possible.

The occurrence of ammonia behind the SCR catalyst (so-called NH3 slip) should be avoided because ammonia in a high concentration has a harmful effect.

Monitoring functions currently known by the market determine the efficiency of a NOx reduction (NOx conversion rate) with the aid of one NOx sensor each upstream and downstream of the SCR catalytic converter. In this case, the upstream SCR catalytic converter can also be replaced by a model-based variable. Due to the aging of the SCR catalyst, the achievable conversion rate decreases with increasing service life, and accordingly the NOx emissions behind the SCR catalyst increase. Based on preset limit values for a permissible NOx emission, it is possible to determine a threshold value for the SCR efficiency, below which a system fault of the exhaust gas system is concluded.

The current procedure is to integrate the NOx emissions upstream and downstream of the SCR catalytic converter over a predefined period of time and to calculate the SCR efficiency achieved from the NOx masses determined as a comparison value. The comparison of the SCR efficiency with a definable threshold serves to classify the SCR catalyst in terms of its functionality. Often it is intended to perform the integration only during stationary driving conditions during which a fluctuating NOx signal before the SCR catalyst (NOx peaks) is not expected, and then to determine a comparison value.

Thus, the performance of the SCR catalyst can be sufficiently evaluated with respect to an average NOx conversion, but it is possible that NOx peaks occurring in a dynamic operation of the internal combustion engine and may no longer be managed by the SCR catalyst remain undetected.

The DE 10 2007 040 439 A1 describes a monitoring strategy for an SCR catalyst in which a NH3 storage capability of the SCR catalyst is determined. Namely, it has been found that the ability of the catalyst to adsorb NH3 (NH3 storage ability) can be used as a characteristic for aging or deterioration of the catalyst. In this strategy, the SCR catalyst is first filled with reductant by a superstoichiometric reductant dosage (overdose) to the maximum achievable NH3 storage capability to provide a defined starting point for diagnosis to reach. Achieving the maximum storage capacity is detected by the breakthrough of ammonia by the SCR catalyst (NH3 slip), which is indirectly measurable due to the cross sensitivity of the NOx sensor for NH3.

Subsequently, the reducing agent dosage compared to a normal dosage is reduced (underdosing) or completely off, so that the stored in the SCR catalyst NH3 mass is gradually reduced by NOx reduction again (so-called emptying test). By determining the SCR efficiency or other NOx conversion rate dependent variables during the purge test, the useful NH3 storage capability may be indirectly determined because a lower amount of NH3 stored can convert a lower NOx mass at the catalyst surface.

During such an emptying test, however, reduction of the reducing agent supply can lead to increased NOx emissions. In addition, for a so-called conditioning of the SCR catalyst by filling the NH3 memory to the maximum storage capacity, an exceeding of the NH3 slip limit is required, whereby unwanted ammonia is released to the environment. Therefore, this "active" - because in the Reduktionsmitteldosierung intervening - method is only applied if the required monitoring accuracy can not be achieved by means of a "passive" monitoring of the NOx conversion penalty.

In order to determine the functionality of SCR catalysts more clearly, so-called plausibility functions are carried out under certain monitoring conditions according to the prior art. In the field of exhaust gas aftertreatment, monitoring is frequently limited to specific value ranges for one or more modeled or determined variables. The following quantities are named as examples: exhaust gas mass flow, exhaust gas volume flow, exhaust gas temperature at any point, operating point (rotational speed, injection quantity), vehicle speed, ambient pressure, ambient temperature, NOx, PM, HC, CO, O 2 signals, EGR rate, Engine operating mode, engine status, engine runtime, and / or engine life.

In the diagnosis of an SCR catalyst, additional variables are used, which are named below by way of example: status of the HNOx sensors, actual and desired level of the reducing agent (NH3) in the catalyst, control deviation of an NH3 level controller, status of a reducing agent metering device, status Dose Rate Feedforward Mode, Adaptation Factor (Reduction Agent Dose Adjustment Factor), Dose Rate Adaptation Status, DPF Regeneration Status, DPF Regeneration Request Number, and / or Hydrocarbon Poisoning (HC) Status.

In addition, monitoring for the same reason is often performed under (quasi) steady state conditions determined by one or more of the above variables.

Disclosure of the invention

The problem underlying the invention is solved by a method according to claim 1 and by a control and / or regulating device and a computer program according to the independent claims. Advantageous developments are specified in subclaims. Features which are important for the invention can also be found in the following description and in the drawings, wherein the features, both alone and in different combinations, can be important for the invention, without being explicitly referred to again.

The inventive method has the advantage that the aging state of an SCR catalyst in an exhaust system of an internal combustion engine can also be evaluated with respect to the conversion of individual and compared to an average value of increased levels of NOx (nitrogen oxides) in the exhaust gas, that is transient or dynamic Operating conditions are taken into account. In addition, the method has the advantage that a so-called emptying test following a previous increase in a reducing agent (NH 3, ie ammonia) may possibly be dispensable.

The invention is based on the consideration that SCR catalysts age more or less continuously during their service life, whereby frequently an average proportion of NOx in the exhaust gas can still be sufficiently converted, but at times no longer increases the NOx flow supplied to the SCR catalyst can be handled, so it comes in dynamic operation to NOx breakthroughs. This is utilized for the diagnosis according to the invention. Therefore, the amount of ammonia fed to the SCR catalyst as a reducing agent is changed, preferably increased so that the ammonia does not completely consumed for the conversion and instead of which, according to the aging-dependent NH3 storage capacity, a certain proportion of the ammonia in the active surface of the SCR catalyst is at least temporarily stored. The amount of ammonia stored in the SCR catalyst is thus increased. At the same time, according to the invention, at least one currently existing increase in the NOx flow (so-called NOx peak) upstream of the SCR catalytic converter is utilized, or such is deliberately brought about. As a result, an SCR efficiency of the SCR catalytic converter is determined for the NOx peak and its time course is evaluated. The SCR efficiency of the catalyst is determined in each case from the ratio of the proportion of NOx in the exhaust gas before and after the SCR catalytic converter.

The terms "SCR efficiency" and "efficiency" or, as used below, "standardized SCR efficiency loss" and "normalized efficiency loss" are used synonymously.

From the thus determined efficiency, a break in efficiency is determined by reference to an efficiency, as it - under otherwise comparable conditions during the supply of the reducing agent - can be determined without the presence of the temporary increases in NOx flow. In turn, the efficiency break is now used as a measure to infer the aging state of the SCR catalyst.

• (a) Es wird ein zeitlicher Verlauf einer NOx-Spitze und des zugehörigen SCR-Wirkungsgrads ermittelt.
• (b) Deren Änderungen über der Zeit werden mittels folgender Formel zu einem normierten SCR-Wirkungsgradverlust (SEL = ”Scaled Efficiency Loss”) zusammengefasst:
  
  der Zähler die Summe von aufeinander folgenden Teilbeiträgen einer zeitlichen Ableitung (Gradient) des Wirkungsgrads darstellt; und

der Nenner die Summe von aufeinander folgenden Teilbeiträgen einer zweiten zeitlichen Ableitung der NOx-Masse, des NOx-Massenstroms darstellt.Furthermore, it is proposed that a gradient of the efficiency is detected and evaluated for the determination of the aging state of the SCR catalyst. Thus, for example, a break in efficiency can be evaluated as a result of a single increase in the NOx flow and proceed as follows:

• (a) A time course of a NOx peak and the associated SCR efficiency is determined.
• (b) Their changes over time are summarized using the following formula to a normalized SCR loss of efficiency (SEL = "Scaled Efficiency Loss"):
  
  the counter represents the sum of successive partial contributions of a time derivative (gradient) of the efficiency; and

the denominator represents the sum of successive sub-contributions of a second time derivative of the NOx mass, the NOx mass flow.

This formula is advantageously suitable for a practical realization, since the time derivative of the efficiency eliminates its constant component, so that after the subsequent summation, only the contribution of the efficiency caused by the respective NOx peak remains.

Furthermore, it is proposed that a quotient SEL = (∫η _SCR dt) / (∫ (dm _Nox / dt) dt) where η is _SCR = SCR efficiency and dm is _Nox / dt = NOx mass flow and that a proper SCR catalyst is closed when the quotient SEL reaches or falls below a lower limit, the lower limit being a minimum required Reductant storage capability of the SCR catalyst characterized.

Together with the molecular formula described above, the normalized loss of efficiency can also be stated as:

This can be used to evaluate individual NOx peaks. If the determined values become ever smaller during an increased supply of the reducing agent ("overdosage phase"), this is an indicator of the amount of reducing agent stored in the SCR catalyst, ie of the ammonia. Since that maximum available reductant supply correlates with the NH3 storage capacity of the SCR catalyst, a lower limit for the normalized loss of efficiency can be defined, below which the SCR catalyst can be classified as proper - so useful -.

In addition, the method provides that the SCR efficiency from a conversion rate of NOx in N ₂ determined degradation product of nitrogen oxides in gaseous nitrogen used as a measure of the functioning of the SCR catalyst.

The method is particularly meaningful if, during a phase of changing the amount of reducing agent stored in the SCR catalyst, a plurality of preferably successive efficiency dips of the SCR catalyst are detected and evaluated with a corresponding plurality of temporary increases in the NOx flow supplied to the SCR catalyst and a course of reduction in efficiency drops is used in the assessment of the aging condition. For example, the amount of reductant supplied or stored in the SCR catalyst ramps up to an individual age-dependent storage capacity. In this case, the method can determine this limit comparatively quickly and reliably by means of the successive NOx peaks and the associated efficiency dips, and from this evaluate the usefulness of the SCR catalytic converter.

In addition, it is provided that - during a phase of the change of the amount of the reducing agent stored in the SCR catalyst - from the course of the reduction of the efficiencies or a corresponding size, or from the absolute value of the detected efficiency break or a corresponding size reaching a saturation of the SCR catalyst is predicted with the reducing agent. Thus, the saturation - ie the storage capacity - of the SCR catalyst can be determined without reaching the saturation limit correlated with the aging of the SCR catalyst and without the ammonia breakthrough associated therewith.

Furthermore, the method takes into account that the sensitivity of monitoring for a breakthrough of reducing agent by the SCR catalytic converter is increased when it can be concluded from the evaluation of the curve or the absolute value or the corresponding variables on the early achievement of a saturation. As a result, it can be avoided that larger amounts of the reducing agent flow through the catalyst unused and leave the exhaust system of the internal combustion engine. This reduces the environmental impact. The breakthrough of the reducing agent is also referred to as NH3 slip.

• – Wirkungsgradeinbruch nach einer zeitlich vorangehenden NOx-Spitze;
• – Wirkungsgradeinbruch nach einer zeitlich nachfolgenden NOx-Spitze;
• – Referenzwerte des Wirkungsgrades, die einem vergleichbaren Betriebszustand der Abgasanlage ohne zeitweise Erhöhung des Stickoxidanteils entsprechen;
• – mindestens ein vorgegebener Schwellwert und/oder Sollwert;
• – eine zugeführte Menge des Reduktionsmittels;
• – eine Dauer der Zuführung des Reduktionsmittels; und/oder
• – ein aktueller Anteil des Reduktionsmittels.

The procedure works better if the efficiency dips are evaluated depending on at least one of the following quantities:

• - efficiency break after a preceding NOx peak;
• - Efficiency break after a temporally following NOx peak;
• - Reference values of the efficiency corresponding to a comparable operating condition of the exhaust system without a temporary increase in the nitrogen oxide content;
• At least one predetermined threshold value and / or setpoint value;
• An added amount of the reducing agent;
• A duration of supply of the reducing agent; and or
• - a current proportion of the reducing agent.

In this way, it is advantageously possible to evaluate a plurality of individual efficiency dips together, thus making the method safer and more accurate.

An embodiment of the method provides that the at least temporary increase of the NOx stream supplied to the SCR catalytic converter is brought about in a targeted manner, in particular by changing an exhaust gas recirculation of the exhaust gas system. Thus, regardless of an operating situation of the internal combustion engine, the method can be carried out selectively, especially when no NOx peaks in the exhaust gas occur over a long time. By an at least temporary change in the exhaust gas recirculation, for example by means of an adjustment of an exhaust gas recirculation valve, NOx peaks can be generated artificially in the appropriate number, strength and duration.

• – ein Abgasmassenstrom;
• – ein Abgasvolumenstrom;
• – eine Abgastemperatur;
• – eine Fahrzeuggeschwindigkeit;
• – ein Umgebungsdruck;
• – eine Umgebungstemperatur;
• – ein Anteil von Stickoxid, Kohlenwasserstoff, Kohlenmonoxid, PM oder Sauerstoff des Abgases;
• – eine Dosierrate des Reduktionsmittels;
• – ein Temperaturgradient der Abgasanlage;
• – eine Abgasrückführrate; und/oder
• – eine Drehzahl, eine Einspritzmenge, eine Betriebsart, ein Betriebszustand, eine Laufzeit und/oder eine Standzeit der Brennkraftmaschine.

The accuracy of the method can be increased if it is carried out only if at least one of the following variables is in a given value range:

• An exhaust gas mass flow;
• An exhaust gas volume flow;
• An exhaust gas temperature;
• A vehicle speed;
• An ambient pressure;
• - an ambient temperature;
• A proportion of nitrogen oxide, hydrocarbon, carbon monoxide, PM or oxygen of the exhaust gas;
• A metering rate of the reducing agent;
• A temperature gradient of the exhaust system;
• An exhaust gas recirculation rate; and or
• - A speed, an injection quantity, an operating mode, an operating condition, a running time and / or a service life of the internal combustion engine.

By limiting various variables acting on the internal combustion engine or the exhaust gas system to predetermined value ranges, their influence on the determination of the SCR efficiency can be reduced. This improves the accuracy of the method.

Hereinafter, exemplary embodiments of the invention will be explained with reference to the drawings. In the drawing show:

1 a schematic of an internal combustion engine and an exhaust system;

2 a time chart of an efficiency of an SCR catalyst during a NH3 overdosage phase;

3 a timing chart of the efficiency of the SCR catalyst; along with a NOx fraction of the exhaust gas upstream of the SCR catalyst, along with a stored amount of NH3 in the SCR catalyst, and along with a normalized efficiency loss of the SCR catalyst;

4 a time chart with examples for the evaluation of NOx peaks in the exhaust gas; and

5 a timing diagram of the efficiency of the SCR catalyst together with a count for the evaluation of a NH3-Schlupferkennung.

The same reference numerals are used for functionally equivalent elements and sizes in all figures, even in different embodiments.

1 shows in the lower part of the drawing a simplified scheme of an exhaust system 10 of a motor vehicle. Above the exhaust system 10 is an internal combustion engine 12 symbolically represented by a pipe connection 14 , Exhaust gas in the exhaust system 10 flows. A control and / or regulating device 16 with a computer program running on it 18 is about incoming and outgoing control lines 20 and 22 with the internal combustion engine 12 as well as incoming and outgoing control lines 24 and 26 with components of the exhaust system 10 connected. The compounds are merely indicated in the drawing and not shown in detail.

In the exhaust system 10 the exhaust gas is passed through substantially from left to right and prepared. In the present case it concerns the exhaust system 10 a diesel vehicle. The exhaust system 10 has in the flow direction of the exhaust gas a diesel oxidation catalyst 28 , a diesel particulate filter 30 a feeder 31 for urea-based additives ("additives"), and an SCR catalyst 32 on (SCR means "selective catalytic reduction"). Upstream of the diesel oxidation catalyst 28 is a lambda sensor 34 arranged in the exhaust stream. Upstream and downstream of the SCR catalyst 32 are each a NOx sensor 36 arranged in the exhaust stream. Furthermore, the exhaust system 10 in this case, five temperature sensors 38 on.

The temperature sensors 38 , the lambda probe 34 and the NOx sensors 36 are with the control and / or regulating device 16 electrically via the incoming and outgoing lines 24 and 26 connected. This is in the drawing of 1 but not shown individually. The lambda probe 34 detected during operation of the internal combustion engine 12 the oxygen concentration in the exhaust gas. The NOx sensors 36 detect a proportion of NOx (nitrogen oxide fraction) in the exhaust gas before and after the SCR catalytic converter 32 and are thus suitable, the functionality of the SCR catalyst 32 to monitor and in particular a so-called To determine SCR efficiency. About the feeder 31 For example, a reducing agent (ammonia) can be introduced into the exhaust gas. Hereinafter, the term "SCR efficiency" is also referred to as "efficiency" and the term "SCR catalyst" is referred to simply as "catalyst".

It is understood that the 1 is only exemplary and that the inventive method is not limited to diesel engines, but also to gasoline engines and other comparable internal combustion engines 12 or on their exhaust systems 10 is applicable.

2 shows a first efficiency 50 a new SCR catalytic converter (catalyst) together with a second efficiency 52 an old (that is long-used) catalyst, and along with a NH3 loading 54 of the catalyst 32 over time t. Under NH3 loading, the process of storing the reducing agent on an active surface of the catalyst 32 Understood. NH3 is the chemical formula for ammonia.

For the first efficiency 50 is an average efficiency 56 of a new catalyst, for the second efficiency 52 is an average efficiency 58 of an old catalyst, and for NH3 loading 54 is a maximum storage capacity 60 specified. These are in the 2 also represented by horizontal dashed lines. Likewise, there is a difference 57 as well as a limit 59 between the average efficiency 56 and the average efficiency 58 entered in the drawing.

Furthermore, for the first efficiency 50 a first size 62 and for the second efficiency 52 a second size 64 indicated, each of which characterize a so-called NH3-slip. The NH3 loading 54 indicates an initial value in the diagram 66 , a peak 68 at a time t1 and a final value 70 on. Accordingly, the period before the time t1 describes an overdosing phase and the period after the time t1 describes an emptying phase of the NH3 loading 54 in the catalyst 32 ,

One recognizes, first, that the first efficiency 50 of the new catalyst in the 2 shown period is greater than the second efficiency 52 of the old catalyst. Second, it can be seen that with increasing NH3 loading, the first efficiency 50 increases faster than the second efficiency 52 , Third, it can be seen that the sizes 62 and 64 , which in each case to a preceding relative maximum of the first and the second efficiency ( 50 respectively. 52 ), have a different amount at time t1, that is, at a same NH3 loading over time 54 breaks the second efficiency 52 earlier and stronger than the first efficiency 50 , Furthermore, by means of the limit value 59 a comparison can be made such that the average efficiency 56 a useful (that is, proper) catalyst 32 characterized and the average efficiency 58 an unusable catalyst 32 characterized.

3 shows in the upper part of a time course of an efficiency 72 with multiple efficiencies 73 How it adjusts to existing catalyst properties, if at the same time the catalyst 32 supplied exhaust gas sequentially multiple NOx peaks 74 a NOx mass flow 75 and if at the same time the NH3 loading 54 the middle of the 3 has shown course. A reference curve or reference values 76 show a course of the efficiency 72 how he is without the in the 3 represented NOx peaks 74 would result. Also entered is a (non-normalized) loss of efficiency 77 that is, the relative minimum relative to the reference values 76 or an average value of the efficiency 72 , An arrow 83 indicates an incipient NH3 slip. At the bottom of the 3 is a normalized loss of efficiency 78 applied. Two vertical dashed lines exemplify a single measurement period 79 ,

The NOx peaks 74 are synonymous synonymous as a temporary increase 74 the NOx mass flow 75 designated.

The 3 represents a possibility, the catalyst 32 in terms of its aging on the basis of its NH3 storage capacity. The NH3 storage capacity is a measure of the property of the catalyst 32 to store ammonia in its active surface and to use it as a reducing agent to catalyze the NO x compounds in the exhaust gas. It can be seen how with the increase in NH3 loading 54 the efficiency drops 73 to become smaller in amount. This is all the more pronounced as the catalyst 32 is able to store NH3 and actively use it for catalysis. The smaller and flatter the break in efficiency 73 with respect to a respective preceding NOx peak 74 and taking into account the respective NH3 loading 54 the more the catalyst is 32 as useful to evaluate. The representations of the 3 are preferably suitable for, a positive statement of a useful catalyst 32 close.

The normalized efficiency loss shown in the lower part of the drawing 78 gets out of the efficiency break 73 taking into account the strength of the associated NOx peak 74 in the control and / or regulating device 16 the internal combustion engine 12 determined. arrows 80 indicate the respective times of this determination, which only after the decay of a respective NOx peak 74 and the associated efficiency break-in 73 can take place, wherein the respectively determined value remains stored until the following determination.

SEL = normierter Wirkungsgradverlust 78; und im ersten Term η_SCR = SCR-Wirkungsgrad 72;
dm_NOx/dt = NOx-Massenstrom 75; und im zweiten Term
der Zähler die Summe von aufeinander folgenden Teilbeiträgen einer zeitlichen Ableitung des SCR-Wirkungsgrads 72 darstellt; und
der Nenner die Summe von aufeinander folgenden Teilbeiträgen einer zweiten zeitlichen Ableitung der NOx-Masse, das heißt einer ersten zeitlichen Ableitung des NOx-Massenstroms 75 darstellt.The normalized loss of efficiency 78 can be determined by the following formula:

SEL = normalized loss of efficiency 78 ; and in the first term η _SCR = SCR efficiency 72 ;
dm _NOx / dt = NOx mass flow 75 ; and in the second term
the counter is the sum of successive partial contributions of a time derivative of the SCR efficiency 72 represents; and
the denominator is the sum of successive partial contributions of a second time derivative of the NOx mass, that is, a first time derivative of the NOx mass flow 75 represents.

A limit 81 allows the usability of the catalyst 32 to rate. The height of the limit 81 may, for example, depend on an initial value of the normalized efficiency loss 78 be elected. If the normalized loss of efficiency 78 at least once during the implementation of the method the limit 81 falls below, so can a useful catalyst 32 be closed, as it is in the presentation of 3 the case is. In this case, the NH3 loading 54 of the catalyst 32 be stopped immediately and be dispensed with a subsequent so-called emptying test.

Will be the limit 81 not below, so there is a possibility that either the catalyst 32 perhaps useful, but where appropriate the measurement conditions were inappropriate, or else the catalyst 32 is considered unusable. These alternatives are with the process steps of 3 but indistinguishable.

4 shows four examples B1, B2, B3 and B4 for the evaluation of individual NOx peaks 74 Examples B1 to B4 are valid individually and independently of one another or can also occur in the order shown. Together with the NOx tips 74 is the respective course of the efficiency 72 shown. areas 84 indicate a period of time for a respective evaluation.

Examples B1 and B2 show discrete NOx NOx peaks 74 which are individually distinguishable. Example B3 shows two short successive NOx peaks 74 , which are not individually distinguishable. Example B4 shows a period of time following a permanent operating point change of the internal combustion engine 12 ,

All examples B1 to B4 can be evaluated by the method according to the invention. This is a time derivative ("gradient") of the NOx mass flow 75 made and compared against thresholds (not shown). If a first threshold is exceeded, the method can be used. Both positive and negative gradient areas are important. At the same time the efficiency 72 and the efficiencies 73 evaluated. It may be to assess the catalyst 32 be sufficient, only a single NOx peak 74 to record and evaluate.

• (a) Es wird abgewartet, bis der Gradient des NOx-Massenstroms 75 den ersten Schwellwert überschritten hat und ein vergleichsweise schneller Anstieg des NOx-Massenstroms 75 und somit ein auswertbares Ereignis vorliegt. Dadurch wird das Verfahren gestartet.
• (b) Es wird die Stärke des Wirkungsgradeinbruchs 73 des Wirkungsgrads 72 über der Zeit ermittelt und bewertet. Dies kann entweder durch einen Bezug auf Referenzwerte 76 erfolgen oder mittels Summierung von Teilbeiträgen des Gradienten des Wirkungsgrads 72, vergleiche dazu die obige Formel für den normierten Wirkungsgradverlust 78, SEL.
• (c) Es wird die Stärke der NOx-Spitze 74 über der Zeit ermittelt.
• (d) Die Erfassung bzw. Ermittlung der im Schritt (b) und (c) behandelten Größen wird beendet, wenn der Gradient des NOx-Massenstroms 75 einen zweiten Schwellwert unterschritten hat und somit ein vergleichsweise schneller Abfall des NOx-Massenstroms 75 erfolgt und damit eine Rückkehr zu einem eher durchschnittlichen NOx-Massenstrom 75 vorliegt.
• (e) Es wird der Quotient aus der im Schritt (b) ermittelten Größe und der im Schritt (c) ermittelten Größe gebildet. Die Größe des Quotienten wird dazu verwendet, um den Katalysator in Bezug auf seine Brauchbarkeit zu bewerten.

Basically, in examples B1 to B4, the 4 proceed as follows:

• (a) Wait until the gradient of NOx mass flow 75 has exceeded the first threshold and a comparatively rapid increase in NOx mass flow 75 and thus an evaluable event is present. This starts the procedure.
• (b) It becomes the strength of the efficiency break 73 the efficiency 72 determined and evaluated over time. This can be done either by reference to reference values 76 or by summation of partial contributions of the gradient of the efficiency 72 compare the above formula for the normalized efficiency loss 78 , SEL.
• (c) It becomes the strength of the NOx peak 74 determined over time.
• (d) The detection of the quantities treated in steps (b) and (c) is terminated when the gradient of the NOx mass flow 75 has fallen below a second threshold and thus a relatively fast drop in the NOx mass flow 75 and thus a return to a rather average NOx mass flow 75 is present.
• (e) The quotient of the variable determined in step (b) and the variable determined in step (c) is formed. The size of the quotient is used to evaluate the catalyst for its usefulness.

5 shows one opposite the 3 and 4 simplified method for evaluating the catalyst properties. It describes a statistical counter evaluation for a NH3-slip detection in a dynamic operation of the internal combustion engine 12 ,

An upper timing diagram shows the efficiency 72 and a lower timing diagram shows a count 90 a statistics counter. A threshold 92 allows evaluation of NH3 slip. A first turn 94 - Which is shown in dashed lines and serves for comparison - shows the course of the efficiency 72 in the event that there is no or little NH3 slip in the catalyst 32 occurs. A second turn 96 shows the course of the efficiency 72 in the event that noticeable NH3 slip occurs. arrows 98 show a recovery in efficiency 72 or to a subsequent decrement of the counter reading 90 after the decay of a NOx peak 74 (not drawn). Both diagrams and the illustrated curves have an equal time scale t.

It can be seen that with increasing time t the efficiency 72 according to the curve 96 is getting smaller. An arrow 100 indicates the case that the threshold 92 from the meter reading 90 is exceeded, indicating a saturation of the catalyst 32 with the reducing agent and according to a NH3-slip in the exhaust system 10 can be closed.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 102007040439 A1 [0009]

Claims (12)

Method for operating an exhaust system ( 10 ) an internal combustion engine ( 12 ), in which NOx by an SCR catalyst ( 32 ) and in which the aging state of the SCR catalyst ( 32 ) is monitored, the amount of SCR catalyst ( 32 ) is changed, characterized in that during a phase of the change in the amount of the in the SCR catalyst ( 32 ) stored reducing agent a break in efficiency ( 73 ) of the SCR catalyst ( 32 ) for at least one temporary increase ( 74 ) of the SCR catalyst ( 32 ) is detected and evaluated and from this to the aging state of the SCR catalytic converter ( 32 ) is closed. Method according to claim 1, characterized in that a degree of SCR efficiency ( 72 ) and for the determination of the aging state of the SCR catalyst ( 32 ) is evaluated Method according to one of claims 1 or 2, characterized in that a quotient SEL = _(SCR ∫η dt) / (∫ (dm _NOx / dt) dt) where η _SCR = SCR efficiency ( 72 ) and the _NOx / dt = NOx mass flow, and that a proper SCR catalyst ( 32 ) is closed when the quotient SEL is a lower limit ( 81 ) is at least equal to or lower than the lower limit ( 81 ), a minimally required reductant storage capacity of the SCR catalyst ( 32 Characterized. Method according to one of the preceding claims, characterized in that the SCR efficiency ( 72 ) is determined from a conversion rate of NOx in N ₂ . Method according to one of the preceding claims, characterized in that during a phase of the change of the amount of in the SCR catalyst ( 32 ), preferably several consecutive efficiencies ( 73 ) of the SCR catalyst ( 32 ) with a corresponding plurality of temporary increases ( 74 ) of the SCR catalyst ( 32 ) are detected and evaluated and a course of the reduction in the efficiencies ( 73 ) is used in the evaluation of the state of aging. A method according to claim 5, characterized in that from the course of the reduction of the efficiencies ( 73 ) or a corresponding size, or from the absolute value of the recorded loss of efficiency ( 73 ) or a corresponding size, the achievement of saturation of the SCR catalyst ( 32 ) is predicted with the reducing agent. A method according to claim 6, characterized in that the sensitivity of monitoring for a breakthrough of reducing agent by the SCR catalyst ( 32 ) is increased when it can be concluded from the evaluation of the course or the absolute value or the corresponding quantities on the early achievement of saturation. Method according to one of the preceding claims, characterized in that the efficiencies ( 73 ) are evaluated depending on at least one of the following variables: - efficiency degradation ( 73 ) after a temporally preceding NOx peak ( 74 ); - efficiency degradation ( 73 ) after a temporally following NOx peak ( 74 ); - reference values ( 76 ) of the SCR efficiency ( 72 ), which corresponds to a comparable operating state of the exhaust system ( 10 ) without temporary increase ( 74 ) of the nitrogen oxide content correspond; At least one predetermined threshold value and / or setpoint value; An added amount of the reducing agent; A duration of supply of the reducing agent; and / or - a current proportion of the reducing agent. Method according to at least one of the preceding claims, characterized in that the at least temporary increase ( 74 ) of the SCR catalyst ( 32 ) is specifically brought about, in particular by an exhaust gas recirculation of the exhaust system ( 10 ) is changed. Method according to at least one of the preceding claims, characterized in that it is carried out only if at least one of the following variables is in a respectively predetermined value range: An exhaust gas mass flow; An exhaust gas volume flow; An exhaust gas temperature; A vehicle speed; An ambient pressure; - an ambient temperature; A proportion of nitrogen oxide, hydrocarbon, carbon monoxide, PM or oxygen of the exhaust gas; A metering rate of the reducing agent; A temperature gradient of the exhaust system ( 10 ); An exhaust gas recirculation rate; and / or a rotational speed, an injection quantity, an operating mode, an operating state, a transit time and / or a service life of the internal combustion engine ( 12 ). Computer program ( 18 ), characterized in that it is programmed for use in a method according to one of the preceding claims. Control and / or regulating device ( 16 ) for an internal combustion engine ( 12 ), characterized in that it is programmed for use in a method according to one of claims 1 to 10.

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