DE102015207670A1 - Method for monitoring an SCR catalyst - Google Patents

Abstract

In a method for monitoring an SCR catalyst, in particular, the storage capacity of the SCR catalyst for NH3 is monitored. For diagnostic purposes, a superstoichiometric metering (42) of reducing agent into the SCR catalyst is provided until a predefinable NH 3 level is reached or until a predefinable reductant metering amount (45) is reached. According to the invention, the phase with the superstoichiometric metering (42) is terminated prematurely as soon as NH 3 slip can be concluded on the basis of increased signals of a NOx sensor arranged downstream of the SCR catalytic converter.

Description

The present invention relates to a method for monitoring an SCR catalyst, in particular by monitoring the storage capacity of the SCR catalyst for ammonia (NH ₃ ), according to the preamble of claim 1.

State of the art

Methods and apparatuses for operating an internal combustion engine, in particular in motor vehicles, are known, in the exhaust gas region of which an SCR catalytic converter (selective catalytic reduction) is arranged, which reduces the nitrogen oxides (NOx) contained in the exhaust gas of the internal combustion engine to nitrogen in the presence of a reducing agent. As a result, the proportion of nitrogen oxides in the exhaust gas can be significantly reduced. For the course of the reaction ammonia (NH ₃ ) is required, which is admixed to the exhaust gas. Stricter laws in the field of diagnosis of emission-related components require the monitoring of all exhaust aftertreatment components and the sensors used in the context of an on-board diagnosis (OBD) with regard to compliance with OBD limit values, which are usually specified as a multiple of the statutory emission limit values.

The basic principle of an SCR catalyst is that nitrogen oxide molecules are reduced on the catalyst surface in the presence of NH ₃ as a reducing agent to elemental nitrogen. The metering of the reducing agent is usually carried out in the form of aqueous urea solution, which is injected via a metering device upstream of the SCR catalyst. The required metering rate is determined as needed in an electronic control unit, with the strategies for operating and monitoring the SCR system generally being stored in the control unit.

The SCR catalysts known today store NH ₃ as a reducing agent on the catalyst surface. The NOx conversion in the SCR catalyst is all the more successful, the greater the amount of reducing agent in the catalyst, that is, the more NH _{3 is} stored in the catalyst. As long as the storage capacity of the SCR catalyst for NH ₃ has not yet been exhausted, unused reducing agent is stored. If the metering unit provides less reducing agent than is spent for the conversion of the nitrogen oxides currently present in the exhaust gas, the NOx conversion which continues to take place on the catalyst surface reduces the NH ₃ level.

Today known dosing strategies for SCR systems have a level control, which sets an operating point in the form of a target value for the NH ₃ level in the SCR catalyst. The term "level" here describes the mass of NH ₃ stored in the SCR catalyst. The operating point (set point) is chosen so that the NH ₃ level is high enough to ensure both a high NOx conversion rate and an NH ₃ buffer for short-term NOx peaks in the engine's raw emissions. On the other hand, the operating point setpoint is chosen to be as far as possible from the maximum storage capability of the NH ₃ SCR catalyst to avoid NH ₃ slip.

An OBD-II capable SCR system has at least one NOx sensor downstream of the SCR catalyst. Currently used NOx sensors usually show a cross-sensitivity to NH ₃ , so that the NOx sensors measure a sum signal of NOx and NH ₃ . An increase in the sensor signal of a NOx sensor arranged downstream of the SCR catalytic converter can therefore be attributed both to a falling NOx conversion rate, ie to an increase in the NOx concentration, and to a breakthrough of pure ammonia, that is to say an increase in NH ₃ . Concentration, point. A direct distinction between NOx and NH ₃ is not possible with such sensors.

It is known that the NH ₃ storage capacity of an SCR catalyst decreases greatly with progressive (thermal) aging. The NH ₃ storage capability is already being used as a diagnostic feature for catalyst monitoring. For example, the German Offenlegungsschrift describes DE 10 2010 029 740 A1 a method for monitoring an SCR catalyst based on the NH ₃ storage capacity, in which initially by a superstoichiometric reductant dosage (overdosing) of the SCR catalyst is filled up to the maximum achievable NH ₃ storage capacity. Achieving the maximum storage capability is detected by breaking pure NH ₃ downstream of the SCR catalyst (NH ₃ slip), whereby a NOx characteristic is continuously detected during the overdose phase and closed upon a drop in NOx conversion in that an NH ₃ slip is present. Subsequently, the reducing agent dosage compared to the normal dosage is reduced (underdosing) or completely switched off, so that in the course of this so-called emptying test, the stored NH ₃ mass is gradually reduced by the NOx reduction again. On the basis of characteristics which are dependent on the NOx conversion rate during this discharge test phase, the usable NH ₃ storage capacity can be indirect can be determined because with less stored NH ₃ mass, a lower NOx mass can be converted at the catalyst surface.

The German patent application DE 10 2012 201 749 A1 also describes a method for monitoring an SCR catalytic converter, in which, prior to an overdosage phase and a subsequent discharge test, a conditioning phase is additionally carried out in advance, in which a predeterminable operating point for the SCR catalytic converter is initially set. This operating point is set in the form of a specific NH ₃ level in the SCR catalyst, wherein a substoichiometric metering of reducing agent is preferably carried out in the conditioning phase until the NO x conversion rate of the SCR catalytic converter is below the NO x conversion rate that occurs with normal metering is to be expected. This so achievable operating point can be predicted with a higher accuracy than the selected in the normal metering setpoint for the NH ₃ level, so made by the thus improved tolerance position already during the phase of overdosing a distinction between a new and an aged SCR catalyst can be and thus can be dispensed with the emptying test.

Disclosure of the invention

Advantages of the invention

The inventive method for monitoring an SCR catalyst uses in particular the storage capacity of the SCR catalyst for NH ₃ as a diagnostic feature. The invention is based on a method in which, for diagnostic purposes, a superstoichiometric metering of reducing agent into the SCR catalyst is provided until a predefinable NH ₃ level is reached or until a predefinable reductant metering amount is reached. This phase of the superstoichiometric dosage is used by determining suitable characteristics for the diagnosis. A subsequent emptying test for diagnostic purposes preferably does not take place. According to the invention, the phase of the superstoichiometric metering is terminated prematurely as soon as NH ₃ slip can be deduced based on increased signals of a NOx sensor arranged downstream of the SCR catalytic converter. If such an NH ₃ slippage is detectable and the phase of the over-stoichiometric dosing is terminated prematurely, it can be concluded that the SCR catalyst is defective or that the NH ₃ storage capacity of the SCR catalyst is no longer sufficient to a satisfactory Implementation of nitrogen oxides in the exhaust gas to ensure. If the phase of the superstoichiometric dosage is terminated prematurely according to the inventive method, so a corresponding error message can be issued.

In a preferred embodiment of the method according to the invention, a conditioning phase for setting a predeterminable operating point of the SCR catalyst is carried out before the phase with the superstoichiometric metering of reducing agent. This conditioning phase avoids tolerance-related uncertainties when evaluating the SCR catalytic converter.

Compared with conventional methods, the method according to the invention has the particular advantage that the time required for the diagnosis of an SCR catalytic converter, in particular in the event of a fault, is reduced in comparison with conventional methods. Under certain circumstances, the diagnosis according to the invention can be based solely on whether or not NH ₃ slippage is detectable during the overdose phase, so that not in each case, if the method is not terminated prematurely, based on the conversion rate or other sizes further diagnosis of the catalyst must be made. In general, however, it is advantageous, in the case where no premature termination of the overdose phase took place, to carry out a further evaluation of characteristic values from the phase of the overdose so that the meaningfulness of the diagnosis can be increased. The reduced diagnostic time makes it possible to increase the frequency of diagnoses, which can improve the ratio between in-use (monitoring) and performance (so-called IUMPR or IUPR) so that legal requirements can be met even better. IUMPR (USA) or IUPR (EU) refers to a standardized calculation of the frequency of diagnosis prescribed by the respective legislation. Furthermore, due to the reduced duration of diagnosis, less NH ₃ slip occurs during the diagnosis of an optionally defective SCR catalytic converter. In addition, the amount of reducing agent is reduced, which is consumed in the diagnosis of an optionally defective SCR catalyst.

If, in the process according to the invention, no NH ₃ slip is detectable during the phase of the overstoichiometric metering, the phase of the superstoichiometric metering is regularly completed, ie until a predefinable NH ₃ level is reached or until a predefinable reductant metering amount is reached. Based on at least one characteristic which is dependent on the NOx conversion rate of the SCR catalyst during the phase of the superstoichiometric metering, the size of the storage capacity of the SCR catalyst for NH _{3 can be} concluded, comparable to conventional methods, this size being a diagnostic feature For the SCR catalyst is used. For example, in the phase of the superstoichiometric metering, the NOx conversion rate of the SCR catalyst, in particular an averaged NOx conversion rate, can be used in order to be able to carry out an evaluation of the SCR catalytic converter. For example, the averaged NOx conversion rate may be compared to a predeterminable minimum NOx conversion rate threshold. The minimum NOx conversion rate is preferably chosen such that this conversion rate represents an acceptable function of the SCR catalyst. Therefore, if this minimum NOx conversion rate is achieved, it can be concluded that the SCR catalyst is sufficiently functional. Otherwise, the SCR catalyst is rated as defective or deficient. In a comparable way, for example, the efficiency of the SCR catalyst can be considered during the overdose phase.

The phase of overdose is carried out in the regular case, that is, if this phase is not terminated prematurely in the manner according to the invention, until a predeterminable NH ₃ level is reached or until a predefinable reductant dosage amount is reached. This predefinable NH ₃ level or this predefinable reductant metering amount is preferably selected in a temperature-dependent manner, since the storage capacity of the SCR catalyst is temperature-dependent. The temperature-dependent selected NH ₃ level is preferably between the maximum storage capacity of the catalyst when new and the maximum storage capacity of an aged catalyst. If no premature termination of the overdosage phase took place according to the method according to the invention, a further diagnostic evaluation of the overdosage phase is preferably carried out. In particular, if a conditioning phase has been carried out in advance, can, comparable to that of the published patent application DE 10 2012 201 749 A1 known methods, the further diagnosis are performed without a subsequent emptying test, as by the start of overdosage at a defined operating point already by observing the course of the NOx conversion rate during the overdosage phase or another characteristic, the NOx conversion rate during the phase depends on the stoichiometric dosage, meaningful information about the storage capacity of the catalyst can be obtained.

In order to determine the characteristic value which is dependent on the NOx conversion rate of the SCR catalytic converter, signals of a NOx sensor arranged downstream of the SCR catalytic converter are preferably included. On the basis of this sensor signal data, the NOx conversion rate or the efficiency of the SCR catalyst can be calculated. For example, an averaged NOx conversion rate can be used for the diagnostic evaluation. The calculation may further include signals from an optional NOx sensor located upstream of the SCR catalyst. Additionally or alternatively, data from a NOx raw emission model for the NOx emissions, in particular upstream of the SCR catalyst can be included. In particular, in those systems where no NOx sensor is provided upstream of the SCR catalyst, the NOx conversion rate or other NOx conversion rate dependent quantities may be included, including calculated NOx emission model values upstream of the SCR catalyst preferably, these model data, together with signals from a NOx sensor downstream of the SCR catalyst, are included in the calculation of the NOx conversion rate or the other variables used.

For setting the predeterminable operating point in the conditioning phase, which precedes the phase with the superstoichiometric metering of the reducing agent, a substoichiometric metering of reducing agent is preferably carried out. For example, this substoichiometric metering of reducing agent takes place until the NOx conversion rate of the SCR catalyst is below the NOx conversion rate that is to be expected for a normal metering of reducing agent. For further aspects related to the conditioning phase, see the Offenlegungsschrift DE 10 2012 201 749 A1 directed.

The monitoring method according to the invention, in which the phase carried out for diagnostic purposes is terminated prematurely with an over-stoichiometric dosing, if an NH ₃ slippage is detectable, is suitable primarily for SCR catalysts with high NH ₃ storage capacity. Particularly in such systems, the advantages of the method according to the invention, which result from the reduced duration of diagnosis with a defective SCR catalyst, have a particularly advantageous effect compared to previously known approaches. Furthermore, the method according to the invention can be very advantageous in connection with an OBD demonstration, because in this case the advantages of the method according to the invention, namely a shortening of the duration of the diagnosis, especially in the event of a defect, come into their own.

The monitoring method according to the invention is suitable in principle for the monitoring of each SCR catalyst. The method may be for systems with one or more SCR catalysts be used. Furthermore, the method can also be used for example for monitoring a diesel particulate filter with SCR coating (SCRF) with advantage.

The invention further comprises a computer program which is set up to carry out the steps of the method according to the invention. Furthermore, the invention comprises a machine-readable storage medium, on which such a computer program is stored, as well as an electronic control device which is set up to perform the steps of the method according to the invention. The implementation of the monitoring method according to the invention as a computer program or as a machine-readable storage medium or as an electronic control unit has the advantage that the monitoring method according to the invention can readily be used, for example, in existing motor vehicles in order to be able to utilize the advantages of the method.

Further features and advantages of the invention will become apparent from the following description of exemplary embodiments in conjunction with the drawings. In this case, the individual features can be implemented individually or in combination with each other.

Brief description of the drawings

In the drawings show:

1 a schematic representation of the components of an SCR catalyst system (prior art);

2A / B are schematic representations of the relationship between the NOx sensor signal downstream of the SCR catalyst and the NH ₃ level in a prior art monitoring process with preconditioning (conditioning phase) in a new catalyst ( 2A ) and an aged catalyst ( 2 B );

3 a schematic representation of the relationship between the NOx sensor signal downstream of the SCR catalyst and the NH ₃ level in a monitoring method according to the present invention and

4 schematic flow diagram of the method according to the invention.

Description of exemplary embodiments

1 shows in a schematic way the known components of an SCR catalyst system. In the exhaust system 10 an internal combustion engine 11 is an SCR catalyst 12 arranged, which selectively reduces nitrogen oxides in the exhaust gas by a selective catalytic reduction (SCR). The reaction uses ammonia (NH ₃ ), which has a reducing effect. NH ₃ is produced by the injection of a liquid urea water solution (reducing agent solution) via the metering device 13 in the exhaust system 10 upstream of the SCR catalyst 12 introduced according to demand. The aqueous urea solution is placed in a reducing agent tank 14 stored, from which the solution by means of a feed pump 15 via the pressure line 16 the actual metering device 13 is supplied. To monitor the nitrogen oxide concentration in the exhaust gas is downstream of the SCR catalyst 12 a NOx sensor 17 intended. In other systems, another NOx sensor may be upstream of the SCR catalyst 12 be arranged. The control of the dosage and the detection and evaluation of the nitrogen oxide values is carried out in an evaluation unit 18 in particular in a control unit of the SCR catalytic converter system or in a control unit of the internal combustion engine.

An already known metering strategy for monitoring the NH ₃ storage capability of a catalyst envisages the setting of a defined operating point in relation to that in the SCR catalyst 12 stored NH ₃ (NH ₃ level) as a starting point for an overdose phase, which is carried out for diagnostic purposes, wherein during the phase of overdose one or more characteristic values are determined, which characterize the NOx conversion rate during the overdose phase. From this a distinction can be made between a new and an aged SCR catalyst. Such a method is for example in German Offenlegungsschrift DE 10 2012 201 749 A1 and is described below with reference to 2A / B explained in more detail. The illustrations illustrate the reference of the current NH ₃ level to the maximum NH ₃ storage capacity, wherein 2A a new or new SCR catalyst and 2 B represent an aged SCR catalyst. Starting from the working point AP, in the 2A and the 2 B corresponds to a normal working point (model value), there is a partial emptying 27 by conversion of the reducing agent dosage to an underdosing, eg with α = 0.5 (conditioning phase). α here describes the reducing agent dosage, where α = 1 would correspond to a stoichiometric dosage. In this way, a defined operating point 28 set. This operating point corresponds to a certain leveled NH ₃ level that can be calculated, for example, based on NOx sensor signals, starting with the model value at the operating point AP. The adjustment of the operating point 28 may also have a rising NOx signal Be done as illustrated here. Subsequently, the filling takes place 29 of the NH ₃ storage (filling test) by an over-stoichiometric dosage (overdose phase). The filling 29 can, for example, up to a certain calculable level 30 take place, or the overdose phase is carried out until a predefinable reductant dosage was metered. During the overdose phase 29 For example, the NOx conversion rate or, for example, the efficiency of the SCR catalyst is observed, in particular based on characteristics that are representative thereof. For example, the average NOx conversion rate can be considered in comparison with predetermined threshold values in order hereby to make an assessment whether the NH ₃ storage capacity of the SCR catalyst is sufficient or not. From this it can be concluded that an over-aged SCR catalyst may be considered defective. An insufficiently functioning SCR catalyst shows during the overdose phase 29 an increasingly worse NOx conversion rate resulting from an increase in the NOx sensor signal downstream of the SCR catalyst (measurement effect 31 ) is derivable.

Based on this known method, the monitoring method according to the invention provides that the overdose phase is prematurely terminated or terminated when an NH ₃ slip downstream of the SCR catalyst is detected. This procedure is in 3 illustrated. Starting from the operating point AP is preferably carried out first a conditioning phase 37 in which the NH ₃ storage of the SCR catalyst is partially emptied. Here, the dosage is changed to underdosing, eg with α = 0.5, until the corresponding level has settled. The fill level can be calculated in particular from the NOx sensor signals, starting with the model value at the operating point AP. The underdosing may optionally be continued beyond the slip limit, wherein the possibly measurable increase of the NOx sensor signal is due to a deteriorated NOx conversion due to the lack of reducing agent. The conditioning phase 37 is continued until the predefinable operating point 38 is reached. This is followed by an overdose phase 39 which would be carried out regularly until a predefinable amount of reducing agent is metered in or until a predefinable NH ₃ level has been reached. If in the course of this phase 39 an increase in the NOx sensor signal downstream of the SCR catalyst can be detected prematurely, this results in an NH ₃ slip 40 closed and the overdose phase 39 is terminated or terminated prematurely according to the inventive method. In this case, it is concluded according to the invention that the SCR catalyst is defective or has insufficient NH ₃ storage capacity. There is no further evaluation of characteristic values, which is the NOx conversion rate during the overdose phase 39 hang up and the diagnostic procedure stops. If no NH ₃ -slip during the overdose phase 39 is ascertainable, the method is continued as in conventional methods, and an evaluation of characteristic values which depend on the NOx conversion rate in the overdose phase is carried out in a manner known per se in order to carry out an evaluation of the SCR catalytic converter.

4 summarizes the process steps of the monitoring method according to the invention in a schematic flow diagram. After the start of the monitoring process for the SCR catalyst, a conditioning phase first takes place 41 in which initially a specifiable operating point of the SCR catalytic converter is set with regard to the NH ₃ level of the SCR catalytic converter. For this purpose, a conversion of the dosage to underdosing, for example, with α = 0.5, in particular first carried out, so that the NH ₃ storage of the SCR catalyst is at least partially emptied and settles a certain level. Then there is an overdose phase 42 , In the course of the overdose phase 42 is in the step 43 checks whether a NH ₃ slippage is detectable. In particular, it is checked here whether the NOx sensor signal downstream of the SCR catalytic converter exceeds a specific value. If this is the case, it is closed on NH ₃ -slip and the catalyst is in the step 44 rated as defective. In this case, the overdose phase 42 aborted prematurely, whereby the duration of diagnosis is significantly shortened, especially in the case of a defective SCR catalytic converter. This has the advantage, in particular in the case of a defective SCR catalytic converter, that during the diagnosis, less NH _{3 is released} in the form of NH ₃ slipping than in conventional methods. In addition, the reducing agent consumption during diagnosis is significantly reduced in a defective SCR catalyst compared with conventional monitoring methods.

If at check in step 43 it is determined that there is no NH ₃ slip, is in step 45 Check for the completion of the overdose phase 42 Reductant metered dose or the predetermined NH ₃ level is reached. If this is not the case, the overdose phase will be 42 continued. If this is the case, the overdose phase is terminated and in step 46 An evaluation of characteristic values from the overdosage phase takes place 42 , In this case, it is checked, for example based on an averaged NOx conversion rate during the overdose phase, whether the NH ₃ storage capacity of the SCR catalyst is sufficient. Therefor will / will be appropriate characteristics or a corresponding characteristic value, which is derived from the operation of the SCR catalyst during the overdose phase, in step 47 compared with appropriate thresholds. For example, if the average NOx conversion rate of the SCR catalyst during the overdose phase above a predetermined threshold value (minimum NOx conversion rate), in step 48 concluded that the SCR catalyst is OK. If, for example, the average NOx conversion rate is below the predefinable threshold value, in step 49 concluded that the SCR catalyst is defective or aged too much.

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 102010029740 A1 [0007]
• DE 102012201749 A1 [0008, 0015, 0026]
• DE 102012201749 A [0013]

Claims (11)

Method for monitoring an SCR catalyst ( 12 ), in particular by monitoring the storage capacity of the SCR catalyst ( 12 ) for NH _3, where for diagnostic purposes, a superstoichiometric dosage ( 39 . 42 ) of reducing agent in the SCR catalyst ( 12 ) until reaching a predefinable NH ₃ level or until a predefinable reductant dosage amount ( 38 . 45 ), characterized in that the phase with the superstoichiometric dosage ( 39 . 42 ) is terminated prematurely, as soon as based on increased signals of a downstream of the SCR catalyst arranged NOx sensor ( 17 ) to a NH ₃ slip ( 40 ) can be closed. A method according to claim 1, characterized in that is evaluated at a premature termination of the phase of the superstoichiometric metering of the SCR catalyst as defective ( 40 ). A method according to claim 1 or claim 2, characterized in that before the phase with the superstoichiometric dosage ( 39 . 42 ) of reducing agent is a conditioning phase ( 37 . 41 ) for setting a predefinable operating point ( 38 ) is carried out Method according to one of the preceding claims, characterized in that based on at least one characteristic value, which is dependent on the NOx conversion rate of the SCR catalyst ( 12 ) is dependent on the size of the storage capacity of the SCR catalyst during the phase of the overstoichiometric metering ( 12 ) is closed for NH ₃ . Method according to one of the preceding claims, characterized in that based on at least one characteristic value, which is dependent on the NOx conversion rate of the SCR catalyst ( 12 ) is dependent on the above-stoichiometric metering, in particular on the basis of an averaged NOx conversion rate, an evaluation of the SCR catalytic converter takes place, whereby the achievement of a predefinable threshold value for a minimum NOx conversion rate is preferably checked ( 47 ). A method according to claim 4 or claim 5, characterized in that for determining the characteristic value, which is dependent on the NOx conversion rate of the SCR catalyst, signals of a downstream of the SCR catalyst ( 12 ) arranged NOx sensor ( 17 ) be included. A method according to claim 6, characterized in that to determine the characteristic further signals of an upstream of the SCR catalyst ( 12 ) arranged NOx sensor ( 17 ) and / or data from a NOx raw emission model for NOx emissions upstream of the SCR catalyst ( 12 ) be included. Method according to one of the preceding claims, characterized in that for setting the predefinable operating point ( 38 ) in the conditioning phase ( 41 ) a substoichiometric metering of reducing agent is carried out until the NOx conversion rate of the SCR catalyst ( 12 ) is below the NOx conversion rate that is expected with a normal dosage of reducing agent. Computer program adapted to perform the steps of a method according to one of claims 1 to 8. A machine-readable storage medium on which a computer program according to claim 9 is stored. An electronic control device adapted to perform the steps of a method according to any one of claims 1 to 8.

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