FR3035442A1 - METHOD OF MONITORING AN SCR CATALYST - Google Patents

Abstract

A method for monitoring an SCR catalyst, in particular by monitoring its NH3 storage capacity in which, for diagnostic purposes, an over-stoichiometric assay (42) of reducing agent in the SCR catalyst is provided until a level of NH3, predetermined or a predetermined amount of reducing agent (38, 45). The superstoichiometric assay stage (42) is prematurely terminated as soon as it is concluded that NH3 has slipped by means of augmented signals provided by a NOx sensor installed downstream of the SCR catalyst.

Description

FIELD OF THE INVENTION The present invention relates to a method for monitoring an SCR catalyst, in particular by monitoring its NH3 storage capacity, and for assaying an oversoichiometric determination of reducing agent in the SCR catalyst. until reaching a predetermined level of NH3 or until reaching a predetermined reducing agent dose. State of the art There are known methods and devices for managing an internal combustion engine, in particular a motor fitted to motor vehicles; this method consists in installing an SCR (selective catalytic reduction catalyst) catalyst in the exhaust in order to reduce the nitrogen oxides (NO x) of the exhaust gases emitted by the engine in the presence of a reducing agent in order to obtain nitrogen. This considerably reduces the nitrogen oxide content of the exhaust gas. For the course of the reaction, ammonia (NH3) is required which is mixed with the exhaust gas. Stricter regulations concerning components related to the emission of pollutants require, as part of an on-board diagnostic (OBD diagnostic), the monitoring of all the aftertreatment components of the exhaust gases as well as the sensors. used to meet the OBD diagnostic limits and are generally indicated as multiples of the emission values set by the regulations.

The basic principle of an SCR catalyst is to reduce the nitrogen oxide molecules on the surface of the catalyst in the presence of ammonia NH 3 constituting the reducing agent to obtain elemental nitrogen. The dosage of the reducing agent is usually in the form of an aqueous solution of urea, which is injected by a metering device upstream of the SCR catalyst. The required dosing rate is determined according to the demand, using an electronic control unit; in general, the operating and monitoring strategies of the SCR system are recorded in the control unit.

The SCR catalysts currently known accumulate ammonia NH3 as a reducing agent on the surface of the catalyst. The conversion of NOx nitrogen oxides in the SCR catalyst is all the more successful as the supply of reducing agent is high in the catalyst, i.e. the more ammonia NH3 is accumulated in the catalyst. As long as the NH 3 ammonia storage capacity in the SCR catalyst is not exhausted, the unconsumed reducing agent will be accumulated. When the metering unit has less reducing agent than necessary for the conversion of the nitrogen oxides presently contained in the exhaust gas, the conversion of the NOx nitrogen oxides to the catalyst surface continues. decrease the level of ammonia NH3. Currently known metering strategies for SCR catalyst systems have level control which adjusts the operating point in the form of a set value of ammonia level NH3 in the SCR catalyst. The term "level" or "fill level" describes the NH 3 ammonia mass stored in the SCR catalyst. The operating point (set point) is chosen so that the level of NH 3 is still sufficiently high to have both a high conversion rate of nitrogen oxides NO x and also a buffer of ammonia NH 3 for peaks. NOx nitrogen oxides which occur briefly in the gross emissions of the internal combustion engine. On the other hand, the operating point setpoint is chosen to be as far away as possible from the maximum NH 3 ammonia storage capacity of the SCR catalyst to prevent slippage of NH 3 ammonia. An SCR catalyst system that meets the OBD-II diagnostics has at least one NOx nitrogen oxide sensor downstream of the SCR catalyst. The NOx catalysts currently used generally have a transverse sensitivity to NH 3 ammonia so that these NO x sensors measure the sum signal of NOx nitrogen oxides and NH 3 ammonia. An increase in the NOx sensor signal installed downstream of the SCR catalyst can thus mean both a decreasing conversion rate of the NOx oxides, that is to say an increase in the NOx concentration, but also a crossing. pure ammonia, that is to say an increase in the ammonia NH3 concentration. It is not possible to directly distinguish NOx and NH3 with such sensors. It is known that the NH 3 ammonia storage capacity of an SCR catalyst decreases significantly with (thermal) aging. The NH3 storage capacity is used as a diagnostic feature for catalyst monitoring. Thus, for example, DE 10 2010 029 740 A discloses a method of monitoring an SCR catalyst from its NH3 storage capacity consisting firstly of performing an over-stoichiometric reductant dosage. (Overdose) of the SCR catalyst up to its maximum storage capacity NH3. It is detected that one is at the maximum storage capacity using the passage of pure ammonia NH3 downstream of the SCR catalyst (slip of NH3); for this purpose, during the overdose phase, a quantity characteristic of the NOx conversion is continuously taken and, when this NOx conversion falls, it is concluded that there is an NH3 ammonia cross-over. Subsequently, the dosage of reducing agent is reduced from the normal dosage (underdosing) or completely cut so that during this emptying test the ammonia mass is gradually decreased again. NH3 stored by the reduction of nitrogen oxides NOx. Using characteristic values which depend on the conversion rate of NOx nitrogen oxides during the emptying test phase, it is possible indirectly to determine the NH3 ammonia storage capacity because, for a reduced mass of d The ammonia NH3 stored will have a lower mass conversion of NOx nitrogen oxides to the catalyst surface. The document DE 10 2012 201 749 A1 also describes a method of monitoring the SCR catalyst according to which, prior to the underdosing phase and the following emptying test, a conditioning phase consisting of adjusting firstly a predetermined operating point of the SCR catalyst. This operating point is set in the form of a determined level of NH 3 ammonia filling in the SCR catalyst and in the conditioning stage a sub-stoichiometric dosage of reducing agent is preferably carried out until the NOx NOx conversion rate of the SCR catalyst is below the predictable NOx NOx conversion rate for a normal assay. The operating point thus obtained can be predicted with a higher accuracy than the set point of the fill level NH3 chosen in normal dosing; thus, thanks to this improved state of tolerance, from the underdosing phase, it will be possible to distinguish between a new SCR catalyst and an aged SCR catalyst, thus avoiding the guiding test. DESCRIPTION AND ADVANTAGES OF THE INVENTION The subject of the present invention is a method for monitoring an SCR catalyst, in particular by monitoring the NH 3 storage capacity of the SCR catalyst, and for assaying an over-stoichiometric determination of the agent. in the SCR catalyst until a predetermined level of NH 3 is reached or until a predetermined reducing agent dose is reached, this process being characterized by prematurely terminating the over-stoichiometric assay stage as soon as it is concluded that NH3 is sliding using augmented signals provided by a NOx sensor installed downstream of the SCR catalyst. The over-stoichiometric assay phase is used to determine characteristic values suitable for diagnosis. We do not perform a dump test for the diagnosis. According to the invention, the over-stoichiometric metering stage is prematurely terminated as soon as using the higher signals provided by a NOx sensor installed downstream of the SCR catalyst, it is concluded that there is an NH3 slip. If such an NH3 slip can be seen and the premature completion of the stoichiometric determination phase, 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 allow satisfactory conversion of the nitrogen oxides contained in the exhaust gas. When the over-stoichiometric assay phase according to the method of the invention terminates prematurely, it is then possible to issue a fault message. According to a preferred development of the process of the invention, before the over-stoichiometric dosing phase of the reducing agent, a conditioning phase is carried out to adjust a predetermined operating point of the SCR catalyst. This conditioning phase makes it possible to avoid the insecurities related to the tolerances for the evaluation of the SCR catalyst. The process according to the invention has the advantage over known processes of reducing the time required for the diagnosis of an SCR catalyst, especially in the case of a defect as compared with the known method. If necessary, the diagnostic according to the invention can be based solely on the observation of an NH3 slip during the overdose phase, so that it is not necessary in all cases when the process is not not stopped prematurely, to make another diagnostic of the catalyst with the conversion rate or other quantities. In general, however, it is advantageous in case there is no premature termination of the overdose phase, to perform another evaluation of the characteristic values from the overdose phase, which increases the character of the overdose. significant diagnostic. The reduced diagnostic time increases the frequency of the diagnosis, which improves the ratio between the "in operation" (control) and performance (IUMPR or IUPR) characteristics, which makes it possible to better comply with the regulations. The references IUMPR 20 (USA) and IUPR (EU) designate a calculation of the diagnostic frequency given by the respective, standardized regulation. In addition, the shorter duration of the diagnosis results in less NH3 slip during the diagnosis in the case of an SCR catalyst possibly defective. In addition, the amount of reducing agent consumed for the diagnosis of a SCR catalyst, if any, is reduced. If, in the process according to the invention, during the on-stoichiometric determination phase no NH 3 slip is observed, the stoichiometric determination is regularly completed, that is to say until reaching a predefined NH3 filling level or until a predefined dose of reducing agent With the aid of at least one characteristic that depends on the NOx conversion rate of the SCR catalyst during the over-stoichiometric assay phase, it can be concluded, in a manner comparable to a conventional process at the capacity dimension of SCR catalyst storage for NH3 ammonia; this size is used as a diagnostic feature of the SCR catalyst. For example, in the over-stoichiometric assay phase the NOx conversion rate of the SCR catalyst is used, in particular the average NOx conversion rate to evaluate the SCR catalyst. The average NOx conversion rate can be compared for example to a predefined threshold of a minimum conversion rate of NOx. The minimum conversion level NOx is preferably chosen so that this conversion rate represents a still acceptable operation of the SCR catalyst. If the minimum conversion level NOx is reached, it can be concluded that the SCR catalyst can still function sufficiently, otherwise the SCR catalyst is considered to be defective or insufficient. In a comparable manner, for example, the yield of the SCR catalyst can be considered during the overdose phase. In a normal case, that is to say when the overdose phase is not prematurely stopped according to the invention, it continues to give a predetermined level of filling NH3 or until a dose of predefined reducing agent. This predefined NHR filling level or the predefined reducing agent dose is preferably selected as a function of temperature since the storage capacity of the SCR catalyst is temperature dependent. The level of filling NH3 chosen as a function of the temperature is preferably between the maximum capacity of the new catalyst and the maximum storage capacity of an aged catalyst if there is no premature stop of the overdose phase. according to the method of the invention, the operation is continued in diagnosis of the overdose phase. In particular, if a conditioning phase has been carried out beforehand, it is possible, by comparison with the known method according to document DE 10 2012 201 749 Ai, to continue the diagnosis without subsequently performing a dump test, because, by starting the phase overdose at an operating point defined by observation of the evolution of the conversion rate of NOx nitrogen oxides during the overdose phase or of another characteristic dependent on the conversion rate of nitrogen oxides NOx during the over-stoichiometric assay phase will yield significant information regarding the storage capacity of the catalyst.

In order to determine the characteristic which depends on the conversion rate of NOx nitrogen oxides of the SCR catalyst, the signals of a NOx sensor installed downstream of the SCR catalyst are preferably used. The yield or NOx conversion rate of the SCR catalyst can be calculated with the sensor signal data. For example, the average NOx conversion rate can be used for operating diagnostics. For the calculation, it is also possible to use the signals of a NOx sensor installed, if necessary, upstream of the catalyst SCR. In addition or alternatively, the raw NOx emission model data can be used for nitrogen oxide NOx emissions, especially upstream of the SCR catalyst. In particular, in such NOx sensorless systems upstream of the SCR catalyst, the NOx conversion rate or other NOx conversion dependent quantities can be determined using model values calculated for the NOx emissions from upstream of the SCR catalyst; these model data are preferably used with signals from a NOx sensor downstream of the SCR catalyst for calculating the NOx conversion rates or other quantities used. In order to set the predefined operating point in the conditioning phase which is upstream of the superstoichiometric dosage stage of the reducing agent, a sub-stoichiometric assay of the reducing agent is preferably carried out. This sub-stoichiometric dosage of the reducing agent is, for example, until the NOx conversion of the SCR catalyst is below the predictable NOx conversion rate for a normal reducing agent assay. For other aspects in connection with the conditioning phase, reference is made to DE 10 2012 201 749 A1. The monitoring method according to the invention consisting in prematurely terminating the on-stoichiometric assay stage performed for the diagnosis. if NH 3 slip is observed, this process is primarily suitable for SCR catalysts having a high NH 3 storage capacity. In particular, for such systems, for the process according to the invention, the advantages of reduced diagnostic time in the case of defective SCR catalyst compared to known solutions have been realized. In addition, the method according to the invention is very advantageous in connection with an OBD diagnosis because thus the advantages of the method of the invention, namely the reduction of the duration of the diagnosis, especially in case of defect, are particularly important. The monitoring method according to the invention is in principle suitable for monitoring any SCR catalyst. The method can be applied to systems having one or more SCR catalysts. In addition, the method can be advantageously applied, for example to monitor a diesel particulate filter with SCR coating (SCRF).

The invention also relates to a computer program for implementing the steps of the method according to the invention as well as a machine-readable memory medium containing the computer program and an electronic control device which perform the steps of the method of the invention.

The implementation of the monitoring method according to the invention in the form of a computer program or on a machine-readable memory medium or in the form of an electronic control device has the advantage of being able to use the monitoring method according to the invention, as such by applying it for example to existing vehicles to benefit from the advantages of the method. Drawings The present invention will be described hereinafter in more detail by way of exemplary embodiments of a method of monitoring an SCR catalyst according to the invention shown in the accompanying drawings, in which: FIG. a diagram of the components of a prior art SCR catalyst system, Figs. 2A / 2B are diagrams showing the relationship between the NOx sensor signal downstream of the SCR catalyst and the NH3 fill level in a monitoring method according to the state of the art with preconditioning (conditioning phase), in the case of a new catalyst (FIG. 2A) and an aged catalyst (FIG. 2B), FIG. 3 is a diagram of the relationship between the NOx sensor signal downstream of the SCR catalyst and the NH3 fill level in a monitoring method according to the invention, and Figure 4 shows a schematic flow diagram of the process of the invention. DESCRIPTION OF EMBODIMENTS FIG. 1 schematically shows the components known per se of an SCR catalyst system. The exhaust gas line 10 of the internal combustion engine 11 comprises a SCR catalyst 12 which selectively reduces the nitrogen oxides of the exhaust gas by selective catalytic reduction (abbreviated SCR reduction). The reaction uses ammonia (NH3) as a reducing agent. Ammonia NH 3 is introduced by the injection of an aqueous solution of urea (reducing agent solution) through the metering unit 13 into the exhaust gas line 10 upstream of the SCR catalyst 12. The aqueous solution of urea is contained in a reducing agent tank 14 in which the solution is taken by a feed pump 15 feeding via the pressure line 16, the metering installation 13 itself. A NOx sensor 17 monitors the concentration of the nitrogen oxides in the exhaust gas downstream of the SCR catalyst 12. In other systems, a NOx sensor can also be located upstream of the SCR catalyst 12. The dosing control and the capture and exploitation of the nitrogen oxide values are done in an operating unit 18, in particular in the control unit 25 of the SCR catalyst system or in the control (or management) apparatus of the internal combustion engine. A previously known metering strategy for monitoring the NH3 storage capacity of a catalyst is to set a defined operating point with respect to the ammonia NH3 (NH3 fill level) stored in the SCR catalyst 12 as the point of departure. starting of an overdose phase performed to make a diagnosis; during the overdose phase one or more characteristic values corresponding to the NOx conversion rate during the overdose phase are determined. This allows recognition of a new SCR catalyst and an aged SCR catalyst. Such a method is for example described in DE 2012012049 A1. It will be discussed in more detail below with reference to FIGS. 2A, 2B. The graphs illustrate the relationship between the current NH3 fill level and the maximum NH3 storage capacity; Figure 2A shows a new SCR catalyst or equivalent to a new catalyst, and Figure 2B shows an aged SCR catalyst. Starting from the operating point AP which corresponds to FIGS. 2A and 2B to a normal operating point (model value) partial emptying 27 is obtained by switching the reducing agent dosage to an underdosing, for example with a = 0 , 5 (conditioning phase). The size described the reducing agent dosage; the value a = 1 corresponds to a stoichiometric dosage. This operating point corresponds to a certain level of filling NH3, stabilized, which can be calculated for example with the NOx sensor signals starting with the value of the modulus. at the point of operation AP. The setting of the operating point 28 can also be done with a NOx signal, increasing as shown. Then, the NH3 ammonia accumulator (fill test) is filled by an over-stoichiometric assay (overdose phase). The filling 29 may be, for example, up to a calculated, predetermined filling level or the overdose phase continues until a predefined dose of reducing agent is metered. During the overdose phase 29 the NOx conversion rate or, for example, the yield of the SCR catalyst is observed, in particular using representative characteristic quantities. For example, the average NOx conversion rate can be considered by comparison with predefined setpoint values to make an evaluation and whether or not the storage capacity NH3 of the SCR catalyst is sufficient. If necessary, it can be deduced that the SCR catalyst is too old and must be considered as defective. An SCR catalyst which is not sufficiently capable of operating has during the overdose phase 29, a conversion rate of NOx nitrogen oxides which is deteriorating increasingly; this conversion rate is deduced from the rise of the NOx sensor signal downstream of the SCR catalyst (measurement effect 31).

Based on this known method, the monitoring method according to the invention provides for prematurely stopping the overdose phase if an NH3 slip is detected downstream of the SCR catalyst. This process is explained in FIG. 3. Starting from the operating point AP, there is preferably a conditioning phase 37 during which the accumulator NH3 of the SCR catalyst will be emptied at least partially. For this purpose the dosage is adjusted to underdosing, for example with a = 0.5 until the corresponding filling level has stabilized. The fill level is calculated in particular from the NOx sensor signals starting with the model value at the operating point AP. Underdosing may, if necessary, continue beyond the slip limit; the measurable rise, if any, in the NOx sensor signal results from a deterioration in the conversion of NOx nitrogen oxides due to a lack of reducing agents. The conditioning phase 37 continues until reaching the predefined operating point 38. Then, an overdose phase 39 is performed which would be executed normally until reaching a predefined dose of reducing agent or until reaching a filling level NH3, predefined. If, during phase 39, a rise of the NOx sensor signal is observed prematurely downstream of the SCR catalyst, it is concluded that there is a glide NH3 40 and the overdose phase 39 is stopped according to the method of the invention, that is to say that it terminates prematurely. In this case, according to the invention it is concluded that the SCR catalyst is defective or that its storage capacity NH3 is insufficient. There is no other exploitation of characteristic values depending on the NOx conversion rate during the overdose phase 39 and the process is terminated. If no NH3 slip is observed during the overdose phase 39, the process is continued as usual and then the characteristic values which depend on the conversion rate in the overdose phase are used in a manner known per se for evaluate the SCR catalyst. Figure 4 shows the steps of the monitoring method according to the invention in the form of a schematic flow chart. After the start of the SCR catalyst monitoring process, there is first a conditioning stage 41 for which first a predetermined operating point of the SCR catalyst is set with respect to the filling level NH3. SCR catalyst. Often the dosage is first changed to underdosing, for example with a = 0.5 to at least partially empty the NH 3 accumulator of the SCR catalyst and a filling level is established. determined. Then there is an overdose phase 42. During the overdose phase 42, step 43 is checked if NH3 slip is observed. It is checked whether the NOx sensor signal downstream of the SCR catalyst has exceeded a certain value. If so, it is concluded that there is slip NH3 and the catalyst will be considered defective in step 44. In a defined case, the overdose phase 42 is prematurely stopped, which considerably shortens the duration of the overdose. of the diagnosis, if the SCR catalyst is defective. This has the advantage, in the case of a defective SCR catalyst, that during diagnosis there has been less ammonia NH3 released in the NH3 slip compared to a conventional process. In addition, the reducing agent consumption for the diagnostic in the case of a defective SCR catalyst is considerably reduced in comparison with the usual monitoring method.

If during the check in step 43 it is found that there is no slip NH3, it is verified in step 45 whether the reduction agent dose intended to complete the phase has been reached. overdose 42 or the predefined NH3 fill level. If this is not the case, the overdose phase 42 is continued. If this is the case, the overdose phase is terminated and, in step 46, the characteristic values of the overdose phase are used. 42. For this, for example using the average conversion rate NOx during the overdose phase, it is checked whether the NH3 storage capacity of the SCR catalyst is reached. For this purpose, corresponding characteristic values or a corresponding characteristic value deduced from the operation of the SCR catalyst during the overdose phase is compared to appropriate thresholds in step 47. If, for example, the average conversion rate NOx of the catalyst SCR during the overdose phase is above a predefined threshold (NOx minimum conversion rate), so in step 48 it is concluded that the SCR catalyst is functioning properly. If, for example, the average NOx conversion rate is below the pre-defined threshold, then in step 49 it is concluded that the SCR catalyst is defective or is too old.

5 3035442 14 NOMENCLATURE OF MAIN ELEMENTS 10 Exhaust gas line 11 Internal combustion engine 5 12 SCR catalytic converter 13 Dosing unit 14 Reducing agent tank 15 Fuel pump 16 Pressure line 10 17 NOx sensor 18 Unit Operation 27 Emptying 28 Defined operating point 29 Filling the NH3 accumulator (filling test / 15 overdose phase) 30 Filling level NH3 37 Conditioning phase 38 Preset operating point 39 Overdose phase 20 41-49 a Stages of the flow chart of the process AP reduction agent dosing coefficient Operating point 25

Claims (5)

CLAIMS 1) Method for monitoring an SCR catalyst (12), in particular by monitoring the NH 3 storage capacity of the SCR catalyst (12), in which, for diagnostic purposes, a superstoichiometric determination is provided (39, 42) of reducing agent in the SCR catalyst (12) until a predetermined level of NH3 is attained or until a predetermined reducing agent dose (38, 45) is reached, characterized in that the reaction phase is terminated prematurely. over-stoichiometric determination (39, 42) as soon as NH3 slip (40) is concluded using augmented signals provided by a NOx sensor (17) installed downstream of the SCR catalyst. 2) Method according to claim 1, characterized in that in case of premature stopping of the stoichiometric dosage phase, the SCR catalyst is considered as defective (40). Process according to Claim 1, characterized in that prior to the over-stoichiometric metering step (39, 42) of the reducing agent, a conditioning phase (37, 41) is carried out to adjust an operating point. predetermined (38). Process according to Claim 1, characterized in that with the aid of at least one characteristic value which depends on the NOx conversion rate of the SCR catalyst (12) during the stoichiometric overdose phase, it is concluded that importance of the SCR catalyst storage capacity (12) for ammonia NH3. Process according to Claim 1, characterized in that with the aid of at least one characteristic value which depends on the NOx conversion rate of the SCR catalyst (12) during the stoichiometric overdose phase, the SCR catalyst, especially using the average conversion rate of NOx, and preferably it is verified (47) that one reaches a predefined threshold of the minimum conversion rate of NOx. Process according to Claim 4 or Claim 5, characterized in that to determine the characteristic value which depends on the NOx conversion rate of the SCR catalyst, the signals of a NOx sensor (17) installed downstream of the SCR catalyst (12). Method according to Claim 6, characterized in that, in order to determine the characteristic value, the signals of a NOx sensor (17) installed upstream of the SCR catalyst (12) and / or the data of FIG. a NOx gross emission model for NOx emissions upstream of the SCR catalyst (12). Method according to Claim 1, characterized in that for adjusting the predefined operating point (38) in the conditioning phase (41), a sub-stoichiometric determination of the reducing agent is carried out until the NOx conversion rate of the SCR catalyst (12) is below the predictable NOx conversion rate for a normal dosage of the reducing agent. 9) Computer program performing the steps of the method according to any one of claims 1 to 8, and memory medium containing the recording of this computer program as well as electronic apparatus for carrying out all the steps of the method . 30