DE102014209960A1 - Method and device for evaluating the functionality of an SCR catalytic converter in an exhaust system of a diesel engine - Google Patents

Abstract

The present invention relates to a method and a device for evaluating the functionality of an SCR catalytic converter in an exhaust system of a diesel engine, the exhaust system comprising a diesel particle filter (13) and an SCR catalytic converter (15) arranged downstream thereof for the selective catalytic reduction of in the SCR -Catalyst supplied exhaust gas contains nitrogen oxides. A method according to the invention has the following steps: measuring at least one temperature value (T2) at a position downstream of the SCR catalytic converter, carrying out a first comparison of the measured temperature value with a first reference value, the first reference value representing an operating state of the exhaust system with a functional SCR catalytic converter Performing a second comparison of the measured temperature value with a second reference value, the second reference value representing an operating state of the exhaust system with faulty SCR catalytic converter, and evaluating the functionality of the SCR catalytic converter based on the results of the first and the second comparison, the first Reference value and the second reference value are determined in a model-based manner, with this model-based determination the influence of a water dam contained in the exhaust gas due to condensation occurring on the surface of the SCR catalytic converter (15) pf caused exothermic response to the temperature downstream of the SCR catalyst is estimated.

Description

The invention relates to a method and a device for evaluating the functionality of an SCR catalytic converter in an exhaust system of a diesel engine.

The introduction of increasingly stringent NO _x limits has led to the development of a variety of exhaust aftertreatment technologies to control NO _x emissions (= nitrogen oxide emissions) in the exhaust of a diesel engine. One of these solutions is the so-called NO _x storage catalytic converter whose operating principle is based on first storing nitrogen oxides (NO _x ) under lean exhaust gas conditions and then converting them into a regeneration phase while adjusting a rich, reducing exhaust gas mixture. A second technology is the selective catalytic reduction in a so-called SCR catalyst (SCR = "Selective Catalytic Reduction"), wherein the nitrogen oxides contained in the exhaust gas are reduced to nitrogen (N ₂ ) on the SCR catalyst by means of ammonia , Moreover, the SCR catalyst is capable of storing, for example, the ammonia fed directly to the exhaust gas stream upstream of the SCR catalyst at low exhaust gas temperatures.

In the case of the use of an SCR catalyst in engine-side or bottom-side construction in the exhaust system advantageously the aging of such an SCR catalyst in the remote engine position is relatively limited, since the SCR catalyst then relative to about the NO _x storage catalyst or compared to a SDPF system (comprising an SCR washcoat on a diesel particulate filter substrate) is less exposed to high temperatures due to the hot exhaust gas flow. However, in order to comply with emission regulations, it is necessary to monitor the functionality or degradation of the SCR catalytic converter.

An exemplary application of an exhaust system in which the monitoring of an SCR catalyst in motor further or bottom-side construction to meet the emission regulations is needed is an arrangement with a NO _x storage catalytic converter (close to the engine construction, in conjunction with a passive SCR catalyst as pSCR catalyst) in bottom-side or motor-distant construction. 3 shows a diagram illustrating the advantage or the need for monitoring the SCR catalyst in this application. This is in 3 the percentage of nitrogen oxide (NO _x ) conversion is applied to the nitrogen oxide catalyst (LNT) as a function of the amount of nitrogen oxide in the supplied exhaust gas, for different NO _x removal rates at the location of the SCR catalyst in the range of 10-30 mg / km. How out 3 As can be seen, increasing the NO _x removal rate at the location of the SCR catalyst to 30 mg / km combined with monitoring of the SCR catalyst enables a more robust diagnostics of the close coupled engine NO _x storage catalyst using two lambda sensors taking into account the realizable (in 3 also indicated and marked with "W") monitoring window.

In the case of application of a so-called SDPF (ie a diesel particulate filter substrate with SCR washcoat coating thereon), the presence of an SCR catalyst in engine-side or bottom-side design downstream of the SDPF also results in more robust compliance with emission regulations and reduction of ammonia slip. 4 shows a further diagram in which the percentage rate of nitrogen oxide (NO _x ) conversion is plotted as a function of the amount of nitrogen oxide in the supplied exhaust gas to illustrate the advantage or the need for monitoring the SCR catalyst in this application. In particular, according to 4 reduces the threshold required for on-board diagnostics (OBD) on SDPF. For example, with an amount of nitrogen oxides (NO _x ) in the supplied exhaust gas of about 210 mg / km, an additional SCR catalyst with a NO _x removal rate of 20 mg / km leads to a decrease in the SDDF component of 60 at the OBD threshold % to 45%, which increases robustness in monitoring the SDPF component.

Conventional approaches for detecting or detecting a failure of an SCR catalyst based z. B. on the thermal inertia of the SCR catalyst, but the realization of a robust detection behavior in on-board diagnostics in practice often proves to be problematic.

Out JP 2010-180847 A Among others, there is known a failure diagnostic apparatus in an exhaust emission control system in which the deterioration degree of the state of an SCR catalyst is determined based on the amount of moisture that can be adsorbed by the SCR catalyst. Specifically, first, the condensed water remaining in the exhaust system is removed, and said deterioration degree is diagnosed based on the correlation between the amount of moisture removed and the temperature in the catalyst bed of the SCR catalyst.

Out EP 1 052 385 B1 is, inter alia, a method for diagnosing a hydrocarbon oxidizing properties exhibiting three-way catalyst in which the temperature profile of the exhaust gas downstream of this three-way catalytic converter is monitored and in the case of a temperature increase remaining below a threshold value, the three-way catalytic converter is diagnosed as defective.

It is an object of the present invention to provide a method and apparatus for evaluating the performance of an SCR catalyst in an exhaust system of a diesel engine, which provides a robust distinction between an operating condition of the exhaust system with a functional SCR catalyst and an operating condition of the exhaust system with faulty SCR -Catalyst allow.

This object is achieved by the method according to the features of the independent patent claim 1 and the device according to the independent claim 9.

• – Messen wenigstens eines Temperaturwertes an einer Position stromabwärts des SCR-Katalysators;
• – Durchführen eines ersten Vergleichs des gemessenen Temperaturwertes mit einem ersten Referenzwert, wobei der erste Referenzwert einen Betriebszustand des Abgassystems mit funktionsfähigem SCR-Katalysator repräsentiert;
• – Durchführen eines zweiten Vergleichs des gemessenen Temperaturwertes mit einem zweiten Referenzwert, wobei der zweite Referenzwert einen Betriebszustand des Abgassystems mit fehlerhaften SCR-Katalysator repräsentiert; und
• – Bewerten der Funktionsfähigkeit des SCR-Katalysators auf Basis der Ergebnisse des ersten und des zweiten Vergleichs;
• – wobei der erste Referenzwert und der zweite Referenzwert modellgestützt ermittelt werden, wobei bei dieser modellgestützten Ermittlung der Einfluss einer durch eine an der Oberfläche des SCR-Katalysators stattfindende Kondensation von im Abgas enthaltenem Wasserdampf bewirkten exothermen Reaktion auf die Temperatur stromabwärts des SCR-Katalysators abgeschätzt wird.

An inventive method for evaluating the operability of an SCR catalyst in an exhaust system of a diesel engine, wherein the exhaust system comprises a diesel particulate filter and a downstream thereof arranged SCR catalyst for the selective catalytic reduction of nitrogen contained in the exhaust gas supplied to the SCR catalyst, comprises the following steps :

• Measuring at least one temperature value at a position downstream of the SCR catalyst;
• - performing a first comparison of the measured temperature value with a first reference value, wherein the first reference value represents an operating state of the exhaust system with functional SCR catalytic converter;
• - performing a second comparison of the measured temperature value with a second reference value, the second reference value representing an operating state of the exhaust system with faulty SCR catalytic converter; and
• Evaluating the functionality of the SCR catalyst based on the results of the first and second comparisons;
• Wherein the first reference value and the second reference value are determined model-based, wherein in this model-based determination of the influence of a taking place on the surface of the SCR catalyst condensation of water vapor contained in the exhaust gas exothermic reaction is estimated to the temperature downstream of the SCR catalyst ,

In particular, the present invention is based on the concept of utilizing a SCR catalyst washcoat coating to adsorb water vapor from the exhaust gas during a cold start phase to monitor an SCR catalyst, with desorption of the water at a later stage in the desorption process Test cycle follows. The condensation of the water vapor from the exhaust gas on the cold catalyst surface leads to a pronounced exothermic reaction, which is associated with an elevated temperature in the exhaust system (in particular downstream of the SCR catalyst). This circumstance is utilized according to the invention in order to obtain information about the operating state of the exhaust gas system or the functionality of the SCR catalytic converter.

According to one embodiment, the model-based determination of the first reference value takes into account a washcoat coating present on the SCR catalytic converter.

According to one embodiment, the model-based determination of the second reference value does not take into account any washcoat coating on the SCR catalyst.

According to one embodiment, the model-based determination of the first and the second reference value takes place on the basis of an estimate of the water vapor concentration in the exhaust gas.

According to one embodiment, the model-based determination of the first and the second reference value respectively estimates the rate of water condensation on the SCR catalytic converter and the heat release.

According to one embodiment, the step of evaluating the operability of the SCR catalyst each comprises determining the mean square deviation for the first and the second comparison within a predetermined time window.

According to one embodiment, the measuring of the at least one temperature value takes place immediately after a cold start of the diesel engine.

According to one embodiment, the method according to the invention further comprises the following steps: measuring at least one temperature value at a position upstream of the SCR catalytic converter, the evaluation of the functionality of the SCR catalytic converter also taking place taking into account this second temperature value.

The invention further relates to an apparatus for evaluating the operability of an SCR catalyst in an exhaust system of a diesel engine, the apparatus being configured to carry out a method having the features described above.

Further embodiments of the invention are described in the description and the dependent claims.

The invention will be explained below with reference to preferred embodiments with reference to the accompanying drawings.

Show it:

1 a schematic representation of the possible structure of an exhaust system in which the invention is feasible;

2 a diagram of an exemplary temperature behavior in an exhaust system according to the invention for different vehicles for explaining the concept underlying the invention; and

3 - 4 Diagrams illustrating the advantages of monitoring the functionality of an SCR catalytic converter in an exhaust system.

1 shows only a schematic representation of a possible construction of an exhaust system of a diesel engine 11 with a NO _x storage catalytic converter (LNT) 12 and an SCR catalyst 15 , According to 1 is also in combination with the NO _x storage catalytic converter (LNT) 12 a diesel particulate filter (DPF) 13 provided, which together in one unit 14 are arranged. The SCR catalyst 15 is downstream of the NO _x storage catalytic converter (LNT) 12 and Diesel Particulate Filter (DPF) 13 is also referred to as a passive SCR system, "pSCR" for short, and is used, among other things, for the purpose of the NO _x storage catalytic converter (LNT) 12 to store ammonia (NH ₃ ) released in rich operating phases. That at the SCR catalyst 15 stored ammonia can be used to convert additional nitrogen oxides (NO _x ) containing the NO _x storage catalyst (LNT) 12 when operating with lean exhaust gas mixture (ie under "lean operating conditions") can break.

The inventive concept enables a robust distinction between an operating state of the exhaust system with functional SCR catalyst 15 and an operating state of the exhaust system with faulty SCR catalyst 15 (Especially without on the SCR catalyst 15 located washcoat coating). In this case, a cold start of the internal combustion engine can be used as the initial state, wherein a measurement of the temperature upstream and downstream of the SCR catalyst 15 he follows. According to 1 is the temperature upstream of the SCR catalyst 15 denoted by T ₁ , and the temperature downstream of the SCR catalyst 15 T ₂ is designated.

In accordance with the invention, to monitor the operability of the SCR catalyst 15 in particular the tendency of the washcoat coating of the SCR catalyst 15 utilized in a cold start phase to adsorb water vapor from the exhaust gas, which then follows in a later phase of operation (or a later stage of the test cycle) on this adsorption desorption of the previously condensed water (so that the water molecules in this later phase, the catalyst surface of the SCR catalyst 15 leave again).

The above-described adsorption or condensation of the water vapor from the exhaust gas on the cold catalyst surface causes an exothermic reaction, which in turn at an elevated temperature downstream of the SCR catalyst 15 accompanied. This circumstance is utilized according to the invention in order to obtain information about the operating state of the exhaust gas system or the functionality of the SCR catalytic converter.

The temperature behavior described above is in 2 for a plurality of different vehicles ("A" to "C") of different size and weight, wherein the curves A1, B1, and C1 respectively represent the temperature value upstream of the SCR catalyst 15 and curves A2, B2, and C2, respectively, are the temperature value downstream of the SCR catalyst 15 describe. In the area marked with the arrow P1, heat is released due to the condensation of the water vapor on the catalyst surface of the SCR catalyst 15 and the associated exothermic reaction. In the area marked with the arrow P2, a shift of the temperature rise edge takes place in the diagram of FIG 2 to the right, indicating the desorption or evaporation of the previously condensed water from the catalyst surface of the SCR catalyst 15 and related energy needs.

According to the invention, the distinction between the operating state of the exhaust system with functional SCR catalyst 15 and an operating state of the exhaust system with faulty SCR catalyst 15 using two reference models, one of which is a reference model for the operating state of the exhaust system with a functional SCR catalytic converter 15 and the other reference model for the operating state of the exhaust system with faulty SCR catalyst 15 is valid. In particular, in the position downstream of the SCR catalyst 15 measured temperature T ₂ each with the reference model for the operating state of the exhaust system with functional SCR catalyst 15 and with the reference model for the operating state of the exhaust system with faulty SCR catalyst 15 (or the resulting from these reference models temperature values) are compared, whereupon each of the mean square deviation (RMS) can be calculated. As a criterion for deciding whether an operating condition of the exhaust system with functional SCR catalyst 15 or an operating condition of the exhaust system with faulty SCR catalyst 15 For example, the minimum of this mean square deviation within a predetermined time window can be used for the monitoring.

In particular, an estimation of the water vapor concentration in the exhaust gas based on the air-fuel ratio and other parameters of the combustion process can take place here. In this case, the saturation partial pressure at the SCR catalyst 15 and calculate the catalyst temperature to determine the rate of water condensation and heat release. Based on this, an estimate of the temperature of the SCR catalyst 15 with and without washcoat coating (ie for the operating state of the exhaust system with functional SCR catalytic converter 15 and for the operating state of the exhaust system with faulty SCR catalyst 15 ) respectively.

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

• JP 2010-180847 A [0007]
• EP 1052385 B1 [0008]

Claims (9)

Method for evaluating the functionality of an SCR catalytic converter in an exhaust system of a diesel engine, the exhaust system comprising a diesel particulate filter ( 13 ) and a downstream thereof arranged SCR catalyst ( 15 ) for selective catalytic reduction in the SCR catalyst ( 15 ), characterized in that the method comprises the steps of measuring at least one temperature value (T2) at a position downstream of the SCR catalyst ( 15 ); Performing a first comparison of the measured temperature value (T2) with a first reference value for the temperature downstream of the SCR catalyst ( 15 ), wherein the first reference value an operating state of the exhaust system with functional SCR catalyst ( 15 represents; Performing a second comparison of the measured temperature value (T2) with a second reference value for the temperature downstream of the SCR catalyst ( 15 ), wherein the second reference value is an operating state of the exhaust system with faulty SCR catalytic converter ( 15 represents; and evaluating the functionality of the SCR catalyst ( 15 ) based on the results of the first and second comparisons; wherein the first reference value and the second reference value are determined model-based, wherein in this model-based determination of the influence of a on the surface of the SCR catalyst ( 15 ) condensation of water vapor contained in the exhaust caused exothermic reaction to the temperature downstream of the SCR catalyst ( 15 ) is estimated. A method according to claim 1, characterized in that in the model-based determination of the first reference value on the SCR catalyst ( 15 ) existing washcoat coating is taken into account. A method according to claim 1 or 2, characterized in that in the model-based determination of the second reference value no washcoat coating on the SCR catalyst ( 15 ) is taken into account. Method according to one of claims 1 to 3, characterized in that the model-based determination of the first and the second reference value is based on an estimate of the water vapor concentration in the exhaust gas. Method according to one of claims 1 to 4, characterized in that in the model-based determination of the first and the second reference value respectively the rate of water condensation on the SCR catalyst ( 15 ) and the heat release can be estimated. Method according to one of claims 1 to 5, characterized in that the step of evaluating the functionality of the SCR catalyst ( 15 ) each comprises determining the mean square deviation for the first and the second comparison within a predetermined time window. Method according to one of claims 1 to 6, characterized in that the measuring of the at least one temperature value immediately after a cold start of the diesel engine ( 11 ) he follows. Method according to one of claims 1 to 7, characterized in that the method further comprises the steps of measuring at least one temperature value (T1) at a position upstream of the SCR catalyst ( 15 ); wherein evaluating the functionality of the SCR catalyst ( 15 ) taking into account this second temperature value (T1). Device for evaluating the functionality of an SCR catalytic converter in an exhaust system of a diesel engine, wherein the exhaust system has a diesel particulate filter ( 13 ) and a downstream thereof arranged SCR catalyst ( 15 ) for selective catalytic reduction in the SCR catalyst ( 15 ), characterized in that the apparatus is configured to carry out a method according to any one of the preceding claims.

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