DE102019122663A1 - Method for monitoring an SCR catalyst - Google Patents

Abstract

The present invention relates to a method for monitoring an SCR catalyst (11) in an exhaust line (1) of a diesel internal combustion engine (2), wherein the exhaust line (1) at least one exhaust gas recirculation (12) with an exhaust gas recirculation valve (13) downstream of the SCR Catalytic converter (11), a NOx sensor (14) for measuring the NOx content in the exhaust gas upstream of the SCR catalyst (11) and a metering device (15) for injecting a reducing agent upstream of the SCR catalyst (11) a measured value of the NOx sensor (14) is compared with a mathematically determined model value of the NOx content in the exhaust gas upstream of the SCR catalytic converter (11).

Description

The present invention relates to a method for monitoring an SCR catalyst in an exhaust line of a diesel internal combustion engine, wherein the exhaust line at least one exhaust gas recirculation with an exhaust gas recirculation valve (low pressure exhaust gas recirculation system) downstream of the SCR catalyst, a NOx sensor for measuring the NOx content in the exhaust gas upstream of the SCR catalyst and a metering device for injecting a reducing agent upstream of the SCR catalyst.

STATE OF THE ART

In the course of the progressive tightening of the exhaust gas legislation for motor vehicles, the vehicle diagnostic systems (on-board diagnostics, OBD) are also subject to increasingly stringent regulations. With regard to the control of nitrogen oxide (NOx) emissions from diesel engines, it is necessary to monitor the functionality of each of the catalysts responsible for NOx reduction and, in the event of a malfunction, to accurately identify the defective catalyst.

The prior art uses catalysts based on the SCR (selective catalytic reduction) principle in which the nitrogen oxides are reduced to nitrogen in the presence of a reducing agent, typically ammonia (NH 3), deposited on the catalyst surface. The reducing agent is introduced upstream of the catalyst by means of a metering device in the exhaust gas. In particular, so-called SCRF catalysts (SCR on filter), which are particle filters with SCR surface coating, see DE 10 323 607 A1 , in combination with a further downstream underbody SCR catalyst used. The term "SCR catalyst" is used analogously below for any catalyst based on the SCR principle, ie in particular also for SCRF catalysts.

As a result of aging and wear processes, the SCR coating can be damaged or partially removed so that the NH3 storage capacity of the SCR catalyst and thus also the NOx conversion rate decrease. The catalyst is considered to be defective if the conversion rate is below a system-specific limit, which is usually derived from a model efficiency of the dosing strategy, which describes the NOx conversion expected at the respective operating point.

Diesel exhaust systems with modern SCR systems based on a SCRF catalytic converter and a downstream SCR catalytic converter in the underbody therefore have at least one NOx sensor downstream of the SCR catalytic converter for measuring the NOx content remaining in the exhaust gas Another NOx sensor immediately downstream of the SCRF catalyst is common. As the emissions legislation progressively stricter, the latter design, ie the use of separate NOx sensors behind each catalyst, will be suggested and accordingly scheduled by OEM equipment. Detailed descriptions of monitoring and testing strategies of SCRF or SCR catalysts using the downstream sensor and defined dosing strategies are found in, for example, the references DE 10 2012 201 749 A1 . DE 10 2015 221 945 A1 and DE 10 2013 203 580A1 ,

DISCLOSURE OF THE INVENTION

The object of the invention is to provide a simplified compared to the prior art method for monitoring and identification of a possible defect of an SCR catalyst in an exhaust system of a diesel internal combustion engine, wherein the considered exhaust system at least one exhaust gas recirculation with an exhaust gas recirculation valve downstream of the SCR catalyst and a NOx sensor upstream of the SCR catalyst. Such a sensor between internal combustion engine and catalyst system is installed as standard in any modern exhaust system and its measurements form the basis for the control of NH3 metering.

This object is achieved on the basis of a method according to the preamble of claim 1 with the characterizing features. Advantageous developments of the invention are specified in the dependent claims.

The invention includes the technical teaching that a measured value of the NOx sensor is compared with a mathematically determined model value of the NOx content in the exhaust gas upstream of the SCR catalytic converter.

The core idea of the invention is to dispense with a measurement of the NOx content remaining immediately downstream of the SCR catalytic converter, which advantageously makes it possible to dispense with the use of the additional NOx sensor customary in the prior art, and instead by means of the latter downstream of the internal combustion engine existing NOx sensor to close indirectly on the NOx content of the recirculated exhaust gas. With precise knowledge of the operating points of the internal combustion engine and the exhaust system, it is possible, based on computer-aided simulation a model value for the NOx content in the exhaust gas immediately behind the internal combustion engine ( Engine-out NOx, EO NOx). The input parameters for the simulation are in particular the vehicle operating data continuously collected by the engine control unit. The simulation, and thus the model value for EO NOx, is typically based on the assumption of a fully functional SCR catalytic converter. In the case of a defective SCR catalyst, an increased NO x content and excess NH 3 (NH 3 slip effect) not stored in the catalyst are returned to the internal combustion engine, causing an increase in EO NO x. Differences between the model value and the measured value for EO NOx can thus be interpreted as deviations in the functionality of the real catalytic converter, provided that all other variables influencing the NOx content of the exhaust gas are correctly recorded in the simulation. This creates a possibility to perform an online monitoring of the functionality of the SCR catalytic converter on the basis of the EO NOx measured values. Thus, in the method according to the invention, the NOx sensor downstream of the catalytic converter used in OBD methods of the prior art need not be accessed. This additional sensor is therefore obsolete and when using the method according to the invention, a correspondingly simpler constructed exhaust system can be used, whereby the operation of the exhaust system is potentially less prone to failure and also reduce the associated production costs.

In an advantageous embodiment of the method according to the invention, when the measured value of the NOx sensor is raised, it actively intervenes in the position of the exhaust gas recirculation valve and / or in the injection of the reducing agent through the metering device. As a result, optimal conditions can be set for measuring the NOx content in the exhaust gas and / or defined schemes can be carried out for functional testing of the SCR catalytic converter. The former can be realized, for example, by an exhaust gas recirculation valve that is as wide open as possible, which maximizes the proportion of recirculated exhaust gas to EO NOx. In analogy to those in the publications DE 10 2012 201 749 A1 . DE 10 2015 221 945 A1 and DE 10 2013 203 580 A1 For example, the NH3 storage capacity of the catalyst can be investigated, for example by selective overcharging and discharging.

In addition, in the method according to the invention in a plurality of operating points of the internal combustion engine and / or of the exhaust gas line, the respective measured values of the NOx sensor and the associated model values of the NOx content can be compared with one another. As a result, the method assumes the character of a continuous function monitoring, in which operating point-independent, and in particular also outside of the aforementioned, specifically conducted test schemes, a permanent comparison of the EO NOx measured values with associated model values takes place. For example, information obtained on a malfunction of the SCR catalyst can lead to the initialization of such a targeted test.

In a further embodiment, the difference between EO NOx measured value and model value is formed and compared with a limit value which, for example, describes a maximum tolerable deviation of the catalyst functionality from a nominal functionality. Such a limit value is preferably defined for each operating point of the internal combustion engine and exhaust system and / or a suitably standardized limit value is used. A secured overshoot of the limit, i. Exceeding the limit at a plurality of successive or within a defined time interval collected EO NOx measured values can then be interpreted as a malfunction of the catalyst and lead, for example, to the activation of the engine control lamp of the vehicle.

The invention further relates to a computer program which is adapted to carry out the steps of the method according to the invention when it is executed on an engine control unit.

In addition, the invention relates to a computer program product with program code, which is stored on a machine-readable carrier, for carrying out the method according to the invention, when the program code is executed on an engine control unit.

The invention further relates to a method for operating an exhaust system of a diesel internal combustion engine, wherein the exhaust system comprises at least a first SCR catalyst, in particular a SCRF catalyst, and a second, further downstream arranged SCR catalyst, and a first metering device for injection a reductant upstream of the first SCR catalyst and a second metering device upstream of the second SCR catalyst; and a first NOx sensor for measuring the NOx content in the exhaust gas upstream of the first SCR catalyst and a second NOx sensor downstream of the second SCR catalyst. Catalyst, and further exhaust gas recirculation with an exhaust gas recirculation valve downstream of the first SCR catalyst and upstream of the second SCR catalyst, wherein an inventive method for monitoring the first SCR catalyst according to one of the aforementioned embodiments is performed and an over monitoring of the second SCR catalyst is performed by means of the second NOx sensor, based on the monitoring Functional capabilities of the first SCR catalyst and the second SCR catalyst are determined and a metering strategy for injecting the reducing agent by means of the two metering devices is adapted to the determined functional capabilities. Such a method makes it possible to monitor the two catalysts independently of one another and to identify, for example, in the case of an exhaust-side increased NOx content in the exhaust gas, which catalyst has a defect or a corresponding impairment of its functionality. The dosing strategy can then be adapted to this finding so that the error-free working catalyst is more heavily loaded with reducing agent in order to compensate for the failure of the defective catalyst.

Furthermore, the invention relates to a control device, in particular an engine control unit, which is designed to carry out a method according to the invention for monitoring an SCR catalytic converter in an exhaust gas line of a diesel internal combustion engine. In particular, the control device is designed to execute the computer program according to the invention.

Finally, the invention relates to a control device, in particular an engine control unit, which is designed to carry out a method according to the invention for operating an exhaust gas line of a diesel internal combustion engine.

PREFERRED EMBODIMENT OF THE INVENTION

• 1 eine schematische Darstellung eines Abgasstrangs zur Durchführung des erfindungsgemäßen Verfahrens.

Further, measures improving the invention will be described in more detail below together with the description of a preferred embodiment of the invention with reference to a single figure. It shows the

• 1 a schematic representation of an exhaust system for carrying out the method according to the invention.

The 1 serves to illustrate the method of the invention with reference to a schematic representation of an exhaust system 1 according to the prior art in arrangement on a diesel internal combustion engine 2 , For exhaust gas purification includes the exhaust system 1 a catalyst system with the oxidation catalyst 16 for removal of carbon monoxide and hydrocarbons, the SCRF catalyst 11 for the combined filtering of fine dust particles and NOx reduction, and the SCR and ammonia slip catalyst usually arranged in the underbody of the vehicle 18 for further NOx and NH3 reduction. The reducing agent for the NOx purification is via the metering devices 15 and 20 upstream of the SCRF catalyst 11 or upstream of the SCR and ammonia slip catalyst 18 injected into the exhaust gas. To determine the NOx content in the exhaust gas along the exhaust line 1 are the NOx sensor 14 immediately downstream of the engine 2 , the NOx sensor 17 downstream of the SCRF catalyst 11 as well as the NOx sensor 19 downstream of the SCR and ammonia slip catalyst 18 arranged. The dashed line of the NOx sensor 17 should clarify that this in OBD method of the prior art for monitoring the SCRF catalyst 11 is common, but when using the method according to the invention as obsolete deleted.

The inventive method is based on the idea of the NOx content of the exhaust gas after flowing through the SCRF catalyst 11 by means of the exhaust gas recirculation 12 in the internal combustion engine 2 be determined by the reclassified share. The exhaust gas recirculation rate is determined by the position of the exhaust gas recirculation valve 13 determined and is in typical driving, for example, a diesel engine up to about 60%. In laboratory tests with a faulty SCRF catalyst 11 was at the NOx sensor 14 Operating point dependent up to 25% increased EO NOx content compared to a test with fully functional SCRF catalyst 11 measured.

The comparison of the measured value for EO NOx with an associated mathematically determined model value for checking the catalyst functionality is most meaningful in those operating points in which high exhaust gas recirculation rates are present and thus a significant proportion of EO NOx originates from the recirculated exhaust gas. This is typically the case in the partial and medium load range of driving operation, but alternatively, for the purpose of catalyst testing, it may also be active in the control of the exhaust gas recirculation valve 13 be intervened.

For the targeted diagnosis of the storage capacity of the SCRF catalyst 11 for the reducing agent, for example, a stoichiometric metering by means of the metering device 15 and then calculate the deviation from EO NOx measurement and model values. As soon as the deviation regularly exceeds a specified limit, there is a defect in the SCRF catalyst 11 go out.

With the method according to the invention can therefore be identified in particular, which of the two in the exhaust system 2 arranged SCR catalysts 11 and 18 has a malfunction or a defect. For example, with the exhaust-side NOx sensor 19 If an increased NOx content is detected, then the method according to the invention can be used to improve the functionality of the SCRF catalyst 11 to be checked separately. If, for example, a faultless operation of the SCRF catalyst 11 can be concluded that there is a defect in the underbody SCR catalyst 18 is present. In particular, the dosing strategy for reducing agent can also be adapted to such findings for the individual functionality of the two catalysts. For example, a reduction in NOx conversion rate on the SCRF catalyst 11 As determined, the engine control can increase the over the rear metering device 20 introduce introduced amount of reducing agent to a corresponding increase in NOx reduction in the SCR catalyst 18 to generate.

The invention is not limited in its execution to the above-mentioned preferred embodiment. Rather, a number of variants is conceivable, which makes use of the illustrated solution even with fundamentally different types of use. Any features and / or advantages resulting from the claims, the description or the drawing, including constructive details, spatial arrangements and method steps, can be essential to the invention, both individually and in the most diverse combinations.

LIST OF REFERENCE NUMBERS

1

exhaust gas line

2

Diesel engine

11

SCRF catalyst

12

Exhaust gas recirculation

13

Exhaust gas recirculation valve

14

NOx sensor

15

metering

16

oxidation catalyst

17

NOx sensor

18

SCR and ammonia slip catalyst

19

NOx sensor

20

metering

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 10323607 A1 [0003]
• DE 102012201749 A1 [0005, 0010]
• DE 102015221945 A1 [0005, 0010]
• DE 102013203580 A1 [0005, 0010]

Claims (10)

Method for monitoring an SCR catalytic converter (11) in an exhaust line (1) of a diesel internal combustion engine (2), wherein the exhaust line (1) has at least one exhaust gas recirculation (12) with an exhaust gas recirculation valve (13) downstream of the SCR catalytic converter (11) , a NOx sensor (14) for measuring the NOx content in the exhaust gas upstream of the SCR catalyst (11) and a metering device (15) for injecting a reducing agent upstream of the SCR catalyst (11), characterized in that a measured value of the NOx sensor (14) is compared with a mathematically determined model value of the NOx content in the exhaust gas upstream of the SCR catalytic converter (11). Method according to Claim 1 , characterized in that when raising the measured value of the NOx sensor (14) is actively intervened in the position of the exhaust gas recirculation valve (13) and / or in the injection of the reducing agent through the metering device (15). Method according to Claim 1 or 2 , characterized in that in a plurality of operating points of the internal combustion engine (2) and / or the exhaust line (1) the respective measured values of the NOx sensor (14) and the associated model values of the NOx content are compared. Method according to one of the preceding claims, characterized in that the difference between measured value and model value is formed and compared with a limit value. A computer program adapted to perform the steps of a method according to any one of the preceding claims when executed on an engine control unit. Computer program product with program code, which is stored on a machine-readable carrier, for performing a method according to one of Claims 1 to 4 when the program code is executed on an engine control unit. Method for operating an exhaust line (1) of a diesel internal combustion engine (2), wherein the exhaust line (1) has at least one first SCR catalytic converter (11) and a second SCR catalytic converter (18) arranged further downstream, and a first metering device ( 15) for injecting a reducing agent upstream of the first SCR catalyst (11) and a second metering device (20) upstream of the second SCR catalyst, and a first NOx sensor (14) for measuring the NOx content in the exhaust gas upstream of the first SCR And an exhaust gas recirculation (12) having an exhaust gas recirculation valve (13) downstream of the first SCR catalyst (11) and upstream of the second SCR catalyst (18), characterized in that a method for monitoring the first SCR catalyst (11) according to one of Claims 1 to 4 is carried out and a monitoring of the second SCR catalyst (18) by means of the second NOx sensor (19) is performed, wherein based on the monitoring functional capabilities of the first SCR catalyst (11) and the second SCR catalyst (18) are determined and a metering strategy for injecting the reducing agent by means of the metering devices (15, 20) is adapted to the determined functional capabilities. Control device, in particular engine control unit, for carrying out a method according to one of Claims 1 to 4 , Control device after Claim 8 to execute a computer program according to Claim 5 , Control device, in particular engine control unit, for carrying out a method according to Claim 7 ,

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