DE102012211709A1 - Method for checking cross sensitivity of ammonia in SCR catalyst system of diesel engine of motor car, involves recognizing fault of nitrogen oxide sensor in predetermined period according to increase in temperature of catalyst - Google Patents

Abstract

The invention relates to a method for checking a NH ₃ -sensitive nitrogen oxide sensor, which is arranged in a catalyst system between two catalysts. This includes monitoring the temperature T of the first catalyst, and detecting the failure of the nitrogen oxide sensor when the concentration [NOx] detected by the nitrogen oxide sensor does not fall within a predetermined period after the temperature T of the first catalyst has increased by at least a predetermined value ΔT _min increases at least by a threshold Δ [NOx] _min .

Description

The present invention relates to a method for testing a NH 3 cross-sensitive nitrogen oxide sensor, which is arranged in a catalyst system between two catalysts. Furthermore, the present invention relates to a computer program that performs all the steps of the method according to the invention and runs in a computing device or controller. Finally, the present 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 is executed on a computing device.

State of the art

To meet the increasingly stringent legislation, paragraph (Euro6, Tier2Bin5 and further emission regulations), it is necessary, nitrogen oxides and nitrogen oxides _(NOx) in the exhaust of internal combustion engines, especially diesel engines to reduce. For this purpose, it is known to arrange an SCR catalytic converter (selective catalytic reduction) in the exhaust area of internal combustion engines, which reduces nitrogen oxides contained in the exhaust gas of the internal combustion engine in the presence of a reducing agent. As a result, the proportion of nitrogen oxides in the exhaust gas can be significantly reduced. At the end of the reduction, ammonia (NH ₃ ) is required, which is added to the exhaust gas. Therefore, NH ₃ or NH ₃ -sabspendende reagents are metered into the exhaust line. As a rule, an aqueous urea solution (HWL = urea water solution) is used for this, which is injected in the exhaust line upstream of the SCR catalytic converter. From this solution forms ammonia, which acts as a reducing agent. A 32.5% aqueous urea solution is commercially available under the trade name AdBlue ^®. In order to achieve high conversion rates of the nitrogen oxides to be reduced in an SCR catalyst system, the SCR catalyst must be operated so that it is constantly filled to a certain level with the reducing agent ammonia. The DE 10 2004 031 624 A1 describes, for example, how such a process for an SCR catalyst system based on the ammonia level can be built. The FR 2 872 544 A1 describes a temperature-dependent desired level specification.

Usually, the SCR catalyst is designed as a single component. However, an SCR catalyst system has also become known comprising a first SCR catalyst and a second SCR catalyst disposed downstream of the first SCR catalyst in an exhaust line. A metering device for metering in a reducing agent solution is arranged upstream of the first SCR catalytic converter in the exhaust gas line. The SCR catalyst system has no device for metering a reducing agent solution into the second SCR catalyst. By means of the at least one metering device, such a large amount of reducing agent solution is metered into the first SCR catalyst that NH ₃ slip occurs in the first SCR catalyst. In the second SCR catalyst, an SCR reaction is performed which reacts the reductant from the reductant slip of the first SCR catalyst with a nitrogen oxide. The first SCR catalyst is thus operated as a source of reducing agent.

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 on-board diagnosis (OBD) to ensure compliance with the OBD limit values, which are usually specified as a multiple of the statutory emission limit. A defective nitrogen oxide sensor should be detected from 2013 on exceeding the 1.5-fold nitrogen oxide limit (FTP75 standard). In addition, its suitability for monitoring the SCR catalyst must be monitored. If an error in a nitrogen oxide sensor is so pronounced that an aged SCR catalytic converter could no longer be detected, it must be detected and displayed. This requirement is called "monitoring capability" in US regulations. So far, however, is still no monitoring function for a nitrogen oxide sensor available, which is arranged between two SCR catalysts.

Disclosure of the invention

The method according to the invention serves to check a NH ₃ cross-sensitive nitrogen oxide sensor which is arranged in a catalyst system between two catalysts. It includes monitoring the temperature of the first catalyst, and detecting a failure of the nitrogen oxide sensor when the concentration detected by the nitrogen oxide sensor does not rise at least by a threshold in a first predetermined time period after a temperature increase of the first catalyst by at least a predetermined value.

The two catalysts are preferably independently selected from the group consisting of an SCR catalyst, a SCR on filter (SCRoF) catalyst, a barrier catalyst, an AMOX catalyst and a catalyst which promotes a reaction between NH ₃ and NO _x .

Is it after a normal catalytic converter operation to a strong warming of the first catalyst by at least the predetermined value, for example, after a strong acceleration of a motor vehicle or after a load change of its internal combustion engine, so the ammonia storage capacity of the first catalyst is reduced. This is to be expected with a significant increase in ammonia slip. Due to the cross-sensitivity of the nitrogen oxide sensor for ammonia, this increase in ammonia slip in an intact nitrogen oxide sensor manifests itself in an increase in the detected nitrogen oxide concentration by at least the threshold value. The threshold value can in particular be a predetermined value. Alternatively, it can also be determined from the signal of an ammonia sensor, which is arranged between the two catalysts. If in the first predetermined period after the rise of the temperature of the first catalyst by at least the predetermined value, the nitrogen oxide detected by the nitrogen oxide sensor does not rise at least by the threshold, it is particularly recognized that the basic functionality of the nitrogen oxide sensor is not present, ie that so-called "stuck in range" error is present.

In order to avoid erroneous fault detection when the first catalyst does not have an ammonia level optimized for nitrogen oxide conversion and an increase in the temperature of the first catalyst by at least a predetermined value would not result in an increase in ammonia concentration downstream of the first catalyst by at least the threshold preferred to suspend the error detection according to the invention when a received from an ammonia level model of the first catalyst ammonia level of the first catalyst falls below a predetermined minimum level.

Preferably, an error of the nitrogen oxide sensor is also detected if in a second predetermined period of time after the temperature of the first catalyst has increased by at least the predetermined value, the nitrogen oxide concentration detected by the nitrogen oxide sensor does not increase at least at a limiting speed. According to the invention, a limiting speed is understood to mean the quotient of a change in concentration and a change in time. The limit speed may in particular be a predetermined value. Alternatively, it can also be determined from the signal of an ammonia sensor, which is arranged between the two catalysts. If in the second predetermined time period after the rise of the temperature of the first catalyst by at least the predetermined value, the nitrogen oxide concentration detected by the nitrogen oxide sensor does not increase at least at the limit speed but has not been detected on a stuck in range error, it is particularly recognized that that the speed of the nitrogen oxide sensor is insufficient, d. H. that there is a so-called "slow response" error.

In order to be able to predict the increase in ammonia slip particularly accurately and thus to be able to select a particularly advantageous threshold value, it is preferable to increase the temperature of the first catalyst by an active intervention in the operating strategy of the catalyst system by at least the predetermined value. In order to reduce the influence of emissions, this active intervention can be used, for example, in a two-stage monitoring method, after a suspected fault has already been detected by passive observation of the nitrogen oxide sensor.

The computer program according to the invention can execute all steps of the method according to the invention when it runs on a computing device or control unit. This makes it possible to implement different embodiments of the method according to the invention in a catalyst system, without having to make any structural changes thereto. For this purpose, the computer program product according to the invention with program code, which is stored on a machine-readable carrier, can carry out the method according to the invention when the program is executed on a computer or control unit.

Brief description of the drawings

Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.

1 shows an SCR catalyst system in which a nitrogen oxide sensor can be checked by a method according to an embodiment of the invention.

2 provides a graph of the temperature profile of an SCRoF catalyst of the SCR catalyst system according to 1 the signal curve of a nitrogen oxide sensor in an embodiment of the method according to the invention.

Embodiments of the invention

1 shows a known SCR catalyst system, which is intended to be arranged in the exhaust system of a diesel-powered motor vehicle. The direction of the exhaust gas flow is in 1 marked with an arrow. In the exhaust line are three catalyst housing in the flow direction one behind the other 1 . 2 . 3 arranged. The first catalyst housing 1 at his entrance one Diesel Oxidation Catalyst (DOC) 11 , Downstream of the DOC 11 is in the case 1 a dosing module 12 arranged, which is adapted to meter urea water solution into the exhaust gas. Downstream of this metering point is a first catalyst 13 arranged, which consists of a particulate filter with SCR coating (SCRoF). The second catalyst housing contains a second catalyst 21 which is an SCR catalyst 21 is. The third catalyst housing contains a clean-up catalyst 31 , Upstream of the first catalyst housing 1 is a first nitrogen oxide sensor 41 arranged. Other nitrogen oxide sensors 44 and 46 are between the SCRoF catalyst 13 and the SCR catalyst 21 and downstream of the clean-up catalyst 31 arranged. Between the SCRoF catalyst 13 and the SCR catalyst 21 is still an ammonia sensor 43 arranged. Between the DOC 11 and the SCRoF catalyst 13 , between the SCR catalyst 21 and the clean-up catalyst 31 and downstream of the clean-up catalyst 31 are three temperature sensors 42 . 45 . 47 arranged. A control unit 5 is with the catalyst housings 1 . 2 . 3 and with the sensors 41 . 42 . 43 . 44 . 45 . 46 . 47 connected (compounds not shown).

For checking the nitrogen oxide sensor 44 between the SCRoF catalyst 13 and the SCR catalyst 21 In one embodiment of the method according to the invention, the temperature of the first catalyst 13 by means of the temperature sensor 42 in the first catalytic converter housing 1 supervised. It is on a "stuck in range" error of the nitrogen oxide sensor 44 between the SCRoF catalyst 13 and the SCR catalyst 21 detected when in a first predetermined period t ₁ + t ₂ after an increase in the temperature T of the SCRoF catalyst 13 by at least a predetermined value ΔT _min that of the nitrogen oxide sensor 44 detected concentration c does not rise at least by a threshold Δ [NOx] _min . If there is no "stuck in range" error, a "slow response" error of the nitrogen oxide sensor is detected 44 detected when in a second predetermined period t ₂ after the rise in the temperature T of the SCRoF catalyst 13 by at least the predetermined value, the concentration detected by the nitrogen oxide sensor does not rise at least at a limiting speed v _min . The threshold Δ [NOx] _min and the limit speed v _min may be predetermined values. In another embodiment of the method according to the invention, these two values are obtained from the signal of the ammonia sensor 43 determined between the SCRoF catalyst 13 and the SCR catalyst 21 is arranged.

2 shows the time course of the temperature of the SCRoF catalyst 13 , In a time period t ₀ , a temperature rise .DELTA.T occurs, which exceeds the predetermined value .DELTA.T _min . That of an intact nitric oxide sensor 44 Measured concentration [NOx] a increases with a time delay with respect to the temperature rise in a period t ₁ at high speed and reaches its maximum in a period t ₂ . The speed v can be determined according to formula 1 and exceeds the limit speed v _min :

The measured change Δ [NOx] a of the concentration due to the cross-sensitivity of the sensor 44 for ammonia exceeds the threshold Δ [NOx] _min . Consequently, in the method according to the invention no error of the nitrogen oxide sensor 44 detected.

The from a defective nitric oxide sensor 44 Measured concentration [NOx] b does not rise significantly in the period t ₁ nor in the period t ₂ after the temperature rise. Consequently, in the method according to the invention, a "stuck in range" error of the nitrogen oxide sensor 44 between the SCRoF catalyst 13 and the SCR catalyst 21 recognized.

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 102004031624 A1 [0002]
• FR 2872544 A1 [0002]

Claims (10)

Method for testing an NH ₃ cross-sensitive nitrogen oxide sensor ( 44 ), which in a catalyst system between two catalysts ( 13 . 21 ), comprising - monitoring the temperature T of the first catalyst ( 13 ), and - detecting a defect in the nitrogen oxide sensor ( 44 ) in a first predetermined time interval t ₁ + t ₂ after a rise in the temperature T of the first catalyst ( 13 ) by at least a predetermined value ΔT _min, the concentration [NOx] detected by the nitrogen oxide sensor does not increase by at least a threshold value Δ [NOx] _min . Process according to claim 1, characterized in that the first catalyst ( 13 ) and the second catalyst ( 21 ) are independently selected from the group consisting of an SCR catalyst, an SCRoF catalyst, a barrier catalyst, an AMOX catalyst and a catalyst which promotes a reaction between NH ₃ and NO _x . A method according to claim 1 or 2, characterized in that the threshold value Δ [NOx] _{min is} a predetermined value. A method according to claim 1 or 2, characterized in that the threshold value Δ [NOx] _min from the signal of an ammonia sensor ( 43 ) between the two catalysts ( 13 . 21 ) is arranged. Method according to one of claims 1 to 4, characterized in that an error of the nitrogen oxide sensor ( 44 ) is detected when in a second predetermined period t ₂ after the rise in the temperature T of the first catalyst ( 13 ) by at least the predetermined value .DELTA.T that of the nitrogen oxide sensor ( 44 ) detected concentration [NOx] does not increase at least at a limit speed v _min . A method according to claim 5, characterized in that the limit speed v _{min is} a predetermined speed. A method according to claim 5, characterized in that the limit speed v _min from the signal of an ammonia sensor ( 43 ) between the two catalysts ( 13 . 21 ) is arranged. Method according to one of claims 1 to 3, characterized in that the temperature T of the first catalyst ( 13 ) is increased by an active intervention in the operating strategy of the catalyst system by at least the predetermined value .DELTA.T _min . A computer program which performs all the steps of a method according to one of claims 1 to 8 when it is stored on a computing device or control device ( 5 ) expires. Computer program product with program code, which is stored on a machine-readable carrier, for carrying out the method according to one of claims 1 to 8, when the program is stored on a computer or control unit ( 5 ) is performed.

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