DE102012220151A1 - Method for checking SCR catalysts in exhaust gas system of internal combustion engine of motor car, involves altering size of ammonia sensor to output signal to ammonium cross-sensitive sensor, which is attenuated below threshold level - Google Patents

Abstract

The method involves detecting operability of an ammonia sensor (43) and an ammonium cross-sensitive sensor (44), which is arranged between two SCR catalysts (13, 21). Size of an internal combustion engine (6) is alternated after-treatment of an exhaust gas system. Nitrogen oxide concentration of exhaust gas is determined. Ammonium storage capacity of the first SCR catalyst is recognized. Size of the ammonia sensor is altered to output signal to the ammonium cross-sensitive sensor, which is attenuated below threshold level. Independent claims are also included for the following: (1) a computer program comprising a set of instruction for detecting catalyst (2) a computer program product has a set of instruction for detecting catalyst.

Description

The present invention relates to a method of verifying a first catalyst disposed in an exhaust aftertreatment system upstream of a second catalyst to which an exhaust gas emitted from an internal combustion engine is supplied. In addition, the invention relates to a method for checking the second catalyst in this exhaust aftertreatment system. Furthermore, the present invention relates to a computer program that performs all the steps of a method according to the invention, when it 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 performing a 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.

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.

The occurrence of ammonia slip downstream of the SCR catalyst must be avoided as much as possible because ammonia in high concentration has a deleterious effect. Several known concepts therefore serve to monitor catalyst systems which usually have an SCR catalyst or space-constrained SCR catalyst volume distributed over two housings. Nevertheless, in these applications, the two catalysts for process control and monitoring are considered largely as a single catalyst.

Currently known and used in series monitoring functions determine the efficiency of nitrogen oxide reduction (NOx conversion rate) using a nitrogen oxide sensor downstream of the SCR catalyst and a nitrogen oxide sensor upstream of the SCR catalyst, which is optionally replaced by a model-based substitute value. Due to the aging of the SCR catalyst, the achievable conversion rate decreases with increasing use time and the nitrogen oxide emission downstream of the SCR catalyst increases. From the maximum permissible nitrogen oxide emissions, a threshold for the efficiency can be determined, below which a system error is reported.

The current procedure is that the nitrogen oxide emissions upstream and downstream of the SCR catalyst are integrated in the presence of definable monitoring conditions over a predeterminable period of time and from the thus determined nitrogen oxide masses the achieved SCR efficiency is formed as a comparison value. The comparison of the SCR efficiency with the definable threshold serves to classify the SCR catalyst as still in order or as defective. In the monitoring function is usually provided during periods of unsteady driving the integration not perform or interrupt, so that only during stationary driving conditions without dynamically fluctuating nitrogen oxide signal upstream of the SCR catalyst, a comparison value is determined.

One from the DE 10 2007 040 439 A1 Known monitoring strategy determines the ammonia storage capability of the SCR catalyst as a feature for aging or damage to the catalyst. In this strategy, the SCR catalyst is first by superstoichiometric reductant dosage up to the maximum achievable Ammonia storage capacity filled with reducing agent to achieve a defined starting point for the diagnosis. Achieving the maximum storage capacity is detected by the breakthrough of ammonia downstream of the SCR catalyst, which is indirectly measurable due to a cross sensitivity of the nitrogen oxide sensor for ammonia. Subsequently, the reducing agent dosage compared to the normal dosage is reduced or eliminated, so that the stored ammonia mass is gradually degraded by nitrogen oxide reduction again (so-called emptying test). By determining the SCR efficiency or other parameters dependent on the rate of nitrogen oxide conversion during the discharge test, the usable ammonia storage capacity can be determined indirectly, since with lower ammonia mass stored, a lower nitrogen oxide mass can be converted at the catalyst surface.

In order to achieve a better separation between the BPU ("best part unacceptable") and the WPA ("worst part acceptable") of the SCR catalyst system, plausibility functions are often only carried out under certain monitoring conditions. In the field of exhaust gas aftertreatment for this monitoring is often limited to certain ranges of values for one or more modeled or measured sizes. These variables are, for example, the exhaust gas mass flow, the exhaust gas volume flow, the exhaust gas temperature at a specific point, the operating point (rotational speed, injection quantity), the vehicle speed, the ambient pressure, the ambient temperature, the signals from sensors for determining the nitrogen oxide, hydrocarbon , Carbon monoxide or oxygen concentration, or particulate mass in the exhaust, the exhaust gas recirculation rate (EGR), engine operating mode, engine status, engine run time, or engine life. Additionally, values are often used in the diagnosis of an SCR catalyst, such as the status of the nitrogen oxide sensors of the exhaust aftertreatment system, actual and desired level of the reducing agent ammonia in the catalyst, control deviations of the ammonia level controller, the status of the reducing agent metering device, the status / mode of the metered quantity precontrol, an adaptation factor, i. H. a reductant metering amount correction factor, metered dose adaptation status, particulate filter regeneration status, particulate filter regeneration request number, or hydrocarbon poisoning status of the SCR catalyst. In addition, monitoring for the same reason is often performed under (quasi) steady state conditions determined by one or more of the above parameters.

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.

In SCR catalyst systems with two SCR catalysts, catalyst monitoring by integrating nitrogen oxide emission upstream and downstream of the SCR catalysts has the weakness of having a low concentration of nitrogen oxide when applied to a second SCR catalyst mounted downstream of a first SCR catalyst is achieved in the second SCR catalyst. Currently applicable nitrogen oxide sensors have a constant tolerance in the lower concentration range, such as up to 100 ppm, so that at relatively low concentrations the relative tolerance increases. The smaller the nitrogen oxide concentration upstream of the second SCR catalyst, the more uncertain the diagnostic result becomes. In addition, the adulteration by the ammonia slip of the first SCR catalyst is relatively larger at smaller nitrogen oxide concentrations.

The use of the surveillance strategy according to DE 10 2007 040 439 A1 represents a possibility to secure a passively calculated monitoring result of the second SCR catalyst. As a result of the active intervention and the evaluation of the storage capability, a higher accuracy and robustness of the monitoring result can be achieved. The control of the level of the second SCR catalyst by the first SCR catalyst, however, brings some difficulties. Whether it succeeds in a test cycle, such as FTP or Carb Unified cycle, the second SCR catalyst thus sufficiently safe to diagnose, is not yet clarified.

Disclosure of the invention

The method of verifying a first SCR catalyst disposed in an exhaust aftertreatment system upstream of a second SCR catalyst to which an exhaust gas emitted from an internal combustion engine is supplied includes checking the operability of an ammonia sensor or an ammonia cross-sensitive sensor interposed between the exhaust gas aftertreatment systems two SCR catalysts is arranged, the change of an operating variable of the internal combustion engine and / or the exhaust aftertreatment system, which influences the nitrogen oxide concentration of the exhaust gas and the detection of insufficient ammonia storage capacity of the first SCR catalyst, when the operability of the disposed between the two SCR catalysts ammonia sensor or ammonia cross-sensitive sensor has been detected and the change in the operating variable leads to a change in the signal of this ammonia sensor or ammonia cross-sensitive sensor, which a Dämpfun g falls below a predetermined first threshold. The inventive method for checking the second SCR catalyst disposed in the exhaust aftertreatment system downstream of the first SCR catalyst to which an exhaust gas emitted from the internal combustion engine is supplied includes checking the operability of the first SCR catalyst, checking the operability of a NOx sensor disposed downstream of the second SCR catalyst, the change of an operation amount of the internal combustion engine and / or the exhaust aftertreatment system, which influences the nitrogen oxide concentration of the exhaust gas and the detection of insufficient ammonia storage capacity of the second SCR catalyst, when the operability of the downstream of the second SCR catalyst arranged NOx sensor was detected and the change in the operating variable leads to a change in the signal of this NOx sensor, which is subject to a damping, which is a predetermined second S below the threshold value. The checking of the functionality of the first SCR catalyst is preferably carried out by the method according to the invention.

The change of the operating quantity may be a single stimulus or a periodic change.

Each of the two SCR catalysts may be a conventional SCR catalyst or an SCR on Filter (SCRoF) catalyst.

The change in the operating quantity leads to a change in the nitrogen oxide emissions in the exhaust gas downstream of the first SCR catalyst. This allows the ammonia storage capability of the first SCR catalyst to be checked by determining whether the change in operating magnitude in the ammonia sensor or ammonia cross-sensor signal is reflected between the two SCR catalysts and is subject to attenuation. When the ammonia storage capability of the first SCR catalyst has been determined, a check of the ammonia storage capability of the second SCR catalyst is possible by determining whether the change in the amount of operation is reflected in the signal from the NOx sensor downstream of the second SCR catalyst. Although the capacity of the first SCR catalyst for nitrogen oxide reduction is disturbed by the change in the operating size, this does not affect the nitrogen oxide emission downstream of the second SCR catalyst, as may be generated between the two SCR catalysts peaks in nitrogen oxide concentration from the second SCR Catalyst to be intercepted. The method according to the invention thus has no significant influence on the composition of the exhaust gas leaving the exhaust aftertreatment system.

In the real driving operation of a motor vehicle, there are always short periods of time in which short metering pauses of a reducing agent in the exhaust aftertreatment system occur. In a preferred embodiment of the invention, the operating quantity is therefore a reducing agent metering into the exhaust gas aftertreatment system. As a result, in particular during these pauses, a short sequence of several dosing pulses of the same size or equally spaced in time can be discontinued with a predetermined frequency.

In another preferred embodiment of the invention, the operating variable is an air supply and / or exhaust gas feed into the internal combustion engine. This exploits that, for example, by suitable control of an exhaust gas recirculation valve, a throttle or an exhaust valve of the internal combustion engine in certain operating points of the internal combustion engine, for example, at idle, nitrogen oxide peaks can be generated in the exhaust gas at a predetermined frequency.

In yet another preferred embodiment of the invention, the operating quantity is a post-injection. By discontinuing post-injection, for example in a particle filter regeneration nitrogen oxide peaks can be generated at the same frequency in certain operating points.

According to the invention, various options are available for checking whether the change in the operating variable leads to a change in the operating quantity Signal of the ammonia sensor or the NH ₃ -sensitive sensor or the NOx sensor leads. In a preferred embodiment of the invention, the change in the operating variable takes place periodically with a predetermined frequency and the signal of the sensor is analyzed with a bandpass filter. If, with appropriate excitation, a signal of the ammonia sensor or of the NH ₃ cross-sensitive sensor at the predetermined frequency is determined, which is subject to sufficient damping, a sufficient ammonia storage capability of the SCR catalytic converter is provided. In another preferred embodiment of the invention, a frequency modulation of a periodic change of the operating variable takes place and the signal of the ammonia sensor or of the NH ₃ cross-sensitive sensor is subjected to a Fourier transformation. This makes it possible to impose different sequences of identical frequencies on the time course of the nitrogen oxide concentration in the exhaust gas. A result of the decomposition of the signal of the sensor by a Fourier transform or Fourier series is an amplitude spectrum which contains all the frequencies which are located in the signal of the ammonia sensor or of the NH ₃ cross-sensitive sensor. If the amplitude spectrum contains the frequency of the periodic change of the operating variable and this is subject to a sufficient attenuation, a sufficient ammonia storage capacity of the investigated SCR catalytic converter is given.

The computer program according to the invention can execute all the steps of a method according to the invention when it runs on a computing device or control unit. This makes it possible to implement different embodiments of a method according to the invention in an exhaust aftertreatment 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 perform a method according to the invention when the program is executed on a computing device 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 connected to an internal combustion engine in which a nitrogen oxide sensor can be checked by a method according to an embodiment of the invention.

2 FIG. 2 is a graph showing the timing of sensor signals of the nitrogen oxides sensors downstream and upstream of a first SCR catalyst in the SCR catalyst system according to FIG 1 which is defective.

3 2 is a graph showing the timing of nitrogen oxide sensors upstream and downstream of the first SCR catalyst in the SCR catalyst system according to FIG 1 which is fully functional.

4 shows in a diagram the timing of nitrogen oxide sensors upstream of the SCR catalysts, downstream of the SCR catalysts and between the SCR catalysts of an SCR catalyst system according to 1 , wherein the second SCR catalyst is defective.

5 shows in a diagram the timing of nitrogen oxides sensors downstream and upstream of the SCR catalysts and between the SCR catalysts of the SCR catalyst system according to 1 with the second SCR catalyst functioning properly.

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 has a Diesel Oxidation Catalyst (DOC) at its entrance 11 on. 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 NH ₃ cross-sensitive nitric oxide sensor 41 arranged. Other NH ₃ -sensitive 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 (connections not shown). An internal combustion engine 6 is upstream of the first catalyst housing 1 arranged and introduces exhaust gas into this.

The inventive review of the SCRoF catalyst 13 of the SCR catalyst 21 by assessing the degree of damping of a change in an operating variable exploits the fact that only in one of the catalysts 13 . 21 bound ammonia can contribute to nitrogen oxide conversion. In addition, the storage capacity of the catalysts decreases 13 . 21 during their aging. For the SCRoF catalyst 13 , which is directly downstream of the dosing module 12 is located in the exhaust system, positive nitrogen oxide sensor signals are observed, since a complete adsorption of ammonia is no longer possible and the nitrogen oxide conversion can not completely eliminate this effect.

2 shows the waveform [NOx] 41 of the upstream of the SCRoF catalyst 13 arranged nitrogen oxide sensor and the sensor signal [NOx] 44 of the between the two catalysts 13 . 21 arranged nitrogen oxide sensor 44 , The positive signal excursion of the ammonia cross-sensitive nitric oxide sensor 44 based on gaseous ammonia, which via the SCRoF catalyst 13 is transported. For a defective SCRoF catalyst 13 this positive nitrogen oxide signal [NOx] 44 is strong, since there is a low ammonia adsorption and thus a nitrogen oxide conversion is less favored. 3 shows the waveforms of the two nitrogen oxides 41 . 44 in the same SCR catalyst system at the same engine operating points and the same Reduktionsmitteldosiermenge for a fully functional SCRoF catalyst 13 , The positive nitrogen oxide signal [NOx] 44 is less pronounced, since the adsorption of ammonia and the conversion of nitrogen oxides still work well enough here.

The SCR catalyst 21 can only via the SCRoF catalyst 13 operate. The SCRoF catalyst 13 serves as a buffer and allows only a "filtered" amount of ammonia from an ammonia dosing pulse. The nitrogen oxide sensor 46 downstream of the SCR catalyst 21 therefore indicates negative oxides of nitrogen [NOx] 46. Depending on how good the storage capacity of the SCR catalyst 21 is, is stored by the offered ammonia more or less in the catalyst and used in the aftermath or at the same time for the nitrogen oxide conversion. 4 shows the signal curves of the nitrogen oxide sensors 41 . 44 . 46 when the SCRoF catalyst 13 okay and the SCR catalyst 21 is defective. 5 shows the waveforms of these three nitrogen oxides 41 . 44 . 46 if both catalysts 13 . 21 work properly. In this case, the negative signal of [NOx] 46 is the downstream of the SCR catalyst 21 arranged nitrogen oxide sensor 46 more pronounced than in the case of the defective SCR catalyst 21 , Sufficient ammonia is provided to the SCR catalyst, as can be seen from the signal [NOx] 44, so that the amount of nitrogen oxide in accordance with the signal [NOx] 41 of the upstream of the SCRoF catalyst 13 arranged nitrogen oxide sensor 41 would have to be greatly reduced. Because in 4 compared to 5 reduced nitrogen oxide is the Ammoiaksticherfähigkeit the SCR catalyst 21 according to 4 insufficient.

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]
• DE 102007040439 A1 [0007, 0011]

Claims (10)

Method for verifying a first SCR catalyst ( 13 ) in an exhaust aftertreatment system upstream of a second SCR catalyst ( 21 ), to which one of an internal combustion engine ( 6 ) emitted exhaust gas, comprising - checking the functionality of an ammonia sensor ( 43 ) or a NH ₃ -sensitive sensor ( 44 ) between the two SCR catalysts ( 13 . 21 ), - change of an operating variable of the internal combustion engine ( 6 ) and / or the exhaust aftertreatment system, which influences the nitrogen oxide concentration of the exhaust gas, and - Recognizing an insufficient NH ₃ storage capability of the first SCR catalyst ( 13 ), if the operability of the between the two SCR catalysts ( 13 . 21 ) arranged ammonia sensor ( 43 ) or NH ₃ -sensitive sensor ( 44 ) and the change in the operating variable to a change in the signal of this ammonia sensor ( 43 ) or NH ₃ -sensitive sensor ( 44 ), which is subject to an attenuation that falls below a predetermined first threshold. Method for checking a second SCR catalyst ( 21 ) in an exhaust aftertreatment system downstream of a first SCR catalyst ( 13 ), to which one of an internal combustion engine ( 6 emitted exhaust gas, comprising - checking the operability of the first SCR catalyst ( 13 ), - checking the functionality of a NOx sensor ( 46 ) downstream of the second SCR catalyst ( 21 ), - change of an operating variable of the internal combustion engine ( 6 ) and / or the exhaust aftertreatment system, which influences the nitrogen oxide concentration of the exhaust gas, and - Recognizing an insufficient NH ₃ storage capability of the second SCR catalyst ( 13 ), if the operability of the downstream of the second SCR catalyst ( 21 ) arranged NOx sensor ( 46 ) and the change of the operating variable to a change of the signal of this NOx sensor ( 46 ), which is subject to an attenuation that falls below a predetermined second threshold. Method according to Claim 2, characterized in that the checking of the functionality of the first SCR catalytic converter ( 13 ) by means of the method according to claim 1. Method according to one of claims 1 to 3, characterized in that the operating variable is a Reduktionsmitteleindosierung in the exhaust aftertreatment system. Method according to one of claims 1 to 3, characterized in that the operating variable an air supply and / or exhaust gas supply into the internal combustion engine ( 6 ). Method according to one of claims 1 to 3, characterized in that the operating variable is a post-injection. Method according to one of claims 1 to 6, characterized in that there is a periodic change of an operating variable with a predetermined frequency and the signal of the sensor ( 43 . 44 . 46 ) is analyzed with a bandpass filter. Method according to one of claims 1 to 6, characterized in that a frequency modulation of a periodic change of the operating variable takes place and the signal of the sensor ( 43 . 44 . 46 ) is subjected to a Fourier transformation. 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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