DE102013204686A1 - Method and device for determining dosing quantity deviations of an SCR catalyst - Google Patents

Abstract

In a method and a device for determining dosing quantity deviations of a reducing agent of an SCR catalyst system for the aftertreatment of exhaust gases from an internal combustion engine, it is provided in particular that data or signals representing the dosing quantity are recorded from at least two different measurement or signal sources (100) and by preferably weighted linear combination of the at least two recorded data or signals, a deviation in the metered quantity is determined (135).

Description

The present invention relates to a method and a device for determining dosing quantity deviations of an SCR catalytic converter for after-treatment of exhaust gases of an internal combustion engine. Further objects of the present invention are a computer program and a computer program product which are suitable for carrying out the method.

State of the art

From the DE 10 2010 028 891 A1 are known a method and an apparatus for operating an internal combustion engine, in the exhaust gas region of an SCR (Selective Catalytic Reduction) catalyst is arranged, which contained in the exhaust of the internal combustion engine nitrogen oxides (NO _x ) in the presence of a reducing agent (or reagent) to nitrogen reduced. As a result, the nitrogen oxides in the exhaust gas can be significantly reduced. It is preferred to reduce nitrogen oxides (NO, NO ₂ ), while undesirable side reactions are largely suppressed. For the course of the reaction ammonia (NH ₃ ) is required, which is admixed to the exhaust gas. The reducing agents used are generally NH ₃ -separating reagents. Often, aqueous urea solution is used.

For storing the reducing agent or the urea solution, a reducing agent tank is provided. The tank is usually equipped with a suction line to aspirate the urea solution from the tank. To promote the urea-water solution, a pump is provided which promotes the solution through a conduit system of the metering device, so that the urea solution can be injected as required via a metering valve under pressure into the exhaust gas line.

The metering device for metering the reducing agent into the exhaust system functions as a hydraulic system. Decisive for the dosage of the reducing agent is the reducing agent pressure, which is regulated to a predefinable target pressure. For example, the opening of the metering valve causes pressure fluctuations in the line system, which are compensated by a change in the speed of the feed pump motor.

In order to enable optimal exhaust aftertreatment, it is necessary that the reducing agent is injected very precisely and precisely as needed. Therefore, the metering valve via signals of a control device, for example, the control unit of the internal combustion engine, driven to meter a certain currently required amount of reducing agent in the exhaust system. Since the entire SCR catalytic converter system is an exhaust gas-relevant part, malfunctions, in particular quantity deviations of the dosed reductant due to disruptions of the metering valves, must be monitored or recognized and reported.

The legislator therefore demands in motor vehicles monitoring of an SCR catalyst system even during the current driving operation of a motor vehicle affected here (so-called "on-board diagnosis" (OBD)).

Like from the DE 10 2008 005 989 A1 As can be seen, such monitoring can take place by means of an evaluation of the pressure drop of the reducing agent carried out on a dosing valve. In this procedure, the pump is shut down and then the dosage continues during a diagnostic dosing operation while monitoring the pressure. In this case, the metered amount of reagent is determined and finished only when reaching a Dosiermengen threshold, the evaluation of the pressure drop.

According to the DE 10 2009 029 408 A1 the monitoring of an SCR catalyst system can also take place in that in a Rücksaugmodus the SCR catalyst system, the construction of a negative pressure is detected by means of a pressure sensor measured pressure values and in a deviation of the detected pressure value of an expected reference value to a malfunction of the function SCR catalyst system is closed.

It is also known to check the function of a said metering valve on the basis of the relative change in the volume flow upon actuation of the metering valve. For this purpose, for example, the speed of the feed pump is monitored during the activation of the metering valve and closed on an error when the control of the metering valve does not lead to the expected change in the pump speed, which characterizes the delivery rate of the pump.

In another known method, the current and expected level of the reducing agent tank is evaluated in the OBD.

• – Das Zeitfenster (bzw. der maximale Zeitraum), innerhalb dessen eine Störung des OBD, d.h. eine Mengenabweichung bzw. ein fehlerhafter Verbrauch an Reduktionsmittel, festgestellt werden muss, beträgt 48 h oder der einem Bedarf an Reduktionsmittel von wenigstens 15 Litern entsprechenden Zeit;
• – Geforderte Erkennung einer Mengenabweichung von 50% (bis zum Jahr 2015) bzw. 20% (ab 2016) zwischen dem durchschnittlichen Verbrauch an Reduktionsmittel und dem durchschnittlichen Bedarf an Reduktionsmittel der Brennkraftmaschine bzw. des Motorsystems.

The OBD also requires a so-called Consumption Deviation Monitoring (CDM) with the following requirements:

• The time window (or the maximum period of time) within which a malfunction of the OBD, ie a quantity deviation or a faulty consumption of reducing agent, must be determined, is 48 hours or the need for Reducing agent of at least 15 liters corresponding time;
• - Required detection of a quantity deviation of 50% (until 2015) or 20% (from 2016) between the average consumption of reducing agent and the average demand for reducing agent of the internal combustion engine or the engine system.

Disclosure of the invention

A disadvantage of the known diagnostic methods is that they insufficiently or not at all fulfill the mentioned OBD requirements due to the tolerance chain and measurement inaccuracies. In addition, these methods require a complex calibration and are therefore relatively expensive to implement. Furthermore, these methods lead to an intervention in the normal metering operation and, if necessary, thereby caused disturbances in the diagnosis.

An object of the present invention was therefore to provide an aforementioned method and apparatus for determining Dosiermengenabweichungen an SCR catalyst, with which the detection of disturbances or quantity deviations can be done with higher accuracy and thus the more stringent from 2016 stricter OBD requirements can be met in an SCR diagnostic system.

This object is solved by the features of the independent claims. Advantageous embodiments are formulated in the respective subclaims.

The inventive idea is to assess a possible quantity deviation not only on the basis of a single measurement on a single measuring or signal source (pressure sensor, tank level sensor, etc.), but on the basis of at least two different measuring or signal sources data or signals. By combining the data or signals acquired at the at least two measuring sources, the measuring error is reduced, as a result of which the quality of evaluation is improved.

The determination of the quantity deviation is preferably carried out by weighted linear combination with error minimization of the data acquired at the at least two different measurement sources. The differently recorded data are preferably total quantity deviation values.

By said linear combination, i. The combination of such individual measurements supplied to a linear estimator makes it possible to significantly increase the quality of assessment of quantity deviations in the metered reducing agent and thus significantly improve the OBD monitoring quality.

In addition, the method proposed according to the invention comes with sensors already present in a motor vehicle or an SCR catalyst system and, in particular, makes a costly and technically complex flow mass meter superfluous.

The consideration or merging of the sensor data provided by at least two different measurement sources makes it possible to increase the accuracy in determining a specified quantity deviation of reducing agent, by means of which the aforementioned OBD requirements, in particular the requirements applicable from 2016, can be met.

The invention can be used with said OBD engine, especially in motor vehicles, with the said advantages.

Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination indicated, but also in other combinations or in isolation, without departing from the scope of the present invention.

Brief description of the drawings

1 shows a schematic representation of the components of a SCR catalyst system according to the prior art;

2 shows a block diagram illustrating a preferred embodiment of the invention.

Description of exemplary embodiments

1 shows in a schematic way the known components of a metering device of an SCR catalyst system.

In the exhaust system 10 an internal combustion engine 11 is an SCR catalyst 12 arranged in which by a selective catalytic reduction (SCR) selectively the nitrogen oxides are reduced in the exhaust gas. For this reaction, ammonia (NH ₃ ) is used as a reducing agent. Because ammonia is a toxic substance, this substance is made from non-toxic Urea material obtained as a liquid urea water solution by means of a metering module, in particular a metering valve 13 , in the exhaust system 10 upstream of the SCR catalyst 12 is injected. The aqueous urea solution is placed in a reducing agent tank 14 stored. To remove the urea water solution is a suction line 15 intended. The reducing agent is via a feed pump 16 from the reducing agent tank 14 conveyed and under pressure in the pressure line 17 to the metering valve 13 directed.

The urea solution is precisely and as needed in the exhaust system 10 injected. This is the pressure of the reducing agent in the pressure line 17 prevail. The reducing agent pressure is adjusted to a predefinable target pressure. This is a pressure sensor 18 in the pressure line 17 provided, the detected pressure signals to a control unit 19 forwards, so that the feed pump 16 via a signaling of the control unit 19 can adjust the predetermined target pressure.

The activation of the metering valve 13 also takes place via a signaling of the control unit 19 , Downstream of the SCR catalyst 12 is a NOx sensor 20 arranged, the nitrogen oxide values of the exhaust gas, which is the SCR catalyst 12 leaves, recorded. These values are sent to the controller 19 and can be used for controlling the dosage and for monitoring the SCR catalyst 12 be evaluated.

The method described uses the control signals for the metering valve 13 and in parallel with the NOx sensor 20 detectable nitrogen oxide values of the exhaust gas downstream of the SCR catalyst 12 In order to draw a conclusion on the functionality of the metering valve 13 to be able to pull. For this purpose, the control signals of the metering valve and the nitrogen oxide values are each compared with predefinable threshold values, and if both threshold values are exceeded, a metering valve which does not function properly is closed, in particular an at least partially or completely closed clamping metering valve.

The invention will be described with reference to an embodiment.

In the present exemplary embodiment, the first measuring or signal source used is a quantity deviation q ₁ calculated from the pressure change during the injection process of the reducing agent (for example in the unit [%]). The calculation of the quantity deviation takes place with the known in the prior art (mentioned above) method. This signal is inherently subject to an error or a variance sigma ² (q ₁ ). Causes of the variance may be, for example, due to usual tolerances of the components, influences of the component life as well as by involved signal chains conditional signal tolerances.

In the exemplary embodiment, a tank level indicator or sensor arranged on the storage tank storing the reducing agent serves as a second measuring or signal source, which supplies a quantity deviation q ₂ or by means of which a quantity deviation q _{2 was determined} by the method known in the prior art. This second signal is naturally also subject to a variance sigma ² (q ₂ ).

In the method described below, the two present signals q ₁ and q _{2 are} coupled according to the embodiment with weighting factors k ₁ and k ₂ , wherein a correspondingly weighted linear combination with error minimization is performed on the signals or the signals are fed to a corresponding linear estimator.

The advantages according to the invention result in particular from the following statistical consideration:

Based on the two quantity deviations, q ₁ and q _{2 are} calculated as estimated quantity deviations x _t2 at time t2 for the named linear combination x _t2 = k ₁ q ₁ + k ₂ q ₂ with k ₁ + k ₂ = 1.

The difference between the estimated quantity deviation x _t2 and the actual quantity deviation is in accordance with equations known per se (see, for example, US Pat Maybeck, Peter, "Stochastic Models, Estimation, and Control, Volume 1, p. 1979, Academic Press, Inc. ) minimal for the following values k ₁ and k ₂ : k ₁ = sigma ² (q ₂ ) / (sigma ² (q ₁ ) + sigma ² (q ₂ )) k ₂ = sigma ² (q ₁ ) / (sigma ² (q ₁ ) + sigma ² (q ₂ ))

With that follows: x _t2 = q ₁ + Gain · (q ₂ - q ₁ ) with Gain = sigma ² (q ₁ ) / (sigma ² (q ₁ ) + sigma ² (q ₂ )).

Taking into account x _t1 = q ₁ follows: x _t2 = q ₁ + Gain (q ₂ - x _t1 ).

For the variance sigma ² (x _t2 ) present at time t2, the following therefore holds: sigma ² (x _t2 ) = sigma ² (x _t1 ) - gain sigma ² (x _t1 ).

Accordingly, the (coupled) variance determined from the two signals q ₁ and q _{2 is} reduced by the second addend Gain sigma ² (x _t1 ) compared to the variance sigma ² (x _t1 ) of the individual values (first addend). By combining the two signals q ₁ and q ₂ , the quantity deviation can thus be resolved better than when using individual signals and thus meet the OBD requirements mentioned with a corresponding system design. If the number of signal sources for determining the quantity deviation is increased, the accuracy can be further increased.

Since the invention can be implemented in the form of a control program and existing algorithms can be used to determine the quantity deviation, the technical implementation can be carried out with considerable cost reduction compared to the prior art. In particular, can be dispensed with a costly flow mass meter.

The determination of said variances can be carried out on an engine test bench with measurements against calibrated reference sensors, wherein the reference sensors can be omitted in the production vehicle.

A possible technical realization of the invention is based on the in 2 shown block diagram illustrates. It is understood that in 2 shown functions or functional sequences can be implemented either in the form of a loaded into a volatile or non-volatile memory program or control code or in the form of a logic device (eg control board or chip).

In block 100 First, a detection of the amount of already metered reducing agent q _tm means of a conventional tank level sensor 105 ,

In addition, the activation time of the SCR system (see 1 ) arranged metering valve 110 the currently theoretically injected amount of reducing agent dq _th calculated or determined. The instantaneous injection quantity dq _th is measured multiple times over a given period of time, such as through the loop 115 indicated, and the values recorded in block 120 each added up or integrated. This results in the sum value q _th = Sum (dq _th ).

In block 125 is the thus summed amount q _th compared with the already detected change in the tank level q _tm or, as in the present embodiment, the quantity deviation q ₁ from the difference q ₁ = q _th - q _tm formed.

The inventively further required quantity deviation q ₂ is present in block 130 via the pressure sensor signal, as in the aforementioned DE 10 2008 005 989 A1 or the DE 10 2009 029 408 A1 described, determined. Instead of these methods, however, it is also possible to use other methods known per se for determining the quantity deviation, for example the "CDM functions" known per se or methods in which the quantity deviation is determined intrusively, ie without metering requirement for the actual reduction of NO _x . In the intrusive process, after the pressure buildup of the dosing agent has been completed, the dosing valve is opened for a predetermined time interval and the resulting pressure drop is measured. The thus detected differential pressure is then compared with a predetermined threshold and determined from the result of the comparison, a value for the quantity deviation.

The values q ₁ and q ₂ thus acquired or determined are finally shown in block 135 the above-described coupling method according to the invention, with the accuracy increase described therein in the determination of the quantity deviation applied.

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 102010028891 A1 [0002]
• DE 102008005989 A1 [0007, 0047]
• DE 102009029408 A1 [0008, 0047]

Cited non-patent literature

• Maybeck, Peter, "Stochastic Models, Estimation, and Control, Volume 1, p. 1979, Academic Press, Inc. [0036]

Claims (8)

Method for determining metering quantity deviations of a reducing agent of an SCR catalyst system for after-treatment of exhaust gases of an internal combustion engine, characterized in that data or signals representing the metered quantity are detected from at least two different measuring or signal sources ( 100 ) and that a deviation of the dosing amount is determined by combining the at least two recorded data or signals ( 135 ). A method according to claim 1, characterized in that the quantity deviation is determined by weighted linear combination of the at least two detected data or signals. A method according to claim 2, characterized in that are used as measuring or signal sources, a pressure sensor and / or a metering valve and / or a tank level sensor. A method according to claim 3, characterized in that a momentarily metered amount of reducing agent is detected over a predetermined period of time ( 100 ) and the quantity values recorded thereby are summed or integrated and from this a sum value q _th = sum (dq _th ) is formed ( 120 ) that the difference q ₁ = q _th - q _{tm is} formed from the formed sum value q _th and the detected tank level q _tm ( 125 ), that a quantity deviation q _{2 is} detected by means of the pressure sensor, and that the values q ₁ and q ₂ are subjected to a weighted linear combination ( 135 ). Control device for determining metering quantity deviations of an SCR catalytic converter for after-treatment of exhaust gases of an internal combustion engine, characterized by computing means and / or control means for carrying out the method according to one of claims 1 to 4. Control device according to claim 5, characterized in that the calculating means are formed by a linear estimator. Computer program that performs all the steps of a method according to one of claims 1 to 4, when it runs on a computing device or a control device according to claim 5 or 6. 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 4, when the program is executed on a computer or a control device according to claim 5 or 6.

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