CN115217599A

CN115217599A - Method and device for diagnosing an SCR system for an internal combustion engine

Info

Publication number: CN115217599A
Application number: CN202210410932.8A
Authority: CN
Inventors: T·布兰兹; 金忱; T·梅尔
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2021-04-19
Filing date: 2022-04-19
Publication date: 2022-10-21
Also published as: DE102021212869A1; DE102021214177A1

Abstract

Method for diagnosing an SCR system for an internal combustion engine, wherein a reducing agent solution is injected before an SCR catalyst (1) and a second NO is injected downstream of the SCR catalyst (1, 9) _x The sensor (3) determines a second NO in the exhaust gas (12) _x Signal (a)

) Wherein the reducing agent solution is injected at a predetermined frequency (F)Wherein a diagnosis of the SCR system for the internal combustion engine is granted if a permission condition for the SCR system is identified, wherein a second NO is used _x Signal (a)

) To determine a frequency component of a predetermined frequency (F) for the injection of the reducing agent solution and to determine a malfunction of the SCR system on the basis of this frequency analysis.

Description

Method and device for diagnosing an SCR system for an internal combustion engine

Technical Field

The present invention relates to a method and a device for diagnosing an SCR system for an internal combustion engine according to the preambles of the independent claims.

Background

A method and a device for diagnosing an SCR system are known from DE 10 2016 203 227 A1, in which a reducing agent solution is injected before an SCR catalyst and NO is arranged after the SCR catalyst _x Sensor, said NO _x The sensor measures NO in the exhaust 12 _x And (4) concentration. For the diagnosis, NO is interrupted in this case _x Regeneration of the catalyst, and thus no diagnosis is made during normal operation.

Disclosure of Invention

In a first aspect, the invention relates to a method for diagnosing an SCR system for an internal combustion engine, wherein a reducing agent solution is injected before an SCR catalyst and a second NO is injected after the SCR catalyst _x Sensor for determining a second NO in the exhaust gas _x A signal, wherein the reducing agent solution is injected at a predetermined frequency, wherein, if an enabling condition for the SCR system is identified, an enable is given for a diagnosis of the SCR system for the internal combustion engine, wherein the second NO is used _x The frequency analysis of the signal determines a frequency component of a predetermined frequency for the injection of the reducing agent solution and determines a malfunction of the SCR system on the basis of this frequency analysis.

The method offers particular advantages, namely: by a second NO arranged downstream _x Second NO of sensor _x Identifying the frequency of injection of the reductant solution before the SCR catalyst in the signal can provide robust and improved diagnostics.

Further advantages are obtained by the features of the dependent claims. A particular diagnostic situation consists in the detection of an intentional actuation of the exhaust system. Such manipulations aim at reducing or regulating the consumption of reducing agent and saving the costs resulting therefrom.

Here, NO is usually the case _x The sensor signal and/or the dispensed signal is/are manipulated, or the dispensing valve is blocked or simulated (emulieren).

Furthermore, the method can be approved if operating states exist for the SCR catalyst in which ammonia slip is present.

This is particularly advantageous because NO is a result of this operating state for the SCR catalytic converter _x Lateral sensitivity of the sensor to enable NO arranged downstream of the SCR catalyst _x NO of sensor _x The signal is used to determine the dosed quantity injected.

Furthermore, a diagnosis can be approved if the active dosing of the reducing agent solution is carried out with a predetermined frequency.

The presetting of the dosing frequency is thus advantageous, so that the second NO arranged downstream of the SCR catalytic converter _x Second NO of sensor _x The frequencies to be found can be robustly implemented in the signal.

Advantageously, the second NO is evaluated before the frequency analysis _x The signal is normalized, since the normalization of the second NO to be analyzed can thus be performed in the controller _x Robust comparison of signals.

It is particularly advantageous to transform the normalized Fourier transformed NO _x In the signal, the maximum intensity is determined at a predetermined frequency or in a range around the predetermined frequency, since the maximum intensity makes it possible to determine a measure for the presence of a match or for detecting a match before the SCR catalyst.

Furthermore, the SCR system can be identified as being able to operate properly if the maximum strength sought exceeds a predefinable threshold value. If the maximum intensity, which represents a measure for the injection of the reducing agent solution at a predefined frequency, exceeds a predefinable threshold value, an injection upstream of the SCR catalyst can be concluded and a robust diagnosis of the SCR system can therefore be carried out.

Furthermore, a malfunction of the SCR system is detected if the maximum strength sought is below a predefinable threshold value. However, if the maximum intensity sought does not exceed a predeterminable threshold value, it is assumed that dosing has not already taken place at the predeterminable frequency or that the SCR system is otherwise handled or damaged.

Furthermore, the actuation of the SCR system can be determined in the event of a malfunction.

In a preferred embodiment, the pairing of the second NO is performed by means of a fast fourier transformation or a discrete fourier transformation _x Frequency analysis of the signal.

Furthermore, if a low storage capacity and a low modeled NH3 fill level are identified for the SCR catalyst, a diagnostic can be granted. This is advantageous because, in this way, operating states exist for the SCR system in which ammonia slip is present as a result of the dosed reducing agent solution upstream of the SCR catalyst passing through the SCR catalyst and NO downstream of the SCR catalyst _x Can be determined in the signal.

Furthermore, if high NO is determined before the SCR catalyst _x The concentration can be given permission.

Furthermore, if a regeneration of the diesel particulate filter of the internal combustion engine is recognized, permission can be given.

In a further aspect, the invention relates to a device, in particular a controller, and a computer program which are configured, in particular programmed, to carry out one of the methods. In yet another further aspect, the invention relates to a machine-readable storage medium having stored thereon a computer program.

Drawings

The invention is explained in more detail below on the basis of embodiments shown in the figures. Wherein:

figure 1 shows schematically an exhaust system with an SCR catalyst,

fig. 2 shows a schematic flow diagram of an exemplary embodiment of a method for diagnosing an SCR system of an internal combustion engine.

Detailed Description

Fig. 1 shows an exhaust gas system in which the exhaust gas 12 of an internal combustion engine is guided in the flow direction of the exhaust gas 12 through a plurality of exhaust gas purification devices.

The exhaust gases 12 are guided along the exhaust line successively through different catalytic converters in order to achieve a purification of the exhaust gases 12. The diesel oxidation catalyst 7 is provided as a first catalyst in which carbon monoxide and hydrocarbons are oxidized to carbon dioxide or carbon dioxide and water. In a next step, the exhaust gas 12 is then conducted through the diesel particulate filter 8, in which soot particles are filtered out of the exhaust gas 12. Then, the soot particles accumulated in the diesel particulate filter 8 are burned through an oxidation process according to the amount of the stored soot. In order to detect a predetermined quantity of soot particles in the diesel particulate filter 8 (DPF), a differential pressure measurement, not shown here, is provided at the diesel particulate filter 8. In a further step, the exhaust gas 12 is then conducted through an exhaust pipe through an SCR catalyst 1 (selective catalytic reduction), in which the Nitrogen Oxides (NO) to be included in the exhaust gas 12 are contained _x ) Reducing to nitrogen and water. In order to enable this reduction in the SCR catalyst 1, an injection part 2 of a reducing agent solution (typically an aqueous urea solution) is provided before the SCR catalyst 1. Then, also following the SCR catalyst 1 is an ammonia slip catalyst 9 (ASC), by means of which excess reducing agent solution, in particular NH3, which may be present after the SCR catalyst 1, is removed from the exhaust gas 12. In an advantageous embodiment, the ammonia slip catalyst 9 (ASC) can be dispensed with, i.e. the SCR system comprises only the SCR catalyst 1.

The exhaust system also has a plurality of sensors. Setting a first and a second NO before and after all SCR catalysts _x Sensors 4, 3, whereby NO before the diesel oxidation catalyst 7 and after the ASC 9 _x The concentration is measured.

Here, the first NO _x Sensor 4 is arranged upstream of SCR catalytic converter 1 and determines a first NO _x Signal

. Second NO _x Sensor 3 is arranged downstream of SCR catalytic converters 1, 9 and determines a second NO in the exhaust gas tract _x Signal

。

Furthermore, a plurality of temperature sensors 5, one temperature sensor 5 each, are arranged upstream of the diesel oxidation catalytic converter 7, upstream of the diesel particulate filter 8 and upstream of the SCR catalytic converter 1. Furthermore, a particle sensor 6 is provided downstream of the ASC 9, by means of which any particles still remaining in the exhaust gas 12 are detected.

Of these catalysts the SCR catalyst 1 is adapted to take a special role, since this catalyst indicates a sufficient injection of the reducing agent solution by means of the injection valve 2. The injected urea aqueous solution generally represents a significant cost factor and corresponding measures are often taken to avoid the costs for the urea aqueous solution or the reducing agent solution. According to the invention, a method and a device for diagnosing an SCR system for an internal combustion engine are therefore proposed, which enable a corresponding malfunction or actuation of the SCR system to be detected.

In most operating states of the internal combustion engine, the SCR catalytic converter 1 is operated such that a certain amount of reducing agent solution, in particular NH ₃ Is stored on the surface of the SCR catalyst 1. This storage behavior of the SCR catalyst 1 for the reducing agent solution has the effect of balancing such that the reactively available amount of the reducing agent solution does not have to be introduced into the SCR catalyst 1 at the same time as the exhaust gas 12. Thus, NO in the exhaust gas stream _x And minor fluctuations in the proportion of the reducing agent solution (the exhaust gas flow being supplied to the SCR catalyst) are thereby balanced. If relative to NO _x An excess of reducing agent solution is injected, and the excess is stored in the SCR catalyst 1. If relative to NO _x If too little reductant solution is injected, part will be missing, and so willThat is to say that the additionally required amount of reducing agent solution is taken off from the SCR catalyst 1. This applies, of course, only if the SCR catalyst 1 has a corresponding capacity at the respective operating point for storage or removal. The SCR catalyst 1 therefore acts as a low-pass or integrating element for the quantity of reducing agent solution. However, in certain operating states, the storage performance of the SCR catalyst 1 is severely limited, that is to say the SCR catalyst 1 is only able to store very small amounts of reducing agent solution. This is particularly the case when the temperature of the SCR catalyst 1 is particularly high, for example above 350 ℃. At such high temperatures, the capacity of the SCR catalytic converter 1 for storing the reducing agent solution is so severely reduced that, without additional injection of the reducing agent solution, in the order of 0.5 to 2 seconds, the second NO is passed _x Second NO of sensor 3 _x Signal

Can measure NO _x A significant deterioration of the reduction. Furthermore, there is an SCR catalyst which has a low storage performance for the reducing agent solution due to the materials used (vanadium SCR catalyst).

The injection valve 2 for the reducing agent solution or the urea-water solution is a dosing valve with an open and closed state. Such valves have only a switching state "open" or a switching state "closed" and the regulation of the amount of injected reducing agent solution can be effected only by corresponding timing. Such control schemes are stored on the controller 10 and are permanently monitored and calculated. In certain operating states, the timing of the injection valve 2 is carried out so slowly that it enters a range in which the reduced storage capacity of the SCR catalyst 1 can be measured. This is in particular the case when a very small amount of aqueous urea solution is injected, that is to say only a small amount of reducing agent solution is required. With such small amounts of reducing agent solution it may happen that the injection valve is actuated with a lower frequency in the order of 0.5 to 2 Hz. However, faster actuation at higher frequencies cannot be achieved with smaller quantities, since no arbitrary actuation is possibleThe injection valve 2 is actuated for a short time, so that a minimum quantity per injection is obtained. When the required amount of reducing agent solution per second fluctuates in the range of the minimum amount per injection, then the injection valve 2 can therefore be operated only with such a low frequency of 0.5 to 2 Hz. When at the same time the capacity of the SCR catalyst 1 for storing the reducing agent solution is also of this order of magnitude, then in the second NO _x Second NO of sensor 3 _x Signal

NO downstream of the SCR catalyst 1, 9 _x It can be verified in the sensor that the measured second NO is _x Signal

With respect to the effect of the injection of the reducing agent solution at the predetermined frequency F.

Fig. 2 shows an exemplary procedure for diagnosing an SCR system of an internal combustion engine.

In a first step 200, the authorization conditions for the method are checked by the controller 10 and, if these authorization conditions exist, an authorization is given for the diagnosis of the SCR system.

In a first embodiment, the method for diagnosing the SCR system is approved if the SCR catalytic converter 1 has a low storage capacity for the reducing agent solution or ammonia until no storage capacity is available and its degree of filling is low and the reducing agent solution is introduced into the exhaust gas tract. That is, there is an operating state in which ammonia slip occurs via the SCR catalyst 1.

The filling degree of the SCR catalytic converter 1 can be determined here preferably by means of a filling model calculated on the control unit 10. The filling model is based on the current temperature of the SCR catalyst 1, the air quantity (preferably measured by an air mass sensor in the intake tract of the internal combustion engine), the NO upstream of the SCR catalyst _x Concentration, and the filling of the SCR catalyst 1 by the injection history of the reducing agent solution. The injection history is preferably stored in the control unit 10 and contains information about the injection into the exhaust gas tractInformation of amount and time point. Exemplary operating conditions are in particular for operating states of the SCR system in which the SCR catalyst 1 has a relatively high temperature, preferably a temperature of more than 350 ℃. As the temperature of the SCR catalytic converter 1 increases, the storage capacity of the SCR catalytic converter 1 for the reducing agent solution or ammonia decreases, so that the probability of ammonia slip increases further as the temperature of the SCR catalytic converter 1 increases.

An exemplary operating state is given, for example, if a diesel particulate filter regeneration is carried out for the diesel particulate filter 8 by the controller 10. The diesel particulate regeneration is preferably carried out by measures inside the engine or by injecting additional fuel into the exhaust system in order to generate high temperatures in the exhaust system in order to burn off the soot particles stored in the diesel particulate filter 8.

The dosing of the reducing agent solution into the exhaust gas tract is carried out by means of an injection valve 2 positioned upstream of the SCR catalyst 1.

Dosing can be requested, for example, by a dosing strategy, so that dosing is achieved by the injection valve 2 at a predetermined frequency F.

Here, depending on the filling level and storage capacity of the SCR catalyst 1, NO upstream and downstream of the SCR catalyst 1 _x First and second NO of sensors 4, 3 _x Signal

、

The dosing strategy is evaluated at the controller 10 in a known manner.

This dosing frequency used here is read in as a predefined frequency F and stored in the control unit 10. When dosing of the reducing agent solution is performed, such dosing can be at the second NO _x Sensor 3, i.e. NO downstream of SCR catalysts 1, 9 _x Second NO of sensor _x Signal

At a corresponding frequency through signal divisionAnd analyzing or frequency analyzing to obtain the target.

If authorization is granted, in an advantageous embodiment a second NO located downstream of the SCR catalytic converter 1 _x Second NO of sensor 3 _x Signal

The determination takes place for a predeterminable time range, preferably 30s, and is stored in the controller 10. Here, the first and second NO _x Signal

、

Is preferably between 1-100 ms. The method then continues in step 210.

In a special embodiment, the dosing of the reducing agent solution at the predefined frequency F can also be actively requested by a diagnostic function. In this case, the predefined frequency F, which is preferably between 0.5 and 2 Hz, can likewise be predefined by the diagnostic function for fitting. If dosing is started, the method can continue in step 210.

Here, the request for dosing is preferably made when the dosing strategy does not request dosing and it is determined by the controller 10 that there is a low storage performance and a low filling level for the SCR catalyst 1.

The method then continues in step 210.

In step 210, the second NO is _x Stored second NO of sensor 3 _x Signal

And (6) standardizing. This can be performed by means of the controller 10, for example, by the following formula:

。

whereinY _nox Is the second NO _x Normalized NO of sensor 3 _x Signal, X is stored NO _x Raw data of the signal, μ is the mean and σ is the standard deviation.

Subsequently, the method can be continued in step 220.

In step 220, the pairing of the second NO is then performed in the controller 10 _x Normalized NO of sensor 3 _x Frequency analysis of the signal. For this purpose, a Fast Fourier Transform (FFT) is used in the following embodiments. Alternatively, a Discrete Fourier Transform (DFT) can also be used.

From a second NO _x Normalized, fourier transformed NO of sensor 3 _x Maximum intensity of predetermined frequency of subsequent fitting operation in signalI _max And stores it in the controller 10.

Alternatively, the maximum intensity can also be determined in a defined frequency range around the predetermined frequency F, preferably in the range F ± 0.5Hz around the predetermined frequency FI _max Because the injection frequency is contained in the second NO _x Signal

The information in (1) may vary slightly.

The method then continues in step 230.

In step 230, the maximum intensity is checked against a predeterminable threshold value SI _max . If the maximum intensity isI _max If a predeterminable threshold value S is exceeded, the diagnosis of the SCR system is recognized as normal or fault-free and this state is stored in the controller 10. If the maximum intensity is foundI _max Below the threshold value S, the SCR system identifies an abnormality or a malfunction or an actuation, and this state is stored in the controller 10.

The maximum intensity thus determinedI _max S below thresholdOne situation can be regarded as an indicator for the manipulation of the SCR system. If in the second NO _x Sensor 3 (for NO downstream of SCR catalyst 3 _x Sensor) second NO _x Signal

It can be ascertained, by means of frequency analysis, that, with a predetermined frequency F, no signal component is dosed in, which can indicate, for example, that dosing of the reducing agent solution is not being carried out by the dosing valve 2.

In a particularly advantageous embodiment, the engine control light is activated in the dashboard in the event of a fault.

Subsequently, the method can start in step 200 from the beginning.

In a special embodiment, the diagnosis of the SCR system can be carried out over a plurality of driving cycles and/or over a plurality of authorizations for the diagnosis of the SCR system 1. For this purpose, the analysis from the individual driving cycles or the admissions or diagnostics is stored as intermediate values or intermediate results in the controller 10 and can subsequently be analyzed jointly.

The method can then end or start from the beginning in step 200.

Claims

1. Method for diagnosing an SCR system for an internal combustion engine, wherein an injection of a reducing agent solution is carried out before an SCR catalyst (1) and a second NO is carried out downstream of the SCR catalyst (1, 9) _x The sensor (3) determines a second NO in the exhaust gas (12) _x Signal (a)

) Wherein a reducing agent solution is injected at a predefined frequency (F), wherein a diagnosis of an SCR system for the internal combustion engine is granted if a permissive for the SCR system is identified,

characterised in that by means of said second NO _x Signal (a)

) To find a frequency component of a predetermined frequency (F) of the injection of the reducing agent solution and to determine a malfunction of the SCR system on the basis of this frequency analysis.

2. Method according to claim 1, characterized in that the method is granted if there is an operating state for the SCR catalyst (1) in which there is an ammonia slip.

3. Method according to claim 1, characterized in that the active dosing of reducing agent solution is performed with a specified frequency (F) when permission is given for the diagnosis.

4. The method of claim 1, wherein the second NO is sought _x Signal (a)

) Normalized prior to the frequency analysis.

5. Method according to claim 4, characterized in that from normalized Fourier transformed NO _x In the signal, the maximum intensity is determined at a predetermined frequency (F) or in a range around the predetermined frequency (F) (F)I _max ）。

6. Method according to claim 5, characterized in that if the maximum intensity (V) is found (V &)I _max ) If a predeterminable threshold value (S) is exceeded, the SCR system is identified as being able to operate properly.

7. The method according to claim 4, wherein if said found maximum intensity (A), (B) is foundI _max ) Lower than energyA predetermined threshold value (S) is determined, and a malfunction of the SCR system is detected.

8. Method according to claim 7, characterized in that in case of a malfunction, an operation of the SCR system is determined.

9. Method according to claim 1, characterized in that the second NO downstream of the SCR catalyst (1, 9) is performed by means of a Fast Fourier Transform (FFT) or a Discrete Fourier Transform (DFT) _x NO of sensor (3) _x Frequency analysis of the signal.

10. Method according to claim 1, characterized in that the diagnosis is granted if a low storage capacity and a low modelled NH3 filling level are identified for the SCR catalyst (1).

11. Method according to claim 1, characterized in that if high NO is determined before the SCR catalyst (1) _x And giving permission for concentration.

12. Method according to claim 1, characterized in that permission is given if a regeneration of a diesel particulate filter (8) of the combustion engine is identified.

13. A computer program arranged to perform the method according to any one of claims 1 to 12.

14. Electronic storage medium with a computer program according to claim 13.

15. Device, in particular a controller, for carrying out the method according to one of claims 1 to 12.