Method and device for detecting ammonia slip in an exhaust gas aftertreatment system

Abstract

Method for detecting an ammonia slip in an exhaust gas aftertreatment system of an internal combustion engine (1), which has an SCR catalyst (80) and in which, for nitrogen oxide reduction, an ammonia-releasing reducing agent solution is metered into the exhaust gas flow (20) by means of a metering unit (110) in the flow direction of the exhaust gas upstream of the SCR catalyst (80), wherein a cross-sensitivity to nitrogen oxides and ammonia of at least one NO x -Sensor formed exhaust gas sensor (90), which is arranged in the flow direction of the exhaust gas behind the SCR catalyst (80) in the exhaust gas duct (30), is used to detect an ammonia breakthrough, characterized in that to check the plausibility of an ammonia slip, a difference signal is formed by taking from the output signal of the exhaust gas sensor (90) behind the SCR catalyst (80) as a measure for the sum of a nitrogen oxide and an ammonia concentration (204, 205) a value for the NO x-concentration is subtracted before the SCR catalyst (80), and the temporal course of the difference signal or the course of variables derived therefrom is evaluated and compared with applicable limit values.

Description

State of the art

[0001] The invention relates to a method and a device for detecting an ammonia slip in an exhaust gas aftertreatment system of an internal combustion engine, which has an SCR catalyst and in which, for nitrogen oxide reduction, an ammonia-releasing reducing agent solution is metered into the exhaust gas flow in front of the SCR catalyst by means of a metering unit, wherein a cross-sensitivity to nitrogen oxides and ammonia of at least one NO x -Sensor designed exhaust gas sensor, which is arranged in the exhaust gas flow direction behind the SCR catalyst in the exhaust duct, is used to detect an ammonia breakthrough.

[0002] The invention further relates to a device, in particular a diagnostic unit, for carrying out the method according to the invention.

[0003] Stricter laws in the area of ​​diagnosis of emission-relevant components require the monitoring of all exhaust gas aftertreatment components and the sensors used for OBD limit values, which are usually specified as a multiple of the emission limit value, as part of on-board diagnostics (OBD).

[0004] In connection with legal requirements regarding nitrogen oxide emissions from motor vehicles, appropriate exhaust gas aftertreatment is required. Selective catalytic reduction (SCR) can be used to reduce NO x-emissions (denitrification) of internal combustion engines, particularly diesel engines, with predominantly lean, i.e., oxygen-rich exhaust gases. A defined amount of a selective reducing agent is added to the exhaust gas. This can be in the form of ammonia (NH3), for example, which is metered directly in gaseous form, or obtained from a precursor substance in the form of urea or a urea-water solution (UWL). Such UWL-SCR systems have been used for the first time in the commercial vehicle segment.

[0005] DE 10139142 A1 describes an exhaust gas purification system of an internal combustion engine in which, to reduce NO x-emission, an SCR catalyst is used which reduces the nitrogen oxides contained in the exhaust gas to nitrogen using the reagent ammonia. The ammonia is obtained from the urea-water solution (HWL) in a hydrolysis catalyst arranged upstream of the SCR catalyst. The hydrolysis catalyst converts the urea contained in the HWL into ammonia and carbon dioxide. In a second step, the ammonia reduces the nitrogen oxides to nitrogen, producing water as a by-product. The exact process has been adequately described in the specialist literature (cf. WEISSWELLER in CIT (72), pages 441-449, 2000). The HWL is provided in a reagent tank.

[0006] For optimal SCR catalyst function, a certain ratio of NO and NO2 to NH3 in the exhaust gas is necessary. A dosing system is required to provide the appropriate NH3 concentration. Malfunctions in this dosing system can lead to undesirable ammonia slip. In addition to its unpleasant odor, ammonia can cause problems even at low concentrations and must therefore be avoided whenever possible.

[0007] In order to comply with the legal requirements regarding NO x -emission and OBD requirements, all vehicles with an SCR system are equipped with a NO x -Sensor downstream of the SCR catalyst is mandatory. All amperometric NO xDue to their design, sensors also exhibit a certain degree of cross-sensitivity to ammonia. Such sensors are typically known as dual-chamber sensors. The nitrogen oxide-oxygen mixture passes through two chambers in this sensor. Since the oxygen influences the nitrogen oxide measurement, it must be removed. This is achieved by an electrical voltage in the first chamber. This voltage breaks down the O2 molecules into ions. These penetrate the solid electrolyte consisting of ZrO2. In the second chamber, the remaining nitrogen oxide is split into nitrogen and oxygen. A current flows that is proportional to the concentration of nitrogen oxide in the exhaust gas. Deposits or contamination can significantly influence the diffusion dynamics in these sensors, so that the resulting sensor signal can be inaccurate.

[0008] The document DE 10 2012 105 953 A1 discloses a method for determining reducing agent slip from an exhaust gas treatment device, in particular for mobile internal combustion engines.

[0009] The document DE 10 2009 012 092 A1 describes a method for use in conjunction with an exhaust gas aftertreatment system for metering an ammonia-releasing reducing agent into the exhaust gas flow of an internal combustion engine installed in a vehicle and operated with excess air, wherein a control unit measures the amount of reducing agent depending on a stored model and during operation of the internal combustion engine the metered amount varies in a defined manner in certain operating phases and the change in the measured value of at least one NO xsensor with an expected value, which the control unit determines from the size of the variation. The degree of agreement between the expected value and the value determined by the NO x -Sensor determines the actual value for the presence of NO x and / or NH3 closed. The variation of the dosing quantity is continued until either the NO x -Target concentration or the NO determined from it x -target conversion is reached or the presence of NH3 is concluded due to the deviation of the actual value from the expected value or the actual value and thus the sum of NO x and NH3 reaches a minimum or a preset value.

[0010] DE 10 2005 050 709 A1 proposes a method for the optimized operation of an exhaust gas aftertreatment system of an internal combustion engine. The exhaust gas aftertreatment system comprises an SCR catalyst for the selective catalytic reduction of nitrogen oxides, a reducing agent addition device upstream of the SCR catalyst, an exhaust gas sensor sensitive to nitrogen oxides and ammonia arranged downstream of the SCR catalyst, and a control device for regulating the reducing agent addition device based on an ammonia fill level model of the SCR catalyst such that the ammonia fill level of the SCR catalyst is adjusted to a predeterminable target fill level.The operating method includes the steps of detecting an output signal of the exhaust gas sensor downstream of the SCR catalyst during overrun operation of the internal combustion engine and, if the detected output signal exceeds a predetermined threshold value, adjusting the current modeled ammonia fill level of the SCR catalyst to a predetermined limit fill level.

[0011] As in DE 10 2009 012 092 A1, DE 10 2005 050 709 A1 also performs a plausibility check before initiating nitrogen oxide-reducing measures to determine whether the exhaust gas sensor signal is nitrogen oxide or ammonia. For this purpose, the nitrogen oxide concentration downstream of the SCR catalyst is evaluated during engine overrun, i.e., during a phase in which no fuel injection occurs. If the measured nitrogen oxide concentration during overrun shows values ​​greater than zero, this clearly indicates ammonia slip.

[0012] WO 2007 028 677 A1 describes, by way of example, another method which is intended to enable the best possible exhaust gas purification result with minimal reagent slippage.

[0013] The disadvantage of all of these previously described methods is the previously mentioned cross-sensitivity of the nitrogen oxide sensor and the associated difficulty in quickly and clearly detecting NH3 slip.

[0014] It is therefore the object of the invention to verify the plausibility of the diagnosis of an NH3 slip as quickly as possible, to conclude from this that an NH3 slip has occurred and to initiate appropriate countermeasures.

[0015] It is a further object of the invention to provide a corresponding device for carrying out the method. Disclosure of the invention

[0016] The problem concerning the method is solved by the features of claims 1 to 11.

[0017] According to the invention, it is provided that a difference signal is formed to check the plausibility of the ammonia slip by taking a value for the NO from the output signal of the exhaust gas sensor behind the SCR catalyst as a measure of the sum of a nitrogen oxide and an ammonia concentration. x-concentration upstream of the SCR catalyst is subtracted, and the temporal progression of the difference signal or the progression of variables derived from it is evaluated and compared with applicable limit values. The advantage here is that this method does not require additional hardware and can therefore be easily implemented in, for example, an existing engine control system. In addition, the method is characterized by a high diagnostic quality, so that ammonia slip can be reliably detected. Since this method does not require correction signal determination, it reacts more quickly than conventional methods for controlling reagent dosing when appropriately designed. The proposed method is independent of the plausibility check of the hardware of the reagent dosing system.

[0018] It can be provided that the NO x concentration before the SCR catalyst is calculated from a model or with a NO x-Sensor, which is arranged upstream of the SCR catalyst in the direction of exhaust gas flow. This allows for a high level of diagnostic accuracy, as interference that already occurs upstream of the SCR catalyst with increased NO x -content can be detected and taken into account during diagnosis. This can help avoid errors.

[0019] One process variant involves considering the expected conversion efficiency of the SCR catalyst as a correction value when calculating the difference. This allows, on the one hand, interference from a potentially operating-point-dependent SCR catalyst efficiency to be taken into account when checking the plausibility of ammonia slip, significantly reducing misinterpretations. On the other hand, it increases the accuracy of the process.

[0020] In a preferred method variant, a temporal gradient is formed from the difference signal and compared with an applicable differential limit value, whereby an ammonia slip is detected if this differential limit value is exceeded. This utilizes the fact that a NO x -Concentration increase in the exhaust gas is not too steep, i.e., the gradient does not exceed certain limits. However, if a steep signal increase is recorded, i.e., the gradient of the differential signal increases significantly, one must assume an NH3 breakthrough. However, the limit value must be applied specifically for each vehicle class.

[0021] Another advantageous method variant provides for the difference signal to be compared with an applicable absolute limit value, whereby an ammonia slip is detected if this absolute limit value is exceeded. This takes advantage of the fact that the NO x-content in the exhaust gas for a regulated area is within e.g. the measuring range for the exhaust gas sensor, whereby the NO x -component is usually much more tightly limited. If the difference signal exceeds such an absolute limit, it can be safely assumed that there is ammonia slip. The limit of the difference to be defined depends on a plausible NO x -proportion in the exhaust gas, the acceptable NH3 slip and the measuring range of the exhaust gas sensors.

[0022] Another advantageous method provides for a temporal integral value to be calculated from the difference signal and compared with an applicable integral limit value. If this integral limit value is exceeded, an ammonia slip is detected. This integral calculation allows for short-term tolerance of higher NH3 concentrations, which are less harmful in the exhaust gas. The limit value of the integral to be defined depends on the plausible NOx -proportion in the exhaust gas and acceptable NH3 slip.

[0023] Combining the aforementioned methods can increase the accuracy of the diagnosis. This allows for special effects that may interfere with one of the evaluation methods to be checked using the other method, and the result can be corrected if necessary.

[0024] It is particularly advantageous if the above-mentioned limit values ​​are applied at least temporarily depending on the operating conditions of the internal combustion engine. This is particularly true during operating phases, e.g. during the regeneration phase of a particulate filter, where a higher NO x -content and / or an NH3 content is common and cannot be avoided, error messages regarding NH3 slip can be avoided.

[0025] Due to the rapid detection of ammonia slip, the process provides that, if ammonia slip is detected, measures to limit the ammonia slip can be initiated immediately before an odor nuisance or even a massive ammonia release can occur.

[0026] The measures are intended to comprise a multi-stage process that is carried out partially or completely, depending on the extent of the detected ammonia slip. The first stage of this multi-stage process can initially involve preventing the addition of reagent. If ammonia slip is still detected, the second stage can generally reduce the exhaust gas temperature, which can be achieved, for example, by limiting the engine's power. If this measure is still not sufficient to prevent the ammonia slip, the third stage can, for example, limit the number of restarts and display a request for a workshop visit as long as the error continues to occur.

[0027] Within the scope of on-board diagnostics, it may be advantageous in another process variant to perform a check of the exhaust gas sensor signals in addition to the ammonia slip diagnosis. For diagnostic purposes, certain states are specified for the dosing unit, at least temporarily. This can be used, for example, to check the functionality of the exhaust gas sensors. Furthermore, certain functions of the dosing unit can also be checked.

[0028] The object of the device is achieved in that a diagnostic unit is provided for detecting ammonia slip, in which the functionality of the method according to the invention and its variants is implemented. The implementation can be at least partially software-based, whereby the diagnostic unit can be designed as a separate unit or as an integral component of a higher-level engine control system. To check the plausibility of ammonia slip, the diagnostic unit has devices, in particular at least one subtraction unit for forming the difference, calculation units for calculating temporal gradients and / or integrals, checking units, and comparators for comparing the signals with applicable limit values.Furthermore, memory units and map memories can be implemented in the diagnostic unit, allowing various limit values ​​to be stored or, using specific engine parameters, operating-phase-dependent limit values ​​for the plausibility check of ammonia slip. Logic units can also be implemented, allowing the results to be evaluated, for example, when combining different evaluation strategies.

[0029] The invention is explained in more detail below using an exemplary embodiment illustrated in the figures. It shows: Fig. 1 an example of a technical environment for the invention, Fig. 2 in a trend diagram the signal curve of an exhaust gas sensor as well as the curve of the nitrogen oxide and ammonia concentration as a function of a dosing quantity for a reducing agent and Fig. 3 shows a functional diagram schematically the detection method for ammonia slip.

[0030] Fig. Figure 1 shows an example of a technical environment in which the method according to the invention can be applied. The illustration is limited to the components necessary for explaining the invention.

[0031] In the Fig. Figure 1 shows, by way of example, an internal combustion engine 1 configured as a diesel engine, consisting of an engine block 10 and an exhaust duct 30 through which an exhaust gas stream 20 is conducted. The exhaust duct 30 has an exhaust gas purification system, which, in the example shown, is a catalytically coated component arranged in the flow direction of the exhaust gas, initially comprising a diesel oxidation catalyst 40 (DOC) and a diesel particulate filter 50 (DPF). This is followed by an SCR catalyst 80, upstream of which a reducing agent can be introduced by means of a dosing unit 110 to reduce nitrogen oxides in the exhaust stream.

[0032] To monitor the nitrogen oxide concentration in the exhaust gas, an exhaust gas sensor 90 designed as a nitrogen oxide sensor is installed in the exhaust duct 30 downstream of the SCR catalyst 80 in the direction of exhaust gas flow. Amperometric dual-chamber sensors, such as those described above, serve as nitrogen oxide sensors. Such sensors are typically calibrated to 100% NO and therefore exhibit a corresponding cross-sensitivity to NO2 and NH3. If necessary, another exhaust gas sensor 90 designed as a nitrogen oxide sensor can also be provided upstream of the SCR catalyst 80.

[0033] The exhaust gas sensors 90 are connected to a diagnostic unit 101 in which the signals are evaluated. The diagnostic unit 101 has corresponding comparison devices (subtraction units, comparators, etc.) and can, as shown in the Fig. 1, be an integral component of a higher-level engine control system 100. The functionality of the diagnostic unit 101 can be implemented in the engine control system 100 using software and / or hardware.

[0034] Also shown is a pressure sensor 60, which can be configured as a differential pressure sensor and serves to monitor the soot load of the diesel particulate filter 50 (DPF). A temperature sensor 70 can also be provided in the exhaust duct 30. The signals from these sensors can also be fed to the diagnostic unit 101.

[0035] In Fig. 2, a characteristic diagram 200 shows a typical sensor signal curve 203 of the exhaust gas sensor 90, designed as a nitrogen oxide sensor, as a function of a metered amount 202 of the metering unit 110 (abscissa), with a sensor signal level 201 plotted as a first ordinate. Furthermore, this characteristic diagram 200 shows the curve of a nitrogen oxide and an ammonia concentration 204, 205, with corresponding concentration values ​​206 indicated on a second ordinate.

[0036] With increasing NH3 content, which correlates with an increasing dosage amount 202 of reducing agent, the nitrogen oxide content is reduced, so that the sensor signal level 201 of the exhaust gas sensor 90 decreases. If there is no longer enough NO xavailable for the reaction, further addition of reducing agent results in NH3 slip, which is accompanied by an increase in the ammonia concentration 205. An increase in the ammonia concentration 205 causes the sensor signal level 201 to rise again. In certain operating conditions, NO x can be excluded and thus NH3 slip can be assumed.

[0037] The detection procedure for NH3 slip is based on different plausibility approaches, which are described below.

[0038] The monitoring function depends on the NO x -Sensor after the SCR catalyst 80 and from a known NO x -Concentration before the SCR catalyst 80. Depending on the operating point, it is checked whether the measured value of the NO x -Sensor rather around NO xor NH3. Depending on applicable limit values ​​and possible exclusion conditions, a corresponding reaction of the dosing unit 110 can be initiated.

[0039] The detection method according to the invention for an NH3 slip is based on a difference determination between the signal of the exhaust gas sensor 90 behind the SCR catalyst 80 and a calculated measure for the NO x -current in the exhaust gas flow 20 of the internal combustion engine 1. Due to the cross-sensitivity, the sensor signal of the NO x -Sensor designed exhaust gas sensor 90 the sum of reagent slip, here ammonia, and NO x -current or ammonia concentration 205. The NO x -Current determination calculated measure for the NO x current is subtracted from the exhaust gas sensor signal in a differential calculation. A difference occurring can mean either reagent slip or high NO x-current has occurred. If available, an additional exhaust gas sensor 90, which is also used as NO x sensor and is located in the flow direction of the exhaust gas upstream of the SCR catalyst 80, directly measures the difference between the NO x -Current before and after the SCR catalyst 80 can be determined and evaluated. Furthermore, the expected efficiency of the SCR catalyst 80 can be taken into account.

[0040] Fig. 3 shows schematically in a functional diagram 300 different process variants of the process according to the invention.

[0041] A NO x -Concentration value before catalyst 302 is subtracted from a measured NO by means of a subtraction unit 303 x -Concentration value after catalyst 301 is subtracted, whereby the NO x -Concentration value before catalyst 302 as a model or by means of another as NO x-Sensor designed exhaust gas sensor 90 in front of the catalyst.

[0042] In a first checking unit 304, a method variant checks whether an increase in the difference, despite a correction within a defined time window, is higher than or reaches an applicable differential limit value. Since the NO x Since the CO2 content in the exhaust gas only increases with a limited gradient, but the NH3 content can increase more rapidly, exceeding this differential limit is an indication of NH3 slip. To evaluate the gradient with which the difference increases, the system behavior must be applied specifically for the vehicle classes.

[0043] In a second verification unit 305, it can be checked whether the difference reaches or exceeds an absolute limit despite a correction. This evaluation variant is based on the fact that the NO x-component in the exhaust gas should be within a regulated range, at least within the measuring range of the exhaust gas sensor 90. Ideally, the proportion is much more narrowly limited. The limit of the difference to be defined depends on the plausible NO x -Proportion in the exhaust gas, the acceptable NH3 slip and the measuring range of the exhaust gas sensors 90.

[0044] In a third verification unit 306, it can be checked whether a time integral of the difference reaches or exceeds an applicable integral limit value despite a correction. This evaluation strategy is based on the fact that higher NH3 concentrations can occur in the exhaust gas for a short time. This fact is taken into account with a time integral. The limit value of the integral to be defined depends on a plausible NO x -proportion in the exhaust gas and acceptable NH3 slip.

[0045] In principle, each of the aforementioned evaluation strategies, based on a difference value, is suitable for detecting NH3 slip. However, according to the invention, a combination of two of these evaluation strategies or a combination of the three evaluation strategies is also suitable as a detection method for NH3 slip.

[0046] Additionally, an adaptation of the above-mentioned limit values ​​can be provided for operating conditions, such as during particulate filter regeneration. During particulate filter regeneration, a short-term NH3 slip is often unavoidable due to a high temperature gradient. To prevent misdiagnosis, a temporary increase in the limit values ​​may therefore be advisable.

[0047] Furthermore, according to the invention, measures are provided to limit NH3 slip when NH3 slip is detected. On the one hand, the supply of further reagent dosage can be prevented. This can be achieved, for example, by back-feeding and switching off the dosing system. In addition, a general reduction in the exhaust gas temperature can help to reduce the conversion of the stored and released reagent and thus the NH3 slip. This can be achieved, for example, by limiting the engine power. As a last resort, if the above-mentioned measures do not work and the error continues to occur, the number of restarts can be limited and / or a reminder to visit a workshop can be provided via a display (e.g., via the engine control light).

[0048] In a further embodiment of the method, a plausibility check of the sensor values ​​for states of the dosing unit 110 initiated for diagnostic purposes can be integrated into this method.

Claims

[1] Method for detecting an ammonia slip in an exhaust gas aftertreatment system of an internal combustion engine (1), which has an SCR catalyst (80) and in which, for nitrogen oxide reduction, an ammonia-releasing reducing agent solution is metered into the exhaust gas flow (20) by means of a metering unit (110) in the flow direction of the exhaust gas upstream of the SCR catalyst (80), wherein a cross-sensitivity to nitrogen oxides and ammonia of at least one NO x -Sensor designed exhaust gas sensor (90), which is arranged in the flow direction of the exhaust gas behind the SCR catalyst (80) in the exhaust gas duct (30), is used to detect an ammonia breakthrough, characterized by that in order to check the plausibility of an ammonia slip, a difference signal is formed by determining a value for the NO from the output signal of the exhaust gas sensor (90) behind the SCR catalyst (80) as a measure for the sum of a nitrogen oxide and an ammonia concentration (204, 205). x-concentration is subtracted before the SCR catalyst (80), and the temporal course of the difference signal or the course of variables derived therefrom is evaluated and compared with applicable limit values. [2] Method according to claim 1, characterized by that the NO x -Concentration before the SCR catalyst (80) calculated from a model or with a NO x -Sensor formed exhaust gas sensor (90), which is arranged in the flow direction of the exhaust gas upstream of the SCR catalyst (80). [3] Method according to claim 1 or 2, characterized by that an expected conversion efficiency for the SCR catalyst (80) is taken into account as a correction value when forming the difference. [4] Method according to one of claims 1 to 3, characterized bythat a temporal gradient is formed from the difference signal and compared with an applicable differential limit value, whereby an ammonia slip is detected if this differential limit value is exceeded. [5] Method according to one of claims 1 to 3, characterized by that the difference signal is compared with an applicable absolute limit value, whereby an ammonia slip is detected if this absolute limit value is exceeded. [6] Method according to one of claims 1 to 3, characterized by that a time integral value is formed from the difference signal and compared with an applicable integral limit value, whereby an ammonia slip is detected if this integral limit value is exceeded. [7] Method according to at least one of claims 4 to 6, characterized by that the evaluation methods according to claims 4 to 6 are applied in combination. [8] Method according to at least one of claims 1 to 7, characterized by that the limit values ​​are applied at least for a limited time depending on the operating conditions of the internal combustion engine (1). [9] Method according to one of claims 1 to 8, characterized by that if ammonia slip is detected, measures are taken to limit the ammonia slip. [10] Method according to claim 9, characterized by that the measures comprise a multi-stage process that is carried out partially or completely, depending on the extent of the ammonia slip detected. [11] Method according to one of claims 1 to 10, characterized by that in addition to the ammonia slip diagnosis, a check of the sensor signals of the exhaust gas sensors (90) is carried out, wherein for diagnostic purposes, certain states for the dosing unit (110) are specified at least temporarily. [12] Device, in particular a diagnostic unit (101), for detecting an ammonia slip in an exhaust gas aftertreatment system of an internal combustion engine (1), which has an SCR catalyst (80) and in which, for nitrogen oxide reduction, an ammonia-releasing reducing agent solution can be metered into the exhaust gas flow (20) upstream of the SCR catalyst (80) by means of a metering unit (110), wherein a cross-sensitivity to nitrogen oxides and ammonia of at least one NO x -Sensor designed exhaust gas sensor (90), which is arranged in the flow direction of the exhaust gas behind the SCR catalyst (80) in the exhaust gas duct (30), can be used to detect an ammonia breakthrough, characterized bythat the diagnostic unit (101) for checking the plausibility of an ammonia slip comprises devices, in particular at least one subtraction unit (303), calculation units for calculating temporal gradients and / or integrals, checking units (304, 305, 306) and comparators, for carrying out the detection method according to method claims 1 to 11.

Images