Method for checking the quality of an ammonia-containing reducing agent

Abstract

Method for checking the quality of a reducing agent containing ammonia in free or bound form, which is added to the exhaust gas emitted by the internal combustion engine upstream of an SCR catalyst (4) arranged in the exhaust gas purification system (1) for the selective catalytic reduction of nitrogen oxides in an exhaust gas purification system (1) of a motor vehicle internal combustion engine, wherein - in a first process step, an addition quantity of the reducing agent is set according to a first dosage ratio (α1) with respect to a nitrogen oxide content in the exhaust gas upstream of the SCR catalyst (4) and a first actual value for a nitrogen oxide conversion of the SCR catalyst (4) caused by this is determined, and - in a second process step, an addition quantity of the reducing agent is set according to a second dosing ratio (α2) and a second actual value for a nitrogen oxide conversion of the SCR catalyst (4) caused by this is determined, characterized in that - an actual value difference between the first actual value and the second actual value is determined and the actual value difference is compared with a target value difference, determined from a first target value for nitrogen oxide conversion assigned to the first dosing ratio (α1) and a second target value for nitrogen oxide conversion assigned to the second dosing ratio (α2), - if the actual value difference deviates from the target value difference by more than a predefined amount, an unacceptably reduced quality of the reducing agent is diagnosed, and - the first and second process steps are carried out during an ongoing thermal soot combustion regeneration of a particulate filter (3) arranged in the exhaust gas purification system.

Description

[0001] The invention relates to a method for testing the quality of a reducing agent containing ammonia in free or bound form according to the preamble of claim 1.

[0002] Aqueous urea solution (HWL) is widely used to remove nitrogen oxides (NOx) from the exhaust gas, particularly from motor vehicle internal combustion engines operated with excess air. This solution is added to the exhaust gas upstream of an SCR catalyst capable of selective catalytic NOx reduction under oxidizing conditions. In the hot exhaust gas, ammonia (NH3) is released from the urea by thermolysis and / or hydrolysis. This ammonia serves as a directly effective reducing agent and reduces NOx present in the exhaust gas at the SCR catalyst to harmless nitrogen (N2). However, another reducing agent containing ammonia in free or bound form can be used instead of HWL. In order to achieve the desired level of nitrogen oxide reduction, it must be ensured that the reducing agent used has a prescribed quality with regard to its ammonia content.For example, the urea solvent used should have the prescribed urea content of approximately 32%. Various methods for directly or indirectly verifying the quality of the reducing agent have already been proposed.

[0003] A generic method is shown in US 2011 / 0 047 964 A1, in which a total of four dosing ratios are set with respect to a nitrogen oxide content in the exhaust gas upstream of the SCR catalyst and in each case an actual value is determined for a nitrogen oxide conversion of the SCR catalyst caused thereby.

[0004] For example, an indirect method is known from US 7 610 750 B2, in which it is determined whether a deterioration in the urea water quality has occurred by comparing NOx conversions achieved at a certain dosing ratio with NOx conversions previously achieved under comparable conditions.

[0005] US 2011 / 0 252 767 A1 describes a method for diagnosing a NOx -sensor and in particular the detection of a reaction difference between two NO x Sensors on each side of a catalyst with ammonia storage capacity. This makes it possible to detect a failed or deviating NO x sensor, allowing the control scheme to provide an alternative NO x -determination and / or set an error indicator.

[0006] DE 10 2008 047 860 B3 relates to a method for leak testing a reducing agent injector of an SCR exhaust aftertreatment system of an internal combustion engine. The method comprises the steps of: a) generating and / or detecting a release state of the SCR system in which no reducing agent is stored in the SCR catalyst; b) determining a target NOx value in the exhaust gas; c) starting the leak test with the reducing agent injector closed by increasing the reducing agent pressure; d) detecting the NOx signal at the NOx sensor; and e) evaluating the NOx signal.

[0007] The object of the invention is to provide an improved method for checking the quality of a reducing agent containing ammonia in free or bound form.

[0008] This object is achieved by the methods having the features of claims 1 and 2.

[0009] In the method according to the invention for testing the quality of a reducing agent containing ammonia in free or bound form, which is added to the exhaust gas emitted by the internal combustion engine upstream of an SCR catalyst arranged in the exhaust gas purification system for the selective catalytic reduction of nitrogen oxides in an exhaust gas purification system of a motor vehicle internal combustion engine, in a first method step, an added amount of the reducing agent is set according to a first dosing ratio with respect to a nitrogen oxide content in the exhaust gas upstream of the SCR catalyst, and a first actual value for a nitrogen oxide conversion of the SCR catalyst caused thereby is determined. In a second method step, an added amount of the reducing agent is set according to a second dosing ratio, and a second actual value for a nitrogen oxide conversion of the SCR catalyst caused thereby is determined.According to the invention, an actual value difference between the first actual value and the second actual value is determined, and the actual value difference is compared with a target value difference. The target value difference is determined from a first target value for nitrogen oxide conversion assigned to the first dosing ratio and a second target value for nitrogen oxide conversion assigned to the second dosing ratio. If a deviation of the actual value difference from the target value difference by more than a predeterminable amount is detected, an impermissibly reduced quality of the reducing agent is diagnosed.

[0010] The method according to the invention preferably checks whether the urea content of urea water used as the reducing agent is within the prescribed range or deviates from it, particularly downwards. However, the method is also suitable for checking the quality of other reducing agents containing ammonia in free or bound form, such as ammonia water, carbamate solutions, or formate solutions.

[0011] As is generally accepted, the dosing ratio is understood here as the calculated molar ratio of the NH3 introduced into the exhaust gas in free or bound form due to a set amount of the reducing agent added to the nitrogen monoxide (NO) or nitrogen dioxide (NO2) present in the exhaust gas, assuming a given NH3 content of the reducing agent. A dosing ratio of one corresponds to a molar ratio of NH3 and NO or NO2 of 1:1 according to the following nitrogen oxide reduction equations: 4 NH3+4 NO+O2→4 N2+6 H2O 2 NH3+NO+NO2→2 N2+3 H2O

[0012] For dosing ratios greater than one, there is a corresponding calculated NH3 surplus, and for smaller dosing ratios, there is a corresponding calculated NH3 deficit.

[0013] The nitrogen oxide conversion n is defined as a NOx content c present on the inlet side of the SCR catalyst. NOx,einrelative decrease Δc NOx of the NOx content in the exhaust gas according to the equation η=ΔcNOx / cNOx,on=(cNOx,on−cNOx,off) / cNOx,on understood, where c NOx,aus the NOx content in the exhaust gas at the outlet side of the SCR catalyst.

[0014] The actual values ​​for the nitrogen oxide conversion are determined, in particular, at dosing ratios that differ significantly from zero. Preferably, the first and second dosing ratios differ by at least a factor of 2. The first dosing ratio is preferably smaller than the second dosing ratio, but the reverse setting can also be used. The smaller dosing ratio is preferably in the range from 0.1 to 0.4. A value of approximately 0.3 is particularly preferred. The larger dosing ratio is preferably in the range between 0.4 and 0.8.

[0015] The actual values ​​for nitrogen oxide conversion are preferably determined by sensors. For this purpose, an exhaust gas sensor sensitive to NOx is preferably arranged at least downstream of the SCR catalyst in the exhaust gas purification system. This sensor can emit a signal from which a connected control unit can determine the NOx content c present in the exhaust gas at the outlet side of the SCR catalyst. NOx,aus The NOx content c present in the exhaust gas at the inlet side of the SCR catalyst NOx,ein can also be determined using such a sensor. However, it is also possible to determine the NOx emissions from engine maps based on the engine's operating conditions.

[0016] The target values ​​for nitrogen oxide conversion assigned to a particular dosing ratio are conveniently taken from corresponding characteristic maps, in which they are stored in a readable format depending on the key operating conditions of the internal combustion engine and / or the exhaust gas purification system. This can also take into account any age-related deterioration in the effectiveness of the SCR catalyst.

[0017] By using NOx conversions of the SCR catalyst determined at two different dosing ratios, improved accuracy in determining the reducing agent quality is possible compared to methods in which NOx conversion is taken into account at only one dosing ratio. In particular, the inventive approach enables a reduction in interference and inaccuracies due to system aging and cross-sensitivities, particularly of the sensors used to HC, NO2, H2, and the like. The verification method is particularly robust against changes in operating conditions and, as has been determined, delivers comparatively more accurate and reliable results.

[0018] According to a first aspect of the invention, the first and second method steps are performed during an ongoing thermal soot burn-off regeneration of a particulate filter arranged in the exhaust gas purification system. During this particulate filter regeneration, elevated temperatures of approximately 700°C to 850°C are set for the particulate filter. The particulate filter regeneration is considered to be ongoing when the particulate filter has the temperature required for soot burn-off at a noticeable rate. Carrying out the first and second method steps during an ongoing particulate filter regeneration has the advantage that, due to the increased temperatures of the particulate filter and thus also of the preferably downstream SCR catalyst and other exhaust gas purification components in the exhaust gas purification system, the NH3 storage capacity of the SCR catalyst is comparatively low.Therefore, only a small amount of NH3 stored in the SCR catalyst can hardly or not at all distort the NOx reduction achieved by the addition of the reducing agent to the exhaust gas. Furthermore, the often disruptive NO2 content in the exhaust gas is low due to the prevailing high temperatures. Therefore, more accurate and reliable results can be achieved.

[0019] According to a second aspect of the invention, the addition of the reducing agent is suspended for a predeterminable period of time immediately before carrying out the first method step. This enables a controlled discharge of any deposits of reducing agent residues present in the exhaust gas purification system. A determination of actual values ​​for NOx conversion influenced by deposits is therefore avoidable. Furthermore, any stored NH3 is removed from the SCR catalyst, bringing it into a defined state for determining its efficiency. The addition of the reducing agent is preferably suspended until the NOx content in the exhaust gas upstream of the SCR catalyst deviates from the NOx content in the exhaust gas downstream of the SCR catalyst by less than a predeterminable amount. This ensures that the discharge processes have at least largely subsided.

[0020] It can also be provided that the addition of the reducing agent is suspended at least until the ratio of the NOx content in the exhaust gas upstream of the SCR catalyst and the NOx content in the exhaust gas downstream of the SCR catalyst deviates from one by less than a predeterminable amount and / or the temporal gradient of the ratio of the NOx contents falls below a predeterminable lower limit value.

[0021] In a further embodiment of the invention, in a third method step following the second method step, the addition of the reducing agent is suspended for a predeterminable period of time. In this case, too, the SCR catalyst is placed in a defined state, and in the third method step, a plausibility check of the obtained result can advantageously be performed. In particular, when using a NOx sensor, its reading accuracy can be verified. It is particularly preferred if the third method step takes place after the completion of the regeneration of the particulate filter.

[0022] In a further embodiment of the invention, in the first and second method steps, a predeterminable number of first and second actual values ​​are determined at a predeterminable time interval and added together to form a respective sum value. From the sum value, an average value for the first and second actual values ​​is determined and the actual value difference is determined from the average values ​​for the first and second actual values. Actual values ​​are preferably determined at a rate of approximately 10 Hz. The number of actual values ​​provided for a sum value can be specified either over a predeterminable period of time from approximately 30 s to several minutes, preferably approximately one minute, or by a target value of recorded and validly evaluated actual values, preferably approximately 300 actual values. As a result of the provided averaging over a large number of recorded actual values, an increase in accuracy and reliability is possible.It is particularly preferred if, in a further embodiment of the invention, actual values ​​are only used to determine the total value if predeterminable stability conditions for an exhaust gas mass flow and / or for the nitrogen oxide content in the exhaust gas upstream of the SCR catalyst are met.

[0023] Advantageous embodiments of the invention are illustrated in the drawings and described below. The features mentioned above and those to be explained below can be used not only in the respective combinations of features specified, but also in other combinations or on their own, without departing from the scope of the present invention.

[0024] It shows Fig. 1 a schematic block diagram of an exhaust gas purification system connected to a motor vehicle internal combustion engine, Fig. 2 a schematic characteristic diagram to illustrate the relationship between nitrogen oxide conversion and dosing ratio, Fig. 3 a schematic flow diagram illustrating the procedure for checking the reducing agent quality, Fig. 4 a diagram for exemplary temporal curves of a ratio of NOx contents in the exhaust gas determined downstream and upstream of the SCR catalyst during system conditioning, Fig. 5 related diagrams for exemplary time courses of a status bit concerning the stability, a set calculated dosing ratio α and a number of determined actual values ​​for the NOx conversion η of the SCR catalyst during a check of the reducing agent quality and Fig. 6 related diagrams for exemplary time courses of a status bit concerning the stability, NOx contents in the exhaust gas determined downstream and upstream of the SCR catalyst and a number of determined actual values ​​for the NOx contents during a plausibility check.

[0025] In Fig. Figure 1 shows a merely rough schematic of an exhaust gas purification system 1 of a motor vehicle internal combustion engine, in particular a diesel engine. Exhaust gas from the internal combustion engine (not shown) enters the exhaust gas purification system 1 in the direction indicated by arrow 10. This system comprises, in the exhaust gas flow direction, an oxidation catalyst 2, a particulate filter 3, and an SCR catalyst 4 arranged one after the other in an exhaust line 9. The SCR catalyst 4 can also be designed as a dual-flow catalyst arrangement comprising two catalyst units connected in parallel.

[0026] The exhaust gas purification device 1 further comprises a fuel addition device 5 upstream of the oxidation catalyst 2. A hydrocarbon- and / or hydrogen-containing fuel can be introduced into the exhaust line 9 via the fuel addition device 5. The fuel addition device 5 is preferably designed to supply a fuel present on board the vehicle, in particular intended for its propulsion, to the exhaust gas.

[0027] A reducing agent containing NH3 in free or bound form can be added to the exhaust gas via a reducing agent addition device 6 for selective catalytic NOx reduction. Without limiting generality, it is assumed below that the reducing agent is an aqueous urea solution (UWL). The UWL preferably has a target urea concentration of approximately 32% and is taken from a storage container (not shown).

[0028] Furthermore, a first NOx-sensitive sensor 7 is provided upstream of the SCR catalyst 4. As shown, this is arranged between the fuel addition device 5 and the oxidation catalyst 2 in the exhaust line 9. An arrangement at a different location upstream of the SCR catalyst 4, for example between the particulate filter 3 and the reducing agent addition device 6, can also be provided. A further, second NOx sensor 8 is arranged downstream of the SCR catalyst 4 in the exhaust line 9. The NOx conversion of the SCR catalyst 4 is preferably determined by evaluating the signals emitted by the NOx sensors 7, 8 in the method according to the invention explained below. For this purpose, a control unit (not shown) is preferably provided, which can receive and process the signals from the NOx sensors 7, 8.This control unit can preferably also control the operation of the addition devices 5, 6 and carry out further control and evaluation functions relating to the operation of the internal combustion engine and the exhaust gas purification system 1.

[0029] It is understood that the exhaust gas purification system 1 may have further catalytic or filter-effective components not specifically shown here for exhaust gas purification. In particular, a nitrogen oxide storage catalyst may be provided upstream of the oxidation catalyst and / or between the oxidation catalyst 2 and the particulate filter 3 and / or between the particulate filter 3 and the SCR catalyst 4. Particularly preferably, an ammonia barrier catalyst for oxidizing NH3 slip is provided downstream of the SCR catalyst 4. Furthermore, additional exhaust gas, pressure, and temperature sensors (not shown) are preferably provided, via whose signals the operation of the exhaust gas purification system 1 and the internal combustion engine can be adjusted as needed. Temperature sensors are preferably provided upstream and downstream of the oxidation catalyst 2 and / or the particulate filter 3 and / or the SCR catalyst 4.

[0030] The oxidation catalyst 2 is preferably designed as a so-called diesel oxidation catalyst with a coating with low or no oxygen storage capacity. A coating with a three-way catalyst function can also be provided. A support for the coating is preferably designed as a ceramic support or as a metal foil support body. A heating element, preferably designed as a coated metal foil support body (so-called E-catalyst), can also be provided directly upstream.

[0031] The particulate filter 3 can be constructed of sintered metal or as a wall-flow filter unit in a honeycomb structure. Preferably, a catalytic coating, for example, with an oxidation-catalytically active material and / or with an SCR catalyst material, is provided for the particulate filter 3. The SCR catalyst material can catalyze a selective, continuous reduction of NOx under oxidizing conditions using stored and / or supplied ammonia (NH3) as a selective NOx reducing agent. A zeolite containing iron or copper is preferred as the catalyst material. The SCR catalyst material can be provided on the raw gas side and / or on the clean gas side of the filter-effective surfaces of the particulate filter 3.In the preferred embodiment of the particulate filter 3 as a conventional wall-flow filter with a honeycomb structure, any coating with the corresponding SCR catalyst material is preferably provided on the channel walls exposed to the raw gas. In this case in particular, it may be advantageous to provide the SCR catalyst coating only in sections on the inlet side or outlet side of the channel walls of the particulate filter 3. For example, a coating with the SCR catalyst material can be provided over a first part of the axial extent of the particulate filter 3, approximately 50% of its length. The rear part, viewed in the axial direction, can be uncoated or provided with an oxidation-catalytically effective coating.

[0032] The SCR catalyst 4 arranged downstream of the particulate filter 3 is capable of selectively reducing NOx with NH3, analogous to any SCR coating present on the particulate filter 3, and is preferably designed similarly with respect to such a coating. Two different SCR coatings with different temperature ranges for maximum effectiveness can also be provided, arranged one behind the other in the direction of exhaust gas flow, on one and the same carrier body or on two carrier bodies arranged one behind the other at a short distance. Zoned coatings comprising a zeolite containing iron and copper are preferred. The SCR catalyst 4 is preferably arranged away from the engine in the underbody area of ​​the vehicle. In any case, an arrangement in the exhaust system 9 such that the temperature of the SCR catalyst 4 does not rise above 650°C during thermal particulate filter regeneration is particularly preferred.By geometrically distancing the particulate filter 3 in conjunction with appropriate dimensioning of the exhaust line 9 with respect to heat dissipation, it is possible to achieve a temperature gradient of more than 200 °C or more between a particulate filter 3 heated to approximately 800 °C and the SCR catalyst 4.

[0033] To perform thermal regeneration of the particulate filter 3 by soot burnoff, the fuel addition device 5 is activated, and fuel, in particular diesel fuel, is added to the exhaust gas. Oxidation of the fuel at the oxidation catalyst 2 heats the exhaust gas supplied to the particulate filter 3 and the particulate filter 3 to a temperature of approximately 650°C to 850°C, the temperature required for soot burnoff. A fuel supply, possibly with interruptions, is usually required for the entire duration of the thermal particulate filter regeneration in order to maintain the temperature of the particulate filter 3 at the temperature required for soot burnoff.

[0034] The principle underlying the method according to the invention for checking the quality of urea is explained below using Fig. 2 explained. Fig. 2 schematically shows a diagram showing the dependence of a NOx conversion η of the SCR catalyst 4 on the dosing ratio α for urea added to the exhaust gas via the addition unit 6. The first lane 21 schematically shows a dependency as it results when the urea added to the exhaust gas has the intended urea concentration of preferably about 32%. The first lane 21 therefore indicates a dependency of target values ​​of the NOx conversion η as a function of the dosing ratio α. In contrast, the second lane 22 schematically shows the resulting conditions when a calculated dosing ratio α based on the intended urea content is set, but the urea added to the exhaust gas has a lower urea concentration and thus a deteriorated quality. The relationship shown in the diagram of Fig. The ratios shown in Figure 2 with the second lane 22 correspond to a dilution of the urea added to the exhaust gas of approximately 45%. Depending on the dilution of the urea, nitrogen oxide conversions η are achieved compared to undiluted urea with the same addition amount or the same calculated dosing ratio α, which are lower than the expected target values ​​according to the first lane 21.

[0035] In the method according to the invention, in a first method step, an addition of urea solvent to the exhaust gas is adjusted via the addition device 6 in accordance with a first calculated dosing ratio α1 and in a second method step, an addition of urea solvent is adjusted in accordance with a second calculated dosing ratio α2. Fig. 2, the first dosing ratio α1 is different from zero and lower than the second dosing ratio α2. A reverse assignment is of course also possible. Preferred values ​​for the dosing ratios α1 and α2 are approximately 0.2 and approximately 0.7. The NOx conversions η resulting from a respective dosing ratio α are determined on the basis of the signals from the nitrogen oxide sensors 7, 8. For the Fig. In the case illustrated by way of example in Figure 2, the actual values ​​η3 and η4 result when appropriately diluted HWL is used, whereas the larger target values ​​η1 and η2 for the NOx conversion η of the SCR catalyst 4 should result when undiluted HWL is used. From the determined actual values ​​η3 and η4, an actual value difference (η4 - η3) is determined and compared with the associated target value difference (η2 - η1). If the actual value difference deviates from the target value difference by more than a specified amount, an error message is output and / or entered in an error log. The critical value for this can be specified as a relative value of, for example, 10% related to the target value difference. A specification of an absolute value of, for example, 0.1 can also be provided. In this way, it is possible to check the quality of the urea wash largely independently of interference.In response to a detected impermissible urea dilution, the vehicle's drivability can also be restricted, for example by limiting the maximum speed or the engine torque to a predefined upper limit. Such a restriction preferably only takes effect after a predefined time has elapsed, with a warning display.

[0036] A preferred procedure for determining the actual values ​​for the NOx conversion η and the entire verification process is described below using the Fig. 3 to 6 are explained in more detail, with reference first to Fig. 3 is referred to.

[0037] In Fig. Figure 3 is a flowchart illustrating a chronological sequence of measures and method steps in conjunction with a preferred procedure for checking the reducing agent quality. According to the invention, this is carried out during ongoing thermal regeneration of the particulate filter 3 by soot burnoff. Regeneration begins at time t0 by heating the particulate filter 3 and ends at time t3, which is indicated by block 30 "DPF regeneration status." For heating purposes, internal engine measures, such as early fuel post-injection, are initially taken. These measures are maintained until time t1, which is indicated by block 33. Once a predeterminable temperature of the particulate filter 3 is reached, the fuel addition device 5 is activated and fuel is supplied to the exhaust gas.This leads to a further heating of the particulate filter 3 to a temperature required for soot burn-off, corresponding to the track 37 for the temperature of the particulate filter 3, by oxidizing the added fuel at the oxidation catalyst 2. The fuel addition remains activated for the duration of the particulate filter regeneration, provided it is required to maintain the soot burn-off temperature, which is illustrated by block 31.

[0038] Upon completion of the internal engine heating measures identified by block 33, a previously active HWL dosing via the reducing agent addition device 6 is terminated, which is identified by block 32a. The HWL dosing remains switched off until time t2, and conditioning is carried out during the time period identified by reference numeral 34 (block "conditioning"), which will be described further below with reference to Fig. 4 will be discussed in more detail.

[0039] After the conditioning is complete, the actual check of the urea water quality is carried out, particularly with regard to any dilution that may be present, which is indicated by block 35 ("Dilution"). This block 35 begins at time t2 and is preferably completed at the latest with the completion of the particulate filter regeneration at time t3. During this check, the Fig. 2 above, and a UWL dosage is carried out with two different dosage ratios, which is symbolically represented by block 32 b. A detailed description of a preferred procedure for the data collection carried out in this case is given below with reference to Fig. 5 is explained in more detail.

[0040] Directly following the actual verification procedure regarding the urea solvent quality, indicated by block 35, and after completion of the particulate filter regeneration, a plausibility check is performed, which includes a review of the signals from the NOx sensors 7, 8, represented by block 36 ("Sensor Check"). During this plausibility check, the urea solvent supply via the reducing agent addition device 6 is shut off. Upon completion of the plausibility check at time t4, normal urea solvent dosing, intended for optimal NOx reduction at the SCR catalyst 4, is resumed, which is indicated by block 32c.

[0041] The following is based on Fig. 4 the preferred procedure for conditioning the exhaust gas purification system 1 in the conditioning block 34 is explained in more detail.

[0042] In Fig. 4 shows two exemplary time curves 41, 42 of a NOx ratio cNOx,aus / c NOx,einof NOx contents downstream and upstream of the SCR catalyst 4, preferably determined by means of the NOx sensors 7, 8. The conditioning performed in the conditioning block 34 primarily serves to break down NH3 stored in the SCR catalyst 4 and to remove any reducing agent or urea deposits present in the exhaust gas purification system 1. These two potential NH3 sources could falsify the urea water quality check performed in the subsequent check block 35 by simulating a dosing ratio α that does not exist mathematically and is not set via the reducing agent addition device, and could lead to an inaccurate or incorrect result. This is avoided by the conditioning with the removal of stored NH3 or NH3 deposited in free or bound form.

[0043] When the addition of urea to the exhaust gas is switched off in the conditioning block 34, NH3 stored in the SCR catalyst 4 is broken down by reaction with inflowing NOx as temperature and time increase. As a result, the NOx content c present at the outlet side of the SCR catalyst 4 approaches NOx,aus in the exhaust gas the NOx content present on the inlet side c NOx,ein This behavior is in Fig. 3 is shown schematically by the course of the traces 38 and 39. The corresponding ratio of the NOx contents therefore approaches with the exhaustion of any existing NH3 sources, as in Fig. 4 schematically represented by the track 42, for example, starting from lower values, approaches the value one. As determined by the inventors, this behavior only occurs in the absence of noticeable reducing agent deposits in the exhaust gas purification system 1 or with comparatively low NH3 storage quantities in the SCR catalyst 4. If, on the other hand, comparatively large quantities of reducing agent are deposited in the exhaust gas purification system 1 or stored in the SCR catalyst 4, these are released as a result of the heating of the system during particulate filter regeneration, and a more or less severe NH3 slip occurs on the output side of the SCR catalyst 4. Since a commonly used NOx sensor 8 is cross-sensitive to NH3, in the event of a correspondingly strong NH3 slip, the output signal of this sensor 8 increases and a value for the NOx ratio c, which may temporarily be above one, is determined. NOx,aus / c NOx,einThis case is in Fig. 4 is exemplified by the course of the track 41. In both cases, however, the NOx contents c present at the output and input sides of the SCR catalyst 4 normally approach NOx,aus and c NOx,ein each other and the NOx ratio approaches the value 1 as shown.

[0044] In this case, the time course of the NOx ratio is to be monitored starting at a time tx at which the NOx ratio, starting from lower or higher values, enters a predefined narrow range of, for example, + / - 5% around the value 1.0. If the NOx ratio then changes by less than a predefined amount of, for example, 10% relative within a predefined time interval of, for example, 120 s, the conditioning is considered complete and the check block 35 is started.

[0045] In a particularly preferred procedure, two different dosing ratios α for the urea added to the exhaust gas via the addition device 6 are set successively in the check block 35 over predeterminable time intervals, and actual values ​​for the NOx conversion η are determined by reading the signals from the NOx sensors 7, 8 at a predeterminable time interval. Preferably, the reading of the signals or a validation of the read-in signals only takes place when predeterminable conditions with regard to stationarity or stability, in particular of the exhaust gas mass flow and / or the NOx content c NOx,ein upstream of the SCR catalyst 4 are met. Preferably, it is required that the exhaust gas mass flow and / or NOx content lie within a predeterminable absolute or relative fluctuation range. A preferred procedure in block 35 ("dilution") is described below using Fig. 5 explained.

[0046] Fig. 5 shows three related diagrams for exemplary time courses of a status bit concerning the stability of operating conditions (upper diagram), a set calculated dosing ratio α (middle diagram) and a number of determined actual values ​​for the NOx conversion η of the SCR catalyst 4 based on reading in corresponding validly evaluated signals of the NOx sensors 7, 8 (lower diagram).

[0047] As shown in the upper diagram as an example, the stability conditions are met in the time intervals t51 to t52, t53 to t55 and t56 to t57, which is why signals are only read in or evaluated as valid in these time intervals and therefore the number of actual values ​​specified in the lower diagram can increase.

[0048] At time t50, a first dosing ratio α1 of preferably approximately 0.2 is set, with actual value determination only being enabled after a waiting time Δt has elapsed. This waiting time has already elapsed at time t51, with stable conditions present, so that the number of determined actual values ​​increases, starting from zero. In the time interval t52 to t53, the actual value determination is interrupted because the stability conditions no longer exist. The number of determined first actual values ​​for the set first dosing ratio α1 therefore initially remains constant and only increases from time t53 onwards, when the stability conditions are met again.

[0049] At time t54, the specified number of first actual values ​​is determined, and a second, higher calculated dosing ratio α2, preferably approximately 0.7, is set. After the waiting time Δt has elapsed, the determination of second actual values ​​begins. Analogous to the procedure described, actual values ​​are only determined if stable conditions exist until the specified total number is reached. At time t57, the specified total number for the second actual values ​​is also reached, and the urea water dosing is stopped, and block 35 ("Dilution") is terminated.

[0050] Averages are then calculated from the determined actual values, and these, in turn, are used to calculate an actual value difference. If the actual value difference deviates by a predefined amount, preferably approximately 5% to 10%, from a target value difference for NOx conversion η determined for the dosing ratios α1, α2, a reduced urea content of the urea-based wastewater is diagnosed.

[0051] In this case, the diagnostic result determined in block 35, especially in the case of a diagnosed poor urea water quality, is only considered valid if a subsequent plausibility check procedure establishes that the nitrogen oxide sensors 7, 8 are functioning correctly. The plausibility check procedure performed in block 36 (“Sensor Check”) with the urea water supply to the exhaust gas switched off is subsequently Fig. 6 is explained in more detail.

[0052] Fig. 6 shows three related diagrams for exemplary time courses of a status bit concerning the stability of operating conditions (upper diagram), NOx contents c detected by the NOx sensors 7, 8 NOx in the exhaust gas (middle diagram) and a number of valid values ​​for the NOx content in the exhaust gas recorded by a respective NOx sensor 7, 8 during the plausibility check (lower diagram). In the upper diagram, according to the upper diagram of Fig. 5 time intervals are shown in which stability conditions, analogous to the explanations for Fig. 5 are present. Accordingly, validly evaluated signals from NOx sensors 7, 8 are recorded in the time periods t60 to t61, t62 to t63, and t64 to t65. This is preferably started at the earliest after a predefined waiting time has elapsed following the completion of the urea addition carried out in block 35. It is intended to record signals from NOx sensors 7, 8 at a frequency of approximately 10 Hz, or to determine the corresponding NOx contents c NOx,ein and c NOx,aus a predefined or predetermined time period of approximately 1 to 10 minutes. A time course for this is shown in the middle diagram of Fig. 6 is schematically represented by tracks 66 and 67.

[0053] Since the urea water supply is switched off during the plausibility check, if the NOx sensors 7, 8 are operating correctly, the traces 66, 67 must approach each other as shown due to a lack of NOx conversion at the SCR catalyst 4. In this case, it is intended to diagnose a fault-free operation of the NOx sensors 7, 8 if a predeterminable minimum number of approximately 600 valid values ​​has been recorded, all of which are within a predeterminable error band F by at least one of the NOx contents c NOx,ein and c NOx,aus To diagnose fault-free operation, it may also be required that the ratio of the recorded NOx levels c NOx,ein and c NOx,ausfor the predeterminable number of valid values ​​within a predeterminable tolerance band by one. If this is the case, the result determined in the previous block 35 regarding the urea wash quality is set as valid, particularly if the urea wash quality is diagnosed as being impermissibly reduced. If the specified number of valid values ​​for NOx contents in error band F cannot be recorded within the specified period, an error message regarding the function of NOx sensors 7, 8 is preferably output or written to an error memory. It is also possible to identify the faulty NOx sensor by comparing it with values ​​stored in a characteristic map for a raw NOx emission of the diesel engine.

[0054] The preferred procedures described above make it possible to detect an inadmissible deterioration of the reducing agent quality or an inadmissibly reduced urea content of the urea wash water used with high reliability.

Claims

[1] Method for checking the quality of a reducing agent containing ammonia in free or bound form, which reducing agent is added to the exhaust gas emitted by the internal combustion engine upstream of an SCR catalyst (4) arranged in the exhaust gas purification system (1) for the selective catalytic reduction of nitrogen oxides in an exhaust gas purification system (1) of a motor vehicle internal combustion engine, wherein - in a first method step, an added quantity of the reducing agent is set according to a first dosing ratio (α1) in relation to a nitrogen oxide content in the exhaust gas upstream of the SCR catalyst (4) and a first actual value for a nitrogen oxide conversion of the SCR catalyst (4) caused thereby is determined, and - in a second method step, an added quantity of the reducing agent is set according to a second dosing ratio (α2) and a second actual value for a nitrogen oxide conversion of the SCR catalyst (4) caused thereby is determined, characterized by , that - an actual value difference between the first actual value and the second actual value is determined and the actual value difference is compared with a setpoint value difference determined from a first setpoint value for the nitrogen oxide conversion assigned to the first dosing ratio (α1) and a second setpoint value for the nitrogen oxide conversion assigned to the second dosing ratio (α2), - if the actual value difference deviates from the target value difference by more than a predefined amount, an inadmissibly reduced quality of the reducing agent is diagnosed, and - the first and second process steps are carried out during an ongoing thermal soot burn-off regeneration of a particle filter (3) arranged in the exhaust gas purification system. [2] Method for checking the quality of a reducing agent containing ammonia in free or bound form, which reducing agent is added to the exhaust gas emitted by the internal combustion engine upstream of an SCR catalyst (4) arranged in the exhaust gas purification system (1) for the selective catalytic reduction of nitrogen oxides in an exhaust gas purification system (1) of a motor vehicle internal combustion engine, wherein - in a first method step, an added quantity of the reducing agent is set according to a first dosing ratio (α1) in relation to a nitrogen oxide content in the exhaust gas upstream of the SCR catalyst (4) and a first actual value for a nitrogen oxide conversion of the SCR catalyst (4) caused thereby is determined, and - in a second method step, an added quantity of the reducing agent is set according to a second dosing ratio (α2) and a second actual value for a nitrogen oxide conversion of the SCR catalyst (4) caused thereby is determined, characterized by , that - an actual value difference between the first actual value and the second actual value is determined and the actual value difference is compared with a setpoint value difference determined from a first setpoint value for the nitrogen oxide conversion assigned to the first dosing ratio (α1) and a second setpoint value for the nitrogen oxide conversion assigned to the second dosing ratio (α2), - if the actual value difference deviates from the target value difference by more than a predefined amount, an inadmissibly reduced quality of the reducing agent is diagnosed, and - immediately before carrying out the first process step, the addition of the reducing agent is suspended for a predeterminable period of time. [3] Method according to claim 2, characterized by that the addition of the reducing agent is suspended at least until the nitrogen oxide content in the exhaust gas upstream of the SCR catalyst (4) deviates from the nitrogen oxide content in the exhaust gas downstream of the SCR catalyst (4) by less than a predeterminable amount. [4] Method according to claim 2, characterized by that the addition of the reducing agent is suspended at least until the ratio of the nitrogen oxide content in the exhaust gas upstream of the SCR catalyst (4) and the nitrogen oxide content in the exhaust gas downstream of the SCR catalyst (4) deviates from one by less than a predeterminable amount and / or the temporal gradient of the ratio of the nitrogen oxide contents falls below a predeterminable lower limit value. [5] Method according to one of claims 1 to 4, characterized by that in a third process step following the second process step, the addition of the reducing agent is suspended for a predeterminable period of time. [6] Method according to claim 5 and claim 1, characterized by that the third process step takes place after completion of the regeneration of the particulate filter (3). [7] Method according to one of claims 1 to 6, characterized by that in the first and second method steps a predeterminable number of first and second actual values ​​are determined at a predeterminable time interval and added together to form a respective total value, and from the respective total value an average value for the first and second actual values ​​is determined and the actual value difference is determined from the average values ​​for the first and second actual values. [8] Method according to claim 7, characterized by that actual values ​​are only used to determine the total value if predeterminable stability conditions for an exhaust gas mass flow and / or for the nitrogen oxide content in the exhaust gas upstream of the SCR catalyst (4) are met.

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