Method for monitoring a storage catalyst of an internal combustion engine and internal combustion engine for carrying out such a method

Abstract

Methods for monitoring a NO x -storage catalyst (6) of an internal combustion engine (1), which is provided in an exhaust gas discharge system (2) for the discharge of exhaust gases, in which - to check the functionality of the storage catalyst (6) the internal combustion engine (1) under defined operating conditions for a predetermined period of time Δt λ<1 > Δt min into a substoichiometric operation (λ rich < 1) is transformed, where Δt min The time period required to convert the nitrogen oxides (NOx) stored in the storage catalyst x and to release the stored oxygen O2 and form hydrogen H2, - the air-fuel ratio λ down,mess downstream of the storage catalyst (6) is measured using a sensor (5), - the measured air-fuel ratio λ down,mess with a reference value λ ref, which is given, is compared, where λ ref the air ratio which is determined downstream of a malfunctioning storage catalyst (6) under the defined operating conditions, wherein the operating point-specific reference value (λ ref ) is stored in an engine control unit and made available for comparison purposes when needed, and - the storage catalyst (6) is assumed to be sufficiently functional if the measured air-fuel ratio λ down,mess the specified reference value λ ref falls below, and - a malfunction of the storage catalyst (6) is assumed if the measured air-fuel ratio λ down,mess the specified reference value λ ref does not fall below a certain threshold.

Description

[0001] The invention relates to a method for monitoring a NO x -Storage catalyst of an internal combustion engine, which is provided in an exhaust gas removal system for removing the exhaust gases, in which - to check the functionality of the storage catalyst, the internal combustion engine is operated under defined operating conditions for a predefined period of time Δt λ<1 > Δt min into substoichiometric operation (λ rich < 1) is transferred, - the air ratio λ down,mess is measured downstream of the storage catalyst by means of a sensor, - the measured air ratio λ down,mess with a reference value λ ref is compared, and - it is assumed that the storage catalyst is functioning sufficiently if the measured air ratio λ down,mess the specified reference value λ ref falls below, and - the storage catalyst is assumed to be inoperable if the measured air ratio λ down,mess the specified reference value λ ref does not fall below.

[0002] Furthermore, the invention relates to an internal combustion engine with at least one cylinder for carrying out such a method.

[0003] Typically and within the scope of the present invention, the air ratio λ is defined as the ratio of the air mass m actually supplied to the at least one cylinder Luft to the stoichiometric air mass m Luft,stöch which would be required to supply the fuel mass m to at least one cylinder Kraftstoff just to be completely oxidized (stoichiometric operation of the internal combustion engine A = 1).

[0004] The following applies: λ=mAir / mAir,sto¨ch or with the mass flows ṁ instead of the masses m: λ=m˙air / m˙air,sto¨ch With the stoichiometric air requirement L stöch , which is defined to: Lsto¨ch=mAir,sto¨ch / mFuel or with the mass flows ṁ instead of the masses m to: Lsto¨ch=m˙air,sto¨ch / m˙fuel Applies to the air ratio λ: λ=m˙air / m˙fuel*(1 / Lstoch)

[0005] According to the state of the art, internal combustion engines are equipped with various exhaust aftertreatment systems to reduce pollutant emissions.

[0006] In gasoline engines, catalytic reactors are used. These reactors ensure the oxidation of HC and CO even at low temperatures by using catalytic materials that increase the rate of certain reactions. If nitrogen oxides are to be reduced, this can be achieved by using a three-way catalyst, which, however, requires stoichiometric operation (λ ≈ 1) of the gasoline engine within narrow limits. In this case, the nitrogen oxides NO x by means of the existing non-oxidized exhaust gas components, namely the carbon monoxide CO and the unburned hydrocarbons HC, whereby these exhaust gas components are oxidized at the same time.

[0007] In internal combustion engines that operate with an excess of air, for example lean-burn gasoline engines, but also direct-injection diesel engines and direct-injection gasoline engines, the nitrogen oxides in the exhaust gas cannot be reduced due to the nature of the engine, i.e. due to the lack of reducing agents.

[0008] As a result, exhaust gas aftertreatment systems must be provided to reduce nitrogen oxides, for example storage catalysts (LNT - Lean NO xTrap). The nitrogen oxides are initially adsorbed in the catalyst during lean operation of the internal combustion engine, i.e. collected and stored, and then reduced during a regeneration phase, for example by means of substoichiometric operation (λ < 1) of the internal combustion engine in the event of oxygen deficiency, with the unburned hydrocarbons HC and carbon monoxide CO in the exhaust gas serving as reducing agents. Further internal engine options for enriching the exhaust gas with reducing agents, in particular with unburned hydrocarbons, are offered by exhaust gas recirculation and - in diesel engines - throttling in the intake tract. The exhaust gas can also be enriched with unburned hydrocarbons by means of post-injection of fuel into at least one cylinder of the internal combustion engine. A disadvantage of the latter procedure is, in particular, oil dilution.Internal engine measures can be dispensed with if the reducing agent is introduced directly into the exhaust tract, for example by injecting additional fuel upstream of the LNT.

[0009] During the regeneration phase, the nitrogen oxides NO x and oxygen O2 stored in the storage catalyst is released and essentially converted into nitrogen dioxide N2, carbon dioxide CO2 and water H2O. The temperature of the storage catalyst should preferably be in a temperature window between 200°C and 450°C, so that on the one hand a rapid reduction is ensured and on the other hand no desorption without conversion of the released nitrogen oxides NO x takes place, which can be triggered by excessively high temperatures.

[0010] One difficulty with using a storage catalyst arises from the sulfur contained in the exhaust gas, which is also adsorbed and must be regularly removed through a process known as desulfurization. For this purpose, the storage catalyst must be heated to high temperatures, typically between 600°C and 700°C, and supplied with a reducing agent.

[0011] The high temperatures required for desulfurization can damage the storage catalyst, contribute to thermal aging of the catalyst, and significantly reduce the desired conversion of nitrogen oxides toward the end of its service life. The storage capacity, or rather the ability to store nitrogen oxides, decreases with increasing operating time, which is due, among other things, to contamination of the storage catalyst with sulfur, i.e., the deposition of sulfur, but also to thermal aging due to the high temperatures.

[0012] The legally prescribed limits for nitrogen oxide emissions require on-board diagnostics (OBD) to monitor and detect the expected reduction in functionality, i.e., the reduction in conversion efficiency, with increasing operating life of the LNT. Consequently, concepts are required to monitor functionality and prevent undesirably high pollutant emissions due to reduced functionality or insufficient conversion.

[0013] European patent application EP 1 936 140 A1 describes a method for monitoring a storage catalyst using two lambda sensors, exploiting a measurement error in the lambda sensors. If the unburned hydrocarbons in the exhaust gas exceed a certain concentration, the sensor delivers a higher value for the air / fuel ratio λ. messthan actually exists, for example in substoichiometric operation (λ rich < 1) of the internal combustion engine and an HC concentration of 10,000 ppm in the exhaust gas an air ratio of λ mess = 0.95 as a measured value, although in fact only an air ratio λ tat = 0.85.

[0014] To test the functionality of the storage catalyst, the HC concentration in the exhaust gas is deliberately increased in such a way that a first sensor located upstream of the storage catalyst malfunctions. If the storage catalyst is inoperative, i.e., its storage capacity is at least limited, fewer or no unburned hydrocarbons are removed by releasing nitrogen oxide NO. xand oxygen O2 is oxidized, and the HC concentration downstream of the storage catalyst is essentially the same as upstream of the storage catalyst, which is why a second lambda sensor located downstream of the storage catalyst provides the same value for the air ratio—with an equally large measurement error. Therefore, the storage catalyst is assumed to be inoperative if the air ratios λ determined by the sensors up,mess , λ down,mess are essentially the same size and λ up,mess / λ down,mess ≈ 1.

[0015] If, however, the storage catalyst continues to function, the unburned hydrocarbons in the exhaust gas will be at least partially oxidized as they flow through the storage catalyst, which is why the HC concentration in the exhaust gas downstream of the storage catalyst will be lower than upstream of the catalyst. Therefore, the storage catalyst is assumed to be at least partially functional if the two air ratios λ up,mess , λ down,mess are of different sizes with λ up,mess / λ down,mess > 1. The air ratio λ determined with the second probe located downstream of the storage catalyst down,mess does not necessarily have to be free from measurement errors. The deviation of the air ratio λ down,mess from the actual air ratio λ tat but is at least lower than upstream of the storage catalyst.

[0016] A disadvantage of the process described in EP 1 936 140 A1 is that a significant enrichment (λ << 1) is required, namely an enrichment of the exhaust gas with unburned hydrocarbons to a degree that leads to a measurement error of the first lambda sensor. This results in significantly more fuel being injected than can actually be combusted with the available air volume. This is considered disadvantageous from an energy perspective, particularly with regard to the engine's fuel consumption, but also fundamentally with regard to pollutant emissions.

[0017] The German patent application DE 10 2012 218 728 A1 also describes a method for monitoring a storage catalyst using two lambda sensors, in which the internal combustion engine is switched to substoichiometric operation (λrich < 1). In contrast to the method described in EP 1 936 140 A1, however, no significant enrichment is performed, but only a moderate enrichment, which does not lead to any measurement errors in the lambda sensors. This is actually a necessary prerequisite of the method according to DE 10 2012 218 728 A1 in order to be able to make a statement about the functionality of the storage catalyst.

[0018] The fact is exploited that when flowing through a storage catalyst with limited functionality, no or fewer unburned hydrocarbons in the exhaust gas are released by the release of nitrogen oxides NO x and oxygen O2 are oxidized, which is why the HC concentration in the exhaust gas does not change or only changes to a minor extent.

[0019] The oxidation processes taking place in the storage catalyst influence an air ratio λ recorded downstream of the second lambda probe down,mess (t). In a fully functional storage catalyst, the unburned hydrocarbons in the exhaust gas as a result of enrichment upstream of the catalyst are completely oxidized as they flow through the catalyst, so that there are no unburned hydrocarbons in the exhaust gas downstream of the catalyst and the second sensor has an air ratio λ down,mess ≈ 1, ie detected as if the internal combustion engine had been operated stoichiometrically, ie without excess fuel.

[0020] If the air ratio λ down,mess (t) is observed over the given period of time, a more or less rapid decrease in the air ratio λ down,mess (t) can be observed, where the course of the function λ down,mess(t) is influenced over time by the storage capacity of the LNT. If the functionality of the storage catalyst is limited, ie the storage capacity is significantly reduced, the LNT will only be able to remove unburned hydrocarbons by releasing NO for a short time after the enrichment has been initiated. x and O2, whereby the downstream measured air ratio λ down,mess (t) decreases comparatively quickly and more or less quickly approaches the air ratio λ measured upstream up,mess (t) approaches.

[0021] Consequently, based on the temporal progression of the air ratio λ down,mess (t) during the enrichment phase, a statement can be made regarding the functionality of the storage catalyst. However, this requires a comparatively long observation period and thus a long-lasting enrichment, which is why DE 10 2012 218 728 A1 proposes using the temporal functions λ up,mess(t) and λ down,mess (t) integrals over a specific period of time. The functionality of the storage catalyst can then be assessed by comparing the two integrals.

[0022] US 2008 / 0 168 824 A1 describes a method for monitoring a storage catalyst of a diesel engine using two lambda sensors, of which a first lambda sensor is located upstream of the storage catalyst for detecting the air ratio λ up,mess (t) and a second lambda probe downstream of the storage catalyst to measure the air ratio λ down,mess (t). To check the functionality of the storage catalyst, the internal combustion engine is operated in substoichiometric mode (λ rich < 1), whereby the downstream measured air ratio λ down,mess (t) with the upstream measured air ratio λ up,mess (t) is compared.

[0023] The storage catalyst is considered to be inoperative if the air conditions λ down,mess (t) and λ up,mess (t) do not differ. However, sufficient functionality of the storage catalyst is assumed if the air ratios λ down,mess (t) and λ up,mess (t) differ.

[0024] The disadvantage of the method described in US 2008 / 0 168 824 A1 is that the upstream measured air ratio λ up,mess (t) is used for comparison purposes. This ignores the fact that oxidation processes also occur in a non-functional storage catalyst and that these oxidation processes influence the air ratio λ measured downstream of the second lambda sensor. down,mess (t).

[0025] A diagnostic module for determining the capacity of a NOx storage catalyst is known from FR 2 950 386 A1.

[0026] Against this background, it is an object of the present invention to provide an improved method with which the functionality of a storage catalyst can be monitored.

[0027] A further sub-object of the present invention is to provide an internal combustion engine for carrying out such a method.

[0028] The first subtask is solved by a method for monitoring a NO x -Storage catalyst of an internal combustion engine, which is provided in an exhaust gas removal system for removing the exhaust gases, in which - to check the functionality of the storage catalyst, the internal combustion engine is operated under defined operating conditions for a predefined period of time Δt λ<1 > Δt min into substoichiometric operation (λ rich < 1), where Δt minthe time required to decompose the nitrogen oxides NO stored in the storage catalyst x and release the stored oxygen O2 and form hydrogen H2, - the air ratio λ down,mess is measured downstream of the storage catalyst by means of a sensor, - the measured air ratio λ down,mess with a reference value λ ref , which is given, where λ ref the air ratio which is determined under the defined operating conditions downstream of a non-functional storage catalyst, whereby the operating point-specific reference value λ ref stored in an engine control unit and made available for comparison purposes when required and - the storage catalyst is considered to be functioning properly if the measured air ratio λ down,messthe specified reference value λ ref falls below, and - the storage catalyst is assumed to be inoperable if the measured air ratio λ down,mess the specified reference value λ ref does not fall below.

[0029] In the method according to the invention, the internal combustion engine is also switched off for a predeterminable period of time Δt to check the functionality of the storage catalyst. λ<1 into substoichiometric operation (λ rich < 1), ie enrichment is carried out under defined operating conditions.

[0030] The duration Δt λ<1 of substoichiometric operation is sufficiently long or large, ie the internal combustion engine is operated substoichiometrically for at least long enough that the nitrogen oxides NO stored in the storage catalyst xand the stored oxygen O2 is largely released and consumed and hydrogen H2 is formed.

[0031] The effect is exploited that with a functioning storage catalyst large amounts of hydrogen H2 are formed, for example through the water gas reaction (CO + H2O → H2 + CO2), as soon as the stored nitrogen oxides NO x and the stored oxygen O2 were released and consumed during oxidation processes. The increase in the H2 concentration in the exhaust gas leads to the air / fuel ratio λ measured downstream of the storage catalyst down,mess Advantageously, the selectivity of the sensor or the lambda probe used with regard to hydrogen H2 is particularly high, so that the change in the air ratio λ down,messwith an increase in the H2 concentration in the exhaust gas is particularly pronounced, ie it is significantly reduced. The latter is particularly advantageous because, within the scope of the method according to the invention, the measured air ratio λ down,mess with a reference value λ ref is compared and significant differences in the air conditions to be compared λ down,mess and λ ref improve the quality of the procedure in a relevant way.

[0032] According to the invention, however, an air ratio λ determined upstream of the storage catalyst is not up or λ rich used for comparison, ie used as a reference value, but a reference value λ ref used and specified, which represents the air ratio that would be or is determined under the defined operating conditions downstream of a malfunctioning storage catalyst.

[0033] The reference value λref was previously generated during calibration work and stored in a motor control system. For comparison purposes, ie if necessary, the reference value λ ref then provided, i.e. specified. The reference value λ ref can be determined empirically, for example, by measuring a lambda sensor on a roller dynamometer or an engine test bench, but can also be calculated using computational models. In each case, it is a reference value λ ref , which is already available and is provided when needed. The reference value λ ref is operating point specific, ie assigned to certain defined operating conditions, which are determined, for example, by the speed n of the internal combustion engine, the load T of the internal combustion engine and / or the air ratio λ rich or λ up are marked as operating parameters.

[0034] In contrast to an air ratio λ determined upstream of the storage catalyst up or λ rich the use of the reference value λ according to the invention ref the fact that oxidation processes also occur in a non-functional storage catalyst and that these oxidation processes influence the air ratio λ down downstream of the storage catalyst. Thus, the oxygen O2 in the exhaust gas reacts with the incompletely oxidized exhaust gas components, especially or preferably with the hydrogen and, after the hydrogen has been consumed, with the carbon monoxide. The consumption of O2 and H2 due to oxidation and the resulting decrease in the H2 concentration in the exhaust gas lead to the air / fuel ratio λ down downstream of the inoperative storage catalyst. That is, the air ratio λ downdownstream of the inoperative storage catalyst is greater than the air ratio λ upstream of the storage catalyst up or λ rich . The following applies: λ down > λ rich and this despite the fact that the storage catalyst is inoperative, i.e. it no longer has the ability to store nitrogen oxides and oxygen and release them again as part of a regeneration or enrichment process.

[0035] The method according to the invention solves the first sub-problem underlying the invention, namely an improved method is shown with which the functionality of a storage catalyst can be monitored.

[0036] In contrast to the prior art, the method according to the invention does not necessarily require a sensor or a lambda probe upstream of the storage catalyst.

[0037] Further advantageous process variants according to the subclaims are explained below.

[0038] Embodiments of the method are advantageous in which the air ratio λ down,mess downstream of the storage catalyst, it is measured using a lambda sensor. Lambda sensors are available at low cost due to high demand and the resulting high volumes. Practical experience is available, particularly with a knowledge of the measurement behavior and special characteristics of a lambda sensor, which advantageously simplifies the handling of this type of sensor. A lambda sensor provides an air / fuel ratio λ as its output signal.

[0039] In principle, a NO sensor serving as an O2 sensor can also x -Sensor downstream of the storage catalyst, which provides an O2 concentration as an output signal, whereby the air ratio λ down,messcan be determined or calculated using the measured O2 concentration.

[0040] Embodiments of the method are advantageous in which the air ratio λ up upstream of the storage catalyst.

[0041] The method according to the invention fundamentally does not require a sensor or lambda probe upstream of the storage catalyst. However, it does result in control-related advantages when setting or monitoring the air ratio λ at which the internal combustion engine is operated. In particular, feedback control of the air ratio λ or λ rich realize.

[0042] In this context, embodiments of the method are advantageous in which the air ratio λ up,mess is measured upstream of the storage catalyst, for example by means of a lambda probe or a NO sensor serving as an O2 sensor x-Sensor. In this context, reference is made to the previous statements.

[0043] In this context, embodiments of the method are also advantageous in which the air ratio λ up upstream of the storage catalyst. The air ratio λ up using the supplied fresh air, the supplied fuel and the stoichiometric air requirement L stöch be calculated. For the air ratio λ up applies: λup=m˙air / m˙fuel*(1 / Lstoch)

[0044] If low-pressure exhaust gas recirculation is provided, the exhaust gas mass recirculated by means of low-pressure exhaust gas recirculation and its composition may also have to be taken into account.

[0045] Embodiments of the method are advantageous in which the internal combustion engine is switched to substoichiometric operation with 1 > λ to check the functionality of the storage catalyst. rich > 0.87 is transferred.

[0046] Embodiments of the method are advantageous in which the internal combustion engine is switched to substoichiometric operation with 1 > λ to check the functionality of the storage catalyst. rich > 0.90, preferably into a substoichiometric operation with 1 > λ > 0.93 or with 1 > λ > 0.95.

[0047] Embodiments of the method can also be advantageous in which the internal combustion engine is switched to substoichiometric operation with 0.99 > λ to check the functionality of the storage catalyst. rich > 0.92, preferably into substoichiometric operation with 0.96 > λ > 0.93.

[0048] The determination of the air ratio λ rich For substoichiometric operation of the internal combustion engine, a compromise is required. On the one hand, enrichment should be as minimal as possible, as measurement errors of the lambda sensor must be avoided and the disadvantages of enrichment become more pronounced with increasing enrichment. On the other hand, the exhaust gas must be enriched with incompletely oxidized combustion products, particularly unburned hydrocarbons, to such an extent that a relevant proportion can be oxidized in the storage catalyst with desorption of nitrogen oxides and oxygen, resulting in a measurable change in the air / fuel ratio λ.

[0049] Embodiments of the method are advantageous in which the internal combustion engine is switched on for a period of time 2 < Δt to check the functionality of the storage catalyst. λ<1< 10 seconds into substoichiometric operation with λ rich is transferred.

[0050] Also advantageous are embodiments of the method in which the internal combustion engine is switched off for a period of time 4 < Δt to check the functionality of the storage catalyst. λ<1 < 8 seconds into substoichiometric operation with λ rich is transferred.

[0051] The duration Δt λ<1 of substoichiometric operation must be sufficiently long or large or selected so that the nitrogen oxides NO stored in the storage catalyst x and the stored oxygen O2 is released and sufficient or as much hydrogen H2 as possible is formed. The invention is intended to utilize the effect that, with a functioning storage catalyst, large amounts of hydrogen H2 are formed as soon as the stored nitrogen oxides NO xand the stored oxygen O2 were released and consumed. The production of H2 leads to the air ratio λ measured downstream of the storage catalyst down,mess noticeably decreases.

[0052] If the enrichment period is too short, no hydrogen H2 or an insufficient amount of it will be formed, making it impossible to check the functionality of the storage catalyst using the method according to the invention. Therefore, the definition of a minimum duration Δt min not only advantageous for the greasing to be carried out, but absolutely necessary.

[0053] On the other hand, the enrichment should be as short as possible so that the disadvantages of enrichment only have to be accepted for as long as absolutely necessary.

[0054] Embodiments of the method are advantageous in which the defined operating conditions under which the internal combustion engine operates substoichiometrically (λ rich < 1), the speed n of the internal combustion engine, the load T of the internal combustion engine and the air ratio λ rich as operating parameters.

[0055] Embodiments of the method are advantageous in which the reference value λ ref is determined empirically.

[0056] In this context, embodiments of the method are advantageous in which the reference value λ ref is measured using a lambda probe that serves as a sensor.

[0057] In this context, embodiments of the method are also advantageous in which the reference value λ ref is measured on a roller dynamometer or an engine test bench.

[0058] Embodiments of the method may also be advantageous in which the reference value λ ref is calculated using calculation models.

[0059] In this context, embodiments of the method are advantageous in which, in the mathematical determination of the reference value λ ref the concentrations of the raw emissions of the internal combustion engine of hydrogen H2 and oxygen O2 are measured and taken into account.

[0060] Embodiments of the method are advantageous in which the internal combustion engine during substoichiometric operation (λ rich < 1) is operated on a stationary basis when the procedure is carried out.

[0061] This ensures that the air ratio λ rich does not vary as a result of a change in the operating parameters and the air ratio λ determined by the sensor down,messonly varies as a result of the reactions taking place in the storage catalyst, ie the temporal change of the air ratio λ down,mess results from the oxidation of incompletely burned exhaust gas components or the production of hydrogen H2.

[0062] It must also be taken into account that the reference value λ ref is operating point specific, i.e. is assigned to certain defined operating conditions or operating parameters.

[0063] Embodiments of the method are advantageous in which the internal combustion engine is transferred to a predeterminable operating point before the internal combustion engine is switched to substoichiometric operation (λ rich < 1).

[0064] Since the parameters relevant for the process, namely the air ratio λ rich and the time span Δt λ<1 for the substoichiometric operation of the internal combustion engine and the reference value λ refmust be specified and are usually operating point-specific, it is sensible and advantageous to select or provide a limited number of operating points in the engine map for carrying out the procedure in order to avoid having to store parameters in the engine control system for all map points.

[0065] In this context, embodiments of the method are advantageous in which the predetermined operating point is selected from a limited list of k predefinable operating points.

[0066] The second sub-task, namely to provide an internal combustion engine for carrying out a method of the type mentioned above, is achieved by an internal combustion engine with at least one cylinder and at least one NO x -Storage catalyst provided in an exhaust gas removal system for removing the exhaust gases from the at least one cylinder, in which - downstream of the storage catalyst, a sensor for measuring an air ratio λ down,mess is arranged, and - an engine control system for processing the measured air ratio λ down,mess is provided, whereby an operating point-specific reference value λ ref stored in the engine control unit, which is used for comparison with the measured air ratio λ down,mess serves.

[0067] What has already been said for the method according to the invention also applies to the internal combustion engine according to the invention, which is why reference is generally made here to the statements made with regard to the method variants.

[0068] Embodiments of the internal combustion engine are advantageous in which an operating point that can be specified for carrying out the method is stored in a list of k operating points stored in the engine control unit.

[0069] Embodiments of the internal combustion engine are advantageous in which the operating point-specific reference value λ ref for a large number of operating points, i.e. for a large number of defined operating points in the engine control system.

[0070] In the following, the invention is described with reference to Fig. 1 and Fig. 2 is described in more detail. Here: Fig. 1 schematically shows an embodiment of the internal combustion engine including exhaust gas removal system, storage catalyst and lambda sensors, and Fig. 2 shows in a diagram the air ratios λ(t) over time t during the testing of the functionality of a storage catalyst.

[0071] Fig. 1 schematically shows an internal combustion engine 1 with four cylinders and an exhaust gas removal system 2 with which the exhaust gases of the cylinders are removed.

[0072] In the exhaust gas discharge system 2, a storage catalyst 6 serving as an exhaust gas aftertreatment system 4 is provided, which has the ability to convert nitrogen oxides NO x in the presence of incompletely oxidized combustion products. Upstream and downstream of the storage catalyst 6, a lambda probe 3.5 is provided to measure the air ratio λ up,mess , λ down,mess arranged in the exhaust gas discharge system 2.

[0073] Fig. Figure 2 shows in the form of a diagram the air ratios λ(t) over time t during the testing of the functionality of a storage catalyst.

[0074] The associated internal combustion engine is regularly lean during normal operation, ie over-stoichiometric with an air ratio λ lean= 1.3. To check the functionality of the storage catalyst, a moderate enrichment is carried out, in which the internal combustion engine is operated substoichiometrically with an air ratio λ rich = 0.9 and does not lead to any measurement errors of the lambda sensors. The stoichiometric air ratio λ stöch = 1.0 is shown as a dashed line for orientation.

[0075] The air ratio λ measured upstream of the storage catalyst by means of a lambda probe up,mess (t) corresponds to the substoichiometric air ratio λ rich = 0.9 of enrichment.

[0076] The reference value for checking the functionality is a previously generated air ratio λ stored in the engine control unit. ref, which was determined downstream of a malfunctioning storage catalyst under the same defined operating conditions as those used during testing or enrichment. The air ratio λ ref downstream of the inoperative storage catalyst is greater than the air ratio λ upstream of the storage catalyst up (t) = λ rich (t). The following applies: λ ref > λ rich This is despite the fact that the storage catalyst is inoperative, i.e., it no longer has the ability to store nitrogen oxides and oxygen and release them again during regeneration or enrichment. This is because oxidation processes also occur in a non-functional storage catalyst, and these oxidation processes influence the air / fuel ratio λ. ref = λ down,mess downstream of the storage catalyst.

[0077] In addition, Fig. 2 the air conditions λdown,1 (t), λ down,2 (t), λ down,3 (t) for different storage catalysts or differently aged storage catalysts, where λ down,1 (t) represents the measured air ratio downstream of a storage catalyst without precious metal coating. Fig. 2, the air ratio λ down,1 (t) time-delayed to the air ratio λ present upstream of the storage catalyst up (t).

[0078] The air ratio λ down,2 (t) is measured downstream of a fully functional storage catalyst with intact precious metal coating.

[0079] In a fully functional storage catalyst, the unburned hydrocarbons in the exhaust gas as a result of enrichment upstream of the catalyst are initially completely oxidized as they flow through the catalyst, so that there are no unburned hydrocarbons in the exhaust gas downstream of the catalyst and the second sensor initially detects an air ratio λ down,2 (t) ≈ 1, ie detected as if the internal combustion engine had been operated stoichiometrically, ie without excess fuel.

[0080] If the air ratio λ down,2 (t) over a given period of time Δt λ<1 observed, a more or less rapid decrease in the air ratio λ down,2 (t) is observed, where the course of the function λ down,2 (t) is influenced by the storage capacity of the LNT over time t. As soon as the stored nitrogen oxides NO xAs the stored oxygen O2 is released and consumed during oxidation processes, hydrogen H2 is produced in a functioning catalyst. The increase in the H2 concentration in the exhaust gas leads to a significant decrease in the air / fuel ratio λ measured downstream of the storage catalyst. down,2 (t).

[0081] In this case, it is assumed that the storage catalyst is functioning sufficiently, since the measured air ratio λ down,2 (t) the reference value λ ref However, the storage catalyst would have to be considered inoperative if the air ratio λ down,2 (t) the reference value λ ref would not fall below.

[0082] The above statements make it clear that the duration Δt λ<1The substoichiometric operation must be sufficiently large. The internal combustion engine must operate substoichiometrically at least until sufficient hydrogen H2 is produced.

[0083] The air ratio λ down,3 (t) is detected downstream of a functional storage catalyst whose functionality is already limited and in which the precious metal coating has already noticeably aged.

[0084] If the functionality of the storage catalyst is restricted, ie the storage capacity is significantly reduced, the LNT is only able to achieve a stoichiometric air ratio λ for a short time after the enrichment has been initiated, or as in this case, is no longer able to achieve a stoichiometric air ratio λ down,3 (t) = 1.0. The air ratio λ down,3(t) decreases continuously from the beginning, whereby the production of hydrogen H2, ie the increase of the H2 concentration in the exhaust gas, leads to a less significant decrease of the air ratio λ down,3 (t). Nevertheless, the air ratio λ down,3 (t) the reference value λ ref temporarily, which is why it can be assumed that the storage catalyst is functioning sufficiently again.

[0085] How Fig. 2 can be taken, applies during the entire enrichment λ down,3 (t) > λ rich , which is why the storage catalyst is or should be considered to be inoperative according to the state-of-the-art procedures for checking its functionality. Reference symbol 1 internal combustion engine 2 exhaust gas removal system 3 first lambda probe 4 Exhaust aftertreatment system 5 second lambda probe 6 Storage catalyst L stöch stoichiometric air requirement m mass m Kraftstoff fuel mass introduced into the cylinders m Luft,stöch stoichiometric air mass m Luft,tat air mass actually introduced into the cylinders m Luft supplied air mass ppm parts per million t time Δt λ<1 specified time period for substoichiometric operation Δt min required time to store the NO stored in the storage catalyst x or to release O2 and form H2, A air ratio λ up,mess Air ratio measured upstream of the storage catalyst using a lambda probe λ down,mess Air ratio measured downstream of the storage catalyst using a lambda probe λ rich Air ratio in substoichiometric operation

Claims

[1] Method for monitoring a NO x -storage catalyst (6) of an internal combustion engine (1), which is provided in an exhaust gas removal system (2) for removing the exhaust gases, in which - to check the functionality of the storage catalyst (6), the internal combustion engine (1) is operated under defined operating conditions for a predeterminable period of time Δt λ<1 > Δt min into substoichiometric operation (λ rich < 1), where Δt min the time required to decompose the nitrogen oxides NO stored in the storage catalyst x and release the stored oxygen O2 and form hydrogen H2, - the air ratio λ down,mess downstream of the storage catalyst (6) is measured by means of a sensor (5), - the measured air ratio λ down,mess with a reference value λ ref, which is given, where λ ref the air ratio which is determined under the defined operating conditions downstream of a non-functional storage catalyst (6), wherein the operating point-specific reference value (λ ref ) is stored in an engine control unit and made available for comparison purposes when required, and - it is assumed that the storage catalyst (6) is functioning properly if the measured air ratio λ down,mess the specified reference value λ ref falls below, and - the storage catalyst (6) is deemed to be inoperative if the measured air ratio λ down,mess the specified reference value λ ref does not fall below. [2] Method according to claim 1, characterized by that the air ratio λ down,messdownstream of the storage catalyst (6) is measured by means of a lambda probe serving as a sensor (5). [3] Method according to claim 1 or 2, characterized by that the air ratio λ up upstream of the storage catalyst (6). [4] Method according to claim 3, characterized by that the air ratio λ up,mess upstream of the storage catalyst (6) is measured. [5] Method according to claim 3, characterized by that the air ratio λ up upstream of the storage catalyst (6). [6] Method according to one of the preceding claims, characterized by that the internal combustion engine (1) is switched to substoichiometric operation with 1 > λ in order to check the functionality of the storage catalyst (6) rich > 0.87 is transferred. [7] Method according to one of the preceding claims, characterized bythat the internal combustion engine (1) is switched to substoichiometric operation with 1 > λ in order to check the functionality of the storage catalyst (6) rich > 0.90 is transferred. [8] Method according to one of the preceding claims, characterized by that the internal combustion engine (1) is switched to substoichiometric operation with 0.99 > λ to check the functionality of the storage catalyst (6) rich > 0.92 is transferred. [9] Method according to one of the preceding claims, characterized by that the internal combustion engine (1) is switched on for a period of time 2 < Δt to check the functionality of the storage catalyst (6) λ<1 < 10 seconds into substoichiometric operation with λ rich is transferred. [10] Method according to one of the preceding claims, characterized by that the internal combustion engine (1) is switched on for a period of time 4 < Δt to check the functionality of the storage catalyst (6) λ<1< 8 seconds into substoichiometric operation with λ rich is transferred. [11] Method according to one of the preceding claims, characterized by that the defined operating conditions under which the internal combustion engine (1) operates substoichiometrically (λ rich < 1), the speed n of the internal combustion engine (1), the load T of the internal combustion engine (1) and the air ratio λ rich as operating parameters. [12] Method according to one of the preceding claims, characterized by that the reference value λ ref is determined empirically. [13] Method according to claim 12, characterized by that the reference value λ ref is measured using a lambda probe that serves as a sensor. [14] Method according to claim 12 or 13, characterized by that the reference value λ ref is measured on a roller dynamometer or an engine test bench. [15] Method according to one of claims 1 to 11, characterized by that the reference value λ ref is calculated using calculation models. [16] Method according to claim 15, characterized by that in the calculation of the reference value λ ref the concentrations of the raw emissions of the internal combustion engine (1) of hydrogen H2 and oxygen O2 are measured and taken into account. [17] Method according to one of the preceding claims, characterized by that the internal combustion engine (1) during substoichiometric operation (λ rich < 1) is operated on a stationary basis when the procedure is carried out. [18] Method according to one of the preceding claims, characterized by that the internal combustion engine (1) is transferred to a predeterminable operating point before the internal combustion engine (1) is switched to substoichiometric operation (λ rich < 1). [19] Method according to claim 18, characterized by that the specified operating point is selected from a limited list of k specifiable operating points. [20] Internal combustion engine (1) for carrying out a method according to one of the preceding claims, having at least one cylinder and at least one NO x -Storage catalyst (6) which is provided in an exhaust gas removal system (2) for removing the exhaust gases from the at least one cylinder, in which - downstream of the storage catalyst (6) a sensor (5) for measuring an air ratio λ down,mess is arranged, and - an engine control system for processing the measured air ratio λ down,mess is provided, wherein an operating point-specific reference value λ ref stored in the engine control unit, which is used for comparison with the measured air ratio λ down,mess serves.

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