DE10305635B4 - Emission control method for lean-burn engines - Google Patents

Abstract

Exhaust gas purification method for a lean-burnable internal combustion engine, in which in a lean operating phase of the internal combustion engine emitted NOx compounds are stored in a located in the exhaust tract of the engine NOx storage catalyst, wherein a loading of the NOx storage catalyst takes place with NOx compounds,
the internal combustion engine is operated temporarily in regeneration operating phases in which the NOx storage catalytic converter catalytically converts stored NOx compounds and is thereby emptied of NOx compounds, and
a value for a storage capacity of the NOx storage catalytic converter is determined using a NOx model describing the raw emission of NOx compounds upstream of the NOx storage catalytic converter and used to evaluate the NOx storage catalytic converter,
characterized in that
a) determining an amount of NOx compounds emptied from the NOx storage catalytic converter during one of the regeneration phases and determining a first value for the storage capacity of the NOx storage catalytic converter using the NOx model,
b) a NOx concentration downstream of the NOx storage catalytic converter is measured and, together with the NOx model, a second value for the storage capacity of the NOx storage catalytic converter is determined,
c) that the ...

Description

The The invention relates to an exhaust gas purification method for a lean-burnable Internal combustion engine according to the preamble of claim 1.

Around to further reduce the fuel consumption of motor vehicles, come more often Internal combustion engines are used with lean fuel / air mixture can be operated since the efficiency of an internal combustion engine in lean operation especially is high. To fulfill required Exhaust emission limits, however, is in the lean operation of an internal combustion engine regularly one specific exhaust aftertreatment required, otherwise limits exceeded in terms of permissible NOx emissions would.

Therefore NOx storage catalysts are used, due to a special Coating are able to adsorb NOx compounds from the exhaust gas, which arise during lean burn. For emptying such NOx storage catalytic converter Regeneration is required in the stored NOx compounds converted into harmless compounds in the NOx storage catalytic converter when a reducing agent is added.

When Reducing agents can Carbon monoxide, hydrogen and hydrocarbons are used. It has proved to be useful, this by short-term operation of the internal combustion engine with a rich Fuel / air mixture to produce, whereby the NOx storage catalyst the necessary reducing agent as part of him anyway supplied Receives exhaust gas and degrades stored NOx compounds so that it again to the renewed Storage of NOx compounds be able to. This short-term operation of the internal combustion engine represents the mentioned Regeneration operation phase is.

By such regeneration operating phases, the efficiency decreases, d. H. the fuel consumption increases. In addition, during the Regenerationsbetriebsphase Although often only lower, however in principle inevitable slip of reducing agent through the NOx storage catalyst, thereby in the regeneration phase of operation the exhaust behavior of the internal combustion engine is deteriorated. Out these reasons would like to to initiate regeneration plant phases only if they are really to Emptying of the NOx storage catalyst are required.

It Therefore, the prior art is known to initiate the regeneration operating phases depending on demand to design. An essential criterion when a regeneration operating phase is initiated should be, is the current one storage capacity of the NOx storage catalyst, in turn, of the currently available Loading the NOx storage catalyst depends. With increasing duration the lean operating phase and thereby successful storage of NOx compounds, the storage efficiency decreases continuously, so that under consideration the exhaust limits a Regenerationsbetriebsphase is required.

To initiate a regeneration operating phase, it is known in the art to measure the NOx concentration, such. B. from the DE 198 44 082 C1 the applicant. Furthermore, the oxygen content in the exhaust gas flow can be measured downstream of the NOx storage catalytic converter, and thus a termination of the regeneration operating phase oriented on the current state of the NOx storage catalytic converter can be effected.

To the Optimal NOx-regenerating operation is usually further a degree of loading of the NOx storage catalyst used, which as a quotient of instantaneous Amount of stored NOx compounds and a maximum amount of storage is defined. It can be seen that to determine this degree of loading one possible exact knowledge of both the current load and the maximum Loading required is. Although the maximum possible Memory amount, which represents a maximum storage capacity, on a test bench by Measurement of the stored amounts of NOx compounds until they are reached a saturation state recorded, such a saturation However, the NOx storage catalyst is in an internal combustion engine in normal operation for emission reasons usually not possible, which is why this maximum storage capacity for the Operation mostly can not be used. In addition, the maximum is subject memory a NOx storage catalyst an aging process, so that it is necessary over the Life of a NOx storage catalytic converter a corresponding adaptation perform.

In this regard, it is known, for example from that in the DE 198 40 082 C1 described approach, during a regeneration phase to detect the amount of regenerant supplied and infer from the emptied in the regeneration phase from the NOx storage catalyst amount of NOx compounds. Comparing the thus determined amount of NOx compounds, which was stored at the beginning of the regeneration phase, and thus at the end of the lean operating phase in the NOx storage catalyst, with the amount of NOx compounds that a model calculation than during the previous lean operation phase indicates emitted from the internal combustion engine, the actual maximum storage capacity of the NOx storage catalyst can be corrected accordingly. A corresponding NOx model is for example from the EP 0 597 106 A1 known; It delivers the raw NOx emission emitted by the internal combustion engine upstream of a NOx storage catalytic converter as a function of operating data of the internal combustion engine and makes it possible to model the amount of NOx compounds absorbed in the NOx storage catalytic converter.

In DE 101 25 759 A1 A method for determining a loading state of an arranged in an exhaust duct of a lean burn combustion engine NOx storage catalyst is described under a lean exhaust gas atmosphere with lambda> 1 nitrogen oxides (NOx) releases and releases under a rich or stoichiometric exhaust gas atmosphere with lambda ≤ 1, the Loading state is determined taking into account a NOx discharge from the NOx storage catalytic converter, and the use of a method for determining a loading state of a arranged in an exhaust passage of a lean burn internal combustion engine NOx storage catalyst. It is provided that the NOx discharge is determined as a function of a measured or modeled concentration of reducing agents in the exhaust gas (reducing agent breakthrough) present downstream of the NOx storage catalytic converter or a variable derived from the reducing agent concentration.

In DE 198 47 874 A1 a method for nitrogen oxide reduction in the exhaust gas of a lean-burn internal combustion engine with a downstream NOx storage catalyst is described, in which the NOx storage favors by exhaust gas temperature increasing and / or mass flow reducing measures and the NOx regeneration of the catalyst is controlled so that there is an optimal emission control , To control the NOx regeneration of the loading state of the catalyst is determined with nitrogen oxides and / or the catalyst activity monitored by an on-board diagnosis. When exceeding a maximum allowable loading or when an irregularity of the catalyst activity occurs, the permissibility of a NOx regeneration is first checked by checking safety-relevant components for proper functioning and / or the current driving situation for compliance with predetermined driving parameters. In addition, it is checked whether it is possible to carry out a NOx regeneration by observing predetermined regeneration parameters. If the conditions for admissibility are met, the necessary regeneration parameters are set, if necessary, and a NOx regeneration is initiated, which is carried out until either a predetermined degree of regeneration has been reached or the current results of the admissibility check require a premature termination or interruption of the regeneration process.

In DE10007049A1 a method and a device for controlling a NOx regeneration of an arranged in the exhaust line of an internal combustion engine for automotive NOx storage catalytic converter is described, wherein the NOx regeneration is at least initiated when a threshold for a loading state of the NOx storage catalyst or a NOx emission downstream of the NOx storage catalyst is exceeded. It is envisaged that (a) it is detected whether the internal combustion engine is switched to idling, and that (b) alternatively or in any combination at idle, the threshold state for the load state or the NOx emission is increased, the NOx regeneration only is initiated after expiration of predetermined time intervals, and / or a current NOx regeneration is interrupted when changing to idle.

In DE 100 08 563 A1 A method for the diagnosis of a NOx storage catalyst is arranged, which is arranged in the exhaust system of an internal combustion engine and operated in response to the lambda value in an absorption and a regeneration mode, wherein the NOx concentration in the exhaust gas downstream of the NOx storage catalyst is measured. It is envisaged that when the NOx storage catalytic converter transitions from the absorption mode to the regeneration mode, the values of characteristic features of a NOx desorption peak in the time course of the NOx concentration are determined, compared with predetermined test patterns, a comparison result is formed and a function of the comparison result Catalyst status signal, in particular an error signal stored and / or displayed. In the apparatus for diagnosing a NOx storage catalytic converter having a lambda sensor having lambda control for measuring and changing the lambda value and a downstream of the NOx storage catalytic converter arranged NOx sensor for measuring the NOx concentration in the exhaust gas, a NOx-control device is provided of which the measured values of the NOx sensor can be supplied and the means for determining the values of characteristic features of a NOx desorption peak in the time course of the NOx concentration during a transition of the NOx storage catalyst from the absorption mode to the regeneration mode.

In DE 100 08 564 are a method and apparatus for controlling the regeneration of a NOx storage catalyst in the exhaust system an internal combustion engine arranged and operable in an absorption and a regeneration mode is described. In this case, a change of operating parameters of the internal combustion engine is made depending on the operating state of the NOx storage catalytic converter. The NOx concentration is measured in the exhaust gas downstream of the NOx storage catalyst. To determine the operating state, in particular damage to the NOx storage catalytic converter, the values of characteristic features of a NOx desorption peak in the time course of the NOx concentration are determined during a transition of the NOx storage catalytic converter from the absorption mode to the regeneration mode, compared with predetermined test patterns and a comparison result is formed, from which a catalyst state signal characterizing the operating state of the NOx catalyst is determined. The change in the operating parameters includes the implementation of a regeneration measure as a function of the catalyst state signal in order to achieve optimum regeneration of the NOx storage catalytic converter. The device for controlling the regeneration of a NOx storage catalytic converter has an engine control unit for detecting and influencing operating parameters of the internal combustion engine.

In DE 100 34 874 A1 A method for adapting a raw NOx concentration model of a lean and rich internal combustion engine is described. During the adaptation process for the modeled NOx raw concentration, the duration of a lean phase is shortened such that no NOx emission occurs during the lean phase. In a subsequent rich phase, the storage amount of the shortened lean phase is calculated and compared with the modeled NOx load for that lean phase. The comparison value is used to adapt the NOx raw concentration.

In DE 10036390 A1 is a method and an apparatus for desulfurizing an arranged in an exhaust passage of an internal combustion engine NOx storage catalyst, wherein during a desulfurization of the NOx storage catalytic converter is at least temporarily acted upon with a rich exhaust gas atmosphere with a predetermined lambda lambda value. It is provided that the fat lambda input for the desulfurization is determined as a function of a NOx storage rate of the NOx storage catalytic converter or a variable derived therefrom in a lean phase preceding the desulfurization phase, the NOx storage rate being a fraction of a stored NOx Amount of an amount of NOx flowing into the NOx storage catalyst.

In the DE 198 23 921 A1 a method for checking the efficiency of a NOx storage catalyst is described, which is arranged in the exhaust tract of an air-operated internal combustion engine. The current storage capacity of the NOx storage catalytic converter is calculated from the amount of NOx stored in the NOx storage catalytic converter and the associated charge level of the NOx storage catalytic converter using the signal of a sensor arranged downstream of the NOx storage catalytic converter and measuring the concentration of at least one exhaust gas component. The current storage capacity is compared with a predetermined minimum capacity and diagnosed falls below the predetermined minimum capacity, a faulty NOx storage catalyst.

In the WO 00/28201 a method is described for adapting the NOx raw concentration of an operating at least in certain operating ranges with excess air internal combustion engine. In an exhaust passage of the internal combustion engine, a NOx storage reduction catalyst is arranged, which adsorbs NOx during a storage phase, when the internal combustion engine is operated with a lean air-fuel mixture and catalytically converts the stored NOx in a regeneration phase with the addition of regeneration. Downstream of the NOx storage reduction catalyst, a NOx sensor is arranged. The raw NOx concentration is stored as a function of operating parameters of the internal combustion engine in a characteristic map of a storage device of a control device controlling the internal combustion engine. During operation of the internal combustion engine, the raw NOx concentration read from the map is adapted during a cycle consisting of the storage phase and the regeneration phase on the basis of the output signal of the NOx sensor.

in the The state of the art is further known approach, the maximum storage capacity under recourse to a reading of the NOx concentration downstream of the NOx storage catalyst to effect. In this case, the measured NOx concentration with a Model value for compared the NOx concentration. For the determination of the actual Loading the NOx storage catalyst is also a the current NOx raw emissions the engine indicating NOx model used.

Both described methods for adapting the maximum storage capacity of a NOx storage catalytic converter thus require a NOx model which displays the raw NOx emission of an internal combustion engine for a respective operating point. Inaccuracies of the known NOx models must be taken into account inevitably in the form of tolerances in the operation of a NOx storage catalyst. This not only leads to increased fuel economy but is also in the event that a NOx storage catalyst must be replaced because of excessive aging-related decrease in the maximum storage capacity, particularly unfortunate, since sometimes the exchange would actually (even) with a closer look not necessary.

Of the The invention is therefore based on the object, a more accurate adaptation the maximum storage capacity a NOx storage catalyst to enable.

These Task is according to the invention in a Emission control method of the type mentioned above achieved in that a) an amount of from the NOx storage catalyst during a the regeneration phases emptied NOx compounds is determined and under recourse on the NOx model, a first value for a storage capacity of the NOx storage catalyst is determined, b) a NOx concentration downstream of the NOx storage catalyst measured using the NOx model a second value for the storage capacity of the NOx storage catalyst is determined, and c) the raw emission NOx model describing NOx compounds upstream of the NOx storage catalyst is corrected using first and second values.

The inventive method So combines two individually known in the art Method and thus achieved that a correction of the raw emission the internal combustion engine indicating NOx model is achieved. The inventors recognized in this regard, that the two part-procedures, one of which in step (a) and the other in step (b) is applied to a deviation of Current raw emissions of the modeled value react in principle opposite. The sub-procedure according to step (a) to determine the current maximum storage capacity of a NOx storage catalyst while determining the amount of emptied from the NOx storage catalyst NOx compounds leads in principle for a raw NOx emission by the descriptive NOx model is displayed too high, to a lower storage capacity. In which On the other hand, the approach followed in step (b) leads to a too low one Indication of the raw NOx emission by the descriptive NOx model a lower storage capacity. By exploiting this, for the first time recognized by the inventors, contradiction, is it possible over one Correction of the NOx emission of the raw emission model in the Steps (a) and (b) performed Sub-procedure or the descriptive used in both sub-procedures NOx model to be corrected so that the NOx model is corrected and so that both sub-processes provide the same value for the storage capacity of the NOx storage catalyst. The initiation of the regeneration operating phase is thus even more needs-based.

The Exhaust gas purification method according to the invention is particularly advantageous if from the current storage capacity of the NOx storage catalyst an aging factor is determined as the ratio of current storage capacity to one predetermined reference storage capacity is defined and a statement about the Due to age deteriorated quality of the NOx storage catalyst allowed. That fulfilled the inventive method the Requirement for a permanent system monitoring (on-board diagnosis).

Both Submodels are known in principle to the person skilled in the art and could also be each alone to control a NOx aftertreatment be used. The approach followed in (a) is to model a NOx load while a lean operating phase of the internal combustion engine with the during the subsequent Regeneration operating phase eventual discharge of NOx compounds from the NOx storage catalyst compared. To capture the latter For example, during the the regeneration operating phase supplied amount of a regeneration agent determined in the form of a so-called Regeneratsmittelintegrals and about known ones relationships between the amount of regenerant and amount of NOx, for example by means of a characteristic curve or a map in the from the NOx storage catalytic converter emptied amount of NOx compounds are reacted. The end of Regeneration operating phase can be known to those skilled in the art Way, for example, by evaluating a signal downstream of the NOx storage catalyst be set oxygen sensor. A deviation between the NOx amount obtained from the NOx model before the start of the regeneration operating phase stored in the NOx storage catalyst, and that from the regeneration agent integral The determined amount is then expediently compared to the Changed starting conditions memory returned and a corresponding correction of the value for the storage capacity of the NOx storage catalytic converter performed.

In this regard is it is advantageous that in the regeneration phase in the NOx storage catalytic converter a Supplied regeneration agent, in step (a), an amount of regeneration agent supplied in the regeneration phase determines, determined therefrom the amount of empty NOx compounds and with recourse to a value supplied by the NOx model for an amount of the NOx storage catalyst stored NOx compounds the first value is calculated.

The approach followed in (b) becomes NOx measured value of a NOx sensor arranged downstream of the NOx storage catalytic converter with a value supplied by a NOx model value for a NOx concentration upstream of the NOx storage catalytic converter. From the comparison of both values, an instantaneous Einspeicherwirkungsgrad the NOx storage catalyst can be determined, which in turn can be converted via a known relationship in a degree of loading. This then provides the storage capacity on the basis of a modeled catalyst loading, which in turn was obtained by using the NO x model indicating the NO x raw emissions.

It Therefore, it is preferable that a step b) a NOx concentration measurement compared with an NOx concentration model value provided by the NOx model and from this a Einspeicherwirkungsgrad the NOx storage catalyst is determined, with which a degree of loading is determined, in turn under recourse to a value supplied by the NOx model for an amount of in the NOx storage catalyst stored NOx compounds is converted into the second value.

The Correction of the model with previous registration of the two values for the NOx Storage Capacity can, in order to achieve a particularly good stability of the process, iteratively executed become. The iteration can be considered successful if the amount the difference between the first and second values for the storage capacity below a certain threshold, making them negligible is. The iteration can be done by a series of iteration steps at the end of a pair of lean / regeneration operating phases as well as just one step per pair. In the latter case it is advantageous, the correction of the NOx model then in the following Lean / regeneration operating phase basis.

alternative for an iteration, the NOx model can also be calculated using a Mean value of the first and second value to be corrected.

In Each iteration step may have a correction factor for the NOx model be determined or a correction factor, anyway in the NOx model Input is corrected per iteration step. A corresponding Increment or decrement (alternatively, it is also a multiplicative change the correction factor is possible) is a function of the difference in the first and second values.

The NOx model can be advantageously corrected so that it is the raw emission at each operating point of the internal combustion engine indicates the NOx compounds in more detail. In this regard, one is the current one Operating point associated adaptation possible with the inventive method.

The inventive method further advantageously allows the request of a regeneration operating phase solely on the signal of the NOx sensor downstream of the NOx storage catalytic converter based. The values supplied by the NOx model are then merely used in addition, the NOx storage capacity for the aging test of the NOx storage catalyst to calculate. An ongoing calculation of the NOx emissions are no longer essential in normal operation required, so that computing time is saved. Furthermore can the for need the NOx model Maps simplified and / or exclusively by taking place in operation Adaptation filled become.

The Exhaust gas purification method according to the invention allows further error checking of the procedure, by an iteration after a given number of iterations aborted and an error condition is detected when a sufficient match between the first and second measured value can not be produced. The the same applies if the difference between the two values is too high ab initio is. In such a fault condition, a large tolerance-taking, the raw emission of NOx compounds upstream of the NOx catalyst NOx model for the subsequent (emergency) operation the internal combustion engine are based on compliance of NOx limits in any case. simultaneously can the determination of the aging state of the NOx storage catalyst by determining an aging factor resulting from the storage capacity of the NOx storage catalytic converter is derived, terminated and instead on a known aging model for the NOx storage catalyst can be switched. One last chance represents the pure operation of the internal combustion engine with stoichiometric mixture dar. d. H. the abandonment of lean operating phases.

The The invention will be described below with reference to the drawings for example, even closer explained. In the drawings shows:

1 a schematic representation of an internal combustion engine with a NOx storage catalyst,

2 a schematic flow diagram for carrying out a method for exhaust gas purification and

3 a flowchart of a portion of a method for emission control.

In 1 is in the form of a block diagram of an internal combustion engine with a Abgasnachbe shown treatment plant, in which a still to be explained method for exhaust gas purification is used. There are in the 1 shown only those components of an internal combustion engine or the exhaust aftertreatment system, which are required for understanding the process.

An internal combustion engine 10 has an intake tract 11 and an exhaust tract 12 on. In the intake tract 11 a fuel metering device is present, for the representative schematically an injection valve 13 is drawn. The injection valve 13 injected into the intake system 11 Fuel. Alternatively to the injection into the intake tract, the fuel can also be introduced directly into the cylinders of the internal combustion engine in the form of a direct injection. In the exhaust tract 12 in which the internal combustion engine 10 gives off their exhaust, an exhaust aftertreatment system is provided. It has a three-way catalyst arranged as close as possible to the internal combustion engine 14 on, in the flow direction of the exhaust gas, a NOx storage catalyst 15 is downstream.

Due to its location in front of the NOx storage catalytic converter 15 becomes the three-way catalyst 14 also referred to as pre-catalyst. The selection and design of this precatalyst is done in terms of fast response and oxygen storage capacity. Thus, during the still to be explained fat operation in a regeneration operating phase provided regeneration as possible unabated the NOx storage catalyst 15 is fed, the precatalyst should have the lowest possible oxygen storage capacity. In addition, he should perform as much as possible oxidation of hydrocarbons and carbon monoxide, since this is in lean operating phases of the internal combustion engine 10 has a favorable effect on the work of the NOx storage catalytic converter.

The NOx storage catalyst 15 stores NOx compounds discharged during lean operating phases of the internal combustion engine. He further has a coating that in stoichiometric operation of the internal combustion engine 10 the NOx storage catalyst 15 also gives catalytic three-way function.

To operate the exhaust aftertreatment system, sensors are provided which include an oxygen sensor 16 upstream of the precatalyst, a temperature sensor 17 near the NOx storage catalyst 15 and another oxygen sensor 18 downstream of the NOx storage catalyst 15 include.

All sensors and the fuel metering device via unspecified lines with a control unit 20 connected, which is a lambda controller 19 contains, which will be explained later.

As an oxygen sensor 16 Preferably, a broadband lambda probe is used which, depending on the oxygen content in the exhaust gas a steady, z. B. outputs a linear output signal. From the signal of this broadband lambda probe 16 determines the controller 20 the air ratio with which the internal combustion engine 10 was operated. This is the lambda controller 19 active, the lambda value in the stoichiometric operation of the internal combustion engine 10 according to target specifications. The regulation in the optimal lambda range of the internal combustion engine by means of the oxygen measuring sensor 18 , which is preferably designed as a binary lambda probe (2-point lambda probe), is a known control to a fixed lambda value close to 1. For this purpose, a arranged between the pre-catalyst and NOx storage catalyst lambda probe can be used.

In the oxygen sensor 18 At the same time a NOx sensor is integrated. Such sensors are known for example from the publication N. Kato et al. "Performance of thick film NOx sensor on diesel and gasoline engines", Society of Automotive Engineers, Publ. No. 970858, known. Such a sensor emits both a signal indicating a lambda value and a corresponding signal for the NOx concentration in the exhaust gas. The following is therefore of a NOx sensor 18 spoken.

The temperature of the NOx storage catalyst, which is evaluated in the subsequent method, is from the signal of the temperature sensor 17 calculated by means of a temperature model, since the temperature sensor 17 indicates only the temperature of the exhaust gas flowing into the NOx storage catalytic converter, which differs from the temperature in the NOx storage catalytic converter. Alternatively, the temperature of the NOx storage catalyst 15 be measured directly by, for example, a temperature sensor directly on or in the housing of the NOx storage catalyst 15 is arranged.

The control unit 20 is further on a unspecified data line with a map memory 21 are connected, in the corresponding maps, which will come to speak, are stored.

The regeneration operating phases of the internal combustion engine 1 in which it is operated with a rich fuel / air mixture, to the regeneration of the NOx storage catalytic converter 15 required regeneration agent in the raw gas of Internal combustion engine 10 to be prepared are initiated on the basis of a NOx model, as for example in the fully used here DE 196 07 151 C1 the applicant is described. A sensor-based introduction is alternatively possible.

The Einspeicherwirkungsgrad the NOx storage catalyst 15 drops with increasing NOx loading level. This NOx loading level is the quotient of the instantaneous absolute NOx load and the maximum NOx storage capacity and can be used to control the lean and regeneration operating phases of the internal combustion engine 10 be used. It can be seen that then to determine the degree of loading as accurate as possible both the current load and the maximum storage capacity of the NOx storage catalyst 15 is necessary. In addition, the knowledge of a remaining storage capacity and thus the maximum storage capacity is also necessary for a request for desulfurization operations. Finally, there is a general monitoring of the NOx storage catalyst by an aging factor is calculated from the maximum storage capacity. The aging factor is determined as the quotient of the current maximum storage capacity and a reference storage capacity. If it falls below a limit, the NOx storage catalytic converter must 15 replaced or switched to homogeneous operation.

The maximum storage capacity for a new NOx storage catalytic converter 15 can be determined on a test bench by measuring the amount of NOx compounds stored per unit time until saturation is reached. However, this value for the maximum storage capacity can not be used directly in normal operation of the internal combustion engine. So saturation of the NOx storage catalyst for emission reasons and technically in normal operation is not possible. Also decreases the current maximum storage capacity of a NOx storage catalyst 15 due to aging processes, so it is necessary to have the maximum storage capacity over the life of the internal combustion engine 10 or the NOx storage catalyst 15 to adapt or to determine and monitor the aging factor.

In addition the in 2 schematically illustrated method used.

The schema representation of 2 starts in a state where the internal combustion engine is in a lean operation phase; This can be done, for example, by the control unit 20 depending on the internal combustion engine 10 introduced performance requirements and performed by the control unit 20 in a step S1 ensures that the internal combustion engine with a superstoichiometric mixture (λ> 1) is operated.

In the lean operating phase, the amount ΣNOx SK of NOx compounds, which is the NOx storage catalyst, is determined in a step S2 15 saved since the last regeneration phase. The starting point for this quantity is that of the internal combustion engine 10 since the initiation of the lean operating phase emitted amount of NOx and the Einspeicherungswirkungsgrad the NOx storage catalyst 15 , To determine this amount is by means of map memory 21 determined via a map KF to the current operating parameters P, the NOx concentration NOxMod in Rohhabgas and converted taking into account other operating parameters, such as the speed of the internal combustion engine in a NOx flow and multiplied by the Einspeicherungswirkungsgrad. The operating parameters P include, among others, the temperature sensor 17 displayed exhaust gas temperature upstream of the NOx storage catalytic converter 15 ,

In a step S3, it is now checked whether the NOx sensor 18 indicates a NOx concentration NOx Sens exceeding a threshold value SW1. The at the NOx storage catalytic converter 15 Exiting NOx amount depends on the degree of loading of the NOx storage catalytic converter, so that the monitoring of the NOx concentration NOx Sens allows a demand-based initiation of a regeneration phase of operation. If no threshold is exceeded (N branch), the lean operation phase is continued by returning before step S1. In the case of a threshold crossing (J branch), however, a regeneration operating phase is initiated. Beforehand, however, the last measured value for the NOx concentration NOx Sens and the last current value for the modeled NOx concentration NOx Mod are stored in step S4, since these are used later to determine the maximum storage capacity and thus ultimately to determine a correction factor for the NOx Model needed.

Then, it is switched to the regeneration mode by the control unit in step S5 20 the internal combustion engine 10 with a rich mixture (λ <1) operates. During the regeneration operation phase, the amount of regeneration agent supplied through this rich mixture is obtained by a suitable summation or integration in a step S6. The amount of regeneration agent Σλ determined in this way will also be used later to determine the maximum storage capacity.

In one step 57 is monitored, whether the oxygen concentration downstream of the NOx storage catalyst 15 a threshold SW2 below below. The monitoring of the oxygen concentration for detecting the end of a regeneration operating phase, that is, for detecting the time at which the rich NOx-storage catalyst rich mixture at the output of the NOx storage catalyst exits because it is no longer needed for the regeneration of stored NOx compounds is known in the art, for example from the EP 0 597 106 A1 , If the oxygen concentration lies downstream of the NOx storage catalytic converter in step S7 15 not below the threshold (N-branch), the regeneration operation phase is continued by a return before step S5.

Otherwise (J branch), the regeneration operation phase is ended. Previously, however, in a step S8, the current storage capacity SK and the currently valid aging factor A for the NOx storage catalyst 15 determined. The sub-steps performed in step S8 will be described in connection with 3 still received. The internal combustion engine 10 then runs again (step S1) in a lean mode of operation.

For determining the storage capacity of the NOx storage catalyst 15 or for the evaluation of the aging state and for comparison of the NOx model, with the NOx concentration upstream of the NOx storage catalytic converter 15 is determined, in step S8 of 2 following sub-steps performed.

First is returned from the amount of regenerant Σλ by a step S9 known, for example stored in a map, the stored amount of NOx ΣNOx_S determined. The relationship is exploited that the amount of regenerant supplied while a regeneration operating phase for emptying the NOx storage catalytic converter is used by the stored NOx compounds.

Subsequently, in a step S10, the stored NOx amount ΣNOx_S determined from the regenerant amount is compared with the amount ΣNOxSK describing the descriptive NOx model as in the NOx storage catalyst 15 stored before beginning the regeneration operating phase. In this case, a deviation F1 is determined in step S10. The deviation F1 between the model value and the measured value is based on a memory capacity differing from the ideal value, which is then determined accordingly in a step S11, so that a first value SK1 for the memory capacity results therefrom.

Parallel to or before and after the steps S9 to S11, in a step S12, the injection efficiency H of the internal combustion engine becomes the measured NOx concentration NOx Sens downstream of the NOx catalyst 15 and the upstream of the NOx catalyst indicated by the descriptive NOx model 15 present raw NOx concentration NOx_Mod determined. step 12 thus provides the Einspeicherwirkungsgrad H, which was present just before the start of the regeneration operating phase. By way of a known relationship, this injection efficiency H is converted into a degree of loading B (step S13), for which purpose a map is used where appropriate.

Out the degree of loading B is in turn based on the descriptive NOx model specified value for the stored amount of NOx ΣNOx_S over a known in the art, optionally under utilization of a corresponding characteristic field, a second value SK2 for the storage capacity is determined (Step S14).

In a step S15, a deviation F2 between the two values for the storage capacity SK1 and SK2 is now determined. A step S16 then supplies a correction factor K which is dependent on the deviation F2 and which is used to correct the NOx raw emission upstream of the NOx storage catalytic converter 15 descriptive NOx model is used, for example, in the map memory 21 Correct the stored data accordingly or incorporate the correction factor in the calculations performed on the NOx model.

After the results of steps S11 and S14 are present, steps S15 and S16 are used which, from the results of steps S14 and S11, correct the NOx model that is the raw NOx emission upstream of the NOx storage catalyst 15 indicates cause. It is exploited that the approaches executed in steps S9 to S11 or S12 to S14 counteract in the event of a deviation of the NOx raw emission concentration delivered by the NOx model, as described in US Pat 4 is shown.

4 shows in two curves 23 and 24 the relationship between model error D and relative storage capacity change rSK. Here, both model error D and relative storage capacity change rSK are to be understood multiplicatively, ie at a value of 1, the NOx model is 100% correct, and at a relative storage capacity deviation rSK of 1, the same holds for the value of the storage capacity SK. The curve 23 , in the 4 is shown by the solid line, represents the dependence of the storage capacity or the relative storage capacity change rSK for the determination by means of steps S9 to S11. The curve 24 in contrast, for the steps S12 to S14.

As can be seen, the dependencies run in opposite directions, so that correction of the NOx model is achieved either by taking into account the difference of the two values for the storage capacity Ski and SK2 or by an iterative method in steps S15 and S16 the model error D. at the intersection of the curves 23 and 24 is returned, ie the NOx model is working error-free. Then not only is a needs-based initiation of the regeneration operating phase in which the NOx model is used, but above all an accurate determination of the aging factor is achieved. The use of two methods as well as an error correction of the NOx model contributes to the latter.

Claims (8)

An exhaust gas purification method for a lean-burnable internal combustion engine, in which stored in a lean operating phase of the internal combustion engine NOx storage in a located in the exhaust tract of the engine NOx storage catalyst, wherein a loading of the NOx storage catalyst takes place with NOx compounds, the internal combustion engine is operated temporarily in regeneration operating phases in which the NO x storage catalytic converter catalytically converts stored NO x compounds and is thereby emptied of NO x compounds, and a value for a storage capacity of the NO x storage catalytic converter based on a NOx emission that describes the raw emission of NO x compounds upstream of the NO x storage catalytic converter. model is determined and used for evaluation of the NOx storage catalyst, characterized in that a) an amount of is determined from the NOx storage catalyst deflated during the regeneration phases NOx compounds and R b) a NOx concentration downstream of the NOx storage catalytic converter is measured, and together with the NOx model, a second value for the storage capacity of the NOx catalyst is determined based on the NOx model. Storage Catalyst is determined, c) correcting the raw emission of NOx compounds upstream of the NOx storage catalyst descriptive NOx model using first and second value. Method according to claim 1, characterized in that that the descriptive model is corrected so that first and second value within certain limits. Method according to claim 1 or 2, characterized that steps a) and b) are carried out iteratively. Method according to claim 3, characterized that iteration after a predetermined number of iteration steps aborted and an error condition is detected if no match of first and second value. Method according to one of the above claims, characterized in that in the regeneration phase in the NOx storage catalytic converter supplied a regeneration agent, in step a) an amount of regeneration agent supplied in the regeneration phase determines, determined therefrom, the amount of empty NOx compounds and thus under recourse to a value supplied by the NOx model for an amount of in the NOx storage catalyst stored NOx compounds, the first value is calculated. Method according to one of the above claims, characterized characterized in that in step b) a NOx concentration measurement with a The NOx-concentration model value supplied by the NOx model is compared and from this a Einspeicherwirkungsgrad the NOx storage catalyst is determined, with which a degree of loading is determined, in turn under recourse to a value supplied by the NOx model for an amount of in the NOx storage catalyst stored NOx compounds is converted into the second value. Method according to one of the above claims, characterized characterized in that as first and second value respectively an aging factor of the NOx storage catalyst is used as the ratio between current storage capacity and a reference storage capacity is defined. Method according to claim 7, characterized in that if the aging factor falls below a threshold, a defect of the NOx storage catalyst is detected.

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