DE102016005968A1 - Method and device for determining a nitrogen oxide storage capability of a catalytic converter of a vehicle - Google Patents

Abstract

The invention relates to a method for determining a nitrogen oxide storage capability of a catalytic converter (14) of a vehicle, in which a concentration of nitrogen oxides in the exhaust gas downstream of the catalyst (14) is measured, wherein in at least a first step (30) a concentration of nitrogen oxides in Is adjusted exhaust gas at which the catalyst (14) absorbs nitrogen oxides. In at least a second step (36), a concentration of nitrogen oxides in the exhaust gas is set at which desorption of the nitrogen oxides from the catalyst (14) takes place. For determining the nitrogen oxide storage capacity of the catalytic converter (14), the behavior of the catalytic converter (14) is taken into account, at least during the desorption of the nitrogen oxides. Furthermore, the invention relates to a device for determining a nitrogen oxide storage capability of a catalytic converter (14) of a vehicle.

Description

The invention relates to a method and a device for determining a nitrogen oxide storage capability of a catalytic converter of a vehicle. In the method, a concentration of a nitrogen oxide in the exhaust gas downstream of the catalyst is measured.

From the prior art, it is known to determine the performance of a catalyst with nitrogen oxide storage capability with regard to the reduction of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in the exhaust gas of an internal combustion engine of a vehicle by means of specially introduced measures. These include, for example, an extra induced exotherm, ie a heating of the catalyst, or a fat jump, ie an operation of the internal combustion engine with a rich air-fuel mixture.

Furthermore, the describes DE 198 52 240 A1 a method for monitoring a NOx storage catalyst, wherein the NOx storage efficiency of the catalyst from the NOx exhaust concentration is determined before and after the NOx storage catalyst. The NOx exhaust gas concentrations before and after the NOx storage catalyst are converted into NOx mass flows, and from these values, the NOx storage efficiency is determined.

The DE 103 18 116 B4 describes a method for operating an internal combustion engine, in which a storage catalytic converter is regenerated. In this case, a storage capacity at the point of inflection of a temporal signal curve of a NOx mass flow downstream of the storage catalytic converter is predetermined.

The DE 10 2006 055 238 A1 describes a method for determining the NOx storage capability of a NOx storage catalytic converter, in which the exhaust gas is supplied to two parallel NOx storage catalytic converters. There is generated a different size loading of both storage catalytic converters. Thereafter, both storage catalysts are loaded parallel in time with nitrogen oxides until the signal of a NOx sensor signals a depletion of the capacity of one of the storage catalytic converters.

The NOx trapping catalyst may be a NOx storage catalyst (NSK) or a Diesel Oxidation Catalyst (DOC). Both the generation of an exotherm and the provision of a fat jump are associated with an additional fuel consumption. Therefore, such measures are advantageously applied only if they are needed to reduce the pollutants in the exhaust. The frequency of the monitoring events, ie the respective investigations of the nitrogen oxide storage capacity of the catalyst, is thus limited. If the NOx storage capability of the catalytic converter is to be determined in the context of an on-board diagnosis (OBD), then this diagnosis is only possible at this interval or at the expense of additional fuel consumption if the monitoring events are additionally requested. For example, the diagnosis of a DOC using exothermicity is carried out as part of a particle filter regeneration. This is usually done in an interval between 500 kilometers and 1500 kilometers of driving distance traveled by the vehicle. In this form of monitoring, information about the performance of the DOC is obtained in terms of reducing hydrocarbons and carbon monoxide in the exhaust gas.

With a nitrogen oxide storage catalyst, the diagnosis is performed more frequently. Such a diagnosis is usually done with the help of a fat jump. Therefore, such an event usually occurs in an interval between 1 kilometer and 5 kilometers of a distance traveled by the vehicle.

This form of monitoring provides feedback on the nitrogen oxide storage capacity of the catalyst. From the nitrogen oxide storage capacity of the catalyst can in turn be concluded on the performance in terms of reducing the content of hydrocarbons or carbon monoxide in the exhaust gas.

However, in particular with novel catalyst technologies such as, for example, a passive nitrogen oxide absorber (PNA), a DOC or a passively operated NSK, no or at least significantly less fat jumps can be provided. Correspondingly, diagnostic events are only possible here by the exothermic reaction in the context of the regeneration of the particulate filter or by extra-requested diagnoses. However, these in turn result in increased fuel consumption.

Object of the present invention is therefore to provide an improved method and an improved device of the type mentioned.

This object is achieved by a method having the features of patent claim 1 and by a device having the features of patent claim 10. Advantageous embodiments with expedient developments of the invention are specified in the dependent claims.

In the method according to the invention, the nitrogen oxide storage capacity of a catalytic converter of a vehicle is determined. Here is a Concentration of nitrogen oxides in the exhaust gas measured downstream of the catalyst. In at least a first step, a concentration of nitrogen oxides in the exhaust gas is set at which the catalyst absorbs nitrogen oxides. In at least a second step, a concentration of nitrogen oxides in the exhaust gas is set at which desorption of the nitrogen oxides from the catalyst takes place. To determine the nitrogen oxide storage capacity of the catalyst, the behavior of the catalyst is taken into account, at least during the desorption of the nitrogen oxides.

In this way, the storage capacity of the catalyst can be qualitatively and indirectly determined quantitatively without specially initiated measures, which have a fuel consumption. In this case, different effects or mechanisms can be used. Nitrogen oxide saturation can be produced in the nitrogen oxide storage capacity catalyst. The maximum storage capacity or stored quantity is dependent on the current NOx partial pressure. By lowering the NOx partial pressure so also the maximum storage capacity or stored amount of nitrogen oxides decreases, and NOx is desorbed. Conversely, by raising the NOx partial pressure, the maximum storage capacity or stored amount of nitrogen oxides increases, and NOx is again absorbed by the catalyst. Thus, by observing the behavior of the catalyst at least during the deliberately induced desorption of the nitrogen oxides, the nitrogen oxide storage capacity of the catalyst can be deduced in an improved manner.

The information regarding the nitrogen oxide storage capability of the catalyst enables a diagnosis of the catalyst with regard to the HC / CO / NOx performance. Furthermore, with the information regarding the nitrogen oxide storage capability of the catalytic converter, the operating strategy of the internal combustion engine or the engine of the vehicle can be optimized for the current state of the exhaust system. The vehicle may in particular be a motor vehicle or a commercial vehicle. Furthermore, it is possible to optimize a strategy for desulfurizing the catalyst (DeSOx strategy). The catalyst is considered to have a NOx storage capability that varies with the age of the exhaust system or with the age of the catalyst.

Preferably, in the at least one first step, a concentration of nitrogen oxides in the exhaust gas is set, at which the catalyst absorbs the nitrogen oxides up to a saturation of the catalyst with nitrogen oxides.

It has been shown to be advantageous if, after adjusting the saturation of the catalyst with nitrogen oxides, a coasting operation of the vehicle is carried out, which leads to the desorption of the nitrogen oxides from the catalyst. This is based on a time course of the desorption of nitrogen oxides on the nitrogen oxide storage capacity of the catalyst closed. For the determination of the NOx storage capacity can thus make the above mechanism advantage that a NOx desorption in overrun operation of the vehicle or in operating conditions of the vehicle is present, in which the raw emission of nitrogen oxides of the internal combustion engine is preferably virtually zero.

According to a further advantageous embodiment, a concentration of nitrogen oxides in the exhaust gas is set in a plurality of first steps, in which the catalyst absorbs nitrogen oxides. In a plurality of second steps, a concentration of nitrogen oxides in the exhaust gas is set, at which the desorption of the nitrogen oxides takes place from the catalyst. Based on the quantities of the nitrogen oxides stored or discharged via the majority of the first steps and the second steps, the nitrogen oxide storage capacity of the catalytic converter is concluded here. In particular, the respectively absorbed amounts of nitrogen oxides and the respectively desorbed amounts of nitrogen oxides can be accumulated separately from one another. From the absorption amounts and the desorption can then be concluded that the storage capacity of the catalyst with respect to nitrogen oxides.

According to a further advantageous embodiment, during the at least one first step, respective first gradients of a temporal course of the concentration of nitrogen oxides in the exhaust gas are determined upstream of the catalytic converter and downstream of the catalytic converter. During the at least one second step, respective second gradients of a time course of the concentration of nitrogen oxides in the exhaust gas are determined upstream of the catalytic converter and downstream of the catalytic converter. Based on the gradients, it is concluded that the nitrogen oxide storage capacity of the catalyst. It is therefore in particular based on a nitrogen oxide gradient detection after the catalyst, in particular nitrogen oxide storage catalyst, closed on the nitrogen oxide storage capacity of the catalyst.

In this case, an average value is preferably formed from a plurality of amounts of the first gradients and from a plurality of amounts of the second gradients. Based on the average can then be concluded that the nitrogen oxide storage capacity of the catalyst. Such With regard to the design of a corresponding control device or a control device of the vehicle for determining the nitrogen oxide storage capacity of the catalytic converter, the method can be carried out in a particularly simple and low-effort manner.

Preferably, the nitrogen oxide storage capacity of a passive nitrogen oxide absorber of the vehicle and / or an oxidation catalytic converter of the vehicle and / or a passively operated nitrogen oxide storage catalytic converter of the vehicle is determined. In the case of a passive nitrogen oxide absorber (PNA), the storage or sorption of the nitrogen oxides takes place, for example, in the zeolite material of the passive nitrogen oxide absorber. However, the nitrogen oxides are not chemically bound. It is therefore not necessary to enrich the air-fuel mixture for reducing chemically bound nitrogen oxides in the course of a chemical reaction. Instead, the passive nitrogen oxide absorber undergoes thermal desorption of the nitrogen oxides.

An oxidation catalyst, in particular diesel oxidation catalyst (DOC), also has a sorption capacity or absorption capacity for nitrogen oxides, for example, through the use of zeolites as support materials of the catalytically active substances of the oxidation catalyst. Again, therefore, a desorption of nitrogen oxides take place without the need for enrichment of the air-fuel mixture, ie the setting of an air ratio λ greater than 1, needs to be made.

Furthermore, a nitrogen oxide storage catalyst can be operated passively, in which the nitrogen oxides are chemically bonded to a corresponding material of the nitrogen oxide storage catalyst. This chemically bound NOx can also be thermally desorbed, for example by raising the temperature of the passively operated nitrogen oxide storage catalytic converter.

In particular, in such catalysts, in which no fat jump in the air-fuel mixture is adjusted in order to achieve a reduction in the content of nitrogen oxides in the catalyst, therefore, the method described above is particularly advantageously used.

The device according to the invention for determining a nitrogen oxide storage capability of a catalytic converter of a vehicle comprises a sensor for measuring a concentration of nitrogen oxides in the exhaust gas downstream of the catalytic converter. The device further comprises a control device, which is designed to set in at least a first step, a concentration of nitrogen oxides in the exhaust gas, wherein the catalyst absorbs nitrogen oxides. The control device is further configured to adjust a concentration of nitrogen oxides in the exhaust gas in at least a second step, in which a desorption of the nitrogen oxides takes place from the catalyst. Furthermore, the control device is designed to take into account the behavior of the catalytic converter, at least during the desorption of the nitrogen oxides, in order to determine the nitrogen oxide storage capability of the catalytic converter.

The advantages and preferred embodiments described for the method according to the invention also apply to the device according to the invention and vice versa.

The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of the figures and / or in the figures alone can be used not only in the respectively specified combination but also in other combinations or in isolation, without the scope of To leave invention. Thus, embodiments are also included and disclosed by the invention, which are not explicitly shown or explained in the figures, however, emerge and can be produced by separated combinations of features from the embodiments explained. Thus, embodiments and combinations of features are also to be regarded as disclosed which do not have all the features of an originally formulated independent claim.

Further advantages, features and details of the invention will become apparent from the claims, the following description of preferred embodiments and from the drawings. Showing:

1 the time course of the nitrogen oxide concentrations upstream of an oxidation catalyst and downstream of the oxidation catalyst during a coasting operation of a vehicle, which takes place following saturation of the oxidation catalyst with nitrogen oxides;

2 the time course of the NOx mass flows upstream and downstream of the oxidation catalyst and the respective sums of the absorbed or desorbed within the considered time amounts of nitrogen oxides; and

3 the slow step response of the nitrogen oxide concentration in the exhaust gas downstream of the oxidation catalyst with changes in the raw nitrogen oxide emissions.

From a vehicle such as a passenger car or a commercial vehicle is in 1 schematically and in part one exhaust system 10 shown. The exhaust system 10 includes an exhaust system 12 in which a catalyst 14 is arranged, which has a nitrogen oxide storage capacity. With the catalyst 14 it may be, for example, an oxidation catalyst, in particular a diesel oxidation catalyst (DOC). In the exhaust system 12 can the catalyst 14 a particle filter 16 and / or an SCR catalyst 18 be downstream. The particle filter 16 Furthermore, it can be used in particular as an SCR-coated particle filter 16 be educated. For reducing nitrogen oxides in a selective catalytic reduction reaction, in the SCR catalyst 18 (SCR = selective catalytic reduction, selective catalytic reduction) nitrogen oxides from the exhaust gas with ammonia are converted to water and nitrogen. For this purpose, by means of a metering device 20 For example, an aqueous urea solution in the exhaust system 12 are introduced, wherein the ammonia is formed from the urea in the exhaust gas.

Downstream of the catalyst 14 and in the present case upstream of the metering device 20 is in the exhaust system 12 a sensor 22 arranged, by means of which the concentration of nitrogen oxides in the exhaust gas downstream of the catalyst 14 can measure.

A corresponding curve 24 in the 1 illustrates the time course of the concentration of nitrogen oxides (NOx) in the exhaust gas downstream of the catalyst 14 , Another curve 26 in 1 illustrates the raw nitrogen oxide, ie the concentration of nitrogen oxides in the exhaust gas, as due to the operation of an internal combustion engine (not shown) of the vehicle upstream of the catalyst 14 available. The NOx raw emission upstream of the catalyst 14 can be detected by means of a sensor or determined on the basis of a model. A control device approximately in the form of a control device 28 the vehicle is present to determine the nitrogen oxide storage capacity of the catalyst 14 used.

In the case of 1 Illustrative methods include desorption of the nitrogen oxides from the catalyst 14 preferably considered in the overrun mode of the vehicle to the NOx storage capacity of the catalyst 14 to investigate. The NOx storage capacity of the catalyst 14 namely, changes with the aging of the exhaust system 10 wherein the nitrogen oxide storage capacity of the catalyst 14 usually with increasing age of the exhaust system 10 decreases.

In the case of 1 illustrated method is in a first step 30 a saturation of the catalyst 14 made with nitrogen oxides. That an at least substantial saturation of the memory or catalyst 14 is present, it can be seen that in the time interval of the step 30 , in which the NOx saturation is established, the NOx raw emissions of the internal combustion engine (curve 26 ) the content of nitrogen oxides in the exhaust gas downstream of the catalyst 14 (Curve 24 ) are the same.

In one on a timeline 32 in 1 applied time 34 is for example by means of the controller 28 set an operation of the internal combustion engine or the engine in which the NOx raw emissions is zero. This is advantageous because then the raw emission is known without measurement error. Such a condition occurs, for example, during overrun of the vehicle. The overrun operation can also be assisted by an electric motor of the vehicle, so that load requirements can be met even without an engine assist.

During a second step 36 which is at the time 34 immediately followed, finds a desorption of nitrogen oxides from the catalyst 14 instead of. Accordingly, the nitrogen oxide concentration in the exhaust gas downstream of the catalyst 14 (Curve 24 ) in which at the time 34 non-zero for the following period, but the concentration decreases. The behavior of the catalyst 14 During this desorption of the nitrogen oxides is used to the nitrogen oxide storage capacity of the catalyst 14 to investigate.

Preferably, the conditions to be met in this case are that of the NOx storage, that is, the catalyst 14 , is saturated and that the temperatures are in front of the catalyst 14 and after the catalyst 14 neither rise strongly nor fall sharply. It is advantageous if the temperature gradient is not too large. This is because the NOx storage capability of the catalyst 14 usually also depends on the temperature. At at least substantially constant temperature, therefore, the desired effect of the desorption is not superimposed too strongly by the temperature effect.

If at the exhaust system 10 an exhaust gas recirculation, in particular high-pressure exhaust gas recirculation is provided, should be dispensed with in the overrun advantageously the exhaust gas recirculation. Otherwise, nitrogen oxide emissions from the internal combustion engine circulate in a circle and the raw emissions decrease correspondingly more slowly. However, as soon as the NOx raw emissions from the engine reaches zero (time 34 ), the exhaust gas recirculation can again made, in particular the high-pressure exhaust gas recirculation rate can be increased.

In order to enhance the desorption, further the exhaust gas mass flow through the catalyst 14 be changed by suitable motor measures. In particular, the exhaust gas mass flow can be reduced to the measurement accuracy when detecting the nitrogen oxide concentration by means of the sensor 22 to increase. The changing of the exhaust gas mass flow can be carried out, for example, by means of a corresponding setting of the engine speed and / or by actuating a throttle valve and / or by changing a high-pressure exhaust gas recirculation rate or low-pressure exhaust gas recirculation rate.

If these prerequisites are met, preference is given to using the nitrogen oxide sensor 22 the nitrogen oxide concentration behind the catalyst 14 measured. If the catalyst 14 even has a nitrogen oxide storage capacity, so the nitrogen oxide concentration drops behind the catalyst 14 that is, downstream of the catalyst 14 , slower than before the catalyst 14 , so as upstream of the catalyst 14 , So it finds in the overrun a desorption of nitrogen oxides from the catalyst 14 instead of.

Preferably, the desorbed amount of nitrogen oxides with a modeled, so expected nitrogen oxide amount is adjusted. Such a model can thus indicate how the nitrogen oxide concentration in the exhaust gas downstream of the catalyst 14 in the presence of zero NOx emissions. So the model can be about the decay of the measurements of the sensor 22 or the presence of less well exhaust gas flow through areas of the exhaust system 10 Accordingly, which accordingly slows down the decrease of the nitrogen oxide concentration downstream of the catalyst 14 being able to lead. Furthermore, the aging of the exhaust system is preferred in the model 10 , in particular the catalyst 14 , considered.

The actually measured course of the decrease of the nitrogen oxide concentration deviates downstream of the catalyst 14 from the expected NOx decay curve, so may a corresponding change, in particular reduction, of the nitrogen oxide storage capacity of the catalyst 14 getting closed.

In particular, from the desorbed amount of nitrogen oxide in the overrun mode and the gradient of the nitrogen oxide decay curve in overrun mode based on a model on the absolute nitrogen oxide storage capacity of the catalyst 14 getting closed. The nitrogen oxide storage capacity of the catalyst used in a currently used model 14 is then corrected up or down when the measured nitrogen oxide decay curve deviates from the target NOx decay curve.

Instead of considering the NOx saturation and the NOx desorption in the overrun operation, the amounts accumulated in a plurality of absorption processes or desorption processes can also be considered. This should be explained with reference to 2 be illustrated.

This variant is based on the finding that by lowering and raising the NOx partial pressure in the region of the catalyst 14 continuously small absorption processes and desorption occur. Such variations in the NOx partial pressure occur, for example, in dynamic driving situations, that is to say when the raw NOx emissions of the internal combustion engine vary as a result of different load requirements on the internal combustion engine. Preferably, the respective amounts of NOx are accumulated separately. The total amount of nitrogen oxides absorbed and the total amount of nitrogen oxides desorbed are then compared to expected levels according to a model.

In 2 is on a first ordinate 38 the NOx mass flow upstream or downstream of the catalyst 14 plotted as a function of time t, which is on the time axis 32 is specified. A first turn 40 thus illustrates the NOx mass flow upstream of the catalyst 14 and a second curve 42 the NOx mass flow downstream of the catalyst 14 , In a number of first steps 44 are the emissions before the catalyst 14 higher than the emissions after the catalyst 14 , Accordingly, it comes here to an absorption of nitrogen oxides. Analogously, it takes place in a plurality of second steps 46 a desorption of the nitrogen oxides from the catalyst 14 instead of. This is the case when the emissions downstream of the catalyst 14 are higher than the emissions upstream of the catalyst 14 ,

In another graph in 2 are on another ordinate 48 the respective sums of the amounts of nitrogen oxides accumulated over time t are plotted. A curve 50 illustrates the accumulation of absorption and a curve 52 the accumulation of desorption. The time t is again on the time axis 32 in the second graph in 2 applied. Within the period of time considered a difference results 54 between the accumulated absorption amounts and the summed desorption quantities. This difference 54 is compared with a setpoint, and based on the comparison is based on the nitrogen oxide storage capacity of the catalyst 14 closed.

In order to determine the nitrogen oxide quantities as correctly as possible, it is important that the NOx concentrations in front of the catalyst 14 or after the catalyst 14 with low fault tolerances are known. If there are measurement errors, these errors should be in front of the catalyst 14 and after the catalyst 14 in the same direction. The presence of errors can be detected, for example, by observing conditions in which there is no storage of nitrogen oxides in the catalyst 14 occurs. Then the readings should be upstream of the catalyst 14 or downstream of the catalyst 14 correspond respectively or the modeled value upstream of the catalyst 14 with the sensor 22 measured value downstream of the catalyst 14 correspond. Furthermore, the nitrogen oxide concentrations should be present at the right time. For this, the values of the sensors before the catalyst 14 or after the catalyst 14 (or the values provided by the raw emission model and those of the sensor 22 after the catalyst 14 supplied values) are timed to each other. Corresponding methods are known to the person skilled in the art.

In order to determine the absorption processes and desorption processes, the nitrogen oxide absorption and the nitrogen oxide desorption of the catalyst continuously become continuous 14 determined. This is done by subtracting the nitrogen oxide mass flows upstream of the catalyst 14 and the nitrogen oxide mass flows after the catalyst 14 from each other. To determine the NOx mass flow downstream of the catalyst 14 is the NOx sensor 22 intended. For the determination of the NOx mass flow upstream of the catalytic converter 14 An emission model can also be used.

The nitrogen oxide absorption mass flows and the nitrogen oxide desorption mass flows are accumulated or added up separately. Furthermore, the raw nitrogen oxide emission of the internal combustion engine is accumulated. In addition, a characteristic value is preferably determined which is representative of the averaged nitrogen oxide gradient upstream of the catalyst 14 is, which therefore indicates the average slope of a curve representing the raw nitrogen oxide emissions of the internal combustion engine. Such a characteristic value can in particular be calculated or specified in ppm _NOx per second. A high characteristic value is accordingly present in the case of strong changes in the raw emissions. In contrast, a lower characteristic value indicates less pronounced fluctuations in the raw emissions of nitrogen oxides.

Furthermore, the difference between the total absorbed amount of nitrogen oxides and the total desorbed amount of nitrogen oxides depends on the temperature profile and on the raw emission during the evaluation period. For example, the nitrogen oxide storage capacity of the catalyst increases 14 usually when the temperature of the catalyst 14 falls. Accordingly, the accumulated amount of absorbed nitrogen oxide is larger than the accumulated amount of desorbed nitrogen oxide. Conversely, an increasing temperature of the catalyst 14 that the accumulated desorption is usually greater than the accumulated absorption.

The difference at the end of the considered evaluation period 54 is compared to an expected, modeled difference. Is the difference 54 greater or smaller than the expected difference, this may be due to drift of the emission model or the sensor 22 indicate. This can be done by a corresponding new calibration of the emission model or the sensor 22 be compensated. If there is no such drift, based on the difference 54 on the NOx storage capacity of the catalyst 14 be inferred.

The diagnosis is preferably carried out considering a past time interval. This is because it can be ensured that predetermined boundary conditions have been present within the time interval. As one of these boundary conditions can be provided that the NOx storage, so the catalyst 14 , in the considered, past time interval was sufficiently saturated with nitrogen oxides. For example, a saturation of at least 80 percent may be provided. Furthermore, it is preferable to consider a time interval in which the temperature was sufficiently stable and within an allowable range. For this purpose, for example, it may be considered whether a temperature change of the exhaust gas upstream of the catalyst 14 there is no or at most a slight change in temperature downstream of the catalyst 14 has come. The specification of this boundary condition is in turn based on the temperature dependence of the desorption.

As a further boundary condition it can be provided that the parameter for the nitrogen oxide gradient upstream of the catalyst 14 within a given range. This area should not be too small. Otherwise, the absorption effects and the desorption effects are very low, so that they can not be detected well by measurement. If the range is too large, however, the tolerances of the emission model and the tolerances of the sensor 22 get too big. Furthermore, it can be checked as a boundary condition whether the difference between the absorption and the desorption in the past time interval is plausible. This can be checked in particular by using a model.

For the evaluation of the diagnosis the height of the accumulated absorption and the amount of accumulated desorption are compared with the modeled absorption and the modeled desorption. If the difference between the accumulated absorption amount and the accumulated desorption amount is less than the modeled difference or the modeled quantity, this is an indication of a reduced nitrogen oxide storage capacity of the catalyst 14 , From the absorption amounts and the Desorptionsmengen can therefore a model on the absolute nitrogen oxide storage capacity of the catalyst 14 getting closed. In the given period, the model preferably includes a setpoint value for the absorption amount and a setpoint value for the desorption amount for the given boundary conditions. These setpoint values are a function of the temperature, the characteristic value of the NOx gradient, the exhaust gas mass flow and the sum of the raw nitrogen oxide emissions. The currently modeled nitrogen oxide storage capacity of the catalyst 14 may be corrected upwards or downwards if the measured absorption amount and the measured desorption amount deviate from the target absorption amount and the target desorption amount.

Another possibility, the desorption of nitrogen oxides from the catalyst 14 as a function of the concentration of nitrogen oxides in the exhaust gas for determining the nitrogen oxide storage capacity of the catalyst 14 to consider with respect to 3 be illustrated. Here is a graph on an ordinate 56 the nitric oxide concentration is plotted and on the time axis 32 turn the time t. A first turn 58 represents the course of the raw emission as a function of time, and a second curve 60 the concentration of nitrogen oxides in the exhaust gas downstream of the catalyst 14 , which by means of the sensor 22 is detected. Here, too, the effect is taken advantage of that resulting by lowering and raising or by variations in the nitrogen oxide partial pressure, as is common in dynamic driving situations, resulting in continuous small absorption processes and desorption processes. Due to these absorption processes and desorption processes, however, the nitrogen oxide concentration after the catalyst changes 14 slower than the nitrogen oxide concentration before the catalyst 14 , The corresponding step response, ie the change in the nitrogen oxide concentration downstream of the catalyst 14 resulting from a corresponding change in the nitrogen oxide concentration upstream of the catalyst 14 results is compared to a step response expected by a model. In particular via a statistical evaluation of several such step responses can by means of a model on the absolute nitrogen oxide storage capacity of the catalyst 14 getting closed.

Preferably, the following procedure is used to determine the characteristic value for the NOx gradient. First, a signal noise of the nitrogen oxide sensor 22 after the catalyst 14 and the (optional) nitrogen oxide sensor upstream of the catalyst 14 averaged out. The gradient or slope of the nitrogen oxide concentration upstream of the catalyst 14 is determined, for example, in ppm _NOx per second. The basis for this purpose is a (not shown here) nitrogen oxide sensor upstream of the catalyst 14 or a nitrogen oxide raw emission model. Furthermore, the gradient or the slope of the nitrogen oxide concentration downstream of the catalyst 14 preferably determined in ppm _NOx per second. The basis for this is the nitrogen oxide sensor 22 ,

Subsequently, for a defined period of, for example, at least 100 seconds, the mean value of the amounts of the nitrogen oxide gradient before the catalyst 14 and after the catalyst 14 educated. This mean value is a characteristic which is representative of the nitrogen oxide gradient.

According to 3 for example, during a first step 62 an absorption of nitrogen oxides in the catalyst 14 instead of. This can be seen from the fact that a strong increase in the raw emission (curve 58 ) a significantly slower increase in the nitrogen oxide concentration downstream of the catalyst 14 (Curve 60 ) occurs. Conversely, for example, it comes in a second step 64 to a nitrogen oxide desorption from the catalyst 14 , Here the nitrogen oxide raw emission decreases accordingly (curve 58 ) rapidly, while the nitrogen oxide concentration downstream of the catalyst 14 decreases more slowly. Because of these slower jump responses of the nitrogen oxide concentration downstream of the catalyst 14 , which by means of the sensor 22 can be detected on the presence of absorption during a plurality of first steps 62 or desorbing during a plurality of second steps 64 getting closed.

Also in the detection of the nitrogen oxide gradient downstream of the catalyst 14 , which has the nitrogen oxide storage capability, the diagnosis is preferably performed on a past time interval. As a boundary condition can also be provided here that the NOx storage or the catalyst 14 was sufficiently saturated with nitrogen oxides in the past time interval. Furthermore, it is preferably detected that the exhaust gas temperature in the past time interval was sufficiently stable and in the permitted range. Furthermore, the parameter for the nitrogen oxide gradient was preferably upstream of the catalyst 14 within a given range. If this range is too small, the tolerances and noise of the sensors are too dominant. If the characteristic value is too large, then The tolerances of the emission model and the tolerances of the sensors become too large.

From the characteristic value, which results from the mean values of the amounts of the NOx gradients before the catalyst 14 and after the catalyst 14 can determine via a model on the absolute nitrogen oxide storage capacity of the catalyst 14 getting closed. For the given boundary conditions, the model preferably has a nominal value for the characteristic value of the nitrogen oxide gradient downstream of the catalyst in the given period of time 14 , However, this is a function of the temperature, the exhaust gas mass flow, the sum of the nitrogen oxide raw emission and the nitrogen oxide gradient upstream of the catalyst 14 , The currently modeled nitrogen oxide storage capacity of the catalyst 14 is corrected upwards or downwards if the measured characteristic value of the nitrogen oxide gradient deviates from the nominal value.

Knowledge about the nitrogen oxide storage capacity of the catalyst 14 can for the operation of the exhaust system 10 be used. For example, can be adjusted via the engine control, the raw nitrogen oxide emissions of the engine. Furthermore, a fat jump dosing strategy and / or a urea dosing strategy can be adapted, or heating measures of the exhaust gas can be made.

From the storage capacity of the catalyst 14 can also look at the current performance of the catalyst 14 in view of reducing the content of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in the exhaust gas. Thus, a diagnosis of the catalyst 14 according to OBD possible. For the diagnosis, an error can be reported when falling below a critical nitrogen oxide storage capacity or amount of storage. This can result in that, for example, an engine control lamp is activated. Additionally or alternatively, further diagnostic measures can be initiated, for example to validate the result. Such validation measures may be made according to the state of the art.

From the change in the nitrogen oxide storage capacity of the catalyst 14 within a period or interval in which desulfurization of the catalyst 14 can also be based on the current sulfur concentration in the fuel and on the sulfur loading of the catalyst 14 getting closed. It is therefore also possible to optimize a DeSOx strategy, ie a strategy of desulfurization of the catalyst 14 , In particular, the interval or the period between two DeSOx measures and the DeSOx intensity can be optimized. The DeSOx intensity manifests itself in particular in a depth of the fat jump, that is to say in the extent or intensity of the enrichment of the air-fuel mixture, in the duration of the fat jump and in the number of fat leaps. Desulphurization of the catalyst 14 can in particular at elevated temperature, for example in the context of a regeneration of the particulate filter 16 , be accomplished by appropriate leaps in fat.

LIST OF REFERENCE NUMBERS

10

exhaust system

12

exhaust gas line

14

catalyst

16

particulate Filter

18

SCR catalyst

20

metering

22

sensor

24

Curve

26

Curve

28

control unit

30

step

32

timeline

34

time

36

step

38

ordinate

40

Curve

42

Curve

44

step

46

step

48

ordinate

50

Curve

52

Curve

54

difference

56

ordinate

58

Curve

60

Curve

62

step

64

step

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 19852240 A1 [0003]
• DE 10318116 B4 [0004]
• DE 102006055238 A1 [0005]

Claims (10)

Method for determining a nitrogen oxide storage capacity of a catalyst ( 14 ) of a vehicle in which a concentration of nitrogen oxides in the exhaust gas downstream of the catalyst ( 14 ), characterized in that in at least a first step ( 30 . 44 . 62 ) a concentration of nitrogen oxides in the exhaust gas is set, at which the catalyst ( 14 ) Absorbs nitrogen oxides, wherein in at least a second step ( 36 . 46 . 64 ) a concentration of nitrogen oxides in the exhaust gas is set, in which a desorption of the nitrogen oxides from the catalyst ( 14 ), wherein to determine the nitrogen oxide storage capacity of the catalyst ( 14 ) the behavior of the catalyst ( 14 ) is taken into account at least in the desorption of nitrogen oxides. Method according to claim 1, characterized in that in the at least one first step ( 30 ) a concentration of nitrogen oxides in the exhaust gas is set, at which the catalyst ( 14 ) the nitrogen oxides up to a saturation of the catalyst ( 14 ) absorbed with nitrogen oxides. A method according to claim 2, characterized in that after adjusting the saturation of the catalyst ( 14 ) is carried out with nitrogen oxides, a coasting operation of the vehicle, which to the desorption of nitrogen oxides from the catalyst ( 14 ), based on a time course of the desorption of the nitrogen oxides on the nitrogen oxide storage capacity of the catalyst ( 14 ) is closed. Method according to one of claims 1 to 3, characterized in that in a plurality of first steps ( 44 ) a concentration of nitrogen oxides in the exhaust gas is set, at which the catalyst ( 14 ) Absorbs nitrogen oxides, and in a plurality of second steps ( 46 ) a concentration of nitrogen oxides in the exhaust gas is set, at which the desorption of the nitrogen oxides takes place from the catalyst, based on the over the majority of the first steps ( 44 ) and second steps ( 46 ) stored and discharged amounts of nitrogen oxides on the nitrogen oxide storage capacity of the catalyst ( 14 ) is closed. Method according to one of claims 1 to 4, characterized in that during the at least one first step ( 62 ) respective first gradients of a time profile of the concentration of nitrogen oxides in the exhaust gas upstream of the catalyst ( 14 ) and downstream of the catalyst ( 14 ) and during the at least one second step ( 64 ) respective second gradients of a time profile of the concentration of nitrogen oxides in the exhaust gas upstream of the catalyst ( 14 ) and downstream of the catalyst ( 14 ), based on the gradients on the nitrogen oxide storage capacity of the catalyst ( 14 ) is closed. A method according to claim 5, characterized in that an average value is formed from a plurality of amounts of the first gradient and a plurality of amounts of the second gradient, wherein based on the mean value on the nitrogen oxide storage capacity of the catalyst ( 14 ) is closed. Method according to one of claims 1 to 6, characterized in that the nitrogen oxide storage capacity of a passive nitrogen oxide absorber of the vehicle is determined. Method according to one of claims 1 to 7, characterized in that the nitrogen oxide storage capacity of an oxidation catalyst of the vehicle is determined. Method according to one of claims 1 to 8, characterized in that the nitrogen oxide storage capacity of a passively operated nitrogen oxide storage catalytic converter of the vehicle is determined. Device for determining a nitrogen oxide storage capacity of a catalyst ( 14 ) of a vehicle with a sensor for measuring a concentration of nitrogen oxides in the exhaust gas downstream of the catalyst ( 14 ), characterized by a control device ( 28 ), which is adapted to, in at least a first step ( 30 . 44 . 62 ) to adjust a concentration of nitrogen oxides in the exhaust gas at which the catalyst ( 14 ) Absorbs nitrogen oxides, and in at least a second step ( 36 . 46 . 64 ) to adjust a concentration of nitrogen oxides in the exhaust gas, wherein a desorption of the nitrogen oxides from the catalyst ( 14 ), and to determine the nitrogen oxide storage capacity of the catalyst ( 14 ) the behavior of the catalyst ( 14 ) to be considered at least in the desorption of nitrogen oxides.

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