Method and Device to Determine the Nitrogen Oxide-Storage Capability of a Catalytic Converter of a Vehicle

Abstract

A method to determine a nitrogen oxide storage capacity of a catalytic converter of a vehicle in which a concentration of nitrogen oxides is measured in an exhaust gas downstream of the catalytic converter includes, in at least one first step, setting a concentration of nitrogen oxides in the exhaust gas at which the catalytic converter absorbs nitrogen oxides, and in at least one second step, setting a concentration of nitrogen oxides in the exhaust gas at which a desorption of nitrogen oxides by the catalytic converter takes place. The nitrogen oxide storage capacity of the catalytic converter is determined by considering a behavior of the catalytic converter at least during the desorption of nitrogen oxides.

Description

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method and a device to determine the nitrogen oxide storage capability of a vehicle's catalytic converter. This method measures the nitrogen oxide concentration in the exhaust gas downstream of the catalytic converter.

It is known from the state of the art that the performance of a catalytic converter having nitrogen oxide storage capacity can be determined by means of specially introduced measures with respect to a reduction of hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in the exhaust gas of the combustion engine of a vehicle. This includes for instance especially introduced exothermic processes, in other words, heating the catalytic converter, or an enrichment step, in other words, operating the combustion engine by means of an enriched air-fuel mixture.

Furthermore, DE 198 52 240 A1 describes a method to monitor a NOx storage catalytic converter whereby the NOx storage efficiency of the catalytic converter from the NOx exhaust gas concentration is identified before and after the NOx storage catalytic converter. The NOx exhaust gas concentrations before and after the NOx storage catalytic converter are converted into NOx mass flows and the NOx storage efficiency is determined on the basis of these values.

DE 103 18 116 B4 describes a process to operate an internal combustion engine whereby a storage catalytic converter is regenerated. For this purpose, the storage capacity is preset at the turning point of a chronological signal curve of a NOx mass flow after the storage catalytic converter.

DE 10 2006 055 238 A1 describes a method to operate an internal combustion engine to determine the NOx storage capacity of a NOx storage catalytic converter whereby exhaust gas is fed to two parallelly arranged NOx storage catalytic converters. Different amounts of loads are generated for both storage catalytic converters. Subsequently, both storage catalytic converters are loaded simultaneously with nitrogen oxide until the signal of a NOx sensor signals that the absorption capacity of one of the storage catalytic converters has been exhausted.

The catalytic converter having NOx storage capacity can be a NOx storage catalytic converter (NSK), or a Diesel oxidation catalytic converter (DOC). Inducing an exothermic process as well as providing an enrichment step are associated with an increased fuel consumption. Thus, such type of measures are advantageously only applied if they are required to reduce the pollutants in the exhaust gas. The frequency of the monitoring events, in other words, of the respective calculations of a catalytic converter's nitrogen oxide storage capacity is therefore limited. Should the NOx storage capacity of a catalytic converter be identified as part of an on-board diagnosis (OBD), this diagnosis will only be possible with this margin, or when accepting additional fuel consumption, if additional monitoring events were requested. For instance, the diagnosis of a DOC using exothermic processes will be performed as part of a particle filter regeneration. This generally occurs at an interval ranging from 500 km to 1,500 km of a distance covered by the vehicle. For this type of monitoring, information on the DOC performance with respect to a reduction of the hydrocarbons and the carbon monoxide in the exhaust gas will be achieved.

The diagnosis is performed more frequently for a nitrogen oxide storage catalytic converter because such type of diagnosis generally occurs by means of an enrichment step. Thus, such type of event generally occurs at a time interval between 1 km and 5 km of a distance covered by the vehicle.

This type of monitoring provides feedback on the nitrogen oxide storage capacity of a catalytic converter. In turn, the nitrogen oxide storage capacity of the catalytic converter is indicative of the efficiency with respect to a reduction of the hydrocarbon or carbon monoxide in the exhaust gas.

Though for the new catalytic converter technologies in particular, as for instance a passive nitrogen absorber (PNA), a DOC, or a passively run NSK no or at least significantly less enrichment steps can be arranged. Accordingly, diagnostic events are only possible by means of exothermic processes as part of the regeneration of the particle filter or by specially requested diagnoses, which in turn lead to an increased fuel consumption.

The task of the present invention thus consists in providing for an improved method and an improved device of the initially mentioned type.

In the method according to the invention, the nitrogen storage capacity of a vehicle's catalytic converter will be determined. For this purpose, a concentration of nitrogen oxides will be measured downstream of the catalytic converter. In at least one initial step, a concentration of nitrogen oxides will be set in the exhaust gas at which the catalytic converter absorbs nitrogen oxides. In at least one second step, a concentration of nitrogen oxides will be set in the exhaust gas at which a desorption of nitrogen oxides from the catalytic converter takes place. To determine the nitrogen oxide storage capacity of a catalytic converter, its behavior will be taken into account at least while the nitrogen oxides are desorbed.

In this manner, the catalytic converter's storage capacity can be determined qualitatively and indirectly even quantitively without any specially induced measures, which would lead to an increased fuel consumption. Hereby, different effects or mechanisms can be used. In a catalytic converter having nitrogen oxide storage capacity, a nitrogen oxide saturation can be produced. The maximum storage capacity or stored quantities depend on the current partial NOx pressure in this case. By lowering the partial NOx pressure, the maximum storage capacity or the stored nitrogen oxide quantiles will also be lowered and NOx will be desorbed. Conversely, when increasing the partial NOx pressure, the maximum storage capacity or the stored quantity of nitrogen oxides and NOx will be reabsorbed by the catalytic converter. Therefore, by observing a catalytic converter's behavior at least during the deliberately induced desorption of nitrogen oxides, the nitrogen oxide storage capacity of a catalytic converter can be inferred in an improved manner.

The information with respect to the nitrogen oxide storage capacity of a catalytic converter allows for a diagnosis of the catalytic converter with respect to the HC/CO/NOx performance. Furthermore, using the information regarding the nitrogen oxide storage capacity of the catalytic converter, the operating strategy of the combustion engine or a vehicle's motor can be optimized towards the exhaust system's current state. The vehicle can be a motor vehicle in particular or a commercial vehicle. Moreover, optimizing a strategy to desulfurize the catalytic converter (DeSOx strategy) will be possible. It is assumed that the catalytic converter has a NOx storage capacity, which changes as the exhaust system of the catalytic converter ages.

It is preferred in at least one first step to set a nitrogen oxide concentration in the exhaust gas at which the catalytic converter absorbs the nitrogen oxides until a saturation of the catalytic converter with nitrogen oxides has been achieved.

It has been shown to be advantageous in this case, to realize an overrun mode of the vehicle after setting the catalytic converter's saturation with nitrogen oxides, which will lead to a desorption of nitrogen oxides from the catalytic converter. Hereby and based on the chronological sequence of the nitrogen oxides' desorption, the catalytic converter's nitrogen oxide storage capacity can be inferred. To determine the NOx storage capacity, the above-mentioned mechanism can therefore be utilized that a NOx desorption exists in the vehicle's overrun mode or in the vehicle's operating state in which the raw emission of nitrogen oxides by the combustion engine is preferably quasi zero.

In accordance with another advantageous arrangement, a concentration of nitrogen oxides will be set in the exhaust gas in a plurality of initial steps whereby the catalytic converter absorbs nitrogen oxides. In a plurality of second steps, a concentration of nitrogen oxides will be set in the exhaust gas whereby the desorption of the nitrogen oxides will be realized by the catalytic converter. Based on the majority of the stored or released amounts of nitrogen oxides in the initial steps and in the second steps, the catalytic converter's nitrogen oxide storage capacity can be inferred. In particular, the respective absorbed amounts of nitrogen oxides and the desorbed amounts of nitrogen oxides can be accumulated separately from each other. The catalytic converter's storage capacity with respect to the nitrogen oxides can be inferred based on the absorbed amounts and the desorbed amounts.

In accordance with another advantageous arrangement, respective initial gradients of a chronological sequence of concentrations of nitrogen oxides in the exhaust gas upstream of the catalytic converter, and downstream of the catalytic converter are determined during at least one initial step. During at least one second step, respective second gradients of a chronological sequence of the concentration of nitrogen oxides in the exhaust gas are determined upstream of the catalytic converter and downstream of the catalytic converter. The catalytic converter's nitrogen oxide storage catalyst can be inferred based on the gradients. Thus, a catalytic converter's nitrogen oxide storage catalyst can be inferred in particular when using a nitrogen oxide gradient identification after the catalytic converter, particularly a nitrogen oxide storage catalyst.

Hereby an average value is preferably generated based on a plurality of values from the first gradient and from a plurality of values from a second gradient. The catalytic converter's nitrogen oxide storage catalyst can then be inferred based on the average value. Such type of method is feasible in a particularly simple and economical manner with respect to the design of an appropriate control system or a control device of a vehicle to determine the catalytic converter's nitrogen oxide storage catalyst.

The nitrogen oxide storage capacity of a passive nitrogen oxide absorber of the vehicle and/or of an oxidation catalytic converter of the vehicle and/or of a passively operated nitrogen oxide storage catalytic converter of the vehicle is preferably determined. In a passive nitrogen oxide absorber (PNA), the nitrogen oxides are stored or sorbed for instance in the passive nitrogen oxide absorber's zeolite material, whereby the nitrogen oxides are not chemically bound, however. Therefore, enriching the air-fuel mixture to reduce chemically bound nitrogen oxides as part of a chemical reaction will be not be required. In fact, a thermic desorption of the nitrogen oxides occurs at the passive nitrogen oxide absorber.

Likewise, an oxidation catalytic converter, particularly a diesel oxidation catalytic converter (DOC) has sorption capacities or absorption capacities for nitrogen oxides for instance by using zeolites as substrates of the catalytically effective substances of the oxidation catalytic converter. In this case as well, a desorption of nitrogen can take place without that the air-fuel mixture would need to be enriched, that is, without that an air ratio λ ratio greater than 1 would need to be set.

Furthermore, a nitrogen storage catalytic converter can be operated passively whereby the nitrogen oxides are chemically bound to an appropriate material of the nitrogen oxide storage catalyst because this chemically bound NOx can also be thermally absorbed by raising the temperature of the passively operated nitrogen oxide storage catalyst for instance.

Particularly in the case of such type of catalytic converters where no enrichment jump was set in the air-fuel mixture to obtain a reduced content of nitrogen oxide in the catalytic converter, the above described method will be applicable in a particularly advantageous manner.

The device in accordance with the invention to determine a vehicle's catalytic converter's nitrogen oxide storage capacity comprises a sensor to measure a concentration of nitrogen oxides in the exhaust gas downstream from the catalytic converter. Furthermore, the device comprises a control device, which has been formed to set a concentration of nitrogen oxides in the exhaust gas in at least one initial step at which the catalytic converter absorbs nitrogen oxides. In addition, the control device is formed to set a concentration of nitrogen oxides in the exhaust gas in at least one second step at which a desorption of nitrogen oxides by the catalytic converter takes place. Furthermore, the control device is formed to take the catalytic converter's behavior at least into account when desorbing the nitrogen oxides to determine the catalytic converter's storage catalyst of nitrogen oxide.

The described advantages and the preferred embodiments for the method according to the invention apply for the device in accordance with the invention and inversely.

The characteristics and combinations of characteristics described above as well as the characteristics and combinations of characteristics described below in the description of the figures and/or in the figures by themselves are not only applicable in the respectively indicated combination but also in other combinations or by themselves without leaving the invention's scope. Thus, embodiments must be considered as being comprised by the invention and as disclosed that were not explicitly shown or explained in the figures, but which follow from separate combinations of characteristics based on the explained explanations and which can be produced. Thus, embodiments and combinations of characteristics can be considered as having been disclosed that do not feature all the characteristics of an originally formulated independent claim.

Additional advantages, characteristics, and details of the invention result from the claims, the subsequent description of preferred embodiments of the invention, as well as from the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the chronological sequence of the nitrogen oxide concentrations upstream of an oxidation catalytic converter and downstream of an oxidation catalytic converter while a vehicle is in overrun mode, which takes place subsequently to a saturation of the oxidation catalytic converter with nitrogen oxides;

FIG. 2 illustrates the chronological sequence of the NOx mass flows upstream and downstream of the oxidation catalytic converter as well as the respective totals of the absorbed or desorbed amounts of nitrogen oxides during the observed time interval; and

FIG. 3 illustrates the decelerated step response of the nitrogen oxide concentration in the exhaust gas upstream of the oxidation catalytic converter in the event of changes of the raw emissions of nitrogen oxide.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts schematically and partially exhaust system 10 of a vehicle such as, for example, a passenger car or of a commercial vehicle. Exhaust system 10 comprises an exhaust gas system 12 in which catalytic converter 14 is arranged having nitrogen oxide storage capacities. Catalytic converter 14 can be for instance an oxidation catalytic converter, particularly a diesel oxidation catalytic converter (DOC). In exhaust gas system 12, particle filter 16, and/or an SCR catalytic converter 18 can be connected in series after catalytic converter 14. Furthermore, particle filter 16 may be particularly formed as an SCR-coated particle filter 16. To reduce nitrogen oxides in a selective catalytic reduction reaction in SCR-catalyst 18 (SCR=selective catalytic reduction), nitrogen oxides can be converted from the exhaust gas into water and nitrogen using ammoniac. For this purpose, an aqueous urea solution may be introduced in exhaust gas system 12 for instance by means of dosage device 20, whereby ammoniac is created from the urea solution in the exhaust gas.

Downstream of catalytic converter 14 and in the present case upstream of dosage device 20, sensor 22 is arranged in exhaust gas system 12 by means of which the concentration of nitrogen oxides in the exhaust gas downstream of catalytic converter 14 can be measured. An appropriate curve 24 in FIG. 1 illustrates the chronological sequence of the nitrogen oxides' (NOx) concentration in the exhaust gas downstream from catalytic converter 14. Additional curve 26 in FIG. 1 illustrates the raw nitrogen oxide emission, in other words, the concentration of nitrogen oxides in the exhaust gas as they exist based on the operation of a combustion engine of the vehicle upstream of catalytic converter 14 (not shown). The raw NOx emissions upstream of catalytic converter 14 can be captured using a sensor or based on a model. A control device such as in form of control device 28 of the vehicle will be used in the present case to determine the nitrogen oxide storage capacity of catalytic converter 14.

In the method to be illustrated with the aid of FIG. 1, a desorption of nitrogen oxides of catalytic converter 14 is observed preferably in the vehicle's overrun mode to determine the NOx storage capacity of catalytic converter 14 because the NOx storage capacity of catalytic converter 14 changes while exhaust system 10 ages, whereby the nitrogen oxide storage capacity of catalytic converter 14 reduces as the age of exhaust system 10 increases.

In the method illustrated by means of FIG. 1, a saturation of catalytic converter 14 with nitrogen oxides is produced in initial step 30. The fact that at least a substantial saturation of the storage or catalytic converter 14 exists, can be recognized by the fact that in the time interval of step 30, in which the NOx saturation is produced, the raw NOx emissions of the combustion engine (curve 26) are the same as the nitrogen oxide content in the exhaust gas downstream of catalytic converter 14 (curve 24).

In point of time 34 applied into timeline 32 in FIG. 1, an operation of the combustion engine or the motor is set using control device 28 for instance, in which the raw NOx emission is zero. This is advantageous because in that case the raw emission will be known without any measuring errors. Such type of condition occurs while the vehicle is in overrun mode for instance. The overrun mode can also be supported by an electrical motor of the vehicle so that any load requirements can be met without support from the combustion engine.

During a second step 36, which takes place immediately after point in time 34, catalytic converter 14 desorbs the nitrogen oxides. Accordingly, the concentration of nitrogen oxides in the exhaust gas downstream of catalytic converter 14 (curve 24) during the time interval following point in time 34 is not zero; the concentration drops, however. The behavior of catalytic converter 14 during this desorption of nitrogen oxides will be used to determine the nitrogen oxide storage capacity of catalytic converter 14.

Preferably, the requirements to be met while doing so consist in that the NOx storage, that is catalytic converter 14, be saturated and that the temperatures before catalytic converter 14 and after catalytic converter 14 neither increase nor drop drastically as it is advantageous that the temperature gradient be not too high. The reason for this is that the NOx storage capacity of catalytic converter 14 also depends on the temperature in general. Thus, with an essentially constant temperature, the desired desorption effect will not be superposed too significantly by the temperature effect.

In the event that exhaust system 10 was provided with an exhaust gas recirculation, particularly a high-pressure exhaust gas recirculation, it is preferable that the exhaust gas recirculation be done without in overrun mode. Otherwise nitrogen oxide emissions of the combustion engine would circulate in a circle and the raw emissions would drop more slowly accordingly. However, as soon as the raw NOx emissions from the engine reach the value of zero (point in time 34), an exhaust gas recirculation can be realized again, and the high-pressure exhaust gas recirculation rate can be increased.

Furthermore, to reinforce the desorption, the exhaust gas mass flow through catalytic converter 14 can be modified by means of suitable motor measures. In particular, the exhaust gas mass flow through catalytic converter 14 can be reduced to increase the measurement accuracy when capturing the nitrogen oxide concentration by means of sensor 22. For instance, changing the exhaust gas mass flow can be realized by appropriately setting the engine running speed, and/or by activating a throttle valve, and/or by changing a high-pressure exhaust gas recirculation rate or low-pressure exhaust gas recirculation rate.

When these requirements are met, the nitrogen oxide concentration will be preferably measured behind catalytic converter 14 using nitrogen oxide sensor 22. In the event that catalytic converter 14 is still capable at all to store nitrogen oxide, the nitrogen oxide concentration behind catalytic converter 14 will drop, in other words, downstream from catalytic converter 14, slower than before catalytic converter 14, that is, than upstream of catalytic converter 14. Thus, a desorption of nitrogen oxides by catalytic converter 14 takes place in overrun mode.

Preferably, the desorbed amount of nitrogen oxides in a modeled, that is, expected nitrogen oxide amount will be adjusted. Such type of model can also indicate how the nitrogen oxide concentration in the exhaust gas downstream of catalytic converter 14 should reduce to zero should a raw emission of nitrogen oxide be present. Thus, the model can for instance take into account the subsiding of measurements of sensor 22 or the existence of areas where the exhaust gas runs through less well in exhaust system 10, which accordingly can lead to a slowed down reduction of the nitrogen oxide concentration downstream of catalytic converter 14. Moreover, the model takes into account the ageing of exhaust system 10, particularly of catalytic converter 14.

If the actual measured course of the downstream nitrogen oxide concentration reduction of catalytic converter 14 deviates from the expected NOx decay curve, it is indicative of an appropriate change, particularly of a reduction, of the nitrogen oxide storage capacity of catalytic converter 14.

In particular, the desorbed nitrogen oxide amount, while in overrun mode, as well as the gradient of the nitrogen oxide decay curve, while in overrun mode by means of a model suggests the absolute nitrogen oxide storage capacity of catalytic converter 14. The nitrogen oxide storage capacity of catalytic converter 14 used as a basis in a currently used model will then be corrected upwards or downwards if the measured nitrogen oxide decay curve deviates from the target NOx decay curve.

Instead of observing the NOx saturation and the NOx desorption in overrun mode, the amounts that accumulated during a plurality of absorption processes or desorption processes can be observed. This is to be illustrated with reference to FIG. 2.

This variant is based on the finding that by lowering and increasing the partial NOx pressure in the area of catalytic converter 14, small absorption processes and desorption processes occur continuously. Such type of variations of the partial NOx pressure occur in dynamic driving situations for instance when the raw NOx emissions of the combustion engine vary due to varying load requirements to the combustion engine. In this case, it is preferred to accumulate the respective NOx amounts separately from each other. The total amount of nitrogen oxides that was absorbed, and the total desorbed amount of nitrogen oxides will then be compared with the quantities that are expected in accordance with a model.

In FIG. 2 the NOx mass flow upstream or downstream of catalytic converter 14 as a function of time t is applied on first ordinate 38, which is indicated on timeline 32. Initial curve 40 therefore illustrates the NOx mass flow upstream of catalytic converter 14 accordingly, and second curve 42 illustrates the NOx mass flow downstream from catalytic converter 14. In a plurality of initial steps 44, the emissions before catalytic converter 14 are higher than the emissions after catalytic converter 14. Accordingly, an absorption of nitrogen oxides occurs. Analogously, a desorption of nitrogen oxides by catalytic converter 14 takes place in a plurality of second steps 46. This is the case when the emissions are higher downstream of catalytic converter 14 than the emissions upstream of catalytic converter 14.

In another diagram in FIG. 2, the respective totals of the accumulated amounts of nitrogen oxides over time t are applied to additional ordinate 48. Curve 50 illustrates the accumulation of the absorption and curve 52 illustrates the accumulation of the desorption. Time t in turn is applied on timeline 32 in the second diagram in FIG. 2. Within the observed time interval, difference 54 between the added up absorption amounts and the added up desorption amounts results. This difference 54 will be compared with an index value and based on the comparison, a conclusion will be made with respect to the nitrogen oxide storage capacity of catalytic converter 14.

To determine the nitrogen oxide amounts as correctly as possible, it is important that the NOx concentrations before catalytic converter 14 or after catalytic converter 14 be known with low margins of error. In the event that measuring errors do exist, these errors before catalytic converter 14 and after catalytic converter 14 should tend to go into the same direction. The existence of errors can be caught for instance by observing conditions in which a storage of nitrogen oxides in catalytic converter 14 does not occur. In that case, the measuring values upstream of catalytic converter 14 or downstream of catalytic converter 14 should match or the modeled value upstream of catalytic converter 14 should correspond to the value downstream of catalytic converter 14 as measured with sensor 22. Furthermore, the nitrogen oxide concentrations should exist at the correct point in time. For this, the values of sensors before catalytic converter 14 or after catalytic converter 14 (or the values provided by the raw emission model and the values provided by sensor 22 after catalytic converter 14) should be geared to each other chronologically. The appropriate procedures are known to the expert.

To determine the absorption processes and desorption processes, the nitrogen oxide absorption and the nitrogen oxide desorption of catalytic converter 14 are continuously ascertained. This takes place by means of a subtraction of the nitrogen oxide mass flows before catalytic converter 14 and the nitrogen oxide mass flows after catalytic converter 14 from each other. NOx sensor 22 has been provided to determine the NOx mass flow downstream of catalytic converter 14. An emission model can also be used to determine the NOx mass flow before catalytic converter 14.

The nitrogen oxide absorption mass flows and the nitrogen oxide desorption mass flow are accumulated or added up separately from each other. Furthermore, the nitrogen oxide of the nitrogen oxide raw emissions of the combustion engine will be accumulated. In addition, a specific value will be determined, which will be representative for the averaged nitrogen oxide gradient upstream of catalytic converter 14, which in other words indicates the averaged increase of a curve, which represents the nitrogen oxide raw emissions of the combustion engine. Such type of specific value can be calculated or indicated in ppm_NOx per second in particular. Accordingly, a high specific value exists in the event of significant changes of the raw emissions. In contrast, a low specific value indicates less strong variations of the nitrogen oxide raw emissions.

Additionally, the difference between the total absorbed amounts of nitrogen oxides and the total desorbed amounts of nitrogen oxides depends on the temperature profile and on the raw emission during the evaluation period. For instance, the nitrogen oxide storage capacity of catalytic converter 14 generally increases when the temperature of catalytic converter 14 drops. Accordingly, the accumulated amount of absorbed nitrogen oxide is larger than the accumulated amount of desorbed nitrogen oxide. Conversely, an increased temperature of catalytic converter 14 will cause the accumulated desorption to be generally larger than the accumulated absorption.

Difference 54, which exists at the end of the observed evaluation period will be compared with an expected, modelled difference. If difference 54 is greater or smaller than the expected difference, it could suggest a drift of the emission model or of sensor 22. This can be compensated by an appropriate new calibration of the emission model or of sensor 22. In the event that no such drift exists, the NOx storage capacity of catalytic converter 14 can be inferred based on difference 54.

The diagnosis will be preferably performed while observing a past time interval. Thus, it can be assured that pre-determined fringe conditions existed within the time interval. It can be provided that one of these fringe conditions be that the NOx storage, in other words, catalytic converter 14, was sufficiently saturated with nitrogen oxides during the observed, past time interval. For instance, a saturation of a minimum of 80 percent can be allowed for. Moreover, a time interval is preferably observed in which the temperature was sufficiently stable, and was within a permitted range. For this, it can be considered for instance whether in case of a temperature change of the exhaust gas upstream from catalytic converter 14 no or at the most a slight temperature change took place downstream of catalytic converter 14. Fixing this fringe condition in turn is founded on the desorption's temperature dependency.

As an additional fringe condition, it can be provided that the specific value for the nitrogen oxide gradient upstream from catalytic converter 14 was within a pre-determined range. This range should not be too small. Otherwise the absorption effects and the desorption effects will be very low so that they cannot be captured very well meteorologically. By contrast, if the range is too wide, the tolerances of the emission model and the tolerances of sensor 22 can become too great. Furthermore, it can be checked whether the difference between the absorption and the desorption of the past time interval is plausible as a fringe condition. This can be checked in particular while consulting a model.

To assess the diagnosis, the amount of the accumulated absorption and the amount of the accumulated desorption will be compared with the modelled absorption and the modelled desorption. In the event that the difference of the accumulated absorption total and the accumulated desorption total is lesser than the modelled difference or the modelled total, it is a sign for a reduced nitrogen oxide storage capacity of catalytic converter 14. Thus, based on the absorption quantities and the desorption quantities, the absolute nitrogen oxide storage capacity of catalytic converter 14 can be inferred by means of a model. The model preferably comprises a target value for the absorption amount and a target value for the desorption amount for the provided fringe conditions in the provided time interval. These target values constitute a function of the temperature, the specific value of the NOx gradient, of the exhaust gas mass flow, and the total of the nitrogen oxide raw emissions. The currently modelled nitrogen oxide storage capacity of catalytic converter 14 can be corrected upwards or downwards when the measured absorption amount and the measured desorption amount deviates from the target absorption amount and the target desorption amount.

An additional possibility of taking into account the desorption of the nitrogen oxides by catalytic converter 14 depending on the concentration of nitrogen oxides in the exhaust gas to determine the nitrogen oxide storage capacity of catalytic converter 14, is to be illustrated while referring to FIG. 3, where the nitrogen oxide concentration is applied in a diagram on ordinate 56, and where time t is applied in turn on timeline 32. Initial curve 58 represents the course of the raw emissions as a function of time, and second curve 60 represents the concentration of the nitrogen oxides in the exhaust gas downstream of catalytic converter 14, which is recorded by means of sensor 22. In this case as well, the effect is used that by lowering or increasing the partial pressure of the nitrogen oxide by means of variations as they are customary in dynamic driving situation, resulting continuous and small absorption processes and desorption processes occur. However, due to these absorption processes and desorption processes the nitrogen oxide concentration after catalytic converter 14 changes more slowly than the nitrogen oxide concentration before catalytic converter 14. The corresponding step response, in other words, the change of the nitrogen oxide concentration downstream of catalytic converter 14, which is the result of a corresponding change of the nitrogen oxide concentration upstream of catalytic converter 14, will be compared with a step response as expected from a model. In particular, the absolute nitrogen oxide storage capacity of catalytic converter 14 can be inferred using a statistic analysis of several of such step responses by means of a model.

Preferably, one proceeds in the following manner to determine the specific value for the NOx gradient. Initially, a signal noise of nitrogen oxide sensor 22 after catalytic converter 14 as well as the (optional) nitrogen oxide sensor upstream of catalytic converter 14 will be averaged out. The gradient or the increase of nitrogen concentrations upstream of catalytic converter 14 will be determined in ppm_NOx per second for instance. As a basis for this, a nitrogen oxide sensor upstream of catalytic converter 14 (not shown) or a nitrogen oxide raw emission model will be used. Furthermore, the gradient or the nitrogen oxide concentration increase downstream of catalytic converter 14 will be preferably ascertained in ppm_NOx per second for which nitrogen oxide sensor 22 serves as a basis.

Subsequently, an average value will be formed for a defined time period of a minimum of 100 seconds for instance based in the totals of nitrogen oxide gradients before catalytic converter 14 and after catalytic converter 14. This average value is a specific value that is representative of the nitrogen oxide gradient.

For instance, according to FIG. 3, an absorption of nitrogen oxide takes place in catalytic converter 14 during initial step 62. This can be recognized by a significant increase of the raw emissions (curve 58) being followed by a significantly slower increase of the nitrogen oxide concentration downstream of catalytic converter 14 (curve 60). Conversely, in second step 64, a nitrogen oxide desorption by catalytic converter 14 occurs where the nitrogen oxide raw emission (curve 58) rapidly drops accordingly, while the nitrogen oxide concentration upstream of catalytic converter 14 decreases more slowly. Due to these slower step responses of nitrogen oxide concentrations downstream of catalytic converter 14, which is recorded by means of sensor 22, the existence of an absorption during a plurality of initial steps 62 or of a desorption during a plurality of second steps 64 can be inferred.

Also, when recognizing the nitrogen oxide gradient downstream of catalytic converter 14, which exhibits a nitrogen oxide storage capacity, the diagnosis is preferably performed on a past time interval. In this case as well, it can be provided as a fringe condition that the NOx storage or catalytic converter 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 within the permitted range. The specific value for the nitrogen oxide gradient was preferably upstream of catalytic converter 14 within a pre-determined range. If this range was too small, the tolerances and the noise made by the sensors dominated excessively. In contrast, if the specific value was too high, the tolerances of the emission model and the sensors' tolerances were too high.

By means of the specific value, which can be ascertained by means of the average values of the totals of the NOx gradients before catalytic converter 14 and after catalytic converter 14, the absolute nitrogen oxide storage capacity of catalytic converter 14 can be inferred using a model. The model preferably has a target value for the specific value of the nitrogen oxide gradient after catalytic converter 14 for the provided parameters in the provided time interval. This is a function of the temperature, the exhaust gas mass flow, the total of the nitrogen oxide raw emission and of the nitrogen oxide gradient before catalytic converter 14, however. The currently modelled nitrogen oxide storage capacity of catalytic converter 14 will be corrected upwards or downwards if the measured specific value of the nitrogen oxide gradient deviates from the target value.

The knowledge of the nitrogen oxide storage capacity of catalytic converter 14 can be used for the operating mode of exhaust system 10. For instance, the engine's raw nitrogen oxide emission can be adapted via the engine control unit. Furthermore, an enrichment step dosage strategy and/or a urea dosage strategy can be adapted or heating measures of the exhaust gas can be taken.

Moreover, based on the storage capacity of catalytic converter 14, the current performance of catalytic converter 14 in respect of a reduction of the content of hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in the exhaust gas can be inferred. Thus, a diagnosis of catalytic converter 14 according to OBD will be possible. For the diagnosis, an error can be reported in the event of a lower deviation of a critical nitrogen oxide storage capacity or storage amount. The consequence of this would be that an engine control light would be activated. In addition, or alternatively, additional diagnostic measures could be initiated to validate the result for instance. Such type of validation measures can be realized based on the state of the art.

Also, the current Sulphur concentration in the fuel and the Sulphur load of catalytic converter 14 can be inferred based on the change of the nitrogen oxide storage capacity of catalytic converter 14 within a time interval or interval in which a desulfurization of catalytic converter 14 takes place. Therefore, an optimization of a DeSOx strategy will be possible, that is, a desulfurization strategy of catalytic converter 14. In particular, the interval or the time interval between two DeSOx measures and the DeSOx intensity can be optimized. The DeSOx intensity is particularly expressed in a depth of the enrichment step, in other words, in the extent or intensity of the enrichment of the air-fuel mixture, in the duration of the enrichment step and in the number of enrichment steps. The desulfurization of catalytic converter 14 can be realized particularly under increased temperatures, for instance as part of a regeneration of particle filter 16, by means of appropriate enrichment steps.

REFERENCE CHARACTERS

• 10 Exhaust system
• 12 Exhaust gas system
• 14 Catalytic converter
• 16 Particle filter
• 18 SCR catalytic converter
• 20 Dosage device
• 22 Sensor
• 24 Curve
• 26 Curve
• 28 Control device
• 30 Step
• 32 Timeline
• 34 Point in time
• 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

Claims

1.-10. (canceled)

11. A method to determine a nitrogen oxide storage capacity of a catalytic converter of a vehicle in which a concentration of nitrogen oxides is measured in an exhaust gas downstream of the catalytic converter, comprising the steps of:

in at least one first step, setting a concentration of nitrogen oxides in the exhaust gas at which the catalytic converter absorbs nitrogen oxides;

in at least one second step, setting a concentration of nitrogen oxides in the exhaust gas at which a desorption of nitrogen oxides by the catalytic converter takes place; and

determining the nitrogen oxide storage capacity of the catalytic converter by considering a behavior of the catalytic converter at least during the desorption of nitrogen oxides.

12. The method according to claim 11, wherein the concentration of nitrogen oxides is set in the exhaust gas at which the catalytic converter absorbs nitrogen oxides such that a saturation of the catalytic converter with nitrogen oxides occurs.

13. The method according to claim 12, wherein after the saturation, an overrun mode of the vehicle is realized, and wherein based on a chronological sequence of the desorption of nitrogen oxides, the nitrogen oxide storage capacity of the catalytic converter is inferred.

14. The method according to claim 11, wherein in a plurality of first steps, the concentration of nitrogen oxides is set in the exhaust gas at which the catalytic converter absorbs nitrogen oxides, and in a plurality of second steps, the concentration of nitrogen oxides is set in the exhaust gas at which the desorption of nitrogen oxides by the catalytic converter takes place, wherein based on a stored and a released amount of nitrogen oxides via the plurality of first steps and second steps, the nitrogen oxide storage capacity of the catalytic converter is inferred.

15. The method according to claim 11, wherein respective initial gradients of a chronological sequence 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 first step, wherein respective second gradients of a chronological sequence of the concentration of nitrogen oxides in the exhaust gas upstream of the catalytic converter and downstream of the catalytic converter are determined during the at least one second step, wherein based on the gradients the nitrogen oxide storage capacity of the catalytic converter is inferred.

16. The method according to claim 15, wherein an average value is created from a multitude of totals of the first gradient and from a multitude of totals of the second gradient, wherein the nitrogen oxide storage capacity of the catalytic converter is inferred on a basis of the average value.

17. The method according to claim 11, wherein the nitrogen oxide storage capacity of a passive nitrogen oxide absorber of a vehicle is determined.

18. The method according to claim 11, wherein the nitrogen oxide storage capacity of an oxidation catalytic converter of a vehicle is determined.

19. The method according to claim 11, wherein the nitrogen oxide storage capacity of a passively operated nitrogen oxide storage catalytic converter of a vehicle is determined.

20. A method to determine a nitrogen oxide storage capacity of a catalytic converter of a vehicle with a sensor to measure a concentration of nitrogen oxides in an exhaust gas downstream of catalytic converter, comprising the steps of:

in at least one first step, setting a concentration of nitrogen oxides in the exhaust gas at which the catalytic converter absorbs nitrogen oxides by a control device;

in at least one second step, setting a concentration of nitrogen oxides in the exhaust gas at which a desorption of nitrogen oxides by the catalytic converter takes place by the control device; and

determining the nitrogen oxide storage capacity of the catalytic converter by considering a behavior of the catalytic converter at least during the desorption of nitrogen oxides by the control device.