DE102015013463A1 - Method for determining the aging state of an oxidation catalytic converter for an internal combustion engine - Google Patents

Abstract

The invention relates to a method for determining the state of aging of an oxidation catalytic converter (12) arranged in an exhaust tract (10) of an internal combustion engine and permeable by exhaust gas of the internal combustion engine for oxidizing constituents present in the exhaust gas, comprising the steps of: - determining the capability of the oxidation catalytic converter (12) to store existing nitrogen oxides in the exhaust gas; and - determining the state of aging as a function of the determined ability.

Description

The invention relates to a method for determining the aging state of an oxidation catalytic converter for an internal combustion engine.

For purifying exhaust gas from internal combustion engines, in particular diesel engines, use is made, for example, of nitrogen oxide storage catalysts (NSK) and of oxidation catalysts, such an oxidation catalyst, if it is used in an exhaust tract of a diesel engine, being referred to as a diesel oxidation catalyst (DOC). NSK can store nitrogen oxides (NO _x ) under oxidizing or lean exhaust conditions, that is to say at a combustion air ratio λ of greater than 1. The nitrogen oxide storage (NO _x storage) is carried out mainly by binding the nitrogen oxides in the form of nitrates to contained in the NSK, formed from alkali and / or alkaline earth metal compounds nitrogen oxide storage components.

By creating reducing or rich exhaust conditions (λ <1) decomposition of the nitrates is achieved, wherein the nitrogen oxides liberated in the nitrate decomposition are reduced by the reducing exhaust gas components at noble metal reaction centers of NSK chemically harmless nitrogen (N ₂ ). This will regenerate the NSK and will then be available again to store NO _x . In the context of regeneration, the NSK releases the nitrogen oxides stored in it. The creation of the reducing or rich exhaust gas conditions is typically carried out by a so-called fat jump, as it enriches the exhaust gas with reducing components, in particular CO and HC, which, for example, by a motor fuel post-injection and / or by a secondary injection of fuel in the exhaust gas is reached. With HC thereby unburned hydrocarbons are referred to, wherein CO is called carbon monoxide.

Oxidation catalysts, in particular DOC, are mainly used for the catalytically assisted oxidation of constituents present in the exhaust gas, in particular pollutants such as carbon monoxide (CO) and unburned hydrocarbons (HC). It is known that oxidation catalysts, in particular diesel oxidation catalysts, are subject to a largely irreversible aging in terms of their catalytic action and, on the other hand, to at least partial reversible passivation or deactivation. The irreversible aging can be at least partially offset by operation of the oxidation catalyst at more or less elevated temperatures. The passivation can be remedied by short-term exposure to reducing exhaust gas, analogous to the above-mentioned fat jump. However, both high temperature increases and fat leaps result in increased fuel consumption.

New types of DOC technologies have the potential to be activated after irreversible aging. When activated, the DOC can reach a similar light-off temperature as the fresh or new DOC. Activation can be effected by short-term exposure to reducing exhaust gas, analogous to the above-mentioned fat jump. The following reversible passivation occurs within a shorter period of time than the irreversible aging in lean operation and can be remedied by repeating the activation.

Furthermore, the DE 10 2010 028 373 A1 a method for monitoring the operating state of a NO _x storage catalytic converter. In the method, a fully loaded state of the NO _x storage catalyst is brought about. Further, the wall temperature of the NO _x storage catalyst is increased to a predetermined value. Furthermore, an existing NO _x storage downstream of the NO _x storage catalyst is determined. In addition, a degree of aging is assigned to the NO _x storage catalyst on the basis of the temporal behavior of the determined NO _x concentration.

Finally, the reveals WO 2008/047170 A1 a method of reducing nitrogen oxides taken up in a lean gas stream.

Against this background, it is an object of the invention to provide a method by means of which the aging state of an oxidation catalyst, in particular a diesel oxidation catalyst, can be determined in an efficient and effective manner.

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

The method according to the invention for determining the aging state of an oxidation catalytic converter, in particular diesel oxidation catalyst (DOC), which is arranged in an exhaust gas tract of an internal combustion engine, in particular a diesel engine, and which can be flowed through by exhaust gas of the internal combustion engine, comprises a first step, in which the ability of the oxidation catalyst to store existing nitrogen oxides (NO _x ), is determined. This ability of the oxidation catalyst to store nitrogen oxides present in the exhaust gas is also referred to as nitrogen oxide storage capacity or as the NO _x storage capacity of the oxidation catalyst. In a second step of the method, the aging state is determined as a function of the ascertained capability, that is to say of the determined NO _x storage capacity.

The invention is based on the surprising finding that, on the one hand, even in oxidation catalysts, especially in diesel oxidation catalysts, nitrogen oxides are stored under lean exhaust conditions, in particular reversibly stored, and on the other hand, the NO _x storage capacity of the oxidation catalyst is a measure of its aging with respect to its oxidation-catalytic activity , By determining the state of aging, it is possible to infer the ability of the oxidation catalyst to oxidize components contained in the exhaust gas, such as CO and HC, so that according to the invention the NO _x storage capacity of the oxidation catalyst is determined and used to determine the state of aging. As a result, in particular, the state of aging which relates specifically to the oxidative activity with respect to CO and HC can be determined particularly effectively and efficiently, that is to say in a manner which is low in fuel consumption.

Furthermore, the inventive method allows the realization of a particularly fuel-efficient operation of the oxidation catalyst or the internal combustion engine, since in this respect the invention is based on the idea to adjust leaps to reactivate the example passivated oxidation catalyst to its aging state. Under such a fat jump is - as described above - a effect of reducing or rich exhaust conditions (λ <1) to understand, so that such a fat jump is realized for example by enriching the mixture of the internal combustion engine. The fat jump can be achieved, for example, by a post-injection engine fuel injection and / or by a secondary injection of fuel into the exhaust gas. The reactivation of the oxidation catalyst is understood to mean that, as described above, its passivation is at least partially remedied by at least short-term exposure to reducing exhaust gas.

It has proven to be particularly advantageous if the oxidation catalyst used is an oxidation catalyst which is free with regard to alkali metals and alkaline earth metals, in particular a diesel oxidation catalyst. In other words, preferably, the oxidation catalyst is free of alkali metals and alkaline earth metals. This embodiment is based on the finding that even oxidation catalysts which are free with respect to alkali metals and alkaline earth metals, optionally can store nitrogen oxides, in particular can store reversibly. This makes it possible to determine the aging state of such an oxidation catalyst in a particularly effective and efficient manner.

A further embodiment is characterized in that at least one reactivation process, in which a passivation of the oxidation catalyst is at least partially eliminated by enriching the exhaust gas with reducing constituents, is carried out as a function of the determined aging state. As a result, the fuel consumption of the internal combustion engine can be kept particularly low, so that a particularly efficient operation can be realized.

In order to be able to determine the nitrogen oxide storage capacity and thus the state of aging in a particularly precise manner, it is provided in another embodiment of the invention that the oxidation catalyst is operated in a saturation phase in which the lean oxidation gas containing nitrogen oxides is applied to the oxidation catalyst. In other words, the internal combustion engine is operated, for example, such that lean exhaust gas is formed in which nitrogen oxides are contained. In the saturation phase, a first content of nitrogen oxides contained in the exhaust gas is furthermore determined, the exhaust gas having this first content of nitrogen oxides upstream of the oxidation catalyst. Furthermore, a second content of nitrogen oxides contained in the exhaust gas is determined in the saturation phase, wherein the exhaust gas has this second content of nitrogen oxides downstream of the oxidation catalyst. Furthermore, the exhaust gas containing the nitrogen oxide is applied to the oxidation catalyst in the saturation phase until the second content deviates from the first content by a predeterminable and preferably low level. This measure is preferably approximately 0. In this saturation phase, the oxidation catalyst is saturated with nitrogen oxides, so that it can not store nitrogen oxides or only to an insignificant extent.

In order to saturate the oxidation catalyst very quickly and thus achieve particularly efficient operation, it is provided in a particularly advantageous embodiment of the invention that the oxidation catalyst in the saturation phase at a temperature in a range of 170 degrees Celsius inclusive up to and including 250 degrees Celsius The nitrogen oxide-containing exhaust gas is applied. Preferably, the temperature is maintained at least substantially constant during the saturation phase.

It has proven to be particularly advantageous if the first content of nitrogen oxides contained in the exhaust gas is detected by means of a sensor arranged upstream of the oxidation catalyst in the exhaust gas tract and / or determined by means of a computing device on the basis of a characteristic map of the internal combustion engine. As a result, the first content and thus the nitrogen oxide storage capacity and the aging state can be determined particularly precisely.

A further embodiment is characterized in that, for determining the NO _x storage capacity of the oxidation catalyst, it is operated after the saturation phase in a desorption phase in which the oxidation catalyst is heated by means of the exhaust gas in such a way that the oxidation catalyst stores the stored nitrogen oxides at least partially, in particular at least predominantly, releases. As a result, the nitrogen oxide storage capacity can be determined very precisely.

By heating the oxidation catalyst in the desorption phase, the oxidation catalyst is subjected to a temperature increase. It is preferably provided to continue to operate the internal combustion engine such that lean exhaust gas is formed. With regard to this temperature increase, it is preferably provided that the temperature increase, that is, the heating of the catalyst, is carried out comparatively rapidly. Preferably, the oxidation catalyst is heated to a predeterminable end temperature in less than eight minutes, in particular within less than five minutes, and preferably within less than two minutes, in the desorption phase.

It has proven to be particularly advantageous if this final temperature is more than 250 degrees Celsius, in particular more than 300 degrees Celsius, and preferably more than 350 degrees Celsius. In other words, it is preferably provided that the oxidation catalyst is heated in the desorption phase to a temperature of more than 250 degrees Celsius, in particular more than 300 degrees Celsius, and preferably more than 350 degrees Celsius, this temperature being the final temperature described above. The fuel consumption can be kept particularly low especially when the final temperature is lower than 550 degrees Celsius.

In a further advantageous embodiment of the invention, a quantity of the nitrogen oxides released by the oxidation catalyst in the desorption phase is determined and compared with a reference value, wherein the aging state of the oxidation catalyst is determined as a function of the comparison. As a result, the state of aging can be determined particularly precisely.

Finally, it has proven to be particularly advantageous if, before the determination of the NO _x storage capacity of the oxidation catalytic converter, it is desulphurized and / or a particle filter arranged in the exhaust gas tract is regenerated. Thereby, the nitrogen oxide storage capacity and thus the aging state of the oxidation catalyst can be determined particularly precisely, since the determination is not affected by an excessive sulfur content and / or by an excessively loaded particulate filter.

Further advantages, features and details of the invention will become apparent from the following description of a preferred embodiment and from the drawing. 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.

The drawing shows in:

1 a diagram illustrating the nitrogen oxide storage capacity and the light-off temperature of an oxidation catalyst in the form of a diesel oxidation catalyst over time or on its aging state;

2 a schematic representation of an exhaust tract of an example designed as a diesel engine internal combustion engine, wherein in the exhaust tract of the diesel oxidation catalyst is arranged;

3 a diagram illustrating a method for determining the aging state of the diesel oxidation catalyst;

4 another diagram illustrating the method;

5 another diagram illustrating the method;

6 another diagram illustrating the method; and

7 a diagram illustrating the relationship between the aging state of the Dieseloxidationskatalysators and the frequency of so-called fat activations or fat jumps.

In the figures, identical or functionally identical elements are provided with the same reference numerals.

The following is a method for determining the state of aging of an exhaust tract 10 ( 2 ) arranged for example as a diesel engine internal combustion engine and can be flowed through by exhaust gas of the diesel engine oxidation catalyst 12 for oxidizing constituents present in the exhaust gas such as CO and HC. Since the internal combustion engine is a diesel engine, the oxidation catalyst is 12 a diesel oxidation catalyst, also referred to as DOC. As will be explained in more detail below, in the context of the method, the ability of the oxidation catalyst 12 to store existing or contained nitrogen oxides (NO _x ) in the exhaust, determined, the aging state is determined depending on the determined ability.

The ability to store nitrogen oxides present in the exhaust gas, as nitrogen oxide storage capacity or as a _NOx storage capacity of the oxidation catalyst 12 designated. For an explanation of the method, reference will be made to an in 1 taken diagram shown, which is only schematically the aging-related decrease in NO _x storage capacity and the aging-related increase in light-off temperature as a function of the period of use of the DOC, which is present in terms of alkali and alkaline earth metals, especially alkali and alkaline earth metal compounds, free. This means that the oxidation catalyst 12 is free of alkali and alkaline earth metals or has no alkali and alkaline earth metals. On the abscissa 14 of the diagram is the time or the aforementioned period of use of the DOC (oxidation catalyst 12 ) applied. On a first ordinate 16 The NO _x storage capacity is plotted in the unit percent, taking on a second ordinate 18 of the graph, the light-off temperature with respect to an oxidation of CO or HC in the unit degrees Celsius is plotted. A solid curve entered in the diagram 20 illustrates the NO _x storage capacity of the DOC as a function of time, with dashed curves entered in the diagram 22 and 24 illustrates the light-off temperature with respect to time.

The DOC serves in the exhaust tract 10 mainly the catalytically assisted oxidation of pollutants present in the exhaust gas such as carbon monoxide (CO) and unburned hydrocarbons (HC). With regard to its oxidation-catalytic effect, the DOC is subject to a mostly largely irreversible aging on the one hand, and to an at least partial reversible passivation or deactivation on the other hand. The irreversible aging can be at least partially compensated for by operating the DOC at more or less elevated temperatures, since an increase in temperature is typically associated with an increase in catalytic activity. The passivation can be remedied by short-term exposure to reducing exhaust gas. The reducing effect of the exhaust gas is adjusted by a so-called fat jump, by means of which reducing or rich exhaust gas conditions are created. The fat jump is also called fat activation. In such a fat jump, the exhaust gas is enriched with reducing components such as CO and HC, which is achieved for example by a motor fuel post-injection and / or by a secondary injection of fuel into the exhaust gas. However, both temperature increases and fat leaps result in increased fuel consumption.

The history 22 illustrates the light-off temperature of a passivated DOC as a function of time during the course 24 represents the light-off temperature of a non-passivated DOC as a function of time. The aforementioned removal of the passivation is also referred to as reactivation or reactivation and is in 1 by a dotted arrow 26 illustrated. By means of the method, a particularly efficient and effective determination of the state of aging can be realized while simultaneously realizing a particularly fuel-efficient operation of the DOC, in particular of the internal combustion engine. With regard to the determination of the state of aging, in particular with regard to the catalytic effect with respect to the oxidation of CO or HC and / or NO, the method is based on the surprising finding that, on the one hand, in particular a DOC free of alkali and alkaline earth metal compounds under oxidizing conditions , ie, can store nitrogen oxides in lean exhaust gas, in particular can store reversibly, on the other hand, the NO _x storage capacity of the DOC represents a measure of its aging or aging state with respect to its oxidation catalytic activity.

With regard to fuel-efficient operation, the method is based on the idea of adjusting the frequency of fat leaps to reactivate the passivated DOC to the state of aging. The NO _x storage capacity decreases with increasing temperature. Typically, it is relatively strong at low temperatures of 170 degrees Celsius to 250 degrees Celsius and only slightly pronounced at temperatures above 300 degrees Celsius. With regard to the light-off temperature, the course represents 22 the Aging behavior of a passivated DOC and the course 24 the aging behavior of a non-passivated DOC. Under the light-off or light-off temperature is understood to mean the temperature at which the DOC can cause a predetermined oxidizing conversion of, for example, 50 percent of CO and / or HC. As indicated by the dotted arrow 26 For example, a passivated DOC can be reactivated by means of a fat jump, that is, converted back into the non-passivated state.

Out 1 it can be seen that a) the aging-related increase of the light-off temperature correlates with the age-related decrease of the NO _x storage capacity and b) the aging-related increase of the light-off temperature is more pronounced in the case of a passivated DOC than in a non-passivated DOC. The process uses the relationship mentioned under a) to characterize the aging state of the DOC through its NO _x storage capacity. For this purpose, for example, the storable nitrogen oxide amount (given in mol or grams per liter of volume of the DOC, or as a percentage of existing in the fresh state storage capacity) can be used directly or converted into an aging index.

In the following, a preferred procedure for determining the NO _x storage capacity is explained, which is also referred to as storage capacity for the sake of simplicity. It is on the in 2 illustrated, exemplary exhaust tract 10 of in 2 Of course, the diesel engine not shown, the process of course not limited to this exhaust tract 10 is.

In the exhaust tract 10 is an emission control system 28 arranged, which the oxidation catalyst 12 Includes (DOC). The exhaust tract 10 is traversed by the exhaust gas, the exhaust gas is the exhaust tract 10 flows through in a flow direction in 2 by a directional arrow 30 is illustrated. Based on the flow direction of the exhaust gas through the exhaust tract 10 is an addition unit downstream of the DOC 32 arranged, by means of which a reducing agent, in particular an aqueous urea solution, can be metered into the exhaust gas. Downstream of the DOC and downstream of the addition unit 32 is a particle filter 34 arranged, which is also referred to as a diesel particulate filter, because the internal combustion engine is designed as a diesel engine. The diesel particulate filter is also referred to as DPF. The present is the particle filter 34 coated or impregnated with an SCR catalyst material (SCR - selective catalytic reduction). Thus, the diesel particulate filter is also referred to as SDPF.

Downstream of the particulate filter 34 is an SCR catalyst 36 arranged, which is also referred to as SCR. Between the DOC and the SDPF and / or downstream of the SCR catalyst and thus downstream of the DOC can each be designed as a nitrogen oxide sensor (NO _x sensor) sensor 38 respectively 40 be arranged. In addition, upstream of the DOC, a sensor designed as a nitrogen oxide sensor may be arranged, which in 2 not shown. By means of the respective nitrogen oxide sensor, a respective content of nitrogen oxides contained in the exhaust gas can be determined or detected. By means of in 2 Not shown nitrogen oxide sensor, it is thus necessary to detect a first content of nitrogen oxides contained in the exhaust gas, the exhaust gas having this first content upstream of the DOC, that is arranged at a first upstream of the DOC. By means of the sensor 38 respectively 40 For example, it is possible to detect a second content of nitrogen oxides contained in the exhaust gas, the exhaust gas having this second content at a respective second location arranged downstream of the DOC. In other words, the exhaust gas has the respective second content downstream of the DOC.

In the present case, the DOC is embodied, for example, as a honeycomb body or comprises such a honeycomb body which has, for example, a platinum and / or palladium-containing coating which is at least substantially free of alkali metals and alkaline earth metals or alkali metal and alkaline earth metal compounds. The SDPF is designed, for example, as a honeycomb body through which the wall flows, with an SCR catalytic coating or impregnation. The SCR is preferably also formed as a coated honeycomb body. The coating may be identical or similar to that of the SDPF. Preference is given to zeolites containing iron (Fe) and / or copper (Cu).

To determine the storage capacity of the DOC this is first in an in 3 Saturated at a preferably comparatively low and preferably at least approximately constant temperature of about 170 degrees Celsius to 250 degrees Celsius with NO _x contained exhaust gas of the diesel engine applied until the downstream sensor 38 respectively 40 indicates a nitrogen oxide content in the exhaust gas, which differs only by a specifiable, low level, preferably approximately 0, from the nitrogen oxide content in the exhaust gas on the input side of the DOC. In other words, the DOC is acted upon in the saturation phase at a preferably comparatively low and preferably approximately constant temperature of 170 degrees Celsius to 250 degrees Celsius with nitrogen oxide-containing exhaust gas of the diesel engine until the means of the sensor 38 respectively 40 recorded, second salary by one specifiable, small amount deviates from the first salary. The second content is usually also referred to as NO _x behind DOC, wherein the first content is usually referred to as NO _x before DOC. The first content is, for example, by means of in 2 not shown, arranged upstream of the DOC nitrogen oxide sensor detected and / or, in particular by means of a computing device such as a control unit, determined from a map relating to the operating point-dependent raw emissions of the diesel engine.

3 shows a diagram on the abscissa 42 the time or the period of use is plotted. On a first ordinate 44 of in 3 In the diagram shown, the nitrogen oxide concentration in the exhaust gas is plotted in units of parts per million (ppm), taking on a second ordinate 46 of in 3 Diagram shown the temperature of the DOC in the unit degrees Celsius is plotted. A solid course 48 illustrates the said first content over time, with a dashed trace 50 illustrates the second content over time. A dotted course 52 illustrates the temperature of the DOC over time.

A stabilization of the temperature of the DOC can be achieved by cooling or heating measures. For heating, for example, a load can be connected, in which the engine is heated, at least one post-injection is set and / or an unillustrated heating catalyst is used. Cooling can be achieved, for example, by increasing the fresh air mass or changing the boost pressure. An increase in the level of raw nitric oxide emissions (for example, by reducing the EGR rate) shortens the saturation phase. For example, a reference value for the NO _x storage capacity can be obtained via a previously empirically determined characteristic map relating to the temperature-dependent NO _x storage capacity of a fresh, ie new DOC. A temperature model, in particular in the radial and axial direction of the DOC, via the DOC also enables a reliable determination of the reference value.

If the DOC is defined as saturated with nitrogen oxides, an in 4 switched desorption phase, which follows in time to the saturation phase. In the desorption phase, the DOC is heated in such a way that the NO _x stored in the DOC is desorbed at least substantially thermally. In other words, the DOC is heated in the desorption phase such that it releases the nitrogen oxides stored in the DOC at least partially, in particular at least predominantly.

4 shows the diagram according to 3 , but not during the saturation phase, but during the desorption phase. The DOC has at least largely released the nitrogen oxides stored in it when, after passing through a desorption maximum present on the input side of the DOC, almost the same nitrogen oxide content in the exhaust gas as upstream of the DOC is determined again behind the DOC. The DOC is then at least largely exempt from stored NO _x . The quantity of nitrogen oxide desorbed in the desorption phase represents the NO _x storage capacity of the DOC at the conditions prevailing in the saturation phase.

The amount of nitrogen oxide desorbed in the desorption phase can be determined, for example, by signal integration, as in 4 is illustrated by the integral sign ∫ by means of the downstream sensor 38 respectively 40 be determined. Of course, the amount of nitrogen oxide which is determined by calculation or measurement during the desorption phase on the basis of the normal NO _x emission of the diesel engine and flowing into the DOC is taken into account. In order to keep this offset low, in the desorption phase, the NO _x raw emission can be reduced by engine measures. Likewise, it is advantageous to keep the exhaust gas mass flow in the desorption phase as low as possible. Appropriate measures are the operation of an exhaust or intake throttle, changing a high pressure or low pressure EGR rate or a ratio of low pressure (ND) to high pressure (HD) EGR rate, replacement of internal combustion engine drive power by an electric auxiliary drive , Choice of a higher gear or a higher gear ratio or the like.

With regard to the temperature increase of the DOC effected during the desorption phase, it is preferably provided to carry out the temperature increase comparatively rapidly, that is to say preferably within less than eight minutes, in particular less than five minutes and preferably less than two minutes. In the desorption phase of the DOC is heated, for example, to a final temperature, which should be achieved as quickly as possible, and preferably within the aforementioned time. The final temperature during the heating is preferably more than 250 degrees Celsius, in particular more than 300 degrees Celsius and preferably more than 350 degrees Celsius.

Is in the saturation phase, which is also referred to as the storage phase, and / or the desorption phase NO _x behind DOC, that is, the second content by means of the sensor arranged downstream of the SCR 40 determined, it is preferably ensured that both the SDPF and the SCR behave at least largely inert with respect to NO _x contained in the exhaust gas. This can be done by timely switching off the metering of the reducing agent and / or by, for example, thermal desorption of any ammonia stored in the SDPF and / or in the SCR. By comparing the quantity of nitrogen oxide desorbed in the desorption phase with the abovementioned reference value for the NO _x storage capacity of, for example, an unaged, fresh DOC, a quantitatively definable state of aging of the DOC can be determined.

In the following, a further advantageous procedure for determining the NO _x storage capacity of the DOC and thus its aging state will be explained. It is assumed that the NO _x storage capacity of a DOC, in particular due to the lack of alkali and alkaline earth metals or alkali and alkaline earth metal storage components, compared to a NSK is relatively low and the DOC therefore, especially at relatively high temperatures, usually is present in a saturated or near-saturated state with respect to the storage of nitrogen oxides, that is, the NO _x storage capacity is usually at least approximately exhausted. On the other hand, since the NO _x storage capacity is temperature-dependent, for example, temperature changes caused by engine operating point changes result in measurable NO _x storage or desorption effects with a temporarily increasing exhaust gas temperature and NO _x storage or absorption effects with a temporarily decreasing exhaust gas temperature 5 is shown schematically.

5 shows a diagram on the abscissa 54 the time is up. On the ordinate 56 the NO _x concentration is plotted in ppm. A solid course 58 characterized as before the content of nitrogen oxides contained in the exhaust gas upstream of the DOC, wherein a dotted course 60 illustrates the content of nitrogen oxides contained in the exhaust downstream of the DOC. One through the course 60 illustrated increase 62 of NO _x behind DOC is caused by a temperature increase, with one passing through the gradient 60 illustrated decrease 64 the NO _x behind DOC is caused by a decrease in temperature. The extent of these effects, that is a respectively desorbed or adsorbed amount of nitrogen oxide, is dependent on the size of the available NO _x storage capacity. Since the latter as above based on 1 in turn, is aging-dependent, a respectively desorbed or adsorbed nitrogen oxide amount can thus be used to determine the aging state of the DOC.

By way of illustration, reference will be made to 6 taken in which the temperature dependence of the NO _x storage capacity κ of a new and an aged DOC are shown diagrammatically in diagram form. 6 shows a diagram on the abscissa 66 the temperature of the DOC is plotted. On the ordinate 68 the NO _x storage capacity κ is plotted in the unit moles per liter volume of the DOC. A solid course 70 illustrates the storage capacity of a new or fresh DOC versus temperature, with a dashed trace 72 illustrates the storage capacity of an old or aged DOC over time.

To determine the state of aging, a nitrogen oxide amount desorbed or adsorbed by ΔT in each case increasing or decreasing the temperature of the DOC, which corresponds to a reduction or increase in the NO _x storage capacity of the optionally aged DOC by Δκ _a , as explained above, by integration of the Signal of the nitrogen oxide sensor arranged downstream of the DOC determined and compared with an expected reference value Δκ _n of fresh, that is unaged and new DOC.

In order to increase the reliability of the determination of the state of aging, it may generally be provided for the determination of the NO _x storage capacity carried out for this purpose following a desulphurization procedure of the DOC and / or following a thermal regeneration of the particulate filter 36 make. In desulfurization, the DOC is adsorbed by an oscillating exhaust atmosphere between lean (λ approximately 1.2, five seconds, T> approximately 350 degrees Celsius) and rich (λ approximately 0.9, five seconds, T> approximately 350 degrees Celsius) Sulfur free. In a thermal particulate filter regeneration at a temperature greater than about 550 degrees Celsius, the SDPF is freed of deposited soot. Desulfurization and particulate filter regeneration can also be performed in parallel.

In order to eliminate a typically occurring reversible passivation or deactivation of the DOC, it is provided, as mentioned above, to perform a so-called fat jump from time to time. For this purpose, the exhaust gas is enriched by means of engine post-injection and / or secondary engine injection for a short time (five seconds to one minute) with unburned hydrocarbons or fuel components (CO, HC) (λ approximately 0.95 to 0.85). In order to keep a resulting fuel consumption as low as possible, it is intended to make this reactivation called fat reactivation first when the DOC has reached a predeterminable state of aging. This is due to the fact that at low aging, a fat reactivation compared to a stronger aging only a comparatively small improvement, that is, lowering the light-off temperature causes (see 1 ). Specially can be provided be set to the frequency of fat activations and the set so-called Anfettungstiefe, that is λ-reduction or richening depending on the aging state. This is in 7 illustrated.

7 shows a diagram on the abscissa 74 the aging or the state of aging of the DOC is plotted. On the ordinate 76 the frequency of fat activations or the enrichment depth or richening time is plotted. Thus, a history illustrates 78 the frequency of fat activations over the increasing aging of the DOC. As shown, it is preferred if no fat activations are carried out with a slight aging, ie an aging state below a predeterminable comparatively low critical aging. In this case, fat activations only take place when the aging of the DOC has reached a level that exceeds the limit age. One criterion for this is, for example, the rise in the light-off temperature beyond a limit temperature.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 102010028373 A1 [0006]
• WO 2008/047170 A1 [0007]

Claims (10)

Method for determining the state of aging of an exhaust tract ( 10 ) of an internal combustion engine and can be traversed by exhaust gas of the internal combustion engine oxidation catalyst ( 12 ) for oxidizing constituents present in the exhaust gas, comprising the steps of: - determining the ability of the oxidation catalyst ( 12 ) to store existing nitrogen oxides in the exhaust gas; and - determining the state of aging as a function of the determined ability. Process according to claim 1, characterized in that as the oxidation catalyst ( 12 ) a with respect to alkali metals and alkaline earth metals free oxidation catalyst ( 12 ) is used. A method according to claim 1 or 2, characterized in that at least one reactivation process in which a passivation of the oxidation catalyst ( 12 ) is at least partially eliminated by enriching the exhaust gas with reducing constituents, depending on the determined aging state. Method according to one of the preceding claims, characterized in that for determining the ability of the oxidation catalyst ( 12 ) is operated in a saturation phase, in which: - the oxidation catalyst ( 12 ) is subjected to lean exhaust gas containing nitrogen oxides; A first content of nitrogen oxides contained in the exhaust gas is determined, the exhaust gas containing the first content of nitrogen oxides upstream of the oxidation catalyst ( 12 ) having; A second content of nitrogen oxides contained in the exhaust gas is determined, wherein the exhaust gas has the second content of nitrogen oxides downstream of the oxidation catalyst ( 12 ) having; and - the oxidation catalyst ( 12 ) is acted upon in the saturation phase with the exhaust gas containing nitrogen oxides until the second content deviates from the first content by a predeterminable amount. A method according to claim 4, characterized in that the oxidation catalyst ( 12 ) is exposed in the saturation phase at a temperature in a range of 170 degrees Celsius to 250 degrees Celsius with the exhaust gas containing nitrogen oxides. A method according to claim 4 or 5, characterized in that the first content of nitrogen oxides contained in the exhaust gas by means of an upstream of the oxidation catalyst ( 12 ) is detected in the exhaust system arranged sensor and / or determined by means of a computing device based on a map of the internal combustion engine. Method according to one of claims 4 to 6, characterized in that for determining the ability of the oxidation catalyst ( 12 ) is operated in time after the saturation phase in a desorption phase in which the oxidation catalyst ( 12 ) is heated by means of the exhaust gas such that the oxidation catalyst ( 12 ) at least partially, in particular at least predominantly, releases the nitrogen oxides stored in it. A method according to claim 7, characterized in that the oxidation catalyst ( 12 ) is heated in the desorption phase to a temperature of more than 250 degrees Celsius. A method according to claim 7 or 8, characterized in that an amount of in the desorption phase of the oxidation catalyst ( 12 ) and is compared with a reference value, wherein the aging state of the oxidation catalyst ( 12 ) is determined as a function of the comparison. Method according to one of the preceding claims, characterized in that prior to determining the ability of the oxidation catalyst ( 12 ) desulfurizes and / or in the exhaust tract ( 10 ) arranged particulate filter ( 34 ) is regenerated.

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