DE10107388B4 - Process for the aftertreatment of an exhaust gas of a lean-running internal combustion engine - Google Patents

Abstract

A method for the aftertreatment of an exhaust gas of a lean-burn internal combustion engine (10), wherein at least one NOx storage catalytic converter (18) is arranged in an exhaust gas duct (14) of the internal combustion engine (10) and the internal combustion engine (10) is in lean operation for the purpose of NOx regeneration of the NOx storage catalytic converter (18) is operated discontinuously in lean-rich lambda intervals, with (a) a NOx storage catalytic converter (18) being used which contains at least one storage material which is suitable for binding nitrogen oxides in the form of such a stable nitrate, that the NOx storage catalytic converter (18) has an upper temperature limit (TH) of a working temperature range of at least 520 ° C, with a minimum NOx conversion rate (KRmin) of at least 80% in the time average of the discontinuous lean operation of the internal combustion engine (10 ) is present, and the internal combustion engine (10) up to the upper temperature limit (TH) of the NOx storage catalytic converter (18) of at least 520 ° C is operated in discontinuous lean operation, unless exceeding load and / or speed thresholds or a need for desulfurization requires a stoichiometric or rich operating mode, (b) a need for desulfurization is detected when the mean minimum NOx conversion rate ...

Description

The invention relates to a method for the after-treatment of an exhaust gas of a lean-running internal combustion engine using NO _x storage catalysts according to the preamble of claim 1.

Modern internal combustion engines are for reasons of consumption optimization over the widest possible operating ranges in a lean operating mode, that is, operated with an oxygen-rich exhaust gas with λ> 1. Particularly lean air-fuel ratios can be realized in internal combustion engines, which are operable in a so-called stratified operation or even stratified charge mode. These are direct-injection gasoline engines, which have a charge movement flap arranged in the air intake duct. The operating point-dependent adjustable charge movement flap causes in its one position flow conditions in the combustion chamber of a cylinder such that the fuel injected into a combustion chamber of the internal combustion engine concentrates in the form of a fuel cloud in the region of a spark plug. At higher load requirements, for example, at vehicle speeds above 70 km / h, the internal combustion engine is switched from the lean stratified mode by corresponding position of the charge motion flap in a so-called homogeneous operation in which a uniform air-fuel mixture with λ ≤ 1 is generated in the entire combustion chamber ,

A problem lean burnable internal combustion engines are nitrogen oxides NO _x , which can not be fully converted to environmentally neutral nitrogen N ₂ in the course of a catalytic reaction of unburned hydrocarbons HC and carbon monoxide CO on conventional 3-way catalysts. To remedy this situation, it is known to arrange NO _x storage catalytic converters in the exhaust gas ducts of internal combustion engines and, at intervals, to apply lean and rich exhaust gas atmospheres in a discontinuous operation. Here, in the lean operating intervals, a storage of NO _x in the form of nitrate and in the shorter, fat operating intervals, a regeneration of the storage catalyst with release and reduction of NO _x to N ₂ . Today's NO _x storage catalysts have an operating temperature range in which they store and convert NO _x with sufficient efficiency from about 200 to 450 ° C. If the catalyst temperature exceeds the upper limit of the temperature window, the lean operating mode must be ended or suppressed and the internal combustion engine switched to a stoichiometric or rich operating mode in order to prevent emission of unconverted nitrogen oxides.

A known problem for NO _x storage catalysts is sulfur contained in current fuels. This is burned in the combustion process to sulfur dioxide SO ₂ and stored undesirable in the form of sulfate in the NO _x storage. Due to its high stability, sulphate formation is not reversible under the conditions of NO _x regeneration, in contrast to the desired nitrate formation. In order to counteract creeping sulfur poisoning, it is therefore necessary to carry out desulfurization at recurring intervals, the catalyst being exposed to a rich exhaust gas atmosphere at high catalyst temperatures. A required minimum desulphurisation temperature of current NO _x storage catalysts is about 630 ° C. If a vehicle is used predominantly in a low-load area, for example in city traffic, then the minimum desulfurization temperature on the catalyst is rarely or even never achieved spontaneously. In this case, active heating measures must be taken to raise the catalyst temperature. For this purpose, for example, motor measures into consideration, with which a combustion temperature and thus an exhaust gas temperature is increased. These measures lead to increased fuel consumption at the expense of the consumption advantage achieved by the lean running capability of the internal combustion engine, which depends inter alia on the sulfur content of the fuel, on the desulphurisation temperature and on the NO _x working temperature range.

The exhaust gas cleaning of direct-injection gasoline engines by means of NO _x storage catalysts and their mode of operation is also described in Glück et al. (Motortechnische Zeitschrift, 2000 (6), 61st year, pp. 402-412). The use of NO _x storage catalysts for the aftertreatment of exhaust gases from diesel engines is described by Bauder (Motortechnische Zeitschrift, 1998 (7/8), Volume 59, p. XI-XVIII).

DE 198 16 276 A1 discloses a method for desulfurizing an internal combustion engine downstream NO _x storage catalytic converter having an operating temperature range of about 200 to 450 ° C, in which takes place an effective NO _x storage. The upper temperature limit of 450 ° C is reached at a vehicle speed of about 70 km / h. The need for desulfurization is detected according to this document by measuring the NO _x concentration downstream of the storage catalytic converter or by means of the course of the oxygen concentration downstream of the storage catalytic converter during its NO _x regeneration. The desulphurization itself is through Exposure of the catalyst with a rich exhaust gas atmosphere at a temperature that corresponds to at least the minimum desulfurization temperature of about 600 ° C, achieved.

According to DE 198 49 082 A1 Desulphurisation of an NO _x storage catalytic converter takes place at an initial temperature in the range from 580 to 620 ° C.

In EP 1 065 351 A1 discloses a working temperature range of a NO _x storage catalyst from 250 to 350 ° C and a minimum desulfurization temperature of 650 ° C.

The invention has for its object to provide a method for the aftertreatment of exhaust gas mauflauffähiger internal combustion engines by means of NO _x storage catalytic converters available, which ensures over the current state of the art, a higher consumption advantage over the vehicle life.

This object is achieved by a method having the features of claim 1.

• (a) ein NO_x-Speicherkatalysator verwendet wird, der eine obere Temperaturgrenze eines Arbeitstemperaturbereiches von mindestens 520°C aufweist, wobei innerhalb des Arbeitstemperaturbereiches eine NO_x-Mindest-Konvertierungsrate von mindestens 80% im zeitlichen Mittel des diskontinuierlichen Magerbetriebs der Verbrennungskraftmaschine vorliegt, und die Verbrennungskraftmaschine bis zu der oberen Temperaturgrenze des NO_x-Speicherkatalysators im diskontinuierlichen Magerbetrieb betrieben wird, sofern nicht eine Überschreitung von Last- und/oder Drehzahlschwellen oder eine Entschwefelungsnotwendigkeit einen stöchiometrischen oder fetten Betriebsmodus erfordert,
• (b) eine Entschwefelungsnotwendigkeit erkannt wird, wenn die mittlere NO_x-Mindest-Konvertierungsrate des NO_x-Speicherkatalysators im diskontinuierlichen Magerbetrieb von mindestens 80% unterschritten wird, und
• (c) eine Entschwefelung des NO_x-Speicherkatalysators bei einer mindestens einer Mindest-Entschwefelungstemperatur entsprechenden Katalysatortemperatur von mindestens 680°C durchgeführt wird und der NO_x-Speicherkatalysator bei der Mindest-Entschwefelungstemperatur und bei einer Abgasbeaufschlagung mit Lambda ≤ 0,99 eine Schwefelaustragsrate von mindestens 0,005 g/s aufweist.

The inventive method is characterized in that

• (a) using an NO _x storage catalyst having an upper temperature limit of a working temperature range of at least 520 ° C, within the operating temperature range having a minimum NO _x conversion rate of at least 80% in the time average of the intermittent lean burn operation of the internal combustion engine, and the internal combustion engine is operated up to the upper temperature limit of the NO _x storage catalytic converter in the discontinuous lean operation, unless an exceeding of load and / or speed thresholds or a desulfurization necessity requires a stoichiometric or rich operating mode,
• (b) a desulphurisation requirement is detected when the average NO _x minimum conversion rate of the NO _x storage catalyst in the batch lean operation is at least 80% less than, and
• (C) a desulfurization of the NO _x storage catalyst is carried out at a minimum desulfurization at least a corresponding catalyst temperature of at least 680 ° C and the NO _x storage at the minimum desulfurization temperature and at a exhaust gas emission lambda ≤ 0.99 a sulfur discharge rate of at least 0.005 g / s.

An essential feature of the invention is compared to the prior art in the direction of higher temperatures extended operating temperature range of the NO _x storage catalytic converter used. In the context of the invention, the working temperature range is defined by a NO _x conversion rate of at least 80%, the NO _x conversion rate comprising a proportion of converted NO _x at a NO _x total amount flowing into the storage catalytic converter in the time average of the discontinuous lean / Indicates rich operation with test parameters described below. Conventional used for exhaust aftertreatment NO _x storage catalysts store up to catalyst temperatures of about 450 ° C nitrogen oxides with high efficiency. The contrast increased by at least 70 ° C upper temperature limit allows maintenance of the low-consumption lean operation of the internal combustion engine into high load ranges, where conventionally already switched to a stoichiometric or rich operating mode to avoid emission of unconverted nitrogen oxides. In this way not only the fuel consumption can be significantly reduced, but also an emission of pollutants.

According to a particularly advantageous embodiment, the upper temperature limit of the working temperature range is at least 560 ° C., so that the discontinuous lean operation of the internal combustion engine, in particular a lean stratified charge mode, can be maintained up to this temperature limit. Depending on the actual position of the upper limit of the operating temperature range of a specific NO _x storage catalytic converter, operation of the internal combustion engine in lean operation can even take place up to a catalyst temperature of at least 600 ° C. The catalyst temperature is determined inter alia by a combustion temperature, which in turn depends on an engine load. Depending on the operating temperature range, the lean operation of the NO _x storage catalytic converter according to the invention can take place up to vehicle speeds of at least 80 km / h, in particular of at least 100 km / h, preferably at least 120 km / h. Since the lean operation and especially the stratified lean operation leads to a high reduction in consumption, a significant consumption advantage over today's conventional exhaust aftertreatment method is achieved by the expansion of the allowable lean operating range according to the invention.

The expansion of the working temperature range according to the invention can be realized by storage materials which form more stable nitrates compared to storage materials customary today. Here, for example, oxides and / or carbonates of the elements potassium, rubidium, strontium and / or cesium come into question. However, the use of other compounds is also conceivable, provided they are able to store nitrogen oxides with sufficient stability. Current NO _x stores usually contain barium oxide BaO and barium carbonate BaCO ₃ as storage material and store nitrogen oxides as barium nitrate Ba (NO ₃ ) ₂ .

Since the property of the NO _x storage catalytic converter to form thermodynamically more stable nitrates generally leads to the formation of more stable sulfates, the NO _x storage catalytic converter used according to the invention is further characterized by a minimum desulfurization temperature of at least 680 ° C., which is significantly higher than today's catalysts , According to specific embodiments, the minimum desulfurization temperature is at least 720 ° C., in particular at least 750 ° C. The minimum desulfurization temperature is here defined by a sulfur discharge rate of at least 0.005 g / s based on elemental sulfur in a rich exhaust gas atmosphere with λ ≤ 0.99. The desulfurization of the NO _x storage catalyst is carried out at catalyst temperatures which correspond to or exceed the concrete minimum desulfurization temperatures of the catalyst and in a rich exhaust gas atmosphere, that is at least 680 ° C, in particular at least 720 ° C. In the case of particularly stable sulphates, desulphurisation takes place at at least 750 ° C. At higher temperatures and with exhaust gas loading at λ ≤ 0.99, the sulfur discharge rate increases significantly and reaches a maximum at about 50 to 70 K above the minimum desulphurisation temperature. Further temperature increases lead to at best slight increase. It is preferably provided to carry out the desulfurization at catalyst temperatures of at least 20 K above the specific minimum desulfurization temperature and an exhaust gas charge with lambda ≦ 0.99 over a period of at least 200 s per liter of catalyst volume. The usual values are ≥ 0.01 g / s sulfur output as maximum sulfur discharge rate.

Since the high desulfurization temperatures required imply a high thermal load for the catalyst system, in particular also for an upstream pre-catalyst, and active measures for increasing the catalyst temperature are also associated with a high fuel consumption, the NO _x storage catalyst should be designed so that an active desulfurization is necessary only at long intervals. In particular, during a lifetime of the NO _x storage catalytic converter, at most 100 desulfurizations, preferably at most 30 desulfurations, should be carried out without and the NO _x minimum conversion rate of the NO _x storage catalytic converter of at least 80% in the time average of the discontinuous lean / rich Operation is not fallen below. The catalyst design should be particularly advantageous in such a way that at most 20 active desulphurations within the lifetime are required. For this purpose, the NOx storage catalyst should be able to store at least 0.5 g of sulfur per liter of catalyst volume before a collapse of the NO _x conversion rate requires desulfurization. Due to the high desulfurization temperatures and the already mentioned tendency to form stable sulfates of said storage materials, which make extensive or complete desulfurization difficult, desulfurization over an assumed lifetime of ≥ 100,000 km should be carried out as rarely as possible. It is particularly favorable, although the dimensioning of the NO _x storage catalytic converter is designed so that no desulfurization is required at the earliest after 100 l, in particular at the earliest after 150 l of spent fuel in lean operation.

According to a particularly advantageous embodiment of the method according to the invention, the combustion engine upstream of the NO _x storage catalytic converter is operated with a low-sulfur fuel, ie with a fuel having a sulfur content of at most 30 ppm, in particular of at most 20 ppm. Preference is given to a maximum sulfur content of 10 ppm, which may be promoted in the foreseeable future due to legal requirements. This comparatively low sulfur content - according to current legislation, a maximum sulfur content of 150 ppm is permissible - leads to a very slow sulfurization of the NO _x storage catalytic converter. As a result, and because of the described preferred catalyst design, desulfurization measures are necessary only after long periods of operation.

Due to the low sulfur content of the fuel and the associated slow sulfurization of the NO _x storage catalytic converter, a desulfurization requirement only arises after long operating _times . With a maximum sulfur content of 10 ppm, this is the case, for example, after 3000 to 15000 km, whereby this route depends on a driver-specific use of the vehicle and average fuel consumption. In contrast, currently available sulfur levels require desulfurization after 200 to 1000 km of driving distance. The long operating phases between two desulphurizations of a NO _x storage catalyst according to the present invention lead to a very strong increase in the probability that during this phase of operation at least passive desulfurization, in which sufficient catalyst temperatures are operating point, occurs spontaneously. Preferably, therefore, an active desulfurization, in which the catalyst temperature by targeted measures to the minimum Desulfurization is raised, carried out only in the unlikely event that there is no sufficient frequency of passive desulfurization. The active desulphurization will therefore be the exception, resulting in further consumption benefits.

A currently common measure for increasing the catalyst temperature for the purpose of active desulfurization is the spark ignition, which leads to an increase in combustion and exhaust gas temperature. Due to the high minimum desulfurization temperature of the NO _x storage catalytic converter according to the invention, however, the delayed ignition is only suitable at high vehicle speeds in order to achieve sufficient temperatures. According to an advantageous embodiment of the method therefore measures are used in addition to or alternatively to the spark ignition, which lead to an increase of the pollutant and / or oxygen content in the exhaust gas and cause an increase in the catalyst temperature by catalytic conversion of pre- and / or NO _x storage , These measures include post-injection of fuel before / during or after a burn; a so-called lambda spread, in which the various cylinders of the internal combustion engine are operated with different combustion lambdas; a fuel injection into the exhaust system; a rich operation of the internal combustion engine and / or a secondary air supply into the exhaust system upstream of the NO _x storage catalytic converter. These measures can also be used in combination with each other.

Further advantageous embodiments of the invention are the subject of the remaining dependent claims.

The invention will be explained in more detail in embodiments with reference to the accompanying drawings. Show it:

1 schematically an internal combustion engine with an exhaust system,

2 a temperature dependence of a _NOx -Konvertierungsrate a conventional and an inventive NO _x storage and

3 Dependence of the fuel consumption on the sulfur content of the fuel and on the temperature characteristic of the NO _x storage catalytic converter according to FIG 2 ,

At the in 1 shown internal combustion engine 10 it is a lean running, direct injection gasoline engine, the one with a total of 12 is assigned designated exhaust system. The exhaust duct 14 houses a catalyst system which contains a small volume, close to the engine pre-catalyst 16 , in particular a 3-way catalyst, and an arranged at a sub-position NO _x storage catalytic converter 18 includes. In the exhaust duct 14 is also a sensor for controlling the internal combustion engine 10 and the catalyst system 16 . 18 arranged. A downstream of the internal combustion engine 10 installed lambda probe 20 measures an oxygen content of the exhaust gas to control the internal combustion engine 10 supplied air-fuel ratio. Downstream of the NO _x storage catalytic converter 18 is a NO _x sensor 22 and serves to control the NO _x regeneration and desulphurisation intervals of the NO _x storage catalytic converter 18 , Upstream of the NO _x storage catalytic converter 18 is also a temperature sensor 24 installed, which measures an exhaust gas temperature and thus conclusions about a temperature of the NO _x storage catalytic converter 18 allowed. The current catalyst temperature can be particularly accurately from a temperature difference between the exhaust gas temperature in front of and behind the NO _x storage catalytic converter 18 determine. This can be done downstream of the NO _x storage catalytic converter 18 present exhaust gas temperature particularly advantageous from an internal temperature signal of the NO _x sensor 22 to be obtained. As an alternative to measuring the exhaust gas temperature, this can also be calculated as a function of selected operating parameters of the internal combustion engine 10 can be obtained in a known manner. Similarly, as an alternative to the NO _x sensor 22 the use of another lambda probe conceivable. The ones from the sensors 20 . 22 . 24 provided signals are sent to an engine control unit 26 which digitizes and processes the signals. In the engine control unit 26 is also a control unit 28 integrated, in which an algorithm for controlling the operation of the NO _x storage catalytic converter 18 is deposited. The engine control unit 26 and the control unit 28 control various operating parameters of the internal combustion engine 10 depending on the signals. In particular, the regeneration and desulfurization intervals of the NO _x storage catalyst become 18 controlled by, according to the algorithm about the air-fuel ratio of the internal combustion engine 10 is specified and measures to increase an exhaust gas and / or catalyst temperature can be initiated.

The one with 30 designated graph in 2 represents the temperature dependence of a NO _x -Konvertierungsrate a storage catalyst according to the prior art by corresponding to the structure 1 , Plotted is the NO _x -Konvertierungsrate as a percentage ratio of a converted amount of _NOx in a flowing into the storage catalytic converter during a measurement period amount of _NOx as a function of the temperature of the storage catalyst. For the measurement, a 3-way pre-catalyst with a Precious metal content of 1.4 to 7.1 g / dm ³ (40 to 200 g / ft ³ ) and a precious metal ratio of 0 to 4 parts platinum Pt to 0 to 15 parts palladium Pd to 0 to 3 parts rhodium Rh used. The volume of the precatalyst 16 corresponded to 15 to 60% of a volume of the NO _x storage catalyst 18 , An exhaust gas flow path between the precatalyst and the storage catalyst was less than 1500 mm. The NO _x storage catalyst had a mixture of barium oxide BaO and barium carbonate BaCO ₃ as the storage-active material. A space velocity of the exhaust gas in the storage catalyst was 10,000 to 30,000 / h. The NO _x conversion rate became the typical discontinuous rich-lean interval operation of the internal combustion engine 10 measured and averaged. For the lean intervals, which lasted 50 to 60 s, a combustion lambda of 1.8 to 2.5 was specified. This corresponded to a proportion of unburned hydrocarbons HC in the raw exhaust gas of 2,000 to 4,000 ppm, a proportion of carbon monoxide CO of 2,000 to 3,000 ppm and an NO _x content of 200 to 500 ppm. During the 4 to 8 second fat intervals, a combustion lambda of 0.75 to 0.85 was specified, the HC content being 5,000 to 15,000 ppm, the CO content 40,000 to 60,000 ppm, and the NO _x content 500 to 1,500 ppm , The catalyst temperature of the NO _x storage catalyst was meanwhile controlled by means of a tempering device. The temperature profile measured for this storage catalytic converter 30 in 2 shows a typical operating temperature window with a NO _x conversion rate of at least 80%, which is limited by a lower temperature limit T _L 'at about 200 ° C and by an upper temperature limit T _H ' at about 450 ° C. Achieved this conventional storage catalyst 18 the upper temperature limit T _H 'of its working range in normal operation due to an operating point-dependent engine load - this is typically the case at vehicle speeds above 70 km / h - the lean operation and optionally the stratified mode must be suppressed and the operation of the internal combustion engine in stoichiometric or rich operation (Homogeneous operation). The present in stoichiometric operation exhaust gas composition then ensures a virtually complete NO _x conversion at the primary catalyst. The suppression of the lean operation must take place when the upper temperature limit T _H 'is reached, even if the engine load itself does not require an increase in the fuel fraction.

This state of the art is an extrapolated temperature profile 32 a NO _x conversion rate of a NO _x storage catalyst according to the present invention is compared, for example, instead of barium lighter alkali and / or alkaline earth metals in the form of their oxides and carbonates as a storage material. Such an NO _x storage catalyst is able to form more stable nitrates, so that its operating temperature range is extended in the direction of higher temperatures by at least 50 K compared to the conventional storage catalyst. In the illustrated example, the upper temperature limit T _{H of} the NOx storage catalyst according to the invention is approximately 560 ° C. This corresponds approximately to a storage material containing at least potassium. This more of the working temperature range can therefore be used to the lean and / or shift operation of the internal combustion engine 10 maintain over higher load ranges. Thus, according to the illustrated example, a suppression of the lean operation would have to take place only at vehicle speeds of approximately above 120 km / h.

The ability to form more stable nitrates leads to the undesirable side effect that the necessary from time to time desulfurization must be carried out at higher catalyst temperatures, since the stored sulfates have a correspondingly higher stability. The hatched areas in the illustration 34 and 36 respectively designate the desulfurization temperature ranges for a NO _x storage catalyst according to the prior art ( 34 ) or a storage catalyst according to the invention ( 36 ). The desulfurization temperature ranges 34 . 36 are each limited by a minimum desulfurizing temperature T _min ', T _min above which a Schwefelaustragsrate of at least 0.005 g / s and the ability of the storage catalyst for at least almost complete discharge of sulfur, that is at least 80%, is present. For the conventional storage catalyst, T _min 'is approximately 630 ° C., while the NO _x storage catalyst according to the invention has a minimum desulfurization temperature T _min of 680 ° C. The desulfurization temperature ranges 34 . 36 are limited to the top by a decomposition temperature of the storage material, which should not be exceeded. As already explained, the high desulfurization temperatures of the NO _x storage catalytic converter according to the invention sometimes require very drastic measures for the active temperature increase. These represent for the entire catalyst system, in particular for the precatalyst, a life-reducing load. For this reason, it is provided to use the NO _x storage catalyst according to the invention using low-sulfur fuel. In this way, the sulfurization of the catalyst is slowed down so that the distances between two active desulfurizations are so great that during this time a passive, operational desulfurization occurs with high probability. Consequently, the active desulfurization of the NO _x storage catalyst according to the invention 18 the exception and only necessary if a vehicle is practically never used in full load range.

In 3 are approximately the realizable fuel consumption changes .DELTA.V _KS depending on the sulfur content of the fuel and the operating temperature window of the NO _x storage catalytic converter 18 shown. For reference - shown in bars 40 - The fuel consumption in the European driving cycle NEDC with a NO _x storage catalytic converter according to the prior art and a fuel with the current maximum statutory sulfur content of 150 ppm S was based. bar 42 and 44 show approximately the consumption reduction ΔV _KS when using fuels with lower sulfur contents of 50 ppm S (bar 42 ) or <10 ppm S (bars 44 ). Due to the lower sulfur content due to the lower sulfur content, consumption savings ΔV _KS of about 1.5% (bar 42 ) or 2.4% (bars 44 ) achieved. If the process according to the invention and a corresponding NO _x storage catalyst is used for sulfur-free fuel with sulfur contents of less than 10 ppm, an additional consumption advantage of about 2.5% according to bars may result due to the broader NO _x working temperature window and the resulting higher lean fraction in the driving cycle 46 will be realized.

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

12

exhaust system

14

exhaust duct

16

precatalyzer

18

NO _x storage catalyst

20

lambda probe

22

NO _x sensor

24

temperature sensor

26

Engine control unit

28

control unit

30

Temperature curve NO _x conversion rate according to the prior art

32

Temperature curve NO _x conversion rate according to the invention

34

Desulfurization temperature range according to the prior art

36

Entschwefefungstemperaturbereich according to the invention

40

Fuel consumption change in NO _x storage catalytic converter according to the prior art and 150 ppm sulfur content in the fuel

42

Fuel consumption change with NO _x storage catalytic converter according to St. d. T. and 50 ppm sulfur content

44

Fuel consumption change with NO _x storage catalytic converter according to St. d. T. and ≤ 10 ppm sulfur content

46

Fuel consumption change according to the invention and ≤ 10 ppm sulfur content

T _H

upper temperature limit according to the invention

T _H '

upper temperature limit according to the prior art

T _cat

Temperature of the NO _x storage catalytic converter

T _L

lower temperature limit according to the invention

T _L '

lower temperature limit according to the prior art

T _min

Minimum desulfurization temperature according to the invention

T _min '

Minimum desulfurization temperature according to the prior art

ΔV _KS

Fuel consumption change related to actual state

Claims (14)

Process for the aftertreatment of an exhaust gas of a lean-running internal combustion engine ( 10 ), wherein at least one NO _x storage catalytic converter ( 18 ) in an exhaust duct ( 14 ) of the internal combustion engine ( 10 ) is arranged and the internal combustion engine ( 10 ) during a lean operation for the purpose of NO _x regeneration of the NO _x storage catalyst ( 18 ) is operated intermittently in lean-fat lambda intervals, wherein (a) a NO _x storage catalyst ( 18 ) containing at least one storage material suitable for binding nitrogen oxides in the form of such a stable nitrate that the NO _x storage catalyst ( 18 ) has an upper temperature limit (T _H ) of a working temperature range of at least 520 ° C, wherein within the operating temperature range a minimum NO _x conversion rate (KR _min ) of at least 80% in the time average of the discontinuous lean operation of the internal combustion engine ( 10 ), and the internal combustion engine ( 10 ) up to the upper temperature limit (T _H ) of the NO _x storage catalytic converter ( 18 ) of at least 520 ° C in discontinuous lean operation, unless exceeding load and / or speed thresholds or a desulphurisation need requires a stoichiometric or rich mode of operation, (b) a desulphurisation need is recognized when the average NO _x minimum conversion rate (KR _min ) of the NO _x storage catalyst ( 18 ) in discontinuous lean operation of at least 80%, and (c) desulfurization of the NO _x storage catalyst ( 18 ) is carried out at a minimum of a minimum desulfurization temperature (T _min ) corresponding catalyst temperature (T _Kat ) of at least 680 ° C and a rich exhaust gas atmosphere and the NO _x storage catalytic converter ( 18 ) at the minimum desulfurization temperature (T _min ) and at a Exhaust gas with lambda ≤ 0.99 has a sulfur output of at least 0.005 g / s. Method according to claim 1, characterized in that a NO _x storage catalytic converter ( 18 ) is used, which has an upper temperature limit (T _H ) of the working temperature range of at least 560 ° C, in particular of at least 600 ° C, and the discontinuous lean operation of the internal combustion engine ( 10 ), in particular a lean stratified charge mode, up to the upper temperature limit (T _H ) of the NO _x storage catalytic converter ( 18 ) he follows. A method according to claim 1 or 2, characterized in that the discontinuous lean operation takes place up to a vehicle speed of at least 80 km / h. A method according to claim 3, characterized in that the discontinuous lean operation up to a vehicle speed of at least 100 km / h, in particular at least 120 km / h takes place. Method according to one of the preceding claims, characterized in that the NO _x storage catalytic converter ( 18 ) a minimum desulfurization temperature (T _min ) at which a Schwefelaustragsrate of at least 0.005 g / s is achieved at an exhaust emission with lambda ≤ 0.99, of at least 700 ° C, in particular of at least 720 ° C. Method according to one of the preceding claims, characterized in that the desulfurization at a desulfurization temperature of at least 20 K above the minimum desulfurization temperature (T _min ) and an exhaust gas with lambda ≤ 0.99 over a period of at least 200 s per liter of catalyst volume is performed , Method according to one of the preceding claims, characterized in that during the operation of the NO _x storage catalytic converter ( 18 ) at most 100 desulphurizations, in particular at most 30 desulphurizations, and the NO _x minimum conversion rate (KR _min ) of the NO _x storage catalytic converter ( 18 ) of at least 80% in the time average of the discontinuous lean-fat operation is not below. Method according to one of the preceding claims, characterized in that up to falling below the NO _x -minimum conversion rate (KR _min ) in the discontinuous lean operation of the internal combustion engine ( 10 ) at least 0.5 g of sulfur per liter of catalyst volume into the NO _x storage catalyst ( 18 ) are storable. Method according to one of the preceding claims, characterized in that the desulfurization need at the earliest after a consumed fuel volume in the discontinuous lean-rich operation of at least 100 l, in particular of 150 l, is detected. Method according to one of the preceding claims, characterized in that the NO _x storage catalytic converter ( 18 ) upstream internal combustion engine ( 10 ) is operated with a fuel having a sulfur content of at most 30 ppm. A method according to claim 10, characterized in that the sulfur content of the fuel is at most 20 ppm, in particular at most 10 ppm. Method according to one of the preceding claims, characterized in that the at least one storage material of the NO _x storage catalytic converter ( 18 ) is selected from the alkali and / or alkaline earth group, in particular from oxides and / or carbonates of these, in particular from the elements potassium, rubidium, strontium or cesium. Method according to one of the preceding claims, characterized in that an active desulfurization, in which the catalyst temperature (T _Kat ) is raised by targeted measures to the minimum desulfurization temperature (T _min ), is only performed if no sufficient frequency of passive desulfurization in which the catalyst temperature reaches the minimum desulfurizing temperature (T _min ) due to a sufficient engine load. A method according to claim 13, characterized in that the measures for raising the catalyst temperature (T _cat ) a spark ignition; a post-injection of fuel before / during or after a combustion; a lambda stimulation; a fuel injection into the exhaust system ( 12 ); a rich operation of the internal combustion engine ( 10 ) and / or a secondary air feed into the exhaust system ( 12 ) upstream of the NO _x storage catalytic converter ( 18 ).

Images