FR2817590A1 - Pre-catalyser desulfurization for IC engine exhaust system expels sulfur at intervals to pass through main catalyser - Google Patents

Abstract

The invention relates to a process for desulphurizing a pre-catalyst disposed in a gas exhaust channel of an internal combustion engine capable of operating with a lean mixture and downstream of which a main catalyst, in particular for storing NOx, is fitted, together with a device for carrying out the desulphurization.It is provided, at periodic intervals, to expel the sulfur deposited in the pre-catalyst (16) at least partially in a form which, under the operating conditions existing, crosses the main catalyst (18) up to 25% at least. The sulfur is preferably expelled primarily as hydrogen sulfide (H2S) while the main catalyst (18) is largely free of oxygen. This makes it possible to slow down the sulphidation of the main catalyst (18) so that the measures of desulphurisation of the main catalyst (18), which consume fuel, become more rarely necessary.

Description

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 The invention relates to a method and a device for desulfurizing a pre-catalyst which is arranged in a gas exhaust duct of an internal combustion engine that can operate with a lean mixture and downstream of which a main catalyst, in particular Nos storage catalyst is mounted.

 The known catalytic systems used for cleaning the exhaust gas are frequently composed of a small volume pre-catalyst placed near the engine and a larger main catalyst further downstream in an escape route. gases. The catalyst components can be arranged as oxidation catalysts for the conversion of unburned hydrocarbons HC and carbon monoxide CO, as reduction catalysts for reducing nitrogen oxides NOx or as catalysts for three ways that simultaneously promote the oxidizing and reducing conversions mentioned. In the case of internal combustion engines that can operate with a lean mixture, the main catalyst can also be equipped with a NOx storage element which, in the poor operating phases in which the internal combustion engine is powered with a fuel. Oxygen-rich air-fuel mixture where X is greater than 1, stores NOx nitrogen oxide surplus to release the latter during richer operating periods and reduce them. Such catalysts are also known as nut storage catalysts.

 A known problem in the purification of exhaust gases is the sulfur contained in the fuels which, in the combustion process, is converted almost completely to SO 2 sulfur dioxide and is deposited in various forms on the various components of the fuel. catalytic system. This problem concerns

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la manière la plus aiguë les éléments de stockage de Nox des catalyseurs de stockage de Nox qui, lors des lambdas de combustion pauvres, stockent le S02 sous forme de sulfate S042-avec un rendement de déposition de presque cent pour cent. La conséquence en est une lente neutralisation de la capacité de stockage de Nox du catalyseur de stockage (empoisonnement par l'acide sulfhydrique), laquelle a entraîné le développement de divers procédés de désulfuration destinés aux catalyseurs de stockage de Noix.

the most acute way the Nox storage elements of NOx storage catalysts which, during poor combustion lambdas, store S02 in the form of S042-sulfate with a deposition efficiency of almost one hundred percent. The consequence is a slow neutralization of the Nox storage capacity of the storage catalyst (hydrogen sulfide poisoning), which has led to the development of various desulphurization processes for Nitrogen storage catalysts.

des gaz d'échappement riches afin de désorber (libérer) le sulfate stocké et de le réduire en S02 principalement. The principle of these processes is to feed the catalyst, at catalyst temperatures of at least 650 C, with

rich exhaust gases to desorb (release) the stored sulfate and reduce it to SO2 primarily.

de désorption spécifique à un composant (400 à 450 oc environ pour les OSC et 500 oc environ pour les métaux précieux) et dans une atmosphère de gaz d'échappement riche, il est possible de ré-expulser le soufre déposé. In addition to the sulfurization of NO x storage elements, there is also, but to a much lesser extent, a deposition of sulfur in other components of the catalyst system. These are substantially precious metals (Pt, Pd, Ru) catalytic coatings and oxygen storage components (OSC). At catalyst temperatures above the temperature

of desorption specific to a component (about 400 to 450 oc for OSC and about 500 oc for precious metals) and in a rich exhaust gas atmosphere, it is possible to re-expel the deposited sulfur.

But the disadvantage is then that the released sulfur from the pre-catalyst is deposited in part in the main catalyst downstream. If it is a NOxi storage catalyst the deposition occurs almost completely. As a result, the main catalyst must often be desulphurised, resulting in increased fuel consumption.

 The object of the present invention is to provide a process for the desulphurisation of a precatalyst, which results in as low a sulphurization as possible of a main catalyst placed downstream.

 This objective is achieved through a process according to

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lambda de combustion riche avec pl -F2 < 'Selon l'invention, il est prévu que le soufre déposé dans le précatalyseur soit expulsé, à intervalles périodiques appropriés, principalement sous une forme lui permettant, dans les conditions d'exploitation existantes, de traverser le catalyseur principal à hauteur de 25 % au moins et de préférence à hauteur de 50 % au moins. Le soufre déposé est de préférence expulsé, au moins partiellement, sous forme d'hydrogène sulfuré H2S pendant que le catalyseur principal est au moins en grande partie exempt d'oxygène. wherein the sulfur stored in the pre-catalyst is at least partially expelled at periodic intervals in a form which passes through the main catalyst by at least 25% under the prevailing operating conditions. The invention also provides a method according to which, when the temperature of the pre-catalyst is at least temporarily greater than or equal to the sulfur desorption temperature of the pre-catalyst, in a first phase, the internal combustion engine is supplied with a first lambda of rich combustion until the oxygen accumulator of the main catalyst is at least largely free of oxygen and, in a second subsequent phase, the combustion lambda of the internal combustion engine is increased until to a second

According to the invention, it is provided that the sulfur deposited in the precatalyst is expelled, at appropriate periodic intervals, mainly in a form allowing it, under the existing operating conditions, to pass through. the main catalyst at least 25% and preferably at least 50%. The deposited sulfur is preferably expelled, at least partially, as hydrogen sulfide H 2 S while the main catalyst is at least substantially free of oxygen.

 According to a particularly advantageous method, when the pre-catalyst has, at least temporarily, a temperature greater than or equal to the sulfur desorption temperature of the pre-catalyst: (a) in a first phase, the internal combustion engine is fed with a first combustion lambda rich until the oxygen accumulator of the main catalyst is at least largely free of oxygen, and (b) in a second subsequent phase, the combustion lambda of the internal combustion engine is increased to a second lambda of combustion

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 rich which is superior to the first lambda of rich combustion and which is less than one.

 The invention takes advantage of the fact that the H2S can not be stored in a significant amount in the oxygen storage components (OSC) and / or the NOx accumulator of the main catalyst in the presence of oxygen, since the storage requires first the oxidation of the H2S. Therefore, in the first phase, the oxygen is largely removed from the entire catalyst system, including the main catalyst oxygen storage. After that, at most 20%, preferably less than 10%, of the oxygen storage capacity of the main catalyst is used. In the first phase, it is advantageous to choose a lambda combustion as low as possible, that is to say the poorest possible oxygen. In this respect, lambda values of 0.7 to 0.95, and preferably 0.8 to 0.9, have been found to be good. These comparatively low lambda values promote, on the one hand, the formation of H2S rather than the predominant 802 formation in less rich exhaust gas atmospheres. On the other hand, the elimination of oxygen from the catalytic system is thus obtained in a particularly rapid and complete manner. The evolution of oxygen removal can be monitored simply by means of an oxygen-sensitive measuring device placed downstream of the main catalyst. This may be, for example, a lambda probe or a NOx sensor equipped with a lambda measurement function.

 In the second subsequent phase, the combustion lambda is increased to a nearest lambda value just below one in order to prevent the passage of harmful substances (CO and HC). The second phase may especially be advantageously carried out at a combustion lambda of from 0.95 to 0.995, and preferably from 0.98 to 0.99. Since, at this

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d'oxygène (OSC) ou dans les éléments de stockage de NOx du catalyseur principal nécessite une oxydation de l'H2S en S03 en passant par le S02 mais qu'il n'y a pas non plus d'oxygène disponible dans le catalyseur principal, un stockage de soufre peut tout au plus avoir lieu avec un très faible rendement. La désulfuration du pré-catalyseur a donc lieu en grande partie sans stockage supplémentaire de soufre dans le catalyseur principal. At this point, the pre-catalyst is substantially free of oxygen and fed with a reducing exhaust gas atmosphere, sulfur release takes place almost exclusively in the form of H 2 S. In addition, since the storage of sulfur in accumulator components

Oxygen (OSC) or in the NOx storage elements of the main catalyst requires oxidation of H2S to SO3 through SO2 but no available oxygen in the main catalyst. , a sulfur storage can at most take place with a very low yield. The desulphurization of the pre-catalyst therefore takes place largely without additional storage of sulfur in the main catalyst.

 According to a particularly advantageous arrangement of the process, it is intended to maintain the second rich combustion lambda until the pre-catalyst is at least largely free of sulfur, before completing the second phase and re-switching the internal combustion engine in ordinary operating mode. For this purpose, it is possible, for example, to model continuously the entry of sulfur into the pre-catalyst and / or the exit of sulfur from the pre-catalyst so as to be able to calculate a total (modeled) sulfur load in the pre-catalyst. -catalyst.

The end of the second phase and the switching of the internal combustion engine in ordinary lean operating mode then advantageously takes place when the modeling calculation shows that the sulfur purge of the precatalyst is substantially complete or has reached a predetermined threshold. The modeling of the sulfur inlet and the sulfur outlet can be carried out in known manner by means of current operating parameters of the internal combustion engine and in particular by means of the combustion parameters.

 The temperature of the pre-catalyst can either be measured by means of a temperature sensor disposed upstream or downstream of the pre-catalyst, or can be determined at

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 modeling calculation taking into account appropriate operating parameters. In addition, in a particularly advantageous arrangement of the process, an extrapolation (forecast) of the precatalyst temperature takes place for a certain period of time to come. If the extrapolation allows to predict that the desorption temperature of sulfur will be reached in the period of time considered by virtue of a particularly rapid increase of the temperature, the first phase of the desulphurisation of the pre-catalyst can be initiated even before the latter has reached the desorption temperature. Thus, the removal of oxygen from the first phase of the process can be carried out partially or completely before even one or all components of the pre-catalyst reach the desorption temperature.

The extrapolation of the temperature of the pre-catalyst is preferably carried out taking into account the position of the pedal value indicator (Pedalwertgeber, PWG) of the accelerator, the dynamics of the pedal value indicator, the engine speed, the amount of fuel injected, the current temperature of the pre-catalyst, the temperature dynamics of the pre-catalyst, the sulfur load of the modeled pre-catalyst and / or the oxygen storage activity of the pre-catalyst.

 According to a particularly advantageous embodiment of the method, the first and / or second lambda (s) rich combustion can be predetermined depending on the previously mentioned parameters. Alternatively or in addition, the combustion lambdas of the first and second phases may be predetermined by considering the state of the main catalyst. In this case, the main consideration is temperature and / or temperature dynamics, modeled sulfur load, conversion activity and / or oxygen storage activity of the main catalyst.

 Since, when we use in the usual way

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 internal combustion engines can operate with a lean mixture, the rich operating periods are regularly required for various reasons, it is preferably planned to arrange such a natural rich operating phase for the desulfurization according to the invention of the pre- catalyst and in accordance with the mentioned indications, provided that the pre-catalyst temperature exceeds the desorption temperature of sulfur. In the case of a main catalyst arranged in the form of Nos storage catalyst, it is in particular possible to use a period of regeneration of NOx nitrogen oxides for the desulfurization according to the invention of the pre-catalyst. But it is also conceivable to use the rich periods of so-called forced amplitude of stoichiometric operation (1 1) in three-way catalytic systems.

 The invention further comprises a device for implementing the method. The device provides means with which it is possible to execute the described process phases. These means comprise a central unit in which a control algorithm of the process phases is introduced in digital form. This control can also be advantageously integrated into a control device of the vehicle engine.

 Other advantageous arrangements of the invention result from the other features mentioned below.

 The invention is explained below in detail by means of embodiments and associated drawings among which: Figure 1 is a block diagram of an internal combustion engine having an exhaust device; FIG. 2 represents the curve, as a function of time, of various parameters relating to the operation and the gases of the exhaust device according to FIG. 1, and

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 during the conventional operation of the exhaust system, and FIG. 3 represents the curve, as a function of time, of various parameters relating to the operation and gases of the exhaust device according to FIG. 1, and this, during a desulphurization according to FIG. the invention of a pre-catalyst.

 An exhaust system generally indicated by the reference numeral 12 is associated with the internal combustion engine 10 shown in FIG. 1. The exhaust device 12 comprises a gas exhaust channel 14 in which are disposed near the engine, a small volume pre-catalyst 16-typically a three-way catalyst-, as well as, under the floor of the vehicle, a large volume NOx storage catalyst 18. In addition to the catalytic system 16, 18, the gas exhaust channel 14 ordinarily contains various gas and / or temperature sensors, not shown in detail, for the adjustment of the internal combustion engine 10. In the present case, only one An oxygen-sensitive measuring device 20 installed downstream of the NO storage catalyst 18 is shown. The measuring device 20 may, for example, be a lambda probe or a NOx sensor equipped with a lambda measuring function. In either case, the measuring device 20 transmits a signal as a function of the proportion of oxygen in the exhaust gas to a motor controller 22 in which it is digitized and processed. A central unit 24, in which an algorithm for implementing a pre-catalyst desulfurization process 16 is introduced, is integrated in the motor control device 22. The motor control device 22 and the central unit 24 are able to influence at least one operating parameter of the internal combustion engine 10, especially on an air-fuel mixture (combustion lambda), in a way that

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 we will explain.

S02 amont sur aval du catalyseur de stockage de NOX 18. To illustrate the problem that is the basis of the invention, we can see in Figure 2 different operating parameters and exhaust gas of the device shown in Figure 1, which correspond to a conventional operation. In this case, the curve of the combustion lambda of the internal combustion engine 10 is represented in the lower diagram of FIG. 1 by the graph 100. The central diagram shows parameters relating to the pre-catalyst 16. More precisely, the graph 102 shows the curve of the pre-catalyst oxygen load 16. Chart 104 gives the ratio of the proportion of 802 in the exhaust gas upstream of the pre-catalyst 16 to the downstream proportion. Values above the time axis t reflect a sulfur deposition in the pre-catalyst 16, while values below the time axis t indicate a sulfur output. In a manner analogous to the above, graph 106 represents the ratio of hydrogen sulfide H 2 S upstream and downstream of pre-catalyst 16. Finally, the upper diagram shows parameters of the main catalyst or NOx storage catalyst. 18. The graph 108 gives the oxygen charge and the graph 110 the ratio

Upstream S02 downstream of the NOX storage catalyst 18.

 Until a time designated by t1, the internal combustion engine 10 is fed with a poor combustion lambda IM (graph 100). Due to the high proportion of oxygen in the exhaust gas during this phase, the oxygen storage capacity of the pre-catalyst 16 but also of the NOx storage catalyst 18 is completely used to such an extent that graphs 102 and 108 are at their maximum value.

At the same time, a continuous storage of SO 2 in the two catalysts 16, 18 (graphs 104, 110) takes place. In this case, a part of the SO 2 emitted by the internal combustion engine 10 is stored on the components in question.

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sulfate par l'accumulateur de NOx du catalyseur de stockage de NOx 18. A l'instant tl, le moteur à combustion interne 10 est commuté dans une phase de fonctionnement riche Tp

avec un lambda de combustion IF < 1. Ceci a par exemple lieu pour effectuer une régénération des oxydes d'azote NOx du catalyseur de stockage 18 dont la nécessité est établie au moyen du dispositif de mesure (du capteur de Noix) 20. precious metal of the pre-catalyst 16 as well as in the oxygen storage components (OSC) of the pre-catalyst 16. Almost all the remainder of the proportion of SO 2 that passes through the pre-catalyst 16 is stored in the form of

by the NOx accumulator of the NOx storage catalyst 18. At time t1, the internal combustion engine 10 is switched into a rich operating phase Tp

with a combustion lambda IF <1. This is for example to carry out a regeneration of NOx nitrogen oxides storage catalyst 18 whose need is established by means of the measuring device (Nut sensor) 20.

(graphe 104). Au cours de cette phase, l'accumulateur d'oxygène du catalyseur de stockage de NOx 18 (graphe 108) est encore presque complètement rempli. Ceci est dû à la durée de parcours des gaz d'échappement, à la capacité d'accumulation d'oxygène du catalyseur principal 18, globalement beaucoup plus élevée, ainsi qu'à la libération de moyens d'oxydation par les processus ayant lieu au niveau du pré-catalyseur 16. La présence d'oxygène a pour effet un stockage presque complet du S02 émis par le précatalyseur 16 dans le catalyseur principal 18 (graphe 110). Due to the lack of oxygen then prevailing in the exhaust gas, the purge of the oxygen accumulator of the precatalyst 16 begins just after this change (graph 102). If-as presupposes here-the temperature of the pre-catalyst corresponds at least to the sulfur desorption temperature, the output of the sulfur stored as SO 2 in the pre-catalyst 16 begins with the transition to the rich operating mode Tp and continues until the oxygen accumulator is completely drained

(graph 104). During this phase, the oxygen accumulator of the NOx storage catalyst 18 (graph 108) is still almost completely filled. This is due to the travel time of the exhaust gas, to the oxygen storage capacity of the main catalyst 18, which is generally much higher, and to the release of oxidation means by the processes taking place at Pre-catalyst level 16. The presence of oxygen results in an almost complete storage of the SO 2 emitted by the precatalyst 16 in the main catalyst 18 (graph 110).

oxydation en S02 des constituants soufrés déposés n'est désormais plus possible. Mais puisque le pré-catalyseur 16 dépasse toujours la température de désorption du soufre, la As soon as the oxygen accumulator of the pre-catalyst 16 is completely emptied (graph 102), the release of SO 2 by the pre-catalyst 16 ceases (graph 104) since a

S02 oxidation of the sulfur-containing constituents deposited is no longer possible. But since the pre-catalyst 16 still exceeds the desorption temperature of sulfur, the

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dIH2S à travers le catalyseur de stockage de NOx 18 est à noter à la fin de la régénération des NOx IF-Cette proportion d'H2S qui passe est indiquée à l'aide de la zone hachurée 112 du graphe 106. A l'issue de la régénération des NOX, à l'instant t2 auquel le moteur à combustion interne 10 est de nouveau commuté en mode de fonctionnement pauvre avec ÂM > 1, un nouveau remplissage de l'accumulateur d'oxygène du pré-catalyseur 16 (graphe 102) débute, et par conséquent une baisse puis la cessation de l'émission d'H2S (graphe 106). Le dépôt de 802 recommence dans les deux catalyseurs 16,18 (graphes 104, 110). The release of stored sulfur then begins as H2S hydrogen sulfide (graph 106). As long as the least amount of oxygen remains stored in the main catalyst 18, oxidation of the H2S emitted by the pre-catalyst 16 as well as its substantially complete deposition in the NOx accumulator but also in the components occurs. oxygen storage (OSC) storage catalyst 18 (graph 110). It is only after the complete purging of the oxygen accumulator that a slight passage

dIH2S through the NOx storage catalyst 18 should be noted at the end of the regeneration of the NOx IF-This proportion of H 2 S that passes is indicated using the hatched area 112 of the graph 106. At the end of the regeneration of the NOX, at time t2 at which the internal combustion engine 10 is switched back to lean operating mode with ÂM> 1, a new filling of the oxygen accumulator of the pre-catalyst 16 (graph 102) begins, and therefore a decline and cessation of the emission of H2S (graph 106). The deposition of 802 starts again in the two catalysts 16, 18 (graphs 104, 110).

stockage de NOx une réduction de l'activité d'accumulation de NO.. Du fait du transfert presque complet du soufre émis par le pré-catalyseur 16 dans le catalyseur principal 18, It is clear from all this that, in the case of conventional operation, almost all of the 802 emitted by the internal combustion engine 10 is stored by the catalytic system 16, 18 not only during the poor operating phases but also in the rich operating phases. While the pre-catalyst 16 still has some desulphurization at the appropriate temperature during the rich operating phases, there is an increasing sulfurization of the main catalyst 18 due to the transfer of sulfur from the pre-catalyst to the main catalyst. . The consequence is a reduction in the conversion activity as well as, in the case of

storage of NOx a reduction of the NO accumulation activity. Due to the almost complete transfer of the sulfur emitted by the pre-catalyst 16 into the main catalyst 18,

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 intervals, in which the active implementation of desulfurization of the main catalyst 18, becomes necessary are reduced. Since these desulphurizations of the main catalyst 18 require very high catalyst temperatures (in the case of Nos storage catalysts, temperatures greater than or equal to 650 ° C.), this goes hand in hand with a high increase in fuel consumption.

catalytique est alimenté dans une première phase TF1 avec une atmosphère de gaz d'échappement riche avec F1 < 1. Les In order to control or, at least, to mitigate this problem, the following way of proceeding represented by FIG. 3 is proposed below:
FIG. 3 shows the same parameters as in FIG. 2, but this time during the desulfurization process according to the invention of the pre-catalyst 16. The internal combustion engine 10 operates first in a lean mode with AM> 1 At a time t1, the need to regenerate the NOx of the NOx storage catalyst 18 is established, for example, based on measurement of the NOx flow. Simultaneously, the temperature of the pre-catalyst is measured by means of a temperature sensor or it is calculated by the central unit 24 according to appropriate operating parameters using theoretical models or stored data matrices. This way of proceeding is familiar to those skilled in the art and, in this case, does not require detailed explanations. Then, it is checked whether the temperature of the pre-catalyst exceeds the sulfur desorption temperature of at least one of the sulfur accumulator components of the pre-catalyst 16. The desorption temperature is about 450 ° C. for the accumulator components. oxygen (OSC) and about 500 OC for the precious metal components of the catalytic coating. If the existence of a sufficiently high temperature is established, the desulfurization according to the invention of the pre-catalyst 16 is triggered. For this, the system

catalytic is fed in a first phase TF1 with a rich exhaust gas atmosphere with F1 <1. The

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 The processes provoked in this way in the pre-catalyst and the main catalyst are similar for the moment to those of FIG. 2: the oxygen accumulator of the precatalyst 16 is purged (graph 102) and the sulfur stored in the pre-catalyst 16 is released as 802 (graph 104) and is almost completely stored in the NOx storage catalyst 18 which is not yet oxygen-free (graph 110). The value of lambda wealth ΔF1 is maintained until the oxygen-sensitive measuring device 20 disposed downstream of the NOx storage catalyst 18 detects a rich exhaust gas atmosphere and therefore the full purge of the exhaust gas. oxygen of the catalytic system, including storage catalyst 18. For this purpose, a suitable lambda value, or a probe voltage, can be predefined as the limit value. During this first phase TF1, one can opt, compared to the conventional NOx regeneration processes, for a lower lambda value rich, that is to say, richer. In particular, a value of lambda λ, pi of 0.8 to 0.9 has proved to be advantageous. Thus, the oxygen purge of the catalytic system 16, 18 is accelerated and the formation of H2S at the level of the pre-catalyst 16 is promoted.

phase TF2 peut tout au plus être stocké avec un faible rendement par le catalyseur de stockage de Nox 18 puisque, pour le stockage du soufre au niveau des éléments de After detection of the rich exhaust gas by the measuring device 20, the combustion lambda is increased to a second lambda value IF2 which is only slightly less than one (graph 100). IF2 is preferably 0.98 to 0.99. The lambda value approach of a stoichiometric exhaust gas composition serves to prevent passage of harmful substances such as carbon monoxide and unburned hydrocarbons. The sulfur which continues to be emitted in the form of H2S by the pre-catalyst 16 during this second

TF2 phase can at most be stored with a low yield by the Nox storage catalyst 18 since, for the storage of sulfur at the level of the elements of

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stockage d'oxygène ou de Nos, une oxydation de l'H2S en 803 en passant par le 802 est nécessaire pour pouvoir former des sulfates (SO'). Cependant, aucun oxygène n'est à disposition pour ces réactions, de telle sorte que le soufre est émis en grande partie sans être stocké. Seul un stockage insignifiant de l'H2S sous forme de sulfures S2peut se produire sur les composants en métal précieux du catalyseur de stockage de NOX 18. La surface hachurée 112 du graphe 106, qui indique les quantités de soufre

traversant le catalyseur de stockage de NOx 18, permet de voir que, selon l'invention, une grande partie du soufre stocké dans le pré-catalyseur 16 est expulsée sans subir aucun nouveau stockage dans le catalyseur principal 18.

Oxygen storage or Nos, 802 oxidation of H2S through 802 is necessary to form sulfates (SO '). However, no oxygen is available for these reactions, so that sulfur is emitted largely without being stored. Only an insignificant storage of H2S in the form of S2 sulphides can occur on the precious metal components of the NOX storage catalyst 18. The shaded area 112 of graph 106, which indicates the amounts of sulfur

passing through the NOx storage catalyst 18, allows to see that, according to the invention, a large part of the sulfur stored in the pre-catalyst 16 is expelled without undergoing any new storage in the main catalyst 18.

This makes it possible to slow down the slow poisoning of the main catalyst 18 with hydrogen sulfide and, consequently, to increase the time intervals between the active desulfurization measurements of the main catalyst 18, which are fuel-consuming. The storage of sulfur in the pre-catalyst 16 and its transfer from the latter are advantageously modeled continuously on the basis of appropriate operating parameters of the internal combustion engine 10 and the exhaust device 12 so that the second phase IF2 is interrupted and a return to normal operation with XM> 1 takes place if the desulfurization of the precatalyst 16 is largely complete according to this modeling calculation.

 Although the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be readily understood by those skilled in the art that changes in form and detail can be made without out of the mind or the field of invention.

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LIST OF REFERENCES 10 internal combustion engine 12 exhaust system 14 exhaust gas channel 16 pre-catalyst 18 main catalyst or NOx storage catalyst 20 oxygen-sensitive measuring device 22 motor control unit 24 unit central 100 lambda combustion 102 oxygen charge, pre-catalyst 104 ratio proportion of 802 in the exhaust gas upstream of the pre-catalyst on downstream proportion 106 ratio proportion of H2S in the exhaust gas upstream of the pre-catalyst on downstream proportion 108 oxygen load, main catalyst 110 ratio SO2 ratio in the exhaust gas upstream of the main catalyst on downstream proportion 112 H2S passage, main catalyst ΔM poverty value lambda ΔF value of wealth lambda t time TF rich operating phase

Claims (21)

 1. A process for desulphurizing a pre-catalyst which is arranged in a gas exhaust duct of an internal combustion engine that can operate with a lean mixture and downstream of which a main catalyst, in particular a catalyst for storing fuel. Nuts, is mounted, characterized in that the sulfur stored in the pre-catalyst (16) is expelled at least partially at periodic intervals in a form which passes through the main catalyst (18) at a level of at least 25% under the conditions existing operating conditions.  2. Method according to claim 1, characterized in that the expelled sulfur form of the pre-catalyst (16) passes through the main catalyst (18) at least 50%.  3. Method according to claim 1 or 2, characterized in that the sulfur is expelled at least partially in the form of hydrogen sulfide (H2S).  4. Method according to claim 3, characterized in that the main catalyst (18) is at least largely free of oxygen during the expulsion of sulfur.  5. A process for desulfurizing a pre-catalyst which is arranged in an exhaust gas channel of an internal combustion engine that can operate with a lean mixture and downstream of which a main catalyst, in particular a Nitrogen storage catalyst. , is mounted, characterized in that, when the temperature of the pre-catalyst (16) is at least temporarily greater than or equal to the sulfur desorption temperature of the precatalyst (16):

 (a) in a first phase (Tp1), the internal combustion engine (10) is supplied with a first rich combustion lambda (if) until the oxygen accumulator of the main catalyst (18) is at less largely oxygen free, and <Desc / Clms Page number 17>  (b) in a second subsequent phase (TF2) 1 the combustion lambda of the internal combustion engine (10) is increased to a second rich combustion lambda (IF2) with Δp1 <ΔP2 <1.

 6. Method according to claim 5, characterized in that the second rich combustion lambda (cap2) is maintained until the pre-catalyst (16) is at least largely free of sulfur.  7. Method according to claim 5 or 6, characterized in that the first lambda of rich combustion (Fl) is maintained until the oxygen storage capacity of the main catalyst (18) is

 at most 20%, in particular less than 10%. 8. Method according to one of claims 5 to 7, characterized in that the purge, at least in large part, of the oxygen accumulator of the main catalyst (18) is monitored by means of a measuring device ( 20) responsive to oxygen placed downstream of said catalyst.  9. Method according to one of claims 5 to 8, characterized in that the first lambda of rich combustion (ka1) is from 0.7 to 0.95, in particular from 0.8 to 0.9.  10. Process according to one of Claims 5 to 9, characterized in that the second rich combustion lambda (IF2) is from 0.95 to 0.955, in particular from 0.98 to 0.99.  11. Method according to one of claims 5 to 10, characterized in that the end of the second phase (sip2) is determined by means of a modeled sulfur charge of the pre-catalyst (16).  12. Process according to claim 11, characterized in that the sulfur charge is calculated as a function of the sulfur input into the pre-catalyst (16) and / or of the sulfur outlet out of the latter, which are modeled based on current operating parameters of the internal combustion engine (10). <Desc / Clms Page number 18>  13. Method according to one of claims 5 to 12, characterized in that the temperature of the precatalyst (16) is measured or in that it is determined by carrying out a modeling calculation.  14. Process according to one of Claims 5 to 13, characterized in that the temperature of the pre-catalyst (16) is extrapolated over a predetermined period of time and in that the first phase is initiated even before the precatalyst (16) ) has reached the desorption temperature of sulfur if it is established, by extrapolation, that the desorption temperature of sulfur will be reached during the lapse of time considered.  15. Method according to claim 14, characterized in that the extrapolation of the pre-catalyst temperature takes place taking into consideration: the position of the accelerator pedal value indicator, the dynamics of the indicator of pedal value, engine speed, amount of fuel injected, current temperature of pre-catalyst, pre-catalyst temperature dynamics, modeled sulfur load of precatalyst (16) and / or of oxygen accumulation of the pre-catalyst (16).  16. Process according to one of Claims 5 to 15, characterized in that the first rich combustion lambda (AF1) and / or the second rich combustion lambda (IF2) are predetermined according to: the position of the indicator accelerator pedal value, pedal value indicator dynamics, engine speed, amount of fuel injected, pre-catalyst temperature, temperature dynamics of the precatalyst, of the modeled sulfur charge of the precatalyst (16), the oxygen storage capacity of the pre-catalyst (16), the temperature and / or temperature dynamics of the main catalyst (18), the Modeled sulfur load of the main catalyst (18), of the conversion activity of the main catalyst <Desc / Clms Page number 19>  (18) and / or the oxygen storage activity of the main catalyst (18).  17. Method according to one of claims 5 to 16, characterized in that the supply of the internal combustion engine (10) with a first lambda of rich combustion (Fl) and a second lambda of rich combustion (F2) takes place at least partially during a NOx regeneration period of the NOx storage catalyst (18).  18. Apparatus for implementing a desulfurization process of a pre-catalyst arranged in a gas exhaust duct of an internal combustion engine that can operate with a lean mixture and downstream of which a main catalyst, in particular Nos storage catalyst is mounted, characterized by means with which the expulsion of the sulfur stored in the precatalyst (16) can be carried out at periodic intervals, in a form which, under operating conditions, passes through the catalyst (18) at least 25%.  19. Device according to claim 18, characterized by means with which, when the temperature of the pre-catalyst (16) is at least temporarily greater than or equal to the sulfur desorption temperature of the precatalyst (16), it is possible to perform the following process steps:

 (a) supplying the internal combustion engine (10) with a first rich combustion lambda (IF1) during a first phase (cF1) until the oxygen accumulator of the main catalyst (18) is at least substantially free of oxygen, and (b) supplying the internal combustion engine (10) with

 during a second subsequent phase (sip2) with a second rich combustion lambda (IF2) with p <. p2 <1.  20. Device according to claim 18 or 19, characterized in that the means comprise a unit <Desc / Clms Page number 20>  central (24) in which a process phase control algorithm is introduced in digital form.  21. Device according to claim 20, characterized in that the central unit (24) is integrated in a motor control device (22).