DE10202935A1 - Operating process for removal of sulfur deposits from the pre-catalyst in an IC engine exhaust system by periodic inducing of high temperature with alternating lean and rich lambda conditions - Google Patents

Abstract

The process for removal of sulfur deposited in the pre-catalyst of exhaust system of an IC engine which also contains a main catalyst unit, wherein the pre-catalyst is heated, at periodic intervals, to at least the desorption temperature for sulfur and: (a) during a first phase (T1), the engine is operated under rich lambda conditions (lambdaF1) until at least the main catalyst unit is essentially oxygen-free, before (b) in a consecutive second stage (T2) operating the engine in an alternating rich and lean sequence (lambdaF1), (lambdaF2) for intervals (tauF), (tauM), with the intervals time controlled so that sulfur is at least 25% converted to sulfur dioxide. An Independent claim is included for an exhaust system for a lean-burn engine implementing the above process.

Description

The invention relates to a method and an apparatus for desulfurization Pre-catalyst of internal combustion engines with the in the preambles of independent claims 1 and 31 mentioned features.

Known catalytic converter systems used for exhaust gas purification often include at least one small-volume pre-catalytic converter arranged near the engine and at least one larger main catalytic converter arranged further downstream in an exhaust gas path. The catalyst components can be configured as oxidation catalysts for converting unburned hydrocarbons HC and carbon monoxide CO, as reduction catalysts for reducing nitrogen oxides NO _x or as 3-way catalysts which simultaneously promote the oxidative and reductive conversions mentioned. In the case of lean-burn internal combustion engines, the main catalytic converter can additionally be equipped with a NO _x storage component which, in lean operating phases in which the internal combustion engine is charged with an oxygen-rich air / fuel mixture with λ> 1, non-convertible nitrogen oxides NO _x in the form of nitrate stored and released and reduced again in intermediate regeneration intervals. Such catalysts are also referred to as NO _x storage catalysts.

A problem known in exhaust gas purification is sulfur contained in fuels, which is almost completely burned to sulfur dioxide SO ₂ in the combustion process and is deposited in various forms on the various components of the catalyst system. This issue affects the _NOx most - save in lean lambdas with a near 100% storage efficiency in the form of sulfate storage components of NO _x storage catalytic converters, the SO _2. Due to its high thermal stability, the sulfate is not removed from the storage in the course of the regular NO _x regeneration. The result is a gradual deactivation of the NO _x storage capacity of the storage catalytic converter (sulfur poisoning), which has made it necessary to develop various desulfurization processes for NO _x storage catalytic converters. For desulfurization, the catalyst is subjected to a rich exhaust gas atmosphere at catalyst temperatures of at least 600 ° C. in order to desorb the stored sulfate and mainly to reduce it to SO ₂ and hydrogen sulfide H ₂ S. In order to counteract an emission of the odor-intensive H ₂ S, it is also known, for example from DE 198 27 195 A or DE 198 35 808 A, to carry out the desulphurization in alternating lean-fat intervals instead of continuously charging the NO _x storage catalytic converter with fat. With a suitable interval design, the slower H ₂ S formation compared to SO ₂ formation can be almost completely suppressed.

In addition to the sulfurization of NO _x storage components, sulfur is also stored, albeit to a lesser extent, in other components of the catalyst system. These are essentially precious metals (Pt, Pd, Rh) of the catalytic coatings and oxygen-storing components OSC ("oxygen storage components"). At catalyst temperatures that are above a component-specific sulfur desorption temperature (about 400 to 450 ° C for OSC and about 500 ° C for precious metals), and under a rich exhaust gas atmosphere, the stored sulfur is expelled. The disadvantage of this is that the sulfur released from the pre-catalyst in the form of H ₂ S and SO _{2 is} partly stored again by the downstream main catalyst due to the oxygen present there. If this is a NO _x storage catalytic converter, the storage is practically complete. The result is a frequent need for desulfurization of the main catalytic converter and a resulting high additional fuel consumption.

From the older patent application DE 100 59 791.2 a process for the desulfurization of a precatalyst is known, in which the stored sulfur is predominantly expelled in the form of hydrogen sulfide H ₂ S, which, under the prevailing operating conditions, in particular in the case of an oxygen-free main catalyst, not in the downstream main catalyst stored, but mostly happens. For this purpose, the internal combustion engine is initially operated with a rich combustion lambda until the main catalytic converter is largely free of oxygen. Then, in the case of a lightly rich combustion lambda, the sulfur stored in the pre-catalyst is driven off. Since the skin catalyst is oxygen-free at this point in time, the sulfur that has been driven off in this way cannot be oxidized and stored by the main catalyst. Disadvantages of this process are a certain breakthrough of the reducing agents HC and CO associated with the rich operating mode and the emission of H ₂ S.

The present invention has for its object to provide a method for desulfurization of a pre-catalyst, which leads to the lowest possible sulfurization of a downstream main catalyst and reduces pollutant emissions of H ₂ S, HC and CO as much as possible. Furthermore, a device that is suitable and as inexpensive as possible for carrying out the method is to be proposed.

• a) in einer ersten Phase die Verbrennungskraftmaschine mit einem ersten fetten Verbrennungslambda so lange betrieben wird, bis ein Sauerstoffspeicher des mindestens einen nachgeschalteten Katalysators zumindest weitgehend sauerstofffrei ist, und
• b) in einer anschließenden zweiten Phase die Verbrennungskraftmaschine alternierend in Magerintervallen mit einem mageren Verbrennungslambda und in Fettintervallen mit einem zweiten fetten Verbrennungslambda betrieben wird, wobei die Magerintervalle und/oder die Fettintervalle zeitgesteuert bemessen werden derart, dass in den Vorkatalysator eingelagerter Schwefel zu mindestens 25%, insbesondere mindestens 50%, in Form von Schwefeldioxid ausgetrieben wird.

This object is achieved by a method and a device for the desulfurization of at least one pre-catalytic converter arranged in an exhaust gas duct of an internal combustion engine, which is followed by at least one further catalytic converter, in particular a NO _x storage catalytic converter, with the features of independent claims 1 and 31. According to the method according to the invention it is provided that when a pre-catalytic converter temperature is reached or with a predeterminable pre-run temperature which is greater than or equal to a minimum sulfur desorption temperature of the pre-catalytic converter,

• a) in a first phase the internal combustion engine is operated with a first rich combustion lambda until an oxygen store of the at least one downstream catalyst is at least largely oxygen-free, and
• b) in a subsequent second phase, the internal combustion engine is operated alternately in lean intervals with a lean combustion lambda and in fat intervals with a second rich combustion lambda, the lean intervals and / or the fat intervals being time-controlled in such a way that at least 25% of the sulfur stored in the pre-catalyst , in particular at least 50%, is expelled in the form of sulfur dioxide.

The process takes advantage of the fact that sulfur-containing exhaust gas components, such as sulfur dioxide SO ₂ , which are released from the pre-catalyst under a rich, low-oxygen exhaust gas atmosphere, only to a significant extent in the presence of oxygen in oxygen-storing components and / or in a NO _x - Storage of the downstream catalyst can be stored, since storage in the form of sulfate first requires an oxidation of the sulfur in the hexavalent oxidation state. Therefore, the desulphurization of the pre-catalyst takes place in two phases, whereby in the first phase of the process oxygen is largely removed from the entire catalyst system, in particular from the oxygen storage of the downstream catalyst, and in the second phase the actual sulfur expulsion from the pre-catalyst, especially in the form of SO ₂ .

In the second phase, the pre-catalytic converter is exposed to a lean and rich exhaust gas atmosphere. This alternating application of lean fat has the advantage over a continuous application of fat that the intermediate lean intervals always introduce a certain amount of oxygen into the pre-catalyst and thus the sulfur released in the fat intervals is mainly discharged in the form of SO ₂ . In contrast, the formation of odor-causing hydrogen sulfide (H ₂ S) can be largely suppressed in this way. The lean intervals are designed in such a way that there is practically no breakthrough of oxygen through the pre-catalytic converter in order to avoid renewed oxygen storage in the downstream catalytic converter (s).

Due to the time control of the lean and rich intervals according to the invention, the use of a measuring device, such as a lambda probe, connected downstream of the pre-catalyst can be dispensed with, as a result of which the method can be implemented particularly cost-effectively. Nevertheless, the intervals can be optimally interpreted with regard to the lowest possible re-storage rate of the released sulfur in the downstream catalytic converter and a low reducing agent and H ₂ S emission by using an oxygen concentration of the exhaust gas measured downstream of the at least one downstream catalytic converter in a manner to be explained the specifications for the duration of the lean and fat intervals are checked and adapted if necessary.

According to a particularly advantageous embodiment of the method, for the durations at least a first lean interval and / or at least a first fat interval in the second phase of the desulfurization, variable pre-control values are specified preferably depending on an oxygen storage capacity of the pre-catalyst determined and stored in operating point-dependent maps. The Oxygen storage capacity of the pre-catalyst, that is its maximum storable Amount of oxygen, depending on the total oxygen storage capacity Catalyst system and depending on a known ratio of Oxygen storage capacities of the pre-catalyst and the at least one downstream catalyst determined. For the total oxygen storage capacity can to the known values of a fresh catalyst system, that is, an oxygen and sulfur-free and undamaged catalyst system. by virtue of However, the higher accuracy is preferably a current one, based on Aging processes mostly reduced the total oxygen storage capacity Catalyst system before and / or during the second phase of the process determine the current oxygen storage capacity of the pre-catalyst using the known and assumed to be constant capacity ratio.

The lean intervals of the second phase are preferably dimensioned in such a way that that as much oxygen as possible is added to the pre-catalyst during a lean interval at the same time, the lowest possible oxygen breakthrough stored by the pre-catalyst becomes. This is necessary in order to introduce oxygen into the downstream catalyst with the consequence of a subsequent sulfur rearrangement in the downstream To avoid catalyst. Preferably, during the lean interval At least 30% and a maximum of 99% oxygen storage of the pre-catalyst, in particular at least 50% and at most 95%, filled with oxygen. It is it is particularly preferably provided that one end of the lean interval essentially corresponds at the latest to a point in time when a breakthrough of lean exhaust gas by the Pre-catalyst used.

On the other hand, a duration of a fat interval is preferably dimensioned such that during the fat interval the sulfur stored in the precatalyst is predominantly expelled in the form of sulfur dioxide SO ₂ . For this purpose, an end of the rich interval should advantageously essentially correspond to a point in time at which the oxygen storage of the pre-catalyst is largely emptied, that is to say is oxygen-free.

Different strategies, which can also be used in combination with one another, can be used to determine the oxygen storage capacity of the entire catalyst system, that is to say the sum of the individual maximum oxygen storage capacities of the individual catalysts. According to an advantageous embodiment of the method, the determination is carried out as a function of a period of time which passes in the case of a practically maximum oxygen-loaded catalyst system before, after the internal combustion engine has been switched from a lean to a rich operating mode, rich exhaust gas is detected for the first time downstream of the at least one downstream catalyst. Current operating parameters of the internal combustion engine, for example an engine load or engine speed, can also be used to determine the total oxygen storage capacity. This type of determination can be carried out particularly advantageously at the end of the first phase and before the alternating exhaust gas treatment in the second phase begins. For this purpose, the catalyst system is preferably initially exposed to slightly lean exhaust gas, in particular λ = 1.005 to 1.05, at least until lean exhaust gas is detected for the first time downstream of the downstream catalyst. The internal combustion engine is then switched over to a slightly rich exhaust gas lambda, in particular to a lambda value between 0.95 and 0.995, until rich exhaust gas is detected for the first time downstream of the downstream catalyst. The total oxygen storage capacity can then be determined from the time delay between switching the internal combustion engine from lean to rich to the breakthrough of the rich exhaust gas by the downstream catalytic converter. This procedure also has the advantage that the downstream catalytic converter is then at least almost oxygen-free, so that when the sulfur is subsequently expelled from the pre-catalytic converter, it is not re-stored in the downstream catalytic converter. It is therefore advantageous to carry out this determination of the oxygen storage capacity of the overall system before the minimum sulfur desorption temperature of the precatalyst has been reached, in particular approximately 20 K below this temperature. With this type of determination of the current total oxygen storage capacity, the pre-control values stored as a function of the operating point can be verified and, if necessary, corrected before the alternating lean-fat loading is started. In this way, a particularly effective suppression of the H ₂ S emission and the exposure to oxygen of the downstream catalyst is possible.

According to another embodiment of the method, one is carried out additionally or alternatively Determination of the oxygen storage capacity of the overall catalyst system during the second phase of desulfurization, that is, during the alternating lean and Fat intervals. This is done after a fat interval, i.e. almost oxygen-free Catalyst system, an extended lean interval performed, and the length of time evaluated between switching the internal combustion engine to lean operation and detection of lean exhaust gas downstream of the at least one downstream one Catalyst or a predeterminable lean threshold value passes. This The lean threshold value is designed such that the time when it is exceeded with an at least approximately complete filling of the oxygen storage of the downstream catalyst correlated.

According to a further advantageous embodiment of the method takes place during the alternating lean and fat loading in the second phase an additional Checking and adapting the pre-control value of the grease intervals. There will be a period of time for this between switching the internal combustion engine from a rich interval to one Lean interval and falling below a predetermined fat threshold value by the falling exhaust gas lambda downstream of the at least one downstream catalyst in Directed to fatter lambda values measured and evaluated. The Fat threshold value designed so that the time it is undershot with a at least approximately complete emptying of the oxygen storage of the pre-catalyst correlated.

In the first phase, the combustion lambda is advantageously chosen to be as low as possible, that is to say as low as possible in oxygen. Lambda values from 0.7 to 0.95, preferably from 0.8 to 0.9, have proven particularly useful. These lambda values lead to particularly rapid and exhaustive oxygen discharge from the catalyst system. In contrast, slightly rich lambda values have proven particularly useful for specifying the second rich combustion lambda of the fat intervals. In particular, lambda values from 0.93 to 0.995, preferably from 0.97 to 0.99, lead to particularly low pollutant breakthroughs and to a virtually complete suppression of H ₂ S. The lean intervals of the second phase are preferably between 1.0 and 1.2 , especially between 1.02 and 1.04.

An oxygen storage capacity of the downstream catalyst should be occupied by at most 20%, preferably less than 10%, after the first phase of oxygen evacuation. The course of the oxygen discharge can be monitored in a simple manner by means of an oxygen-sensitive measuring device arranged downstream of the downstream catalyst. This can be, for example, a lambda probe or, in the case of a NO _x storage catalytic converter in particular, a NO _x sensor equipped with a lambda measuring function. During this first phase, the pre-catalyst does not necessarily have to have reached its sulfur desorption temperature.

According to a particularly advantageous embodiment of the method, the to maintain the second phase until the pre-catalyst is at least largely is sulfur-free before desulfurization has ended and the internal combustion engine is switched back to the regular operating mode. For example, the Sulfur input and / or the sulfur output of the pre-catalyst is continuously modeled are so that a total sulfur loading of the pre-catalyst can be determined. Completion of the second phase and switchover of the internal combustion engine to Regular lean operation is advantageously carried out when the model calculation predetermined, for example largely complete, sulfur emptying of the Indicates pre-catalyst. The modeling of the sulfur input and the sulfur output can in a known, not explained here based on current Operating parameters of the internal combustion engine, in particular based on the Combustion parameters.

The temperature of the pre-catalyst can either be by means of an on, before or after Pre-catalytic converter arranged temperature sensor can be measured or using a Model calculation can be determined taking into account suitable operating parameters. According to a particularly advantageous embodiment of the method, a Extrapolation (forecast) of the pre-catalyst temperature for a certain future Period of time. If the extrapolation shows that the sulfur Desorption temperature within the period due to a particularly fast Predicted temperature rise, the first phase of desulfurization of the Pre-catalyst are introduced before it reaches the desorption temperature Has. In this way, oxygen removal can be the first phase of the process already below the desorption temperature of one or all of the pre-catalyst components partially or even completely. The extrapolation of the Pre-catalyst temperature is preferably carried out taking into account a position of a Pedal value transmitter (PWG) of an accelerator pedal, a dynamics of the pedal value transmitter, one Engine speed, an injected fuel quantity, the current Pre-catalyst temperature, a dynamic of the pre-catalyst temperature, the modeled Sulfur loading of the pre-catalyst and / or an oxygen storage activity of the Pre-catalyst.

Since in normal operation of lean-burn internal combustion engines, rich operating intervals are regularly necessary for various reasons, it is preferably provided to use such a "natural" rich operating phase for the desulfurization of the pre-catalyst according to the invention and to design it in accordance with the specifications mentioned, provided the pre-catalyst temperature exceeds the sulfur desorption temperature , In this way, the desulphurization of the pre-catalyst can be carried out with the lowest fuel consumption. In the case of a main catalytic converter configured as a NO _x storage catalytic converter, in particular a NO _x regeneration interval can be used for the desulphurization of the precatalyst according to the invention or the desulphurization can be carried out immediately after the regeneration. However, it is also conceivable to use fat intervals of a so-called forced amplitude of a stoichiometric operation (λ = 1) in 3-way catalyst systems.

Another advantageous embodiment of the method provides a fuel cut-off in particular at an operating point at which a driver requests Desired driving torque is less than a thrust torque of the vehicle, and / or in Shift pauses during a gear change in gearboxes with traction interruption suppress during the desulfurization of the pre-catalyst. That way an exposure of the exhaust system to the very oxygen-rich exhaust gas from the Thrust cut-out can be prevented.

The invention further comprises an apparatus for performing the method, the one downstream of the at least one downstream catalyst (main catalyst) arranged oxygen-sensitive measuring device provides, and means with which the described process steps are executable. These means include a control unit, in which an algorithm for performing the method is stored in digital form. This Control unit can also be integrated into a motor control in a particularly advantageous manner.

According to a particularly preferred embodiment of the device, the precatalyst is equipped with a storage component which allows sulfur to be stored reversibly from a lean exhaust gas. By using such a "sulfur trap", which is desulfurized at regular intervals by the process according to the invention, targeted protection of a downstream catalytic converter, in particular a NO _x storage catalytic converter, is achieved for the first time against the particularly disadvantageous sulfur poisoning for it.

Further advantageous refinements of the invention result from the features of other subclaims.

The invention is described below in exemplary embodiments on the basis of the associated Drawings explained in more detail. Show it:

Fig. 1 is a schematic block diagram of an internal combustion engine with an exhaust system and

Fig. 2 temporal courses of an engine lambda value, a downstream of a pre-catalytic converter and downstream of a main catalytic converter according to Fig. 1 measured exhaust gas lambda and courses of the oxygen loads of the pre-and main catalytic converter during desulfurization of the pre-catalyst according to the inventive method.

The internal combustion engine 10 shown in FIG. 1 is assigned an exhaust system, designated overall by 12. The internal combustion engine 10 is particularly advantageously equipped with a direct injection, not shown, with which a fuel to be supplied to the cylinders is injected directly into the cylinder combustion chambers via high-pressure injection valves. Furthermore, the internal combustion engine 10 is preferably capable of stratified charge, the injected fuel being concentrated at an ignition point essentially in the area of a spark plug of a cylinder in the form of a stratified charge cloud in stratified charge operation. In stratified charge mode, particularly lean air-fuel mixtures can be represented, as a result of which very low fuel consumption can be achieved.

The exhaust system 12 comprises an exhaust duct 14 , in which a small-volume pre-catalytic converter 16 , typically a 3-way catalytic converter, in a position near the engine, and a large-volume main catalytic converter in an underbody position of the vehicle, in particular in the case of a lean-running internal combustion engine 10, a NO _x storage catalytic converter 18 , is arranged. The pre-catalytic converter 16 is also equipped with a sulfur storage component 20 , which is able to store sulfur-containing exhaust gas components in lean operating phases of the internal combustion engine 10 . Suitable sulfur storage components known per se include, for example, barium salts. The storage component 20 can be mixed with the catalytic components of the coating in a homogeneous distribution or can be designed as a spatially separated catalyst component.

In addition to the catalytic converter system 16 , 18 , the exhaust gas duct 14 houses various gas sensors 22 , 24 , which detect a concentration of at least one exhaust gas component in the exhaust gas and serve to regulate the internal combustion engine 10 . In detail, a lambda probe 22 , preferably a broadband lambda probe, is arranged upstream of the storage catalytic converter 16 . This measures an oxygen concentration of the exhaust gas and serves to regulate the air-fuel mixture (combustion lambda) to be supplied to the internal combustion engine 10 . Another gas sensor 24 is an oxygen-sensitive measuring device and is arranged downstream of the NO _x storage catalytic converter 18 . This can also be a lambda sensor, in particular a step response lambda sensor, or, in particular if the main catalytic converter 18 is a NO _x storage catalytic converter, as in this case, a NO _x sensor that has a lambda measuring function. The oxygen-sensitive measuring device 24 serves, in a manner still to be explained, to control the method according to the invention for the desulfurization of the pre-catalyst 16 or its sulfur storage component 20 .

The exhaust gas duct 14 can also house temperature sensors that measure an exhaust gas or a catalyst temperature. Alternatively, the catalyst temperatures can also be modeled as a function of suitable operating parameters of the internal combustion engine 10 . All signals provided by the sensors and selected operating parameters of the internal combustion engine 10 are input into an engine controller 28 , which digitizes and processes the signals and controls an operating mode of the internal combustion engine 10 , in particular the combustion lambda, the stratified charge mode and an exhaust gas recirculation rate, depending on the signals. A control unit 26 is integrated in the engine control 28 and comprises a stored algorithm for carrying out the method according to the invention for the desulfurization of the pre-catalyst 16 .

During lean operation of internal combustion engine 10 , in particular lean stratified charge operation, sulfur components contained in the fuel are converted almost completely to sulfur dioxide SO ₂ . The SO ₂ is further oxidized on the catalytic precious metal components of the pre-catalyst 16 in the oxygen-rich exhaust gas and is stored with approximately 100% efficiency in the storage component 20 or in other sulfur-storing components of the pre-catalyst 16 in the form of sulfate. If a temperature of the pre-catalytic converter 16 is recognized, which approximately corresponds to a sulfur desorption temperature of the storage component 20 or will shortly reach it, the control unit 26 initiates desulfurization of the pre-catalytic converter 16 .

The control of the method in FIG. 2 is based on the time profiles of the combustion lambda λ _mot (engine lambda) measured with the lambda probe 22 , the exhaust gas lambda λ _nVK present downstream of the pre-catalytic converter 16 (and not measured in the course of the method) and that with the lambda probe 24 shown exhaust gas lambda λ _nHK measured downstream of the main catalytic converter 18 . In addition, the middle part of FIG. 2 shows the course of a current oxygen loading O _{VK of} the pre-catalyst 19 and the lower part shows the course of the oxygen loading O _{HK of} the NO _x storage catalytic converter 18 .

First, the internal combustion engine 10 is operated in the usual stratified charge mode with a lean combustion lambda (or, if appropriate, in the stoichiometric homogeneous operation). With a certain advance before reaching the sulfur desorption temperature of the pre-catalyst 16 , a first phase T _{1 of} the desulfurization is initiated and the internal combustion engine 10 with a motorized lambda λ _mot corresponding to a first rich combustion lambda λ _F1 , which is preferably between 0.8 and 0.9 lies, operated (time t ₀ ).

Immediately after switching the internal combustion engine 10 to λ _F1 , the oxygen loading O _{VK of} the pre-catalytic converter 16 begins to drop, starting from an oxygen storage capacity OSC _{VK of} the pre-catalytic converter 16 corresponding to a maximum fill level. In this phase, the stored oxygen is used for the oxidation of the reducing agents (HC, CO) of the exhaust gas. As soon as the oxygen reservoir of the pre-catalytic converter 16 is emptied (O _VK = 0), rich exhaust gas breaks through the pre-catalytic converter 16 for the first time, so that the exhaust gas lambda λ _nVK downstream of the pre-catalytic converter 16 falls below λ = 1. As a result, the oxygen storage of the NO _x storage catalytic converter 18 also begins to be emptied and its current O ₂ loading O _{HK also} drops to 0. Only then is a rich exhaust gas lambda λ _nHK detected downstream of the NO _x storage catalytic converter 18 . The first phase T ₁ is ended as soon as the exhaust gas lambda λ _nHK measured with the gas sensor 24 downstream of the NO _x storage _{catalytic converter} 18 falls below a first fat _{threshold value} S _F1 (time t ₁ ). At this time, it is ensured that the entire catalyst system is at least largely or completely oxygen-free, in particular the oxygen loading O _{HK of} the NO _x storage catalytic converter 18 is at most 20%, in particular less than 10%, of its maximum oxygen storage capacity OSC _HK . S _F1 is designed taking into account an exhaust gas runtime between internal combustion engine 10 and NO _x storage catalytic converter 18 so that the lean exhaust gas present after switching at time t ₁ reaches the NO _x storage catalytic converter 18 when it is completely free of oxygen.

In a subsequent second phase T ₂ , in which the pre-catalytic converter 16 has at least almost reached the minimum sulfur desorption temperature so that the actual sulfur discharge takes place, the internal combustion engine 10 is operated alternately in lean intervals τ _M and rich intervals τ _F. During the lean intervals τ _M , the engine lambda λ _{mot is set} to a lean combustion lambda λ _M , which is advantageously 1.0 to 1.2, preferably 1.02 to 1.04, in order to ensure a certain oxygen storage in the pre-catalytic converter 16 cause. As a result, the oxygen load O _{VK of} the pre-catalyst 16 and - with a certain delay - the exhaust gas lambda λ _nVK after the pre-catalyst 16 _begins to rise. Only after oxygen breakthrough through the pre-catalytic converter 16 (λ _nVK > 1) does a slow filling of the oxygen store of the NO _x storage catalytic converter 18 and thus a slight increase in O _HK begin. Again with a certain time delay, an increase in the exhaust gas lambda λ _nHK measured behind the NO _x storage catalytic converter 18 _begins .

Even before the increase in λ _nHK begins, the internal combustion engine 10 is at a time t ₂ in a rich interval τ _F with a second rich combustion lambda λ _F2 , which is advantageously 0.93 to 0.995, preferably 0.97 to 0.99, switched. As a result, the oxygen load O _{VK of} the pre-catalyst 16 drops. In this phase, the sulfate stored in the sulfur storage component 20 is reduced and released by means of the reducing agents present in the exhaust gas. The oxygen stored in the previous lean interval τ _M ensures that the reduction remains at the oxidation level of the sulfur dioxide SO ₂ (+ IV) and does not proceed completely to the level of the hydrogen sulfide H ₂ S (-II). From a certain point in the fat interval τ _F , the reducing agents of the exhaust gas are no longer completely used to convert the sulfate, so that the exhaust gas lambda λ _nVK behind the pre-catalyst 16 gradually drops to a rich lambda value <1 in order to empty the oxygen store of the NO _x - Storage catalyst 18 (O _HK sinks) to cause.

The SO ₂ discharged from the pre-catalytic converter 16 during the second phase 12 or in the rich intervals τ _F does indeed reach the downstream NO _x storage catalytic converter 18 , but can pass through it without being stored. The storage of sulfur in the NO _x storage catalyst 18 in the form of sulfate namely requires the oxidation of SO ₂ to SO ₃ and thus the presence of oxygen. However, this oxygen is not available because during the first phase T ₁ the oxygen store of the NO _x storage catalytic converter 18 was emptied and even in the subsequent lean intervals τ _{M of} the second phase T ₂ almost no oxygen was stored in the storage catalytic converter 18 .

The durations of the lean intervals τ _M and rich intervals τ _F are initially precontrolled, at least at the beginning of the second phase T ₂ , by specifying stored precontrol values as a function of current operating parameters of the internal combustion engine 10 , for example an engine load or engine speed. The pilot control values depend in particular on the oxygen storage capacity OSC _{VK of} the pre-catalytic converter 16 and can correspond, for example, to a fresh state of the catalytic converter 16 or to a current storage capacity determined in a previous desulfurization. The durations of the lean intervals τ _{M are} dimensioned such that one end of a lean interval τ _M coincides as precisely as possible with a lean breakdown through the pre-catalytic converter 16 . In this way, only very little oxygen reaches the NO _x storage catalytic converter 18 , so that it experiences only a small amount of oxygen storage. The pilot control values for the grease intervals τ _F are dimensioned such that, taking the exhaust gas runtime into account, the oxygen store of the NO _x storage catalytic converter 18 is practically completely emptied.

The stored pilot control values for the durations of the lean and rich intervals τ _M , τ _F are checked and, if necessary, adapted by measuring the time delay between switching the internal combustion engine 10 from a lean interval τ _M to a rich interval τ _F (for example at time t ₂ ) up to a _falling below a first predetermined fat threshold value S _F1 by exhaust gas lambda λ _nHK behind the NO _x storage catalytic converter 18 in the direction of lower lambda values. S _F1 is designed taking into account the exhaust gas runtime so that a practically complete emptying of the oxygen storage of the pre-catalyst is guaranteed. If it turns out that the measured time delay is greater than the previous pre-control value for the duration of the fat intervals τ _F , an adaptation is made by shortening the future fat intervals τ _F by a fixed or variable difference value. If, in the opposite case, the measured time delay is shorter than the previous duration of the fat intervals τ _F (in this case, the value does not fall below S _F1 ), the subsequent intervals τ _{F are} extended by the difference value.

In order to take into account aging of the catalysts and a decrease in the oxygen storage capacities associated therewith, the oxygen storage _capacity OSC _{Σ of} the entire catalyst system 16 , 18 is determined in the course of the second phase T ₂ and / or at the end of the first phase T ₁ . For this purpose, an extended lean interval τ _M 'is carried out within the second phase T ₂ until the exhaust gas lambda λ _nHK behind the storage catalytic converter 18 reaches a predetermined lean threshold S _M. The lean threshold value S _M is designed such that its exceeding corresponds to 100% oxygen loading of the NO _x storage catalytic converter 18 and thus of the entire catalytic converter system 16 , 18 . In fact, the extended lean interval τ _M 'is ended and the internal combustion engine 10 is switched to λ _F2 when λ _{nHK exceeds} a second fat _{threshold value} S _F2 . As a result, the lean threshold value S _M is corrected for the exhaust gas runtime in order to avoid an excessive introduction of oxygen into the storage catalytic converter 18 . The oxygen storage capacity OSC _{Σ can be determined} as the sum of the storage capacities (OSC _VK + OSC _HK ) from the measured time delay between engine initiation of the extended lean interval τ _M 'until the lean breakthrough measured by the gas sensor 24 through the NO _x storage catalytic converter 18 . The maximum oxygen storage capacity OSC _{VK of} the pre-catalyst 16 can be determined as a function of OSC _Σ and a ratio of the oxygen storage capacities known from the fresh catalysts 16 , 18 . The durations of the lean intervals τ _M and rich intervals τ _F are then corrected depending on the OSC _{VK of} the pre-catalytic converter 16 determined in this way. The lambda threshold values S _M , S _F1 and S _{F2 are also} adapted.

It is particularly preferred to carry out a determination of the oxygen storage capacity OSC _{Σ of} the entire catalyst system 16 , 18 (not shown) before the beginning of the alternating exhaust gas application at the end of the first phase T. For this purpose, preferably 20 K below the minimum sulfur desorption temperature of the pre-catalyst 16, the catalyst system is initially subjected to a slightly lean exhaust gas between 1.005 and 1.05 until a probe _jump from λ _nHK into the " _lean " behind the storage _catalyst 18 is detected. If this is the case, there is a maximum oxygen loading of the catalyst system 16 , 18 . This is followed by actuation of the system with a slightly rich exhaust gas at lambda values from 0.95 to 0.995, to downstream of the storage catalyst 18, a probe jump of the gas sensor 24 to the "fats" is detected. With this procedure, the time delay from switching the engine lambda from lean to rich to the detection of the fat breakthrough downstream of the NO _x storage catalytic converter 18 is evaluated in order to determine the total oxygen storage capacity OSC _Σ . The advantage of this procedure is that the duration of the lean and rich intervals τ _M , τ _F can be adapted to the current catalytic converter state, in particular the pre-catalytic converter 16 , already at the beginning of the second phase T ₂ .

Although the method was explained using the particularly advantageous example of a pre-catalyst 16 designed with a sulfur storage component 20 , the invention is not restricted to this. The implementation of the process with conventional pre-catalysts without a "sulfur trap" practically does not differ from the procedure described above.

It is achieved by the method according to the invention that almost all of the sulfur stored in the pre-catalyst 16 is released in the form of SO ₂ , without subsequently leading to a renewed sulfur incorporation in the NO _x storage catalyst 18, which is particularly sensitive to this. As a result, the extremely fuel-consuming desulphurization of the NO _x storage catalytic converter 18 , which, because of the very high catalytic converter temperatures required for this, always require measures to reduce engine efficiency and thus lead to a considerable increase in fuel consumption, are necessary at larger intervals. Furthermore, the discontinuous exposure to lean fat during the second phase T ₂ almost completely suppresses the formation and emission of odor-intensive and harmful H ₂ S and also minimizes the breakthrough of the reducing agents HC and CO. Overall, the process is characterized by extremely low pollutant emissions. In addition, the method allows for the first time a targeted design of the pre-catalyst 16 as a "sulfur trap". In this way, the NO _x storing catalyst 18 can be almost fully protected from sulfur poisoning and virtually completely to dispense with the otherwise necessary desulfurization of the storage catalyst 18th REFERENCE SIGN LIST 10 internal combustion engine
12 exhaust system
14 exhaust duct
16 pre-catalytic converter
18 downstream catalytic converter / NO _x storage catalytic converter
20 sulfur storage component
22 Oxygen-sensitive measuring device / lambda probe
24 oxygen-sensitive measuring device / NO _x sensor
26 control unit
28 Engine control unit
λ air-fuel ratio lambda
τ _F fat interval
λ _F1 first rich combustion lambda
λ _F2 second rich combustion lambda
λ _M lean combustion lambda
τ _M lean interval
τ _M 'extended lean interval
λ _mot combustion lambda / motor lambda
λ _{nHK exhaust} lambda downstream of the NO _x storage _catalytic converter
λ _nVK exhaust lambda downstream of the pre-catalyst
O _HK oxygen loading of the NO _x storage catalytic converter
OSC _Σ oxygen _storage capacity of the entire catalyst system
OSC _HK Oxygen storage capacity of the NO _x storage catalytic converter
OSC _VK oxygen storage capacity of the pre-catalyst
O _VK oxygen loading of the pre-catalyst
S _F1 first fat threshold
S _F2 second fat threshold
S _M lean threshold
t time
T ₁ first phase (oxygen removal)
T ₂ second phase (desulfurization)

Claims (34)

1. A method for desulfurizing at least one pre-catalytic converter ( 16 ) arranged in an exhaust gas duct ( 14 ) of an internal combustion engine ( 10 ), which is followed by at least one further catalytic converter ( 18 ), wherein when a pre-catalytic converter temperature is reached or with a predeterminable flow ( 16 ), which is greater than or equal to a minimum sulfur desorption temperature of the pre-catalyst ( 16 ), a) in a first phase (T ₁ ) the internal combustion engine ( 10 ) is operated with a first rich combustion lambda (λ _F1 ) until an oxygen store of the at least one downstream catalyst ( 18 ) is at least largely oxygen-free, and b) in a subsequent second phase (T ₂ ), the internal combustion engine ( 10 ) is operated alternately in lean intervals (τ _M ) with a lean combustion lambda (λ _M ) and in fat intervals (τ _F ) with a second rich combustion lambda (λ _F2 ), the lean intervals (τ _M ) and / or the fat intervals (τ _F ) being timed in such a way that at least 25% of the sulfur stored in the pre-catalyst ( 16 ) is expelled in the form of sulfur dioxide (SO ₂ ). 2. The method according to claim 1, characterized in that during the second phase (T ₂ ) for the durations of at least a first lean interval (τ _M ) and / or at least a first rich interval (τ _F ) variable pre-control values are specified. 3. The method according to claim 1 or 2, characterized in that the pilot control values are determined as a function of an oxygen storage capacity (OSC _VK ) of an oxygen store of the pre-catalyst ( 16 ). 4. The method according to claim 3, characterized in that the oxygen storage capacity (OSC _VK ) of the pre-catalyst ( 16 ) in dependence on an oxygen storage capacity (OSC _Σ ) of the entire catalyst system ( 16 , 18 ) and in dependence on a ratio of the oxygen storage capacity (OSC _VK ) of Pre-catalyst ( 16 ) is determined to an oxygen storage capacity (OSC _HK ) of the at least one downstream catalyst ( 18 ). 5. The method according to any one of claims 2 to 4, characterized in that the Pilot values are saved in operating point-dependent maps. 6. The method according to any one of the preceding claims, characterized in that in the second phase (T ₂ ) the duration of the lean intervals (τ _M ) is dimensioned such that during a lean interval (τ _M ) the oxygen storage capacity (OSC _VK ) one of the pre-catalyst ( 16 ) at least 30% and a maximum of 99%, in particular at least 50% and a maximum of 95%. 7. The method according to claim 6, characterized in that the duration of the lean intervals (τ _M ) is dimensioned such that an end of the lean interval (τ _M ) essentially corresponds at the latest to a point in time at which a breakthrough of lean exhaust gas by the pre-catalyst ( 16 ) starts. 8. The method according to any one of the preceding claims, characterized in that in the second phase (T ₂ ) a duration of the rich intervals (τ _F ) is dimensioned such that the sulfur stored in the pre-catalyst ( 16 ) during a rich interval (τ _F ) predominantly in the form of sulfur dioxide (SO ₂ ). 9. The method according to claim 8, characterized in that the duration of the rich intervals (τ _F ) is dimensioned such that an end of the rich interval (τ _F ) essentially corresponds to a point in time at which the oxygen storage (OSC _VK ) of the pre-catalyst largely free of oxygen is. 10. The method according to any one of the preceding claims, characterized in that the oxygen storage _capacity (OSC _Σ ) of the entire catalyst system ( 16 , 18 ) is determined as a function of a period of time that passes with a practically maximum oxygen-loaded catalyst system ( 16 , 18 ) before after a Switching the internal combustion engine ( 10 ) from a lean to a rich operating mode, for the first time, rich exhaust gas downstream of the at least one downstream catalytic converter ( 18 ) is detected. 11. The method according to claim 10, characterized in that the determination of the oxygen storage _capacity (OSC _Σ ) of the catalyst system ( 16 , 18 ) before the start of the second phase (T ₂ ), in particular at the end of the first phase (T ₁ ), is carried out. 12. The method according to claim 10 or 11, characterized in that the determination of the oxygen storage capacity (OSC _Σ ) of the catalyst system ( 16 , 18 ) before reaching the minimum sulfur desorption temperature of the pre-catalyst ( 16 ), in particular about 20 K below the minimum sulfur Desorption temperature is carried out. 13. The method according to any one of claims 1 to 9, characterized in that the oxygen storage _capacity (OSC _Σ ) of the catalyst system ( 16 , 18 ) is determined as a function of a period of time that passes with at least approximately oxygen-free catalyst system ( 16 , 18 ) before switching of the internal combustion engine ( 10 ) from a rich to a lean operating mode downstream of the at least one downstream catalytic converter ( 18 ), for the first time lean exhaust gas is detected or a predefinable lean threshold value (S _M ) is exceeded. 14. The method according to claim 13, characterized in that the predetermined lean threshold value (S _M ) is designed such that the time it is exceeded by the exhaust gas lambda (λ _nHK ) downstream of the at least one downstream catalyst ( 18 ) with an at least approximately complete filling of the Oxygen storage (OSC _HK ) of the catalyst ( 18 ) correlated. 15. The method according to claim 13 or 14, characterized in that the determination of the oxygen storage _capacity (OSC _Σ ) is carried out during the second phase (T ₂ ). 16. The method according to any one of the preceding claims, characterized in that during the second phase (T ₂ ) as a function of a measured period of switching of the internal combustion engine ( 10 ) from a rich interval (τ _F ) in a lean interval (τ _M ) until falling below one the first predetermined fat threshold value (S _F1 ) is adjusted by the falling exhaust gas lambda (λ _nHK ) downstream of the at least one downstream catalytic converter ( 18 ) in the direction of richer values the pilot control value for the duration of the fat intervals (τ _F ). 17. The method according to claim 16, characterized in that the first predetermined fat threshold value (S _F1 ) is designed such that the point in time when it is undershot by the exhaust gas lambda (λ _nHK ) downstream of the at least one downstream catalyst ( 18 ) with an at least approximately emptying of the Oxygen storage (OSC _HK ) of the main catalyst ( 18 ) correlated. 18. The method according to any one of the preceding claims, characterized in that the exhaust gas lambda downstream of the at least one downstream catalytic converter ( 18 ) is measured with an oxygen-sensitive measuring device ( 22 ), in particular with a NO _x sensor or a lambda probe. 19. The method according to any one of the preceding claims, characterized in that the second rich combustion lambda (λ _F2 ) of the fat intervals (τ _F ) of the second phase (T ₂ ) is 0.93 to 0.995, in particular 0.97 to 0.99 , 20. The method according to any one of the preceding claims, characterized in that the lean combustion lambda (λ _M ) of the lean intervals (τ _M ) of the second phase (T ₂ ) 1.0 to 1.2, in particular 1.02 to 1.04, is. 21. The method according to any one of the preceding claims, characterized in that the first rich combustion lambda (λ _F1 ) of the first phase (T ₁ ) is 0.7 to 0.95, in particular 0.8 to 0.9. 22. The method according to any one of the preceding claims, characterized in that the first phase (T ₁ ) is maintained until the oxygen storage capacity (OSC _HK ) of the at least one downstream catalyst ( 18 ) is at most 20%, in particular less than 10% is occupied. 23. The method according to any one of the preceding claims, characterized in that the second phase (T ₂ ) is maintained until the pre-catalyst ( 16 ) is at least largely sulfur-free. 24. The method according to claim 23, characterized in that an end of the second phase (T ₂ ) is determined on the basis of a modeled sulfur loading of the pre-catalyst ( 16 ). 25. The method according to claim 24, characterized in that the sulfur loading is determined as a function of a sulfur input and / or sulfur output of the pre-catalyst ( 16 ) modeled on the basis of current operating parameters of the internal combustion engine ( 10 ). 26. The method according to any one of the preceding claims, characterized in that the temperature of the pre-catalyst ( 16 ) is measured or determined using a model calculation. 27. The method according to any one of the preceding claims, characterized in that the temperature of the pre-catalyst ( 16 ) is extrapolated for a predetermined period of time and the first phase (T ₁ ) is initiated before the pre-catalyst ( 16 ) reaches the minimum sulfur desorption temperature has if the extrapolation recognizes a future reaching of the sulfur desorption temperature within the time period. 28. The method according to claim 27, characterized in that the extrapolation of the pre-catalyst temperature taking into account a position of a pedal value transmitter of an accelerator pedal, a dynamic of the pedal value transmitter, an engine speed, an injected fuel quantity, a current pre-catalyst temperature, a dynamic of the pre-catalyst temperature, the modeled sulfur loading of the pre-catalyst ( 16 ) and / or an oxygen storage activity of the pre-catalyst ( 16 ). 29. The method according to any one of the preceding claims, characterized in that the desulfurization of the pre-catalyst ( 16 ) at least partially falls into a NO _x regeneration of the downstream NO _x storage catalyst ( 18 ) or is carried out immediately afterwards. 30. The method according to any one of the preceding claims, characterized in that a fuel cut-off is suppressed during the desulfurization of the pre-catalyst ( 16 ). 31. Device for desulfurization of at least one pre-catalytic converter ( 16 ) arranged in an exhaust gas duct ( 14 ) of a lean-burn internal combustion engine ( 10 ), which is followed by at least one further catalytic converter ( 18 ), in particular a NO _x storage catalytic converter, with a downstream of the at least one Downstream catalyst ( 18 ) arranged oxygen-sensitive gas sensor ( 22 ) and means with which, when reached or with a predetermined flow before reaching a temperature of the pre-catalyst ( 16 ), which is greater than or equal to a minimum sulfur desorption temperature of the pre-catalyst ( 16 ), the steps a) operating the internal combustion engine ( 10 ) in a first phase (T ₁ ) with a first rich combustion lambda (λ _F1 ) until an oxygen store of the downstream catalyst ( 18 ) is at least largely oxygen-free, and b) in a subsequent second phase (T ₂ ) operating the internal combustion engine ( 10 ) alternately in lean intervals (τ _M ) with a lean combustion lambda (λ _M ) and in fat intervals (τ _F ) with a second rich combustion lambda (λ _F2 ) Time-controlled measurement of the lean intervals (τ _M ) and / or the fat intervals (τ _F ) such that at least 25% of the sulfur stored in the pre-catalyst ( 16 ) is driven off in the form of sulfur dioxide (SO ₂ ), are feasible. 32. Apparatus according to claim 31, characterized in that the pre-catalyst ( 16 ) is equipped with a sulfur storage component ( 20 ) for the reversible storage of sulfur from a lean exhaust gas. 33. Device according to claim 31 or 32, characterized in that the means comprise a control unit ( 26 ) in which an algorithm for carrying out the method is stored in digital form. 34. Device according to claim 33, characterized in that the control unit ( 26 ) is integrated in a motor control ( 28 ).