EP0931922B1 - Method and apparatus to purify the exhaust gas of an internal combustion engine - Google Patents

Description

The invention relates to a method for cleaning exhaust gases an internal combustion engine with the features of the preamble of claim 1. In addition, the invention relates to a Device for cleaning exhaust gases from an internal combustion engine.

In order to reduce the pollutant emissions of an internal combustion engine, such an engine, for example a diesel or gasoline engine, can be equipped with an exhaust gas cleaning device through which the exhaust gases of the internal combustion engine flow. NO _x adsorber systems are particularly suitable for cleaning the combustion engine exhaust gases. Exhaust gas cleaning elements of this type, also referred to as NO _x adsorber catalysts, store the nitrogen oxides (NO _x ) from internal combustion engines under certain conditions, provided they are operated "leanly". Such lean operation occurs when the combustion-air ratio lambda (λ) is greater than 1, that is to say when there is over-stoichiometric combustion in which large amounts of oxygen are present in the exhaust gas. Such For regeneration, due to their storage capability NO, also referred to as a storage catalyst _x -Adsorbersysteme is reducing effect exhaust gas requires high as possible reducing agent content, so that the NO stored in the NO _x adsorber catalyst _x is released and can be converted to nitrogen N _2. An internal combustion engine produces exhaust gas with a reducing action if there is a "rich" combustion, that is to say a substoichiometric combustion with λ <1, in which there is little or no residual oxygen in the exhaust gas.

The internal combustion engines equipped with such a NO _x storage catalytic converter must accordingly have an engine control which enables a change between lean operation and rich operation of the internal combustion engine.

Due to the lean operation, the exhaust gases of the internal combustion engine contain sulfur oxide compounds (SO _x ), preferably sulfur dioxide (SO ₂ ), which react with the storage material of the NO _x storage catalyst during lean operation and thereby sulfates form. Such sulfate formation leads to a reduction in the NO _x storage capacity of the NO _x storage catalytic converter, which is also referred to as "sulfur poisoning" of the NO _x storage catalytic converter.

In order to maintain the function of the NO _x storage catalyst, it can be freed from attached sulfate from time to time and thereby regenerated. To achieve effective regeneration, it is known to set elevated exhaust gas temperatures of, for example, over 550 ° C. and a rich exhaust gas composition. Depending on the sulfur content of fuel and engine oil as the main sulfur sources, the regeneration of the NO _x storage catalytic converter must be carried out in more or less short intervals. These intervals are typically in the range of a few hours of operation.

For this purpose, a method for sulfate regeneration of a NO _x storage catalytic converter is described in US Pat. No. 5,657,625 A, which, in order to achieve the regeneration conditions, involves setting different air ratios in different cylinders of a multi-cylinder internal combustion engine and adjusting the ignition timing of the cylinders concerned includes. However, this method is associated with a high level of control engineering and undesirably interrupts the normal operation of the internal combustion engine.

In order to reduce the frequency of the required regeneration processes, fuels and motor oils with a low sulfur content can be used, on the one hand, and, on the other hand, the sulfur poisoning of the NO _x storage catalyst can be avoided, in which a so-called "SO _x trap" in the literature SO _x storage catalyst is used, which is arranged in the exhaust line before the NO _x storage catalyst.

When the exhaust gases flow through the SO _x storage catalytic converter, a large part of the sulfur compounds emitted by the engine are absorbed and stored therein and in this way the sulfur poisoning of the NO _x storage catalytic converter is avoided.

However, the NO _x storage capacity of such a SO _x trap or SO _x storage catalytic converter is limited, so that regeneration or desulfation of the SO _x storage catalytic converter must be carried out for continuous operation. Desulfation of this type can be achieved with the aid of an exhaust gas which contains reducing agents (for example CO, H ₂ , HC) and has a relatively high temperature. Under these conditions, the previously stored amounts of sulfur are mainly desorbed and released as SO ₂ and H ₂ S, the SO _x storage capacity of the SO _x storage catalyst being restored.

The use of a SO _x trap and a method for desulfating it is described in European patent application EP 0 625 633 A1. However, EP 0 625 633 A1 does not contain any information as to how the temperature increase in the exhaust gas required for desulfating the SO _x trap can be achieved in a targeted and simple manner.

The present invention deals with the problem of designing a method of the type mentioned at the outset such that the exhaust gas composition and exhaust gas temperature required for the desulfation of the SO _x storage catalyst can be provided using technically simple measures or devices.

According to the invention, this problem is solved by a method solved the features of claim 1.

The invention is based on the general idea of using the engine control system to influence the exhaust gas composition in such a way that they have a reducing atmosphere which can release the SO _x compounds in the SO _x storage catalytic converter. The high exhaust gas temperature also required for this is achieved in a simple manner — with the aid of the proposed supply of secondary air into the exhaust line, after the engine and before the SO _x storage catalytic converter. Here, the knowledge is exploited that the exhaust gas enriched with reducing agents contains a high chemical energy, which can be converted into thermal energy by supplying oxygen by means of appropriate chemical reactions. The oxygen required for this is provided with the secondary air. In the SO _x storage catalyst a catalytic combustion can take place of a portion of the entrained in the exhaust gas reducing agent with oxygen in the secondary air, liberated in the thermal energy, and is preferably transferred to the surface material of the SO _x storage catalyst. The high temperature in the SO _x storage catalyst required for the sulfate decomposition can thus be generated by this chemical reaction in the SO _x storage catalyst itself and therefore does not require an additional energy source.

An atmosphere containing reducing agent is in the exhaust gas provided in a simple manner by the engine control from lean operation to rich operation of the internal combustion engine is converted.

In order to be able to achieve optimal desulfation, a temperature of more than 550 ° C. is preferably set in the SO _x storage catalyst.

In order to be able to achieve such a high temperature in the SO _x storage catalyst and to achieve an optimal composition of the exhaust gases for the desulfation of the SO _x storage catalyst, the combustion / air ratio of the exhaust gases mixed with the secondary air is from a range of λ = 0.75 to λ = 0.99 selected.

According to a preferred embodiment of the method according to the invention, the setting of these preferred values for the combustion-air ratio of the exhaust gas mixed with secondary air and for the temperature prevailing in the SO _x storage catalytic converter is achieved by the amount of secondary air supplied by the engine control during the desulfurization and / or the combustion air ratio of the exhaust gases coming from the engine is influenced or varied. The proposed measures make it possible to regulate or control the parameters which are characteristic of the course of the desulfation in a simple manner.

In an exhaust gas purification device in which the SO _x storage catalyst is arranged in the exhaust line upstream of the NO _x storage catalyst, reach the released during the desulfurization of the SO _x storage catalyst sulfur compounds in the NO _x storage catalyst and can there form connections with the NO _x storage material and form sulfates. As a result, the NO _x storage capacity of the NO _x storage catalytic converter is reduced.

The problem thus arises of carrying out the desulfation of the SO _x storage catalyst in such a way that the storage capacity of the NO _x storage catalyst is not impaired. This is made possible by the fact that a bypass that bypasses the NO _x storage catalytic converter is provided in the exhaust line and is activated by the engine control during the desulphation. With the help of this bypass, the exhaust gases loaded with sulfur compounds are guided past the NO _x storage catalytic converter during desulfation, so that no sulfate formation can occur in the NO _x storage catalytic converter.

In another particularly advantageous embodiment of the method according to the invention, the adsorption of sulfur compounds in the NO _x storage catalytic converter during the desulfation of the SO _x storage catalytic converter can also be prevented by switching the lean mode to the rich mode of the internal combustion engine a regeneration of the NO _x storage catalytic converter is first carried out, the engine control monitoring a parameter correlating with the degree of regeneration of the NO _x storage catalytic converter and only initiating the supply of secondary air into the exhaust gas line when a predetermined threshold value for this parameter has been reached. Through this upstream regeneration phase, the amounts of oxygen and nitrates stored in the SO _x storage catalytic converter and in the NO _x storage catalytic converter are converted with the aid of the reducing agent emitted by the engine during rich operation. The two catalysts (SO _x and NO _x storage catalyst) are thereby brought into a reduced state in which - apart from the sulfates in the SO _x storage catalyst - there are approximately no more oxygen-containing atoms or molecules in the catalysts. After such regeneration, in particular of the NO _x storage catalytic converter, the actual desulfation of the SO _x storage catalytic converter can then take place by supplying secondary air. In this immediately downstream desulphation, the sulfur compounds adsorbed and stored during lean operation are desorbed and released from the SO _x storage catalyst. The released sulfur compounds can flow through the reduced NO _x storage catalyst without the sulfur compounds being adsorbed or stored. Sulfur poisoning or sulfation of the NO _x storage catalytic converter can thus be prevented during the desulfation of the upstream SO _x storage catalytic converter, and only through the choice of a particularly skillful sequence of the control processes. An exhaust gas purification device operating according to this method has few moving components, is therefore robust, less prone to failure and inexpensive.

Fig.1

eine Prinzipskizze eines Verbrennungsmotors mit einer Abgasreinigungseinrichtung, die einen einen NO_x-Speicher-Katalysator umgehenden Bypass aufweist und mit zwei Schließorganen ausgestattet ist,

Fig. 2

eine Prinzipskizze eines Verbrennungsmotors mit einer Abgasreinigungseinrichtung wie in Fig. 1, jedoch mit nur einem Schließorgan, und

Fig. 3

eine Prinzipskizze eines Verbrennungsmotors mit einer Abgasreinigungseinrichtung wie in den Fig. 1 und 2, jedoch ohne Bypass.

Further important features and advantages result from the subclaims, from the drawings and from the following description of preferred exemplary embodiments with reference to the drawings. Each shows schematically

Fig. 1

1 shows a schematic diagram of an internal combustion engine with an exhaust gas cleaning device which has a bypass that bypasses a NO _x storage catalytic converter and is equipped with two closing elements,

Fig. 2

a schematic diagram of an internal combustion engine with an exhaust gas purification device as in Fig. 1, but with only one closing member, and

Fig. 3

a schematic diagram of an internal combustion engine with an exhaust gas purification device as in FIGS. 1 and 2, but without bypass.

1 to 3, an internal combustion engine 1, which can be both a diesel and a petrol engine, Air via an electronically or electrically adjustable Throttle valve 2 supplied. The throttle valve 2 is included an electronic engine control 3 connected via a Computer, a memory with data and corresponding programs disposes.

The exhaust gases formed by the engine 1 during combustion occur into an exhaust line 4 of an exhaust gas purification device 5 of the Motors 1 on. Here, the exhaust line 4 is shown in FIG Embodiment already in the outlet area of the exhaust gases from the internal combustion engine 1, a secondary air supply 6 is connected, the one controlled by the engine control 3 Secondary air pump 7 secondary air in the exhaust line 4 for Mixing with the exhaust gases can bring.

After the connection points of the secondary air supply 6 to the exhaust line 4, a λ probe 8 is arranged in the exhaust line, which is connected to the engine control 3. A SO _x storage catalytic converter 9, which is preferably designed as a SO _x trap, is arranged in the exhaust line 4 after the λ probe 8.

After the SO _x storage catalytic converter 9, a temperature sensor 10 connected to the engine control 3 is arranged in the exhaust line 4. The temperature sensor 10 measures a temperature that correlates with the temperature prevailing in the SO _x storage catalytic converter 9.

In the embodiment according to FIG. 1, the exhaust line 4 branches off in its further course, a NO _x storage catalytic converter 11 being arranged in a first partial line 4a. In front of the NO _x storage catalytic converter 11, a closing element 12 designed as an exhaust gas flap is arranged in this first branch line 4a, which is connected to the engine control 3 and can be adjusted between a passage position and a blocking position by this.

A second partial line 4b formed after the branching forms a bypass 13 bypassing the NO _x storage catalytic converter 11. In this bypass 13 there is also a closing element 14 designed as an exhaust flap, which is also connected to the engine control 3 and between an open position and a blocked position is adjustable.

The sub-lines 4a and 4b of the exhaust line 4 are brought together again to form a common exhaust line 4 after the NO _x storage catalytic converter 11.

The method proposed according to the invention works as follows:

The engine control 3 monitors the storage capacity of the SO _x storage catalytic converter 9 and determines when regeneration of the SO _x storage catalytic converter is required. In order to be able to determine the current storage capacity of the SO _x storage catalytic converter 9, it can be provided that sensors, not shown here, are arranged in the SO _x storage catalytic converter 9 or in the exhaust line 4, which sensors, for example, show an increase in the content of sulfur compounds in the exhaust gas or detect another parameter that correlates with the SO _x storage capacity. It is also possible to determine the current storage capacity of the SO _x storage catalytic converter 9 on the basis of maps stored in a corresponding storage, in which, for example, the SO _x storage capacity is dependent on the operating time of the internal combustion engine 1 and the sulfur content of the Exhaust gases coming from engine 1 are stored.

After the engine control 3 has determined that the SO _x storage capacity has dropped to or below a predetermined threshold value, it influences the operating behavior of the internal combustion engine 1 in that it is switched from a lean operation to a rich operation. It can be provided that any change in the engine power, in particular the engine torque, which occurs during the changeover between the two operating modes (lean or rich) is compensated, for example, by a corresponding change in the position of the throttle valve 2, so that the driver makes the change between the operating modes.

With the change to rich engine operation or to it in time the secondary air pump 7 is activated with a delay, so that secondary air is blown into the exhaust line 4. Mixed up the exhaust gas coming from engine 1 with the secondary air. Due to the substoichiometric in fat mode Combustion with λ <1 are the exhaust gases coming from engine 1 loaded with reducing agent. By supplying secondary air the exhaust gases are also enriched with oxygen.

With the help of the λ probe 8, the engine control 3 measures the current λ value upstream of the SO _x storage catalytic converter 9, that is to say the combustion air ratio of the exhaust gases mixed with the secondary air. In order to be able to set a predetermined λ value of the exhaust gases, in which an optimal sequence of the desulfation of the SO _x storage catalytic converter 9 can be ensured, the engine control 3 influences the exhaust gas composition. According to the invention, several options are proposed for this:

According to the first possibility for influencing the λ value of the exhaust gases supplied to the SO _x storage catalytic converter 9, the amount of secondary air supplied is controlled by a corresponding control of the secondary air supply, with the combustion-air ratio of the exhaust gases coming from the richly operated engine 1 remaining constant 6 or their secondary air pump 7 varies.

According to a second possibility for influencing the exhaust gas composition before it enters the SO _x storage catalytic converter 9, the combustion air ratio of the exhaust gases generated by the engine 1 can be varied by the engine control 3, with the amount of secondary air supplied remaining constant, by the engine control 3 engages in the operation of the engine 1.

In a third possibility for influencing the λ value of the exhaust gases supplied to the SO _x storage catalytic converter 9, the abovementioned possibilities are combined, that is to say both the combustion air ratio of the exhaust gases generated by the engine 1 and the amount of secondary air supplied are controlled by the engine controller 3 appropriately influenced.

The combustion air ratio aimed for desulfation of the SO _x storage catalytic converter 9 is preferably selected from a range from λ = 0.75 to λ = 0.99.

The exhaust gases entering the SO _x storage catalytic converter 9 have a high content of reducing agents (for example CO, H ₂ , HC). In addition, these exhaust gases are enriched with oxygen after the secondary air supply 6, so that in the SO _x storage catalytic converter 9 catalytic combustion can take place. In this reaction, the chemical energy stored in the reducing agents is converted into thermal energy by oxidation. In this way, the SO _x storage catalyst 9 is heated and can reach an optimal temperature for the desulfation.

With the aid of the temperature sensor 10, the heating of the SO _x storage catalytic converter 9 is monitored. This heating of the SO _x storage catalytic converter 9 can be regulated by influencing the combustion air ratio of the exhaust gases supplied to the SO _x storage catalytic converter 9. The engine control 3 regulates or sets in the SO _x storage catalytic converter 9 with the aid of the temperature sensor 10 an optimum temperature for desulfurization, preferably more than 550 ° C. In addition, the temperature sensor 10 enables effective protection against overheating of the SO _x storage -Catalyst 9 or the other components of the exhaust gas purification device 5.

During the normal operating phase of the internal combustion engine 1 or its exhaust gas purification device 5, in which sulfur compounds are adsorbed and stored in the SO _x storage catalytic converter 9, the exhaust gas flap 14 of the bypass 13 is closed, while the exhaust gas flap 12 in which the NO _x storage Partial line 4a of the exhaust line 4 containing catalyst 11 is opened. The exhaust gases cleaned of sulfur compounds flow through the NO _x storage catalytic converter 11 and are freed of nitrogen oxides (NO _x ) there.

During the desulfation, simultaneously with the activation of the secondary air supply 6 or with a time delay, the exhaust flap 12 is closed and the exhaust flap 14 is opened, so that the exhaust gases only flow through the bypass 13 while bypassing the NO _x storage catalytic converter 11. In this way it is ensured that the sulfur compounds released during the desulfation of the SO _x storage catalytic converter 9 cannot be transported into the NO _x storage catalytic converter 11 by the exhaust gas flow. Thus, sulfate formation in the NO _x storage catalytic converter 11 and consequently its poisoning or capacity reduction can be effectively prevented.

In order to avoid sulfur poisoning of the NO _x storage catalytic converter 11 during the desulfation of the SO _x storage catalytic converter 9, in contrast to the embodiment according to FIG. 1, in another construction of the exhaust gas purification device 5 according to FIG. 2, only one closing element 15 designed as an exhaust gas flap is required Provided, which is arranged in the bypass 13 and is adjustable via a connection to the engine control 3 by this between a through position and a blocking position. During normal operation of the internal combustion engine 1 or the exhaust gas cleaning system 5, the exhaust gas flap 15 is in its closed position, so that the sulfur-free exhaust gases have to flow through the NO _x storage catalytic converter 11. In contrast to this, the exhaust flap 15 is switched to pass during the regeneration phase or desulfation of the SO _x storage catalytic converter 9. Although two flow paths are possible in this embodiment according to FIG. 2 when the exhaust flap 15 is open, namely through partial line 4a and through partial line 4b, the exhaust line 4 is designed in terms of flow in this area such that when the exhaust gas flap 15 is open, the exhaust gases are only or at least largely flow through the bypass 13 and no sulfur-containing exhaust gases or only negligibly small proportions flow through the NO _x storage catalytic converter 11. This is achieved, for example, by increasing the flow resistance in sub-branch 4a, for example by means of a throttle point. 2 is due to its design with only one exhaust flap 15 cheaper and less prone to failure than the embodiment according to FIG. 1.

According to FIG. 3, in another variant, protection of the NO _x storage catalytic converter 11 from sulfur poisoning during the desulfation is achieved even without a bypass. This is made possible by the fact that, in such an exhaust gas purification device 5, the engine control 3 performs a regeneration of the NO _x storage catalytic converter 11 before the SO _x storage catalytic converter 9 is actually desulfated.

The entire desulfation process takes place in one arrangement 3 as follows:

After the engine controller 3 has determined that the SO _x storage capacity of the SO _x storage catalytic converter 9 has dropped to or below a predetermined threshold value, it causes a change from lean to - as in the embodiments according to FIGS. Operation on rich operation of the internal combustion engine 1, but in this case without activating the secondary air supply 6. The internal combustion engine 1 then generates exhaust gases with a relatively high reducing agent content, which trigger a reduction reaction in the NO _x storage catalytic converter 11, in which the nitrogen oxides adsorbed in the NO _x storage catalytic converter 11 are reduced and in the form of harmless compounds such as N ₂ , CO ₂ , H ₂ O are released. The regeneration of the NO _x storage catalytic converter 11 brings it into a reduced state in which there are no longer any oxygen-containing species in the NO _x storage catalytic converter 11.

During this regeneration of the _NOx storage catalyst 11 and the SO _x storage catalytic converter 9 by flowed from the reductive exhaust gases of the fat-fueled internal combustion engine 1, so that even in the SO _x storage catalytic converter 9 can take place a reduction in which, apart from the sulfur oxide compounds (SO _x ), releases oxygen-containing compounds.

The end of the regeneration process for the NO _x storage catalytic converter 11 is determined by the engine control 3. For example, the regeneration process takes place on the basis of parameters stored in characteristic diagrams or with the aid of an additional sensor 16 arranged in the exhaust line 4 after the NO _x storage catalytic converter 11. This sensor 16 is connected to the engine control 3 and, according to a preferred embodiment, can be used as a λ probe be trained. The end of the regeneration phase can be detected by the sensor 16, for example, in that the reducing agents contained in the exhaust gas flow through the NO _x storage catalytic converter 11 unchanged to an increasing extent.

After the regeneration phase of the NO _x storage catalytic converter 11 has ended, the actual desulfation of the SO _x storage catalytic converter 9 begins, in which secondary air is introduced into the exhaust gases coming from the engine 1 with the aid of the secondary air supply 6. With the aid of the combustion air ratio upstream of the SO _x storage catalytic converter 9, the optimal conditions for the desulphation are set or regulated by the engine control 3. It is entirely possible that rich operation with a different λ value is set for the regeneration of the NO _x storage catalytic converter 11 than for the desulfation of the SO _x storage catalytic converter 9.

The sulfur compounds released during the desulfation are fed to the NO _x storage catalytic converter 11 by the exhaust gas flow. However, since the latter is in a reduced state, the sulfur compounds contained in the exhaust gas cannot be adsorbed and stored by its adsorber material, so that the sulfur compounds flow through the NO _x storage catalytic converter 11 unchanged. With the help of this skillful control process proposed according to the invention, sulfation or sulfur poisoning of the NO _x storage catalytic converter 11 can thus be effectively avoided during the desulfating of the upstream SO _x storage catalytic converter 9.

An exhaust gas purification device 5 corresponding to FIG. 3 has compared to the previously described embodiments 1 and 2 no exhaust flaps, so that the Overall structure of the exhaust gas purification device 5 considerably more robust and less susceptible to faults, therefore easy to maintain and overall is inexpensive.

The end of the desulfation of the SO _x storage catalytic converter 9 is determined in all the exemplary embodiments shown by the engine control 3, for example on the basis of parameters stored in characteristic maps. Additionally or alternatively, according to FIG. 3, a further sensor 17 can be located between the SO _x storage catalytic converter 9 and the NO _x storage catalytic converter 11 in the exhaust line 4, in particular in the examples according to FIGS. 1 and 2 before the bypass 13 be arranged, which is connected to the engine controller 3. This sensor 17 can, for example, detect a decrease in released sulfur compounds in the exhaust gases or, in another embodiment, can be designed as a λ probe and monitor the combustion air ratio of the exhaust gases after the SO _x storage catalytic converter 9.

Claims (17)

1. A process for purifying the exhaust emissions of an internal combustion engine with a engine control system which permits the internal combustion engine to switch between lean operation and rich operation and an exhaust emission control system in which a λ-sensor, a SO_x storage catalytic converter and a NO_x storage catalytic converter are positioned one after another in the exhaust train,
   characterised by the following stages:

   A

   a parameter correlating with the current SO_x storage capacity of the SO_x storage catalytic converter (9) is generated by means of a sensory mechanism connected to the engine control system (3),

   B

   when the engine control system (3) detects a reduction in SO_x storage capacity to below a pre-set parameter value, it starts desulphatisation of the SO_x storage catalytic converter by initiating a switch in the internal combustion engine from lean operation to rich operation,

   C

   secondary air is introduced into the exhaust train (4) after the engine (1) and before the λ-sensor (8) by means of a controlled secondary air feed system,

   D

   the current combustion air ratio of the exhaust emissions mixed with secondary air is detected by the λ-sensor (8) and set to a pre-determined value by the engine control system (3),

   E

   a signal value correlating with the temperature prevailing in the SO_x storage catalytic converter (9) is generated by means of a temperature sensor (10), with the temperature prevailing in the SO_x storage catalytic converter (9) being set to achieve a pre-determined signal value by the engine control system (3),

   F

   upon reaching a pre-determined threshold value for the parameter correlating with the SO_x storage capacity of the SO_x storage catalytic converter (9) the engine control system (3) ends desulphatisation by initiating a switch in the internal combustion engine (1) from rich operation to lean operation.

2. A process in accordance with claim 1,
   characterised in that
   during the desulphatisation process in order to set the pre-determined value for the combustion air ratio of the exhaust emissions mixed with the secondary air and for the temperature prevailing in the SO_x storage catalytic converter (9), the engine control system (3) varies the amount of secondary air supplied and/or the combustion air ratio of the exhaust emissions coming from the engine (1).
3. A process in accordance with claim I or 2,
   characterised in that
   a bypass (13) avoiding the NO_x storage catalytic converter (11) is provided in the exhaust train (4) and is activated during the desulphatisation process by the engine control system (3).
4. A process in accordance with claim 3,
   characterised in that
   means (12, 14) are provided to guide the flow of exhaust emissions which guide the emissions through the bypass (13) and block the supply to the NO_x storage catalytic converter (11) when the desulphatisation process is active and guide the emissions through the NO_x storage catalytic converter (11) and block any flow through the bypass (13) when the desulphatisation process is deactivated
5. A process in accordance with claim 4,
   characterised in that
   a switch positioned in a fork of the exhaust train (4) into the NO_x storage catalytic converter (11) and the bypass (13) is provided as the means of guiding the flow of exhaust emissions.
6. A process in accordance with claim 4,
   characterised in that
   a first closing member (12) is provided in the feed flow to the NO_x storage catalytic converter (11) and a second closing member (14) is provided in the bypass (13) as the means of guiding the flow of exhaust emissions, with the closing members (12, 14) being switched altemately to open and closed.
7. A process in accordance with claim 3,
   characterised in that
   a closing member (15) is positioned in the bypass (13) and that the arrangement of the bypass (13) and the NO_x storage catalytic converter (11) in the exhaust train (4) is designed in terms of flow technology in such a way that when the closing member (15) is switched to open the exhaust emissions flow exclusively or essentially through the bypass (13).
8. A process in accordance with claim 1 or 2,
   characterised in that
   once stage B is complete, a regeneration of the NO_x storage catalytic converter (11) is carried out with the engine control system (3) monitoring a parameter which correlates with the degree of regeneration of the NO_x storage catalytic converter (11) and not initiating the start of stage C until a pre-determined threshold value for this parameter is reached.
9. A process in accordance with claim 8,
   characterised in that
   in order to detect the parameter correlating with the degree of regeneration of the NO_x storage catalytic converter (11), a sensor (16), in particular a λ-sensor, is provided which is positioned after the NO_x storage catalytic converter (11) in the exhaust train (4).
10. A process in accordance with one of the preceding claims,
    characteri sed in that
    the parameter correlating with the SO_x storage capacity of the SO_x storage catalytic converter (9) is calculated by means of a characteristic diagram dependent upon the period of operation of the internal combustion engine (1) and the composition of the exhaust emissions coming from the engine (1).
11. A process in accordance with one of the preceding claims,
    characterised in that
    in order to detect the parameter correlating with the SO_x storage capacity of the SO_x storage catalytic converter (9) a sensor (17), in particular aλ-sensor, is positioned between the SO_x storage catalytic converter (9) and the NO_x storage catalytic converter (11) in the exhaust train (4).
12. A process in accordance with one of the preceding claims,
    characterised in that
    the value of the combustion air ratio of the exhaust emissions mixed with secondary air pre-determined for the desulphatisation process is selected from a range of between λ = 0.75 and λ= 0.99.
13. A process in accordance with one of the preceding claims,
    characteri sed in that
    the value of the temperature prevailing in the SO_x storage catalytic converter (9) predetermined for the desulphatisation process corresponds to a temperature of more than 550°C.
14. A process in accordance with one of the preceding claims,
    characterised in that
    during the switch in the internal combustion engine (1) from lean operation to rich operation the engine control system (3) varies a supply of air to the internal combustion engine (1) in order to generate constant engine torque or constant engine output by means of a controlled throttle valve.
15. A device for purifying exhaust emissions in an internal combustion engine (1) with an engine control system (3) which permits a switch in the internal combustion engine (1) between lean operation and rich operation in which a secondary air feed (6), a λ-sensor (8), a SO_x storage catalytic converter (9), a temperature sensor (10) and a NO_x storage catalytic converter (11) are positioned one after the other after the engine in an exhaust train (4).
16. A device in accordance with claim 15,
    characterised in that
    a bypass (13) to avoid the NO_x storage catalytic converter (11) is provided in the exhaust train (4).
17. A device in accordance with claim 16,
    characterised in that
    means (12, 14) are provided in the exhaust train (4) to guide the flow of exhaust emissions and which essentially guide the flow of exhaust emissions either through the NO_x storage catalytic converter (11) or through the bypass (13).

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