GB2383004A

GB2383004A - An exhaust gas cleaning apparatus

Info

Publication number: GB2383004A
Application number: GB0223869A
Authority: GB
Inventors: Josef Guenther; Dirk Voigtlaender; Marko Weirich
Original assignee: DaimlerChrysler AG
Current assignee: Mercedes Benz Group AG
Priority date: 2001-10-23
Filing date: 2002-10-14
Publication date: 2003-06-18
Anticipated expiration: 2022-10-14
Also published as: GB2383004B; GB0223869D0; FR2831077B1; DE10152187A1; FR2831077A1

Abstract

An exhaust-gas cleaning apparatus for cleaning the exhaust gas from an internal combustion engine which is operated predominantly in lean-burn mode, in particular in a motor vehicle, having an exhaust pipe, in which a three-way catalytic converter 2, a nitrogen oxide storage catalytic converter 3 and an SCR catalytic converter 4 are arranged in series in the direction of flow, has a bypass line 5, which can be used to feed exhaust gas which flows out of the three-way catalytic converter 2 to the SCR catalytic converter 4, bypassing the nitrogen oxide storage catalytic converter 3. It is provided that an exhaust-gas part-stream of the exhaust gas which flows out of the three-way catalytic converter is fed via a bypass line to the SCR catalytic converter, bypassing the nitrogen oxide storage catalytic converter, the exhaust-gas part-stream being adjusted as a function of a temperature of the exhaust-gas cleaning apparatus.

Description

1 2383004

Exhaust-gas cleaning apparatus The present invention relates to an exhaust-gas cleaning apparatus for exhaust gases, particularly but not exclusively of an internal combustion engine, having a nitrogen oxide storage catalytic converter and SCR catalytic converter, and to a method for reducing the levels of nitrogen oxides in the exhaust-gas.

An exhaust-gas installation in which a nitrogen oxide storage catalytic converter is arranged between a three-way catalytic converter and an SCR catalytic converter in the exhaust pipe of an internal combustion engine, is known from EP 0 802 315 A2. The operation of the internal combustion engine alternates between lean and rich. In the lean periods of the alternating lean/rich operation, the nitrogen oxides (NOx) emitted by the internal combustion engine flow through the three-way catalytic converter and are for the most part stored in the nitrogen oxide storage catalytic converter. A proportion of the NOx, which is dependent on the degree of saturation of the nitrogen oxide storage catalytic converter, flows into the SCR catalytic converter and is reacted with ammonia (NH3), which has previously been stored there, to form nitrogen (N2). During the rich periods of the alternating lean/rich operation, the NOx emitted by the internal combustion engine are for the most part reduced to form NH3 in the three-way catalytic converter. This NH3, like the other reducing agents which are also present in the exhaust gas (carbon monoxide (CO), hydrogen (H2) or hydrocarbons (HC)), reduces the NOx stored in the nitrogen oxide storage catalytic converter to form N2. Excess NH3 flows onwards into the SCR catalytic converter, where it is stored. In this way, NOX can be removed from the exhaust gas emitted by an internal combustion engine which is alternately operated in lean and rich mode.

Since the lean periods of the internal combustion engine typically last much longer than the rich periods and the internal combustion engine can be operated so as to reduce fuel consumption in the lean periods, the overall result is internal combustion engine operation with a low fuel consumption combined, at the same time, with effective exhaust-gas cleaning.

However, the above-described effect of the nitrogen oxide storage catalytic converter and of the SCR catalytic converter is restricted to a material-dependent temperature range. The cleaning action of the nitrogen oxide storage catalytic converter and/or the SCR catalytic converter drops substantially above and below an optimum temperature range (temperature window) of typically 250 C to 420 C and 200 C to 400 C, respectively. If the cleaning action of the said catalysts is no longer present, in particular on account of the respective temperature window being exceeded, to avoid high levels of NOX emissions, the internal combustion engine has to be operated with a stoichiometric air/fuel mix (1 = 1). In the process, the exhaust -gas is cleaned by the three-way catalytic converter, which under these conditions has a good cleaning action even at elevated temperatures. In this case, however, the consumption benefits associated with alternating lean/rich operation are lost. Since the temperature of the catalytic converters is determined predominantly by the exhaust-gas temperature, and the latter is dependent on the load range of the internal combustion engine, in the exhaust-gas cleaning installation which has been described, therefore, the consumption-

saving alternating leanlrich operation is also limited to a certain load range of the internal combustion engine.

By contrast, the present invention seeks to provide an exhaust-gas cleaning installation and a method with which effective lowering of the levels of nitrogen oxides is achieved in a widened load range of an internal combustion engine which is operated predominantly in lean-burn mode.

According to the invention there is provided an exhaust gas cleaning apparatus for cleaning the exhaust gas from an internal combustion engine which is operated predominantly in lean-burn mode, having an exhaust pipe, in which a three-way catalytic converter, a nitrogen oxide storage catalytic converter and an SCR catalytic converter are arranged in series in the direction of flow, wherein the exhaust-gas cleaning apparatus has a bypass line, through which exhaust gas which flows out of the three-way catalytic converter to the SCR catalytic converter, bypassing the nitrogen oxide storage catalytic converter.

The present invention also provides a method for operating an exhaust-gas cleaning apparatus for cleaning the exhaust gas from an internal combustion engine optionally

operable in a lean combustion mode or in a rich combustion mode or a stoichiometric combustion mode and has an intake-air throttling element, and having - a three-way catalytic converter, - a nitrogen oxide storage catalytic converter which is connected downstream of the three-way catalytic converter, - and an SCR catalytic converter which is connected downstream of the nitrogen oxide storage catalytic converter, wherein an exhaust-gas part-stream of the exhaust gas which flows out of the threeway catalytic converter is fed via a bypass line to the SCR catalytic converter, bypassing the nitrogen oxide storage catalytic converter, the exhaust-gas part-stream being adjusted as a function of a temperature of the exhaust-gas cleaning apparatus.

According to the invention, the exhaust-gas cleaning device has a bypass line, which can be used to feed exhaust gas which flows out of the threeway catalytic converter to the SCR catalytic converter, bypassing the nitrogen oxide storage catalytic converter. In this context, the term three-way catalytic converter is to be understood as meaning a catalytic converter which is usually used to clean the exhaust gas from internal combustion engines which are operated with a stoichiometric (I = 1) air/fuel ratio.

Under these conditions, catalytic converters of this type catalyse both the oxidation of CO and HC and the reduction of NOx, and can therefore effectively clean the exhaust gas from internal combustion engines operated in this mode. Lean-burn operation of the internal combustion engine (air/fuel ratio I > 1) results in an oxidizing exhaust gas, i.e. an exhaust gas which has an excess of oxygen. Under these conditions, the oxydizable pollutants CO and HC are removed from the exhaust gas by the three-way catalytic converter, but the pollutant component NOx cannot be reduced. Conversely, in rich-

burn mode of the internal combustion engine (air/fuel ratio I < 1), the result is a reducing exhaust gas, i.e. an exhaust gas which has an excess of reducing agents. Under these conditions, CO and HC can only be removed from the exhaust gas to a very incomplete extent by the three-way catalytic converter, but the pollutant component NOX is reduced. The reduction products are N2 and/or NH3.

A nitrogen oxide storage catalytic converter is to be understood as meaning a catalytic converter which, in the case of oxydizing exhaust gas, collects and stores nitrogen oxides, releases them again under reducing conditions and, at the same time, at least

partially reduces them to form N2 and/or NH3. Furthermore, in the present context, as is customary, an SCR catalytic converter (SCR = selective catalytic reduction) is to be understood as meaning a catalytic converter which is able to store NH3 under reducing conditions and to reduce nitrogen oxides which are supplied under oxidizing exhaust-

gas conditions using stored or supplied NH3, predominantly to form harmless N2. The nitrogen oxide storage catalytic converter and SCR catalytic converter therefore serve primarily to remove nitrogen oxides when the associated internal combustion engine, in this case in particular a direct-injection spark-ignition engine, is in lean-burn mode. To enable this consumption-saving operation to take place within the widest possible load range, combined, at the same time, with an effective lowering of the levels of nitrogen oxides, therefore, optimum deployment of these catalytic converters is of particular importance. The inventive use of a bypass line enables the exhaust gas which flows out of the three- way catalytic converter if necessary to be passed completely or partially around the nitrogen oxide storage catalytic converter, so that only some or none of the exhaust gas acts on the nitrogen oxide storage catalytic converter. The bypass line does not include any exhaust-gas cleaning components, so that in these cases the exhaust-gas cleaning is carried out completely or partially by the three-way catalytic converter and the SCR catalytic converter. Therefore, depending on the operating conditions of the internal combustion engine, the exhaust-gas cleaning capacity of the catalytic converters which are connected downstream of the three-way catalytic converter can be utilized individually or together in an optimum manner.

In a preferred configuration of the invention, the proportion of the exhaust gas which flows out of the three-way catalytic converter which is formed by the exhaust gas which flows through the bypass line can be adjusted as a function of a temperature of the exhaust-gas cleaning device. This measure allows the further exhaust-gas cleaning which is required after the exhaust gas has flowed through the three-way catalytic converter can be distributed between the nitrogen oxide storage catalytic converter and the SCR catalytic converter according to the temperature. Therefore, an optimum exhaust-gas cleaning can be achieved within a wide temperature range or load range.

The location at which the temperature used to control the exhaust-gas bypassing is taken is selected appropriately according to the geometric configuration of the exhaust-gas cleaning installation and according to the design of the nitrogen oxide storage catalytic converter and SCR catalytic converter.

s In a further embodiment of the invention, the proportion of the exhaust gas which flows out of the three-way catalytic converter which is formed by the exhaust gas which flows through the bypass line can be adjusted as a function of a temperature of the nitrogen oxide storage catalytic converter. With this configuration of the invention, it is possible, for example when the temperature of the nitrogen oxide storage catalytic converter approaches the upper limit of its temperature window, to take account of the associated fall in its cleaning action. In this case, the nitrogen oxide storage catalytic converter is exposed to a lower flow of exhaust gas in accordance with its lower performance, and the cleaning capacity of the SCR catalytic converter arranged further downstream is utilized to a greater extent. It is preferable for the temperature of the nitrogen oxide storage catalytic converter to be determined by direct measurement at a suitable location, but it can also be determined from a temperature model by calculation on the basis of the current internal combustion engine operation.

In a further configuration of the invention, above a predeterminable threshold value TS I for a temperature T1 of the nitrogen oxide storage catalytic converter, the proportion of the exhaust gas which flows out of the three-way catalytic converter which is formed by the exhaust gas which flows through the bypass line is at least approximately 100%.

Therefore, above the threshold temperature TS I, scarcely any further exhaust gas is fed to the nitrogen oxide storage catalytic converter, with the result that, for example, overheating of and damage to this catalytic converter are avoided.

The method according to the invention is distinguished by the fact that an exhaust-gas part-stream of the exhaust gas which flows out of the three-way catalytic converter is fed via a bypass line to the SCR catalytic converter, bypassing the nitrogen oxide storage catalytic converter, the exhaust-gas part-stream being adjusted as a function of a temperature of the exhaust-gas cleaning installation. This inventive procedure allows the distribution of the exhaust-gas stream to be matched to the temperature in the exhaust-gas cleaning installation and therefore makes it possible to react to a cleaning capacity on the part of the nitrogen oxide storage catalytic converter and SCR catalytic converter which differs as a function of temperature. The overall result is improved reduction in the levels of nitrogen oxides over a widened load range of the internal combustion engine.

In one embodiment of the method according to the invention, if a temperature T1 of the nitrogen oxide storage catalytic converter exceeds an associated predeterminable threshold value TS1, the exhaust gas which flows out of the three-way catalytic converter is fed to the SCR catalytic converter completely or almost completely via the bypass line. As a result of this measure, when the upper threshold temperature TS1 of the nitrogen oxide storage catalytic converter is reached, this converter is disconnected from the further supply of gas and heat. This on the one hand avoids overheating of the nitrogen oxide storage catalytic converter and on the other hand results in the catalytic converter quickly returning to the temperature range of its full efficiency as a result of heat being released to the environment, so that it is correspondingly quickly available again for exhaust-gas cleaning.

In a further embodiment of the method according to the invention, if the temperature T1 of the nitrogen oxide storage catalytic converter exceeds the threshold value TS1 and a temperature T2 of the SCR catalytic converter exceeds an associated predeterminable threshold value TS2, the internal combustion engine is operated exclusively in rich or stoichiometric combustion mode. Ending the alternating lean/rich operation at high exhaust-gas temperatures in accordance with the temperature threshold values TS1 and TS2 counteracts the associated deterioration in the cleaning action of the nitrogen oxide storage catalytic converter and of the SCR catalytic converter and therefore an undesirable increase in the NOx emissions. With the transition to the rich or stoichiometric combustion mode under these conditions, the exhaust-gas cleaning is performed almost exclusively by the three-way catalytic converter, which is fully effective even at high exhaust temperatures.

In a further configuration of the method according to the invention, if a temperature T1 of the nitrogen oxide storage catalytic converter exceeds the threshold value TS 1 and a temperature T2 of the SCR catalytic converter exceeds an associated predeterminable threshold value TS2, when the internal combustion engine is in overrun mode, the intake-air throttling element is opened and the exhaust gas which flows out the three-

way catalytic converter is fed completely or almost completely to the nitrogen oxide storage catalytic converter. Opening the throttling element in the intake line in over run mode results in ambient air passing through the internal combustion engine according to

the current rotational speed. As a result, the fresh air which is sucked in passes at relatively low temperature into the exhaust-temperature cleaning installation and cools the catalytic converters. The latter are then quickly available again for exhaust-gas cleaning. In the text which follows, the invention is explained in more detail with reference to drawings and associated examples. In the drawings: Fig. 1 shows a diagrammatic block diagram of an exhaust-gas cleaning installation, Fig. 2 shows a further diagrammatic block diagram of an exhaust-gas cleaning installation. The exhaust-gas cleaning installation which is diagrammatically depicted in Fig. 1 is used to clean the exhaust gas from an internal-combustion engine which is not shown here, as is used, for example in a motor vehicle and is designed, for example, in the form of a direct-injection spark-ignition engine. In an exhaust pipe 1, the exhaustgas cleaning installation has a three-way catalytic converter 2 and, downstream of this, a nitrogen oxide storage catalytic converter 3 and a SCR catalytic converter 4. The catalytic converters are preferably designed as honeycomb bodies. The direction in which the exhaust gas emitted by the internal combustion engine flows in is indicated by the arrow in the drawing. Furthermore, between the three-way catalytic converter 2 and the nitrogen oxide storage catalytic converter 3, the exhaust pipe 1 has a branch leading to a bypass line 5 which leads back into the exhaust pipe 1 between nitrogen oxide storage catalytic converter 3 and SCR catalytic converter 4. Therefore, the exhaust gas which flows out of the three-way catalytic converter 2 can be fed to the SCR catalytic converter 4, bypassing the nitrogen oxide storage catalytic converter 3, by means of the bypass line 5. The temperature of the nitrogen oxide storage catalytic converter 3 and/or of the SCR catalytic converter 4 can be recorded by temperature-

measuring points which are denoted by T1 and by T2. Furthermore, a control member 6, which is designed as a throttle valve and is used to throttle the exhaust-gas stream flowing into the nitrogen oxide storage catalytic converter 3 and/or to distribute the exhaust-gas stream to the bypass line 5 and the exhaust pipe 1 further downstream of the branch, is arranged in the exhaust pipe I downstream of the branch. Lee

temperature-measuring points T1 and T2 are arranged in such a way that the temperature of the nitrogen oxide storage catalytic converter 3 and of the SCR catalytic converter 4 can be recorded reliably. This can be achieved by arranging them at the entry to each catalytic converter or directly in the catalytic converter. The temperature-

measuring points T1, T2 and the control member 6 are connected via signal lines to a control unit (likewise not shown here). The control unit also holds information about the operating state of the internal combustion engine. Linking this information to the values of recorded temperatures T1, T2 results in the operation of the internal combustion engine and the position of the control member 6 being influenced by means of the functionality implemented in the control unit. In particular, the position of the throttle valve 6 can be set as a function of the temperature T}, T2 and the operating state of the internal combustion engine. It will be understood that the exhaust-gas cleaning installation may include further components, such as for example exhaust-gas sensors, which are not of primary interest in the present context and are likewise not illustrated for reasons of clarity.

The exemplary embodiment illustrated in Fig. 1 shows the control member or the throttle valve 6 in the fully open state. In this position, the exhaust gas flows completely or almost completely through the nitrogen oxide storage catalytic converter 3 and the downstream SCR catalytic converter 4. This is preferably achieved by means of a suitable flow configuration of the bypass line 5 and/or the branch located between three-

way catalytic converter 2 and nitrogen oxide storage catalytic converter 3, but may also be effected by an additional blocking member in the bypass line 5. The setting which is shown corresponds to the standard operating state of the internal combustion engine, in which the latter is operated alternately in lean-burn and rich-burn modes. Both the nitrogen oxide storage catalytic converter 3 and the SCR catalytic converter 4 are at a temperature within their respective temperature windows and are fully effective.

The nitrogen oxide storage catalytic converter 3 typically has a temperature window of approximately 250 C to 420 C, and the SCR catalytic converter 4 has a temperature window of approximately 200 C to 400 C. Since the SCR catalytic converter 4 is arranged a certain distance downstream of the nitrogen oxide storage catalytic converter 3, in normal driving mode there is a certain temperature drop between these catalytic converters. Therefore, the SCR catalytic converter remains fully effective even if, for

example on account of an increased engine load, the nitrogen oxide storage catalytic converter 4 has reached the upper limit of its temperature window and is therefore no longer operating optimally.

The catalytic coating of the three-way catalytic converter 2 shown in the Fig. 1 can be specially optimized, at least in a defined V range, within the reducing conditions of rich operation of the internal-combustion engine, to chemically reduce the NOx present in the exhaust gas primarily to form NH3. Suitable catalytic coatings are known to the person skilled in the art and therefore require no further explanation at the present point.

The NOx storage catalytic converter 3 is designed in the usual way, such that under the oxidizing conditions of lean operation of the internal combustion engine 1 it takes up the NOx present in the exhaust gas, primarily by chemical bonding as nitrate to the coating material, and releases it again under reducing conditions, converting it into nitrogen and/or NH3. In this case, too, the coating can be optimized in such a way the NH3 content is as high as possible. The result of this is that the downstream SCR catalytic converter 4 is supplied with sufficient NH3 during the rich operating phases.

The SCR catalytic converter 4 arranged downstream of the nitrogen oxide storage catalytic converter 3 is likewise of standard design, i.e. for example as a solid catalyst based on vanadium pentoxide or as a Pt-doped, coated zeolite-containing catalyst. As such it has the property, which is also known from power plant engineering, for example, of being able to store NH3 under reducing conditions and to make use of this stored NH3 under oxidizing conditions as a reaction partner in a selective catalytic reduction reaction so as to form nitrogen, in order to chemically reduce NOx.

The latter property is used in particular to render NOx harmless by means of the abovementioned selective reduction reactions. In the exhaust-gas cleaning installation illustrated in Fig. 1, an NOx supply to the SCR catalytic converter 4 results primarily from the increasing exhaustion of the NOx storage material during the NOx storage-

during lean operation of the internal combustion engine (increasing NOx slippage).

However, a precondition for NOx reduction by the SCR catalytic converter 4 is that corresponding quantities of NH3 have previously been made available for it to store.

This takes place in the rich phases of the alternating lean/rich operation as a result of the abovementioned formation of NH3 at the three- way catalytic converter 2 and at the nitrogen oxide storage catalytic converter 3. Depending on the demand for NH3, it may

be necessary to increase the formation of NH3 by increasing to a greater or lesser extent the relatively low formation of NOX which is customary during rich operation of the internal combustion engine. This is achieved by increasing the combustion temperatures in the combustion chamber, which in turn can be achieved, for example, by shifting the ignition angle to an earlier point in a spark-ignition internal combustion engine or by shifting the injection of fuel to an earlier point.

If the level of NOX slippage in lean operation of the internal combustion engine has risen to an unacceptable level, the NOx storage catalytic converter 3 is subjected to nitrate regeneration as a result of the provision of reducing agent. This is effected by switching the internal combustion engine from lean operation to rich operation.

Provided that the temperature of nitrogen oxide storage catalytic converter 3 and SCR catalytic converter 4 lies within the corresponding temperature windows, with the operating variant of the exhaust-cleaning installation according to the invention which is shown in Fig. 1 it is possible to achieve -effective removal of NOX from the exhaust gas from the internal combustion engine which is alternately operated in lean-burn and rich-

burn modes.

If the temperature T1 of the nitrogen oxide storage catalytic converter 3 rises, for example on account of an increased engine load, so that its efficiency is reduced, or the upper limit of the temperature window of, for example, 420 C of the nitrogen oxide storage catalytic converter 3 is reached or exceeded, the throttle valve 6 is partially or completely closed. Therefore, the supply of gas to the nitrogen oxide storage catalytic converter 3 is throttled or completely suppressed. The extent of throttling in this case preferably takes place in accordance with a pre-programmed, temperature-defined dependency. This can be effected by continuous or stepwise changing of the opening width set by the throttle valve 6. At elevated exhaust-gas temperatures, therefore, the nitrogen oxide storage catalytic converter 3 is acted on by a lower exhaust-gas stream, so that despite the elevated temperature it is able to clean NOx out of the exhaust-gas stream, which is now lower.

On the other hand, the NOX load on the SCR catalytic converter 4 is increased, since, as the temperature Tl rises, increasing amounts of NOxcontaining exhaust gas are fed to

it directly via the bypass line 5 on account of the increasing closure of the throttle valve 6. The fact that the SCR catalytic converter 4 is arranged downstream of the nitrogen oxide storage catalytic converter 3 means that the latter is naturally at a slightly lower temperature than the nitrogen oxide storage catalytic converter 3. Since, moreover, the exhaust gas which is passed via the bypass line 5 is cooled to a certain extent during this bypassing, the SCR catalytic converter 4 is still at operating temperature even if the upper limit of the temperature window of the nitrogen oxide storage catalytic converter 3 has been exceed to a considerable extent. Therefore, a good reduction in the levels of nitrogen oxides can be achieved during lean operation or alternating lean/rich operation within a wider load range of the internal combustion engine.

If the engine load or exhaust-gas temperature rises further, the throttle valve 6 is ultimately closed completely. This preferably takes place when the temperature T1 of the NOX storage catalytic converter 3 has reached an upper threshold value TS I of, for example, 420 C. The corresponding conditions are illustrated in Fig. 2. In this operating mode of the exhaust-gas cleaning installation, lean operation or alternating lean/rich operation of the internal combustion engine is still possible and appropriate, provided that the temperature T2 of the SCR catalytic converter 4 is still within the associated temperature window. However, the SCR catalytic converter 4 alone now has to deal with the removal of NOx from the exhaust gas. It may therefore be advantageous for the supply of NH3 in the rich operating phases of the internal combustion engine to be increased by increasing the untreated emissions of NOx from the internal combustion engine with subsequent reduction to form NH3 in the three-way catalytic converter 2 with the aid of the measures which have already been described.

If sufficient reduction in the levels of NOx is no longer possible as a result of the exhaust-gas temperature having risen excessively or as a result of an excessive load on the SCR catalytic converter 4, the internal combustion engine is switched over to stoichiometric or slightly rich operation. This switching may be coupled to a defined load characteristic diagram area of the internal combustion engine being reached or to the occurrence of specific limit values for the temperatures T1 and T2 of nitrogen oxide storage catalytic converter 3 and SCR catalytic converter 4. When a standard SCR

catalytic converter is being used, by way of example in the event of the limit value of TS2 = 400 C for the temperature T2 of the SCR catalytic converter 4 being exceeded, the engine can be switched to stoichiometric or slightly rich operation. The throttle valve 6 preferably remains closed in the associated load range of the internal combustion engine, in order to prevent the nitrogen oxide storage catalytic converter 3 from beingheated. Therefore, after a change to a characteristic diagram point with a lower engine load, the nitrogen oxide storage catalytic converter 3 is quickly available again for reducing the levels NOx.

If the internal combustion engine is switched from a high load range to overrun mode, this can be utilized to rapidly cool the catalytic converters. Load change situations of this nature occur, for example, when the motor vehicle is travelling downhill after it has previously been travelling uphill. To accelerate the cooling of the nitrogen oxide storage catalytic converter 3, for example in driving situations of this nature, in overrun mode the intake-air throttling element in the intake line of the internal combustion engine is opened and at the same time the throttle valve 6 is opened. Since in overrun mode of the internal combustion engine the supply of fuel is stopped (overrun cutoff), only moderately heated fresh air is passed to the catalytic converters. Therefore, opening the throttle valve in the intake section of the internal combustion engine and the throttle valve 6 in overrun mode results in correspondingly accelerated cooling of the catalytic converters. It will be understood that the abovementioned threshold values TS1 and TS2 are dependent on the catalytic converter technology used and may therefore differ from the values which have been mentioned. Furthermore, these threshold values are dependent on the installation position of the temperature measurement points T1 and T2 if the temperature of the nitrogen oxide storage catalytic converter 3 and of the SCR catalytic converter 4 are determined by measurement. Furthermore, it will be understood that, to control the bypassing of the exhaust gas around the nitrogen oxide storage catalytic converter 3 through the bypass line 5, there are other suitable options in addition to the use of a throttle element 6 between bypass-line branch and nitrogen oxide storage catalytic converter 3. By way of example, for this purpose, a throttle element may be fitted exclusively or additionally in the bypass line 5.

Claims

1. An exhaust gas cleaning apparatus for cleaning the exhaust gas from an internal combustion engine which is operated predominantly in lean-burn mode, having an exhaust pipe, in which a three-way catalytic converter, a nitrogen oxide storage catalytic converter and an SCR catalytic converter are arranged in series in the direction of flow, wherein the exhaust-gas cleaning apparatus has a bypass line, through which exhaust gas which flows out of the three-way catalytic converter to the SCR catalytic converter, bypassing the nitrogen oxide storage catalytic converter.

2. An exhaust gas cleaning apparatus according to Claim 1, wherein the proportion of the exhaust gas which flows out of the three-way catalytic converter which is formed by the exhaust gas which flows through the bypass line is adjustable as a function of a temperature of the apparatus.

3. An exhaust gas cleaning apparatus according to Claim 1 or 2, wherein the proportion of the exhaust gas which flows out of the three-way catalytic converter which is formed by the exhaust gas which flows through the bypass line is adjustable as a function of a temperature of the nitrogen oxide storage catalytic converter.

4. An exhaust gas cleaning apparatus according to any one of Claims 1 to 3, wherein, above a predeterminable threshold value TS1 for a temperature Tl of the nitrogen oxide storage catalytic converter, the proportion of the exhaust gas which flows out of the three-way catalytic converter which is formed by the exhaust gas which flows through the bypass line is at least approximately 100%.

5. An exhaust gas cleaning apparatus according to any one of Claims 1 to 4, adapted for use in a motor vehicle.

6. A method for operating an exhaust-gas cleaning apparatus for cleaning the exhaust gas from an internal combustion engine optionally operable in a lean combustion mode or in a rich combustion mode or a stoichiometric combustion mode and has an intake-air throttling element, and having

- a three-way catalytic converter, - a nitrogen oxide storage catalytic converter which is connected downstream of the three-way catalytic converter, - and an SCR catalytic converter which is connected downstream of the nitrogen oxide storage catalytic converter, wherein an exhaust-gas part-stream of the exhaust gas which flows out of the three-way catalytic converter is fed via a bypass line to the SCR catalytic converter, bypassing the nitrogen oxide storage catalytic converter, the exhaust-gas part-stream being adjusted as a function of a temperature of the exhaustgas cleaning apparatus.

7. A method according to Claim 6 wherein, if a temperature T1 of the nitrogen oxide storage catalytic converter exceeds an associated predeterminable threshold value TS1, the exhaust gas which flows out of the three-way catalytic converter is fed to the SCR catalytic converter completely or almost completely via the bypass line.

8. A method according to Claim 6 or 7, wherein, if a temperature T1 of the nitrogen oxide storage catalytic converter exceeds the threshold value TS1 and a temperature T2 of the SCR catalytic converter exceeds an associated predeterminable threshold value TS2, the internal combustion engine is operated exclusively in rich or stoichiometric combustion mode.

9. A method according to Claim 6 or 7, wherein, if a temperature T1 of the nitrogen oxide storage catalytic converter exceeds the threshold value TSI and a temperature T2 of the SCR catalytic converter exceeds an associated predeterminable threshold value TS2, when the internal combustion engine is in overrun mode, the intake-air throttling element is opened and the exhaust gas which flows out the three-

way catalytic converter is fed completely or almost completely to the nitrogen oxide storage catalytic converter.

10. An exhaust gas cleaning apparatus for cleaning the exhaust gas from an internal combustion engine, substantially as described herein with reference to, and as illustrated in, the accompanying drawings.

11. A method for operating an exhaust-gas cleaning apparatus for cleaning the exhaust gas from an internal combustion engine, substantially as described herein with reference to, and as illustrated in, the accompanying drawings.