EP1837497B1 - Exhaust gas purification system comprising an absorber catalyst and a particulate filter as well as a method to produce such a system - Google Patents

Description

The invention relates to an exhaust aftertreatment system comprising a storage catalytic converter and a particle filter for after treatment of the exhaust gas of an internal combustion engine, which flows through the exhaust aftertreatment system substantially in the direction of its longitudinal axis, wherein the storage catalytic converter for storing and reducing the nitrogen oxides (NO _x ) and exhaust gas in the exhaust gas Collecting and burning the soot particles located in the exhaust gas are integrally formed as a structural unit, wherein the particulate filter is a honeycomb filter and this honeycomb filter also serves as a carrier substrate for forming the storage catalyst.

Furthermore, the invention relates to a method for producing such an exhaust aftertreatment system.

In the prior art, internal combustion engines are equipped with various exhaust aftertreatment systems to reduce pollutant emissions. Although there is no additional measures during the expansion and expulsion of the cylinder filling at a sufficiently high temperature level and the presence of sufficiently large amounts of oxygen oxidation of the unburned hydrocarbons (HC) and carbon monoxide (CO) instead. However, these reactions come to a halt quickly due to the rapidly decreasing exhaust gas temperature downstream and consequently rapidly decreasing reaction rate. Any lack of oxygen can be compensated by a secondary air injection. However, special reactors and / or filters usually have to be provided in the exhaust tract in order to noticeably reduce pollutant emissions under all operating conditions.

Thermal reactors attempt to achieve a substantial post-oxidation of HC and CO in the exhaust system by providing heat insulation and a sufficiently large volume in the exhaust pipe of the exhaust system. The thermal insulation should ensure the highest possible temperature level by minimizing the heat losses, whereas a large exhaust pipe volume ensures a long residence time of the exhaust gases. Both the long residence time as well as the high temperature level support the desired post-oxidation. The disadvantage is the poor efficiency in substoichiometric combustion and high costs. For diesel engines, thermal reactors are not effective because of the generally lower temperature level.

For the reasons mentioned above, prior art gasoline engines use catalytic reactors which, using catalytic materials which increase the speed of certain reactions, ensure oxidation of HC and CO even at low temperatures. If, in addition, nitrogen oxides (NO _x ) are to be reduced, this can be achieved by using a three-way catalytic converter, which, however, requires a narrow-flow stoichiometric operation (λ≈1) of the gasoline engine.

The nitrogen oxides NO _{x are} reduced by means of the existing unoxidized exhaust gas components, namely the carbon monoxides and the unburned hydrocarbons, wherein at the same time these exhaust gas components are oxidized.

In internal combustion engines, which are operated with an excess of air, so for example in lean-burn gasoline engines, but in particular direct-injection diesel engines but also direct injection gasoline engines, the nitrogen oxides located in the exhaust gas can by principle - d. H. due to the lack of reducing agents - can not be reduced.

For oxidation of the unburned hydrocarbons (HC) and carbon monoxide (CO), an oxidation catalyst is provided in the exhaust system. In order to realize a sufficient conversion, a certain operating temperature is required. The so-called light-off temperature can be 120 ° C to 250 ° C.

To reduce the nitrogen oxides, selective catalysts - so-called SCR catalysts - are used, in which targeted reducing agents are introduced into the exhaust gas to selectively reduce the nitrogen oxides. As a reducing agent, not only ammonia and urea but also unburned hydrocarbons are used. The latter is also referred to as HC enrichment, the unburned hydrocarbons are introduced directly into the exhaust system or by internal engine measures, namely be supplied by a post injection of additional fuel into the combustion chamber after the actual combustion. In this case, the nacheingespritzte fuel is not ignited in the combustion chamber by the still running main combustion or by - even after completion of the main combustion - high combustion gas temperatures, but are introduced during the charge exchange in the exhaust system.

In principle, the nitrogen oxide emissions can also be reduced with so-called nitric oxide storage catalysts (LNT - L ean N O _x T rap). The nitrogen oxides are first - during a lean operation of the internal combustion engine - absorbed in the catalytic converter, ie stored and stored, and then reduced during a regeneration phase, for example by means of a substoichiometric operation (for example, λ <0.95) of the internal combustion engine in deoxygenation (deNO _x ) ,

Further internal engine options for realizing a rich ie a stoichiometric operation of the internal combustion engine provides the exhaust gas recirculation (EGR) and - in diesel engines - the throttling in the intake system. In-engine measures can be dispensed with if the reducing agent is introduced directly into the exhaust tract, for example by injecting additional fuel. During the regeneration phase, the nitrogen oxides are released and converted essentially into nitrogen dioxide (N ₂ ), carbon dioxide (CO ₂ ) and water (H ₂ O. The frequency of the regeneration phases is determined by the total emission of nitrogen oxides and the storage capacity of the LNT.

The temperature of the storage catalyst (LNT) should preferably be in a temperature window between 200 ° C and 450 ° C, so that on the one hand a rapid reduction of nitrogen oxides is ensured and on the other hand no desorption takes place without converting the re-released nitrogen oxides, which is triggered by excessive temperatures can be.

A difficulty in the use and in particular in the arrangement of the LNT in the exhaust system results from the sulfur contained in the exhaust gas, which is also absorbed in the LNT and in a so-called desulfurization (deSO _x ) ie a Desulphurization must be removed on a regular basis. For this purpose, the LNT to high temperatures, usually between 600 ° C and 700 ° C, heated and supplied with a reducing agent, which in turn can be achieved by the transition to a rich operation of the internal combustion engine. The higher the temperature of the LNT, the more effectively the desulphurisation will take place, whereby a maximum permissible temperature should not be exceeded, because then the desulfurization of the LNT contributes to the thermal aging of the catalyst due to too high temperatures. As a result, the desired conversion of the nitrogen oxides towards the end of the life of the catalyst is adversely affected, in particular, the storage capacity decreases due to thermal aging.

To minimize the emission of soot particles so-called regenerative particulate filters are used in the prior art, which filter the soot particles from the exhaust gas and store, these soot particles are intermittently burned during the regeneration of the filter (deSoot). The intervals of the regeneration are determined, inter alia, by the exhaust backpressure, which occurs as a result of the increasing flow resistance of the filter due to the increasing particle mass in the filter.

The high temperatures for the regeneration of the particulate filter - about 550 ° C in the absence of catalytic support - are achieved in operation only at high loads and high speeds. Therefore, additional measures must be used to ensure regeneration of the filter under all operating conditions.

The combustion of the particles can be effected by additional combustion burner provided in the exhaust duct or by a post-injection of additional fuel into the combustion chamber, the nacheingespritzte fuel is already ignited in the combustion chamber, resulting in the expiring main combustion or the present in the combustion chamber towards the end of the combustion high temperatures can happen, so that the exhaust gas temperature of the exhaust gases pushed into the exhaust tract is raised by the engine. Disadvantages of this procedure are, in particular, the heat losses which are to be feared in the exhaust gas tract on the way to the filter and the associated temperature reduction of the hot ones Exhaust gases. The filter can easily be located a meter and more away from the outlet of the combustion chamber in the exhaust system.

The compensation of the heat losses by the generation of correspondingly high exhaust gas temperatures are limited by the temperature resistance of other components provided in the exhaust system, in particular the temperature resistance of an exhaust gas turbine arranged in the exhaust gas turbocharger, a three-way catalyst or a storage catalyst. Usually, the turbine is subjected to the highest temperatures, since it is located closest to the outlet of the combustion chamber.

The nacheingespritzte fuel can also be unburned and possibly already processed pushed out into the exhaust system and then selectively oxidized locally there in the exhaust system, where high exhaust gas temperatures are necessary, namely in the particulate filter or in its immediate vicinity. The combustion of the post-injected fuel can be catalytically initiated by means of a catalyst positioned in front of the filter. But it can also be provided an electric ignition in or on the soot filter.

As already proposed for the reduction of nitrogen oxides, fuel can also be introduced directly into the exhaust gas tract. The further procedure corresponds to that described above, in which the additionally injected fuel enters the exhaust system unburned and is specifically oxidized in the vicinity of the particle filter.

It must be taken into account that the use of additional fuel, whether due to a transition to a rich engine operation or as a result of the enrichment of the exhaust gas with fuel, inherently affects the fuel consumption of the internal combustion engine adversely. In particular, the frequency with which the particle filter regenerates or the LNT is purified has a significant and direct influence on the amount of fuel used for these purposes and thus on the total consumption.

Since both the exhaust gases of gasoline engines and the exhaust gases of diesel engines - albeit in different quantities and qualities - unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO _x ) and soot particles, come in the prior art in the Usually combined exhaust aftertreatment systems are used, the one or more of the catalysts, reactors and / or filters described above.

The present invention has, for example, a combined exhaust aftertreatment system, ie a system in which the storage catalytic converter for storing and reducing the nitrogen oxides (NO _x ) present in the exhaust gas and the particle filter for collecting and burning the soot particles located in the exhaust gas are formed integrally, ie as a structural unit , The particle filter is a honeycomb filter and this honeycomb filter also serves as a carrier substrate for forming the storage catalyst. The honeycomb filter is coated with a catalytic material which is suitable for storing and reducing the nitrogen oxides (NO _x ) present in the exhaust gas. Such an exhaust aftertreatment system is for example in EP 1 486 248 described.

Such a system is distinguished from conventional systems in which the particulate filter and the storage catalyst each form an independent component, by its compact design. In addition, the number of carrier substrates can be halved, since the carrier substrate of the particulate filter is used simultaneously to form the storage catalyst.

The disadvantages of the system arise partly but also from the described dual function of the carrier substrate. The honeycomb filter serving as a particle filter comprises a multiplicity of channels, which are generally mutually closed, ie in a checkerboard pattern, so that the exhaust flows into the channels open at the inlet of the honeycomb filter and flows through the channel walls of these channels which are closed towards the exit on the way to the outlet in order to get into a channel that is open to the outlet of the honeycomb filter (see also FIG. 1 ). In the case of surface filters, the pore diameters of the carrier substrate are so small that the soot particles do not penetrate into the filter material, but deposit themselves as filter cakes on the surface of the filter.

With the increasing deposition of soot particles, the effective available flow cross-section decreases, that is, the flow resistance increases, which is why the exhaust gas counterpressure increases as the load increases. To counteract this effect, the largest possible filter surface is sought. The current exhaust back pressure or the non-exceeding of a maximum allowable exhaust back pressure essentially form the relevant parameters by which it is decided whether or when a particulate filter regeneration is performed.

Is now on the - previously described - honeycomb filter an additional coating of catalytic material for storing and reducing nitrogen oxides. applied to the formation of the storage catalyst, the exhaust gas backpressure caused by the honeycomb filter increases in addition, which is why less catalytic material is used to form a storage catalyst of the type in question as in a conventional storage catalyst, which does not simultaneously play the role of a particulate filter in the context of a dual function. In general, less material - for example one third less - used to coat a storage catalyst of the generic type, in order not to increase the exhaust back pressure in comparison to conventional storage catalysts in unfavorable manner. However, this means at the same time that the storage capacity of the catalyst in question decreases in proportion to the amount of catalytic material used. The storage catalyst of the generic system - assuming the same catalyst volume - therefore basically has the disadvantage of a reduced storage capacity.

In addition, it proves to be problematic to achieve a uniform temperature over the entire filter or catalyst, which is noticeable in particular in the filter regeneration and desulfurization in a disadvantageous manner.

If the storage catalytic converter is to be heated, for example, to the high temperatures required for the desulfurization between 600 ° C. and 700 ° C., an inhomogeneous temperature distribution in the system is generally present during the heating of the system. The temperature inside the honeycomb filter can already be 650 ° C, while in the outer regions of the filter temperatures around 550 ° C prevail.

The desulphurization of too low temperatures in the outer regions of the system means that these areas of the storage catalyst are not cleaned of sulfur, resulting in a decrease in the storage capacity of these outer areas and thus a decrease in the storage capacity of the LNT with increasing operating time or with increasing age leads. Comes the storage capacity of the outer areas Completely stopped, the exhaust gases flow through these outer areas, without the nitrogen oxides contained in the exhaust gas are stored or converted.

Alternatively, the heating process could be continued until the temperature in the outer regions is sufficiently high for desulfurization. For this, however, the temperature inside the exhaust aftertreatment system would have to be raised to a level which would at least lead to the thermal deterioration of the honeycomb filter for thermal destruction. Therefore, the prior art dispenses with excessive heating of the system and instead accepts the decrease in storage capacity, especially in the outer regions.

The resulting inhomogeneous temperature distribution as a result of heating in the system also proves disadvantageous in terms of filter regeneration. If the filter is heated for the purpose of a filter regeneration, the temperature of about 550 ° C. required for the soot oxidation can already be present in the interior of the system, while the outer regions do not yet have a sufficiently high temperature for the regeneration. Consequently, the soot in the outer regions is not oxidized, the exhaust back pressure increases, and the storage capacity of the outer regions also decreases. But then the soot that has accumulated in the outer regions over a long period of time, actually ignited and burned during a regeneration, triggered for example by the initiation of a desulfurization process, locally such high temperatures can occur that the storage catalyst in the corresponding areas thermally overloaded or destroyed.

Another effect that adversely affects the operation of the honeycomb filter or system, resulting from the ash constituents, which carries the exhaust gas and which are deposited in the system or honeycomb filter. Here, the ash deposits at the closed ends of the inlet channels of the honeycomb filter, where builds up with increasing age of the system, a steadily growing ash deposit, which reduces the effectively usable channel length increasingly and continuously, so that even in this area, the applied catalytic material layer only initially contributes to the conversion and reduction of pollutants, but later remains unused. This is particularly disadvantageous because catalytic materials are very expensive and these materials in the production of an exhaust aftertreatment system represent a crucial cost factor.

Against this background, it is the object of the present invention, an exhaust aftertreatment system of the generic type d. H. According to the preamble of claim 1, which is optimized with regard to the problems known from the prior art - inhomogeneous temperature distribution, thermal aging or damage, cost efficiency, pollutant conversion.

Another object of the present invention is to provide a method for producing such an exhaust aftertreatment system.

The first sub-task is solved by an exhaust aftertreatment system comprising a storage catalytic converter and a particle filter for the aftertreatment of the exhaust gas of an internal combustion engine, which flows through the exhaust aftertreatment system substantially in the direction of its longitudinal axis, wherein the storage catalytic converter for storing and reducing the nitrogen oxides (NO _x ) and the particulate filter for collecting and burning the soot particles located in the exhaust gas are integrally formed as a structural unit, wherein the particulate filter is a honeycomb filter and this honeycomb filter serves as a carrier substrate for forming the storage catalyst, and which is characterized in that an inner to the longitudinal axis of Systems arranged portion of the honeycomb filter for storing and reducing the exhaust gases in the nitrogen oxides (NO _x ) is coated with a catalytic material, whereas an outer portion of the honeycomb filter such Beschic does not have.

In the exhaust aftertreatment system according to the invention is not the entire honeycomb filter coated with catalytic material, but only an inner area, which is characterized, inter alia, that it can be heated more easily ie faster and with less effort than the outlying areas of the system. Only the inner region of the honeycomb filter has a dual function as a particle filter and storage catalyst, whereas the outer region merely serves as a particle filter due to the lack of a catalytic coating.

The well-known from the prior art and already discussed above conflict, namely on the one hand the outer areas - especially in the context of desulfurization - to heat to the required high temperatures, but on the other hand not to overheat the inner area, ie thermally not to overload, is eliminated or obsolete in this way. A desulfurization of the outer areas is no longer necessary because these areas are not used for storing and converting the nitrogen oxides (NO _x ) located in the exhaust gas.

The exhaust aftertreatment system according to the invention has further advantages resulting from the targeted coating of only the inner region and the deliberate omission of the coating in the outer region.

The fact that the outer region of the honeycomb filter is not coated, the flow resistance of the outer regions of the honeycomb filter decreases, which is basically to be regarded as advantageous, since the exhaust back pressure also decreases. In addition, the saved catalytic material, which is used in the prior art for coating the outer regions, at a cost advantage over conventional exhaust aftertreatment systems.

Due to the lower flow resistance of the outer regions compared to the inner region, a larger part of the hot exhaust gases will pass through the outer regions as it flows through the honeycomb filter and a smaller exhaust gas partial flow will flow through the inner region. This results in a much more homogeneous temperature distribution in the system during heating of the system by means of hot exhaust gases, since the areas of different amounts of heat, which correlate with the amounts of exhaust gas, are supplied. That is, the outer regions, which - as known from the prior art - are more difficult to heat, are occupied or subjected to a larger exhaust gas mass flow. This offers particular advantages in the regeneration of the particulate filter because the high temperature of about 550 ° required for the ignition and oxidation of the filter is achieved comparatively quickly in all areas of the exhaust aftertreatment system.

The fact that the outer regions for the exhaust gases are easier to pass, due to the lack of catalytic coating or flow resistance, can also be used to apply more catalytic material to the interior of the system, i. E. to provide a thicker or thicker coating. Although this increases the flow resistance of the inner region and thus the flow resistance of the entire system, whereby the advantage of a lower exhaust back pressure is at least partially lost again. By virtue of this measure, however, the storage capacity of the storage catalytic converter can be increased or the decrease in the storage capacity caused by the omission of the coating in the outer regions can be compensated for again.

The exhaust aftertreatment system according to the invention also allows a higher loading of the outer regions with soot or soot particles.

Thus, the first object of the invention is achieved, namely to provide a system according to the preamble of claim 1, which is in terms of known from the prior art problem - inhomogeneous temperature distribution, thermal aging or damage, cost efficiency, pollutant conversion - optimized.

It should be noted that within the meaning of the present application, the inner region and the outer region do not necessarily have to form a coherent filter volume in each case, but can be composed of a plurality of non-interconnected partial regions or partial volumes. For example, the outer region can be made up of two separate, i.e. be constructed not contiguous subvolume. In addition, however, there will always be talk of an inner area and an outer area.

Further advantageous embodiments of the system according to the invention according to the subclaims are described and explained below.

Embodiments of the exhaust gas aftertreatment system in which the honeycomb filter is provided with an additional coating at least in the outer region, which accelerates the ignition and / or the oxidation of the soot or the soot particles or reduces the ignition temperature for the regeneration of the filter, are advantageous. especially the outer regions which, depending on the particular embodiment of the system, are heated to a greater or lesser extent than the inner region, ie reach a predetermined temperature with respect to the inner central region of the honeycomb filter with a time delay, benefit from a catalytic coating , which supports the regeneration of the particulate filter.

Embodiments of the exhaust aftertreatment system in which the honeycomb filter is of cylindrical shape and has an outer diameter D are advantageous. A cylindrical shape of the honeycomb filter offers advantages in the heating, thereby advantages in the regeneration or cleaning and thus turn basically advantages in terms of pollutant conversion. The cylindrical shape has proven to be particularly favorable to make a given system volume usable as effectively as possible. Systems which have an oval cross-section have the advantages mentioned in a slightly attenuated form. However, in comparison to systems with polygonal, for example rectangular or square, cross sections, they again offer noticeable advantages with regard to the relevant and relevant temperature distribution within the system. The decision as to whether a system having a cylindrical or oval cross section is used is also determined by the available installation space or the arrangement of the system, which is usually arranged below the floor of the motor vehicle.

Embodiments of the exhaust aftertreatment system in which the inner coated region forms a tubular section which has an outer diameter d <D and is surrounded by the outer uncoated region are advantageous. To form an outer region, the inner region must have a smaller outer diameter than the honeycomb filter.

Preferably, the inner region is tubular and thus symmetrical to the longitudinal axis of the system. Assuming a temperature distribution at which the temperature assumes its maximum value in the center of the system ie on the longitudinal axis and decreases continuously towards the outer regions, an approximately equal local temperature prevails over the entire circumference at the transition from the inner region to the outer region.

The advantages of this embodiment become apparent when it is considered that the transition from the inner region to the outer region defines the limit at which the coating of the honeycomb filter with catalytic material ends, and to protect this coating, a maximum allowable temperature is set, which must be observed , By means of the proposed embodiment, the entire inner area can be brought to maximum temperatures or heated.

The tubular formation of the inner region is fundamentally advantageous d. H. an inner region arranged symmetrically about the longitudinal axis offers advantages in terms of temperature distribution, regardless of the specific outer shape of the honeycomb filter.

However, embodiments of the exhaust gas aftertreatment system in which the inner region has a shape similar to the honeycomb filter, that is to say, can also be advantageous. the inner region in a cylindrical honeycomb filter tubular or cylindrical and oval in a honeycomb filter with oval cross-section and optionally aligned accordingly.

In this case, embodiments of the system are advantageous in which d / D> 0.5 or d / D> 0.65, but also embodiments in which d / D> 0.85 applies.

The concrete ratio of the outer diameter D of the honeycomb filter to the outer diameter d of the inner region depends on the individual application and often results from the primary objective.

If a reduction of the exhaust backpressure or a reduction of the component costs is in the foreground, the outer range can be extended comparatively far by the ratio d / D being chosen as small as possible. This means that only a small portion of the surface of the honeycomb filter is coated with a catalytic material, which on the one hand reduces the flow resistance and on the other hand, the costs - by saving catalytic material - lowers.

But it is also to be considered that the exhaust stream, which flows through the outer region of the exhaust aftertreatment system, not freed or purified from nitrogen oxides due to the lack of coating. In the outer area, nitrogen oxides are neither stored in lean operation, nor are nitrogen oxides reduced under oxygen deficiency in rich operation. The nitrogen oxides can escape unhindered - after flowing through the outer area - in the environment or the environment. The proportion of these nitrogen oxides in the total nitrogen oxide emission of the internal combustion engine increases in principle with increasing outer area. with a reduction of the ratio d / D to.

The definition of the size of the inner and the outer region thus also depends on the original emissions (engine-out emissions) of the internal combustion engine and the statutory requirements to be observed.

Advantageous embodiments of the exhaust aftertreatment system, in which the carrier substrate is formed essentially of silicon carbide and / or cordierite. Investigations have shown that carrier substrates produced from these materials can be heated or heated comparatively quickly due to the low specific heat capacity, which is advantageous with regard to the required energy requirement for heating the system. In addition, the heat is distributed more rapidly in the honeycomb filter or in the carrier substrate, which is useful for the most homogeneous possible temperature distribution in the system. Both offer advantages in terms of efficient pollutant conversion.

Embodiments of the exhaust aftertreatment system in which the carrier substrate has a porosity of 50 to 70% are advantageous.

Advantageous embodiments of the exhaust aftertreatment system, in which an oxidation catalyst, in particular for the oxidation of the exhaust gas in the carbon monoxide (CO) and the unburned hydrocarbons (HC), is provided, which forms a four-way catalyst together with the system according to the invention.

An exhaust aftertreatment system for the oxidation of CO and HC not only reduces the emissions of incompletely or not oxidized pollutants, but leads due to the Exothermic reactions in the context of the oxidation to a heating of the exhaust gas and the exhaust gas aftertreatment components through which the exhaust gas flows.

Advantageous embodiments are those in which the oxidation catalyst and the system according to the invention are connected in series, wherein the oxidation catalyst is arranged upstream of the system.

This embodiment is advantageous with regard to the temperatures which are required for the reduction of the individual pollutants.

Both the oxidation catalysts used for diesel engines and the three-way catalysts used in gasoline engines require a certain operating temperature in order to sufficiently convert the pollutants and to noticeably reduce pollutant emissions. The three-way catalysts are to be counted in the context of the present invention to the oxidation catalysts.

By providing the oxidation catalyst upstream of the system, the oxidation catalyst is the exhaust aftertreatment component located closest to the engine exhaust and first traversed by the hot exhaust gases. Consequently, the heat losses and the associated reduction in temperature of the hot exhaust gases on the way to the oxidation catalyst are comparatively low. Accordingly, the oxidation catalyst reaches its so-called light-off temperature of about 250 ° C even after a cold start within a relatively short period of time.

The exothermic reactions taking place in the oxidation catalytic converter bring about heating of the exhaust gas flowing through them and thus heating of the downstream exhaust gas after-treatment systems arranged downstream of the catalytic converter, which is advantageous for the tasks assigned to these components. Thus, both the preferred operating temperature of the storage catalyst and the regeneration temperature of the particulate filter is above the light-off temperature of the oxidation catalyst, so that here an additional heating of the exhaust gases in the oxidation catalyst is expedient or beneficial.

Embodiments of the system in which an end section of predefinable length .DELTA.l of the honeycomb filter is uncoated are advantageous.

As already stated in the introduction above, the exhaust gas carries ash, which deposits on the closed ends of the inlet channels of the honeycomb filter. The growing ash deposit at the channel ends shortens the usable channel length increasingly and thus also the usable length of the honeycomb filter. The ash-covered area of the honeycomb filter can neither be used as a particle filter nor as a storage catalyst. The catalytic material layer applied here can only contribute to the conversion and reduction of pollutants because of ashing.

A disadvantage - in addition to the reduction of the storage capacity of the system due to ashing - in particular that the catalytic materials for forming the storage catalyst - as already mentioned - are very expensive and due to the deposition of ash at the closed channel ends in these areas are of limited use, i. The coating is after deposition of ash without value for the aftertreatment of the exhaust gases of the internal combustion engine.

For these reasons, it is considered expedient not to coat the areas of the honeycomb filter threatened by ash deposition with a catalytic material for storing and reducing the nitrogen oxides (NO _x ) present in the exhaust gas. Consequently, embodiments are advantageous in which an end portion of predeterminable length .DELTA.l of the honeycomb filter remains uncoated.

Embodiments of the system are advantageous in which: Δ1 / L> 0.75 or Δ1 / L> 0.9.

• ■ als Partikelfilter ein Wabenfilter eingesetzt wird,
• ■ dieser Wabenfilter als Trägersubstrat zur Ausbildung des Speicherkatalysators verwendet wird, wobei
• ■ ein innerer, um die Längsachse des Systems angeordneter Bereich des Wabenfilters zum Speichern und Reduzieren der im Abgas befindlichen Stickoxide (NO_x) mit einem katalytischen Material beschichtet wird, wohingegen ein äußerer Bereich des Wabenfilters unbeschichtet bleibt.

The second of the invention underlying subtask is achieved by a method in which

• ■ a honeycomb filter is used as particle filter,
• ■ This honeycomb filter is used as a carrier substrate for forming the storage catalyst, wherein
• An inner region of the honeycomb filter arranged around the longitudinal axis of the system for storing and reducing the nitrogen oxides (NO _x ) present in the exhaust gas is coated with a catalytic material, whereas an outer region of the honeycomb filter remains uncoated.

What has already been said for the exhaust aftertreatment system according to the invention also applies to the method according to the invention.

According to the invention, not the entire honeycomb filter, but only a selected central region in the interior of the honeycomb filter is coated with a catalytic material for storing and reducing the nitrogen oxides.

Embodiments of the method in which the outer region of the honeycomb filter is provided with an additional coating which promotes the ignition and / or the oxidation of the soot or of the soot particles or reduces the ignition temperature for the regeneration of the filter are advantageous.

Embodiments of the method in which an end section of predefinable length Δl of the honeycomb filter is not coated are advantageous. This takes into account the problem of ashing.

Fig. 1

schematisch in der perspektivischen Darstellung einen Wabenfilter nach dem Stand der Technik,

Fig. 2a

schematisch eine erste Ausführungsform des Systems im Längsschnitt,

Fig. 2b

schematisch die in Figur 2a dargestellte erste Ausführungsform des Systems im Querschnitt,

Fig. 3a

schematisch eine zweite Ausführungsform des Systems im Querschnitt,

Fig. 3b

schematisch die in Figur 3a dargestellte zweite Ausführungsform des Systems im Längsschnitt,

Fig. 4a

schematisch eine dritte Ausführungsform des Systems im Querschnitt, und

Fig. 4b

schematisch die in Figur 4a dargestellte dritte Ausführungsform des Systems im Längsschnitt.

In the following the invention with reference to three embodiments according to the Figures 1 to 4b described in more detail. Hereby shows:

Fig. 1

schematically in the perspective view of a honeycomb filter according to the prior art,

Fig. 2a

schematically a first embodiment of the system in longitudinal section,

Fig. 2b

schematically the in FIG. 2a illustrated first embodiment of the system in cross section,

Fig. 3a

schematically a second embodiment of the system in cross section,

Fig. 3b

schematically the in FIG. 3a illustrated second embodiment of the system in longitudinal section,

Fig. 4a

schematically a third embodiment of the system in cross section, and

Fig. 4b

schematically the in FIG. 4a illustrated third embodiment of the system in longitudinal section.

FIG. 1 schematically shows in perspective view a honeycomb filter 4 according to the prior art.

The honeycomb filter 4, which serves as a particle filter and as a carrier substrate for the storage catalyst, comprises a plurality of channels 6, which alternately d. H. are closed in the checkerboard pattern, so that the exhaust gas flows into the open at the inlet 7 of the honeycomb filter 4 channels 6 and on the way to the outlet 8, the channel walls of these channels 8 closed to exit 8 must flow through to get into a channel 6, the to the outlet of the honeycomb filter 4 is open. The sealed at the outlet 8 of the honeycomb filter 4 channel ends bear the reference numeral. 5

FIG. 2a schematically shows a first embodiment of the system 1 in longitudinal section. FIG. 2b shows this first embodiment in cross section.

The exhaust gas of the internal combustion engine to be cleaned of pollutants enters the exhaust aftertreatment system 1 at the inlet 7, flows through it in the direction of the longitudinal axis 9 and leaves the system 1 again at the outlet 8.

The illustrated exhaust aftertreatment system 1 or system 1 has a honeycomb filter 4 - as in FIG. 1 illustrated - on, which acts as a particle filter 3 and at the same time serves as a carrier substrate for forming a storage catalyst 2.

The storage catalyst 2 for storing and reducing the nitrogen oxides (NO _x ) present in the exhaust gas and the particle filter 3 for collecting and burning the soot particles located in the exhaust gas are integrally formed as a structural unit.

In this case, an inner, arranged around the longitudinal axis 9 of the system 1 region 10 of the honeycomb filter 4, which can be heated faster due to its central location within the system 1 or exhaust gas flow, coated to store and reduce the nitrogen oxides with a catalytic material 11, whereas an outer region 12 of the honeycomb filter 4 does not have such a coating.

Only the inner region 10 of the honeycomb filter 4 functions - in a double function - both as a particle filter 3 and as a storage catalyst 2. The outer region 12, however, acts only as a particle filter 3 due to the lack of catalytic coating. Desulfurization of the outer region 12 is therefore not more necessary because this area 12 is not used for storing and converting nitrogen oxides.

The problem known from the prior art, namely on the one hand the outer region 12 - in particular in the context of desulfurization - to heat to the required high temperatures, on the other hand, the inner region 10 does not overheat, ie. thermally not overloaded, is taken into account in this way.

The flow resistance of the outer region 12 is comparatively low due to the lack of catalytic coating for forming a storage catalyst. This reduces the exhaust back pressure. Due to the lower flow resistance of the outer region 12 compared to the inner region 10, a larger part of the hot exhaust gases will flow through the outer region 12. This results in a much more homogeneous temperature distribution in the system 1, since in this way the areas 10,12 different amounts of heat - are supplied - according to need. The more difficult to heat outer regions 12 are occupied by a larger exhaust gas mass flow. The required for the ignition and the oxidation of the filter 3 high temperature of about 550 ° for the purpose of filter regeneration is achieved comparatively quickly.

The honeycomb filter 4 has a cylindrical shape, which offers advantages in the heating. The inner coated portion 10 forms a tubular portion which is formed symmetrically to the longitudinal axis 9 of the system 1 and surrounded by the outer uncoated portion 12 and the outer diameter d is smaller than the outer diameter D of the honeycomb filter 4. For the diameter ratio of the first, in the FIGS. 2a and 2b illustrated embodiment is: d / D ≈ 0.77.

At a temperature distribution at which the temperature in the center of the system 1 d. H. on the longitudinal axis 9 assumes the maximum value and decreases continuously towards the outer regions 12, then prevails over the entire circumference at the transition from the inner region 10 to the outer region 12 of an approximately equal temperature.

FIG. 3a schematically shows a second embodiment of the system 1 in cross section. FIG. 3b shows this second embodiment in longitudinal section.

At this point, only the differences to those in the FIGS. 2a and 2b Otherwise, reference will be made to the previously described embodiments FIGS. 2a and 2b and the statements made in connection with these figures. The same reference numerals have been used for the same components.

In the in the FIGS. 3a and 3b shown system 1 is an end portion 13 of predeterminable length .DELTA.1 of the honeycomb filter 4 uncoated.

The ash carried by the exhaust gas deposits at the closed ends of the inlet channels of the honeycomb filter 4. The growing ash deposit at the channel ends increasingly shortens the usable channel length and thus also the useful length of the honeycomb filter 4. The ash-covered surface of the honeycomb filter 4 can be used neither for collecting soot particles nor for storing nitrogen oxides. An application of catalytic material in this asbestos-threatened area is therefore not useful.

FIG. 4a schematically shows a third embodiment of the system in cross section. FIG. 4b shows this third embodiment in longitudinal section.

At this point should only on the differences to the in the FIGS. 3a and 3b illustrated embodiment, reference is otherwise made to the FIGS. 3a and 3b and the statements made in connection with these figures. The same reference numerals have been used for the same components.

Unlike in the FIGS. 3a and 3b embodiment shown in the FIGS. 4a and 4b embodiment shown on a honeycomb filter 4 with square cross section. In addition, the inner region 10 also has a square cross-section.

reference numeral

1

Exhaust aftertreatment system, system

2

storage catalytic converter

3

particulate Filter

4

honeycombs

5

channel

6

closed channel end

7

inlet

8th

Outlet, exit

9

longitudinal axis

10

inner area

11

catalytic material

12

outer area

13

end

AGR

Exhaust gas recirculation

CO

Carbon monoxide

CO ₂

carbon dioxide

d

Outer diameter of the inner coated tubular portion

.DELTA.d

Diameter difference

D

Outer diameter of the honeycomb filter

deNO _x

Cleaning of the storage catalytic converter

deSoot

Regeneration of the particle filter

deSO _x

Desulfurization, desulfurization

H ₂ O

water

HC

unburned hydrocarbons

N ₂

nitrogen dioxide

NO _x

Nitrogen oxides

.DELTA.l

Length of uncoated end section

₁

Length of the coated section of the honeycomb filter

L

Length of the system, length of the honeycomb filter

LNT

Lean NO _x trap

SCR

Selective Catalytic Reduction

SO _x

sulfur oxides

λ

air ratio

Claims (11)

1. Exhaust-gas aftertreatment system (1) comprising a storage catalytic converter (2) and a particle filter (3) for the aftertreatment of the exhaust gas of an internal combustion engine, which exhaust gas flows through the exhaust-gas aftertreatment system (1) substantially in the direction of the longitudinal axis (9) thereof, in which exhaust-gas aftertreatment system (1) the storage catalytic converter (2) for storing and reducing the nitrogen oxides situated in the exhaust gas, and the particle filter (3) for collecting and burning the soot particles situated in the exhaust gas, are integrally formed as a structural unit, with the particle filter (3) being a honeycomb filter (4) and with said honeycomb filter (4) simultaneously serving as a support substrate for forming the storage catalytic converter (2), characterized in that an inner region (10), which is arranged around the longitudinal axis (9) of the system (1), of the honeycomb filter (4) is coated, for storing and reducing the nitrogen oxides in the exhaust gas, with a catalytic material (11), whereas an outer region (12) of the honeycomb filter (4) does not have a coating of said type.
2. Exhaust-gas aftertreatment system (1) according to Claim 1, characterized in that the honeycomb filter (4) is of cylindrical design and has an outer diameter D.
3. Exhaust-gas aftertreatment system (1) according to Claim 2, characterized in that the inner, coated region (10) forms a tubular section which has an outer diameter d < D and which is surrounded by the outer, non-coated region (12).
4. Exhaust-gas aftertreatment system (1) according to Claim 3, characterized in that d/D > 0.5.
5. Exhaust-gas aftertreatment system (1) according to Claim 3 or 4, characterized in that d/D > 0.65.
6. Exhaust-gas aftertreatment system (1) according to one of Claims 3 to 5, characterized in that d/D > 0.85.
7. Exhaust-gas aftertreatment system (1) according to one of the preceding claims, characterized in that the support substrate is formed substantially from silicon carbide and/or cordierite.
8. Exhaust-gas aftertreatment system (1) according to one of the preceding claims, characterized in that the support substrate has a porosity of 50 to 70%.
9. Exhaust-gas aftertreatment system (1) according to one of the preceding claims, characterized in that an oxidation catalytic converter, in particular for oxidizing the carbon monoxide situated in the exhaust gas and the unburned hydrocarbons, is provided, which oxidation catalytic converter, together with the system (1), forms a four-way catalytic converter.
10. Exhaust-gas aftertreatment system (1) according to one of the preceding claims, characterized in that an end section (13) of predefinable length Δl of the honeycomb filter (4) is non-coated.
11. Method for producing an exhaust-gas aftertreatment system (1) according to one of the preceding claims, characterized in that

    • a honeycomb filter (4) is used as a particle filter,

    • said honeycomb filter (4) is used as a support substrate for forming the storage catalytic converter (2), with

    • an inner region (10), which is arranged around the longitudinal axis (9) of the system (1), of the honeycomb filter (4) being coated, for storing and reducing the nitrogen oxides in the exhaust gas, with a catalytic material (11), whereas an outer region (12) of the honeycomb filter (4) remains non-coated.

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