Exhaust System For An Internal Combustion Engine, and Method For Operating Such An Exhaust System

Abstract

An exhaust system of an internal combustion engine includes a first filter device and a second filter device located downstream of the first. Both filter devices filter soot particles out of the exhaust gas. At least the first filter device has a catalytic material, which promotes an exothermic soot burnoff in such a way that as a result of the corresponding heating up of the exhaust gas, the soot burnoff in the second filter device is set in motion. The materials of the two filter devices are selected such that the soot burnoff in the first filter device ensues at a lower temperature and/or at a similar temperature is stronger than in the second filter device.

Description

PRIOR ART

[0001] The invention relates first to an exhaust system for an internal combustion engine, in particular in a motor vehicle, having a first filter device and a second filter device, located downstream of the first, which each filter soot particles out of the exhaust gas, and at least the first filter device has a catalytic material which promotes an exothermic soot burnoff in such a manner that as a result of the corresponding heating up of the exhaust gas, the soot burnoff in the second filter device is set in motion. The invention also relates to a method for operating an exhaust system of this kind.

[0002] An exhaust system and a method of the type defined at the outset are known from German Patent Disclosure DE 101 56 191 A1. This shows an exhaust system which has a main filter device and a prefilter device preceding it. In both the prefilter device and the main filter device, soot from the exhaust gas is accumulated. For regeneration, the soot accumulated in the prefilter device is burned off. As a result, so much heat is released that the exhaust gas flowing through is also heated to the soot ignition temperature, which serves to initiate the burnoff of the soot that has accumulated in the main filter device. It is also known to provide both the main filter device and the prefilter device with a catalytic coating, by which ignition temperature of the soot is lowered. For heating the soot that has accumulated in the prefilter device, a heater is provided, for instance a heating element protruding into the filter body of the prefilter device, or an external burner by which a hot gas is generated.

[0003] However, it is a problem that many catalytic materials that promote the soot burnoff lead to corrosion problems in the filter structure onto which they are applied. This necessitates the use of special filter materials, which increases the production costs.

[0004] It is therefore the object of the present invention to refine an exhaust system and a method of the type defined at the outset such that the exhaust system can be produced as inexpensively as possible.

[0005] This object is attained, in an exhaust system of the type defined at the outset, in that the materials of the two filter devices are selected such that the soot burnoff in the first filter device ensues at a lower temperature and/or at a similar temperature is stronger than in the second filter device. In a method of the type defined at the outset, the stated object is attained accordingly.

ADVANTAGES OF THE INVENTION

[0006] It is true of many catalytic materials that promote soot burnoff that their corrosive aggressiveness increases with the intensity of the catalytic property. Since according to the invention that catalytic activity in the second filter device is less than in the first filter device, the corrosive activity in the second filter device is also reduced, so that a less-expensive substrate or filter material can be used there. As a result, the production costs of the exhaust system are reduced.

[0007] Because the catalytic activity of the first filter device is greater than in the second filter device, soot burnoff there can already be set into motion with a comparatively slight energy input. As a result of the soot burnoff, which is an exothermic reaction, the exhaust gas flowing through the first filter device is heated so markedly that even in the second filter device, the soot ignition temperature, which is higher there, can be reached and the soot accumulated there can be accommodated. In the exhaust system and the method of the invention, reliable regeneration of both the first and second filter devices is thus possible, despite lower production costs.

[0008] The exhaust system of the invention is especially appropriate if the first filter device has a filter efficiency of at most 90%, because soot particles in the second filter device can then be accumulated and burned off to an adequate extent.

[0009] Advantageous refinements of the invention are recited in the dependent claims.

[0010] For instance, it is proposed that in the second filter device, no catalytic material that promotes the soot burnoff is present. This not only makes it possible to use a comparatively inexpensive material for the structure of the second filter device, but the comparatively complex process of coating with a catalytic material is also dispensed with, which additionally reduces costs. Furthermore, damage to the filter structure from excessively strong soot burnoff is avoided. Since the "ignition temperature" in the second filter device is then comparatively high, however, a correspondingly active catalytic material must be used in the first filter device, so that the required heating of the exhaust gas can be attained with a reasonable energy input.

[0011] It is especially advantageous if the first filter device includes a depth filter, preferably comprising an open-pore ceramic foam, and the second filter device includes a surface filter, preferably of metal and even more preferably of special steel. Because of the large effective surface area, an especially good catalytic action can be attained with a depth filter. This is especially true when so-called molten salt catalysts are used, which are in liquid form in the operating range in question. A surface filter in turn has a very good filter action and can be inexpensively made from metal, especially special steel, while at the same time having good high-temperature strength and thermal shock resistance.

[0012] A further advantageous feature of the exhaust system of the invention is distinguished in that the second filter device includes a catalytic material which promotes the oxidation of CO to CO.sub.2 and/or the oxidation of NO to NO.sub.2. By the likewise exothermic oxidation reaction of CO to CO.sub.2 in the second filter device, heat is additionally liberated, which contributes to attaining the ignition temperature required for burning off the soot in the second filter device. The ignition temperature in the second filter device thus need not be achieved solely by way of the heating of the exhaust gas. Moreover, whenever in the second filter device there is no catalytic material that promotes the soot burnoff, this embodiment has the advantage of spatially separating the various oxidation processes. As a result, unwanted interactions are prevented.

[0013] A catalytic material that is simple to process and promotes the oxidation of CO and NO is platinum and palladium. These materials can be substrated onto a large-surface-area oxide or directly onto a filter body, which keeps the production costs low.

[0014] It is especially advantageous if the first filter device and the second filter device are located immediately adjacent one another. This prevents the exhaust gas, heated in the soot burnoff in the first filter device, from cooling down excessively along its way to the second filter device. In this embodiment of the exhaust system of the invention, the efficiency of the soot burnoff is thus especially high. The first filter device and the second filter device can be integrated into a filter unit, which is accommodated in a common housing, for instance. It is even possible for a single filter to be embodied in its upstream region like the first filter device and in its downstream region like the second filter device. This makes for an especially compact design.

[0015] To assure the high catalytic activity, provided according to the invention, in the first filter device, a catalytic material can be used which is a member of the following group: 1 to 50 weight % Ag on an oxidic substrate, and/or V and/or Mo and/or W and/or Mn and/or Cu on an oxidic substrate, the substrate including: Al.sub.2O.sub.3, TiO.sub.2, CeO.sub.2, ZrO.sub.2, and/or SiO.sub.2; Ag in chemical combination with oxides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Al, and/or Zr, in particular Ag.sub.2CrO.sub.4 or AgMn.sub.2O.sub.5; metal vanadate; molybdate; tungstenates; metal-V mixed oxide, in particular Cs.sub.2V.sub.2O.sub.4; Cr mixed oxide; Mo mixed oxide; manganese oxide; rhenium oxide; metal manganate; metal rhenate.

[0016] In an advantageous refinement of the method of the invention, it is proposed that for regenerating the filter devices, the exhaust gas temperature upstream of the first filter device is elevated to a first level, at which in the first filter device a soot burnoff ensues to such an extent that as a result the exhaust gas temperature is additionally elevated to a second level, at which in the second filter device a soot burnoff is likewise set into motion. An exhaust system operated in this way can be produced very inexpensively, since the use of a separate heater, with which the first filter device is heated to the ignition temperature required for the soot burnoff, can be dispensed with. Instead, the soot burnoff is set into motion by provisions associated with the motor.

DRAWINGS

[0017] Especially preferred exemplary embodiments of the present invention are described in further detail below in conjunction with the accompanying drawings. In the drawings:

[0018] FIG. 1 is a schematic illustration of an internal combustion engine with an exhaust system having a first and a second filter device;

[0019] FIG. 2 is an enlarged view of the first and second filter devices of FIG. 1;

[0020] FIG. 3 is a detail of the first filter device in FIG. 1;

[0021] FIG. 4 is a detail of the second filter device in FIG. 1; and

[0022] FIG. 5 is a view similar to FIG. 1 of an alternative embodiment of an exhaust system.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0023] In FIG. 1, an exhaust system of an internal combustion engine 10 is identified by reference numeral 12. It includes an exhaust tube 14, which delivers the exhaust gases from combustion first to a first filter device 16 and after that to a second filter device 18. The engine 10 operates on the diesel principle. Soot particles produced in combustion are at least partially filtered out of the exhaust gas in the two filter devices 16 and 18.

[0024] As FIG. 2 shows, the first filter device 16 includes a housing 20, in which a filter structure in the form of an open-pore ceramic or metal foam 22 is located. The first filter device 16 is accordingly a depth filter. The second filter device 18 likewise includes a housing 24, in which a filter structure 26 in the form of alternatingly closed honeycombs or conduits 28 is located.

[0025] A detail of an internal region of the filter structure 22 of the first filter device 16 is show in FIG. 3. As shown, this filter structure 22 has pores 30, which are present in the material comprising the filter structure 22. This material is identified in FIG. 3 by reference numeral 32. Here, it is silicon carbide. However, in principle, all open-pore bodies and bulk materials comprising ceramic and metal material, and combinations of the two, which are suitable for filtering soot particles out of the exhaust gas that flows through the exhaust tube 14, can be considered for the filter structure 22.

[0026] As also seen from FIG. 3, the silicon carbide material 32 of the filter structure 22 of the first filter device 16 is provided with a catalytic material 34. This material is selected such that it promotes the oxidation or burnoff of soot particles (reference numeral 36 in FIG. 3), which have accumulated in the first filter device 16, at comparatively tow temperatures. This kind of soot burnoff prevents the first filter device 16 from becoming clogged over time.

[0027] The catalytic material 34 used in the first filter device 16 is highly active in view of the promotion of soot burnoff and reduces the ignition temperature at which the exothermic soot burnoff in the first filter device 16 is set into motion by from 100 to 250 K or even more. It is also especially advantageous if the catalytic material 34 promotes the soot burnoff in such a way that CO.sub.2 is also formed (see hereinafter).

[0028] The second filter device 18, show in detail in FIG. 4, is a surface filter, which is embodied here as a wall flow filter. It has a filter structure 38, which is honeycomblike as viewed in the flow direction and is made from special steel. Some of the honeycombs are open on the side toward the engine 10 and closed on the side away from the engine 10 (the exhaust gas stream is represented by arrows 40 in FIG. 4). These honeycombs are identified by reference numeral 28a in FIG. 4. Other honeycombs 28b, adjacent to the honeycombs 28a, are closed on the side toward the engine 10 and open on the side away from the engine 10.

[0029] The filter action is due to the porosity of the walls of the filter structure 38 that are located between the honeycombs 28a and 28b. The exhaust gas stream accordingly passes through the wall surfaces from the honeycombs 28a into the honeycombs 28b, as indicated by the arrow 42 in FIG. 4. The filter efficiency of the second filter device 18 is approximately 95% to 99%. The filter action is selected such that small soot particles are preferentially filtered out of the exhaust gas stream. The filter efficiency of the first filter device 16 is approximately 50%. Through this filter device 16, large soot particles are preferentially filtered out of the exhaust gas stream. The result is accordingly a total efficiency of over 99%.

[0030] In the course of time, more and more soot particles 36 are deposited in the first filter device 16 and the second filter device 18. To prevent clogging of the exhaust system 12, the temperature of the exhaust gas flowing from the engine 10 into the exhaust tube 14 is raised from time to time by motor provisions, specifically to a temperature in the range from 350 to approximately 550.degree. C.

[0031] It is true that normally, soot burnoff to the relevant extent does not occur until temperatures of above approximately 600.degree.. Because of the presence of the catalytic material 34 in the first filter device 16, however, the aforementioned temperature level does suffice to put the burnoff of the deposited soot particles 36 into motion in the first filter device 16. The minimum temperature required for this is also called the "ignition temperature". Burning off the deposited soot particles involves an exothermic oxidation reaction. By the energy released as a result, the exhaust gas flowing through the first filter device 16 is heated.

[0032] Because of the high catalytic activity of the catalytic material 34 in the first filter device 16, the ensuing exothermic reaction is very strong at the temperature level of the exhaust gas attained, which leads to a correspondingly strong increase in the temperature of the exhaust gas flowing from the first filter device 16 to the second filter device 18. A typical temperature of the exhaust gas flowing from the first filter device 16 to the second filter device 18 during an ongoing soot burnoff in the first filter device 16 is approximately 600 to 700.degree. C. This temperature is above the ignition temperature of the soot particles 36 in the second filter device 18, in which no catalytic material that promotes the soot burnoff is present. Thus even in the second filter device 18, the soot burnoff of the deposited soot particles 36 is set into motion.

[0033] As a result of the soot burnoff in the first filter device 16, which because of the use of a special and highly active catalytic material 34 proceeds quickly and with high energy output once ignition has occurred, the exhaust gas temperature is accordingly increased such that the soot particles 36 themselves, located in the second filter device 18, ignite. This effect is enhanced still further in the exemplary embodiment shown in FIG. 4 because the filter structure 38 of the second filter device 18 is also provided with a catalytic material 43, which, however, is of a completely different type from the catalytic material 34 of the first filter device 16:

[0034] In the soot burnoff in the first filter device 16, specifically, CO is also released, which is now oxidized into CO.sub.2 in the second filter device 18, thanks to the catalytic material 43 present there. This oxidation is likewise an exothermic reaction, or accordingly one in which heat is released. This energy can be utilized to reach the ignition temperature in the second filter device 18, which in turn makes it possible to use a less-active catalytic material 34 in the first filter device 16, or in all less catalytic material 34 in the first filter device 16. Platinum or palladium can be considered, as an example, for the catalytic material 43 in the second filter device 18. It can be substrated on a large-surface-area oxide or can be located directly on the special steel filter structure 38.

[0035] As FIG. 1 shows, the first filter device 16 and the second filter device 18 are located immediately adjacent one another. As a result, the exhaust gas heated in the soot burnoff in the first filter device 16 is prevented from cooling down excessively along its way to the second filter device 18. In this respect, the embodiment of FIG. 5, in which the first filter device 16 and the second filter device 18 are integrated into a filter unit 44, is optimal. It should be pointed out that in FIG. 5, those regions and elements that have equivalent functions to elements and regions in the preceding drawings have the same reference numerals and are not described again in detail. It can be seen from FIG. 5 that the filter structure 22 of the first filter device 16 is integral with the filter structure 38 of the second filter device 18 (the dividing line between the two filter structures). The first filter device 16 is accordingly distinguished from the second filter device 18 only in that it includes a catalytic material, not shown in FIG. 5, which is especially active and which promotes the soot burnoff in the first filter device 16.

Claims

1-11. (canceled)

12. In an exhaust system for an internal combustion engine, in particular in a motor vehicle, having a first filter device and a second filter device, located downstream of the first, each of which filters soot particles out of the exhaust gas, and at least the first filter device has a catalytic material which promotes an exothermic soot burnoff, in such a manner that as a result of the corresponding heating up of the exhaust gas, the soot burnoff in the second filter device is set in motion, the improvement wherein the materials of first and second filter devices are selected such that the soot burnoff in the first filter device ensues at a lower temperature and/or at a similar temperature is stronger than in the second filter device.

13. The exhaust system as defined by claim 12, wherein essentially no catalytic material that promotes the soot burnoff is present in the second filter device.

14. The exhaust system as defined by claim 12, wherein the first filter device comprises a depth filter, preferably comprising an open-pore ceramic foam, and the second filter device comprises a surface filter, preferably of metal and even more preferably of special steel.

15. The exhaust system as defined by claim 13, wherein the first filter device comprises a depth filter, preferably comprising an open-pore ceramic foam, and the second filter device comprises a surface filter, preferably of metal and even more preferably of special steel.

16. The exhaust system as defined by claim 12, wherein the second filter device comprises a catalytic material which promotes the oxidation of CO to CO.sub.2 and/or the oxidation of NO to NO.sub.2.

17. The exhaust system as defined by claim 13, wherein the second filter device comprises a catalytic material which promotes the oxidation of CO to CO.sub.2 and or the oxidation of NO to NO.sub.2.

18. The exhaust system as defined by claim 14, wherein the second filter device comprises a catalytic material which promotes the oxidation of CO to CO, and/or the oxidation of NO to NO.sub.2.

19. The exhaust system as defined by claim 16, wherein the second filter device comprises a catalytic material, which includes platinum and/or palladium.

20. The exhaust system as defined by claim 17, wherein the second filter device comprises a catalytic material, which includes platinum and/or palladium.

21. The exhaust system as defined by claim 18, wherein the second filter device comprises a catalytic material, which includes platinum and/or palladium.

22. The exhaust system as defined by claim 19, wherein the catalytic material is coated on a large-surface-area oxide or directly on a filter body.

23. The exhaust system as defined by claim 20, wherein the catalytic material is coated on a large-surface-area oxide or directly on a filter body.

24. The exhaust system as defined by claim 21, wherein the catalytic material is coated on a large-surface-area oxide or directly on a filter body.

25. The exhaust system as defined by claim 12, wherein the first filter device and the second filter device are located immediately adjacent one another.

26. The exhaust system as defined by claim 14, wherein the first filter device and the second filter device are located immediately adjacent one another.

27. The exhaust system as defined by claim 16, wherein the first filter device and the second filter device are located immediately adjacent one another.

28. The exhaust system as defined by claim 12, wherein the first filter device and the second filter device are integrated into a filter unit.

29. The exhaust system as defined by claim 12, wherein the first filter device comprises at least one catalytic material, which is a member of the following group: 1 to 50 weight % Ag on an oxidic substrate, and/or V and/or Mo and/or W and/or Mn and/or Cu on an oxidic substrate, the substrate including: Al.sub.2O.sub.3, TiO.sub.2, CeO.sub.2, ZrO.sub.2, and/or SiO.sub.2; Ag in chemical combination with oxides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Al, and/or Zr, in particular Ag.sub.2CrO.sub.4 or AgMn.sub.2O.sub.5; metal vanadate; molybdate; tungstenates; metal-V mixed oxide, in particular Cs.sub.2V.sub.2O.sub.4; Cr mixed oxide; Mo mixed oxide; manganese oxide; rhenium oxide; metal manganate; metal rhenate.

30. A method for operating an exhaust system for an internal combustion engine, in particular in a motor vehicle, having a first filter device and a second filter device, located downstream of the first, each of which filter soot particles out of the exhaust gas, and at least the first filter device has a catalytic material which promotes an exothermic soot burnoff in such a manner that as a result of the corresponding heating up of the exhaust gas, the soot burnoff in the second filter device is set in motion, the method comprising selecting the materials of the two filter devices are selected such that the soot burnoff in the first filter device ensues at a lower temperature and/or at a similar temperature is stronger than in the second filter device.

31. The method as defined by claim 30, wherein, for regenerating the filter devices, the exhaust gas temperature upstream of the first filter device is elevated to a first level, at which in the first filter device a soot burnoff ensues to such an extent that as a result the exhaust gas temperature is additionally elevated to a second level, at which in the second filter device a soot burnoff is likewise set into motion.