EP1687086A1

EP1687086A1 - Exhaust system for a combustion engine

Info

Publication number: EP1687086A1
Application number: EP04798134A
Authority: EP
Inventors: Markus Widenmeyer
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2003-11-15
Filing date: 2004-11-02
Publication date: 2006-08-09
Also published as: US20070081925A1; JP2007514523A; WO2005046869A1; DE10353426A1

Abstract

An exhaust system for a combustion engine comprises a deep-bed filter (16) for removing soot from the exhaust gas. The deep-bed filter (16) comprises a catalyst material (18) that promotes soot oxidation. The invention provides that an inner pore structure (28) of the deep-bed filter (16) is provided with a catalyst material (18) that is liquid beyond a temperature of no more than approximately 400 DEG C, preferably of no more than approximately 350 DEG C.

Description

Exhaust system for an internal combustion engine

State of the art

The invention relates to an exhaust system for an internal combustion engine, with a depth filter for removing soot from the exhaust gas, at least the depth filter comprising a catalyst material which promotes soot oxidation.

An exhaust system of the type mentioned is known from DE 101 30 338 AI. In this an exhaust system for a diesel engine is described. The exhaust system should also be able to filter out soot particles from the exhaust gas.

One possibility for this are surface filters, which are also implemented as wallflow or inflow filters with a honeycomb structure, the flow channels of which are mutually closed, so that the exhaust gas has to flow through the porous filter walls. Around To counteract a blockage of the filter with increasing soot loading, such a filter must, however, be continuously or cyclically freed from the soot that has accumulated or has already accumulated. This is possible using thermal combustion and / or catalytic processes.

When using thermal processes, nitrogen monoxide must first be converted from the exhaust gas into nitrogen dioxide. This in turn can oxidize diesel soot from temperatures of approx. 300 ° C. However, since modern internal combustion engines only emit very little nitrogen monoxide, there is often not enough nitrogen dioxide available to convert the diesel soot. In contrast, catalytic processes have so far often been comparatively inefficient because the contact areas between the soot and the catalyst are small.

A depth filter is also known from DE 101 30 338 AI. This has an open pore system, which is designed in such a way that even larger soot particles can be separated within the filter body. The filter effect of this depth filter therefore extends over its entire volume or its entire surface. Here too, constipation of the

Depth filter counteracted by the fact that it is provided with a catalyst which should enable soot oxidation in the depth filter even at comparatively low temperatures. For this purpose, catalyst particles are finely dispersed on the surface of the depth filter.

The object of the present invention is to develop an exhaust system of the type mentioned at the outset in such a way that on the one hand the greatest possible proportion of soot particles the exhaust gas can be filtered out, and that, on the other hand, simple and effective regeneration of the filters used is possible.

This object is achieved in an exhaust gas system of the type mentioned in the introduction in that an inner pore structure of the depth filter is provided with a catalyst material which, at an operating temperature of the depth filter, in particular from a temperature of approximately at most 400 ° C., more preferably of at most approximately 350 ° C, is liquid.

Advantages of the invention

The use of a liquid catalyst in the depth filter used has the advantage that the catalyst material can flow to the soot particles in the liquid state, even cover them, and can oxidize soot that has accumulated at much lower temperatures than previously. This creates a much larger contact area between the soot particles and the catalyst material, which significantly increases the oxidation rate of the soot particles deposited in the depth filter. The liquid state is already at the operating temperature of the depth filter, which is normally related to the exhaust gas temperature. The operating temperature can again be reached either in normal operation or in special operating phases.

Advantageous developments of the invention are specified in the subclaims. First, it is proposed that the catalyst material of the depth filter comprises a "molten salt" material, in particular Cs ₂ S0 ₄ V ₂ 0 ₅ or Cs vanadates or Ag compounds, in particular Ag vandanates. These materials are in liquid form at temperatures above approx. 350 ° C.

These materials can optionally be combined with other catalytically active substances, for example: Rh and / or Pd, on supports such as aluminum, zirconium, cerium oxides and / or mixed oxides such as Ce / Zr0 ₂ , or without a support; Group 11 elements (Ag, Au, and / or Cu) on supports such as aluminum, zirconium, cerium oxides and / or mixed oxides such as Ce / Zr0 ₂ , or without supports; Oxygen-storing and releasing materials, for example compounds of Mn, Fe, Ce, Pr; under

Exhaust gas conditions nitrate-forming materials (NO _x storage), in particular elements of the alkaline earth group, and of group 3 and the rare earths; and / or materials which are characterized by high acidity, for example zeolites and the following oxides or oxide mixtures: Ti0 ₂ , Zr0 ₂ , Si0 ₂ , Al ₂ 0 ₃ , boron oxides.

An exhaust system in which the depth filter comprises an open-pore silicon carbide foam filter with pore diameters in the range from approximately 40 μm to approximately 1000 μm and a porosity of at least approximately 60% is particularly advantageous. Such a depth filter is comparatively inexpensive and easy to manufacture and has a sufficiently good filter performance.

It is also proposed that the exhaust system comprises a downstream surface filter and that a catalyst device is arranged upstream of the surface filter, by means of which nitrogen dioxide is formed from the exhaust gas. This exhaust system according to the invention makes it possible to filter out more than 99% of the soot particles contained in the exhaust gas. Although a depth filter can only filter out a maximum of approximately 90% of the soot particles contained in the exhaust gas, this is made possible by the downstream surface filter, which in turn can filter out up to 95 to 99% of the approximately 10% soot content passing through the depth filter. However, since overall only a small amount of soot reaches the surface filter at all, it can be operated for a comparatively long time without having to be freed from the filter cake that forms there. Overall, an exhaust system is created that works simply, has a long service life, and one

Filtering almost all of the soot from the exhaust gas allows.

The proposed catalyst device can in particular work with a platinum catalyst material. As a result, nitrogen dioxide is formed during operation of the internal combustion engine, which burns off the soot on the surface filter at a corresponding temperature. This is also possible continuously and allows the surface filter to be kept completely free, since only a comparatively little soot reaches the surface filter from the depth filter, so that only a comparatively small amount of soot has to be burned off there.

It is also possible that the exhaust system comprises a downstream surface filter and that a structure of the surface filter is provided with a catalyst material. Particles that are not retained by the depth filter are at least to a large extent removed by the surface filter filtered out of the gas stream. Due to the upstream depth filter, however, the surface filter only has to cope with part of the total particle mass, which enables its continuous regeneration, for example by means of nitrogen oxides contained in the exhaust gas. By the

Catalyst material further improves the oxidation of the soot particles on the surface filter.

It is particularly advantageous if the catalyst material of the surface filter comprises a material from the extensive list above.

Alternatively or additionally, the catalyst material of the surface filter can also be a conventional NO _x storage catalyst material, a conventional NH ₃ -SCR-

Catalyst material, and / or include another material for reducing nitrogen oxide emissions. The surface filter thus has an additional function, namely it also serves as a catalyst device for reducing further emissions, in particular nitrogen oxide emissions.

A comparatively inexpensive surface filter is a cordierite filter with a cell number of approximately 50 to 300 cpsi, a porosity of approximately 50% and a pore diameter of at most approximately 100 μm, preferably at most approximately 40 μm, even more preferably at most approximately 10 μm.

The exhaust system according to the invention is particularly compact if the surface filter has a Pt

Catalyst material, in particular Pt-Ce / Zr0 ₂ , and a conventional NO _x on its outflow side

Storage catalyst material includes. In this case on

Input of the surface filter generates the nitrogen dioxide required for soot combustion, and on the On the outflow side, the surface filter works as a NO _x storage catalytic converter, which reduces nitrogen oxide emissions.

The invention also relates to a method for operating an internal combustion engine with an exhaust system of the type in which a surface filter is connected downstream of the depth filter. It is proposed that soot that is deposited in the surface filter is continuously oxidized. This is possible because only comparatively few soot particles reach the surface filter, and because of the comparatively large volume flow that passes through the surface filter. In this way, the surface filter always remains as continuous as possible, which is optimal for the efficiency of the exhaust system.

drawing

Below is a particularly preferred one

Embodiment of the present invention explained with reference to the accompanying drawings. The drawing shows:

Figure 1 is a schematic representation of an exhaust system with a depth filter and a downstream surface filter;

FIG. 2 shows a schematic section through an area of the depth filter from FIG. 1;

Figure 3 shows a detail III of Figure 2; and

FIG. 4 shows a schematic section through an area of the surface filter from FIG. 1. Description of the embodiment

In FIG. 1, an exhaust system of an internal combustion engine bears the reference number 10 overall. The internal combustion engine itself is only shown schematically and is designated by reference number 12. The hot combustion exhaust gases are discharged from the internal combustion engine 12 via an exhaust pipe 14. This initially leads to a depth filter 16 which is provided with a catalytic device 18. This will be described in more detail below. The exhaust pipe 14 leads from the depth filter 16 to a surface filter 20. This is provided on its inflow side with a catalytic device 22 and on its outflow side with a further catalytic device 24. It is also explained in great detail below.

The internal combustion engine 12 is a diesel internal combustion engine. Soot particles, which are filtered out of the exhaust gas stream by the two filters 16 and 20, are initially contained in their exhaust gas, especially during certain operating phases. With the depth filter 16, the soot particles are deposited inside the filter. The depth filter 16 thus has a filter effect over its total volume or its total surface area. A section of an inner region of the depth filter 16 is shown in FIG. 2. Thereafter, the depth filter 16 has pores 26 which are formed between a structure 28 of the depth filter 16. In the present exemplary embodiment, this structure is produced from silicon carbide, so that a so-called open-cell silicon carbide foam filter 16 is formed. Basically, however, for the depth filter 16, all open-porous bodies and fillings made of ceramic and metallic material or combinations of both are suitable, which are suitable for filtering out soot particles from the exhaust gas that flows through the exhaust pipe 14. In particular, comparatively large soot particles should also be able to be filtered out by the depth filters. The pore diameter varies in the range from approximately 40 μm to approximately 1000 μm. Overall, the depth filter 16 has a porosity of more than 60%.

As can be seen from the enlarged detailed view of FIG. 3, the silicon carbide structure 28 of the depth filter 16 is provided with a catalytic material 18. The catalytic material is selected such that it promotes the oxidation or combustion of the soot particles (reference numeral 30 in FIG. 3) at comparatively low temperatures, thereby preventing the depth filter 16 from becoming blocked. The catalytic material 18 which is used in the depth filter 16 has the special feature that it is liquid at a comparatively low temperature. This temperature can be in the range of the normal operating temperature of the depth filter 16, so that the catalytic material 18 in the

Normal operation of the internal combustion engine 12 is always fluid, but it can also be so high that the catalytic material 18 is only fluid when the internal combustion engine 12 is in a special operating phase, in which the

Internal combustion engine 12 through the exhaust pipe 14 exhaust gas has a correspondingly high temperature.

In the case of so-called “molten salt catalyst” materials, for example CS ₂ SO _{4 2} O ₅ or in the case of Cs vanadates, this lies Temperature at which the catalytic material 18 is liquid, at approximately 350 to 400 ° C. Due to the liquefaction of the catalytic material 18, the soot particles 30 deposited in the depth filter 16 are contacted very intimately by the catalytic material 18, in some cases even completely enclosed at least temporarily. As a result, a very high conversion rate is achieved with the same energy input. Such liquid catalysts can also be combined with other of the catalytically active materials described at the outset.

In principle, the depth filter 16 shown has only a maximum filter efficiency of 90%. This means that at least 10% of the soot particles 30 pass through the depth filter 16 and reach the surface filter 20. The surface filter 20 is a wall flow or wall flow filter. This has a honeycomb-like structure seen in the flow direction, which is made of cordierite and bears the reference number 32 in FIG. Some of the honeycombs are open on the side facing the internal combustion engine 12 and closed on the side facing away from the internal combustion engine 12 (the exhaust gas flow is indicated by arrows 33 in FIG. 4). These honeycombs have the reference number 34 in FIG. 4. Other honeycombs 36 adjacent to the honeycombs 34 are on the same

Internal combustion engine 12 side closed and open on the side facing away from the internal combustion engine 12.

The filter effect results from the porosity of the structure 32 of the surface filter 20, so the exhaust gas flow passes through the wall surfaces from the honeycomb 34 into the honeycomb 36, corresponding to the arrow 38 in FIG. 4. The surface filter shown in FIG. 4 has a cell number from 50 to 300 cpsi, a porosity of approximately 50% and a pore diameter of 10 to 30 μm. The porosity of the Structure 32 is selected such that surface filter 20 preferably filters out small soot particles from the exhaust gas flow. The filter efficiency of the surface filter 20 is 95 to 99%. Overall, because of the combination of the depth filter 16 and the surface filter 20, more than 99% of the soot particles are filtered out of the exhaust gas.

Over time, the soot particles 30 deposited on the surface filter 20 form a so-called "filter cake" which, if it is too large, can impair the permeability of the surface filter 20 and thus increase the exhaust gas back pressure. However, this filter cake from the soot particles 30 forms only very slowly, since there are only very few soot particles overall

30 ever get to the surface filter 20. Nevertheless, the surface filter 20 is freed from the soot particles either continuously or cyclically by a so-called CRT process.

For this purpose, the surface filter 20 has on its side facing the internal combustion engine 12 (inflow side) a structure 22 with a platinum catalyst material, in the present case a Pt-Ce / Zr0 ₂ mixture. As a result, nitrogen monoxide contained in the exhaust gas is oxidized to nitrogen dioxide anyway. By means of this nitrogen dioxide, the soot 30 deposited on the surface filter 20 can also preferably be oxidized continuously at comparatively low temperatures (below 400 ° C.).

The surface of the structure 32 of the surface filter 20 facing away from the internal combustion engine 12 is provided with a catalyst layer 24, which in the present exemplary embodiment consists of a conventional NO _x storage catalyst material. As a result, Exhaust nitrogen oxides are reduced. This has nothing to do with the actual function of the surface filter 20, but saves installation space. In principle, however, it is also conceivable to use other catalytically active materials as mentioned at the beginning.

Claims

Expectations

1. Exhaust system (10) for an internal combustion engine (12), with a depth filter (16) for removing soot (30) from the exhaust gas, the depth filter (16) comprising a catalyst material (18) which promotes soot oxidation, characterized that an inner pore structure (28) of the depth filter (16) is provided with a catalyst material (18) which, at an operating temperature of the depth filter (16), in particular from a temperature of approximately at most 400 ° C., more preferably of at most approximately 350 ° C, is liquid.

2. Exhaust system (16) according to claim 1, characterized in that the catalyst material (18) of the depth filter (16) "Molten Salt" material, in particular Cs ₂ S0 ₄ V ₂ O _s or Cs vanadates or Ag compounds, in particular Ag-Vandanate.

3. Exhaust system according to claim 2, characterized in that the catalyst material additionally comprises: Rh and / or Pd, on supports such as aluminum, zirconium,

Ceroxides and / or mixed oxides such as Ce / Zr0 ₂ , or without a carrier; Group 11 elements (Ag, Au, and / or Cu) on supports such as aluminum, zirconium, cerium oxides and / or mixed oxides such as Ce / Zr0 ₂ or without Carrier; Oxygen-storing and releasing materials, for example compounds of Mn, Fe, Ce, Pr; under exhaust gas conditions, nitrate-forming materials (NO _x storage), in particular elements of the alkaline earth group, as well as of group 3 and the rare earths; and / or materials which are characterized by high acidity, for example zeolites and the following oxides or oxide mixtures: Ti0 ₂ , Zr0 ₂ , Si0 ₂ , A1 ₂ 0 ₃ , boron oxides.

4. Exhaust system (16) according to one of the preceding claims, characterized in that the depth filter (16) comprises an open-pore silicon carbide foam filter (28) with pore diameters in the range from approximately 40 μm to approximately 1000 μm and a porosity of at least approximately 60% ,

5. Exhaust system (16) according to one of the preceding claims, characterized in that it comprises a downstream surface filter (20), and that upstream of the surface filter (20) a catalyst device (22) is arranged, through which nitrogen dioxide is formed from the exhaust gas ,

6. Exhaust system (16) according to one of the preceding claims, characterized in that it comprises a downstream surface filter (20), and that a structure (32) of the surface filter (20) is provided with a catalyst material (24).

7. Exhaust system (16) according to claim 6, characterized in that the catalyst material (24) comprises a material from the list of claim 2.

8. Exhaust system (16) according to claim 6 or 7, characterized in that the

Catalyst material (24) a conventional NO _x storage catalyst material, a conventional NH ₃ -SCR- Catalyst material, and / or another material for reducing nitrogen oxide emissions.

9. Exhaust system (16) according to one of claims 6 to 8, characterized in that the surface filter (20) is a cordierite filter (32) with a cell number of approximately 50 to approximately 300 cpsi, a porosity of approximately 50% and a pore diameter of at most about 100 μm, preferably at most about 40 μm, more preferably at most about 10 μm.

10. Exhaust system (16) according to one of claims 6 to 9, characterized in that the surface filter (20) on its inflow side a Pt catalyst material (22), in particular Pt-Ce / Zr0 ₂ , and on its outflow side a conventional NO _x - Storage catalyst material (24) comprises.

11. A method for operating an internal combustion engine with an exhaust system (16) according to one of claims 5 to 10, characterized in that soot (30) deposited in the surface filter (20) is continuously oxidized.