WO1993014304A1

WO1993014304A1 - Soot filter for internal combustion and especially diesel engines

Info

Publication number: WO1993014304A1
Application number: PCT/DE1993/000003
Authority: WO
Inventors: Chu-Wan Hong
Original assignee: Tuhh - Technologie Gmbh
Priority date: 1992-01-11
Filing date: 1993-01-07
Publication date: 1993-07-22
Also published as: DE4200552A1; DE4200552C2; AU3447793A

Abstract

Improvements are to be made in the sturdiness of a soot filter for internal combustion and especially diesel engines with adjacent, substantially straight channels separated by porous material, the ends of which are alternately sealed to form in and outlet channels and are hexagonal in cross-section. This applies particularly to the axial end regions of the filter and the use of a ceramic as the porous material. To this end such a soot filter has the following construction: (a) the number of in and outlet channels is substantially the same; (b) channels completely surrounded by adjacent channels have four permeable and two impermeable walls; (c) channels of the same kind are arranged in planes perpendicular to the channel axes aligned in sections; (d) the impermeable walls of a channel cross-section are diametrically opposite. In a particularly advantageous embodiment the impermeable channel walls are arranged in substantially concentric circles.

Description

Soot filter for internal combustion, especially diesel engines

The invention relates to a soot filter for combustion, in particular diesel engines according to the preamble of claim 1.

In such filters, soot is collected on the upstream channel walls, which burns there at high temperatures to clean the channel walls. Due to the burning off and the associated different temperature distribution over the filter volume, the filter material is subjected to extremely different thermal loads, which can lead to mechanical destruction of the filter material. This applies particularly to ceramic as a filter material.

To explain the thermally unequal loading of the individual filter sections, a soot filter with a cylindrical shape is considered below.

The soot particles to be separated accumulate in the inlet channels on the connecting walls there to the outlet channels. As soon as a suitable state with regard to a sufficient temperature and a sufficient amount of oxygen is reached, a combustion process begins in the soot particle layer, by means of which the filter is freed of the deposited soot. This process is referred to as regeneration of the filter. The combustion temperatures are very high and are around 2400 degrees C. In contrast, the exhaust gas temperature is approx. 250 - 600 degrees C relatively low. During the combustion process, exhaust gas continues to flow through the particle layer, the heat of combustion being largely absorbed by the exhaust gas and transported through the channel walls to the outflow side of the filter. In addition, heat also passes directly into the channel walls. Because of the much lower temperature of the exhaust gas compared to the temperature of the burning particle layer and because of the large amounts of exhaust gas that pass through the individual channel walls, the heat removal by the exhaust gas can be referred to as cooling of the channel walls.

The flow through the filter body usually takes place in such a way that the particle deposits are unevenly strong over the cross section, the deposits usually being larger in the middle than in the peripheral zones. This results in an uneven temperature distribution during the burning process with maximum values of approximately 1200 degrees C in the area of the cylinder axis and lower values down to 200-400 degrees C in the outer zone. This drop in temperature is exacerbated by heat losses to the environment. The combustion process usually begins near the inflow side of the filter and propagates in the longitudinal direction. The burning rate decreases as the incoming exhaust gas escapes through the already burned channel walls instead of through the particle layer, thereby reducing the oxygen supply. At the same time, however, the cooling of the combustion zone also decreases. In addition, the particle deposits on the outflow side are stronger. As a result, the greatest thermal load on the channel walls occurs in the last phase of a filter regeneration and essentially stresses the channel walls close to the stopper on the outflow side serving as channel closures. The temperature-related deformations correspond to the temperature distribution. The hot inner area expands radially against the resistance of the less strongly heated edge zone. Accordingly, compressive stresses occur on the inside, tensile stresses on the outside in tangential and also axial directions. The area provided with sealing plugs is not heated directly and therefore behaves like an edge zone.

The cross-sectional shape of the channels is important when considering the thermal stress and its consequences on the mechanical strength. For example, square channel cross sections that have been frequently used so far are unfavorable. Such filters have, in particular, very different strengths under radial loading, depending on whether the loading is directed parallel to the walls or not. There can therefore be up to seven-fold differences in strength. In the case of the thermal deformation described above, such an anisotropy property causes an uneven expansion in the radial direction (“cloverleaf” shape) and consequently leads to a deformation of the originally square channel cross sections to form rhombuses. This results in a bending load on the channel walls. However, the end area provided with sealing plugs has isotropic strength properties, since the "compartments" of the "framework" are filled. Therefore, at the transition from free channels to the respectively closed end regions, ie where the different types of deformation meet and have to be compensated for, a complex load of bending and shear stress on the walls arises. Consequently, mechanical damage to ceramic filters often occurs in these. With regard to the anisotropic strength and deformation properties described above, the use of channels with hexagonal cross sections, as is known per se from EP-B-0089751, already brings a significant improvement. Through such a channel In cross-sectional form, more favorable isotropy properties can already be achieved, by means of which the bending load on the channel walls and the combined bending and shear load in the transition zone from free to closed channels can be reduced. In terms of strength, the channel distribution in the filter cross-sectional planes perpendicular to the channel axes is not favorable in the known filter designs with hexagonal channel cross sections.

Proceeding from this, the invention is concerned with creating a further improvement in this regard.

This problem is solved by designing a soot filter according to the characterizing features of patent claim 1.

Appropriate refinements and developments of this basic solution are the subject of the subclaims.

In the case of hexagonal channel cross sections, no arrangement can be found in which all channel walls are flowed through. With a ratio of the number of flowed through to non-flowed through walls of 2: 1, there are walls with different thermal loads in each channel cell, since the deposition, regeneration and cooling processes are different. These locally different stresses cause local deformations, which in turn can lead to structural damage in unfavorable cases. If the inlet and outlet channels are aligned with one another in accordance with the invention in such a way that in the individual channels the two walls not flowed through are diametrically opposite, the channel cells can deform oval, but not to an oblique hexagon, which would be particularly unfavorable in terms of strength. Embodiments of the invention are shown in the drawing.

Show it

1 shows a section through an essentially schematically illustrated cylindrical soot filter,

2a-d different channel arrangements in a filter body in section along line II-II in Fig. 1,

Fig. 3 is an enlarged schematic representation of a

Channel arrangement according to the filter body shown in cross section in FIG. 2b,

FIG. 4 shows an alternative channel arrangement in a representation according to FIG. 3.

The soot filter according to FIG. 1 has a cylindrical housing 1, into which a cylindrical filter body 2 made of porous ceramic with rectilinear inlet and outlet ducts 4, which are mutually closed at the ends, is inserted.

2 shows various channel arrangements distributed over the cross section of the filter body.

The hexagonal channel cross sections marked with crosses represent the inlet channels 3 and the white channel cross sections represent the outlet channels 4. It is important for the function according to the invention that channels 3 or 4 of the same type are arranged adjacent to one another in a plane perpendicular to the channel axes on line sections 5, 6.

The arrangement of the channels 3, 4 according to FIG. 2 b is particularly expedient. Then the individual channels lie in the cross-sectional planes of the filter in each case on approximately concentric circles 5 to 10, the channel type changing from circle to circle. This arrangement is particularly expedient in relation to the temperature load described at the outset and the mechanical load on the filter body resulting therefrom. This can be explained quite clearly by the enlarged representation of the channel arrangement from FIG. 2b in FIG. 3. 3, the channel walls 11 not flowed through are blackened in each case. All other channel walls 12 are flowed through and hatched. As can be clearly seen, these extend practically radially outward from the central axis of the cross section of the filter body, whereby they lie circumferentially on concentric circles. In the case of the problem underlying the invention which was explained in detail at the beginning, such a radial arrangement of the channel walls through which there is no flow has a considerable advantage, namely because of the center-weighted particle loading of the filter body, which causes a corresponding radial expansion. With regard to the remaining channel cross-section and manufacturing requirements, the channel walls and their wall thicknesses can be optimally adjusted to the essentially concentric mechanical load in this embodiment.

Another also particularly favorable channel arrangement is shown in FIG. 4, in which the display bolik corresponds to that of FIG. 3. In the execution there lie the channel walls 11 not flowed through again essentially aligned in concentric circles, but in this case tangential to them. This is achieved by the mutual assignment of the inlet and outlet channels shown in that FIG.

According to the invention, the wall thicknesses of the through-flow and non-through-flow channel walls 11 can be designed differently, depending on the mechanical loads to which they are exposed.

In the embodiment according to FIG. 3, it is advantageous to give the walls 11 not flowed through a greater wall thickness.

Also in the arrangement of the channels according to FIG. 4, it is advisable to make the thickness of the channel walls 11 not flowed through larger than that of the flowed through channels 12. This may take into account a different combination of radial and tangential stresses, such as it could occur with a filter with a larger diameter (and consequently also larger voltages in the outer region).

Claims

1. Ru filter for internal combustion engines, in particular diesel engines, with channels lying next to one another and separated from porous material, essentially rectilinearly parallel, which are mutually closed at their ends to form inlet and outlet channels and have a hexagonal cross section, characterized by di M note:

(a) the number of inlet (3) and outlet channels (4) is essentially the same,

(b) channels surrounded on all sides by adjacent channels generally have four flow-through (12) and two non-flow-through channel walls (12),

(c) channels of the same type are lined up in lines perpendicular to the longitudinal axis of the channels at least in sections,

(d) the channel walls (11) not flowed through are generally diametrically opposite each per channel cross section.

2. Soot filter according to claim 1, characterized in that the channel walls (12) not flowed through are arranged in planes perpendicular to the longitudinal axis of the channels on essentially concentric circles (5 to 10) or polygons.

3. soot filter according to claim 2, d a d u r c h g e k e n n z e i c h n e t that the polygons are hexagons.

4. Soot filter according to claim 2 or 3, so that the channel walls (11) through which the flow does not flow are aligned essentially radially to the central axis of the filter cross section.

5. soot filter according to claim 4, d a d u r c h g e k e n n z e i c h n e t that on the concentric circles (6 to 10) or polygons each have only inlet or outlet channels and that the type of channels varies from circle to circle.

6. soot filter according to claim 2, so that the channel walls (11) lying on essentially concentric circles and not flowed through flow tangentially to these circles.

7. A soot filter according to one of the preceding claims, that the flow through (12) and non-flow through channel walls (11) have different wall thicknesses corresponding to their respective stresses.