Ceramic filter for exhaust gas emission control Number:7,393,376 from the United States Patent and Trademark Office (PTO) owispatent A ceramic filter assembly that resists cracking. The ceramic filter assembly is formed by integrating a plurality of columnar honeycomb filters made of a porous ceramic sintered material with a ceramic sealing material layer and formed to have a substantially elliptical cross sectional shape. The honeycomb filters includes square columnar honeycomb filters in which the ratio between the lengths of their long sides and short sides is between 1.1 and 3.0. The honeycomb filters are arranged so that the long sides and the short sides are respectively parallel to the major axis and the minor axis of the assembly.<H emission, control, Ceramic, filter, , 7393376.html, Patent, pto, 7393376

Title: Ceramic filter for exhaust gas emission control

Abstract: A ceramic filter assembly that resists cracking. The ceramic filter assembly is formed by integrating a plurality of columnar honeycomb filters made of a porous ceramic sintered material with a ceramic sealing material layer and formed to have a substantially elliptical cross sectional shape. The honeycomb filters includes square columnar honeycomb filters in which the ratio between the lengths of their long sides and short sides is between 1.1 and 3.0. The honeycomb filters are arranged so that the long sides and the short sides are respectively parallel to the major axis and the minor axis of the assembly.

Patent Number: 7,393,376 Issued on 07/01/2008 to Taoka, et al.

Inventors: Taoka; Noriyuki (Gifu, JP), Yoshida; Yutaka (Gifu, JP)
Assignee: Ibiden Co., Ltd. (Ogaki-shi, JP)

Appl. No.: 10/502,044

Filed: March 17, 2003

PCT Filed: March 17, 2003

PCT No.: PCT/JP03/03183

371(c)(1),(2),(4) Date: January 24, 2005

PCT Pub. No.: WO03/078026

PCT Pub. Date: September 25, 2003

Foreign Application Priority Data


Mar 15, 2002 [JP]			2002-072847

Current U.S. Class: 55/523 ; 55/282.3; 55/385.3; 55/484; 55/524; 55/DIG.10; 55/DIG.30; 60/311

Current International Class: B01D 46/00 (20060101); F01N 3/021 (20060101)

Field of Search: 55/282.2,282.3,385.3,482,483,484,523,524,DIG.5,DIG.10,DIG.30 60/311 428/116,117,118

References Cited [Referenced By]

U.S. Patent Documents


4276071	June 1981	Outland
4417908	November 1983	Pitcher, Jr.
4693338	September 1987	Clerc
4698213	October 1987	Shimozi et al.
4782661	November 1988	Motley et al.
5242871	September 1993	Hashimoto et al.
5273724	December 1993	Bos
5456965	October 1995	Machida et al.
5686039	November 1997	Merry
5914187	June 1999	Naruse et al.
5930994	August 1999	Shimato et al.
5943771	August 1999	Schmitt
6159578	December 2000	Ichikawa
6317976	November 2001	Aranda et al.
6447564	September 2002	Ohno et al.
6565630	May 2003	Ohno et al.
6656564	December 2003	Ichikawa et al.
6669751	December 2003	Ohno et al.
6770116	August 2004	Kojima
7138002	November 2006	Hamanaka et al.
2004/0031264	February 2004	Kojima
2004/0033175	February 2004	Ohno et al.
2004/0055265	March 2004	Ohno et al.
2004/0093858	May 2004	Aoki
2004/0161596	August 2004	Taoka et al.

Foreign Patent Documents


43 41 159	Jun., 1995	DE
296 11 788 U 1	Oct., 1996	DE
0 299 626	Jan., 1989	EP
0 816 065	Jan., 1998	EP
1 142 619	Oct., 2001	EP
60-65219	Apr., 1985	JP
62-114633	May., 1987	JP
2-4500	Jan., 1990	JP
6-241018	Aug., 1994	JP
7-204500	Aug., 1995	JP
2001-162119	Jun., 2001	JP
2001-170426	Jun., 2001	JP
2002-054422	Feb., 2002	JP
2002-273130	Sep., 2002	JP
2003-117320	Apr., 2003	JP
WO 03/021089	Mar., 2003	WO
03 071105	Aug., 2003	WO
03/074848	Sep., 2003	WO
03 080219	Oct., 2003	WO
03 081001	Oct., 2003	WO
03 084640	Oct., 2003	WO
03 093657	Nov., 2003	WO
03/093658	Nov., 2003	WO
2004-031100	Apr., 2004	WO
2004/031101	Apr., 2004	WO
2004-076027	Sep., 2004	WO
WO 2004/106702	Dec., 2004	WO
WO 2004/111398	Dec., 2004	WO
WO 2004/113252	Dec., 2004	WO
WO 2005/000445	Jan., 2005	WO
WO 2005/002709	Jan., 2005	WO
WO 2005/005018	Jan., 2005	WO
WO 2005/044425	May., 2005	WO

Other References

US. Appl. No. 09/926,795, filed Jan. 12, 1996. cited by other .
U.S. Appl. No. 10/129,126, filed Jan. 13, 2000. cited by other .
U.S. Appl. No. 10/787,089, filed Mar. 17, 2003. cited by other .
U.S. Appl. No. 10/490,206, filed Sep. 2, 2004, Hong et al. cited by other .
U.S. Appl. No. 10/490,205, filed Sep. 9, 2004, Komori et al. cited by other .
U.S. Appl. No. 10/493,056, filed Aug. 17, 2004, Hong et al. cited by other .
U.S. Appl. No. 10/502,054, filed Jul. 30, 2004, Kudo et al. cited by other .
U.S. Appl. No. 10/502,045, filed Jul. 29, 2004, Kudo et al. cited by other .
U.S. Appl. No. 10/502,044, filed Jul. 29, 2004, Taoka et al. cited by other .
U.S. Appl. No. 10/506,247, filed Sep. 9, 2004, Kudo et al. cited by other .
Patent Abstracts of Japan, JP 2002-060279, Feb. 26, 2002. cited by other .
Patent Abstracts of Japan, JP 2000-279728, Oct. 10, 2000. cited by other.

Primary Examiner: Greene; Jason M
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.

Claims

The invention claimed is:

1. A ceramic filter assembly comprising: major and minor axes; a plurality of columnar honeycomb filters adhered together; the plurality of columnar honeycomb filters being made of a porous ceramic sintered material with a ceramic sealing material layer and having end faces and a generally elliptical cross sectional shape when cut parallel to the end faces of the plurality of honeycomb filters; the plurality of honeycomb filters including a honeycomb filter having a rectangular cross sectional shape when cut parallel to the end faces and provided with a long side having a length B1 and a short side having a length B2 in which the ratio B1/B2 is between 1.1 and 3.0; the honeycomb filter being arranged so that the long side and the short side of the honeycomb filter are respectively parallel to the major axis and the minor axis of the assembly.

2. A ceramic filter assembly comprising; major and minor axes; a plurality of columnar honeycomb filters adhered together; the plurality of columnar honeycomb filters being made of a porous ceramic sintered material with a ceramic sealing material layer and having end faces and a generally elliptical cross sectional shape when cut parallel to end faces of the plurality of honeycomb filters; each honeycomb filter including a plurality of rectangular cells extending along an axis of the filter with each cell provided with a long side having a length C1 and a short side having a length C2 in which the ratio C1/C2 is between 1.1 and 3.0; and the plurality of honeycomb filters being arranged so that the long sides of the cells are parallel to the major axis of the assembly and the short sides of the cells are parallel to the minor axis of the assembly.

3. A ceramic filter assembly comprising: major and minor axes; a plurality of columnar honeycomb filters adhered together; the plurality of columnar honeycomb filters being made of a porous ceramic sintered material with a ceramic sealing material layer and having end faces and a generally elliptical cross sectional shape when cut parallel to the end faces of the plurality of honeycomb filters; each honeycomb filter including an axis and a plurality of rectangular cells extending along the axis of the filter and defined by relatively thick cell walls and relatively thin walls that are orthogonal to each other; and the plurality of honeycomb filters being arranged so that the relatively thick cell walls are parallel to the major axis of the assembly and the relatively thin cell walls are parallel to the minor axis of the assembly.

4. The ceramic filter assembly as claimed in claim 3, wherein when the thickness of the relatively thick cell walls is represented by D1 and the thickness of the relatively thin cell walls is represented by D2, D1 and D2 are within a range of 0.1 to 0.5 mm, and the ratio D1/D2 is 3 or less.

5. A ceramic filter assembly comprising: a major axis; a plurality of columnar honeycomb filters including outer surfaces adhered together; the plurality of colunmar honeycomb filters being made of a porous ceramic sintered material with a ceramic sealing material layer and having end faces and a generally elliptical cross sectional shape when cut parallel to the end faces of the plurality of honeycomb filters; and the ceramic sealing material layer including, a first sealing material layer extending parallel to the major axis of the assembly, and a second sealing material layer extending orthogonal to the major axis of the assembly, wherein the first sealing material layer is thicker than the second sealing material layer.

6. The ceramic filter assembly as claimed in claim 5, wherein when the thickness of the first sealing material layer is represented by E1 and the thickness of the second sealing material layer is represented by E2, E1 and E2 are between 0.3 mm to 3 mm, and the ratio E1/E2 is 1.05 or greater and 5 or less.

7. A ceramic filter assembly comprising: a major axis; a plurality of columnar honeycomb filters adhered together; the plurality of columnar honeycomb filters being made of a porous ceramic sintered material with a ceramic sealing material layer and having end faces and a generally elliptical cross sectional shape when cut parallel to the end faces of the plurality of honeycomb filters; and the ceramic sealing material layer including, a first sealing material layer parallel to the major axis of the assembly, and a second sealing material layer orthogonal to the major axis of the assembly, the first sealing material layer having thermal conductivity that is lower than the thermal conductivity of the second sealing material layer.

8. The ceramic filter assembly as claimed in claim 7, wherein when the thermal conductivity of the first sealing material layer is represented by G1 and the thermal conductivity of the second sealing material layer is represented by G2, the ratio G1/G2 is 0.2 or greater and 0.7 or less.

9. A ceramic filter assembly comprising: an outer periphery, major, and minor axes; a plurality of columnar honeycomb filters adhered together; the plurality of columnar honeycomb filters being made of a porous ceramic sintered material with a ceramic sealing material layer made of ceramic and having end faces and a generally elliptical cross sectional shape when cut parallel to the end faces of the plurality of honeycomb filters; an outer sealing material layer made of ceramic and formed on the periphery of the assembly; and the outer sealing material layer including a first portion located along an extension of the major axis of the assembly that is thicker than a second portion located along an extension of the minor axis of the assembly.

10. The ceramic filter assembly as claimed in claim 9, wherein when the thickness of the first portion is represented by H1 and the thickness of the second portion is represented by H2, the ratio H2/H1 is 0.06 or greater and 0.95 or less.

11. The ceramic filter assembly as claimed in claim 9, wherein the outer sealing material layer is formed from two or more types of a coating material having different thermal conductivity.

12. A canning body comprising: a ceramic filter assembly including major and minor axes and a plurality of columnar honeycomb filters adhered together, the plurality of columnar honeycomb filters being made of a porous ceramic sintered material with an inner sealing material layer made of ceramic and having end faces and a generally elliptical cross sectional shape when cut parallel to the end faces of the plurality of honeycomb filters; a tubular casing for accommodating the ceramic filter assembly; and a thermal insulation material arranged between the casing and the ceramic filter assembly, the thermal insulation material including a first portion located along an extension of the major axis of the assembly and a second portion located along an extension of the minor axis of the assembly, wherein the first portion is thicker than the second portion.

13. The canning body as claimed in claim 12, wherein when the thickness of the first portion is represented by I1 and the thickness of the second part is represented by 12, the ratio I2/I1 is 0.30 or greater and 0.91 or less.

14. The canning body as claimed in claim 12, wherein the thermal insulation material is made of two or more types of material having different thermal conductivity.

15. A columnar honeycomb filter comprising: a plurality of rectangular cells extending along an axial direction of the honeycomb filter; each rectangular cell being defined by a relatively thick cell wall and a relatively thin cell wall that are orthogonal to each other, and being made of a porous ceramic sintered material; and the relatively thick cell walls having a uniform wall thickness and the relatively thin cell walls having a uniform wall thickness.

16. The columnar honeycomb filter as claimed in claim 15, wherein when the thickness of the relatively thick cell wall is represented by D1 and the thickness of the relatively thin cell wall is represented by D2, the ratio D1/D2 is 3 or less.

17. The ceramic filter assembly as claimed in claim 1, wherein the porous ceramic sintered material includes silicon carbide and metal silicon.

18. The ceramic filter assembly as claimed in claim 1, further comprising a catalyst.

19. A ceramic filter assembly comprising: a plurality of columnar honeycomb filters adhered together; the plurality of honeycomb filters being made of a porous ceramic sintered material with a ceramic sealing material layer and having end faces and a generally elliptical cross sectional shape when cut parallel to the end faces of the plurality of honeycomb filters, wherein when a hypothetical first straight line intersects the generally elliptical contour at two points in which the distance therebetween is maximum and a hypothetical second straight line orthogonal to the first straight line intersects the generally elliptical contour at two points in which the distance therebetween is maximum, the number of sealing material layers the first straight line of the assembly traverses is less than or equal to the number of sealing material layers the second straight line traverses.

20. The ceramic filter assembly as claimed in claim 2, wherein the porous ceramic sintered material includes silicon carbide and metal silicon.

21. The ceramic filter assembly as claimed in claim 2, further comprising a catalyst.

22. The honeycomb filter as claimed in claim 15, wherein the porous ceramic sintered material includes silicon carbide and metal silicon.

23. The columnar honeycomb filter as claimed in claim 15, further comprising a catalyst.

Description

TITLE OF THE INVENTION

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from Japanese Patent Application No. 2002-72847, filed on Mar. 15, 2002, the contents of which are hereby incorporated herein reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to ceramic filters for exhaust gas emission control, and more particularly, to a ceramic filter assembly in which a plurality of filters made of a ceramic sintered material are integrated, a canning body, and a columnar honeycomb filter that may be used when manufacturing the same.

DESCRIPTION OF THE RELATED ART

The number of automobiles is dramatically increasing, and in proportion thereto, the amount of exhaust gas exhausted from internal combustion engines of automobile is also rapidly increasing. Various substances contained in the exhaust gas, especially from diesel engines cause pollution, and thus presently, are seriously affecting the world environment. Reports have been recently made on study results that fine particles (diesel particulate) in the exhaust gas may sometimes cause allergic symptoms or reduce sperm counts. A measure for eliminating the fine particles in the exhaust gas is thus an urgent problem that must be coped with for the sake of mankind.

Accordingly, a variety of exhaust gas purifying devices have been proposed in the prior art. A typical exhaust gas purifying device has a configuration in which a casing is arranged on an exhaust pipe coupled to an exhaust manifold of an engine, and a filter including fine holes is arranged therein. The filter may be made of, besides metal and metal alloy, ceramic. A known example of a filter made of ceramic includes a honeycomb filter made of cordierite. Recently, a porous silicon carbide sintered material is often used as the material forming the filter because of the advantages of, for example, high thermal resistance, high mechanical strength, high collecting efficiency, chemical stability, and small pressure loss (e.g., Japanese Laid-Open Patent Publication No. 2001-162119).

The honeycomb filter has multiple cells (through-holes) extending in an axial direction thereof. When exhaust gas passes through the filter, the fine particles are trapped at the cell walls of the filter. As a result, the fine particles are removed from the exhaust gas.

However, since the honeycomb filter made of a porous silicon carbide sintered material has large thermal expansion, as the size of the filter increases, cracks tend to occur in the filter during use at high temperature. Thus, a technique for manufacturing one large ceramic filter assembly by integrating a plurality of small filter pieces has been recently proposed as a means for avoiding damage caused by cracks.

A general method for manufacturing the above mentioned assembly will now be briefly introduced.

First, a square columnar shaped honeycomb molded product is formed by continuously extruding a ceramic material through a metal mold die of an extruder. After cutting the honeycomb molded product into equal lengths, each cut piece is sintered to produce a filter. After the sintering, the outer surfaces of the filters are adhered to each other by a ceramic sealing material layer to bundle and integrate the filters. Consequently, the desired ceramic filter assembly is completed. A mat thermal insulation material including ceramic fibers and the like is wrapped around the outer surface of the ceramic filter assembly. The assembly in such state is accommodated within a casing arranged on the exhaust pipe.

[Patent Publication 1]

Japanese Patent Publication No. 2001-162119

In case of a filter having an integrated structure and a cross section that is oblong, such as, a substantially elliptical shape, it is found that cracks are more likely to occur in filters located at peripheral portions rather than at central portions of the assembly. When observing the filter assembly after repeating reproduction a number of times and dividing the filter assembly, a slight amount of burnt residue of soot was found for the first time in filters located at the peripheral portions. It can thus be presumed that a temperature difference exists between individual honeycomb filters. This causes a difference in the reproduction level during a single reproducing process. Further, the soot residue causes a difference in the subsequent collecting amount, and the temperature stress due to the difference in the amount of soot during reproduction causes cracks in the honeycomb filter.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a ceramic filter assembly having superior strength. It is a further aspect of the present invention to provide a columnar honeycomb filter suitable for manufacturing such a ceramic filter assembly.

The inventor of the present invention has recognized that in an exhaust gas purifying device connected to an engine through a pipe having an inner diameter smaller than the filter, a temperature difference is produced between a central portion and a peripheral portion of the filter assembly when the pipe is conically enlarged just before the filter.

Further, the inventor of the present invention has recognized that in a filter assembly having an oblong shape such as substantially elliptical shape, a large temperature difference is produced between the peripheral portion in the major axis direction and the peripheral portion in the minor axis due to difference in the distance from the central portion. The inventor has confirmed that this temperature difference prevents uniform reproduction, produces residual soot, and cause cracking when the filter exceeds its strength limit.

Based on the above knowledge, the inventor has conducted tests and research to manufacture an oblong filter assembly enabling uniform temperature rise. As a result, it has become understood that if the ceramic filter assembly is manufactured so as to satisfy certain conditions, thermal stress would be absorbed and a ceramic filter assembly having superior strength would be manufactured.

Accordingly, a conclusion has been reached that in order to uniformly transfer heat from the central portion to the peripheral portion, a condition in which the thermal conductivity in the major axis direction is higher than the thermal conductivity in the minor axis direction, particularly, a condition in which the heat insulating effect is increased at the peripheral portion of the assembly in the major axis direction compared to the peripheral portion in the minor axis direction should be satisfied.

The present inventions provides a ceramic filter assembly integrated by adhering together a plurality of columnar honeycomb filters made of a porous ceramic sintered material with a ceramic sealing material layer and having a substantially elliptical cross sectional shape when cut parallel to end faces of the plurality of honeycomb filters.

In a first aspect, the plurality of honeycomb filters include a honeycomb filter having a rectangular cross sectional shape when cut parallel to the end faces and provided with a long side having length B1 and a short side having length B2 in which the ratio B1/B2 is between 1.1 and 3.0. The honeycomb filter is arranged so that the long side and the short side of the honeycomb filter are respectively parallel to the major axis and minor axis of the assembly.

In a second aspect of the present invention, each honeycomb filter includes a plurality of rectangular cells extending along an axis of the filter with each cell provided with a long side having length C1 and a short side having length C2 in which the ratio C1/C2 is between 1.1 and 3.0. The plurality of honeycomb filters are arranged so that the long sides of the cells are parallel to the major axis of the assembly and the short sides of the cells are parallel to the minor axis of the assembly.

In a third aspect, each honeycomb filter includes a plurality of rectangular cells extending along an axis of the filter and defined by relatively thick cell walls and relatively thin walls that are orthogonal to each other. The plurality of honeycomb filters are arranged so that the relatively thick cell walls are parallel to the major axis of the assembly and the relatively thin cell walls are parallel to the minor axis of the assembly.

In a fourth aspect, the ceramic sealing material layer includes a first sealing material layer extending parallel to the major axis of the assembly and a second sealing material layer extending orthogonal to the major axis of the assembly. The first sealing material layer is thicker than the second sealing material layer.

In a fifth aspect, the ceramic sealing material layer includes a first sealing material layer parallel to the major axis of the assembly and a second sealing material layer orthogonal to the major axis of the assembly. The first sealing material layer has a thermal conductivity lower than the thermal conductivity of the second sealing material layer.

In a sixth aspect, the ceramic filter assembly further includes an outer sealing material layer made of ceramic and formed on the periphery of the assembly. The outer sealing material layer includes a first portion located along an extension of the major axis of the assembly that is thicker than a second portion located along an extension of the minor axis of the assembly.

In a seventh aspect, a ceramic filter assembly integrated by adhering together a plurality of columnar honeycomb filters made of a porous ceramic sintered material with an inner sealing material layer made of ceramic and has a generally elliptical cross sectional shape when cut parallel to end faces of the plurality of honeycomb filters is provided. A tubular casing accommodates the ceramic filter assembly. A thermal insulation material is arranged between the casing and the ceramic filter assembly. The thermal insulation material includes a first portion located along an extension of the major axis of the assembly and a second portion located along an extension of the minor axis of the assembly. The first portion is thicker than the second portion.

In an eighth aspect, a columnar honeycomb filter made of a porous ceramic sintered material is provided. The honeycomb filter has a rectangular cross sectional shape when cut parallel to an end face thereof and is provided with a long side having length B1 and a short side having length B2 in which the ratio the B1/B2 is 3.0 or less.

In a ninth aspect, the columnar honeycomb includes a plurality of cells, extending along the axial direction thereof, and an end face. Each cell has a rectangular cross sectional shape when cut parallel to the end face. Each cell is provided with a long side having length C1 and a short side having length C2 in which the ratio C1/C2 is 3.0 or less.

In a tenth aspect, a columnar honeycomb filter made of a porous ceramic sintered material includes a plurality of rectangular cells extending along the axial direction of the honeycomb filter. Each rectangular cell is defined by a relatively thick cell wall and a relatively thin cell wall that are orthogonal to each other.

In an eleventh aspect, in a ceramic filter assembly having a substantially elliptical cross sectional shape, when a hypothetical first straight line intersects the generally elliptical contour at two points in which the distance therebetween is maximum and a hypothetical second straight line orthogonal to the first straight line intersects the generally elliptical contour at two points in which the distance therebetween is maximum, the number of sealing material layers that the first straight line of the assembly traverses is less than or equal to the number of sealing material layers that the second straight line traverses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an exhaust gas purifying device including a ceramic filter assembly according to one embodiment of the present invention;

FIG. 2 is a perspective view showing the ceramic filter assembly of FIG. 1;

FIG. 3(a) is a perspective view showing a honeycomb filter having a rectangular cross section;

FIG. 3(b) is a perspective view showing a honeycomb filter having a rectangular cell;

FIG. 3(c) is a perspective view showing a honeycomb filter having a plurality of cells partitioned by cell walls that are orthogonal to each other and have different thicknesses;

FIG. 4 is a cross sectional view showing the exhaust gas purifying device of FIG. 1;

FIGS. 5(a) to 5(e) are views showing cross sectional shapes of the ceramic filter assembly;

FIG. 6(a) is a side view of the filter assembly formed from a honeycomb filter having a rectangular cross section;

FIGS. 6(b) and 6(c) are side views of the filter assembly formed from a honeycomb filter having a square cross section;

FIGS. 7(a), 7(b), and 7(c) are side views of the filter assembly formed from a honeycomb filter having cells of different shapes;

FIGS. 8(a), 8(b), and 8(c) are side views of a filter assembly formed from a honeycomb filter having walls of different thicknesses;

FIGS. 9(a), 9(b), and 9(c) are side views of the filter assembly integrated with a sealing material layer of different thickness;

FIG. 10(a) is a side-view of the filter assembly integrated by a sealing material layer of different thermal conductivity;

FIG. 10(b) is a side view of the filter assembly including an exterior sealing material layer of uneven thickness; and

FIG. 10(c) is a side view of the filter assembly including a thermal insulation material of uneven thickness.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exhaust gas purifying device 1 according to one embodiment of the present invention will now be described.

As shown in FIG. 1, the exhaust gas purifying device 1 is a device for purifying exhaust gas discharged from a diesel engine 2 serving as an internal combustion engine. The diesel engine 2 includes a plurality of cylinders (not shown). Each cylinder is connected to a branch pipe 4 of an exhaust manifold 3 that is made of a metal material. Each branch pipe 4 is connected to a single manifold body 5. Accordingly, the exhaust gas discharged from each cylinder pipe is concentrated at one location.

A first exhaust pipe 6 and a second exhaust pipe 7 made of a metal material are arranged downstream of the exhaust manifold 3. An upstream end of the first exhaust pipe 6 is coupled to the manifold body 5. A tubular casing 8, also made of a metal material, is arranged between the first exhaust pipe 6 and the second exhaust pipe 7. An upstream end of the casing 8 is coupled to a downstream end of the first exhaust pipe 6, and a downstream end of the casing 8 is coupled to an upstream end of the second exhaust pipe 7. The exhaust gas flows through the first exhaust pipe 6, the casing 8, and the second exhaust pipe 7.

As shown in FIG. 1, a central portion of the casing 8 has a greater diameter than the exhaust pipes 6, 7. That is, the interior of the casing 8 is larger than that of the exhaust pipes 6, 7. A ceramic filter assembly 9 is accommodated in the casing 8. The exhaust gas purifying device 1 accommodating the ceramic filter assembly 9 in the casing 8 is referred to as a canning body.

A thermal insulation material 10 is arranged between the outer surface of the assembly 9 and the inner surface of the casing 8. The thermal insulation material 10 is a mat-shaped body formed from ceramic fibers, and has a thickness of 2 mm to 60 mm. The thermal insulation material 10 preferably has an elastic structure and has a function for releasing thermal stress. The thermal insulation material 10 minimizes energy loss during reproduction by preventing heat from escaping from the outermost portion of the assembly 9. Further, due to the elastic structure, the ceramic filter assembly 9 is prevented from being displaced by the pressure of the exhaust gas and vibrations when the vehicle is traveling.

The ceramic filter assembly 9 of the present embodiment is for removing diesel particulates as mentioned above, and is thus normally referred to as a diesel particulate filter (DPF). As shown in FIG. 2 and FIG. 4, the assembly 9 of the present embodiment is formed by bundling and integrating a plurality of honeycomb filters F1. Among the plurality of honeycomb filters F1, the honeycomb filters F1 located at the central portion of the assembly 9 each has a square columnar shape, as shown in FIGS. 3(a) to 3(C). Honeycomb filters F1 having a shape other than a square columnar shape are arranged around the square columnar shaped honeycomb filters F1. As a result, when seen as a whole, the ceramic filter assembly 9 has a substantially elliptic cylinder shape with a substantially elliptical cross sectional shape.

The cross section of the assembly 9 of the present invention is substantially elliptical. "Substantially elliptical" is not limited only to an ellipse configured only by curves, as shown in FIG. 5(a). An oblong elliptical shape partially having, for example, straight lines as shown in FIG. 5(b), more specifically, a pair of straight parallel lines to each other is also included. The straight portion may only be at one section, or may be at more than three sections. "Oblong" includes shapes as shown in FIG. 5(a), FIG. 5(b), FIG. 5(c), FIG. 5(d), and FIG. 5(e). The lengths of the major axis and minor axis of the assembly 9 are defined as A1 and A2 (A1>A2), respectively. If the substantially elliptical shape is an ellipse, a long axis passing a focal point is the major axis, and a short axis orthogonal thereto is the minor axis. The dimensions A1 and A2 are preferably 500 mm or less. If the dimensions A1 and A2 are greater than 500 mm, it becomes difficult to manufacture the assembly with sufficient strength.

The length L (mm) of each honeycomb filter F1 is defined as the dimension of the direction in which the exhaust gas or fluid subjected to treatment flows (direction orthogonal to the end face of the filter). When each honeycomb filter F1 is cut perpendicular to the flow direction of the exhaust gas (that is, cut parallel to the end face of the filter), the cross section is rectangular. The lengths (outside dimension) of the long side and the short side of the cross section of the honeycomb filter F1 are defined as B1 and B2 (B1.gtoreq.B2), respectively. Each of the dimensions B1 and B2 are preferably 110 mm or less. This is because the strength of the filter F1 decreases significantly when the dimensions B1 and B2 are greater than 110 mm.

It is preferred that the ratio of B1/B2 be 3 or less. This is because if the ratio of B1/B2 is greater than 3, thermal stress is more likely to act on the filter F1 due to thermal shock, and cracks are more likely to occur.

The honeycomb filter F1 is made of a porous silicon carbide sintered material, which is one type of porous ceramic sintered material. The silicon carbide sintered material is used because of its excellent thermal resistance and heat conductivity when compared to other ceramics. Instead of silicon carbide, the sintered material may be made of, for example, silicon nitride, sialon, alumina, cordierite, mullite, and the like.

Silicic ceramics in which metal silicon is mixed to the above mentioned ceramic, and ceramics bonded with silicon and silicate compound may also be used. This is because the metal silicon prevents cracks caused by thermal shock and the like.

It is preferred that 5 to 50 parts by weight of metal silicon is included per 100 parts by weight of silicon carbide. If the amount of metal silicon is too small, the adhesive strength of the filter F1 decreases, and if the amount is too large, the filter F1 becomes dense and the properties necessary as the filter cannot be obtained.

As shown in, for example, FIG. 3(a) to FIG. 3(c), each honeycomb filter F1 has a so-called honeycomb structure. The honeycomb structure is adopted because the pressure loss is small even if the collected amount of fine particles increases. Each honeycomb filter F1 includes a plurality of cells 12 (through-holes) having a rectangular cross section, regularly formed in the axial direction thereof. The lengths of the sides (inner diameter) of the rectangular cross section of each cell 12 are defined as C1 and C2 (C1.gtoreq.C2). The cells 12 are partitioned from each other by thin cell walls 13a and 13b. The thickness of the cell walls 13a, 13b are defined as D1 and D2 (D1.gtoreq.D2), respectively.

The ratio C1/C2 is preferably 3 or less. This is because if the ratio C1/C2 is greater than 3, thermal stress is more likely to act on the filter F1 due to thermal shock, and cracks are more likely to occur.

The ratio D1/D2 is preferably 3 or less. This is because if the ratio D1/D2 is greater than 3, thermal stress is more likely to act on the filter F1 due to thermal shock, and cracks are more likely to occur.

An oxidation catalyst made of platinum group elements (e.g., Pt) and other metal elements and oxides thereof are carried by the cell walls 13a and 13b. Each cell 12 is sealed with a plug 14 (made of a porous silicon carbide sintered material in this embodiment) at either one of the end faces 9.alpha. and 9.beta. of the filter F1. A checker-board like pattern is formed on the end faces 9.alpha. and 9.beta. by the sealed cells 12. The density of the cell 12 is preferably approximately 200 cells/square inch. About half of the cells 12 are open at the upstream side end face 9.alpha. and the remaining cells 12 are open at the downstream side end face 9.beta.. The lengths C1, C2 of the sides of the cell 12 are preferably set between 0.5 mm and 5.0 mm. If the dimensions C1, C2 are greater than 5.0 mm, the filtering surface area of the cell walls 13a and 13b becomes small. This lowers the performance of the filter F1. On the other hand, if the dimensions C1 and C2 are smaller than 0.5 mm, the filter F1 becomes very difficult to manufacture. The thicknesses D1, D2 of the cell walls 13a, 13b are preferably set between 0.1 to 0.5 mm. This is because if the dimensions D1 and D2 are greater than 0.5 mm, the fluid resistance (pressure loss) produced by the filter F1 becomes high and is thus not satisfactory. If, on the other hand, the dimensions D1 and D2 are smaller than 0.1 mm, the strength of the filter F1 becomes insufficient.

The average pore diameter of the honeycomb filter F1 is preferably between 1 .mu.m and 50 .mu.m, and more preferably, between 5 .mu.m and 20 .mu.m. If the average pore diameter is less than 1 .mu.m, the honeycomb filter F1 would often become clogged by the deposition of fine particles. If, on the other hand, the average pore diameter exceeds 50 .mu.m, small fine particles cannot be collected. This would reduce the collecting efficiency.

The porosity of the honeycomb filter F1 is preferably between 30% and 80%, and more preferably, between 40% and 60%. If the porosity is less than 30%, the honeycomb filter F1 becomes too dense and may not allow the passage of exhaust gas. If the porosity exceeds 80%, the amount of gaps formed in the honeycomb filter F1 becomes excessive. This would weaken the strength and decrease the collecting efficiency of the fine particles.

When a porous silicon carbide sintered material is selected, the thermal conductivity of the honeycomb filter F1 is preferably between 5 W/mK and 80 W/mK, and more preferably, between 30 W/mK and 70 W/mK.

As shown in FIG. 2, FIG. 4, and FIG. 10(a), the outer surfaces of the honeycomb filters F1 are adhered to each other by means of ceramic sealing material layers 15a and 15b. The ceramic sealing material layers 15a and 15b are defined to be of the same type for those that are parallel to each other. Hereinafter, the ceramic sealing material layers parallel to the long side of the assembly 9 are denoted by 15a, the thickness of which is E1, the thermal conductivity of which is G1. The ceramic sealing material layers parallel to the short side of the assembly 9 is denoted by 15b, the thickness of which is E2 (E1.gtoreq.E2), and the thermal conductivity of which is G2. The ratio of E1/E2 is preferably equal to or less than 5. If the ratio El/E2 is greater than 5, the heat conduction reverses between the short side direction and the long side direction, and thus a uniform temperature rise of the assembly 9 becomes difficult. The ratio E1/E2 is preferably 1.05 or greater. If the ratio E1/E2 is less than 1.05, thermal conduction in the direction of the long side becomes difficult. Thus, uniform temperature rise of the assembly 9 becomes difficult. This produces soot and cracks become likely to occur.

If the thicknesses of the sealing material layers 15a and 15b are the same, the thermal conductivity G1 and G2 of both sealing material layers 15a and 15b may be adjusted by differing the compositions (compound) of the sealing material layers 15a and 15b from each other. In this case, the ratio G1/G2 is preferably 0.20 or greater. If the ratio G1/G2 is smaller than 0.20, the thermal conduction reverses between the short side direction and the long side direction, and thus a uniform temperature rise of the assembly 9 becomes difficult. The ratio G1/G2 is preferably 0.7 or less. If the ratio G1/G2 is greater than 0.7, the thermal conduction in the direction of the long side becomes difficult. This produces soot and cracks become likely to occur.

The ceramic sealing material layers 15a, 15b of the present invention will now be described in detail.

The thicknesses E1 and E2 of the sealing material layers 15a and 15b are preferably between 0.3 mm and 3 mm, and more preferably between 0.5 mm and 2 mm. If the thicknesses E1 and E2 exceed 3 mm, the thermal resistance of the sealing material layers 15a and 15b become large even if the thermal conductivity is high, thus inhibiting the thermal conduction between the honeycomb filters F1. The percentage of the honeycomb filters F1 occupying the assembly 9 also relatively decreases, thus leading to lower filtering performance. If, on the other hand, the thicknesses E1 and E2 of the sealing material layers 15a and 15b are less than 0.3 mm, the thermal resistance will not be large but the force adhering the honeycomb filters F1 to each other becomes insufficient, and thus the assembly 9 is likely to break.

The sealing material layers 15a and 15b include at least inorganic fibers, an inorganic binder, an organic binder, and inorganic particles. Further, the sealing material layers 15a and 15b preferably made of an elastic material formed by bonding the inorganic fibers and the inorganic particles with the inorganic binder and the organic binder.

The inorganic fiber contained in the sealing material layers 15a and 15b include at least one or more types of ceramic fiber selected from silica-alumina fiber, mullite fiber, alumina fiber, and silica fiber. Among these fibers, silica-alumina ceramic fiber is particularly preferable. This is because silica-alumina ceramic fiber has excellent elasticity and exhibits thermal stress absorbing performance.

The content of the silica-alumina ceramic fiber in the sealing material layers 15a and 15b is 10% by weight to 70% by weight, preferably 10% by weight to 40% by weight, and more preferably, 20% by weight to 30% by weight in solid content. If the content of the silica-alumina ceramic fiber is less than 10% by weight in the solid content, the effect as an elastic body decreases. If the content of the silica-alumina ceramic fiber exceeds 70% by weight, not only does the thermal conductivity decrease, but elasticity also decreases.

Shot content in the silica-alumina ceramic fiber is 1% by weight to 10% by weight, preferably 1% by weight to 5% by weight, and more preferably, 1% by weight to 3%. If the shot content is less than 1% by weight, manufacturing becomes difficult. If, on the other hand, the shot content exceeds 50% by weight, the outer surface of the honeycomb filter F1 tends to be damaged.

The fiber length of the silica-alumina ceramic fiber is 1 .mu.m to 100 mm, preferably, 1 .mu.m to 50 mm, and more preferably, 1 .mu.m to 20 mm. If the fiber length is shorter than 1 .mu.m, an elastic structure cannot be formed. If the fiber length exceeds 100 mm, fuzzballs of fibers are formed. This lowers dispersion of the inorganic fine particles. Further, it becomes difficult to make the sealing material layers 15a and 15b less than or equal to 3 mm, and the thermal conductivity between the honeycomb filters F1 cannot be improved.

The inorganic binder contained in the sealing material layers 15a and 15b is preferably at least one or more types of colloidal sol selected from silica sol and alumina sol. Among these sols, silica sol is particularly preferable. This is because silica sol is easy to obtain, easily becomes SiO.sub.2 by performing sintering, and is thus suitable as an adhesive agent under high temperatures. Furthermore, silica sol has superior insulation.

The content of the silica sol in the sealing material layers 15a and 15b is 1% by weight to 30% by weight, preferably, 1% by weight to 15% by weight, and more preferably, 5% by weight to 9% by weight in solid content. If the content of the silica sol is less than 1% by weight, the adhesion strength decreases. If the content of the silica sol exceeds 30% by weight, this may reduce thermal conductivity.

The organic binder contained in the sealing material layers 15a and 15b is preferably a hydrophilic organic macromolecule, and more preferably, at least one or more types of polysaccharide selected from polyvinyl alcohol, methyl cellulose, ethyl cellulose, and carboxymethyl cellulose. Among these, carboxymethyl cellulose is particularly preferable. This is because carboxymethyl cellulose produces suitable fluidity for the sealing material layers 15a and 15b and thus exhibits excellent adhesiveness under normal temperatures.

The content of the carboxymethyl cellulose in the sealing material layers 15a and 15b is 0.1% by weight to 5.0% by weight, preferably, 0.2% by weight to 1.0% by weight, and more preferably, 0.4% by weight to 0.6% by weight in solid content. If the content of the carboxymethyl cellulose is less than 0.1% by weight, migration can not be sufficiently suppressed. "Migration" is a phenomenon in which the binder in the sealing material layers 15a and 15b migrates as the solvent is dried and removed when the sealing material layers 15a and 15b filled between the subjected sealing body cure. If the content of the carboxymethyl cellulose exceeds 5% by weight, the organic binder is burnt by the high temperature and the strength of the sealing material layers 15a and 15b is lowered.

The inorganic particles contained in the sealing material layers 15a and 15b is preferably an elastic material using a whisker or at least one or more types of inorganic powder selected from silicon carbide, silicon nitride, and boron nitride. Such carbides and nitrides have very large thermal conductivities, and are arranged on the ceramic fiber surface or on the surface and the inside of the colloidal sol and contribute to the enhancement of thermal conduction.

Among the inorganic particles of the above carbides and nitrides, silicon carbide powder is particularly preferable. This is because silicon carbide has an extremely high thermal conductivity, and in addition, has affinity for ceramic fiber. Moreover, this is because the honeycomb filter F1 serving as the subjected sealing body is of the same type, in other words, is made of porous silicon carbide in the present embodiment.

The content of silicon carbide powder is 3% by weight to 80% by weight, preferably, 10% by weight to 60% by weight, and more preferably, 20% by weight to 40% by weight in solid content. If the content of the silicon carbide powder is less than 3% by weight, the thermal conductivity of the sealing material layers 15a and 15b decreases and causes the sealing material layers 15a and 15b to remain as a large thermal resistance. If, on the other hand, the content exceeds 80% by weight, the adhesion strength under a high temperature decreases.

The particle diameter of the silicon carbide powder is between 0.01 .mu.m and 100 .mu.m, preferably, between 0.1 .mu.m and 15 .mu.m, and more preferably between 0.1 .mu.m and 10 .mu.m. If the particle diameter exceeds 100 .mu.m, the adhesive force and thermal conductivity decrease. If the particle diameter is less than 0.01 .mu.m, the cost of the sealing material layers 15a and 15b increases.

The procedures for manufacturing the above mentioned ceramic filter assembly 9 will now be described.

First, a ceramic ingredient slurry used in an extrusion molding process, a sealing paste used in an end face sealing process, and a sealing material layer forming paste used in a filter adhesion process are prepared in advance.

The ceramic ingredient slurry is formed by mixing and kneading a predetermined amount of silicon carbide powder, organic binder, and water (in some cases, metal silicon is also added). The sealing paste is formed by mixing and kneading silicon carbide powder, organic binder, lubricant, plasticizer, and water. The sealing material layer forming paste is formed by mixing and kneading predetermined amounts of inorganic fibers, inorganic binder, organic binder, inorganic particles, and water.

Next, the ceramic ingredient slurry is charged into the extruder, and is continuously extruded through a metal mold die. The extruded honeycomb molded product is cut into equal lengths to obtain cut pieces of square columnar honeycomb molded products. Further, a predetermined amount of sealing paste is filled into an opening on one side of each cell of the cut piece to seal both end faces of each cut piece.

Subsequently, sintering temperature, sintering time and the like are set to a predetermined condition to perform main sintering, and the honeycomb molded product cut piece and the plug 14 are completely sintered. To have the average pore diameter be 6 .mu.m to 100 .mu.m, and the porosity be 30% to 80%, the sintering temperature is set to 1400.degree. C. to 2300.degree. C. in the present embodiment. The sintering time is set between 0.1 hour and 5 hours. The atmosphere within the furnace during sintering is inactive, and the pressure of the atmosphere is normal.

Next, after a base coating layer made of ceramic is formed on the outer surface of the honeycomb filter F1 if necessary, the sealing material layer forming paste is applied thereto. Then, 4 to 130 of such honeycomb filters F1 are used to adhere the outer surfaces of the honeycomb filters F1 with each other and integrate the honeycomb filters F1.

In the subsequent outer shape cutting process, unnecessary parts of the peripheral portion of the assembly 9 having a square cross section obtained through the filter adhering process is ground and removed, the ceramic sealing material layer forming paste is applied to the peripheral portion to form an outer ceramic sealing material layer. This adjusts the outer shape. As a result, the ceramic filter assembly 9 having a substantially elliptical cross section is manufactured.

The outer ceramic sealing material layer will now be described. The thickness of a normal outer ceramic sealing material layer is uniform. In the present embodiment, as shown in FIG. 10(b), in the outer ceramic sealing material layer, the portion located along an extention of the major axis of the assembly 9 is denoted by 15c, the portion located along an extension of the minor axis of the assembly 9 is defined as 15d, and the thickness of portion 15c is denoted by H1, and the thickness of portion 15d is denoted by H2.

Depending on the type of assembly 9, the cell 12, or recesses, are exposed from the peripheral surface of the assembly 9 by the grinding process. In this case, the thickness of the ceramic sealing material layer is defined as the distance from a curve surface connecting the cell walls 13a and 13b of the exposed cells 12.

The ceramic sealing material layer forming paste is applied so that the thickness of the middle part between the portion 15c and the portion 15d changes gradually. The adjustment of the thickness of the ceramic sealing material layer is possible by performing machining after the application of the paste. Alternatively, the sealing material layer may be formed by injecting and drying the ceramic sealing material in the mold so that the sealing material layer has such thickness.

The ratio H2/H1 is preferably 0.95 or less. If the ratio H2/H1 is greater than 0.95, the filter in the long side direction easily cools, and uniform temperature rise of the assembly 9 becomes difficult. This causes soot to remain and cracks tend to occur.

The ratio H2/H1 is preferably 0.06 or greater. If the ratio H2/H1 is less than 0.06, the release of heat reverses between the short side direction and the long side direction. Thus, a uniform temperature rise of the assembly 9 becomes difficult.

The assembly 9 is wrapped by the thermal insulation material 10 (refer to FIG. 1 and FIG. 10(c)) and is accommodated in the casing 8. The thermal insulation material normally has a uniform thickness. In the present embodiment, the thickness of the thermal insulation material differs between portion 16a, which is located along an extension of the major axis of the assembly 9, and portion 16b, which is located along an extension of the minor axis of the assembly 9. In the following description, the thickness of portion 16a is denoted by I1, and the thicknessof portion 16b is denoted by I2.

The ratio of I2/I1 is preferably 0.91 or less. If the ratio I2/I1 is greater than 0.91, the filters F1 near the outer side in the direction of the long side cools easily, uniform temperature rise of the assembly 9 becomes difficult, and soot remains thereby causing cracks to easily occur. It is preferable that the ratio I2/I1 be 0.30 or greater. If the ratio I2/I1 is less than 0.30, the heat release rev