Exhaust gas filter and apparatus for treating exhaust gases using the same

An imaginary plane dividing a profile of a rough surface of a filter into halves with equal volume is defined as a mean plane. Assuming cutting the filter with the mean plane, a ratio of the total cross-sectional area of the recesses appearing to the whole area of the mean plane is defined as a Valley Level. An exhaust gas filter having a surface with a Valley Level of at most 20%, a porosity of 40% to 55% and an average pore diameter of 5 .mu.m to 50 .mu.m effectively collects fine particles contained in exhaust gases discharged from internal combustion engines, such as diesel engines, with little pressure loss, and has an improved releasability of deposited fine particles and can be readily regenerated by flow of blowback air, with high efficiency. When the filter has a specified two-layer structure, the Valley Level is readily controllable.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to exhaust gas filters for collecting fine 
particles which are contained in exhaust gases discharged from internal 
combustion engines, such as diesel engines, and apparatuses for treating 
exhaust gases with such filters. 
2. Description of the Prior Art 
Exhaust gases generally contain fine particles comprising, as a main 
ingredient, carbon, other than nitrogen oxides NO.sub.x, carbon monoxide 
CO, hydrogen carbides HC or the like. These fine particles per se not only 
cause air pollution but also deteriorate, as poison, catalytic activity of 
catalysts for purifying NO.sub.x, CO, HC or the like. Therefore, various 
exhaust gas filters for collecting those fine particles have so far been 
proposed. 
Exhaust gas filters require characteristics, such as low pressure loss, 
high efficiency of collecting fine particles, high compressive strength, 
high thermal shock resistance or the like. Additionally, it is important 
that the exhaust gas filters can be regenerated with high efficiency, 
because the filters, since fine particles deposit thereon during 
filtration, require an intermittent regeneration by removing the deposits. 
If the regeneration efficiency is low, long use of the filter will result 
in increase of its pressure loss. 
Japanese Patent Application Laid-open No. 03-47,507 discloses a technique 
for obtaining an excellent filter by superimposing a filter layer having 
an average pore diameter of 0.2-10 .mu.m on a filter substrate having an 
average pore diameter of 10-100 .mu.m and a ratio of the pore diameter in 
the position of 75 vol. % to that in the position of 25 vol. %, with 
respect to a cumulative pore distribution, of at least 1.3, which filter 
layer is fixed on the filter substrate in such a manner that the filter 
layer may block open-pores on the surface of the filter substrate. 
As a process for regenerating filters, it has been known that collected 
fine particles are burnt up in situ on the filters by raising the 
temperature of the filters. Alternatively, there has also been known 
another conventional process wherein collected fine particles on filters 
are blown away by injecting blowback air into the filters counter to 
exhaust gas flow, and then the fine particles are burnt up. The latter 
process wherein the fine particles are blown away by blowback air has the 
advantage in that the life of the filters is generally extended, as 
compared with the former process wherein the fine particles are burnt up 
in situ on the filters. 
However, the above conventional blowback process has posed a problem of an 
insufficient ability of regenerating filters during the blowback, 
resulting in increase of pressure losses with the lapse of time of 
collection operation, though it may partly depend on the properties of the 
filters. Alternatively, in the case where the filters are formed into a 
two-layer structure such as disclosed in Japanese Patent Application 
Laid-open No. 03-47,507, even with such a filter, a problem of increase in 
pressure loss has been posed, though it may depend upon the material that 
forms the filter layer. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an exhaust gas filter 
having regeneration efficiency improved by blowback air and exhibiting 
little increase in pressure loss even after long use, and an apparatus for 
treating exhaust gases with such filters. 
The above object is achievable by a first embodiment of the present 
invention, that is, an exhaust gas filter for collecting fine particles 
contained in exhaust gases discharged from internal combustion engines, 
characterized by a Valley Level as defined hereinafter of a surface of the 
filter of not more than 20%, a porosity of the filter of between 40% and 
55%, and an average pore diameter of the filter of between 5 .mu.m and 50 
.mu.m. 
Alternatively, the object of the present invention also can be attained by 
a second embodiment of the present invention, that is, an exhaust gas 
filter for collecting fine particles contained in exhaust gases discharged 
from internal combustion engines, comprising a filter substrate and a 
filter layer provided on the surface of the filter substrate, which gas 
filter is characterized in that the above filter layer has a surface with 
a Valley Level as defined hereinafter of not more than 20%, and the above 
filter substrate has a porosity of between 45% and 60% and an average pore 
diameter of between 10 .mu.m and 80 .mu.m. In this second embodiment, the 
filter layer is preferred virtually not to block open-pores on the surface 
of the filter substrate. 
According to the present invention, the exhaust gas filter is preferred to 
comprise a ceramic material comprising at least one main crystalline 
component selected from the group consisting of cordierite, mullite and 
alumina. 
Further, the exhaust gas filter of the present invention is preferred to be 
composed of a honeycomb structure. 
Furthermore, the exhaust gas filter of the present invention is preferred 
to comprise a ceramic material comprising, as the main crystalline 
component, particularly cordierite, and have a coefficient of thermal 
expansion, along a direction of exhaust flow, of at most 
1.0.times.10.sup.-6 /.degree. C. between 40.degree. C. and 800.degree. C.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
In the present invention, conditions of the surface of a filter is 
evaluated by means of "Valley Level". 
The term "Valley Level" used throughout this specification will be 
explained hereinafter. 
The surface roughness of a filter is determined by means of an instrument 
for the measurement of surface roughness by the stylus method according to 
JIS B-0651. The obtained data is three-dimensionally analyzed and a plane 
dividing the profile of the filter surface into halves having an equal 
volume: an upper half of projections and a lower half of recesses, is 
imagined. This imaginary plane is defined as a mean plane. When the filter 
is assumed to be cut on the level of the mean plane, a ratio of the total 
cross-sectional area of the recesses appearing on the mean plane to the 
whole area of the mean plane is defined as a Valley Level. 
In FIG. 1 is two-dimensionally shown and illustrated how to find the Valley 
Level. A mean plane is set so as to equalize the sum in volume of the 
projections above with the sum in volume of the recesses below, with 
respect to the mean plane, within the range of measurement, S. Namely, the 
mean plane is set to satisfy the following equation (1): 
EQU (V.sub.11 +V.sub.12 +V.sub.13 +V.sub.14 +V.sub.15)=(V.sub.21 +V.sub.22 
+V.sub.23 +V.sub.24) (1) 
wherein V represents volume of projections or recessions. 
Recesses having cross-sectional areas, s.sub.1, s.sub.2, s.sub.3 and 
s.sub.4, respectively, appear when the surface of the filter is cut on the 
level of the mean plane. The ratio of the sum of the cross-sectional areas 
of the recesses on the level of the mean plane to the whole area of the 
mean plane in the measurement range S is defined as the Valley Level which 
is represented by the following formula (2): 
EQU Valley Level=(s.sub.1 +s.sub.2 +s.sub.3 +s.sub.4)/S.times.100(2) 
It should be noted that, apart from the cross-sectional area of the 
recesses used in computing the Valley Level the concept of which has been 
introduced into the present invention, a usual cross-sectional area of 
pores is found by means of image analysis, such as SEM or the like, and 
its obtained value is larger than a cross-sectional area of recesses 
appearing on the level of the mean plane which is used in computing the 
Valley Level, as shown in FIG. 1. 
During collecting fine particles, though they may be collected on the whole 
surface of the filter, the fine particles are, in particular, 
preferentially collected in the pores on the surface. This is because the 
fine particles are collected and deposit selectively in the pore portions 
on the surface where pressure loss is low. Since it is difficult to remove 
thoroughly the deposits of the fine particles from the pore portions of 
the surface by means of blowback air, the effective area of the filter 
becomes decreased with the consequence that the pressure loss is 
increased. 
In this case, pores in which fine particles are preferentially collected 
are those opening on the surface lower than the mean plane which has been 
set by the measurement of the surface roughness. Namely, amongst the 
cross-sectional areas of the pores on the surface, those to have an effect 
on collection and release of fine particles are the cross-sectional area 
of the pores on the level of the mean plane and not the whole area of the 
pores opening on the surface which is derived from image analyses, such as 
SEM or the like. 
If the cross-sectional area of the pores on the level of the mean plane, 
i.e., Valley Level, is decreased, the portions in which fine particles are 
preferentially collected are decreased. Therefore, the collected fine 
particles are improved in releasability during blowback procedures with 
the consequence that the effective area of the filters is scarcely 
decreased. Accordingly, with decreasing the Valley Level, the regeneration 
efficiency of the filters will increase. 
The present invention has been achieved by the above findings. Namely, as 
described above, the exhaust gas filter of the first embodiment of the 
present invention that is used for collecting fine particles contained in 
exhaust gases discharged from internal combustion engines is characterized 
by a Valley Level of the surface of not more than 20%, a porosity of 
between 40% and 55% and an average pore diameter between 5 .mu.m and 50 
.mu.m. 
When the Valley Level is 20% or less, releasability of fine particles 
collected on the surface of the filter will be improved and, therefore, 
efficiency of regeneration of the filter by means of blowback air is 
improved as well. In order to further decrease pressure losses, the Valley 
Level is preferred to be not more than 10%. If the Valley Level exceeds 
20%, the releasability of the collected fine particles from the surface of 
the filter is so low during blowback that the pressure loss may be 
increased. Additionally, even when the Valley Level is 20% or less, if the 
filter has a porosity of less than 40%, the blowback air flows too slowly 
to release thoroughly the collected fine particles, thereby also causing 
pressure loss increase. On the other hand, if the porosity exceeds 55%, 
the mechanical strength of the filter will be decreased undesirably. 
Additionally, even when the Valley Level is 20% or less, if the filter has 
an average pore diameter of less than 5 .mu.m, the blowback air flows too 
slow to release thoroughly the collected fine particles, thereby also 
causing pressure loss increase. On the other hand, if the average pore 
diameter exceeds 50 .mu.m, the efficiency of collecting fine particles 
will be decreased. 
Alternatively, the exhaust gas filter of the second embodiment of the 
present invention also used for collecting fine particles contained in 
exhaust gases discharged from internal combustion engines, has a two-layer 
structure comprising a filter substrate and a filter layer provided on the 
surface of the filter substrate, and is characterized by a Valley Level of 
a surface of the above filter layer of not more than 20%, a porosity of 
the above filter substrate of between 45% and 60%, and an average pore 
diameter of the above filter substrate of between 10 .mu.m and 80 .mu.m. 
The technique of the present invention to improve in releasability of 
collected and deposited fine particles and increase in regeneration 
efficiency of the filters, by means of lowering the Valley Level, is 
particularly effective when it is applied to the filters of the two-layer 
structure comprising a filter substrate and a filter layer. This is 
because, in usual monolayer filters, it is difficult to control 
concurrently three parameters: Valley Level, porosity and average pore 
diameter, and further achieve the decrease of the coefficient of thermal 
expansion. The decrease of the Valley Level of the surface of the filter 
layer to 20% or less is facilitated by forming the filter into the 
two-layer structure, fabricating the filter substrate with attention being 
paid to air-permeability, mechanical strength, heat resistance and the 
like, and the filter layer with attention being paid to the Valley Level. 
Additionally, when the filter layer is formed to have a Valley Level of 
20% or less and, at the same time, so as not to block open-pores on the 
surface of the filter substrate, the pressure loss can be decreased 
without negatively affecting the collection efficiency, so that such 
filters are more preferable. 
The filters of two-layer structure, since the filter layer, in general, has 
a mechanical strength higher than the filter substrate, exhibit a 
sufficient mechanical strength as compared with filters of monolayer 
structure, even when the filter substrate has a somewhat high porosity. 
Therefore, an appropriate porosity of the filter substrate is in the range 
between 45% and 60%. Furthermore, since the filter layer adds an 
air-permeation resistance, the open-pores on the surface of the filter 
substrate are preferred to have a larger diameter as compared with filters 
of monolayer structure. However, diameters of more than 80 .mu.m are not 
preferred, because which will allow particles forming the filter layer to 
enter into the filter substrate, resulting in high pressure losses. 
In the above second embodiment, it is preferred that the filter layer 
virtually does not block open-pores on the surface of the filter 
substrate. 
If the filter layer blocks the pores opening on the surface of the filter 
substrate, the porosity of the whole two-layer filter including the filter 
layer becomes lower than that of the filter substrate alone and, moreover, 
particles which form the filter layer may enter into the filter substrate, 
resulting in high pressure losses. 
In both the above first and second embodiments of the present invention, 
the exhaust gas filter is preferred to comprise a ceramic material 
comprising at least one main crystalline component selected from the group 
consisting of cordierite, mullite and alumina. 
Further, the exhaust gas filter according to the present invention is 
preferred to be composed of a honeycomb structure. 
Furthermore, the exhaust gas filter according to the present invention is 
preferred to comprise, particularly, as a main crystalline component of 
the filter or filter substrate, cordierite, and has a coefficient of 
thermal expansion, along a direction of exhaust flow, of at most 
1.0.times.10.sup.-6 /.degree. C. between 40.degree. C. and 800.degree. C. 
If the coefficient of thermal expansion is in excess of 1.0.times.10.sup.-6 
/.degree. C., the thermal shock resistance of the filters will decrease to 
such a degree that the filters cannot be adapted for application in an 
exhaust gas filter for diesel engines. In order to maintain the thermal 
shock resistance for a long period of time, the coefficient of thermal 
expansion is more preferably not more than 0.8.times.10.sup.-6 /.degree. 
C. 
According to the present invention, the exhaust gas filters are improved in 
regeneration efficiency by virtue of a synergetic effect of adequate 
Valley Level, porosity and average pore diameter provided therein. 
Particularly in the second embodiment of the present invention, the exhaust 
gas filters of two-layer structure are easy to control concurrently three 
parameters thereof: Valley Level, porosity and average pore diameter. 
Further in this second embodiment, if the filter layer virtually does not 
block open-pores on the surface of the filter substrate, pressure losses 
can be kept low. 
Furthermore, according to the present invention, the exhaust gas filters 
can have sufficient thermal shock resistance and mechanical strength by 
virtue of using a ceramic material comprising at least one main 
crystalline component selected from the group consisting of cordierite, 
mullite and alumina. Particularly with a ceramic material comprising 
cordierite as a main crystalline component, and with a coefficient of 
thermal expansion in the direction of exhaust flow of at most 
1.0.times.10.sup.-6 /.degree. C., the filters according to the present 
invention have an excellent thermal shock resistance. 
Furthermore, the exhaust gas filter according to the present invention, 
since it comprises a honeycomb structure having a large surface area per 
volume, can be formed into a compact size with a sufficient mechanical 
strength. 
Additionally, the present invention is further embodied in an apparatus for 
treating exhaust gases, comprising the above-described filters of the 
first or second embodiment of the invention, which is characterized in 
that blowback air is used to regenerate the filters. 
The above apparatus for treating exhaust gases is used with a diesel engine 
mounted on motor vehicles. 
In the apparatus for treating exhaust gases according to the above 
embodiment of the present invention, the filters having a releasability of 
fine particles improved by lowering the Valley Level is regenerated by 
means of blowback air. Therefore, the apparatus for treating exhaust gases 
comprising the filters according to the present invention has an excellent 
regeneration efficiency of the filters. 
Furthermore, the apparatus for treating exhaust gases mounted on a diesel 
engine can collect efficiently fine particles which are exhausted from the 
diesel engine and cause environmental disruption, such as air pollution, 
and decrease catalytic activity. 
The present invention will be explained in more detail by way of examples 
hereinafter. 
In the examples, the physical properties of the filters were determined 
according to the following methods. 
Physical Properties 
(1) Porosity 
The porosity was determined by the Boiling Method shown in JIS R-2206. 
(2) Average pore diameter 
The average pore diameter was determined by the Mercury Injecting Method. 
(3) Valley Level 
By an instrument for the measurement of surface roughness by the stylus 
method, with a diamond stylus having a tip curvature radius of 2 .mu.m, a 
surface roughness was measured under the conditions of: a measuring field 
of view of 0.8 mm.times.0.8 mm; a measuring pitch of 1.5 .mu.m; and a 
stylus load of 85 mgf. Then the Valley Level was determined, based on the 
above-described definition, as a mean value of 5 measurements. 
(4) Coefficient of thermal expansion 
With a sample 50 mm long in the direction of exhaust gas flow, and 5 mm 
wide, an average coefficient of thermal expansion from 40.degree. C. to 
800.degree. C. (referred to as "CTE" in Table 2 below) was determined. 
Characteristics 
(a) Pressure loss 
Using a 2,000 cc diesel engine as an exhaust gas supply source, fine 
particles were collected under the running conditions of: an exhaust gas 
temperature of 400.degree. C.; an average amount of generating fine 
particles of 17 g/hr; and an exhaust gas flow rate of 3 m.sup.3 /min., 
while the filters were regenerated under the conditions of: a blowback air 
pressure of 6 kg/cm.sup.2 ; a blowback interval of 5 min.; and a blowback 
time of 0.5 sec. Under these conditions, the engine was continually run 
for 20 hours and then the pressure loss was substantially stabilized. 
Therefore, after 20 hour running, the change of the pressure loss was 
considered to be minute. Accordingly, the value of the pressure loss 20 
hours after the commencement of the test was used for appraisal of 
performance. 
The pressure loss is desired to be at most 1,000 mmH.sub.2 O from the 
practical point of view. 
(b) Collection efficiency 
The amount of fine particles recollected in a receiving reservoir was 
measured after 3 hours from the commencement of the test running of the 
engine under the same conditions as in the measurement of the pressure 
loss. The ratio of the amount of the recollected fine particles measured 
to the amount of fine particles generated from the exhaust gas supply 
source represented a collection efficiency. Calculation of the collection 
efficiency is shown in the following formula (3): 
EQU (Amount of recollected fine particles/generated fine particles).times.100(3 
) 
The collection efficiency is desired to be at least 90% from the practical 
point of view. 
(c) A-axis compressive strength 
The axial direction of a cylindrical sample of 2.5 cm dia..times.2.5 cm 
length was assumed to be an A-axis. The compressive strength in the A-axis 
direction was determined and unit conversion was made. 
The compressive strength is desired to be at least 100 kg/cm.sup.2 from the 
practical point of view. 
(d) Thermal shock resistance 
A sample was placed in an electric oven and heated from 500.degree. C. with 
a 50.degree. C. step-up, each step being kept for 30 min. At each 
temperature step, the sample was taken out to room temperature and tested 
by knocking or observed visually. Until thick sound was heard by knocking 
or a crack was observed, the step-up was repeated. The maximum temperature 
before crack development was assumed as a measured value of the thermal 
shock resistance (referred to as "TSR" in Table 2 below). 
The TSR is desired to be at least 700.degree. C. from the practical point 
of view. 
EXAMPLE 1 
Filter sample Nos. 1-15 having various Valley Levels, porosities and 
average fine particle diameters as shown in Table 1 were manufactured 
according to the following method: 
Manufacture of Ceramic Filters 
Blending talc, kaolin, alumina, silica and other materials for forming 
cordierite, each in the range of amount for cordierite-formation to 
progress satisfactorily, and the blend was admixed and kneaded with 
shaping aids, such as methylcellulose, surfactants or the like, and 
solvents, such as water, alcohols or the like. The resultant blend was 
extruded and shaped into a honeycomb structure of 118 mm dia..times.152 mm 
length, having a partition wall thickness of 430 .mu.m and a cell density 
of 15.5 cells/cm.sup.2. This honeycomb structure was fired at temperatures 
for a cordierite-formation reaction enough to progress. Then, the 
throughholes of this honeycomb structure were sealed in a so-called 
"zigzag fashion" such that adjacent throughholes were sealed alternately 
at one end and the other. Thus, a ceramic filter of wall-flow type was 
manufactured. 
The properties and characteristics of the resulting ceramic filters were 
appraised according to the above-described methods. The results are shown 
in Table 1 below. 
TABLE 1 
__________________________________________________________________________ 
Average A-axis 
Valley pore Pressure 
Collection 
Compressive 
Sample 
level 
Porosity 
diameter 
loss efficiency 
strength 
No. (%) (%) (.mu.m) 
(mmH.sub.2 O) 
(%) (Kg/cm.sup.2) 
__________________________________________________________________________ 
1 20 40 15 990 97 138 Example 
2 20 50 16 970 95 120 
3 20 55 15 970 94 101 
4 20 50 5 990 98 124 
5 20 50 20 970 94 118 
6 20 50 48 970 92 112 
7 15 50 14 870 95 118 
8 10 50 15 780 94 115 
9 5 50 15 660 95 121 
10 1 50 14 570 96 117 
11 20 38 14 1030 97 140 Comparative 
12 20 57 15 970 94 95 Example 
13 20 50 3 1020 99 130 
14 20 50 52 950 89 110 
15 22 50 15 1070 95 120 
__________________________________________________________________________ 
As is apparent from Table 1, the filter samples having a porosity of 40% to 
55%, an average pore diameter of 5 .mu.m-50 .mu.m and a Valley Level of 
20% or less (Sample Nos. 1-10) had an excellent performance characteristic 
of low pressure loss, improved collection efficiency and high A-axis 
compressive strength. 
In contrast therewith, the sample having a Valley Level of more than 20% 
(Sample No. 15), showed a poor releasability of deposited fine particles 
during blowing-back and increased pressure loss, so that it was found to 
be not adaptable for practical use. Alternatively, the sample having a 
porosity of less than 40% (Sample No. 11), since the blowback air flowed 
therethrough too slow for deposited fine particles enough to release, also 
increased in its pressure loss. Even when the Valley Level was lowered to 
improve the releasability of the fine particles, the pressure loss could 
not be kept low, still due to a poor releasability. Alternatively, the 
sample having a porosity of more than 55% (Sample No. 12) decreased in its 
mechanical strength shown as an A-axis compressive strength, so that it 
could not possess even a minimal strength necessary for being mounted on 
motor vehicles or the like. Alternatively, the sample having an average 
pore diameter of less than 5 .mu.m (Sample No. 13), since the blowback air 
flowed therethrough too slow as in the case of too low porosity, also 
increased in its pressure loss due to a poor releasability of fine 
particles, even when the Valley Level was lowered to improve the 
releasability. On the other hand, the sample having an average pore 
diameter of more than 50 .mu.m (Sample No. 14) decreased in its collection 
efficiency, so that its performance as a filter was found to be 
insufficient. 
EXAMPLE 2 
Filter sample Nos. 16-19 having various Valley Levels, porosities and 
average fine particle diameters as shown in Table 2 were manufactured in 
the same manner as Example 1 and appraised according to the 
above-described methods. In addition to the appraisal items in Example 1, 
the average coefficient of thermal expansion (CTE) and thermal shock 
resistance (TSR) were also appraised. The results are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Average A-axis 
Valley pore Pressure 
Collection 
compressive 
Sample 
level 
Porosity 
diameter 
loss efficiency 
strength 
CTE TSR 
No. (%) (%) (.mu.m) 
(mmH.sub.2 O) 
(%) (Kg/cm.sup.2) 
(.times. 10.sup.-6 /C..degree. 
) (.degree.C.) 
__________________________________________________________________________ 
16 10 50 14 790 95 114 1.0 700 Example 
17 10 50 14 780 94 110 0.8 750 
18 10 50 15 790 94 110 0.7 750 
19 10 50 15 780 94 115 1.3 600 Comparative 
Example 
__________________________________________________________________________ 
In the installing position of filters generally used in diesel engines, the 
maximum temperature is about 700.degree. C. and a maximum temperature 
difference undergoing during rapid cooling is considered to be 700.degree. 
C. Therefore, it is desired that the filters exhibit a thermal shock 
resistance of at least 700.degree. C. As is clear from Table 2, the 
samples having an average coefficient of thermal expansion of not more 
than 1.0.times.10.sup.-6 /.degree. C. (Sample Nos. 16-18) exhibited a 
thermal shock resistance of 700.degree. C. or more. Additionally, in order 
to maintain a high thermal shock resistance for a long period of time, it 
is considered that an initial thermal shock resistance of at least 
750.degree. C. would be required. It is found from Table 2 that the 
samples having an average coefficient of thermal expansion of not more 
than 0.8.times.10.sup.-6 /.degree. C. (Sample Nos. 17 and 18) satisfy this 
requirement. 
As is apparent from the above, filters to be mounted on motor vehicles are 
required to have a high thermal shock resistance other than a low Valley 
Level, and in order to satisfy this requirement, it will be necessary that 
the average coefficient of thermal expansion is not more than 
1.0.times.10.sup.-6 /.degree. C., preferably not more than 
0.8.times.10.sup.6 /.degree. C. 
EXAMPLE 3 
In this Example, ceramic filters of two-layer structure, Sample Nos. 20-33, 
were manufactured according to the following method: 
Manufacture of Ceramic Filters of Two-layer Structure 
Blending talc, kaolin, alumina, silica and other materials for forming 
cordierite, each in the range of amount for cordierite-formation to 
progress satisfactorily, and the blend was admixed and kneaded with 
shaping aids, such as methylcellulose, surfactants or the like, and 
solvents, such as water, alcohols or the like. The resultant blend was 
extruded and shaped into a honeycomb structure of 118 mm dia..times.152 
mm, having a partition wall thickness of 380 .mu.m and a cell density of 
15.5 cells/cm.sup.2. This honeycomb structure was fired at temperatures 
for a cordierite-formation reaction enough to progress. Then, the 
throughholes of this honeycomb structure were sealed in a so-called 
"zigzag fashion" such that adjacent throughholes were sealed alternately 
at one end and the other. Thus, a filter substrate was manufactured. The 
surface of this filter substrate was coated with silica having an average 
particle diameter of 10 .mu.m, by utilizing an alumina sol, which silica 
coating formed a filter layer 50 .mu.m thick. 
The properties and characteristics of the resulting two-layer filters were 
appraised according to the above-described methods. The results are shown 
in Table 3 below. 
TABLE 3 
__________________________________________________________________________ 
Filter substrate 
Filter 
Whole body of 2-layer filter 
Average 
layer A-axis 
pore Valley Pressure 
Collection 
compressive 
Sample 
Porosity 
diameter 
level 
Porosity 
loss efficiency 
strength 
No. (%) (.mu.m) 
(%) (%) (mmH.sub.2 O) 
(%) (Kg/cm.sup.2) 
__________________________________________________________________________ 
20 45 34 9 43 940 96 136 Example 
21 60 36 10 54 780 95 115 
22 55 10 11 55 900 96 120 
23 55 80 10 53 960 95 103 
24 55 36 20 54 990 94 117 
25 55 35 5 55 650 96 116 
26 55 36 10 56 730 95 116 
27 55 35 11 58 680 94 116 
28 55 35 10 50 810 96 120 
29 43 35 10 42 1020 96 140 Comparative 
30 63 35 10 59 720 95 98 Example 
31 55 8 9 54 1050 96 120 
32 55 83 9 52 1030 96 112 
33 55 36 23 55 1080 95 116 
__________________________________________________________________________ 
As is apparent from Table 3, the filter samples having a porosity of 45% to 
60%, an average pore diameter of 10 .mu.m-80 .mu.m and a Valley Level of 
20% or less (Sample Nos. 20-28) had an excellent performance 
characteristic of low pressure loss, improved collection efficiency and 
high A-axis compressive strength. 
In contrast therewith, the sample having a Valley Level of more than 20% 
(Sample No. 33), showed a poor releasability of deposited fine particles 
during blowing-back and increased pressure loss, so that it was found to 
be not adaptable for practical use. Alternatively, even though having a 
Valley Level of less than 20%, the sample having a porosity of less than 
45% (Sample No. 29), since the blowback air flowed therethrough too slow 
for deposited fine particles enough to release, also increased in its 
pressure loss. Alternatively, the sample having a porosity of more than 
60% (Sample No. 30) decreased in its mechanical strength, so that it could 
not possess even a minimal strength necessary for being mounted on motor 
vehicles or the like. Alternatively, the sample having an average pore 
diameter of less than 10 .mu.m (Sample No. 31), since the blowback air 
flowed therethrough too slow, also increased in its pressure loss due to a 
poor releasability of fine particles. 
Additionally, when the filter layer was formed on the surface of the filter 
substrate in such a manner that the filter layer might not block 
open-pores on the surface of the filter substrate, the resulting two-layer 
filters (Sample Nos. 26 and 27), as a whole, had a porosity generally 
higher than the porosity of the filter substrate alone. It was found that 
such samples had a lower pressure loss, as compared with the two-layer 
filter sample having pores on the surface of the filter substrate blocked 
with the filter layer (Sample No. 28). 
Therefore, in order to prevent increase of pressure losses in the filter of 
two-layer structure, it is preferred that the open-pores on the surface of 
the filter substrate are not blocked with the filter layer. However, it is 
much more difficult to form a filter layer on the surface of the filter 
substrate without blocking than with blocking (as Sample No. 28) the 
open-pores on the surface of the filter substrate with a filter layer. 
Particularly, it is true when the open-pores have a large average pore 
diameter, because the larger the pore diameter, the more fine particles of 
the filter layer readily enter and are apt to block the open-pores, 
resulting in a pressure loss of even more than 1,000 mmH.sub.2 O. As a 
result of investigation, it has been found that an average pore diameter 
to virtually keep the fine particles of the filter layer out of open-pores 
of the filter substrate should be at most 80 .mu.m in the two-layer 
filter, in order to prevent increase of pressure losses. 
EXAMPLE 4 
In FIG. 2 is shown an example of a diesel engine mounted on a motor 
vehicle, equipped with an apparatus for treating exhaust gases wherein the 
exhaust gas filters manufactured in Examples 1-3 of the present invention 
were used. 
In the apparatus for treating exhaust gases 10 shown in FIG. 2, during 
usual exhaust gas filtration (the usual exhaust gas filtration is referred 
to as "collection mode" hereinafter), the exhaust gases flow from an 
exhaust gas pipe 11 into each of exhaust gas filters 12. During the 
collection mode, since each exhaust valve 13 is opened, the exhaust gases 
flow into each exhaust gas filter 12 where fine particles mainly 
comprising carbon, contained in the exhaust gases, are collected, and then 
exhaust gases are discharged from the exhaust gas treating apparatus 10. 
During blowback-to-regenerate (the blowback-to-regenerate is referred to as 
"blowback mode" hereinafter), an exhaust valve 13 on the regeneration 
side, such as the lower exhaust valve 13 in FIG. 2, is closed to stop 
flowing of the exhaust gases into exhaust gas filters 12 to be 
regenerated, and a solenoid valve 14 is opened to inject blowback air into 
the exhaust gas filters 12. Thus, the gas filters are regenerated. Fine 
particles discharged are pneumatically conveyed to a collector tank 15, 
i.e., a device for receiving the recollected fine particles. The conveyed 
and recollected fine particles are disposed of by burning with an electric 
heater, burner or the like (not shown), or recovered by dismounting the 
collector tank 15 from the exhaust gas treating apparatus 10. 
According to this example of the present invention, since exhaust gas 
filters having releasability of collected and deposited fine particles 
improved by controlling the Valley Level, porosity and average pore 
diameter of the exhaust gas filter 12 are regenerated by means of blowback 
air, the exhaust gas filters have an excellent regeneration efficiency.