Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~'Z~ 539
CLEANSING OF EXHAUST GASES
1 Back~round of the Invention
This invention relates to the cleansing or purification
of exhaust gases, particularly for diesel engines in motor
vehicles.
Althou~h the exhaust gases of diesel engines contain car-
bon particles, which are recognized as noxious, they otherwise
contain only relatively small ounts of carbon monoxide, hydro-
carbon compounds, and nitro~en oxide. With known exhaust gas
filters ~or diesel engines the primary objective, therefore, has
been to intercept the carbon particles and burn them. In so
doinq, certain emission limits must be observed as to the hydro-
carbons must be observed as to the hydrocarbon component in the
exhaust gas. These limits are prescribed by national directive
such as those of the Environmental Protection Agency (EPA) o~
the United States of America.
Environmental protective directives specify the allow-
able emission of carbon particles for passenger vehicles at a
level such as 0.6 g/mi ~grams per mile) for the model year 1982
and at 0.2 g/mi for the model year 1985. The present objective
for the year 1990 is to reduce carbon particles by 75~, relative
to the total diesel exhaust. The purpose of these directives
is to achieve a considerable reduction in environmental pollu-
tion. It has been calculated that, for example, in the year
1985, about 8 million motor vehicles will be produced equipped
with diesel engines, and that a diesel passenger car without an
exhaust gas filter will expel, over a driving distance of
100,000 km., an average of 30 to 40 kg. of carbon particles
into the environment.
Technical experts have agreed that the emission ~imits
for 1985, mentioned above, can be achieved only by additional
filtering of the diesel exhaust gas. It has been ~enerally
assumed that suitable filter devices require equipment for
regeneration, i.e., the collection and burning of incident car-
bon Particles as soon as the exhaust gas temperature has
reached the combustion temperature of carbon, which i5 about
~50 to 50~C.
53~
1 ~ well~knowll ~arbon filter for diesel exhaust gases
is of monolithic porous ceramic, formed ~y mutually parallel,
thin-walled channels running over its entire length. On
their frontal sides, the channels are closed and opened in
a checkerboard pattern, and every channel has an open end
and a closed end. ~hen such a ceramic filter is used with
diesel exhaust qases, the exhaust which enters the channels
that are open on the inlet side is forced to flow throu~h
the porous channel walls into the neighboring channels, which
are o~en on the outlet side. Since about 50~ of the carbon
particles have a si~e exceeding 0.5mm, they consequently
cannot pass through the channel walls and collect in the
channels that are open upstream. The carbon collected in
such a filter is not burned off before its ignition tempera-
ture is reached, and only then if there is sufficient concen-
tration of oxygen in the exhaust gas.
Between the two such regeneration processes,in such
a filter, there is an in¢reased loadin~, and associated with
this, a not inconsiderable rise in the counterpressure of
the exhaust gas. If the load is especially high, the carbon
burn~off can be quite intense and can occur with the generation
of undesirably high temperatures. Under unfavorable conditions
- there is even a risk of exceeding the melting point of the
ceramic material. Although a high degree of filterinq,of
about 70%,can be achieved with such a filter, with only 30%
of the particles in the exhaust gas actually exhausted -to
the outside, the burn~off conditions need improvement. This
is in part because of the high counterpressure of the exhaust
gas, which even rises gradually from its basic level as the
operating time increases, and in part because of the sudden
and severe temperature rise following ignition of the carbon.
A more homogeneous burn~off of carbon is achieved by
another well-~nown filter device as disclosed in DE-OS 30
24 491, where the exhaust gases are transported through a
chamber with a catalyst. Here, the catalyst is a substrate
3~
~3-
1 of a thread-like metallic material in a meshed, woven, or fis-
sured~form, which is coated with catalytic material consisting
of metals, metal alloys, metal oxides. The catalyst causes a
catalytic oxidation of the smoke-generating particles in the
exhaust gases. Due to the action of the catalyst, the oxida-
tion occurs below temperatures above 405C,beginning in fact
at tem?erat7~res as low as 200~C. The resulting homogenization
of the combustion of the carbon particles in the diesel exhaust
gas makes it possible to o~erate the catalytic filter at
a relatively low counterpressure of the exhaust gas. The
operating temperature of 400C, which is usual with diesel
engines, is quite adequate for the combustion process in the
catalyst; the only precondition for the catalytic oxidation
is an excess of air in the exhaust gas and the presence of
hydrocarbons which burn exothermally under the action of
the catalyst. The resulting filter efficiency, however,
is lower than with the ceramic filter discussed above, namely
about ~0 to 50%.
(}~i3~
1 Summary of the Invention
By contrast with the prior art t the present invention
provides a filter for diesel en~ine exhaust gases which i5 com-
parable in filtering act-on with the well-known ceramic filters,
5 but simultaneously achieves a uniform particle combustion with
relatively low exhaust-gas counterpressure at a relatively low
temperature.
According to the invention, this is achieved by the
use of a large number of macrocells in a substrate of porous
lOceramic, connected into the flow of the exhaust gas. The macro-
cells always have an opening connected with the flow of the
exhaust gas, and all or some of them are wholly or partly filled
with a gas-permeable catalyst support, having a surface coated
with catalytic material.
In accordance with one aspect of the invention, the
catalytic material can be a metal, alloy or intermetallic com-
pound with an oxidizin~ effect.
In accordance with another aspect of the invention,
the gas permeable catalyst support is of metal threads in the
20form of wire or tape and is compacted to an insert correspond-
ing to the hollow shape of the macrocells.
In accordance with still another aspect of the inven-
tion, the macrocells are formed in a monolithic ceramic body.
The resulting filter combines the advantages of uni-
25form catalytic combustion of exhaust gases and the partialoxidation of particles at tem~eratures lower than 450C, with
the advantage of filtering even the finest carbon particles
in the areas of the walls of the macrocells of a ceramic sub-
strate, where the macrocells contain the catalyst. The carbon
30particles that are collected in the pores of the walls and on
their surface, and which have ~assed the catalyst without com-
bustion, are burned uniformly and at a temperature below 450C,
in contrast to the well-known ceramic filter. Evidently, there
is a reduction in energy because of the catalytic reaction.
r~ 3~3
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1 This result is furthered by the fact that the catalyst
support, which is present preferably in the form of metal
threads, fills the cavity of the macrocells to the walls,
so that the catalytic energy can act directly,or in the form
of radiation,on the porous walls of the macrocells, where-
the volatile carbon particles have been trapped. The catalytic
energy is essentially generated in the oxidation of hydro-
carbons, which are present in the exhaust gas either in free
form or as deposited solid carbon particles.
Accordin~ to another aspect of the invention, the
cells, which are filled with catalyst, are designed as lonqi-
tudinally extended channels, open on one side, in a mono-
lithic ceramic body. The channels are alternatively open
and closed upstream or downstream respectively, more or less
accordin~ to standard ceramic filter construction of cellular
ceramic discussed above.
However, in contrast with the well-known ceramic filters,
the channel cross sections in the present invention are con-
siderably larger. Instead of 50 to 200 open channels per
square inch. the ceramic structure accordin~ to the invention
provides only for about 5 to 15 channels per square inch,
that is about 0.5 to 4 channels per s~uare centimeter. These
channels are filled with insert catalytic bodies. The cata-
lyst support consists of metal threads in the form of wire
or tape material, which is pre-compressed in accord with
the hollow shape of the channels that are filled.
Also according to the invention, the wall thickness
of the channels is less than 1 mm, preferably between 0.3
and 017 mm.
A preferred embodiment of the invention provides that
only one complete channel cross section exists per square
centimeter frontal surface of the ceramic body. It has already
been noted that the catalytic insert bodies desirably have
a cross section correspondinq with the hollow shape of the
channels. Although the bodies can fill the channels wholly
(}~3~
1 or partially, it is preferred for them to extend over the
entire length of their channels.
To attain a desirable filtering action oE 60 to 80~
it may be sufficient in accordance with the invention, that
only channels which are open upstream are provided with an
insert body. The prime consideration is that only a small
fraction of the carbon particles, and of this fraction only
those wi-th the smallest dimensions, can travel from the open
upstream channel, through its walls, into a channel that is
open downstream. Another advantage is that, except for the
customary fastening of the ceramic monolith, no special fas-
tening is necessary for the insert bodies in the interior
channels. They are held at the channel walls through surface
friction of the woven wire material. This provides a clear
improvement over the well-known catalytic filter of the prior
artfor which expensive devices are required to fasten the
catalyst support to the interior of a combustion chamber~
With respect to exhaust-gas counterpressure, a clear
reduction is achieved compared to the well-known ceramic
filter which operates without any catalytic action. ~nly
the catalytic combustion effects continuous carbon burn-
off in the area of the catalyst insert bodies. The result-
ing energy is again directly used to burn off the carbon
particles that have been caught at the channel walls7 parti-
cles which were able to pass the catalyst insert body withouthaving been burned, so that no carbon collects. Consequently
the channel walls or the channels themselves do not become
plugged. They always remain free for the unhindered passage
of the exhaust gases. As a result, the exhaust-gas counter-
pressure has a desirably low level of about 0.1 to 0.2 bar.
53~
1 Description of the Drawings
Other aspects of the invention will become apparentafter considering several illustrative embodiments taken
in conjunction with the drawing which shows an axial longi-
tudinal section through a filter device that is connectedto a diesel enqine.
r s(}~39
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1 Detailed Description
As shown in the drawing, a cylinder head 1 of a diesel
engine is connected to an exhaust gas manifold 2. The latter
is in turn connected to the outlet 3 of the engine. Prom
the exhaust manifold 2, a pipe socket 4 feeds the exhaust
gases throuqh a connection sleeve 5 into the housing 6 of
an exhaust gas filter according to the invention. The flow
direction of the exhaust gases is indicated by arrows. Down-
stream of the exhaust gases in the filter,the housin~ 6
has a sleeve 7 to connect it to the remaining exhaust line.
In the interior of the filter housing 6, a monolitic
ceramic body 8 is supported in shock-proof fashion by a woven
wire casing 9, which is situated between the housing 6 and
the circumferential surface of the ceramic body. A gas tight
ring lOa is integrated into the woven wire casing 9 so that
no exhaust gases can pass through. The monolithic-ceramic
body 8 is~ofiporous ceramic material, e.g. Kordierit ceramic,
which is well known. The monlith has axial channels 10,11,
which are mutually parallel, and bound one another by thin
~ walls. These channels are closed either at the upstream or
at the downstream end, alternately as viewed in cross section
and in checkerboard fashion as in a top view on the ~rontal
side of the monlith. The channels 11, which are o~en upstream,
and the channels 10, which are open downstream, are closed
respectively downstream and upstream by a floor 12, which
likewise consists of ceramic material. The floor 12 can also
be thicker than the channel walls. The exhaust gases which
flow into the channels 11, which are open upstream, will
diffuse, as in~icated by the arrows, through the channel walls
into the neiahboring channels 10, which are open on the exhaust
side.
It is now important that the channels 10~11 are filled
with catalyst inserts 13, which are placed into the channels
from their open ends. The catalyst inserts 13 are form-pressed
woven wire bodies, with the wire being coated with a catalyt-
3~
g
1 ically effective oxidation sur~ace. The inserts 13 have across section coxresponding to the hollow shape of the channels,
or example, square, so that they can be inserted and can
be fixed within the channels without special mounting. The
inserts in the channels 10, which are o~en do~stream, have
been shown without cross-hatchinq only to make the drawing
clearer. However, it is also possible to leave the downstream
open channels 10 empty, i.e. not to place any inserts 13 in
them.
The catalytic combustion of the carbon particles in
the exhaust gas takes place in the region of the catalyst
inserts 13. Here the hydrocarbon compon~nts which are deposited
as the carbon particles are burned with the oxygen contained
in the exhaust gas, to form carbon dioxide and water, i.e.
pure carbon is burned to carbon dioxide. The particles which
have not alrea~y been burned in the region of the catalyst
inserts 13 are deposited in the pores of the channel walls
and on their surfaces. There they are likewise burned by
the energy derived from the catalytic combustion.
As a result only a very small portion of the carbon
particles contained in the exhaust gas, namely those with
very small dimensions, will pass through the channel walls
into the channels that are open on the exhaust side. From
their =, if no catalytic after-buring takes place there, they
pass into the exhaust gas flowing out of the filter. The
proportion o these particles lies between 20 and 30%.
Naturally, a cer-tain exhaust-gas temperature is required
for the catalytic reaction in the area of the carbon filter.
As a consequence of the catalytic combustion, however, this
temperature is reached by all types of diesel engines, even
those with relatively low combustion temperatures. Normally,
the temperature in the exhaust gas emerging ~rom the enqine
combusiton sapce is about 400C. The catalytic xeactions,
however, already occur beginning at 200C.
The woven wire material of the insert body 13 prefer-
~.~?,rt(~s3~
-10-
1 ably is of a corrosion-resistant metal alloy, with nickel
and chromium portions. Such materials are well know. The
catalytically active coating, which is composed of the most
various metals, their alloys, and their intermetallic com-
pounds, is also well known.
As far as the cermaic monolith is concerned, thefigure in the drawing corresponds to only one embodiment.
Smaller and larger channel cross sections may be used. The
density of the channels, measured per unit area on the frontal
side, desirably lies between 0~5 and 4 channels per square
centimeter of frontal surface and the drawing corresponds
to the upper limit of this range. The wall thickness between
the channels is approximately between 0.3 and 0.7 mm. The
prosity of the ceramic material is between about 35 and 55%,
with an average pore diameter of 10 to 35 um.
While various aspects of the invention have been set
--- forth by the drawing and specification, it is to be under-
stood that the foregoing detailed description is for illustra-
tion only and that various changes in parts, as well as the
substititution of equivalent constituents for those shown
and describea may be made without departing from the spirit
and scope of the invention as set forth in the appended claims.
