Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a catalytic filter
for purifying diesel exhaust gas
Since the worldls oil reserves are shrinking, petroleum
products and thus also fuels for internal combustion engines
are becoming increasingly more expensive. Therefore, it is
aim to construct enyines having a fuel consumption as low as
possible. The Diesel engine having an efficiency which is higher
by approximately 5% than that of engines operated with Otto
fuels is an alternative solution to this problem. So-called
lean concepts for Otto engines constitute an exception therefrom,
but constructionally they are not yet fully developed.
Without a catalyst all the exhaust gas limiting
standards applicable at present must be maintained for the
majority of Diesel engines. However, this does not exclude
the fact that Diesel engines frequently constitute a burden on
the environment and this miyht cause a real problem in case of a
percentage increase in the number of vehicles on the road.
Not only do Diesel engines emit deleterious substances
such as CQ, hydrocarbons and nitric oxides, but because of their
mode of operation they also emit carbon black particles and mlnute
condensate droplets or a conglomerate thereof ("particulates").
These particulates are very rich in condensed polynuclear
hydrocarbons, some of which are known to be carcinogenic.
Aldehydes also are among the emitted substances and contribute
to the odorous annoyance of the environment.
In many countries, as for example, in the United States,
it is intended to introduce limiting values for deleterious
substances and particulates in Diesel exhaust gases which can no
longer be attained by measures concerning the engine. In fact,
by recycling the exhaust gas (EGR) it is possible, for example,
to reduce the emission of NOX, but this simultaneously results in
an out-of-proportion increase in the amount of the particulates.
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Other solutions for reducing the emission of deleterious
substances from exhaust gases of Diesel engines propose the
interception of carbon black and condensate particles in particu-
late traps.
However, since the temperatures of Diesel exhaust gases
normally are not sufficiently high to burn off carbon black,
for which temperatures of at least ~50~C are required, the exhaust
gas temperature for regenerating the trap must be increased in
order to avoid an accumulation of carbon black and condensate
and thus clogging. This can be brought about, for example, by
periodically fattening the air-fuel mixture so that higher
exhaust gas temperatures are obtained. However, this is
associated with additional fuel consumption and would partially
cancel an advantage of the Diesel engine.
Furthermore, the ignition of an accumulation of carbon
black and condensate particles results in an uncontrollable
and even explosive combustion and thus in the destruction of the
particulate trap or of the entire exhaust system. Of course, the
volatile components, carbon monoxide and the malodorous substances,
0 such as aldehydes, contained in the Diesel exhaust gas cannot
P
t~, be separated by means of traps. ~e catalytic reaction is
required for their removal.
It might be possible to use a catalyst on the basis of
a monolithic ceramic reinforcing body in order to remove said
deleterious substances (see US Patent No. 3,597,165), but even
after a short operating time these kinds of systems are gradually
covered with carbon black since they are relatively remote from
the engine and thus rapidly lose their activity in many cases.
However, the particulates can pass unimpededly through the
3a channels of the reinforcing body without being degraded.
Other devices have, in addition to a catalyst, a filter
for separating carbon-black and condensate particles. This filter
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is cleaned mechanically ~y shak~ng off the particles (Bxitish
Patent No, 1,425,386), However, there still remains the
proble~ that the parti~cles are practically not reacted on a
succeeding catalyst.
The present invention provides a screen cloth system
of metal which can oe used as a highly effecti~e filter and/or
as a catalyst support and forces the exhaust gas to flow through
the filter surfaces.
This screen cloth system is made of high-temperature
resistant and corrosion resistant metal and comprises alterna-
tely at least one layer of a corrugated or concentrina s~reen
cloth or of sheet metal and at least one flat, closed or per-
forated cover layer. These layers are piled as a stack which is ~able
to gas or wound spirally so as to form a cylindrical, oval, rectangular or
polyg~ bcdy which ls ~meable to gas. This screen cloth system is
characterized in that the stack or the coiled ~ closed on the opposite
ends such ~hat a closed e~ porti.on is opposite an open end portion.
The flat cover layer can ~e a smooth metal sheet
or a metallic screen cloth.
A U-shaped edging may be slipped over one screen
cloth layer, or in case of identical geometrical shape over
several screen cloth layers, on the face end and secured to
said layer, said edging being adapted to the shape of the
layer concerned.
For fixing the screen cloth system it has been found
to be favourable to spot weld or lap weld or solder the
edging to the layers. A stack or coil produced from alter-
nately at least one layer of a corrugated or concentrina
screen cloth and at least one flat closed perforated cover
3Q layer is suitably welded or soldered on the end face for further
mechanical strengt~ening.
The screen clotfi systems thus produced can be
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coated ~y mean~; of conYentiOnal proce~ses, ~or ex~mple, hy
electroplatin~
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or impregnating with noble metal and/or base metal. However, they
can also be processed into a support matrix coated with a
conventional supporting material for catalysts and impregnatable
with solutions of active catalyst metals. According to an
embodiment of the invention the individual layers are coated on
their surfaces with a catalysis-promoting supporting material,
usually a metallic oxide of high surface area, for example, active
AQ2O3, such that a portion of the opening cross section of the
perforated layers, i.e., of the screen cloth layers, remains
intact. The filtering action is then adjusted with the aid of the
mesh width.
A further embodiment having a particularly good
filtering action is characterized in that the screen cloth system
is coated with a catalysis-promoting metallic oxide while filling
the opening cross section of the perforated layers, the oxide
layer being such that it is permeable to gas. In this case the
filtering action is adjusted by means of the porosity of the
oxide layer.
The elements cerium, zirconium, iron, nickel, rare
earths or a combination thereof can also be incorporated in this
metallic oxide as catalysis-promoting additives. The intermediate
layer of supporting material then is provided with the actual
catalyst by means of conventional methods.
The agent used for covering the front ends is a high
temperature-resistant and corrosion-resistant material. The
covering agent may be a ceramic material, a weld or solder surface
or a fitted and/or fastened stencil, which, when required, forms
a perforated mask embracing a portion of the jacket of the parcel or
of the coil.
Plugs of a ceramic material consisting of talc and
sodium tetrasilicate are particularly suitable as the means for
covering the face end portions. With this material the individual
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channels on the screen cloth s~stems descxibed are so plug-
ged that a closedchannel end altern~tely opposes an open
channel end, ~.e., that in ~ s~iral arran~ement of the screen
cloth layers a closed front end portion is adjacent to two
front end portIons and vice versa.
The end portions can also be covered with a mask
resting against the front ends and~or secured thereto which,
when required, may be a perforated, or slotted mask embracing
a portion of the ~acket of the parcel or of the coil, said
mask closing the corresponding channel ends.
The mask can also be such that in the case of a
spirally wound matrix made exclusively of screen cloth alter-
nately covers portions and leaves them open in the width of
twice the amplitude of the corrugation or fold, open and
closed portions being opposite on opposite front ends. This
assures that exhaust gas entering an open front end portion
cannont leave on the front end opposite this portion but must
pass the matrix partitions which are permeable to gas.
The spiral opening sey-ments may extend closely around
the circumference of a coil or of a parcel, but the closest
front end portions must always overlap correspondingly in
order to prevent a by-pass effect. A segmental division of
the spirally slotted surfaces by odd numbers is also possible.
In a further em~odiment of the invention the front
ends can be closed by weld or solder coating in the manner
described hereinbefore.
The screen clot~ used may be a web having a width of
mesh of 0.25 to Q.Q25 mm, preferabl~ 0.1 to 0.05 mm and a thick-
ness of wire of 0.15 to Q.Q25 mm, preferabl~ 0.1 to Q.05 mm,
particularly 0.07 mm. Met~llic ri~bons having a wall thic~-
ness lower than 0.15 mm are used as closed cover ~ayers.
Width and height of the
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cells encompassed by the smooth and corrugated or folded layers
f~ should not be ~ than 0.5 mm.
J
The corrugated layer may vary in shape. It is favour-
able when it is sine-shaped or has the shape of an involute or
has a rectanulgar or quadrangular shape. Sheet metal and screen
cloth consisting of an alloy of iron, chromium, aluminium, and
when required cerium or yttrium, and the usual alloying
constituents, and other heat-resistant and nonscaling materials
are particularly suitable for the screen cloth system used.
The screen cloth systems according to the invention, i.e.,
in the form of stacks or coils, can be disposed in a steel jacket
and rigidly connected thereto by soldering or welding in order to
stabilize them mechanically. The heat insulation of the steel
jacket with high-temperature insulating material may be
advantageous.
When the filter catalyst must be installed at points
of colder exhaust-gas temperatures for reasons of space or when
the degradation of carbon black and particulates is to be
commenced as early as during the cold starting period, then it
?0 may be advantageous to install, for the filter catalyst, current
supply leads for the continuous or periodic galvanic heating of
the layers.
The screen cloth system according to the invention can be
used in exhaust-gas purifying plants, particularly for internal
combustion engines, as filters for dust and/or aerosols and/or as
a support matrix for catalysts. The purification of exhaust
gases from Diesel engines is a preferred field of application.
For this purpose a filter catalyst is placed in the exhaust
pipe directly on the engine block. The oxidizable volatile
components of the exhaust gas can be degraded by providing a
suitablecatalytic coating parallel thereto. However, a standard
catalyst or a catalyst according to the invention can also be
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connected in series to an embodiment designed exclusively as a
filter unit.
The present invention will be further illustrated by
way of the accompanying drawings in which:-
Figure la is a cylindrical coil body in plan view,a front end portion closed with a plug being opposite on open
front end portion;
Figure lb is a section of the coil body of Figure l;
Figure 2a illustrates one form of a cover mask when a
wound matrix, consists entirely of screen cloth excepting the
outer sheath;
Figure 2b shows a partial view of a section through
coiled cylinder of Figure 2a;
Figure 2c is a similar view of another form of the
cover mask when wound around a matrix;
Figure 3 illustrates various possibilities of com-
bining screen cloth and sheet metal; and
Figure 4 is a diagrammatic representation of a screen
cloth system according to the invention with current supply leads
for heating the layers galvanically.
Referring to Figure 1, the filter consists of spirally
wound layers of smooth screen cloth or smooth metal foil and
corrugated screen cloth. The channels thus formed are closed
at one front end at 1 and open at the other front end at 2. A
plug 3 of ceramic material consisting, for example, of a hardened
mixture of talc and sodium tetrasilicate, serves as the seal.
The exhaust gas 4 enters through the front end at the
open channels of the filter and penetrates the porous side walls
'~' 5. It leaves at the other end at the open sL~c~ L__ channels.
For use in exhaust systems of motor vehicles the coiled body is
fitted into a metal jacket 6 and rigidly connected thereto.
However, as shown in Figure 2 perforated or slotted masks are
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also suitable as cover means for the partial closing of front
end portions. The Figures 2a and 2c represent alternative forms
of a slotted mask. They are used to cover segments 7 and 11 of
the front ends while the non-covered segments 8, 10 on the
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bl
opposite face end are closed. Figure 2b shows a section through
a cylindrical arrangement of screen cloths, wherein the flow
of the exhaust gas 4 through the screen cloth layers is shown
diagrammatically. The slotted mask used for covering may be
f' ~ de~ a cap 9 and is rigidly connected to the coil body
by welding or soldering.
As shown in Figure 3 various possibilites of combining
screen cloths 12 and metal sheets 13 exist. Thus, for example,
the corrugated or folded layers may consist of screen cloth and
the cover layers of smooth metal foil or the corrugated or
folded layers and the cover layers may consist of screen cloth.
However, it is also possible to produce the corrugated or folded
layers of sheet metal and to use a corresponding screen cloth
as the cover layer.
As mentioned hereinbefore, it can be advantageous to
preheat the filter catalyst via an ignition lock and timing relay
prior to starting the engine and thus to degrade the amounts of
carbon black and particulates encountered particularly in the
cold starting phase of a Diesel engine. This can also become
necessary during normal operating phases, for example, when
the exhaust gas temperature is too low or when the accumulation
of carbon black on the filter is too intense and an increased
pressure loss thus results in the exhaust gas system.
The possibility of galvanically heating a filter
catalyst according to the invention with the aid of à standard
automobile battery is shown diagrammatically in Figure 4. Thus,
for example, during the start the electric circuit may be
switched on automatically, for example, for 30 seconds.
Subsequently either a timing relay or a differential pressure
switch assumes the function of controlling the flow of current.
