Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 022463~3 l998-08-ll
33785
An apparatus for the cleaning of exhaust gases from
internal combustion engines
The invention relates to an apparatus for the cleaning of
exhaust gases from internal combustion engines, in
particular a diesel exhaust particulates filter, pursuant
to the preamble of claim 1.
The disadvantages of this filter for diesel exhaust
particulates known from EP-A 332609 or EP-A 537219 for
example are that the exhaust particulates deposited in the
particulates filter outside of the ducts of the ceramic
body form conducting bridges between the inner electrode
and ground after some time which lead to parasitic currents
and permanently forming spark gaps.
It is the object of the present invention to eliminate
these disadvantages by constructional measures.
This is achieved in an apparatus of the kind mentioned
above by the features as mentioned in the characterizing
part of claim 1.
By closing off the hollow chamber which comprises the inner
electrode which is under high voltage, the permanent
formation of conducting exhaust particulates deposits
outside of the ducts of the ceramic body can be prevented.
It is preferably provided that the hollow inner chamber of
the ceramic body is also sealed off on the rear side by an
insulator, preferably a ceramic plug, which has a pass-
CA 022463~3 1998-08-11
through of a diameter of preferably 1 to 2 mm through which
the inner electrode is supplied with high voltage.
The supply of high voltage on the rear side has the
advantage that in this zone already very small exhaust
particulates deposits are present and, in addition, the
field strength at the pass-through is already so high as a
result of the small diameter of the feed line that there
occurs an immediate incineration of the exhaust
particulates deposited there, which again prevents the
formation of conducting bridges of exhaust particulates.
In order to insulate the discharge electrode especially,
the discharge electrode can be carried by the ceramic body
and be at the same high voltage potential as the inner
electrode.
The inclination towards the formation of sparks in the zone
of the discharge electrode by precipitated exhaust
particulates is counteracted in accordance with the
invention in such a way that the backplate electrode
opposi~e of the discharge electrode is provided with a
ceramic coating with a high electric resistance.
It has proved to be advantageous that the teeth of the
discharge electrode are provided at their tips with a
ceramic coating with a thickness of between 0.05 mm and 0.2
mm and have an electric volume resistance per tip of
between 1 megohm and 1 gigohm, preferably between 10 megohm
and 100 megohm.
It can also be advisable in accordance with the invention
that the ceramic coating of the backplate electrode has a
thickness of between 0.1 and 0.5 mm and has an electric
volume resistance of between 1 megohm.cm2 and 1 gigohm.cm2,
preferably between 10 megohm.cm2 and 100 megohm.cm2.
CA 022463~3 1998-08-11
Preferably, the coating of the discharge electrode and/or
backplate electrode consists of one of the materials Al203,
TiO, ZrO and CrO or mixtures thereof.
It is provided in accordance with a further feature in
accordance with the invention that the inner electrode
arranged on the inner side of the ceramic body is arranged
at a distance from the inlet side and pre~erably also from
the outlet side of the ducts of the ceramic body. In this
way the formation of bridges of exhaust particulates in the
inlet and outlet zone o~ the ducts o~ the ceramic body is
prevented.
Pursuant to a further embodiment o~ the invention it is
provided that between the inner electrode which is at high
voltage and the inner cylindrical surface of the ceramic
body there is arranged a posistor. Pre~erably, the posistor
increases its volume resistance from values below 10
megohm.cm2 to at least 100 megohm.cm2, preferably 300
megohm.cm2, at a temperature rise of 100~C to 500~C.
If at higher temperatures the resistance of the ceramic
body decreases too strongly, the high voltage on the inner
electrode needs to be reduced because only a limited amount
of power can be taken from the onboardmain power supply
which supplies the high voltage power supply unit. In this
way the discharge or backplate electrode which is switched
electrically parallel to the inner electrode would cease to
~ operate in the case o~ a lack of a posistor. The posistor,
on the other hand, compensates by increasing its resistance
the resistance o~ the ceramic body which decreases at
higher temperatures, thus ensuring that the function of the
discharge or backplate electrode is not impaired. If an
inhomogeneous current distribution occurs in the ceramic
body there will further be a local heatup of the ceramic
body which can lead to thermal damage to the ceramic body.
The local heatup regulates the local current supply back by
CA 022463~3 1998-08-11
way of the rising resistance of the posistor, thus leading
to an even distribution of the supplied power.
The invention is now explained in closer detail by
reference to the drawings, in which:
Fig. 1 shows a longitudinal sectional view through a first
embodiment of an apparatus in accordance with the
invention.
Fig. 2 shows a longitudinal sectional view through a
further embodiment of an apparatus in accordance with the
lnvent lon .
Fig. 3 shows a sectional view along the line III-III in
fig. 2.
A ceramic body 1 of annular shape is fastened in a
cylindrical tube 2 made of metal by press mats, wire meshes
3 or the like. The hollow cylindrical inner chamber 22 of
ceramic body 1 is closed off on either side by plugs 4, 4'.
A preferably metallic layer 5 which is electrically
conducting is arranged on the inner wall 21 of the ceramic
body 1, which layer is used as an inner electrode which is
connected with high voltage. A metallic layer 6 is arranged
on the outer cylinder wall of ceramic body 1, which layer
is used as an outer electrode and is connected to ground.
The ceramic body 1 is provided with continuous ducts 20
which extend in the longitudinal direction and which
preferably are provided with the brick structure known from
EP-A 537219. The two plugs 4, 4' are each provided with a
pass-through 23, 23', with an axially extending metallic
tube 7 reaching through the same, which tube is as thin as
possible in its diameter and carries on the inlet side the
backplate electrode 28. In order to be able to positionally
fix the tube 7, inserts (not shown) with waves or ribs
extending in the axial direction of the tube are provided
CA 022463~3 1998-08-11
in the pass-throughs 23, 23' between tube 7 and insulators
4, 4'. Tube 7 tapers on the outlet side towards a
connecting end 12 which engages in a receiving opening 13
of a cylindrical ceramic holding means 10 and is supplied
with high voltage by way of a conduc~or 11 guided in the
holding means 10. The inner electrode 5 is connected with
high voltage via the conductor 11, the connecting end 12,
the tube 7 and a contact spring 9 attached on tube 7. An
electric field builds up in the ceramic body 1
transversally to the continuous ducts 20 between the inner
electrode 5 which is at high voltage and the outer
electrode 6 connected to ground. In order to support this
field the tube 7 can be arranged between the insulators 4,
4' as an emission electrode. The ceramic body 1 is
preferably made of a cordierite mass by high-pressure
extrusion and is then ~ired at high temperatures. The
ceramic body 1 is to have a very low porosity, preferably
lower than 0.5 %. The height of the ducts is usually
between 0.6 and 1 mm and the width of the ducts 20 is
between 3 and 6 mm, depending on radial position.
The discharge electrode is ~ormed by a cylindrical tube
body 8 which emits electrons, is provided with spray teeth
24 and rests on tube 2. The backplate electrode 28 which is
opposite o~ the discharge electrode 8 comprises a
cylindrical basic body which tapers conically on the inlet
side. The backplate electrode 28 comprises a ceramic
coating 14. The coating has a thickness of 0.1 to 0.5 mm
and comprises an electric volume resistance relating to cm2
o~ 1 megohm.cm2 to 1 gigohm.cm2, preferably 10 megohm.cm2
to 100 megohm.cm2. The high voltage at the inner electrode
5 and thus the backplate electrode 28 is approx. + 8 to 12
kV. Pre~erably, the high voltage is regulated proportional
to the volume or mass flow o~ the exhaust gas within an
interval from 2 kV/cm to 6 kV/cm relating to the distance
between inner electrode 5 and outer electrode 6.
CA 022463~3 1998-08-11
The exhaust gas flowing in ~rom the inlet side A and being
charged with diesel exhaust particulates flows into the
ring duct 26 against the inlet openings of the ducts 20 o~
the ceramic body 1, which ring duct 26 is formed by the
discharge electrode 8 and the backplate electrode 28. The
exhaust gas particulates are ionised in the ring duct 26
and penetrate the ducts 20 of the ceramic body 1. As a
result of the electric field which is built up
transversally to the ducts 20, the exhaust particulates
which are contained in the exhaust gas and are charged by
the discharge electrode 8 are deposited on the wall
sur~aces o~ the ducts 20 and are oxidised electrochemically
by a gas plasma of emitted electrons formed as a result of
the high electric field strength. Exhaust particulates o~
the exhaust gas leaving the ring chamber 26 cannot reach
the inner chamber 22 of ceramic body 1 and thus the inner
electrode 5 as a result o~ the plug 4. The majority o~ the
exhaust particulates contained in the exhaust gas will
penetrate the ducts 20 and will be oxidised by the gas
plasma a~ter being deposited on the walls o~ the ducts 20.
Exhaust particulates which deposit on the outer side of the
pass-through 23 on plug 4 or tube 5 and thus form
conducting bridges o~ exhaust particulates are incinerated
by spark formation as a result of the small diameter of
tube 7 and the thus prevailing high field strength, so that
no longer conducting bridges o~ exhaust particulates can
form there. The inner electrode 5 which is at high voltage
is also protected ~rom the outlet side B by the plug 4'. At
the outlet side B the exhaust gas exiting the ducts has
already substantially been cleaned from exhaust
particulates. However, i~ residues of exhaust particulates
are deposited on tube 7 or closing end 12 at outlet side B,
high ~ield strengths will occur as a result of the small
diameter o~ tube 7 or the closing end 12, as a result o~
which the exhaust particulates deposited there will
incinerate by spark formation. As is shown in fig. 1, the
inner electrode 5 and the outer electrode 6 do not extend
CA 022463~3 1998-08-11
over the entire length of ceramic body 1, so that a
virtually ~ield-~ree zone o~ ~low will be retained in the
inlet and outlet zone of ceramic body 1. In this way any
short circuiting of the inner electrode 5 with the outer
electrode 6 by way o~ any brldges of exhaust particulates
occurring at the inlet or outlet openings of the ducts is
prevented.
Fig. 2 shows a sectional view along the main axis of
another embodiment o~ a converter ~or diesel exhaust
particulates. In the converter ~or diesel exhaust
particulates pursuant to fig. 2 the ceramic body 1 is
electrically and mechanically separated from the discharge
electrode 29. The ceramic body 1 having the continuous
ducts 20 for the diesel exhaust particulates is also
provided with an annular diameter and is ~astened by press
mats or wire meshes 3 in an extended tubular portion of the
exhaust gas tube 2. The hollow inner part 22 of the ceramic
body 1 is closed o~ on the inlet side by a non-conducting,
preferably ceramic plug 4. An electrically conducting layer
is arranged on the inner and outer cylinder jacket of the
ceramic body 1, which layer is used as an inner electrode 5
which is at high voltage or an outer electrode 6 which is
at ground. The hollow inner chamber 22 of the ceramic body
1 is closed of~ on the outlet side by a non-conducting,
preferably ceramic plug 4'. Plug 4' comprises a thin bore
through which extends a metallic tube 7 which is as thin as
possible and per~orms the contacting of the inner electrode
5 with the help of a contact spring 9. The high voltage is
supplied to tube 7 by a conductor 11 arranged in a ceramic
cylindrical holding means 10. The rear-sided end of the
tube 7 tapers into a pin 12 which is electrically connected
with the conductor 11 and engages in a recess 13 of the
holding means 10. The high-voltage values are substantially
identical with those of the embodiment according to fig. 1,
but the high voltage is provided with a negative polarity
on the inner electrode 5 and on the discharge electrode 29.
CA 022463~3 1998-08-11
The discharge electrode 29 is arranged electrically and
mechanically separate from the ceramic body 1 in tube 2 of
the exhaust gas strand. The discharge electrode 29
comprises a basic body 25 which carries cylindrical spray
teeth 24 and is provided on either side with thin pins 18,
18' which preferably have a thickness o~ 2 to 4 mm and by
which the discharge electrode 8 is supported in recesses
19, 19' of ceramic holding means 15, 16. The high voltage
is supplied to the discharge electrode 29 via pin 18 by a
conductor 17 guided in the holding means 16. The backplate
electrode 30 encompassing the discharge electrode 29 is
formed by a ceramic coating applied to tube 2 which has a
thickness of O.lmm to 0.5mm. The electric resistance values
correspond to those of the backplate electrode 14 in the
embodiment pursuant to ~ig. 1.
A posistor 27 is arranged between the inner electrode 5 and
the inner wall 21 of the ceramic body 1, which posistor
increases its resistance in the case of any increase of
temperature. By the increase of its resistance, the
posistor 27 compensates the resistance of the ceramic body
1 which decreases at higher temperatures.
The exhaust gas entering at A is ionised in the ring
chamber 26 between the discharge electrode 29 and the
backplate electrode 30 and flows through the ducts 20 of
ceramic body 1 and leaves the exhaust particulates filter
at B. As a result of the electric field which is built up
between the inner electrode 5 and the outer electrode 6,
there will be a separation of the exhaust gas particulates
contained in the exhaust gas on the side walls of ducts 20.
Electrons will emit from the walls of ducts 20 as a result
of the temperature, which electrons are accelerated in the
direction towards the deposits of exhaust particulates by
the electric field prevailing there and trigger an
CA 02246353 1998-08-11
oxidation of the deposits of exhaust particulates on
impact.
