FR2608941A2 - Descending granules forming enclosed filter for exhaust gas - Google Patents

Abstract

Exhaust gas from a diesel engine passes from a soiled gas chamber through an annular chamber formed by concentric sieve surfaces resp. of fine and coarse metal mesh, into the central clean gas exit chamber. During passage the gas traverses a charge of refractory non-metallic granules descending under gravity, and on which soot particles from the gas are deposited. Soot collected by the filter material is burned away in controlled conditions by heat developed from a pair of electrodes, pref. formed by the metal sieves, with the coarse mesh acting as earthed electrode. To ensure the granules are packed to effective density while descending, they are subjected to radial pressure from air compressed in an outer annular chamber surrounding the filter chamber.

Description

The invention relates to a device for
Elimination of the black smoke from the exhaust gases of an internal combustion engine, in particular of an engine
Diesel, comprising a metal body with inlet and outlet pipes for the exhaust gas stream and a filter cartridge arranged in a body which contains a non-metallic filter medium, resistant to high temperatures, consisting of a granular material in bulk, which is arranged in a retaining element, the filter cartridge separating in the filter body a raw gas chamber from a filtered gas chamber, and a source of electric current being used for the controlled combustion of the black smoke deposited on the filter medium, in accordance with the indications given in Le Brevet principal NO 87 09479 filed on July 3, 1987.

 According to the main patent, the black smoke itself plays the role of electric heating conductor. The filters used can be surface filtration filters (for example non-woven fiber filters or fabric filters) or deep filtration filters (for example bulk material filters, i.e. granules).

 Surface filtration filters run the risk of being clogged with combustion residues such as ash particles. On the other hand, the granule filters undergo by the effect of the particles of ash of this kind only an increase in volume, insofar as these particles have not been carried away by the current of exhaust gas passing through the volume of permeable material made up of granules. Pellet filters, when produced in non-compacted mass as is usually the case, generally require a large volume and therefore have a high weight which makes their use often unattractive with a motor vehicle engine. .

 The reasons why bulk filters require a large volume is the relatively large length of the average path which must pass through the granules The exhaust gases loaded with black smoke, so that a desired probability is obtained determined separation of the black smoke contained.

 What is called the "length of the separation course is directly a function of the average size of the granules, if, for the moment, the effect of the speed of the exhaust gases through the granules is not taken into account. is that the more the diameter of the granules increases, the more the average free microscopic passage section available for the exhaust gases also increases, which ultimately leads to a reduction in the efficiency of the separation, which can then only be improved by an increase in the volume of the bulk material (which corresponds to an increase in the length of the separation path).

 The conclusion which follows, that is to say the choice for a smaller dimensioning of the filter of small granules, has however limits, because it is necessary at the same time to seek to lower the speed of passage of # exhaust gas (in order to avoid loss of material by entrainment) which leads to an increase in the surface of the perforated bottom through which the exhaust gases are conducted.

 The reduction in the height of the bulk material layer with, simultaneously, the increase in the area of passage of the gas stream and the reduction in the diameter of the granules therefore follow a law of formation which does not lead to the expected reduction in the volume or, consequently, weight, which are essential conditions for the use on a motor vehicle of a bulk material filter.

 One way to use, despite everything, lightweight pellet structures in the production of a deep filtration filter is the immobilization of the bulk material.

 The basic idea is as follows. In the usual granule filters, the volume flow per unit of passage surface is, roughly, a function of a determined value of the ratio between the two forces which act on each elementary granule of the bulk material during operation. On the one hand, it is the driving force (acting against gravity), dependent on the flow speed and applying to the surface of the grain, which is antagonistic to the weight of this same grain. When the driving force is greater than the weight (ratio between the two forces greater than 1), this results in the undesirable entrainment of the granule. If, on the other hand, a barrier against the entrainment is provided by means of a sieve, the presence of this allows considerably higher flow speeds inside the filter, up to the value of the pressure drop which is still at the limit of tolerance by the motor as component of the back pressure in the exhaust gas stream.

It is not difficult to understand that such a filter is distinguished by a comparatively low need for bulk material, as well as by its suitability for use on a motor vehicle.

 The invention, which relates to a smoke black filter of the type indicated above, aims to achieve immobilization of your bulk material which is particularly effective, of safe and relatively simple operation, making it possible to obtain in the filter a very high degree. high separation of smoke black.

 This result is obtained according to the invention with a device of the type already indicated thanks to the fact that the retaining element comprises two perforated tubular bodies arranged vertically and coaxially, limiting a first annular space, while the surfaces of the perforated tubular bodies turned towards this first annular space are each lined with a fine mesh screen and a large mesh screen which are connected, the exhaust gases passing through the filter cartridge thus formed in a transverse direction relative to the action of the gravity on the granules and Said first annular space which can be supplied for compacting the granules with compressed air by means of a second annular space extending it provided in the upper part of the filter body.

 Thanks to the invention, a double immobilization of the bulk material is obtained, firstly by the sieves installed and, secondly, by the action of compressed air. This is further reinforced by the special arrangement and structure of the filter cartridge as well as by the described guidance of the exhaust gas flow. In this way, the pressure drop undergone by the exhaust gases in the bulk material no longer has the possibility of going beyond the latter, which would result in undesirably weakening the degree of separation of the black smoke.

 According to an improvement of the invention, the pressurized air is directed to the second annular space by a pipe equipped with a current type regulator.

When neither the tremors produced by the natural mechanical vibrations of the exhaust, nor those produced by the pressure waves of the exhaust gases, are sufficient to prevent the formation of plugs in the granules when the feed is put into service in compressed air, this can be remedied by means of an ultrasonic transmitter tuned to the resonant frequency and mounted on the base plate of the filter body by means of a solid waveguide. In the event that the addition of smoke black and ash by the exhaust gases results in an undesirable increase in the resistance to flow through the filter as a result of the narrowing of the pores, it is possible, by cutting off the supply of compressed air by means of a solenoid valve, and at the same time switching on the ultrasonic transmitter, causing
The disintegration and therefore the expansion of the volume of the granules. By opening the compressed air supply again,
Bulk granules can be compacted again, the ultrasonic transmitter can stay on for
The short duration of the training operation.This process can be controlled for example using a metal bellows dynamometer with spring which, when a critical exhaust gas backpressure is exceeded ( closing a contact) activates the following chain of operations: cutting the compressed air - switching on the ultrasonic transmitter - opening the compressed air - stopping
The ultrasonic transmitter.

 For the electrical elimination of the black smoke in accordance with the method of main patent NO 87 09479, the first annular space filled with granules is provided with an electrically conductive electrode, cylindrical with passage openings, made of wire mesh with large mesh, energized, which is placed near the side of the gas flow inlet, where the maximum amount of smoke black deposit is found, and which is fixed in position by means of insulating spacers (by example of the balls). This cylindrical perforated electrode is connected to an electrical power source by means of a target which is held by an insulating element provided in the base plate of the filter body. Intermediate pieces in plate, which can be made of several parts or a single part, provide relief from the insulator against pressure forces.

 Particularly advantageously, the large mesh screen which follows the fine screen acts as an electrode for the ground potential of the connected current source. This sieve is arranged in such a way that any risk of passage of the lines of the electric flow towards the fine sieve is practically excluded, which would destroy its destruction.

Other characteristics and other advantages of the invention will appear during the description which follows of an exemplary embodiment. Preferably, a varistor inserted on the discharge line of a pulse emitter has the role of producing a shaping in time of the discharge current in order to avoid
Thermal destruction of granules or electrodes.

The invention will be described below with reference to the accompanying drawings in which:
- Figure 1 is a longitudinal sectional view of a bulk material filter according to the invention;
- Figure 2 is a partial view on a larger scale of the filter cartridge of Figure 1 traversed by the flow of exhaust gases, showing the electrical phenomena in the vicinity of the surface of the filter which receives the flow of exhaust gases ;;
- Figure 3 is a diagram of the electrical circuit for controlling the triggering of the ignition for the electrical combustion of the black smoke by means of a current supply working with an intrinsic power independent of the load (in the spirit of main patent no. 87 09479 which can be referred to for a more detailed explanation).

 The smoke black filter 1 comprises a filter body 2 made of a cylindrical sheet metal envelope which is closed at its ends by end plates 2a and 2b. Plate 2a is the cover plate and plate 2b is the bottom plate. Inside the body is a filter cartridge 5. This is constituted as follows. Two tubular bodies 10 and 11 with perforated walls, arranged vertically and coaxially, compose a retaining element 7 for fine granules 6 in bulk.

The loose granules (which are made of non-electrically conductive and heat-resistant material, for example SiO2), are housed in a first cylindrical annular space 12 limited by the two perforated tubular bodies 10 and 11. To prevent the granules to escape in the radial direction, the perforated walls of the tubular bodies 10 and 11, the faces of which face towards the cylindrical annular space 12 are furnished with one and the other with a sieve 13,13a, with fine fine mesh in the heat. The size of the mesh openings is chosen such that the smallest particles of the granules, in relation to the width of the band of the particle size distribution, are also surely retained.

 The supply of exhaust gases to the filter cartridge 5 described above, which is arranged parallel to the action of gravity on the granules, is effected by an inlet pipe 3 disposed in the vicinity of the bottom plate. 2b on the filter body 2 tangentially thereto to promote uniformity of the current and avoid rebounding. The exhaust gases laden with smoke black pass by creating a gyratory current first in a raw gas chamber with a cylindrical annular configuration, pass through the filter cartridge 5 then arrive in a central cylindrical collecting chamber (filtered gas chamber 9 ), and escape through an outlet tube 4 embedded in the center of the cover plate 2a, leaving the filter after being filtered and purified (the arrows 26 materialize the flow of exhaust gases).

In the upper part of the filter body 1, follows the first cylindrical annular space 12 a second cylindrical annular space 15 which extends it by surmounting it. This space 15 represents a clearance volume for the volume of the granules which are continuously enriched in ash and which, consequently, are expanding. Light but permanent. This second annu
the cylindrical tube 15 is constituted by the extension towards the inside of the cylindrical outlet tube 4 and by a support ring 16 which has the role of holding the perforated tube 11. The other perforated tube 10 is held by the tube outlet 4 * The two tubes 10 and II are joined to the bottom plate 2b.

 This second cylindrical annular space 15 is supplied with compressed air by means of a conduit 17 for compacting the loose granules 6. This compressed air comes from a regulator not shown and the conduit can be opened or closed by a solenoid valve 18. L compressed air supply can operate in conjunction with a common type ultrasonic transmitter, not shown, mounted on the bottom plate 2b of the filter body 2, which has the role of disaggregating the granules. Naturally, the ultrasonic transmitter can also work alone.

 For the electrical elimination of the smoke black which takes place by direct heating of the smoke black by electric energy, the first cylindrical annular space 12 is provided with an electrode 19 constituted by a cylindrical sieve with large meshes which is fixed in position. by means of insulating intermediate pieces 20. In the example of embodiment shown, these intermediate pieces are balls which are housed in hemispherical recesses formed in the cylindrical electrode 19 in sieve fabric. By an electrical conductor 23 which passes through the bottom plate 2b inside an insulator 22 and which is connected to the electrode 19, an electrical power supply unit is connected to the latter. So that the pressure which the electrode 19 can exert does not act on the insulator 22, there are provided in the lower part of the cylindrical annular space 12 ceramic intermediate plates 21.

With fine mesh cylindrical screens 13 and 13a
(granule retaining sieve) of the cylindrical annular space 12 are connected directly towards the inside of the cylindrical sieves 14, 14a with large meshes (mesh width: about 0.8 mm). These cylindrical screens are set to the potential of the mass of the connected current source. The large sieves 14 and 14a prevent by
Their presence the current flow to reach the fine screens 13, 13a, as shown in Figure 2.

 This figure shows on a larger scale a part of the filter cartridge with a sketch of the electrical phenomena in the vicinity of the surface of the filter encountered by the current of exhaust gases. From a macroscopically homogeneous distribution of the black smoke deposited inside the granules, a distribution of the density of the electric current is created as shown in FIG. 2. The important thing is that the field lines are terminate in their great majority on the surface of the son of the screen 14 which is capable of withstanding a high thermal load. If, on the contrary, they ended on the fine mesh retaining screen 13, one would have to fear its destruction under the effect of a too high thermal load (electric power density microscopically too high during the formation of the current path d The smaller the width of the meshes or the larger the diameter of the wires of the sieve 14, the less the danger of an "electrical penetration" through the sails of the sieve 14 deriving the potential. On the other hand, the diameter of the metal wire should not be chosen larger than required by the conditions of current density on the surface of the wire in the area of the ignition zone. Thus it is obtained, as it is desirable, that the volume of the space 24 devoid of field lines is as small as possible. At the same time, is obtained the certainty that the combustion which is started electrically and which becomes autonomous, by moving in the opposite direction to that of the flow of the exhaust gases, will act in the spaces 24 up to the screen of retainer 13 and remove the smoke black deposits therefrom.

In the same way, a distribution of the heating current is established in the direction of the screen wall 14a which is set to earth potential. Since the profile of the smoke black deposit accumulating in the cylindrical annular space 12 presents its maximum on the side of
The arrival of the gas flow, it is advantageous to give the cylindrical electrode 19 a diameter such that its spacing in the radial direction on the side of the gas flow arrival is less than its spacing on the side of the current outlet gases.

 To safely start the electrical combustion of the black smoke, an electrical circuit is provided as shown in FIG. 3.

 A transient current unit 25 is connected in parallel to the current source Q required in the present case, which operates at an intrinsic power independent of the load. This is essentially constituted by a capacitor C which, of a switching means S with timed control, is connected via a varistor R2 (resistance in silicon carbide variable with the voltage) to the set of electrodes of the pellet filter, and therefore finally to the resistance R1 representing that of smoke black.

 By means of the decoupling diode D2 the capacitor C is permanently maintained in the charged state using the high voltage source H. Another decoupling diode D1 prevents any unwanted discharge of C in the source of current Q. For the capacitor discharge cycle sequence, a frequency range of 1 to 0.2 Hz can be considered, which must be taken into account for the corresponding setting of the time control (multivibrator at free speed).

 The reason for using the transient current generator is as follows. When the smoke black filter must be activated from the fully charged state, this can correspond to a very low ohmic resistance of the equivalent resistance of the smoke black, in particular when the presence of the smoke black results a long period of operation at full load of the engine.

 A qualitative characteristic of the black smoke emitted under these conditions is its "dryness". this term must be understood as meaning The absence of insulating envelopes of hydrocarbon condensate which, when they exist, give rise to a high contact resistance between the elementary particles of black smoke and which, even in the case of a layer thick deposit of "wet smoke black", lead to comparatively high equivalent resistances (engine working at idle). But, in the case mentioned above of dry smoke black, these contact resistances are absent, which explains the low ohmic value mentioned.

 But as the supply bLoc Q operating at an applied intrinsic power independent of the load (i.e. at a limited current) is only capable of supplying weak currents (which are entirely sufficient to maintain what is called "combustion zone"), it follows that it cannot create within the locally almost homogeneous distribution of the resistance of the smoke black a current density sufficient to produce a sudden rise in temperature resulting in the formation of a combustion zone.

 With an auxiliary current source, such as the capacitor C shown, which gives rise in the deposition of smoke black for a very short time, at a current density high enough to initiate the formation of a combustion zone < this can be attributed on the one hand to the lack of micro-homogeneity in the distribution of the resistance before the instant of ignition and on the other hand to the negative temperature coefficient of the resistivity of the smoke black having an effect of local concentration on the current distribution) it is possible to overcome the difficulty explained above. When the combustion zone is produced in this way, it establishes in the distribution of the resistance between the surfaces of the electrodes a point of discontinuity of value extremely low ohmic resistance which, due to the negative temperature coefficient of the resistivity, develops the property of gathering in itself in an avalanche The electrical energy of the external regions have to the combustion zone and thus increase in a very short time the energy conversion as well as the temperature so that, If no previous measure was taken to turn off the current, we should expect heat destruction of the combustion zone concerned.

 To avoid this danger safely, the variation over time of the capacitor discharge current is controlled as follows. A short-lived peak current first creates the combustion zone; immediately afterwards, this current must drop to a value just sufficient to maintain the combustion zone with its current concentration effect, but this with a local energy conversion which ensures thermal compatibility with the environment.

 The descent as a function of time of the current to zero value must be characterized by a duration which is greater than the inverse value of the frequency of emission of pulses by the current supply Q.

Thus, it follows from this the maintenance of the combustion zone thanks to the fact that the production of energy in this same zone is assured with certainty by the current coming from the current production unit Q. The variation over time of the transient current mentioned above is obtained by the passage of the discharge current through the varistor R2. This resistance made of silicon carbide has the property that the instantaneous value of its resistance is a function of the instantaneous value of the discharge voltage of the capacitor C. A high initial charge voltage of the capacitor means a lower value of the resistance of R2, that is, a desired high transient current; as the charging voltage decreases, the resistance increases and, consequently, the current decreases but lasts longer <to compare with the discharge via a normal resistance with constant ohmic value)
Given the final value of the equivalent resistance of the combustion zone, which is at the same time dependent on the current, it is to be expected that, for very large volumes of granules surrounded by the surfaces of the electrodes, the equivalent resistance of black of smoke from the non-lit deposit is smaller than the equivalent resistance of the ignition zone while the latter is supplied with heating current at the highest thermally permissible value possible.

With such a system, the formation of a combustion zone cannot be achieved, even with a transient current source, otherwise one can expect destruction by heat. This can be remedied by splitting it into several electrically separated electrode zones which, by means of a timed switch (for example a relay chain) can themselves be connected one after the other in rapid succession ( repeatedly without end) to the parallel connection of the power supply unit Q and the transient current transmitter. Dividing the electrode area into several small fractions of the same size can go hand in hand with dividing the filter into several filter units.
null, mounted in parallel from the point of view of La circu
gas lation, housed in a common filter body.

Each filter element represents at the same time one on
partial face of electrode that can be switched on.

This procedure is also rational in that
which concerns the realization.

Finally, it is still necessary to report a
other use of shutdown of compressed air supply
mé, for example in the working conditions of the Diesel engine
which are characterized by smoke black emission if
low that, according to legal obligations, filtering
it's not mandatory. If, during such operating conditions, driving with non-compacted granules, one obtains
Desirably, as a result of less pressure drop across the filter, reduced engine losses. It would be
conceivable, by acting by means of the solenoid valve by a suitable regulation, of not activating the filter in the
above meaning only when it is really necessary.

It would also be possible to add to the
filters, upstream of this, an electrostatic coagulator. As a result of the formation of agglomerates there (from the smoke black particles appearing first at the outlet of the engine, new smoke black particles are formed with a much larger diameter), could size the filter of
so as to ensure a reduction in the pressure drop across the volume of the granules.

This is achievable either by decreasing
the cylindrical annular space 12, with the same grain diameter of the granules, either by the use of a larger grain diameter of the granules while maintaining a constant value of the size of the cylindrical annular space (the diameter microscopic pore size can be increased). The first of these measures has the advantage of causing a reduction in weight; with the second measurement we have 13,13a retaining sieves with larger meshes which makes them more stable.

Claims (5)

 1. Device for the elimination of smoke black from the exhaust gases of an internal combustion engine, in particular of a diesel engine, according to claim 2 of the main patent, comprising a metal body with inlet pipes and outlet of the exhaust gas stream and a filter cartridge arranged in a body which contains a non-metallic filter medium resistant to high temperatures, consisting of a loose granular material, which is arranged in a retaining element, the filter cartridge separating in the filter body a raw gas chamber from a filtered gas chamber, and a source of electric current being used for the controlled combustion of the smoke black-deposited on the filtering means, characterized in that the retaining element comprises two tubular bodies <10,11) perforated arranged vertically and coaxially, limiting a first annular space (12) while the surfaces of the tubular bodies <10,11) perforated turned towards this first annular space (12) are each furnished with a fine mesh screen (13,13a) and a large mesh screen (14,14a) which are connected, the exhaust gases passing through the cartridge filter (5) thus formed in a transverse direction relative to the action of gravity on the granules (6) and said first annular space (12) which can be supplied for the compacting of the granules (6) with compressed air via a second annular space (15) extending it, provided in the upper part of the filter body (2).  2. Device according to claim 1 characterized in that the compressed air arrives through a pressure reducer via a line (17) and a valve (18) and its action can be coupled to that of a ultrasonic transmitter mounted on the bottom plate t2b) of the filter body (2).  4. Device according to claim 1 characterized in that, for the electrical elimination of the black smoke, the first annular space (12) is provided with a cylindrical electrode with passage openings (19) which is centered with respect to La side wall of the exhaust gas flow by means of insulating inserts <20) and, in the vicinity of the body bottom plate (2b), by ceramic plates (21), an insulator (22) passing through the bottom plate of the filter body (2b) for the isolation of an electrical supply conductor (23) leading to the electrode (19).  4. Device according to claim 3 characterized in that the large mesh screen (14,14a) acts as an electrode for the ground potential and at the same time prevents the current from reaching the fine mesh screen (13 , 13a) in the form of electrical aromas.  5. Device according to any one of claims 3 and 4, characterized in that a transient current emission unit (25) is mounted in parallel via a diode <D1) with a block of power supply (Q) working at a power independent of the load which is connected to the electrodes (14,14a, 19)  6. Device according to claim 5 characterized in that the transient current emission unit (25) consists essentially of a capacitor (C) supplied from a high voltage current source <H) via a diode (D2) and which, by means of a switching means (S) with timed control, is connected via a varistor R2 to the electrodes of the pellet filter and therefore to the resistance equivalent (R1) of the existing smoke black deposit.