EP0307414B1 - Exhaust gas cleaning system for diesel engines - Google Patents

Abstract

Exhaust gas cleaning system for diesel engines, with a filter unit to separate soot from the exhaust gases and with a regenerating device for the filter unit. The filter unit is provided with filter tubes (2) which on one side are closed. In this way, the exhaust gases can be fed in at the other side and penetrate the filter partition walls. During this, the soot is deposited on the inner surface of the filter tubes (2). A burner (11) is provided to produce the combustion gases for the soot, whereby the combustion gases are, as a result of suitable arrangements, made to impinge on only one sector of the filter unit at a time. The inlet (15) and outlet (17) of the exhaust gases are so selected that the cleaned exhaust gases flow around the filter tubes (2), which are subjected to regeneration. The flow direction of the exhaust gases can also be reversed, so that the exhaust gases first flow around the filter tubes (2) and reach, via the partition walls, the interior, and flow from there via the tube openings to the outlet. During this, the soot is deposited on the outside of the filter tubes.

Description

The invention relates to an exhaust gas purification system for diesel engines according to the preamble of the main claim.

The filtering of the diesel exhaust gases is necessary because engine measures to reduce soot alone are not sufficient. A wide variety of devices with filters have already been proposed for this aftertreatment of the exhaust gases, ceramic or electrostatic filters being used, for example. These filters filter out the soot particles in the exhaust gas that are deposited on or on the filter. This results in the need to remove the deposited soot from time to time in order to maintain the functionality of the filter, which is generally referred to as regeneration of the filter.

For example, SAE paper No. 850015 describes a device with monolithic ceramic filters and their regeneration through the targeted selection of engine setting parameters in conjunction with the filter being installed close to the engine. In this device, however, the filter regeneration is subject to large coincidences, with the risk, among other things, that the filter block is thermally overloaded.

An improvement in the regeneration can be achieved by metering in metal-containing additives which are added to the fuel, as is shown, for example, in SAE publication no. 860137. Here, however, there is the further problem that metal-containing compounds are emitted.

A fundamentally different solution for the regeneration of the filter is to burn off the soot by means of an additional burner, which is switched on when necessary and burns the soot through the burner gases (compare, for example, DE-OS 3219948). However, the particular problem here is to adapt the burner function to the respective operating state, namely the amount of exhaust gas, the exhaust gas temperature, and the exhaust gas pressure of the engine gases. Even slight deviations from the required coordination can lead to thermal damage or destruction of the filter.

To avoid these difficulties, switching devices with two identical filters have been proposed, in each of which one filter is located in the exhaust gas stream and the other is regenerated independently of it with the aid of a burner (DE-OS 3204176). The disadvantage of such a device lies in the large construction volume, construction effort and in the necessary switching elements that are exposed to the hot corrosive fuel gases.

It has therefore already been proposed to carry out the regeneration of the filter sector by sector, by passing an appropriately constructed combustion chamber sector-wise over the filter block (compare, for example, US Pat. No. 4,481,767). A major disadvantage here is that the burner hood warps due to the different heat development and the leverage of the fastening device, which in turn means that the hood cannot be properly sealed against the parts of the filter that are not currently undergoing regeneration.

To avoid this disadvantage, the invention in the preamble of claim 1 is based on the kinematic reversal of this known proposal, the filter being rotated in sectors (US Pat. No. 4,573,317) into the burner area in a known manner. Apart from the large construction costs, this generic device has considerable disadvantages. This particularly includes the cooling problem. Since the burner gases and soot represent an additional heat source, a direct-acting heat sink should be available to protect the components against temperature overload. In all known devices there is also a heat sink, but this is insufficient, so that a heat build-up arises which can very soon lead to the destruction of the filter.

Here, the invention with the characterizing features of claim 1 is intended to remedy the fact that the proposed exhaust gas purification system no longer has the disadvantages mentioned and which also brings significant advantages.

Advantageously, the filter elements are designed as tubes, the wall of which the exhaust gas flows radially, except that a large filter area is present. The exhaust gases can be fed into the second or alternatively into the first subspace. The supplied exhaust gases to be cleaned thus flow through the filter tubes and through their walls from one part of the room to the other and, when the chamber is arranged in the second part of the room, also paint around the filter tubes assigned to the regeneration sector, which are themselves decoupled from the exhaust gas cleaning by the chamber. As a result, a heat exchange process takes place between the filter tubes in front of the chamber and the filtered exhaust gas flow. This process achieves that the filter tubes of the regeneration sector are either kept at temperature and thus already have the exhaust gas temperature when swiveling into a filter sector or are cooled if the hot burner or. Soot exhaust gases flow through the filter tubes and heat them up too much. One therefore satisfies the requirement that the heat source and heat sink are spatially together. It is also advantageous that the heat exchange takes place along the entire length of the filter tubes.

Advantageously, according to the invention, the regeneration burner can also be switched on when the internal combustion engine is started, so that the cold exhaust gases are heated by the heated tubes, so as to prevent the usual burst of smoke when the machine is still cold. This advantage does not exist in known devices.

It is obvious that by designing the filter block from several spatial ones separate radially flowed tubes avoids heat accumulation, as can occur with the known monoliths (filter candles). It is also advantageous here that the individual filter tubes can be replaced if necessary. Ceramic wound filters or steel wool filters, which are designed as filter tubes or are arranged in these, can serve as filter elements.

According to an advantageous embodiment of the invention, a burner can be used as the regeneration means for generating hot combustion gases which are generated in the chamber. In principle, however, catalytically active substances can be used for regeneration in the chamber or chemical oxidants can be sprayed in.

According to a further embodiment of the invention, it is particularly advantageous if the filter tubes are bundled rotationally symmetrically around a central axis and are accommodated in the cylindrical drum, the central axis being mounted in the housing as the axis of rotation of the drum.

According to a further embodiment of the invention, the dividing walls are arranged in the second partial space, extending radially from the axis of rotation to the inner wall of the housing, so that a sector-shaped chamber is created.

According to an advantageous embodiment of the invention, the filter tubes are connected to one another on the closed side via a disk-shaped wall of the drum and are supported in a rotationally fixed manner by means of this wall. The filter tubes can be mounted on one or two sides in the drum. With one-sided clamping, the filter tubes can expand freely and thereby reduce stress; with double-sided mounting of the filter tubes with fixed and loose sides, the filter tubes can also expand freely and there are no additional stresses on the ceramic material due to vibrations which could affect the housing. The type of storage of the filter tubes is adapted to the need.

According to a further advantageous embodiment of the invention, the cleaning capacity can be adapted to the actual accumulation of residual quantities in that the clock frequency can be changed as the filter block continues to rotate as a function of the motor load averaged over time.

According to a method according to the invention, the filtered exhaust gases for cooling or temperature control are advantageously passed over the regeneration area, thereby avoiding temperature overload and avoiding dew point problems when the temperatures are too low. It is continuously regenerated within the tubular filter element and outside the filter element is continuously flushed with the already cleaned exhaust gas stream and thus cooled. The heat exchange process and the regeneration process both take place continuously and in parallel, unaffected by one another. This heat exchange process is therefore stationary. The temperature field is constant with all advantages for the process and the material stress.

Further features and advantages of the invention result from the subclaims and the following description of exemplary embodiments.

• Fig. 1 einen Schnitt durch das erfindungsgemäße Abgasreinigungssystem
• Fig. 2 einen Schnitt nach Linie 11-11 in Fig. 1
• Fig. 3 eine perspektivische Darstellung des neuen Abgasreinigungssystems
• Fig. 4-6 weitere Ausführungsbeispiele der Erfindung.

Show it.

• Fig. 1 shows a section through the exhaust gas purification system according to the invention
• 2 shows a section along line 11-11 in FIG. 1st
• Fig. 3 is a perspective view of the new exhaust gas purification system
• Fig. 4-6 further embodiments of the invention.

Figure 1 shows a preferred embodiment of the invention.

The filter block is accommodated in the cylindrical housing and consists of the individual filter tubes 2, which are arranged rotationally symmetrically and axially parallel around the central axis 3. The central axis 3 is mounted as a shaft in the housing 1. The left ends of the filter tubes 2 in the illustration in FIG. 1 are inserted into the disk-shaped wall 4 and either as shown in the dash-dotted circle, thereby closed on one side, or the tubes 2 penetrate the wall 4 and are closed in themselves. The walls 4 and 5 are connected in a suitable manner to the shaft 3 in a rotationally fixed manner, so that the filter tubes 2 also rotate when the shaft 3 rotates.

The housing 1 is further divided by the partition 5 into two sub-rooms 6 and 7 sealed against each other. The free ends of the filter tubes 2 are mounted in the partition 5 in such a way that in this case their openings are aligned with the end face of the partition 5 facing the partial space 6, as can be clearly seen from FIG. 3.

The sub-space 6 serves as a supply space for the exhaust gases to be cleaned, while in the sub-space 7 the exhaust gases are filtered by means of the filter tubes 2. The subspace 6 is further divided by the partitions 8 and 9, which extend radially from the shaft 3 to the inner wall of the housing 1 and are stationary. A sector-shaped space 10 is thus obtained which is sealed off from the residual space and which is considerably smaller than the remaining filter area. The burner 11 projects into the sector space 10 so that it can be referred to as a regeneration space. Fig. 2 clearly shows the formation of the regeneration room. The exhaust gases to be cleaned are fed in via the connector 12 and the cleaned exhaust gases are removed via the connector 13. The filter block consisting of the individual filter tubes and the walls 4 and 5 can be rotated in the housing 1 by means of the schematically indicated motor 14.

The operation of the exhaust gas purification system shown will now be described.

It is assumed here that the filter block is in the position shown in FIG. 1. The exhaust gas flows in the direction of arrow 15 through the nozzle 12 into the space separated from the regeneration space and from there via the openings 16 of the filter tubes 2. Since the filter tubes 2 are closed at the opposite end, the exhaust gases are forced to penetrate through the filter walls, namely over the entire length of the filter tubes, so that you have large effective filter areas. When the gases flow through, the suspended particles, ie the soot, settle on the inner wall of the filter tubes. The exhaust gases cleaned by the filter tubes flow around the filter tubes belonging to the regeneration sector on their way to the drainage connection 13, have a cooling effect on them and combine with the regeneration gases in order to flow off with them in the direction of arrow 17. No exhaust gases can get into the filter tubes 2 belonging to the regeneration sector 10 because of the sealing by the partition walls 8 and 9. Instead, the burner gases enter them, which heats them up until the ignition point of the deposited soot is reached and this is ignited and burned. It may be advisable to switch off the burner after the start of the burning process.

After the filter tubes concerned have been regenerated, the filter block is rotated further by a sector width by means of the motor 14. It is of course also possible to continuously rotate the filter block instead of the cyclical rotation.

Sealing means 18 are arranged between the filter block and the housing in order to prevent uncleaned exhaust gases from entering the regeneration space. This also applies to the slots between the fixed partition walls 8 and 9 and the movable filter block, for which there is a strip seal, not explained in more detail, over which the end face of the partition wall 5 slides. To reduce the wear between the seals and the partition 5, it is advantageous to relieve the seals during the rotation by slight axial displacement. This can be done, for example, by axially displacing the shaft 3 using suitable means that are not described in detail. However, the lifting can only be kept insignificant, or be completely omitted, so that the rotating surfaces rub against one another, so that the soot is scraped off at the end faces of the filter tubes and cleaning of these surfaces is also achieved. In the latter case, the seals 18 described must be correspondingly resistant.

In the example described, the filter tube is cleaned in cycles; it is possible to use the cycle time, i.e. to determine the time of the regeneration and to make it dependent, for example, on the state of loading of the filter tubes with soot. For this purpose, the loading condition must then be checked and measured, which can be done by pressure measurements, in that the pressure difference between the static pressures in front of and behind the regeneration sector is determined. 1, the measuring probes 19 and 20 serving for this purpose are indicated schematically. The pressure difference can be measured when the burner has just started (hot process) or the burner air has just started (cold process). The differential pressure measurement is safer by having a defined burner air flow and not being dependent on the changing operating conditions of the engine exhaust gases.

Another measure known per se can also be applied to the new exhaust gas purification system, namely coating with catalytically active substances. This has the advantage that the hydrocarbons adsorbed in the soot are completely burned. This is particularly necessary if the desorption of the hydrocarbons by heat conduction takes place faster than the burning process, i.e. the exhaust gases would otherwise not have been burned sufficiently. It is advantageous and expedient if the catalytic coating is provided on the outflow side of the filter.

It is readily apparent that various changes and additions to the described exhaust gas purification system remain to the person skilled in the art without departing from the scope of the invention.

A variation useful for certain applications may be to reverse the direction of flow of the exhaust gases, i.e. that the exhaust gases are introduced into the nozzle 13 and flow out cleaned from the nozzle 12. Here, the exhaust gases flow around the filter tubes 2 and penetrate through their walls, so that the soot is deposited on the outer surface of the filter tubes 2.

This reversal has the effect that, in contrast to FIG. 1, where the deposition of the residual particles and the supply of the burner gases take place in cocurrent, these two processes now take place in countercurrent.

FIG. 4 shows such an exemplary embodiment, only the inlet connection 21 being offset relative to the outlet connection 13 compared to FIG. 1. Otherwise, this system works exactly like the system described in FIG. 1, so that a further explanation is not necessary.

On the other hand, it is possible in the example of FIG. 4 to reverse the direction of flow of the exhaust gases. In this case, the left ends of the filter tubes 2 must then be open and the right ends of the filter tubes 2 must be closed, as can be seen from the flow arrows in FIG. 5, which is not to be described in more detail. Here, too, soot is deposited on the outer surfaces of the filter tubes 2, but the supply of the hot burner gases also takes place, so that a direct current is again formed. In order to separate the regeneration sector from the rest of the filter block, in this example there are radial partition walls in the filter block which divide the filter block into segments whose cross-section corresponds to that of the combustion chamber.

It is also possible to arrange the burner 11 differently than in FIGS. 1 to 5. Fig. 6, which corresponds to Fig. 4, shows such a modification in which the burner 22 acts on the circumference of the filter block. Here too, partitions are required in the filter block, as indicated in the previous example. Deposits of the residual particles and supply of the hot burner gases are also carried out in cocurrent again. A detailed description is unnecessary since the operation of this example corresponds to the previously described embodiment.

All of the features shown in the description, the following claims and the drawing can be essential to the invention both individually and in any combination with one another.

Claims (15)

1. An exhaust purification system for diesel engines having a filteration means to separate soot particles out of the exhaust gases and a regeneration means for the filteration means that comprises a separate regeneration sector with the input of regeneration agents for the soot, whereby the filteration means comprises a number of filter elements, preferably equidistant from one another, carried in a rotatable drum contained in an outer housing having an inlet and outlet but being sealed against the escape of exhaust gases whereby one end wall of said drum divides said housing into two chambers with openings for the axial flow of exhaust gases from a first header chamber providing entry into the filter elements and a second chamber into which the gases passing through the filter elements are discharged and whereby depending on the rotational position of said drum the filter elements are regenerated in succession and in only a suitably partitioned-off region of the filteration means, characterized by the filter elements (2) being tubular in form and being closed at one end, by the exhaust gases passing radially through the filter tubes (2) and axially through the perforated end wall (5) and by partition walls (8 and 9) running parallel to the filter tubes (2) between the perforated end wall (5) of the drum and the opposing wall at the end of the housing (in the header chamber 6) or between the end walls of the drum (in the filter chamber 7) to form a chamber (10) for the regeneration means in at least one of the chambers (6, 7).

2. An exhaust purification system in accordance with claim 1, characterized by the regeneration means being a burner (11) to produce hot combustion gases that are generated in the chamber (10).

3. An exhaust purification system in accordance with claims 1 or 2, characterized by the filter elements (2) being arranged rotationally symetri- cal and axially parallel in the drum around a shaft (3) with said shaft (3) being firmly attached to the drum.

4. An exhaust purification system in accordance with claim 3, characterized by the partition walls (8 and 9) being in the second chamber (6), being fixed to extend radially from the shaft (3) to the inner wall of the housing (1) and being sealed against or joined to said inner wall to form a sector shaped combustion chamber (10).

5. An exhaust purification system in accordance with claim 4, characterized by the combustion chamber (10) being stationary and by the drum with the filter elements being capable of being rotated in front of the combustion chamber (10), preferably in steps equal to the size of the sector.

6. An exhaust purification system in accordance with claim 5, characterized by the frequency of the cyclic rotation of the drum (filter block) being variable in relation to the periodically determined engine load.

7. An exhaust purification system in accordance with one of the foregoing claims, characterized by the axis of the burner (11) being parallel to the axis of the filter elements (2).

8. An exhaust purification system in accordance with claims 4 to 6, characterized by one of the connections (13) for the discharge of the filtered exhaust gas being provided and by the axis of said connection (13) being approximately symet- rically axial to the sector shaped combustion chamber and being located in the center of the first chamber (7).

9. An exhaust purification system in accordance with one of the foregoing claims, characterized by the closed ends of the filter elements (2) all being secured to a disc-shaped wall (4) of the drum and being caused by the said wall (4) to rotate with the shaft (3).

10. An exhaust purification system in accordance with one of the foregoing claims, characterized by the point at which regeneration is initiated being controllable in relation to a drop in pressure of the exhaust gases in the sector to be regenerated.

11. An exhaust purification system in accordance with one of the foregoing claims, characterized by the filter elements (2) being coated on their outer surface relative to the direction of flow with substances having a catalytic action.

12. An exhaust purification system in accordance with one of the foregoing claims, characterized by seals (18) being provided between the housing (1) and the end wall (5) and by the end wall (5) being lifted off the seals (18) during the intermittent rotation of the drum (filter block).

13. An exhaust purification system in accordance with claim 1, characterized by provision being made for reversing the direction in which the exhaust gases flow.

14. An exhaust purification system in accordance with claim 2, characterized by the burner (22) being so located in the housing (1) that its flame is directed vertically downwards against the filter elements (2) (Fig. 6).

15. A process for the purification of diesel exhaust gases in accordance with claims 1 to 10, characterized by provision being made for the exhaust gases from which soot particles have already been filtered out to be directed for a heat exchange process through a thermal regeneration region.

Images