WO2005047662A1

WO2005047662A1 - Particulate filter for exhaust gases of diesel engines

Info

Publication number: WO2005047662A1
Application number: PCT/IB2004/003397
Authority: WO
Inventors: Paolo Della Porta; Giovanni Salvago; Stefano Zilio; Giorgio Vergani
Original assignee: Saes Getters S.P.A.
Priority date: 2003-11-14
Filing date: 2004-10-18
Publication date: 2005-05-26
Also published as: ITMI20032211A1

Abstract

It is described a particulate filter (10) for exhaust gases of diesel engines, comprising a housing (11;52) which includes a filtering body (14;50) and has at least one inlet opening (12) for the exhaust gases to be filtered, as well as with at least one outlet opening (13) for the gases filtered by the filtering body(14;50), in which the filtering body (14; 50) comprises a plurality of panels (15) made of metal fibers, which panels are arranged with one edge facing the inlet opening (12) and another edge facing the outlet opening (13), said edges being mutually joined two by two so as to form two alternate series of spaces, the first series open at the inlet side of the filter and closed at the outlet side of the filter (10), and the second series closed at the inlet side of the filter (10) and open at the outlet side of the filter (10).

Description

PARTICULATE FILTER FOR EXHAUST GASES OF DIESEL ENGINES

The present invention relates to a filter for diesel engines, and in particular to a filter which can be employed for filtering the particulate from the exhaust gases of these engines and is characterized by an easy regenerability. As known, one of the main problems posed by the use of diesel engines is the emission of particulate, also known in the field as "soot"; the soot consists of particles having a size below about 100 nanometers (nm), made up of a central portion of unburned hydrocarbons surrounded by carbonaceous particles formed by the fuel combustion; these particles are notoriously responsible of respiratory problems. In particular, the problem is caused by the vehicle engines, due to the number of circulating vehicles with a diesel engine (even if there are diesel engines employed for other uses, for example in uninterruptible electric power generators). For reducing the problem, the antipollution regulations require that soot- retaining filters (known as "Diesel Particulate Filters" or DPF) are employed downstream the diesel engines; these regulations are increasingly stringent and require a constant technical evolution of these filters. The DPF filters employed nowadays are practically all of the ceramic kind, i.e. consisting of a ceramic substrate, on the surface of which is generally present a catalytic active material, suitable for decreasing the soot conversion temperature. The DPF filters must be periodically regenerated for removing the soot accumulation which causes their obstruction. The regeneration operation is generally controlled in an automatic way by the vehicle electronics when it evaluates, through a sensor measuring the pressure upstream and downstream the filter, that the pressure drop between these two points exceeds a preset threshold value. Regeneration can be carried out by sending to the DPF nitrogen dioxide (NO₂), which is a gas more oxidizing than oxygen and thus favors the combustion of the hydrocarbons and the carbon forming the soot, thereby generating carbon dioxide (CO₂). Nitrogen dioxide is a gas which is generally already emitted by the engine, but mixed with the monoxide, NO, and the quantity of dioxide is not generally sufficient for a complete regeneration of the DPF. It has thus been proposed that, when the regeneration is needed, a complete conversion of NO to NO₂ is provoked, but this requires the use of additional system components; furthermore, the ratio between NO and NO₂ emitted by the engine strongly depends on the running condition of the same, so that it is difficult to completely control the conversion. In the newest engines the so-called "post-injection" concept has been proposed. According to this working scheme, when the regeneration of the DPF is needed, some fuel is injected into a cylinder during its exhaust stroke, therefore with the exhaust valves opened; in this condition, the fuel crosses the cylinder without exploding, and reaches a first gas converter, called CCC (from the English "Close Coupled Catalyst") which is always present and is used for the complete oxidation of the unburned gases. The CCC, also thanks to its closeness to the engine, has a high temperature, which is sufficient for causing the catalytic oxidation of the fuel; the gases leaving the CCC after this combustion have a particularly high temperature, and by reaching the DPF they can carry out an efficient soot combustion, which, in turn, generates further heat. The overall result is that, after the post-injection, the temperature on some points of the DPF can easily exceed 1000 °C. However, the use of the post-injection, and the high temperature gradients associated therewith, causes problems to the traditional ceramic filters, which have a poor resistance to the thermal unhomogeneity and can thus break because of this. It is therefore preferable to use filters with a metal base, characterized by a thermal resistance which is intrinsically higher than that of the ceramic filters. EP-A-1317950 discloses a filtering material consisting of two overlapping layers of metal fibers, which is preferably folded like a bellow and can be arranged in the flow of the exhaust gas of a diesel engine for retaining the particulate. Since the metal fibers are mutually interlaced and sintered, this filtering material is relatively rigid, so that the angle comprised between the pleats of the bellow cannot be lower than a given limit value. As a consequence, given a maximum overall volume, the total filtering surface is relatively limited, and consequently the efficiency and the capacity of the filter are limited. It is therefore an object of the present invention to provide a filter which overcomes said disadvantage. Said object is achieved with a particulate filter for exhaust gases of diesel engines, comprising a housing which includes a filtering body and is provided with at least one inlet opening for the exhaust gases to be filtered, as well as with at least one outlet opening for the gases filtered by the filtering body, characterized in that said filtering body comprises a plurality of panels made of metal fibers, which panels are arranged with one edge facing the inlet opening and another edge facing the outlet opening, said edges being mutually joined two by two so as to form two alternate series of spaces, the first series open at the inlet side of the filter and closed at the outlet side of the filter, and the second series closed at the inlet side of the filter and open at the outlet side of the filter. According to a particular aspect of the invention, the panels of metal fibers are catalytically active, so that the filter can carry out, besides a mechanical filtering, also a conversion of the exhaust gases of the diesel engines and assist the soot combustion during the filter regeneration. Further, the deposition treatment of a catalytic layer is more advantageous onto the single panels to be mutually joined as in the present invention rather than onto a continuous folded structure as in the prior art, since in this latter case the inner zones of the corners act as accumulation zones for the catalytic material and therefore as zones which can be quickly obstructed during the particulate filtering. Further advantages and features of the filter according to the present invention will become clear to those skilled in the art from the following detailed and non-limiting description of one embodiment thereof with reference to the attached drawings wherein: - figure 1 shows a perspective view of the filter of the invention;

- figure 2 shows an exploded view of a filter of the invention, in a first embodiment thereof;

- figure 3 shows a perspective view of the filter of figure 2, longitudinally sectioned;

- figure 4 shows an enlarged cross-section view of a detail of the filtering body of the filter of figure 2 ;

- figure 5 shows an exploded view of another embodiment of filter of the invention;

- figure 6 shows a sectional view of the filter of figure 5; and

- figure 7 shows an enlargement of a component of the filter of figure 5. The arrangement of panels in the filtering bodies of the invention is such that gases, in their path between inlet and outlet of the filter, are forced to pass across the panels thickness; in this way, particles are retained by the panels, while gases are free to pass through said panels and emerge at the outlet of the filter. Figure 1 shows in perspective a complete filter according to the invention. Filter 10 comprises a casing 11 having an inlet 12 and an outlet 13, and inside the casing, a filtering body 14. Filtering body 14 comprises a plurality of panels produced with fibers made of steel or, preferably, of an alloy containing iron, chrome and aluminum, plus small percentages of other elements, known as Fecralloy^® (trademark registered by UKAEA, Didcot, Great Britain). This alloy turned out to be particularly suitable for the prolonged use at high temperatures, as in the foreseen use in filters for internal combustion engines. Panels made of Fecralloy^® fibers can be obtained from the company N.V. Bekaert SA, of Zwevegem, Belgium, and are marketed with the name Bekipor^®. The surface of the metal fibers of these panels is generally oxidized by means of a thermal treatment in oxidizing atmosphere (e.g., air) at temperatures of about 1000 °C, thus creating a layer of dense alumina whiskers a few micrometers long; preferably, onto this layer another oxide layer is grown, in particular a mixed oxide of cerium and zirconium (with the optional addition of lanthanum), which is particularly efficient in catalyzing the soot oxidation reaction. Other catalytic elements can also be added to this catalyst, for example a noble metal chosen among those of the eighth group of the periodic table, preferably platinum. The construction of a first embodiment of body 14 is illustrated in detail in figures from 2 to 4. According to this first embodiment, filtering body 14 suitably comprises a plurality of panels 15 made of metal fibers, which panels are arranged with one edge facing inlet 12 and another edge facing outlet 13, said edges being mutually joined two by two so as to form at least one bellow. A flange (not shown in the figures) can be arranged around the outlet opening 13 for locking filtering body 14 into housing 11. With reference to figures 3 and 4, it is seen that the adjacent edges of two panels 15 are preferably joined to each other by means of a C-shaped metal section bars 16 arranged astride said edges. Angle α comprised between panels 15 is lower than 20°, in particular lower than 10°. Thanks to the use of a plurality of panels of metal fibers mutually joined two by two for forming a bellow, the filter according to the present invention can comprise a filtering body in which the angle formed by the pleats is very low, even lower than 10°, so as to increase the filtering surface and thus the efficiency and the capacity of the filter, with the same size of the filtering body. A second, preferred embodiment of the invention, is shown in figures from 5 to 7. Figure 5 shows an exploded view of this preferred filter. The filter comprises a filtering body 50 inside a casing 51; in turn, casing 51 is inserted in housing 52 (analogous to housing 11 of figure 1); the filter is completed by an inlet flange 53 and an outlet flange 54, for connection to other elements of the exhaust pipe of the vehicle. Inlet flange 53 holds tightly against filtering body 50 a gasket 55, the function of which will be illustrated in the following. Figure 6 shows the filter of figure 5 in a cross-section view, taken along the direction of gases inside the filter. In this embodiment, the panels 15 are essentially parallel to each other, forming spaces 60, and are kept in the desired configuration by the metallic spacers 70. Spacers 70 are shown in greater detail in figure 7; these spacers present two deep grooves, 71, 71', for receiving the edges of panels 15; spacers 70 are preferably fixed to panels 15 mechanically, for instance by crimping; to improve the fixing of panels 15 in grooves 71, 71', a high-temperature resistant glue is preferably inserted into these grooves; an example of such a glue is Durabond 954, sold by COTRONICS Corp. of Brooklyn, NY (USA). Spacers 70 are fixed alternately to panels 15, in such a way that if a first space 60 defined by two adjacent panels is closed at the inlet side, the next space (having one panel in common with first space) will be closed at the outlet side. Further to grooves 71, 71', the section of spacers 70 defines a wide channel 72. Particles in the exhaust gas to be filtered could accumulate in channel 72 of the spacers 70 at the inlet side of filtering body 50. To avoid this, metallic elements 61 are added on the inlet side of body 50. Elements 61 have a thickness equal to that of spacers 70 and a section such as to convey gases from inlet 12 into spaces 60; besides, the shape of these elements is such to create turbulence by compression in the flow of gas entering filtering body 50, thus causing the gas to contact panels 15 already at the inlet side of the same panels and increasing the efficiency of the filter. Elements 61 are shown in figure 6 as solid elements, but could as well be void, as long as the outer section is maintained. In operation, elements 61 are maintained in place by the pressure of gas coming from inlet 12; anyway, it's better to ensure their positioning by mechanical fixing, by welding or by using high-temperature resistant glues, such as Durabond 954 cited before. To improve structural stability of the filter, panels 15 are preferably inserted in two lateral holding members (not shown in the figure) having the shape of a Greek fret, and fixed thereto e.g. by mechanical crimping. These holding members are made of a metallic sheet and have the further advantage of having a low weight, and thus a low thermal inertia, so that during post-injection these members (being the lateral walls of the filter) reach a high temperature more quickly than would be the case with thicker walls, thus avoiding the presence of "cold-spots" where soot could accumulate unburned. Finally, gasket 55 has the main function of closing at the inlet side the perimeter of filtering body 50, in order to avoid the presence of leaks that could constitute ways for gases to enter this body and by-pass it, without being forced to pass through the thickness of panels 15. The filtering bodies of the invention achieve, in different ways, the same effect of an improved performance of the filtering panels compared to the systems of the prior art, per unit volume of filter; with filtering body 14 this effect is obtained thanks to the particularly high packing (and thus surface area) of panels 15 in the available volume, while in filtering body 50 the effect is obtained because its geometry allows an optimized use of the whole surface of the panels. This leads to remarkable advantages in the working of the filter according to the invention: the wide surface offers a greater section for "capturing" soot, and consequently the same amount of soot, distributed on a wider area, will cause less frequently the filter occlusion; the final result is that the filter according to the invention requires a lower regeneration frequency. Alternatively, with the same regeneration frequency, the amount of soot to be burned will be lower, and therefore the heat generated by its combustion and the temperature reached by the filter will be lower. Both these effects ensure a longer life of the filter according to the invention with respect to the known metal filters. This feature allows the construction of filters of reduced dimensions compared to the filters of the prior art, and this brings about another remarkable advantage, i.e., an increased freedom of design for engineers in positioning the filter: filters of the prior art are rather

, bulky and, due to limited space available amongst the other components of the engine, need be positioned rather away from the engine, where the temperature of the gases has already decreased. The filters of the invention, instead, may be positioned closer to the engine and possibly even connected directly downstream the "Close Coupled Catalyst" converter (also known in the field as CCC), whose function is to complete oxidation of gases coming from the engine; an example of CCC converter suitable for use together with the filter of the present invention is described in co-pending application entitled "CONVERTER FOR EXHAUST GASES OF INTERNAL COMBUSTION ENGINES", filed concurrently with the present application and in the name of the same Applicant. With this positioning, thanks to the high temperature, a DPF produced according to the invention works anytime at its best possible performance.

Claims

1. Particulate filter (10) for exhaust gases of diesel engines, comprising a housing (11; 52) which includes a filtering body (14; 50) and is provided with at least one inlet opening (12) for the exhaust gases to be filtered, as well as with at least one outlet opening (13) for the gases filtered by the filtering body, characterized in that said filtering body (14; 50) comprises a plurality of panels (15) made of metal fibers, which panels are arranged with one edge facing the inlet opening and another edge facing the outlet opening, said edges being mutually joined two by two so as to form two alternate series of spaces, the first series open at the inlet side of the filter and closed at the outlet side of the filter, and the second series closed at the inlet side of the filter and open at the outlet side of the filter.

2. Filter according to claim 1, wherein said panels (15) comprise fibers made of an alloy containing iron, chrome and aluminum.

3. Filter according to claim 2, wherein said fibers are covered with a layer of alumina whiskers.

4. Filter according to claim 2 or 3, wherein said metal fibers of the panels are catalytically active.

5. Filter according to claim 4, wherein said metal fibers are coated with a catalyst comprising a mixed oxide of cerium and zirconium or a mixed oxide of cerium, zirconium and lanthanum.

6. Filter according to claim 5, wherein said catalyst also comprises an element chosen among the noble metals of the eighth group of the periodic table.

7. Filter according to claim 1, wherein said filtering body (14) is formed by mutually joining the edges of two adjacent panels (15) by means of a C-shaped metal section bars (16) arranged astride said edges.

8. Filter according to claim 7, wherein said angle (α) comprised between two adjacent panels is lower than 20°.

9. Filter according to claim 8, wherein said angle is lower than 10°.

10. Filter according to claim 1, wherein said filtering body (50) is formed by a plurality of panels (15) essentially parallel to each other forming spaces (60) and kept in the desired configuration by metallic spacers (70).

11. Filter according to claim 10, wherein said metallic spacers (70) present two grooves (71, 71') for receiving the edges of said panels.

12. Filter according to claim 11, further comprising metallic elements (61) at the inlet side of said filtering body, said metallic elements (61) having the same thiclcness of said spacers and a section such as to convey gases from inlet 12 into spaces 60.

13. Filter according to claim 12, further comprising a gasket (55) between said filtering body (50) and an inlet flange (53).

14. Filter according to claim 13, further comprising two metallic members having the shape of a Greek fret holding the sides of said panels parallel to the gas flow direction in the filter.