EP2047073A1

EP2047073A1 - Manufacture-friendly silencer

Info

Publication number: EP2047073A1
Application number: EP07785765A
Authority: EP
Inventors: Svend Frederiksen
Original assignee: Silentor Holding AS
Current assignee: Silentor Holding AS
Priority date: 2006-07-14
Filing date: 2007-07-13
Publication date: 2009-04-15
Anticipated expiration: 2027-07-13
Also published as: WO2008006383A1; EP2047073B1; ATE525551T1

Abstract

Among others in order to provide efficient and manufacture-friendly configurable silencers, with preferred noise attenuation and pressure-drop characteristics, there is disclosed a silencer with a casing (3) having a longitudinal centre axis and at least one inlet (8) and at least one outlet (9) and at least one internal silencing section (2) extending between the inlet (7) and the outlet (8). The silencing section is along the longitudinal centre axis geometrically delimited by an inlet face (12) and an outlet face (13) and, in a direction transverse to, preferably perpendicular to a longitudinal centre axis, comprising a core region. The silencing section further comprising at least three oblong slots (18) extending outwards from the core region towards the casing (3) and extending from the inlet face (12) to the outlet face (13). The side walls of the slots (18) being essentially plane along an outwards extension.

Description

MANUFACTURE-FRIENDLY SILENCER

Summary of the invention

The invention relies on fitting into silencers silencing sections comprising radially extending, through-flowed slots, being constituted by solid or perforated slots to create Low-Pass filter sections or sound-absorptive sections. These slots and further elements of a completely or partly flat shape that results in efficient and manufacture-friendly silencers, can be made in many configurations, to achieve preferred noise attenuation and pressure-drop characteristics. Although the invention is not restricted to round shell, many of the particularly preferable embodiments of the invention are adapted to such shells, in particular to the very common shapes of a cylindrical or nearly cylindrical shell. Furthermore, many configurations using sound-absorptive material in a simple manner prevent contact corrosion of the inner surface of the shell. Part of the manufacture- friendliness can be attributed to the fact that manufacturing of preferred embodiments are not sensitive to manufacturing tolerances. Further, silencers according to the invention can be adapted to be inserted into new or existing chimneys, for instance dispersing exhaust gas flow from an internal combustion engine.

Background of the invention

Gas flow silencers come in various basic forms that are often combined to achieve a good broad-banded attenuation of noise; that is noise reduction at low, medium, and high frequencies. A first basic form is the through-flowed, sound-absorptive type of silencer that is predominantly efficient at higher frequencies and sometimes mid-range frequencies as well. A second form is the Low-Pass filter silencer, which in particular is able to reduce noise at low frequencies. Low-Pass filter silencers are sometimes instead termed expansion chamber silencers, reactive silencers, or reflection-type silencers.

A common form of an absorptive silencer is the splitter-type silencer. This type above all results in shells of a rectangular shape. It can of course also by adapted to cylindrical shells, but at higher manufacturing cost. As with all absorptive silencers, their performance drops at low frequencies. Although this can be compensated for by combining them with Low-Pass filter silencers, there would be an overall improvement of silencing if the function of absorptive silencers could be extended further down in the frequency range, in particular when space is scarce. The invention makes this possible.

When combined, usually absorptive and Low-Pass filter silencing sections are designed in ways that require quite different elements to compose each type of section. This contributes to relatively high manufacturing costs of most high- performance silencers. In contrast, the invention makes it possible to use very similar elements in the two types of silencing sections.

Many silencers of both the absorptive and Low-Pass filter types fitted into cylindrical shells are composed of internal parts, many of them being round. Such elements are rather costly, not least when shells are not perfectly cylindrical, but slightly oval, as is often the case in practice, where silencers, not allowing for costly manufacture of accurate and expensive shells unavoidably will be oval by several units of percentage. By first inspection, such a deviation may not seem great, but in practice it will, in many types of prior art silencers, call for a lot of work to adapt internal elements to such tolerances.

An unfortunate phenomenon that often occurs with sound-absorptive material, such as mineral or glass wool used in silencers and being in contact with the outer shell, is that corrosive vapours of exhaust gas will condense onto the inner surface and cause corrosion.

Many stationary combustion engine plants are fitted with a silencer and a downstream arranged stack to disperse exhaust gas into the atmosphere. Often there will be a too high noise level, the silencer not having been designed to provide sufficient noise reduction. In such cases, but also when plants are designed from the beginning, there is a need for a silencer type which is suited for insertion into such a chimney.

As will be demonstrated, the present invention is capable of fulfilling all the needs and overcoming all these difficulties in particularly manufacture-friendly silencers. Detailed description of the invention

A detailed description of the invention will now be made by reference to the following figures:

Figs. Ia - Ic show a first embodiment, and fig. Id shows a typical accompanying noise reduction spectrum.

Figs. 2a and 2b, for comparison, show a common type of prior art silencer.

Fig. 3 shows a cross-sectional view of a second embodiment of the silencer.

Fig. 4a shows a cross-sectional view of a third embodiment of the silencer. Figs. 4b, 4c, and 4d show a suitable way of manufacturing elements of the third and other embodiments.

While the previously shown embodiments refer to sound-absorptive silencers, figs.

5a, 5b, and 5c show a fourth embodiment of the silencer combining a Low-Pass filter section with a sound-absorptive section. Fig. 6 shows a fifth embodiment of the silencer, adapted to a slightly oval shell.

Figs. 7a - 7d show a sixth embodiment of the silencer, being fitted into a chimney, combined with four diagrams of sound wave forms, used to support an understanding of the functionality of this embodiment.

Fig. 8 shows a cross-sectional view of a seventh embodiment of the invention. Figs. 9a - 9e show an eighth embodiment of the invention where silencing is combined with SCR-denox.

Figs. Ia-Ic show a first embodiment of the invention, this first embodiment being an engine exhaust silencer 1 comprising a single silencing section 2, being a through-flowed sound-absorptive section. Fig. Ia is a longitudinal section, and fig. Ib a cross-section, along B-B. Fig. Ic is an enlarged view of part of the cross- section, and fig. Id is a typical noise reduction spectrum of the first embodiment. In the discussion of other embodiments of the invention, further reference will be made to this kind of spectrum.

As is seen from fig. Ia, a silencing section 2 is comprised within a casing 3 being constituted by a cylindrical shell 4 and end flanges 5 and 6 at opposite ends of the casing. Gas is led into the silencer via an inlet opening 7, from an inlet pipe 8 leading gas up to this opening. Similarly, an outlet opening 9 leads gas from the silencer, via an outlet pipe 10, discharging gas to the atmosphere at an opening 11. In the longitudinal direction, a silencing section 2 is delimited by a plane inlet face 12 and a plane outlet face 13. Between a silencer inlet 6 and a section inlet face 11, a cavity 14 is through-flowed in such a manner that gas flow entering this cavity will diverge axi-symmetrically, as indicated by velocity arrows.

This divergence is prompted by the fact that the inlet face has been divided into two parts: an outer, through-flowed and annular part 15 and an inner, not through-flowed, circular-cylindrical core part 16. A cylindrical, solid shell 17 constitutes the division between these two silencer section parts. The arrangement of forcing flow within the cavity 14 to diverge in this manner serves the purpose of ensuring an approximately uniform flow into and within the annular section. Without such a core part forcing the flow to spread out radially, there would be a tendency for flow into the silencing section 2 to be concentrated around to the centre, especially if the cavity 14 is of a short length, as is the case of the shown embodiment. Below it will be demonstrated how such a concentration can be avoided in other ways within the scope of the invention.

Downstream of the silencing section 2 there is a similarly through-flowed cavity 17 within which gas leaving the outer section annular part 15 converges before entering the silencer outlet opening 9.

As can be seen from fig. Ib, within the outer, annular part 15 there are in total eight radially and longitudinally extending slots 18 diving the annular part into eight identical segments 19. Within each of these segments there are a segmental body part 20 and an outer void part 21. Plane, perforated plates 22 constitute the divisions between these two segment parts. Due to this arrangement, gas will be transcending section 2 in a predominantly longitudinal fashion, the gas being divided into in total sixteen parallel part flows, eight of these within the slots 18 and the rest within the voids 21.

Fig. Ic shows an enlargement. Here, some further elements have been designated. Thus, the innermost delineation of the segment body is arc 24 of the previously mentioned solid, cylindrical shell 17. Slot 18 is sidewise delimited by plane, perforated plates 23, the lower plane, perforated plate 23 belonging to the adjacent, segmental body (see fig. Ib), situated below the segmented body shown in fig. Ic. Innermost, the slot is supplemented by a small distance- controlling member 25 adjacent to a very small arc part 27 of the cylinder. Outwardly, a likewise small distance-controlling member 28 is arranged adjacent to cylindrical shell 4. The two small members will ensure a well-defined width s of 5 slot 18.

Sound-absorptive material 28 is comprised within each sector body. The embodiment shown is an exhaust silencer, in which this material will typically be heat-resistant mineral or glass wool. If the silencer had instead operated at low0 temperature, such as for instance atmospheric air in a ventilation system, alternative types of absorptive materials, such as open-celled polymeric foam, could have been used.

The sound-absorptive effect will manifest itself fully from a frequency in the order5 of:

fD = c/D

where: O c = speed of sound D = diameter of shell

Descending in frequency below this value, the absorptive effect will gradually diminish. In fig. Ib a full wave w has been indicated to transverse across the5 entire cross-section. As the person skilled in art will know, this simple way of handling sound-corresponding transverse waves is somewhat simplified, but since the attenuation curve is rather crude, such a simplification is permissible from a practical point of view. 0 At the inlet face 12, a non-perforated plate 29 is arranged, blocking flow from entering the core part 16. Also, segmental bodies 20 delimit each of the eight sectors as well as the core region, such that neither the interior of the sector bodies nor the core part will be entered by any gas flow or by any acoustical energy propagating with the gas. Plate 29 is provided with in total sixteen5 openings giving gas flow free entrance to all eight slots 18 and all eight outer region void spacings 21. Along contour lines where an inlet face intersects with slots 18 and voids 21, there may be (not shown) roundings, for instance made by impressions onto inlet plate 29. Such roundings will prevent vena contracta flow phenomena and will thus lower flow resistance of the silencing section 2.

Similarly, at silencing section outlet 13 a plate 30 with in total sixteen openings is provided. The shape of this plate resembles that of plate 29, but where plate 29 delimits the core region, plate 30 is not full, but instead perforated to constitute an inner, circular part 31. This will admit sound waves travelling against the overall flow direction within the silencer to enter the core region. Further, a perforated, circular plate 32 has been arranged within the core part 15, dividing it into an upper part 33 and a lower part 34, respectively. The upper part has been filled out with sound-absorptive material 28.

Such a division will be appropriate whenever the silencing section of the first embodiment is of a length substantially exceeding an order of roughly 0.5 meter. This rule-of-thumb is based on the following reasoning : it is well-known in acoustics that sound-absorptive material will not constitute additional sound- absorptive effect when the size / thickness of the absorptive layer, as measured in the direction of sound penetration into the material, exceeds what can be termed the "penetration depth" of sound waves. From a theoretical point of view, this concept is vague, since even in extremely thick insulation layers, at no point even very weak waves are blocked from penetrating deeper into the material. However, in practice the concept is very useful, especially to be used within such silencers, where the total available space is restricted.

As example, if the thickness of the sound-absorptive material 28 between plates 31 and 32 is in the order of 0.5m, adding further sound-absorptive material into the empty spacing 34 would cause the core section to provide less sound- absorptive effect, "wasting" volume from an acoustical point of view. Instead, a quarter-wave (w) will extend right down to the bottom of the core part 16, i.e. down to plate 29, to absorb sound waves around a frequency:

f4 = c / (4U) where: c = velocity of sound and

L₅ = length of the pipe-like core part 16, in this case substantially equal to the length of section 2.

As an alternative to the arrangement within core part 16 of sound-absorptive material between plates 32 and 33, the entire core body 16 could have been void. The effect of this would be that at f4 there would be a more pronounced noise- reducing effect, but only in a small frequency range.

In relation to the dimensioning of segments 20, the concept of sound penetration depth also serves as valuable design tool: sound waves will penetrate distances (cf. fig. Ic) P = Penetration depth inwards from all three perforated plates 22, 23. With the selected dimensions, it can be seen that no inner point of the segmental body, being completely filled out with sound-absorptive material, is left isolated from penetrating waves, so that no segment part is lost from a sound-absorptive point of view. If the cross-sectional size had been bigger, alternative / supplementary designs according to the invention could be made in order to avoid wasted space at the segment cores. For instance, one could divide section 2 into more segments. Fig. 8 below will show a further method of handling this problem, which is present in silencers of big diameter.

In prior art splitter-type silencers it is recommended not to make splitters thicker than twice the penetration depth. Imagining the circular cross-section of the first embodiment of the invention to have been provided with such splitters reveals two attractive features of the embodiment. A fair comparison would be to imagine a set-up with a multitude of parallel splitters of differing lengths substantially spanning the entire cross-section in the longitudinal direction of the cross-sections of splitters. If an absorptive surface of the same order of magnitude as that of the embodiment of the invention were to be attained, a multitude of splitters would be needed. This would imply a splitter thickness substantially thinner than corresponding to twice the penetration depth. Also, the mean distance between the splitter surfaces and the shell would tend to be bigger than in the embodiments of the invention. Compared to such a splitter silencer, the following two advantages of the embodiments of the invention can be identified: First, while the splitters are of differing lengths, the segmental bodies of the present invention are identical, which is a simplification from a manufacturing point of view.

Secondly, in the embodiments of the present invention, voids 21 extend substantially all the way around the periphery of the silencer, as seen in fig. Ib, implying that all waves of a length in the order of diameter D, will penetrate sound-absorptive material radially inwards in differing directions, across the entire cross-section. At first sight, the core member 16 may seem to represent an obstacle to such sound behaviour, but as the person skilled in the art will appreciate, the diameter of the core body here is too small to represent a significant impediment in this respect. Furthermore, the design could be altered to eliminate the core body, which would surely eliminate any possible such inhibiting effect on waves spanning the entire cross-section.

In the splitter solution, by contrast, waves in the direction of the splitters would not meet sound-absorption material. In the transverse direction, they would of course meet sound-absorptive material, but such waves would have to transverse a multitude of void slots and their adjacent surfaces, where sound reflection would take place. Thus, for such waves the effective damping of sound would become less effective.

In sum, the shown embodiments of the invention, or variations of the segmental configuration, will provide a better sound absorption of noise of relatively low frequencies, corresponding to wavelengths in the order of magnitude corresponding to the diameter D of the cross-section.

Apart from the sound-absorptive function of the silencer embodiment, noise- reducing effects will be caused by sound reflections, in particular at the big, sudden cross-sectional changes at inlet 7 and outlet 9. This will cause the inner volume of the silencer and the pipe 11 to constitute a Low-Pass filter, i.e. a noise- reducing effect above a certain lower frequency which, according to well-known acoustic theory would be in the order of: where: c = velocity of sound a = cross-sectional area of pipe 8 Lp = length of pipe 8

V = acoustically effective volume of casing interior space.

In the case of dimensions being as indicated in the drawings, this frequency will be rather low.

If the entire silencing section had been left out, there would have been a similar Low-Pass filter effect. However, the noise reduction spectrum would have been much poorer, due to a multitude of "dips" at resonance frequencies corresponding to waves set up across the volume, both transversely and longitudinally. Instead, sound-absorptive material will now cancel out such resonances.

What has been termed "acoustically effective volume" in the above will substantially be equal to the combined volumes of annular section 15 and voids 14 and 17, that is a major part of interior of the silencer. Thus, the volume of annular section 15 would not be lost from a LP-filter effect point of view. A way of illustrating this is to imagine a sound wave being reflected at cross-sectional change from shell diameter to pipe diameter at the opening 9. Such a wave will travel backwards across the silencing section outlet 13. Even though a small sound reflection will occur at the relatively small effective cross-sectional change at the annular part of the outlet section 13, this will only cause a modification of the LP-filter effect; it will not cancel it.

Fig. Id is a schematic diagram of a noise attenuation spectrum of the first embodiment of the invention. The previously mentioned frequency fL_p can be seen at the bottom end of the noise reduction spectrum. At a somewhat higher frequency, f4, a relatively high level of noise reduction has been reached. The beneficial effect of f4 manifests itself as a small peak, which may be beneficial if the noise spectrum to be attenuated also exhibits a peak at this frequency. Provided that volume V and pipe length L_p are not too small, the LP-filter effect can be made effective from a sufficiently low frequency to become effective in a frequency in the range of 100 - 300 Hz, a normal "firing" frequency of a combustion engine. Taking the sizes of penetration depth P (see fig. Ic) to be around the previously mentioned value of 0.5 meter, not only will this be possible, but sound-absorptive effects will not be insignificant at the firing frequency, which can often be considered to be the most prominent one in the noise spectrum of an engine.

A last characteristic frequency indicated at a frequency value effectively constituting a drop in attenuation towards very high frequencies is:

fs = c/s

corresponding to a transverse wave w of full wavelength s within slots 18, indicated in fig. Ic. Such a wave has been indicated in the slot shown in fig. Ic. The correctness of this postulate can be shown by reference to the chapter on silencers in the well-known acoustical textbook authored by Beranek & Vir, although in corresponding diagrams another notation has been used.

The fall-off of the attenuation curve at high frequencies is a consequence of a phenomenon known as "beaming" : at frequencies significantly exceeding c/s, sound will propagate predominantly in the longitudinal direction, resembling that of a beam of light, to an extent by-passing sound-absorption from the side walls. The higher the frequency, the smaller will be the sidewise sound propagation into the adjacent sound-absorptive structures; a prerequisite for these structures to fulfil their sound-absorptive function. The changes of direction within cavities 14 and 17 to some extent counteract beaming within the straight silencing section. Further embodiments of the invention will demonstrate how some embodiments of the invention in a surprisingly simple manner can be adapted to further suppress beaming.

Fig. 2a and 2b depict a prior art sound-absorptive silencer configuration that in its essential form is well-known, for instance from Beranek & Vir. This silencer can be viewed as a variation of what is sometimes referred to as "a lined duct". The circular-symmetrical configuration with a casing 4 of length L and a diameter D, comprises a lining in the form of sound-absorptive annular body consisting of an inner, perforated cylinder 5 which is surrounded by sound-absorptive material 6. In the absence of further silencing elements, it would have been a proper lined duct silencer. However, in the present case, the silencing capacity has been improved by supplementing the lining with an inner body consisting of a somewhat smaller, perforated cylinder 7 harbouring further sound-absorptive material 8. Between the two perforated cylinders 5, 7 a rather narrow annular slot 9 of width s leads flow through the sound-absorptive section.

Beranek & Vir for this and other prior art absorption silencers specify the attenuation frequency curve with an ordinate that may essentially be expressed as L/s, although the notation is also different in this case. That is, the attenuation is proportional to the length of the section and inversely proportional to the width. Thus, narrowing down the width for a given length will not only extend the attenuation curve (cf. fig. Id) towards higher frequencies, but will also increase the overall attenuation level. An intuitive consideration can explain this: if a slot is made narrower, the mean distance from acoustic energy, distributed across the slot, will be brought closer to the sound-absorptive surface.

However, there is a limit to how narrow a slot is allowable for a given gas flow rate due to increased velocity that will increase pressure drop (roughly by the square of the velocity) and may incur a risk of sound-absorptive material being drawn out through perforations.

Based on this understanding, additional advantages of the first embodiment can be pointed out. Due to the in total eighth slots 18 and voids 21 being through- flowed, widths of slots and spacings can be made relatively small. Concerning voids 21 the width can be interpreted as the mean value s', taken in the peripheral direction. Admittedly, within the voids 21 there is only sound attenuation on one side. As a general guideline, s' should not exceed s excessively, since this would lead to a situation where a major part of the total flow will pass through parts of the silencing section having a relatively great slot width. By making a comparison of fig. Ia with figs. 2a and 2b, a further attractive feature of the first embodiment of the invention can be pointed out:

The prior art silencer represented in figs. 2a and 2b will be found in many ventilation systems and in many exhaust systems of engines. In the latter case, corrosion will often take place on the inside of shell 4: accompanying the beneficial effect of sound waves penetrating into the sound absorptive material 6, water and acid vapour will also penetrate this material. For the material to function in a sound-absorptive manner, it must be of an open structure, due to which such transport of matter towards the shell is avoided. Moreover, sound- absorptive material will also function as a heat insulator, so that the surface of shell 4 will attain a relatively low temperature. Depending on the degree of exterior (not shown) heat insulation, this easily leads to more or less serious condensation on the inside of the shell, creating a basis for so-called contact corrosion that can be most destructive, sometimes even if the shell is made of stainless steel. By contrast, in the configuration shown in the first embodiment (and all further embodiments), there will be no direct contact between the sound- absorptive material and the shell.

Fig. 3 shows a cross-section of a second embodiment of the invention with only three slots 18, three sound-absorptive segments 19, and three outer voids / supplementary 'slots' 21. Comparing with fig. Ib, it can be seen that in the second embodiment widths s and s' are significantly bigger, which will lead to a smaller attenuation per unit length. A relatively open structure may be necessary in some cases to avoid a too big flow velocity. But if ample length is at disposal, attenuation can still be significant.

An example of such a situation is when the interior of an existing, long chimney is supplemented by sections to attenuate noise. Further below in this description we shall see an example of such chimney silencer, although it will be more complicated than a single, through-flowed sound-absorptive silencer.

In case of a shorter silencer, often calling for a smaller slot width, the third embodiment of the invention, shown in fig. 4a, can be represented as an attractive configuration. Here, instead of a single, flat and perforated plate, delimiting the voids 21 of sound-absorptive material 28, two plates 20, creating a V-shape with an apex 3, have been inserted. Thereby, a quadruple segmental body has been created. Additionally, an internal, perforated plate 29, to assist in manufacturing an absorptive structure of great mechanical robustness has been inserted.

Figures 4b, 4c, and 4d illustrate a step in manufacturing procedure applied to the V-shape constituted by plates 23 with apex 1 in the core region, with plate 29 in a position ready to be lowered. With slight modifications, the procedure being explained here can be applied to all polygonal, outer shells of bodies adopted according to the invention, for example to the triangles of fig. 3.

Plates 23 have been arranged onto a supportive structure 5. At apex 1 in the bottom there may be a sharp connection (as shown in the figure) or a rounding, which may contribute to a simple manufacture. For instance, the V-shape could be manufactured by bending a single sheet of perforated metal. Sound-absorptive material 28 has been arranged onto the plate V-shape. More sound-absorptive material than needed to fill out the potential triangle has been brought in place. The reason for this is that when the triangle has been completed by welding at ends 2, there will be a compression of the absorptive material, which will increase the mechanical stability of the absorptive material during service: Firstly, a tendency for fibres to be sucked out of perforations will be smaller. Secondly, the risk of random variations in density inside the enclosure developing over time will be smaller. For instance, sometimes mineral wool will gradually sink down or even tumble around among major holes in the absorptive structure. The risk of such phenomena must be considered especially with big structures of absorptive material and/or when silencers are exposed to vibrations, such as for instance in automotive applications.

The next step to be performed from the situation shown in fig. 4b will be to press plate 29 down and weld corners together at ends 2.

Fig. 4c, which is an enlargement of part of the triangular body at a perforated wall, shows that as a refinement, a thin layer of, for instance steel wool, 6 can be interposed between perforated plate 23 and mineral wool 28. Such a thin structure should be open so as to represent no major resistance to penetrating sound waves, but of greater strength than mineral wool. Such a refinement reducing the risk of absorptive material being pulled out during service is per se known in prior art, but the manufacturing procedure described here will provide a more sophisticated structure of the interior of the absorptive body.

All in all, arrangement of sound absorptive material by the method described will make manufacture easier, especially compared to common cases where absorptive material has to be stuffed in from the end of a closed structure, open for a while at the end where material is being inserted. In particular this will be the case when long, narrow cavities are to be filled up. Here, perforations in the plates will resist absorptive material to be inserted. Sometimes great care may be exercised to avoid that corners are left empty of material.

Comparing figs. Ib, 3, and 4, it will be appreciated that a great many variations of polygonal or polygonal-like (for instance with rounded corners) of cross-sectional shapes can be made rather simply from sheets of metal plate.

Fig. 4d shows a sectional view taken across B-B in fig. 4b; that is if one looks down upon sound-absorptive material and ends 2 of the V-shape. This figure illustrates a further feature of the invention, viz. that silencing sections to a great extent can be produced from metal sheets 7 of identical width that may simply be of a width that is offered as standard widths in the market. This will greatly reduce the amount of cutting. At intersections 8, one may arrange transverse walls 8 that together with internal plates 29 will further contribute to keep the absorptive structure coherent during long time of service.

The manufacture-friendly kind of silencing sections described not only permit manufacture to be performed by relatively un-skilled labour and/or with a high degree of automation, but also offers possibilities of dividing manufacture of various parts of silencers into work performed in various geographical locations. This will sometimes be both cost-saving and will simplify logistics to make possible swift arrangement of tailor-made silencers all around the world. For instance, parts of the internals, such as V-shapes can be manufactured in a first place, for instance a low-cost country. V-shapes can be stacked onto each other in a very compact way, which will reduce shipping costs. In a second place, closer to the final destination, all internals of the silencer can be assembled. When silencers are big and silencing equipment is intended to be installed into a pipe system in place at the final site, i.e. a third place, assembled internals can be brought to this place. The fourth embodiment, to be explained here below, will provide a typical example where such logistic facilities can be very convenient and cost-saving.

Fig. 5a - 5d show a fourth embodiment of the invention. Here there are two internal noise reduction sections, 1 and 2. The upper section 2 is a through- flowed, sound-absorptive section like section 2 in fig. 1. The lower section 1 is instead a through-flowed Low-pass filter section, where flows inside passages 18, 19 are not delimited by perforated plates, but by full plates 23. By virtue of this, flow widths here can generally be made narrower compared to providing perforated plates, where the risk of absorptive material being sucked out through perforations exists. From fig. 5a it can be seen that there are small, longitudinally clearances 30 between plates 23 and shell 4. Passages 18, 19 will be closed off by long strips (not shown), arranged adjacent to the clearances, so that the passages are completed closed off radially.

As previously pointed out, a Low-Pass (LP) filter section, apart from a general increase in noise reduction, can extend noise reduction spectrum significantly towards lower frequencies. Especially with silencers of small diameter, such a function can be indispensable. Especially the shown type of a Low-Pass filter with built-in diffusers will create a relatively low pressure drop. Significant low frequency noise reduction can indeed be created in a relatively narrow space by strongly throttling flow, but this will represent a significant drawback in terms of pressure drop across the silencer.

As can be seen, section 1 by a division line 18 is divided into a lower part 1' with constant slot width, while in the upper part 1" the width of the slots 19 gradually increases in flow direction, to constitute pressure-recovering diffusers.

The characteristic frequency of the LP section, having the character of a cut-off frequency can be calculated by a formula that is akin to that previously given for frequency fL_p wherein:

L = length of acoustically effective passage = length of slots = length of section in present case

Vl = acoustically effective volume upstream of LP-section V2 = acoustically effective volume downstream of LP-section a = acoustically effective total (sum for all four slots in the present case) flow area = approximate mean value of local a-values, taken in the longitudinal extension.

In a rough calculation, V2 can be interpreted to include volumes of cavities 16 and 17, as well as the volume of a major part of section 2, since sound-absorptive material in relation to the LP filter effect can be taken as void space.

At inlet face 17 to the LP-section there is a circular plate with such openings only allowing passage of gas from cavity 14 to enter into the - in this embodiment - four radially extending passage entrance parts 18. That is, when comparing the LP-section with a sound-absorptive section, for instance section 2 in the present embodiment, voids 21 adjacent to the shell are not through-flowed, and perforated plates 22, constituting divisions between these voids and the inner, sound-absorptive sectional bodies 20, are not swept by gas flow. Still, there will be an absorptive effect of these segments, in addition to the LP-filter effect, since sound in a gas flow not only propagates in the direction of gas flow, but also in the opposite direction, that is backwards from the downstream cavity 15 into voids 21 and further into sound-absorptive material 20 of section 1.

Use of diffusers in LP-sections of silencers has been adopted in many prior art silencers. To become effective as a pressure -recovering feature, a diffuser of the kind shown requires that there is only a rather small increase in flow area inside each passage per length unit in flow direction. Thus, splitting up the total flow within the passage into several smaller parallel passages, allows for diffusers of a given length to attain a bigger outflow / inflow area ration than if the entire flow were concentrated to a single passage. Thereby, the pressure-recovering effect will become bigger. Especially when there are limitations to the length of a silencer, this feature is of importance.

When comparing cross-sectional views 5b and 5c, it should be noted that the two sections have been turned 45 degrees relative to each other, whereby slots in section 2 are not aligned with slots in section 1. Thereby, gas flows within the transitional cavity 15 are forced to change direction. By this arrangement, which is easy to accommodate, there will also be a change in direction of sound waves which will contribute effectively to suppress the previously mentioned beaming effect, reducing the fall-off in the noise reduction curve at very high frequencies. This effect will be the more prominent the shorter the distance is between the two sections, that is the more abrupt the forced change of direction is within cavity 15. If this distance is made very short, however, the pressure drop will be affected, having to be balanced against the reduction in beaming when designing for an optimal distance.

A similar change of direction to reduce the beaming effect could of course have been made if both sections 1 and 2 had been of the type of a through-flowed sound-absorptive type. In addition the LP-section type by itself contributes to a reduction of noise at very high frequencies.

All in all, the present embodiment demonstrates how a through-flowed sound absorptive section and a through-flowed LP-section according to the invention can be combined in a simple way to supplement each other excellently. The LP-section will extend the noise reduction spectrum at very low and very high frequencies, while the sound-absorptive section will help suppress standing waves that would otherwise lead to significant noise reduction at corresponding frequencies.

Fig. 6 is a cross-sectional view of a fifth embodiment of the invention, comprising an LP-section of a constitution both bearing similarities to and representing differences compared to section 1 of the previous embodiment. Instead of perforated plates 22 there are full plates 22, and the not shown plate 17 will be shaped with additional openings in such a way that voids 21 are being through- flowed supplementary to passages 18. Thereby, the entire flow will be divided into as much as eight flow parts. As the section shows, enclosures 20 comprise sound- absorptive material. This material will be brought to contribute to noise reduction by arranging a transverse, perforated plate, so that the enclosures of the present embodiment will function similarly to the core part of the first embodiment, by way of sound waves penetrating the cavities from the rear end of the section.

The shape of the present LP-section can be made in such a way that one could arrange an absorptive section with rather equal quadruple segments downstream of an LP-section represented by the present embodiment, so that flow areas will be unchanged or increase somewhat when passing from the first to the second section. Plate 17 could be perforated adjacent to sound-absorptive material, and a full, transversely arranged plate akin to plate 17, but without perforations, could be arranged at the transition.

The fifth embodiment has been adapted to retain simplicity of manufacture in the case of a slightly oval shell. Distance-keeping elements 24 are seen to be shorter than elements 25, and all four segments are substantially identical, one (but not the only) feature that will contribute to simplify manufacture. Thus, the design has been made in such a way that identical segments have been accommodated inside a slightly oval shell.

Round shells of silencers are usually made by rolling up flat sheets of metal and joining ends by welding. Although there will usually be an ambition to achieve a shape as close as possible to a cylinder, this simple manner of manufacture will in fact seldom yield a perfect, circular symmetrical shape. Experience shows that ovalities of a magnitude of several percentage units of the diameter are more common than not. Of course, if in the previously shown embodiment the shape of the shell had been slightly oval, the contour of plate 17 could also have been made slightly oval to match the ovality of the shell. However, since the degree of ovality will often change slightly in longitudinal direction, a simple oval plate arrangement may cause difficulties when arranging the plate inside the shell. As the person skilled in the art will appreciate, this problem can be taken care of when designing an arrangement akin to that shown in figs. 5a-c, but the fifth embodiment can sometimes represent a cheaper solution. Provided the ovality is not excessive, choosing slot widths smaller than with a cylindrical shell shape can compensate, so that the total flow area is retained. Alternatively, the shapes of the quadruples can be made slightly unsymmetrical.

Figs. 7a - 7d show a sixth embodiment of the invention, supplemented by four diagrams 7e - h. Here, as much as five through-flowed silencing sections have been arranged within a chimney 1. In addition, at the bottom there is a non- through-flowed silencing section 10.

Sections 11, 13, and 15 are all LP-filter sections, while sections 12 and 14 are through-flowed sound-absorptive sections. Section 10 at the bottom serves as a resonator and sound absorber.

The seven sections are of partly differing lengths, for reasons to be explained. The reader will appreciate that manufacture of the embodiment can be rationalised to a great extent, not least because individual lengths (exemplified as part-sections 28 of section 11) of sections have been chosen in a modularised way, as previously shown.

The height of a chimney will typically be chosen so as to effectuate a sufficient distribution of discharged gas to the surroundings. In case of the chimney to be manufactured along with its interiors, the length could sometimes be adapted to better suit noise reduction features. But an attractive feature of the present embodiment is that sufficient noise reduction capacity and low pressure drop can often be provided without any change of chimney height. This feature will be especially appreciated when an existing chimney is being retro-fitted by inserting noise reduction elements.

When fitting-in silencing sections into a long chimney, the designer is offered degrees of freedom that are not generally available. Thus, there will often be a relatively large silencing volume at disposal, although the diameter of the chimney may be comparatively small, which will sometimes represent a challenge. The embodiment shown appears to be rather crammed with internal silencing elements, which will be representative of a case where significant overall noise reduction capability and tailoring to a certain noise reduction spectrum are required.

An interpose: Fig. 8 shows a cross-section of a seventh embodiment of the invention, adapted to a shell that is not of a small diameter, but on the contrary as big as for instance 4 meter. Here, hollow, not through-flowed triangles 1 made up of identical, perforated plates 2, 3, and 4 have been arranged inside sound- absorptive sections 5. By this arrangement, all sound-absorptive material can be penetrated by a simple arrangement. For example, with a velocity of sound of 400 m/s, according to the formula for fD, full sound-absorptive effect will be attained from a frequency around 100 Hz, which may suffice to provide sufficient noise reduction capacity in the entire frequency range down to firing frequencies of combustion engines, reciprocating engines as well as gas turbine engines.

In such a case, a single, long through-flowed sound-absorptive silencing section may provide sufficient noise reduction. Such a design will indeed be of low manufacturing cost.

All embodiments of the invention shown, including the seventh embodiment, display the feature of eliminating contact corrosion between sound-absorptive material and the internal surface (2 in fig. 7a) of the shell. A chimney weakened by such corrosion could disintegrate completely in the event of a storm causing bending load onto the chimney.

Many features already shown in previous embodiments of the invention have been re-iterated in the present embodiment. There are, however also some important variations and refinements, as well as other features that deserve mention:

The relatively many sections inserted, combined with the previously mentioned modular composition facilitates possibilities for tailoring the noise attenuation spectrum to the particular needs in various circumstances, at relatively cheap manufacturing cost. In the sixth embodiment this facility has been drawn upon to attain a noise reduction spectrum that will be remarkable even within the entire frequency range of attenuation, as will appear from what will be explained here below. One such point has been pointed at already in the previous embodiment: By arranging slots belonging to consecutive sections in a not aligned way according to the invention, the manufacture can be done cheaply and effectively and the phenomenon of beaming can be avoided.

Another point is that differing lengths of sections will even out the noise reduction spectrum and minimise the risk of this spectrum suffering from significant 'dips' at frequencies where unfortunate, but unavoidable resonances will occur inside the silencer assembly. This point will be demonstrated by reference to figs. 7e, f, and

As has been pointed out, LP-filter sections are efficient in enhancing attenuation at low frequencies. In addition they contribute to attenuation within a wide range of mid-frequencies and high frequencies. However, LP sections also suffer from the disadvantage that they are more prone than pure absorptive sections to perform dips in attenuation, in particular due to half-wave resonances λ/2, cf. figs. 7e, f, and g. These dips are here counteracted by supplementing LP-filtering with sound-absorptive features, both within the LP-sections and within the sound- absorptive sections. Further, coincidences of dip frequencies to a significant extent have been avoided by designing passages of LP-sections to be of unequal lengths.

In the of LP-sections, as has also been illustrated in the mentioned wave-curve figures there will be beneficial noise reducing capacity represented by waves λ/4. These quarter-waves represent resonator capacity, supplemented by sound- absorptive capacity that will broaden out the frequency ranges within which these resonators will be active.

Likewise, the quarter-wave indicated in fig. 7h represents such capacity, and indeed with a rather long quarter wave-length and thus a low centre frequency of sound absorption. A perforated floor 25 serves the double purpose of permitting sound to be transmitted into section 10 and as a solid floor whereupon a man 33 can walk around. The point of this possibility will be explained here below. The man has been indicated by dotted lines to indicate that of course he will not be present when the plant is in operation. It can be seen that inlet pipe 8 has a right- hand part 8' that is directed partly upwards. This serves the triple purpose of facilitating incoming flow to change direction in an ordered manner inside cavity 16, increasing the length of section 10, and making it possible for the man to enter the chimney via opening 7 (cf. sidewise view in right-hand direction shown in fig. 7b), by disassembling the pipe arrangement.

Instead of using prismatic sound-absorptive parts of LP-filter segments, the effect of resonances in slots can be further ameliorated by adopting other shapes. For instance, as illustrated in fig. 7c, completely open inlet faces will make sound penetration easier. Flat plates 22 arranged in pyramidal fashion can accommodate sound-absorptive material 20.

As another variation of design details illustrated by the shown embodiment, pertaining to all through-flowed sections, the sections have been attached to fixture elements as well as transverse plates inhibiting flow through voids 21 at the top of the LP-sections, instead of at the inlet (bottom here), as shown previously. In a vertical arrangement of a chimney, this will facilitate insertion of elements, especially if the sections are to be inserted by being lowered from the top of the chimney. At temperature variations, sectional elements can expand and contract freely downwards and upwards. This latter facility in the embodiment shown also pertains to through-flowed absorptive sections 12 and 14.

A further additional facility of the present embodiment can be seen in fig. 7d, combined with inspection of fig. 7a, where, in addition to the previously mentioned man 33, another man 35 at the top of the chimney is shown: Movable wing-like sectional parts 29 can be turned around the longitudinal axis and relative to fixed sectional parts 30, so that cross-sectional areas of slots 31 can be varied, both in section 11 and in section 15. Not shown elements will permit the movable sectional parts to be fixed during operation and released when adjustments are made. Man 35 has been positioned at the top of the chimney by a vehicle equipped with a telescopic arrangement 37. This man is equipped with a tool 36 permitting him to turn the movable section element by making the turning operation by attachment of the tool to central element 37. As a simpler measure, slot widths of LP-sections 11 and 15 could be made smaller by inserting plane plates into the slots.

Figs. 9a - 9e show an eighth and final embodiment of the invention, including an SCR-denox facility. Fig. 9a is a longitudinal view of the embodiment, including exhaust piping 8 connecting a silencer with an engine 9 of which only a small part is depicted, including a turbo-supercharger 10. Fig. 9b is an enlarged longitudinal view of a passage. Figs. 9c and 9d are two cross-sectional views, and fig. 9e is a folded-out side view of a circular arrangement where the last-mentioned cross- sectional view has been arranged.

As fig. 8a shows, at the top of the silencer interior a standard honeycomb SCR catalyser section 11 has been arranged inside casing 4. Urea is injected from the outside, via a pipe 12 that leads a urea flow radially (13) inwards to the core region, where the pipe bends upwardly to form a short, vertical part 14, which is a nozzle for injecting urea via holes 15 into slots 18 of an LP filter section 17 having been arranged. As has been indicated by arrows, urea is being injected radially outwardly into the slots, but the gas flow will gradually change the direction of urea flow into an approximately longitudinal flow direction, following the main gas stream. At the same time, the initially quite different flow directions of gas and urea in the centre parts of the slots will promote mixing of urea with gas as well as evaporation of urea in addition to the residual mixing and evaporation (if necessary) taking place in the downstream region. The arranged is supposed to be designed in such a way that all urea will have evaporated before entering the catalyser.

This embodiment represents a very compact solution to a combined problem of reducing noise and NOx, within a limited total space available, especially adapted to a case where this space is provided immediately above the engine. In other circumstances, it will still be advantageous to arrange the catalyser into the silencer, but there may be a significant length of upstream piping, for instance to include a horizontal pipe, at the inlet of which urea can be injected. If the piping is long enough, the injected amount of urea may have evaporated already upstream of the silencer. When injection of urea into the silencer is instead performed as in the shown embodiment, the arrangement with injection from the core part of the LP filter section eliminates a necessity for several injection nozzles, to achieve good urea distribution.

Fig. 9b shows a sidewise view of a slot 18 of the LP filter section. Here it can be seen that there is a stepwise increase in width s of the slot, from a more narrow slot part 20 to a wider part 21, and thus of the total flow area. From diffuser theory it is well-known that a moderate, stepwise increase of flow area, although slightly less efficient in terms of pressure recovery, is in fact quite good; it is known as a Borda diffuser. Use of such a stepwise increase instead of a gradual increase of slot width, as shown in the first embodiment of the invention, has the advantage of being cheaper to manufacture, in particular when made from standardised (modular) widths of sheets of metal as previously explained.

In the particular embodiment with urea injection, there will be an additional effect: At the step there will be a recirculation of flow, which will promote urea mixing and evaporation. In a more sophisticated variation (indicated by dotted lines 22) of the design there could be a rounded notch, to intensify the recirculation phenomenon.

As in previously shown embodiments of the invention, and as shown in fig. 9c, sound absorptive material 23 has been arranged inside of segments of the LP filter section, shielded behind perforated plates 24. As previously explained, sound will propagate in reverse direction to that of the gas flow, so that these segments will contribute to overall noise reduction. A concern with combined apparatuses for reducing both noise and NOx according to the SCR-urea-injection principle is that inflected urea which might not have evaporated completely, could deposit onto sound-absorptive elements of the apparatus, and/or penetrate the sound- absorptive material to condense inside this material.

As an extra precaution against this unwanted phenomenon, one could have designed the segments without any sound-absorptive elements. By doing so, the segments would still serve a noise-reducing purpose, but within a more narrow part of the frequency spectrum. However, according to the invention, the risk of such unwanted phenomena is to a large extent inherently prevented: As previously pointed out, any condensation of corrosive gas components can generally be prevented by arranging sound- absorptive material away from the inside of the shell which, due to heat loss to the surroundings, will attain a temperature that is lower than that of the gas flow. This is avoided, as also pointed out in the present embodiment. Sound absorptive material has been arranged away from the outer shell and will thus be exposed to relatively high temperatures, which will act against condensation or urea. Furthermore, since in the present case sound-absorptive material has been arranged within segments that are not through-flowed by gas, the sound- absorptive segments only to a small degree can be continuously fed with ever new amounts of urea.

A problem that is important to handle appropriately in all systems with urea injection as part of an SCR-denox system applied to engine exhausts, is to avoid any backflow of urea that could cause serious damage to the engine. Urea can 'creep' along surfaces to produce a nasty 'salmon effect', that is propagating opposite to a general flow direction. In an exhaust system where one or more bends are interposed between the engine and urea injection, this problem will usually be easy to handle. But when urea injection is made close to and above the engine, as in the embodiment shown, via vertical pipe 8 leading up to the silencer, this problem becomes less easy to handle, especially when space for noise reduction is scarce, and only a moderate back-pressure to the engine can be accepted. When considering this problem it is important to take not only the case of steady-state operation into consideration, but also situations where there may be some not-intended urea injection at stand-still.

The bottom section 25 of the embodiment solves this problem by a design of a double LP-filter section that will both prevent urea back-flow and contribute significantly to noise reduction, in a flow-friendly way that will require a minimum of pressure drop. Figs. 9c and d together depict this part of the embodiment, fig. 9d as previously mentioned representing a folded-out side view of this section

A circular inlet plate 26 blocks flow, except for flow parts entering each of four slots 27. Flow leaving these slots will enter an interior chamber 28 of the section. From here, flows enter four further slots 29 attached to a top plate 30 arranged at the outlet of the section, blocking all flow, except for the four flow parts leaving slots 30. The two mentioned groups of four slots are off-set from each other by 45 degrees. In the longitudinal direction, the two groups of slots overlap each other, so that flow parts inside chamber 28 will generally follow a route that turns the flow parts two times 180 degrees.

By this arrangement, drops of urea that may enter chamber 28, via back-flow through slots 29, are caught inside chamber 30.

A further facility of section 25 is that it will function as a spark-arrestor, i.e. sparks that may leave the engine and enter the apparatus will be caught so that they will not leave the apparatus or even propagate up to catalyser 11 that may be sensitive to sparks. Since both sections 17 and 25 reduce sound, they will also reduce gas-dynamic, vibratory forces from the gas onto the catalyser, such forces often being seen to reduce the life-time of a catalyser.

Claims

1. A silencer with a casing (3) having a longitudinal centre axis, said silencer being intended for being through-flowed by a gas entering said silencer via at least one inlet (8) and leaving said silencer via at least one outlet (9), - said silencer comprising at least one internal silencing section comprising both solid parts and void parts (21), said silencing section extending between the inlet (7) and the outlet (8) of said silencer, and said silencing section along the longitudinal centre axis being geometrically delimited by an inlet face (12) and an outlet face (13), and - said silencing section in a direction transverse to, preferably perpendicular to the longitudinal centre axis comprising a core region, said core region extending around said longitudinal centre axis and outwards towards an outer part of the core part,

- said silencing section further comprising at least three oblong slots (18) extending outwards from said core region towards said casing (3), and

- said at least three oblong slots (18) extending from said inlet face (12) to said outlet face (13), the side walls of said slots (18) being essentially plane along the outwards extension.

2. A silencer according to claim 1, wherein at least one of said slots (18) intersect within the core region, either at an angle between the radial extensions of the slots (18) and/or such that one or more pairs of slots (18) together is viewed as a single slot extending across said core region.

3. A silencer according to claim 1 or 2, wherein at least one core body (16) is provided within said core region.

4. A silencer according to claim 3, wherein said core body (16) is intended for essentially blocking through-flow of gas between said slots across said core region.

5. A silencer according to any of the preceding claims,

- wherein said silencing section is a through-flowed sound-absorptive section, and wherein said slots (18) are transversely and longitudinally delimited by side walls, and

- where said slots (18) are at least partially perforated and at least partly essentially plane plates, and wherein sound-absorption is obtained by arranging sound-absorptive material (6) on an opposite side of said plates, as seen from said slots (18).

6. A silencer according to any of the preceding claims, wherein the total mean flow area within said silencing section, taken as a mean value along the longitudinal extension of said section, occupies less than p% of the total cross- sectional area of said section, and wherein p is at the most 50.

7. A silencer according to claim 6, wherein p is at the most 30.

8. A silencer according to claim 7, wherein p is at the most 20.

9. A silencer according to any of preceding claims, wherein said silencing section (1) is a through-flowed Low-Pass filter section, and wherein at least part of the walls of said slots (18) are constituted by walls that are essentially impervious to sound.

10. A silencer according to any of the preceding claims, wherein said section (1) is a not through-flowed section being in acoustic contact with said gas flow, and being designed to provide an acoustic resonator function and/or an acoustic sound-absorptive function.

11. A silencer according to any of the preceding claims, wherein at least one section (2) is a through-flowed sound absorptive section and at least one other section (1) is a through-flowed Low-Pass filter section, and where said two sections (1,2) are joined, such that flow will pass directly from the one section (1,2) into the other section (1,2), or wherein there may be an interspace between the two sections (1,2).

12. A silencer according to claim 11, wherein said slots (18) belonging to one section are off-set as related to slots from another section, such that flow passing between the two said sections (1,2) is forced to deviate substantially from a direct longitudinal flow within interspace(s) between the two said sections (1,2).

13. A silencer according to any of the preceding claims, wherein said slots (18) 5 are designed to be pressure-recovering diffusers by flow area increase in the direction of gas flow, either by gradual increase of flow area and/or by step-wise increase of flow area.

14. A silencer according to any of the preceding claims, wherein said slots (18) 10 and the delimitation of said voids (21) together form segments essentially shaped as polygons.

15. A silencer according to claim 14, wherein said polygons are essentially triangles.

15

16. A silencer according to claim 14, wherein said polygons are essentially quadruples.

17. A silencer according to any of claims 14-16, wherein said segments or parts of 20 said segments are essentially identical.

18. A silencer according to any of the preceding claims, wherein at least part of said shell (4) is round.

25 19. A silencer according to claim 18, wherein at least part of said shell (4) is essentially circular cylindrical.

20. A silencer according to claim 21 or claim 22, wherein at least part of said shell (4) has another shape than a circular cylindrical.

30

21. A silencer according to any of the preceding claims, wherein sound-absorptive material (6) constitutes part of said silencing section, and wherein essentially no sound-absorptive material belonging to said section is in direct contact with said shell (4).

35

22. A silencer according to any of claims 1-20, wherein essentially no sound- absorptive material is in direct contact with said shell (4).

23. A silencer according to any of the preceding claims, wherein at least one said 5 silencing sections is fitted into a stack of multiple silencing sections.

24. A silencer according to claim 23, wherein said stack is intended for being essentially vertically aligned during use and is intended for being part of a stationary plant, and where said stack is intended for being directly or indirectly

10 founded onto solid ground.

25. A silencer according to any of the preceding claims, wherein said silencer comprises a catalyser (11) for converting noxious components of said exhaust flow.

15

26. A silencer according to claim 25, wherein the silencer comprises means (12) for injecting into said silencer at least one substance promoting the function of said catalyser.

20 27. A silencer according to claim 26, wherein a Low-Pass filter section (17) of said silencer is provided with means (12) for injecting said substance into said silencer.