FIELD OF THE INVENTION

The invention relates to a silencer with an outer shell and provided with one or more inlet conduits for leading a gas into the silencer and with one or more outlet conduits for leading the gas from the silencer. The invention also relates to such a specific silencer being designed so as to provide means intended for acting as a spark-arrestor. Also, the invention relates to a combustion engine provided with such silencer.

BACKGROUND OF THE INVENTION

Reactive silencers for gas flows comprise one or more through-flowed chambers. It is known in the art of silencer design to supplement such silencers in various ways to improve a generally broad-banded noise reduction spectrum at various frequencies, where such improvement is warranted, e.g. because the un-silenced noise spectrum exhibits a peak at one or more frequencies, or because the silencer would otherwise exhibit dips in the attenuation spectrum, typically because of harmful resonance waves set up in chambers or passages of the silencer.

A general feature of such supplementary methods relies on the use of various types of essentially not though-flowed elements.

One such method is fitting elements containing a sound absorptive material into the silencer, such as mineral wool. Such elements will supplement silencing in a wide frequency range roughly above a certain lower limit frequency, which can be determined by analyzing standing waves set up within cavities containing the absorptive material.

A further aspect of methods of supplementary silencing relies on the use of various types of resonators providing added noise attenuation at one or more selected frequencies. One type of such resonators is the Helmholtz resonator, which is connected to a through-flowed passage or chamber via a neck section, which in turn is connected to a not-through-flowed cavity. The peak attenuation frequency of such a Helmholtz resonator can be calculated approximately by a mass-spring analogy, considering the mass of gas present by the neck as a stiff, concentrated mass, and the flexibility of the not-through-flowed cavity and the flexibility of a cavities on the opposite side of the neck as springs, each spring being connected at one end to the mass and at the other end to a steady wall. If the last- mentioned cavity is much bigger than the not-through-flowed cavity, it will act as a relatively much softer spring whose stiffness may be omitted in a rough calculation of the peak attenuation frequency.

A third aspect of supplementary silencing element is the side-branch resonator with a closed-end pipe in which standing waves are set up to absorb sound at corresponding noise frequencies. Such a resonator will provide added noise reduction at frequencies corresponding to 0.25, 0.75, 1.25, etc. waves set up in the pipe. The frequency corresponding to 0.25 wave length will be the lowest (and usually the most prominent) one.

It is further known to design silencers in various ways so as to act as spark arrestors, i.e. to prevent any significant amount of glowing particles (sparks) from passing the silencer, thereby reducing the risk of causing harm to human beings or causing fire or explosion in case of any inflammable substances being situated close to the exhaust.

It is easy to fit into a silencer a screen or some other particle obstructing means, but commonly known means of this kind generally significantly augment the pressure drop across the silencer. There is a need in the market for a silencer which can produce a significant, not necessarily a maximum, spark arresting effect with a low or a very low additional pressure drop across the silencer.

Although many silencers of known designs, even though not having been designed for such a purpose, will in fact have some spark-arresting effect, a gas flow, such as exhaust from a diesel engine, containing particles, may gradually compromise a silencer in its acoustic function. One important reason for this can be that perforations in walls of such a silencer, allowing noise to be transmitted into sound absorptive material, may in the course of time become clogged, whereby the desired sound absorptive effect may become gradually obstructed.

SHORT DESCRIPTION OF THE INVENTION

It may be an object of the invention to provide a silencer of the reactive type with a resonator and having an increased noise attenuation within confined dimensional restraints. It may also be an object of the invention to provide a silencer being based on already applied techniques and being redesigned easily and cheaply.

These objects and possible other objects are obtained by a silencer with - an outer shell and provided with one or more pipes for leading gas flow into the silencer and with one or more outlets for leading gas from the silencer, the silencer comprising - at least one chamber intended for being trough-flowed by gas entering the chamber and at least one perforated conduit section adjacent to which is provided sound absorptive material, and - said sound absorptive material, in order to constitute a closed absorptive material containing cavity, except for said perforations, said sound absorptive material and conduit section together constituting a protrusion extending from part of said outer shell, such as an end cap, or from an internal member, such as a baffle, said internal member causing separation of two through-flowed chambers of the silencer, and where - at least one not-through-flowed cavities (NTC), at least partly surrounding said protruding sound absorptive material, thereby constituting one or more resonators acoustically communicating with said at least one through-flowed chamber.

The invention combines, in a basically reactive type silencer, the above-mentioned absorptive type of element with either a Helmholtz resonator or with a side-branch resonator in a way, which is particularly compact and cheap to manufacture. In several preferred embodiments of the invention, an annular cavity, surrounding the absorptive element, is utilised as a resonator, to absorb energy at one or more selected noise frequencies, which by further, simple design features, can be selected rather freely, as will be shown and explained here below.

In order for the at least one through-flowed chamber to truly act to reflect sound waves at inlet passages to and outlet passages from the chamber, and for the chamber to function to reduce noise efficiently from a not too high lower cut-off frequency, it is desirable that the cross-sectional area ratios of the passages in respect to the chamber cross-sectional area, and the outlet passages in respect to the chamber are not too big. It is also desirable that the chamber is of a not too small volume. These objects may be obtained by said at least one through-flowed chamber being provided with one or more inlet passages for leading gas to said chamber, and being provided with one or more outlet passages for leading gas from said chamber, and where the sum aιn of acoustically representative cross- sectional areas of said inlet passages is fulfilling the condition ain < A/3C, and/or where the sum aout of acoustically representative cross-sectional areas of said outlet passages is fulfilling the condition aout < A/3C, and/or where the volume V of said chamber is fulfilling

the condition V > 6C (-Ji(Ci1n + aoul )l2)γ , A being an acoustically representative cross-

sectional area of said chamber (CHA) and C being a constant at least taking the value C=I. In alternative embodiments, the constant C may take the value C=2 or C=3.

According to one possible embodiment of the invention, said at least one not-through- flowed cavity is of a longitudinal shape along a longitudinal extension of the silencer, said cavity thereby acting as a resonator attenuating noise at frequencies corresponding to standing waves in the longitudinal direction of the cavity, the lowest of said frequencies corresponding to a standing quarter-wave. This feature has the advantage of the silencer being tuned to specific frequencies, thus obtaining maximum noise attenuation in the number of not-through-flowed cavities. Also, any one and selected wavelength of the frequency may be used for designing the length of the not-through-flowed cavity, but still maintaining the noise attenuating properties of the remaining elements of the silencer, and not necessarily having to re-design these elements, because of a chosen length of the not- through-flowed cavity. Thus, the individual design features may be selected and designed rather freely and not necessarily dependent on possible mutual relationships.

As will be demonstrated, such cavities of a longitudinal shape can be accommodated in remarkably simple and compact designs, making the silencer cheap to manufacture. Also the silencer is well-suited for augmenting the noise reduction effect of the silencer at one or more particular noise frequencies in situations, where noise reduction would otherwise be insufficient, if one were to rely solely on noise reduction caused by noise reducing effects caused by a combination of the reactive/reflective effect of through-flowed chambers and the noise absorptive effect achieved by means of sound absorptive material.

According to another possible embodiment of the invention, said not-through-flowed cavity is acoustically communicating with said at least one through-flowed chamber either directly or via a neck section provided between the at least one not-through-flowed cavity and the chamber intended for being through-flowed by gas. This feature has the general advantage of achieving additional noise reducing effects at one or more target frequencies by simple designs. For instance, in the case of a silencer with several through-flowed chambers, it will be possible to design the noise reduction characteristic of the silencer within wide limits, without resorting to complicated and costly designs.

Depending on the specific design of the silencer, on the possible dimensional restraints and on the intended and/or demanded noise attenuating properties, the acoustically effective length, L, of said not though-flowed cavity is essentially equal to, or smaller than, or longer than a length of said protruding sound absorptive material seen along the longitudinal extension of the silencer.

When a not-through-flowed cavity is used as a side-branch resonator, it should be of a longitudinal shape in order to achieve a distinct resonance effect in the longitudinal direction. This may be achieved by ensuring that a mean distance between walls in radial direction, integrated over the entire longitudinal and circumferential extension of the acoustically effective length of the not-through-flowed cavity is at the maximum a ratio 1/3 of said acoustically effective length, alternatively the mean distance being at the maximum a ratio 1/5 of said acoustically effective length, even alternatively the mean distance being at the maximum a ratio 1/10 of said acoustically effective length.

When a not-through-flowed cavity is of a more complex shape, the longitudinallity of the cavity can be quantified by referring to a volume, V, of the cavity and to summed surfaces, S, of side walls of the cavity. In such a case, the acoustically effective length, L, of said at least one not-through-flowed cavity is at least 6 times the ratio V/S between the volume of said cavity, and the summed surface areas of the side walls of said not-through-flowed cavity. Alternatively, the acoustically effective length, L, of said at least one not-through- flowed cavity is at least 10 times the ratio V/S, and even alternatively said acoustically effective length, L, of said at least one not-through-flowed cavity is at least 20 times said ratio V/S.

The shape of said cavity may at least partly constitute a helical winding around said protruding sound absorptive material. A helical winding entails the advantage that the acoustic effective length may be longer than a length of said protruding sound absorptive material seen along the longitudinal extension of the silencer, however, without taking up as much dimensional space seen in either an axial or a radial direction in comparison with the actual acoustic effective length obtained by a helical winding.

If the silencer is provided with at the least one neck section acoustically connecting said at least one not-though-flowed cavity with said at least one through-flowed chambers, the neck section may be designed for creating a Helmholtz-type resonator. Thereby a very pronounced additional noise reduction effect at a frequency, which can be selected within a very broad range of frequencies, can be attained. For instance, it becomes possible to extend the noise reduction spectrum down to very low frequencies, which may otherwise be difficult to attenuate with a silencer to be accommodated within a limit space restriction, i.e. if the silencer has to be compact.

Furthermore, said at least one protruding sound absorptive material and said at least one essentially surrounding not-through-flowed cavity may be of substantially circular- cylindrical configuration. This is an example of how the invention makes it possible to accommodate three, mutually supplementing noise reduction effects in a particularly simple design. Those three effects are, noise reduction by: 1) sound reflection at cross- sectional changes at inlets and outlets of through-flowed chambers, 2) sound absorption in sound absorptive material, and 3) resonance sound absorption, either by a longish quarter-wave sound absorber, or by a Helmholtz sound absorber, all those types having been described above. A possible embodiment of a silencer comprises at least two not-through-flowed cavities, the one not-through-flowed cavity having an acoustically effective length Ll and the at least one other not-through-flowed cavity having an acoustically effective length L2, and where said length Ll is different form said length L2. This is a demonstration of a rather simple way of 'design-making' the noise reduction characteristic of a silencer according to the invention. If more than two lengths are selected, a more sophisticated 'design-making' can be attained. One may target very different frequencies in some cases. In other cases, two or more targeted frequencies may differ only slightly, whereby the rather narrow- banded sound-absorptive effect of a single resonator can be extended in terms of frequency range effect of the sound absorptive effect.

In order to design the outer shape of a big silencer, for instance the silencer of a big diesel engine serving the electricity needs of an island, where the allowable length of the silencer is limited, but it is possible design for a certain width and a certain height of a horizontally disposed silencer, at least three of said protruding sound absorptive materials are arranged to establish at least three parallel gas flows inside said protruding sound absorptive material, and wherein said at least one not-through-flowed cavity is constituted by a spacing between said protruding sound absorptive material.

This cavity may be a residual cavity found between the three or more protrusions. The cross-sectional shape of this residual cavity may appear complicated, but since it is a residual, it can be accommodated by a very simple design, as will be demonstrated here below. If the residual spacing is used as a quarter-wave resonator, it should be of a rather longish shape, which can be obtained simply by designing the protrusions to be of a certain minimum length and by accommodating the protrusions close to each other, so that the mean transverse direction between the protrusions becomes rather small.

If at least three protruding sound absorptive materials are provided, more of said cavities are formed by sub-dividing said spacing, possibly sub-dividing of said spacing by establishing insertions of one or more walls between said protruding sound absorptive material. Said sub-dividing of said spacing may be established by inserting one or more walls between said protruding sound absorptive material and another member of the silencer, e.g. the outer shell of the silencer. The sub-division can be used for having different lengths of different quarter-wave resonators. Walls for subdivision may be called for anyhow, to provide sufficient stiffness to the assembly of protrusions. At least one said resonator of said silencer is constituted by at least one not-through- flowed cavity (NTC) at least partly surrounding protruding sound absorptive material, is tuned so as to target a peak frequency of an un-attenuated noise spectrum to be attenuated by said silencer.

According to an aspect of the invention, the silencer is designed so as to provide means intended for acting as a spark-arrestor, said means comprising said at least one not- through-flowed cavity capable of collecting particles separated from the gas flow as the spark-arresting function. Preferably, at least one of the following flow motions: radial outward flow motion, radial inward flow motion and swirling flow motion is enforced upon the gas flow passing through the silencer, thereby promoting particles contained in the gas flow being collected in said at least one not-through-flowed cavities designed as a spark- arrestor.

Spark-arresting function is commonly called for in exhaust systems, especially exhaust systems of diesel engines, where the exhaust will contain particles of various sizes. Such particles will tend to clog any perforations in communication with the sound absorptive material, especially in cases where the perforations are not continuously being swept by a gas flow. Such clogging can gradually compromise the sound absorptive function of the sound absorptive material. Larger particles may be glowing for a long time after having been exhausted form the engine as such, and if such glowing particles are transmitted to the environment, they may cause harm such as fires or even explosions. Thus, according to another aspect of the invention, the invention also relates to a combustion engine being provided with a silencer according to any of the preceding claims, said combustion engine preferably being a diesel engine.

It will be demonstrated here below how the invention makes it possible to attain a significant spark-arresting effect without resorting to design elements that would cause a significant increase of pressure drop across the silencer.

It will also be demonstrated how the invention can be adapted to instead contain one or more screens inside a silencer in a way that provides enhanced spark arresting function in such a way that there will be less accumulation of soot or other particles onto such screens, compared to prior art silencers providing spark arresting by means of screens. DETAILED DESCRIPTION OF THE INVENTION

Hereafter, the invention will be described in more detail by reference to the figures:

Fig. 1 shows a longitudinal cross-section of a first embodiment of the invention, Fig. 2 shows a longitudinal cross-section of a second embodiment of the invention, Fig. 3a shows a longitudinal cross-section of a third embodiment of the invention, Fig. 3b shows a transverse cross-section of the third embodiment of the invention, Fig. 4 shows a longitudinal cross-section of a fourth embodiment of the invention, Fig. 5 shows a longitudinal cross-section of a fifth embodiment of the invention, Fig. 6a shows a longitudinal cross-section of a sixth embodiment of the invention, Fig. 6b shows a transverse cross-section of the sixth embodiment of the invention, Fig. 7a and 7b show cross-sections of a seventh embodiment of the invention and Figs. 8a and 8b show cross-sections of an eighth embodiment of the invention.

Fig. 1 shows a longitudinal cross-section of a first, essentially circular-cylindrical embodiment of a silencer according to the invention. The silencer has at least one casing 3. A first pipe 1 leads a gas flow into the silencer, and a second pipe 2 leads gas from the silencer. The casing 3 is composed of a cylinder 4 fitted with a conical end cap 5,6 at each end of the silencer. The end caps 5,6 may have other shapes such as plane or spherical, and the end caps may also have individually different shapes. The first pipe extends into the silencer at least partly along a perforated pipe section 7 leading up to a radial diffuser 8, composed of two circular-symmetrical baffles 9,10, held together by radially extending ribs 11. In the cross-section shown, two such ribs 11 are shown to fall within the sectional plane - typically further ribs will be provided, e.g. in total eight ribs positioned with intersecting angles of 45 degrees. From the diffuser 8, gas continues to flow inside a through-flowed chamber 12.

From a position 14 in the immediate vicinity of the end cap 5, alternatively at a position more or less remote from the end cap 5, the perforated pipe section 7 is surrounded by sound-absorptive material 13 contained between the pipe and a cylindrical shell 15. The whole assembly of pipe section 7, sound absorptive material 13, cylinder 4 and radial diffuser 8 all together provide a protrusion 16 from the end cap 5 at the first pipe 1 into the silencer interior. The protrusion 16 may also be constituted only by at least the pipe section 7 and the sound absorptive material 13, thus omitting either the cylinder 4 or the radial diffuser 8, or omitting both the cylinder 4 and the radial diffuser 8. The protrusion 16 is surrounded by an annular spacing, providing a not through-flowed cavity 17 between the cylindrical shell 15 and the outer casing 3 of the silencer. The not through-flowed cavity 17 acts as a side-branch resonator acoustically communicating with the through-flowed chamber 12 through an opening 18. Inside the resonator, during operation of the silencer, standing waves are set up, said standing waves having the effect of absorbing sound energy from the through-flowed chamber 12 at 5 corresponding frequencies. The lowest of these frequencies is the one whose quarter-wave pressure amplitude p variation from the closed end of the non-through-flowed cavity 17 to the opening 18 is shown adjacent to the silencer. In the embodiment shown, the resonator length Ll is essentially the same as the length Pl of the protrusion 16.

10 The second pipe 2 extends backwards into the silencer by a perforated pipe section 27 which is surrounded by sound absorptive material 23 extending from a position 24 in the immediate vicinity of the end cap 6, alternatively at a position more or less remote from the end cap 6. The sound absorptive material 23 is surrounded by a cylindrical shell 25. Smooth gas flow from the through-flowed chamber 12 into the second pipe 2 is provided 15 for by means of a conical baffle 19, which together with the cylindrical shell 25 acoustically closes off sound absorptive material 23 against the through-flowed chamber 12. Between cylindrical shell 25 and the cylindrical silencer casing 4, a not through-flowed cavity 27 is formed, which is an annularly shaped side-branch, communicating acoustically with the through-flowed chamber 12 via an opening 28. 20 The perforated pipe section 27, the sound absorptive material 23, the cylindrical shell 25, and a conical baffle 19 together constitute a backward protrusion 26 into the silencer. The length L2 of not-through-flowed cavity 27 has been made shorter than length P2 of the backward protrusion 26 by inserting an annular baffle 29 between cylindrical shell 25 and 25 the cylindrical casing 4 of the silencer.

This feature has the effect of shortening the length of the quarter-wave, as can be seen from the small diagram of pressure amplitude, shown adjacent to the silencer. Corresponding to the shortened quarter-wave is a higher peak frequency of sound 30 absorption associated with resonator, which could be desirable to target a certain peak in the un-attenuated noise frequency spectrum, or a dip in the attenuation spectrum of a corresponding silencer without the annular baffle 29.

Summing up, fig. 1 is an example of a silencer comprising two not-through-flowed cavities, 35 17 and 27, whose acoustically effective lengths can be adapted to achieve sound absorption, peaking at two different sound frequencies. Naturally, the one peaking sound frequency associated with the one not-through-flowed chamber 17 could be raised by inserting an element like the annular baffle 29 in this cavity as well. Fig. 2 shows a second embodiment of the invention in which the acoustically effective length L of a not-through-flowed cavity 17 has been made significantly longer than a length P of a protrusion 16 PRO into the silencer. This has been achieved by fitting a helically extending, annular wall 30 inside the not-through-flowed chamber 17. This feature creates a significant lowering of standing-wave frequencies, starting with the quarter- wave.

Figs. 3a and 3b show a third embodiment of the invention. This silencer has been designed to act, in addition to noise reduction, as a spark-arrestor in which particles 31 are collected at the bottom of an annular, not through-flowed cavity 17. The particles accumulating at the bottom of the not through-flowed cavity 17 can be sucked out of the cavity 17 via a suction pipe 32. A radial diffuser 8 has been fitted with bending ribs 11 as can be seen from fig. 3b. These bending ribs 11 will cause the radial flow between the baffles 9 and 10 to bend inside the diffuser 8 to leave the diffuser 8 with a tangential component, imparting a centrifugal force on the gas and on particles contained in the gas.

Thereby, especially larger particles, e.g. those whose glow is potentially the most harmful to the environment if not retained, are forced towards the inner side of the silencer casing, from which the particles, due to gravity, will fall downwards to be collected at the bottom of the cavity as indicated by dotted arrows in fig. 3a. Baffle 9 has been shown to be tapered at its periphery, facilitating a 180 degree turning of flow direction of particles. Full arrows show paths of gas flow and of small particles following the gas flow. A conical baffle 33 extending inwardly into the through-flowed chamber 12 assists in separating bigger particles from the gas flow through the silencer, at the same time being shaped so as not to create too great a flow-restriction on gas flow.

It is to be noted that the radial diffuser 8, as indicated in fig. 3a, has been designed to cause the gas to flow in an outward, perpendicular direction to the longitudinal axis of the pipe section 1 and the to the whole silencer. This differs from flow conditions in the (also radial) diffuser shown in fig. 1, in which the radial flow between baffles 9 and 10 has a small axial component, i.e. in the embodiment shown in fig. 1 the change of direction of flow from the incoming axial direction in the first pipe 1 is somewhat smaller. A design as shown in fig. 1 tends to improve the degree of pressure recovery in the diffuser. The 90 degree change of flow direction in the diffuser 8 shown in fig. 3a instead represents some sacrifice of pressure recovery for the benefit of more efficient particle separation.

For a given distance between baffles 9 and 10 in the embodiment shown in fig. 3a, the tangential component of flow exiting the diffuser 8 increases the dynamic pressure of flow leaving the diffuser. Since this dynamic pressure is virtually dissipated inside the through- flowed chamber 12, the tangential flow component tends to decrease the pressure recovery in the diffuser. This could be compensated for by increasing the distance between the baffles, but would simultaneously cause a less efficient separation of particles. Thus, there is a trade-off between pressure recovery and efficiency of particle separation. When a maximum of particle separation is the objective, the diffuser DIFF may be designed with such a big absolute velocity of flow leaving the diffuser DIFF that there is a positive pressure drop across the diffuser 8 (no net pressure recovery).

Fig. 4 shows a fourth embodiment of the invention, featuring both another spark-arresting arrangement and a previously not shown feature of a not through-flowed cavity acting as a side-branch resonator.

In fig. 3a, the not-through-flowed cavity 17 in which particles are retained surrounds the protrusion comprising the diffuser 8 causing flow to change direction. By contrast, in fig. 4, where there are a first and a second not-through-flowed cavity 17 and 27, particles are collected in the second cavity 27, which does not surround the first protrusion comprising diffuser 8. Such an arrangement facilitates an overall design for low pressure drop, across the silencer, still causing a particle separation effect, since the radial diffuser forces the flow to bend twice, and bigger particles, due to their inertia, will tend to be caught by the second cavity 27, in which they are collected at the bottom, noticing that in fig. 4 the silencer has been arranged horizontally. Two dotted lines indicate flow paths of bigger particles. Some particles enter the upper part of the second cavity 27 and thereafter follow a course of turning around the axis of symmetry of the annular cavity. The corresponding curved, dotted line is a projection of the three-dimensional particle flow path in the longitudinal section shown on the drawing.

It should be noted that, although, as mentioned, the flow through the silencer has been forced to follow rather abrupt changes of flow direction, there will not be any big pressure drop across the silencer. One important reason for this can be found in the use of the radial diffuser, which can be designed for a negative pressure (i.e. a pressure recovery) across this particular design element. Also, 'catching' the particles in the second cavity 27 is associated with a minimal pressure loss, since the flow, when forced inwardly from the outer shell has a rather low flow velocity.

The second feature illustrated in fig. 4 is a continuous, angular variation of length L of the first not-through-flowed cavity 17 between a minimum length Lmin and a maximum length Lmax both found on the top of left-hand side of the drawing. A helically extending annular wall 30 extending 360 degrees round, causes this angular variation of L. A plane, longitudinally extending member 34 positioned in the drawing plane in the upper-left part of the annular spacing between cylinder 15 and the silencer cylindrical casing 3 connects the two ends of the annular wall 30 and closes off the resonator towards the residual, annular spacing to the left of the annular wall 30.

This variation of length L increases the band-width of attenuation, caused by the side- branch, at the expense of a lowered peak attenuation. Such increase of band-width may be desirable if the frequency where added attenuation is needed is not known exactly and/or if it varies, e.g. due to varying speed of an engine.

Fig. 5 shows a fifth embodiment according to the invention. Here, a Helmholtz resonator surrounds a protrusion 16 extending into the interior of the silencer, and comprising sound-absorptive material 13 as in all the previously shown embodiments. The Helmholtz resonator consists of a resonator volume provided by a not through-flowed annular cavity 17 and a likewise annular neck section 35, which communicates acoustically, both with the through-flowed chamber 12 at opening 18A and with the not through-flowed cavity 17 at opening 18B. The neck section 35 is created by adding a short cylinder 36 whose diameter is larger than that of the cylinder 15. As previously mentioned, the Helmholtz resonator allows for frequency-targeted noise reduction within a particularly broad range of frequencies to be selected as target frequency.

Figs. 6a and 6b show a sixth embodiment of the invention, being a silencer with a first and a second through-flowed chamber 12 and 22, separated by a baffle wall 40. Gas flow is led into a first through-flowed chamber 12 via a first, conical diffuser 41. The connection between the two chambers is made of in total twenty small absorptive silencers, each providing a protrusion from the baffle wall BAF towards a second through-flowed chamber 22. Each small silencer is made of a perforated pipe 42 ending in a small, conical diffuser 43. Between the protrusions and the inner side walls of the silencer casing 4 a residual spacing is created, constituting a not through-flowed cavity assembly 17, which acts as a collective side-branch resonator, as illustrated by the quarter-wave pressure diagram in fig. 6a. Unlike all the previous embodiments, the silencer casing 4 is rectangular, and not circular. This last embodiment will typically be used when bigger silencers are called for.

In fig. 6, since the protrusions are positioned slightly apart, the not-through-flowed chamber 17 is essentially a single cavity. By moving the protrusions to be in contact with each other, or by inserting division walls, several or even many smaller cavities 17 could be created instead. Further, by insertion of walls analogous to annular baffle 29 of the embodiments shown in figs. 1 and 3, two, more, or even many resonators for differing resonator frequencies could be created. A similar effect could be achieved by varying the lengths of differing protrusions in the embodiment shown in fig. 6. Quater-waves are most prominently set up in side-branch resonators when such resonators are pipes or function as pipes by being of a longitudinal, rather narrow shape, to create a rather distinct resonance in the longitudinal direction. Thus, in the embodiments shown in figs. 1-4, all annular or helically extending cavities are significantly longer than the width of their annular gaps. Also, in fig. 6, the not-through-flowed cavity 17 will act as a longitudinal quarter-wave resonator. In cases like the last-mentioned one, where the cavity is of a complex shape, Mongitudinality' can be defined as prescribing that the length L of the cavity, should be at least a certain multiplication factor, such as 10 or 20, times the ratio V/S between a volume V of the cavity, and the (summed) surface(s) of the side-wall(s) of the cavity or cavities.

In all previously shown embodiments, the perforated pipes and the protruding sound absorptive material have been shown to be circular-cylindrical, and the sound absorptive material as well as the not-though-flowed cavities have been shown surrounding the perforated pipes.

Fig. 7a and 7b in contrast hereto show a seventh embodiment with a rectangular shaped silencer, where there are perforated conduits, 44 and 45, of a rectangular cross-sectional shape. The sound absorptive material is contained within adjacent cavities on the side of which material not-through-flowed cavities, 17 and 27, have been arranged. In the embodiment shown, due to neck sections, 35 and 45, the cavities are providing parts of Helmholtz resonators.

In all the embodiments shown, the silencers are shown to be designed with a simple outer casing. It should be understood that acoustical and spark-arresting effects similar to those demonstrated in the embodiments can be achieved, also with more complicated designed silencer casings. For instance, for the sake of heat insulation, it may be preferable to design the casing as a double-wall design or multiple-wall design, possibly with heat- insulating material in spacings between the walls.

Further, in all the embodiments, sound absorptive material 13,23 is shown to surround perforations 7,27 of pipes in immediate vicinty of the pipes. For the sake of minimizing the amount of sound absorptive material drawn out into the pipe at large gas velocities, protective open-structured (not preventing sound propagation) intermediate layers may separate the perforations 7,27 and the sound absorptive material 13,23.

Figs. 8a and 8b show cross-sections of an eighth embodiment of the invention. Here, a per se known type of screen 47 has been fitted onto cylinder 25 in such a way that all gas flowing through the silencer is forced to pass the screen 47. The screen 47 may as example be provided by a perforated plate or by one or more woven, metallic thread networks. The sizes of openings in the screen 47 will be chosen in such that they will not allow any particles greater than a certain size, for instance 0.5 mm, to pass through the screen.

Flow enters the silencer tangentially to the cylindrical shell 4 via inlet pipe 1 so that a rotating flow pattern is set up inside the chamber of the silencer to the left of the assembly 26 extending backwards from silencer outlet into the interior of the silencer. This rotational flow, by centrifugal forces acting on particles, will cause bigger particles to move towards the inner surface of the shell, so that such bigger particles, following paths indicated by to dotted curves, will enter not through-flowed cavity 27 via annular opening 28, to accumulate within this cavity.

Smaller particles will be subject to less strong centrifugal forces and will therefore be more prone to follow the gas stream which will converge towards inflow to 2 via the screen. Any particle that is small enough to not being captured in NTC, but is bigger than the size of the openings of the screen, will be retained on the upstream side of the screen. The left- hand end cap 5 of the silencer is removable (as has been indicated by flanges and bolts), which permits access to the screen for cleaning it from the outside.

Compared to a prior art silencer providing spark-arrestor function by using a screen, the eighth embodiment of the invention has the advantage that, since a significant part of soot is retained within NTC, there will be less build-up of soot on the screen, causing less increase of pressure drop and/or permitting less frequent cleaning of the screen.

Although all the previous embodiments shown in fig. 1 to fig. 7 of the invention, when properly designed, in many cases can provide satisfactory spark arresting function, insertion of a screen, as shown, can be motivated to improve the spark arresting function in various circumstances. Conversely, the eighth embodiment of the invention can be preferable to other types of silencers comprising a screen, due to better acoustical performance, as well as further considerations. The screen shown in fig. 8A may nonetheless be provided in any of the embodiments shown in fig. 1 to fig. 7.

By using small openings of the screen even very small particles can be retained. When spark-arresting relies on a screen only, the designer will tend to not choose too small openings, since that could result in rather rapid build-up of soot on the screen. When the silencer is designed to be relatively small, particles following the gas flow will reside inside the silencer only during a short time interval. This may call for use of a screen with small openings. Another example when adding a screen can be motivated is when an engine sometimes runs at low rotational speed, in which case gas velocities within the silencer will be small, causing a diminishing of centrifugal forces on particles.

A further aspect is that some regulations specifically demand that spark-arresting effect be attained by use of screens with a specific geometry. In such cases, the formal demand can be fulfilled without heavy build-up of soot or other particles onto the screen, by using the last shown embodiment of the invention. A still further aspect is that in the eighth embodiment of the invention, even if a screen happen to break down mechanically, the silencer will retain is significant spark-arresting function, i.e. the double spark-arresting function represents a non-trivial redundancy.

The drawing shows a preferred embodiment where a screen has been arranged inside a silencer according to the invention. Since that screen has a much bigger diameter than both pipes, PIPEl and PIPE2, the pressure drop across the screen will be relatively small. The effective area of the screen can be increased further by shaping it in various ways apart from a plane shape. Thus, for instance, the screen can be made dome-shaped, which adds the advantage of a greater mechanical stability of the screen.