Structures and methods based on a plurality of different phenomena are available in order to attenuate the sound caused by pressure variations in a gas flowing in the piping, both into the piping and out from the piping, such as in the exhaust piping of combustion engines, in suction air purifiers, in compressors and corresponding ma- chines. A flow which due to such pressure variations varies continuously can best be described with the term"pulsative flow", because the total flow of a gas is generally formed by an average flow of a certain magnitude and positive pressure pulses and/or negative pressure pulses, which at intervals act on the average flow. The question is of course about relative positive pressures and negative pressures. The pressure pulses are typically repeated, for instance when a combustion engine is running or when a compressor is in operation, at rather even intervals as the running speed is kept constant. Normally the strength of the pulses will vary when the load- ing of the machine changes, even if the running speed, such as the speed of rotation of a combustion engine of a compressor, would be kept constant. In most machines and engines there are further made provisions for variations of the running speed, which means that at least the intervals between the pressure pulses often vary in a wide range. In order to facilitate the treatment the pulses appearing at intervals in the piping can be approximated by a closely corresponding frequency, even though the pressure pulses often are asymmetric. The running speeds vary depending on the machine or engine type, for instance between 1000 and 6000 rpm in cars, and be- tween 1500 and 10,000 rpm in motorcycles, whereby the corresponding frequency of the pressure variations in the exhaust gas and the suction air lie in an interval of

about 67 to 400 Hz for a four-cylinder engine, and in an interval of about 25 to 167 Hz for a one-cylinder engine, and in an interval of about 50 to 333 Hz for a two- cylinder engine. Thus a problem of silencers is to provide a structure which effec- tively attenuates the sound caused by such pressure variations both at the lower end of the frequency range, i. e. typically at frequencies 20 to 100 Hz, and at the higher end of the frequency range, i. e. typically at frequencies 300 to 500 Hz, and in addi- tion at all intermediate frequencies which are generated when the running speed of the engine or machine is changed. In addition, the gas flow can simultaneously contain a plurality of pressure variations at different frequencies, which also must be possible to attenuate. Particularly it must be observed that in addition to the basic frequencies determined by the speed of rotation pressure pulses also occur at their harmonic frequencies, whose sufficient attenuation is often quite problematic. The frequency range where the attenuation must be possible is substantially widened due to these harmonic frequencies, as compared to the frequency range caused only by the basic frequencies. The above described operation on a wide frequency range is the most common situation. Very seldom a machine or engine would be used at only one running speed, whereby the attenuation could be designed only regarding this frequency.

One attenuation possibility is to direct the gas flow past a material which is porous or non-porous, whereby a part of the flowing medium and at least the pressure fronts of the pulses penetrate into the mass of the attenuating material, where the sound energy is transformed via friction into heat. Such a structure is presented in the pub- lication US-4 362 223. Disadvantages of such a silencer of the by-pass type is a poor sound attenuation capability, the short life of the attenuating material for instance due to the pores becoming clogged, and the decreasing elasticity of the attenuating material which will be totally lost in the long run. As a result of these phenomena the attenuation capability will further decrease, and the renewal of the attenuating material causes both material costs as well as costs for the installation work. As a modification of the absorption silencer the flow can be directed through the attenuat- ing material, whereby a part of the sound energy is lost as friction losses. The most serious drawback of an absorption silencer of the flow-through type is its high flow resistance. Further disadvantages are also the short life of the attenuating material, i. a. due to the clogging of the pores, whereby already a partial clogging impedes the operation of the engine or machine, and a total clogging will completely prevent the operation of the engine or machine. Thus this latter solution also always acts as a filter of a kind, so that it is used only in such cases which also otherwise require fil- tering of the gas flow, such as at the suction air side of engines or machines.

Another alternative is to direct the flow through a chamber assembly so that there are generated as much reflections as possible. In this reflection silencer the sound is attenuated as differently directed pressure waves collide with each other. For in- stance the exhaust gas silencers of combustion engines in cars, motorcycles and other devices are currently mainly of this type. In the publications EP-0530 493 and DE-34 06 507 the sound attenuation characteristics of the structure are obtained mainly by following this principle, even though the form and location of the cham- bers are slightly different from the most common embodiments in use. The DE pub- lication further uses the above mentioned absorption method in order to improve the attenuation. A disadvantage is that it increases the periodical motion forward and backward of the gas flow, i. e. a periodical backward directed flow, and the poor flow characteristics or the relatively high flow resistance.

A third alternative is a resonator attenuator, i. e. a Helmholz resonator, which is formed by a channel and a chamber, whereby the entity formed by them has a natu- ral frequency which is as far from the natural frequency of the sound source as pos- sible. Such resonator silencers are described in the publications EP-0 709 555 and NO-31414. With this arrangement the resonator can be used as a flow-through reso- nator, and the attenuation functions on a rather wide range, as long as the frequen- cies to be attenuated substantially differ from the natural frequency of the device, at which a flow-through resonator acts as an amplifier.

A fourth alternative is to distribute the incoming flow into channels which have dif- ferent lengths and which are combined at their other ends, at a position where the pressure difference pulses have different phases due to the propagation paths with different lengths. The publications EP-0 421 724 and GB-2 056 555 describe silenc- ers of this type. The publication EP-0 421 724 describes an arrangement where the cross-sectional flow areas of the inlet duct and the outlet duct are equal, and where, between the inlet and the outlet, the total cross-sectional flow area of the silencer tubes with different lengths is equal to the inlet duct's cross-sectional flow area or slightly larger and equal to the outlet duct's cross-sectional flow area or slightly larger. The length difference of the silencer tubes is not mentioned in the publica- tion, but on the basis of the figures the difference seems to be about 10 %. The pub- lication GB-2 056 555 describes a corresponding structure, where however all tubes are shaped as flat tubes in order to cool the exhaust gas passing through the tubes. A disadvantage of the described solutions is their very large size and weight, particu- larly when attenuation is required on a large frequency range and/or at low frequen- cies. The large size and weight are due to the fact that the length difference between

the silencer tubes must be at least approximately as large as half of the wavelength of the frequency to be attenuated. On the other hand, in the solutions according to these publications the wavelength depends on the sum of the sound velocity and the flow velocity, whereby the wavelengths of the above mentioned low frequencies are of the order 17 m (20 Hz), 13.6 m (25 Hz), 6.8 m (50 Hz) and 5 m (67 Hz). In these cases the half wavelength, and thus the length difference of the silencer tubes is 8.5 m, 6.8 m, 3.4 m and 2.5 m, respectively. These are so great lengths that they can not be mounted at least in passenger cars and motorcycles. As the frequencies in cars and motorcycles will vary during a ride it would require a large number of silencer tubes with substantially differing lengths, and at the same time long tubes with a to- tal weight which would be unacceptably high, perhaps a hundred kilograms or sev- eral hundred kilograms. Also the flow resistance of such long tubes is so high that it substantially will impede the operation of the engine. The length difference of the silencer tubes according to the figures in the above mentioned reference publica- tions will not provide any noticeable attenuation in any conventional and generally used engine or machine. Thus the realisation is so difficult that this silencer type has hardly been used.

At present most countries require, in addition to the silencing of the exhaust gases of combustion engines, also effective purification of the exhaust gases with the aid of catalytic afterburning. The catalysers are generally arranged in the exhaust pipes, separated from the silencers, and particularly between the main silencer and the en- gine, rather close to the engine, so that the temperature of the catalyser can be rap- idly raised after a start to the operating temperature and kept there, even if a wind with a subzero temperature would cool the device. The publications EP-0 530 493 and DE-34 06 507 propose to use a catalyser also as the silencer of the exhaust gases, whereby the device units on the exhaust gas side would be cheaper and smaller. As already stated above, disadvantages of a silencer built in this way are an occasional backward flow and a high flow resistance. In addition, the capability to attenuate the sound is rather poor, particularly in relation to the high flow resistance.

Thus the object of the invention is a device and a method for effectively attenuating sound generated by the pressure variations of a gas or similar medium flowing in a space defined by walls, such as in a channel or a tube or a chamber. Another object of the invention is to realise this effective sound attenuation so that the flow resis- tance caused by the attenuating members is as low as possible, and that backward flow of the gases or periodical backward and forward of the gases should not occur at all, or at most to a very low extent. A third object of the invention is to realise this

sound attenuation so that the attenuation is effective also at the frequencies of low pressure variations as well as on the harmonic frequencies. A fourth object of the invention is to realise this effective sound attenuation so that the size and weight of the silencer unit are as small as possible, most preferably so that they are smaller than the size and weight of known silencer units having a corresponding attenuation effect. A fifth object of the invention is to provide a silencer unit which fulfils the above defined requirements and which can be used in the exhaust pipes of combus- tion engines, in the suction air ducts of combustion engines, in different compres- sors, as well as in other similar engines and machines, and which when required also could be combined with at catalyser, at the same time at least maintaining both the advantages of the catalyser and the advantages of the silencer. The silencer unit should also be advantageous regarding the manufacturing costs.

In order to eliminate the above described disadvantages and to achieve the above defined objects the silencer according to the invention is characterised in what is presented in the characterising clause of claim 1, and the sound attenuation method according to the invention is characterised in what is presented in the characterising clause of claim 9.

An essential advantage of the invention is that a silencer according to the invention can effectively attenuate also low frequencies, such as the sound from the exhaust gases and/or suction air of combustion engines at normally used speeds of rotation, and at the same time this silencer according to the invention is light-weight and small-sized, whereby it is suitable for mounting for instance at conventional loca- tions in cars, motorcycles and in other working machines. An advantage of the in- vention is also a low flow resistance, and for instance that the backward flow of the exhaust gases is prevented or very low, which enables an even flow through the si- lencer without any substantial power losses. In addition, a silencer according to the invention has a simple structure, it is easy to mount due to its small size and low weight and due to its adaptable shape. An advantage of the invention is also that a silencer and a catalyser can be effectively combined, whereby according to the in- vention the catalytic members of the catalyser are actually shaped as members realis- ing the attenuation, whereby it provides an effective silencer having good flow resis- tance properties. In the case of a catalyser-silencer the catalytic members can, when required, be arranged according to the invention so that they will be rapidly heated and maintained at a suitable temperature.

The invention is described in detail below with reference to the enclosed figures.

Figure 1 shows in a side view, in the direction I of figures 2A and 2B, one em- bodiment of a silencer according to the invention when the outer casing of the si- lencer is removed from the viewing side. In this embodiment the gases flow from the silencer tubes immediately into a mixing chamber which is common to the tubes.

Figures 2A and 2B show in cross section two different ways according to the in- vention to shape the silencer tubes, as applied to the positions of figure 1 and shown along the plane II-II of figure 1.

Figure 3 shows in a side view another embodiment of a silencer according to the invention, seen in the same way as in figure 1, when the outer casing of the silencer is removed from the viewing side.

Figure 4 shows in a side view a third embodiment of a silencer according to the in- vention, seen in the direction III of figure 5, when the outer casing of the silencer is removed from the viewing side. In this embodiment the gases flow from the silencer tubes immediately into a mixing chamber which is common to the tubes.

Figure 5 shows the silencer of figure 4 in a top view, in the direction IV of figure 4, when the outer casing of the silencer is removed from the viewing side.

Figure 6 shows in a side view a fourth embodiment of a silencer according to the invention, in the same view as in figure 1, when the outer casing of the silencer is removed from the viewing side. In this embodiment the gases flow from the silencer tubes immediately into a mixing chamber which is common to the tubes.

Figure 7 shows in a side view a fifth embodiment of a silencer according to the in- vention, in the same view as in figure 1, when the outer casing of the silencer is re- moved from the viewing side. In this embodiment the gases flow from the silencer tubes immediately into a mixing chamber which is common to the tubes.

Figure 8 shows in a side view a sixth embodiment of a silencer according to the in- vention, in the same view as in figure 1, when the outer casing of the silencer is re- moved from the viewing side. In this embodiment the gases flow from the silencer tubes immediately into a mixing chamber which is common to the tubes.

Figure 9 shows in a side view a seventh embodiment of a silencer according to the invention, in the same view as in figure 1, when the outer casing of the silencer is removed from the viewing side. In this embodiment the gases flow from the silencer tubes first to an additional members formed by a catalyser, and then into a mixing

chamber which is common to the tubes.

Figure 10 shows in a side view an eighth embodiment of a silencer according to the invention, in the same view as in figure 1, when the outer casing of the silencer is removed from the viewing side. In this embodiment the gases flow from the silencer tubes first to an additional member formed for instance by a particle filter, and then to a mixing chamber which is common to the tubes.

Figure 11 shows in a side view a ninth embodiment of a silencer according to the invention, in the same view as in figure 1, when the outer casing of the silencer is removed from the viewing side. In this embodiment the gases flow from the silencer tubes into a mixing chamber which is substantially formed by the volume of a par- ticle filter or a catalyser and which is common to the silencer tubes.

Figure 12 shows in a side view a part of a tenth embodiment of a silencer according to the invention, at the position V of figure 1, when the outer casing of the silencer and also any external casings of the silencer tubes are removed from the viewing side. In this embodiment the gases flow through such silencer tubes which contain catalyser material, and then to a mixing chamber which is common to the tubes.

The figures show a silencer according to the invention for a pulsative gas flow, whereby the silencer at one end is connected to the inlet duct 1 or ducts la, lb, and at its other end to the outlet duct 2 or ducts 2a, 2b. The inlet duct 1 has a gas flow F1 which is distributed into a plurality of mutually separate silencer channels 3 of dif- ferent lengths, through which the gas passes as a flow F3. Regarding the through flowing flow F3 the silencer channels 3 are coupled in parallel. From the silencer channels 3 the gas flow F3 discharges, either immediately or via any additional de- vice into a mixing chamber 4 which is common to all these silencer channels, and in this chamber the gas propagates as the flow F4. This gas flow F4 moves further to the outlet duct 2, where it moves as the flow F2. Thus the mixing chamber 4 is be- tween the plurality of silencer channels 3 and the outlet duct 2. The inlet duct has a first cross-sectional flow area A1, which is defined by the wall 20 of the channel, such as a duct. In the corresponding way the outlet duct 2 has a second cross- sectional flow area A2, which is defined by the walls in the direction of the flow F2.

Said plurality of silencer channels 3 have a total combined cross-sectional flow area A3, which is not substantially smaller than the first cross-sectional flow area Al of the inlet duct 1. In many applications the cross-sectional flow area Al of the inlet duct 1 and the cross-sectional flow area A2 of the outlet duct 2 are substantially equally large, but depending on the application of the silencer according to the in-

vention they may also have different sizes, or substantially different sizes. Thus the first cross-sectional flow area Al of the inlet duct may be either larger or smaller than the second cross-sectional flow area A2 of the outlet duct. The gas 10 coming as a flow Fl in the inlet duct may originate in a combustion engine, whereby the question is about exhaust gases and typically a piston operated combustion engine, whereby there occur pressure pulses in the inlet flow F1 caused by the engine's ex- haust strokes, and the propagation of these pulses to the outlet duct must be pre- vented, i. e. they must be attenuated or silenced. In this case the outlet duct 2 dis- charges either directly into the environment 22, as in figure 8, or it continues as an outlet flow F2 into an extension (not shown in the figures) of the outlet duct, or to some other device not shown in the figures. If the silencer according to the inven- tion is used for silencing the suction air, then the outlet duct 2 is connected to the combustion engine, such as for instance on the suction side of a piston engine, whereby there occur pressure pulses in the outlet flow F2 caused by the suction strokes of the engine, and the propagation of these pulses to the inlet flow Fl at the inlet duct 1 must be prevented, i. e. attenuated. The silencer according to the inven- tion operates quite as well on the exhaust gas side, where the pressure pulses propa- <BR> <BR> <BR> gate in the same direction as the flows F1, F3, F4, F2 of the gas 10, as on the suction side of the combustion engine, where the pressure pulses propagate in the opposite direction as the flows Fl, F3, F4, F2 of the gas 10.

Above we discussed attenuating sound in a flow of gas 10, with which in this appli- cation is meant any gas or gas mixture, and moreover also steams or steam mixtures, either as such or mixed in any gases. The silencer according to the invention oper- ates also quite as well for attenuating the sound effects of the flow of any flowing medium in which there occurs pressure pulses, so that the invention also relates generally to all such embodiments or applications where the sound effects of pres- sure pulses in the medium flow must be attenuated or silenced. Thus the medium can also be a liquid. However, primarily and mainly the invention relates particu- larly to the attenuation of the sound effects of pressure pulses contained in gases, gas mixtures and steam containing gases and steam mixtures.

A particular feature of the invention is thus the above mentioned mixing chamber 4 which is located between the outlet openings 5 of the silencer channels 3 and the inlet openings 6 of the outlet duct 2. According to the invention the silencer chan- nels 3 generally discharge immediately into the mixing chamber 4 which is common to the silencer channels, whereby the cross-sectional flow area A4 in this chamber is substantially larger than the total combined cross-sectional flow area A3 of the si-

lencer channels 3. The recombination of the flows F3 occurring immediately after the silencer channels 3 is very advantageous. Thus the total cross-sectional flow area A4 regarding the flow F4 in the mixing chamber is substantially larger than the total combined cross-sectional flow area A3 of the silencer channels 3 regarding the flow F3. According to the invention the cross-sectional flow area A4 in the mixing chamber is at least two times, preferably four times, and typically ten times or more times the total combined cross-sectional flow area A3 in the silencer channels. As the lengths L2, more particularly the lengths L2,... L2n, of the silencer channels 3 are different, as can be seen in the figures, the sub-flows of the flow F3 branched into these silencer channels 3 have phase shifts regarding the pressure pulses when they are discharged from the outlet openings 5 of the silencer channels 3. When these sub-flows F31... F3n are discharged from the openings 5 into the mixing cham- ber 4 there occurs a substantial reduction of the flow velocity from the flow velocity V3 prevailing in the silencer channels 3 to the flow velocity V4 of the flow F4 in the mixing chamber 4. This substantial reduction of the flow velocity V3-> V4 has a considerable effect in increasing the attenuation.

According to the present opinion the efficiency of the attenuation in the silencer and the method according to the invention is based on the following phenomenon. When the gas 10 flows in the inlet duct 1 and there occur pressure pulses at the frequency (p, then the wavelength X of these pressure pulses is determined in a known way by the equation X = v (p, where v is the sum of the sound velocity (about 330 m/s) and the flow velocity. Half of this wavelength X/2 is that factor which determines the length difference of the silencer tubes according to the prior art and described in the general part of this application. In contrast to that, in this silencer and method ac- cording to the invention the length difference AL2 of the silencer tubes, at which certain frequencies and the corresponding wavelengths are attenuated, is now de- termined according to the equation AL2 = v (p V3/V4, where V3 is the average velocity of the flow F3 in the silencer tubes, and V4 is the average velocity of the flow F4 in the mixing chamber 4. If thus the cross-sectional flow area A4 in the mixing chamber is for instance ten times that of the total combined cross-sectional flow area A3 in the silencer channels 3 the length difference of the silencer channels 3 is AL2 = L2n-L2"in other words the length difference between the longest and shortest tube is only one tenth of that which would be required in corresponding structures without a mixing chamber. Thus the mixing chamber 4 according to the invention considerably shortens the silencer channels 3, compared to the lengths of the prior art silencer channels 3 at the same lowest frequency (p to be attenuated. If the ratio A4/A3 of the cross-sectional flow areas is ten, the silencer channels are

shortened to one tenth, if the ratio A4/A3 of the cross-sectional flow areas is twenty, the silencer channels are shortened to one twentieth, and if the ratio A4/A3 of the cross-sectional flow areas is thirty, then silencer channels 3 are shortened to one thirtieth. Ratios A4/A3 of the cross-sectional flow areas of this order can in practice be achieved easily, even when the outer dimensions of a silencer according to the invention are of the same order as the outer dimensions of any currently used si- lencer. Thus the silencer and the silencing method according to the invention are ve- ry efficient and small-sized, irrespective of whether the velocity determining the wavelength of the pressure pulses is the sound velocity, the flow velocity, or their combination. In any case the length of the silencer channels 3 and thus the size of the whole silencer is substantially smaller than that of any known silencer with the same effect. The shortening of the silencer channels 3 to the above mentioned frac- tion also substantially reduces their flow resistance, as is understandable. In a si- lencer and the method according to the invention the mixing chamber 4 seems to eliminate, in at least some degree, the effect of the sound velocity on the wave- length, whereby in a solution according to this invention the flow velocity of the gas 10 would act as a factor determining only the effective wavelength X of the pressure pulses, or as a factor which is substantially significant in forming the effective wavelength X. This effect further shortens the length L2 of the silencer channels 3 by at least one or two orders, or to one tenth or one hundredth, compared to the si- lencer channel lengths required by prior art solutions. When both these above men- tioned factors are combined we arrive in the solution according to the invention at silencer channels 3 whose length L2 is between 1/10 and 1/500 of the lengths of known silencer channels. It is understandable that silencer channels having a very reasonable length, having a length difference AL2 of the order of less than one me- tre, perhaps half a meter or 20 cm, can almost completely attenuate pressure pulses with a frequency below 30 Hz. If the lowest frequency to be attenuated is higher, for instance 100 Hz, an effective attenuation can be obtained already with a length dif- ference AL2 of the silencer tubes 3 of only a few centimetres.

In order to retard the flows, coming from the silencer channels 3 and their outlet openings 5 to the mixing chamber 4, from the velocity V3 to the velocity V4 in a similar way and efficiently, it is advantageous that the cross-sectional flow area A4 of the mixing chamber 4 is continuous and undivided, such as shown in the figures.

It is of course possible to arrange plates, blades or similar in the mixing chamber for directing the flow F4, but however, according to the present understanding this is not advantageous. Further there is a distance LI between the outlet openings 5 of the silencer channels and that opposite wall 7 of the mixing chamber 4, which is

closest to said outlet openings 5 in the main direction of the gas flow F3 discharging from the silencer channels into the mixing chamber. This distance LI is preferably larger than the smallest cross-section Wl of the silencer channels 3. The object of this dimensioning is to prevent the flow F3, as it is discharged from the outlet openings 5 of the silencer channels, to hit a solid surface too early, which could re- flect the pressure pulse, and which could prevent a rapid reduction of the flow ve- locity V3 to the flow velocity V4 prevailing in the mixing chamber. Further it is ad- vantageous to arrange an inlet opening 6 of the outlet duct 2 in such a position of the wall of the mixing chamber that the second cross-sectional flow area A2 of the out- let duct 2 is located at least partly elsewhere or in some other place than in the area defined by the extensions of the silencer channels 3 or in the area defined by the extension of the main direction of the gas flow F3 discharging from the silencer channels 3. The purpose of this is to prevent the gas flow F3 discharged from the silencer channels to continue, due to the inertial forces, directly into the outlet duct, which would impair the retardation in the mixing chamber 4. In the embodiment of figure 3 the inlet openings 11 of the downstream silencer channels 3b form at the same time the opposite wall 7 and the inlet opening of a virtual outlet duct for the first silencer channels 3a. The inlet openings 11 of the downstream silencer chan- nels 3b are partly located in the region of the outlet openings of the first silencer channels 3a and partly outside them. The outlet openings 5 of the second silencer channels 3b are also located partly in the region of the outlet ducts 2a and 2b, of which there are two in this case, and partly outside them. In the other figures the in- let opening 6 of a single outlet duct 2 is located wholly outside the are defined by outlet openings 5 of the silencer channels 3.

Because the speed of rotation of most combustion engines and other machines, such as compressors, will vary and change during operation, also the frequencies (p of the pressure difference pulses of said pulsative gas flow F1 or correspondingly F2 will vary during the operation of the engine of machine. This results in that a particular silencer must attenuate pressure difference pulses occurring at different frequencies, and often on a quite wide frequency range. In these circumstances the attenuation can be realised by arranging in a silencer according to the invention at least five si- lencer channels 3 and preferably at least ten silencer channels 3. Obviously there may be even more silencer channels, such as 20,50,100, or even thousand or thou- sands, depending on the cross-sectional flow area of the individual silencer channels and on the manufacturing techniques of the silencer channelling. The lengths L2l...

L2n of the individual silencer channels 3 are arranged to be distributed, either at even intervals or according to a predetermined function between the shortest si-

lencer channel with the length L2, and the longest silencer channel with the length L2n. With this length distribution and the length difference AL2 between the longest silencer channel and the shortest silencer channel the silencer and correspondingly the method according to the invention can be made to operate so that they attenuate the sound caused by pressure pulses over a desired frequency range. The lengths L2z... L2n of the silencer channels, the length difference AL2, and the cross- sectional flow area A4 in the mixing chamber 4, and the total cross-sectional flow area A3 of the silencer channels 3, and then of course also their ratio A4/A3, are defined in advance to suit the particular application of the silencer, i. a. on the basis of the frequencies (p to be attenuated.

According to the present understanding it is advantageous to arrange the flow F1 in the inlet duct 1 to be evenly distributed and without any substantial velocity changes from the first cross-sectional flow area A1 to the combined cross-sectional flow area A3 of the silencer channels 3. This can be arranged with the aid of a flow distributi- on chamber 8 described below. Thus it is assumed that the cross-sectional flow area A3 of the silencer channels and the first cross-sectional flow area A1 of the inlet duct preferably differ at most 30 %, and if possible, at most 15 % from each other.

The combined cross-sectional flow area A3 in the silencer channels and the first cross-sectional flow area Al can of course be equally large. No advantage, but on the other hand no disadvantage, is seen in the fact that the total cross-sectional flow area A3 of the silencer channel and the first cross-sectional flow area A1 of the inlet duct would differ more than that mentioned above. The engine or machine, to which the silencer according to the invention is connected, may require a departure from the differences given above.

The outlet flow F2 from the mixing chamber 4 can be arranged through the second cross-sectional flow area A2'or A2 or A2"of the outlet opening 6, which is either as large as the largest cross-sectional flow area A4 of the mixing chamber 4, such as in figure 8, or smaller than the largest cross-sectional flow area A4 of the mixing chamber. Thus in an extreme case the mixing chamber 4 has a free connection to the environment 22 or an almost free connection to the environment 22, as in figure 8, or via an outlet duct 2 with a more restricted cross-sectional flow area to the envi- ronment 22 or to some other device not shown in the figures.

Figure 1 shows an advantageous embodiment of the invention where the gases 10 are supplied through the flow distribution chamber 8 as the flow F5 to the inlet openings 11 of the silencer channels 3. In this case the flow distribution chamber 8

has a toroidal cross section and is conically widening. The cross section in a trans- versal direction regarding the flow F5 is a circular annulus, and the total cross- sectional flow area A5 of the distribution chamber 8 is preferably kept substantially constant over the whole length L3 of the flow distribution chamber 8, as described above. The silencer channels 3 are arranged against the inner surface of the outer casing 23 of the silencer, as can be seen in the figures 2A and 2B. Then the silencer channels form a peripheral succession of silencer channels 3 on the inner surface of the casing 23, whereby the silencer channels open their inlet openings 11 to the flow distribution chamber 8 and their outlet openings 5 to the mixing chamber 4. The guiding wall 24 with the form of a cone tip in the flow distribution chamber 8 forms a guide for the flow F5 in the flow distribution chamber, and at the same time it closes the area defined by the inner casing 25 of the inner sides of the silencer chan- nels 3 facing each other, such as can be understood from the figures 2A and 2B.

Then the flow F5 reaches the mixing chamber 4 only through the silencer channels 3. The total cross-sectional flow area A4 of the mixing chamber 4 is then formed by the total sum of the cross-sectional flow area of the guiding wall 24 and the total combined cross-sectional flow areas of the silencer channels 3, or by the area de- fined by the silencer's outer casing 23. As described above, this total cross-sectional flow area A4 is substantially larger than the total combined cross-sectional flow area A3 of the silencer channels, which on the other hand is approximately as large as the total cross-sectional flow area A5 of the flow distribution chamber and the first cross-sectional flow area Al of the inlet duct. The mixing chamber 4 extends in the flow direction F4 at the conically tapering opposite wall 7, which changes into the outlet duct 2 at the inlet opening 6, through which inlet opening the gases 10 flow into the outlet duct.

In this solution the silencer channels 3 can be formed in many different ways, of which two are shown in figures 2A and 2B. The silencer channels can be made be- tween the outer casing 23 and inner casing 25 of the silencer, either with radial par- titions 26 or by a corrugated plate 27 pushed in between the outer casing 23 and the inner casing 25, whereby this plate is supported on the corrugation tops 28a and the bottoms 28b against the outer casing and the inner casing 25, respectively. Both ways create silencer channels 3 separated from each other. In the case of figure 2B it is further arranged so that the cross-sectional flow area of the silencer channels with lengths longer than the length L2 (the lengths = L2n, L2n l, L2n 2, etc.) is larger than the lengths of the silencer channels with shorter lengths (the lengths = L2l, L22, etc.). With this arrangement the flow resistance of the longer silencer channels can be kept generally the same as the flow resistance of the shorter silencer channels. In

the embodiment of figure 2A the cross-sectional flow areas of all individual si- lencer channels 3 are again mutually equal, which also gives a quite acceptable so- lution. In this case the distances of the inlet openings of the silencer channels from the inlet duct 1 are very accurately equal, whereby the desired phase difference be- tween the gas subflows F31... F3n is created in a controlled manner due to the propagation time difference.

The figures 4 and 5 show a solution which in a way is similar to that in the figures 1 -2B, but here the silencer tubes 3 are arranged as a rectangular silencer. The si- lencer channels are arranged against the inner surfaces of the parallel inner walls 29a, 29b of the outer casing 23 of the silencer. The area defined between these si- lencer tubes is closed in the direction of the flow distribution chamber 8 for instance by a straight guiding wall 24, or by a guiding wall which is cylindrically convex which is not shown in the figures. In this case the flow distribution chamber 8 is rectangular, and it receives two inlet ducts la and lb, of which the first one la is close to the upper side 30a of the casing and the other one close the lower side 30b of the casing, whereby these sides are perpendicular to the side walls 29a and 29b.

The outlet duct 2 with its inlet opening 6 is in this case arranged at the lower side 30b of the casing, so that the outlet flow F2 is transversal regarding the inlet flow F1. Also in this case there is a distance LI between on one hand the end wall of the casing 23, which is transversal regarding the direction of the flow F3 in the silencer channels 3, and on the other hand the outlet openings 5 of the silencer channels, whereby this distance LI is at least as large as the transversal dimension Wl of the silencer channels. In this case the distance LI is substantially larger than the meas- ure Wl. The mixing chamber 4 is formed by the space close to the side walls and between the plurality of silencer channels and by the space as an extension to them, whereby the cross-sectional flow area A4 of this space is substantially larger than the total combined cross-sectional flow area A3 of the silencer channels 3, in the manner described above.

In both embodiments described above the silencer channels are located on the inner surface of the silencer's casing, in other words far from the central parts of the si- lencer. This solution provides in an easy way a large cross-sectional flow area for the mixing chamber, but the arrangement results in that the environment of the si- lencer can affect the temperature of the gas 10 flowing through the silencer channels 3, which again can be either advantageous or disadvantageous. The figures 6 to 8, and the figure 3, show embodiments of the invention where all silencer channels 3 are arranged close to each other. Such an embodiment is advantageous, for instance

when the silencer channels are formed by silencer channels between the catalytic surfaces of a catalyser cell.

Figure 6 shows an embodiment where the individual silencer channels 3 are abutting each other, whereby they are separated only by the wall thickness of their partitions.

As it is preferable to arrange the wall thicknesses to be as small as possible, observ- ing of course the durability of the silencer channels in the silencer, it was in this case possible to connect the inlet duct 1 to the silencer channels 3 via a flow distribution chamber 8 which is very small, almost infinitesimal. The radial widening P caused by the flow distribution chamber corresponds only to the face surface caused by the partitions of the silencer channels in a direction perpendicular to the inlet flow Fl.

In this case the mixing chamber 4 is formed partly at the extension of the flow di- rection F3 of the silencer channels 3 and partly outside the area defined by these extensions, and particularly on that side which in the transversal direction regarding the length of the silencer channels projects outside the longest channels, as can be seen in the figure 6. Then the outlet duct 2 is arrange in the opposite wall 7 particu- larly in that area which is outside the extensions of the silencer channels in the flow direction, in other words the axis of the outlet duct is at a distance H from the long- est silencer channels 3.

Figure 7 shows an embodiment where the inlet duct 1 and the silencer channels 3 are arranged mutually and regarding each other in the same way as in the embodi- ment of figure 6. However, in this case the mixing chamber 4 is arranged partly outside the plurality of silencer channels 3 to surround them, and the outlet duct 2 is arranged at that end of the outer casing 23 of the mixing chamber where the inlet flow Fl comes into the silencer. Thus the outlet flow F2 in the silencer is generally opposite to the inlet flow Fl in the ducts, which of course are arranged next to each other, as can be seen in figure 7. In this solution the whole mixing chamber 4 sur- rounds the plurality of silencer channels 3, whereby the silencer channels very well will maintain the temperature of the gas 10, if this is required. In the mixing cham- ber 4 the mixing flow F4 certainly has to make an essential change of the direction close to the opposite wall 7, but this does not critically increase the flow resistance, as long as the distance LI between the opposite wall 7 and the outlet openings 5 of the silencer channels 3 is sufficiently long, in this case preferably a multiple of the transversal dimension Wl of a single silencer channel. After the changed direction and retardation the mixing flow F4 propagates in the opposite direction regarding that of the flow F3 in the silencer channels, and finally it reaches the inlet opening 6 of the outlet duct 2. A silencer of this type can be placed for instance in the engine

compartment of a car, and then it is particularly advantageous to arrange the silencer channels 3 so that they are formed by the flow channels between the catalytic sur- faces of a catalyser cell 9.

Figure 8 shows an embodiment where the inlet duct 1, the flow distribution chamber 8 and the silencer channels 3 are arranged in the same way as in the embodiment of <BR> <BR> <BR> figure 6 regarding the flows F1, F5 and F3. However, in this case the silencer chan- nels 3 are different from the other embodiments regarding the outlet openings 5 such that the outlet openings are located in the side surface of each silencer channel 3, as can be understood from the figure on the basis of the lines representing the flow 3 and the reference numerals 5 of the outlet openings. When using such outlet openings 5 or openings in the sides of the silencer channels the inlet opening 6 of the outlet duct 2 can be arranged directly in register with the mechanical direction of the silencer channels 3, because the flow coming from the outlet openings 5 anyhow meets the above described requirement, in other words that the outlet opening of a silencer channel is not in the direction of the extension of the flow F3 from the si- lencer channels 3. This can be clearly seen in figure 8, because the discharge direc- tion of the flows F3 is pointing downward or obliquely downward, and the inlet opening 6 of the outlet duct is directly on the right side. In this embodiment the outer casing 23 of the mixing chamber 4 is further shaped so that it is widening in a horn-like manner from the region 35 toward the inlet opening 6 of the outlet duct.

Different auxiliary means can be located in the region 35 or outside it. Then the outlet duct 2 may open for instance without any obstacles directly into the surround- ing external space 22 with a cross-sectional flow area A2', as shown with a continu- ous line in figure 8, or alternatively it may be redirected to a tapering extension of the outlet duct 2 with a cross-sectional flow area A2", as shown with dotted lines in the figure. The extension of the outer casing 23 can also be omitted, whereby the mixing chamber is completely or approximately straight in the direction of the flow in it. In those embodiments of the invention where the outlet duct 2 discharges di- rectly into the external space 22 the free edge 33 of the mixing chamber which joins the external space will form the effective inlet opening. Also these structures oper- ate as effective silencers.

Figure 3 shows an embodiment where two sets of silencer channels 3a and 3b are arranged in series in the direction of the gas flows F5, F3a, F4a, F3b, F4b. As can be seen in the figure the first silencer channels 3a are located in a similar manner as in figure 1, so that they begin inwards from the silencer's outer casing 23. The differ- ence is that in this case there are silencer channels 3a in the direction of the radius

R, if the silencer has a circular cross section, or generally from the outside to the in- side in many layers. The space left empty by the silencer channels 3a is also in this case closed by a guiding wall 24 on the side of the flow distribution chamber 8, whereby the flow F5 is distributed approximately evenly into the first silencer chan- nels 3a. From these first silencer channels with different lengths the flow F3a is re- tarded to a flow F4a in the first mixing chamber 4a having the cross-sectional flow area A4a, which again is substantially larger than the cross-sectional flow area A3a of the first silencer channels 3a. From this first mixing chamber 4a the flow F4a contracts again into the second set of silencer channels 3b, as the flow velocity in- creases, whereby the channels contain a flow F3b. From these second silencer chan- nels 3b the flow is again retarded to a flow F4b when it discharges into the second mixing chamber 4b. This retardation is caused by the fact that the cross-sectional flow area A4b of the second mixing chamber 4b is substantially larger than the total combined cross-sectional flow area A3b of the second silencer channels 3b. The second silencer channels 3b are arranged at the central parts of the silencer, and an annular flange 34 closes the gap between their outer edges and the outer casing 23.

This arrangement, where the first silencer channels 3a are located only close to the casing 23, whereas the second silencer channels 3b are close to the central parts of the silencer, will cause a change of the direction of flow F4a in the first mixing chamber 4a, which makes the mixing more effective and retards the flow. From the second mixing chamber 4b there are two outlet ducts 2a and 2b separated by the op- posite wall 7. With this arrangement the flow F4b in the second mixing chamber 4b can be changed, which makes the mixing and retardation of the flow more effective.

In addition, in the case of figure 3 the silencer channels 3 are formed by flow chan- nels between the catalytic surfaces of a catalyser cell 9. It is obvious that, when re- quired, there may be also arranged more sets of silencer channels 3 in series in the direction of the flow, whereby there may be one or two sets as shown in the figures, and moreover also three, four or more sets. Of this plurality of silencer channel 3 sets one or some may be formed by flow channels between the catalytic surfaces of a catalyser cell, an a part may be formed by chemically passive channels, which are described in connection with the figures 1-2B, figures 4-6, and figure 8.

The figure 9 shows an embodiment where the inlet duct 1, the flow distribution chamber 8, and the silencer channels 3 are of the same type as those which were presented in connection with figure 8. In this embodiment a catalyser cell 31 is placed immediately after the silencer channels, whereby the cell preferably com- prises plate-like catalytic surfaces in the direction of the flow, although it would be also possible to use a catalyser of the mass type. In this case the channels between

the catalytic surfaces of the catalyser cell 31 are equally long and thus they are not silencer channels, and immediately downstream of this cell there is a mixing cham- ber 4 according to the invention where the flows F3 with different phases coming from the silencer channels 3 finally are retarded in the large cross-sectional flow area A4 and mixed in the manner described above. As the catalyser cell does not op- erate here to generate phase differences in the individual sub-flows of the gas this embodiment can also use catalyser cells comprising a porous mass or similar. As the retardation and mixing of the flows coming from the silencer channels 3 are only slightly delayed compared to the above described embodiments it is assumed that this structure is at least approximately as efficient as the above described. In other respects the earlier described features are applicable to this embodiment.

Figure 10 shows an embodiment where the inlet duct 1, the flow distribution cham- ber 8, and the silencer channels 3 are of the same type as those presented in connec- tion with figure 6. In this embodiment a particle filter 32 is located immediately af- ter the silencer channels, the filter comprising a suitable porous filter material. The actual mixing chamber 4 according to the invention is downstream from the particle filter 32, which thus does not contain silencer channels, and in the mixing chamber the flows F3 with different phases discharged from the silencer channels 3 are fi- nally retarded in the large cross-sectional flow area A4 and mixed in the manner de- scribed above. Here it must be noted that in the embodiment of the figure the cross- sectional flow area of the particle filter 32 increases in the flow direction F3-> F4, so that the flow coming from the silencer tubes will be retarded, and the different phases are mixed already in the particle filter, so that the particle filter 32 must be regarded as a part of the mixing chamber 4. In the figure the rest of the mixing chamber 4 is continuous and open, but it could also contain a partition or similar for preventing the gas flow directly to the outlet duct 2 due to the inertia forces, as was described above. As the retardation and the mixing of the flow from the silencer channels 3 is only slightly delayed compared to the above described embodiments it is assumed that this structure is also an at least approximately as efficient silencer as they are. In other respects the above described is applicable to this embodiment.

Figure 11 shows an embodiment which is quite close to the embodiments of figures 9 and 10. There the inlet duct 1, the flow distribution chamber 8, and the silencer channels 3 are of the same type as was shown in connection with figures 6 and 10.

In this embodiment there is located after the silencer channels a particle filter 32 or catalyser 31, which comprises a suitably porous filtering material. This particle filter 32 and the catalyser cell 31 have an internal structure which is of the same type as in

the figures 9 and 10 and as is described later. In this embodiment the particle filter 32 or catalyser cell 31 form the mixing chamber 4, either totally or mainly, as in the figure 11. The mixing chamber 4 is thus formed by the space defined by the particle filter or catalyser cell, and thus this does not necessarily require any other substan- tially open space for the mixing chamber, and the particle filter 32 and/or catalyser cell 31 have the described large cross-sectional flow area A4. The sub-flows of gas F3 with different phases coming from the silencer tubes 3 are retarded and substan- tially mixed to the mixing flow F4 inside the particle filter or catalyser cell. In this embodiment both the particle filter 32 and the catalyser cell 31 can be of any type.

In other respects the earlier described is applicable to this embodiment.

Figure 12 shows an embodiment of the invention which in other respects has si- lencer channels 3 of the basically same type as was described in connection with the figures 1-2B, 4-6 and 7-11, except that the interiors of the silencer channels are filled with catalytic material 33. Thus the structure has silencer channels tightly ad- jacent but separate from each other and having different lengths L2, in each of which the interior generally comprises catalytic material 33, which may be of any type which is suitable for the purpose and enables the flow-through of gases, such as the types described in the paragraph below. If the catalytic material 33 comprises plate-like or tube-like parts, i. e. contains channels, it is appropriate in this embodi- ment that these catalyser channels have transversal dimensions which are substan- tially smaller than the actual silencer channels 3. It also possible to use catalytic material 33 of the mass type.

Above it was noted that the silencer channels according to the invention can be formed by flow channels between catalytic surfaces of a catalyser, and particularly in order to purify the exhaust gases of combustion engines and at the same time to attenuate sound in the same multi-purpose channel members, as was described in connection with the figures 3 and 7. Here it must be specified that for this multi- purpose part of the silencer, i. e. a combined structure of a catalyser cell and silencer channels, there must be used a catalyser cell of the plate or sheet type which com- prises catalytic material coated on metal plates or ceramic plates or on any other possible plate-like components, between which there are left identifiable openings, holes, channels or corresponding flow passages. Thus the silencer channels in appli- cations of this type can be shaped to have different lengths according to the inven- tion. For instance, a large number of such plates are arranged in a pile in the cata- lyser cell, which pile in addition can be turned around one or more lines parallel to the planes of the plates so that a spiral shape is obtained. The above mentioned

catalyser cell means particularly a cell which comprises thin plates or tubes, which are coated with catalytic material and between which plates or within which tubes there are left distinct flow channels, even if they often would be very small. Obvi- ously such a cell can be manufacture in many different ways, but as this invention does not primarily relate to catalytic cells they will not be described in greater detail here. It will only be noted that a cell of the described type is suitable to be used in connection with the invention. In such embodiments of the invention where a cata- lyser cell is located downstream of the silencer tubes, or alternatively before the si- lencer tubes, before the silencer, as seen in the flow direction of the gases, it is pos- sible to use a catalyser cell of any type, such as a cell comprising plate-like catalytic components described above, or alternatively a porous catalytic mass through which e. g. the exhaust gas passes. When required, it is of course necessary to observe the disadvantages of a catalyser of the mass type, which were mentioned in the general part of this application in connection with the absorption silencers. The above de- scribed particle filter can be of a type which collects small and/or very small parti- cles from for instance exhaust gases, and because a silencer according to the inven- tion placed in a suitable place can be made to operate at a very high temperature, this combination results in that the collected particles will burn in the particle filter.

As the particles are generally mainly soot or carbon, the combustion products are discharged in a gaseous form. Then the micro particles can be removed from the ex- haust gases with the aid of a self-cleaning particle filter, which is also one of the ad- vantages obtained with the invention, when required.

Obviously the details of the invention presented in this description in connection with the different figures can be combined with other silencer solutions, and they can be used in silencers according to the invention which are of another type. In the above described silencers according to the invention the silencer channels are straight, but it must be understood that they can also be arcuate, meandering, or have other shapes or shapes varying, either mutually differently or in the same way.

Similarly the cross-sectional shape of the silencer channels 3 and the mixing cham- ber 4 can vary in a wide range and have a very different shape.