The invention relates to a silencer for internal combustion engines with outer walls consisting at least partly of a fireproof and corrosion resistant material and a surrounding cover of a weatherproof 5 and impact resistant material.

It is well-known that usual silencers for internal combustion engines, e.g. in automobiles suffer from considerable corrosion problems entailing a relatively short life-time of such silencers. 10 In order to remedy these problems silencers have been suggested in which the shield or outer wall con¬ sists entirely of a ceramic material. It has been suggested to let the sound absorption rely solely on the sound-absorbing properties of the ceramic 15 material itself. However, usually a better sound absorp¬ tion will be obtained with an internal arrangement of labyrinth-forming members extending the flow path of the combustion gas. It has been suggested to use transverseceramic dampingplates as such members. 2.0 Even if silencers of this kind solve the corrosion problems they have in another respect proved less suitable in practice because a sintered ceramic material is very sensitive to the mechanical influences in the form of impacts or shocks towhich a silencer is exposed 5 under normal operation. As a result of such influences cracks or fractures often arise in the ceramic shield of the silencer.

In aiolder ceramic silencer which is known from US-A-3,227,241 the sensitivity to mechanical shock 0 influences has been reduced by enclosing the ceramic outer wall with an external shield of a corrosion resistant metal. In order to improve the sound absorbing properties it is prescribed for this known silencer that a number of flow resisting members shall be 5 arranged in its internal space,said members being

a sintered ceramic material and designed with a honeycomb structure-In a manner known per se the flow resisting members shall be ^' designed and arranged in such a way that a meandering possibly labyrinth-like flow path for the- combustion gas is formed. This design complicates the construction and makes it more expensive and as a result thereof silencers of this type have appeared to be without interest for use as standard and replacement components for automobiles. Neither has this known silencer been effective to remedy the drawback of all-ceramic silencers caused by the inferior resistance of sintered ceramic materials against the influences of various and shifting weather conditions, in particular the alternation of frost and thaw.

With a silencer according to the invention excellent sound absorbing properties as well as a great resist¬ ance to impact influences and other dynamic loads are obtained in combination with a substantially improved resistance to weather influences by means of a simple and cheap construction which according to the invention is characterized in that said fireproof and corrosion resistant material comprises a form stable internal lining of a high-insulating, thermo-resistant fibre- based material.

With the use of said internal lining which may be produced in a vacuum forming process a considerable degree of freedom is obtained with respect to the design of the silencer in contradiction to the known ceramic silencers which are limited to relatively simple geometrical shapes. In addition, an effective protection of the surrounding impact proof cover against shock-like temperature influences is provided by the lining- Thus a lining produced in this way may be applied as an internal lining in a silencer with a conventional metallic outer wall of arbitrary form. Thereby the

possibility is opened in a simple manner for the intro¬ duction of sound absorbing linings according to the invention for the replacement of standard silencers in the market. In a preferred embodiment of the silencer accord¬ ing to the invention, the lining consists mainly of aluminium silicate fibres. In practice, this material is shrinkproof even at temperatures above 1000 C and in spite thereof elastic, as a result of which an external cover produced by the above mentioned direct spraying will give no occasion to harmful tensions which might otherwise be caused by differences between the thermal coefficients of expansion of the materials.

In a silencer with a conventional design having a mainly cylindrical wall and mainly flat end walls in connection with inlet and outlet pipes .for combustion gas connected thereto, a preferred embodiment is further characterized in that the lining at its ends comprises conical wall parts directed towards the interior of the silencer.

Such conical wall parts around the silencer's inlet and outlet for combustion gas will be effective to damp the transmission of the sound waves and thereby the sound level will typecally be reduced to half of that of known exhaust systems.

According to a further preferred embodiment of the silencer according to the invention, the external impact, proof cover consists of a fibre-reinforced concrete or cement. Thereby the impact proof cover may - be formed directly on the lining by spraying and/or bandaging of glass, steel, plastics or coal fibre- reinforced concrete or cement.

In connection with such production of the impact proof cover by direct spraying and/or bandaging of fibre-reinforced concrete or cement, the above mentioned design with conical wall parts opens the possibility to avoid use of separate and rather expensive gaskets

between the cylindrical wall and the flat end walls in that said parts are formed by separate vacuum-formed bodies of said fibre based material having edge portions functioning as ^" gaskets located between the ends of the cylindrical wall and the flat end walls.

Moreover, the invention relates to a method for manufacturing a silencer of the above mentioned pre¬ ferred embodiment, said method being characterized according to the invention in that the cylindrical wall of the lining is manufactured by vacuum forming from a liquid slurry of said fibre-based material on a tubular core corresponding to the internal space of the silencer, the lining thus produced being connected after curing with the conical wall parts and the flat end walls, the external cover of said fibre-reinforced concrete or cement being ^* applied to the cylinder wall between said edge portions of the. conical wall parts.

In the following, the invention will be explained further with reference to the schematical drawings in which. Figs. 1 and 2 are longitudinal sectional views of two embodiments of a silencer according to the inven¬ tion.

The silencer shown in Fig. 1 which may be of conventional form with a circular or oval cross section comprises in accordance with the invention a form stable internal lining of a fire proof and corrosion resistant fibre-based material and an externally surrounding weather proof and impact proof cover of fibre-reinforced cement or concrete. Preferably the lining consists of aluminium silicate fibres and comprises a cylindrical part 1 manufactured by a vacuum forming process from a liquid slurry on a perforated tubular core matching the internal cross section of the silencer. In the embodiment shown, the lining comprises, in addition, inwardly protruding conical wall parts 3 and 4 at the ends of the silencer, said parts being manufactured as separate vacuum formed parts of the same fibre based material

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as the wall 1 and being designed with flange-forming edge portions 12 and 13, which after connection of the wall parts 3 and 4 with the cylindrical wall 1 will extend outside the external side of the latter to an extent corresponding to the desired thickness of the surrounding impact proof cover 2. At the ends, the silencer is externally finished with flat end walls 5 and 6, which may be of a corrosion resistant metal such as stainless steel and may be welded on to the combustion gas inlet pipe 7 and outlet pipe 8 of the silencer. In the-embodiment shown, pipes 7 and 8 are formed as a single pipe which is blocked in the middle by a transverse partition 9 so that there is no direct communication throughout the pipe. The pipes are of a corrosion resistant metal such as stainless steel and have each a number of openings 9a through which combustion gases are communicated to and from the interior of the silencer. However, the blocking between the inlet and the outlet pipes is not a necessary σon- dition. By other embodiments there may be a continuous perforated pipe communication effecting inside the silencer an expansion of the combustion gases.

In the embodiment shown, each of the conical wall parts 3 and 4 of the lining is formed by a solid conical body 10 and 11, respectively, which are engaged on the external side by the flat end walls 5 and 6 and have towards the interior of the silencer a short tubular piece surrounding the inlet and outlet pipes 7 and 8, respectively. With the design of the lining illustrated and described above, the external cover 2 may be provided by direct spraying and/or bandaging of glass, steel, plastics or coal fibre-reinforced cement or concrete on the external side of the cylindrical wall 1 of the lining. Using this form of application of the external - cover 2, there will in connection with the above mentioned conical bodies 10 and 11 be obtained a

sealing between the external cover 2 and the flat end walls 5 and 6, which is fully sufficient in practice, since the edge portions 12 and 13 of the conical wall parts 3 and 4 of the lining will form stops for the sprayed material during application of the external cover 2 - and will thereby function as gaskets between the end edges of the cover 2 and the flat end walls 5 and 6. Thus, no need will exist for separate and rather expensive gasket members such as graphite gaskets between the wall 1, 2 of the silencer and the flat end walls 5 and 6. However, a ceramic adhesive 14 may possibly be applied between the opposing sides of the end walls 5 and 6 and the bodies 10 and 11 as well as between the latter and the inlet and outlet pipes 7 and 8.

For a silencer as shown in Fig. 1 having a diameter of 150 ms and a length of 300 ms, the thickness of the cylindrical part 1 of the lining may be 10 mms e.g. and of the external cover 2 about 4 mms, whereas the conical wall parts 3 and 4 may have an apical angle of e.g. 90°.

In addition, in order to obtain a further improved strength and resistance towards dynamic influences, a covering la of a vibration and flow resisting material may be applied to the internal side of the lining and on the wall parts 3 and 4.

This covering may be applied with a relatively small thickness prior to the assembling of the cylin¬ drical wall 1 of the lining with the conical bodies 10 and 11 and the end walls 5 and 6 from a ^' liquid slurry of aluminium silicate compounds or by application of a fibre reinforced cement containing e.g. alumunium s-ilicate fibres.

In the embodiment shown in Fig. 2, the conical wall parts 16 and. 17 of the lining 15 are formed by double-conical solid bodies 18 and 19. Thereby, the external cover 20 may be applied not only to the

cylindrical part 15 of the. lining, but also on the external conical sides 21 and 22 of the bodies 18 and 19, so as to avoid the arrangement by welding of the relatively large flat end walls 5 and 6 of stainless steel shown- in Fig. 1. Since the external conical sides 21 ^■ and 22 may be formed in the same way as the internal conical wall parts 16 and 17 with short tubular pieces, an excellent securing on the inlet and outlet pipes 23 and 24 may be obtained by means of caps 25 and 26 of stainless steel secured on to the tubular pieces by clips 27. The con¬ siderable axial length of the double-conical bodies 18 and 19 provides in itself an excellent sealing.

In Fig. 2, a covering corresponding to the cover- ing la of flow resistant material in Fig. 1 is desig¬ nated with - 15a.-

Also in Fig. 2, a layer of a ceramic adhesive between the ^" external cover 20 and the bodies 18 and 19 as well as between the latter and the inlet and outlet pipes 23 and 24 is indicated by a thick line at 14 in Fig-. 2.

The invention is not limited to the embodiments illustrated in the drawings, since the manufacture of the lining by vacuum forming implies a considerable degree of freedom with respect to the geometrical shape of the silencer. Thus, as already mentioned, the inter¬ nal lining may also be utilized in connection with a conventional metallic external ^' wall.

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