The present invention relates to a sound absorption system intended particularly for combustion engines and especially for use on automobiles.

For the suppression of pulsations and insulation against noise in exhaust systems, for example for combustion engines, the use is known of sound absor¬ bers or silencers of various designs to reduce the otherwise necessary length of the exhaust pipe. Common to them all is that in order to insulate against the noise produced by the engine, they are equipped with guideways, baffle plates or sound-ab¬ sorbing plates with the object of preventing noise and sound pressure from passing out to the surround¬ ings, in that they must be designed while taking into account that the resistance against the passage of the exhaust gases must not be too great.

The construction of conventional sound absorbing systems is complicated and therefore expensive. At the same time, the conventional sound absorbing sys- terns, especially for automobiles, are greatly expos¬ ed to corrosion, both externally due to the use of thawing agents for roads, and internally because of aggressive components in the exhaust gases, espec¬ ially in connection with moisture.

Surprisingly, it has now proved that one can achieve a very strong sound-insulation and at the same time an expedient heat-insulation of an exhaust system by insulating, according to the invention, at least a

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part of the system with a muff of firmly-pressed mineral wool.

By providing at least a part of the exhaust system with such a muff of mineral wool, it becomes pos¬ sible to considerably reduce the system's dimen¬ sions and, moreover, there is also achieved hereby a heat-insulation which reduces the tendency towards internal corrosion in the exhaust system, the reason being that this gets hotter, whereby the aqueous va¬ pour formed by a combustion engine is led completely out of the system without being condensed, which otherwise gives rise to accelerated corrosion.

When the muff of firmly-pressed mineral wool is pro¬ vided with a protection casing as presented in claim 2, stabilization ^" of the mineral wool muff is achiev¬ ed and at the same time a protection against exter¬ nal influences, especially against moisture and the effects of impact where automobiles are concerned, and thus the advantageous characteristics of the mineral wool muff are safeguarded and improved.

It is known from US-PS 3,233,699 to provide an ex- haust system on a car with a protection casing which is provided internally with soft and flexible min¬ eral wool for the heat-insulation of the exhaust system, to protect it against external influences and to achieve a further sound absorption of a con- ventional exhaust system with a conventional silen¬ cer.

In. accordance with the present invention, a muff of firmly-pressed mineral wool is used, and surprising-

ly it hereby becomes possible to suppress exhaust noise, particularly from combustion engines which are very difficult to insulate against noise, by means of very simple and compact silencers in the exhaust system, in that use can be made of silenc¬ ers without guideways, baffle plates, sound-absor¬ bing plates or other sound pressure reducing ar¬ rangements and of small dimensions, whereby the silencers can be greatly standardized in relation to the silencers known at present. At the same time, also achieved hereby is that the whole of the ex¬ haust system becomes more simple, less expensive, more efficient and of far greater durability than the. known exhaust systems.

By making the exhaust system and casisg of metal, as presented in claim 3, a system which is very stable in form is achieved. Moreover, due to the heat capacity in the wall of the exhaust system it- self, a particularly desirable and beneficial in¬ fluence on the lifetime of the system is achieved, in that the exhaust system becomes relatively hot¬ ter than a conventional system because of the heat insulation, and also remains hot for a longer time after stopping due to the amount of heat which is contained by the system. Therefore, even after stop¬ ping of the engine, to a great degree the aqueous vapour is still actively conveyed out of the sys¬ tem, hereby reducing the risk of condensation and thus corrosion. The lifetime of the whole of the sound absorption system is hereby extended.

When provided with a silencer as presented in claim 4, the sound absorption system becomes more compact.

Compared-with conventional silencers, a silencer which is provided with a muff of firmly-pressed min¬ eral wool can be executed in considerably smaller dimensions and with a much more simple construction.

A particularly simple method of making a silencer for a sound absorption system according to the in¬ vention is as presented in claim 5. By using simple, standard components, the possibility is thus pro- vided of constructing a very effective, durable and compact sound absorption system. In its simplest embodiment, the system's exhaust pipe is merely led into a silencer, where it ends, and from where a continuation of the pipe extends, preferably offset in the silencer, so that the sound pressure cannot pass straight through the silencer.

A preferred embodiment of the sound absorption sys¬ tem is as presented in claim 6. This embodiment is very compact and simple and is particularly exped¬ ient for smaller automobiles, where there is re¬ stricted free height under the car. The number and the size of the holes in the wall of the pipe in that part of the exhaust pipe which lies in the si- lencer is adapted to provide a suitable resistance to the flow of the exhaust gas.

A second preferred embodiment of the sound absorp¬ tion system is as presented in claim 7. This embod- iment is especially suitable for use in connection with slightly larger automobiles or engines, in that the silencer in this embodiment is given a short building-in length.

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By forming the sound absorption system as presented in claim 8, it can be used in a simple manner for, for example, large V-type engines, which typically have an exhaust pipe on each side of the engine. It is similarly possible, for sound absorption systems for engines which have a high performance, to form that part of the exhaust pipe which leads from the engine as several pipes, whereby one can lower the resistance to through-flow and at the same time re- duce the risk of condensation of water in the sys¬ tem's extreme end, the reason being the better pos¬ sibilities of leading away the aqueous vapour.

The sound absorption system according to the inven- tion can be produced in a particularly simple manner from quite simple component parts. To the extent that it is desirable to insulate the exhaust pipe¬ lines, one can insert firmly-pressed and tight-fit¬ ting (form-pressed) mineral wool sleeves with pro- tection casings on the pipes. By using short leng¬ ths of mineral wool sleeving with bevelled end sur¬ faces, by the turning of the sleeves the mineral wool muffs can be fitted so that they close tightly together, also at the bends. For example, straight mineral wool sleeves in lengths of 25-30 cm or more can be used, while the sleeves used at the bends are preferably shorter. Both the short and the long sleeves can be bevelled. After assembly, the joints between the sleeves can be sealed or left as they are.

A silencer unit for use in the system can be formed, for example, from a pipe with a larger diameter than the actual exhaust pipe, and two end walls each pro-

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vided with a lead-in for the exhaust pipe. The end walls are fixed by welding to the slightly larger pipe, and the inlet and the outlet pipes are led through the end walls and welded. A mineral wool muff is then disposed on the outside as described above. If that part of the exhaust pipe which lies in the actual silencer is required to be provided with holes, this is effected before the assembly, and that end of the pipe with holes is closed with a plate. The pipes are inserted through the firmly -welded end walls until they abut against the other end wall, to which they are secured by welding through a small hole in this, and the lead-in thro¬ ugh the end walls is welded. The insulation of the silencer is expediently effected with two form -pressed end walls and a similarly form-pressed sleeve of firmly-pressed mineral wool.

The mineral wool muff which comprises the outside of at least a part of the sound absorption system ac¬ cording to the invention has a thickness of from approximately 10 mm to approximately 50- mm, in that a certain thickness is necessary to achieve the de¬ sired sound and heat insulation, while at the same time there is often limited space, for example when building into automobiles. Very good results can be achieved by using mineral wool thicknesses of, for example, 30 mm.

The outer protection casing is expediently made of metal which is painted, or possibly of armoured plastic material. When using steel plate, it is expedient to utilize plate thicknesses of 0.3 to 1.2 mm, appropriately approximately 0.6 mm.

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The pipes themselves and possibly the silencer in the sound absorption system according to the inven¬ tion are executed in metal such as iron, prefer¬ ably steel. For the pipe connections, pipe with a wall thickness of 1-2 mm is used, preferably ap¬ proximately 1.5 mm, whereby a good heat capacity is achieved and yet at a reasonable price. The holes in that part of the pipes which extends into the silencer have a diameter of 1-3 mm, preferably ap- proximately 2 mm, and the number of holes is adap¬ ted to provide a resistance to flow which is suit¬ able, for the engine with which the sound absorption system is to be used. When used for automobiles, the pipes are 10-50 mm, all depending on the des- ired capacity, but considerably larger pipe dimen¬ sions are foreseen for use with larger engines, whether these are stationary links in, for example, ^• a power supply or the like, c-r mounted in a loco¬ motive or ship. For automobiles, pipe dimensions of 25-35 mm are preferred. Sound absorption systems of the kind discussed here can also be used for motor cycles, scooters and mopeds.

A silencer is produced from a pipe and two end walls, each provided with a lead-in for the exhaust pipe. For private automobiles, for example, cir¬ cular pipes with a diameter of 50-150 mm are used, or pipes with a'n oval cross-section with a height of 50-100 mm and a breadth of 80-200 mm. For larger vehicles, for example trucks or buses, locomotives or marine engines, it will be typical to use larger pipe dimensions than those stated above for private cars, while for motor cycles, scooters and mopeds, for example, smaller dimensions can be used. The

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silencers, made of metal, for example iron, prefer¬ ably steel, with a wall thickness of 0.5-3 mm, preferably 1.5-2 mm.

For reasons of its great sound-insulating capacity, the sound absorption system is particularly suitable for use in connection with combustion engines, es¬ pecially internal combustion engines where there are particularly difficult suppression conditions, but the special advantages achieved with the sound ab¬ sorption system make it equally usable in connec¬ tion with, for example, turbines or ventilation units, where one has a need for the suppression of noise and insulation against noise from ventilators etc., and also resonances in the system.

In the production of silencers where the greatest demands are placed on their use, it is preferable to use stainless steel as. the material for the pipe system, and as firmly-pressed mineral wool the use is preferred of Rockwool fire-batts, Rockwool mar- ine-batts or Rockwool No. 2 pipe shells, which have proved to have particularly good characteristics for the purpose according to the invention.

The invention is described in more detail with ref¬ erence to the drawing, where

fig. 1 shows a longitudinal section through an embodiment of a silencer for a sound absorption system according to the invention,

fig. 2 is a longitudinal section through a

second embodiment of a silencer,

fig. 3 is a longitudinal section through a third embodiment of a silencer, seen from above,

fig. 4 is a section along the line IV-IV in fig. 3, and

fig. 5 is a section along the line V-V in fig. 3.

In fig. 1 is shown a simple form of silencer for a sound absorption system according to the invention. The firmly-pressed mineral wool muff 2 is placed on a pipe 3 and provided with a protection casing 4. The _v end walls 8 are provided with mineral wool end pieces 9 and lead-ins for the exhaust pipe 1. This pipe 1 is led through the end wall and terminates in the chamber in the pipe 3, from where a contin¬ uation 1u leads out. 1u is preferably displaced axially in relation to 1i.

In fig. 2 is shown an embodiment of a silencer for the sound absorption system for use with small building-in breadths. The exhaust pipe 1 is led through the end walls 8 in a cylindrical pipe 3. The pipe 1 is blocked off at the middle with a wall 6, and that part of the pipe 1 which extends into the pipe 3 is provided with holes 7. The pipe 3 is covered with firmly-pressed mineral wool 2, which is provided with a protection casing 4.

The embodiment of the silencer for the sound absorp-

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tion system shown in figs. 3-5 has a pipe 3 and end walls 8 of oval cross-section. The exhaust pipe's inlet part 1i is led through the one end and welded firmly to same, and the other end is welded to the other end wall. There are holes 7 in that part of the pipe which extends into the silencer. That part of the exhaust pipe 1u which leads out of the sil¬ encer is executed in a manner corresponding to 1i, and is mounted in an axially displaced position herefrom.

Practical testing of the sound absorption system.

Example 1. A silencer of the form shown in fig. 2 was made of a 25 mm steel pipe with a wall thickness of 1.5 mm (pipe 1 ^" ) , a 60 mm steel pipe with a wall thickness of 2 mm (pipe 3) , a firmly-pressed mineral wool muff of 30 mm Rockwool No. 2 pipe shell, and a pro- tection casing of 0.6 mm painted body-plate. The total length was 660 mm. It was tested on a Morris Marina 1300 for more than two years, after which there was no sign of internal corrosion, which must be attributed to the higher operating temperature and slow cooling, which reduces the condensation of aqueous vapour in the silencer. The silencer sup¬ pressed at least just as much as a conventional si¬ lencer.

Example 2. ^'

A silencer of the form shown in figs. 3-5 was made of a 32 mm steel pipe (pipe 1) with a wall thick¬ ness of 1.5 mm, an oval steel pipe with a breadth of 130 mm, a height of 60 mm and a wall thickness

of 2 mm, a pressed mineral wool muff of 30 mm cut out of Rockwool marine-batts, and a protection cas¬ ing of 0.6 mm painted body-plate. The length of the inner pipe 3 was 300 mm (total length 360 mm) .

Three such silencers were mounted on a Ford Granada V6, two in front and an outlet silencer which was modified so that it had two inlet pipes and an out¬ let pipe. The results achieved corresponded to those of example 1.

Example 3.

A silencer was made as in example 2, except that it was made 100 mm longer, i.e. with a total length of 460 mm. Such a silencer was mounted on a Peugeot

404 diesel, where there are normally .two silencers. With regard to corrosion, the results achieved here also corresponded to those of examp ^' le 1, and even with one silencer the noise suppression was at least just as good as with Peugeot's original equip¬ ment.

Even after a drive of 100 km, the silencers tested in examples 1-3 had such a low surface temperature that it was immediately possible to touch them with an unprotected hand, without them feeling hot. This low temperature on the outside of the sound absorption system also contributes towards the ex¬ tension of lifetime, the reason being that a cas- ing of steel does not get so hot that the protect¬ ive paint is "burned" off, nor a casing of plastic ruined by the heat.

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