WO2001069051A1

WO2001069051A1 - A gas-flow silencer

Info

Publication number: WO2001069051A1
Application number: PCT/IB2001/000373
Authority: WO
Inventors: Dennis Kappetijn; Louis Jacobus Heyns Du Plessis
Original assignee: Coetzee, Magdalena, Petronella
Priority date: 2000-03-17
Filing date: 2001-03-15
Publication date: 2001-09-20
Also published as: AU2001240973A1

Abstract

A gas-flow silencer (14) is provided which includes an elongate rigid conduit (12) and a sound absorbing body (14) substantially covering at least a major portion of an interior surface (16) of the conduit (12) and defining a gas-flow passage (20) which is spaced from the conduit (12) by the sound absorbing body (14).

Description

A GAS-FLOW SILENCER

THIS INVENTION relates to gas-flow silencers or exhaust apparatus.

In particular_^ it relates to a gas-flow silencer and to a method of manufacturing

a gas-flow silencer, to a combination of an internal combustion engine and said

gas-flow silencer, and to a method of fitting said gas-flow silencer to a vehicle.

According to one aspect of the invention, there is provided a gas-

flow silencer which includes

an elongate rigid conduit; and

a sound absorbing body substantially covering at least a major

portion of an interior surface of the conduit and defining a gas-flow passage

which is spaced from the conduit by the sound absorbing body.

The conduit may be cylindrical. Typically, the conduit is circular

^•cylindrical and the gas-flow passage is coaxial with the conduit. The conduit is

typically a mild steel or stainless steel pipe. Preferably, the conduit has constant

transverse dimensions, e.g . a constant diameter, over substantially its entire

length. The sound absorbing body may include a sound absorbing material

which is resistant to heat and fatigue from vibration and resistant to corrosion and

chemicals which may be present in gas flowing through the silencer. Examples

of suitable sound absorbing material include glass wool, mineral wool, steel wool

and rovings, woven rovings and stitched rovings of glass and mineral or steel strands.

The sound absorbing material of the sound absorbing body may have

a density of between about 60 git and about 1000 git, typically between about

1 00 gi t and about 500 git , e.g . about 300 git .

The sound absorbing body may include a composite of two or more

materials, at least one of which is sound absorbing .

In one embodiment of the invention, the sound absorbing body comprises a woven body of wire and fibre glass rovings. Such a body is fairly

rigid and cohesive.

In another embodiment of the invention, the sound absorbing body

comprises braided bands of a sound absorbing material and a more rigid

supporting material.

In a further embodiment of the invention, the sound absorbing body

comprises a tubular base with fibres or pile projecting radially inwardly or radially outwardly from the base. The base may be woven and may include a stiffening material, e.g . steel wire.

In yet another embodiment of the invention, the sound absorbing

body comprises an outer layer of fibrous sound absorbing material against the

interior surface of the conduit with an inner layer of a perforated continuous

material, such as an open weave woven roving glass fibre tube, defining the gas-

flow passage. The glass-fibre tube may have a basis weight of about 600 g/m

to about 1 200 g/m².

The sound absorbing body may thus include an outer layer of sound

absorbing material adjacent the interior surface of the conduit and an inner sound

permeable layer. The inner and outer layers may be in strip form and wound in

a helix, with adjacent windings of the inner layer overlapping each other and

adjacent windings of the outer layer abutting each other. The outer layer may be

of a non-woven material and the inner layer may be of a woven material. The

outer layer is typically thicker than the inner layer.

The sound absorbing body may include a hollow or tubular core supporting sound absorbing material.

The core, when present, typically has sufficient acoustic permeability

to allow for sound wave movement into the sound absorbing body and has

enough rigidity to support the sound absorbing material in position and to maintain the definition of the gas-flow passage. Preferably, the core has

sufficient flexibility to allow the rigid conduit to be bent without collapse of the

core and resultant restriction of the gas-flow passage. The core is preferably also

of a material which is resistant to heat, corrosion and chemicals which may be

present in gas flowing through the silencer.

The hollow core may be selected from the group consisting of a

tubular wire mesh, a thin-walled perforated tube, a helical coil, and a helical coil

covered by a tubular foraminous material. The mesh, tube, coil, or the like of the

hollow core may be of stainless steel.

In a preferred embodiment of the invention, the hollow core is in the

form of a helical steel wire coil.

A ratio between the cross-sectional area of the volume occupied by

the sound absorbing body and the cross-sectional area of the gas-flow passage

may be between about 1 00: 1 and about 0.1 : 1 . Preferably the ratio between the

cross-sectional area of the volume occupied by the sound absorbing body and the

cross-sectional area of the gas-flow passage is between about 3: 1 and about

0,2: 1 , e.g . about 0,8: 1 .

The core, when in the form of an apertured tubular body, may have

a void ratio of between about 0.4 and about 8. Typically, the apertured tubular body, when present, has a void ratio of between about 2 and about 6, e.g . about

2,5.

The silencer may include noise reducing or modifying elements, e.g .

a venturi tail piece, a baffle tail piece, or the like, in addition to the sound

absorbing body. Such a tail piece may have substantially the same outer diameter

as the rigid conduit, and may be fitted to the conduit by means of grub screws, welding, press fitting, or the like.

The rigid conduit may include more than one rigid conduit portion,

the portions each being attached to an adjacent portion in end-to-end relationship,

e.g . by welding or with clamps.

The silencer may include a retaining formation on at least one end

of the conduit retaining the sound absorbing body in position in the conduit.

Typically, the silencer includes a retaining formation at each end of the rigid

conduit or at each end of each rigid conduit portion, as the case may be.

Each retaining formation may project into the conduit and may define

a central gas-flow passage. Each retaining element may be in the form of a

roughly frusto-conical or bell-shaped body defining a central gas flow passage,

and may be attached to the rigid conduit or the rigid conduit portion, as the case

may be, by means of welding or press fitting . The retaining elements retain the sound absorbing body in position in the rigid conduit and inhibit loss of material from the sound absorbing body.

The silencer may include a flange at one end of the rigid conduit for

attaching to an object. The object may be an internal-combustion engine, and the flange may be configured to attach the silencer of the invention to an exhaust

manifold of the engine.

The silencer may also include mountings for fitting the silencer to a motor vehicle structure in conventional manner.

According to another aspect of the invention, there is provided an

internal-combustion engine and gas-flow silencer combination, which includes a

gas-flow silencer as hereinbefore described for conducting and silencing exhaust

gas produced by the internal combustion engine.

The volume occupied by the sound absorbing body of the gas-flow

silencer may be between about 0,3 and about 25 times the displacement volume

of the internal combustion engine. Typically, the volume occupied by the sound absorbing body of the gas-flow silencer is between about 1 and about 9,5 times

the displacement volume of the internal combustion engine, e.g. about twice the

engine displacement volume. The diameter of the gas-flow passage may be the diameter required

to obtain the desired back pressure prescribed for the internal combustion engine.

On larger engine sizes, e.g. above 2000 cm , or when required for any reason,

the gas-flow silencer may be multiplexed, i.e. it may be in the form of two or

more co-running pipes, using appropriate Y-fittings or multiplexers, in order to reduce back pressure or improve silencing.

According to a further aspect of the invention, there is provided a

method of fitting a gas-flow silencer to a vehicle, the method including

bending a gas-flow silencer as hereinbefore described to fit in

a prescribed vehicle exhaust system path; locating the bent gas-flow silencer in the prescribed vehicle

exhaust system path; and attaching the bent gas-flow silencer to the vehicle.

The bending of the gas-flow silencer may be manually effected .

Instead, more preferably, the bending is effected by a CNC (computer numerical

control) bending apparatus. The CNC bending apparatus may include a database

of bending profiles for a plurality of vehicle models. Thus, bending the gas-flow silencer may include loading an initially linear gas-flow silencer in a computer

numerically controlled (CNC) bending machine set up with an appropriate bending

profile selected from a database of bending profiles, and bending the linear gas-

flow silencer with the CNC bending machine. The fitting method may include splitting the linear gas-flow silencer

into two or more linear gas-flow silencer portions prior to bending at least one of

the linear gas-flow silencer portions, and then attaching the gas-flow silencer

portions to the vehicle and to each other, or first to each other and then to the

vehicle.

The method may include attaching mountings to the rigid conduit of

the gas-flow silencer from which the rigid conduit can be suspended or which can

be used to attach the rigid conduit to the vehicle. The mountings may be welded

to the rigid conduit.

Locations at which the mountings are to be attached to the rigid

conduit, and/or the location or locations at which the linear gas-flow silencer must

be split may be included in the database for a plurality of vehicle models.

The method may include attaching a noise reducing element, e.g . as

hereinbefore described, to the rigid conduit.

According to yet another aspect of the invention, there is provided

a method of manufacturing a gas-flow silencer, the method including

forming an elongate sound absorbing body with . a passage

extending through the body and opening out in the ends of the body; and

inserting the elongate body into an elongate rigid conduit. Forming the elongate sound absorbing body may include applying a

sound absorbing material over a hollow core which defines the passage.

The method may include forming the hollow core prior to applying

the sound absorbing material over the hollow core. Forming the hollow core may

include winding an elongate thiή Tlexible^~element around a mandrel to form a

helically wound core.

The helically wound core may be resilient. The method may include

initially preventing the helically wound coil from expanding to a diameter larger

than the diameter of the mandrel, but allowing the helically wound coil to expand

once the sound absorbing body has been inserted into the elongate rigid conduit.

Preventing the helically wound coil from expanding may include

wrapping the sound absorbing body in a constraining material which constrains

the sound absorbing body. Allowing the helically wound coil to expand once the

sound absorbing body has been inserted into the elongate rigid conduit may

include treating the constraining material inside the elongate rigid conduit to

release the sound absorbing body.

The constraining material may be a synthetic plastics or polymeric

material, and treating the constraining material may include heating the

constraining material to weaken or destroy it. Applying the sound absorbing material may include winding a strip which includes the sound absorbing material over the hollow core, e.g. in the

same direction as the elongate thin flexible element is wound around the mandrel.

The strip may include a length of a sound permeable material with

a layer of the sound absorbing material on one side of the length of the sound

permeable material. The length of sound permeable material is typically thinner than the layer of the sound absorbing material, but wider than the layer of the

sound absorbing material, the strip being wound over the core in a helix, with

adjacent windings of the length of the sound permeable material overlapping each

other and adjacent windings of the layer of the sound absorbing material abutting each other.

The invention will now be described, by way of example, with

reference to the accompanying diagrammatic drawings in which

Figure 1 shows a longitudinally sectioned view of a portion of an elongate

gas-flow silencer in accordance with the invention;

Figure 2 shows a cross section of the gas-flow silencer of Figure 1 ;

Figure 3 shows a cross section of another embodiment of a gas-flow

silencer in accordance with the invention;

Figure 4 shows a partial longitudinally sectioned view of an end of the gas-

flow silencer of Figure 1

Figure 5 shows a partial longitudinally sectioned view of the gas-flow

silencer of Figure 1 where two rigid conduit portions are attached to each other; Figure 6 shows a partial longitudinally sectioned view of another end of the gas-flow silencer of Figure 1 ;

Figure 7 shows a longitudinally sectioned view of an end portion of a gas-

flow silencer in accordance with the invention which includes a noise reducing

element in the form of a venturi tail piece;

Figure 8 shows a longitudinally sectioned view of an end portion of a gas-

flow silencer in accordance with the invention which includes a noise reducing

element in the form of a baffle tail piece; and

Figure 9 shows a longitudinally sectioned view of a portion of another embodiment of an elongate gas-flow silencer in accordance with the invention.

Referring to Figures 1 , 2 and 4 to 6 of the drawings, reference

numeral 1 0 generally indicates a gas-flow silencer in accordance with the

invention. The silencer 1 0 includes an elongate rigid conduit, in the form of a

mild or stainless steel pipe 1 2 comprising two pipe portions 1 2.1 and 1 2.2 (see

Figure 5) . The pipe 1 2 has an outside diameter of 57 mm and a wall thickness

of typically 1 ,6 mm to 2 mm. The pipe 1 2 is circular in cross section. The

silencer 1 0 further includes a sound absorbing body 1 4. The sound absorbing

body 1 4 includes a layer of glass wool with a bulk density of about 300 git .

The sound absorbing body 1 4 includes a hollow core, in the form of

a tubular wire mesh 1 8, also of mild or stainless steel. The tubular wire mesh 1 8

has a void ratio of about five. A gas-flow passage 20 is defined by the sound absorbing body 1 4.

The gas-flow passage 20 is co-axial with the pipe 1 2, is also circular in cross

section, and has a diameter of 40 mm. As will be appreciated, the ratio between

the cross-sectional area of the volume occupied by the sound absorbing body 1 4

and the cross-sectional area of the gas-flow passage 20 is thus about 0,75 : 1 .

Referring to Figure 4 of the drawings, the silencer 1 0 includes a

retaining formation 22 at one end of the pipe 1 2, which in use is a free end. The

retaining formation 22 retains the tubular wire mesh 1 8 and the sound absorbing

material in position in the pipe 1 2 and inhibits loss or damage of sound absorbing

material from the sound absorbing body 1 4 in use. The retaining formation 22

is in the form of a roughly frusto conical body having a curved outer surface, or

a roughly bell-shaped body having a truncated end, which defines a central flow

passage 24. The retaining formation 22 is welded to the pipe 1 2 along its outer

circumference at 26.

The retaining formation 22, instead of being welded to the pipe 1 2,

may be pressed from a thin section (typically 0,5 mm stainless steel) sheet metal

and expanded against the pipe 1 2. A custom-made expansion tool may be used

to fit the retaining formation 22 in position and to expand it.

The two pipe portions 1 2.1 and 1 2.2 are attached or joined to each

other by a clamp 28, as shown in Figure 5 of the drawings. Where the pipe

portions 1 2.1 and 1 2.2 are joined, a retaining formation 22 is provided in each pipe portion 1 2. 1 , 1 2.2. However, in one embodiment, the retaining formations

22 are not welded to the pipe portions 1 2.1 , 1 2.2. Instead, the retaining

formations 22 are welded to each other on their maximum diameter as indicated

at 27. In another embodiment, the retaining formations 22 are expanded, as

described above, into position, whereafter the pipe portions 1 2.1 and 1 2.2 are

joined to each other, e.g . by welding or by a clamp 28.

Referring to Figure 6 of the drawings, another ertd of the gas-flow

silencer 1 0 is shown. In addition to a retaining formation 22, the silencer 1 0 at

this end includes an apertured flange 30 welded to the pipe 1 2. The purpose of

the flange 30 is to facilitate location and attachment of the gas-flow silencer 1 0

to an engine exhaust manifold in conventional manner.

Referring to Figure 3 of the drawings, a cross section of another

embodiment of a gas-flow silencer in accordance with the invention, generally

indicated by reference numeral 1 1 0, is shown. The gas-flow silencer 1 1 0 is

similar to the gas-flow silencer 1 0, and unless otherwise indicated, the same

reference numerals are used to indicate the same or similar parts or features.

Unlike the gas-flow silencer 1 0, the silencer 1 1 0 does not include a

core, such as the tubular wire mesh 1 8. Instead, the sound absorbing body 1 4

comprises a woven body of wire and fibre glass roving . The sound absorbing

body 1 4 is thus fairly rigid and cohesive and does not require a core to define the passage 20 or to hold the sound absorbing material in position against the interior surface 1 6 of the pipe 1 2.

With reference to Figures 7 and 8 of the drawings, gas-flow silencers

in accordance with the invention which include different tail pieces, are generally

indicated by reference numerals 1 20 and 1 30 respectively. The silencers 1 20,

1 30 are similar to the silencer 1 0, and unless otherwise indicated, the same

reference numerals used hereinbefore are used to indicate the same or similar

parts or features.

The gas-flow silencer 1 20 includes a noise reducing . or modifying

element, in the form of a venturi tail piece 1 22. The venturi tail piece 1 22 is

clamped to the steel pipe 1 2 by means of a clamp 1 24.

The gas-flow silencer 1 30 includes a noise reducing or modifying

element, in the form of a baffle tail piece 1 32. The baffle tail piece 1 32 is

similarly clamped to the stainless steel pipe 1 2 by means of a clamp 1 34.

With reference to Figure 9 of the drawings, yet another embodiment of a gas-flow silencer in accordance with the invention is generally indicated by

reference numeral 1 40. The gas flow silencer 1 40 is similar to the silencer 1 0,

and unless otherwise indicated, the same reference numerals used hereinbefore

are used to indicate the same or similar parts or features. The hollow core of the sound-absorbing body 1 4 of the gas-flow silencer 1 40 is in the form of a helically wound steel wire core 142 with a pitch

of about 6 to 7 mm. In the embodiment shown in Figure 9 of the drawings, the

core 1 42 is wound from right to left across the paper and clockwise when seen

in end view from a right hand end of the gas flow silencer 140.

The sound absorbing body 1 4 comprises an outer layer 144 of a non-

woven glass fibre sound absorbing material adjacent the interior surface 1 6 of the

pipe 1 2, and an inner sound permeable layer 1 46 of a heavy woven glass fibre

material . The outer layer 1 44 has a thickness of about 6 to 9 mm and is thicker

than the inner layer 1 46. The outer layer 1 44 has a density of about 300 git as supplied, i.e. before being employed in the silencer 1 40. The inner layer has a

basis weight of about 800 g/m .

The sound absorbing body 1 4 is formed by first winding a length of

steel wire about a mandrel to form the helically wound steel wire coil 1 42. The

coil 1 42 is prevented from expanding away from the mandrel by restraining free ends of the coil 1 42 and the layers 1 44 and 1 46 are applied . This is achieved by

winding an elongate strip, which comprises the woven glass fibre material and the

non-woven glass fibre material, over the helically wound core 142, in the same

direction that the helically wound core 1 42 is wound . The strip thus comprises

two layers, one consisting of the non-woven glass fibre sound absorbing material

and the other of the woven glass fibre sound permeable material. The woven

glass fibre sound permeable material layer has a width of about 1 50 mm. The non-woven glass fibre sound absorbing layer has a width of about 1 00 mm and

one longitudinally extending edge thereof is in register with one longitudinally

extending edge of the woven glass fibre layer. The strip is thus wound such that

adjacent windings of the inner woven glass fibre sound permeable layer overlap

each other and adjacent widings of the outer layer of the non-woven glass fibre

sound absorbing material abut each other.

A thin layer of a synthetic plastics or polymeric material is wound

over the non-woven glass fibre sound absorbing layer 1 44 to constrain the sound

absorbing body 1 4 and thereby preventing the helically wound coil 142 from

expanding before the sound absorbing body 1 4 has been inserted into the steel

pipe 1 2. The sound absorbing body 1 4 is then removed from the mandrel as a

unit and inserted into the pipe 1 2 by sliding it into the pipe 1 2, whereafter heat

is applied to an external surface of the pipe 1 2, to weaken or destroy the

synthetic plastics or polymeric layer. The helically wound core 142 and also the

layers 1 44 and 1 46 are thus allowed to expand radially to fit snugly inside the

pipe 1 2 and to compress the non-woven sound absorbing outer layer 1 44

between the woven glass fibre inner layer 1 46 and the pipe 1 2.

The sound absorbing body 1 4 of the gas-flow silencer 1 40 has the

particular advantage that the gas-flow silencer 1 40 can be bent without adverse

effects. When being bent, the helically wound core 1 42 along an outer radius of

the bend is able to open up, whilst it compresses along an inner radius of the

bend . The windings of the woven glass fibre material layer 1 46 are able to move slightly relative to each other, bunching up on the interior radius of the bend and

moving slightly away from each other along the outer radius of the bend, whilst

still remaining overlapping . The gas-flow passage 20 is thus not restricted at the

bend and the non-woven fibre glass sound absorbing layer 1 44 is not directly

exposed to gas-flowing along the gas-flow passage 20.

The Applicant expects that the invention will not only find

application in the market for retrofitting vehicles with worn gas-flow silencers or

exhaust apparatus with new gas-flow silencers or apparatus, but also in the

original equipment market.

Advantages of the gas-flow silencer of the invention, as illustrated,

includes less raw material usage during manufacture, resulting in lower cost, good

mass manufacturability, low levels of labour and machine time required, a low

component count, the absence of complex parts, standardization (it is foreseen

that a range of about three different pipes sizes will accommodate most internal

combustion engine sizes) , and ease of manufacture, requiring equipment of low

complexity.

Further advantages of the gas-flow silencer of the invention, as

illustrated, includes low stock level requirements and less storage space

requirements as a result of the modularity of the silencer and the standardization

of raw material usage, acceptable sound damping characteristics, significantly

lower corrosion due to quick and even heating during use, and less susceptibility to water entrapment when installed on an internal combustion engine, cost and

weight reduction, lower engine backpressure providing opportunity for improved

engine thermal efficiency, low levels of noise transmission to a vehicle passenger

compartment due to the fact that the whole gas-flow silencer is inherently

damped, less chance of holing of the gas-flow silencer due to road debris and

other obstacles, in the light thereof that the gas-flow silencer does not include

mufflers, inherent thermal insulation, and no adverse affect on closed loop engine control systems.

The method of fitting a gas-flow silencer to a vehicle in accordance

with the invention may include advantages such as less tools, jigs and fixtures

required to install the system compared to conventional systems, quick bending,

assembly and installation of the gas-flow silencer according to customer

requirements, quick and accurate bending and assembly due to the use of CNC

equipment and a database catering for a large variety of vehicles, low labour and

machine time requirements, short turnaround times resulting in lower fitment

equipment requirements such as lifts, welding tools and hand tools, simple

material planning and control requirements, and easy maintenance of high levels

of quality assurance.

Claims

CLAIMS:

1 . A gas-flow silencer which includes

an elongate rigid conduit; and

a sound absorbing body substantially covering at least a major

portion of an interior surface of the conduit and defining a gas-flow passage

which is spaced from the conduit by the sound absorbing body.

2. A gas-flow silencer as claimed in claim 1*, in which the conduit is

circular cylindrical and the gas-flow passage is coaxial with the conduit.

3. A gas-flow silencer as claimed in claim 1 or claim 2, in which the

sound absorbing body includes a hollow core supporting sound absorbing

material.

4. A gas-flow silencer as claimed in any one of the preceding claims,

in which the hollow core is selected from the group consisting of a tubular wire mesh, a thin-walled perforated tube, a helical coil, and a helical coil covered by

a tubular foraminous material.

5. A gas-flow silencer as claimed in any one of claims 1 to 3 inclusive,

in which the hollow core is in the form of a helical steel wire coil.

6. A gas-flow silencer as claimed in any one of the preceding claims,

in which a ratio between the cross-sectional area of the volume occupied by the

sound absorbing body and the cross-sectional area of the gas-flow passage is

between 1 00: 1 and 0. 1 : 1 .

7. A gas-flow silencer as claimed in claim 6, in which the ratio between

the cross-sectional area of the volume occupied by the sound absorbing body and the cross-sectional area of the gas-flow passage is between 3: 1 and 0,2: 1 .

8. A gas-flow silencer as claimed in any one of the preceding claims,

in which the sound absorbing body includes an outer layer of sound absorbing

material adjacent the interior surface of the conduit and an inner sound permeable

layer.

9. A gas-flow silencer as claimed in claim 8, in which the inner and

outer layers are in strip form and wound in a helix, with adjacent windings of the

inner layer overlapping each other and adjacent windings of the outer layer

abutting each other.

1 0. A gas-flow silencer as claimed in claim 8 or claim 9, in which the

outer layer is of a non-woven material and the inner layer is of a woven material and in which the outer layer is thicker than the inner layer.

1 1 . A gas-flow silencer as claimed in any one of the preceding claims,

which includes a retaining formation on at least one end of the conduit retaining the sound absorbing body in position in the conduit.

1 2. A gas-flow silencer as claimed in claim 1 1 , in which the retaining

formation projects into the conduit and defines a central gas-flow passage.

1 3. An internal-combustion engine and gas-flow silencer combination,

which includes a gas-flow silencer as claimed in any one of the preceding claims

for conducting and silencing exhaust gas produced by the internal combustion

engine.

1 4. An internal-combustion engine and gas-flow silencer combination as

claimed in claim 1 3, in which the volume occupied by the sound absorbing body

of the gas-flow silencer is between 0,3 and 25 times the displacement volume of

the internal combustion engine.

1 5. An internal-combustion engine and gas-flow silencer combination as

claimed in claim 1 4, in which the volume occupied by the sound absorbing body

of the gas-flow silencer is between 1 and 9,5 times the displacement volume of

the internal combustion engine.

1 6. A method of fitting a gas-flow silencer to a vehicle, the method

including bending a gas-flow silencer as claimed in any one of claims 1

to 1 2 inclusive to fit in a prescribed vehicle exhaust system path;

locating the bent gas-flow silencer in the prescribed vehicle exhaust system path; and

attaching the bent gas-flow silencer to the vehicle.

1 7. A method as claimed in claim 1 6, in which bending the gas-flow silencer includes loading an initially linear gas-flow silencer in a computer

numerically controlled (CNC) bending machine set up with an appropriate bending profile selected from a database of bending profiles, and bending the linear gas-

flow silencer with the CNC bending machine.

1 8. A method of manufacturing a gas-flow silencer, the method including

forming an elongate sound absorbing body with a passage

extending through the body and opening out in the ends of the body; and inserting the elongate body into an elongate rigid conduit.

1 9. A method as claimed in claim 1 8, in which forming the elongate

sound absorbing body includes applying a sound absorbing material over a hollow

core which defines the passage.

20. A method as claimed in claim 1 9, which includes forming the hollow

core prior to applying the sound absorbing material over the hollow core, forming the hollow core including winding an elongate thin flexible element around a mandrel to form a helically wound core.

21 . A method as claimed in claim 20, in which the helically wound core

is resilient, the method including initially preventing the helically wound coil from expanding to a diameter larger than the diameter of the mandrel, but allowing the

helically wound coil to expand once the sound absorbing body has been inserted

into the elongate rigid conduit.

22. A method as claimed in claim 21 , in which preventing the helically

wound coil from expanding includes wrapping the sound absorbing body in a

constraining material which constrains the sound absorbing body, and in which

allowing the helically wound coil to expand once the sound absorbing body has

been inserted into the elongate rigid conduit includes treating the constraining

material inside the elongate rigid conduit to release the sound absorbing body.

23. A method as claimed in claim 22, in which the constraining material

is a synthetic plastics or polymeric material, and in which treating the constraining

material includes heating the constraining material to weaken or destroy it.

24. A method as claimed in any one of claims 1 9 to 23 inclusive, in

which applying the sound absorbing material includes winding a strip which

includes the sound absorbing material over the hollow core.

25. A method as claimed in any one of claims 20 to 23 inclusive, in which applying the sound absorbing material includes winding a strip which

includes the sound absorbing material over the helically wound core in the same

direction as the elongate thin flexible element is wound around the mandrel.

26. A method as claimed in claim 24 or claim 25, in which the strip

includes a length of a sound permeable material with a layer of the sound

absorbing material on one side of the length of the sound permeable material.

27. A method as claimed in claim 26, in which the length of the sound

permeable material is thinner than the layer of the sound absorbing material, but

wider than the layer of the sound absorbing material, the strip being wound over

the core in a helix, with adjacent windings of the length of the sound permeable material overlapping each other and adjacent windings of the layer of the sound

absorbing material abutting each other.

28. A gas-flow silencer as claimed in claim 1 , substantially as herein

described and illustrated .

29. An internal-combustion engine and gas-flow silencer combination as

claimed in claim 1 3, substantially as herein described and illustrated.

30. A method as claimed in claim 1 6 or claim 1 8, substantially as herein

described and illustrated .

31 . A new gas-flow silencer, a new inter-combustion engine and gas-

flow silencer combustion, or a new method of fitting or manufacturing a gas-flow

silencer, substantially as herein described.