GB2479787A

GB2479787A - Exhaust silencer with acoustic reducer cup

Info

Publication number: GB2479787A
Application number: GB1006811A
Authority: GB
Inventors: Peter Jarvis
Original assignee: EXHAUSTS UK Ltd
Current assignee: EXHAUSTS UK Ltd
Priority date: 2010-04-23
Filing date: 2010-04-23
Publication date: 2011-10-26
Also published as: GB201006811D0

Abstract

An exhaust silencer 1 with a silencer chamber 10 having an exhaust inlet 20 and an acoustic reducer 30 within the chamber, where the acoustic reducer is in the form of a cup and has a cup mouth 32, a cup base 34, and at least one cup wall 36 extending between the cup mouth and cup base, the cup wall(s) comprise a narrowed portion 38 that is narrower than the cup mouth. The cup may cup incoming exhaust gases and reflect incoming sound waves so that outgoing sound waves destructively interfere with incoming sound waves, thus providing noise attenuation. Preferably the cup mouth is flared outwardly and the exhaust inlet is facing the cup mouth. An acoustic reducer and a method of modifying an exhaust gas silencer is also claimed.

Description

Exhaust silencer

Introduction

The present invention relates to an exhaust silencer, particularly an exhaust silencer for a motor vehicle. The invention also relates to an acoustic reducer for an exhaust silencer, a method of modifying an exhaust silencer, and a motor vehicle comprising the exhaust silencer.

Description of the Background Art

Exhaust gases produced by an internal combustion engine of a motor vehicle flow through an exhaust system before being emitted into the atmosphere. Such exhaust gases tend to carry along with them acoustically displeasing sound waves produced by the engine. If not dealt with inside the exhaust system, these sound waves are also emitted from the exhaust system, often producing undesirable noise of a high amplitude. There is therefore a widespread need for exhaust silencers which allow the overall sound emitted from an exhaust system to be reduced.

These problems are in part solved by absorption silencers, whose walls generally comprise fibrous or porous sound-absorbing materials. Sound waves propagate within passages or pores of the sound-absorbing materials and are converted into heat as a result of friction arising as gas particles oscillate against the sound-absorbing materials within the pores. As such, sound is converted to heat, and noise is thereby attenuated. A problem with these silencers is that sound reduction is usually inadequate.

Reactive silencers generally provide for much greater noise reduction than absorption silencers owing to the plurality of various pipe segments and chambers throughout the silencer. Sound waves are thereby reflected within the reactive silencer to prevent some of the sound waves being transmitted beyond the silencer. A problem inherent with reactive silencers is backpressure caused by the silencer's resistance to the flow of exhaust gases.

This leads to increased fuel consumption and poor power transmission. Another problem with reactive silencers is that they are complex to manufacture in view of the plurality of components.

An object of the present invention is to solve at least one of the problems of the prior art.

Another object is to provide an exhaust silencer with better noise attenuation which does not suffer from significantly increased backpressure.

Summary of the Invention

According to a first aspect of the present invention there is provided an exhaust silencer comprising a silencer chamber, wherein the silencer chamber comprises: an exhaust inlet for channelling exhaust gases into the silencer chamber; an acoustic reducer arranged within the silencer chamber to form a cup having a cup mouth, a cup base, and a cup wall(s) extending between the cup mouth and the cup base; wherein the cup wall(s) comprises a narrowed portion that is narrower than the cup mouth.

The present invention provides silencers which are simple to manufacture, and give excellent noise attenuation without causing increased exhaust flow resistance and backpressure. Moreover, current silencers can be easily adapted/modified by simply installing the acoustic reducer.

The cup mouth is preferably defined by a mouth rim, wherein the cup mouth is the empty space within and on a plane defined by the mouth rim. Where the mouth rim is not entirely in the same plane, the plane of the cup mouth may be defined by an average of all the mouth rim points.

The cup formed by the acoustic reducer is preferably arranged to receive the exhaust gases via the cup mouth. Preferably the exhaust gases can exit the acoustic reducer via the mouth, but preferably only after having first entered via the mouth.

The narrowed portion may comprise a neck portion or, where the acoustic reducer has a cup mouth flared outwardly, the narrowed portion may comprise a portion of the acoustic reducer between the flared cup mouth and the cup base. The narrowed portion may comprise both of these.

The cup base is preferably reflective to sound waves. Preferably the cup base reflects at least 80% of sound waves, more preferably at least 90% of sound waves, most preferably 95% of sound waves. Preferably the cup base is arranged to reflect sound waves back into the path of incoming sound waves (i.e. back from whence they came). This allows for destructive interference between outgoing and incoming sound waves, which in turn attenuates noise. Preferably the cup base does not deflect sound waves away from incoming sound waves, and preferably does not deflect incoming sound waves towards walls of the silencer chamber.

The acoustic reducer may be welded within the silencer chamber, preferably to an internal wall of the silencer chamber. The acoustic reducer may be removably fixed within the silencer chamber. The acoustic reducer may be integral to the silencer, but is preferably not integral. Most preferably the acoustic reducer is a separate part to the silencer.

The acoustic reducer may be comprised of stainless steel, mild steel, aluminium or titanium. Preferably the acoustic reducer comprises the same material as the internal wall(s) of the silencer chamber, most preferably stainless steel.

Preferably the cup wall(s) are the same as the wall(s) of the acoustic reducer -i.e. preferably wherever cup wall(s) are referred to, this also relates to the wall(s) of the acoustic reducer. However, the benefits of the invention may still be achieved where the acoustic reducer provides only some of the cup wall(s) such that other components within the silencer chamber, or the wall(s) of the silencer chamber itself, contribute to the remaining cup wall(s).

Preferably the cup mouth is the same as a mouth of the acoustic reducer. Herein, all parts of the cup may be parts of the acoustic reducer.

The acoustic reducer is preferably a generally cylindrical-shaped hollow body, in which case the acoustic reducer and cup has a single wall. However, the present invention may be realised where the acoustic reducer is a generally non-circular-based prism-shaped hollow body, such as a regular or irregular triangular, square, pentagonal, or hexagonal-based hollow prism, in which case there will be a plurality of cup walls. Generally "cylindrical-shaped" or polygonal "prism-shaped" refers to the general cross-sectional shape of the hollow running through the hollow body of the acoustic reducer, and is not necessarily limited to a shape whose width (or diameter) is uniform along its length.

The narrowed portion is preferably a neck portion. The neck portion is preferably an indentation or undulation along the length of the cup wall(s) -i.e. preferably the neck is characterised by a narrowing and rewidening along the wall(s) between the cup mouth and the cup base. Preferably the neck is formed in a curved profile, preferably a pseudo-sigmoidal profile. Preferably an internal region of the cup/acoustic reducer between a narrowest part of the neck portion and the cup mouth defines a mouth region. Preferably an internal region of the cup/acoustic reducer between a narrowest part of the neck portion and the cup base defines a base region. The neck portion is advantageous because, in use, (i.e. when exhaust gases are flowing) it can provide a pressure differential between the mouth region and base region, which can help encourage sound waves to enter the base region and thereby reflect off the cup base back into incoming sound waves. Preferably the acoustic reducer is arranged, in use, to provide a base region which is a low-pressure region relative to the mouth region. This maximises destructive interference of sound waves.

A minimum internal diameter of the acoustic reducer, as defined by a narrowest part of the neck portion, is preferably less than 95% of the internal diameter of the cup mouth, more preferably less than 90% of the internal diameter of the cup mouth, most preferably less than 85% of the internal diameter of the cup mouth. The minimum internal diameter is preferably more than 50% of the internal diameter of the cup mouth, more preferably more than 70%, most preferably more than 75%. This optimises the pressure differential between the mouth and base regions, thereby maximising noise attenuation.

The internal diameter of the cup mouth is preferably the same or smaller than the internal diameter of the cup base, most preferably smaller. The internal diameter of the cup mouth is preferably between 30 and 100% of the internal diameter of the cup base, more preferably between 60 and 95%, most preferably between 80 and 90%.

Preferably the average internal diameter of the base region is larger than the average internal diameter of the mouth region. Preferably the volume of the base region is larger than the volume of the mouth region. This helps maximise the pressure differential between the relatively low-pressure of the base region and the relative relatively high-pressure of the mouth region.

Herein, where a shape is non-circular based, "diameter" refers to the longest distance between two points.

The neck portion is preferably nearer to the cup mouth than the cup base. This maximises the funnelling effect upon exhaust gases and thus maximises pressure differentials within the acoustic reducer and cup. Preferably the distances along a central axis of the acoustic reducer defined respectively by the narrowest part of the neck portion to the cup mouth (i.e. the depth of the mouth region) and the narrowest part of the neck portion to the cup base (i.e. depth of the base region) are in a ratio of between 1:20 and 1:1.1, more preferably between 1:10 and 1:2, and most preferably between 1:5 and 1:3. Herein, depth within the cup or acoustic reducer relates to the position along the length of the central cylindrical or prismatic axis of the cup.

The cup base region preferably comprises at least one hole in the cup wall(s). This helps to maintain low pressure in the base region and thereby maintain the required pressure differential which encourages sound waves to enter the base region and reflect of the cup base. Preferably the base region comprises a plurality of holes, preferably evenly spaced, preferably substantially at the same depth within the base region. The total surface area of the hole(s) is preferably less than 20% of the overall internal surface area of the cup wall(s), more preferably less than 10%, more preferably less than 5%, most preferably less than 2%. The hole(s) are preferably spaced from the cup base by at most 40% of the total depth of the cup, more preferably by at most 30%, most preferably at most 25%. The hole(s) are preferably spaced from the cup base by at least 5% of the total depth of the cup, more preferably at least 10% of the total depth of the cup, most preferably at least 15% of the depth of the cup. The depth of the cup (or acoustic reducer) is defined as the total length of the central cylindrical or prismatic axis between a central point of the base and a central point of the mouth. Preferably the hole(s) constitute a sufficient surface area of the cup wall(s) to maintain, in use, a lower pressure in the base region than in the mouth region for a given flow rate of exhaust gases.

Preferably the hole(s) are not the only route exhaust gases can take through the silencer chamber. Preferably exhaust gases can flow around the acoustic reducer between the exhaust inlet and an exhaust outlet. Preferably the mouth region is free of holes in the cup wall(s).

The acoustic reducer may itself comprise the cup base. The cup base may be a continuation of the cup walls, but is preferably sharply angled relative to the cup walls (e.g. at 90°) in order to provide the best reflective qualities for sound waves. The cup base may be integral to the acoustic reducer. However, preferably the cup base comprises at least part of an internal wall of the silencer chamber. This allows for more facile and cheaper manufacturing of the acoustic reducer, which may then be open at both ends and thus easily moulded -as such the acoustic reducer has an open mouth and an open base. It furthermore makes use of a wall which is already made in the current manufacturing process for silencers.

Preferably the entire cup base comprises at least part of the internal wall of the silencer chamber. The part of the internal wall of the silencer chamber forming the cup base may be less absorbent of sound waves (i.e. more reflective of sound waves) than other parts of the internal wall or other walls within the silencer chamber.

The cup mouth is preferably flared outwardly. This helps catch incoming exhaust flow so as to create the required pressure differential. It also helps catch incoming sound waves which may otherwise pass outside the acoustic reducer. Moreover, it adds to the noise attenuation effect. Such flaring may give rise to and even constitute the narrowed portion.

Preferably, however, any such flaring is in addition to a narrowed portion, which is preferably the neck portion. This allows for better noise attenuation. The angle of flaring at the mouth rim, relative to the central cylindrical or prismatic axis, is preferably between 1° and 80°, preferably between 5° and 60°, more preferably between 20° and 50°, and most preferably between 25° and 45°.

Preferably the exhaust inlet is facing the cup mouth so as to allow direct channelling of at least some of the exhaust gases into the cup. This maximises noise attenuation by allowing the acoustic reducer to act on exhaust gases at there noisiest. It also maximises reflection of sound waves and hence destructive interference effect on incoming/outgoing sound waves.

Direct channelling means that the exhaust gases enter the mouth of the acoustic reducer directly from the exhaust inlet without being first deflected by any other wall(s). The cup mouth is preferably directly opposite and facing the exhaust inlet. The plane of the mouth of the exhaust inlet is preferably substantially parallel to the plane of the cup mouth. The plane of the mouth of the exhaust inlet is preferably substantially parallel to the plane of the cup base. By substantially parallel is preferably meant relatively inclined at an angle of +/-100, more preferably +/-50, most preferably +/-2°.

The acoustic reducer is preferably arranged relative to the exhaust inlet such that a plane of the cup base is substantially perpendicular to the direction of flow of exhaust gases from the inlet. Preferably the plane of the neck (at its narrowest part) is substantially perpendicular to exhaust gas flow. By substantially perpendicular is preferably meant relatively inclined at an angle of between 80° to 1000, more preferably 85° to 95°, most preferably 88° to 92°. The cup base is preferably flat. Preferably the plane of the cup base is substantially parallel to the plane of the cup mouth.

The size of the exhaust inlet is preferably the same or greater in size than the narrowest part of the narrowed portion (or neck portion). This maximises noise reduction and helps maintain the pressure differential, in use, between the mouth region and base region. By size is hereby meant the area or diameter of the respective openings/hollow portions. By exhaust inlet is meant a mouth of an exhaust inlet conduit or pipe. Most preferably the size of the exhaust inlet is substantially the same as the narrowest part of the narrowed portion. Where the narrowed portion is a neck portion, the diameter of the narrowest part of the neck portion is preferably between 50 and 120% of the diameter of the exhaust inlet, more preferably between 70 and 110%, most preferably between 90 and 100%. The cup mouth is preferably larger than the exhaust inlet, preferably having a diameter 100 to 200% of the diameter of the exhaust inlet, more preferably 110 to 150%, most preferably 120 to 130%. Having a cup mouth which is wider but a neck portion which is the same as or narrower than the exhaust inlet in size maximises funnelling and the consequent pressure differential between the mouth and base regions of the cup.

An interference space is defined by a volume between the exhaust inlet and the cup mouth encapsulated by a hypothetical straight hollow tube of uniform internal diameter (preferably cylindrical but possibly polygonal-based prism depending on the cross-sectional shape of the exhaust inlet) continuing from the exhaust inlet to the cup mouth (preferably intersecting the plane of the cup-mouth substantially at a right angle). The volume of the interference space (in mm3) is preferably greater than the cross-sectional area of the exhaust inlet (in mm2), preferably at least 25% greater, more preferably at least 50% greater, and most preferably 80% greater. The volume of the interference space (in mm3) is preferably at most 300% greater than the cross-sectional area of the exhaust inlet (in mm2), more preferably at most 200% greater, and most preferably at most 120% greater. Preferably the interference space also includes the volume of any holes in the cup wall(s), and the preferred ratios of interference space to the cross-sectional area of the exhaust inlet remain as defined above.

The cup mouth is preferably spaced from the exhaust inlet by a distance of at least 20% of the diameter of the exhaust inlet, more preferably at least 50%, most preferably at least 70%. The cup mouth is preferably spaced from the exhaust inlet by a distance of at most 150% of the diameter of the exhaust inlet, more preferably at most least 100%, most preferably at most 80%. Preferably the cup mouth is spaced from the exhaust inlet by a distance of at least 10% of the depth of the cup/acoustic reducer, more preferably at least 20%, most preferably at least 30%. Preferably the cup mouth is spaced from the exhaust inlet by a distance of at most 100% of the depth of the cup/acoustic reducer, more preferably at most 60%, most preferably at most 40%.

The exhaust silencer is preferably an absorption silencer. This allows for a smoother flow of exhaust gases, minimal backpressure, and better performance in terms of fuel consumption and power transmission. They are also cheaper and easier to manufacture. The present invention allows absorption silencers to be used where reactive silencers would have been otherwise necessary to keep noise levels down. However, acoustic reducers of the present invention still provide the above mentioned benefits to reactive silencers, especially where the acoustic reducer is positioned to provide pre-noise reduction before the exhaust gases and sound waves enter the various chambers of the reactive silencer.

According to a second aspect of the present invention there is provided an acoustic reducer as described in the first aspect. All features of the first aspect described in relation to the cup may apply to the acoustic reducer of the second aspect. This applies to the mouth, wall(s), base, narrowed portion, mouth region and base region, etc. Preferably the acoustic reducer is arrangable within an exhaust silencer to provide a cup as described in the first aspect. Preferably, the acoustic reducer has a mouth and a base, both of which are open.

The acoustic reducer is preferably open-ended at both ends.

The mouth of the acoustic reducer is preferably defined by a mouth rim, wherein the mouth is the empty space within and on a plane defined by the mouth rim. Where the mouth rim is uneven, the plane of the mouth is preferably defined by an average position of the rim.

The base may be defined likewise where the base of the acoustic reducer is open.

The depth of the acoustic reducer is preferably between 1 and 20 cm, more preferably between 2 and 10 cm, most preferably between 3 and 7 cm. The diameter of the mouth of the acoustic reducer is preferably between 1 and 20 cm, more preferably between 3 and 15 cm, most preferably between 5 and 10 cm. The diameter of the narrowest part of the neck portion is preferably between 1 and 20 cm, more preferably between 2 and 10 cm, most preferably between 4 and 9 cm. The diameter of the base of the acoustic reducer (whether open or otherwise) is preferably between 1 and 20 cm, more preferably between 3 and 15 cm, most preferably between 4 and 12 cm. The thickness of the acoustic reducer is preferably 0.1 mm to 10mm, more preferably between 0.2mm and 5 mm, most preferably between 1mm and 2mm.

According to a third aspect of the present invention there is provided a method of modifying an exhaust silencer, comprising installing the acoustic reducer, as described in any previous aspect, within the exhaust silencer. The method preferably comprises affixing the acoustic reducer to an internal wall of the silencer chamber, preferably facing the exhaust inlet.

Affixing preferably comprises welding.

According to a fourth aspect of the present invention there is provided a motor vehicle comprising the exhaust silencer as described in the first aspect. The motor vehicle may be a car, van, lorry, marine vehicle, or aircraft.

Preferred features of any aspect are also preferred features of any other aspect.

Brief Description of the Drawings

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which: Figure 1 is a side cross-sectional view of an exhaust silencer relating to an embodiment of the present invention; Figure 2 is an overhead perspective view of an acoustic reducer relating to an embodiment of the present invention.

Detailed Description of the Exemplary Embodiments of the Invention The exemplary embodiments of the present invention will be discussed in detail in relation to an exhaust silencer for a motor car, in this case an absorption silencer, which provides significant noise reduction whilst avoiding backpressure which compromises the efficiency of an exhaust system. However, the teachings, principles and techniques of the present invention are also applicable in other exemplary embodiments. For example, embodiments of the present invention are also applicable to other silencer/muffler systems, such as those for aircraft.

FIG. 1 shows a side cross-sectional view of an absorption silencer I with a silencer chamber 10. The silencer chamber 10 has internal walls 12 (or baffle plates 12), a circular exhaust inlet 20 which channels exhaust gases from an inlet baffle tube 22 into the silencer chamber 10, an acoustic reducer 30 which directly receivers incoming exhaust gases from the exhaust inlet 20 and provides noise attenuation, and a circular exhaust outlet 40 for channelling noise attenuated exhaust gases from the silencer chamber 10 into an outlet baffle tube 42. Behind the baffle plates 12, and surrounding the silencer chamber 10 and baffle tubes 22, 42, is a ceramic fibre filling 50. The inlet baffle tube 22 is part of an exhaust system which delivers exhaust gases from the motor car's combustion engine to the silencer chamber 10 before ejecting the exhaust gases into the atmosphere via the outlet baffle tube 42 and ultimately out of a tail pipe 60.

The acoustic reducer 30, shown in cross-section in FIG. 1, is generally cylindrical-shaped with an open mouth 32 and an open base 34 defined by walls 36 (i.e. a single wall owing the general cylindrical shape) extending between the mouth 32 and base 34. The walls also define a narrowed neck 38 lying between the mouth 32 and base 34 but nearer to the mouth 32. The neck 38 essentially divides the interior of the acoustic reducer between a mouth region (near the mouth 32) and a base region (near the base 34). The mouth region flares outwardly to give a flared mouth 32. The base region has no flaring at the base 34. The walls 36 of the base region have small holes 39 very near the base 34 itself.

The acoustic reducer 30 is weldably fixed within the silencer chamber 10 with its base 34 against a portion of the baffle plate 12 opposite the exhaust inlet 20. As such, the acoustic reducer forms, within the silencer 1, a cup having a cup mouth 32, cup walls 36, and a cup base 14, where the cup base 14 is formed by a part of the baffle plate 12 enclosed by the open base 34 of the acoustic reducer 30. The cup base 14 part of the baffle plate 12 is therefore flat. The mouth 32 is directly facing the exhaust inlet 20 (i.e. the cylindrical axes of both the inlet baffle tube 22 at the inlet 20 and the acoustic reducer 30 are co-aligned) so exhaust gases can be channelled via the inlet baffle tube 22 directly into the mouth 32 of the acoustic reducer 30. The mouth 32 is wider than the inlet 20 so that all exhaust gases can be captured by the acoustic reducer 30 via the mouth 32. The neck 38 is, however, the same width (diameter) as the inlet 20 so as to funnel exhaust gases from the mouth region into the base region of the acoustic reducer 30. The base 34 of the acoustic reducer and the corresponding cup base 14 is wider than the mouth 32 such that the base region is on average wider (in diameter) than the mouth region. This allows for a pressure differential between the mouth and base regions when exhaust gases are flowing into the acoustic reducer 30 -i.e. the mouth region is a relatively high pressure region whilst the base region is a relatively low pressure region. The small holes 39 in the base region help to maintain a relatively low pressure in the base region.

FIG. 2 shows the acoustic reducer 30 in more detail. The acoustic reducer is symmetrical about its cylindrical axis. The outwardly flared mouth 32 is defined by a circular mouth rim 32a, and the non-flared base 34 is defined by a circular base rim 34a. The neck 38 is formed by a pseudo-sigmoidal narrowing and rewidening of the walls 36. The base region has four evenly spaced holes, at the same depth (i.e. same distance from the base rim 34a), around the circumference of the acoustic reducer 30 close to the base rim 34a.

Various sizes of acoustic reducer 30 may be used depending on a number of factors, including the size of the silencer chamber 30, the size of the exhaust inlet 20, the pressure of the exhaust gases emerging from the exhaust inlet 20, and the sound frequencies of the noise to be reduced. Calculations are used to optimise the design of acoustic reducer (including the curved profile of the walls 36). In this example, the silencer chamber is 6.5 cm wide (i.e. distance between the baffle plates 12), and the exhaust inlet 20 and its corresponding baffle tube 22 has an internal diameter (bore) of 5.08 cm. The length of the acoustic reducer 30 (i.e. depth of the cup/cylindrical height), which is the distance along the cylindrical axis between the centre of the mouth 32 and the centre of the base 34, 14, is 5 cm. As such, the mouth 32 is positioned 1.5 cm from the exhaust inlet 20 (the mouth is considered to be a plane defined by the mouth rim 34a). The mouth 32 of the acoustic reducer 30 and cup has an internal diameter of 6.35 cm. The angle of flaring at the mouth rim 34a, from the cylindrical axis, is 30°.

The narrowest part of the neck 38 is 5.08 cm in diameter, whereas the base 34, 14 has an internal diameter of 7.62 cm. Each of the four holes 39 are 3 mm in diameter, the centre of the holes being located 1 cm from the base rim 34a. The depth of the mouth region (i.e. the along the cylindrical axis between the centre of the mouth and the centre of the narrowest portion of the neck) is 1cm whereas the depth of the base region is 4cm. The thickness of the walls of the acoustic reducer is uniform throughout, and is 1.5 mm.

An interference space is defined by a volume between the exhaust inlet 20 and the cup mouth 32 encapsulated by a hypothetical straight hollow cylindrical tube of uniform internal diameter extending from the exhaust inlet 20 to the cup mouth 32 so as to insect the cup mouth at right angles. Since the distance between the exhaust inlet 20 and the cup mouth 32 is 1.5 cm and the diameter of the exhaust inlet is 5.08 cm, the interference space has a volume of 30.4 cm3. The cross-sectional area of the exhaust inlet is 16.0 cm2. The volume of the interference space (in cm3 or mm3) is 90% greater than the cross-sectional area of the exhaust inlet (in cm2 or mm2).

The acoustic reducer 30 is made of the same material as the internal parts of the silencer chamber 10 to which it is welded, namely stainless steel. The acoustic reducer 30 is moulded from a tube by using a hydrasizer on hydraulic tooling.

The above described absorption silencer 1 is produced by taking a standard absorption silencer, and affixing the acoustic reducer 30 within the silencer chamber 10 so that it forms a cup whose mouth faces the exhaust inlet 20, as described above. Affixing is performed by welding the stainless steel acoustic reducer 30 to the stainless steel silencer wall 12 or baffle plate 12. However, it will be understood that other equally appropriate methods of fixing may be employed. For instance, the absorption silencer may be pre-fitted with special clips arranged to removably receive the acoustic reducer 30. This way an acoustic reducer 30 may be replaceable, which could allow for different acoustic reducers to be used depending on the conditions. For instance, retuning an engine may precipitate the need to replace the acoustic reducer 30 with one more appropriate for the particular engine tuning.

In use, exhaust gases generated by a car engine are continuously pumped through the exhaust system into the silencer 1, at a flow rate generally determined by the revolutions per minute (rpm) of the car engine (i.e. fuel combustion rate). These exhaust gases also carry powerful sound waves produced by the car engine. Such exhaust gases pass through the inlet baffle tube 22 and into the silencer chamber 10 via the exhaust inlet 20. Due to the arrangement of the acoustic reducer 30 within the silencer chamber 10, the exhaust gases directly enter the mouth 32 of the acoustic reducer and are compressed in the mouth region owing to the funnelling effect of the neck 38 so as to give a relatively high-pressure mouth region. The pressurized exhaust gases then pass through the neck 38 of the acoustic reducer and into the base region which, due to being more expansive than the mouth region, gives rise to a relatively low-pressure region. The continuous flow of exhaust gases from the exhaust inlet 20 combined with turbulent flow within the acoustic reducer 30 allows exhaust gases to be exchanged within the acoustic reducer such that exhaust gases generally progress through the silencer chamber 10, out of the exhaust outlet 40, through the outlet baffle tube 42 and out of the tail pipe 60 into the atmosphere. Whilst flow of exhaust gases occurs, sound waves continuously flow into the silencer chamber 10 and into the acoustic reducer 30. The base of the cup 14, which is necessarily reflective to sound waves, reflects these sound waves back onto incoming sound waves and, in so doing, causes destructive interference which reduces overall sound levels. A small amount of exhaust gases pass through the holes 39 at the base 34 of the acoustic reducer 30 so as to maintain a relatively low pressure in the base region, thus continually encouraging incoming sound waves to enter thereinto so that they can be duly reflected to give rise to the desired noise attenuation effect.

Back pressure is avoided due to the gap between the mouth 32 of the acoustic reducer 30 and the exhaust inlet 20. As such, exhaust gases can easily pass around the the sides of the acoustic reducer whilst the sound waves continue to travel straight into the acoustic reducer 30 where they are destroyed/reduced.

Testing TEST 1 The above described absorption silencer was tested on a SubaruTM Forester 2.0 motor car with and without the above described narrow-necked acoustic reducer. Tests revealed that the noise reduction achieved by silencers fitted with the acoustic reducer was 8 decibels greater than that achieved by silencers without the acoustic reducer. In both cases the backpressure, fuel consumption, and power transmission were equal. Therefore the acoustic reducer provides significant noise attenuation without compromising performance in other aspects.

TEST 2 The above tests were repeated using the same absorption silencers and same motor car, this time comparing silencers with the above described narrow-necked acoustic reducer with silencers having an acoustic reducer that is perfectly cylindrical (i.e. having no neck).

Silencers fitted with the narrow-necked acoustic reducers gave a noise reduction of 8 decibels as compared to silencers with no acoustic reducer, whereas silencers fitted with the perfectly cylindrical acoustic reducers gave a noise reduction of 1 decibels as compared to silencers with no acoustic reducer. Therefore, the narrow-necked acoustic reducers out-performed those with no neck by 7 decibels. Therefore, an acoustic reducer having a narrowed portion relative to the mouth is very important to achieve the desired effect.

Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

CLAIMS: 1. An exhaust silencer comprising a silencer chamber, wherein the silencer chamber comprises: an exhaust inlet for channelling exhaust gases into the silencer chamber; an acoustic reducer located within the silencer chamber to form a cup having a cup mouth, a cup base, and a cup wall(s) extending between the cup mouth and the cup base; wherein the cup wall(s) comprises a narrowed portion that is narrower than the cup mouth.
2. The exhaust silencer as claimed in claim 1 wherein the narrowed portion is a neck portion.
3. The exhaust silencer as claimed in claim 2 wherein the minimum internal diameter of the acoustic reducer, as defined by a narrowest part of the neck portion, is less than 95% of the internal diameter of the cup mouth.
4. The exhaust silencer as claimed in any of claims 2 to 3 wherein the neck portion is nearer to the cup mouth than the cup base.
5. The exhaust silencer as claimed in any of claims 2 to 5 wherein the acoustic reducer comprises an internal cup base region between a narrowest part of the neck portion and the cup base, and an internal cup mouth region between the narrowest part of the neck portion and the cup mouth; wherein the wall(s) of the cup base region comprises at least one hole.
6. The exhaust silencer as claimed in any preceding claim wherein the cup base comprises at least part of an internal wall of the silencer chamber.
7. The exhaust silencer as claimed in any preceding claim, wherein the cup mouth is flared outwardly.
8. The exhaust silencer as claimed in any preceding claim, wherein the exhaust inlet is facing the cup mouth so as to allow direct channelling of at least some of the exhaust gases into the cup.
9. The exhaust silencer as claimed in claim 8 wherein the acoustic reducer is arranged relative to the exhaust inlet such that a plane of the cup base is substantially perpendicular to the direction of flow of exhaust gases from the inlet.
10. The exhaust silencer as claimed in any preceding claim wherein the size of the exhaust inlet is larger than the size of the narrowed portion.
11. The exhaust silencer as claimed in any preceding claim, wherein the exhaust silencer is an absorption silencer.
12. An acoustic reducer as described in any preceding claim.
13. The acoustic reducer as claimed in claim 12, wherein the acoustic reducer has a mouth and a base, both of which are open.
14. A method of modifying an exhaust silencer, comprising installing the acoustic reducer as described in any preceding claim within the exhaust silencer.
15. A motor vehicle comprising the exhaust silencer as claimed in any of claims 1 to 11.
16. An exhaust silencer, an acoustic reducer, a method of modifying an exhaust silencer, or a motor vehicle as substantially hereinbefore described with reference to the diagrammatic drawings.