GB1594212A

GB1594212A - Exhaust silencers application

Info

Publication number: GB1594212A
Application number: GB5142276A
Authority: GB
Original assignee: Victoria University of Manchester; University of Manchester
Current assignee: Victoria University of Manchester; University of Manchester
Priority date: 1978-03-09
Filing date: 1978-03-09
Publication date: 1981-07-30

Description

(54) IMPROVEMENTS IN EXHAUST SILENCERS (71) We, THE VICTORIA UNIVERSITY OF MANCHESTER, an institution established according to Royal Charter of Oxford Road Manchester M13 9PL, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to exhaust silencers.

According to the present invention there is provided an exhaust silencer comprising inner and outer tubes co-axially spaced from one another, the inner tube being closed at one end and the outer tube being open adjacent said one end of the inner tube, the inner and outer tubes defining between them a substantially unobstructed gas outlet passage, the height of said passage being such as to cause attenuation of the noise of gas flowing along the passage and an exhaust gas inlet being provided at the other end of the outer tube, the length of the inner tube between the exhuast gas inlet and its said closed end being divided axially into two or more substantially closed chambers, the circumferential wall of the inner tube being perforated, the perforations constituting a small fraction of the wall area, so as to tune one or more said chambers as a Helmholtz cavity and cause further attenuation of the noise of gas flow along said passage and in which the chambers contain no fibrous or other sound absorbent material for noise attenuation purposes.

In one preferred form of the exhaust silencer the inner tube has an exhaust gas inlet at the other end, the outer tube being closed adjacent said other end of the inner tube and the inner tube being perforated in a region downstream of said closed end of the outer tube so that gas flowing into the inner tube through said inlet can pass through said perforations and along the gas outlet passage.

In another preferred form of exhaust silencer the inner tube is closed at one end by a streamlined cap and is closed at its other end, the outer tube being open at its other end to define an exhaust gas inlet onto the streamlined end of the inner tube.

The invention in either of these preferred forms is not limited to the gas flow passage between the inner and outer tubes being of constant height. It includes within its scope the possibility of the gas flow passage increasing in height as the gases flow along the silencer, since this would counteract the tendency for progressive boundary layer build-up along the walls of the gas flow passage.

We have verified that a silencer with progressively increasing gas flow passage height gives improved performance. Preferably therefore the height of the gas flow passage increases as the gases travel along it.

The passage height may for example increase progressively from the base of said streamlined cap to the other end of the inner tube.

Preferably again the cross-sectional area of the gas flow passage, at any point along its length, is greater than the cross-sectional area of the exhaust gas inlet.

The cap closing off the upstream end of the inner tube need not be, literally, "streamlined" in the strict, aerodynamic sense. It could, for example, by conical, concave, hemispherical, or even simply dished circumferentially around a basically flat central portion, as long as its resistance to flow is noticeably lessened in the region where it joins--or blends intthe inner tube.

In any embodiment of the invention, the end walls of each said chamber, as well as the circumferential wall of the chamber, may also be perforated to "tune" the attenuating effect of the chambers.

The outer tube may continue axially beyond the downstream end of the inner tube in order to define a final outlet shroud for the exhaust gases.

The invention will now be described further, by way of example, with reference to the accompanying drawings. in which: Figure I is a partial axial section of a silencer embodying the invention; Figure 2 is a radial section of a modification of the silencer of Figure 1; and Figure 3 is an axial section of another silencer embodying the invention.

One design of exhaust silencer embodying the invention is shown in Figure I of the accompanying drawings. This particular silancer, in use, fits over the exhaust pipe of a racing motor cycle and has been especially designed for that particular use. It will be appreciated that the silencing of motor cycles is necessarily something of a compromise, since power loss and weight must both be minimised and design freedom is restricted because the silencer can only be of relatively small physical size. In the case of a competition machine, very small increases in power loss and weight will significantly affect the competitiveness of the machine as a whole.

In addition, competition silencers should be relatively simple, robust, and able to be quickly removed from the machine and/or taken apart, for example for scrutineering purposes.

Figure 1 shows an embodiment of the silencer in cross-section. In this embodiment, inner and outer circular-cylindrical tubes 10, 12 are co-axially spaced from one another so as to define a basically annular gap, or gas outlet passage 14, between the outer wall of the inner tube and the inner wall of the outer tube, the gap being substantially free of obstruction except for three bosses 16 which are welded to the outer wall of the inner tube and are equally radially spaced about its circumference. These bosses act as spacers toward one end of the combined overlapping length of the two tubes, and hexagonrecessed socket screws co-operate with threaded bores in the bosses to secure the outer tube 12 to the inner tube 10 at these points.

The inner tube 10 is closed at its downstream end (the right-hand end in the longitudinal cross-section of Figure 1) by a circular end cap 18. A cap screw 20 is secured to the outer face of the end cap 18, and its head is drilled so that a rod or wire can be inserted and used to pull the inner tube 10 out for checking and cleaning. At its upstream end, the tube changes from a circularcylindrical to a conical form 22 and is thus again effectively closed off. Thus, at the point where the exhaust gases enter the gas outlet passage, the diameter of the inner tube is increasing progressively down the tube.

Radially spaced projections 24, or alternatively a circumferentially continuous spacing ring 26, (Figure 2) are welded to the outer wall of the inner tube to locate its upstream portion in place inside the outer tube.

The outer tube 12 also changes, at its lefthand end when viewed as in the drawing, from cylindrical to frusto-conical form. The tube tapers down to the outer wall of a gas inlet feed pipe 28, and the feed pipe, in use of the silencer, fits over the exhaust pipe of a motor cycle internal combustion engine and so constitutes a gas inlet for the exhaust gases from the engine. The outer tube 12 remains open at its other end 30, i.e. around the region of the circular end cap 18 which closes off the downstream end of the inner tube.

The height of the annular gas flow passage 14 between the two tubes increases progressively, in the direction of gas flow, from 4.75 mm at the base of the conical end cap 22 at one end of the inner tube to 9.5 mm at the circular end cap 18 at the other end of the tube 10. The dimensions of the passage are so chosen that a significant attenuation of exhaust noise takes place as the gases flow down the progressivley widening annular passage. In all practical embodiments of the invention which can be foreseen at the moment, the gap between the two tubes at outlet from the silencer will not be greater than three-eights of an inch (9.5 mm) given the diameters of silencers which are currently acceptable on motor cylces.

It will be seen that these particular silencers are relatively short in relation to their outer diameters. This, however, is due to their mounting on a competition machine, and the length of the gas flow passage is thought to be a less important parameter for silencing purposes than the passage height and the number and individual dimensions of the tuned chambers which will be described shortly. On a road-going machine, where there is more space along the machine for the silencer to run, the silencer could be longer than the ones shown and still embody the principles of the invention.

The inner tube 10, between its conical end cap 22 and the end cap 18 which closes off the other end of the tube, is divided into an axially successive series of two substantially closed chambers 32 and 34. Each of these chambers has a number of relatively small holes 36 drilled in its circumferential wall at specified points. In addition, the circular disc 38 which divides these chambers could be similarly drilled. Each chamber acts as a Helmholtz cavity, causing further attenuation of the gases flowing along the annular passage between the tubes, and the holes 36 are selected to "tune" the chambers to differing frequencies within the frequency band of the exhaust noise to be silenced.

An alternative design of silencer embodying the invention is shown in Figure 3 of the accompanying drawings. This silencer is similar in its noise attenuating principles to the silencers of Figures 1 and 2, but has the following constructional differences: The inner tube 10 is closed only at one end the right-hand end in the longitudinal crosssection of Figure 3 by an end cap 18. At its other end, the tube changes from a cylindrical to a conical form and is welded to the cylindrical feed-pipe 28. The feed pipe 28 in use of the silencer, fits over the exhaust pipe of the motorcyle internal combustion engine and so constitutes the gas inlet for the exhaust gases from the engine.

The outer tube 12 also changes, at its lefthand end when viewed as in Figure 3, from cylindrical to conical form. The tube tapers down to the outer wall of the gas inlet feed pipe 28, and is effectively closed off at that end by a correspondingly-tapered ring 40 and a high-temperature-resistant asbestor string seal 42. The tapered ring 40 acts to maintain the spacing between the inner and outer tubes at the end opposite the three bosses 16. The annular passage 14 between the two tubes does not expand, but is of substantially constant height throughout the combined overlapping lengths of the tubes.

The wall of the inner tube 10 is perforated, in a region axially downstream of the tapered ring and asbestos string seal between the feed pipe and the outer tube. The perforated region 44 consists of two axially successive circumferentially-spaced rows of holes, and occupies most of the conical section of the inner tube. Thus, exhaust gases flowing into the feed pipe (i.e. from left to right in Figure 3) can pass through these two rows of holes in the inner tube wall and out into the substantially unobstructed annular passage 14 between the inner and outer tubes, to emerge eventually at the outlet 30 around the inner tube end cap.

The height of the passage again is so chosen that a significant attenuation of exhaust noise takes place as the gases flow down the annular passage. In this particular example, the constant annular gap between the two tubes is one quarter of an inch (about 6 mm).

It will be noted that there is no other sound-absorbent material, for example fibrous wadding or packing, in any of these silencers for the purposes of noise attenuation.

It is within the scope of the invention for the outer tube to continue, axially beyond the end cap which closes of the downstream end of the inner tube, so as to define a final outlet for the exhuast gases. The outer tube could, for example, taper into a generally conical or a bell-shaped open-ended shroud.

It is alternatively possible for the end cap 18 closing off the downstream end of the inner tube, to protrude beyond the end of the annular gas flow passage. If the end cap is dished so that it protrudes beyond the end of the outer tube, noise attenuation is improved.

The end cap could alternatively be dished inwardly, and could be made a positive styling feature of the silencer.

It is possible to modify the construction shown in Figure 1 of the drawings, by extending the gas inlet feed pipe farther into the outer tube of the silencer, and perforating the extended portion within the outer tube.

With silencers of the dimensions shown, the feed pipe could extend about four inches to eight inches inside the outer tube, and be perforated with e.g. quarter-inch holes. This has the effect of diffusing the resonance in the otherwise finite length of the primary exhaust pipe from the engine to the silencer.

It spreads the resonance by removing the precise resonance length.

It will be appreciated that a silencer whose inner and outer tubes comprises slow-tapered cones, rather than cylinders, is fully within the scope of the invention. The cones of such a silencer could be of differing diameters but identical taper, thus defining between them a gas flow passage of constant height; or the outer cone could be more steeply tapered than the inner one, so that a progressive increase in gas flow passage height takes place from the inlet to the outlet end of the silencer.

It is further within the scope of the invention for each chamber to be adjustable in length, for example by a threaded rod being secured to the end cap of the inner tube and passing centrally along the tube with the chamber end wall discs 38 threaded onto it.

The discs would, of course, have to be a tight circumferential fit within the inner tube: they could be located in position on the rod by being bored and threaded themselves, or being bored and having a nut on either side of them on the rod.

Although the flow of exhaust gases in all the illustrated embodiments is entirely in one axial direction, it is within the scope of the invention for the gas flow to be reversed, within the silencer, and then reversed again so as to continue in its original direction before reaching the final outlet. Several pairs of co-axially spaced inner and outer tubes would then be used, provided of course that the essential requirements of the invention were still present. Such a silencer could, for instance, be used on a non-racing road-going motor cycle, where power losses and weight gain are more readily sacrificed to very significant gains in noise attenuation.

In all these silencers, the cross-sectional area of the gas flow passage at any point along its length is at least equal to the crosssectional area of the exhaust gas inlet.

WHAT WE CLAIM IS: 1. An exhaust silencer comprising inner and outer tubes co-axially spaced from one

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Claims

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the silencers of Figures 1 and 2, but has the following constructional differences: The inner tube 10 is closed only at one end the right-hand end in the longitudinal crosssection of Figure 3 by an end cap 18. At its other end, the tube changes from a cylindrical to a conical form and is welded to the cylindrical feed-pipe 28. The feed pipe 28 in use of the silencer, fits over the exhaust pipe of the motorcyle internal combustion engine and so constitutes the gas inlet for the exhaust gases from the engine.

The outer tube 12 also changes, at its lefthand end when viewed as in Figure 3, from cylindrical to conical form. The tube tapers down to the outer wall of the gas inlet feed pipe 28, and is effectively closed off at that end by a correspondingly-tapered ring 40 and a high-temperature-resistant asbestor string seal 42. The tapered ring 40 acts to maintain the spacing between the inner and outer tubes at the end opposite the three bosses 16. The annular passage 14 between the two tubes does not expand, but is of substantially constant height throughout the combined overlapping lengths of the tubes.

The wall of the inner tube 10 is perforated, in a region axially downstream of the tapered ring and asbestos string seal between the feed pipe and the outer tube. The perforated region 44 consists of two axially successive circumferentially-spaced rows of holes, and occupies most of the conical section of the inner tube. Thus, exhaust gases flowing into the feed pipe (i.e. from left to right in Figure 3) can pass through these two rows of holes in the inner tube wall and out into the substantially unobstructed annular passage 14 between the inner and outer tubes, to emerge eventually at the outlet 30 around the inner tube end cap.

The height of the passage again is so chosen that a significant attenuation of exhaust noise takes place as the gases flow down the annular passage. In this particular example, the constant annular gap between the two tubes is one quarter of an inch (about 6 mm).

It will be noted that there is no other sound-absorbent material, for example fibrous wadding or packing, in any of these silencers for the purposes of noise attenuation.

It is within the scope of the invention for the outer tube to continue, axially beyond the end cap which closes of the downstream end of the inner tube, so as to define a final outlet for the exhuast gases. The outer tube could, for example, taper into a generally conical or a bell-shaped open-ended shroud.

It is alternatively possible for the end cap 18 closing off the downstream end of the inner tube, to protrude beyond the end of the annular gas flow passage. If the end cap is dished so that it protrudes beyond the end of the outer tube, noise attenuation is improved.

The end cap could alternatively be dished inwardly, and could be made a positive styling feature of the silencer.

It is possible to modify the construction shown in Figure 1 of the drawings, by extending the gas inlet feed pipe farther into the outer tube of the silencer, and perforating the extended portion within the outer tube.

With silencers of the dimensions shown, the feed pipe could extend about four inches to eight inches inside the outer tube, and be perforated with e.g. quarter-inch holes. This has the effect of diffusing the resonance in the otherwise finite length of the primary exhaust pipe from the engine to the silencer.

It spreads the resonance by removing the precise resonance length.

It will be appreciated that a silencer whose inner and outer tubes comprises slow-tapered cones, rather than cylinders, is fully within the scope of the invention. The cones of such a silencer could be of differing diameters but identical taper, thus defining between them a gas flow passage of constant height; or the outer cone could be more steeply tapered than the inner one, so that a progressive increase in gas flow passage height takes place from the inlet to the outlet end of the silencer.

It is further within the scope of the invention for each chamber to be adjustable in length, for example by a threaded rod being secured to the end cap of the inner tube and passing centrally along the tube with the chamber end wall discs 38 threaded onto it.

The discs would, of course, have to be a tight circumferential fit within the inner tube: they could be located in position on the rod by being bored and threaded themselves, or being bored and having a nut on either side of them on the rod.

Although the flow of exhaust gases in all the illustrated embodiments is entirely in one axial direction, it is within the scope of the invention for the gas flow to be reversed, within the silencer, and then reversed again so as to continue in its original direction before reaching the final outlet. Several pairs of co-axially spaced inner and outer tubes would then be used, provided of course that the essential requirements of the invention were still present. Such a silencer could, for instance, be used on a non-racing road-going motor cycle, where power losses and weight gain are more readily sacrificed to very significant gains in noise attenuation.

In all these silencers, the cross-sectional area of the gas flow passage at any point along its length is at least equal to the crosssectional area of the exhaust gas inlet.

WHAT WE CLAIM IS: 1. An exhaust silencer comprising inner and outer tubes co-axially spaced from one

another, the inner tube being closed at one end and the outer tube being open adjacent said one end of the inner tube, the inner and outer tubes defining between them a substantially unobstructed gas outlet passage, the height of said passage being such as to cause attenuation of the noise of gas flowing along the passage and an exhaust gas inlet being provided at the other end of the outer tube, the length of the inner tube between the exhaust gas inlet and its said closed end being divided axially into two or more substantially closed chambers, the circumferential wall of the inner tube being perforated, the perforations constituting a small fraction of the wall area, so as to tune one or more said chambers as a Helmholtz cavity and cause further attenuation of the noise of gas flow along said passage and in which the chambers contain no fibrous or other sound absorbent material for noise attenuation purposes.
2. An exhaust silencer according to claim I in which the inner tube has an exhaust gas inlet at the other end, the outer tube being adjacent closed adjacent said other end of the inner tube and the inner tube being perforated in a region downstream of said closed end of the outer tube so that gas flowing into the inner tube through said inlet can pass through said perforations and along the gas outlet passage.
3. A silencer according to claim I in which the inner tube is closed at one end by a streamlined cap and is closed at its other end, the outer tube being open at its other end to define an exhaust gas inlet onto the streamlined end of the inner tube.
4. A silencer according to claim 3 in which the cap closing off the upstream end of the inner tube is conical or frusto-conical in shape.
5. A silencer according to any of the preceding claims in which the exhaust gases enter the gas outlet passage at a point where the diameter of the inner tube is increasing progressively down the tube.
6. A silencer according to any of the preceding claims, in which the height of the gas outlet passage increases progressively as the gases flow down the tube.
7. A silencer according to claim 6 in which the inner tube, or the outer tube, or both, are conical throughout substantially the whole of the combined overlapping lengths of the tubes.
8. A silencer according to any of the preceding claims, in which the cross-sectional area of the gas outlet passage, at any point along its length, is greater than the cross-sectional area of the exhaust gas inlet.
9. A silencer according to any of the preceding claims in which the height of the gas outlet passage is between 4.75 mm and 9.5 mm.
10. A silencer according to claim 9 in which the height of the gas outlet passage is about 6 mm.
11. An exhaust silencer substantially as herein before described with reference to and as illustrated in Figures 1, 2 or 3 of the accompanying drawings.