Silencer cartridge

Details Silencer cartridge Silencer cartridge

1999-06-04

2001-03-06

Nappi, Robert E. (Department: 2837)

Acoustics

Sound-modifying means

Muffler, fluid conducting type

C181S256000, C181S282000

Reexamination Certificate

active

06196351

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a silencer cartridge and particularly, although not exclusively, to a silencer cartridge or a silencer in a vehicle exhaust assembly. The invention also extends to a silencer which includes such a silencer cartridge.
BACKGROUND OF THE INVENTION
The silencer of a vehicle exhaust assembly may comprise an outer wall and porous exhaust pipe which defines an inner wall of an annular cavity. Typically, the cavity is randomly packed with sound absorbent material, for example, a glass fibre or basalt fibre material is positioned around the perforated pipe so as to fill the void between the pipe and the outer wall of the silencer and absorb sound waves which pass thereinto.
Since sound absorbent material in the form of glass fibre and basalt fibre is unpleasant to handle, suppliers of such fibrous materials supply the material to vehicle exhaust manufacturers in a suitably shaped and sized pack which is then inserted into the cavity of the silencer. Although the inner wall of the silencers are typically circular in cross section, the outer wall may be circular or oval and, increasingly, the oval outer wall is becoming more common. Numerous attempts have been made to improve the acoustic performance of silencers and it is one of the objects of the present invention to provide a silencer with a silencer cartridge with an improved acoustic performance.
A further problem with silencers of this type is that they are continuously subjected to directional gas flow which tends, over the course of time, to pack down the sound absorbent material or blow it out of the silencer altogether. Furthermore, the sound absorbent material is constantly vibrated so that the silencer material loosens over time and any structure present in the material collapses. The vibrational forces together with the uni-directional flow of gas through the silencer cause rapid degradation of the sound absorbent material leading to the necessity for its eventual replacement. It is a further object of the present invention to overcome these problems.
A still further problem with existing silencers, is that they are typically random packed with sound absorbent material. The random packing process tends to lead to a relatively wide variation in packing densities leading to difficulties in re-producibility and quality control in the final silencer product.
It is a still further object of the present invention to overcome this problem.
U.S. Pat. No. 5,670,756 discloses a silencer with the sound absorbing material wound directly around the outside of the inner sleeve with two or more turns so that the thickness of wound sound absorbing material around the inner sleeve is greater than the dimension of the gap between the inner sleeve and the outer sleeve to give an interference fit between the sound absorbing material and the inner and outer sleeves. The sound absorbing material with the inner sleeve is press fitted into the outer sleeve. The sound absorbing material is prevented from being blown out by exhaust gas flowing through the inside of the inner sleeve so that the silencer is improved in durability. This is provided by the press-fitting of the sound absorbent material which prevents vibration of the sound absorbing material during use. In addition, some stability in density is achieved by this technique compared with random packed silencers. However, although the vibrational problem is reduced, no attempt is made to solve the problem of the long term effect of uni-directional gas flow on the sound absorbing material.
U.S. Pat. No. 5,718,045 describes a similar technique where a sound absorbing material is formed of a heat-resistant fibre layer which is wound around an outer peripheral surface of the porous inner pipe with a binder inter-posed therebetween, and an outer peripheral surface of the heat-resistant fibre layer is press-fitted to an inner peripheral surface of the outer pipe with a predetermined interference. However, again, no attempt is made to address the problem of silencer degradation due to uni-directional gas flow.
BRIEF SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a silencer for a vehicle exhaust assembly comprising an outer wall, a porous exhaust pipe which defines an inner wall and a sound absorbent fibrous material located between the inner and the outer wall, wherein the sound absorbent fibrous material is substantially unidirectional and parallel with the direction of gas flow through the porous exhaust pipe.
According to a second aspect of the present invention there is provided a silencer cartridge for packing into the cavity between the inner and outer wall of a silencer comprising sound absorbent fibrous material which is substantially unidirectional, the cartridge being capable of being inserted into a cavity between the inner and outer wall of a silencer in such a manner that the direction of the fibres is substantially parallel with the direction of gas flow through the silencer.
Preferably, the cartridge includes an outer casing to hold the fibrous material in the shape required. Preferably, the casing is made from a flexible material such as needlemat.
Preferably, the cartridge has a hollow tubular core designed to be close fitting with the outer wall of an exhaust pipe tube.
Preferably, the fibres are repeatedly wrapped around and directed back at the longitudinal limits of the said unidirectional fibres so that substantially endless unidirectional fibres are present.
Preferably, the wrapping of the fibres is carried out in such a manner that successive fibres are generally displaced laterally from the previously wrapped fibre so that a continuous bed of fibres is built up.
Preferably, the bed of fibres is formed by wrapping substantially continuous fibre around a bed shaped former, beginning at one end thereof and gradually turning the fibres around the former and displacing fibres along the former until a bed of fibres is built up. Thereafter, a bed of fibres may be removed from the former so that a folded bed of unidirectional fibres is formed.
The bed of fibres so formed may be easily shaped along the axis perpendicular to the direction of the fibres to fill a correspondingly shaped cavity on a silencer.
Accordingly, the sound absorbent material comprises a unidirectional substantially continuous structure, preferably formed by repeatedly folding the unidirectional fibres back in the opposite direction at the longitudinal limits of extension of the fibres and, preferably, generally directing successive fibres slightly laterally so that a bed of fibres is built up.
Preferably, at least some of the fibres in the product are directed slightly laterally with respect to the substantial direction of the fibres. Preferably, the fibres are continuous and folded back in the opposite direction at the longitudinal limit of the cartridge. Preferably, the fibres are repeatedly folded back in the opposite direction as they successively meet the longitudinal limit of the cartridge.
Advantageously, the unidirectional fibres are found to have improved sound absorbing characteristics when parallel with the gas flow through the silencer. It is not conclusively clear why the directional fibre has better acoustics than randomly packed fibre but this may be due to the closer packing of the fibres which is possible with directional fibre. The unidirectional nature of the fibre bed allows the use of higher packing densities closer to the gas flow than has previously been considered practical. This is especially significant in promoting the locational stability of the fibrous sound absorbing material and resistance to vibration.
Furthermore, the circumferential geometry of the fibre bed allows the density of the fibrous sound absorbing material to be controlled in a manner proportional to the depth of the material. The ultra high packing density close to the central aperture of the cartridge is progressively reduced, as a function of the cartridge depth, towards the point of contact with the inside of the oute

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