GB2287752A

GB2287752A - Gas flow silencer

Info

Publication number: GB2287752A
Application number: GB9405462A
Authority: GB
Inventors: Richard John Llewelyn Evans
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1994-03-19
Filing date: 1994-03-19
Publication date: 1995-09-27
Also published as: GB9405462D0

Abstract

A large number of parallel, thin-walled passageways 22 are arranged in a silencer body so that noise in the gas flow is silenced by viscous energy dissipation in the boundary layers on the passageways. The metal or moulded plastics defining the passageways 22 in an i.c. engine exhaust or intake silencer may have a wall thickness of 0.04 to 0.2 mm, with a passage length from 100 to 600 mm and a passage cross-sectional area from 1 to 3 mm<2>. <IMAGE>

Description

SILENCER This invention relates to a silencer, for use either in the intake or in the exhaust gas system of a motor vehicle or in a similar duct noise situation. The invention is particularly suitable for attenuating noise which includes a strong plane wave content as found in the exhaust and air induction systems of internal combustion engines.

According to the invention, there is provided a silencer comprising an inlet, an outlet and a plurality of small diameter, long, axial passages arranged to allow gas flow through the silencer from the inlet to the outlet such that the noise of the gas flow is reduced by viscous dissipation of acoustic energy in the boundary layer formed by the walls of the passages.

A silencer as set forth above can be used either in the air intake duct leading to an engine, or in the exhaust gas duct leading from the engine (or in both the intake and exhaust ducts).

The invention therefore also provides an exhaust pipe for mounting in a motor vehicle wherein a silencer is mounted in line in the tailpipe of the exhaust pipe, the silencer comprising an inlet, an outlet and a plurality of small diameter, long, axial passages arranged to allow gas flow through the silencer from the inlet to the outlet such that the noise of the gas flow is reduced by viscous dissipation of acoustic energy in the boundary layer formed by the walls of the passages.

In order to form the required boundary layer at the walls of the passages, the passage walls should be generally smooth and continuous, ie without any openings in the walls which might interrupt laminar flow of the gases.

The thickness of the walls forming the passages should be as small as possible, to avoid creating unnecessary pressure drop across the silencer.

The cross-sectional area of each passage will be optimised to allow flow through the passages at an acceptable rate, while creating a relatively high volume of 'boundary layer' in which noise can be absorbed.

The mechanism by which this silencer is believed to work is described below.

Exhaust gas noise and air intake noise results from the pulsed nature of gas flow from and to an internal combustion engine. The resulting pressure fronts produce audible noise.

All silencers aim to smooth out these pressure fronts to create a more uniform flow.

When gas molecules move relative to a fixed wall, a boundary layer is formed due to friction between the gas and the wall surface, and due to the viscosity of the gas.

Acoustic energy may be dissipated by viscous damping in an acoustic boundary layer resulting in noise reduction.

The thickness of the acoustic boundary layer is substantially independent of the cross-sectional area of the passage and is of the order of imam, but noise reduction depends on the area of the boundary layer.

The invention therefore requires that the area of 'boundary layer' be increased, by increasing the area of passage walls. This means that there should be a large number of passages.

It is desired that the volume of effective boundary layer should be maximised within a device of compact external dimensions.

On the other hand, if the number of passages is made too large, the pressure drop across the silencer will be unacceptably increased, both by the blocking effect of the passage walls themselves and by the retarding effect of the gas flow induced boundary layers. The invention therefore aims to optimise the balance between these two aspects. The balance between these two aspects will depend on the degree of attenuation required and the gas flow rate, so the balance will vary between applications.

In one embodiment, which is given by way of example only, but which has given satisfactory results located in an exhaust system tailpipe, the silencer intake had a crosssectional area of 1200mm2, the outlet had a crosssectional area of 1260mm2, the chamber occupied by the silencer body had a cross-sectional area of 1260mm2, and the passage walls occupied only 8.4% of this crosssectional area, the remainder being open for gas flow.

The number of passages per square inch was 400 (0.64 passages per square millimetre). The passage walls had a wall thickness of 0.04mm.

It is anticipated, from empirical data and from the results of the test work so far carried out, that satisfactory results would also be achieved with the following parameter ranges Passage cross-sectional area from lmm2 to 3mum2 Passages per square millimetre from 0.3 to 1 Passage wall thickness from 0.04mm to 0.2mm Passage length from 100mm to 600mm The passage walls may be made of metal, so that they can be thin yet rigid and durable and capable of withstanding high temperatures encountered in the exhaust system.

Other materials are also possible, particularly in air induction system applications.

The passage walls are preferably all parallel to each other and all parallel to the silencer axis.

The silencer of the invention can be used in an exhaust gas pipe either instead of, or in addition to, a conventional exhaust gas silencer. The silencer of the invention is however preferably the last element through which the exhaust gases pass before leaving the pipe. The silencer of the invention can also be used in the air induction system, preferably as the first element.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a silencer body in accordance with the invention; Figure 2 is a detail of a small section of the silencer body, on a much enlarged scale; Figure 3 shows an engine exhaust pipe installation in accordance with the invention.

Figure 4 shows a second form of silencer body in accordance with the invention; Figure 5 is a detail of a small section of the silencer body of Figure 5, on a much enlarged scale; and Figure 6 shows an engine air intake installation in accordance with the invention.

Figure 1 shows in dotted outline the tailpipe 10 of an exhaust pipe. The pipe is of cylindrical section. Within the pipe there is a silencer body 12. The body 12 is a close fit within the pipe 10, so that all the gas flowing along the pipe passes through the silencer body.

The silencer body is formed by a large number of longitudinally folded and joined walls which thereby form a large number of gas passages through the body. Figure 2 shows individual folded walls 14, 16, 18 where the folds are parallel corrugations alternating with one or more plane strips 20 joined to one another to form a single body with passages 22. The walls 14, 16, 18 and 20 are very thin and could be described as foils.

Figure 3 shows an internal combustion engine 24 with an intake manifold 26 and an exhaust manifold 28. The exhaust manifold leads first to a catalytic converter 30 which is followed by an intermediate pipe 32. A silencer box 34 containing conventional silencer components is located near the remote end of the exhaust duct, and a silencer body 12 is fitted within the tailpipe 10.

Figures 4, 5 and 6 show the invention applied to an air intake installation. Figure 4 shows a silencer body 112, and Figure 5 shows a small part of the silencer body on a much larger scale, indicating that the passages through the body are of hexagonal section.

Because the temperatures in the air intake are much lower than those in the engine exhaust, it is possible to make a silencer for use in the air intake of, for example, a plastics material and if a plastics material is used, then it becomes possible to form the silencer body by moulding or by extrusion.

It should be noted that if the body is formed by moulding, then it is likely to be necessary to form a draft on the passage walls to allow the moulded product to be removed from the mould tool. References in this specification to the passages being parallel to each and other and parallel to the silencer body access do not exclude the fact that the passages may have some taper on their walls to allow for demoulding.

The use of a moulding technique allows the silencer body to be made in any desired shape to fit within an air intake duct. In Figure 4 the silencer body has an overall generally rectangular shape with rounded corners.

In Figure 5 the silencer body passages are indicated with the reference 122.

Figure 6 shows the arrangement of the silencer body 112 in an air intake duct 38, with the silencer body being closer to the upstream end of the duct 38 and an air cleaner 36 also mounted in the duct.

Claims

1. A silencer comprising a housing with an inlet and an outlet for the flow of gases, and a silencer body incorporating a plurality of small diameter, long, axial passages arranged to allow gas flow through the silencer body from the inlet to the outlet such that the noise of the gas flow is reduced by viscous dissipation of acoustic energy in the boundary layer formed at the walls of the passages.

2. A silencer as claimed in Claim 1, wherein the passage walls are smooth and continuous.

3. A silencer as claimed in Claim 1 or Claim 2, wherein the total cross-sectional area of all the passages in the silencer body is from 85% to 120% of the cross-sectional area of the housing inlet.

4. A silencer as claimed in any preceding claim, wherein the silencer body has from 0.3 to 1 passages per square millimetre.

5. A silencer as claimed in any preceding claim, wherein the passage wall thickness is between 0.04 and 0.2mm.

6. A silencer as claimed in any preceding claim, wherein the passage length lies in the range from 100 to 600mm.

7. A silencer as claimed in any preceding claim, wherein the passage walls are of metal.

8. A silencer as claimed in any preceding claim, wherein the passage walls are all parallel to each other and are all parallel to the silencer axis.

9. An exhaust pipe for mounting in a motor vehicle, the exhaust pipe including a tailpipe and a silencer mounted in line in the tailpipe, the silencer comprising an inlet, an outlet and a plurality of small diameter, long, axial passages arranged to allow gas flow through the silencer from the inlet to the outlet such that the noise of the gas flow is reduced by viscous dissipation of acoustic energy in the boundary layer formed by the walls of the passages.

10. An air induction system for mounting in a motor vehicle, the induction system including an inlet duct and a silencer mounted in line in the duct, the silencer comprising an inlet, an outlet and a plurality of small diameter, long, axial passages arranged to allow gas flow through the silencer from the inlet to the outlet such that the noise of the gas flow is reduced by viscous dissipation of acoustic energy in the boundary layer formed by the walls of the passages.

11. A silencer for silencing a gas flow, substantially as herein described, with reference to the accompanying drawing.