EP0020823A1

EP0020823A1 - Engine exhaust silencer

Info

Publication number: EP0020823A1
Application number: EP79301033A
Authority: EP
Inventors: Lionel Fothergill
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-04-10
Filing date: 1979-06-01
Publication date: 1981-01-07

Abstract

in an engine exhaust silencer the gases are first led along an elongate annular passage, defined between two chambers mounted one within another, and are then funnelled into and along an elongate exhaust tube whose longitudinal wall is perforated and is surrounded by one or more noise attenuating chambers. The outer wall of the annular passage converges towards, but ends just short of, the elongate exhaust tube, and the gap between the two is surrounded by and opens into an expansion chamber which joins and seals the annular passage exit to the exhaust tube inlet.

Description

The invention relates to apparatus for attenuating the poise caused by a moving stream -of gases.
Such apparatus is most commonly embodied in the form of an engine exhaust silencer, and will be referred to as such throughout the rest of this specification. It is to be understood, however, that the term "engine exhaust silencer" is used for convenience only and is not to be used to limit the scope of the invention unduly.
It is known to attenuate the noise of the exhaust gases of an internal combustion engine by leading the gases along an elongate annular passage defined between two chambers mounted one within another. It is also known to'attenuate exhaust gas noise by leading the gases through an exhaust tube whose longitudinal wall is perforated and is surrounded by one or more noise attenuating chambers containing sound absorbing material.
Each of these constructions has its respective advantages and drawbacks. The annular passage type of silencer absorbs relatively little of the power of the engine, whereas the perforated exhaust type of silencer tends to absorb relatively more of the engine power; but the latter type of silencer tends to provide much greater sound attenuation when used on vehicles (in which the overall length of the silencer is necessarily limited).
A primary object of the invention is to provide an improved exhaust silencer which, when installed for example in the exhaust outlet of an internal combustion engine, will provide substantial reduction of exhaust gas noise without unduly affecting the engine performance.
In its broadest aspect the invention provides an engine exhaust silencer in which the gases are first led along an elongate annular passage defined between two chambers mounted one within another, and are then funnelled into and along an elongate exhaust tube whose longitudinal wall is perforated and is surrounded by one or more noise attenuating chambers, the outer wall of the annular passage converging towards but ending just short of the exhaust tube, and the gap between the two being surrounded by and opening into an expansion chamber.which joins.and seals the annular passage exit to the exhaust tube inlet.
It is found in practice that such a silencer gives satisfactory noise attenuation whilst being characterised by an abnormally low back pressure at its inlet. The invention thus makes it possible for an engine to be satisfactorily silenced without the silencer absorbing an undue proportion of the engine power. The overall noise attenuation can probably be attributed to the combination of the annular passage followed by the perforated exhuast tube and its surrounding chambers: the especially low back pressure, however, is thought to be a direct result of converging the annular passage outlet towards the exhaust tube inlet (thus effectively defining a venturi which pulls. the gases through into the exhaust tube) and at the same time . providing a gap surrounded by an expansion chamber (thus relieving the build up of back pressure caused by the convergence).
The size of the gap, the area of the expansion chamber, and the rate of convergence of the annular passage-exit, will of course be balanced against one another in order to achieve a practical embodiment. For example, if the gap were to be made unsuitably large, the "venturi" effect would be lost; whereas if the gap were to be abnormally small, the back pressure at the silencer exhaust would rise unacceptably.
If the inner chamber defining the annular passage, or the sound attenuating chamber or chambers surrounding the exhaust tube, is filled with sound absorbing material such as steel wool or fibre glass, the noise attenuation will be further improved. This art is already known in itself. However, if the inner chamber defining the annular passage contains sound absorbing material and has a down stream wall which is perforated to allow the gas flowing over the chamber to communicate with said sound absorbing material, the efficiency is improved still farther. The perforated end wall in conjunction with the converging outer wall of the annular passage creates gas flow conditions which materially increase. the attenuation properties of the silencer.
If the downstream end of the inner chamber defining the annular passage is streamlined, irrespective of whether or not it is also perforated, further benefits are obtained. In one such construction, the streamlined end of the inner chamber could continue approximately parallel with the converging annular passage outer wall, and could be unperforated. It would then assist the gas flow towards the exhaust tube, as well as improving the sound attenuation by effectively prolonging the annular pas.sage. In another possible construction, the downstream end of the inner chamber could be streamlined and could be perforated as well. The chamber could then either be filled with sound absorbing material, or could be without such material and could constitute a sound attenuating resonating chamber. In-a third possible construction, the downstream end of the inner chamber could taper towards the exhaust tube inlet but could be in the form of an open ended cone, with the inner chamber being closed off inside that cone by a separate perforated end wall. This construction is especially advantagous since it provides an expansion area within the walls of the open ended tapering cone, thus lowering the back pressure at the eventual exhaust.
The noise attenuating properties of a silencer embodying the invention can be further increased, if the expansion chamber joins and seals the annular passage exit to the exhaust tube through the intermediary of a further tube, the further tube also having, part way along its length, an annular gap which is surrounded by and opens into a sealed expansion chamber. With such a construction, the gases leave the annular passage exit and are funnelled into the further tube from whence they pass straight into the exhaust tube. The presence of the annular gap and the expansion chamber of the intermediate tube tends to remove very deep resonant notes. This construction is thus especially useful on silencers which are intended for fitment to large diesel engines.
In any silencer embodying the invention, the cross-sectional area of the annular passage is never less than the cross-sectional area of the silencer inlet. This again avoids build up of back pressure.
Several silencers each embodying the invention are shown in the accompanying drawings. They will now be described, by way of example only, and with reference to those drawings.
Each drawing is a diagrammatic side section, and in order the drawings illustrate:

Figure 1 - the basic principle behind the invention;
Figure 1A --a diesel engine silencer;
Figure 2 - another diesel engine silencer of modified design;
Figure 3 - the first portion of a petrol engine silencer; and
Figure 4 - the second portion of the silencer of Figure 3.

Referring first to Figure 1, exhaust gases from an engine(which is not shown inthe drawings) enter the silencer through an inlet 11. They are then, as shown by the arrows, forced to take an annular path defined between inner and outer chambers respectively 12, 13 which are mounted one within another and which - in this particular example - are circular cylindrical and co-axial. As the radial distance between the chambers 12, 13 is small in comparison to the combined overlapping length of the chambers, the exhaust gases are constrained to flow in contact with or close to the wall of the outer chamber 12, causing some cooling of the gas and a reduction in the volume thereof.
The front end wall 14 of the inner chamber, onto which the incoming gases impinge, is generally convex. The downstream end wall 15 of the chamber is perforated, as indicated in broken line in Figure 1, and the chamber is tightly packed with sound absorbing material such as steel wool or fibre glass. The longitudinal wall of the cylindrical chamber 13 is not perforated.
The outer chamber 12 narrows at its downstream end. This narrowing is achieved by welding an open ended cone frustum 16 inside the chamber so that the cone frustum 16 effectively constitutes an extension of the outer chamber 12. The gases from the annular passage are thus forced to follow the taper of the cone frustum 16, and in the area of this frustum shaped channel 16 considerable noise attenuation is achieved by gas and pressure waves passing through the perforations in the wall 15 of the inner chamber 13 and contacting the sound absorbing material within that chamber.
The cone frustum 16 converges towards, but stops just short of, the inlet of an exhaust tube 17 which constitutes the second portion of the silencer. The longitudinal cylindrical wall of the tube 17 is perforated. Respective end walls 18a, 18b seal the radial gap between the tube 17 and the outer chamber 12 of the silencer, and the annular chamber so defined is again tightly packed with sound absorbing material. As the gases are funnelled into and along the exhaust tube 17, the annular chamber between the walls 18a and 18b acts as a noise attenuating chamber.
Because the cone frustum 16 stops just short of the inlet to the exhaust.tube 17, there is a gap between the two. This gap opens into what is effectively an expansion chamber, defined by the surfaces 12, 16, 18a, and further reduction of noise is obtained as the gas passes over the gap between the narrow end of the cone frustum 16 and the inlet to the exhaust tube 17.
Referring now to Figure 1A, the silencer shown therein embodies the principles of Figure 1 in a practical form. Parts of this silencer which correspond to the same parts of Figure 1 have been given the same reference numbers as in Figure 1. Thus, respective inner and outer chambers 12, 13 define between them an annular passage into which the gases entering at 11 and impinging on the end wall 14 are forced to flow and are then funnelled down into the cone 16 and swirlaround against the perforated downstream end wall 15 to flow along the exhaust tube 17 which is perforated and surrounded by sound absorbing material. In the Figure 1A design, however, the end wall 14 of the inner chamber is a streamlined cone, and the annular passage is noticeably short. The inner chamber 13 is held coaxially within the outer chamber 12 by three equally circumferentially spaced supporting webs 19. The webs 19 are longitudinally streamlined so as to lessen their resistance to the gas flow around them.
It will be noted that the silencer of Figure 1A is remarkably short. It takes up relatively little space, whilst achieving a very acceptable ndse attenuation. Non-public tests indicate that the silencer of Figure 1A fitted to a Bedford Model 500 sixteen ton lorry powered by a 150 brake horsepower diesel engine reduced the engine noise level at exhaust below the legal limit, whilst giving a substantial fuel saving of the order of 11 or 12% over the silencer fitted as standard to the lorry.
Figure 2 illustrates a silencer which is similar to that of Figure 1A but which includes a third . section inbetween the annular passage section and the perforated exhaust tube. This third section consists of inner and outer chambers respectively 21, 22 which are so sized that the inner chamber 21 joins or fits within the perforated exhaust tube 17; whilst the outer chamber 22 fits closely within the chamber 12 of the annular passage section. The silencer can thus very easily be taken apart into its three component sections. The tube 21 is split, part way along its length, by an annular gap 23 which is surrounded by and opens into the expansion chamber defined between the two tubes 21, 22.
In this particular silencer, the perforated end wall 15 of the inner chamber 13 has a minimum of 40% of its surface area perforated.
The effect of the additional intermediate section 21, 22 is to cut out very deep resonant notes. The whole silencer is only just over 3½ feet long, yet non-public tests with the silencer fitted to a Daimler Ileetline bus powered by a Gardner 6LXB diesel engine resulted in an abnormally quiet exhaust. On tickover, the back pressure in-this silencer was only ½" water gauge - approximately half the value obtained on tickover with the standard silencer supplied with the engine. At maximum engine speed, the standard silencer indicated 14" water gauge pressure at exhaust: the silencer of Figure 2 indicated only 4" water gauge pressure whilst cutting out almost entirely the deep resonant notes which the standard silencer is unable to get rid of.
The silencers described so far are intended for use with diesel engines. The silencer illustrated in Figures 3 and 4 is, by comparison, intended for use with a petrol engine. Non-public tests of one such silencer have been carried out. The following results were observed.
The silencer of Figures 3 and 4 was fitted to a Ford Granada motor car powered by a 2 litre petrol engine. The standard silencer of this car is in two successive sections, which are separate from one another and are in the form of elliptical boxes filled with sound absorbing material. The gas flow from the first box is led to the second box by a length of standard tubing.
The annular passage silencer shown in Figure 3 replaced the first of the boxes, whilst the perforated exhaust portion shown in Figure 4 replaced the second box. The composite silencer thus embodied the principles of the invention, but instead of the two portions being joined directly to one another they were joined by the length of tubing previously mentioned.
At 1000r.p.m. the standard Granada silencer showed i" water gauge pressure at exhaust, as did the silencer illustrated in Figures 3 and 4.
At 2000 r.p.m. the standard silencer indicated 1" water gauge pressure, the illustrated silencer indicated ¼" water gauge pressure.
At 3000 r.p.m. the standard silencer indicated 1½" water gauge pressure at exhaust, the illustrated silencer indicated 1" water gauge pressure at exhaust.
At 4000 r.p.m. the standard silencer indicated 3½" water gauge pressure at exhaust, the illustrated silencer indicated 2½" water gauge pressure at exhaust.
In all these situations, the noise attenuation given by the illustrated silencer was at least comparable with that given by the standard silencer.
In summary, it can be seen that the invention is applicable to a wide range of situations and results in acceptable noise attenuation whilst absorbing remarkably little of the power generated by the engine to which it is fitted.
The drawings show that silencers embodying the invention are compact and simple in construction, and are likely to weigh no more than silencers of conventional type.

Claims

1. An engine exhaust silencer in which the gases are first led along an elongate annular passage defined between two chambers mounted one within another, and are then funnelled into and along an elongate exhaust tube whose longitudinal wall is perforated and is surrounded by one or more noise attenuating chambers, the outer wall of the annular passage converging towards but ending just short of the exhaust tube, and the gap between the two being surrounded by and opening into an expansion chamber which joins and seals the annular passage exit to the exhaust tube inlet.

2. A silencer according to claim 1, in which the inner of the two chambers defining the annular passage contains sound-absorbing material, and has a downstream end wall which is perforated to allow the gas flowing over the chamber to communicate with said sound-absorbing material.

3. A silencer according to claim 1 or claim 2, in which the downstream end of the inner of the two chambers defining the annular passage is streamlined.

4. A silencer according to claim 3, in which the streamlined downstream end of the inner chamber continues approximately parallel with the outer wall of the annular passage as said wall converges towards the exhaust tube.

5. A silencer according to claim 4, in which the streamlined end of the inner chamber takes the general form of an open-ended cone, the inner chamber being closed off inside that cone by a separate perforated Wall.

6. A silencer according to any of the preceding claims, in which the expansion chamber joins and seals the annular passage exit to the exhaust tube through the intermediary of a further tube, the further tube also having, part-way along its length, an annular gap which is surrounded by and opens into a sealed expansion chamber.