GB2071209A - Gas flow silencer - Google Patents

Abstract

The perforated pipe 20 within the sound-absorbing material 4 has a larger diameter than the inlet and outlet pipes 21, 22 and the outlet pipe extends into the pipe 20 for a distance between one quarter and one half of the total length of the pipe 20. The part 24 of the outlet pipe 22 projecting into the pipe 20 may have various configurations, including Venturi-nozzles and slots, Figs. 5 to 7 (not shown). <IMAGE>

Description

SPECIFICATION Silencer The invention relates to silencers, and is applicable to absorption silencers comprising a casing with one or more chambers, which contain absorption material and through which extends a gas channel, for example in the form of a perforated pipe.

Generally, adequate design of an exhaust gas silencer especially for high performance motors, necessitates very costly constructions. Known such silencer constructions rely upon at least one of two basic principles. On the one hand there are reflection silencers and on the other hand absorption silencers. Usually a good silencer unit contains both a large number of reflection elements, and chambers filled with absorption material and thus relies upon a combination of these two basic principles.

In addition to adequate silencing, and again this is particularly valid for high performance motors, the flow resistance encountered by the exhaust gases ejected from the motor into the silencer unit, should be kept as low as possible. Then not only motor performance but also specific fuel consumption can be favourably influenced.

Absorption silencers are known which comprise a simple single channel which, if necessary, can be equipped with a Venturi-nozzle or nozzles, inserted in the pipe. The Venturi-nozzles reduce low frequency humming noises. It has been proposed, instead of a straight-through pipe, to provide a multi-curved gas channel, for example formed by three parallel pipe or channel parts, connected to one another through return elbows.

In this way a total length of the passage is attained which is about two and a half times that of a silencer with a single pipe. Negative feedback and additional reflection and interference effects can further increase the efficiency of such a further-developed absorption silencer relative to a silencer with a single pipe passing through the casing. However, such a multi-curved pipe arrangement can only be housed in one plane, thus requiring a suitably flat casing. Additional flow resistance may also result from the multireturn pipe arrangement to the extent that, especially for high performance motors, it may lie outside acceptable tolerances.

The present invention seeks to overcome these disadvantages whilst taking advantage of the simple construction of an absorption silencer, with reference to its flow characteristics.

According to the present invention there is provided a silencer comprising a casing having one or more chambers containing absorption material and a gas channel extending therethrough, wherein a medial part of the gas channel within the or a chamber has a larger cross-section that its outlet end portion, and an outlet pipe projects into the medial part for a distance of substantially one quarter to one half of the length of the enlarged part of the channel.

The widening of the gas channel increases considerably the absorption available along the total length of the silencer. Accordingly the speed of flow of the exhaust gas is retarded, so that better realisation of the absorption potential of the silencer is possible. Extension of the outlet pipe into the widened gas channel gives a lower resonance level for the silencer and thus a better silencing of low frequencies is obtained. The acoustic efficiency of such a silencer, used as a reverberation silencer, reaches approximately the levels for the above described silencers equipped with three pipes interconnected by return elbows, without sharing the disadvantages of their increased flow resistance, yet may be less costly to make.

The cross-sectional widening of the gas channel preferably amounts to at least 40% in relation to that of the inlet and outlet pipes.

The region of the outlet-pipe projecting into the channel may also be shaped as a Venturi-nozzle.

In narrowing the free flow cross-section the Venturi-nozzle improves the silencing efficiency, especially in the low frequency region.

The flow resistance is increased only insignificantly by the Venturi-nozzle.

Finally, the region of the outlet pipe projecting into the channel may have longitudinal slots for example in the part formed by the Venturi-nozzle, or between this and the end of the gas channel.

Additional side flow is possible through these longitudinal slots so that further flow resistance behind the nozzle breaks down and yet again the acoustic behaviour of the silencer can be improved.

The simplest embodiment of the invention can be used with low performance motors. With increasing motor performance it is preferable to provide additionally at least the Venturi-nozzle, if not also the additional slotting.

Instead of a Venturi to reduce the cross-section of the outlet pipe in the silencer, the inner end of the outlet pipe can be rolled inwardly, in accordance with a then prescribed residual diameter of the total silencing system.

Embodiments of the invention will now be described, by way of example only and with reference to Figures 4 to 7 of the accompanying schematic drawings, Figures 1 to 3 of which illustrate known silencers. In the drawings Figure 1 shows one known embodiment; Figure 2 another known construction; and Figure 3 a further known form of silencer construction; Figure 4 illustrates one embodiment of the invention; Figure 5 shows a modified embodiment; Figure 6 shows another variant; and Figure 7 shows yet another possible embodiment.

The known silencer shown in Figure 1 consists of a casing 1 ,which in the example illustrated has two chambers 2 and 3, which are filled with absorption material. The casing 1 is penetrated in the longitudinal direction by a pipe 5, through which the exhaust gases (arrow 6) coming from the motor are conveyed. In order for it to correspond with the individual chambers, the pipe 5 is provided in the region of these chambers with perforations 7 and 8. It is thus an absorption channel of the simplest type.

As likewise known, this absorption channel can be improved as shown in Figure 2 by the introduction of a Venturi-nozzle channel section 9.

Otherwise this silencer has the same structural parts and features as the silencer illustrated in Figure 1.

Figure 3 illustrates an embodiment of an absorption silencer which in its silencing properties, is better than the two above described embodiments. However, because of its construction, it has a correspondingly higher flow resistance. The pipe 5 here forms three parallel parts 10, 11 and 12. The silencer is equipped with three chambers 13, 14 and 15. The separating walls 16 and 17 between the three chambers serve also as supporting means for the pipe parts 10,11 and 12 and return elbows 18 and 19 connecting those pipe parts.

The embodiment of the invention illustrated in Figure 4, provides a silencing effect which approximates somewhat to that of the silencer shown in Figure 3.

In the silencer shown in Figure 4, the pipe part 20 within the silencer is widened, preferably to such an extent that the increase in cross-section of the pipe part 20 in relation to the cross-section of the inlet pipe 21 and the outlet pipe 22 is about 40%. In the specific example shown, the silencer is again constructed as a two-chamber-silencer with chambers 2 and 3, which are filled with absorption material 4. The end 23 of the outlet pipe 22 projects into the widened pipe zone 20 by an amount which lies approximately between one quarter and one half of the total length of the pipe part 20. The length of the inserted end 23 depends in the individual case on the characteristics of the motor, the exhaust gas from which is to be silenced. Nevertheless, as stated, practically the same silencing is attained here as with the complicated structure according to Figure 3, but without encountering the not insignificant flow resistance that entails. If the silencing requirements increase with increasing motor performance, the outlet pipe 22, can be as shown in Figure 5, with an end zone 24, which corresponds somewhat to a Venturi-nozzle. Yet another embodiment is shown in Figure 6, in which longitudinal slots 25 are provided in the outlet pipe 22 just after the Venturi-nozzle 24.

Finally, as shown in Figure 7, in place of the Venturi-nozzle, the end 26 of the outlet pipe 22 can simply be rolled inwardly, in order to attain a certain narrowing of the cross-section.

Claims (7)

1. A silencer comprising a casing having one or more chambers containing absorption material and a gas channel extending therethrough, wherein a medial part of the gas channel within the or a chamber has a larger cross-section that its outlet end portion, and an outlet pipe projects into the medial part for a distance of substantially one quarter to one half of the length of the enlarged part of the channel.

2. A silencer as claimed in claim 1 , wherein the cross-section of the medial part is at least 40% greater than that of the outlet pipe.

3. A silencer as claimed in claim 1 or 2, wherein the part of the outlet pipe projecting into the channel comprises a Venturi.

4. A silencer as claimed in claim 3, wherein the outlet pipe has longitudinal slots in the part comprising the Venturi or between that and the end of channel in the silencer.

5. A silencer as claimed in claim 1 or 2, wherein the end of the outlet pipe in the channel is inwardly-turned.

6. A silencer as claimed in any preceding claim wherein the gas channel comprises a pipe perforated in the region of the or each chamber.

7. A silencer substantially as described herein with reference to, and as shown in, any one of Figures 4 to 7 of the accompanying drawings.