GB2041083A - Silencer for an internal combustion engine - Google Patents

Abstract

An exhaust silencer has a central duct 13 to receive exhaust gas at 14 and pass it to atmosphere at 15, and a surrounding annular passage closed at its upstream and downstream ends and divided into a series of chambers 20, 21, 22 by perforated spaced annular walls 16, 17. Perforations 23, 24, 25 in the central duct place it in communication with each of said chambers. The central duct has within it at a location corresponding to one of chambers downstream of the most upstream chamber a convergent nozzle 26 and a divergent nozzle 27 each comprising a small cylindrical section joined to a frustoconical section. Preferred ratios are given for the diameter of the central duct, the distance between the cylindrical sections of the two nozzles and the diameters of the cylindrical sections. <IMAGE>

Description

SPECIFICATION Silencer for an internal combustion engine This invention relates to a silencer for the exhaust system of an internal combustion engine.

The exhaust gas of internal combustion engines is subject to pressure pulsations having an amplitude and frequency which vary greatly with the working conditions of the engines, and which give rise to uncomfortable noise if not effectively silenced before discharge to atmosphere. One or more silencers are therefore inserted into engine exhaust pipes, their type depending on the required performance.

In designing silencers for internal combustion engines, various requirements have to be satisfied such as low cost, minimum overall size and reliability and duration of operation, but generally the main requirement is for a high degree of silencing with the smallest possible increase in the exhaust pipe back pressure, so as not to reduce the power delivered by the engine to an unacceptable degree.

In practice, it is not easy to attain satisfactory designs from all points of view, because the pressure and frequency characteristics of the exhaust gas change greatly with the loading and speed of the individual engine, and also vary greatly from one engine to another.

Generally, for damping relatively low frequencies use is made of resounding devices, reflection chambers and interference systems, while acoustic absorption materials are used for damping high frequencies. Moreover, in the commonly used silencers, the gas flow can be either practically continuous or can be subjected to interruptions, deviations and division. In the first case, silencers offer a very low resistance to gas passage, but the damping level is limited to very restricted frequency bands, while in the second case silencers have a high flow resistance, but give damping over a side frequency range.

According to this invention there is provided a silencer for the exhaust system of an internal combustion engine comprising metal walls defining an annular passage closed at its ends by annular end walls and divided into a plurality of chambers by perforated internal annular walls spaced along the length of the passage and a central duct connectdd at one end to said exhaust header and at the other end to said pipe which conveys the gas to atmosphere, said duct having perforations along its length through which the duct communicates with each of said chambers, having therein, at a location radially inward of the chamber or one of the chambers downstream of the most upstream of the chambers, an upstream convergent nozzle and a downstream divergent nozzle, each nozzle being constituted by a frusto-conical section and a cylindrical section which is of lesser crosssection than said duct.

The two cylindrical sections tend to damp the higher frequencies, and induce damping by interference because they create two zones in which the pressure drop sucks into the duct the gas which flows through the silencer chambers, so assisting its mixing with the gas stream flowing through the duct.

These positive effects from the point of view of acoustic damoing are obtained with a low pressure drop because of the presence of the frusto conical sections which give a gradual variation in cross-section between the duct and the cylindrical sections.

Moreover, the acoustic mass of the gas contained in the two cylindrical sections in conjunction with the capacity of the silencer chambers constitutes a system which is tuned for damping determined frequencies of the exhaust gas.

It has been found that the dimensional proportions of the cylindrical sections of the nozzles are very important for silencing purposes. In a preferred embodiment of the invention the ratio d/D, where d is the diameter of said cylindrical sections to the diameter D of the duct, is in the range 0.50 to 0.75, the ratio of the length I to the diameter d of said cylindrical sections l/d'0.4, and the ratio L/d is in the range 2.5 to 3.5, where L is the distance between the cylindrical sections of said nozzles and drn is the smaller of the diameters d of the cylindrical sections.

The invention will now be described in more detail with reference to the accompanying drawing which shows a preferred embodiment of the invention by way of example.

In the drawing, the reference numeral 10 indicates a tubular silencer shell, and the reference numerals 11 and 1 2 indicate annular walls which close it at its ends. A duct 1 3 extends through the entire length of the silencer and is coaxial with the shell 10. The ends of duct 1 3 are coupled to pipe portions 14 and 1 5 which are connected respectively to the engine exhaust gas manifold and the pipe for conveying the gas to atmosphere, which is generally fitted with at least one further silencer.Two internal annular walls 1 6 and 1 7 are provided with holes 1 8 and 19, and are fixed to the shell 10 and to the duct 1 3 to divide the internal volume of the silencer into three annular chambers 20, 21 and 22.

The duct 1 3 is provided with a set of holes indicated generally at 23 situated to open to the chamber 20, a set of holes indicate generally at 24 opening to chamber 21, and a set of holes indicated generally at 25 opening to the chamber 22.

Two nozzles 26 and 27 are inserted in duct 13 where it bounds the chamber 21 and project towards each other. Nozzle 26 is convergent and nozzle 27 divergent in the direction of the gas flow. The nozzles have frustoconical sections 28 and 29 respectively, and cylindrical sections 30 and 31 respectively, the latter being of small cross-section relative to the cross-section of the duct 1 3. The outer edge of each cylindrical section may terminate in a slightly flared portion to reduce the outlet and inlet losses of the gas passing through the two nozzles 26 and 27 respectively. The engine exhaust gas enters the silencer through the duct portion 14, and divided into two gas streams, of which one flows through the duct 1 3 and the other passes through the holes 23 into the chamber 20.This second gas stream passes through the holes 18 of the annular wall 1 6 and into the chamber 21. From this chamber, part of the gas flows back into the duct 1 3 through the holes 24 to mix with the gas stream flowing through the duct 1 3 and a part flows into the chamber 22 through the holes 1 9 in the annular wall 1 7 and is finally returned from chamber 22 to the duct 1 3 through the holes 25.

The overall silencing effect on the silencer is due to the contribution of various phenomena: attenuation of the lower frequencies is due to the successive expansions of the gas stream flowing through the silencer chambers, and attenuation of the higher frequencies is due to the presence of the small cross-sections of the cylindrical sections 30 and 31 of the nozzles 26 and 27 in the duct 13.

A further silencing effect is due to interference between the gas stream flowing from the chamber 21 and the gas stream flowing through the duct 13, in those zones close to the cylindrical sections 30 and 31, due to the aspiration effect deriving from the pressure drop which the gas undergoes through the small cross-section of said cylindrical section.

A final silencing effect is due to the acoustic mass of the gas contained in the two cylindrical sectors 30 and 31, in conjunction with the capacities of the chambers 20, 21 and 22 and of the portions of the duct 1 3. These together constitute a tuned system for damping determined frequencies of the exhaust gas.

The best response was obtained when the silencer was designed according to the following dimensioning criteria: the ratio d/D is in the range 0.50 to 0.75, where d is the diameter of the cylindrical section (30 or 31) and D is the diameter D of the duct 13, the ratio I/d = 0.4, where I and d are respectively the length and diameter of the cylindrical section (30 or 31), and finally the ratio L/dm is in the range 2.5 to 3.5 where L is (as shown) the distance between the cylindrical sections (30 and 31) and drn is the smaller of the diameters d of the cylindrical sections.

The frusto-conical sections 28 and 29, which produce gradual cross-section variations between the duct 1 3 and cylindrical sections 30 and 31, enable the pressure drop to be kept within acceptable values, such that the back pressure in the exhaust manifold upstream of the silencer is not influenced adversely.

Claims (5)

1. A silencer for the exhaust system of an internal combustion engine comprising metal walls defining an annular passage closed at its ends by annular end walls and divided into a plurality of chambers by perforated internal annular walls spaced along the length of the passage and a central duct connected at one end to said exhaust header and at the other end to said pipe which conveys the gas to atmosphere, said duct having perforations along its length through which the duct communicates with each of said chambers, having therein, at a location radially inward of the chamber or one of the chambers downstream of the most upstream of the chambers, an upstream convergent nozzle and a downstream divergent nozzle, each nozzle being constituted by a frusto-conical section and a cylindrical section which is of lesser crosssection than said duct.

2. A silencer as claimed in claim 1, wherein the ratio of the diameter d of said cylindrical sections to the diameter D of the duct d/D is in the range 0.50 to 0.75.

3. A silencer as claimed in claim 1 or claim 2, wherein the ratio of the length I to the diameter d of said cylindrical sections l/d = 0.4.

4. A silencer as claimed in any one of claims 1 to 3, wherein the ratio of the distance L between the cylindrical sections of said nozzles and the smaller of the diameters d of the cylindrical sections L/dm is in the range 2.5 to 3.5.

5. A silencer for the exhaust system of an internal combustion engine substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.