Low Back Pressure in a New Silencer
Page 52
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HAVING for some years specialized in the design and Iproduction of air, fuel and lubricant filters, C. G. Vokes, Ltd., 95, Lower Richmond Road, London, S.W.15, has now turned its attention to the development of an improved form of silencer for internal-combustion engines, aiming at efficient silencing, low back pressure and reduced weight and size.
The basic principle adopted has been to turn the pulsating movements of gases under pressure into a steady stream, and reference to an accompanying sketch shows one of the simplest of several forms of silencer that have been produced.
It wilt be seen that the exhaust gas strikes an inclined • p38
deflector plate, so shaped as to throw the gas tangentially against the inside of a cone. This causes a whirling, spiral column of gas to descend the cone towards its smaller outlet, and a partial vacuum is created at the axis of the cone.
The cone is surrounded by an outer cylinder forming the silencer case, and is provided with a number of holes, the aggregate area of which is at least equal to the area of the outlet aperture. These holes enable the outer chamber to act as an equalizing space for .the pulsations from the individual engine cylinders. At the same time, the high speed of the gas emitted at the smaller end of the cone tends to lower the average pressure in the annular surrounding chamber. Sound is thus absorbed by the central or axial part-vacuum and by the annular part-vacuum.
The gas does not actually lie dormant in the annular
chamber, but is subject to continual inflation at the large end of the cone and extraction at the small end. Flame does not come into contact with the outer case; this is the reason why the " tinny " drumming and the running-down noise are not experienced.
The design permits of oval, circular or rectangular silencers, to suit various engine layouts, also special shapes to suit aircraft engines, etc. A virtue claimed for the design is that the performance will not vary after prolonged use, because there is nothing to wear out or to choke up.
A modification of the idea is being employed in an e7perirnental silencer for aeroplanes, a drawing of which also accompanies this article.