From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article from 'The Autocar' magazine discussing the design and principles of various automobile silencers with diagrams.
| Identifier | ExFiles\Box 152\3\ scan0013 | |
| Date | 13th July 1912 | |
| THE AUTOCAR, July 13th, 1912. 71 Silencers. It has often been said that the reason an engine fitted with a silencer (provided the latter be properly designed) will give more power than one exhausting direct into the atmosphere is that the exhaust from one cylinder acts like a pump upon that of another, and consequently sucks it out, and that a similar effect obtains with the successive explosions which emanate from one individual cylinder. This idea, though more or less founded on fact, is fallacious in the form in which it is generally expressed. What happens is this. When the engine exhausts direct into the atmosphere the discharged gas has to accelerate a mass of air from rest up to its own speed, whereas if it exhausts into a pipe which contains a moving column of air, the power required for such acceleration is less, and as a consequence the resistance to the egress of the exhaust is less. To argue that any “pumping” effect exists suggests that the speed of the gases inside the exhaust pipe is greater than that with which they are issuing from their exhaust port, which is, of course, absurd. The minimum loss of power in this respect will obviously occur when the final-exit speed of the exhaust gas is the highest, and that being so it is clear that for a given degree of silence of two engines, one six-cylinder and the other four, the former will gain the most from its silencer, since the exhaust delivers more impacts in unit time, and the speed at which the gas becomes free from impacts and moves with uniform velocity is higher. Similarly an eight-cylinder will be better than a six, and a two-cylinder better than a single. Not only so, but it is clear from what has been said above that the greater the number of cylinders in the engine the simpler can the silencer be made. To get the same effect a 12 h.p. single-cylinder requires a silencer much larger than a 12 h.p. four-cylinder—about four times as large, in fact. An engine with an infinite number of cylinders would exhaust silently if all its cylinders could be made to discharge into a short plain pipe. The gas would issue from it at the same velocity as it came from the cylinders, but since the velocity of the flow would be constant there would be no noise. We now illustrate and describe a few of the silencers which are in use at the present time. In most cases the sketches are almost self-explanatory. Fig. 1 represents a type of silencer (sic) which is used on quite a number of racing cars. It is good in respect of ease of manufacture and simplicity, but bad in this, that if long pipes are used from the engine to the expansion chamber a very complicated fluid-motion will take place therein. The gases are given no particular direction, and after striking against the wall of the expansion chamber, tend to raise pressure in the other pipes which are temporarily inactive. Unless the chamber be made of thick material it will act to a certain extent as a sounding-box. The silencer used on Calthorpe cars is of the simple type illustrated in fig. 2. To judge by results it is, however, quite effective. The conical ends of the box enhance the radiating effect of the surface, as the gas in expanding naturally impinges against the walls, whereas, in the square ends, surfaces of discontinuity would be formed in the corners. The function of the baffles is two-fold: to offer a braking effect and to direct the gas against the cool walls. It will be noticed that these baffle plates, by considerably overlapping one another, do not allow a sound wave to pass direct from one end of the box to the other. The silencer shown in fig. 1 will allow sound waves to pass unchallenged unless its farther end be shaped as shown in fig. 3, which represents a type of orifice which is in fairly general use. Whatever kind of silencer be used, such an orifice is well worth while. It could, however, be improved by being at one point flattened vertically, and another point a little further on flattened horizontally. It would thus arrest sound waves in any plane. The above and the silencers which immediately follow may be described as of the “simple deflection” type, whereof there are a number of examples. Fig. 4 shows the Nest o’ Cups device, which consists of a number of baffle plates pierced with large-sized holes, of which the edges are so set as to cause the gases to adopt a very indirect course. The baffles and the shell of the contrivance are built up in a series of cups, which enables the contrivance to be enlarged if necessary to almost any length, at the same time facilitating cleaning out. It will be noticed that the trumpet-shaped holes in the baffles stiffen them against vibration. Direct sound waves are also interrupted by the baffle plates. Fig. 6.—The concentric chambers in the Cadillac silencer. Fig. 7.—The Star silencer, showing the helical spring. Fig. 8.—The multiple cone type of silencer (Binks). Fig. 9.—The Panhard silencer consisting of two plain baffle plates. Fig. 10.—The Rolls-Royce silencing arrangement. | ||