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 on the theory and construction of exhaust silencers from 'The Autocar' magazine.
| Identifier | ExFiles\Box 152\3\ scan0011 | |
| Date | 13th July 1912 | |
| THE AUTOCAR, July 13th, 1912. 69 Silencers. The Theory and Construction of Devices Designed to Reduce the Noise of the Exhaust. THE recent Local Government Board pronouncement that the use of a cut-out on motor cars will in future be visited by pains and penalties has had the natural effect of turning the attention both of the buying public and the automobile manufacturer towards the design of silencers rather more markedly than probably would have been the case had matters been allowed to remain as they were. We do not intend to convey by this that car designers have had to improve their exhaust boxes in order to comply with the new statute, for one must admit that the majority of autocars to-day are, if not absolutely silent—that would be a mechanical impossibility—at all events comfortably and reasonably so. We would not be taken to suggest that there is no further room for improvement, but in the average case a satisfactory standard of quietness has been set and upheld, at any rate so far as exhaust noises are concerned. This being so, the designer would not be likely to take much notice of any additional regulations were it not that the buyers of his cars have a right to interest themselves in the matter. In a certain number of cases they have, in the past, been able to obtain a little extra power by opening a cut-out. This advantage they must now deny themselves, and the result is a demand for a silencer which not only does not belie its name, but by enabling the engine to yield its maximum power, renders a cut-out, illegal as it is, also unnecessary. It has often struck us that if ever there were a “mystery-box” on a car it is the silencer. Essentially a passive instrument with no, or few, working parts, it rarely requires attention, and probably even more rarely receives it. The average owner, even if he be otherwise thoroughly au fait with the mechanism of his car, very seldom knows what type of silencer is used. His chauffeur will probably only know if continued excess of lubrication has entailed its cleaning out. Provided, of course, the exhaust box gives no trouble and performs its functions with regularity and efficiency, there is no need for anyone to pry into its internal economy. At best it has no interesting motions, no complicated mechanism, its workmanship is commonplace, and its mien, both exterior and interior, unattractive. What more natural then than that it should be regarded merely as a necessary attachment which is utterly devoid of interest. The last supposition, that it is uninteresting, is quite undeserved, as we hope to show. Let us first consider the primary significance of a silencer, its requirements, and its mode of operation. When an engine exhausts into the atmosphere direct it gives rise to a noise by reason of the fact that the rapid discharges of gas at high velocity set up sound waves in the air which are duly recorded by our eardrums. The noise is essentially due to variations in local pressure in the atmosphere immediately surrounding the exhaust pipes. At the instant of discharge this pressure is relatively high, whereas immediately afterwards it falls again to normal, being similarly raised and lowered at a very high rate. In other words, it is the impact of the rapidly-moving particles of gas upon these stationary particles of the surrounding air that produces, through a series of atmospheric waves, similar impacts upon our hearing organs, in exactly the same way as the impact of a hammer upon an anvil gives rise to noise. The primary function of a silencer is to get rid of this noise by preventing these sudden impacts from taking place except upon air, or gas, which is in some way insulated from the outer atmosphere. If, for instance, the engine were caused to discharge into a thickwalled chamber which was closed to the atmosphere no noise would be set up, but obviously unless some means of relieving the pressure which would develop inside the chamber were used the engine would very soon stop. If the pressure were relieved by a pump which withdrew as much gas from the chamber as the engine put in, such a device would act as an excellent, though, needless to say, unpractical, silencer. (It has been sometimes suggested, as an explanation of the fact that air-cooled motor cycle engines are nearly always relatively noisier than water-cooled motors of a similar size and power, that the actual noise of the explosion inside the cylinder is heard more readily through a thin cylinder wall than through the double wall of cylinder and jacket. Such an idea is utterly wrong. The action which takes place inside the combustion chamber is purely a chemical one and cannot directly give rise to noise; it does not, in fact, do so indirectly until the exhaust valve opens and the pressure in the cylinder can react on the atmosphere.) The same end, viz., that of getting rid of the impacts, or sharp peaks in the pressure curve, can be compassed in a much easier and more practical way, all that is required for this purpose being a pipe of considerable length and having thick walls. Now if this pipe be sufficiently long the air or gas it contains will have sufficient inertia and elasticity to absorb the discharge impacts and convert them into continuous motion. When the exhaust valve of the engine opens the outcoming gas will very rapidly accelerate that part of the column of air in the tube which is nearest the engine; this acceleration will be communicated to the whole of the contents, but owing to the elasticity and inertia of the fluid, the pulsating acceleration (as it were) at the engine end of the pipe will become a steady velocity at the other. If the pipe be long enough it will thus have at one end a series of violent gushes of pressure and the other a perfectly steady flow. The case is analogous to that of a row of men arranged in a narrow corridor in single file. The front man can be made to progress at a steady uniform speed by virtue of a series of pushes maintained on the person of the rear man! It is true that a plain pipe will act as a silencer, but the length it would require to be would be altogether out of the question as far as automobile work is concerned. If it be not very, very long, then the whole contents will tend to be accelerated all together and thus simply transmit the impact almost unaffected. In like manner, the file of men above referred to would need to be very long in order to damp out the oscillations produced by the pushes. But if the front man of a comparatively short file had to push a hand cart before him, this additional resistance would greatly enhance the damping effect, and he would progress with a steadier motion. The long plain pipe can be made an effective silencer if this analogy be pursued. If the orifice at its farthest end be constricted, a slight resistance is placed on the gas and a steady outflow is the result. Even so, however, the pipe would have to be prohibitively long | ||