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 discussing exhaust silencing using a Servais sound-absorption silencer and a related propeller shaft issue.
| Identifier | ExFiles\Box 152\3\ scan0211 | |
| Date | 19th June 1936 | |
| The Light Car 150 JUNE 19, 1936. PRACTICAL ASPECTS BY 'SHACKLEPIN' Exhaust Silencing by Sound Absorption—A Convincing Demonstration—A Possible Propeller Shaft Trouble TIME was when a noisy exhaust was regarded by some drivers as a kind of hall-mark of engine efficiency. Many people, particularly the anti-motorist section of the community, have never agreed with this, and, indeed, it must be admitted that a noisy exhaust can be very objectionable. If, however, silence can be obtained only by loss of efficiency, there is an excuse for a certain amount of noise. Motorists have always received more than their fair share of blame with regard to general traffic noises, but I do not propose here to embark upon any discussion on that aspect of the matter. My purpose is to point out that practically all modern cars achieve a perfectly reasonable degree of exhaust silence, thanks to the very considerable study which has been made of the problems relating to silencer design. I was present last week at a very interesting demonstration in London organized by Servais Services, Ltd., of 8, Old Pye Street, Westminster, London, S.W.1. This concern manufactures the Servais silencer, which operates on a sound-absorption principle. Reference to it has already been made in The Light Car, but, as no very precise details were available at the time, it seems fitting now to deal with it more extensively. An accompanying illustration shows that the silencer is of the straight-through type, which means that there are no baffles of any kind, the exhaust gases passing unobstructed through the centre portion. The silencing medium consists of various grades of sound-absorbing material held between the layers of a rolled-up expanded-mesh framework, the whole assembly being contained in an asbestos-lined steel casing. This cut-away view of the Servais silencer shows the layers of sound-absorbing material held on an expanded-mesh framework. The exhaust gases pass unobstructed through the central passage. In action the exhaust gases pass freely through the centre passage, but the sound waves spread upwards and are damped out, or absorbed, by the porous lining. It should be pointed out that the lining is free to vibrate within the casing, as this has a very considerable bearing upon the effectiveness of the silencer. Furthermore, the vibration ensures that a dirty exhaust cannot give rise to the formation of carbon deposits within the silencer, as the soot and so on is shaken clear and blown out with the exhaust. The Servais silencer has, of course, been thoroughly tested on the bench and on the road. Bench figures taken on a six-cylinder 2-litre engine show that there is an improved power output as compared with two other types of silencer, whilst there is also a very considerable reduction in back pressure, which means that, in spite of the high degree of silence achieved, the engine runs more efficiently and with a reduced fuel consumption. The symbols X and X+ denote how the length of a propeller shaft is altered when the rear axle is deflected. In a bad case the splined end may be pulled out of its housing. The purpose of the U-shaped bracket is explained in the text. Modern science assesses sound in terms of phons and decibels. I must confess to having not the slightest idea as to the difference in these terms or, indeed, as to what exactly either of them is intended to convey; nevertheless, as a basis for comparison, I am told that on the test engine the noise made by the Servais silencer was 65 phons, whilst, in the comparison types, the noise was 80 phons and 92 phons respectively. Therefore, whatever a phon may be, we are left with no doubt that the Servais is a very effective silencer. At the demonstration which I attended this effectiveness was made clear in a rather interesting manner. The demonstrator had a pistol firing blank cartridges, and the discharge of one of these was undoubtedly very noisy. With a very small Servais silencer (about 10 ins. long by 2 ins. diameter) slipped over the barrel the discharge was literally inaudible and one heard only the click of the hammer as it fell on to the cartridge. Incidentally, I understand that Servais silencers have been fitted experimentally to machine-guns with remarkably satisfactory results. They are also being used on Diesel engines, the silencing of which, in the ordinary way, is a by no means easy problem. I have dealt with this silencer at some length because I was impressed by its efficiency and I know that many of my readers run old cars on which the silencing arrangements are not so good as might be desired. It is extremely difficult effectively to clean out a welded-up silencer of normal type when it becomes choked with carbon, and the practice of knocking holes through the baffle plates, with a long, pointed drift, in order to provide a free exhaust, almost invariably leads to excessive noise. Servais silencers are not expensive; the prices range from 27s. 6d. E38 | ||