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).
Calculations and design considerations for carburettor air silencers on the Phantom II.
| Identifier | ExFiles\Box 13\3\ 03-page421 | |
| Date | 2nd May 1932 | |
| To E.{Mr Elliott - Chief Engineer} From Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} 87080. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}3/MJ.2. 5. 32. AIR SILENCERS FOR R.R. CARBURETTERS. We have been doing some calculations on the air intake silencers for the R.R. semi-expanding carburetter on Phantom II. We think that the Americans have established the necessity for having an adequate size of air silencer if good results are to be obtained without loss of power. Generally speaking, the freer the carburetter the more noise it makes. The old R.R. carburetter was not very noisy because it worked at about 5" of Mercury depression at full throttle. The size of silencer, however, was totally inadequate; so much so that it produced a further 3 HP. loss on top of the already restricted carburetter, which can only be considered very inefficient. While I was in the States I managed to get approximate outside dimensions of a number of American A.C. air silencers, which are almost without exception, the standard equipment of the industry. A typical silencer is that of the 'Nash' which is 12 3/4" long 7" dia., and the capacity of the engine for which this type of silencer is designed is under 5 litres. We have a 7.6 Litre 6 Cylinder engine from which we require superlative silence and efficiency. We have conducted tests on the test bed to ascertain the frequency of the note at which the intake roar occurs. We find that the length of the closed pipe which will damp out this roar is 3 ft. 3 ins., which gives a frequency of 89/sec. A resonator to deal with frequencies as low as these should have a capacity of 450 cu.ins. and for convenience of construction should have a length of not less than 12". In calculating this capacity we have allowed for a 3 ins. intake pipe as we have proved by previous experiments that such a size drops 2% of max power. Will you please consider what size of silencer you are prepared to fit to the semi-expanding carburetter having regard to the facts enumerated above, and having received the dimensions we shall proceed with the experimental work and evolve the most efficient silencer which can be obtained within practical limitations imposed. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} | ||