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).
Technical description and analysis of an Alfa-Romeo 1750 cc Roots type supercharger.
| Identifier | ExFiles\Box 124\5\ scan0085 | |
| Date | 9th July 1932 | |
| X 4482. R.{Sir Henry Royce} From Hs{Lord Ernest Hives - Chair}/Aln. c. Sg.{Arthur F. Sidgreaves - MD} Wor.{Arthur Wormald - General Works Manager} c. E.{Mr Elliott - Chief Engineer} Da.{Bernard Day - Chassis Design} c. By.{R.W. Bailey - Chief Engineer} Hdy.{William Hardy} (WW). X4550. Hs{Lord Ernest Hives - Chair}/Aln.3/MA. 9.7.32. ALFA-ROMEO SUPERCHARGER. The 1750 cc Alfa-Romeo car is fitted with a Roots type supercharger driven direct from the front end of the crankshaft. This embodies a cast aluminium casing, ribbed for cooling, and steel rotors of the two-lobed design providing a displacement of 1420 cc per revolution. The mass of one rotor with its gear-wheel is 5.2 lbs and the moment of inertia 6.2 lb in2 the radius of gyration being 1.09". The total weight of the blower is 32 lbs. The machining and general finish of the blower are good, but there are signs of scoring on the rotors and the inside of the casing. The rotors are geared together by a pair of very well made wheels which are located on the rotor shafts by taper pins driven through at right angles. The lobes of the rotors are of cylindrical form, eased off slighting at the tips. The hub portion is also a concave cylindrical surface on each side. The clearances between rotors and from rotors to casing are in the neighbourhood of .006". The gear backlash is about .004" at 1.275" radius allowing only about 11.0 minutes of arc relative movement. There is a passage cast around the casing which communicates direct from the carburetter port (which is the intake side of the blower) to the delivery port feeding the mixture pipe, thus by-passing the blower and supplying the engine direct. A light unsprung mushroom valve prevents any flow in the reverse direction. The object of this is evidently to deal with the conditions at very low speeds where the volumetric efficiency of the blower is so low that it sets up a back pressure instead of a boost. When the delivery pressure rises the valve will close and the by-pass cease to act. | ||