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).
Inspection report on a two-stage roots blower supercharger from Melbourne Engineering Co.
| Identifier | ExFiles\Box 140\1\ scan0230 | |
| Date | 5th October 1938 | |
| Lr{Mr Ellor}/Ain.10/GC.5.10.38. MELBOURNE ENGINEERING CO'S SUPERCHARGER. We called at the Standard Garage, Alfreton Rd., yesterday to inspect a supercharger sponsored by a Mr. Winterbottom of the above concern. It is a special arrangement of roots blower, there being a central rotor having four lobes working in conjunction with two two-lobed rotors, one on each side of it and running at twice its speed. There is also a partition in the plane of rotation which divides the working compartments so as to provide two stages of compression. Whereas in centrifugal blowers, multi-stage working usually involves some loss of efficiency, in a roots type it may give an improvement since there is a definite theoretical limit to the stage efficiency depending on the compression ratio. Thus the theoretical maximum adiabatic efficiency for a 4/1 compression in a single-stage roots is 57% whereas two stages of 2/1 each would be 74.8% overall. As these values cannot be reached in practice however, it is doubtful whether the arrangement would be of use for aero work. The blower inspected is represented diagrammatically in the attached sketch Ain.70. Two carburetters are fitted at the two intake ports of the first stage as indicated. The two delivery ports then lead by more or less diagonal passages cast in the casing to the two inlet ports of the second stage, the two deliveries of which feed the engine. It may be seen by following the action of the blower that the mixture from the two carburetters intermingles and therefore, as they point out the carburetter settings need not be perfectly balanced. The casing is of aluminium, fairly well ribbed. All ports are nearly the full axial width of the rotor compartments. The first stage appeared to be about 1 1/4 times the length (and hence the displacement) of the second; the rotor sections being identical in both stages. The first stage was said to be of 1 1/2 litre displacement per revolution. The lobes are of involute form and clearances were said to be about .005". The driving gears appeared very substantial and of good finish. There were 26 teeth on the side shaft wheels, which were mounted with 26 splines on the hubs. This provides a vernier adjustment for meshing the rotors when assembling. | ||