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).
Issues with a diffuser supercharger, concluding its unsuitability for cars due to noise and performance characteristics.
| Identifier | ExFiles\Box 140\1\ scan0191 | |
| Date | 30th October 1935 | |
| -2- The characteristic curve falls away more steeply as the output is increased than when no diffuser is present. This resulted in the boost curve flattening off badly at high speeds as shown on Hs.{Lord Ernest Hives - Chair}2213. This feature could no doubt be improved by increasing the depth of the rotor, but we think that it cannot entirely be avoided in a diffuser type supercharger working over such a wide speed range as is met with in car work. The diffuser supercharger was exceedingly noisy, producing a piercing whistle at full throttle which could not be overcome by the use of air silencers. This was attributed to interference between the rotor blades and diffuser vanes, since it disappeared if the diffuser was removed. The rotor had 12 blades and there was no difference in either pitch or intensity of noise between a 9 and a 7 bladed rotor. We do not think, therefore, that the trouble can be overcome by any such means as adopting unsymmetrical blade spacing. In view of the above and other considerations, we have concluded that we do not want a diffuser for car work for the following reasons :- (a) A flat supercharger characteristic is desirable when the blower is to operate at full throttle over a very wide speed range. (b) A diffuser makes the supercharger noisy. (c) From the design point of view the supercharger should be compact, and the rotor speed should be kept down. If a diffuser is incorporated either the overall diameter of the blower or the rotor speed must be increased. | ||