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).
Carburettor performance, comparing expanding versus fixed jet types and their effect on power output and engine flexibility.
| Identifier | ExFiles\Box 182\M19\ img249 | |
| Date | 2nd October 1931 | |
| Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}1/AD2.10.31 contd. -2- All we are trying to indicate by sending you these curves is that there is a very real difference between 3" and 2" induction pipe depression and that there are expanding carburetters in existence which will work satisfactorily on the road with depressions of little over 2" of mercury beyond the throttle (see curve Rm.{William Robotham - Chief Engineer}27) at maximum power output. These same carburetters, of course, give a depression of over 1/2" of mercury at full throttle 1000 R.P.M. which is considerably higher than the average fixed jet carburetter and assists them to get snap acceleration without using accelerating pumps. The advantage of having a single variable jet is, of course, that the change over position is avoided which, from the carburetters we have been trying, seems to be the Achilles heel of the majority. There is no doubt that our present method of increasing the power output by raising the compression ratio progressively increases our difficulties with regard to maintaining Rolls-Royce sweetness and flexibility in the power unit and for this reason we are more than anxious that any new type of carburetter shall give us maximum cylinder filling at high speeds. With regard to carburetter dimensions for the power outputs we are getting on P.II and hope to get on J.III. We think the 1 7/8" throttle dia. carburetter you have at W.W. will be about the right dimensions for a single expanding carburetter for J.III, whilst for 180 B.H.P. on P.II we shall want something considerably larger, the approx. size of which we hope to be able to indicate by trying a bigger carburetter which we have. We hope this information will be of some use to you and if there is anything further we can tell you will you please let us know. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} | ||