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).
Performance of the autovac fuel system versus the S.U. carburetter on the 20/25 model.
Identifier | ExFiles\Box 31\5\ Scan125 | |
Date | 26th October 1933 | |
X1487. To E.{Mr Elliott - Chief Engineer} from Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}5/KT.26.10.33. 1 3/4" RR. S.U. CARBURETTER FOR 20/25. Replying to E/PSN.1/HP.11.10.33. (a) From dynamometer tests we find that the autovac will not keep pace with the engine requirements when the S.U. is used, but it loses ground so slowly that there is little chance of failure occurring on the road. Under the worst conditions one might have to run 20 mins. full throttle at 70 m.p.h. before drying up commenced. The reason we are not worse off is because, owing to the passage through the block, we get a much higher depression in the induction pipe than we do on the Bentley. At low speeds the S.U., of course, runs at a higher depression than the R.R. We should be glad to be rid of the autovac because it makes the worst of a flooding carburetter, and if the final edition of the electric pump is satisfactory on the Bentley, the more we can use the better will be the price. However, it appears we can use which ever we please. (b) With regard to the petrol level, we can go down 1/4" below the normal S.U. level of 1/16" beneath the top of the jet without suffering from loss of "pick up". Actually, even with the normal level, the rate at which petrol is lost in a 1 in 5 gradient is very slow indeed. We cannot imagine anyone leaving their car on a 1 in 4 gradient, and actually on such a slope, facing downhill, petrol used to come out of the top of the autovac on the Phantom. The electric petrol pumps of course eliminate the trouble. (c) With regard to slow running, so far we have not yet been able to get away with a bigger jet than .025" dia. With our edge filter, however, it should be impossible to choke this size of jet. The trouble about using a big jet and a big hole through the throttle body is that it spoils starting up unless, a means is supplied for cutting off the slow running air which is an undesirable complication, or a separate starter carburetter is fitted. Our present method on the S.U. of dropping the main piston, is actually less effective than on a normal S.U. even when the slow running jet is only .025" which we are at present using. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} | ||