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 comparison between Stromberg and S.U. carburettor systems, focusing on part-throttle correction and the effects of mixture pulsation.
| Identifier | ExFiles\Box 99\1\ scan0014 | |
| Date | 10th May 1938 | |
| -2- RM{William Robotham - Chief Engineer}/CTS.{C. Trot Salt - Carburation}8/JH.10.5.38. The two different types of Stromberg carburettors can be initially set to give whatever correction is needed at part throttle, and the twin choke Stromberg does not give sufficient increase in low speed M.E.P. to warrent further development of its type on the particular engines we are producing at the present time. From our experience with the S.U. carburettor todate, we know two facts which are very important. (A) That a single S.U. (horizontal) on our six cylinder engine gives no correction at all for part throttle mixture. (B) Two S.U.'s working on one pipe on our six cylinder engine can be so arranged to give correction for part throttle. Graphs No. 34 shows the comparison between our downdraught Stromberg system on the Wraith and a single horizontal S.U. fitted to a similar pipe. Graph No. S.33 shows how the part throttle correction on a 2 S.U. carb. scheme is affected by altering the size of balance hole connecting the two halves of the pipe. This fact is explained as follows - In the case of the single S.U. system, the effective pulsations are smaller than with the two carb. system. On the latter, the centre of the induction pipe is partly blanked, and the smaller the hole connecting the two halves of the pipe, the greater the pulsations acting on each carburettor. This pulsation is causing a "stop and go" effect on the mixture leaving the carburettors and due to the petrol having the higher inertia, the pulsation has most effect on the air. As the throttle is closed, so that effect of pulsation decreased in the carburettor, and therefore more air will be taken in, for a given quantity of petrol. | ||