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).
Comparison of Stromberg and S.U. carburettors, focusing on part throttle correction and pulsation effects.
| Identifier | ExFiles\Box 127\3\ scan0244 | |
| Date | 10th May 1938 | |
| -2- The two different types of Stromberg carburetters 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 MEP. to warrent further development of its type on the particular engines we are producing at the present times. From our experience with the S.U. carburetter 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.S.34 shows the comparison between our down-draught Stromberg system on the Wraith and a single horizontal S.U. fitted to a similar pipe. Graph No.S.35. 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 carburetter. This pulsation is causing a "stop and go" effect on the mixture leaving the carburetters 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 carburetter, and therefore more air will be taken in, for a given quantity of petrol. This means therefore that for a given full throttle mixture setting, a weakening effect is evident at part throttle. | ||