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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).
Investigation into carburetter mixture weakening, automatic correction, throttle characteristics, and potential modifications.

Identifier  ExFiles\Box 31\5\  Scan279
Date  20th December 1934
  
-3- Hs{Lord Ernest Hives - Chair}/Tmr.2/MJ.20.12.34. Cont'd.{John DeLooze - Company Secretary}

It was found that the mixture weakened off excessively as the intake pressure was reduced when the altitude correcting needle was in operation. When the aneroid was disconnected, allowing a fixed jet carburetter it was found under these conditions i.e. Rated Boost Climb, that the carburetter was still automatic and corrected itself up to full throttle height, and then commenced to richen up from that position.

As the carburetter corrected itself without the use of the altitude needle it was assumed that it must be due to the air balance of the carburetter in the respect that one section may influence another when the intake pressure is reduced, so the whole system was analysed from every point likely to be influenced but the results prove that this was not the case.

A considerable amount of investigation was carried out to trace the reason for the automatic action but it was eventually found to be due to the throttle characteristic.

The slope of the throttle curve from rated boost up to full throttle, weakened the mixture just sufficient to balance out the richening effect due to reduced density. This is shewn on the Curve No.3 in which the throttle curve is indicated on a percentage weakening basis and is shewn to balance out the approximate richening curve to 5/8. This is borne out by the necessity for correction when running with fixed throttle opening as shewn previously.

A comparison of the S.U. carburetter throttle characteristic compared with the Kestrel VI. carburetter is shewn on Curves No.4 and 5. Several attempts were made to alter the S.U. characteristic to follow the Kestrel VI. The results of these attempts will form a separate report owing to the fact that under no circumstances have we been able to make the slightest difference to the shape of the curve.

This is to be discussed with the S.U. Company - their assistance in carrying out the investigations to alter the curve to suit our requirements being essential.

The throttle spindles of the S.U. carburetter are bored out to allow the return oil from the scavenge pump to pass through and heat the throttles, at the same time it is claimed that a substantial temperature drop will occur due to the oil passing through and heat being absorbed due to the throttles being in the wet fuel stream.

Observations have been made but up to the present we have never recorded more than 1° centigrade temperature drop across the oil passing through the spindles and this only occurs at full throttle running when the maximum amount of fuel is impinged on the throttles.

This is to be further investigated.
  
  


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