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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).
Carburation, hot air intakes, and a comparison between S.U. and Stromberg carburettors in cold weather.

Identifier  ExFiles\Box 123\1\  scan0018
Date  22th January 1940
  
To Ev.{Ivan Evernden - coachwork} from Rm{William Robotham - Chief Engineer}/TAS.{T. Allan Swinden}
c. Jer.
c. Rm{William Robotham - Chief Engineer}/YD.
c. Rm{William Robotham - Chief Engineer}/Wym.{G. Harold Whyman - Experimental Manager}
c. Rm{William Robotham - Chief Engineer}/CTS.{C. Trot Salt - Carburation}
c. Rm{William Robotham - Chief Engineer}/RC.{R. Childs}

CARBURATION.
Some notes on hot air intakes.

Nobody has yet told us of any modern car, save our own, which has a similar arrangement of carburetter, cooling and manifolding. The nearest approach, as quoted by Fisher of Stromberg's, is on one of the big Daimlers. Here there is a model only differing in one detail, namely, that although a water heated hot spot is used, both manifolds are on the same side of the engine. This car is not good for carburation, however, and is liable to the trouble which we have run into.

There is little doubt that the low grade Pool petrol has accentuated the cold weather trouble. This if a "percent-age vaporization" curve was obtained for Pool and compared with the known curve for Ethyl, the disadvantage of Pool would be shown immediately. It would show what difference in air temperature the change in fuel represents, and approximately what rise in temperature of the manifold would be necessary to overcome the misfortune. In effect, for a given manifold temperature Pool could be calibrated to represent a drop in air temperature.

With reference to the superior behaviour of S.U. carburetters in cold weather, there appear to be two outstand-ing advantages over the Stromberg fixed choke type.

(1) Under normal conditions, when the throttle is fully depressed, though even quite slowly, things are done to compensate for the sudden drop in depression which ensues. The S.U. carburetters have damped pistons, and the Stromberg has an accelerator pump. In cold weather the damped pistons may even do a bit more damping which is all to the good. The accelerator pump, however, merely shoots cold Pool petrol into a not very hot induction pipe where it apparently stays as one big heavy end until the acceleration is over.
  
  


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