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).
Notes on carburation, choke sizes, and throttle bias, with reference to an article by Louis Mantell.
| Identifier | ExFiles\Box 123\1\ scan0079 | |
| Date | 27th January 1941 | |
| To Rm.{William Robotham - Chief Engineer} Ev.{Ivan Evernden - coachwork} Jnr.{Charles L. Jenner} from Rm{William Robotham - Chief Engineer}/TAS.{T. Allan Swinden} Copy to Rm{William Robotham - Chief Engineer}/CTS.{C. Trot Salt - Carburation} & Rm{William Robotham - Chief Engineer}/Wym.{G. Harold Whyman - Experimental Manager} Rm{William Robotham - Chief Engineer}/TAS.{T. Allan Swinden}3/ET.27.1.41. NOTES ON "CARBURATION AND ENGINE" by Louis Mantell. (The Automobile Engineer, Jan. 1941). -------------------------------- AIR FLOW AND CHOKE SIZE. The jet choke size curve in Fig.1 is useful for illustrating some of our past trouble, when bad consumption and flat spots on pick up have gone hand in hand. The worst case was when the 42 mm. Stromberg carburettor with a 1¼" choke was used on the 3¾" bore engined 12.B.V. This choke was far in excess of the critical size, the jet curve ran right away, and a very large main jet resulted in 10 M.P.G. cruising consumption and flat spots. The choke was ultimately reduced to 1¹/₃₂". With our unfortunate combination of shuttering and hot spotting, it is quite impossible for us to venture above the critical choke size. We are rather compelled to choose the smallest choke which will not result in appreciable top speed loss on the road, even though we are a few H.P. down on the test bed. DISTRIBUTION AND THROTTLE BIAS. As usual much play is made of the effect of throttle bias on distribution and the resulting economy. Economy from good distribution is commonsense, but we should like to shew that the problem may not be so simple on the road as it is on the test bed. Let us consider the DC.42 Stromberg carburettor as mounted on Wraith, with float chamber to the front. We know that this unit is obsolete on senior range engines, but not all of its features are dead. Part throttle the car will accelerate from 10 to 20 M.P.H. in 5 seconds; i.e. mean acceleration is about 3 ft/sec². When the car is accelerated in this way, four things happen to the mixture. 1. An acceleration of 3 ft/sec² is equivalent to a free descent of a 5½° gradient, and vice versa. Thus the petrol level in the carburettor will take up approximately this angle, rising towards the back of the carburettor and raising the level in the discharge tube 0.2". This would be almost sufficient to cause flooding if the car was stationary, therefore an uncommonly rich mixture must be provided while the acceleration lasts. | ||