Carburettor jet choke size, throttle stall, and acceleration characteristics from a 1941 article.
| Identifier | ExFiles\Box 122\3\ scan0244 | |
| Date | 27th January 1941 | |
| 1033 To Mr. Dr. Jnr. {Charles L. Jenner} From Ra/T.M.S. Copy to Rm {William Robotham - Chief Engineer} /OTS. & Rm {William Robotham - Chief Engineer} /Wym. {G. Harold Whyman - Experimental Manager} Ra/T.M.S./T.27.1.41. DETAIL CARBURETTORS AND ENGINES 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 m.m. Stromberg carburettor with a 1 1/2" choke was used on the 3 1/2" bore engine; 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. causing consumption and flat spots. The choke was ultimately reduced to 1"/32". With our unfortunate combination of stuttering 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 STALL. As usual much play is made of the effect of throttle bias on distribution and the resulting economy. Economy from good distribution is common-sense, but we should like to show that the problem may not be so simple on the road as it is on the test bed. Let us consider the D.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 We know that this unit is obsolete on senior range engines, but not all of its features are dead. Fast throttle the car will accelerate from 10 to 20 M.P.H. in 3 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 3½º 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. | ||
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).