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).
Carburettor performance issues regarding fuel mixture, low temperatures, and potential solutions.
| Identifier | ExFiles\Box 178\4\ img026 | |
| Date | 26th August 1929 | |
| -2- FW/CRW/LGM. 26.8.29. Cont'd This theory was upheld until it was pointed out that if the diffuser did fill with petrol, the engine would be cut right out due to excessive richness when the machine was dived to get the revs up. Up to now it was assumed that the carburetter was delivering the correct mixture, and that as the engine was weak at low r.p.m. (2/300) something was happening between the carburetter and the cylinders. It then occured to me that the carburetter may not be delivering the correct mixture, and that this is where the fault lay. The high petrol level influenced by the fuel pressure and also the low temperatures reached causing a restriction to the petrol flow in the diffuser, and therefore weaknings. An increase in r.p.m. would increase the choke velocity thus increasing the fuel/air ratio, because the fuel flow increases at a greater rate than the air flow through the choke. This accounts for the resultant richening up and increase in performance. The application of heat to the petrol which resulted in a slight improvement at part throttle would lower the viscosity and so richen up slightly. We discussed this with Mr. Rowledge and Dr. Griffiths, and they thought that this was quite feasible adding that petrol might be separating out and forming crystals helping to clog the diffusers still more, also pointing out that the greater viscosity of the fuel would cause the fuel/air ratio to increase more rapidly than would otherwise be the case at low altitudes and normal temperatures. A flight was done with the heated elbow in use, and this enabled the pilot to use more ✓, and the engine would not cut out even with the r.p.m. at 18° and full ✓ could be used at 11,000 ft - 40°. It is hoped that altering the diffuser alone will richen up the mixture at the lower r.p.m. sufficiently, without having to use a heated elbow. HM{Capt. W. Hallam - Head Repairs}/ L.W.Barker. | ||