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).
Optimizing engine ignition timing, including advance and retard mechanisms under various throttle and compression conditions.
| Identifier | ExFiles\Box 41\4\ Scan021 | |
| Date | 28th January 1930 guessed | |
| - 3 - The setting is obtained by trial, giving the engine the advance which will give smoothest operation without dropping power. A new feature is shown in cover 6 over the "retard" (T.D.C.) end of the control cylinder. By changing the shape of this cover the volume of the retard side can be varied. It is found on modern high compression engines of 5 to 1 or more compression, which tend to run "hard" or detonate on sudden throttle-opening, that this volume should be reduced to a minimum. When idling the ignition will be fully advanced and the piston all the way over to the left. (See curve SK-286). On opening up this piston has to go all the way back as much as 40° (crank) retard before stable conditions with freedom from detonation are reached. On "softer" engines it may be desirable to actually increase this volume by a domed cover to prevent too rapid retard on opening up. The best combination is found by trial. The desirable size of the device is not quite certain. About a 1" control cylinder with .750 to 1" stroke is enough for an ordinary ignition head (with 7 or 9 H.T. cables attached) which has to be turned through say 30° to 35°. On an aircraft engine two magnetos have been handled fairly well by a 1-1/2" cylinder with 1-1/2" stroke, but the control was not quite certain under idling conditions. This could have been improved by better workmanship. Freedom in the mechanism and the piston a good enough fit to bounce a little "on compression" on the retard side is desirable as a rough indication of desired workmanship. At full throttle pressure-differences of 50 or 60 lbs/sq.in. are produced by comparatively slight inaccuracies in timing and the device is very positive in action. Under idle conditions these forces may drop to 20% and the need for reasonable freedom of the mechanism and a fair size of control pistons becomes noticeable. In some engines it has been found necessary to allow much more advance than the makers had first contemplated, because of the great advance required (but never previously provided) under part-throttle conditions. - continued - | ||