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).
Magneto ignition systems, advance mechanisms, and sparking requirements for an engine.
| Identifier | ExFiles\Box 4\5\ 05-page277 | |
| Date | 29th May 1924 | |
| Contd. -2- 29.5.24. One of these methods is, of course, the movable sleeve between the pole pieces and armature of the magneto, which in magnetos subject to the hand advance only is turned round by the hand advance lever, but which in this instance might also be caused to move by the automatic advance mechanism. This sleeve does, to a large extent, give the same effect as advancing and retarding the whole drive. If the automatic advance is made to move a sleeve, since the sleeve itself is subject to some amount of impulsive torque, this method would not entirely relieve the advance mechanism of having to pass through itself some appreciable amount of torque variation. Appreciable partial equalisation of the time functioning might be secured by the use of specially shaped trailing pole tips, and/or still further by the use of copper damping conductors encircling these tips as in a Conner magneto which we tested. In our experience of this machine, this appeared to be quite an effective method of bringing about a result similar to that attained with a sleeve, namely, that of securing a lowest sparking speed at retard no greater than the lowest sparking speed at advance. It is most desirable, we might almost say essential, in the proposed magneto by some means or other to attain this end, in which the full retarded position (hand and automatic) gives the lowest low sparking speed, i.e. lower than for any other degree of advance. The second principal requirement of the magneto is arrived at as follows :- In order that we may have the excellent slow running on magneto ignition which is obtainable on the battery ignition (only limited by carburation), (and which is one of the special features of the R.R. 40/50 engine) it is necessary to be able to run with relatively wide gaps at the sparking plugs. It has been proved that this renders the firing of the mixture for a given total energy of spark more certain, the ignition of the mixture, when properly vaporized, depending more upon instantaneous heat density (which quantity is greater in the initial oscillatory spark than in the ensuing arc) than upon the total quantity of heat in the spark. Contd. | ||