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 and spark plug performance, analysing spark gap size and voltage requirements to prevent misfires.
| Identifier | ExFiles\Box 4\5\ 05-page278 | |
| Date | 29th May 1924 | |
| 29.5.24. -3- Contd. With a long spark gap, the current amplitude of the initial oscillatory spark is greater, and the volume of the ensuing arc correspondingly less, and vice versa. In a short gap, the heat of the spark (though the same amount of heat may be generated) is also more rapidly abstracted by the electrodes, so that the greater incendivity of the long spark is explained. The use of relatively long spark plug gaps, however, may sometimes result in a condition in which the so-called 'resistance' of the sparking plug gaps is excessive for ordinary magnetos, resulting in the fact that the discharge sometimes takes place at the safety gap of the magneto instead of, as desired, at the plugs. This total condition may occur with all or some of the partial conditions of:- (1) Fully retarded ignition. (2) Weak mixture. (3) Full throttle. In order to guard against misfires, resulting as above described, it is necessary to use a magneto capable of satisfactorily withstanding and furnishing a higher secondary voltage than is usual in magnetos, and to this end we tentatively specify a figure of 12000 volts for the high tension safety gap of the magneto, corresponding, roughly speaking, to a high tension safety gap in the magneto of certainly not less, and probably somewhat greater than 8.5 mm. The more ordinary magnetos whose available secondary voltage is less than this, may be capable (according to the size of secondary wire used in, and space allotted the secondary winding) of coping with spark plugs which are subject to bad leakages, because they can supply a fair amount of quantity of secondary discharge to build up sparking over voltage, in spite of leaky conditions. In fact they may be able to spark over a moderate gap, with a fair leak across, more readily than a magneto which is capable of sparking a longer length of gap, under well insulated conditions. The ordinary run of magnetos may provide what is required in this respect and be capable of sparking under adverse (i.e. leaky plug) conditions, but since on our engine bad leaky plug conditions do not occur, it is not so necessary to pay attention to this point, and therefore it is not necessary to increase the size of the magneto to the extent of attaining the desired voltage, keeping the available quantity of discharge the same, but merely to effect a Contd. | ||