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).
Letter to Messrs. North & Sons Ltd. clarifying specifications for aero and chassis magnetos, specifically regarding the point at break and make variations.
| Identifier | ExFiles\Box 43\4\ Scan110 | |
| Date | 4th January 1927 | |
| X4021 extract EFCL/T. 4th January, 1927. X.4021 X.7791 Messrs. North & Sons Ltd., Whippendell Road, Watford. Dear Sirs, We thank you for your two letters ref. CHK/DEW&2 of the 22nd. December, dealing with aero and chassis magnetos respectively. We note that you are obtaining satisfactory experimental results in increasing the range of aero magnetos to 30°. With regard to the chassis type EW6 magnetos, we feel that you have misunderstood the point at which we are aiming. We have given in our specification that the point at break must not be varied more than ± 3° by a variation of the platinum point gap of ± .004". We think that you have taken us thus to have accurately defined the period of break, whereas it was our intention only to define the instant of break. We think we have already given you the information that in regard to the make, this should be allowed to vary ± 10° for a variation of ± .004" of the platinum point gap. This means that in the two extreme cases, the period of break would vary ± 13°. The point we have tried to stress is that whereas the actual instant of break matters, within comparatively narrow limits, from the point of view of timing the engine, the instant of make does not so matter because if the average instant of make is reasonably arranged, a variation of 10° either way is, owing to the shape of the open circuit volt wave of the magneto, relatively unimportant. Thus, though the instant of break is important, the period of break need not be of the same degree of accuracy. We still can see no reason why the shape of the cam at make should not be a good deal slower than that at break, giving a relatively slow make, but one the instant of which is more susceptible to the variation of the platinum gap than in the case of the break. Contd. | ||