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).
Article from 'The Motor' magazine about a new C.M.I. magneto designed for effective ignition at low engine speeds in luxury cars.
| Identifier | ExFiles\Box 48\3\ Scan065 | |
| Date | 14th June 1920 | |
| S N JUN 14 1920 Reprinted from The Motor May 12, 1920. X 4270 M.D. 14 JUN 1920 (filed 19.6.20) S.N. JUN 14 1920 RECEIVED A MAGNETO FOR THE CAR DE LUXE New Type of C.M.I. Machine, Providing Effective Ignition at Extremely Low Engine Speeds. Novel Feature of Construction. RECEIVED THE general standard of performance aimed at by the manufacturers of better class cars to-day is very high indeed, as everyone can appreciate who has had any experience of the latest models de luxe. It is, therefore, only natural that such manufacturers should require a correspondingly high standard in all those accessories with which their cars are fitted, but which they do not make themselves. This is especially noticeable in the case of the magneto. In the field of construction to which we refer it is nowadays a sine qua non that engines should possess exceptional slow-running capabilities in order to permit slow-speed top-gear work. The attainment of this object depends very largely upon the ability of the ignition apparatus employed to deliver an effective spark at low speeds of revolution. Moreover, the spark must be obtainable with the ignition considerably retarded. Ease of starting is also dependent largely upon ignition efficiency at low speeds, a very important point when self-starters are in universal use. In the past the magneto was so admittedly imperfect in meeting these conditions that battery ignition was generally provided in addition on powerful high-grade cars. Not only was it difficult to get effective sparking at low speeds, but the possibility diminished as the angle of retard was increased to the position desirable for slow speed running under load. The new type of machine which is being introduced by Conner Magneto and Ignition, Ltd., is made to give a wider range of spark control than has hitherto been customary, there being a 50 degrees angle between the full advance and retard positions, as against about 35 degrees or 36 degrees provided by the average machine, and it is further designed so that the spark is practically of constant intensity at any position of advance or retard at about 80 revolutions per minute, which, on a six-cylinder engine, corresponds to approx. 43 engine revolutions per minute. Thus it is particularly suitable for use on modern high-class cars, and has, in fact, been specially designed to meet the requirements of the modern six-cylinder engine. The chart here reproduced shows in diagrammatic form the slow speed characteristic of this type of machine, as compared with average machines of the ordinary type. The method of construction by which these results are obtained is comparatively simple. As will be seen from the illustration, the upper edges of one pole piece and the lower edges of the other are extended to approximately the centre line of the magneto. Each of these extended pole pieces are grooved, above and below, to receive a rectangular copper strip which is slotted down the centre. The object of the pole piece extensions and the function of the copper strip may be briefly explained as follow :- As is generally known, the rotation of the armature between the pole pieces of the field magnets produces a distortion of the lines of force which constitute the magnetic field, and the most effective spark is produced when the arrangement is such that the circuit is broken by the contact breaker at the moment when the armature is cutting the maximum number of lines of force. As the armature revolves, the lines have a tendency to follow it round and to “bunch up,” as it were, at the tip of the pole pieces, so that the maximum magnetic flux occurs, and the maximum number of lines of force are being cut, just about as the armature is passing the pole piece tip, falling off after it has been passed. The extension of the pole pieces, therefore, enables the angle of retard to be increased, because the spark can take place later than if it were not there. Now, with a pole piece of this description the “bunching up” tendency increases at high armature speeds, with the result that the magnetic field in the sector behind the extension is liable to be weakened, and when the circuit is broken at the position of full advance the armature would be cutting few lines of force and the voltage produced might be insufficient to produce a spark across the plug points. This is where the piece of copper comes. Image Captions: [Diagram] Copper Strips. The extended pole pieces as employed on the latest type C.M.I. magneto, showing the position of the copper strips to which reference is made in the text. [Photograph] Aluminium Damper. The C.M.I. machine with the distributor cover removed to show the gears. An aluminium ring is riveted to the gear wheel to assist in securing absolute silence. RECEIVED. | ||