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 'The Electrical Review' comparing coil versus magneto ignition, including a wiring diagram and operational description.
| Identifier | ExFiles\Box 124\3\ scan0074 | |
| Date | 12th December 1919 | |
| 740 THE ELECTRICAL REVIEW. [Vol. 85. No. 2,194, DECEMBER 12, 1919. COIL v.{VIENNA} MAGNETO IGNITION. By A.{Mr Adams} C. BOOTH, M.I.E.E. I HAVE read with very much interest the article by Mr. C. Sylvester, A.M.I.E.E., in your issue No. 2,189 of November 7th, regarding coil v.{VIENNA} magneto ignition, and can thoroughly endorse, as the result of practical experience (on a motor car, not an aeroplane), all that he states in favour of the former. In this respect it may be of interest not only to your readers but to the British motor industry as well if I describe the arrangement that has proved so satisfactory in actual use. To avoid any misunderstanding I must preface the information with a statement that I have no interest in any part of the equipment other than that of a satisfied user, and that I would prefer to see British industry in the forefront. On the other hand, I have no hesitation whatever in giving credit where credit is due, even though it be to those who are not Britishers, primarily in justice, secondarily with a view to encouraging our own people to do likewise if they are unable to do better. In 1915 I purchased an American car fitted with electric lighting and an electric starter, by secondary cells charged from a dynamo which could also be used as a motor. This equipment has given me complete satisfaction over a period of nearly five years. Energy is provided for starting the engine from “cold,” even in winter, for lighting seven lamps, for firing the engine and for actuating the horn. Up to date the car has run some 9,000 miles, mostly on short journeys, and the engine has never once been started by the handle. The sparking plugs have never been changed, and their points have been re-set only two or three times. The small mileage is, of course, due to the war, but the short journeys have meant very frequent engine-starting and a consequent heavy tax on the batteries, but the three cells have stood up to the work and have not so far required any attention other than a little distilled water once a month. They, of course, require charging twice a month either by running the engine as on ordinary journeys or by other means. I enclose an explanatory diagram of the equipment showing details of the connections which may be of use to your readers. A reserve battery of five dry cells dried up two or three years ago, and as it was never required I removed it and its wiring. A great point in favour of the equipment is that no voltmeter or ammeter is fitted to bother the non-electrical driver. The charging is quite automatic, and there are no switches that have to be operated according to the speed of the engine. The coil is of the non-trembler type, giving only one spark per cylinder on firing, and I attribute my freedom from plug trouble largely to this fact. The “make-and-break” contact below the distributor is of substantial size, and has been re-set not more than four times. Not that it was necessary even then. The reason why this point has given so little trouble is, I believe, due in a great measure to the correct wiring. In the old days of coil ignition in this country almost every British car had the connections so arranged that the path of the secondary current was through the break of the primary. This led to rapid burning of the contact and consequent trouble, and was a great factor in favouring magneto ignition in which the connections were correctly made. The diagram is almost self-explanatory, but it is not quite in accordance with the original wiring, as I have made some minor alterations. It will be noticed that the wiring is single, and that the frame of the car and engine are used as the “earth” or return circuit. This method considerably simplifies the wiring, and although there have been no shorts, I should imagine that faults, when they do develop, will be much easier to locate than with double wiring, which, generally speaking, requires two faults before any indication is given that trouble has developed. The two faults as a rule occur in different places, and the indications are often very misleading and puzzling. The lamps are fitted with one central contact, the other end of the filament being joined to the brass sleeve. There are many of these sets in use in this country, but for those who are not acquainted with the operation a brief description may be useful:— (1) To Start Engine.—(a) Pull out button M.{Mr Moon / Mr Moore} This starts the dynamo revolving slowly as a motor due to the current having a path from the cells through the armature and field coils. (b) Next slowly depress the starting pedal. This action pushes the motor pinion into gear with the fly-wheel, then removes one dynamo brush and drops the raised motor brush, resulting in the motor revolving rapidly and consequently turning over the engine at a speed somewhat greater than can be done by hand. (c) When engine fires, release immediately the starting pedal. This raises the motor brush and drops the dynamo brush as well as releasing the gear. The dynamo then charges the cells, besides supplying energy for ignition and lighting. (2) To Stop Engine.—Push in button M.{Mr Moon / Mr Moore} There is nothing else to be done. The combined motor and dynamo has three field windings, two armature windings and two commutators. Each commutator has one brush permanently on, while the other two brushes act as the switch for joining up as required either the motor or dynamo according to whether the starting pedal is in action or released. The dynamo begins to charge the cells when the speed of the engine is at about 350 revolutions. Excess current is avoided by the “reverse series winding.” Excess current through the spark-coil is avoided by the bare wire resistance which heats up and increases in resistance if the current is prolonged. The magnetic circuit breaker does not act until about 25 amperes flow through the coil, when it starts vibrating, and cuts down the current at once to about 4 amperes. In addition, the noise it makes in vibrating calls attention to the fact that there is trouble which must be attended to immediately. [DIAGRAM CAPTION] A, Dynamo; B, Motor; c, Shunt winding; D, Reverse series winding; E, Motor winding; F, Three secondary cells; G, Magnetic circuit breaker; H & s, Spark coil; J, Resistance; K, Condenser; L, Make and break; M, Switch for dynamo and spark coil; N, Secondary coil; o, Distributor; P, Switch for tail lamp and red pilot lamp on dashboard; Q, Switch for two side lights; R, Switch for two head lights; s, Switch for dashboard light; T, Electric Horn. vvv indicate framework of car or engine. FIG. 1.—DIAGRAM OF CONNECTIONS. | ||