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).
Technical article about the Liberty Engine, focusing on its ignition system and electrical diagrams.
| Identifier | ExFiles\Box 37\2\ scan 082 | |
| Date | 5th February 1919 | |
| X.3306 582 (Supplement to THE AEROPLANE) Aeronautical Engineering FEBRUARY 5, 1919 THE LIBERTY ENGINE.—II. BY G.{Mr Griffiths - Chief Accountant / Mr Gnapp} DOUGLAS WARDROP, M.S.A.E., A.S.M.E. (Managing Editor of "Aerial Age Weekly," New York.) IGNITION SYSTEM. The ignition system used on the Liberty Twelve is known as the generator-battery type. The system comprises two independent breaker and distributor mechanisms, identical in every respect and each one firing all twelve cylinders. For starting and for idling speeds up to 650 r.p.m. current is drawn from a specially constructed four cell storage battery. The battery is very light and carries very little liquid or electrolyte. Nevertheless, it has sufficient capacity to ignite the engine at full speed for three hours. It is so constructed that, even though it be turned upside down, it will still continue to function properly. In addition to the battery, a positively driven generator is provided, so geared that it runs at one and one-half times crankshaft speed. Electrical energy for starting and idling speeds is supplied by the battery. As the engine speed is increased the generator “builds up” and its output grows greater, until at about 650 r.p.m. the generator voltage equals that of the battery. The maximum generator output exceeds the requirements for ignition so that at speeds above 650 r.p.m. the direction of flow of current is reversed and the excess output of the generator goes to recharge the battery. With an almost discharged battery the rate will be about 10 amperes, but will diminish as the battery voltage rises until the battery is completely charged, when the charging rate will be just sufficient to maintain it in a properly charged condition. The generator is controlled by a “voltage regulator” which prevents the output exceeding a pre-determined figure. In view of this fact, the generator will supply current for ignition indefinitely, without the battery, so long as the engine speed is not allowed to drop below 500 r.p.m. A duplex ignition switch is provided which will permit either one or both distributors being turned on. This switch is so constructed that either set of ignition alone can be used without connecting in the generator. In starting, only one side should be used, as, with both switches “on,” the generator is connected to the battery. Under these conditions the discharge from the battery through the generator before the engine is started would be an excessive drain on the battery. It is essential that both switches be “on” at all flying speeds. The ignition switch has an ammeter incorporated in it and this ammeter should be watched occasionally, as it indicates the amount of current flowing to or from the storage battery. If the ammeter shows a discharge at any speed above 650 to 700 r.p.m. with both switches “on,” it is an indication that something is wrong with the generator circuit, and that all electrical energy is being supplied by the storage battery. If the ammeter stands at zero under the same conditions it indicates that the storage battery is not receiving a charge, but that the ignition is being carried by the generator. To return to the distributors—the circuit-breaker mechanism for each head is identical with that used in any high-grade magneto, with two exceptions, as follows :— Two main circuit-breakers, connected in parallel, are provided instead of one. The two breakers are timed to operate simultaneously and are provided in duplicate as a precautionary measure. An auxiliary circuit-breaker, the function of which is to prevent the production of a spark when the engine is turned backward or “rocked” is also provided. The auxiliary breaker is connected in parallel with the other two through a resistance unit which reduces the amount of current flowing through it. The breaker is so timed that it opens slightly before the other two when the engine is turned in a forward direction. The opening of the main breakers then results in the production of a spark. When the engine is turned in a backward direction the two main breakers open first and no spark is produced, due to the fact that the current continues to flow through the coil through the auxiliary breaker, but in diminished quantity, due to the resistance unit. By the time the circuit is opened at the auxiliary breaker the intensity of the magnetic field of the coil has weakened to such an extent that no spark is produced. A transformer coil is incorporated in the Bakelite cover of each distributor head. Image Caption: The Liberty "Twelve" Distributor End. Diagram Caption: Circuit diagram of ignition on the Liberty Twelve Diagram Labels: Generator Shunt Field Coil Noninductive Resistance Reverse Coil Regulator Voltage Coil Resistance Unit Switch Blades Separately Insulated Storage Battery Left Distributor Right Distributor Labels around Left Distributor: 1L, 6R, 5L, 2R, 3R, 4L, 5R, 2L, 1R, 6L, ROTOR Labels around Right Distributor: 1L, 6R, 5L, 2R, 3R, 4L, 5R, 2L, 1R, 6L, ROTOR | ||