THE fundamental electrical and mechanical requirements of ignition equipment for aircraft engines are outlined and the special requirements peculiar to this service and that apply, in general, equally to military and commercial aircraft, are described. Brief descriptions are given of various new types of both magneto and battery ignition and the developments in each are pointed out. Characteristics of an ideal ignition system are enumerated as a basis for further development. Among the general requirements reliability is given place of first importance, followed by light weight, compactness, low cost and adaptability of a single model to engines of different types. The chief design-requirements are speed, ruggedness, simple mounting, light rotating-parts, resistance to vibration, ample lubrication, protection against moisture, and fire-proof ventilation. Each of these subjects is dealt with specifically.
Difficulties of meeting the exacting electrical requirements are explained and means employed to overcome them are described. Too great spark energy may cause “overlapping,” which, with battery ignition, results in burning of the breaker contacts, and, in magneto ignition, reduces the intensity of alternate sparks. For ignition of supercharged engines at high altitude where the air density is much reduced, the air insulation of the ignition system is much less effective than at sea-level, and a flash-over distance to ground of roughly 0.75 in. is required. Coil failures will result unless the length of the coil is increased to provide this gap or all the air-spaces are filled with some insulating material.
The principles of shielding the ignition system to prevent interference with radio communication are explained and complete shielding of the system for a Liberty-12 engine, as developed by the Signal Corps and Radio Unit at McCook Field, is shown. As a result of tests to determine fire hazards, the Experimental Engineering Section developed a type of magneto vent for ventilation and drainage which is shown.
Several types of two-spark or double magnetos for supplying sparks to two sets of spark-plugs are illustrated and described, as is also a pivotless type of high-speed breaker-mechanism developed by the Materiel Division at McCook Field. Several new battery-ignition distributors as developed for use on airplanes that carry equipment requiring a generator and battery are described, and the author lists the relative advantages of battery and magneto ignition. He then tells the requirements of an ideal airplane ignition-system, states those that have been met satisfactorily, but concludes with the statement that development of ignition equipment especially adapted to aircraft engines has only begun.