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).
Paper on recent aircraft-ignition developments, discussing design and electrical requirements.
| Identifier | ExFiles\Box 37\1\ scan 160 | |
| Date | 1st July 1927 | |
| Vol. XXI July, 1927 No. 1 RECENT AIRCRAFT-IGNITION DEVELOPMENTS 29 it does not add to the structural strength of the airplane. Compactness is important, for it usually means simplicity as well as reduction in weight. As a matter of production and maintenance cost, the question of adaptability of one model to different types of engine is of interest to the manufacturers both of ignition equipment and of engines. DESIGN REQUIREMENTS The chief requirements of mechanical design are speed, rugged construction, simple mounting, light rotating parts, resistance to vibration, ample lubrication, freedom from moisture, and fire-proof ventilation. The first problem encountered in developing aircraft ignition-apparatus is that of operating continuously at high speed. A 12-cylinder aircraft-engine cruising at 1500 r.p.m. requires 9000 sparks per min., or 150 per sec. In pursuit planes, when diving at full throttle, the engine speed may easily reach 3000 r.p.m., requiring 18,000 sparks per min., or 300 per sec., whereas the normal speed for such engines is approximately 2000 r.p.m. There is good reason to believe that normal speeds of 3000 r.p.m. will be reached in a few years and that the requirements for high speed are still rising. In engines weighing 2 lb. per hp. or less, the mass of the engine is small, compared with the inertia forces and torque reactions; consequently, vibrations of very high frequency are likely to be encountered. Such vibrations require only a short time in which to develop fatigue failures in parts such as mounting flanges, coil leads, condenser terminals, and the like, unless great care is taken to prevent repeated reversals of the stresses near the fatigue limit of the material. Experience has shown that ordinary automobile ignition-equipment is entirely unsuited to aircraft service on this account. The construction throughout must be of the most rugged type. Whenever possible, integral construction should be used, such as casting the body, pole-pieces and end frame of a magneto as a single unit. Studs with castellated nuts are preferable to screws and lock-washers for joining major parts. Apparently, every increase in engine speed produces a new list of vibration failures in equipment that had previously given no trouble whatever at the lower speeds. As a result of the necessity for low weight, the crankshaft and the accessory drives are as small as possible and are therefore subject to rapid torsional vibrations. The twist of a 12-cylinder engine crankshaft at full throttle may exceed +-2 deg. at each power-impulse. Because of such variations in angular velocity transmitted to the ignition drive, it is essential that the moment of inertia of the rotating parts be kept as low as possible, to reduce the size and weight of the drive required. In some cases, it is necessary to introduce a flexible coupling between the crankshaft and the magneto. This distortion is so serious with the Liberty-12 engine that an attempt to use magnetos was abandoned on account of drive failures. A multitude of flexible magneto couplings are available but few are free from trouble in service. Too much flexibility allows the spark-advance to wander about over a considerable range above and below the desired point. This may cause engine roughness, as the sparks that occur in the extreme-advance position cause those particular cylinders to detonate. If the flexible coupling can be dispensed with, a simple flange-mounting, with a driving-spline or gear directly on the rotor shaft, can be adopted. This reduces the necessity for a heavy shelf or bracket that is required with the conventional base-mounting, and the direct drive eliminates the use of an intermediate driveshaft assembly. By slotting the holes for the magneto flange-bolts, a simple means is provided for the accurate adjustment of the timing. It is a very difficult problem to assure ample lubrication of the bearings, cam and breaker-arm while preventing oil from creeping into the distributor or on to the contacts. Both magnetos and distributors, as ordinarily installed, run at a fairly high temperature and the oil creeps wherever possible. The presence of parts carrying high voltage causes corona discharges, which, through their action on the oxygen and moisture in the air, cause the oil or grease to form gummy deposits on the bearings and exposed surfaces, unless the apparatus is well ventilated and the oil-supply replenished. The presence of moisture inside a magneto or distributor is objectionable on account of the leakage of electricity across the moist insulation, and the rusting of the metal parts and bearing-surfaces. The crankcase vapors are completely saturated with water vapor, which condenses as soon as it strikes a cool surface; and great care is necessary to prevent leakage of these vapors into the interior of the ignition system, particularly when a direct flange-mounting is used. Moisture may also form on the interior surface because of “breathing,” or the passage of air into and out of the magneto as it cools off or becomes heated by the operation of the engine. To prevent this formation of moisture and to eliminate the corrosive gases, a certain amount of ventilation is required through properly located ventilating-holes. But these ventilating holes are themselves not entirely innocent of trouble. They must be properly shielded to prevent flames from being blown out through the vent holes in case a leakage of gasoline occurs and the sparks in the distributor ignite the vapors in the magneto, thus causing an explosion. This topic will be discussed in detail later. ELECTRICAL REQUIREMENTS The electrical requirements for aircraft ignition-systems are considerably more exacting than those for other types of engine. The importance of engine reliability and the necessity for at least two spark-plugs in each cylinder, to assure rapid combustion without detonation, require two independent electrical sources of sparks. The high operating-speeds of the engine require the magnetic and the electrical circuits to be designed for high-frequency operation. At these frequencies, the hysteresis and eddycurrent losses in magnetos may cause an undue temperature-rise in the coil and pole-pieces, unless the magnetic circuit is carefully laminated. Since many engines operate more smoothly when the two spark-plugs fire at slightly different degrees of advance, it is necessary to provide means for staggering the sparks. In some types of magneto, the eddy currents in the rotating parts and the high-frequency static discharges roughen the bearing-surfaces and seriously shorten the life of the bearings. This necessitates insulating the ball-bearing races. It is generally thought that the greater the energy of the sparks, the better the ignition; and this is true for ordinary magneto or coil systems at moderate sparking rates. Some slight trouble may result from rapid burning of the electrodes of the spark-plugs when the sparks are very “fat” and the electrodes operate at a high temperature; but this in itself is not a sufficient reason for limiting the spark energy. In high-speed multiple-cylinder engines, the interval between successive sparks is so short that the duration of the spark discharge must be considered. If the spark energy is too great and the secondary is not completely discharged when the primary circuit is broken, the voltage of the succeeding sparks will be seriously reduced. | ||