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).
Journal page discussing ignition systems, insulation for supercharged engines, and radio shielding.
| Identifier | ExFiles\Box 37\1\ scan 161 | |
| Date | 1st July 1927 | |
| Vol. XXI July, 1927 No. 1 30 THE JOURNAL OF THE SOCIETY OF AUTOMOTIVE ENGINEERS circuit is again closed, there is a residual voltage in the primary winding which affects the next succeeding spark. This trouble is called overlapping and is the principal factor in limiting the spark energy of battery ignition-systems, for overlapping causes an excessive primary-current that rapidly burns the breaker contacts. In magnetos, because of the alternating polarity of the primary current, the effect of overlapping is to reduce the intensity of each alternate spark. Any excessive spark-energy over that required for satisfactory ignition is therefore undesirable, for it not only indicates excessive weight in unnecessarily large and heavy magnetic and electrical circuits, but may actually cause ignition trouble. Aircraft engines ordinarily do not start on the magneto directly; at cranking speeds, therefore, a hot spark is not required. Sufficient experience has been obtained with air-gap distributors to demonstrate in many ways their superiority over the carbon-brush type. Tracking, due to carbon dust, moisture and oil, is practically eliminated and the necessity for special track-insulation and frequent inspection and cleaning has been eliminated. The loss of spark energy is not serious when the air-gap is small. The usual method of obtaining ignition for starting, when magnetos are used, is to provide a hand-magneto with a starting-electrode in the distributor that trails the main electrode by from 30 to 60 deg. This gives the necessary retarding-effect by firing the cylinder that has passed top center. The weight of the starting-magneto must be charged to the magneto ignition. INSULATION FOR SUPERCHARGED ENGINES One of the electrical requirements peculiar to ignition for aircraft engines is the ability to operate at high altitudes on supercharged engines. For military purposes, a great strategical advantage is secured in being able to fly at high altitudes. The density of the atmosphere above the earth decreases rapidly and, as a consequence, the power of the engine decreases in approximately the same proportion and limits the ceiling of the airplane. By using an air-compressor or supercharger, the air entering the engine is maintained at sea-level pressure and the engine power is kept at approximately its ground value, enabling the airplane to fly at much higher altitudes. Most present-day superchargers maintain sea-level air-pressure up to about 20,000 ft., where the air-density is approximately one-half that at the ground. It has been found that the dielectric strength of air varies roughly as the density, throughout the range encountered in flight, so that, at 20,000 ft., the air insulation of the ignition system is only one-half as effective as at the ground. The sparking voltage, however, is unchanged, as the air-density in the cylinders is the same as at sea-level. Laboratory tests show that a sparking voltage of at least 8000 volts is required, which is equivalent to the sparking voltage of a 0.2-in. (5-mm.) three-point air-gap at sea-level air-density. With insulating surfaces rather than air spaces, the flash-over distance required may be double that for air, or 0.4 in. Assuming a 20,000-ft. supercharger, it is seen that a flash-over distance to ground of roughly 0.75 in. is required. This requirement affects the design of the ignition at every point in the secondary circuit. The secondary-coil windings are separated by layers of insulation, and the windings of each layer stop at from 3/8 to 5/16 in. from the ends of the coils, leaving air-spaces between the insulation at the ends of the coil. This air-insulation is reduced at high altitudes and coil failures will occur, unless the length of the coil is increased to give the required 3/4-in. air-insulation, or all the air-spaces are filled with some solid insulating-material. Obviously, the latter is the most desirable solution. The safety-gap of most ignition systems is set at from 3/8 to 7/16 in. and flash-over will occur on a supercharged engine unless the safety-gap is increased, or some means is provided for keeping it at sea-level pressure, or independent of the air-density. The next place for flash-over is from the distributor-rotor to the trailing-electrode for the starting-magneto, from one distributor segment to another, or to the ground. Flash-overs are particularly liable to occur inside the distributor, because of the ionized condition of the air owing to the constant sparking; and a generous flash-over distance is required. The secondary cable-terminals at both the distributor and the spark-plug are a source of trouble because of possibility of the effectiveness of the insulation being reduced by moisture or dirt. The proximity of carburetors, controls and cowling to the spark-plugs is also a source of trouble. At present, no magnetos are entirely satisfactory for use on supercharged engines. The Liberty Delco battery-ignition has proved to be satisfactory on account of its generous size and the inherently low voltage available from the secondary. A method of maintaining the air insulation of the magneto by connecting its interior to the supercharger has been tried, but this adds complication to both the magneto and the engine and is only an expedient to allow the use of magnetos now available. The logical solution is the use of solid insulation that is not affected by atmospheric pressure. This requirement is likely to change materially the detailed design of aircraft ignition-apparatus. RADIO SHIELDING Radio communication is undoubtedly one of the most valuable and necessary aids to aviation that have been developed. It is a fortunate coincidence that radio has become available at about the same time that air transportation is being developed, for any extensive use of aircraft makes the use of radio communication imperative. It is possible to obtain radio communication without ignition shielding by using a powerful sending-station and an insensitive receiver, but this requires large and heavy equipment and is limited in distance to the point at which the ignition noise drowns out the signal. By the use of shielded ignition, the power of the sending-station can be reduced, a more sensitive receiver can be used, and communication can be obtained over greater distances. In order to understand the problem of shielding the ignition system to prevent radio interference, a simple case will be explained by means of the diagram in Fig. 1. Assume that the point a is a source of electrical radiation. As the potential of a varies, the electrostatic and electromagnetic fields about a vary in like manner. If a closed circuit, such as the ring b, is placed around a, a current will be induced in the ring by the varying field about the point a.{Mr Adams} This induced current in the ring b, in turn, sets up a field opposed to that of the point a.{Mr Adams} To neutralize the field of the point a in all directions, the closed circuit b must also lie in all directions; in other words, the point of radiation must be completely surrounded by a conducting-surface. If the neutralizing field is to be equal and opposite to that of a, the resistance of the paths b must be zero. Obviously, the resistance of these paths cannot be reduced to zero, so that absolute | ||