Aircraft technical Basics: Aircraft Engines - RAF Flying Training Manual - Chapter VII.- Engines: Ignition

IGNITION

Induced Currents

40. The burning of the charge in the cylinder is started by an electric spark, which jumps between two points set close together in a sparking plug fitted in the cylinder head. As air, especially when compressed, has a high electrical resistance it is necessary to have a very high electric pressure or voltage to overcome the resistance of even a small spark gap. This high voltage, which may amount to more than 30 times as much as that of the average electric light main, is generated by electro-magnetic induction.

41. The region in which a magnet exerts its well-known attractive and repulsive force is called a magnetic field. If a closed coil of wire is situated in a magnetic field and the field through it changes either in direction or strength, an electric current is produced in the wire. The voltage depends on the number of turns of wire in the coil and the rate of change of the field. This principle is the basis of all electric generators and transformers. The change of field which induces a current may be brought about either by moving a magnet in the neighbourhood of the coil, or by varying the intensity of a field produced electrically. In fig. 112 two coils are shown near to one another. Suppose a current to flow in the one marked P (primary) ; as is well known, a magnetic field will be set up which will pass through the coil marked S (secondary), but as long as the current in P remains steady no current will be induced in S. If the current in P be suddenly stopped by opening the switch shown in the circuit the magnetic field produced by it will collapse and the sudden change from its previous strength to zero will be very rapid. This rapid change will induce a correspondingly high voltage in the coil S, and, if the coils are large enough and the initial field strong enough, this induced voltage may be high enough to make the current jump across a spark gap. This is the principle of a coil ignition system, such as is fitted to most cars. The primary current is produced by a battery, the circuit is broken periodically by a contact breaker operated by a cam and rocker arm driven by the engine, and by timing the cam correctly the spark is produced inside the cylinder at the right moment.

42. In order that the primary coil should have the greatest possible inductive effect on the secondary, the two must be close together, and in fact are wound on the same former, the magnetic field being intensified by making the centre part of soft iron, which is, as it were, a good "conductor" of magnetism. The two coils are not similar, for in order to get the high voltage necessary for sparking the secondary must have many turns of wire, each turn contributing its small share, just as each cell of a battery contributes its share of the total voltage ; it is therefore made of very fine wire. The primary, on the other hand, has to carry a comparatively heavy current and so must he of low resistance ; it is therefore made up by a smaller number of turns of thicker wire—for the same reason it is important that the resistance of the whole primary circuit should be as low as possible. This is why it is so important to keep the points of the contact breaker clean, for any resistance weakens the primary current and hence the secondary voltage. When the circuit is closed the field is built up again, and this also induces a voltage in the secondary, but this time only a weak one, for the magnetic field cannot be built up nearly as quickly as it can be broken down ; it has, so to speak, a kind of inertia so that the rate of change, and therefore the induced voltage, are comparatively small, and no spark occurs.

43. In order to provide sparks in all the cylinders in turn, a distributor is introduced between them and the coil. This consists of a pointed arm rotating inside an insulating cover which carries around its inside circumference a number of metal segments, each connected to one cylinder. The rotating arm is connected to the secondary coil and geared to the engine in such a way that it passes close to each of the segments just as the spark is required in the cylinder it serves. The circuit breaker is also geared so that it breaks at the moment the distributor arm passes a segment ; it is in fact often driven by the same spindle. One other item completes this simple coil ignition system, and that is the condenser. When the primary circuit is broken, not only is a voltage induced in the secondary coil but in just the same way a "self-induced" voltage is generated in the primary coil. This tries to make the current go on flowing through it. The self-induced current is, of course, only momentary, but it causes a spark at the contact points which delays the breaking of the circuit and also burns the metal of the points. To prevent this, a condenser is connected across the contact points ; it reduces the sparking to a negligible amount.

The magneto

44. Instead of using a battery, the primary current may be generated by moving the coil through a magnetic field produced by a permanent magnet, or conversely by moving a magnet in the neighbourhood of the coil, and this is the principle of the magneto. It is not necessary, however, to move the whole magnet about and it is usual to keep the main part of the magnet stationary and direct the magnetic field through the coil by a rotating soft iron member known as an "inductor". The lay-out of the inductor, the magnet, and the manner in which the field is directed through the coil is shown in figs. 113 and 114. As the inductor rotates, the direction of the field through the coil is reversed four times in each revolution, and since the rate of change of the field strength is greatest at the moment of reversal, the primary current will also be greatest at that moment. At its maximum intensity the primary current is broken by a contact breaker. The system is otherwise the same as that already described for a coil ignition system. It is usual for the magnets, coils, con-denser and distributor to be housed together in one machine, but sometimes the distributor is mounted and driven separately. In order to put the whole magneto out of action, a switch is fitted in the cockpit of the aeroplane and connected across the contact breaker, so that when it is closed the primary current cannot be interrupted and no secondary voltage is generated. To avoid having two wires to each plug, one end of the secondary winding and one point of the plug are connected to the engine itself, and the current flows back through the metal of the engine.

Variable Timing

45. The correct timing of the spark depends on the speed of the engine and the throttle opening and therefore some timing control must be provided on the magneto. The control for speed is effected by interposing a special coupling in the drive between the magneto and the engine, which works on a governor principle. It has two bob weights which fly outwards as the speed rises and by a suitable linkage cause the spark to be advanced in relation to the crankshaft. The control for throttle opening is arranged by making the contact breaker base so that it can be rotated through a few degrees with respect to the spindle and the cam meets the rocker arm earlier or later as required. This movable part is connected to the throttle in such a way that the required firing point is given. Another way of doing the same thing is to have two rocker arms at different positions, and switch one or the other of them into the circuit according to the throttle position, the spark being retarded when the throttle is opened wide.

Sparking Plugs

46. The actual spark which fires the charge in a cylinder occurs in the sparking plug, two of which are fitted in each cylinder. The construction of these plugs varies somewhat, but in principle they all consist of a hollow body, which is screwed into the cylinder, containing a central conductor passing right through the inside and insulated from it by some substance which is impervious to heat. A typical plug is shown in fig. 115. The central conductor or electrode is insulated from the body by a mica wrapping and a series of mica washers, and its inner end is flattened. Projecting from the inner end of the body, and ending near the flat face, are a series of fine platinum wires and the spark passes between them and the central electrode. Platinum is used because of its high melting point and resistance to erosion ; the central electrode, with its insulator, is made detachable for cleaning. It will be realised that the inner end of the plug becomes very hot, and, as electrical insulators are usually bad conductors of heat, the plug must be carefully designed in order to provide adequate cooling of the central electrode. If the plug becomes too hot it may ignite the charge before the spark passes, and this "pre-ignition" (which may also be caused by any other incandescent body in the cylinder, such as a particle of carbon) has very much the same effect as detonation. On the other hand, if a plug keeps too cool oil may accumulate on its points and prevent the passage of the spark ; or soot may collect on the insulator, causing a short circuit and consequent plug failure. Plugs must therefore be carefully chosen, to suit the engine in which they are to be used, and maintained in the best possible condition.

Screening

47. Just as the primary coil of a magneto induces a voltage in the secondary coil near it, so other electrical apparatus can be affected although to a lesser degree at a greater distance. To prevent the ignition currents interfering with the wireless apparatus on the aeroplane it is necessary to surround the whole of the ignition apparatus—magnetos, sparking plugs, and wires connecting them—with a metal covering which is earthed. This acts as a screen, and prevents the electrical influence radiated from the magneto, etc., from reaching the wireless sets.