Ignition systems can be divided into two classifications:  magneto-ignition systems or electronic Full Authority Digital  Engine Control (FADEC) systems for reciprocating engines.  Ignition systems can also be subclassified as either single or  dual magneto-ignition systems. The single magneto-ignition  system, usually consisting of one magneto and the necessary  wiring, was used with another single magneto on the same  engine. Dual magnetos generally use one rotating magnet  that feeds two complete magnetos in one magneto housing.  An example of each type is shown in  Figure 4-1 . Aircraft magneto-ignition systems can be classified as either  high-tension or low-tension. The low-tension magneto  system, covered in a later section of this chapter, generates a  low-voltage that is distributed to a transformer coil near each  spark plug. This system eliminates some problems inherent in  the high-tension system that was containing the high-voltage  until it passed through the spark plug. The materials that were  used for ignition leads could not withstand the high-voltage  and were prone to leak to ground before the spark would get  to the cylinder. As new materials evolved and shielding was  developed, the problems with high-tension magnetos were  overcome. The high-tension magneto system is still the most  widely used aircraft ignition system. Some very old antique aircraft used a battery-ignition system.  In this system, the source of energy is a battery or generator,  rather than a magneto. This system was similar to that used in  most automobiles at the time.  Figure 4-2  shows a simplified  schematic of a battery-ignition system. The magneto, a special type of engine-driven alternating  current (AC) generator, uses a permanent magnet as a source  of energy. By the use of a permanent magnet (basic magnetic  field), coil of wire (concentrated lengths of conductor), and  relative movement of the magnetic field, current is generated  in the wire. At first, the magneto generates electrical power  by the engine rotating the permanent magnet and inducing  a current to flow in the coil windings. As current flows  through the coil windings, it generates its own magnetic  field that surrounds the coil windings. At the correct time,  this current flow is stopped and the magnetic field collapses  across a second set of windings in the coil and a high-voltage  is generated. This is the voltage used to arc across the spark  plug gap. In both cases, the three basic things needed to  generate electrical power are present to develop the high- voltage that forces a spark to jump across the spark plug gap  in each cylinder. Magneto operation is timed to the engine  so that a spark occurs only when the piston is on the proper  stroke at a specified number of crankshaft degrees before the  top dead center piston position. The high-tension magneto system can be divided, for  purposes of discussion, into three distinct circuits: magnetic,  primary electrical, and secondary electrical circuits. The magnetic circuit consists of a permanent multi-pole  rotating magnet, a soft iron core, and pole shoes.  [Figure 4-3] The magnet is geared to the aircraft engine and rotates in the  gap between two pole shoes to furnish the magnetic lines of  force (flux) necessary to produce an electrical voltage. The  poles of the magnet are arranged in alternate polarity so that  the flux can pass out of the north pole through the coil core  and back to the south pole of the magnet. When the magnet is  in the position shown in  Figure 4-3A , the number of magnetic  lines of force through the coil core is maximum because two  magnetically opposite poles are perfectly aligned with the  pole shoes. This position of the rotating magnet is called the full register  position and produces a maximum number of magnetic lines  of force, flux flow clockwise through the magnetic circuit  and from left to right through the coil core. When the magnet  is moved away from the full register position, the amount of  4-2 Figure 4-1.  Single and dual magnetos. Figure 4-2.  Battery-ignition system. Magneto-Ignition System Operating  Principles Magnetic Circuit High-Tension Magneto System Theory of Operation Battery S P Ignition coil Condenser Distributor Mechanical linkage Cylinder  & piston Cam Breaker contact points