Combustion Ignition System The typical ignition system on WSC aircraft provides the  spark that ignites the fuel/air mixture in the cylinders and  is made up of magneto/generators, control boxes, spark  plugs, high-voltage leads, and the ignition switch. For most  LSA engines designed specifically for aircraft, a magneto/ generator uses a permanent magnet to generate an electric  current independent of the aircraft’s electrical system, which  might include a battery. The aircraft electrical system can  fail—the battery can go dead. However, this has no effect  on the ignition system. The electricity from the ignition magneto/generator goes into  the ignition control box where the correct voltage is produced  and timed to fire the spark plugs at the proper time. Modern  WSC aircraft use an electronic capacitance discharge system  that operates without any moving parts to increase reliability  and efficiency. Capacitance Digital Systems (CDI) operate  similarly but they have the ability to change the timing of the  spark for different rpm. Consult the POH for the particular  system for each engine. The system begins to fire when the starter is engaged and the  crankshaft begins to turn. It continues to operate whenever  the crankshaft is rotating. Most WSC aircraft incorporate  a dual ignition system with two individual magneto/ generators, separate sets of wires, separate sets of control  boxes, and separate sets of spark plugs to increase reliability  of the ignition system. Each magneto/generator operates  independently to fire one of the two spark plugs in each  cylinder. If one of the systems fails, the other is unaffected.  The engine will continue to operate normally, although a slight  decrease in engine power can be expected. The operation of the magneto/generator output to the ignition  system is controlled in the flight deck by the ignition switch.  Since there are two individual ignition systems, there are  normally two separate ignition toggle switches or separate  positions on the ignition control, as shown in  Figure 4-16. Identification of a malfunctioning ignition system during  the pretakeoff check is observed by the decrease in rpm that  occurs when first turning off one ignition switch, turning it  back on, and then turning off the other. A noticeable decrease  in engine rpm is normal during this check. If the engine  stops running when switching to one ignition system or if  the rpm drop exceeds the allowable limit, do not fl y until  the problem is corrected. The cause could be fouled plugs,  broken or shorted wires between the magneto/generator and  spark plugs, or improperly timed firing of the plugs because  of a defective control box. It should be noted that “no drop”  in rpm is not normal, and in that instance, the aircraft should  not be flown. Following engine shutdown, keep the ignition  switches in the OFF position. Even with the battery and  master switches OFF, the engine can fire and turn over if  an ignition switch is left ON and the propeller is moved  because the magneto/generator requires no outside source  of electrical power. The potential for serious injury in this  situation is obvious. Standard category aircraft engine systems are described in  the Pilots Handbook of Aeronautical Knowledge; however,  these engines are not typically used on WSC. Automobile  engines or other non aircraft engines may be used on WSC  where the ignition system runs off the battery rather than a  magneto/generator system. In this case if the battery system  fails, the engine ignition system will fail and the engine will  stop. During normal combustion, the fuel/air mixture burns in  a very controlled and predictable manner. Although the  process occurs in a fraction of a second, the mixture actually  begins to burn at the point where it is ignited by the spark  plugs, then burns away from the plugs until it is consumed  completely. This type of combustion causes a smooth buildup  of temperature and pressure and ensures that the expanding  gases deliver the maximum force to the piston at exactly the  right time in the power stroke. Detonation is an uncontrolled, explosive ignition of the  fuel/air mixture within the cylinder’s combustion chamber.  It causes excessive temperatures and pressures which, if not  Figure 4-16. Keyed ignition system with integral starter. 4-12