Chapter 4 Engine Ignition and  Electrical Systems Reciprocating Engine Ignition Systems The basic requirements for reciprocating engine ignition  systems are similar, regardless of the type of engine. All  ignition systems must deliver a high-tension spark across the  electrodes of each spark plug in each cylinder of the engine  in the correct firing order. At a predetermined number of  degrees ahead of the top dead center position of the piston,  as measured by crankshaft travel in degrees of rotation, the  spark occurs in the cylinder. The potential output voltage of  the system must be adequate to arc the gap in the spark plug  electrodes under all operating conditions. The spark plug is  threaded into the cylinder head with the electrodes exposed  to the combustion area of the engine’s cylinder.