One of the requirement for an efficient engine is the correct amount of heat shock, delivered at the right time. This requirement is the responsibility of the ignition system. This ignition system supplies properly timed, high-voltage surges to the spark plugs. These voltage surges cause cause combustion inside the cylinder. For each cylinder in an engine, the ignition system has three main jobs. First, it must generate an electrical spark that has enough heat to ignite the air/fuel mixture in the combustion chamber. Secondly, it must maintain that spark long enough to allow for the combustion of all the air and fuel in the cylinders. Lastly, it must deliver the spark to each cylinder so combustion can begin at the right time during the compression stroke of each cylinder.

When the combustion process is completed, a very high pressure is exerted against the top of the piston. This pressure pushes the piston down on its power stroke. This pressure is the force that fives the engine power. For an engine to produce the maximum amount of power it can, the maximum pressure from combustion should be present when the piston is at 10 to 23 degrees after top dead center (ATDC). Because combustion of the air/fuel mixture within a cylinder takes a short period of time, usually measured in thousandths of a second (milliseconds), the combustion process must begin before the piston is on its power stroke. Therefore, the delivery of the spark must be timed to arrive at some point before the piston reaches top dead center.

Determining how much before TDC the spark should begin gets complicated by the fact that as the speed of the piston as it moves from its compression stroke to its power stroke increases, the time needed for combustion stays about the same. This means the spark should be delivered earlier as the engine¡¯s speed increases. However, as the engine has to provide more power to do more work, the load on the crankshaft tend to slow down the acceleration of the piston and the spark should be somewhat delayed.

Figuring out when the spark should begin gets more complicated with the fact that the rate of combustion varies according to certain factors. Higher compression pressures tend to speed up combustion. Higher octane gasoline ignite less easily and require more burning time. Increased vaporization and turbulence tend to decrease combustion times. Other factors, including intake air temperature, humidity, and barometric pressure, also affect combustion. Because of all of these complications, delivering the spark at the right time is a difficult task.