Ignition timing (or spark timing) controls when the spark plug fires during the Compression Stroke. Ignition timing is measured in degrees of crankshaft rotation, before top dead center (BTDC).
In a perfect world,
- The spark plug fires.
- The flame travels through the combustion chamber, igniting the air/fuel mixture.
- The burning gases expand, building pressure in the cylinder.
- The pressure is maximized just as the piston hits top dead center (TDC).
- The pressure pushes down as hard as possible on the piston, creating maximum power.
However, conditions inside the engine are always changing. Different cylinder head and piston designs change how fast the flame travels. So, the spark needs to fire at different times, to create maximum pressure at the right time. The solution is to advance or retard the timing.
Ignition Advance
Advancing the timing means the plug fires earlier in the compression stroke (farther from TDC). Advance is required because the air/fuel mixture does not burn instantly. It takes time for the flame to ignite the all the mixture.
However, if the timing is advanced too far, it will cause an Engine Knock. Engine speed (rpm) and load will determine how much total advance is required.
Ignition Retard
Retarding the timing means the plug fires later in the compression stroke (closer to TDC). Retarding the timing can help reduce Detonation.
However, if the spark happens too late, you will lose power. This is because the cylinder pressure won't reach it's max until the piston is already headed back down on the Power Stroke. Engine damage and overheating can also be a problem.
How is it controlled?
In most modern engines, ignition timing is controlled by the engine's computer. In engines with a distributor, timing can be controlled in a variety of ways. Follow the links below for more information.