Ignition timing

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Ignition timing

Ignition timing is the time at which the spark plug is fired. It is almost always expressed as crankshaft degrees before the piston reaches top dead center (BTDC).

At first glance it may appear the spark plug should not be fired before the piston reaches top dead center as the combustion would push against the motion of the piston as it travels upwards. However, since the combustion process takes time, the spark plug must be fired earlier.

The goal of proper ignition timing is to maximize the average cylinder pressure created by combustion while working within the limits of gasoline stability.

For a given engine, there is single point after top dead center (ATDC) at which it is optimal for maximum cylinder pressure to be reached. This point varies very little

To reach the optimal cylinder pressure point, ignition timing must be varied based on the amount of air and fuel in the cylinder as well as engine RPM. Typical trends found will be an increase in ignition advance as RPM rises and a decrease as more air is added.

[Example timing map with trends]

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