When does ignition occur?

The spark must jump at exactly the right moment. However, determining when this right moment is, is not so simple.

Combustion is most effective if the air-fuel mixture ignites at the moment it is most compressed. In a four-stroke engine this maximum compression is reached when the piston, at the end of the compression stroke, changes direction from upward to downward movement.

Before the piston reverses direction, it pauses for a fraction of a second at the highest point of its travel. This point is called top dead centre (TDC).

Returning to the definition of the ignition moment: the ideal moment for the ignition spark is therefore the moment when the piston is just beginning its downward movement. The mixture is at maximum compression, and the piston can be driven down with force.

Nevertheless, it would still be incorrect to set the ignition timing exactly at top dead center. The air-fuel mixture requires a certain amount of time (exactly 1/3000 second) to ignite and develop full combustion pressure. For this reason, the ignition point is moved to an earlier time. This is called "ignition advance". The spark "start" therefore occurs while the piston is still moving up; combustion pressure reaches its peak only a moment after the piston passes TDC.