When does ignition occur?

The spark should jump at the right moment. Combustion of the air-fuel mixture occurs most efficiently at the moment when it is compressed to its smallest volume. In a four-stroke engine, this maximum compression is reached the moment the piston, at the end of its compression stroke, (2nd measure) wants to move from forward to reverse motion (3rd) working stroke. Before the piston starts moving in the opposite direction, for a tiny fraction of a second, it freezes at the highest point in the trajectory of its movement. This point is called «top dead center» (TDC). The corresponding opposite moment - the lower point of the reverse movement - is called «bottom dead center» (NMT), which is of no interest to us in this case.

Let's go back to the correct ignition spark moment: so, the ideal ignition moment is the moment when the piston is just starting its reverse movement. The compression is maximum, and the piston can be pressed against the engine block with force. Despite this, it would be incorrect to set the ignition timing exactly at top dead center, because. air-fuel mixture takes a certain time (about 1 /3000 sec) until it ignites and develops full combustion pressure. Therefore, the ignition timing is set earlier. We're talking about «ignition timing». Therefore, the starting shot for the spark sounds even during the upward movement of the piston, and the combustion pressure develops immediately after TDC.

Top dead center and ignition timing

With an increase in the engine speed, the ignition spark should jump earlier, because. - we already talked about this in the last section - the air-fuel mixture always needs the same time to ignite. Only in this way is the combustion process carried out exactly at the right time, namely when the piston is just starting to move back again.


Combustion of the air-fuel mixture also depends on its composition. With slightly depressed gas pedal (at «partial load») the mixture in the combustion chambers has a reduced ability to ignite; so it burns out more slowly. In this case, the ignition must also work with an earlier lead.

Late ignition

In other situations, it is necessary to move the ignition in the direction «later». In this case, ignition occurs when the piston has long passed the top dead center, i.e. ignition occurs almost on the exhaust stroke, which improves the composition of the exhaust gases, but degrades engine power. In this regard, it is correct to have a late ignition when the engine is running without load in forced idle mode (e.g. when driving downhill without gas).