Table of contents: TDC and ignition timing ↓ Late ignition ↓
The spark must fly at exactly the right moment. However, determining when that very right moment will be is not so easy.
Combustion is most efficient when the air/fuel mixture ignites at its maximum compression. This maximum compression in a four-stroke engine occurs when the piston switches from an upward to a downward motion at the end of the compression process. Before the piston reverses its direction of motion, it pauses for a fraction of a second at the highest point in its trajectory. This point is called top dead center (TDC).
Returning to the definition of the moment for the ignition spark: the ideal ignition moment is therefore the moment at which the piston is just beginning its downward movement. The mixture is maximally compressed, and the piston can be forced downwards. Despite this, it would still be incorrect to set the ignition moment exactly at the top dead center. After all, the mixture of fuel and air needs a certain amount of time (exactly 1/3000 of a second), to ignite and develop full combustion pressure. That is, the ignition moment is moved to an earlier time. And this is called "ignition advance". The "starting shot" of the spark therefore occurs even during the piston's upward movement, the combustion pressure is reached only a moment after TDC has passed.
TDC and ignition timing
As the engine speed increases, the ignition spark must jump earlier and earlier, because - as we mentioned in the previous section - the fuel-air mixture needs the same amount of time to ignite. Only in this way does combustion occur at the right moment, namely when the piston is just beginning to move downwards.
The figure explains the concepts of "Top" and "Bottom Dead Center". The space between them is called "Volume". The combustion chamber is located between the upper position of the piston (2) and the lower edge of the cylinder head (1).
The combustion of the fuel-air mixture depends, however, also on its composition. If the accelerator pedal is depressed very slightly ("partial load"), the ability of the mixture in the combustion chambers to ignite is reduced: therefore, it burns more slowly and for this reason must ignite earlier.
Late ignition
In other situations, it is necessary for the ignition timing to shift towards late ignition. In this case, ignition occurs only when the piston has long passed TDC. Thus, ignition occurs almost in the exhaust stroke, which improves the composition of exhaust gases, but worsens engine power. According to the above, late ignition is the only correct one, for example, when the engine is running in forced idle mode (for example, when driving downhill with the accelerator pedal released).