Combustion will be most beneficial if the mixture of air and fuel ignites at the moment when it is maximally compressed. This highest compression in a four-stroke engine is reached at the moment when the piston, at the end of the compression process, changes from an upward movement to a downward movement. Before the direction of movement of the piston is reversed, it freezes for a small fraction of a second at the highest point of its trajectory. This point is called top dead center (TDC).
Returning to the definition of the moment for the ignition spark: the ideal moment of ignition is therefore the moment at which the piston is just beginning its downward movement. The mixture is compressed as much as possible, and the piston can be forced downwards. Despite this, it would still be incorrect to set the ignition timing exactly to top dead center. After all, a mixture of fuel and air needs a certain time (exactly 1/3000 of a second), to ignite and develop full combustion pressure. That is, the moment of ignition is transferred to an earlier time. And it's called «ignition advance». «Start shot» sparks occur, therefore, even during the upward movement of the piston, the combustion pressure is reached only an instant after the passage of the TDC.
TDC and ignition timing
As the engine speed increases, the igniting spark must jump faster and faster, because - we already mentioned this in the last section - the mixture of fuel and air needs constantly the same time to ignite. Only in this way does combustion occur at the right moment, namely when the piston is just starting to move down.
The figure explains the concepts «Upper» And «bottom dead center». The space between them is called «Volume». The combustion chamber is between the top position of the piston (2) and lower edge of the cylinder head (1).
Combustion of a mixture of fuel and air depends, however, on its composition. If the accelerator pedal is depressed quite a bit («partial load»), the ability of the mixture to ignite in the combustion chambers is reduced: therefore, it burns more slowly and for this reason should ignite earlier.
Late ignition
In other situations, it is necessary that the ignition timing be shifted towards late ignition. In this case, ignition occurs only when the piston has long passed TDC. Thus, ignition occurs almost on 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).
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