TDC and ignition advance

The illustration explains the terms "top dead center" and "bottom dead center". The distance between them is the stroke (height of piston travel). The combustion chamber is located between TDC (2) and the lower edge of the cylinder head (1).

As engine speed increases, the ignition spark must occur earlier, because, as mentioned in the previous section, the air-fuel mixture requires a constant amount of time to ignite. Only in this way does combustion occur at the correct moment, namely when the piston is just beginning to move downward.

However, combustion of the air-fuel mixture also depends on its composition. If the accelerator pedal is pressed only slightly ("part load"), the ability of the mixture in the combustion chambers to ignite decreases: it therefore burns more slowly and for this reason must be ignited earlier.