The figure illustrates the concept «Top dead center» And «bottom dead center». The space between them is the working volume of the cylinder. On the contrary, between the highest position of the piston (2) and the bottom edge of the cylinder head (1) the combustion chamber is shown.
The main element of the ignition system of the four-cylinder engine with a displacement of 1.6 liters is a separate ignition coil (1) and final stage (3) with connector (2). The distribution of the ignition voltage over the cylinders is carried out by the ignition distributor.
Six-cylinder engine ignition coil pack with engine cover removed (static ignition distribution). The numbers on the high voltage connectors explain which cylinder the wire goes to.
In the 1.8l 92 kW engine, the ignition coil on cylinder 3 also serves cylinder 1, and the ignition coil on cylinder 4 also serves cylinder 2. The final stage is mounted on the ignition coils at the top.
Ignition of the 1.8L 110 kW turbo engine: each cylinder has its own ignition coil (1–4).
The spark must jump into the combustion chamber at the right moment. Combustion of the fuel-air mixture occurs most efficiently at the moment when it is compressed to its smallest volume. This maximum compression is reached at the moment when the piston, at the end of the compression stroke, wants to switch from forward to reverse movement of the working stroke. Before the piston begins to move in the opposite direction, for a fraction of a moment it is at the highest point in the trajectory of its movement. This point is called «top dead center» (TDC).
So, the ideal moment of ignition 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. Since the fuel-air mixture needs a certain time (about 1/3000 s) 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. - as already mentioned - the fuel-air 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 fuel-air 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 more slowly and for this reason the ignition must also work with a large advance.
Delayed ignition setting
In other situations, you need to shift the ignition timing in the direction «delays». Ignition occurs when the piston has long passed top dead center. Those. ignition is carried out almost on the exhaust stroke, which improves the composition of the exhaust gases, but worsens engine power. Therefore, late ignition may be correct when the engine is running unloaded in roll mode (e.g. downhill without gas).
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