Table of contents: Top dead center and ignition timing ↓ Setting the ignition with delay ↓
The figure illustrates the concept of "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 lower edge of the cylinder head (1) the combustion chamber is shown.
The main element of the ignition system of a four-cylinder engine with a working volume of 1.6 liters is a separate ignition coil (1) and a final stage (3) with a connector (2). The ignition voltage is distributed among the cylinders here by the ignition distributor.
Ignition coil pack of a six cylinder engine with the engine cover removed (static ignition distribution). The numbers on the high voltage connectors indicate which cylinder the wire goes to.
In the 1.8 l 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 top of the ignition coils.
Ignition of the 1.8 l 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. The combustion of the fuel-air mixture occurs most efficiently when it is compressed to its minimum volume. This maximum compression is achieved at the moment when the piston, at the end of the compression stroke, wants to switch from forward motion to the reverse motion of the power stroke. Before the piston switches to reverse motion, it spends a fraction of an instant at the highest point of its trajectory. This point is called the "top dead center" (TDC).
So, the ideal ignition moment is when the piston is just beginning its return movement. Compression is at its maximum, and the piston can be pushed back against the engine block with force.
Despite this, it would be wrong to set the ignition timing exactly at top dead center. Since the fuel-air mixture requires a certain amount of time (about 1/3000 sec) before it ignites and develops full combustion pressure. Therefore, the ignition timing is set earlier. We speak of "ignition advance". Therefore, the starting shot for the spark sounds while the piston is still moving up, and the combustion pressure develops immediately after TDC.
Top dead center and ignition timing
As the engine speed increases, the ignition spark must jump earlier and earlier, because – as already mentioned – the fuel-air mixture always needs the same time to ignite. Only in this way can the combustion process occur at exactly the right time, namely when the piston is just beginning to move back again. The combustion of the fuel-air mixture also depends on its composition. When the gas pedal is slightly depressed (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 greater lead.
Setting the ignition with delay
In other situations, it is necessary to shift the ignition timing in the direction of "retardation". Ignition occurs when the piston has long passed the top dead center. That is, ignition occurs almost on the exhaust stroke, which improves the composition of the exhaust gases, but worsens engine power. Consequently, late ignition can be correct when the engine is running without load in the rolling mode of the car (for example, downhill without gas).
[The article is a reprint of material from AudiManual.ru]