Audi 80/Avant (B4)

since 1991-1995 release

Repair and operation of the car

The system of ignition provides the correct passing of process of combustion in the engine thanks to purposefully "shot" igniting sparks. And everything occurs precisely because at average 3000 rpm of the engine each cylinder needs 25 sparks a second. So the 4-cylinder engine will need 100 sparks a second, 6000 in a minute, 360 000 in an hour and so on.

That ignition can

In order that in a spark plug in the combustion chamber in general the spark could slip, between electrodes of a spark plug there has to be tension of at least 30 000 Century. The battery gives, however, only 12 V of tension. Thus, tension of the accumulator needs to be increased repeatedly. Flash duration not the tenth, and not 100-th, but thousand fractions of a second. Once it works a little sooner or later, and efficiency of ignition, and thereby and engine capacity will be lowered.

When there is an ignition?

The spark has to slip precisely at the right time. However to establish when there is this most necessary moment, not so simply.

Combustion will be the most useful in case mix from air and fuel flares up while it is most compressed. This highest compression in the four-cycle engine is reached at the moment when the piston at completion of process of compression moves up from the movement to the movement down.

Before the direction of the movement of the piston exchanges on the return, it on a small fraction of a second fades in the highest point of the trajectory. This point is called as the top dead point (TDP).

Coming back to definition of the moment for the igniting spark: the ideal moment of ignition, therefore, is that moment in which the piston only begins the movement down. Mix is most compressed, and the piston can be directed with a force down.

Despite it, nevertheless would be incorrect to determine the moment of ignition precisely on the top dead point. After all mix from fuel and air needs certain time (exactly 1/3000 seconds) to ignite and develop a total pressure of combustion. That is the moment of ignition is postponed to earlier time. And it is called as "an ignition advancing". "The starting shot" of a spark occurs therefore even during the movement of the piston up, pressure of combustion is reached only a moment after passing of VMT.

VMT and advancing of ignition

With increase of turns of the engine the igniting spark has to jump over everything earlier as – we already mentioned it in the last section – mix of fuel and air needs constantly same time to ignite. Only thus combustion happens at the right time namely when the piston only begins the movement down.

Combustion of mix of fuel and air depends, however, and on its structure. If the pedal of an accelerator is squeezed out very little ("partial loading"), ability of mix in combustion chambers to ignition decreases: therefore it burns down more slowly and for this reason has to ignite earlier.

Late ignition

In other situations it is necessary that the moment of ignition moved towards late ignition. Ignition in that case happens only when the piston passed VMT for a long time. Thus, ignition happens practically in a release step that improves composition of exhaust gases, but worsens engine capacity. According to the aforesaid, late ignition is only correct, for example, during the operation of the engine in the mode of compulsory idling (for example, at the movement under a hill with the released accelerator pedal).