Audi 80/Avant (B4)

Since 1991-1995 of release

Repair and car operation

The ignition system provides correct passage of process of combustion to the engine thanks to purposefully "shot" igniting sparks. And all occurs precisely because at averages of the engine of 3000 rpm each cylinder needs 25 sparks in a second. So 100 sparks in a second, 6000 in a minute, 360 000 at an o'clock and so on is required to the 4-cylinder engine.

That ignition can

That in a spark plug in the combustion chamber the spark in general could slip, between spark plug electrodes there should be a pressure a minimum 30 000 V.Akkumuljatornaja the battery gives, however, only 12 In pressure. Thus, pressure of the accumulator needs to be raised repeatedly. Duration of flash not the tenth, and not the 100-th, but thousand fractions of a second. It is necessary to it to work a bit earlier or later, and efficiency of ignition, and thereby and capacity of the engine will be lowered.

When there is an ignition?

The spark should slip precisely in the fullness of time. However to establish, when there will be this most necessary moment, not so is simple.

The most useful combustion will be in the event that the mix from air and fuel flares up while it is as much as possible compressed. This highest compression in the four-cycle engine is reached during the moment when the piston at end of process of compression passes from movement upwards to movement downwards.

Before the direction of movement of the piston will exchange 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 (ВМТ).

Coming back to definition of the moment for an igniting spark: the ideal moment of ignition, hence, is that moment in which the piston only begins the movement downwards. The mix is as much as possible compressed, and the piston can be directed with force downwards.

Despite it, nevertheless would be incorrect to establish the ignition moment precisely on the top dead point. After all the mix from fuel and air needs certain time (exactly 1/3000 of second) to ignite and develop a combustion total pressure. That is the ignition moment transfer for earlier time. And it is called as "an ignition advancing». «The starting shot» sparks occurs therefore even during piston movement upwards, pressure of combustion is reached only an instant after passage ВМТ.

ВМТ and an ignition advancing

With increase of turns of the engine the igniting spark should jump over all earlier as – we already mentioned it in last section – the fuel and air mix needs constantly same time to ignite. Only thus combustion occurs in the fullness of time namely when the piston only begins movement downwards.

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

Later ignition

In other situations it is necessary, that the ignition moment moved towards late ignition. Ignition in that case occurs only when the piston passed for a long time already ВМТ. Thus, ignition occurs practically in a release step that improves structure of the fulfilled gases, but worsens capacity of the engine. According to the aforesaid, later ignition is is unique correct, for example, at engine work in a mode of compulsory idling (for example, at movement under a hill with the released pedal of an accelerator).