Audi 80/Avant (B4)

since 1991-1995 of release

Repair and car operation

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 in a second. So the 4-cylinder engine will need 100 sparks in a second, 6000 in a minute, 360 000 at an o'clock and so on.

That ignition can

In order that in a spark plug in the combustion chamber the spark in general could slip, between electrodes of a spark plug there should be a tension of at least 30 000 Century. The storage battery gives, however, only 12 In tension. Thus, tension 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 little earlier or later, and efficiency of ignition, and thereby and engine capacity will be lowered.

When there is an ignition?

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

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

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 (VMT).

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

Despite it, nevertheless would be incorrect to establish the moment of ignition precisely on the top dead point. After all the 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 postpone to earlier time. And it is called as "an ignition advancing». «The starting shot» sparks occurs therefore even during piston movement up, pressure of combustion is reached only an instant after VMT passing.

VMT and ignition advancing

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

Combustion of a 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 a mix in combustion chambers to ignition decreases: therefore it burns down more slowly and for this reason should ignite earlier.

Late ignition

In other situations it is necessary, that the moment of ignition moved towards late ignition. Ignition in that case occurs only when the piston passed for a long time VMT. Thus, ignition occurs 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, at operation of the engine in a mode of compulsory idling (for example, at movement under a hill with the released pedal of an accelerator).