Table of contents: Knock control function ↓ Knock sensor ↓
Detonation ignition, which occurs when the ignition timing is set too far in advance, is harmful to the engine and results in overheating, bearing damage and piston damage.
On the other hand, the engine's greatest power output occurs when the ignition timing is set as far ahead as possible, i.e. the engine is running hard on the detonation limit. This detonation limit is influenced by too many factors (fuel quality, deposits in combustion chambers, etc.), so that it could be determined precisely in advance.
Therefore, with a traditional ignition setup, it is necessary to have a large "safety distance" or "observation post" that establishes - if detonation combustion occurs - detonation control.
Knock control function
Combustion in the cylinders is monitored by a so-called knock sensor, which is screwed to the left of the engine block. Two sensors are installed in engines with a working volume of 1.8 liters (on the left on the block) and in six-cylinder engines (right and left at the top of the block).
The knock sensor "feels" when combustion in the cylinders, instead of the usual uniform oscillations, produces a picture of uneven oscillations. The sensor transmits this information to the electronic control unit.
What happens there is this: when the signal "detonation combustion at the last ignition" arrives, the next cylinders in line continue to receive the ignition moment calculated for them. But in the cylinder where detonation was detected, the ignition moment is shifted back by about 3°. As noted above, this only happens in one cylinder. In the others, the ignition moment calculated initially is retained.
If detonation combustion in a given cylinder is maintained, then during the next working stroke the ignition timing is again shifted back by 3°. This can be done up to a maximum of 15° (the reference point is the nominal ignition timing).
If ignition occurs normally again in the corresponding cylinder, then after a short time the ignition timing is again advanced. This is done by about 0.5°, then there follows a pause of several working strokes until the advancement of 0.5° occurs again.
This continues until the originally scheduled ignition timing is reached or until the knock sensor again signals knock combustion.
Knock sensor
It listens for detonation: the detonation sensor in four-cylinder engines is located on the left side of the engine. In a six-cylinder engine, there is one detonation sensor (arrow) on the right and left at the top under the intake manifold. Important: the detonation sensor mounting bolts must be tightened to exactly 20 Nm, otherwise the sensor will not work.
A piece of "piezoceramics" is inserted into this sensitive element, a material we know well from gas piezo lighters. Mechanical forces (stretching, pressure), acting on the piezoelectric ceramics are converted by it into electrical voltage.
To activate the sensor, the uneven vibrations produced by detonation combustion are sufficient.
(Information obtained from this resource AUDIMANUAL)