On the other hand, the greatest output of engine power occurs when the ignition timing is set to the maximum advance, i.e. the engine runs hard at the edge of detonation. Too many factors influence this detonation limit (fuel quality, deposits in combustion chambers, etc.), so that it can be determined exactly in advance.
Therefore, with a traditional ignition installation, it is necessary to have a large «safety distance» or «observation post», which sets - if knock combustion occurs - the knock control.
Knock control function
The combustion in the cylinders is controlled by a so-called knock sensor, which is screwed to the left side of the engine block. Two sensors are installed in engines with a displacement of 1.8 liters (on the left of the block) and six-cylinder engines (top left and right on the block).
knock sensor «feels», when combustion in the cylinders, instead of the usual uniform oscillations, gives a picture of uneven oscillations. The sensor transmits this information to the electronic control unit.
The following happens there: when a signal is received «detonation combustion at the last ignition» the following cylinders continue to receive the ignition timing calculated for them. But in the cylinder where detonation was set, the ignition timing is shifted about 3°back. As noted above, this only occurs in one cylinder. In others, the ignition timing, calculated initially, is retained.
If the knocking combustion in this cylinder has been preserved, then during the next operating cycle, the ignition timing is again shifted back by 3°. This can be done up to a maximum of 15° (the reference point is the rated ignition moment).
If the corresponding cylinder ignites normally again, the ignition timing is advanced again after a short time. This is done by about 0.5°, followed by a pause of several working cycles, until again there is a shift of 0.5°towards the advance.
This continues until the originally scheduled ignition timing is reached or until the knock sensor again signals knocking.
Knock sensor
He listens for knock: the knock sensor in four-cylinder engines is on the left side of the engine. The six-cylinder engine has one knock sensor each (arrow) top right and left under the intake manifold. Important: The knock sensor mounting bolts must be tightened to exactly 20 Nm, otherwise the sensor will not work.
A piece is inserted into this sensing element «piezoceramics», a material that we know well from gas piezo lighters. Mechanical forces (stretch, pressure), acting on the piezoceramics are converted by it into electrical voltage.
To activate the sensor, the uneven vibrations that detonation combustion produces are sufficient.
Visitor comments