On the other hand, the highest power output of the engine is 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. Therefore, with a traditional ignition installation, it is necessary to have a large «safety distance» or «observation post», which determines the knock control if knock combustion occurs.
Knock control function
The combustion in the cylinders is electronically controlled. To do this, a so-called knock sensor is screwed to the engine block at the height of the generator between the 2nd and 3rd cylinders. 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 next in line 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. It is called «selective» ignition control.
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 specified theoretical ignition timing).
If the corresponding cylinder ignites normally again, the ignition timing is advanced again after a short time. This is done by about 0.3°, followed by a pause of several working cycles, until again there is a shift of 0.3°towards the advance.
This continues until the originally scheduled ignition timing is reached, or until the knock sensor again signals knocking.
Shown here is the knock sensor (arrow), located on the right side of the engine.
Knock sensor
A piezo-ceramic part is inserted into this sensitive element - a material that you 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