Description And Operation
The knock sensors are piezoelectric type; they are used by the Engine Management Node to recognise any knock in the combustion chamber. The sensors are fitted on the rear of the block and measure the intensity of the vibrations caused by knock in the combustion chambers.
Characteristics and operation
The molecules of a quartz crystal are characterised by electrical polarisation. In rest conditions (A) the molecules are not arranged in a particular way. When the crystal is subjected to pressure or an impact (B), the molecules orient themselves proportionally to the pressure to which the crystal is subjected. This orientation generates a voltage at the ends of the crystal. This voltage is interpreted and adjusted over time (drift due to engine ageing), by the Engine Management Node. This allows engine knock to be recognised and ignition advance can then be reduced until the phenomenon disappears. Following this, the advance is gradually restored to a low value.
A. Rest position
B. Position under pressure