Knock Sensor
The function of the Knock sensor is to monitor combustion knocking from the engine. Knocking may damage the engine and reduces the efficiency of engine combustion.
If the Engine Control Module (ECM) registers knocking from any of the cylinders, the ignition will be retarded for that cylinder at the next combustion stage. If repeated ignition retardation does not prevent knocking, the injection period will be increased. This has a cooling effect.
The sensor is made of piezo-electric crystal. When the engine knocks, vibrations (sounds waves) are generated that are transmitted through the engine block to the knock sensor. This mechanical stress which then arises in the knock sensors' piezo-electric material results in them generating a voltage. This signal is sent to Engine Control Module (ECM). The signal corresponds to the sound waves' frequencies and amplitudes.
Thereby, the Engine Control Module (ECM) can determine whether the engine knocks. Using the camshaft position sensor/engine speed (RPM) sensor, the engine's operating cycle can be determined (which cylinder ignites) and thus which cylinder is knocking.
The Engine Control Module (ECM) can diagnose the knock sensor.