Knock Control
Knock occurs in the combustion chamber when the fuel and air mixture self ignites. This can occur either before or after the spark plug has produced an ignition spark. In both cases the gas in two or more places ignites in the combustion chamber.
This results in an extremely fast combustion process with flames from several directions. When these flames collide, the pressure in the cylinder increases rapidly and there is a mechanical knocking sound.
If one of the cylinders starts knocking there will be a certain type of vibration in the engine block. This vibration is transferred to the knock sensor, which are screwed to the engine block. One knock sensor detects knocking on cylinders 1, 2, 3 and 4. The other one detects knocking on cylinders 5, 6, 7 and 8. The mechanical stress that arises in the piezoelectric material of the knock sensors makes them generate a voltage. With the help of the camshaft position sensor and the impulse sensor, the Engine Control Module (ECM) can then determine which cylinder is knocking.
The knock sensors (KS) also interpret a proportion of normal engine sound. The control module is able to recognize the vibrations which correspond to knocking by filtering, amplifying and using software to evaluate the signal.
If the knock sensors (KS) detect knocking in the engine above a certain threshold value, the ignition timing is first retarded and then the fuel/air mixture is enriched to eliminate knocking.