Knock Control

Knocking occurs in the combustion chamber when the fuel/air mixture self-combusts. This can occur before or after the spark plug has produced an ignition spark. In either case the mixture is ignited at two or more points in the combustion chamber.

This results in an extremely rapid combustion process, characterized by several flame fronts. Pressure increases rapidly and a metallic knocking noise is heard.

Knock in a cylinder produces a certain type of vibration in the cylinder block. The mechanical stresses caused to the piezo-electric crystal in the knock sensor causes the device to generate a voltage. This signal, with the signals from the camshaft position and engine speed sensors, is used by the engine control module to identify which cylinder is knocking.

The knock sensors also detect some normal engine noise. Filtering, amplifying and processing the signal allows the engine control module to recognize vibrations corresponding to engine knock.

If the knock sensors detect knock above a threshold value, ignition is retarded and the fuel/air mixture is enriched to eliminate knocking.