Knock Control

KNOCK CONTROL





Knock is caused by auto-ignition of the air/fuel mixture in the combustion chamber. This may occur either before or after the plug has delivered a spark. In either case, the mixture is ignited at two or more points.

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

Knock in a cylinder produces a certain type of vibration in the block. The mechanical stresses which this imposes on the piezoelectric crystal in the Knock Sensor (KS) causes the device to generate a voltage. Together with the signals from the Camshaft Position Sensor and Engine Speed sensor, this signal is used by the ECM to identify which cylinder is knocking.

The KS sensors also detect some of the normal engine noise. However, filtering, amplification and the software used by the ECM to evaluate the signal enable the latter to recognise the vibrations responsible for knock.

B5234T / B5234T5: If the knock sensors detect that the engine is knocking beyond a specific threshold value, the timing will be initially retarded and the fuel/air mixture will be subsequently enriched to eliminate knocking. If the knocking does not stop despite these measures, the control module will reduce the boost pressure until the knocking ceases.

B5254S: If the knock sensors detect that the engine is knocking above a specific threshold value, the timing will be initially retarded and the fuel/air mixture will be subsequently enriched to eliminate knock.