Knock Control

Knock occurs in a cylinder when the fuel and air self ignites at the wrong timing. This can occur either before or after the spark is produced. The fuel mixture can ignite in different areas of the combustion chamber and results in a fast combustion process creating several separate fuel combustions which together combine to produce a mechanical knocking sound. The sounds produce a certain type of vibration through the engine cylinder block and these are detected by the knock sensors. The two knock sensors detect knocks on cylinders 1, 2 and 3 and 4, 5 and 6 respectively.

The vibrations act upon the piezo crystals within the sensors which results in a voltage being produced which is sensed by the ECM. The ECM, using the CMP sensors and the CKP sensor, can determine which cylinder(s) are knocking. The ECM is able to filter the signal to detect vibrations created during normal engine operation and discard them from the knock detection. The ignition timing is gradually advanced until the knocking is detected once again.

Once the ECM has determined knocking is occurring using other inputs such as catalytic converter temperature for example in addition to the signals from the knock sensors, it first retards the ignition timing and subsequently richens the air/fuel if required.