Knock Sensor: Description and Operation

KNOCK SENSOR:



The Boxster's motor has 2 knock sensors. Knock sensor 1 monitors cylinder line 1 - 3 and is attached to the crankcase level with the 2nd cylinder. Knock sensor 2 is attached to the crankcase at cylinder 5 and monitors cylinder line 4 - 6.

The knock sensors are new. They function according to the piezo-electric principle and are tuned to the frequency of the Boxster's engine. The plug contact is moulded onto the knock sensor housing and the plug contacts are protected against corrosion by a special coating. As with other knock sensors, it must also be ensured that the correct mounting torque is applied.

Function:
If the knock sensor voltage together with an amplification factor calculated by the control unit reaches a maximum voltage threshold, the DME control unit interprets this as "knocking combustion." If knocking is detected, the calculated ignition angle is reset by a crank angle of 2.25° at the appropriate cylinder (cylinder selected). The maximum late setting per cylinder is dependent on the engine speed and may be up to 15° CS towards "late." If knocking combustion is no longer detected, the ignition angle is returned to its optimum value in small steps.

Safety Function:
If an error has been detected by the DME control unit at knock sensor 1 or knock sensor 2, at the Hall sensor, or when the engine load signal was calculated, the calculated ignition angle is reduced by 9° CS once the condition "knock regulation active" applies and a defined engine and intake-air temperature has been reached.

The following must apply before knock regulation is activated:
Engine temperature > 42.7 C.
Engine load > 1.7 ms/revolution.