Knock Sensor: Description and Operation

Knock Signal:

PURPOSE
Knock sensors detect and transmit acoustic vibrations emitted from the combustion chambers.

Knock Sensor:

CONSTRUCTION
The knock sensors (C), which are located on the engine block, consist of a housing (3), a piezoelectric crystal (4), mounted on a threaded sleeve (5), between two connector strips (6). One side of the assembly is fitted with a damping weight (7), and spring washer (8), secured by a nut (9).

OPERATION
The sensors are subjected to the vibrations generated by knock at the combustion chambers. The vibrations cause a small distortion on the surface of the piezoelectric crystal, which consequently generates a small voltage signal. The signals are compared to set values in the DME programming. When these predetermined values are reached, that combustion process is viewed as "knocking" and the timing is immediately retarded 3°. This can occur for one or more cylinders as needed. The ignition retard occurs well before the knock poses a hazard for the engine.

Following knock retardation, the ignition is advanced step by step until the optimum timing point is reached, or until knock is again detected.