MoTeC Knock OKM - Manual (Page 7)

MoTeC Configuration 7
With increasing engine RPM, the higher engine vibrations will result in an
increase in the amplitude of the knock sensor signal. A race engine operating
at high RPM will show high amplitudes making it difficult to detect knock.
Knock sensor signal with increased engine RPM
Knock Modules
The SKM/OKM modules improve knock detection by reducing the influence of
background engine noise using a combination of data gating and frequency
filtering.
Data Gating
The knock window is the period in the
combustion cycle during which knock is likely
to occur, normally between 10 and 50 degrees
after TDC (top dead centre). The ECU sends a
knock window signal to the Knock Module to
mark the start and end angle in the engine
cycle.
The SKM/OKM will measure the knock sensor
signal during this window and sends the
information to the ECU. The ECU uses the
measurement to calculate the knock
percentage for each cylinder of the engine.
Knock window
Frequency Filtering
To correctly identify knock, the SKM switches must be set to match the
theoretical best centre frequency. The centre frequency in the SKM/OKM is
normally set to the frequency of the engine that shows the greatest energy
difference between normal engine operation and active detonation.