Knock Sensors

[Jeffrey S Armfield]

Knock sensors are basically just cheap accelerometers. They respond to vibration
by generating a voltage. There are three basic types - resonant , flat response,
and broadband resonant - which differ in their frequency response 
characteristics. Resonant sensors have a sharp response at a given frequency -
if the engine does not knock at that frequency then you'll never know it. Flat\
response knock sensors are just that - they have low output (typically a few
mV/g accel.) over a broad frequency range. They usually require amplification
of some sort and more extensive filtering. Broadband resonant sensors are an
attempt to take the best of the flat response and resonant sensors and combine 
them into one package. Broader response than resonant (peak response is spread
over 2-3 kHz) with higher output that flat response sensors. I believe AC-Delco
makes these type of sensors.

As far as a knock control system goes, knock occurs somewhere between 5 kHz and
8 kHz (from my experience) and the most important item to locate the sensor in a
place that won't pick up stray vibration. A crude knock control algorithm listens for output from the sensor of sufficient amplitude and then alters the timing.

There's not really any black magic in knock detection and control - just some
signal processing and good engineering in locating the sensor.

Jeff Armfield