Basics:
A
Knock Sensor is basically a mintaure
microphone that
is attached to a portion of the
engine block that picks
up most of the vibrations from engine fuel
combustion;
typically near the
intake manifold, or the first or last
pistons. Locations may vary.
Componets:
A Knock Sensor is made up of a piezoelectric element
surrounded by a coil wire. This element is
tuned to the knock frequency of the engine. (typically around 5Khz) The knock sensor is connected via one wire to the ECU (Engine Control Unit).
Purpose:
Knock sensors detect pre-ignition. (also called detonation, premature combustion) Which can occur
due to over-advanced ignition timing, "bad gas", or underrated
octane for the design of the car. Putting 89 octane
fuel in a car that requres 93 octane will cause engine knock. As the engine vibrates, the Knock Sensor sends voltages back to the computer. The computer analyzes these voltages and determines if they meet a pattern that is equal to engine knock. If so, the computer reduces timing by as much as 5* to try and stop it.
There are various flavors of Knock Sensors, typically
the ones on newer model vehicles work both ways, if the
computer does not detect knock, it advances timing slightly
to try and increase performance. You will find a knock
sensor on every vehicle sold today, and on most older cars,
as long as they're equipped with an ECU.