In article a1caa092-5fa6-43f0-a5e4-756870757db9
@x27g2000prf.googlegroups.com, says...


The knock sensor is to detect detonation, which occurs in the upper
combustion chamber. Knock sensors should go on the heads.




Which is not correct: a knock sensor can be placed anywhere on the motor
where detonation can be detected.

Due to the speed of sound in metals and the detection hysteresis on
electronics there is no gain in time if the sensor is placed on the
head. Let for memory that if your presumption where correct a sensor per
combustion chambre would be needed as the combustion chambers are
individual on most aero-engines.

On automotif engines like Peugeot Mi16 and Porsche (flat6 / V8) engines
the sensors are on the crackcase. For reduncency reasons knocks sensors
are mostly placed in pairs.

The main reason why they are on the crankcase is that they (and their
cabling) are better protected from heat and allow easier mechanical
intervention on the head.

As to detonation itself (experience on race-engines, not aero): it is
common practice to tune race-engines so that mild detonation occurs. The
reason is that detonation occurs with high compression ratio-engines or
engines, able to run on different octane-level fuels, when the ignition
is advanced (which yields more power).

Close to all automotif engines with knocksensors work along the same
principle: if no detonation is detected, the ignition is electronically
advanced. Upon detection the ECU retards the ignition.

The fact that detonation is detected, implies that it happened but if
action is performed within 10-20 enginecycles no damage occurs.

Regards,


Tom De Moor