Peter Skelton wrote:


Why must knock be supersonic?

Peter Skelton

Well, it's not exactly supersonic actually, it's more like
'instantaneous'.

As the normal flame front progresses across the combustion
chambre, it moves the piston and progressively compresses the
REMAINING fuel/air mixture raising it's pressure and temperature
until it reaches the limit of the fuel/air mixture to resist
detonation...THEN, the remaining area DETONATES ALL AT ONCE,
because almost all of it has reached the critical temp/press.
THAT'S why it results in an extremely sharp spike of pressure and
is so devastating. The piston can cope quite well with the
comparatively slow rise of pressure of a normal flame-front, but
the instantaneous extreme spike of pressure from the exploding
amount of fuel/air is so fast rising that the piston's inertia
won't allow it to 'get out of the way' (so to speak).

Someone on here awhile ago mentioned how devastating detonation
is and I entirely agree. I used to fly the Argus as a flight
engineer and had first hand experience with that phenomenon out
of Keflavik. On reducing power from wet power (full) to METO
(climb) I noticed that number 3 engine's torque was about 20
pounds high (other engines were at ~140 pounds), by the time I
looked at the fuel flows and noticed that number 3 was much lower
than the others that engine started popping and banging and the
torque started to fall off rapidly. We punched it out and
continued climbing. Seeing as how the weather was fine all over
we continued to Summerside.

I found out much later that that engine was completely trashed,
and that the problem was that the 'derichment valve' had failed
and didn't restore normal fuel flow when the 'water injection'
was turned off, so the engine was operating at 'best power' at
'climb power' with no excess fuel for cooling...a disasterous
combination. Less than a half minute with heavy detonation
wrecked it. Don't futz around with detonation!. It'll bite yer
butt!.
--

-Gord.