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Charlie, I agree with your
assessment.
Certainly is possible to have pre-ignition without
detonation. Pre-ignition simply means something is firing the combustible
mixture before its intended ignition point - most of the time this is caused by
some element (carbon, metal) retaining sufficient heat and temperature from the
last combustion event to prematurely fire the next combustion
event.
Pre-ignition generally results in higher combustion chamber
pressure and heat as frequently the combustion event is started long before its
intended and may occur as the piston/rotor is coming up on its compression
stroke. The piston trying to compress this burning mixture adds to the
pressure and heat load. Continued pre-ignition
continues to build up the heat load until seals, sparkplugs, etc start to
deteriorate - if continued it frequently leads to detonation which
can quickly lead to destruction of the engine (at least in piston
engines). So pre-ignition can raise indeed raise coolant temps due
to this increased heat load. When the combination of heat load and
pressure reaches a certain point the combustible mixture can explode rather than
burn - detonation has begun.
Detonation does not necessarily add greatly
to the heat load above that already caused by pre-ignition - but since
detonation is essentially a situation where the combustible mixture is no longer
burning but actually exploding. The combination of elevated
heat/temperature and shock of this exploding (rather than
burning) mixture can in short order destroy an engine.
Now our
rotary engines appear to be less prone to detonation and the quenching effect of
its less efficient combustion chamber shape may be to our advantage in this
case. I have experience SAG a number of times and I never got the
impression the engine was under stress (same can not be said for me), in fact if
anything the lesser rpm and EGT always let me to believe it was producing less
power and therefore under less stress. In otherwords, my impression was
the problem was the engine was failing to light off the combustible mixture and
therefore was producing less power (and thereby less stress on the
engine). So while something to be concerned about, I had not felt SAG was
dangerous (other than to seat cushions).
I have had detonation completely
destroy my spark plugs in very short order - but, it took extreme missetting of
my static ignition point to do this. I have never detected any
damage (even to the plugs) through the normal SAG event. So in our case, I
guess its possible that pre-ignition in the rotary is not as conducive to
detonation - or perhaps we are not getting pre-ignition?
It's always
great to understand what is happening to our engines - but, regardless of the
ultimate cause of SAG, we do know that replacing the plugs does provide a fix
(if only temporarily {:>)) and a fairly inexpensive one. I am awaiting
Steve's experiment with colder plugs - even extending the my current SAG
interval from 25- 50 hours would be worthwhile
That's my take on
it.
Ed
--------------------------------------------------
From:
"Charlie England" <ceengland@bellsouth.net>
Sent: Friday, October 01,
2010 9:59 AM
To: "Rotary motors in aircraft"
<flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: SAG from
Paducah
>
> I've only been scanning this series of emails, but
it looks like both
> terms pre-ignition & detonation have been
applied to the SAG symptom.
> Isn't it possible to have pre-ignition
(causing elevated water temps,
> etc and declining EGT) without
detonation, and the resulting destruction
> of plugs, seals, etc?
>
> Charlie
>
> --
> Homepage:
http://www.flyrotary.com/
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http://mail.lancaironline.net:81/lists/flyrotary/List.html
>
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