Old message:
Lots more where this one came from. What do you want to know? :-)
Subject: Knock sensors and 13b REW notes:
Date: Fri, 02 Jul 1999 21:44:18 -0700
From: Paul <rotaryeng@earthlink.net>
To: zz <rotaryeng@earthlink.net>
-- David Carter wrote:
snip..
Paul, and others, when you get
pinging, it is usually as you are advancing throttle and reach a
manifold pressure which, in combination with all the other detonation
variables, is the "final straw that broke the camel's back", i.e.,
started
the detonation or pinging. The detonation started at a certain MAP and
it will stop when you retard the throttle - SLIGHTLY - below that MAP.
We are not talking about chopping to idle or half throttle. It should
be the same in an airplane as we've experienced in a car: Notice the
sound, SLIGHTLY reduce the throttle. In a car, you might slow down 2 or
3 MPH. In a plane, you might cut your 1700-1900 FPM RV-6 rate of climb
to 1600-1800 - NOT A SAFETY HAZARD, and NO EXCUSE for not retardintg the
throttle SLIGHTLY until pinging stops.
The main reason detonation damage happens in airplanes is that the
pinging and worse is masked by the prop noise and louder exhaust noise,
compared to a car. We hear it in a car and back off the throttle
slightly. In an airplane, you don't hear it and you fry your engine. I
have personally seen one V-6 fried by detonation this year and heard of
at least 3 other engines fried (including Ed Anderson's). It is vital
for experimenters to have knock sensors. And it is important to know
what to do when you hear it coming into the new knock sensor audio
circuit: Retard the throttle slightly - no excuses. End of comment on
that.
I agree that the sensors - whether 1 or 2 - can be used to
tell you if you are getting ping or detonation, and that you
could, AFTER RETARDING THE THROTTLE, use your on board
programming system to change a detonation parameter (richen the
fuel/air ratio is about all we can do, since I believe all of
our electronic ignition systems at this time are fixed - not
computer controlled).
I am not sure about that. Tracy's may not be but some are using other
computer systems that may be ignition advance programmable. PL
However, I really disagree that you could
logically use observed onset of detonation to actu
The best way to set fuel/air ratio is with an
oxygen sensor with indicator. Get it right with that sensor, NOT the
knock sensor. If f/a is right, then if you get detonation, you need to
avoid any throttle settings that give detonation, land, and retard spark
timing or increase gasoline octane rating. Those are all detonation
variables you can change, short of replacing hight compression rotors
with low compression rotors.
In a turbo the temperature of the intake air is an issue for knock.
How well your intercooler is working is very important. PL
I should mention, that another easily controlled variable is RPM -
if detonation occurs at a certain RPM and MAP, then initially retard
throttle. Then, if you have a controllable pitch prop, you could
increase RPM a couple of hundred and you'd find that detonation would
occur at a slightly higher MAP reading. HOWEVER, with fixed pitch prop,
and with narrow range of RPM available in aircraft, the RPM variable is
not really a variable - it is more of a given of the particular
airplane-engine-prop combination, more like compression ratio. The "car
standard transmission scenario" that most of us have experienced, I
assume, is this: You get ping when taking off from a stop when the rpm
is initially low, but as you accelerate, the pinging goes away.
So, I stick to my case that you don't do inflight tuning to
eliminate detonation - you do inflight tuning to refine f/a ratio, for
variations in injector flow rates and in mass airflow in different
runners of the induction system. You use the knock sensor to tell you
something is WAY OUT OF ADJUSTMENT (typically ignition timing) - not to
set an adjustment.
Finally about two atmospheric variables: If you never experience
detonation in normal warm weather, and thought you were all finished and
out of danger, wait until you fly into extreme cold weather, with high
pressure dominating your current position, and, you might go over the
edge, and get detonation at near max power if you were just under the
detonation limits at other less dense atmosphere conditions - this is
really a variation on the MAP limit, except you are getting more MAP
than normal due to atmosphere's help, not due to turbo-charging. This a
a classic case why you need a knock sensor -all the time.
Paul Lamar
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