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Knock sensors

 
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dean.psiropoulos(at)veriz
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PostPosted: Fri Oct 12, 2007 6:53 pm    Post subject: Knock sensors Reply with quote

I'm not aware of anyone who's tried this on a big bore aircraft engine and
I've been wondering why also. My 1981 Turbocharged Trans Am had a rather
simple knock sensing system. There was a piezo-electric sensor screwed into
the engine block. The signal from the sensor was amplified and converted to
digital to be analyzed by the microprocessor in the crude engine controllers
of the time. If the engine started knocking the microcontroller would
detect the spike in the voltage of the piezo sensor and send a signal to the
distributor to retard the spark. This wouldn't be real easy to do with
magneto ignition but folks like e-mag/p-mag and Lightspeed should be able to
incorporate the amplifier and A/D convertor into their boxes and sell them
along with a commercially available piezo sensor. One would have to find a
suitable place on the crankcase to solidly mount the sensor but there is no
reason it shouldn't work. Even if the sensor output could be sent to a
display that helped the pilot identify detonation would be better than no
detection at all. Then more of us could run mogas with better piece of mind
and stop using it if we get a bad batch.

Dean Psiropoulos
RV-6A IO-360

____________________ Original Message _______________________________

From: Fiveonepw(at)aol.com
Subject: Knock sensors?

We've had knock sensors on cars for many years that retard timing in the
event of detonation to keep engines running at peak fuel efficiency. I'm
kinda guessing that it is difficult in an air-cooled engine or because of
the large cylinder displacement.

Anyone aware of any attempts to do this on Lycs or Conts?

Mark Phillips


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AzevedoFlyer(at)aol.com
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PostPosted: Sat Oct 13, 2007 5:51 pm    Post subject: Knock sensors Reply with quote

Dean and All,
No question that an electronic means of knock detection is the preferred route, given its sensitivity and ease of integration to the electronic ECU. For altogether different reasons, I developed (...and have a Form 337 granted by FAA) what I call an Engine Health Monitor Indicator (EHMI) and have it in my instrument panel.
Among several other factors, detonation beyond light (light as defined as a ping or two here and there or continuous, barely audible in a test cell) will cause ring flutter. Flutter will change blowby emission. The EHMI monitors (passively) the blowby emitted. On a modified meter, there are bands colored Green / Yellow and Red. During take off you have the worst condition in which detonation could become a problem. That sets your upper limit of your Green band. If, at any time, you see a trend towards this upper value, for sure something is steadily deteriorating and you should do something about it regardless of the source. You know it is affecting your PCU (power cylinder unit), so its bad news.
The great thing about monitoring BBY is that it is the first variable to indicate something going wrong. For example, our engines are prone to ring and valve sticking. If a ring is intermittently stuck in its groove, not even JPI exhaust temperature indicators will catch it, as exhaust temps will remain the same. But the BBY will be affected...and EHMI will catch it! I've been flying with this indicator in my plane for the last two years.
I can tell when oil reaches the upper parts of the cylinder after a cold, long inactivity.
I can tell when oil has reached an ideal operating temperature.
I can use it as a leaning indication instrument.
I can tell if a plug has shorted or I left mags on Left or Right and not in BOTH.
I can distinguish between Left and Right mags firing.
I can...(still learning...)
The guys at the local FAA FISDO were sufficiently impressed to say "...you have turned $4-$5K instruments out there obsolete! Yours should be a required instrument in every new piston engined plane". Felt complimented!
Anyway, this is not a commercial/propaganda email. Purpose is to say that if we satisfy ourselves with less than perfection, something can be done on the cheap to improve engine health monitiring.
Sorry for the lengthy mail. Just got carried away I guess.
Miguel
PA22/20-150
N8714D

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glcasey



Joined: 01 Jul 2007
Posts: 23

PostPosted: Sun Oct 14, 2007 5:37 am    Post subject: Knock sensors Reply with quote

Very interesting. Does it measure pressure or flow? At my day job
we build both for large engines, typically multi-thousand hp mining
truck engines. The flow is still measured by pressure - pressure
drop in a venturi. I agree, one nice thing about measuring crankcase
pressure is that it is totally non-invasive and doesn't affect the
base engine operation at all. Having such a thing would have stopped
me from taking off when a ring broke on my IO-360. Turns out I was
stopped by someone driving up and blocking my way, telling me I was
blowing smoke (come to think of it, I've had people tell me that a
number of times..Smile.
Gary Casey

Quote:

Time: 06:51:36 PM PST US
From: AzevedoFlyer(at)aol.com
Subject: Re: Knock sensors

Dean and All,
No question that an electronic means of knock detection is the
preferred
route, given its sensitivity and ease of integration to the
electronic ECU. For

altogether different reasons, I developed (...and have a Form 337
granted by
FAA) what I call an Engine Health Monitor Indicator (EHMI) and have
it in my
instrument panel.
The EHMI monitors
(passively) the blowby emitted. On a modified meter, there are
bands colored
Green / Yellow and Red. During take off you have the worst
condition in which

(snip)

Quote:
Miguel
PA22/20-150
N8714D


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Gary Casey
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AzevedoFlyer(at)aol.com
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PostPosted: Sun Oct 14, 2007 4:42 pm    Post subject: Knock sensors Reply with quote

Gary,
It translates pressure measured through a modified Pitot Tube into flow.
Thanks,
Miguel

See what's new [quote][b]


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glcasey



Joined: 01 Jul 2007
Posts: 23

PostPosted: Mon Oct 15, 2007 5:43 am    Post subject: Knock sensors Reply with quote

Miguel,
Thought about taking it commercial? Sounds possible.
Gary
On Oct 14, 2007, at 11:56 PM, Engines-List Digest Server wrote:

Quote:


Time: 05:42:14 PM PST US
From: AzevedoFlyer(at)aol.com
Subject: Re: Re: Knock sensors

Gary,
It translates pressure measured through a modified Pitot Tube into
flow.
Thanks,
Miguel



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Gary Casey
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PostPosted: Mon Oct 15, 2007 5:05 pm    Post subject: Knock sensors Reply with quote

Yes Gary.
However, right now I do not have the time!
Maybe a few years from now when I retire?
FAA people that supervised development is most enthusiastic and want me to proceed to an STC and get it into the marked. From their point of view, safety is greatly enhanced and that is enough incentive for them...
Cheers,
Miguel

See what's new [quote][b]


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