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Chuck,
I'm sorry if my posting was
disappointing.
But I tried to make sure it was factual and
informative. It may not be helpful - - for those that want to
go ahead and hope that the automtoive plugs will work in their
application.
I am NOT asserting that the automotive splugs are, in fact,
unsatisfactory.
What I AM asserting is that unless someone has
meaesured the actual temperature of the ceramic in the automotive plug inserted
into the aircraft air cooled cylinder then the people using the automotive spark
plug are relying on "hope as a method" to establish safety for
the configuration of their engines.
Given that there are apparently reliable reports of
pre-ignition with the use of automotive spark plugs in common aircraft engines -
- then there is hard evidence that the heat range of the plugs is marginal at
best and unsuitable in at least some portions of the normal operating
envelope.
>> What is it about auto plugs that are
inadequate? Peak temp tolerance? Temp rate of change? Cylinder
compression? 100LL gas? Where do they fall short,
...<<
I don't know where they fall short in any or all of the
areas about which you inquire. But none of those are the one about which I
am most concerned.
The operating temperature of the spark plug ceramic under
hot day high power conditions across the full range of allowable combustion A/F
ratios is the first area of concern. If it doesn't qualify there - - then
nothing else matters.
And so far as I can tell - - nobody in the experimental
aircraft world knows the answer to that question. They may be just
fine and run along at a cool 1200d F. Or not. But everybody keeps hoping
"yes" but nobody knows and nobody has the data. I assume that you agree
that hope should not be a method of assurance for safety critical items in
aircraft.
The fact that some people have run automotive plugs
successfully in some engines - - is a lot like saying that some people
have successfully used car gas in their aircraft engine.
Unless you know that they have used that gasoline under the
worst case conditions that YOU will ever encounter with your engine and that
your engine is "less critical or the same" as the engine that someone else used
- - and that when you buy your automotive gas from your local dealer that it
will be the same or equivelent gas bought from the earlier supplier for the
earlier test - - - then you really are just following the "hope as a
method" concept for assuring that your use of similar car gasoline would
work in your particular application.
I can go out and run a 350 Hp Navajo Chieftan
turbocharged engine all day at 210 Hp and do it on premium car gas.
But if I try to run it at 230 Hp on a hot day with hot
cylinders in a single engine climb - - it is going to
detonate.
Or if I try to run it at 270 Hp on any day, it is going to
detonate.
But if I just told people that " I have run a TIO-540J2Bd
engine on car gas" - - - then it is likely that someone else will decide
to follow the "hope as a method" concept and try the same thing - - with
disasterous results.
I know what the actual heat range for the aircraft plugs is
in the high performance aircraft engine and the temperature margins that are
present.
But if I were to insert an automotive plug into a bushing
screwed into the cylinder head of my air cooled aircraft
engine I would have to merely "hope" that it was going to work based
on a mixed set of anecdotal testimonials about their successful and unsuccessful
use in aircraft engines reported by others.
That is all I am saying.
Hope is not really a method.
Regards, George
From: Lancair Mailing List
[mailto:lml@lancaironline.net] On Behalf Of Chuck Jensen
Sent:
Friday, May 30, 2008 6:06 PM
To:
lml@lancaironline.net
Subject: [LML] Re: Electronic ignition spark
plugs
George,
Your
postings are quite surprising, and a bit disappointing. You point out that
auto plugs are different that aviation plugs. You point out that you know
the temperature of the aviation plugs during operations. You point out
that auto plugs enjoys a heat sunk environment. You point out that the
aviation plug was designed for the aviation environment.
All of
these are true, but unfortunately, you leave us to 'conclude' that that somehow
means that the auto plug are unsatisfactory in the aviation
environment. I don't like having to bridge such gaps in data.
What is it about auto plugs that are inadequate? Peak temp
tolerance? Temp rate of change? Cylinder compression? 100LL
gas? Where do they fall short, other than they are 'different' from
aviation plugs?
Personally, I run aviation plugs on both my mag and EI, but many have run
auto plugs without problems. Why, or how, were they successful when there
are so many implied deficiencies with auto plugs?
Thanks,
Chuck
Jensen
Colyn,
Other than curiosity - - it is not,
frankly, important that I know what the number is supposed to be - -
What is important is that nobody has a clue
as to what the number is for the automotive spark plug mounted in the
aircraft application.
Keep
in mind that the automotive spark plug enjoyes being "heat sunk" into a
massive water cooled cylinder head that is held at constant temperature from a
thermostat.
By
contrast, the aircraft spark plug is desgined for the much more variable
environment of the air cooled cylinder head which can have temperatures more
than twice as high as the automotive cylinder head.
There is a casual acceptance of a substantial
level of "under-investigation" that is associated with
some of these often routinely accepted recommendations to use components
that have not undergone thoughtful testing at the boundary
conditions of realistic operating
environments.
Regards, George
PS> The answer to your question is in some
of the APS class materials. We made a power point slide that detailed
the aircraft spark plug ceramic temperature as a function of A/F
ratio.
George said
I can tell you the answer to that question with
respect to an aviation spark plug.
okay, what's the answer? say, 50 dF LOP 32" TSIO-550
....or whatever you actually have.
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