Fwd: [Diy_efi] Re: Burning Aluminum etc.

John Gross jogross3
Mon Apr 18 19:25:04 UTC 2005


In terms of the pressures seen under detonation, on the rare occasion that 
the transducer DOES pick it up, I have done some IMEP work on NASCAR 
engines.  With peak pressures in the ballpark of 1100-1200 psi, and sensors 
with max readings of 2000 psi, every time we noted detonation on the trace, 
the pressure was pegging the sensors' scale.

An interesting point on the "energy release" comment at the end of your 
email...because the spikes are so vertical, there is essentially no area 
*observed* under the curve during one of these events.  We KNOW that there 
is energy released during detonation, as you are combusting air and fuel 
(though at an extremely rapid rate).  Therefore, if the during of burn (and 
time for energy release) is exceedingly short, that provides some insight 
into the magnitude of the instantaneous release of energy.

John

>From: Adam Wade <espresso_doppio at yahoo.com>
>Reply-To: diy_efi at diy-efi.org
>To: John Deakin <jdeakin at advancedpilot.com>
>CC: diy_efi at diy-efi.org
>Subject: Re: Fwd: [Diy_efi] Re: Burning Aluminum etc.
>Date: Mon, 18 Apr 2005 09:31:01 -0700 (PDT)
>
>--- John Deakin <jdeakin at advancedpilot.com> wrote:
>
> > I'm flying a little blind here, with no idea who any
> > of you are, or what the genesis of this
> > "conversation" is!
>
>Whoops!  As we discussed, I posted our email thread to
>the list, and discussion was thus spawned.  There was
>a question (not yet in your email, since I have yet to
>answer your last in response) regarding what happened
>to pistons that caused the holes they sometimes
>acquired (whether it was vaporizing, burning, or being
>hammered away).  Phil I mentioned in the first email I
>sent you rgd. gary's thread.
>
> > I speak only of aircraft engines, almost always
> > air-cooled, driving constant-speed props, with each
> > cylinder having two widely-separated spark plugs.
>
>I understand the redundancy factor and all, but man,
>it sure cuts down on the operating margin between
>"won't run" and detonation something fierce to have
>two widely-separated plugs.  :(
>
> > We have an enormous amount of data from the world's
> > finest engine test stand in Ada OK, at the "General
> > Aviation Modifications, Inc" test stand that has
> > over 100 sensors on the present engine, the big TIO-
> > 540 J2BD engine that powers the Piper Navajo,
> > arguably the most "difficult" engine in the general
> > aviation fleet.
>
>Wowzers.  I am salivating at the thought of having
>access to such a rig, or even the data taken with it
>under varying conditions.  You make it actually
>tempting to return to the midwest!  :D
>
> > The data we see corresponds very closely to what is
> > in the older textbooks from the older engine
> > manufacturers (Pratt & Whitney, Curtiss-Wright,
> > etc.) and some older engineering data from the "big
> > two" today, Teledyne-Continental, and Textron
> > Lycoming, as well as the magnificent two-volume set
> > from Charles Fayette Taylor, "The Internal
> > Combustion Engine in Theory and Practice."
>
>I have had CFT's two-volume set for quite a long time,
>and it truly is a comprehensive and invaluable
>resource for anyone wanting to understand ICEs.  I
>refer to both volumes on a nearly daily basis.
>
> > The discussion of whether the aluminum burns or
> > melts, and talk of weighing them is simply silly.
> > Either way, the lost material is going out the
> > exhaust port,
>
>That's something I'd question.  How much ends up in
>the oil pan?  How much goes out the breather (and if
>the breather doesn't attach to the airbox, or a main
>seal blows, then it's not a closed loop).  I'm
>definitely interested in where the metal goes and how
>it gets there.  Further, many of the the ways to
>combat the damage differ with different methods of
>removing the metal, so from an engineering standpoint,
>it definitely helps to know what is actually going on.
>
> > I believe the violence of pre-ignition (or heavy
> > detonation) is creating hot spots that are intensely
> > hot, and the aluminum simply melts and blows out the
> > exhaust.
>
>Looks like things are pointing solidly in that
>direction, from the conversation so far today.
>
> > Once the smooth surface is breached, further
> > "events" will take advantage of the rough spots, and
> > they will continue to be eaten away.
>
>I would think that this might not be the case.
>Increasing average piston temps would increase the
>likelihood of detonation, whether preignition
>continued or not, but that would be "global", not
>localized in the pits.  Perhaps Phil can weigh in on
>this one as well.
>
> > Detonation can be demonstrated and measured, and it
> > is NOT the instant "explosion" of all the remaining
> > fuel and air so commonly believed!  It begins
> > with "light" detonation, tiny explosions of local
> > pockets of mixture that are "perfect" to self-ignite
> > under the local pressure and temperature
> > conditions.  These cause shock waves that can easily
> > be measured with two pressure transducers (one in
> > each spark plug cavity), and these bounce back
> > and forth within the combustion chamber at the local
> > speed of sound.
>
>Check pages 40 and 41 of CFT's book, volume 2.  There
>he discusses "autoignition" as being the cause of
>detonation, and while the "flash-over" is not always
>100% instantaneous, it is nearly so in almost all
>conditions.  Since the volume of the combustion
>chamber changes only negligibly during the burn, it
>can be said to be "instantaneous" for practical
>purposes.  Combustion always takes SOME amount of
>time, but if it happens so quickly that nothing else
>in the system changes for the duration of the event,
>it is essentially "instantaneous".  Check out the
>time-lapse photo montage on pg. 47; note how there is
>essentially no combustion showing in the frame
>immediately before total "flashover".  That mixture
>just went from "not burning" to "all burning" in less
>than 0.000125 sec.  That's 125 microseconds.  All the
>standard modifiers for burn rate are still in play,
>only on a much finer scale.  Proportionally the
>factors are still the same, but the actual change in
>burn rate is much, much smaller.  Look at the slopes
>shown in the graph on page 55; two of three of those
>pressure curves go STRAIGHT up.  The last is not quite
>straight, likely due to movement of the piston rapidly
>increasing the volume of the combustion chamber, and
>the minimal heat energy remaining to be unleashed in
>the much smaller amount of "end gas".
>
> > "Light" detonation is harmless, and can be
> > beneficial, for it cleans deposits off the piston
> > head marvellously.
>
>It takes very little to cross that threshold and move
>into heavy detonation, which can cause nearly
>instantaneous damage to the engine.  It's a very, very
>touchy thing to try and stay in that exceedingly thin
>"border area"; I would not rely on it, myself!
>
> > An aircraft engine could probably run at 60 to 90%\
> > of rated power for its entire life (usually
> > something under 2,000 hours, or about 500,000 miles)
> > in "Light" detonation 100% of the time, provided the
> > cylinder heads are kept from getting too hot.
> > Generally, 400F is a good upper limit, though
> > factories allow 460F and more.
>
>Yep, I remember your diagram with the yield point for
>aluminum based on temperature.  Has your experience
>been, then, that one can reliably manage to balance an
>engine's many factors right on that threshold of
>detonation, without "going over" and damaging the
>engine?
>
> > We have one aircraft engine that has about 2 hours
> > of "heavy" detonation in short bursts, terminated
> > before the CHT could rise too much.  Probably 200
> > hours of "light," 50 of "medium" along with that,
> > and when the engine was finally torn down, no
> > problems were found.
>
>I'd be interested to know what the overheating of the
>pistons had done to their structure.  Obviously you
>allowed them to cool by restricting the extent of the
>heavy detonation, which is why the engine lasted "so
>long".
>
> > It is pre-ignition that is the monster in all this,
> > and we demonstrate and record that too, in every
> > seminar we do.  Primary cause in aircraft engines
> > is probably a "helicoil tang," a small sliver of
> > metal from the helicoil (spark plug insert) that
> > projects into the cylinder.  A cracked spark plug
> > ceramic will prevent heat transfer from the spark
> > plug tip to the cylinder, and that is probably a
> > major cause of pre-ignition.  We've seen no evidence
>
> > that "deposits" cause pre-ignition, though that is
> > widely believed.
>
>Something has to superheat the carbon to get it hot
>enough to be an ignition source.  I'd wager that
>something else would have to introduce substantial
>heat to get to that point, thus carbon deposits would
>be a "contributing factor" in a catastrophic cascade
>failure of a destroyed engine.  But without other
>events to initiate things, the carbon would sit there
>without incident indefinitely (unless the compression
>ratio raised enough from the presence of the deposits
>to compression-ignite the mixture).
>
> >>> Detonation is on the other hand an extremely
> >>> violent and unforgiving condition. It is the
> >>> result of spontaneous combustion of the air fuel
> >>> mixture. There is no flame front to be seen as
> >>> it occurs everywhere at the same time.
>
>CFT's writings back the above part almost 100%.
>
> >>> The pressure rise traces are vertical
>
>This as well, from his own graphs.
>
> > No.  Absolutely not correct.  I believed this myself
> > until a few years ago, and it a very, very common
> > misconception, even among engineers.  Detonation
> > shows on the pressure trace first as little
> > "jaggies" superimposed on a normal trace.
>
>I believe CFT indicated that the huge instantaneous
>spike in pressure was too sudden to show up on
>pressure traces, and the "spikes" were in fact shock
>waves ricocheting around the combustion chamber, and
>varying based on the local speed of sound through the
>different "gas pockets" in the cylinder, with varying
>amounts and types of combustion having occurred in the
>already-burned, partially-normally burned, and
>auto-combusting areas of the cylinder's contents.
>
> > In these aircraft engines, normal combustion is
> > around 800 PSI max, with a few engines seeing 1,000
> > PSI at full power.  Heavy detonation may drive that
> > up a few hundred PSI.
>
>CFT indicates that the actual peak pressure is well in
>excess of the figures you mention, which makes sense,
>since he claims the pressure transducer cannot react
>quickly enough to capture the huge and sudden spike at
>the point of auto-ignition.  The fact that much more
>radical and complete combustion happens much closer to
>the auto-ignition point is what makes "heavy"
>detonation more damaging.  Note that it's not the
>SPREAD of combustion that is at issue; it's how
>quickly the already burning mixture gives up all its
>energy and finished combusting.  Or that is how I
>understand it from CFT's writings, anyway.
>
>| 82 Honda CX500 Turbo (Cassandra)  90 Kwak Zephyr 550 (Daphne) |
>| "It was like an emergency ward after a great catastrophe; it  |
>|   didn't matter what race or class the victims belonged to.   |
>|  They were all given the same miracle drug, which was coffee. |
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>
>
>
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