[Diy_efi] Re: Burning Aluminum etc.

Mon Apr 18 13:24:45 UTC 2005

I'm kind of jumping in the middle here but you're mentioning holes in
pistons and appearance of aluminum disappearing on failed pistons.  It
actually sounds more like pre-ignition than detonation.  Pre-ignition of
course is caused by some hot-spot in the cylinder. The combustion can
actually start during the later stages of the intake stroke, pressure starts
rising dramatically once the intake valve closes.  The pressures and
temperatures reached are extremely high, completely capable of
melting-vaporizing aluminum.  The typical final failure is a hole in the
piston since the high heat has softened the piston crown and with
ridiculously high pressures, the weakest point of the soft piston crown
gives way in the form of a hole.  Don't think this heating takes long, it
can happen very quickly under load and there might not be any specific
noises like detonation knocks to make you aware of a problem.

Detonation as you have been discussing normally shows a more "impact" style
failure when breaking parts, typically ring lands  cracked off etc.

So my vote is for pre-ignition if your used pistons are showing loss of
metal.

A lot of people lump pre-ignition and detonation together but they are two
different processes and usually have different symptoms.

My comments above are based on what I've read not on my own lab tests and no
I can't cite the text.

You mentioned pilots setting fuel mixtures by EGT.  This works in their case
because so few variables are at play, the procedure in this case has been
tested in the manufacturer's engine cells and verified to produce the
desired results.  Also, keep in mind the more common of these engine don't
make very high Hp per cubic inch; normally .67 Hp/Cid for NA and just over 1
HP/Cid for turbo engines.

Interesting discussion.

Dan Nicoson

> -----Original Message-----
> From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]On
> Behalf Of Adam Wade
> Sent: Monday, April 18, 2005 6:50 AM
> To: diy_efi at diy-efi.org
> Cc: jdeakin at advancedpilot.com
> Subject: Re: [Diy_efi] Re: Burning Aluminium etc.
>
>
> --- Phil Lamovie <phil at injec.com> wrote:
>
> > Of course all that is required is a set of scales.
> > Perhaps someone who has a partially ventilated set
> > might do the experiment and help enlighten the list.
>
> I'd definitely be interested.  I've seen pistons where
> visually it was obvious that they much have lost
> material, and I certainly didn't find much in the
> crankcase.  Wish I had a set around to weigh, since it
> didn't occur to me to check last time I tore down an
> engine.
>
> > A quick check of the "blue bible" gives the boiling
> > point of pure Al as 2467 Degrees C at STP. As no
> > combustion could occur prior to vaporization I would
> > posit that what is occurring is melting as a result
> > of the pressure created during a detonation or two.
>
> That was what I thought for a long time, but it
> certainly appears that there is less aluminum than
> there should be after all is said and done.  Could be
> an "optical illusion" due to the hole itself.  Again,
> interested to have more data on the subject.
>
> *snip bit on pre-ignition*
>
> > Detonation is on the other hand an extremely violent
> > and unforgiving condition. It is the result of
> > spontaneous combustion of the air fuel mixture.
>
> Do you ever see situations where the mixture is
> non-homogenous and detonation occurs fairly late in
> the power stroke, such that some areas do not "light
> off" during flashover?  I'd think that it would only
> occur under a very narrow set of circumstances, but in
> theory, you should be able to see such results.  Or
> are modern engines too good at homogenizing the
> mixture for this to occur?  I understand that
> regardles of local variations in the mixture, a
> typical detonation event will raise the temperature so
> high and so quickly that all the available fuel and
> air will combust, regardless of local variations in
> mixture.
>
> > Of course giving the operator (pilot) a manual
> > air/fuel mixture control and reassuring them that
> > Lambda A/F (not sure why this is the A/F target
> > under these conditions) is denoted by peak exhaust
> > temps may lead directly to the impact site.
>
> Well, pilots only "know" EGT.  They don't use O2
> sensors and don't really know what lambda they are
> running at any given time.  Ignition timing, as John
> noted in his emails to me, is often fixed for all
> throttle positions, engine speeds, and mixture
> settings.  In his last email to me, he indicated that
> there was a company coming out with a feedback system
> to best time the PPP automatically; sounds very much
> like they must be using ion sensing for this, as it
> would be the least expensive and most feasible method.
>  That would help things a great deal, I'd say,
> especially WRT setting mixtures that could cause
> elevated EGTs.
>
> | 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. |
> |   The catastrophe in this case, of course, was that the sun   |
> |     had come up again."                    -Kurt Vonnegut     |
> | M/C Fuel Inj. Hndbk. @ Amazon.com -  http://tinyurl.com/6o3ze |
>
>
>
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