[Diy_efi] Re: Burning Aluminium etc.

[John Gross]

Under piston-erosion situations, the missing aluminum is not combusted, but 
rather exhausted.  In racing engines that have melted down, you'll see this 
clearly in the headers, and is referred to as "grey death".

Under lean-out conditions, it happens because there is no excess fuel in the 
cylinder to provide evaporative cooling to the pistons, which is why 
lean-out failures take more time than detonation failures.  Also remember 
that oil-cooling on the underside of the piston crown also provides 
substantial cooling to the pistons.

Under detonation conditions, where the end gas explodes rather than burns, 
piston erosion is the result of instantaneous and infinite LOCAL spikes in 
temperature and pressure.  If you've ever seen a cylinder IMEP trace, 
detonatation spikes are so steep that they rarely will show up on these 
pressure traces, even with a resolution of 1/2 degree of crankshaft 
rotation.  (if you don't know what IMEP traces are, they are in-cylinder 
pressure measurements taken on a running engine using piezo-crystal pressure 
transducers, where the measurement signal is triggered off of an optical 
encoder, or position sensor, bolted to the end of the crankshaft.  IMEP 
stands for indicated mean effective pressure).  When these spikes DO show up 
on IMEP traces, they show up as veritcal lines, with no area under the 
curve.  In this situation, the aluminum is literally blasted out of the 
piston under intense localized pressure and temperature.  That is why 
detonation is identified on a piston by the pock-marked appearance of the 
piston.

While pre-ignition is premature burn of the air-fuel mixture, usually due to 
a hot spot somewhere in the combustion chamber, excessive pre-ignition or 
pre-ignition in an engine with borderline compression ratios can very easily 
lead to detonation part-way through the burn.  It is this result, not the 
pre-ignition itself, that causes the damage to the pistons.  The other 
significant result of excessive pre-ignition is a tendency to hammer the rod 
bearings out of the motor, due to the early combustion's significant 
pressure rise before TDC trying to reverse the mechanical motion of the 
piston prior to the crankshaft reaching TDC.

John

>From: Adam Wade <espresso_doppio at yahoo.com>
>Reply-To: diy_efi at diy-efi.org
>To: diy_efi at diy-efi.org
>CC: jdeakin at advancedpilot.com
>Subject: Re: [Diy_efi] Re: Burning Aluminium etc.
>Date: Mon, 18 Apr 2005 03:50:01 -0700 (PDT)
>
>--- 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 |
>
>
>
>__________________________________
>Do you Yahoo!?
>Plan great trips with Yahoo! Travel: Now over 17,000 guides!
>http://travel.yahoo.com/p-travelguide
>_______________________________________________
>Diy_efi mailing list
>Diy_efi at diy-efi.org
>http://lists.diy-efi.org/mailman/listinfo/diy_efi