[Diy_efi] Re: Burning Aluminium etc.

Phil Lamovie phil
Mon Apr 18 10:00:38 UTC 2005



Hi Guys,

While I have seen and created my very own "low windage pistons"
I have not seen any data that would confirm that "combustion" of
the Aluminium (Aluminum) has taken place.

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.

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.

(Bosch Automotive Handbook ISBN 1-56091-918-3)


Here things get murkier.

>>> "Almost all the failures we see probably start with
>>> preignition, which is far more devastating to an
>>> engine than detonation.  If fact, we believe that most
>>> of these engines would make TBO, running 100% of the
>>> time in "light" detonation."

There is a very large difference between the nature of the two
events. Pre ignition is mostly caused by spark plug tip anomalies
such as "glowing" due to incorrect heat range or having deposits
on the tip that are acting as glow plugs.

Occasionally a bad exhaust valve seat to valve contact can also
produce the local hot spot required

The subsequent combustion is indeed combustion in a normal sense.
It starts at the spark plug tip or other glowing spot and radiates
outward as best it can.

Though it may be earlier  than intended and may cause the very
familiar knocking or pinging noises (piston thrust face reversal syndrome)
it will most often cause no more that skirt cracking on the thrust side
as well as all the by products of too much advance.

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. The pressure rise traces are vertical and the first
sound you hear  is usually the front timing seal being blown out or
some other blow through related failure.

That is not to say that the only way to hole a piston is through
detonation but it is still the easiest and the best.

You could of course do something silly like run your engine at
80 to 90% of full load, the rpm at 90 to 100% of max. with the fuel
leaned off to Lambda 1.1 and the spark over advanced just right
and the ring tension a little low and the ring grooves full of carbon
build up to get the pistons crown to overheat to the point of liquefaction.

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.

YMMV....

Hope this helps,

Regards,

Phil







More information about the Diy_efi mailing list