BS ENGINES

[Stephen Lamb]

Hi, I'm new to this list, but perhaps as a metallurgist I can help with
some of this:

>Clint Corbin wrote (in part):

>>All in all, aluminum is not the best material for a piston.

Ed Lansinger replied (in part):

>As you allude, heat transfer is very important.  Aluminum is a nice
>material from that...

Actually this is probably about the most important factor, but there is
also the fact that aluminium alloys have a thin naturally forming
protective coating of aluminium oxide which has a far higher melting point
than the metal itself.  Aluminium actually 'burns' in a similar manner to
magnesium (ie exothermically) - remember the British ships with aluminuim
superstructures which 'burned' for days after being hit with Exocet
missiles during the Falklands war ??

 >standpoint, much nicer than steel.  It's not clear to me that simply
redoing the same
>aluminum design in steel will improve durability - strength at the same
>temperature goes
>up, but the piston temperature goes up because it can't transfer heat as
>quickly through
>itself to the walls, wrist pin, oil, intake air, etc., plus you have much
>greater
>stresses from the greater mass you are throwing around.

Exactly, steel certainly has poor thermal conductivity relative to
aluminium (and stainless steel is even worse).  The greater reciprocating
mass would mean lower engine speeds and performance, etc.  Vehicles built
in the early part of this century actually used cast iron pistons because
aluminium metallurgy and particularly casting metallurgy, was in its
infancy.  But the speed of these engines was quite low, so the stresses on
the cast components was kept to manageable levels.

>I have heard of people having lots of success with aluminum pistons and
>heads that use a
>(ceramic) thermal barrier coating.  Maybe this would be a good solution to
>the original
>question (which I missed).

Just for interest, modern piston materials are actually a special
aluminium/silicon alloy although engine builders have been experimenting
with ceramics for a number of years now - I believe some diesel engines now
use pistons 'crowned' with a ceramic barrier.

Cheers


Stephen Lamb
Dept. of Defence DSTO, AMRL
506 Lorimer Street
Fishermans Bend  VIC  3207 Australia
Tel: +61 3 9626 7525
Fax: +61 3 9626 7089

IZCC #180