BS ENGINES

[Stephen Lamb]

>From pistons to valves in one fell swoop.........

>> Exactly, steel certainly has poor thermal conductivity relative to
>> aluminium (and stainless steel is even worse).
> [ munch ]
>greetings:
>  perhaps, then, you could explain why going to stainless steel valves
>is an 'upgrade'?  in light of it's poor heat transfer, what might be a

It's amazing the mystique and myths surrounding stainless steels.     These
materials benefit from their high chromium content (>10%) which generates a
strongly adhereing oxide and imparts benefits similar to the oxide
'coating' on aluminium alloys.   Thus, in their austenitic form (don't ask
me to explain that !), they have good high temperature oxidation resistance
and this is good for use in engines.   However, there are a couple of
problems with their use in engines.   One is relatively poor thermal
conductivity which gives a greater risk of burnt valves.  The other is that
they have thermal expansion rates approximatelt twice that of alloy steels
which means that, all other things being equal, greater clearances must be
used to compensate.

So it's a balance really, but unless your exploring the limits of
combustion (eg a really 'hot', turbo/super-charged engine) then I'd
recommend sticking with a quality, austenitic steel valve.   Certainly for
street performance engines these would be more than adequate.

>reasonable material for an exhaust valve (due to excessive heat)?  yea
>i know titanium is the 'best' due to weight & fatigue factors, but how
>well does it conduct heat?  (anna one-a, anna two-a...<cue bubbles> :)

 (Very) Generally speaking, titanium has about the same thermal
conductivity as stainless.   The advantage of titanium would be in the
lower inertia of the valve system, but the cost would be considerably
greater.   My titanium mets' a bit rusty but I suspect that high
temperature characteristics may be less than satisfactory.

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