Lawrence L'Hote wrote:
"Trevor Jones" wrote in message
...

Artemia Salina wrote:

Dumb question I guess, but why do engine manufacturers make
valve stems that are filled with sodium (I think only exhaust
valves are sodium filled). Is it a heat expansion issue?

Thanks.

Better heat conductivity aids in transferring the heat away from the
valve head.


Curious that aluminum and copper both have a much higher thermal
conductivity than sodium
http://web.mit.edu/3.091/www/pt/pert14.html

But I have a feeling that the relatively low melting point of sodium(371K)
with copper(1356K) has something to do with transfering heat better. I
dunno.
http://web.mit.edu/3.091/www/pt/pert12.html
the sodium would be a liquid at internal engine temperatures.




AFAIK sodium filled valve stems operated on the thermosyphon principle.
The sodium "absorbed" heat by evaporating at the part of the stem near
the valve head and delivered it to the other end where the sodium
condensed and the heat it gave off was transferred out through the valve
guides.

Somewhat similar to "heat pipes" but without the wick inside:

http://www.thermacore.com/hpt.htm

Jeff





--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"