playing with stirling engine

Today I replaced the closely wound copper (?) wire regenerator in my Ross type stirling engine with a much looser one (lower fill factor, more flow area) made of knitted stainless wire (actually one of those kettle lime-scale traps). With the inner sleeve and regenerator fitted into the hot cap I performed a flow test (alright, I blew down it!) which showed that the looser mesh did indeed give significantly lower resistance to air flow.
Once started and thoroughly warmed through (this takes some time, I think due to the substantial lump of iron on the top end of the inner sleeve) the engine ran up to a higher speed than before, no doubt due to lower pumping losses, it did not however feel significantly more torquey.
After a good long run I took the hot end apart and was rewarded with a nice range of oxide colours on the regenerator mesh indicating the temperature gradient through the engine. If I could remember the colour/temperature relationship from school metalwork (and wasn't colour blind) I might even be able to estimate the temperatures involved.
Next material I have to hand is pan scourer made of coiled up stainless strip.
Photo of regenerator and sleeve after run at :-
http://community.webshots.com/user/n_highfield
(last pic in 'all hot air' album)
Nick H.

Nick, look here http://www.technologystudent.com/equip1/heat1.htm
Nick H wrote:
  Quoted Text Here  

Thanks for that Martin. As I said, my colour vision is not all that it might be, but Helen reckons that the full range of tempering colours are there with blue (300 C) just about half way up the matrix. Given that the hot cap is probably at about 6-700 C and the cooling water at 60-70 C, then that would make sense.

Nick H wrote:
  Quoted Text Here  
Oxide colours vary with temperature AND time, which is why eye-ball hardening and tempering is an art.
It also mean you can't really infer you engine temperatures.
BugBear

  Quoted Text Here  
Oh well another theory bites the dust!