Ernest Christley wrote in news:47142123$0$32479
:

Bertie the Bunyip wrote:

It's the same either way. Cooling and heating are two sides of th
esame coin. It takes time to disapate heat and it's not so much the
passage of heat from one area to another (or the disappation, it's
irrelevant) but the speed at which the cooling or heating is taking
place and thus the gradient across the material.
In short, you take a frozen lump of metal and apply a torch to one
side you have a problem.
Take a cherry red pice of metal and put some ice on side and you have
the same problem (more or less, and disregading crystalisation)
It is the same if the same delta T is present, but my point is that it
is easier to heat something quickly than cool it quickly. Even at 250
C, you are only 523 degrees above absolute zero. So, this the
absolute largest delta T you can induce for cooling, and it is very
hard to get absolute zero, so you are more likely to have a cool temp
closer to 0 C yielding a delta T of only 250 degrees.

On the hot side things are more open-ended. It isn't too hard to get
450 C exhaust gas temperatures. For an engine that is started at say
20 C ambient temperature, you now have a delta T of 430 degrees which
is much greater than the 250 likely on the cooling side of the cycle.


With the heating, you only have the few hundred CFM of air passing
through the engine to heat it. With the cooling, you have all of the
great outdoors to do the trick. To tie it into your anology, you have a
butane lighter to heat the metal, and the Atlantic Ocean to cool it.


Kind of besides th point. you coudl say the same thing about an oxy
acetylene setup and we all know what that will do to a bit of metal.


Bertie