Engine out practice

Ernest Christley wrote in
:

Matt Whiting wrote:
Ernest Christley wrote:
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.

The heat doesn't come from the air, but from the fuel.

Matt

- Heat comes from the reaction of the fuel vapors with the oxygen in
the air.
- Once the fuel is vaporized, isn't it also part of the air.

Semantics aside, the point is, you have a limited amount of BTU
available from the fuel-air mixture. Since some of those BTU's are
carried away even as the engine is warming up, the heating will be
gradual. It will heat until a dynamic equilibrium is reached between
the heat from combustion and the cooling from air flow. Hopefully at
350 degrees F or less.

Pull the heating part of the equation out, and all you have is
cooling.
All the air around you is a really large heat sink to dump into.
Push
that engine through the air at 100mph, and the heat will come out
FAST!!

When you cut the power, you cut the heat, but the pistons are still
moving. The cylinders cool quickly. They're exposed to the air, and
have lots of vanes designed to give up that heat. The piston is
insulated...by the cylinder, coatings of oil, etc. The cylinder
shrinks, clamps the moving piston, and parts give up shortly
thereafter.
I'm not brave/fool (you pick) enough to test this, but the engine
might never crack if you stopped the windmilling when you chop the
power.

Your welding torch example is not germane.


You have to pump pure
oxygen into an acetylene flame to get welding temps. Acetylene gives
up more BTUs that gasoline, and it won't work with normal atmosphere
which is mostly nitrogen. You won't ever be able to reach the 6000
degree max temp of a welding flame inside a normal combustion engine.
Even then, try to weld a dirty piece of metal sometime. Even the
thinnest coat of crud is enough to insulate the metal enough to make
welding a frustrating experience.




That's beside the point.


Bertie