Jerry E. Howell Model Project Plans & Kits
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Stirling Engine Heat Absorbers (Hot
Cap)
The heat input area of a stirling engine is generally called
the "hot cap" or "hot end" of the engine.
As the engine operates due to the temperature difference between
the hot end or hot cap and the cool end or displacer cylinder
of the engine, we want to do what we can to prevent heat from
conducting directly to the cool end without doing any work for
us. In electrical terms that would be called a "short circuit".
The material used to make the hot cap must conduct heat through
the wall to the air inside the engine while at the same time conducting
a minimum of heat to the cooler (displacer cylinder) area of the
engine. Historically, mild steel has been the most widely used
material in model engines. It is a fair conductor of heat as metals
go - that is, it is a poorer conductor than brass or aluminum
and most other common metals. Right about here you might be thinking
that if we use a metal that is a poor heat conductor then we won't
get much heat to the inside of the hot cap. The heat will only
slightly be hindered going through the thin wall, but will be
greatly hindered traveling the length of the tube.
To further minimize conduction of heat toward the displacer
cylinder the wall is made as thin as practical. This really does
help. An example would be that fewer cars can cross a single lane
bridge at a given speed than can cross a bridge having say 4 lanes
at the same given speed. The number of lanes equals the thickness
of the hot cap wall.
Thin wall tubing is usually selected to make the hot cap. A top
flange is welded or brazed to the tube for mounting to the displacer
cylinder and a thin plug is similarly attached to close the bottom
end. I machine my hot caps from solid rod. This eliminates the
welding or brazing and I can make the ID any size I want. I leave
the bottom and part of the side wall from .025" to .050"
thick. To minimize the conduction to the displacer I greatly reduce
the thickness of the upper portion of the wall. This creates a
narrow waist just below the flange. The length of the waist is
from 1/3 to 1/2 of the length of the hot cap.
I use stainless steel because it is not as good a conductor as
other common metals. Before beginning to machine a hot cap, I
make a plug .001" smaller than the inside diameter of the
hot cap is to be and as long as the hot cap is deep inside. One
end is drilled for a center and the other end is chamfered. After
the hot cap is bored and turned on the outside, but before the
waist is machined, I insert the plug and bring up the tail stock
center as a support. Now it is possible to reduce the waist to
a very thin wall without danger of distorting or otherwise ruining
the work. The plug also prevents the wall from collapsing from
the force of the cutting tool. Use a truly sharp tool bit with
a small radius at the tip (around .010"). Take lighter cuts
as the wall gets thinner and use fine feeds. I routinely produce
hot caps with walls at the waist as thin as .006". I don't
try to get thinner than this as I want to leave some metal for
strength to survive bumps etc.!
Titanium is even a poorer heat conductor than stainless steel.
Since it is not much different to machine than stainless steel
and the fact that it is beginning to become readily available,
I have been experimenting with it for hot caps and I like it.
It is a better hot cap material. If you can get titanium at a
reasonable price, use it because you will like it too. My "Beamer"
and "Vintage" engines have titanium hot caps and they
are the coolest running flame powered engines I have - the hot
cap flange and displacer cylinders run at LESS than luke warm.
© Copyright 1999 - 2007 Jerry
E. Howell - All rights reserved