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.