Two-Can Stirling Engine

By lindseymnorth
Category: Machining
Time Required: 1 day
Difficulty: Easy

The Stirling engine has long captivated inventors and dreamers. Here are complete plans for building and operating a two-cylinder model that runs on almost any high-temperature heat source.

Stirling engines are external combustion engines, which means no combustion takes place inside the engine and there’s no need for intake or exhaust valves. As a result, Stirling engines are smooth-running and exceptionally quiet.

Because the Stirling cycle uses an external heat source, it can be run on whatever is available that makes heat — anything from hydrogen to solar energy to gasoline.

Our Stirling engine consists of two pistons immersed in two cans of water. One can contains hot water and the other cold. The temperature difference between the two sides causes the engine to run. The difference in the hot and cold side temperatures creates variations in air pressure and volume inside the engine. These pressure differences rotate a system of inertial weights and mechanical linkages, which in turn control the pressure and volume of the air cylinder.

Parts
/
Tools

Steel cans (2) At least 4' in diameter. Large juice cans or 1lb. coffee cans work; 13 oz. coffee cans are too small.
Copper gauze (1) Such as â€œChore Boyâ€Â pot scrubber.
Soda cans (2) Aluminum.
Rubber stopper (1) #3 size, to fit middle opening of the copper tee.
Plastic spacers (2) 1' long, the spacerâ€™s outside diameter must match the inside diameter of the sheave, while its inside diameter must just fit the rod used for the crank. Look in hard- ware stores, in the small parts bins that contain specialized fasteners.
Copper tee (1) 3/4'.
Copper pipe (1) 3âÂ„4' copper pipe, about 18' long. Cut as follows: 2 3âÂ„4' (2), 5' (2).
Metal diecast sheaves (2) 5'-diameter metal diecast sheaves or pulleys. Such as McMaster- Carr #6245K45
Wood (1) 2'Ã—4', 4' long, Piece D.
Metal rod (1) about 20' For the crankshaft. I used a .14'-diameter iron rod, 19 1âÂ„2' long. Other diameters may work as well, depending on ductility and strength. Metal rods come in different tempers, some more springy and more difficult to bend. Select one that bends easily, yet is strong enough to support the flywheels without excessive bowing.
Hardwood dowels (2) 25'-long, 3/8'-diameter.
Flat corner braces (2) 4' steel, with screws, such as Stanley Hard- ware #306560.
Metal screw eyes (2) #214.
Copper elbows (2) 3/4'.
Drywall screws (8) 2'.
Thumbtacks (2)
Pipe Clamps (2) 3/4'.
Glue (1) Cyanoacrylate glue and accelerator spray, available in hobby stores or online.

Allen wrench to fit sheave setscrew.
Drill and bits.
Hacksaw
Propane torch
Ruler and tape measure.
Sandpaper
Screwdriver
Utility knife
Vise vise-grips, needlenose pliers for rod bending.

Steps

Step #1: Make the piston subassemblies.

There are two pistons in this engine, one for the hot side and one for the cold side.
With a hacksaw, carefully remove the top end of each soda can. Cut the can at the point where the flat side of the can curves to meet the top, resulting in a 4"-long piston. Sand the cut edge to remove burrs, then wash and dry the interior.
Locate the center of the can bottom as accurately as possible. Push the thumbtack through the can bottom at that point. Remove the thumbtack.
From the interior of the can, re-insert the thumbtack through the hole you just made.
It helps to stuff a rag into the can when pushing the tack through. This will stabilize the sides of the can and prevent buckling.