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Two-Can Stirling Engine

Redlining at 20 RPMs.

Two-Can Stirling Engine

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.

Steps

Step #1: Make the piston subassemblies.

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  • 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.

Step #2: Make the piston subassemblies, continued.

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  • Locate the center on the end of the 2"-diameter dowel and push the thumbtack into the wood. Carefully remove the thumbtack and coat the bottom of the dowel and the tack with super glue. Press into place and apply the super glue accelerator spray to hold fast.
  • Test the can for watertightness. If it leaks, apply more glue.
  • Locate the center of the opposite end of the 2"-diameter dowel, and drill a pilot hole and screw the #214 screw eye into the center. Apply super glue and accelerator spray.

Step #3: Fabricate the crankshaft.

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  • The crankshaft consists of a metal rod bent in a precise way that holds the piston connecting rods in alignment.
  • Lay out bend lines on the rod as accurately as possible using a permanent marker, as shown on the bend diagram.
  • Using a hammer, vise-grips, and vise, bend the metal rod as shown. Use special care when bending the rod to make the bend sizes and shapes correspond closely to the diagram. The 2 bends (the cranks) must be offset by exactly 90 degrees, and the distance from the end of the crank to the centerline of the crank- shaft must be 3⁄4".
  • Insert the plastic spacers into the sheaves. Tighten the setscrew inside the collar of the sheave to lock the plastic spacer in place. Do not put flywheels on the crankshaft yet.

Step #4: Assemble the air cylinder.

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  • Before soldering or gluing, cut down the 23⁄4" pipes if necessary, so that the overall distance of the finished assembly will be 71⁄2", center-to-center.
  • Solder or epoxy the copper pipes and fittings together as shown, making certain the connections are airtight and leak-free. Note the alignment: the copper tee is rotated 90 degrees from the plane formed by the other 2 holes in the assembly.
  • Place the rubber stopper into the middle hole, in the tee. This is the system’s water drain.

Step #5: Assemble the water reservoirs.

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  • Remove the top from each steel can, leaving the bottom intact. Sand edges smooth.
  • Mark a 3⁄4"-diameter circle in the center of the bottom of each can.
  • With a utility knife, carefully make 8 to 12 radial slits on the bottom of the can, but within the 3⁄4" circle. The slits should form a star shape, radiating out from the center.
  • If you are soldering the pipe into the can, the bottom of the can should be very heavily scored with a file to provide a toothy surface that the solder can stick to.

Step #6: Assemble the water reservoirs, continued.

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  • Push the 5" copper pipe into the can’s bottom, through the hole formed by the slits. Slide the pipe until just 1" of pipe still extends out the bottom.
  • With the pipe concentric and parallel to the sides of the can, solder the pipe in place. (Alternatively, you can seal the pipe-to-can connection with slow-curing, waterproof epoxy glue, taking care to seal the pipe carefully so it will not leak. Allow to dry completely.)
  • Do not give up hope on the soldering. It is very difficult to do, but perseverance will pay off.

Step #7: Make the frame.

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Using deck screws or nails, assemble wooden pieces A-D to form a frame, as shown.

Step #8: Assemble the Stirling engine.

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  • Insert the water reservoir assemblies into the air cylinder assembly. Fill the reservoir cans with water and check for leaks. Repair leaks with epoxy and let dry.
  • Measure and then mark a spot on each 1"×2" frame piece, 33⁄4" from the back edge of the frame. Place the combined water reservoir and air cylinder assembly on the 1"×2" frame pieces at the marked spots. Now place the 3⁄4" copper pipe clamps over the assembly. Screw the pipe clamps into the 1"×2" pieces. The clamps must hold the combined assembly firmly in place.

Step #9: Assemble the Stirling engine, continued.

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  • Slide the screw eyes on the connecting rods onto the crankshaft, so that 1 screw eye is on each of the 2 cranks. Place the soda-can pistons inside each of the water reservoirs so that each soda can rests on copper pipes. Turn the crankshaft so that one of the cranks is pointing downward.
  • Holding the crankshaft level, lift the crankshaft until the can corresponding to the bottomed crank is about 1⁄2" above the top of the copper pipe. This is the desired height for the crank- shaft. Mark this height on the upright 2"×4" and attach the angle bracket at this point, making sure that the hole through which the crankshaft will pass is located 33⁄4" from the back of the 2"×4".
  • Slide 1 flywheel onto each end of the crankshaft. Position the flywheels so that they are as far inboard as possible without interfering with the cranks or piston rods. Glue the flywheels onto the crankshaft using super glue and accelerator spray.
  • You’re done!

Step #10: Use it.

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  • To start your Stirling engine, turn the crankshaft until both cranks are tilted upwards at 45-degree angles to the vertical. With the stopper removed from the drain, fill each side with water, until a trickle runs out the drain. Dry it and replace the stopper.
  • Designate one side as the hot side, then heat the water on that side to boiling with a propane torch. This takes a while, depending on the heat output of the torch. Be patient.
  • When the water is ready, start the engine by giving the flywheels a small push. The rotation is determined by this rule: the cold side is 90 degrees behind the hot side.
  • If built properly, your engine will dip and lift, dip and lift, 20 to 30 times per minute to the chuff-chuff beat of Robert Stirling’s ancient idea.

Step #11: Troubleshooting.

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  • Make sure the engine is level. The crankshaft must revolve freely, and the connecting rods should stay in the middle of each crank as it rotates. Use shims or cardboard to level the system. If the connecting rods will not stay centered on the cranks, you can add a small wire loop or small nut to the rod on either side of the eye screw, fastening them into place with super glue.
  • You may have to experiment to find the best flywheel weight. If the flywheels are too heavy, the metal rod will bow, interfering with the crankshaft’s rotation. But if the flywheels are too light, there won’t be enough inertia to carry the crankshaft past the volume compression phase and into the next expansion stroke. If this happens, the engine will pulse but not run cyclically. You can add weight to the flywheel by simply taping bolts or other weighty objects to its perimeter.
  • Large steel cans full of water take time to heat. Be patient, and let the water heat to 200°F or more.
  • Minimize friction and interference. Friction is your engine’s greatest enemy. Minimize rubbing between pistons and water cans, between connecting rods and cranks, and between the crankshaft and the metal support angles that attach it to the wooden frame.
  • Add a regenerator. A regenerator consists of a small piece of heat-conducting metal gauze placed in the air cylinder just behind the rubber stopper. A regenerator will improve cycle efficiency and make the machine turn faster. The copper gauze sold for cleaning kitchen pots (“Chore Boy”) works well.

Conclusion

This project first appeared in MAKE Volume 07, pages 90-101.


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