Simple Stirling Engines

Schematic diagram of a generic Stirling engine.
  The air inside is continuously heated and cooled, which makes it expand and
contract repeatedly. The expansion and contraction moves the piston. The
displacer moves the air back and forth from the cold side to the hot side.
One-moving-part, test-tube Stirling engine.
  Home Shop Machinist ran a story about this in their Sept./Oct. '97 issue.
Parts of the test-tube Stirling engine.
  This is a redesign of the now common test-tube version except ours has only
one moving part.  It uses a Pyrex test tube, electrical tape, two jumbo paper
clips, two #2 single-hole stoppers, glass or copper tubing, a piece of heavy
strap, a corner brace, two bolts with wing nuts, three marbles, the end off of
a balloon, a base of any sort, a copper-wire ball, counter-balance "washer"
and an alcohol burner.
  Cloth tape holds the paper-clip pivot axle to the top of the test tube.
The metal strap is bent and drilled 1/16" for the pivot bearings.  The
copper wire ball in the test tube acts as both a heat exchanger and a cushion.
Glass tubing needs to be bent using a propane torch, or use copper tubing.
The big "washer" is a counterbalance.
  Balance it to tip counterclockwise. As the burner heats the air, the balloon
expands out, pushing on the corner brace.  This tips the  test tube clockwise.
The marbles roll to the right.  The air is then forced to the left, getting
cooler.  The cool air shrinks. The balloon is now sucked in. The test tube now
tips counterclockwise. The marbles roll left. The air is pushed right, to the
hot side. The balloon expands.  And so on...  It takes a little time to adjust
everything right.
A homemade alcohol burner.
   This was made from a metal-lid jar 2" in diameter and 2" high.  The wick
holder is a modified air-hose fitting with a 5/32" hole JB-Weld-epoxied over
the same size hole in the lid.  The wick is a length of first-aid gauze.
                         A BASIC STIRLING ENGINE
The Basic Stirling engine.
  This design is elegant, but not the easiest one to make and adjust properly.
It has a balloon rubber power piston epoxied to a steel washer caulked (or
epoxied) to the aluminum cold plate.  (Always solvent-clean aluminum before
using adhesives.) The piston cork is contact-cemented to the rubber power
piston.  The opening for the piston is 7/8". The wide end of the cork is 1/2".
The piston pushrod going from the cork to the crankshaft Teflon bearing is a
paper clip.  The crankshaft is 3/64" brass rod.
The aluminum cold plate and bearing mounts.
  Use sheet metal brake Vise Grips to bend the aluminum after cutting it out
and drilling.
The displacer piston.
  Use common foamboard, made for signs and posters, for the displacer.  The
plastic cover over DVDs makes a good cylinder when cut to the right height.
Use contact cement to bond the DVD cover to aluminum.  The displacer pushrod
goes through a 3/16" brass tube with the same Teflon bearings inside it.
  The crank offsets are 1/8" so that piston and displacer travel is 1/4". The
cranks must be at 90 degrees to each other.  That will determine the direction
of rotation.  Slots in the aluminum going to the bearing holes are so that the
crankshaft can be removed for precise adjustment.  At full travel the
displacer should be 1/16" from the hot and cold plate and there should be
1/16" clearance between the displacer and cylinder wall.
The Basic Stirling Engine in operation.
                    THE EASY STIRLING ENGINE
   This design was developed so that making a Stirling engine from scratch
could be done by anyone who cared to.  Only common home shop and hobby tools
are needed.  The design is similar to the simple Stirling above, but larger.
   The square hot plate and round cold plate are cut from 5 & 1/4" wide .028"
aluminum.  The cold plate has a 1 & 3/8" hole for the 1 & 3/4" rubber piston
and a 3/64" hole for the displacer pushrod guide. The displacer cylinder is a
DVD case with the center cut out. The displacer is 1" styrofoam 4 & 5/8" in
diameter.  The crankshaft bearings are .082" O.D. brass tube.  The legs are
10-24 bolts 3" long.  The cold-water tub is a tin can 3 & 3/8" in diameter and
height with the top and bottom cut out.  Brass-tube lengths are soldered into
the can for crankshaft bearings.  The power piston is a piece of a balloon
with a cork 1/2" wide at the top contact-cemented in the center.  The
crankshaft is .047" (3/64") brass rod.  The .047" O.D. guide tube for the
displacer .032" rod is 2" long.  Adhesives are JB Weld epoxy and Weldwood
contact cement.  Epoxy is used to secure the tin can and the displacer rod
guide to the cold plate.  Contact cement must be used to bond the DVD case to
the cold and hot plates.  The rubber power piston was epoxied to the cold
plate.  The crank arm hole to the piston connecting rod is 3/16" from the
crankshaft axis, so the rubber-piston stroke is 3/8".
The stock parts.
   Don't use rubber cement as shown here, use contact cement such as Weldwood.
The major parts after modification.
The displacer pushrod and crankshaft.
Some small parts required.
   The power crank is a 1/16" collar with a piece of .030" brass soldered to
it.  The .052" (#55 dill) hole is 3/16" from the crankshaft axis.  The roller
is a piece of round pencil with a .052" hole drilled into the graphite "lead".
The cold plate with the displacer pushrod guide.
The displacer pushrod being epoxied in place.
   There are 1/16" spacers under the displacer temporarily. The roller on the
crankshaft is at its highest position (lowest position while upside down).
The crankshaft in the cold can.
Sketch of Easy Stirling.
The Easy Stirling Engine.
    A counterweight was added to the crankshaft between the can and the crank
    to offset the weight of the displacer and its connecting rod.
The Easy Stirling in operation.
                      THE FOREVER STIRLING ENGINE
   The life of any Stirling engine that uses a balloon rubber power piston is
limited to the life of the rubber.  So this design allows it to be replaced.
It also has no displacer rod guide, just a hole in the cold plate.
The modified parts of the FOREVER Stirling.
   The hot plate is 6" by 6" with legs 3" high.   The cold plate is 5 & 1/4"
in diameter.  The 1/2" thick displacer is 4 & 5/8" in diameter.  Its stroke is
1/4" so the crank offset is 1/8".  The DVD case is 1" high and 5" in diameter.
The cold can is 3 & 3/8" in height and diameter.  The prop is from a Guillow's
Jet Stream balsa airplane.  The displacer pushrod, crankshaft and piston rod
are .048" brass rod.  All the bearings are white plastic tube .118" (approx.
7/64") outside diameter and approx. .052" inside diameter.  The bearings are
epoxied into the cold can.
   The rubber piston is in a 1 & 3/8" hole and is secured to the cold plate
and to the cork by contact cement.  The cork is 5/8" at the wide end.  The
rubber piston stroke is 1/4" so the offset is 1/8".  The displacer cylinder is
contact-cemented to the hot plate and the cold plate.  (Do NOT put the cement
on both parts and wait to join them. Put the cylinder in place and daub cement
onto where the two join.)
Some parts prepared for final assembly.
   The blue cap was to be a sliding piston and the white tube was to be a
power cylinder, but that didn't work.  Aand a displacer made of black 1/2"
Elmer's Foam Board with paper front and back was used instead of styrofoam.
Minor parts for the FOREVER Stirling.
   The circle guide is for marking balloon rubber for the power piston.  Some
slack needs to be allowed for free travel.
The FOREVER Stirling.
   Bolts were used for legs after sheet aluminum sides drained heat off.
The FOREVER Stirling running.
                      THE SPEEDY STIRLING
The Speedy Stirling running.
   This design was a combination of the other designs with the cold can wider
in order to accept full-size ice cubes.  It turned out to also have minimum
friction and so ran fastest of them all.  The power piston balloon rubber is
inside a funnel in the cold can and can't be replaced when it deteriorates.