Pringles Stirling Engine



The amazing pringles Stirling engine, made from cardboard Pringles tube. When I saw this video by MarekSHayward I just had to have my own! It impressed me seeing a Stirling engine made mostly out of cardboard, something which wouldn't normally be workable in an external combustion engine.  This is a really easy to build Stirling that should be accessible to almost anyone.

How does it work ?

This engine uses air which is repeated heated and cooled. To allow the air to be heated and cooled the Pringles tube contains a displacer which is like a loose piston that can move up and down forcing the air around the engine. When the air is heated it expands pushing the diaphragm (balloon) outward which turns the cranks. When the cranks turn they move the displacer down so that the air is near the top where it is cooled causing it to shrink and pull the cranks back, which of course moves the displacer upwards allowing the air to be heated at the bottom, this repeats over and over!


 Materials :  Tools :
 
  • 4 x Pringles tubes, clean and dry
  • 1 x Balloon
  • 1 x Air freshener lid/ small aerosol lid
  • Cardboard from an old shipping box
  • Several plastic straws
  • General purpose adhesive
  • Hot melt glue
  • Fishing line 0.55mm (approx 40lb rated weight?) the creator of the original engine used cotton thread instead.
  • Cotton wool
 
  • Scissors
  • Compass
  • Drill (with 2mm and 5mm drill bits)
  • Marker pen
  • Pencil
  • Ruler
  • Pliers
  • Box cutter
  • Hot melt glue gun





 

Step 1: The displacer

The finished displacer will look something like this.

The displacer is made from a cut down Pringles tube that is glued onto a few cardboard discs. I cut the cardboard disks out to about 6mm smaller than the diameter of the Pringles tube, the cardboard I used was quite thin so I glued three disks together to make sure the Pringles tube was properly supported. The pringles tube is cut to about 9cm and open at both ends.






       
The top set of discs has the fishing line threaded through it's centre. To stop the fishing line being pulled through the cardboard I tied and glued it onto a piece of a plastic drinking straw . I also glued the knot to make sure that it couldn't come undone.

With that done, you can glue the cut down Pringles tube to the cardboard discs. You'll have to cut it down the side so you can make it into the smaller diameter.Glue the bottom cardboard disc down and then fill the space inside of the displacer with cotton wool to take up some of the dead air space.

Glue the top disc with the fishing line on and you're done.

Step 2: Make the displacer cylinder

The displacer cylinder is just another  Pringles tube cut down to about 14cm high. Drill a hole the same size as the drinking straw about a 1cm from the top.

Cut the pringles tube down to 14cm
Drill a hole for the straw.



Step 3: The shelf for the diaphragm

Mark out and cut out the cardboard parts for the shelf as shown in the diagram below. Fold them over and glue them together.

This doesn't show the tabs.
Cut out for the shelf support
This bit will hold the plastic lid.

Drill a hole in the plastic lid the same size as the straw. Glue the lid onto the cardboard shelf. Make sure that the hole for the straw is pointing towards the Pringles tube.


Cut a piece of the straw about 1" long, long enough to connect between the lid and the displacer cylinder. Fit one end of the straw into the plastic lid. Now you can apply glue to the edge of the support for the shelf  Align the other end of the straw with the hole in the displacer cylinder and push it through. I used an elastic band to hold everything in place whilst the glue set.





Seal all around the straw using hot melt glue, apply plenty to be sure that you'll get an air tight seal. Make sure that the straw is flush with the side of the displacer cylinder, or your  displacer will catch on it.



Step 4: The diaphragm

The diaphragm is a straw glued to a cardboard disc, which is glued to a balloon. Cut out the cardboard disc and glue a straw down in the centre. I added some extra support pieces around the straw, but I don't (now) think that these are necessary. 


Inflate a balloon and tie it off. You can now glue the cardboard disc and straw to the balloon.



When the glue has set, cut the neck off the balloon and stretch it over the plastic lid. It's should be tight, but not too loose either. You can always adjust it later .



Step 5: The cranks and displacer rod.



I made the cranks from 2mm galvanised wire - you could use almost any wire though so long as it's not too thin. I think the the minimum you could get away with is about 1.2mm. The part of the cranks that the displacer is attached to needs to bent out about 20 - 24mm. The part that the diaphragm connects to should be bent out about 8 - 12mm. The sizes don't need to be exact though. The diaphragm end of the cranks is rotated through about 90 degrees, this is really important so make sure you get it right. The starting piece of wire for the cranks should be around 20cm long.



Once you've formed the cranks, you can make the connecting rod for the displacer.  It's just  a piece of thin wire with a zig zag shape bent into it, and two hooks on either end . The purpose of the zig zag is so that you can adust the height of the displacer to prevent it from hitting the bottom of the displacer cylinder. This zig zag piece is fitted onto the displacer part of the cranks.



Step 6: Make the top Pringles tube

The top Pringles tube holds the cranks, and of course, seals up the displacer cylinder. This next part is the most important step in the building of this engine. You need to pierce a tiny hole in the base of the top Pringles tube. This needs to be just big enough for the fishing line, if it's too big, it'll leak too much, and the engine won't work. The fishing line should slide through the hole with any noticeable friction. A little patience here, will go a long way towards a working engine! I marked the centre of the Pringles tube using a compass and then pierced the hole using a sewing needle held with pliers.




Once you've done that  you can line the bottom of the top Pringles tube with the top of the bottom Pringles tube and mark the position for the bearing holes. These should be marked on opposite sides of the Pringles tube and about an inch away from the top of the diaphragm straw.




Between the two bearing holes, mark and cut an archway, this is so that you can get to the cranks.



Step 7:  Seal it up

Thread the displacer fishing line through the hole you made in the previous step and put the displacer inside of the displacer cylinder.  Now line the top can up with the displacer cylinder like you did to mark the bearings hole.

Double check the displacer is inside of it's cylinder :) Now seal away with copious amounts of hot melt glue. It must be airtight, so don't hesitate to go crazy with the glue !


Once the glue is set (cooled) you can test if it's airtight by pushing down on the diaphragm and holding it there for about a minute, if it quickly springs right back where it started, there's probably a large air leak somewhere. If it slowly returns to it's starting place, you're good to continue. The reason for this is because of the small air leak around the fishing line, when you push down on the diaphragm, the air will slowly leak out, once you let go, it slowly pulls air back inside.  If there's a big air leak, it can quickly  return to where it started.


Step 8: Connecting it all together



Now you can thread the cranks through the bearing hole. It's a tight fit, but you should be able to squeeze them in.  Turn the cranks to check the turn freely, if not make adjustments to the bearings.


Stretch the zig zag displacer rod out by about 1". Make sure that the displacer is at the bottom of it's cylinder, then tie the end of the fishing line onto the end of the zig zag piece.



Adjust the position of the displacer so that it doesn't hit the bottom of it's cylinder by compressing the zig zag.

Line up the straw that is glued to the diaphragm and drill a 2mm (or whatever size wire you used for the cranks) through it and push the straw onto the cranks. push it onto the cranks.



Step 9: Make the flywheel

Mark 3 or four circles about 6" diameter for the flywheel, then glue them together. It's a good idea to thread a piece of wire through the centre to keep them in line whilst the glue sets.





Once the glue has set, push the flywheel onto the cranks. Bend a right angle on the end of the cranks to secure the flywheel.  This is to give the glue something to grip to. Glue the flywheel in place with more hot melt glue.




Once that's cooled, you can add some counterweights to counter balance the displacer. I found about 4p to be right, but you'll have to experiment.


Step 10: The candle holder

Cut a Pringles tube down to about 2" high and cut a small slot into the side so the candle can get some oxygen.


It's finished now. Just light a candle and place it in the candle holder and place the engine  on top of the candle holder. Leave it for about 2 minutes to heat up properly. Give the flywheel a flick, it should start turning away merrily.


If it doesn't work, there's either too much friction or an air leak, you'll have to look at those things.






Engines that people have built using this design


This one was built by Cyborg2004 :


YouTube Video


This one was built by glome4d

YouTube Video



Back to top