How I built a hot air engine almost entirely from junk. I've wanted to build one of these ever since discovering stirling engines in about 2003. Spending the weekend making things in Steveastrouk's workshop gave me the opportunity to make the precision parts I'd need, and I knew most of the engine was going to be made of improvised materials, so decided to see if I could make the entire thing out of scrap or unwanted materials.
If you aren't familiar with how Stirling engines work, there are plenty of resources online- the Wikipedia page is a good place to start.
I found that an aluminium drinks can would fit neatly inside a steel soup can, so decided to use that for the displacer on the grounds of being cheap, light and easy to work with. The steel soup can provides a sturdy body for the engine and also meant I could ake the bottom removable to allow maintenance.
I found some decking to use as the base, some 1" x 1/2" pine to use as supports for the moving parts, and a piece of 3/4" aluminium strip to use as the beam. The engine is a messy combination of metric and imperial units- that's what happens when you use scrap materials!
There are several ways of getting a power cylinder and piston for your stirling. At one end of the spectrum, you can buy graphite pistons and glass cylinders online. This guarantees you will get good quality parts but feels a bit like cheating :) At the other end of the spectrum, you can attach a rubber membrane made of a piece of balloon over the end of the cylinder, like scraptopower's engines. This is probably the easiest to make but limits travel and introduces friction.
Others have made pistons by casting JB Weld inside their chosen power cylinder. You can read more about that here- I didn't do this because I had the machined piston/cylinder combo, but it seems to be quite a popular method.
The piston is a 20mm x 20mm solid cylinder with a 1/8" hole bored in one end. A short section of steel rod was glued into the hole, and a section of brass tube glued over that. The end of the tube was flattened and drilled to 2.5mm to accept a conrod made of coathanger wire. In engine terms, this is the small-end bearing.
The flywheel is a wheel that I think came from a VCR. I used it because the bearing ran smoothly and the wheel looks quite nice as a flywheel :) The axle hole for the bearing was conveniently almost exactly the size of the brass tubing I had, so that would work for an axle.
I initially tried gluing a short piece of brass tubing to the surface of the wheel to act as a crank, but the glue bond between the metal surfaces was weak and kept breaking. Drilling a small hole into the wheel and gluing a piece of coathanger wire into that made a much stronger crank. A piece of wood with a short section of brass tubing fixed into a hole drilled through it provided a support for the flywheel.
The displacer is a section cut from an aluminium beer can, with lids taken from other cans glued over the ends to make a flat-ended cylinder. This made the displacer air-tight, so to avoid the risk of it pressurising and exploding in the engine I made a small "breather" hole with a needle. The top end plate was drilled in the middle and a section of steel rod glued into it. The steel rod I had is a sliding fit in the brass tubing, which offers low friction but an almost airtight seal, so was ideal for the displacer rod seal.
The length of the displacer is important- it should take up about two thirds of the space inside the cylinder. I worked out how far the crank on my engine was going to move (the "throw", about 20mm), and subtracted that from the length of the displacer cylinder (90mm) to find the largest size the displacer could be (70mm).
I actually glued the rod into the displacer with it inside the engine once I had assembled the displacer cylinder to make sure the two lined up- if everything was measured with sufficient precision this would be unnecessary. I used JB Weld for this join as fast-setting epoxies tend to soften at high temperatures.
To make the displacer cylinder, I cut the bottom off a can with a can opener which removes the entire end of the can. I then cut the bottom two inches off another can of the same size with a tapered end, which I could force into the first can to form an airtight seal, but which could be removed if I needed access to the inside of the cylinder later.
The top of the displacer cylinder was drilled in the centre and a section of brass tubing glued in with JB Weld to form the displacer rod seal. A hole large enough to accommodate the power cylinder was cut into the side using a rotary tool and the power cylinder glued in.
When assembled, a short section of brass tube was glued onto the end of the displacer rod, flattened and drilled like the power piston to provide a joint for the beam conrod.
I also cut out the bottom of a tuna can and glued that around the top of the displacer cylinder to use as a water jacket for cooling, but this was actually less effective than plain air cooling, and developed a leak that was letting water into the cylinder so I removed it.
The conrods are made of coathanger wire. Where a joint is only required to rotate a small amount these are simply 90 degree bends in the wire fit into 2.5mm holes, with a dab of hot glue on the end to keep the wire in place. Where the conrods make a continuously rotating joint (where the power piston and beam conrods attach to the crank on the flywheel) these were made of drilled brass pieces from a section of "terminal block" electrical connector.
These are a very convenient size and shape to join the end of a piece of wire perpendicularly to another piece. One of the screws was removed and a hole drilled sideways all the way through the brass, and the other screw used to clamp the end of the conrod in the connector. I got this idea from reukpower's Coke Can Stirling Instructable.
The conrods all have a Z-shaped bend in the middle which allows for length adjustment by tweaking the bends.
Before building anything I drew a diagram and worked out the necessary dimensions of the engine so that nothing would collide, then dry fit all the pieces before cutting and gluing. The base is made from a piece of scrap decking. The beam is a 250mm section of aluminium strip with a hole drilled for a pivot in the middle and two smaller holes near the ends to accept the conrods. The beam support and engine support were made of more pine, screwed into the base.
Despite working out all the dimensions beforehand, my apparent inability to drill a hole straight through a piece of wood coupled with my cheap wobbly power drill resulted in a certain amount of trial and error being applied to the placement of the engine parts.
In case the video embed doesn't work, the first run video is here on YouTube.
After the engine had run successfully, I addressed some of the issues that had become apparent. The flywheel support wasn't in quite the right place, so the piston conron was running at an angle creating excessive friction on the crank. The flywheel itself wasn't really heavy enough to sustain rotation of the engine, so I added the CD and pennies to give it more momentum. The beam and flywheel tended to wobble on their mounts slightly so I added some spacers on their axles to keep them in place better.
After all these mods, the engine runs more reliably than any fuel source I have to test it with :) It will run at approximately 150-200rpm on a decent candle flame, and has run at nearly 500rpm when I accidentally overheated it slightly with a large meths burner. The engine is mechanically sound at that speed- my concern is the displacer overheating and the epoxy that keeps it attached to the rod failing.
that is great. I like the themo acoustic engines. The hard engine to make is a rotary stirling with a rotary displacer and a thermo acoustic heating tube.
This is a great project! You have done particularly well to make it mostly from scrap.
Hey thanks for this instructable. I have a really cool idea which I dont want to reveal intil I build it and I will post it. I am curious if this can handle a load. Do you know or have you tested what this design can handle?
It can handle a load. Just not a very big one :) I connected the flywheel up to a DC motor running as a generator, and running the engine at full power it was just about able to light a red LED. That's an electrical output power of about 50mW, and if we assume a candle flame outputs 75 watts of heat the efficiency is 0.07%. For generating electricity you'd do better with a Peltier/Seebeck module over a tea light candle.
In terms of mechanical power, it's pretty bad as well :) a very small torque will make it stall, so you'd need lots of reduction gearing to get a usable torque out of it but I'd guess that (say) mounted on a model train carriage it could propel itself at a quarter of a mile per hour.
tin can stirling engines, getting a few hundred milliwatts out of an engine with a similar (or simpler) design.
do u put anything inside the can??
and how much time does it take for the engine to start running??
hey what is the difference between displacer cylinder and displacer,?
LOL!!! Put down your handbags guys.
As far as I am aware Sterlings are used today in pretty much all modern nuclear submarines to help generate power from the heat of the reactor. Think of the heat differential between a nuclear reactor and the depths of the sea!
I think a big factor behind the lack of practical sterling engines is that they are largely unheard of by the general public something which the internet and youtube is now helping along. I also know that some manufacturers have tried and failed to install them into cars as early as the 1940s, probably due to the problem with ramping up the power you get from them. Mainly caused by the fact they will run at a steady rate and accelerating / decelerating can be tricky.
Now a sterling vs a solar panel is an interesting match and I wouldn't be surprised if over the next 10 years or so we start to see some really nice green generators based on Stirlings coming out. Especially when petrol and diesel start to become so expensive that they become no longer feasible for the general population.
Step 3 links to some ways of making pistons- you can make one out of glue, or a balloon, or (the way I did) machining it out of metal. I'm not good enough at machining to write a guide on how to do that, so I skipped over that part.
There is only one piston, which has to be a very close fit in the small cylinder but slide freely, and the displacer inside the engine which needs a small gap around the sides. To understand how the engine works I'd suggest reading the Wikipedia page about them. Briefly, the displacer is there to move air around inside the engine, and the piston is pushed in and out by air pressure. This explains why the parts need to fit the way they do, but to understand the entire cycle you should read more about Stirling engines- there is plenty of information on the internet about them.
Cool, but I'm not sure is it stirling.
Well, as I understand it this engine uses the Stirling cycle (heat, expand, cool, contract), so I think it is. What part are you unsure about?
I get that for the power piston to be able to move, it has to be smaller in diameter than the power cylinder, but the thing that i don't understand is that if the piston is smaller than the cylinder won't it cause air leakage??
thanks for ur response! This is the only thing thats bothering me, otherwise i'm really looking forward to making this!!!
That is the biggest problem with building solid power pistons, they have to move freely but fit very closely. Mine, thanks to a very big lathe, is smaller than the inside of the cylinder by something like a quarter of the thickness of a human hair...!
Unless you can make the cylinder and piston perfectly smooth you are aiming for the best compromise between it being too loose (and leaking air) and too tight (and friction taking power away from the engine). I believe loose is generally better for a hand-made first engine. How you do this is entirely up to you, there are a number of possible approaches- check out the many other Instructables on stirling engines.
Nicely done sir, 5 stars.
Very cool little project. Thanks. Oh, and Sterling engines have enormous real world application without any further development. They already work so they aren't just "curiosities". These engines are highly efficient (and they are also typically used with free or very cheap energy sources anyway) and very low maintenance & cheap to operate. Without any further development whatsoever, these engines are already available for purchase from numerous sources, especially relating to electricity producing solar concentrators and co-generation applications where new or existing heating furnaces & incinerators are fitted to also produce electricity. Co-generation engines are not only available for large commercial applications, but also for residential and small commercial applications, requiring as little as 500 degree Celsius to operate. Comparing petroleum diesel engine applications to sterling engine applications is utter nonsense. Diesel has absolutely no application where sterling engines are designed to be used. These applications are specific to producing very cheap electricity, so implying that diesel generators could somehow be used is ridiculous. I strongly suspect the poster makes his living from either petroleum or combustion engines and is making a feeble attempt to misinform. Now to be fair, algae generated diesel may be something to show enormous promise in the relatively near future with some further technological development, but petroleum based diesel is a non-argument. And even having given a nod to algae generated diesel, it doesn't appear that it will be cost effective enough to produce anytime soon to render sterling engines obsolete. Note: Hebinho posted some relevant links above in regards to this argument for anyone who missed them and would like to see just how advanced sterling engine technology already is and what it has to offer.