Tin Can Stirling Engine

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.

Step 1: Design

The "core" of a gamma stirling engine of this design is a large cylinder that holds the displacer, connected to a smaller power cylinder providing the power strokes.  I decided to follow Darryl Boyd's "walking beam" design, where the power cylinder sticks out of the side of the dis[placer cylinder, because it suited the materials I had and it looks cool  :)

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!

Comments

[eyesee]

Piston seal problem

[jeveda12]

can you explain a little more about this engine, like how to build the piston and why there is to pistons.how this works please

[PKM]

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.

[lonely164]

good

[asdasd]

Cool, but I'm not sure is it stirling.

[PKM]

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?

[deep92]

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!!!

[PKM]

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.

[yokozuna]

Nicely done sir, 5 stars.

[ravenheart_tinkers]

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.

[Raven Destroyer]

it's hard to make a sterling really work but when it works it shouldn't stop working

[Barfight76]

Excellent work! I want to build one using clear materials (i.e. glass, pvc, etc.), but I can't seem to find what kind of hot temperatures that are dealt with. The pvc specs I'm looking at have a max temp of around 150 deg F. My question really is, what kind of temperatures are you getting from your hot air portion?

[charris7]

can someone please explain the power piston and cylinder to me? what do I need, and how do i position it to make it work???

[PKM]

Someone else has probably explained the cycle more concisely than I have, but in essence:

The displacer pushes the air in the engine to the hot end, where it heats up and expands. This expansion pushes the power piston outwards, because it's the only part of the engine that can move to let the air inside expand. The power piston moving outwards turns the rotating parts around, which moves the displacer and so moves the air inside the engine to the cold end. The air cools down and contracts, and so sucks the power piston back inwards.

The power piston itself needs to be able to move in the cylinder freely, but also to move in the cylinder with a change of air pressure. The simple way to do this is to just glue a cut-out circle of balloon rubber over a hole in the engine, so it will move in and out with changing air pressure. I did it by making a smooth brass tube and a piston to fit very closely in that tube.  That way is more fun and can make a more powerful/efficient engine but needs workshop tools whereas the balloon method might only need a knife and some pliers.  Check out reukpower's instructables for more on how you can build stirling engines without precision engineering tools.

[furyy]

That is soo cool!

[wiinick]

What is that metal thing holding the cd, i see them in many of these projects and i cant seem to find one, help me.

[steve-lane]

you could use one of those sterno cans.
I picked up some at a camping store

[tzq33tdq]

i want one!!!

[fail]

Ive got 2 soup cans and there is a gap of half a centimetre between the middle can and the outer one, all of the way around. Is that too much?

[PKM]

No, that sounds about right. The gap should be wide enough to allow air to flow around without restricting it, so less than a millimetre or so would be too close, but not so wide that it creates "dead space" which doesn't contribute to the power cycle. Having a slightly wider gap does give you more leeway if not everything aligns perfectly (which I was glad of!)

Ideally the displacer should be light, so if you can find one an aluminium drinks can might be better than a thicker steel soup can, but I don't see any reason the soup can wouldn't work.

[fail]

Thanks!
I can't seem to find any drink cans around, so I'm going to use these cans instead, now that I know that it should work. I shall inform you on how well it runs.

[wiinick]

Video pleeeeaaaassseeeee

[Biotele]

video please. Can you calculate power and efficiency?

[PKM]

I'll post a few more videos when I have time to take them. I'm not sure of the power, but I hooked up the flywheel pulley to a motor running as a generator and it would just about light up one LED :(

I think it was overheating at the time- with better cooling and a more suitable generator I figure I could get perhaps one or two hundred milliwatts out of it.

Efficiency is pretty poor, I think an average candle puts out something like a hundred watts of heat so we are talking about something like 0.1%. Not great!

[alexsparrow]

that thing runs hecka fast

[PKM]

The video doesn't show it running that fast- what you don't see is it stalling just after the end of the video because that's about as slowly as it will run :)

That was only running on one candle, though. Heating it with a small cup of meths, it will run at several revolutions per second- it might go faster but I'm afraid to let it run that fast in case something gets damaged in the process. I'll take a video of it running fast and post it in the comments when I have time.

[Timofte Andrei]

you should put some oil on joints and on the pistons! you can minimise the friction in this way ;)

[PKM]

I did actually consider that, but the piston is machined with such a smooth surface and such a tight fit that oil actually increases the friction. The forces involved are small and the piston has quite a large area, so the drag of the film of oil tends to slow the engine down.

I do oil the crank bearings and the sliding displacer rod- it's possible I hadn't done that when I recorded the video, but it doesn't squeak as much now.

[pfred2]

I did a school report on these when I was in 7th grade back in the 70s. Since then I've always still been a bit intrigued by them. I've seen examples that can run on the warmth of the palm of someone's hand holding them! Yours is neat for different reasons of course.

[juanvi]

(removed by author or community request)

[Hebinho]

Hi juanvi,
just go to YouTube and search for KS90 and you will see some nice examples!
They also start running when the upper surface of the displacer cylinder is being heated by just letting the sun shine on it!
But they are a bit difficult to align and when building one, you have to avoid everthing which could cause some unneeded friction.

[pfred2]

When I was there they seemed to be rather commonplace items today. Stirling engines are at best a curiosity though.

[Vengence]

Wrong. Currently they're more efficient than solar cells. They can be up to 40% efficient(not the type in this instrucable though). The company Stirling Energy Systems Inc. has been contracted to build a solar farm in California...

[Hebinho]

First of all we should distinguish between "toy" or "demonstartion" stirling engines and real serious stirling engines! Scottsdale's Sterling Energy Systems Inc. is running the "Suncatcher" which has a mirror dish of 11.5 m diameter and produces 25 kW of eletric power at an efficiency of 31.25 % (world record in 2008), while solar cells rarely are getting beyond 15 %!

Europe has a lot of CHPs (Combined Heat and Power plants) installed, using biomass to generate heat and electrical energy in housing areas, based on stirling engines. Of course these engines by no way can be compared to the ones you will find at instructables.com, but these are real hitech products, running 30,000 hours and more needing only a small amount of maintenance.
There are small CHPs (15 kW heat and 3 kW electrical energy), just take a look at http://ec.europa.eu/environment/life/project/Projects/index.cfm?fuseaction=home.showFile&rep=laymanReport&fil=LIFE99_ENV_D_000452_LAYMAN.pdf

Hungary based FlexEnergy is builing a generator (38 kW output) running on landfill gas (methane) only.

Now you can continue to state "...Stirling engines are at best a curiosity though..."

[pfred2]

Thank you for allowing me to continue stating the obvious.  A few obscure projects referenced does not make you right. There is likely more steam piston engines still in operation than Stirling engines today.

A 40kW diesel generator isn't newsworthy, its a pallet item.

http://www.google.com/images?um=1&hl=en&safe=off&tbs=isch:1&&sa=X&ei=s7FrTY7PPIO88gaU4LyQCw&ved=0CC8QBSgA&q=40+kW+diesel+generator

About 153,000 results ...

I think we need to distinguish between fantasy and reality but that's just me.

[Vengence]

[pfred2: A few obscure projects referenced does not make you right.]
Yes, it most certainly does. It's called proof, and it's not an opinion. You have been proven wrong.

[pfred2: There is likely more steam piston engines still in operation than Stirling engines today.]
Of course there is. Steam is still one of the most efficient and most powerful systems there is. If there wasn't a problem with steam production and storage we'd still be using them in our cars. You're not making a case for yourself. How exactly do you think nuclear power plants work? That's right...steam...

[pfred2: I think we need to distinguish between fantasy and reality but that's just me.]
I guess the sources that Hebinho mentioned are fantasy and not reality? I don't think "we" need to distinquish between fantasy and reality, I think just "you" do.

You may now continue to post opinion...

[pfred2]

You just keep on believing that efficient equals practical. As an example fusion is amazingly efficient, just not very practical. Nuclear power plants do not use pistons except to raise and lower the control rods. But yes turbines are fairly efficient, and practical as well.

No I'm not making a case with you! Because you are an unreasonable individual.

I'm supposed to accept a few sources but it is OK for you to ignore the overwhelming number I present? At this point I really don't care what you think.

[Vengence]

[pfred2: As an example fusion is amazingly efficient, just not very practical.]
Fusion isn't even done at a stable level yet. Once fusion reactors are perfected it will be extremely practical and will take the place of current nuclear reactors. You're still wrong in your example.

[I'm supposed to accept a few sources but it is OK for you to ignore the overwhelming number I present?]
Overwhelming number of what? You have produced zero evidence that "stirling engines are at best a curiosity".

[pfred2]

Sure it is it, just takes more power to operate than can be gotten from it. If fusion reactor engineering impossibilities are ever overcome I'm sure it'll rain pennies from heaven on that day!

Here are 153,000 examples

http://www.google.com/images?um=1&hl=en&safe=off&tbs=isch:1&&sa=X&ei=s7FrTY7PPIO88gaU4LyQCw&ved=0CC8QBSgA&q=40+kW+diesel+generator

That took all of a tenth of a second for our favorite search engine to cough up but you conveniently choose to ignore. Go argue something you've a chance with like extra terrestrials or the Easter Bunny. That ought to be a good one, least you'll have baskets pretty soon as proof!

[komodoboyx5]

you're also ignoring the fact that diesel is a non-renewable resource. The sun will burn on for many, many, many, many, many, many, many more days. In fact, by the time the sun does burn out, we'll have found more suns to juice for energy.

& Don't argue the stupid position of proposing ethanol, that would require energy to make energy, wasting time & power.