# How to build and run a Stirling engine the first time out



## smuttley (Nov 3, 2011)

How to build and run a Stirling engine the first time out...............

PLEASE NOTE: This is not an explanation on how Stirling engines work just some main tips, construction details and running them with fewer hassles.

OK you want to build a Stirling engine and the first time (most builders) you try and run it, it fails to run and you then apply more heat than can power you home for a week and the blasted thing will still not turn over ....................... WHY NOT?

1/ First make sure the flywheel will turn freely on giving it a good flick with your finger; it should run for several minutes without wobbling and stopping. This is done before connecting the push or con rods. Using good quality bearings is a must, or very well machined bronze or brass bushings with precision ground shaft. This also applies to connecting push rods; make sure they are as free from friction as possible and FREE running as you can.
Unlike powerful steam and IC engines, the Stirling engine is low powered and does not like friction to hinder it from running.

2/ Try and build an engine that has distance from the HOT and COLD cylinders and pistons????? What's this? 
Well the Stirling engine or hot air engine needs a differential temperature to run, that is the hotter the hot side is and the colder the cold side is will help the engine to run faster, with more power and run longer or if you follow my advice will or can run continuously without stopping and on lower flame temperatures. So the further your cold cylinder is from the hot the better. But not all is lost if you want to build a hot and cold back to back engine as many want to build............. Read on dudes.

3/ Make sure that the hot displacer piston has a very small clearance in both directions, in other words if the hot cylinder has a bore depth of say 50 mm, and you have a 10 mm crank through, then you will need a piston that is 39 mm in length .............. 
50 mm  10 mm (the crank through)  1 mm (for gap of .5 mm on both ends) = 39 mm (god I hope I get this right). 
You can push it out a little, but if yah want an engine that runs like a dream on a smaller flame, then get with the small distance thing. Same goes for the bore size (although I am working on trying smaller bore diameters) and keep this as small as possible without rubbing against the cylinder sides.

Why so smaller gap size????? 
Well you are talking air expansion and contraction (hot and cold air), if the gaps are large, then the air can and will compress and this will act as a damper, we want the gasses to grow on being heated and shrink on cooling (hot air balloons or crashing to the ground), so trust me on this, more time spent on gap sizes will pay dividends on you first or even tenth build.

4/ Do the same for the cold side power piston, it will need less than 1 mm gape between the bottom of the piston and cylinder (.5 or less would be better) to run perfectly. 

Very important NOTE on cold side piston and probably the most important thing of all points 
Take your time machining the cold cylinder, it has to be smooth and free from burs and also symmetrical. Once you are happy with the cylinder, then go make a piston to fit perfectly, yes perfectly.........................it has to slide under its own weight FREELY within the cylinder, NO jamming or stopping under gravity if held between thumb and forefinger without sealing off the air. Now if it passes the free movement test, then you need to also check to see if it will not leak air past the piston. Hold the cylinder vertically and seal the bottom off with your thumb, then gently place the piston in the top of the cylinder and it should stay there without dropping to the bottom. It is ok if it slowly sinks to the bottom with a very small leak, but if it leaks too much, then it will be very hard to run the engine or need a lot of heat energy to get it working. 

Warning: Dont drop the piston testing this out as I did once.

You can use brass cylinder with an aluminium piston, but one of the best cylinder and piston sets you can use is glass cylinder with a graphite piston; we use them for our cold side as friction is so low and will run for ever. We get them specially made by Airpot corp in the US and you can obtain a free sample from them off the shelf at http://www.airpot.com/html/piston_cylinder.html

I recommend spending some time on this site, as it will give you a much better understanding of friction and the magic of glass/graphite combinations. 

Can I machine my own graphite piston with a borosilicate glass cylinder? YES YOU CAN ................ obtain the highest standard test tube that is used in very accurate measuring labs of the approx size you want to use. Cut this to length using a diamond cut off wheel (fleBay for them) in a dremel tool and check the inside for roundness. Then obtain some mechanical graphite bar (High-purity graphite rod/bar) and machine to exact fit to cylinder. Jan Ridders has a good site with details of using this method http://ridders.nu/ You may have to translate this website.

5/ the hot displacer piston shaft will need to glide freely and not bind, this will also need to have a very small tolerance on the shaft and bush to stop as much air leakage as possible and keep the working air trapped within the engine and allow the air to expand and then contract without leaks.

6/ ONLY use the smallest amounts of oil on the moving parts, never use grease or thick oil. Sowing machine oil is one of the light oils that I always use. If you havent got a precision oiler, then a good tip is to use a piece of thin wire and dip this into the oil leaving a very small droplet on the end, touch this on the part you want to oil and only that much.

*NOTE: If you use glass cylinder with graphite piston, then don't oil them, as the graphite is self lubricating.*

To recap major points:
Friction is a Stirling or vacuum engine killer
Keep the pistons gap on the working gas side as small as possible
NO air leaks or as small as possible
Keep the cold working cylinder side as cool as possible (differential temperature)
Try and use cooling fins to assist with heat reduction on cold sides to help with the last tip
Never over oil your engine

Hope that helps and if anyone sees spelling mistake or need to help my grammar, then please chip in, as I am not that good at it.

Please feel free to contribute and join in and I am always happy to help if anyone gets stuck.
Kind regards,

Steve


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## Ken I (Nov 4, 2011)

Applause for that post - a Stirling engine is well up on my to do list and I have been gathering info - yours goes into the folder.

Welcome to the forum - post an intro and some pictures of your Stirlings.

Thanks
     Ken


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## smuttley (Nov 5, 2011)

Ken I  said:
			
		

> Applause for that post - a Stirling engine is well up on my to do list and I have been gathering info - yours goes into the folder.
> 
> Welcome to the forum - post an intro and some pictures of your Stirlings.
> 
> ...


Thanks for the kind words ken and always happy to help out if you get stuck with your up coming build.

I don't have a lot of photos of my past engines, plus most of my builds are not pretty looking, as I wanted to concentrate on internal workings and get them to run as perfect as possible before making them look nice.

But here is a video of my first Stirling engine that I built. It is so ugly only a mother could love it (that's me), I did not even trim the bolts or clean the mark-up blue off it and it is still like that today, but still runs like a Swiss watch, well almost. 
http://www.youtube.com/watch?feature=player_detailpage&v=kJ0V9iTR9sA

And this is a pic of the engines I build today with my friend and business partner 







Regards

Smuttley


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## Rocket Man (Nov 5, 2011)

That is some pretty good information. I want to add one more thing to #3. When the displacer piston is at top dead center or bottom dead center it should be very close to the end about .010" on both ends. The move air you can displace the more efficient your engine will be. 

Assume you have a 1" bore 1" stroke piston. Use cylinder calculator to find the volume at the ends. http://www.calculatorsoup.com/calculators/geometry-solids/cylinder.php  Volume will be V = 0.7854 cu in. The space between the displacer piston and the cylinder wall must be small enough for the gas at the ends to move completely around the displacer piston to the opposite end of the cylinder. The more air you can move from one end to the other end the greater the pressure change and the more power your engine will have.  It is impossible to move all the air from one end to the other but you need to try and get at least 50% so the volume of air between the piston and cylinder should be .7854 / 2 = .3927 cu. in.

Use cylinder calculator to calculate the space around the displacer piston. If you gap between the piston and cylinder is too large the air will never move all the way to the other end. If the clearance between the displacer piston and cylinder all is too long the air will never move all the way around the piston. It is impossible to move all the air but you need to move as much as possible. The shorter you make the piston the more volume it will displace but don't make it too short because the larger temperature difference between the cold and hot end gives the engine more power. It is hard to insulate the hot end from the cold end. You have to calculate how may cu. in. of air is in the space around the piston, trial and error will soon give you the best calculations. 

Here are 3 sterling engines that I built all are 2" bore 1.250" stroke.

[ame]http://www.youtube.com/watch?v=iSBXcn4u4es&feature=mfu_in_order&list=UL[/ame]

[ame]http://www.youtube.com/watch?v=NBpRl5eF3fc&feature=mfu_in_order&list=UL[/ame]

[ame]http://www.youtube.com/watch?v=57GFrxz_mnw&feature=mfu_in_order&list=UL[/ame]


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## smfr (Nov 5, 2011)

smuttley  said:
			
		

> And this is a pic of the engines I build today with my friend and business partner



That's a nice-looking one. Do you sell those?


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## smuttley (Nov 6, 2011)

smfr  said:
			
		

> That's a nice-looking one. Do you sell those?



Hi smfr,

That question is a bit awkward for me to answer here, as I very stupidly posted a link to our website here on the forum in the links section of the site. I have seen the errors of my mistake, as the forum rules state that you can not promote commercial products.

So in answer to your question, yes we do sell them and others to follow very soon, can I give you details of our information, sorry I can,t but you are welcome to email or PM me and I will privately pass on the details. 

Just on a note here: I love to share information and guide others on Stirling builds, I am helping a chap in the Victoria university in Australia with his build and trying to help him get his first hot air engine up and running via email. I can share his excitement as I can for other home builders, it is a fantastic feeling to get any model running and go on and build more,

Regards,

Smuttley


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## smuttley (Nov 6, 2011)

Rocket Man  said:
			
		

> That is some pretty good information. I want to add one more thing to #3. When the displacer piston is at top dead center or bottom dead center it should be very close to the end about .010" on both ends. The move air you can displace the more efficient your engine will be.
> 
> Assume you have a 1" bore 1" stroke piston. Use cylinder calculator to find the volume at the ends. http://www.calculatorsoup.com/calculators/geometry-solids/cylinder.php  Volume will be V = 0.7854 cu in. The space between the displacer piston and the cylinder wall must be small enough for the gas at the ends to move completely around the displacer piston to the opposite end of the cylinder. The more air you can move from one end to the other end the greater the pressure change and the more power your engine will have.  It is impossible to move all the air from one end to the other but you need to try and get at least 50% so the volume of air between the piston and cylinder should be .7854 / 2 = .3927 cu. in.
> 
> Use cylinder calculator to calculate the space around the displacer piston. If you gap between the piston and cylinder is too large the air will never move all the way to the other end. If the clearance between the displacer piston and cylinder all is too long the air will never move all the way around the piston. It is impossible to move all the air but you need to move as much as possible. The shorter you make the piston the more volume it will displace but don't make it too short because the larger temperature difference between the cold and hot end gives the engine more power. It is hard to insulate the hot end from the cold end. You have to calculate how may cu. in. of air is in the space around the piston, trial and error will soon give you the best calculations.



Nice engines dude,

Thanks for contributing with the post, please feel free to add more tips if you think of any.

I hope the Fan in the second video is bolted down, as it looks like it could easily power an aircraft the way that is turning, nice............ very nice

Smuttley


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## Qtron (Jul 1, 2022)

smuttley said:


> How to build and run a Stirling engine the first time out...............
> 
> PLEASE NOTE: This is not an explanation on how Stirling engines work just some main tips, construction details and running them with fewer hassles.
> 
> ...


"Sowing machine oil" U may want to correct, some may wonder what U mean.. (Not being facetious here).
Appreciate the tips.
Am hoping to build a useful 5 kW unit, but getting plans for one with real efficiencies (>50%) is not so easy! 
cheers,


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