Stirlingengineforum.com

I’ve been a follower of your site for some time now – appreciation all the eduction and tools !
For the past year or so I’ve been working on building a low cost Stirling Engine for electrical generation.
Just got it running last week – has taken me a long time. Like some feedback on it if anyone is willing?




Basic specs are:
Beta design
Piston bore – 8.5 ”
Piston stroke – 5″
Displacer bore – 9″
Displacer stroke – 14″
Displacer cylinder – 9.7″
Displacer weight – 11 lbs
Piston weight – 7 lbs
Flywheel – 5 lbs @ 22″ radius
Speed = 60 RPM
Hot End at 260 C
Cold End at 29C
Pressure 0-2.6 PSI

I did a flywheel deceleration analysis for friction and best I can figure is 9.6Watts (piston and displacer) – 4.8 Watts (piston alone)

Our goal is to reach 240 RPM and get 1KW out of the engine with a 300 degree C temp delta.

For a video of the engine running and more info check out our blog at
http://volodesigns-sterlingproject.blogspot.com/

Let me know what your thoughts – strengths, weakness, etc.

We are working to get weight out to the displacer and a reduction in the piston ring friction (poorly seated delrin rings)

Thanks,
Yours,
Tim Sefton

Congradulations,

You have it up and running! I take it this is a commercial unit you want to be able to sell in the future? Questions, How do you picture these engines being powered? Are they design for a specific application? What cost would you like to see these units come in out? I hope you the very best!!!

Quick look before I head off to work.... CONGRATULATIONS! I cant fathom the work you all had to do before making even a test run! I do know a taste the joy on a much smaller scale. Your long term endurance putting things together is amazing. It shows in everyone's the excitement. Looking forward to all your tests and thank you for sharing this all!
Again ...CONGRATULATIONS!

Tim, cold cylinder assy., from what I see it is made of some sort of aluminium alloy, could this be bored and sleeved with preferably cast iron, or steel. A cast iron piston would be preferable to aluminium, and can be made very light, the skirt only would need be 1/4" thick at the most. If piston rings are to be used, absolute minimum pressure on the cylinder walls should be aimed at. If you use an aluminium piston, one or two rings at the crown end, and one at the bottom of the skirt, you want the minimum possibility of metal contact. Have you thought of making the rings from graphite?
It would be possible to use an aluminium cylinder, and have it hard chrome plated, or hard anodised, or treated with PTFE, think the process it called "Xylan" its a Teflon resin paint that is baked on.
The piston crown could be in the form of a cup seal made from "Rulon" a filled Teflon. Ian S C

fullofhotair wrote:Congradulations,

You have it up and running! I take it this is a commercial unit you want to be able to sell in the future? Questions, How do you picture these engines being powered? Are they design for a specific application? What cost would you like to see these units come in out? I hope you the very best!!!

We are working to make it a commercial unit - somewhat based like the open source software model - we want people to utilize what we've done so far and add improvements. Working on several kits, where we'd provide the base components and then the end user could locally purchase the stock parts to complete the engine. The material cost that we are at today is about $600, that with us doing the casting ourselves. We have an engine price today at $1,250 but like to get that down to below $1,000.

Thanks for the well wishes - it has taken us a long time to get to this point and we still have a ways to go !

Ian S C wrote:Tim, cold cylinder assy., from what I see it is made of some sort of aluminium alloy, could this be bored and sleeved with preferably cast iron, or steel. A cast iron piston would be preferable to aluminium, and can be made very light, the skirt only would need be 1/4" thick at the most. If piston rings are to be used, absolute minimum pressure on the cylinder walls should be aimed at. If you use an aluminium piston, one or two rings at the crown end, and one at the bottom of the skirt, you want the minimum possibility of metal contact. Have you thought of making the rings from graphite?
It would be possible to use an aluminium cylinder, and have it hard chrome plated, or hard anodised, or treated with PTFE, think the process it called "Xylan" its a Teflon resin paint that is baked on.
The piston crown could be in the form of a cup seal made from "Rulon" a filled Teflon. Ian S C

Hi Ian,
Thanks for the information, sounds good.
The reason for the Aluminum piston was to utilize it as a heat transfer source at the cold end, hopefully improving the engine efficiency. If you can make out the shape of the piston you'll notice we put a 4" wide by 6" deep nipple in the middle of it to add to its cooling capacity (the shape is mirrored on the displacer). We did have a galling problem and had to open the piston clearance up a bit. We are using Delrin rings right now, and they have an awful fit (basically crafted them with a dremal tool in the garage) - looking to swap those out for PTFE rings in the next couple of weeks.

Right now, our performance is below our target - about 60RPM and we want to get to 240RPM - thinking the biggest factor is the weight of the displacer, it is about 11lbs - any thoughts on improvements for better output ?

Thanks again,
Tim

As far as your pistons go, you do not want metal to metal contact when using aluminium. The rings can be more simple than the ones you have made. Get a sheet of the plastic you intend to use, to the thickness of the depth of the ring, cut a strip to width, and a bit longer and cut a step in each end (look up stepped joint piston rings), wrap the ring around the piston groove. With the stepped ends you can afford to be generous with the ring gap, and still get a good seal. Your piston needs at least two grooves, one at the top/crown end, and the other at the bottom of the skirt, so that the piston is clear of the bore.
Look at the old engines, what do you see with most of them. Yes they are vertical, there by solving the problem of the displacer sagging, also less friction in the gland/bush for the displacer rod, yes I know it would make your motor fairly tall.
I think you may end up with full flywheels, you might be a bit light. If you can turn the motor over quick enough, it will probably run without a flywheel at all, but with very little power.
I have not done any calculations, but I suspect your displacer stroke is too long, most BETTA motors have the same bore right through, and if yours was, going by the power piston stroke of 5", the displacer would have a stroke of 7.5", but as you have different bores you'll have to work it out your self, the ratio of swept volumes is 1.5:1 displacer to power. Ian S C