Low Temperature Differential Stirling engine plans

[Ben]

Hello,

This is my first time making a Stirling engine and I am trying to make the low temperature differential gamma model. I recently finished my own 3D design but I still have many questions and I would really appreciate someone with more experience to look over what I have so far. Hopefully I am not overlooking something important.

Thanks in advance for your help!

[Ian S C]

Ben, looks ok, you need toto keep weight down in the moving parts, the friction down, and minimum air leakage at the displacer rod.
You might need to put a counter balance in the form of a weight on the flywheel to balance the weight of the displacer and the piston.
You don't have to, but it would be interesting/nice if the cylinder around the displacer was transparent(mine isn't).
Ian S C

[Ben]

Thanks Ian!

I have now completed machining the project! I am currently in testing/finishing the project and it is not working, even at high input heats (I am using a hot plate). That being said I plan on continuing to sand down friction areas and use lubricant for the shaft on my final presentation. Still, the rotation seems pretty smooth and I am confused to what went wrong.

I have attached a picture of what I have so far, and I would truly appreciate any feedback. Thanks in advance!

[Ian S C]

What is the displacer made of? The top and bottom plates could do with being at least half as thick, the piston should slide down the cylinder with very little friction, and vertually no air leak. I presume that the cylinder is glass tube. This motor should run when sat on a bowl of boiling water.
Ian S C

[Ben]

Thank you for your quick response. The displacer is made of polyurtherane foam. The cylinder's are both polycarbonate. I have found no air leak in testing and the piston seems to have little friction. The biggest problem at this point might be the thickness of the plates. I don't understand why that would be a problem if I leave the plates under constant heat long enough to heat up.

Ben

[Ian S C]

I have doubts about the polycarbonate power cylinder, I don't think you would get it accuate enough.
With a temperature much above boiling the polyurthane fome will melt, I know because mine did when the bowl of water that I had on the stove boiled dry.
Ian S C

[auret]

I have built a similar machine that worked. I used a graphite piston, the shield snapped to one plate first then the other. I could help you make it work. my piston went thonk thonk thonk in and out, I had no flywheel.

[auret]

also as you are running low pressure, you are correct, change the plates for 3mm.

[auret]

Also for pretty much the same dimensions (relative), my piston had half the diameter. I once made the mistake of using a piston displacement that was greater than the expansion, you may have done the same

[Ian S C]

My LTD motor has a diameter of 160 mm, with a 23 mm power piston made of cast iron, running dry in a cast iron cylinder, it runs at 120 rpm on a bowl of boiling water, it fits nicely on the pot I use for cooking my dinner.
How good is the fit of the piston in the glass cylinder, is the glass cylinder absolutely round, glass syringes were ground on the inside, the plunger was ground, and the individual parts matched up, and not interchangable. In other words, your piston may be round after you take it off the lathe, and it may slide into the cylinder, but if the cylinder is not round there will be gaps. Also your piston looks very heavy, you can lighten it by boring out the centre part to make it look similar to the piston on an IC motor, my pistons usually have a skirt thickness of about 1 mm, and a crown of 3 mm. You can make a graphite piston with a 3 mm skirt, and 5 mm crown.
Ian S C

[auret]

Ok I found your problem ....hehehehe ....... your piston requires a power stroke in it's current position, therefore you need your displacer in it's top position, exposing the working fluid to the lower hot plate. ffs you had me going for a while there dude

[auret]

I probably said enough, but hey from the throw of you pin rotation, it looks like your machine has a very thick displacer, check what Ian used, I used a 5mm thick Styrofoam, some support, and tinfoil both sides. I hope that helped.

[Ian S C]

The displacer in my machine is made of Styrofoam, did have a melt down when the bowl of water boiled dry.
Ian S C

[Bumpkin]

Hi Ben. From the picture, it looks like the wrist-pins might be higher than they need to be. This could cause a bit of extra rocking friction and/or possibly leakage. Also, if it takes longer to heat the bottom plate there's more time for the heat to migrate to the top. You might try cooling the top with a wet cloth as an experiment.

Bumpkin

[Ian S C]

If I want mine to go faster, I just grab a couple of cool pads from the fridge, and put them on top.
Ian S C