Stirlingengineforum.com

Hi everybody,

i'm designing a beta stirling engine at school for my last year project, which will work with 600°C at the hot end. I've got some questions about the design rules. Hope you can help me !!

1. I've found this information in one of the posts:

" i. length of displacer chamber L = 3 times its diameter.
ii. length of heater chamber = 2/3L
iii. length of cooler = 1/3L
iv. swept volume of displacer = 1.5 times swept volume of piston cylinder.
v. length of displacer = 2/3L and stroke = 1/3L. "

So if you solve this equations, they stand that the length of the displacer should be 2 times the diameter, while I've read several times that it should be three times its diameter. A longer displacer would transfer less heat between the cold and hot side but it would also be heavier. So which relation should i go for to get more power ?
It also says that the displacer stroke should be 1/3L (that equals the diameter), and if I design the power piston with the square stroke that is usually recommended, the volume ratio wouldn´t be 1:1,5.

2. Which would be the diameter of the displacer rod for a displacer diameter of 59mm ? Or how can I calculate it ?

3. Does it change much of my efficiency if I use just the annular gap between the displacer and the cylinder as the regenerator (instead of using tubes coming from the hot end cap) ? How much big should be that gap for my 60mm diameter cilynder ? Should I stuff that volume with something ?

4. I'm planning to use a selfmade gas burner as the heat source. How big should be the gap between the heater chamber and the burner ?

That's all for now, I hope you can answer them (even though my english isn't very good). I've seen great projects in this forum and I wish you could give me some of your wisdom. Next week I'm starting the CAD's Design so I will probably share them to help other people that is interested.

Thanks in advance,

Lucas.

Lukas, that's a big motor with 60 mm dia hot cap, my biggest is about 55 mm, but I usually go to 30 mm to 40 mm(its cheaper). If you are going to make a ring/donut style burner, the gap is probably between15 mm and 20 mm, mine are built from what ever material I can find, most of my burners have three rows of twelve holes 1.5 mm dia each.
Have you built a smaller motor in the past, I'm sure that a small motor that works well would impress your tutor more than a big one that wont be that much more powerful. Below is my second motor built in 1994, and based on James G. Rizzo's Dyna, it has a 1 3/4"/43 mm bore, the power piston has a stroke of 3/4"/20 mm, the displacer stroke is 1 3/16"/30 mm. This motor produces about 5 Watts of power at the crankshaft, and will rev up to 1500rpm, and the best power under load is about half that speed.
Ian S C
[img][/img]

Hi Ian, thank you for your reply. My tutor insisted on using the materials that we had on the workshop so we finally designed the motor with a 60mm bore. We had doubts about where should the strokes of the power piston and the displacer start and end.

Power Piston Stroke 50mm
Displacer Stroke 75mm
Displacer Length 150mm (approx 2.5 times its diameter)

This is how I placed the power piston and the displacer inside the cylinder. Both are on their BDC (bottom dead center) in order to understand it better.
Do you think that this is ok or should I modify their positions?

Hi Lucas
Personally I would like to see more cooling area - approx 25% towards the heater end.
The rest looks OK - Should be a powerful engine when finished.
Regards Trev

The cooling system on a Stirling Engine is possibly more important than the heating, that's why most of my motors are water cooled, even one of my small ones has water in a hopper. That motor has a power piston of 5/8" diameter and a stroke of 1/2". the displacer is 5/8" bore by 3/4" stroke.
Ian S C

Speaking of cooling area, this is a dual gamma engine that I recently purchased from Germany. It runs good but I think it could run even better. I think there should be fins or a water jacket on the upper part of the displacer. I see no reason for that area to be "bare". Do others agree? CBStirling2

I agree, but what are the ? 4 black rings around the middle of the top half of the displacer cylinders? These commercially produced motors seem to be more ornamental than productive power producing machines.

For cooling, some aluminium bar bored through for a light fit over the displacer cylinders, with fins cut in with a parting tool on the lathe. The over all diameter --- as big as you can fit in the space, even if you have to cut sections out of the fins to fit it in. Use some of the white silicon paste used on transistor heat sinks in the bore of the fins to conduct the heat. This is what I did with my Alpha motor after I took the two water jackets off. Next thing to do is make a regulated power supply for the generator, I had it out for a demonstration run on Sunday, and wanted 9 volts to run a radio, but had a hard job , it kept creeping up to 12 volts, but after about 6 hours running I could still hold a finger on the cooling fins, so they must work, the hot end was glowing red.

Ian S C

After receiving it in the mail, I was puzzled as well. Why on Earth, is there so few cooling fins and not all the way up?
I wish I had a lathe! Installing aluminum fins could be done in a clamping fashion so they could be removed if need be.
Maybe I could drill out finned computer heat sinks?
Take 2 per cylinder...

If you don't have a lathe the cooling fins can be made up from aluminium discs, all with the same size hole then alternate large and small outside diameter. One of my motors has fins like that, with three thin, long bolts through all the fins and spacers. The aluminium came from a surplus pot from the kitchen, by it's age the pot was probably a WW2 aircraft, here in New Zealand about 500 fighters and bombers were reduced to scrap between 1945, and 1950, lots of aluminium pots.

Ian S C