2-cylinder Gamma - project

[Triangle.Stirling]

Hello everybody,

We're doing our final assignment about Stirling Engines.
We will start very soon on building a 3-cylinder water-bottle stirling engine.

First of al, the water bottles:

We've ordered five of these, just in case we mess it up
Water Bottle

The displacer will be made of steel wool, i'm currently searching for stainless-steel wool, because i think that's better.
The engine will be build up with threaded rods and wood. Cylinders will be closed with wood, with under it a layer of pond-liner as a sealing.

Our biggest inspiration so far is Approtechie and his six-cylinder engine.

All moving connections will be made with bearings to ensure there is very less friction.

Main axis and connecting rods will be made of 4mm diameter stainless steel rod.

Power piston will be made to an idea of Makerj101 , it actually is a diaphragm. We've chosen this, because it is relatively easy to make, and we can ensure an almost air-tight seal.

We have a few questions so far:
- Does anybody have dimensions wich are best to use: displacer travel, diaphragm diameter/travel etc.?
- The hose connecting the displacer piston and the diaphragm what is the minimum diameter?

Let us hear what you think! Al comments are very welcome!

[Ian S C]

One question, why 3 cylinders? Ian S C

[Triangle.Stirling]

we want to get a bit of power from it.
When we use one cylinder, power is limited. 2 cylinders is a little bit more, and 3 seems better.
When we use 4 cylinders, we think it will have less power, due to friction.

Also bending an axis for it will be complexer with 4 cylinders. We thought 3 is as much as we can handle.

Do you recommend another number of cylinders ?

Thanks,

Krijn and Lukas

[fullofhotair]

I love that diaphragm. I really like Approtechie engines. One thing that bothered me on his 3 piston engine was the cylinders are connected with plastic hose. I can just see quite a bit of loss, by the hose flexing. I think copper tubing would be better. Please post pics or a video when you finish. Do know yet what you are going to use as heat source ?

[Triangle.Stirling]

Today i received 2 water bottles in the mail :)
Onfortunately they are different :( One is the right one, but the other is double-walled and smaller. Luckily i ordered 5, the other 3 are coming soon.

a heat source is also a problem i don't know the best solution for.

I most like to use a bunsen-burner (gas-burner), but they're expensive and hard to get.
I've tried alcohol-stoves, but i don't think they put out enough heat.

Any suggestions on this?
And on my other 2 questions about dimensions and connecting of displacer cylinder with power-diaphragm?

[Triangle.Stirling]

I've made a few calculations.

The inner diameter of my can is 72 mm.
If i have a 40 mm stroke then, 20 mm up, and 20 mm down, i have a displacement of 90.5 cc

The displacement of the power piston then must be 60 cc.

if i give the power piston the same stroke, it must be 219 mm in diameter. I will have to expiriment with this, when i get the pond-liner.


Any thoughts about the dimensions?

[Ian S C]

What are the drink bottles made of? If they are aluminium, then a spirit burner is about as hot as you can go, even then you stand a chance of melting it, if they are stainless steel, a bunsen burner, or a gas cooking burner(better), is the way to go.
With the correct proportions 2 cylinders will give more power because of less friction, also the crankshaft will be shorter, and therefore more ridged. Unfortunately a bent up crank cannot have the accuracy of a built up, or machined one.
The ratio for a high temperature motor is 1.5 : 1, displacer : to power cylinder. With a diaphragm, it might pay to have a short stroke compared to the displacer, I would think 20 mm stroke (in your terms 10 mm up, 10 mm down) would be better, I'll leave you to work out the diameter, best wishes. Ian S C

[Triangle.Stirling]

Drink bottles are made of Stainless steel of course. The aluminium ones i have also, but aluminium deforms at high temperatures, and is too soft.
I have to search for a cooking burner, but we can test it with Bunsen-burners at school.

For the diaphragm i am going for a shorter stroke. It is difficult to give a diaphragm a big stroke, but i have to test that when i have the pond-liner.

Do you really think that 2 cylinders is the way to go? We thought that 3 cylinders was possible. The crankshaft can be supported with bearings between each cylinder, so that it is ridged enough.
The only way we know to make a crankshaft is bending. Do you recommend an other way of making it?

Because it is an Gamma-type engine, the displacer cylinder and diaphragm will be connected with a hose. Does anybody know what the minimum diameter needs to be? I was thinking of 8 mm tube?

Thanks for the help so far! When we start building, we will post photo's here.

Krijn and Lukas

[Ian S C]

The ideal bearings for your crankshaft are ball bearing races, but plain bearings well made will work OK. It is difficult to aline more than two bearings perfectly.
If you look in my gallery you'll find a rough diagram of how to fabricate a crankshaft. The other way is to make it out of solid, by turning it in a lathe, this is a bit more complex, but is probably the best way.
Your crankshaft will need to be as ridged as you can make it if you want more power than a simple tin can motor.
To connect the two cylinders, 8 mm or 10 mm would be OK. Copper tube would be a good choice for this, but a flexible tube, plastic or rubber with reasonably thick walls is quite good also
One main reason for suggesting two cylinders is that it may mean a bit less work bending the crankshaft, unless of course the two power cylinders operate on the same crank. Ian S C

[Triangle.Stirling]

Thank you for your answers Ian!

we're going to use these bearings

Thank you for the suggestion of making a crankshaft. I've seen the picture before, but always thought: how do you ensure the pieces are exactly in line?
Now i see the picture again, i see how you did that. I totally forgot that.

I think we're going for your idea of putting it together. We will use 6 mm steel then, thats stronger and rigid. The bearings will be good aligned then. Turning it in a lathe is a step to far for us now, maybe in the future...

We're going for pvc tube, that's easy to get, and easy to fit, a copper tube is more complex to seal. We're going to use clear PVC tube, i think 10 mm. It must have a good airflow, so that the air can easily flow between the displacer cylinder and diaphragm, but a bigger tube is also more dead space.

We will post photo's here during the build, we will start very soon, Lukas is now going to make a few drawings, so we get the dimensions right.

Thank you for your help!

Krijn and Lukas

[Ian S C]

Don't get too worried about the dead space in the connecting tube, one of the Sterling Engine builders in UK, published an article in the Model Engineer magazine, back in the 1970s, he experimented with one of his motors, and ended up with a 6 foot length of rubber tube, there was virtually no difference except that the tube pulsed with each power stroke.
I think you'll find the fabricated crankshaft easier to make than an accurate bent up one. Ian S C

[Triangle.Stirling]

thank you for your help Ian!

Today i got the other bottles, 2 wrong ones, and 1 good. So i ended up with 2 good, and 3 wrong.
Therefore we are now going for a 2 cylinder layout.
I've made sketches of the parts, and now they will be drawn in a cad program so we get nice building plans.

More building-news coming soon!

[Ian S C]

When you make the crankshaft, make the sides/webs of the crank reasonably thick, 6 mm would do OK, make the holes so that the shaft will slide through, fit it in place with a shaft lock adhesive such as Loctite high strength shaft lock, this should be strong enough. Ian S C

[Triangle.Stirling]

Ok i'll keep the dimensions in mind when i go searching for the materials.

We've started on making a 3d-model of the engine. In this way we can make nice detailed plans to make all the parts. Currently we just designed the base, and we're going to make it very soon.
I have already in my mind how to make the rest of the design, just have to put it in the model.

here are 2 screenshots of the basic setup:

https://www.dropbox.com/s/iol7dpquyun3o5e/%231.png
https://www.dropbox.com/s/zme58smievxpak5/%232.png


Let us hear what you think!

[Triangle.Stirling]

Some progress here in the Netherlands:

Just made the diaphragms.

Here are photo's how i made it. The first 3 pieces were bad, the fourth was good, but not the right size. Then i made 2 of the right size:






We're currently working on the plans, and improved them a lot.
Tomorrow i'm starting with building the frame. Keep you updated!

We'd like to hear what you think!