My first LTD stirling - engine performance update

[bmj27]

Hello,

I'm a new member and I've recently built my first model of an LTD Stirling engine, just for fun.
A short video of it running can be found here:

http://www.youtube.com/watch?v=bc8kf9mh5Lo

The displacer chamber is made of two tin can halves and a transparent plastic tube.
The displacer is a 20 mm thick styrofoam piece with a diameter of 80 mm.
The power piston is a short brass tube filled with epoxy glue. It goes into another brass tube, the cylinder.
The displacer throw is 20 mm, and the power piston throw is 14 mm.

I built the crank shaft of M4 screws and nuts, with ball bearings to keep the friction low.
The crank rods are M3 threaded steel rods, with terminal connectors in each end.
The terminal connectors and a short brass tube forms a plain bearing.

The displacer rod also a thin brass tube that is mounted in another brass tube of the next larger size.

On a hot cup of water, it runs for 20-30 minutes. It also runs on the heat of a hand, but needs ice cubes on the cold side.

The ratio between the displacer and power piston displacement volume is 50:1.

The major obstacle I had was to minimize the friction in the plain bearings.
I was sloppy when I glued the brass tubes to the top tin can, so I couldn't get the crank shaft to be exactly perpendicular
to the cylinder and displacer gland.

Does anybody have any advice on the ratio of the displacement volume, is it too low? Is it possible to get the engine
to run on lower temperature differences by increasing the ratio? (I could shorten the power piston throw).

Thanks
/Björn

[Ian S C]

Sounds as if your not doing too bad. The ratio I,v seen quoted for these motors is around 20:1, but yours seems OK. Personally I would not run brass against brass, the only metal that may be run together are cast iron. Normally like metals have high friction, and some, ie aluminium try to weld them selves together, but the wear rate will be relatively high although that may not matter in a motor that is not in constant use.Keep going your doing OK. Ian S C

[jimlarsen]

Your engine seems to run well. Most engines that run from heat of the hand have a larger surface area for the hot and cold surfaces. It also helps to make the top and bottom plates from a good conductor like aluminum or copper. They have a faster pass-through rate so more heat moves through your motor.

[jaket]

nice build and I think it runs very nicely.

try how fast it turns with ice on the cold side and submerge the bottom to hot water.

[bmj27]

I figured I would post some updates of my LTD engine performance after a couple of months, for those people that want to take inspiration from my design.

The little device still performs on a cup of hot water, but now it requires a higher temperature differential to run.

The crank shaft bearings still run smoothly.
The choice of brass tube material in the power piston and cylinder still works ok, the friction has not increased here.
I think that the closer to the center of the piston the crank rod is connected, the less friction it will generate.

However, the friction has increased greatly in the small brass tube that the displacer rod runs through.
When running the engine it makes some noise here, and I can also feel the friction when pushing the rod with my fingers.

So for my next engine, I will need to find a better solution for the displacer gland.

Either I will shorten the displacer gland brass tube, but I think a better solution would be to decrease the throw of the displacer.
This would reduce the sideways forces from the crank shaft.
I'm also thinking of choosing other materials for the displacer gland.

/Björn