Stirling trouble shooting - need a little help

[David Morrow]

I've just finished my Coss Horizontal except for some clean up and colour anodizing. Functionally, it's all set to go. The problem is that it won't.

I can feel no friction when I turn it over by hand. All of the moving parts are running on bearings. Everything feels fine and I feel a tiny bit of kickback which should be the power cylinder compression. I think I have the timing correct. As the power piston reaches bottom dead center, the displacer is just heading back towards the hot end and is 90 degrees ahead on the crank shaft.

I heated the hot end with a little butane torch so there's no shortage of heat.

What's the normal troubleshooting sequence you other Stirling builders follow ?

Any easy and obvious issues that I should be looking for ?

 

[joeby]

David,

Stirling engines can be a challenge to get running. There isn't an excess of power there to help you.

The first thing I look for is ANY potential air leaks. Make sure that all joints fit tightly together and have a good surface finish and no burrs or nicks to keep them from sealing. I've used aquarium sealer on LTD Stirlings, and the half-size Moriya fan I built has thin gaskets between the displacer cylinder parts to help with this.

The gland on the displacer cylinder is another place to check, as it is difficult to get a good sliding fit, yet prevent air from leaking past. I try to make the gland as long as possible to get a better seal and help prevent any binding from the sideways pressure that the connecting rod places on it.

If you can move your crank "throws", try adjusting the displacer timing a little from the 90 degrees you have it set at. It's hard to tell from your pictures if the connecting rods are the same length, if they aren't, it seems to help to adjust the phase angle a little. The last Stirling I built has a very short displacer connecting rod compared to the power piston connecting rod, and it runs better with the displacer timing retarded a bit.

Keep on plugging along, it will run, just takes a little patience.

Kevin

 

[shred]

Also try disconnecting the power cylinder rod and spinning the engine by hand-- it should spin several cycles at least. On my little fan, I had a tiny scraping contact on the displacer of my 1/2 size fan that caused no end of issues until resolved.

On timing, you want all the air around the cold end when the piston is out so it can be sucked down by the low pressure caused by the cooling air. I found the 'suck-in' stroke to be more important than the 'push out' on mine and it took a while of dinking with the timing to get it going.

 

[myrickman]

Have you tried turning it in the opposite direction?

 

[cfellows]

If everything is leakproof, you should get a pretty good kick back on the power cylinder. If you spin it hard enough to turn over several times, it should stop with the power piston right in the middle of the stroke. That usually indicates it's pretty air tight and at them same time, that is has very little friction.

Chuck

 

[NickG]

David,

As everybody has already said, I think it'll be a leak if the friction is low. Your cranks look 90 deg apart which should be fine and from the view you've taken the flywheel should spin clockwise.

What is the clearance like between your displacer and hot cap wall? Also at each end? There should be only a very small clearance. On mine it's about 1/32" in radius and about the same at each end. How thick is your hot cap? Again, the walls should be very thin to maximise heat transfer to the air inside and minimise conduction along the wall into the cold end.

The trouble shooting sequence I used (i had the same problem initially) was to take the whole cylinder assembly off but with the displacer and rod still in place. Dunk the whole thing into a sink of water just with the power cylinder sticking out and blow into it. Look for any bubbles, a few are acceptable around the displacer rod and will be expected. I had a big leak around my hot cap which was causing the problem.

If there are no leaks and friction is down, it can only really be the fit of your power piston in its cylinder.

Even if the cooling of the cold end is not sufficient it should run for a short while to start with until the temp difference gets too small. To check whether it's this you can try putting an ice cube on the cooling fins whilst heating the hot cap.

Hope this helps.

Nick