LTD won't run

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LTD won't run

Postby wa3bm » Mon Feb 11, 2008 1:15 pm

Can anyone tell me what is going on with my homemade ltd engine. It
doesn't run at all. In trouble shooting the engine I disconnected
the piston connecting rod and the displacer connecting rod. I heated
the bottom surface on a cup of hot water and manually moved the
displacer while watching the piston. The piston draws in but never
out. The inward pull is fairly strong. Also how large should the
displacer actually be. My displaced cylinder is 6" in diameter and
1" thick. I believe the displacer should take up approx 50% of the
chamber or be 1/2" thick and about 5 7/8" in diameter. Am I correct
in this thinking? Any help would be appreciated.
Bernie
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Response to LTD won't run

Postby stan.hornbaker » Tue Feb 12, 2008 10:44 am

I would have expected the piston would have been pushed out when placing the cold engine on a hot source. In any event you engine is pressure bound. A model LTD Stirling engine must be air tight EXCEPT for a very slight leakage at the displacer gland to allow the engine to "breathe" and maintain an average/mean internal pressure at atmospheric.

A small hole may be drilled and tapped in the upper plate. This will allow for providing and adjustable leakage and allow for another test for a successfully running engine.

The engine should turn over very freely with no signs of binding or friction. The flywheel will have to be weighted to balance the weight of the displacer and possible the piston.
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Response to LTD won't run

Postby sgraehl » Sun Feb 17, 2008 11:10 pm

Try holding your piston down long enough to bleed out some of the pressure as you run your hot test. It will likely go in and out then. It already sounds like you have a working cycle and your engine may have too much friction to run on it's own. If you have a way to open your system and give it a hand spin without the piston compressing the air it should spin freely and rotate many revolutions (a dozen or more). Linkages (connecting rods) are places that often have this extra friction. Make sure they run true and don't bind as they change directions. Have another look at your timing too, the displacer should lead the power piston by 90 degrees. Your displacer dimensions sound okay, how big is the piston and it's stroke?
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Response to LTD won't run

Postby sgraehl » Sun Feb 17, 2008 11:10 pm

Try holding your piston down long enough to bleed out some of the pressure as you run your hot test. It will likely go in and out then. It already sounds like you have a working cycle and your engine may have too much friction to run on it's own. If you have a way to open your system and give it a hand spin without the piston compressing the air it should spin freely and rotate many revolutions (a dozen or more). Linkages (connecting rods) are places that often have this extra friction. Make sure they run true and don't bind as they change directions. Have another look at your timing too, the displacer should lead the power piston by 90 degrees. Your displacer dimensions sound okay, how big is the piston and it's stroke?
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Response to LTD won't run

Postby wa3bm » Mon Feb 18, 2008 6:40 am

Steve
I have it now at least trying to work. It will spin maybe 10 revolutions the correct way and only 2 or 3 backwards when sitting on a cup of hot water. That tells me that friction is now my enemy. I am thinking my piston is too small. My chamber is 6" x 1" and my displacer is 5 3/4" x 1/2". My piston is an airpot glass cylinder .627 diameter 1 1/2 long. I have the hole in the top plate choked down to 1/4" where the piston moves air into and out of the cylinder. I noticed that my displacer is bottoming out and that is causing some friction. I also have the axle running in two ball bearings and I am thinking that if I reduced this to a single bearing it would reduce friction a little. I also used 1" aluminum rod turned down to make my connections to the displacer and piston and I am using 2 CD's for the flywheel. My linkages are model airplane ball links connected to .256 aluminum rods for the piston and a little smaller brass rod for the displacer. I think now that all of this is adding too much weight and friction to the engine. I didn't have a chance to work on it this weekend but I think I am going to turn down the spindles as much as possible to reduce weight, fix the displacer from bottoming out and see what happens. I am wondering what your thoughts on the piston size and stroke vs the chamber size and the displacer dimensions. By the way the stroke on the piston is 1".

Bernie
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Response to LTD won't run

Postby sgraehl » Mon Feb 18, 2008 2:04 pm

It sounds like your stroke is fine, I have used a 2K160 Airpot cylinder set with about that stroke and about that much displacement chamber size. I used skateboard bearings for the crank but I removed the cage inside and removed all but three balls from each ot the bearings (I used two)I run them with light oil and no seals. I also used bearings for the connecting rods where they connect to the crank. For them I used 4X8MM R/C car wheel bearings (about 3.50 at the hobby shop) I took out the seals with a pin and washed the grease out with brake clean, used light oil on them too. Your flywheel could be a bit too light for the size engine you have. Mine used a 6 inch aluminum 1/8 plate and I added four golf club stick on weights to it's outter rim (that did help) I'm thinking CD discs are too light and not enough mass at the outter rim. Disc flywheels work but it's best to have more mass at the outter rim than a plain disc. The weight is more effective there. Reduce your friction and I think you will have it.
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Response to LTD won't run

Postby wa3bm » Wed Feb 20, 2008 5:07 am

It WORKS!!! Got it running last night thanks to all the help I got here. I reduced the throw on the displacer. It still bottoms out but just barely. I will reduce the thickness of the displacer this weekend so that it does not touch at all. That will help a lot. The engine runs now very slowly but it ran for 5 minutes on a cup of hot water. I can see that I need a heaver flywheel and I still have some friction reduction to do. Got some great ideas from everyone that responded to my questions. I can't thank you enough for the help. I was about to give it up.
Bernie
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Response to LTD won't run

Postby sgraehl » Wed Feb 20, 2008 1:46 pm

I've found a good clue that the flywheel is too light is that the engine speed (when it's slow) surges considerably, nearly stopping etc. You can always drill a ring of holes at the outter rim and add short nuts and bolts if you have clearance in the connecting rod linkage to pass by them.

Warning: A running engine is likely to inspire another one...:)
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