LTD Pancake question

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LTD Pancake question

Postby zorad » Fri Nov 28, 2008 3:18 pm

Does anyone know if using the rotating motion of the LTD pancake
type displacer make the engine more or less efficient than a linear
motion would be? In other words, with the LTD the displacer has a
TDC and a BDC so it is transferring air from one side of the
displacer to the other at varying rates as its connecting rod moves
about a circle. I realize that with a more linear movement you
would have a higher loss at the start of movement but you'd
recapture most of that in a flywheel as you stopped it. Anyway,
from a purely theoretical standpoint does using the rotary motion
provide benefit or detriment to the cycle over a more linear
movement?
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Response to LTD Pancake question

Postby stan.hornbaker » Fri Nov 28, 2008 7:34 pm

Rotation of the displacer about its axis should have no beneficial effect and would require complications of the mechanical design. This would also tend to reduce the output power which is normally quite low.

The sole purpose of the displacer is to move the air from one side of the displacer cylinder to the other. Some slight improvement to create turbulence of the air can be achieved by machining groves like on a phonograph record on the inner faces of both the upper and lower plates. Boring several holes in the displacer and fitting pieces of open cell foam to fill the holes creates, in effect, a regenerator to improve efficiency.
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Response to LTD Pancake question

Postby zorad » Sun Nov 30, 2008 10:19 am

I don't think I phrased the question right. I'm not talking about rotating the displacer about its axix. As the displacer moves up and down it is not at a linear speed since it is connected via a rod to a rotating piece. This means it moves at a sinusoidal speed if you graph it (I think) since, like a regular automive engine piston it is restricted to up and down movement but the drive mechanism is in a circular path. As the displacer connecting rod (not the displacer end) approaches top dead center of the disk it is connected to it is slowing down. As it leaves TDC it is speeding up until the connecting rod connection is 90 degrees from TDC at which point it starts slowing down as it approaches BDC. Then it speeds up again and slows down as it moves back to TDC. This means the displacers speed as it moves up and down is not linear. I'm wondering if this is a theoretical efficiency advantage or disadvantage as it relates to the movement of the air from the hot area to the cold. Would a linear, more constant speed, displacement of the air be more efficient?

Thanks,

Mark
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Response to LTD Pancake question

Postby stan.hornbaker » Sun Nov 30, 2008 5:38 pm

The constant velocity of the displacer from one end of the cylinder to the other, instantly reversing its direction to return it to the other is a very abnormal situation and there is no way to achieve this mechanically that I am aware of. Sinusoidal and UA&R - Uniformaly Accelerated and Retarded motions and mechanisms are widely used.

Bigger gains in efficeincy are to be sought in better heat transfer in and out of the engine and use of thermal regeneration.
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Response to LTD Pancake question

Postby brian_clayton2000 » Sun Nov 30, 2008 8:16 pm

Maybe this is what you mean,

Last Friday, I took one of my small walking beam stirlings (4" displacer bore/1.125 power piston) and disconnected the walking beam at the crankshaft. I rigged up a starter solenoid off of a hitachi starter to a rod and connected it to the walking beam. I made a crude "camshaft" and attached it to the flywheel, and mounted a microswitch and wired up a relay to the starter solenoid. My thinking was that the motion of the displacer would be more square (faster movement and longer at TDC and BTC) and of course free up the friction/mass from the power piston.

Amazingly, it ran on the very first try.

Lemme tell you, it is a strange looking contraption running.

While of course it picked up power and rpms, it did allow me to very quickly play with unswept space and various strokes. It will only run as fast as the solenoid will move, so until I get the displacer to move faster, I am stuck on trying to make more power with it. I think this week, I will rig up a pnumatic solenoid, so I will not be limited in rpms. One other thing I noticed, this particular Stirling is a air cooled version. The motor runs no faster or when you first start it up or when it has been running for 30 minutes and the top of the cold end is so hot you cant touch it. I even cooled the top with nitrous oxide (the only super cooler I had lying around) and the motor ran no different. While I realize that the rpms were limited by the movement of the displacer, my suprise was that the motor never lost power when the cold side was very hot. It just kept on running until the methanol burner ran out of fuel.

IF nothing else, it makes it very easy to see that the motor likes, and doesnt like. Plus it has the added feature of looking really awesome!
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Response to LTD Pancake question

Postby bptdude___2569 » Tue Dec 02, 2008 6:34 am


Very interesting stuff you are playing with.

If you are connecting or not connecting the beam, is really a matter of how to extract the power, and should give some insight into what really affects timing. Timing is not used as much as I personally think it needs to eventually in Stirling engines. It looks like you are beginning to unravel that.

Also, the cold end ...

By pure theory, make the cold end colder should make more work. In your case you did not see that.

One thing is that your liquid nitrogen is not really cooling the gas inside the chamber. Here you have hit upon two issue.

a. you need to thermally seperate the hot and cold ends. any chance to buy a small piece of Space Shuttle tile on ebay? :)

if all you are doing is fighting your hot end with your liquid nitrogen, it may actually make things worse, and that is SO not right.

you may also need to find a way to get the extreme cold into the chamber center, besides the engine casing, which is already being heated at the other end.

a cooling coil thermally insulated at the entry and exit points?

b. the cold end is suppose to collapse the volume of gas, to prepare it for expansion on the hot end. well, at the very least, it needs to do this. even at this, the engine is unbalanced. a good Stirling design will not only push on the hot side, but pull on the cold side. many engines run on only the expansion on the hot side, probably most. this is a drag in many ways including requiring more flywheel to get past the low power spots in the curve. if you want speed, you will want to lighten the weight.



But, yeah, you need to focus on playing with the thermal cycle, before moving on to methods of extraction and engine RPM. You will only be able to remove the power level produced.

Please keep us informed as you go.
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Response to LTD Pancake question

Postby brian_clayton2000 » Tue Dec 02, 2008 5:31 pm

Yesterday, I changed to a pnumatic cylinder to run the displacer. However, I didnt get a chance to play with it any, other than to check and see if it worked right. I am in the process of building two larger engines, that would provide more output, and hopefully be a little more durable. I keep thinking that it needs more time on the cold side of the displacer movement, to cool the gas longer. Of course you can add more heat, and it speeds up, but the same should be able to made true on the cold side. I am going to try fiddling the the cold cycle duration (instead of 50% hot/50% cold, maybe 30% hot/70% cold) and see how that works out next. Maybe in the morning...
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Response to LTD Pancake question

Postby glidingdoc » Wed Dec 03, 2008 5:13 am

If the cold side is getting so hot, is it because of heat conduction through the metal that makes the cylinder. Try putting in a ceramic heat break between the hot and cold ends. Might drastically improve efficiency.
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