1st post, Never built anything before, have a few questions on Stirling LTD engines

[DrJekyll]

Hello, I am happy that I have finally found a sterling engine forum with what appears to be an active community since the other forums I have seen have no activity.

So I have no engineering experience and never built anything ever but I have this urge to build a LTD stirling engine and have found that I am fascinated by it but I want to understand it better before I build it. I have not been able to find the answers to my questions which may mean my questions have easy answers but I am not an engineer lol. All my questions deal with a LTD Stirling engine with this shape :
(http://www.grand-illusions.com/acatalog ... gine_1.jpg)

Questions

I see that the majority of these LTD stirling engines with a base that is a circle in shape are small and run off the heat of someones hand. Now from my understanding so far, the gas that is used to push the displacer up and down that runs off of a temp. difference is either oxygen, air or helium gas.

1) from those 3 gasses which is the best gas that would require the least amount of temp. difference to push the displacer the most efficiently ?

2) has anyone tried any other gas that perhaps has a lower boiling point ?

3) how does someone get that gas inside and seal it in the casing the displacer is in ?

4) why do people say graphite is the best displacer material ?

5) From my understanding, the more surface area we add to the disk then the amount of heat energy becomes less as the surface area increases since more chemical bonds can be broken faster, is this correct? If it is correct then how come all these toy models online have the same general surface area disk shape? wouldent a much larger circle be better since we would increase surface area which mean the engine could run with a smaller amount of heat under it to start it. ?

[Alfista]

DrJekyll,
you are ready for Dr. Senft's book on LTD engines published by Moriya Press. This process is more easily understood through thermodynamics. A rapid heating and cooling of a fixed volume of air creates pressure and vacuum alternately. These forces are then captured in the "power cylinder". The ratio of swept volumes was found in part through theory, in part through trial and error. There are other important authors but this book is available and easy to read and understand.

[Ian S C]

Dr Jekyll, Don't worry about other gasses, These are for sophisticated, pressurised, high temperature motors. air is the magic stuff. Graphite is not used for displacers, it is used for the power piston because it has very low coefficient of friction, it must be used in a very accurately made cylinder, with a highly polished bore, special glass cylinders are available, with fitted graphite pistons.
My LTD motor is a bit over 6" diameter, and made from junk found around the workshop.
Ian S C
DSC01237 (1024x768) (640x480).jpg

[DrJekyll]

Thank you for the replies. I was actually looking at some research papers now and came across the thermoacoustic Stirling engine? What do you think would be more efficient an LTD sterling or a thermoacoustic Stirling engine, both being run on heat from solar mirrors aimed from the sun? At the youtube videos I have seen of the thermoacoustic Stirling engine, it looks way smaller in height (but longer width) BUT it looks to be WAY WAY more powerful while having a smaller wheel (https://www.youtube.com/watch?v=CDCTANU8Tfk) the thermoacoustic Stirling engine also seems to have much less parts needed ,

so what am I missing as the downside of the thermoacoustic in comparison to the LTD in your opinions?

[Alfista]

With so many questions on such a wide array of topics, it is difficult to focus on the fundamentals. If you are able to focus enough heat over a large enough area, then you would probably be better off with the thermosacoustic engine which I will class as a high temperature differential engine. Keep in mind that this would almost certainly require a heliostat.

If you have a (very) large surface area for collecting heat, good heat storage and long hours of sunlight (no heliostat needed), and you can build a rather large engine, then you may be better off with a low temperature differential engine.

It is rather difficult to focus enough heat on a large enough surface to run a high temperature differential engine. I would recommend reading the article on Ericsson's solar engine, an account of which can be found in Sier's History or in Ivo Kolin's book on the evolution of the heat engine.

Building a large ltd engine certainly is challenging. I have not tested these calculations yet in a project but they suggest that with a power piston of 12" X stroke of 12" with a displacer of 118" in diameter (X stroke of 7.5") One could expect about 1.23 ihp (indicated horsepower) at 60 rpm and 1.85 ihp at 90 rpm. A heat differential of 180 degrees F.
Please feel free to critique these figures, they are only meant to show how big the engine would have to be to produce a very usable amount of power.

[DrJekyll]

I really appreciate all the help. let me explain my vision and perhaps that will make this much simpler and you can tell me if what I imagine is possible or if what I imagine requires to much energy or is impossible for some other reason

Imagine a bright sunny day, all of the following takes place outside...

So outside I have a glass tank filled with normal fresh water . Then I have an LTD sterling engine with magnets on the flywheel , pushed super close to the dry side of one side of the glass tank. Now inside the glass tank that is filled with water , I will have a propeller with opposite magnet poles of the fly wheel attached. The total propeller size inside the tank will be more or less: Wet-Side: 3.00 x 3.00 inches (76.0 x 76.0 mm) .... And they will be made of light material ....Now I have Mylar or some other type of reflective material aimed at the base of the LTD sterling so that the sterling base is powered by the Suns heat. And the sterling let's imagine, has helium gas and some how I use the water as the heat sink to cool the top..... And Lets imagine (since I Don't know yet if it will affect it) that the sterling LTD is made with the least amount of magnetic material as possible so that it does not affect the fly wheel magnets ...... So the ideal scenario is , the sterling powers on, which spins the flywheel with the magnets , which due to the fly wheel being really close to the glass (and the glass being a normal thickness so not super thick) and due to the magnets strength , the flywheel will move the propeller on the wet side which holds opposite magnets at the ends of its propellers.


So if the flywheel will have to be connected to something smaller in order for the magnets to be perfectly spaced , so the dry side magnets match with the wet side magnets , I understand that but to make it simple lets just say the flywheel has the magnets on it spaced perfectly to match the wet side magnets on the propeller.

Sooooooooooooo can this idea work? And how large of a LTD do you think will need to be created to make the propeller on the wet side move a normal noticeable amount of water???

[Ian S C]

I think I know what you are trying to explain. To power anything, a LTD motor has to be quite big.
Ian S C

[DrJekyll]

I want to try to understand how big you mean?

Would something built slightly taller in size then the sterling ltd fan in the following video be enough to move the propeller with magnets attached that would be underwater https://m.youtube.com/watch?v=3gOCs6GGuUA ?


Or do you mean like it would need to be wayyyy larger then the engine fan shown in the YouTube link to have enough power to spin the propeller underwater.... And the size of the propeller underwater would be something around this small size :

https://goo.gl/images/WgDYNU

at more or less the speed shown in the sterling fan video underwater ?

[Ian S C]

Why would you want to drive a fan under water?, My 6"/ 150 mm diameter LTD motor runs on a bowl of boiling water, and only has enough power to drive it's self.
I have run one of my high temp motors using a large freznel lens (it was used for placing in front of a TV screen), although the point heated was only about 1 mm dia, the motor produced more power than with it's normal LPG fuel. The heat from the sun was enough to etch the stainless steel hot cap.
Ian S C

[DrJekyll]

Why drive a fan underwater ? Well I wanted the fan to be propeller blades. My idea was to find a way to oxygenate water in a around 20 gallon glass tank (water gets oxygen through water agitation) during the day only using the sun and freznel lens.....

So since your way more experienced then me when it comes to this , I assume using a ltd steerling as a way to drive the propeller you believe is a bad idea ?

[Alfista]

As I have read and reread your posts I have come to think that you would be much better off using a fluidyne to drive an airpump, the airpump being responsible for the oxygenation. A fluidyne is only about 100 times simpler to build than the ltd Stirling. It is effective, nearly foolproof to build and can easily be made to pump air.
Andy Ross' book "Stirling Cycle Engines" offers a nice set of building instructions. It is a great project for someone new to Stirling Engines. It scales nicely and I have seen one run from solar heat.
Sier's "History ..." presents some nice, more sophisticated variants such as the Montgolfier pumping engine which is somewhat reminiscent of Newcomen's atmospheric engine, although the principles are a bit different. I cannot say whether the Montgolfier design would work well as a solar engine but it seems like it may be a good prospect. Also worth mentioning from Sier's book is the Liquid Piston Engine with Rotary Displacer. This engine is solar powered as designed. None of these engines above were really designed to produce horsepower per se. They are all for pumping water, but if you can pump water, then pumping air is a breeze.

Alfista

[DrJekyll]

As I have read and reread your posts I have come to think that you would be much better off using a fluidyne to drive an airpump, the airpump being responsible for the oxygenation. A fluidyne is only about 100 times simpler to build than the ltd Stirling. It is effective, nearly foolproof to build and can easily be made to pump air.
Andy Ross' book "Stirling Cycle Engines" offers a nice set of building instructions. It is a great project for someone new to Stirling Engines. It scales nicely and I have seen one run from solar heat.
Sier's "History ..." presents some nice, more sophisticated variants such as the Montgolfier pumping engine which is somewhat reminiscent of Newcomen's atmospheric engine, although the principles are a bit different. I cannot say whether the Montgolfier design would work well as a solar engine but it seems like it may be a good prospect. Also worth mentioning from Sier's book is the Liquid Piston Engine with Rotary Displacer. This engine is solar powered as designed. None of these engines above were really designed to produce horsepower per se. They are all for pumping water, but if you can pump water, then pumping air is a breeze.

Alfista

Thank you for your comment. Hmm But all the youtube videos I have seen on fluidyne use an open flame and not just regular concentrated heat to power it, which makes me think that the fluidyne would require way more heat then a LTD to power ? or perhaps their are no solar fluidyne videos but it can run on low heat concentrated from a lens ?

[Alfista]

"The drinking bird" really works on the same principle. It can be made to operate at a very small temperature differential. Here is a Youtube example, a Solar Fluidyne Stirling Wasserpumpe pumping very nicely !

[youtube]https://www.youtube.com/watch?v=2HoyAgELfbw&app=desktop[/youtube]

[Ian S C]

The original Fluidine pump was developed by Dr Colin West at the AERE, Harwell. Later it was further developed in India, to pump up to 2500 gal per hour, it quite happily works on solar power.
Ian S C

[Ian S C]

Just a thought, I had a look at some pumps on Google, and the smallest used an electric motor about 30 Watts, my biggest high temperature motor will hardly get to 10 Watts.
Ian S C