LTD Ringbom Stirling from scrap

[mklotz]

I had a dashpot lying around and decided it would make a perfect piston-cylinder for an LTD (Low Temperature Differential) Stirling engine. I wanted to build a Ringbom engine but, in a personal communication with Dr. James Senft, the guru of all LTD Stirling, I was advised that it was a tricky proposition and would only run in a very narrow temperature range if, indeed, it ran at all.

Despite this good advice, I cobbled together a Ringbom using proportions that 'looked right to me'. Well, the good Doctor was right and it didn't want to run - a fact that didn't surprise me given the (non)effort I had expended in its design.

Then, while fooling around with it, I decided that it didn't run because I made the displacer a tad bit heavy and it couldn't generate enough gas pressure to lift it if run in the position pictured below. So, no worry - I turned it on its side (flywheel horizontal and displacer chamber vertical), heated it gently with a small alcohol lamp, and it took off very nicely.

Ringboms will run with the flywheel rotating in either direction and this one does that, making a tiny tump-tump sound as the displacer is driven in and out by the changing gas pressure. Their utter mechanical simplicity is mesmerizing.

Note that the flywheel is made from a Microsoft software CD. This is the first occasion in recorded history where Microsoft software has done something reliably - even if it's just going in circles. Even the bloat of MS wasn't sufficient, though, and I had to add some pocket change to the disk to get enough inertia.

Don't ask me for plans for this engine. There aren't any because I built it from a mental image I had and, no, I won't take it apart and draw plans from the pieces.

 

[Bogstandard]

Hi Marv,
Never really had a look at this side of small engines.
So basically, when heated the air expands in the large cylinder and this is then fed to the displacement cylinder that pushes the piston out, cools the air slightly, forms a vacuum and sucks the piston down again, or is it a bit more complicated than that. If it is like that, how does it work with an ice bag?. Or is that a different type?

John

 

[mklotz]

Stirling engines have two chambers. The smaller one (the dashpot in the case of this engine) houses the power piston which, as the name suggests, powers the flywheel and anything else powered by the engine.

The second chamber (the volume between the two aluminum plates in this engine) is where the expansion and contraction of the working gas (air for this engine) occurs. One side of this chamber is heated and the other side is cooled. This chamber is conneceted via a small passageway to the power chamber so gas pressure in this chamber can act on the power piston.

Inside this chamber is a loosely fitting piston called the displacer. Because of the loose fit, as the displacer is moved, the gas flows around it and is 'displaced' from the hot end to the cold end or vice versa.

In a conventional (non-Ringbom) Stirling, the displacer is driven back and forth by a mechanical crank arrangement off the crankshaft. When the gas is at the hot end it expands and pushes on the power piston (which is phased to be at TDC when this happens). As the displacer moves, the gas is driven to the cold side where it contracts, thus lowering the internal pressure in the engine. Atmospheric pressure then pushes on the backside of the power piston to complete the cycle.

For LTD (i.e., Low Temperature Differential) engines, the mechanical linkage needed to drive the displacer is an unwelcome source of friction. It turns out that, with a bit of cleverness, the displacer can be made to drive itself via the internal gas pressure variations. Such an implementation is called a Ringbom (after the inventor) Stirling engine.

All Stirlings, Ringbom or not, depend on the temperature *difference* between the hot and cold ends of the displacer chamber to drive them. The greater this difference, the greater the engine power. It doesn't matter how this difference is attained. Cooling the cold end below ambient is just as efficient as heating the hot end.

LTD Stirlings, because of their low temperature difference, have only miniscule amounts of power - barely enough to overcome friction and spin their flywheels.
(Some LTD engines operate on as little as one or two degrees C difference.)

Stirlings are also one of the few truly reversible engines. If you drive a Stirling's piston up and down with an external power source, the hot end of the displacer chamber will rise in temperature and the cold end will grow cooler.
This principle is used to build refrigerators (for cooling electronics) on spacecraft.

 

[Bogstandard]

Thanks Marv,
I will just swap my eyeballs over again, I will have to read that a few times to get my brain cells to inwardly digest.
It is what you can't see that is the mystery.
Must try one sometime.

John

 

[mklotz]

Here are several animations of the Stirling cycle...

http://www.cs.sbcc.net/~physics/flash/heatengines/stirling.html
http://www.keveney.com/Stirling.html
http://www.stirlingengine.com/Displacer-Anim.adp

Perhaps one of these will help you to visualize what is going on inside the engine.

It's hard to believe that this engine was invented by a Scots minister, of all people. He was tired of having his parishoners blown to bits by boiler explosions (in the Victorian era of poor boiler materials) and decided to do something about it. (Those were the days when it was still possible to be a "Renaissance man".)

By its design, the Stirling engine only develops very low pressure so confining that pressure, even with inferior materials, is very doable. The price one pays, of course, is low power output. Nevertheless, the engines were used in the UK and the USA (and probably elsewhere) for mundane chores such as powering washing machines, cream separators and corn mills.

I've heard stories of a British Stirling locomotive but I've never been able to find any information on it.

 

[Bogstandard]

Many thanks Marv,
Now I fully understand, it is what they say about a picture being worth a thousand words.
So basically, the big cylinder can be made of almost anything as long as you are not using a naked flame.
I will have to get some plans and see if it is within my capabilities to get it to run, I will try anything once.
Many thanks again for the explanation.

John

 

[Cedge]

Marv...
Stirlings have alays fascinated me. Your's is the first low temp version I've had time to really study and I think I get it. I've seen a few really nice ones at Cabin Fever and would love to build one when I get to it on the list of projects I want to make. The list just keeps on growing and I keep getting older and further behind...LOL

Steve

 

[mklotz]

If you're going to try building a Stirling, I suggest, for your own sanity, that you proceed in the following order...

First build a conventional Stirling (there are plans galore available) that uses an alcohol lamp for the hot end and fins for air-cooling the cold end. The motion of the displacer must be ninety degrees out-of-phase with the motion of the power piston. The easiest way to accomplish that is to mount the two perpendicular to each other. The more conventional design uses an awkward linkage arrangement to get the phasing, as below.

Second, if you want to get into LTD, build something like Jerry Howell's Miser design.

Lots of tiny, fiddly parts and be prepared to spend hours getting the last vestige of friction out of the product. Properly done, it will run for hours on a plate of ice cubes.

Ringboms are a step up in complexity. While they're simpler in design, sealing becomes an issue. I built Jerry Howell's Ringbom pumping engine before I started on my own design.

Note that both Jerry's design and mine use graphite for the power piston and displacer seal. Graphite is easy to turn but very messy. As expected, the finished product is fragile and must be handled carefully.

If you like to take your engines to shows, having at least one LTD Stirling is an asset. Seeing the thing sitting there running with no air or electrical connection will really get people curious. Watching their eyes glaze as you attempt to explain elementary thermodynamics to them is fun too.

 

[Bogstandard]

Hi Marv,
Me again, searching for information.
My oppo has just got me some scrap lumps of graphite that were used for 'burning' out profiles on aero engines.
I have got a couple of questions for you to answer if possible.
Would this stuff be ok for the power pistons, and the second is that I am still slightly confused over the the method of operation. It works on differential heat, so is it that the bottom plate can be cooled OR heated and the same effect is produced, or does the engine have to be built either as a hot OR cold machine?

John

 

[shred]

The basic-est Stirling for me to wrap my brian around was the Test Tube Stirling: http://www.nmri.go.jp/eng/khirata/stirling/testtube01/index_e.html

This guy did a really nice engine on the principle: http://www.stirlingsouth.com/Roy/others/dunman/barry_dunman.html

One day I'm going to give that a go. I've got test tubes and marbles (though some say I may have lost them in the meantime), how much more work could it be? :wink:

 

[Bogstandard]

Hi Shred,
Have a look at his article on Jan Ridders site

http://heetgasmodelbouw.ridders.nu/Webpaginas/pagina_stirling_onrust_jan/onrustjan_frameset.htm

Choose the English conversion, towards the bottom of the page it tells of his problems with plain glass marbles.

John

 

[mklotz]

John,

If by "burning out" you mean EDM electrodes, the answer is almost certainly yes.
The topmost engine above has an aluminum piston running in a steel cylinder but the other two engines both have graphite pistons. Graphite is a good material because it's self-lubricating. A flea power engine like the Miser can't generate enough power to overcome the 'stickiness' of conventional lubricants. Larger Stirlings, like the first one shown, are lubricated with conventional oil.

Stirling engines are completely reversible. All that is required is a temperature DIFFERENTIAL. A conventional Stirling, such as the Miser shown above, can be set on a cup of coffee (tea for you Brits, your coffee is too corrosive) or on a plate of ice cubes. The only difference will be the direction in which the engine runs. At outdoor meets in sunny California, I run the Miser sitting on ice. With the sun heating the upper plate, it runs wildly.

A Ringbom, on the other hand, has no mechanical connection to the displacer. As a consequence it will run in either direction, regardless of which way the heat differential is applied. (The Ringbom pumping engine shown above must be run in a certain direction in order for the coolant pump to operate properly but this is a condition imposed by the presence of the pump, not a consequence of the engine mechanics.)

 

[Bogstandard]

Many thanks for the info Marv, I can now start to get my head and hands around them. Jan Ridders has some nice ones on his site so I think I will make one of those in a couple of months time. I have an English designed normal one that is used for college projects and runs on a spirit lamp, so I will take your advice and make one of those first, to start on the learning curve.
They are EDM blocks, they can only be re-machined so many times then they are thrown away, so I can have as many as I want. But I think this is enough for now, the surface plate they are on is 8" wide, so going by the size I should be able to get one or two pistons out of them.
By the way, I hate tea, Columbian or Kenyan coffee are the only ones I drink.

John

 

[shred]

Hi Shred,
Have a look at his article on Jan Ridders site

http://heetgasmodelbouw.ridders.nu/Webpaginas/pagina_stirling_onrust_jan/onrustjan_frameset.htm

Choose the English conversion, towards the bottom of the page it tells of his problems with plain glass marbles.

John

Cool, thanks for the link. Those plans will be most useful to work from.

I was plotting to anneal my marbles as I have a small glass kiln sitting around waiting for cool weather and I really like the look of clear colored marbles in the tube.

 

[mklotz]

Hi Shred,
Have a look at his article on Jan Ridders site

http://heetgasmodelbouw.ridders.nu/Webpaginas/pagina_stirling_onrust_jan/onrustjan_frameset.htm

Choose the English conversion, towards the bottom of the page it tells of his problems with plain glass marbles.

John

I made a rolling marble Stirling once.

If you build one, be sure to use a Pyrex test tube and put a bit of steel wool in the bottom of the test tube to cushion the impact of the balls as they roll to the bottom of the tube. DAMHIKT - I still occasionally find test tube shards among the tools on my workbench.

 

[tattoomike68]

mklotz if you can post video or e-mail it to us admins we can host the file. I would like to see video of anything you make.

I would like to see your work running. You are good.

 

[mklotz]

I don't have a video camera but both my SILs do. I'll ask them the next time I see them.