LTD Stirling engine with vertical shaft - perhaps

[picclock]

Hi

I decided to make a vertical shaft coffee cup stirlings for my kids for next christmas (I like to plan in advance  ).

This would involve the conventional displacer but with the power piston at right angles to it. The displacer would be driven by a bellcrank from the vertical shaft, and its weight could be counterbalanced by extending the bellcrank and weighting it.

It then occurred to me that if the top arm of the bellcrank to crankshaft joint were made to have a lot of play this would result in the displacer staying in the hot or colder position for longer intervals, increasing the power of the machine.  The displacer piston would stay in position over the slop period because the air pressure encourages it to do so.

Any thoughts on this concept and its snags ?

picclock

[picclock]

As a further thought, has anyone tried sticking aluminium foil to either side of the displacer piston ?

This should increase the speed of heating and cooling of the gas by near doubling the surface contact area. The foil would be reheated/cooled as it comes close to or in contact with the hot or cold plates. Possibly foil from a pie case or similar would be about the correct thickness (and a good excuse to eat pies to find the correct tray )

As a further bounus could probably be made to look cool  

picclock

[Bogstandard]

Picclock,

I don't think the reflectiveness or heat absortion of the displacer piston comes into the operation, it is there just to separate the hot and cold areas.

But I have painted some of my displacers with water colour acrylic paints without adding much to the overall weight. The usual red/white/blue combination.



Bogs

[picclock]

bogs, your engineering parts always look so good - a sort of in built bling - I'm definitely envious .

The rational behind the foil idea is that it will increase the energy available to the engine by near doubling the heating/cooling energy available to the gas. As the current design goes when the displacer is at the bottom - hot end- it prevents the heat from warming the gas, but does little else.  By adding the sheet of foil at the bottom of the displacer, it will become warmed by convection/radiation , possibly conduction if it touches.  When the stroke of the displacer is at the top - cold end- and the gas flows over it, far more energy will be available to heat the gas.

The same is true of foil on the the cold end of the displacer, basically more efficient cooling.

One reason for posting this before proving it, is that if I am correct, a lot of almost running heat engines could be made to work with just the addition of some pie dish foil on either side of the displacer.

I think the end result will be a faster more powerful engine, or an engine which will run from lower temperature differentials.

It will also cool the coffee more quickly  . 

picclock

[BillTodd]

I don't think the reflectiveness or heat absortion of the displacer piston comes into the operation, it is there just to separate the hot and cold areas.

Interesting, the books on Stirling engine design I read as a lad, usually suggested using a porous or perforated displacer, the idea being that the heat would be transferred to and from the displacer as it did its job. Yet, most of the model designs I see now have a light weight thermally neutral displacer.

Bill

[kwackers]

The problem with giving the displacer thermal mass is the time it'll take to heat/cool. Unless it's a very slow engine the displacer will simply take on an average value of temperature and hardly change.
Quite likely giving the displacer a thermal lag will make the engine run worse, since it'll take heat from the warm air cooling it and giving it back to the cool air warming it which would basically try to keep the air temp in the displacer chamber constant, I think in essence you want as little thermal mass in the displacer chamber as possible (except for the hot and cold plates which ideally would never change temperature...)

Better would be to increase the surface area of the hot/cold surfaces to faster raise/lower the air temperature - but then you need to increase the rate you can move heat to/from them otherwise you very quickly end up with an engine who's temperature difference drops too low to work.

[picclock]

Hi Kwackers

The displacer is still very much an insulater - its just that the foil either side of it will decrease the thermal impedance to the gas. One side of the displacer would be cold and the other hot.

Before I make the ones for my kids I will make an experimental one to check it out and sort out the bugs. Using the bellcrank idea it should be easy to balance any displacer weight difference.

I was thinking of making a zoetrope (?) to go round on the top with perhaps pictures of a cat chasing a mouse or something similar.

Best Regards

picclock



 

[John Hill]

I think a porous displacer of the right material would work as an economiser taking heat from the hot air as it is heading to the cold end and adding heat to the cold air on its way to the hot end.

[cidrontmg]

I think a porous displacer of the right material would work as an economiser taking heat from the hot air as it is heading to the cold end and adding heat to the cold air on its way to the hot end.

Indeed. That was actually R.Stirling´s first patent, an economizer, for glass and other furnaces, and for the hot air engine. The economizer is also known as regenerator, and it is usual in "serious" alpha and beta Stirlings, which aim for good fuel efficiency and high power output. In a gamma type Stirling, it´s not common practice, but could be achieved with suitable (porous) displacer piston material. The piston would be at some temperature between the cold and hot side, and act alternately as a pre-cooler or pre-heater for the passing air. But in a "toy" engine, I think it is not worthwhile to worry about economy. Just make sure there´s no unnecessary friction anywhere, and heat it with a bigger flame if it doesn´t want to run... And in an LTD Stirling, a smaller power piston, and a bigger displacer (swept volume)...
 

[John Hill]

Cidrontmg,  indeed, somewhere I read that in fact technically speaking it is not a 'Stirling' engine unless it has an economiser.

The point about this for the gamma engines is that the displacer could be made from a bunch of tightly compressed metal wool or somesuch. You could jam the wool inside a thin walled metal tube to make the displacer, for example.

[reflad]

Picclock,
I have seen designs that have steel wool and or metal screen material placed inside holes in the styrofoam displacer, and the steel wool acts as the regenerator.  Not sure how this would react with a small size, but a person could certainly do some trial run to see if it would pay off. 

I think the gift idea for the kids is a great idea!  Good luck with the build, and keep us and the HMEM crew filled in.  Take plenty of pics!

Ronald

[picclock]

@ reflad

Well I think I have enough time (13 months or so), but as I only plan to do this in my spare time, past experience suggests that this will fly all too quickly. Once I have all the bits together I will start a build log to expose my triumphs and disasters to all, and possbly encourage someone else to have a go.

For the initial version I will try out all of my crackpot ideas and document the results. I figure that balancing the displacer by weighting the bellcrank should be a lot easier than the flywheel method, and allow changes to be made quite easily. My main concern is getting the graphite piston fit correct.

I'm very much a newbie to all of this and am still finding my way, thanks to the help of forums like this.

Best Regards

picclock

[picclock]

This is my current state of LTD thinking. I think I've got a bit more figured out now.

First a nice simple optimum ratio equation:-

optimum temperature differential = 273 / (power cylinder volume/displacer volume)

In Jan's design (latest LTD coffee cup) with a ratio of 51-1 the temperature change for optimum work is 273 / 51  or only 5.4C.   

So why do Jan and others use such a high ratio, and optimise for such a low temperature differential ?. The answer is that during the time that the displacer is in a heating or cooling position, the maximum change of temperature that can be made to the working fluid is limited. These limitations are caused by the low thermal conductivity of the working fluid, and the small surface area available to heat/cool the fluid.

So despite a fairly large temperature differential, I'm guessing 35C, only 5.4C makes it to the gas in the time of 1 revolution, so anything which improves this will help a great deal.

The problem with the current design is that a large amount of gas must be heated and cooled for only a small amunt of gas used. In the above case 50 times the volume of gas is heated and cooled for the small expansion/contraction of the power cylinder.

@ John, Billtodd
I think the 'economiser' idea is spot on. A hollow displacer filled with fine aluminium wool (if there is such a thing), with two diametrically opposed holes in the top and bottom surface would improve things dramatically.  The constructor would then have the choice of leaving the expansion ratio's the same with the engine working at a far lower differential temperature, or decreasing the ratio and extracting more power from the device at the current teperature differential. 

Currently the gap around the displacer works out to 314 mm2, so the two holes would need a diameters of 20 mm. Smaller may be OK because the 1mm cylinder gap exposes a large volume of gas to the boundary layer effect. On the other hand for a heat exchanger to work well would involve high boundary layer contact.

The only downsides to this I can see is that resistance to airflow will be slightly higher, and some additional driven mass will be added to the displacer.

Many thanks for the helpful comments and insights, now I just have to hone my newly learned engineering skills ( to produce something.

Best Regards

picclock






 

[kwackers]

My (albeit limited) experience of Stirling engines suggest that the biggest issue is often the cold side warms up too much and the engine stops running.
So it's not just a case of heating one side, it's also a question of how to cool the other. It's much easier with 'hot' Stirling's driven by direct heat, since the high temperatures increase the rate heat can be dumped on the far side, but low temperature engines tend to fail fairly quickly.

The importance of this is related to the fact that if you increase the amount of heat transferred then the problem of dumping it on the far side becomes more pronounced.

[picclock]

Hi Kwackers

Using the regenerator means that less heat and less cooling is needed. The hot air is cooled by warming up the regenerator, effectively storing the heat in it, and pre cooling it for the cold part of the cycle. When the cold air blows through the regenerator, it warms from the heat stored, pre heating it.

Overall there is less heat transfer from the hot and cold ends.

Best Regards

picclock

[madjackghengis]

Hi picclock, not having built one myself, this information is entirely out of an LTD stirling engine book focused on so called "coffee cup engines" and the like.  The author suggests in a stirling with about a six in dia displacer cylinder, with about half an inch of movement, using styrofoam for the displacer, cutting out four holes about an inch in diameter, and filling each with a wad of stainless steel wool, which has been squashed and formed to fit the holes, and retain its shape so it doesn't contact anything.  The whole principle is based on the poor thermal conduction of stainless, with the displaced air going through the regenerator wads primarily, and as it travels from the hot to cold side transferring substantial percentages of its heat to the steel wool, so it has already lost much of the heat and concurrent volume, and a moment later, travelling through the same wads picking up the heat which was "stored" in the wool, and being pre-heated before it gets more heat.  He claims substantial improvement in running, he is using a comercial made power piston/cylinder assembly, which could be got as a "sample" from a dashpot manufacturer for free, about half in diameter piston, about an in and a quarter stroke in a very fine lapped tempered glass cylinder the whole of which normally retails for about thirty dollars but free samples were available for the asking, if one appeared to be manufacturing something.  I got one of the "dashpots", and it is a work of beauty, but have held off on using it, as I didn't make it.  The engine built around this dashpot assembly had about a six inch flywheel of plastic, like a CD, about a sixteenth in displacer rod in a bronze bushing about an inch long and reamed out, for a snug fit, with the cylinder (dashpot) right on the top plate of aluminum, with the two plates separated about an inch or so, by plexiglass tube and held together with nylon screws and nuts to minimize heat transfer.
    By "Mark's handbook of Engineering", with silver as 100 as heat conducting, and copper being 97 or so, stainless steels are all down in the below 20 range, with some being about 12 or so.  As I see it, the most important factor in adding regenerator material to the displacer is keeping it in place, and shape, not interfering with movement or fit, and getting the displacer properly accounted for in the balance factor, with the steel wool be a substantial percentage of the mass of the displacer.  In full sized stirling engines regeneration can account for up to thirty percent power factor, so it could be a very important issue in an LTD type I think.  I've put off building one of these for the lack of displacer cylinder material, but I have a plastic CD package that is essentially a four inch tall round tube of plastic with a closed top, that fits CDs inside, that ought to be a working displacer cylinder.  My own thoughts on making the regenerator plugs would be to take a proper sized tube and piece of stock to act like a piston, figure out the optimum amount of stainless wool, put it in the tube, pack it down with the piston/stock, and pack until it retains its shape well enough. I suspect some super glue is used to keep stray wire in place, and to keep the regenerators in place as well.  I hope this helps open the air up some, and when I get a chance, I will build an LTD stirling according to the ideas in the book I've got.  I will also find the book so I can reference it next time I read this post.    mad jack

[picclock]

@madjackghengis

The problem I see with stainless as a heat exchanger material is that because its thermal conductivity is poor only the surface of the wire will absorb the heat from the gas, so little energy will be stored. Ideally a long path with thermal breaks should give the best result.
 
IMHO a good regenerator will probably require a hollow displacer cylinder with gas transfer holes diametrically opposite, on opposite sides of the displacer piston. I have some aluminium screen door material which I will try to flatten to give the best surface area. Alternately kitchen foil strips may prove a better filling material. This will give a thermal path length of 3" plus. It will also give a large surface area of material for the gas to pass through, with low resistance to gas flow. If the piston is thicker than the stroke distance the walls of the cylinder will not have to change temperature improving efficiency further.

At the moment I am up to my eyes with other things, so no real progress to report .. .

Best regards

picclock