Stirlingengineforum.com

I'm new to Stirling Engine Building. I live in a rather remote area and I've been trying to design a Home Built Stirling Engine for charging 12 volt Deep Cycle batteries. And at that, one powered by solar energy.

In reading through this forum, I noticed that there are many others with the same basic idea or need.

There are so many different Stirling Engine types and designs that It took a great deal of study of the various plans and diagrams and explanations just to get an idea of the basic operating principles.

What I've concluded is that the basic idea is to have a sealed rigid container of air the interior of which is alternatively heated and cooled (from the outside) causing the air to expand and contract and to use the change in pressure to push and pull a movable piston or diaphragm attached to the air chamber.

The problem in terms of my own needs at the moment is that the plans available are not intended for generating electricity. Also most of the designs do not promise the kind of power output that would be capable of turning a generator or alternator under the load of a 12 volt battery under charge, even if only a trickle charge.

From what I can gather, power output might be increased by more rapid heating and cooling. The faster and more effectively the air (or other gas) can be heated and cooled the greater the pressure exerted on the piston.

The role of the "regenerator", it seems, as far as its construction (usually a wire mesh) is to provide a large surface area. If a Stainless Steel Scrubbing pad, as is sometimes used, for example, were stretched out into a single strand it might stretch a hundred yards and that is all surface area making for rapid absorption and rapid release of stored heat energy.

The Air chamber, or cylinder walls, on the other hand, of all the engines I've looked at are perfectly smooth with relatively little surface area.

I reasoned that If there were some way to increase this air chamber wall surface area there would be a higher surface to air, and air to surface heat exchange rate. The problem seemed then, how to increase the interior surface area of the air chamber without increasing the air volume.

The designs combining the "regenerator" and "displacer" in a single unit seemed the most likely to hold some promise in this direction, at least in terms of a low-budget do-it-yourself type project.

I thought about the possibility of making the air chamber wider or taller, perhaps with a thicker displacer/regenerator so as to maintain the same air volume but increase the solar collector side surface area.

I found that the mechanical aspects of the Stirling engine linkage resulted in some limitation in this area. The solar collecting panel would have to be close enough to the piston or flywheel to attach a drive rod of some sort, the larger the panel the more difficult this seemed, especially if the panel or unit has to be moved in order to track the sun.

I thought if this is going to generate electricity, perhaps a solenoid of some sort to actuate the displacer/regenerator would eliminate this problem. The ability to generate more power would, hopefully more than compensate for the loss due to powering the solenoid.

This solenoid might also provide an "electric start" mechanism.

I was inspired by this project:

http://www.otherpower.com/hamster.html

If a hamster wheel can be made to generate electricity, surely so could a the flywheel of a Home Built Stirling Engine!

Of course the voltage would have to be stepped up to at least about 15 or 18 volts in order to charge a 12 volt battery.

If an adjustable solenoid actuator of some sort could be associated with the turning of the flywheel, perhaps by a micro-switch ridding on a cam, then the "timing" of the engine could be adjusted for maximum power output while in operation, much along the lines of setting the timing on an internal combustion engine. What a difference that might make in terms of power output!

Anyone who has ever had the timing go out on their car knows the power loss that can result from the timing being off by even a few degrees.

I thought another possible advantage might be that rather than the "displacer" movement being limited by the movement of the piston or flywheel, a solenoid would result in the displacer/regenerator snapping back and forth with a greater rapidity than if it had to follow the motion of the other mechanics of the engine such as the piston or flywheel.

I suspect that such a snapping action would result in more torque as the pressure change would be that much more rapid.

This is all theoretical. I have yet to begin building my first Stirling Engine but I have made some rough drawings of these ideas and posted them here:

http://members.tripod.com/prc_projects/stirling.html

These drawings are not intended to represent any kind of working model. Though I can't really see why they wouldn't work.

It should be noted that in these drawings the "pins" protruding from the Air chamber walls are supposed to pass completely THROUGH the regenerator/displacer, and that these holes also serve as air passages.

The idea is to get as much air as possible in contact with as much surface area as possible as quickly as possible. In practice the pins might be thinner and more closely spaced than what is illustrated. The smaller and more closely spaced the pins and corresponding holes the greater the surface area.

Also, I thought a regenerator consisting of an INSULATING MEDIUM sandwiched between two aluminum plates (with the air holes drilled through the whole assembly) would help to reduce heat transfer between the Hot and Cold sides of the regenerator, while the aluminum would throw off or absorb heat very rapidly, the insulating material would help to keep the cold aluminum plate cold and the hot side hot.

Having the hot side on top (toward the sun) might be somewhat more efficient than having the hot side down over a flame, as, generally speaking heat, or at least hot air, tends to rise.

Now before I go and waste a lot of time on this project I thought it might be best to make sure it has some chance of working. As it stands I'm not 100% certain I even understand the whole Stirling Engine operating principle. I'm not a hobbyist, I just want to charge my batteries. I'm either going to build an engine that at least has some chance of at least trickle charging a battery or more than likely I'm not going to bother.

Any thoughts or recommendations in regard to the feasibility of this project or the designs posted would be much appreciated.

Tom

I too have thought about increasing the surface area of the displacer but I had more of a cogged wheel or sunburst shaped displacer piston in mind. While I'm not an engineer and have only one fairly simple working model of my own design, I think your posted plans are good. Built on a fairly large sized engine might have a few HP. Setting up the hot side like a solar panel and cooling the shaddy side (if water is available perhaps a small pump sraying water on it?) with extra fins. Rather than take on the expense of a large, hopefully efficent working engine, I think you will need to build a smaller version to test your principle. If your living off the grid, I'm guessing that might be difficult to build test models. Also, other systems of solar/electric panels and or wind generation would likely work better and are already available. the cost of making a working Stirling system will probably be more in the long run. As far as a electronically actuated displacer goes, I think you lose more converting motion to electricity and back again than you would lose in mechanical linkage. It would make a fun model project though, as long as the current to run it is a external source.

Just my two cents. Good luck which ever way you go.

Hi Tom,

You have some neat ideas there! Here are a couple of random thoughts I have about what you have proposed:

- You're basic notion of how a Stirling engine works is correct. Please note however that you sketch has the displacer and power pistons 180 degrees out of phase; they should be 90 degrees out with the displacer leading.

- Aluminum is not quite as useful as stainless steel if you are building your displacer in one piece. Very thin stainless allows the heat to pass through it and does not transfer it along its length as well as aluminum (this is why aluminum or brass is used for automobile radiators). Currently, the best inexpensive material for a displacer cylinder is very thin stainless steel (thinner walls conduct less heat).

- Theoretically, you do NOT have to connect the displacer to the flywheel by linkage. The change in internal pressure can actually force out a secondary "piston" connected to the displacer. Look into Ringbom Stirlings for more information. Using a linear actuator has been accomplished in the past. I read an article in which the builder had linked the actuator to a computer and was able to "throttle" the engine by changing the speed of the actuator's movement. And yes, it was self-starting.

- I believe that the collector (either mirror or lens) need not be close to the hot cap: only the focal point. Changing the curvature of your collector might get you a bit more breathing room for your design.

- While there has been quite a bit of discussion about different ways of increasing the internal surface area of the displacer cylinder (much of what you suggest is on the right track), I belive that in the interest of simplicity you might direct your efforts towards increasing the temperature difference by means of a cooling system. Provided that you have an air-tight system and low friction, increasing the temperature differential can be the simplest way to increase your power output (though not necessarily _efficient_). I always try to remind myself the the internal combustion engine has remained so successful because it has a high power output for its size/weight and cost, _not_ because of its efficiency. Water jackets and cooling fins are two straightforward ways of keeping that cool side cool.

- It should also be noted that increasing the internal pressure of your engine will also increase its output. Small pumps are used on many larger engines to maintain this pressure and are found to very beneficial despite their parasitic power requirements. (Think of these as the supercharger of Stirlings).

Hope some of this helps and please don't hesitate to post some more ideas, arguments, or suggestions!

-Stefan

Hi, thanks for the rapid replies, I was not expecting a response for a another day. I'll have to mull over the suggestions for a while.

I was however just on this site"

http://www.stirlingsouth.com/richard/de ... opment.htm

Where it is stated:

Improved displacer movement means simply that the displacer piston should spend more time at each end of its travel (dwell) and less time getting from one end to the other.

I'm not sure, but I don't think a solenoid draws a tremendous amount of current, at least not one designed to just move a few ounces. I could be wrong of course, I don't really know what current a solenoid would draw in such an application, nevertheless I thought it would provide an ideal "Dwell" as described above, as the solenoid could snap the displacer from one side to the other almost instantaneously, and it would remain in this "dwell" position until ready to snap back to the other side.

Initially I tried to come up with mechanical means of accomplishing this, but the solenoid idea seemed like the easiest and most effective solution and making adjustments in the timing would be very much simplified I would think.

There are some aspects of Stirling Engine operation that are still over my head. Some of the designs and models I still have difficulty comprehending. Therefore some of the suggestions and explanations in the two replies I've seen so far are over my head and I will have to give them more consideration before I can make a further response.

In the meantime, thanks.

I too have thought about increasing the surface area of the displacer but I had more of a cogged wheel or sunburst shaped displacer piston in mind

I mostly chose the "pin" design as I don't have any machining or casting equipment other than a drill, which I can run off an inverter.

It would be very tedious and time consuming, but at least possible to drill holes in some metal plates and insert pins.

I've been considering that shorter pins might be desirable rather than long pins that go all the way through the displacer/regenerator, as there would be no advantage to having the hot pins transferring heat to the cold side of the regenerator. The long pins pictured in the drawing might partly defeat the purpose of the insulation layer at the center of the displacer as they would bypass it.

One thing good about the long "dwell" period is that the regenerator would have time to pick up some heat as it sat against the hot side, then as it "snapped back" to the cold side the air in the chamber would be heated from both the chamber "wall" and the displacer. However this might not be good if the pins are transferring heat to the wrong side!

It would require very close tolerances and probably some form of guides for the displacer to get all those pins to pass through all those holes without friction but still get good heat transfer to the regenerator. Tapered or pointed pins into tapered holes might make this less of an issue. as the tapered pins would be more or less self guiding as they passed into the funnel shaped holes, but I was thinking of just using some rod cut in small sections for the pins, but ideally the tapered pins would probably be better.

I'm hoping that the various advantages would outweigh the losses due to the solenoid drawing off a percentage of the current generated by the flywheel/generator.

In this design the back of the power piston is exposed to the outside air.

I was thinking that helium would be a relatively easy working gas to get hold of, from shops that sell party supplies (for blowing up balloons) except the helium would likely dissipate past the piston, though I think the back of the piston could be sealed by a very loose fitting "diaphragm" of sorts that could be completely sealed around the open end of the piston cylinder and the piston connecting rod (in the second drawing).

By "loose fitting I mean the diaphragm would have enough slack to move with the connecting rod, but hermetically sealed where it contacts the cylinder rim and the connecting rod.

I'm assuming from what I've been reading that the use of Helium would increase the power output.

The only other area for potential loss would be the rod for the solenoid, but this too could be hermetically sealed, or the solenoid itself would form the seal.

The solar reflectors in the drawing are not intended in any way to be to scale or of proper curvature. These are very imprecise drawings just intended to get across the general idea.

- Aluminum is not quite as useful as stainless steel if you are building your displacer in one piece. Very thin stainless allows the heat to pass through it and does not transfer it along its length as well as aluminum (this is why aluminum or brass is used for automobile radiators). Currently, the best inexpensive material for a displacer cylinder is very thin stainless steel (thinner walls conduct less heat).

I've been trying to wrap my brain around this. I'm not sure I understand the logic of stainless being preferable to aluminum.

In this application or design there are several different situations or different functions going on.

1 Absorption of heat from the sun.

Here I'm assuming we want a "pass through" situation, but at the same time I think some heat retention would be beneficial. I was thinking something like cast iron as this would absorb a lot of heat and tend to hold onto it until it got very hot and would maintain a steady temperature.

Of course it would take much longer for the heavy cast iron to reach operating temperature, but with solar I don't think this would be objectionable as you want some heat retention so as to maintain some heat in the event of the sun being momentarily blocked by passing clouds. The greater the thermal mass that is solar heated the more steadily the engine should run.

With a thermal mass big enough, the thing might run through the night until the sun comes back up the next day!

2 Transfer of heat from solar heated exterior to interior.

3 Transfer of heat from interior side of heated wall to air in Hot side of chamber.

4 TEMPORARY Transfer of heat from interior hot air to hot side of displacer/regenerator and hopefully "reflected" back to air in hot side of air chamber VERY RAPIDLY.

Here is where I thought the properties of Aluminum would come in especially handy for the hot side of the regenerator. as from what I know about aluminum it takes up heat VERY RAPIDLY but will tend to reflect or throw the heat off again just as rapidly

5. Transfer of heat from hot side of displacer to cold side.

Here as far as the solid material of the regenerator is concerned, I think we want to, as far as possible eliminate all heat transfer (hot side to cold side) while still allowing rapid REGENORATOR TO AIR heat transfer as air passes THROUGH the regenerator.

Theoretically I suppose it would be nice if the heat would just disappear at this point, but of course heat from the jets of hot air entering the cold side from the hot side through the holes in the regenerator will be absorbed by the chamber walls. On the cold side, my understanding is that we want to throw this heat off as quickly as possible so as to keep the cold side of the chamber cold.

Here again I thought Aluminum would be the preferable material, at least for the backplate as in this circumstance we want to conduct heat from the interior air space and throw it off to the outside air (or water jacket or refrigerant or whatever) faster than it can build up.

I also thought relatively heavy aluminum would have less tendency to buckle or bend.

In a typical displacement chamber there is relatively little stress on the chamber walls. The pressure is rather easily and steadily transfered to the piston in one way or another.

I guess I'm imagining that in the design I've come up with there will be great pressure changes that will occur before the piston has time to react, resulting in greater torque, but also causing greater stress on the chamber walls. With a flat plate type chamber wall, I'd be afraid of the chamber bowing inward, or the whole thing crumpling up like a tin can during the cold phase or blowing up like a balloon during the hot phase if the walls are too thin.

Even with very thick aluminum or other metal, there would no doubt still be some flexing and so turning a few of the "Pins" into "posts" that pass all the way through the displacer, for extra support is something I've thought might be necessary, as I've seen this in other similar flat plate-solar type designs.

Putting some non-heat conducting "spacers" between the ends of all the "pins" for increased rigidity of the whole structure might be another option. They would have to be securely fastened in some way to prevent ballooning as well, but this is going quite a bit beyond the relatively "low-tech" simple design I was hoping to achieve. Perhaps a few long nuts and bolts for "posts" passing through interior sleeves. These would add to the rigidity of the chamber and act as "guides" for the displacer/regenerator at the same time.

Anyway, I'm rather pleased that there are no major objections to the various proposed ideas that might end up being incorporated. I was afraid I might be overlooking something major and have it all horribly wrong and hopelessly unworkable.

Building a small model, I think, might be more difficult than building a full scale power generating engine.

I can't really see how this project could possibly end up costing more than photovoltaic panels. With a little scrounging I can build this out of scrap metal and used car parts and such, in other words, for "FREE" other than TIME, which I happen to have plenty of lately. There is no way for me to make my own photovoltaic cells, though an entirely home built wind generator is certainly a real possibility. I still have a couple sets of plans for home built wind generators from Mother Earth News.

Right now, cost isn't really an issue. Building an inexpensive, relatively easy to build Stirling Engine that actually puts out some usable power has become something of an obsession.

"Usable power" in my circumstance means charging the 12 v batteries in my camper with intermittent use of an inverter running off the batteries when I need 110v.

In general it is as easy, if not easier to obtain and work with large scrap or second hand items that I can just drill or saw and hammer or screw together than to make a small scale model or miniature version that would take more precision or custom metalworking.

Old refrigerators and freezers are rather easy to come by. The Freezer compartment of some of them looks like a potential source for aluminum plate and aluminum tubing.

I've been considering building some "tin can" Stirling engines, just for the fun of it, though I'm not sure any useful power could be exstracted,...

Unless the "cans" are old 55 gallon drums!

Perhaps something like this:

http://www.rotarystirlingengines.com/rotacola.htm

but made from old 55 gallon drums instead of soda cans!

It's encouraging to know at least that I'm somewhere in the ballpark of a theoretically viable solution.

I'm getting a better picture now, you have been bit by the Stirling bug... I know it well. It's become more than powering the batteries and that's a good thing. Models are a place to start, not sure if I would go full scale for my first engine unless I was sure it would work. With the right materials and proper sweep volumes etc. I think sucess is certainly possible without a smaller test engine. I am curious what you will use for a power piston and cylinder? It will need to be a pretty large and well fitting assembly. Keep us posted on how it goes.

Yea, it certainly seems I've been bitten. I'm having trouble throwing away any empty cans lately. Instead I've been washing them out as potential Stirling engine parts. Even those round tear-off plastic lids that come on fruit cups, instead of looking like garbage, look like potential Stirling engine diaphragms

Looking around the house for something to use as a base for a model I noticed an old shoe polish tin and thought this might be a place to start for a little palm size engine.

I'll be doing some grocery shopping tomorrow, and I have a feeling I'll be selecting items based more on the packaging than on the food value.

For some reason I've also been visualizing it would be quite interesting and beautiful to see a little model made entirely of blown glass.

Also out of glass, something like the "RotaColaSola" containing colored, sunlight and heat absorbing liquid so it could be set on a sunny window sill and one could watch the whole internal and no doubt rather amazing fluid transport process, like a living, self moving stained glass window.

I've always had an interest in hand blown glass artwork.

Ive been working on two different "model" Stirling Engine designs, and went out and scrounged up most of the materials today. There are a few more small items I forgot and will have to go back to the hardware store tomorrow.

I found a solenoid on one of my very old photo mechanical typesetters yesterday, I got this typesetter for parts anyway and it has just been sitting around collecting dust, but the solenoid is much too large for either of these models so I spent most of last night trying to devise some sort of mechanical replacement or alternative dwell mechanism that might work on a small model.

What I came up with is based in part on the linkages I'm familiar with for some lawn mower governors, but also a mechanism that is used on the sump-pump in my basement, which I had to adjust while installing the pump.

In a way, it is a similar type problem with the sump pump.

The rod that controls the pump also has a "dwell" adjustment in that the pump is in the off or "down" position until the float rises high enough to hit a rubber "stop" on the float rod.

In a way the rising of the pump float as the water rises is like the turning of the flywheel or the movement of the rocker arm to which a displacer is attached by a rod.

Anyway, I drew another picture of the concept. In this case the flywheel doubles as a fan to move air over a reservoir of water for evaporative cooling.

It is on the same page as the other drawings so if someone wants to study this they will have to scroll down to the bottom of the page to view.

In the box labeled "LINKAGE DETAIL".

The bottom Dwell is controlled by a rubber stop much like the sump pump arrangement while the Top "dwell" is achieved by means of a spring, somewhat similar to the lawn-mower governor linkage.

The spring is attached to the end of the rod so that the rod is indirectly attached to the flywheel through the spring.

When the displacer reaches the top of the chamber, the flywheel might be at approximately ten or three o'clock depending on the direction of rotation. At this point the displacer has already reached the top of the cylinder so the rod attached to the displacer can not go any higher.

As the flywheel continues to rotate, the spring stretches (hopefully) while the displacer remains in its upper "dwell" position.

On the down-stroke the spring contracts until it is fully contracted at either ten or three o'clock (approximately?) , again depending on the direction of rotation.

At about four or eight o'clock, on the down-stroke the displacer has already reached the bottom of the chamber. At this point the bottom rod stops while the top rod continues downward through the "sleeve" and the "stop" descends with the top rod below the "stop" the top rod attached to the flywheel now moving independent of the other rod attached to the displacer.

This allows the displacer to "dwell" at the bottom of the chamber until the flywheel continues its rotation, about another 90 degrees and once again picks up the displacer rod due to the "stop coming again into contact with the "sleeve".

I have yet assembled this so as to test this out, but I went and bought all the parts I couldn't scrounge up around here.

A great deal of time was spent in selecting just the right spring.

It had to be strong enough to just barely lift the weight of the entire displacer and linkage assembly without stretching, but not so tense as to stop the flywheel when the displacer reaches the top of the cylinder the slight added pressure allowing it to stretch.

The spring length or tension might be adjusted, or the Top "dwell" angle might be reduced if necessary, or it may be necessary to eliminate the Top dwell altogether if this turns out to be troublesome.

I don't see any problem with the bottom dwell arrangement however. This can also be adjusted by moving either the "sleeve" or the "stop" or both.

At least this linkage assembly should provide some means of setting various "dwell" angles and seeing what if any effect it has on the performance of the engine. If it is substantial, the added resistance of the spring tension may be negligible.

I think even a split second of added "dwell" time with the displacer in full contact with the chamber top or bottom might result in improved heat transfer.

Perhaps I will have some real data on this in a few days if all goes well in assembling one of the engines.

The engine in the new "LINKAGE ASSEMBLY DETAIL" drawing consists of an empty baked beens can (the displacer chamber) and an empty tuna-fish can (the water "evaporative cooler").

The other model will be a Plexiglas box similar to the design in the previous drawings with a view of the interior workings.


http://members.tripod.com/prc_projects/stirling.html

I've also been contemplating the possibility of burying a large Stirling engine of some design similar to the "tin can" design with only the top exposed.

In the winter months the ground not far below the frost line stays at about 50 degrees while the air may be in the sub-zero range.

Could this not be used as a kind of shallow geothermal generator?

I'm thinking something like a 55 gallon drum about 80 % buried in the ground or possibly a large pan shaped design which might be more suitable for the low temperature differential, Though I'm not sure 50 degrees F and say 25 degrees F is all that low.

Has anyone ever experimented with anything like this?

Hi Tom,

Alright, I'm gonna break out the big guns here. I think there is a much simpler, mechanical means of generating the "dwell" you are hoping for. In an earlier post, we had a discussion about "intermittent" displacer movement. The idea was, just as you described, to have the displacer pause at each end of its path allowing air to more effectively heat and cool. My solution is a slotted connecting rod.

http://kmoddl.library.cornell.edu/model.php?m=443

By doubling the crank length and creating a slot in the connecting rod of half the crank length, you should be able to generate a "dwell" of approximately one-fourth the entire cycle.

As always, I'm looking for the mechanically simplest solutions in order to reduce the likelihood of failure, decrease maintenance, and ease of manufacture.

Let me know what you think.

-Stefan

That had crossed my mind, and it would work for the bottom "dwell", where the flywheel is above the displacer but on the up stroke the flywheel pin or whatever would already be at the top of the slot, in the process of lifting the displacer. No room left for "dwell" at the top of the cycle. I couldn't figure out any way to make it work, though there may be a way with some other arrangement.

I'm not really sure that having any "dwell" at the top would make any appreciable difference anyway, since hot air rises, the unwanted heat would tend to dissipate out the top anyway? maybe. but I wanted some way of testing it at least and the spring idea was all I could come up with.

I couldn't figure out any combination slot and spring that would work either.

If you have something that is working on both ends, I need a better picture of what you have in mind.

On another topic, I spent the day designing and building the innards of the displacer/regenerator.

It came out different than what I set out to do, but given the materials at hand, it kind of morphed into what you see in the photos.

I made a drawing and took some pictures and posted them here:

http://members.tripod.com/prc_projects/photos_1.html

I used a shelf out of an old refrigerator, some scraps of aluminum siding and a small size (three ounce) Tuna can. And super glue and some sort of epoxy putty, similar to JB Weld but another brand.

The refrigerator shelf was made of aluminum rods riveted inside an aluminum frame.

I cut out the rods from the frame and cut them into little pieces to use as spacers to separate the little squares of aluminum siding.

My idea is that this little gem will sit at the bottom "dwell" position and "cook" like a pot of stew for a while. With an insulating "lid" on the pot, there wont be anywhere else for the heat to go but into this "regenerator". Though I'm not sure regenerator is exactly the right word for it. More like a pre-heater. On the up stroke it would pre-heat the air going through it very quickly and this hot air will get even hotter when it hits the bottom of the heating chamber, then presumably this preheater/displacer/regenerator thing will reabsorb some of that heat on the way back down.

It ended up looking like an automotive type air cleaner.

It is supposed to have an insulating top that goes on it to help retain the heat while "cooking", but this is not shown as I wanted to take some pictures of the guts first.

By the way, I bought a tube of super glue, thinking it would last through the whole project, but on trying to squeeze out some glue all I got was air and a few drops of glue that barely lasted me through making this regenerator. I needed another drop or two but the tube was already empty!

Is this normal?

P.S. Regarding cost of manufacture, I was actually looking at some ready made rods with slotted ends at the hardware store when I went to buy supplies for this thing, but they were kind of pricey and too big for this little baked beans can engine.

The "stops" on the other hand I made out of some sort of clear tubing I saw for sale in the plumbing section. I only needed about a 1/4 inch of the stuff but when he told me the price, 25 cents/foot I told the guy, "give me three feet worth!

I had a feeling I'd be making a lot of these things.

I suppose I could have hammered and drilled my own slotted rod but I opted for the 1/2 cent worth of tubing.

I believe the slotted connecting rod would have to be horizontal in order to work without modification.

Can't wait to see your matching displacer cylinder!

-Stefan

One other thought, the stops are adjustable. I was thinking of metal sleeves with allan screws, but couldn't find any the right size.

Having it adjustable is at least an advantage in the testing phase. I guess slotted rods could be adjustable if they were made with over-sized slots and with some sort of nut and bolt in the slot to take up the slack

My sump pump uses rubber sleeves for stops. They haven't moved at all on the sump pump since I got it about a year ago, but I'm not sure if they will withstand the constant punishment they would get on this little contraption, if I ever actually get it running that is.

I'm not sure how long that patch of JBWeld stuff will hold out either or even super-glue with the constant heating and cooling. Hopefully it will do for a test run at least. I was going to try solder, but I wasn't sure if solder would stick to aluminum and the super glue was cheaper than a bottle of propane. (or so I thought at the time. As it turned out I was getting a lot less glue for my money than what I thought) Does anyone know, will solder stick to aluminum?

I forgot to mention that there is a washer jammed in tight between the aluminum fins under the JBWeld patch. I squished the JBWeld down in between all the fins in hopes it will hold. I need to devise some more positive form of reinforcement, but haven't figured out exactly what yet.

Also, this little "displacer" is rather heavy with all that metal inside it. Much heavier than a typical displacer (just an empty can) but with efficient heating and cooling, ??? I'm hoping this little engine will be able to lift all that weight.

I've seen a metal can fold up like it got run over by a truck before just from air pressure.

When I was a kid we had this book of science projects. One of the projects was to set a metal can on the stove, then after it got good and hot and all the expanding air went out of it we screwed on the lid and put the can in some ice water. The thing imploded almost instantly. Crumpled right up.

Of course that experiment involved several minutes of heating and very rapid cooling.

Possibly that kind of force could be generated with the addition of some appropriate gas other than plain air, but I'm pretty well convinced that rapid heating and cooling is the way to get more juice out of one of these things. A little water would probably work if the thing could be kept at an operating temperature somewhere above the boiling point.

I read on one of the Stirling engine websites I was looking at somewhere that adding a little water to the chamber will make a Stirling engine run better.

JB weld holds up under low flame and super glue almost immediately burns up. Soldering the aluminum will probably be a more permanent solution, but it will require the appropriate solder and flux (it's one more reason I tend to stay away from aluminum in my construction... aluminum requires a special type of solder).

As for putting water in the system... liquid water is less compressible than air and also has a higher degree of thermal retention ... both are properties directly opposed to what you want. Let's not even talk about what water, heat, and air do to metal parts.

-Stefan

Water in displacer chamber...bad, very bad. On my first test run I had an ice cube on the upper suface of my displacer and while I had very pleasing results with the running, I also discovered I had a tiny leak at the seam of the chamber. Well guess what? All that air expanding and contracting does an exellent job of sucking water in. From there you find it's impossible to dry out without disassembly and the foam of my displacer took a tall drink and warped too. On the bright side I discovered from viewing that first run video and the one after I fixed it, I had a faster engine due to my displacer getting reworked and more air could bypass it. (I was aware it had more gap and was afraid it was too much)

There is so much to learn building these seamingly simple things that I think a first time full scale engine could really be a disaster unless one is willing to keep tinkering and remaking large and possibly expensive parts. Imagine milling a three foot connecting rod and disovering it 'MUST" be three feet, one inch. :)