Stirlingengineforum.com

Hi everyone!

I have been in a process of designing and building a small alpha-configuration stirling engine for couple of months now. The primary design feature was to utilise the machine tools I had access to: stic welder, metal lathe and bench drill. The original design includes a regenerator and miniature cold and hot heat exhangers.

The cylinders are turned from 50mm brass bars, and have both a bore of 25mm (1"). The hot cylinder is open from both ends while the cold cylinder has the cold heat exhanger drilled to its head. There's also a chamber for the regenerator cartridge (a piece of pipe filled with steel wool). Both cylinders are supposed to be air cooled. The hot piston has 40mm high steel head to reduce the amount of heat transferred to the sliding surfaces.

The frame of the engine is made of 6mm thick aluminium, and all the parts of it are screwed together with 3mm bolts. It is supposed to hold the cylinders and crank mechanism in place. The crank mechanism itself consists of an applied ross-yoke linkage. All of the pivots have ball bearings to minimize friction. Crank levers are made of aluminium, and piston rods from long 3mm steel bolts. These rods allow for adjustment in piston positions in the cylinder, so it is possible to adjust the compression to some extent. The crank also has several holes with differing placement, thus the stroke length can also be adjusted, by being 15, 30 or 45 millimeters.

I'm almost done with the engine, the cylinders, heat exhangers and pistons are all done, there's only a flywheel and the cooling fan to be finished. I was a little impatient, however, and had to try heating the engine to see if it had any kick in it. It sure has a good compression ratio, but the cold piston seems to be too leaky, since there wasn't notable effet on heating the hot end.

The pistons are turned out of polyacetal, which I thought would be a good material for it's hardness and low friction ratio. It didn't turn out to be that good it seems, mainly because the thermal expansion ratio of the polyacetal seems to be greated than that of a brass. Thus when heating the engine, the hot piston binds to the cylinder creating way too much drag.

The problem here is now following: I need to be able to find a commonly available material for the piston. It should be able to withstand temperatures of about 100°C without binding to the cylinder. The piston material needs to be softer than brass since a new piston is always easier to make than a new cylinder. The piston also needs to be extremely airtight in the cylinder for this one to work. Copper might work, and graphite would work wonders (if I knew where to get it from). What do you think?

I'm also worried about the compression ratio: despite the air leaks the crankshaft is hard to turn when both pistons move up. I quess I would need a large, preferrably huge flywheel. Is there any sense (at this state) to reconfigure the engine to gamma or beta style? with ready cylinder this shouldn't be to hard, but it will take some time and reduce the output power :-x

I will appreciate any ideas and tips you have!

Hi,

I've had great success using the glass (pyrex-borosilicate) cylinder and graphite sets available from airpot.com.

There is a thread here regarding this very issue. My postings about the "Ugly Duckling" document the use thereof. Builder and contributor here, Cartech, provided very helpful advice.

As for coverting to other type, if your current engine won't run the conversion to another style engine that will run will gain power not lose it.

Passing along the same advice I was given, "read the postings already here" as these will prove a valuable resource.

Good luck, hope this is helpful, and keep on trying.

Roger

Those glass-graphite sets would be fine, but they only ship to US it seems.
Besides, I have had problms with the customs in the past for ordering "extraordinary items".

My brass cylinders are fine, no complaints about them. It's only the piston that give me gray hair. Polyacetal won't work it seems, so i would like to know about the alternatives. Copper maybe? Brass with Teflon rings? Graphite (I quess I won't be able to get it unless I order it from china or something)?

Then about the alpha configuration itself: It works in theory, but in practice the mechanical issues have started worrying me. The compression ratio with 30mm stroke is so high that it can easily twist the piston rods out of shape. Furthermore, the engine would require a huge flywheel to kick it over the compression stroke, preferrably several times. It just might be reasonable to adjust to the 15mm strokes or to build a whole new cylinder to act as a displacer cylinder, converting the engine to gamma type.

Edit: Bad news. The lathe I was allowed to use in nearby machine shop is out of order. I won't be able to turn any cylinders, but I could make pistons with the electric drill methode... So the Gamma conversion will have to wait for now.

Hi,

Well, I didn't understand your location issue, sorry for the assuption. Do you mind telling us where you are?

I feel the need for a lathe to accurately turn parts is needed only for high RPM operation. As has been posted in anther thread here a little bit of air leaking is needed to get an engine to run. My walking beam leaks around the displacer gland a tiny bit and until the heat builds in the displacer I too see some compression. Many wlaking beam type engines have been built and run very well with drill press fabricated parts. I don't even have a tool like that so my use of the glass/graphite was about my only option.

I feel what is more critical than absolute sealing is the reduction of friction via either binding in the power segment or other parts like bearings in the crank or elsewhere. Friction is the enemy! By no means am I an "expert" in this area but I've figured out what works and what doesn't.

If I can make a Stirling engine anyone can! Just keep on trying and don't get discouraged by failures, as these often teach us more than success.

Have fun!
Roger

Hi!
For your information, I live in the EU.

The friction of my mechanical parts is as low as it can be by this sitting. All of the joints have ball bearings mounted, and the mass of the moving parts is as low as possible without compromising strength.

My only problem now it so make pistons that are tight enough for the cylinders not to leak too much, but also loose enough not to create too much friction. I have carefully turned and honed my cylinders, so I wouldn't like to scratch them with too hard piston material. Is standard water pipe copper softer than brass? or do I need to turn a complicated brass piston with ultra thin teflon seals?

The graphite would be the best solution, but graphite bars are unavailable in my country. I could order them directly from a manufacturer in China, but I would be bound to run into problems with the customs. ](*,)

Another problem with my engine is the fact that I'm building an alpha stirling. It has much higher compression ratio than gamma engines, and would require a huge flywheel. I'm prepared to make a 2kg flywheen out of steel, but more than that would be too much for the two ball races I have for the main bearings.

I will try to post some pictures of the unfinished engine, but this forum doesn't allow for direct posting, and I don't yet have my own server up and running...

Today I got the flywheel attached. it's bout 1 lb steel piece. The engine turns very smoothly with it, and runs through several cycles when the cylinder heads aren't in place (ie. air can move freely in and out from the engine). But when the cylinder heads are screwed in place, and the system is sealed, it seems to be very hard to turn the flywheel. The engine has a tight spot at the compression stroke, and it is simply caused by the compressing air that acts like a spring under the pistons. In addition to this, the cold piston is too loose in its cylinder. It simply lets the air out too quickly.

I tried to run the engine for the first time (still with polyacetal pistons). With 30mm stroke the pressure ratio is simply too high for that flywheel, and I could hear air hissing out from the cold cylinder. I then adjusted it to 15mm stroke, and it seemded much better. When the engine was cold, it could turn two cycles with the flywheel. When I started to heat the hot end it turned much better to the spinning direction, but seemed to be much harder to turn agains the spinning direction. This is a good sign, but the engine did not run.

I simply need better pistons built from better materials. Any ideas? What would work in brass cylinder?

There are now two alternatives for the engine: Either I build a much larger flywheel, try to minimize all friction and build very accurate pistons OR convert this one to gamma type by replacing the cold cylinder with displacer cylinder. What do you think?