Help with aluminum piston

[wardle3]

I just bought a 1" aluminum rod that i want to make into a piston. How long should i make it and should i drill any holes in it to make more of a vaccum or what is the best way to make it with just basic shop tools... thanks for everybodies help and soon one day i will have mine running . i am using a head that came out of a computer hard drive it is about 3" in size, do i need to add counter weights to make it work. it turns great.

[Administrator]

The piston length.........Too short and it will try to turn in the cylinder.......Too long and you are wasting energy moving it. On a 1"long piston I would pick around 3/4" long plus or minus.

Drilling holes in it to make more of a vacuum?...... Sorry I am not sure of the question here. we drill a large hole in the rear going forward to eliminate weight and give room for the "wrist pin" and connection of the push rod, but I am not sure if this is what you are talking about. Also drilling that hole for the "wrist pin" is done a little towards the top from center. This helps keep the piston traveling down the cylinder straight without "cocking" in the cylinder.

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As for shop tools.........depends on how clever and patient you are. Go slow and try to think it out. Can be done with a common electric drill, but not easy to drill wrist pin hole. Another way is like this. Drill hole in top of piston and use a threaded rod or machine bold and cut a slot in it. See below



The advantage is mainly you can do it off the piston and if you goof it up....make a new one without messing up the piston.

As for the flywheel weight issue. Depends on the mass (weight) of the flywheel being used. Also the diameter of the flywheel. I would try it. If the engine seems to stall then it needs to be heavier to keep the momentum going. Too heavy (I don't think your will be) and you are wasting energy.

If you add weights, add them slowly to the flywheel. It is always a tuff thing to explain how and where to add weights to a flywheel to a beginner. You have to watch the engine to tell where they need to be. If the engine is built to specifications, you will not need weights. They will help to "pick up rpms" but not until after it is already running. The walking beam tin can engine does not need eights to run. It will run faster and better with them, but it was designed to not need them.

The question I always get is "how tight should the piston be in the cylinder". The answer is "not too tight" and "not too loose". I know that doesn't really answer the question but.........The piston should move in the cylinder without any restriction or resistance. But not sloppy. I make my pistons just a little too tight to go in (I mean just a thousands too big) and hand finish them untill they just fit in with no resistance. If you are a little too small for the cylinder, some of that can be tolerated with oil before starting your engine. The oil will "seal" the sides up a little, but don't ask it to do too much! Oil will also cause friction! (really viscosity).

DB

PS wardle3 if you need a machined 1" alum piston (made from a 1" rod) let me know. I have some machined out with wrist pin hole in them. They need to be finished on the outside.

[alpha stirling]

Aluminium is a bad material for piston/cylinder assemblies. The problem of aluminium is its high friction ratio and high thermal expansion rate.

I'm sorry to spoil your fun, but copper, brass or even some plastics would work better as a piston than aluminium. I personally chose polyacetal, but it seems to be only moderate option. My cylinder is turbed from brass bar, so the friction between the cylinder and piston is minimal.

I strongly suggest trying some other piston material. Aluminium will work, but it will require constant oiling. If you are up to it, feel free to try :)

[Administrator]

I disagree with your findings about alum and your idea about polyacetal. I also happen to know he is building a walking beam engine.

The best piston material is graphite. Harder to get. Hard to machine in a garage without a lathe. Very light weight. Self oiling. Very minimal expansion . Will tolerate a lot of heat. But if you want to make your own, (not buy one off the shelf) very difficult. You can buy a piston and cylinder off the shelf that is excellent. You’d be hard pressed to find anything better for the job.

Alum is next best. A walking beam engine is not run all day. Oiling is not a problem. It’s easy to get. Easy to work with ordinary tools and certainly lighter than brass as a piston. Brass is very heavy. Weight is a big deal in the piston. Anything going one direction then having to stop and go the other direction takes a lot of energy to do that. As far as thermal expansion, that can be taken into consideration without any problem at all. Almost any type engine uses alum.

As far as polyacetal………I have no experience. But if it’s got to be machined with a lathe smoothly (which it does) I’d go with graphite. How ever, I have never used polyacetal. Maybe it is better than graphite?

Bottom line
This engine will run very well with an alum piston. When it was designed, it was designed to be as simple as possible, and from as common of material as possible. Without a lathe and minimal power tools. So those not normally able to machine a Stirling engine could build one and understand the engine. The piston in the original plans is from a brake cylinder of a car. Can it be improved on? Heck yes. For a tin can engine and a new beginner, keep it simple and easy.

[tmk]

Brass is very heavy. Weight is a big deal in the piston. Anything going one direction then having to stop and go the other direction takes a lot of energy to do that.

I talked to someone who built stirlings professionaly at the Maker Faire, and (assuming i interpreted him correctly) he was saying that you can basically consider the piston weight part of the flywheel.. imagine the weight being attached at the point where the piston attaches to the flywheel, and the power piston is weightless..

since from the perspective of the flywheel or the engine pressure, it just exerts/accepts a force and tries to move whatever it can. the inertia should be transferred to the flywheel.

He compared it to a pedulum.

He may have been talking about a special case though, but he clearly said the power piston weight didn't matter much.

As far as polyacetal………I have no experience. But if it’s got to be machined with a lathe smoothly (which it does) I’d go with graphite. How ever, I have never used polyacetal. Maybe it is better than graphite?

Polyacetal is a plastic, and has a low melting point. I had the idea that if you could get a block of polyacetal, you could match it exactly to say a copper pipe power cylinder by just getting the copper rocket hot and 'cutting' out a circle of polyacetal with it. Just jam it on there like a cookie cutter.

Should end up with a polyacetal cylinder with the exact inside diameter of the copper pipe, and if it's as low-friction as is claimed, should work really well. If not it should be a simple matter of a little sanding.

just an idea

-tmk

[Administrator]

I would still disagree with the piston being part of the momentum of the flywheel or part of the flywheel. :eek:

Maybe I’m wrong here but when the piston get to the most outward position something has to stop it in it’s track and get it to go the other way. That takes energy to do that. That energy is from the momentum of the flywheel were it is stored. That is the job of the flywheel. The heavier the piston the more energy it takes. This is also the case in an internal combustion engine. Same principle. Also clearly some of the energy is from the second part of the cycle where the air cools rapidly and collapse creating a vacuum. That vacuum “sucks” (for a lack of a better word) the piston back in. Both of these forces are at work on the piston. But the weight of the piston is definitely a factor. I've see people hollow out their piston and it runs faster.or the engine can do more work.

“He compared it to a pedulum.”
What stops a pendulum and makes it go back the other direction is gravity. That is the force that turns it. That is the energy. Without that energy it would continue in one direction and make a circle around. Without the flywheel and the attachment and the energy in the flywheel the piston would continue out ward and come out of the cylinder. The momentum in the weight of the flywheel is what turns the piston the other direction (stops it in its travel and reverses it) Take the flywheel away and the piston will not return.

I’ve built many Stirling engines. Lighten the flywheel too much and the engine will stall. Usually with the piston out (but not always). Make it too heavy and the engine will not start and keep going because too much energy is being used to turn the flywheel. Just my observations and as always, I could be wrong!

As for the polyacetal idea. Sounds like an idea to try out. The low melting point would concern me a little. How low it? Maybe a good and easy alternative if it doesn’t melt.

DB

[tmk]

you're probabaly right about the piston weight thing

according to this site (which sells the stuff?) http://www.apikolon.com/index.asp?page=kocetal_grade

melting point is 166 C = 330 F

so some heat resistance but not that much.

here, 'alpha stirling' notes that the thermal expansion is the biggest problem with polyacetal:

http://boydhouse.com/forums/viewtopic.php?p=573#573

-tmk

[alpha stirling]

Polyacetal HAS larger thermal expansion ratio than brass or copper (which you guys seem to use a lot with walking beams). If your piston fits the cylinder snugly in the room temperature, operation temperature can make the piston stuck inside. I have solved the problem by having the power cylinder isolated from the heat source and by having turned cooling fins for it. Thus, it's operating temperature never exeeds 50°C (120°F)

One good thing about polyacetal is that it doesn't require any oiling. My gamma engine (I designed and built it from scratch) has polyacetal power piston, which fits the cylinder snugly. The cylinder is made from brass, turned, drilled and fine sanded to accurate bore (well... at least it seems to be accurate enough :???: ), and the piston inside is very gastight. the fit is actually so accurate, that the engine will stop if the power cylinder temperature goes above 50°C. Oiling the piston has similar effects (oil in that very thin gap creates more friction than lubrication).

Polyacetal is hard and heavy, but it can be easily machined. the drawback is that it brobably cannot be sanded (mine have sufferet the "porous white crap coating"-effect). There are different size polyacetal bars available, and I think 25mm bar (1") would fit inside 25mm ID copper pipe, so machining might not be necessary. On the plus side, polyacetal makes perhaps even lighter pistons than solid aluminium

Myself... I dislike using polyacetal no matter the good properties. The thermal expansion and low melting point really give me too much gray hair. I will intend to make a new piston out of brass (graphite would work best but it is so darn hard to obtain)...

[SScandizzo]

Hi Guys,

I'd actually believe the notion that the displacer piston is functionally part of the flywheel - most of the time. Darryl is right to say that it interferes with the motion of the flywheel at the extremes. But, during the rest of the cycle, the mass of the piston is following the law of conservation of momentum (a body in motion tends to stay in motion, a body at rest tends to stay at rest). Just as the flywheel is trying to establish a constant rotational momentum the piston is trying to establish a constant linear momentum (though with a simple crank design this will not occur).

Conclusion? Because of those two short periods at the extremes of the displacer's motion, we want to keep it as light as possible relative to the flywheel. Conservation of momentum by a massive displacer in the wrong direction will get us nowhere!

This leads me to another conclusion: what if we didn't need to move the displacer in a reciprocating fashion? Stay tuned, as I am working on a Stirling engine with a rotating displacer. (It's been done before, but not often, and not commonly without a machine shop!)

-Stefan

[alpha stirling]

There are engines that use rotary displacer, which really isn't much more than eccentricly assembled, hollow drum rotating in the displacer cylinder. It's weight is countered by the eccentric flywheel...

[Soeren]

...
The best piston material is graphite. Harder to get. ...

What do you think about the idea to use carbon brushes from electric motors as graphite source (maybe from old hand drills or a from a vacuum cleaner)?
For example I consider to use it for the bearings in my next LTD stirling project, but maybe such brushes are large enough to be used as piston material.

regards
Soeren

[speedless]

Hi
Alu-piston is ok as long as your cyl. material is harder and about the same expansion cooff.
I have good experience with alu. piston and brass cyl. Running without lubrication.
Jan.