Help Needed Building a Gamma Type Engine to Run a Model Car

[Rad]

Hi all,

I have started building a gamma type engine for my final year project. The aim of the project is to run a model car with a chemical reaction. I figure a stirling engine would be a perfect way to demonstrate my proposed reaction and get people excited about these wonderful little engines as well as creating some more awareness.

I have built the basic engine, although from posts in the forum, I see I have some changes to make. At the moment, I have a 22 mm diameter power cylinder made of copper, with a throw on the piston of 58.5 mm. The hot side cylinder is aluminium, with an internal diameter of 36 mm. The throw on the displacer is 38.9 mm. My displacer to power piston ratio is 2:1, I see mention of 1.5:1 as a better alternative. I believe my displacer may be a bit short too. Why should it be 3 times the length of the diameter? I also have questions about the weight of the fly wheel. Should it go a whole revolution or more with the cylinder under compression fairly easily? There is a very slow leak in the hot side cylinder. What is the allowable air leakage? Would you advise a displacer made of tightly compressed wire wool or rather a high density foam or sealed can?

I'd very much appreciate some help, the engine does not run at the moment. I'll upload a couple of photos too. I have a very interesting idea for heating the engine that'll be my chemical reaction to run the engine. Once I get the gosh darned thing running, I'll upload a video of it and another with my proposed heating element on it (assuming it's gonna work).

[Triangle.Stirling]

whow! congratulations on your engine! It looks really nice and clean, nice finishing!
How have you built it, from a plan, of figured out the dimensions from the internet?

I can't answer your questions, but i'm really curious about the answers. Just a few questions i wondered about myself. For example the 1.5 : 1 ratio, is there some evidence for that, some research? I see it mentioned everytime, but don't know where it comes for.

Good luck, and keep us updated, curious what heat source your going to use.

[Ian S C]

The 1.5:1 ratio dates back to Robert Stirling. Your ratio of 2:1 would probably work, but not too efficiently.
The copper power cylinder is not a good idea, brass would be OK, but copper is not a metal used in slidingor bearing situations. The hot end would be better made of steel, or even better stainless steel. The displacer is made three times the diameter to keep the hot end as far as possible from the cold end, it also works as a regenerator. The best displacer I find is a light weight sealed steel, or stainless cylinder, I'v not had great success with steel wool.
I feel that you would be better with about half of the stroke.
What sort of bearings do you have in the crankshaft, and big ends, they really need to be ball bearings.
The leak, is that at the gland where the rod exits the cylinder? If so, a little leak there is OK. If the displacer is a can, make sure that it doesn't leak.
Ian S C

[Triangle.Stirling]

Ian, short question: what if you make the displacer say 4 times the diameter? Will the efficiency decrease, or will it be even better because of better separation of the cold and hot side? however, the air has to travel a longer way, and maybe more wear from the displacer?

[Rad]

Hi, thanks for your quick replies! My dimensions and plans were a combination of calculations and other engines found on the internet and were also influenced by the materials I had available to me. While the engine does look rather nice, It still needs to run! Do you think that ratio was a trial and error by Robert Stirling to find that 1.5:1 works so well? Nice question on the displacer.

Thanks Ian, I will definitely try change the cylinder to steel or stainless steel and certainly change the stroke and ratio. May I ask why you suggest the shorter stroke? The air leak is a minor one around the piston gland, so that should be alright. I believe my flywheel needs to be heavier although it spins really well, I've just used brass rods that fit snugly into a piece of brass tube. A little bit of q20 in there keeps it spinning very well. I do have bearings now, and I'll certainly implement them in my design if the friction seems to be an issue, which is probably likely. The copper power cylinder has a very good interaction with the piston I built for it, but I will keep your words in mind.

Thanks again! Hopefully I'll have an update real soon.

[Ian S C]

I think the 1.5 : 1 ratio was derived by mathematical methods, If you get too long there is too much weight compared to any gain achived.
If you hold the cylinder vertical, and take the piston, without it's con rod, and put it in the cylinder, it should drop through, but if you cover the end of the cylinder (say the top)with your hand, the piston should stop, then slowly slide down, this must be done with no oil and dust. Do this and your well on the way, but copper is not an ideal metal for engineering things like this.
As your bearings go, like metals should not be run together, they try and weld themselves together, oil does not really help. The only metal that can be run like to like is cast iron. You will be OK, but you might be surprised at the wear, keep them lubricated with a light oil/sewing machine oil. Don't use WD40 or similar, it will work the first time, but it leaves a layer of(??) on the metal surface, and the bearings will sieze

[Rad]

Hey, I've made some progress! Thanks again for the advice.

I've now corrected the ratio to 1.5:1 and shortened up the stroke to about 145 mm on the power piston. I have also lathed a new piston for the cylinder, it slides wonderfully. The displacer is now a sealed can, and 3 times it's diameter in length, with a 1.3 mm gap all around. It's actually an empty deodorant can.

A good flick on the flywheel while the displacer (and not the piston) is coupled up makes about 10 revolutions. I'm having some slight friction issues when I couple up the piston to the fly wheel, I'm tinkering with that to try get it really smooth. Could i please get some advice on that? I don't know if it's generally a struggle to get such a tight fitting piston well lined up in the cylinder.

When I have just the displacer connected and the piston sitting uncoupled in the power cylinder, I can apply heat and spin the flywheel causing the piston to move in it's cylinder. I can get it to move about 8 mm in total. Sometimes a bit more, sometimes a bit less. Is this too little movement? Should it move as far as the stroke? I've been tinkering quite a bit and there's been a little improvement, but I know when I need an expert opinion or two. Or three or four or five. There's been a marked improvement since following the other projects and advice on this forum.

I get the feeling it's close to working, i just have to clear up some of the issues. And are there any other Pretoria based stirling enthusiasts? The heat source seems to work as well as any other heating I can apply, for an update on that, although obviously I can't test that fully until the engine is running.

Thanks,
Richard

[Ian S C]

Rad, If you disconnect the displacer from the crankshaft, and have the power cylinder disconnected from the displacer cylinder, ie,. no compression, how much does the flywheel turn with just the piston sliding in it's cylinder, you should get a few turns out of it with a flick on the flywheel.
Ian S C

[Rad]

Hi all.

I apologise for the late reply. I've adjusted a lot of little things on the engine, realigning the conrods and changing the points they were mounted at. A little bit of work with 2000 grit sandpaper and good cleaning out. Now a flick of the wheel with everything connected except the two cylinders (i.e. no compression) will cause 3 or 4 turns.

The next thing I want to do is replace the hot side cylinder with a steel one. At the moment, it's still an aluminium one. I worry about melting the aluminium and also keeping the temperature difference. My proposed heating solution should produce around 600 degrees Celsius and I think aluminium melts at about 650 degrees Celsius. And aluminium also doesn't glow before melting.

I'm still going to tinker a bit with the current setup, but there are tests and such coming for uni. I should be able to lathe a new cylinder next week :)

[Ian S C]

Rad, I know about aluminium displacers. On one engine I melted three of them before I saw the light, well the just collapse, and the motor stops, I changed over to stainless steel, and no trouble ever since. Leave aluminium for the cold end of the motor, it does not belong near high temperatures. Ian S C

[Triangle.Stirling]

Hi Rad,

Do you have further news about your engine?
I'd like to see a photo or hear about how it's going, is it already running?
I'm also a bit curious about your heat source, how's that going?

Regards,

Krijn

[Rad]

Hi all,

The engine is running! The stainless steel cylinder and displacer certainly do the job, thank you for the advice. Hopefully I can upload a video soon, probably during the week. Thank you for the help and interest, I have certainly enjoyed this project. Now I will be testing and optimizing, and I'll bring up another video of my engine with my heating solution. Assuming that solution works. This engine seems to run happily and smoothly at 300 rpm. I have exams to work on (I got in for them all, somehow) so I'm afraid the engine may have to take a break for a while. I may be able upload my research project when it's completed, if anyone would like to see it. Thanks again!

[Rad]

http://m.youtube.com/watch?v=Sl-bG31SXYY

Hey, that's a YouTube link of a video my brother got on his phone. It's not bad but I will try get a better one up and another of my heating solution. I will say it involves red hot copper, acetone vapour and atmospheric oxygen. The red hot copper acts as a catalyst.

[Rad]

Hey again,

So my brother and I used ice as a cooling source and tested the engine again. We really tried to up the rpm. It was accidentally tested to the point of failure. This thing goes a lot faster than 300 rpm! The piston con rod tore it's mounting point off the fly wheel rod. Just 20 minutes of disassembly and work with the soldering iron brought the engine back to life. The alignment is slightly out now and it wobbles rather excessively at high rpm, but I'll be able re align that. My heating solution of red hot copper kept that way by catalysing the dissociation of acetone does work, although I haven't got going reliably yet. More updates in the future :)

[Triangle.Stirling]

Hi Rad!

Just saw your video today. Your engine runs really great!
Like to hear how it went from that video, and also like to see your research project if possible. Very interesting!

Regards,

Krijn