Is this a working design

[Floid]

Hello All! my first post here

I'm building my first stirling engine.

gamma1.jpg (140.89 KiB) Viewed 6232 times

It is my own design but one quite similar can be seen here http://www.youtube.com/watch?v=GomgnkMB ... re=related

Do you think this is a working consept?

cylinders are of stainless steel, for pistons i'm planning to use aluminium. displacer cylinder diameter is 33.7 mm and power cylinder 26.9 mm

If it is not necessary please not suggest major changes. I have already made about half of the parts :)

i can gladly give more info if necessary

-floid

[Ian S C]

Floid,Not as is, the displacer is a bit out of propportion, The displacer its self should be a cylinder approximately three times as long as its diameter. The displacer to power cylinder ratio should be 1.5 : 1. If you make it with the thick plug in the bottom, it will take a bit of time to heat up, it should be about the same thickness as the stainless tube. I would recomend either steel, or stainless steel for the displacer, aluminium does not stand the heat too well. For the power piston I would use cast iron, aluminium has a high coefficient of friction, and tends to gall,ie., it trys to weld its self to other metals. One of things one should aim for is running without oil, as oil tends to drag causing friction. If you have them ball bearing races on the big ends of the con rods could be a usefull addition.
The ratios are fairly basic Stirling Engine design. The rest is from my research in metalergy.
OPPS, just spotted one major boo boo, the cranks must be at 90*, not 180* as you have shown them in you diagram. Ian S C

[Floid]

Ian S C thanks for reply!

First - i know about the 90 degree offset. The model is not perfectly constructed in PRO/E so while i rotate it the offset is not allways keeping up. Don't let that bother you.

So are you saying this is not going to run? or that it will not run at maximum efficiency.

But anyway i wanna improve the design. So i should make the displacer cylinder and piston longer, right?

Material choices are under consideration. I think i try to do wihtout the additional ball bearings.

anything else critical?

-floid

[Ian S C]

The displace was the only thing that really worried me, getting the ratio right, but I don't know if your drawing program perhaps distorts things a little, according to your dimentions, the displacer should be about 90 mm long, and with up to 1.5, or so gap in the cylinder.
You should be OK without the ball races in the big ends.
I get most of my bearings from old VHS tape machines, and similar mechanical/electronic equipment. If you want slightly larger, look for roller skate/ skate board replacement bearings (dead cheap)or sometimes even used ones if they are not to bad. I'v got one motor running with used bearings, and they are that warn that I worry that all the balls might fall out. Tried it with new bearings, no improvement, so I put the old ones back again. Ian S C

[Floid]

Thanks again!

So when you talked about bearings it got me thinking and i added bearings to my design!

I found suitable bearings from RC car netshop quite cheap i think. I ordered 8 pcs of bearings which are of diamension 6*10*3. This cost me about 28 € total.

I made the displacer cylinder and piston longer. The piston is now 45 mm. Could that be long enough? I dont want to make this too high.

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If you think the displacer is still too short is the some way to circumvent. I was wondering if i cut the displcer cylinder to two pieces would that make any difference. Two pieces would be joined together with a material that has a lower thermal coefficient. But this is just a thought.

Comments appreciated

-floid

[Ian S C]

floid, first lose the ends of the pipe nipples where they enter the cylinders, the piston and displacer should clear the ends of the cylinders by about 1 mm. You shoud try and get as near to the 3:1 length to diameter ratio for the displacer as you can, even if you need to put spacers under the pillars that support the motor.
You don't have much cooling, so it would be good if the center transfer tube was made of aluminium (with fins if possible)also the crank support would do well being aluminium, all to radiate heat. Its just as important to get rid of the heat at the cold end as it is to get the heat in the hot end. Ian S C

[Floid]

Hello again!

update to this-

My engine is now finished! The only problem is that it won't run

I have pretty much done every improvement i can think off but no luck.

I made a video today about my project:
http://www.youtube.com/watch?v=mhcnrsAzlOE

So now i would need help from you guys

What do you think is the problem??

-I think the friction issue is ok. This can be seen on the video
- The structure itself has no leaks. I have tested it several times with soapwater
- displacer rod sealing is not perfect but ok i guess.
- power piston is not perfect but ok i guess.

pipe nipples ends have been cut. Center transfer tube is made of aluminum.
Power piston and power cylinder are off stainless steel

Ian S C,
about your comment - 'the piston and displacer should clear the ends of the cylinders by about 1 mm'

are you saying the power piston should also come close to the end of the cylinder and if so why?

If you think the diagnosis needs more info e.g. dimensions please just ask.

by the way
I'm getting a bit frustrated.. But then again if i get it running i'm probably even more happy :)

-floid

[Jerry]

It's definitely trying to work, which is a good sign. At first glance, it doesn't look like your displacer crank throw is long enough. It should move the displacer all the way from top to bottom with maybe 1-2mm clearance at the ends. The clearance around around the circumference of the displacer tube and the displacer piston should also be about 1mm. The offset crank web you have on the displacer doesn't appear to move it enough, but I can't tell for sure just from watching the video.

When you have the balloon on it for testing, the balloon should mostly deflate when the displacer is down at the hot end, and mostly inflate when the displacer is up at the cold end. If it does, but you still get the tiny amount of motion from the power piston itself, then there is probably not a tight enough seal on the power piston.

A good quote about Stirling Engines from a builder at a Maker Fair:
"Don't ask me how long it took to build it, as me how long it took to make it work!"

Don't give up, you're close.

[Floid]

Thanks Jerry

I agree that displacer travel should be a bit longer. It is now about 20 mm. The problem is that it was quite hard to find a suitable displacer and now that i have found it i cant make it shorter. Also it is quite hard to make cylinder longer which would be the other way to enable longer travel. But if this is the main reason for not running then i'll figure out some way to fix the problem. Another side to this coin is that the engine runs better with shorter strokes. I mean that with shorter strokes it rotates more easily.

Clearance between displacer and cylinder is about 1.5 mm on both sides now.

About the balloon testing-
On the video it can be seen that the displacer rod sealing is quite good. Otherwise the balloon would not stay pressurized. When i shot the video the balloon did not inflate/deflate that much. Other times i have seen more movement. But it has never 'lived' considerably.

"Don't ask me how long it took to build it, as me how long it took to make it work!"
This is nice one! gives me hope i guess

-floid

[Jerry]

With the balloon attached, and heat applied to the hot end, when you move the displacer back and forth you should get inflation/deflation of the balloon. If it's not, your most likely culprit is too much dead space, since the balloon is retaining it's inflated state.

[vamoose]

Floid, Nice job for a first effort. I'm still trying to scrape together the right parts for my first attempt (an Alpha configuration). I think I’ll end up having to make a few unwanted compromises if I want to get a move on..


Sounds like you've tried plenty of things to get to the problem, but I might throw a couple more into the pot, (take them or leave them). To get it going maybe you could use a cold gel press wrapped around the cold end, (those blue freezer packs that are used in lunch boxes and on sports injuries) or some other cold source. This active cooling will give you a better Delta T. Also If it is possible there is leakage around your power piston you could use a drop or two of sewing machine oil, or something more viscous like engine oil to help 'fill the gap' so to speak and make a potentially better seal. Although you don't want the oil to migrate to the hot end in your displacer, as it may gasify and Flash 'Uh Oh'.
Alternatively you could just chuck the engine in the freezer for 30 minutes or so and and pull it out and try and get it going strait away, (maybe with a wet rag wrapped around the cold end so it freezes and gives you a cold source for a little bit).
This is just a Jerry Rigged solution that may possibly get it running to help dull the frustration..

vamoose

[Floid]

thanks vamoose

I have tried different oil grades. Too thick oil will increase friction so it is a compromise thing. Maybe i try to make one more cylinder/piston now that i have practised mAking those. Then i might do without oil

I have tried electrical cooling spray. It makes no big difference.

But putting the whole unit to freezer - that i had not thought off. Great idea! Of course this is not the final solution but would help troubleshooting and such. I think i'll give it a try.

-floid

[Ian S C]

What is the stroke of the displacer crank, the gap inside the cylinder, should be that plus about 2 mm/3mm, the same gap at the bottom of the power pistom stroke, this reduces the dead space. The fitting of the power piston is important. To get the right ratio of volume for the two chambers, it may end up in you having to shorten the power piston stroke. The approximate ratio is 1.5 : 1 the displacer having the larger volume. This is the ratio the Robert Stirling used on his motors. Ian S C

[Floid]

Hello :)

update to this again

http://www.youtube.com/watch?v=jkYBffsUEyQ

So it runs better again. But still no continuous operation.

To get to this point I-
- Increased the displacer stroke 20 mm --> 30 mm. This made a big difference. Only problem is that with longer stroke i should have a longer conrod. And that would require major changes. I think i try to do with 30 mm stroke

- made the power piston lighter. I removed material where it is not needed. This is good thing to do and i will do it even more.

-floid

[Ian S C]

floid, I'v got a couple of ideas that may help, just been looking at your vidio. First, I would use a heavier flywheel. second, I would put some spacers under the pillars that hold the crankshaft, an raise it quite a bit, say 50 mm, then make new con-rods to suit, this will allow room for a larger flywheel (you can make it wider too), but more important, the longer con-rods reduce the thrust angle, and reduce the side pressure, there by reducing friction.
Not too important, but on engines of mine, of similar design, the displacer rod would be no more than 5 mm diameter.
You could try wrapping a wet rag around the block that the transfer tube passes, and see if that helps.
See if you can get the flywheel reasonably well ballanced.
You'v got the thing turning, thats a very good start, now comes the interesting time, developing the motor to see how much you can improve it.
If you look at my gallery, you will see a piston, and con-rod, that piston has a skirt that is 1 mm thick, the crown,(the top with the threaded hole) is 3 mm thick. The dark coloured disc is a leather cup seal, these bits are for a stove top fan motor, this too can be seen in the gallery. Ian S C