'60' Sterling engine questions

[nautilus29]

Hello everyone! I've been reading this forum for awhile now, but this is my first post. I decided to attempt to make J Jonkmans 60 Sterling engine that I found on this site. It's the first engine I've made, and I'm having problems getting it to run. I was wondering if you guys had any suggestions and tips to help me trouble shoot it.

If I spin the flywheel by hand it feels really free and smooth. There is one spot that is slightly tighter then the rest, but I emphasize slightly. If I give it a good spin it will to 3 to 5 revolutions before stopping (this is without a heat source). I made the crank to print and there is a balancing problem, but I'm not sure if this would affect it from running? I reamed the piston bore and displacer bushing undersized and lapped them to size eventually using #3 lapping compound until the piston and displacer shaft had a tight but smooth fit.

Now that I have everything assembled when I light the tea candle it won't run. If I put the crank so the piston is in the down position it will slowly push the piston up (which is actually moving the heavier side of the crank up, so I know it must be from the candle heat) but It won't run.

Thanks for any suggestions! Maybe I should have tried an easier engine first. From what I've read so far it seems like you want the cold bore to be close to the size of the hot. On this engine design the cold bore is 5mm by 27.5 and the glass tube has a 13mm I.D. and is 54mm long. I thought about making the cold bore bigger, but since his engine runs I'm weary about changing it.

 

[fcheslop]

Hi, I made the Stirling 60 a few years ago and can confirm she runs straight from the drawings
By what you are saying it sounds like you simply have to much friction
The piston should glide in its bore or a simple test when one end is blanked usually by my thumb the piston should not fall through ,when the thumb is removed the piston should slowly drop through.Again with one end blanked and the piston pulled down the bore it should snap back up.
I would also have a look for the other slight tight spot you mention as any friction simply kills these little engine and this design is a smaller one
I have found that you need to lubricate the displacer bush with a little light oil
The two main problems with these engines are friction and air leaks
Have a look on Jan Ridders site he has some good fault finding tips
When building my own engines depending on the type of flame I go for a ratio of 1.5 to 1 upto 2 to 1
It sound like you are close to a runner
Good luck
frazer

 

[nautilus29]

Thanks for the information! I'm glad I've been heading in the right direction as I've spent the last couple of days trying to get it to turn as smooth as possible.

I removed the pin that connects the piston to the rocker arm, and then lit the candle. As I spin the flywheel the piston moves up and down in the bore depending on where the displacer is. I'm guessing that means my air pressure is good, but I haven't checked the displacer bushing yet since I have to remove the hinge from the displacer shaft to check it.

When you are talking about ratios is that the hot to cold cylinder ratio? If so are you just talking bore diameters, or is that total volume of each?

 

[fcheslop]

Hi , this may explain it better than my one finger typing skills will allow
http://ridders.nu/Webpaginas/pagina_ervaringen_tips_stirlings/ervaringenstirlings_frameset.htm
Just use the English option
The bushing is a place for air leaks and a little soapy water should show up any probs. Iv found despite what the designer says this displacor bush need a little light oil or that maybe just my engine.
If as you say the power piston is moving when you operate the displacer then you are not far away from a runner, In the past Iv used a small spirit (meths) type of burner to get them away and then swapped back to a candle another thing Iv found is dont use cheap candles as the flame is cooler.My candle of choice is beeswax or those smelly Yanky ones burn ok. I only found this out after some frustrating moments with an earlier engine using those £1 per hundred type
Keep going you are nearly there and these engines do make me doubt my sanity at times
cheers
frazer

 

[nautilus29]

I have to admit it is reassuring to see there's s little more to these then throwing them together and they run perfect.

I found out what is causing the slight drag! I noticed from J.J.s pictures that it looked like he slightly belled the end of the pin that holds the arms to the crank to make sure they stayed on. So reluctantly I did the same. After doing some troubleshooting and figuring out that the drag wasnt coming from the cylinder side I decided to remove that pin even though it meant making a new one. Sure enough without the arms on there the flywheel moves freely. Kind of a relief since I didn't want to have to check my flywheel spindle.

This time around I'm going to make a shoulder on the outside if the pin to stop the arms from falling off.

I'll try that soapy water method to check the bushing.
should I expect no air coming out, or will there be a slight air loss?

 

[minh-thanh]

Can you show some picture of your engine?

 

[nautilus29]

I'm trying this from my phone, so hopefully they attach.

 

[minh-thanh]

how many millimeter stroke of piston ?? you design this engine ??

 

[nautilus29]

I spoke too soon about the drag... I replaced the pin and that definitely helped, but there is still some drag and it's coming from the displacer. It's a possibility that when it is fully extended the weight of it is putting a little pressure on the bushing and creating a slight drag.

I made the bushing counterbore in the brass using an endmill since a reamer wouldn't make a flat bottom, and I didn't have a boring bar that small. I noticed by pinning it that the bottom of the hole was around .0015" (.038mm) smaller in the back of the hole. When I pressed the bushing in it closed slightly in the back and I had to lap it back to a smooth slide. If I grab the displacer shaft I can rock it up and down since there is some taper in the bushing hole.

Also the print called for a 2mm displacer rod, but I ended up using a 3mm center drill since it was a straight hardened shaft.

Do you think the tapered bushing is the culprit, or do I possibly just need to reduce the weight of the displacer?

 

[nautilus29]

No I found the design on this forum. It's called the stirling '60' and it was designed by J. Jonkman. It has an 18mm (.708") stroke.

 

[minh-thanh]

I don't no why ...I dont no why ...I replay alway wrong ...

 

[minh-thanh]

And you need to reduce the weight of the displacer !!

 

[minh-thanh]

Basic information for stirling engine !
1. length of displacer chamber L = 3 times its diameter.
2. length of heater chamber = 2/3L
3. length of cooler = 1/3L
4. swept volume of displacer = 1.5 times swept volume of piston cylinder. (****)
5. length of displacer = 2/3L and stroke = 1/3L.
6. diameter displacer = diameter of the displacer cylinder displacement - 1 to 2 mm.
4-****. for a low temperature engine, the ratio might be anywhere from 10:1, to 20:1. A high temperature engine should be around 1.5:1, unless it's an ALPHA engine, in which case the ratio is 1:1.

You need to reduce the weight of the displacer

 

[nautilus29]

Thanks for the help, good information to know. Where you saying that the displacer should move 1.5 times what the piston moves or 10 times if it's a low temp?

I think before I do anything major to it I'm going to try a different heat source since I'm using the cheapo not hot enough candles. The drag is so slight I'm starting to have doubts that it is the issue.

 

[minh-thanh]

"" swept volume of displacer = might be anywhere from 10:1, to 20:1 for a low temperature engine ""

 

[fcheslop]

Hi, the displacor on my Stirling 60 is alloy and from memory around 20thou thick
As Minh_thanh shows the place most likely for an air leak on this engine
The stroke for the displacor also depends on its diameter
cheers

 

[nautilus29]

Here is an update on the engine. I remade the displacer, displacer shaft, and displacer bushing. I used aluminum for the displacer this time to lighten it up, and I have a .01 inch sidewall. The displacer bushing was lapped to size and is no bigger the .0002" over the rod size. Could be closer but that is as close as I can gage it. The rod slides really smooth in the bushing.

I still only have the cheap candles to test it with. It now spins a couple of times before stopping, but still won't continuously run. I have some higher quality candles ordered, so hopefully when they come in I can finally see it run!

 

[minh-thanh]

Hi nautilus29 !
Why do not you use alcohol ?

 

[nautilus29]

Hi nautilus29 !
Why do not you use alcohol ?

It looks like the original designer used a tea candle, so I figured I would use one too. If my next attempt doesn't work I will try that.

 

[fcheslop]

Keep the dead space to a minimum . How much clearance have you got at the front of the hot cap?. The flame needs to right at the end of the glass
Probably telling my granny how to suck eggs and just a few thoughts that come to mind
cheers