Trouble-shooting a new Stirling

[Harry Mueller]

After 4 successful steam (air) engine builds I thought I was ready for a stirling. Despite having 2 plans from Jan Ridders and a plan for the Stirling 60 I was having trouble assembling all the required materials so I just decided to start building something to keep gaining experience. I saw a picture of a Chinese stirling being sold on a website and started from there. The results are shown above. All seems OK except it won't work.
With the tubing between the cylinders off it will rotate 7 or 8 times with a flick of the flywheel. With it attached I'll only get 1 to 2 revolutions. That was after I used some fairly large tubing to replace the narrow tube that really restricted the motor. I'm now burning methel hydrate after trying paraffin oil (not good).
Displacer is built from 2 test tubes and power cylinder is brass tube with aluminum piston. You get a nice "pop" when the piston is removed. The stroke and bore are about 1/2". The displacer piston has about .040" clearance. Nothing seems to be rubbing. There are no bearings but all joints were polished with Brasso. I like the engine but...IT WON'T RUN!

I've tried everything I can think of and am hoping someone here can offer something else I can try or point out where I've gone wrong. It doens't even seem to try to run even after pouring the heat to it for 5 minutes so I'm starting to suspect a design flaw.

Thanks,
Harry

 

[TonyM]

Difficult to tell from the photos but to me it looks like you have made the displacer piston/cylinder smaller than the power piston/cylinder.
This may help take some of the guesswork out.
http://www.solarheatengines.com/2012/02/09/power-piston-sizing-for-stirling-engines/

 

[minh-thanh]

Hi Harry !

[QUOTE ]
The stroke and bore are about 1/2"
[/QUOTE]
stroke and bore is of power piston? I think the diameter of the piston power is large, and do you follow the drawing board or do you design yourself?

 

[Harry Mueller]

Thanks for the response. I designed it myself. Should the power piston be smaller than the displacer?

Harry

 

[minh-thanh]

Yes ,the power piston should be smaller than the displacer .
You should try with a piston with a diameter 8 -> 10 mm
I think it should be 8 mm

 

[Harry Mueller]

Thank-you so much TonyM and Minh Thanh! That looks like it could be the problem and I’ll be heading to the basement shortly to try to fix it.

Harry

 

[TonyM]

The displacer is the glass tube try a power piston and cylinder of around 5/16 (8mm). It's easier to make bigger if you need to.

 

[Harry Mueller]

Well, I think I'm making progress but it's still not working, although now it seems like it wants to run but then slows down and stops. I reduced the power cylinder down to 3/8" ID (9.5mm) because that was the the closest brass tubing size I had. If I had to, I guess I could turn down some brass rod so I can fit an 8mm piston but I'm wondering if 1/16" would make a difference. The motor spins really well now with the smaller piston. It seems really close but something is still missing.

Harry

 

[minh-thanh]

I think , personal opinion , with 3/8" ID (9.5mm) diameter and stroke about 1/2" is too much , With this type of engine, you can only reduce the diameter of the power cylinder.
'" I'm wondering if 1/16" would make a difference.'''
yes, it will change the air volume in the engine
with the power piston-power cylinder, displacer bearing-displacer rod must fit, but move lightly, reducing friction in other parts is also an important

 

[Cogsy]

I'm wondering if 1/16" would make a difference.

That 1/16" reduction in diameter reduces the volume by almost 1/3 of its current volume, at the sizes you're working with.

 

[Hopper]

You might take a look at the relative bores and strokes of power and displacer cylinders on established designs that are known to work. I sat down and did this once and came up with some rules of thumb about relative sizes but all I remember is that yes, displacer is much bigger than power piston. Also the length of the displacer and length of its stroke can be important too. Dr Senft's plans for the Moriya stirling fan is a good starting point to get an idea of proportions.

 

[minh-thanh]

Cogsy !

That 1/16" reduction in diameter reduces the volume by almost 1/3 of its current volume, at the sizes you're working with.

Thank you for the clear answer .
Hopper !

Also the length of the displacer and length of its stroke can be important too.

Exactly .

 

[Harry Mueller]

Thanks everyone for your comments, I obviously still have a lot to learn. But I’m willing. I’ll bore out an 1/8” cylinder insert out of brass and reduce the aluminum piston again. If that doesn’t work I can try a piston out of graphite instead of aluminum. I also wonder if the flywheel has enough mass to move the engine through the dead points.

I should really be working from a set of established plans but I’m sure this even this exercise will help me move toward my ultimate goal of completing an IC engine.

Harry

 

[minh-thanh]

I also wonder if the flywheel has enough mass to move the engine through the dead points.

Flywheel diameter? flywheel made of? How much thick flywheel ?

 

[Harry Mueller]

The flywheel is made out of aluminum and is .25” x 2.5”.

 

[minh-thanh]

My engine is the similar, diameter 75 mm , thick 6mm , aluminum flywheel ( made of brass will be better )

 

[Harry Mueller]

Your diameter is 1/2” larger but fairly close. I can’t get 2.5” diameter brass rod where I live.

I just looked at the Senft engine and noticed the ratio of displacer dia/ power piston dia is almost identical to what mine is now. However, with those bearings and the vertical placement of the cylinders I’d assume the friction is significantly lower than that of my engine.

All very interesting!

Harry

 

[minh-thanh]

If I remember correctly, I tried with same your flywheel and it was hard to run, of course maybe you would do better ..., but I think you maybe make aluminum flywheel but bigger diameter or Thicker (8 or 10 mm) because you won't mind with it, right?
With little engine experience (stirling, flame and IC engine), I think bigger flywheels are always better, personal opinions.

 

[Harry Mueller]

First I’ll try an 8mm piston. If it still doesn’t run, I’ll try a bigger flywheel. I think I’ll have to buy more aluminum for a new flywheel so the piston is easier.

Harry

 

[Harry Mueller]

8mm piston with the same result. I don’t think the flywheel size is the problem because it revolves about 10 times with just a good flick. I also doubt if a graphite piston would help since plugging the air hole holds the piston in and letting go causes it to instantly drop out of the cylinder. Perfect fit, I would think.

Time to give it a rest and think about what might be required to fix this.

Thanks for the suggestions,
Harry