Author Topic: Coffee cup stirling engine question  (Read 3040 times)

Offline Allen Smithee

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Coffee cup stirling engine question
« on: December 16, 2016, 11:35:04 AM »
I'm looking at the Jan Ridders Coffee Cup Stirling Engine as a possible project for the Xmas break, and I have one question:

He has chosen a brass power cylinder with a power piston from either graphite or steel (I think the brass cylinder is as much for esthetic as technical reasons, because it looks pretty against the polished alloy of the upper plate).

Now I gather with these low-pressure heat engines (stirlings, flame-lickers etc) it's crucial to keep the friction as low as possible whilst minimising any pressure losses. To this end the cylinder probably needs to be well polished to size and the piston fit needs to be very close.

Is there any reason why I shouldn't try a PTFE piston, or would it just be too difficult to get it accurately to a diameter (as I guess I couldn't lap a PTFE piston!). Similarly - is the reason for not using an aluminium piston the same problem of beiung unsuitable for lapping, so it's difficult to get it a close enough fit in the cylinder?

AS
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Offline b.lindsey

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Re: Coffee cup stirling engine question
« Reply #1 on: December 16, 2016, 01:50:10 PM »
Allen, I have used aluminum, steel, and cast iron all with success. I do recommend using graphite for the piston however, due mainly to its self-lubricating properties.

Bill

Offline Vixen

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Re: Coffee cup stirling engine question
« Reply #2 on: December 16, 2016, 01:53:18 PM »
What is wrong with doing it the way the designer, Jan Ridders, has designed it?
He has a lot of experience with these engines and has chosen the materials wisely.
It is the journey that matters, not the destination

Offline Bluechip

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Re: Coffee cup stirling engine question
« Reply #3 on: December 16, 2016, 02:02:05 PM »
I've made a Stirling with a PTFE piston, no trouble machining to size and a good surface.

PTFE expands a LOT !!!! so it won't be to size for long .... Had to be quite loose when cold  :thinking:. Still ran though.

Suggest you make to spec. then maybe try a PTFE piston, agree with Bill and Vixen.

Dave


Offline Allen Smithee

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Re: Coffee cup stirling engine question
« Reply #4 on: December 16, 2016, 12:02:13 PM »
What is wrong with doing it the way the designer, Jan Ridders, has designed it?
He has a lot of experience with these engines and has chosen the materials wisely.

Essentially just that I'm trying to adapt his design to the materials I have to hand, plus adding a few personal touches. For instance my con-rods will be 1.5mm carbon fibre rods bonded into brass clevises because I don't have any 1mm brass sheet, and my mainshaft diameters will be different to suit ballraces I have in my spares box and wooden props I have by the small bucket-load. I'm also thinking of using 5/16"sq brass rod rather than flat disks for the throws of the crank area to make it look more "crankshafty" as I think the resilting motion would look more interesting.

I wasn't aware that personalising designs was a crime!

Allen, I have used aluminum, steel, and cast iron all with success. I do recommend using graphite for the piston however, due mainly to its self-lubricating properties.

If I had or could easily get any graphite rod I'd use it, but it's a pain to get hold of!

AS
Quidquid latine dictum sit altum sonatur

Offline Allen Smithee

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Re: Coffee cup stirling engine question
« Reply #5 on: December 16, 2016, 12:04:16 PM »
I've made a Stirling with a PTFE piston, no trouble machining to size and a good surface.

PTFE expands a LOT !!!! so it won't be to size for long .... Had to be quite loose when cold  :thinking:. Still ran though.


Thanks for that Dave - how "loose" could you get away with? I was thinking that as this is a coffee-cup engine the temps would never be particularly high.

AS
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Offline Bluechip

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Re: Coffee cup stirling engine question
« Reply #6 on: December 16, 2016, 02:26:21 PM »
From memory ( I scrapped the engine, melted the silver soldered hot-end cap off  :shrug: ) the power bore was 20mm ish in a brass cylinder and the cold clearance had to be about 0.006" and still it scored a bit IIRC .... Sorry for mixed dimensions .

The piston just fell through, no question whatever of putting a thumb over it and stopping the drop.

On a LTD engine you have so little poke at the best of times, I suggest it just won't get going ... just my feelings .. but as you say, there won't be a lot of heat so maybe you'll get away with it ..

I would do as Bill suggested. Make it with an alum. piston.
When ( if   :lolb: ) it's a goer then fool about with a PTFE piston. Just have the one variable to play with ...

Dave
« Last Edit: December 16, 2016, 02:30:51 PM by Bluechip »

Offline Vixen

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Re: Coffee cup stirling engine question
« Reply #7 on: December 16, 2016, 02:42:04 PM »
Not suggesting for one moment that it is a crime to personalize any engine. Just want you to avoid a disappointment.

Have you tried looking on e-bay? Graphite electrode rods are readily available and inexpensive But messy to machine.

http://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xgraphite+electrode+rod.TRS0&_nkw=graphite+electrode+rod&_sacat=0
It is the journey that matters, not the destination

Offline Allen Smithee

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Re: Coffee cup stirling engine question
« Reply #8 on: December 16, 2016, 02:11:03 PM »
Not suggesting for one moment that it is a crime to personalize any engine. Just want you to avoid a disappointment.

Have you tried looking on e-bay? Graphite electrode rods are readily available and inexpensive But messy to machine.

http://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xgraphite+electrode+rod.TRS0&_nkw=graphite+electrode+rod&_sacat=0

The Ridders design has a 13mm diameter piston - so all those 6.5, 8 and 10mm rods are too small... :'(

AS

Quidquid latine dictum sit altum sonatur

Offline Bluechip

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Re: Coffee cup stirling engine question
« Reply #9 on: December 16, 2016, 04:37:28 PM »

Thought everyone knew  ....  :thinking:

https://www.nogginend.com/product/Graphite-Round-Bar

Or is it too late to get it by Crimbo ??

Dave

Offline mklotz

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Re: Coffee cup stirling engine question
« Reply #10 on: December 16, 2016, 03:26:08 PM »
Another vote for a graphite piston here.  It's hard to believe how little power these engines have.  Keeping the mass of the moving parts as small as possible is important.  Graphite is low mass as well as being self-lubricating and dimensionally stable.
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Offline steam guy willy

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Re: Coffee cup stirling engine question
« Reply #11 on: December 16, 2016, 05:59:55 PM »
I tried to make one but never managed to get it to work .......it does make a nice ornament though  and is a "stationary"  engine to add to my immortality !!
stationary
ˈsteɪʃ(ə)n(ə)ri/Submit
adjective
not moving or not intended to be moved.
"a car collided with a stationary vehicle"
synonyms:   motionless, parked, halted, stopped, immobilized, immobile, unmoving, still, static, stock-still, at a standstill, at rest, not moving; More
antonyms:   moving
ASTRONOMY
(of a planet) having no apparent motion in longitude.
not changing in quantity or condition.
"a stationary population"
synonyms:   unchanging, unvarying, invariable, constant, consistent, uniform, unchanged, changeless, fixed, stable, steady, undeviating
"a stationary population"
Origin

late Middle English: from Latin stationarius (originally in the sense ‘belonging to a military station’), from statio(n- ) ‘standing’ (see station).
Translate stationary to
Use over time for: stationary

Translations, word origin, and more definitions

Offline Vixen

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Re: Coffee cup stirling engine question
« Reply #12 on: December 16, 2016, 06:23:22 PM »
You can always use a 'T lite' candle to give a bigger temperature differential than a cup of coffee.
It is the journey that matters, not the destination

Offline fumopuc

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Re: Coffee cup stirling engine question
« Reply #13 on: December 16, 2016, 07:34:53 PM »
Not suggesting for one moment that it is a crime to personalize any engine. Just want you to avoid a disappointment.

Have you tried looking on e-bay? Graphite electrode rods are readily available and inexpensive But messy to machine.

http://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xgraphite+electrode+rod.TRS0&_nkw=graphite+electrode+rod&_sacat=0

The Ridders design has a 13mm diameter piston - so all those 6.5, 8 and 10mm rods are too small... :'(

AS

I know, the wrong language but may be the right Material.
http://www.bengs-modellbau.de/material/halbzeuge/graphit-rundmaterial
Kind Regards
Achim

Offline yogi

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Re: Coffee cup stirling engine question
« Reply #14 on: December 16, 2016, 10:41:37 PM »
Allen, I have successfully used aluminium piston in a steel cylinder. I would say the material you choose, is not as important as the fit. You have it right when you say: "it's crucial to keep the friction as low as possible whilst minimizing any pressure losses". Friction is your enemy.

We hope to see your progress of the build...  ;)