Need help with my Stirling engine fan

Discussion on Stirling or "hot air" engines (all types)
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StirlingMoss
Posts: 4
Joined: Mon Jul 04, 2016 4:42 am

Need help with my Stirling engine fan

Post by StirlingMoss »

Hi Guys

New here, struggling with my Stirling engine, came across this forum while trying to trouble shoot my engine, seems this a great place for all things Stirling!

My engine does not start when i wind it to start the motion, i thought it was a air leak so sealed it tighter and now it air tight 99%. i re-machined a new power piston (due to air leakage) the problem i have now is that when i try to turn it over there is resistance from the air tight chamber. So when heat added the power piston will rise and stay risen.

From what i understand a bit air leakage is a good thing to equalise the pressure but when i tried it the first time (with leaks) no joy. At present scratching my head, hope you guys can help.

When i heat the base plate to 170C half way up the chamber temp is 74C top plate is 53C is this enough temperature change? Should i extended the chamber in length is there a equation i a missing on volume?

Is there any tests or photos ect i can do to trouble shoot this?
Ian S C
Posts: 2218
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Need help with my Stirling engine fan

Post by Ian S C »

Start with the power piston. The piston should slide down the open cylinder under it's own weight, and when the end is blocked it should come to almost a full stop. The metal combination used for the piston and cylinder can effect the performance, what are you using?
The only place you will have a small but constant leak is at the gland on the displacer rod, there should be virtually non from the power cylinder.
Is your motor timed correctly, the displacer near enough to 90* ahead of the power piston.
What are you using to heat the motor?
Ian S C
StirlingMoss
Posts: 4
Joined: Mon Jul 04, 2016 4:42 am

Re: Need help with my Stirling engine fan

Post by StirlingMoss »

Hi Ian

Thanks for helping.

I made a lightweight brass piston in a glass tube that does as you say takes 3 seconds to slide down and hardly any drop when the end stopped.
I have a slight leak at the brass to stainless rod on the gland on the displacer rod ( i can adjust it to be even tighter)
Timing i have just off 90 degrees and tried many minor adjustments.
Heat source is a camp stove hob, runs on gas

I am not sure on the friction i take the mechanism off that powers the fan but due to it being air tight pushing the power piston i am getting resistance due to it being airtight and will slowly push down and pull up, is this too much f a airtight chamber? Is it about right?

I don't know really where to troubleshoot it and where to start should i lengthen the chamber?
Ian S C
Posts: 2218
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Need help with my Stirling engine fan

Post by Ian S C »

Is the glass cylinder ground, and polished, so that it is perfectly round, and parallel, as in a glass syringe, or door closer? The ideal piston in these cylinders is made of Graphite. A plain drawn glass tube is rarely very accurate.
Could you describe the displacer?
Ian S C
StirlingMoss
Posts: 4
Joined: Mon Jul 04, 2016 4:42 am

Re: Need help with my Stirling engine fan

Post by StirlingMoss »

Hi Ian

I put some images up to help us (picture is a thousand words they say) as you can see i have based my design of proven designs, why i am scratching my head why i had no joy.

The plate is now warped and i will have to make a new one, i have bolted it down as i was convinced it was air loss that was the issue (hence the bolts and lots of them :D)

It is a ground tube yes from a broken fan i bought at a car boot. no graphite in the power piston though. The can is aluminium pot with 6mm drill holes in the top and bottom attached to a stainless rod to the top and have put stainless swarf shaving inside it, it is light weight. Can take a image soon i take the 50 bolts out if needed :big smile: .

When i push down on the piston there is pressure due to being air tight and slowly escaping via the displacer hole, is this right that it hard to push down and slowly goes down or too airtight?

Where should i start to trouble shoot first and checks i can do to eliminate certain aspects you think?

Thanks in advance to a reply
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Ian S C
Posts: 2218
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Need help with my Stirling engine fan

Post by Ian S C »

It might just be your displacer. If you could it would be a good idea to make it from stainless steel, a thin walled tube, or better still a cup shape, the cold side can be sealed with an aluminium disk that can be fitted in with high temperature, shaft lock Loctite, this disc is where the displacer rod is fitted, drill and tap it to suit. The displacer should be air tight.
You would be better off with much lighter bolts holding the hot cylinder together, 5 mm / 3/16" is ample big enough, and get the holes top and bottom in line.
Is there bearings at each end of the crankshaft? And what sort of bearings are you using, also what weight and diameter is the flywheel/fan?
StirlingMoss
Posts: 4
Joined: Mon Jul 04, 2016 4:42 am

Re: Need help with my Stirling engine fan

Post by StirlingMoss »

Hi Ian

The displacer chamber is stainless steel, 1mm wall thick the cup shape you mention that is a slight cut dish in the end the hot chamber tube so when tighting down you use a aluminium washer to squash and seal the chamber better? have i got the wrong end of the stick?

I am using bearings from the car boot fan and the mechanism and the blade.So if it worked on a Stirling fan presume should be ok weight wise and mechanism wise.

I think you`re right about the bolts being to big and transferring a lot heat to the cold-side plate.
I will re-machine a new bottom plate and fit smaller bolts to stop the heat transfer.
Ian S C
Posts: 2218
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Need help with my Stirling engine fan

Post by Ian S C »

To make the displacer it's self, you really need another stainless cup that will fit inside the chamber.
Keep the bolt circle in close to the chamber. The bearings can be fairly cr8p, I used the used bearings from a skate board wheel on one of my fan motors, they wobbled that much I was scared that the balls would fall out. Make the base plate from steel plate, aluminium will bend with only a mild amount of heat. Ian S CTest 039 (640x480).jpg
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Ian S C
Posts: 2218
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Need help with my Stirling engine fan

Post by Ian S C »

Here is a close up of the working end of the stove top fan, the main bearings and the big end of the con rod on the power piston are 8 mm bore ball races, the bearing for the displacer big end is bronze. Ian S C
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