Moriya Stirling Engine Build

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rmac
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Moriya Stirling Engine Build

Post by rmac » Tue Feb 16, 2021 11:48 pm

I've been toying with the idea of trying to make a small model Stirling engine. It looks like the trickiest part is getting a (nearly) airtight fit between the power piston and its cylinder while at the same time keeping that same fit (nearly) free of friction.

It seems that there are a number of ways to go about this. In one video, our friend Mr. Pete bored his cylinder, then honed it with a brake hone, then turned the piston to fit.

I've seen where other guys polish the cylinder bore with a dedicated lap, then turn the piston to fit. The lapping tools they use tend to be copper cylinders of one description or another that are sized (or made adjustable) to fit the cylinder at hand. Apparently it can also work to use a wooden dowel for a lapping tool?

Then I saw another guy who used the cylinder and the piston to sort of mutually lap each other.

I'm having a hard time figuring out if these are all valid approaches to the same problem, or if maybe each one is suited to a particular set of circumstances that I haven't identified. It would be nice to at least start with an appropriate method, since I'll surely have enough difficulty with the actual execution.

For the record, I think I understand that "honing" uses abrasive stones to remove material, and that "lapping" involves abrasive powder that's supposed to sort of embed itself into the lapping tool.

Can anybody shed some light on all this? What would be the best way to start?

Thanks,

-- Russ

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NP317
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Re: Fitting Stirling Engine Cylinders and Pistons

Post by NP317 » Wed Feb 17, 2021 2:27 am

I oversaw the building of MANY Stirling fan projects by my Univ. of Washington students, during their Manufacturing Processes Class shop work.

My home shop wood stove has one sitting on the hot top and even today was spinning along helping move the heat into the shop.
The power cylinders are made from brass, and the power piston turned from mild steel. The cylinder was turned first, then the piston turned for a snug fit.
We honed them together, often using tooth paste as the mild abrasive! I'd tell the students to take the parts home and hone them together while watching TV, or whatever.
Today I would use TimeSaver compound, while turning the cylinder in the lathe.

The results worked every time. My stove fan piston and cylinder continued to "seat" with use, and today, a drop of 3-in-1 oil to the open top of the piston/cylinder gap provides the gas seal to work. No more wear, lots of use. Several 100 hours per year for the past 5 years!
Fun and useful stuff.
RussN
ShopHeatFan.jpg

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liveaboard
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Re: Fitting Stirling Engine Cylinders and Pistons

Post by liveaboard » Wed Feb 17, 2021 7:11 am

That's really cool heat!

Technical Ted
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Re: Fitting Stirling Engine Cylinders and Pistons

Post by Technical Ted » Wed Feb 17, 2021 7:25 am

NP317 wrote:
Wed Feb 17, 2021 2:27 am
I oversaw the building of MANY Stirling fan projects by my Univ. of Washington students, during their Manufacturing Processes Class shop work.

My home shop wood stove has one sitting on the hot top and even today was spinning along helping move the heat into the shop.
The power cylinders are made from brass, and the power piston turned from mild steel. The cylinder was turned first, then the piston turned for a snug fit.
We honed them together, often using tooth paste as the mild abrasive! I'd tell the students to take the parts home and hone them together while watching TV, or whatever.
Today I would use TimeSaver compound, while turning the cylinder in the lathe.

The results worked every time. My stove fan piston and cylinder continued to "seat" with use, and today, a drop of 3-in-1 oil to the open top of the piston/cylinder gap provides the gas seal to work. No more wear, lots of use. Several 100 hours per year for the past 5 years!
Fun and useful stuff.
RussN
ShopHeatFan.jpg
Can you please supply drawings for that fan engine? That would be great on my workshop wood stove!

Thanks,
Ted

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NP317
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Re: Fitting Stirling Engine Cylinders and Pistons

Post by NP317 » Wed Feb 17, 2021 11:55 am

The drawings for that Stirling Fan engine are in one of the first two "Steam and Stirling" books, available from Village Press:
https://secure.villagepress.com/store/i ... /group/124

I have those books but they are buried and boxed in my storage area. And the drawings are most likely owned by the publisher.
Hopefully someone else here can tell you exactly which book that fan is in. Totally worth owning the book.

The base that supports the fan on my stove top started as a big cutoff of 5" or 6" bronze shaft that I had laying around. I lathed it square, and bored the center hole for a close slip fit over the displacer cylinder. It provides support and heat-collecting volume for the fan. Works great.
RussN

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rmac
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Re: Fitting Stirling Engine Cylinders and Pistons

Post by rmac » Wed Feb 17, 2021 12:21 pm

The power cylinders are made from brass, and the power piston turned from mild steel. The cylinder was turned first, then the piston turned for a snug fit. We honed them together, often using tooth paste as the mild abrasive! I'd tell the students to take the parts home and hone them together while watching TV, or whatever. Today I would use TimeSaver compound, while turning the cylinder in the lathe.
Thanks, RussN. That's exactly the kind of info I'm looking for!

Ted:

As it happens I have the book containing the plans for the fan engine. It's the first of the "Steam and Stirling Engines You Can Build" series, edited by William C. Fitt, ISBN number 0-914104-06-03. Besides the link that RussN posted, both new and used copies are listed for sale on Amazon.

-- Russ

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rmac
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Re: Fitting Stirling Engine Cylinders and Pistons

Post by rmac » Wed Feb 17, 2021 6:46 pm

RussN:

I spent some time this afternoon studying the plans for the fan engine and I have a couple more questions if you don't mind ...

1. One of the parts is a 3/16" thick gasket between the cold and hot ends of the displacer cylinder. There aren't any drawings of this thing, but it's pretty obvious what it should look like. The problem is that the text says it should be made of "compressed asbestos", which I'm guessing is not available these days. Any hints on a suitable material for this part?

2. The plans show a 1/8" steel rod attached to the displacer piston running through a bronze gland with a 1/8" reamed hole in it. This seems like another place where you need a (nearly) airtight seal with a (nearly) frictionless fit. Do you remember doing anything special here to make that work?

Thanks again,

-- Russ

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NP317
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Re: Fitting Stirling Engine Cylinders and Pistons

Post by NP317 » Thu Feb 18, 2021 12:13 am

For the Stirling Fan manufacture:
1) We made the 3/16" "gasket" from Bakelite. It properly insulates the upper and lower cylinder parts from each other, and tolerates the heat without deformation.

2) We used 1/8" diameter drill rod to the displacer piston rod. The gland was simply brass (bronze is better) machined, drilled and reamed for a close sliding fit with the rod.
I also put a drop of light oil on those parts before starting the engine turning. It provides lube and gas leakage seal, along the long gland length.
The Stirling engine does work better with a tight fricton-free sliding fit (as you noted) between those two parts.
Be sure the rod is solidly attached to the displacer piston. Occasionally one would come loose which would start to "click", and then stop the engine when the piston hit the top or bottom of the displacer cylinder. A near-complete engine disassembly is required to re-attach the rod and piston. Not fun.

The key to operation of these Stirling engines is the temperature difference between the heated lower displacement cylinder, and the upper finned (cooler) displacement cylinder. The fan facilitates the cooling, so i's a pretty nifty design.

My students successfully built these as teams of 4-5 people, 10 to 15 fans per academic year. I predict you will have great success building one, following your machining experience and instincts. I hope you keep us informed of your progress!
RussN

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Re: Fitting Stirling Engine Cylinders and Pistons

Post by jscarmozza » Thu Feb 18, 2021 8:55 am

Everything Russ said...just make sure the rod is perfectly straight. It took me many hours of trouble shooting to find a very slight bend in the displacer rod was binding the engine.

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rmac
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Re: Fitting Stirling Engine Cylinders and Pistons

Post by rmac » Thu Feb 18, 2021 9:46 am

Thanks, gentlemen, for the extra hints. As for reporting progress, my first step (as always) will be to make a CAD model of the engine. Sometimes this seems like an extra step, but I've found that it often helps me 1) get my head around the project, 2) sometimes discover errors or omissions in the plans, 3) cook up a shopping list for materials, and 4) remain halfway proficient with the CAD program.

-- Russ

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Re: Fitting Stirling Engine Cylinders and Pistons

Post by NP317 » Thu Feb 18, 2021 10:05 pm

RussMac:
Whatever it take to absorb the project to get machining. Enjoy!
RussN

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Re: Fitting Stirling Engine Cylinders and Pistons

Post by rmac » Sun Feb 21, 2021 12:01 am

Progress report #1

I finished my 3D CAD model this afternoon. I didn't find any mistakes in the original drawings, so there's no need to make 2D prints from the model--I'll just use the original plans.

I'm planning one slight deviation from the original design. That's to use disks on the crankshaft instead of the narrow little crank pieces Dr. Senft calls for. I figure the disks will provide a bit of flywheel effect, and it'll be easier to get the holes in them square to the side surfaces.

-- Russell Mac (some friends and family actually call me this!)

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