Power piston Temparture

[gwill67]

Hi all, first post on this forum. I am fascinated with Stirling engines.
I recently completed the 10" Moriya Fan MK1, my first Stirling engine and it runs really well especially now I have replaced the displacer with a very thin walled stainless steel one. The main change I made to James Senft's design was to add a P.T.F.E spacer between the cold cylinder head and the intermediate mounting plate in an effort to try and decrease the heat flow into it.
I understand the the principle of keeping the cold end and hot end insulated from each other. I have gathered a collection of books now but the one area that I don't really understand is what is the ideal temperature of the power piston and cylinder.
My instinct is that the PP should be hot so that the air is not contracting as it enters the PP and so giving up its heat before doing any work. But some designs having cooling fins on the PC and some physically link the cold cylinder and PP.
With the Moriya design there is thick insulation between the hot end and the intermediate mounting plate which also contains the transfer port and I have a thin piece of insulation between that plate and the PP.
It takes quite a long time to heat but runs really well (very encouraging). But I am thinking it can be improved by allowing a good heat path between the hot cap and the power cylinder with the only part being really well thermally isolated being the cold cylinder.
I think the long warming time is due to the time it takes for the power cylinder to come up to temperature as it is mainly only warmed by the hot air entering via the port, a lot of the heat conduction having been eliminated with thermal insulating gasket material. Therefore a lot of that hot air is "condensing" as it enters the cylinder.
Runs great once warm but is this loosing power unnecessarily?
Rather a long winded way of asking "what is the ideal power cylinder temperature?"
Graham W

[VincentG]

Rather a long winded way of asking "what is the ideal power cylinder temperature?"

The short answer is, as high a temperature as mechanically possible but not higher than half way between the cold end and the hot end temperature during operation(in Kelvin), IMO.

Long answer is the power piston and transfer port should have no effective temperature and be perfect insulators, not removing or adding any heat to the working fluid. The reality is that is not possible, so you have to make a design choice. The transfer port should be made as small as possible, and you would be surprised how small you can get away with.

The biggest bottleneck is the volume of cold sink space that does not go to zero when the displacer is exposing the hot cap. As you suggested, the gas is cooled by these areas before it can expand into the power piston, where it is inevitably cooled even further. The Essex hot air engine does this well, and in conjunction with a hot end connected power piston, has almost no warm up time at all. If you can modify the displacer position or size to bring the cold space volume down as much as possible(including clearance volume around the displacer), you will see much quicker "warm up" times and much better performance. This will also serve to keep the cold end colder by not allowing heat to short circuit through the gas and into the cold end. I put warm up in quotes because an exceptionally designed engine only needs to warm the hot end and not the cold end.

[Tom Booth]

Hi all, first post on this forum. I am fascinated with Stirling engines.
I recently completed the 10" Moriya Fan MK1, my first Stirling engine and it runs really well ...

I'm curious, what you would calculate the cost, in material, hours etc.

Would you consider it a viable potential business to build and sell such fans?

[gwill67]

Vincent, thank you for such a thorough answer, a lot of interesting things there. Regards your comment about transfer port size. Is there a rule of thumb for arriving at a suitable size port? The Moriya is about 5.5mm diameter which seems quite large (it's also quite long).

[gwill67]

Hi all, first post on this forum. I am fascinated with Stirling engines.
I recently completed the 10" Moriya Fan MK1, my first Stirling engine and it runs really well ...

I'm curious, what you would calculate the cost, in material, hours etc.

Would you consider it a viable potential business to build and sell such fans?

"I'm curious, what you would calculate the cost, in material, hours etc.

Would you consider it a viable potential business to build and sell such fans?"

That's an interesting question Tom. I'm in the uk and I don't believe it would be a viable business. I have no idea of cost. Material cost would be quite low but labour cost would be quite high unless good production techniques employed. I certainly couldn't entertain it as I'm retired and my output is now pretty slow especially on my manual machines.
Obviously in cheaper labour countries may be a different story. I believe there might be a replica of the old Ky-Ko fan made originally in the UK in about the 1920's being made in India. And of course there is the question of selling price.....it would be a pretty expensive fan so comes down to how much would people pay for the novelty value. I'm not sure if this properly answers your question?

[VincentG]

Vincent, thank you for such a thorough answer, a lot of interesting things there. Regards your comment about transfer port size. Is there a rule of thumb for arriving at a suitable size port? The Moriya is about 5.5mm diameter which seems quite large (it's also quite long).

https://www.gregraven.org/hotwater/calc ... izing.html

I hope it is helpful. No rule of thumb that I know of. I like to use this calculator, enter your bore and stroke along with the rpm it runs at now. For a Stirling engine, there is far less mass flow than ICE, so you can go even smaller. Reduce the port size with epoxy, start small and go bigger if it runs worse.

I have had a little high temperature model engine running 2000rpm with a .0625" port. I'm confident it could have been even smaller. Length of course plays a role.

[gwill67]

Vincent, thank you for info. That's a useful little calculator :-) I just put in guess of 400 rpm until I can measure it and it came out at 4.9mm so pretty close to what it is (5.5mm). I guess with a 10" fan this is quite a slow running engine.

[Tom Booth]

[...I'm not sure if this properly answers your question?

I was just wondering, since you have the experience in successfully building one, just hypothetically, what would you sell it for, or charge to build another one, or what would you be forced to charge, in order to be able to make it worth the while.

I'm not making an offer, but I am definitely going into the business, viable or not.

I have, relatively speaking, minimal machine shop experience. I'm really thinking seriously about bringing down the cost and complexity by making the bulk of the engines from ceramic. I have quite a bit of experience in plaster casting ceramics and thermodynamically speaking, I think it is ideal, as it relates to your question, what VincentG said:

Long answer is the power piston and transfer port should have no effective temperature and be perfect insulators, not removing or adding any heat to the working fluid.

I think ceramics can come as close as just about anything to achieving that and also withstand very high temperatures.

And dirt from my back lot is relatively inexpensive.

[VincentG]

I think ceramics can come as close as just about anything to achieving that and also withstand very high temperatures.

I agree, if they can be made durable and to reasonable tolerances I'd like to give it a try.

[gwill67]

I was just wondering, since you have the experience in successfully building one, just hypothetically, what would you sell it for, or charge to build another one, or what would you be forced to charge, in order to be able to make it worth the while.

OK, thanks for clarifying. It Depends. No simple answer to this. For example wether I am just making a bit of pocket money to subsidise my small pension or I need to make a living, wether I am making a one off or small batches etc, etc.

I'm not making an offer, but I am definitely going into the business, viable or not.

I have, relatively speaking, minimal machine shop experience. I'm really thinking seriously about bringing down the cost and complexity by making the bulk of the engines from ceramic. I have quite a bit of experience in plaster casting ceramics and thermodynamically speaking, I think it is ideal, as it relates to your question, what VincentG said:

Long answer is the power piston and transfer port should have no effective temperature and be perfect insulators, not removing or adding any heat to the working fluid.

I think ceramics can come as close as just about anything to achieving that and also withstand very high temperatures.

And dirt from my back lot is relatively inexpensive.
[/quote]
Interesting idea. Not sure I would class ceramic as a good insulator though - small thermal expansion but good conductor (min 80 W/mk??) Totally agree with ideas to bring cost down.
I see a lot of small novelty Stirling and Hit & Miss engines etc are made in China now and seem pretty good and cheap. Makes it very difficult for a small enterprise in Europe or USA unless people willing to pay for home grown, handmade.

[gwill67]

I need some help with quoting from previous posts I think.

[gwill67]

Tom, I have just spotted your topic for using ceramics for Stirling engines, I will read with interest...

[Fool]

gwill67, welcome to the forum. Building and getting running a 10" Moriya Fan MK1, is quite and accomplishment.

I think VincentG nailed with the idea everything has its compromises. For a power piston and cylinder and a separate displacer and cylinder one must understand that for the power/forward stroke, the gas should be as hot as possible, in all spaces. During the compression/reverse stroke the gas should be as cold as possible in all spaces. It will inevitably be a compromise from this.

All gas not being actively heated doing the expansion, or cooled during the compression, doesn't help as much as it could to total power output for a full cycle. It is called dead space. There is a theory that the Essex is 'better' because it has a hot power piston and cylinder space. I do not know.

MattBrown makes that claim and backs it with theory. He is above me in theory. VincentG is above me in experimentation. And Tom is above me in creativity and experimentation. You are above me in accomplishment. I try to supply some classical engineering to the site, hoping it will clarify what classical theory is, and help.

If you are having problems with the forum tools, I found it helpful to click on the " < quotes symbol in the upper right corner of each person's post. Do it for features you want to know. Adding photos is done with the file attachment feature in the editor. It's a little tricky to figure out. The preview option lets you experiment without making it become a post. The edit feature only is available for a short time.

The features are sensitive to forward verses back slash '/ \', and meticulous coding. That can cause it to skip and do what happened to you. Also when trying a new feature practice with short messages. The code is surrounded by [codde] words intended [\codde].

Had to spell code wrong because code is a code. LOL

[gwill67]

Thanks Fool (that sounds wrong!) for the advice and thank you for the introduction to who does what.
I am currently making a 1/4 scale Hit & Miss engine (Jerry Howell Farm Boy) and after that or when I fancy a change I am going to make the Moriya Major fan which is half as big again spinning a 15" diameter fan.
Very interesting about the Essex engine having hot power piston and cylinder space. Not an engine I know much about (although I have seen one running). My memory might be getting muddled but isn't that the engine that's a Beta design but the push rod to move the displacer goes along the outside rather than through the power piston? I believe Myford boy on Youtube may have made one. As I say my memory is terrible
...
When I get more used to how this forum works I will try and post a picture or video? (how do I do that?) of Moriya running if interested.

[Fool]

There is an Essex thread here, not too long ago. You have it correct, as far as I remember.