Practical Stirling engine design

[Tom Booth]

After thinking about it for a long long time, off and on for years now, if I had a practical yet versatile Stirling engine for general use, something I could carry around to use on various available heat sources, what would such an engine look like?

I've kind of settled on "something like" Tubalcain's "over & under" engine:

https://youtu.be/kR6pBZLGyNs

What I like about it:

A basic proven design.

Having the displacer and power cylinders over and under each other gives it a narrow footprint which could be important for many circumstances. It also provides balance and would make it easy to carry in one hand, if it had a handle on top like a suitcase.

The raised long displacer cylinder "nose" would make it versatile, It could be put over a campfire, stuck into a fireplace or the back or side of a rocket stove etc.

As far as his use of an aluminum displacer, I think that is a no no. Bad material choice, but this is more about general design considerations rather than specific thermodynamic considerations or efficiency issues.

Anyone have any other ideas what would constitute a really practical Stirling engine for general use?

Not really sure about the scotch yoke, though it does make for a more compact package.

[spinningmagnets]

There is a discussion in this forum about the Essex style engine. I thought a valuable tidbit was when someone mentioned that the odd Essex configuration drew the power-piston hot-air from the hot end of the engine, which sounded intriguing, and most models on youtube draw the air from the cool end. Perhaps this is for construction simplicity, or to make the shaft seal easier to deal with, since a hot seal is difficult to get it to seal well using common materials.

This hasn't been addressed (as far as I know) except by the "diaphragm" designs that do not use a piston inside a cylinder. The vast majority of desktop Stirlings are made to create a fun toy, or to sell kits for a profit. By this I mean, I've only seen a few that were hooked up to a small generator for charging a phone, or some other small device. Efficiency and power were un-necessary.

If the hot-end of a displacer is inserted into a camp-fire, we face a lot of bad options if we don't want to draw power from the cold end. The problem I am addressing is that when the displacer forces the working air into the hot end, the expanding heated air typically passes through the cool end to get to the power cylinder. This means a small amount of the potential power is immediately lost just before it reaches the power cylinder.

Its not a lot of loss, and clearly the many youtube desktop Stirlings do still "spin". But...if 10% of the power is being lost this way, it may be worth investigating. I am now convinced that it would be useful for the power cylinder to draw off the expanding hot air from the center of the displacement cylinder, and doing so would allow the hot tip to remain uncluttered for heat harvesting.

xxxxxxxxxxx

If camping, the water-jacket around the cold-end of the displacer would not be cumbersome to keep filled with water. The heating of the water would not be a bad thing either. That being said, many of the common Gamma Stirlings have a a few aluminum fins around the cool end. The aluminum quickly absorbs any heat, and the fins allow heat to rise off of them.

I've often felt that if you want to cheaply and reliably increase air-flow across those find, then the sides of the fins should be encapsulated, with an opening on the bottom and top. This would focus the air-stream and reduce turbulence, and would increase the flow. The improvement would likely be very minor, but it is simple and reliable, although...adding a some kind of belt-driven air-fan nearby would not be too complex either.

Another idea is to enshroud the hot end, and provide it with a "chimney". Then, enshroud the cold end with an exhaust that forms a concentric chimney, which means the rising column of hot air in the center would pull ambient air through the cold fins with no air-fan needed. Just a thought...

A

StirlingShroudedDisplacer1.png (4.36 KiB) Viewed 1493 times

[staska]

Problem with any bigger heater around displacer is heat itself. It will deform it a lot due to different temperatures at atband bottom.

[matt brown]

There is a discussion in this forum about the Essex style engine. I thought a valuable tidbit was when someone mentioned that the odd Essex configuration drew the power-piston hot-air from the hot end of the engine, which sounded intriguing, and most models on youtube draw the air from the cool end. Perhaps this is for construction simplicity, or to make the shaft seal easier to deal with, since a hot seal is difficult to get it to seal well using common materials.

Both gamma and beta can be either hot or cold PP, but both are usually cold PP. There's pros and cons to hot vs cold PP and an 'order of merit' usually is led by Tmax of cycle where sealing and lube issues favor cold PP. Meanwhile, hot PP has some major advantages with more power per gas volume and more efficiency due to a regen nuance. I posted this graphic a while back on another thread and shown as a gamma whereas the Essex is a beta (as Vincent pointed out).

http://www.stirlingengineforum.com/view ... 532#p21532

[Tom Booth]

Anticipating moving beyond efficiency and/thermodynamic issues and we have a good, compact Stirling engine with some reasonable or useful power output, what might it actually be used for, and how might it be necessary to modify it or make it modifiable for various practical uses?

[matt brown]

I'd definitely stay with cold PP since this solves most of the secondary nasties (sealing, lube, etc) and allows repurposing various parts. Even if hot PP has far greater output per given size (~2x within typical DIY values) and reduces regen loss by same as if magic, the wide range of design/build options favors a cold PP gamma, and it's more budget friendly.

The major downside to cold PP gamma is cold space dead volume, but this might be over rated due to Pmax at TDC and low torque. However, both of these shortcomings are due more to typical piston/slider-crank setup than the cycle itself. Like ICE, the piston dominates due to side seal and end seal are the same (like a bullet in a barrel). So, a good solution would most likely nix common piston stuff and solve inherent phasing and torque issues via a novel mech. I'm hanging in with conventional piston stuff, but easily admit that this is extremely limiting.

[Tom Booth]

...
The problem I am addressing is that when the displacer forces the working air into the hot end, the expanding heated air typically passes through the cool end to get to the power cylinder. This means a small amount of the potential power is immediately lost just before it reaches the power cylinder.

Its not a lot of loss, and clearly the many youtube desktop Stirlings do still "spin". But...if 10% of the power is being lost this way, it may be worth investigating.

I am now convinced that it would be useful for the power cylinder to draw off the expanding hot air from the center of the displacement cylinder, and doing so would allow the hot tip to remain uncluttered for heat

Generally speaking, I recall many old timers on the forum years ago claiming basically that pressure is uniform even if temperature isn't, so it's debatable if this is actually an issue.

I'm on the fence myself, but definitely something worth investigating. One of those things that would need to be resolved through experiment, have an engine where the transfer tube can be attached various different locations and see exactly what difference if any that makes. You could be right.


Once that and other similar issues are resolved, what would you actually use a Stirling engine for?

[staska]

I'd definitely stay with cold PP since this solves most of the secondary nasties (sealing, lube, etc) and allows repurposing various parts. Even if hot PP has far greater output per given size (~2x within typical DIY values) and reduces regen loss by same as if magic, the wide range of design/build options favors a cold PP gamma, and it's more budget friendly.

The major downside to cold PP gamma is cold space dead volume, but this might be over rated due to Pmax at TDC and low torque. However, both of these shortcomings are due more to typical piston/slider-crank setup than the cycle itself. Like ICE, the piston dominates due to side seal and end seal are the same (like a bullet in a barrel). So, a good solution would most likely nix common piston stuff and solve inherent phasing and torque issues via a novel mech. I'm hanging in with conventional piston stuff, but easily admit that this is extremely limiting.

Or if take this one differently - one will build engine twice as big for cold PP gamma, which will have twice as many friction. Which will rob a lot of power.

Essex stirling is a way to go for non pressurised engine with at least some power. Make displacer rod thicker as in ringbom stirlings - say 1/4 or even 1/3 of diameter, it will propel itself, and use long conrod only as a brake.

Where cold pp gamma need 0.5 MPa or more pressure to have power at higher order of internal friction. Twin acting pressurised gamma.