Is a high power unpressurised stirling engine possible

[dean48]

Is it possible for a stirling engine to be built that is relatively compact say around the size of the 65cc stirling engine made by Andy Ross but give more than 1 watt per cc for an unpressurised engine. It would be very impressive if It's possible to create a stirling engine that gives 4+ watts/cc unpressurised so that an engine this size could give say 300+ watts at atmospheric pressure. In the article making stirling engines andy ross seems to hint that it may be possible to get lots out power (10 watts/cc) at atmospheric pressure by increasing the compression ratio and lowering the phase angle could possibly work but I am curious if anyone has tryed to build an engine like this or has any ideas of how this could work.

[Tom Booth]

I've had a number of bright ideas, none actually proven, for increasing the power of a Stirling engine. That is, for many who have come to this forum, the Holy Grail.

There are several ideas I woul like to try, but time and resources being limited, don't know if any would work.

I've been an engine mechanic (internal combustion) since graduating high school, and since learning about Stirling engines, the question on my mind has always been the same. How could a Stirling put out as much horsepower as an IC of comparable size? Or why can't it?

I think it could.

The main problem IMO is IC engines have a very well timed, sudden, explosive release of heat. Stirling's pretty much all introduce heat gradually or steadily and there is always, as far as I'm aware, a mixture of heating and cooling going on simultaneously.

So in my mind, improvement could be made to a Stirling by finding a way to introduce heat at just the right time with a BANG! or sudden blast, rather than the rather lackadaisical way a Stirling generally shunts hot air back and forth with a lot of mixing and no real well defined moment when a real blast of heat is introduced.

[Tom Booth]

Another problem is dead air space.

The displacer requires extra room to move back and forth from the hot side or end to the cold and back. An Internal combustion engine has no such need for a displacer in the head of the cylinder, so the piston can move all the way up into the cylinder to Top Dead Center with just enough room left for clearance, so nearly all the air (air/gas mixture) in the cylinder can be compressed into a very small space. The more the air can be compressed, the greater and more powerful the subsequent expansion when heat is delivered by igniting the fuel mixture, or, in a Stirling, when the displacer moves air to the hot heat exchanger or hot side.

So, I personally think such problems are not insurmountable, but there are some mechanical and material issues. In the end I think some other heat delivery system is needed other than a displacer, but then this heat engine would probably no longer be considered a Stirling.

[Sockmonkey]

AFIK the only way to get that sudden burst of heat is with internal combustion.
Coming at the problem from the other direction and giving the air time to heat up before getting to the cylinder seems the only way to solve it.

Granted, the engine has to "breathe" external air now, but the tubing can be as long as needed to make sure the air gets as hot as possible.

[Tom Booth]

I don't believe heating the air like that would solve the problem. I would think the air would have to be heated inside the cylinder to produce any power.

The temperature of the air entering the cylinder is largely irrelevant, it is how much it expands inside the cylinder that determines the power. Preheating the air would, I think be really counter productive.

But I'm open to correction or explanation. How could such a thing possibly work? I don't see how it could.

Aside from that, I think there are a number of ways to, possibly, produce a sudden burst of heat into a sealed, valveless, Stirling type hot air engine.

It is a rather difficult problem, but certainly improvements to the standard Stirling technology are possible.

One would be to decouple the displacer from the piston/crank and have it's movement controlled by a cam.

Attached directly to the crank, the displacer can only move sinusoidally following the relatively slow up and down motion of the crank.

Controlled by a cam or other independent mechanism, the movement of the displacer, allowing the introduction of heat could be made more sudden, and possibly be modified as the engine is running for maximum power and efficiency.

The ignition timing of an IC engine in a car, for example, is advanced and constantly adjusted as the engine accelerates, it does not just lazily follow the revolving crankshaft.

But, there is still the problem of dead air space. A displacer requires dead air space to move through, but a displacer is not the only way to introduce heat.

The surface area of any heat exchanger is another factor.

Given the limited space in a cylinder head, how can the surface to air heat exchange be maximized?

Cutting groves in the heat exchange surface is one possibility.

unnamed_crop_95_resize_22.jpg (184.22 KiB) Viewed 7340 times

This does not introduce additional dead air space if the groves in the heat exchanger are matched to fit similar groves in the displacer.

[dean48]

Wow these are awesome ideas, I think the cam idea might be worth trying because although the engine design would be more complex It wouldn't require an air compressor or pump and the engine could be a small size yet produce a lot of power. I do plan on testing various designs of the stirling engine in the future, I'll probably use a service like xometry or emachineshop for parts since I don't have a machine shop but it will be expensive so I'll have to come up with a good design first.

[Tom Booth]

AFIK the only way to get that sudden burst of heat is with internal combustion.
Coming at the problem from the other direction and giving the air time to heat up before getting to the cylinder seems the only way to solve it.

Granted, the engine has to "breathe" external air now, but the tubing can be as long as needed to make sure the air gets as hot as possible.

I'm now having second thoughts. Now that I've looked at the Ericsson with the Huge regenerator.

viewtopic.php?f=1&t=2669

The design of the Ericsson appears remarkably similar to the one you posted above.

[Tom Booth]

..the cam idea might be worth trying because although the engine design would be more complex It wouldn't require an air compressor or pump and the engine could be a small size yet produce a lot of power....

There is this Solar flat plate Stirling that uses a cam.

This cam, though the text and video explain the theoretical advantage of using a cam, the cam design on this engine still appears rather slow and not far from sinusoidal. I would have given it a much sharper incline to raise and lower the displacer more suddenly.

https://contest.techbriefs.com/2016/ent ... ogies/6615

https://youtu.be/8QE-CmKxz40

There is another kind of linkage that can accomplish much the same result:

https://youtu.be/9Ckd7N3MxUU

I mean the displacer actuator rod with the two adjustable bumpers.

Screenshot_20200616-235918_crop_33_resize_40.jpg (32.94 KiB) Viewed 7314 times

For a small solar Stirling, this engine appears to run fast and strong with such a mechanism, though, as far as I can tell, it is likely overcast, and the plate being vertical, it could hardly be receiving direct sunlight, even if it wasn't cloudy. It results in the displacer snapping from one side to the other quite rapidly and suddenly with some "dwell" or rest period between, more effectively than the cam in the previous video, though I think a cam could work just as well if given a sharper incline.

I think this could work with an LTD Stirling with a regular vertical displacer rod. I've been meaning to give that a try sometime. Easier to implement than a cam I think.

LTD engines using a magnet to actuate the displacer seem to produce a similar action.

I like this engine because it is so small yet runs so we'll. It uses a small magnet to actuate the displacer. There are many similar magnetic Stirlings these days. Unfortunately this little engine seems to be no longer available.

https://youtu.be/czVQVCIi7CU

[Tom Booth]

There is some nice detail about the Flachplatten-Stirlingmotor at this Wikipedia page:

https://de.m.wikipedia.org/wiki/Flachpl ... rlingmotor

Kolin-Engine-Animation-100ms.gif (105.97 KiB) Viewed 7307 times

[Tom Booth]

IMO, or I might be safer saying that after thinking on it for a while, I have an idea that, or thought, tentatively held in mind, awaiting experimental conformation (or refutation as the case may be)

An "unpressurized" Stirling, is perhaps inherently or potentially more powerful than a sealed pressurized design because, a pressurized engine has to work against the artificial high pressure on BOTH sides of the piston. Or I could be wrong, but as far as I know, this is how the engines are pressurized. The whole engine is sealed and pressurized, so there is no exposure anywhere to the open atmosphere. Doesn't the "outside" high pressure work against the piston during heating/expansion?

Anyway, COMPRESSION is pressurization, but only on the inside of the cylinder. So compression, where the piston travels inward, is pressurization in a sense. But compression does not have the drawback that there is higher pressure for the piston to work against during expansion. Perhaps it balances out, so overall, pressurization produces an advantage, I don't know, but I suspect that the real advantage of a sealed engine is that use of purified exotic gasses like helium, with a higher expansion per unit of heat ratio becomes possible.

At any rate, I do think, or agree, that increasing the compression ratio should produce more power in an unpressurized engine. With all the required dead air space in a Stirling to acomodate displacer movement though, high compression is impossible.

Well, not impossible, but it might take a radical redesign of the engine.

Another disadvantage of a compact pressurized engine is the reduction in surface area.

An IC engine doesn't depend on surface area for introducing heat, as the fuel is burned internally.

I've been thinking of a way to eliminate dead air space, increase compression, reduce size, and still have an enormous amount of surface area for heat exchange, by eliminating the displacer and replacing it with a kind of accordion heat exchanger.

During reheating the heat exchanger collapses down. It may be rings like one of those telescoping drinking glasses, or a stack of thin plates that nest together, or a honeycomb, like those paper seed planting trays.

While compressed, dead air space can be entirely eliminated, but when needed, surface to air contact area can be greatly expanded.

Telescoping teacup:

https://youtu.be/uQYJYJnoW4k

Paper-chain seed pots:

https://youtu.be/Lvaw3H7yoGQ

Telescoping ladder:

https://youtu.be/m8sDfFBBVuw

Honeycomb ball:

https://youtu.be/G-hUYBgvBSA

Something like that attached to the hot end inside the cylinder. Compact during compression. Expanded for heat delivery with lots of hot surface area.

During compression, dead air space can be entirely eliminated by collapsing the heat exchanger. Very compact, yet with many many times the surface area of a conventional Stirling.

[Sockmonkey]

I don't believe heating the air like that would solve the problem. I would think the air would have to be heated inside the cylinder to produce any power.

The temperature of the air entering the cylinder is largely irrelevant, it is how much it expands inside the cylinder that determines the power. Preheating the air would, I think be really counter productive.

But I'm open to correction or explanation. How could such a thing possibly work? I don't see how it could.

It's got valves. First piston sucks in air, valve A closes, valve B opens, piston pushes air into the tube, valve B closes, air is heated, heated air wants to expand, valve D opens, expanding air pushes the piston, valve D closes, valve C opens, piston expels spent air through exhaust tube E.

[Tom Booth]

Without knowing exactly what the valve cam arrangement is or how it's driven, but just going by your basic description, assuming the cycle continues in the same way, I would be afraid back pressure would prevent the engine running.

Wouldn't valve B (pumping air into heating tube) and D (receiving air from heating tube) both end up being open at the same time?

So an air pump, by appearances, pushes air through a tube into an air motor, which air motor drives the air pump.

Added heat in the tube would create additional pressure but the pressure would be acting in reverse in opposition to the air pump as much as forward to drive the air motor.

That is what it appears like to me at the moment, going only by what is shown.

[Sockmonkey]

Without knowing exactly what the valve cam arrangement is or how it's driven, but just going by your basic description, assuming the cycle continues in the same way, I would be afraid back pressure would prevent the engine running.

Wouldn't valve B (pumping air into heating tube) and D (receiving air from heating tube) both end up being open at the same time?

So an air pump, by appearances, pushes air through a tube into an air motor, which air motor drives the air pump.

Added heat in the tube would create additional pressure but the pressure would be acting in reverse in opposition to the air pump as much as forward to drive the air motor.

That is what it appears like to me at the moment, going only by what is shown.

No, valve D doesn't open until a bit after valve B has closed. The valves are specifically there to prevent back pressure problems. Air goes into the tube, is sealed off by the valves, heated, then the valve to the power piston opens.

[Tom Booth]

How, and/or why does air "go into the tube"?

The first piston, pump/compressor must force air into a hot tube. The air will be heated immediately as the air goes in while the valve is still open.

Aside from what is powering this pump/compressor, enabling it to do this, why would there not be considerable back pressure while the valve B is open and air "goes into the tube."?

[Sockmonkey]

How, and/or why does air "go into the tube"?

The first piston, pump/compressor must force air into a hot tube. The air will be heated immediately as the air goes in while the valve is still open.

Aside from what is powering this pump/compressor, enabling it to do this, why would there not be considerable back pressure while the valve B is open and air "goes into the tube."?

Valve A closes, valve B opens, piston goes up. The air is going to pick up some heat on the way in, but it's not instant. The whole point of the tube is to let the air hang around in the hot place long enough to get to it's maximum temperature. There's going to be a little back pressure, because that's a thing with all air-breathing engines, but not terrible. The way this operates is closer to an internal combustion engine than a stirling.