Stirlingengineforum.com

matt brown wrote: ↑Thu May 18, 2023 7:00 pm ...
Note that if ANY displacer is truely a 'displacer' than the gas flows around the displacer. So, in my mind, the only way for gas to flow thru this regenerator is for the 'displacer' to act as a positive displacement piston...

The air is still flowing freely around the displacer, so the displacer is not acting as a piston compressing or expanding anything, it is just moving within a close fitting but open ended sleeve or tube, but there is usually still a slight clearance between the displacer and sleeve so there is no friction, other than some air drag perhaps. Most of the air is forced through the regenerator but it is still basically acting as a displacer, IMO.

Similar to this:


Except that the regenerator is integrated into the sleeve rather than isolated in a separate tube.

stephenz wrote: ↑Thu May 18, 2023 2:05 pm I am pretty sure this (those last 2 sketches) would only cause the crank to spin in the other direction.

The only thing you are changing here is instead of +90 deg phase, you now have -90 deg phase, hence while I think this will work, I think it will only work when you spin the flywheel in the other direction.

The only thing I've changed is the location of the port joining the power cylinder to the displacer cylinder. Nothing regarding the crank, direction of rotation of phase is any different.

But the expanding hot gas now has a direct path to the power piston rather than having to expand backwards, back through the regenerator and past the cooling jacket.

VincentG wrote: ↑Thu May 18, 2023 3:52 pm Tom I agree completely that the Nasa engine is the worst offender here. The power piston is ensuring that some hot air is drawn past the cold end at the worst time.

To me it makes the most sense to put the regenerator in the displacer and move the transfer port to dead center between the hot and cold sides. Note that this arrangement is easily achieved by the standard LTD layout, in addition to the (to me) all important benefit of completely shutting off each respective heat exchanger when the other is exposed. The importance of which I don't think can be overstated as shown by my videos demonstrating the relationship of displacer position to power piston travel.

It's ideal to think of a total system pressure change in motion but in reality I think the pressure(temperature) propagates outward from the heat exchanger similar to an internal combustion event.

Placing the regenerator in the displacer at least forces the majority of gas to flow through. In the standard location I agree that it's questionable as to whether there is really a flow of "air"or "energy".

As far as LTD, I've been thinking of trying something else (but similarly, the power cylinder associated with the hot rather than the cold side.

Actually, I might try a diaphragm instead of a rigid displacer. That would seal off the cold side altogether so the cold side would serve as an air spring. The displacer connection with the crank could be eliminated so it would work as a kind of diaphragm/Ringbom but with an air spring rather than a cold side.

There is of course clearance between the bottom of the power piston and the bottom hot plate so the working fluid can expand right off the hot plate up into the power cylinder directly.

Tom Booth wrote: ↑Thu May 18, 2023 8:26 pm The only thing I've changed is the location of the port joining the power cylinder to the displacer cylinder. Nothing regarding the crank, direction of rotation of phase is any different.

But the expanding hot gas now has a direct path to the power piston rather than having to expand backwards, back through the regenerator and past the cooling jacket.

I know you haven't changed anything, but I think that one change would make the crank spin the other direction.


edit: that sketch with the power piston going through the displacer of the LTD would also result in the flywheel spinning in the other direction I think.

stephenz wrote: ↑Thu May 18, 2023 9:01 pm

Tom Booth wrote: ↑Thu May 18, 2023 8:26 pm The only thing I've changed is the location of the port joining the power cylinder to the displacer cylinder. Nothing regarding the crank, direction of rotation of phase is any different.

But the expanding hot gas now has a direct path to the power piston rather than having to expand backwards, back through the regenerator and past the cooling jacket.

I know you haven't changed anything, but I think that one change would make the crank spin the other direction.


edit: that sketch with the power piston going through the displacer of the LTD would also result in the flywheel spinning in the other direction I think.

OK, you can think so, that's alright.

I don't, and I'm not sure why you do, but I guess we could see by trying it.

With the diaphragm, the hot expanding air is not wasted by expanding into the cold side at all, it expands the diaphragm like a balloon, the energy is stored in the "air spring" and returned as the piston executes the power stroke

The gas is not cooled by the cold top plate but rather by heat conversion to power output.

All the energy of expansion goes to the piston rather than being lost (or intentionally dumped as a result of misguided theory IMO) to the cold side.

Tom Booth wrote: ↑Thu May 18, 2023 9:13 pm
OK, you can think so, that's alright.

I don't, and I'm not sure why you do, but I guess we could see by trying it.

I'm not trying to be difficult but for some reason that's what my instinct tells me.

I understand why you are doing this, but the way I see it you are only making the heater the cooler, and the cooler the heater. But because you are still planning of applying heat to the system at the same surface, I think the only way the engine will be able to operate is if you excite the flywheel in the opposite direction.


Trying to force the flywheel the normal way, will mean you you are compressing gas in the displacer volume that is getting heated, and conversely you will expanding gas in the volume that is being cooled, which is exact reverse of running the engine, which is why I think the engine will still run but in the opposite direction.

In other words, I see you connecting power piston to the other side of the displacer as either the mathematical equivalent of reversing the location of the heater and cooler, or simply reverving the direction of rotation of the crank.


My brain may just be too soft from working all day on this swashplate design.

stephenz wrote: ↑Thu May 18, 2023 9:39 pm

Tom Booth wrote: ↑Thu May 18, 2023 9:13 pm
OK, you can think so, that's alright.

I don't, and I'm not sure why you do, but I guess we could see by trying it.

I'm not trying to be difficult but for some reason that's what my instinct tells me.

I understand why you are doing this, but the way I see it you are only making the heater the cooler, and the cooler the heater. But because you are still planning of applying heat to the system at the same surface, I think the only way the engine will be able to operate is if you excite the flywheel in the opposite direction.


Trying to force the flywheel the normal way, will mean you you are compressing gas in the displacer volume that is getting heated, and conversely you will expanding gas in the volume that is being cooled, which is exact reverse of running the engine, which is why I think the engine will still run but in the opposite direction.

In other words, I see you connecting power piston to the other side of the displacer as either the mathematical equivalent of reversing the location of the heater and cooler, or simply reverving the direction of rotation of the crank.


My brain may just be too soft from working all day on this swashplate design.

A Stirling engine is completely sealed, regardless of how many apparent "expansion chambers" and "compression chambers" or "spaces" or whatever, it's still just one volume of gas.

So when the displacer moves and the air heats up and expands it will push the piston at that time regardless of what path it takes through what chambers or ports or pipes or regenerators or whatever.

There may be less lag, as the path is more direct and with a 90° advance, you could be absolutely right. The gas might expand before the piston reaches TDC and push the piston backwards.

However, if that did happen, you might have to make a timing adjustment and reduce the advance from 90° to maybe 45° or something.

Tom, if you consider my foam displacer with the flow port machined into it, the power piston becomes associated with both the hot and cold sides at the appropriate times.

I think the Stirling cycle requires absolute symmetry to work to its full potential, so any one sided layout will naturally produce unbalanced results. To me the beauty of the cycle is the cold power stroke, giving us 4 power pulses per 2 stroke otto's 2 power pulses, or 1 for a 4 stroke.

VincentG wrote: ↑Fri May 19, 2023 6:55 am .... To me the beauty of the cycle is the cold power stroke, giving us 4 power pulses per 2 stroke otto's 2 power pulses, or 1 for a 4 stroke.

I agree with the statement there, just not with the underlying assumption. (I'm assuming an assumption, correct me if I'm off base).

The assumption being that the "cold power stroke" must be accomplished by cooling the working fluid by heat transfer to the "cold side" of tbe engine, cold heat exchanger, "sink" or "cold reservoir".

My theory is, for the heated gas molecules to bounce their way around, through chambers, walls, tubes, regenerators or whatever they may encounter along the path to the inner surface of the piston, and not loose energy before arriving at the piston every surface encountered needs to be as hot as the heat source. If not, then heat/energy is lost to the surface and goes to waste.

Making the path from hot plate to piston as short and free of obstacles as possible is a good start. As pictured in the engines above though, more often than not, the opposite is the case.

Further, the objects in the path are actively cooled - yikes!!! What were the designers thinking???

The return power stroke has nothing to do with the unheated side of the engine. It has only to do with atmospheric pressure or atmospheric heat on the outside of the piston. Kinetic theory would equate heat and pressure as simply molecular motion.

IMO it is absolutely impossible for heat to be transfered from the working fluid inside the engine to any cold sink when an engine is running at say, 3000 RPM. The working fluid would have to be repeatedly heated up completely and also cooled back down completely in just 10 milliseconds or 50 heating and cooling cycles per second.

My conclusion then, is that cooling is taking place, and the "cold power stroke" is being effected by heat conversion to work with cooling from expansion.

Such cooling does not depend upon the temperature of the surroundings.

In these engines by direct observation, the RPM increases as the whole engine gets hotter and hotter, at least up to a point. I've demonstrated many times that the engine, actually runs better with the cold side insulated so it does not loose heat.

IMO the working fluid can be cooled below the cold side temperature by expansion and power output alone, making a cold side superfluous.

Logically, if the working fluid is becoming colder than the cold side of the regenerator due to work output and expansion, these conductive "cooling" measures are actually only hampering cooling by work output, that is, by adding heat back to the cooling/contracting gas. (Hysteresis).

Ideally, the heat transfer path should be heat reflecting, like plated with gold foil or the equivalent, rather than is often done, (in the NASA engine for example) line the heat transfer path with heat conducting copper with a cooling jacket.

Tom Booth wrote: ↑Fri May 19, 2023 2:11 am
A Stirling engine is completely sealed, regardless of how many apparent "expansion chambers" and "compression chambers" or "spaces" or whatever, it's still just one volume of gas.

So when the displacer moves and the air heats up and expands it will push the piston at that time regardless of what path it takes through what chambers or ports or pipes or regenerators or whatever.

There may be less lag, as the path is more direct and with a 90° advance, you could be absolutely right. The gas might expand before the piston reaches TDC and push the piston backwards.

However, if that did happen, you might have to make a timing adjustment and reduce the advance from 90° to maybe 45° or something.

Not sure what I am missing here, I would really like to understand though.

Just to be clear my comment is about the image you posted here, where you are blocking the pathway joining the power piston to the compression space and instead you are joining it to the expansion space.
download/file.php?id=2385

The change you made will make the piston compress gas in the heater (and expansion space) rather than the cooler (and expansion space).
This to me will just make the crank spin in the other direction.

Agree completely that the nasa layout seems wrong in many ways. We can agree to disagree on the driving factor of the working gas cooling down to power the cold stroke. I'll start another thread with hopefully compelling evidence as I've already derailed Stephen's thread enough from his original topic.

Tom Booth wrote: ↑Fri May 19, 2023 7:47 am IMO it is absolutely impossible for heat to be transfered from the working fluid inside the engine to any cold sink when an engine is running at say, 3000 RPM. The working fluid would have to be repeatedly heated up completely and also cooled back down completely in just 10 milliseconds or 50 heating and cooling cycles per second.

Why impossible? Of course 50 cycles per second is fast, but the mass of the gas is small, its specific heat is not very high, and the gas velocity you are dealing with are increasingly higher as the RPM increase.

VincentG wrote: ↑Fri May 19, 2023 9:30 am Agree completely that the nasa layout seems wrong in many ways. We can agree to disagree on the driving factor of the working gas cooling down to power the cold stroke. I'll start another thread with hopefully compelling evidence as I've already derailed Stephen's thread enough from his original topic.

You're fine. Conversations are needed, and since everything is so intertwine it's difficult to remain on topic without missing out on important factors.