Stirlingengineforum.com

There have been some discussions on this topic previously, here for example: viewtopic.php?f=1&t=377

But in all the years I've been here, or anywhere else for that matter, searching for actual examples of a successful compressor to Stirling conversion, I have not found any real examples of what was rumored to be a potentially easy conversion.

The few I have seen involved some rather extensive modification.

It appears though, that the conversion of a V compressor to a Ringbom type Stirling could be fairly straightforward.

I just came across this example of a V twin type Ringbom model engine.

https://youtu.be/id-NBC6UW2g

I think it may also be possible to use just one displacer cylinder that could do double duty, working both pistons, according to some of the rather sparse literature on the subject.

I think there must be some way to cut down on the noise

A twin compressor Ringbom conversion would really be two separate cylinders each with its own displacer.

Anyway, I've been thinking in terms of a single cylinder of late, and supposing a sealed, pressurized system, I thought crankcase back pressure could be an issue.

So, I thought of a kind of round about way of relieving the crankcase pressure somewhat, while also letting it serve as an "air spring" of sorts for the displacer canister.

Kind of, but not quite a duel acting cylinder.

I'm not entirely sure how the dynamics of such an arrangement would work out in practice.
The circle in the middle is just an interior view of the displacer canister on the right.

There is a rather thick heavy silicone diaphragm dividing the top "air spring" from the main displacer chamber.

During compression, high pressure in the displacer canister, and low pressure in the buffer space, connected to the crankcase, lifts the displacer, introducing heat, expanding the gas/air driving the piston down for the power stroke, simultaneously pressure rises in the crankcase, but is transmitted to the buffer space above the displacer diaphragm which holds the displacer down to discontinue heat addition until the next compression / power stroke.

A simplified arrangement occurs to me.

Alternative, simplified arrangement.:

The displacer canister I had in mind is one of these stainless steel "time capsules":

Regenerator?

I'm somewhat skeptical of the usefulness of a regenerator, but it would be easy to incorporate into the power transfer tube.

I'm thinking that displacer/diaphragm movement would be relatively slight and intermittent, with the displacer held down by an adjustable spring in the upper buffer chamber area. Limiting movement (and therefore heat input) to neat TDC. At higher speeds this would settle into a more or less sinusoidal "vibration" I suppose.

Another alternative arrangement that crossed my mind would be a "pancake" style displacer/displacer chamber.

Same basic idea, but a lot more surface area for heat exchange.

Instead of the tall or long time capsule, that would probably be suitable for sticking in a fire, something like, or an actual pressure cooker might be used, relatively shallow with a wide flat bottom. Not sure another drawing is necessary.

Maybe some heating coils spiraled around right inside the chamber on the bottom of the pot circulating hot fluid from solar, hot process water or whatever, or hot condenser coils from a thermodynamic solar assisted heat pump panel.

Lots of possibilities, and variations, if the basic concept pans out.

There is a bit of a problem with pressurization in that the gas would probably eventually leak at the crank seal.

I did post this idea a while back on this other thread.

viewtopic.php?f=1&t=5395&p=16882#p16882


Just put the whole engine inside a sealed drum with the displacer sticking out so it can be heated.
That should be even easier using a Ringbom type displacer. It could be extended out with a pipe or even flexible SS tubing, and doesn't require any connecting rod.

A 30 gallon or whatever drum couldn't be pressurized much, so, weld together a chamber out of some plate steel. Make a tight fitting lid or hatch that can be bolted down tight.

Well that was supper fast. UPS just dropped this off at the door, seems like I just ordered it yesterday

The body is cast iron, but it seems like the head is aluminium.
But that will more than likely be coming off anyway, but I might be able to use it. Need to remove the leaf valves and plug the exhaust or something.

Seems really tight, but there is no lube visible
So I assume some Superzilla will loosen it up. LOL.

Here's a V-2 SE that actually runs...

https://www.youtube.com/watch?v=IgWHuhVcLKA

Watch until end and you'll see he dedicates this to John Archibald (aka Arch) who was, well, you either know or you don't...

matt brown wrote: ↑Mon Dec 12, 2022 8:58 pm Here's a V-2 SE that actually runs...

https://www.youtube.com/watch?v=IgWHuhVcLKA

Watch until end and you'll see he dedicates this to John Archibald (aka Arch) who was, well, you either know or you don't...

Not to be overly critical, I applaud the effort, but since you mention John Archibald, I think it was his contention that the Stirling engine expansion and compression is adiabatic rather than isothermal. Engines simply go to fast for isothermal heat transfer to have any significant effect. Wasn't that the point of his experiment?

https://youtu.be/HMnZrPKjleE

The air in that pressure cylinder took at least 5 seconds to cool down under cold running water. Not sure that actually proves anything because it is the thermal mass of the metal cylinder itself that takes a long time to cool down. The air cannot cool any faster than the cylinder, but he was trying to make a point. It takes a relatively long time for the air to cool by conductive heat transfer, and engines generally run much too fast for that. The power stroke is complete within a fraction of a second.

If we assume John Archibald was right, and for the record, I tend to concur, that expansion and contraction in a Stirling is adiabatic (without significant heat transfer) what does all that active water cooling in that SV-2 actually accomplish?

An air molecule is excited, energized in that heater to a very high temperature, it then needs to travel to the piston in order to impart that energy to the piston, does it not?

Or perhaps the kinetic theory is wrong. Is pressure really a manifestation of the direct impact from a particle? Assuming however, for the moment, that the kinetic theory is at least effectively true, let's follow the particle from the heater to the piston.

What chance is there that the partial will retain it's newly acquired energy as it makes its way to the piston.

There is a water cooled matrix of narrowly spaced cooling fins to get through. How many air molecules manage to pass straight through the narrow gaps without contacting the fins and loosing all their energy just to make some hot water? Relatively few I should think.

After that, there is a fairly long and narrow, as well as bent copper connecting tube. Copper is an excellent conductor of heat, one of the very best.

If the few remaining energized molecules that have gotten that far make it through the bent copper tube, miraculously somehow getting around the bend without contacting the copper tube, they face the water cooled power cylinder itself that will likely suck away any remaining energy before it can have any effect on the cold face of the piston, which being cold will also absorb and conduct away heat rather than reflecting it.

I think the air molecule has to bounce off the surface of the piston face pushing the piston rather than having it's energy absorbed to become latent and of no effect as far as actually moving the piston.

Oh and I completely forgot about that regenerator stack of finely spaced metal foil full of perforations that takes away nearly all the energy the particle acquired before it is able to go anywhere.

I would think the engine might rather be dedicated to Sadi Carnot than John Archibald.

At least the transfer tube appears to be copper, which is not effective as a regenerator, so heat loss there is unrecoverable

Maybe use stainless steel which, although metal is much less conductive and the placement I should think would be better if it were joined to the displacer chamber at the hot end rather than the cold.

If adjoined to the hot end, there is a more or less direct path for the hot gas to travel to the piston

At the cold end, the air must first pass through all the obstacles previously described, loosing energy every step of the way entirely defeating the purpose of the regenerator.

I'm not making these comments to be insulting, though I suppose it might come off that way, so I apologize for that, but I would like to see people improve these engines and come up with effective designs and I'm afraid that is never going to happen if the design work is based on the fallacious Carnot theory that an engine is somehow powered by simply transporting heat through and dumping it to a sink. That notion, if heat is energy and not a fluid, a violation of the first law of thermodynamics, for how can energy be used for power output and also be used to make hot water?

I thought the little guy seemed lonely, so I went and got his big brother
I went to Harbor Freight, mostly just to get a infrared camera, but while looking around, saw this and figured why not?

Rated 5 horsepower.

What sold me on it was the one they had on the shelf for display turned over so easy and smooth, with one finger.

I don't really see why one of these should not run on a candle or two.

On the drive home I was thinking Ringbom "rice" engine.

Maybe an old 20lb propane tank for a displacer chamber. (Or rice/gravel boiler/condenser)

Time to get out the angle grinder I guess. Cut the tank in half and weld on a flange so it could be bolted back together.

A few minor problems with work on converting the V twin so far.

#1) The head and valve plate gaskets adhered very well. Probably at least partially because of being brand new without ever having undergone any heating and cooling.

They would not loosen up with any amount of hammering with a big rubber mallet. Heating the aluminium head for a while (after removing the 4 bolts in each head of course) with a propane torch, both to expand the metal (just the aluminium, avoiding the cast iron cylinder as much as possible), as well as to soften the glue, they pretty much popped right off.

#2) The valve plate, which is not needed is quite thick, about 7/16" without the gaskets. (Including gaskets 1/2 inch.) And the bolts have shoulders. So after the valve plate is removed, the bolts don't have enough thread, so most likely the bolts will need to be replaced.

#3) the head contains large voids that are 1 and 3/8 inches deep. For an engine, just a lot of unnecessary air space that would tend to reduce compression/power output. Not really unanticipated. I had figured on probably replacing the cylinder heads anyway.

#4) The pistons are attached to the connecting rod on the same shaft. Why this might be an issue was brought up recently on another thread: viewtopic.php?f=1&t=2198&start=15#p18430 In this conversion though, my intention is to use two Ringbom type displacers that would work independently, mostly from the pressure from the piston compression so I'm not sure just how much of a problem, if any, this might be. Removing the breather and holding a finger over the hole and turning the crank, it seems, most of the time one piston is going up while the other is going down, but there are some moments of overlap. The crankcase is fairly large, however, and the pressure build up and vacuum seem relatively slight. Even with the breather hole plugged, the crank still turns over rather freely.

I'm thinking that with two displacers, each with their own adjustable air spring tensioners, the previously mentioned method for relieving crankcase pressure might just work good enough. Otherwise, perhaps the crankcase could be equiped with a kind of air spring of its own to level out the pressure in the crankcase, or just keep the existing breather. If I were going to try to pressurize it at all, My plan would be to just enclose the entire compressor in a pressure chamber

Maybe something like:
Probably the generator could also serve as a starter?

I'm pretty sure a starting mechanism of one sort or another could be rigged up, or some access hatch could be bolted down after starting manually. But before all that, I'll just see how well it would run without pressurization, if at all.

With the heads off, and viewing the piston action, the distance between TDC for one cylinder and the other does not really seem that great, 90° I would imagine, which sounds like a lot, but looked at this way (video) doesn't seem that far apart. Which has me wondering:

Could one large displacer be used to supply the expanding gas to both cylinders of a V twin.

Basically what I have in mind would be to have two pistons share one extended power stroke.

Pressure from the first cylinder would lift the displacer which would initiate expansion. The pressure would be sustained and expansion continue beyond the point at which the second piston passes TDC as well.


https://youtu.be/TV5cCFceAc4


Anyway, while observing the piston movement slightly ahead of, and slightly behind a kind of "average" TDC, (marked by the piece of masking tape), it seems like a real possibility to me.

I'm wondering if these cheap Harbor Freight compressors are repurposed old refrigerator compressors.

There are frequent complaints in YouTube video-reviews about the weird, useless "refrigeration" type connectors that don't fit anything, and how to replace them:
Also, the shaft seals are minimal, by being entirely closed on one side:
The other side seems to have a rather heavy duty seal. I think, designed to be easily serviceable.
The older Freon refrigerant did not require as high a pressure, so such crankshaft seals were adequate. Years ago I think this type of compressor was rather common on commercial refrigeration units, such as used for beer coolers in grocery stores. I think I saw one very much like it outside a country store about 25 years ago, but when I became interested in Stirling engines, I went back there to see If I could buy it, but by then it had already been scrapped.

Here is my theory:

The big Ringbom displacer is activated by the increasing pressure. #1 on the left 90° ahead of #2 but both in the compression stroke.

Withe #1 at TDC - high pressure drives #1 down while #2 is still in compression. So pressure is maintained.

Due to the arrangement of pipes, there is a slight delay in the delivery of pressure to cylinder #2 giving time for it to reach TDC

As both pistons are driven down, displacer drops discontinuing heat addition. Process is reversed.

A vacuum develops at BDC first drawing up #1 then #2.

Kind of like a two cylinder thermal lag effect

If that works, it would be like 2 two-cycle engines giving effectively 2 power strokes every revolution of the crank, or it might be thought of as one power stroke extending through a full 270° (about 180° + 90°) using a two piston delivery