Portable generator to Ringbom converstion

[Tom Booth]

I'm thinking lately that there may be a way to simplify, reducing the number of moving parts in this generator conversion by using something along the lines of TK Motors engine.

Rather than a large displacer to move up and down his inner canister simply has a kind of Ringbom (pressure actuated) diaphragm that moves the working fluid up and down and around through a regenerator.

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[Tom Booth]

I had some concerns about making a connection between the canister "buffer" and the crankcase, in effect making the crankcase itself the buffer space, but also, by using the crankcase as buffer, the "back pressure" from the piston will tend to assist the displacer/diaphragm motion.

My concern was that it might assist too much, actuating the displacer too soon causing the piston to be driven backwards.

I spent some time checking the actual pressures involved and I'm happy to report that IMO the crankcase is so large relative to the piston the "assist" pressure is actually quite minimal, I don't think the back pressure (or vacuum depending on the direction of piston travel) is enough to actually move the diaphragm much at all. Compared with the pressure and vacuum generated in the small space above the piston, the crankcase pressure is negligible, but enough to be worth using as an assist. That is, as the piston pressure is pushing the diaphragm, the crankcase vacuum will be pulling from the other side, and vice versa.

At this point I don't really see any other hitches, and should be able to determine if this setup has any chance of actually working, within a few more days.

Another issue I think will be mostly averted by this design is the oil in the displacer chamber possibly burning or exploding. For the most part, the oil should remain in the crankcase remote from the displacer chamber. Still, I'll likely use some non-flammable lubricant.

Also with the crankcase connected to the "air spring" the engine is pretty effectively sealed, no need for any "breather" to outside atmosphere, so the engine could be filled with some inert gas, if not actually pressurized. I now have plenty of helium to give it a try anyway. Probably the seal around the crankshaft is not designed to take high pressure, but should at least contain some inert gas long enough to see if the engine will run.

[Tom Booth]

I'm going to try using the engine design with a diaphragm and stationary regenerator rather than bulky displace.

Not that the regenerator is not itself bulky, but it has the advantage of not needing to be lifted or moved, there will be greater flexibility in orienting the canister as without the movement, binding, wear and friction are not an issue. Another advantage is it makes it easier to line the canister/regenerator with some heat resistant refractory. Since there is no movement, there are more options for refractory material, it does not have to be of such great strength or so shock resistant, as it will not be in motion, you could get away with some cracking and there is no danger of a displacer coming loose and falling off its connecting rod, side clearance and other issues associated with displacer movement are eliminated, actual construction is greatly simplified

I think the design will allow a greater compression ratio than what is possible with most other Stirling engine designs and, though not completely eliminated, "dead air space" will be significantly reduced virtually all the air from tbe power cylinder will be transfered to the hot canister and back.

I may put something like a cotton filter in the transfer line to absorb any oil that gets past the rings.

[Tom Booth]

Earlier I removed the governor, which on the IC engine maintains a constant speed by slightly opening or closing the throttle according to the RPM since this engine will have no carburetor, however I'm now thinking that it could possibly be useful depending on what fuel is used.

If propane or another gas, it could be used to open and close a valve to regulate the gas flow. If solid fuel like wood, coal, pellets etc. It could be used to open and close a damper. If solar, it might be used to adjust a reflector to admit more or less sunlight or adjust a shutter to provide more or less shade at a focal point.

At this point however, I've already tapped out and plugged the hole where the governor shaft once passed through the crankcase, and I'll likely be experimenting with different heat sources so I'll leave it as is for now. Something to take into consideration for future conversions, or this one, if it is ever set up for continuous use, if we ever get that far.

[Tom Booth]

Those stainless steel time capsules are made from some really hard metal. Almost too much for my little Harbor Freight drill press.

https://youtu.be/-qpiQ7-NT0s

Nice to know they are good and strong.

[Tom Booth]

https://youtu.be/l2j-jm6EhQI

[Tom Booth]

Making and assembling the air-spring diaphragm:


https://youtu.be/aJEG0ZlEZfA


For the rubber diaphragm I"m using a section cut from an old tracker trailer truck inner tube I got for free from the local tire repair shop. I don't know how well it will hold up.

[Tom Booth]

Pressure testing, extra bracing and reassembly:


https://youtu.be/sbWozcFQ_hg


There are still a few more things to do. I have not yet made a regenerator, though I'm wondering if the engine would do something without it, just from "adiabatic bounce". But at this point, I think that would leave too much dead air space in the canister.

Turning the engine over a few times with the pressure gauge, I really couldn't tell if there was any compression. The needle didn't seem to be moving.

That was before making the video, and I was turning the engine over fast. Either the pressure was fluctuating so fast the needle on the gauge didn't have time to respond or the needle was moving so fast I couldn't see it.

Even adding slow motion to the video, the needle movement is a blur. It does look like there is over 10 lbs pressure.

In the video, apparently at least once when turning it over, the frame rate must have coincided with the needle movement, because it appears there is no movement at all, but I'm very sure there actually was.

I haven't attached the crankcase breather to the air spring, I can't really decide if that would help or not. It might defeat the purpose of the air spring somehow, so I figured I'd try it without it first.

I'm also probably going to tear the whole engine down again and give it a thorough cleaning, wash out the crankcase and replace the oil, change some gaskets and tighten all the bolts, get it ready to run.

The canister with the regenerator I can work on separately.

I should be able to swap out canisters if necessary to try different designs. With or without a displacer, different regenerator materials etc.

I'll probably also install some kind of heat shield. My hope is that this thing could be parked with the end of the canister in a campfire, but the engine casing appears to be made of fiberglass.

As a portable generator it is actually quite a bit lighter without all the parts that are no longer needed. There might even be room for a small battery where the very large muffler used to be but that would add to the wright. Maybe some super caps.

It could double as a picnic basket if I hinged the top where the gas tank used to be.

[Tom Booth]

Although having decided to design the heat canister for this engine more or less around a stationary TK Motors type regenerator design, I just realized that this is also quite similar to Ted Warbrooke's thermal lag "Stirling 1" or at least similar to one of the illustrations from the book:

Screenshot_20220201-174129.jpg (103.19 KiB) Viewed 3853 times

I've also concluded that the diaphragm (air spring) will not really do much of anything. There would be an "air spring" with or without a physical diaphragm, but it does insure that there will not be any inadvertant mixing of air between the two volumes; the actual heated hot air and the theoretically unheated "air spring".

[Tom Booth]

While giving this machine a more thorough cleaning I found a number of electrical/electronic components hidden underneath the motor/generator

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Also an oil sensor of some sort inside the crankcase

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With it's own electronic controller brain or whatever

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And another component attached to the starter, I suppose some kind of safety relay gizmo

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Hopefully it's all working and I won't have to mess around with it, but it shouldn't prevent the engine itself from running, I don't think.

A generator that doesn't generate won't be so revolutionary but it would be something.

One step at a time.

[Tom Booth]

Not much to report other than I cleaned up the engine real good, and used compressor oil in the crankcase.

I also put the "unnecessary" ignition components back on, because I don't think the thing I thought was a magneto was actually a magneto. The thing has some Kind of electronic ignition and I'm guessing the thing I thought was a Briggs and Stratton style magneto was actually just a position sensor, not sure really, but there is no actual magnet in the flywheel anywhere.

I think while browsing around I saw an old technical manual for this generator.

Anyway I did give the engine a spin, all put together with heat applied to the bottom of the canister with a propane torch, but with nothing inside by way of a regenerator or displacer.

I thought maybe I felt something like the engine turned over easier a few times but not sure.

What I do know is when I took the canister apart the piece of truck tire inner tube had been pulled so that nearly all the bolt holes I had drilled were elongated to the point where they could not hold air.

I was rather shocked when I opened up the canister. I was checking as I did so that the diaphragm was still in position, at least on the outside, and it was, but on the inside it had apparently been stretched so far that the center section had pulled away from the bolts until the holes were so elongated that they allowed air passage through between the bottom chamber and air spring.

I suppose the damage is a good sign??? An indication that enough pressure developed to tear the diaphragm, which was pretty tough truck tire inner tube.

The inner area of the rubber was also discolored - whitish. Crackled looking. I assume due to being stretched? Heat radiating from the bottom? Not really sure.

Anyway the generator showed no definite signs of wanting to run.

My impression so far is, old truck inner tube is not flexible enough and or requires some kind of reinforcement around the bolt holes. Maybe the silicone oven mat would hold up better.

After putting the ignition parts back on, there IS spark, so it seems the ignition system is working, which hopefully is an indication that the rest of the electrical/ generator system is functioning, but no guarantee.

The oil sensor, I think, is a combination simple float switch and possibly a heat sensor to detect either low oil or overheated oil. That bit of electronics is situated in a way, with a metal shroud so that it benefits from cooling from the flywheel fan.

I did try putting a kind of regenerator in the displacer/regenerator can without any diaphragm, but that produced no result.

One problem I think is that the lid on the canister has a raised ridge to help clamp down the silicone seal, but the ridge is on the outside side of the bolt holes leaving the inside not clamped down as tight... Therefore the diaphragm allowed to pull away from the inside stretching the bolt holes.

I may try some fixes, and try making a more proper displacer Ringbom, (rather than just regenerator) but my few days of free free time to experiment is not as free at this point as other pressing matters need doing so progress may be delayed.

I'm undecided about pressure, is there too much or too little?

Too much for the diaphragm but too little to run the engine.

I'm inclined to think the transfer pipe is too large in diameter. On my model Ringbom the transfer tubing is a very narrow diameter. I'm thinking the pipe on this generator is too large a diameter and/or too long so there is too much pressure drop, or dead air space, but until the diaphragm issue is fixed... though I had already concluded that the diaphragm probably didn't do anything (without an actual displacer to lift)

Anyway the pressure relief valve I installed in the line didn't pop. But that was from an old compressor, so, is probably rated for at least 100 psi or more.

[Tom Booth]

Taking a second look at the diaphragm It seems it did stretch and/or was not aligned properly in the first place and was very likely not holding air, but was also pretty much rulled out as THE primary problem.

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After some further experimenting and pressure testing I"ve come to the conclusion that empty, with just a diaphragm, or full with a regenerator (thermal lag type with no displacer) the main problem is very low compression due to an excessive volume of air in the canister.

I made a video of how I tested for this using a barbecue syringe thing I had originally picked up to inject epoxy to reinforce some split beams.


https://youtu.be/15em-6vzLiU


My plan now is to make the canister more along the lines of one of these toy Ringbom type displacers: using a very close fitting propane canister as the displacer to take up as much of the dead air space as possible.


https://youtu.be/55WfzFj-Vwo


The meat injector syringe was actually rather difficult to move manually. Far from frictionless, so I thought made a reasonably good compression tester.

The length of pipe did not seem to be a source of too much "dead air space" but with the canister in the line, compression was too weak to move the syringe plunger.

I tested the canister itself for leaks.

It does seem to hold compressed air and I could not find any signs of any air leaks anywhere, (other than the probable internal diaphragm), so I believe the volume of air in the canister is just so large the great volume of air does not build up hardly any compression, the swept volume of the power piston being quite small by comparison.

My inclination would have been to suppose I needed a bigger canister, but perhaps the opposite is true.

What were those old engines called that had a rather small compartment for heating/combustion above the power cylinder?

[Tom Booth]

A "hot bulb" engine:

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Generally the hot "bulb" on these engines was rather small relative to the power piston.


https://youtu.be/1Rzutde1nGo

[Tom Booth]

So the basic idea, I think, at this point is to have as much surface area as possible, but at the same time try to keep the actual air volume as low as possible.

These are conflicting objectives.

I'm also partly going on the advice in this video:


https://youtu.be/QcppEhp2RfA


He recommends just one millimeter of clearance, which also is somewhere around what Robert Stirling found in his experiments someone cited in here somewhere a while back.

I think the propane bottle probably leaves a little more than 1 millimeter but it's close enough to feel some vacuum when lifting it out of the canister. However his engine (in the above video) "vibrates" very rapidly and the displacer barely has to move. For this conversion, though, the displacer likely needs to move further with greater air flow, so maybe a bit more clearance will be allright.

I will probably hang the canister vertically with a 90° elbow as the "new" design with the propane bottle displacer is likely to rub and/or bind if oriented horizontally.

Some kind of internal flexure would solve that, but I'm not cutting open the propane bottle just for experimenting. If this works it might be worth doing.

The propane bottle is rather heavy, which is a worry, but I do plan on suspending it with the failed rubber diaphragm using the rubber disk as a kind of flexure spring, both to suspend the displacer, taking some of the weight, as well as hopefully, helping to keep the displacer centered to prevent excessive rubbing/friction/wear, with so little clearance

[Tom Booth]

As much as I would like, there is just no way the propane bottle will fit inside this canister.

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It's the perfect diameter but a little too tall, so, I suppose if I can't find something else I'll have to cut it open after all .