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Hi all, I have made a gamma type Stirling engine that uses ross yoke mechanism. I have had difficulties getting it to run and was hoping to get some suggestions on how to solve this. Its specification is;

Power piston:
Stroke = 3.5 mm
Bore = 12.5 mm
Length = 17 mm
Material = Steel

Power cylinder
Material = Brass

Displacer piston:
Stroke = 3.5 mm
Bore = 15.5 mm
Length = 35 mm
Material = Steel

Displacer cylinder:
Internal length = 46.6 mm
Internal diameter = 15.3 mm
Material = Steel

Ross yoke:
Reference image
https://images.squarespace-cdn.com/cont ... -asset.png

d1 = 30 mm
d2 = 27 mm
z = 15 mm
XF = 30 mm
YF = 27 mm
Xpp = 15 mm
Xpd = 15 mm

Notes:
A small methanol burner was used as heat source.
When testing spinning the flywheel the correct way had less resistance. (This did not feel like resistance due to friction)
No bearings were used in the ross yoke mechanism.
Rubbing surfaces were made smooth.
Joint board sealed with silicone sealant.

I have attached an image of the engine below.

Thank you for any suggestions.

Is the displacer steel steel, not stainless ?

You say a "small" methanol burner.

Assuming there is nothing mechanically wrong. Steel would not be my first choice for a displacer. It might be conducting too much heat, so that heat is conducting into and through the displacer so quickly that it (the heat) hardly has opportunity to heat up and expand the air inside the engine.

Like electricity, heat will take the easiest path. So I suspect the steel displacer may be causing a "short circuit". The steel displacer is a "better" path for the heat to follow than the air surrounding it.

It might run with more heat like a propane torch or something.

With a steel displacer cylinder as well, that could be another "short circuit" and if the big plates used for conduit, between the displacer and power cylinders, aluminium is probably worse than steel, add the steel piston and brass...

All that metal is likely absorbing and conducting away all the heat so rapidly there is not enough heat to heat and expand the air to power the engine.

The engine may itself be sinking all the heat and conducting heat to the cold side too rapidly so there is not enough of a temperature differential.

As a kind of diagnostic, heating the outermost tip of the displacer cylinder to a high heat while cooling the back of the cylinder with a piece of wet cloth might at least provide enough temperature difference to get it going.

If the steel displacer could be replaced with something non-heat conducting, that would likely help tremendously.

I did this with a similar engine kit that came with an aluminium displacer.

The engine required an enormous amount of heat to get started.

When I replaced the aluminium displacer with a displacer made of wood, it ran fairly easily on a tea candle with the wick trimmed so it only had a very tiny flame.

Before with aluminium displacer:


https://youtu.be/R_QB5amihko

After, with wood displacer:


https://youtu.be/bQ44Rm40unA

Same engine except for the displacer.

If on the other hand, you are using stainless steel, I'm probably barking up the wrong tree as stainless is, I think, much less heat conductive.

Ideally, the displacer should be less heat conducting than the air, so the heat chooses the path of the air, expanding it in the process.

Conductivity of steel is about 50.0 W/m, oK

Air about 0.025

Wood is at least comparable at about 0.04

Aluminum is abysmal at about 200.0

Foamed glass or pumice is perhaps slightly better than wood. (0.035)

If some helium could be gotten into the engine, that is about 5X more heat conductive than air (0.15) but that still cannot compete with the conductivity of steel, aluminium and brass.

If the heat engine is running very fast, the highly conductive engine body is not as much of a problem, because heat can expand the gas relatively quickly, while conductivity is relatively slow.

So give the engine a couple of good spins of the flywheel and that can get it going, but with a Ross yoke and a small flywheel, the engine can't get up to speed enough to compensate.

At least it looks, from your photo, that that would be rather difficult, and the Ross yoke does not allow the development of enough momentum.

Thank you for replying.

The displacer is steel, not stainless. So I will try changing the displacer material for one less thermally conductive. Unfortunately, I do not have stainless steel so I will try wood first.

Also, given what you said about the thermal conductivity of aluminium, I am wondering if it would help to add insulation between the conduit layers to separate it from the air inside?

12345 wrote: ↑Wed Sep 14, 2022 9:26 am Thank you for replying.

The displacer is steel, not stainless. So I will try changing the displacer material for one less thermally conductive. Unfortunately, I do not have stainless steel so I will try wood first.

Also, given what you said about the thermal conductivity of aluminium, I am wondering if it would help to add insulation between the conduit layers to separate it from the air inside?

The heat may not even be getting that far. Personally I'd try just increasing the temperature difference at the displacer chamber before taking the engine apart or changing anything.

Radiant heat can transfer to the working fluid more quickly.

So getting the "nose" really hot and keeping the other end cool with a cold wet cloth might get it started.

The actual problem could be something else entirely. But really, the heat should not be conducting as far as the aluminum IMO.

Also how much clearance do you have between the displacer cylinder and displacer? (At the tip or nose in particular, it should be minimal) The displacer stroke seems rather small compared with the available space. Just 3.5 millimeters may not be moving enough air between the hot and cold ends of the displacer chamber, while it looks, from your specs, that there is about 10mm. So, that leaves about 7 mm of working fluid, essentially stagnant, not changing temperature, so not expanding and contracting, not contributing any power.

I think the stroke of the displacer could be made greater to some advantage, possibly.

With just a 3.5 mm stroke, the displacer would hardly be displacing much at all.

The connecting pin on the Ross yoke seems like it may be too close to the center of the flywheel. It looks like it is barely off center.

Ok, I will try increasing temperature difference first. I will probably not be able to use propane torch as I do not have anything to isolate the nose of the displacer. But I will try increasing the wick length and using a wet cloth.

I have attached an image with the displacer clearances at TDC below. Did not realise the nose clearance had to be minimal, so I will adjust the displacer accordingly. Thank you for mentioning that.

I originally made the displacer with a 17.5 mm stroke and length of 19 mm (This is why there is an extra hole in the crank disk). After, I read that the displacer length needed to be x3 the dimeter. Hence, to accommodate the longer length I changed the stroke to 3.5 mm. I also thought that it might not make much difference as the swept volume ratio would be kept the same. But I see what you mean about the stroke possibly being too small.

The connecting pin is 1.5 mm away from the centre of the fly wheel. With a stroke of 17.5 mm the connecting pin is 7.5 mm away.

(P.S. Just realised that the displacer cylinder, internal diameter should be 16.3 mm not 15.3 mm in the spec.)

If your displacer is actually flat on the end, that could be part of the problem.

The displacer should be contoured to fit the inside of the hot nose of the chamber as closely as possible

Like shown by the green line:

and at the extremity of the stroke should just "kiss" the inside end of the chamber so that as much of the working fluid as possible is evacuated or "displaced" away from, or out of the heated end.

The whole displacer could be shortened, I don't know where you heard that the displacer length must be 3x the diameter, but IMO, that is wrong. Any such ratio is irrelevant. Consider an LTD "pancake" type Stirling for example. The diameter is 50x the "length" from top to bottom.

The blue X |. was intended to show shortening of the displacer, but rounding/contouring the end to fit would likely effect enough shortening. (Red line)

Basically, judging by the discoloration of the outside of the displacer chamber in your photo, and the positioning and length of the displacer, and the short distance traveled by the displacer, not much is happening except the middle of the displacer is getting cooked.

I originally made the displacer with a 17.5 mm stroke and length of 19 mm (This is why there is an extra hole in the crank disk). After, I read that the displacer length needed to be x3 the dimeter. Hence, to accommodate the longer length I changed the stroke to 3.5 mm. I also thought that it might not make much difference as the swept volume ratio would be kept the same. But I see what you mean about the stroke possibly being too small.

The connecting pin is 1.5 mm away from the centre of the fly wheel. With a stroke of 17.5 mm the connecting pin is 7.5 mm away.

Where you had it to begin with should be about right, I think.

Did you ever try running the engine before making that change?

I can see by the diagram you provided:

There looked like there was some deviation from the sketch. Your original position would certainly give better displacement.

And with the Ross yoke,the power piston would be constrained as well it would seem. Moving that pin so close to center is just locking up the engine. It has no real freedom to move much at all, I think.

Yes, I did a run before the change, but it did not run.

Also, I tried a test using a larger flame and wet towel. But still could not get it to run.

Though, you may be right about the engine not having enough displacement / the stroke being too small as it seemed to get closer to running after I went back to the 17.5 mm stroke and contoured the end of the displacer. However, it has not run yet.

I will try again later using a wet cloth to see if I can get it to run.

Do you have a way to post a video ?

Seeing how it turns over might reveal something.

Also, is the contoured displacer the original steal, or have you tried wood or something less conductive.

Wood worked quite well, but did char with very high heat, however, for a test it should be ok.

The displacer stroke might be extended even further.

This little engine for example, the stroke is the entire length of the displacer, which is quite a distance. It disappears entirely then extends all the way to the end of the tube.

https://youtu.be/E0XQ1qZzZx4

Looks like probably more than 17.5mm more like 25 or 30? Since increasing it showed some improvement.

Actually, now that I look at it, your displacer chamber seems to be all hot side with no, or not much room for the displacer to withdraw to. No real cold side or sink.


The displacer chamber, it looks like, should maybe be about 2X longer? With a cold side ?

Apologies, not sure how to attach a video. I attach a gif of the engine but it is heavily compressed due to the small file size that can be attached. The original video is .mp4 if you know away around this.

Yes, the contoured displacer is steel. I did try PLA with air pockets inside, but it was not able to withstand the heat long enough for a proper test.

Also, I did a test using an ink covered aluminium strip to see if it could show anything further about where the hot side ended. Results showed it was about where the oxide layer ends (Images of results attached below). Though, I am not sure if this is a reliable method of testing.

I looked into how long the displacer cold side should be and stumbled across a forum post (viewtopic.php?t=48) mentioning "The displacer cylinder is ideally divided into 1/3 cool and 2/3 hot. The displacer itself should be about 2/3 the total length of this cylinder". Not sure how I could include this without completely redesigning the displacer cylinder though.

12345, one thing you you could do for a short test would be to wrap some wet cloth around the cool end of the chamber. It does look like you need better isolation between the hot/less hot zones. I’m sure in your original post you inverted the displacer and displacer chamber diameters. It looks like you have about a 1.8/1 compression/expansion ratio before figuring in dead space. Figure maybe 1.5/1 overall. Looking at most other versions, that’s pretty high.

Bumpkin

Hi Bumpkin, I will give wrapping wet cloth around the cold end of the displacer a try, as I have been wrapping it around the power cylinder in previous tests.

Apologies, the displacer and displacer chamber is not inverted. This is a typo, the displacer chamber diameter should be 16.3 mm, not 15.3 mm.

Also, could you please explain what is the compression : expansion ratio. Particularly, what it typically is and how it is worked out?

12345 wrote: ↑Sat Sep 17, 2022 9:12 pm Apologies, not sure how to attach a video. I attach a gif of the engine but it is heavily compressed due to the small file size that can be attached. The original video is .mp4 if you know away around this.

Typically, upload the video to YouTube or possibly some other video hosting service comparable with phpbb. Then just include the video url in your post and it will be embedded automatically.

It appears to me that your linkage is a bit out of alignment, like the overhead connecting arm may be a little long. Possibly that could cause excess friction or binding as well as throwing off the timing. Or, matbe it's just the camera angle?