Jeroen Jonkman's Sterling 60

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Hoping for some help despite old thread. Have completed most parts but being fairly new to machining I wondered -

How do you cut the slots etc in the connecting rods and get them parallel?

What work holding device do you use?

Hi Mike, first, using the pillar drill, after carefully setting the rod in the macine vice perpendicular to the drill axis, and then drilling a hole whose diameter is that of the width of the slot. I have simply used a hacksaw, from the end to the hole, holding the rod vertically in soft (fibre) jaws in the engineers vice. Then carefully open the slot and finish with very small thin files. (WARDING files). Yes, it is a test of your hand skills. Oft forgotten when many just try and use machines.
When I made mine, I drilled the cross holes for the connecting pins and then without changing the setup used a fine slitting saw to make the slot. In the mating part I followed the same procedure using the slitting saw to cut away the top and bottom, leaving a tongue to fit smoothly into the mating slot. With no change of setup there is no risk of not being perfectly perpendicular and perhaps binding when assembled.
Mike, I agree with Almega above - do the drilling and milling in one set up to ensure perpendicularity.

I made my ends separately and soldered them on to tubes - I felt it was too long and spindly to machine from solid.

It's do-able but you would probably have to turn it around and loose your reference for both ends to be true to each other in any case- so I figured solder would work just as well and if you did make a pig's ear of the alignment it would be fixable.

I soldered one end first - no need to reference yet.

Then I clamped the other end vertical in a vice using MDF either side to clamp against the milled flat thus aligning it to the vice.

Then I put a piece of solder in the tube (with the first end already soldered) and stood it upright on the other end in the vice - "eyeballed" it parallel with a ruler - heated it with a butane torch until the solder showed up at the tube end - et voila - worked fine.

Both ends being effectively soldered from the inside by the expedient of placing a piece of solder in the tube before assembly and heating.

Regards, Ken
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A couple of good ideas here. I am not sure why the slots and blades have to be precise? I understand the holes for big end and gudgeon pin MUST be parallel, but as the bearings and pins should be the ONLY contact points then clearance, although small, on any alignment (well, say within a couple of degrees) is all that matters. My Hand-fettled parts - being clearance - are trouble free on all my engines. The only engine where there is axial motion along a big-end is on a fabricated crank, where 1 reamed hole was oversized, so a tiny clearance for the big-end cantilever pin to go off alignment... but this is now a fun talking point when I run the engine, as few understand why the rod moves on the pin! The engine is one I run on miniscule air, on just a few psi, at less than 100rpm. It is a table engine, and many like to watch it...
I made mine from 12L14 rather than brass and I did make them of one piece which worked surprisingly well, with no bending although I did take very light cuts. For the drilling and slotting, each part was done with a single setup by holding in a small aluminum "jaw" tightened on the center part of the part, leaving the ends accessible to machine and then that fixture is clamped into the mill vise.

Hot Air engine parts.jpg
Wow thanks everyone for the ideas so quickly (you know how it is when you are getting to the end of a project and get stuck :) ).

I have, with a number of failures, machined the parts in one piece and the 3mm center section is not strong, hence the work holding question as I realised being able to access both ends on the mill would be the ticket.

If it fails I will try the tube and solder trick, I like that!

Also never too old (75 yrs) for an old dog to learn new tricks. 2 new things today 12L14? and WARDING files?

Really enjoying my retirement hobby and the help from this forum with replies from all over the world.

Thanks again
Warding files are small files for getting into small places, sometimes mis-labelled as jeweler's files. 12L14 is leaded, free machining steel that cuts like butter with sharp tooling and can polish up nicely. The nomenclature for 12L14 may be different in Tasmania than here (I am in the mid-western US), you will have to check with your supplier about that. Just a note, I chose to make my version of the Sterling 60 engine (still in process) of steel rather than brass because I intend to paint parts of the support structure, steel is actually less dense than brass and a lot stronger, I don't like the look of tarnished brass, I like the look of polished steel, and brass has become ridiculously expensive in my area. Good luck making your parts; just take your time and enjoy the process.
I finally made a Boo-Boo with this motor - so just a heads up.

Starting & Running The Motor:​

Firstly - Viewed from the flywheel side (left hand image on page 1)

the flywheel turns AntiClockwise

I have run this motor many times and many hours at shows but recently after nearly two years of not using it I tried to start it in the clockwise direction – idiot.

Here’s the problem – I often use a butane flame to light the wick and to pre-heat the cylinder. By turning the motor in the wrong direction the thermal cycle is reversed and it drives up the displacer volume temperature.

Additionally I was applying way too much heat and of course it refused to start in that direction consequently the displacer bulb unsoldered itself.

So – It runs anticlockwise, apply heat judiciously, remember the correct rotation.

Maybe add a direction indicating arrow to the flywheel.

Regards, Ken I
Steamchick, you might like to hear about my further problems and (accidental) improvements.

Although I got it running it was not as it should be - it turns out it had been dropped and was misaligned.

Anyway I resoldered the displacer but it did not run as it should and I ended up overheating it and suffering the same failure - so I added a couple of screws but this made things worse.

So I made a solid Aluminium displacer and it refused to run - I noticed a distinct "moaning" from the engine which appeared to be the displacer shaft running in its cast iron bush.

Ah-Ha - is doesn't like the mass cantilevered over the bearing - so I figured lets make a lighter piston that does not require soldering so I designed a lightweight crimped and spun closed displacer thus :-

Original weighed 6.9g - the solid Aluminium weighed 13.4g and the lightweight replacement only 3.0g :-

Various turning ops :-

First turned the outer shell parted off square - then locktite to a mandrel to machine the radiused outer end.
I turned the radius as a series of flats at 22.5° increments and then finished by file and emery. Note: I am turning in reverse - its easier to get at all the angles that way.

After crimping the end to the shaft, I spun that into the end of the displacer shell :-

I ran the ball-bearing aginst a bar and polished up and improved the corner rad with a diamond file before spinning the 0.25mm thick wall. I used a tailstock bush to align and press the assembly together during spinning.

Put the motor back together and the damn thing still didn't run.

So why this post ?

It was not a failure, the problem was misalignment all along (like I said I think it had been dropped) so once I got that sorted all was well.

Only Better.

See the video (ignore the bench clutter) :-

It will now run super slowly on a low flame.

Here's what I think: The motor needs to be absolutely free running (nothing new there) however the bending moment on the displacer rod bush generates friction - my bush is 1mm longer than Jerome's original design and that alone will help. However the radically reduced displacer mass is an even greater improvement.

Jerome's design advocates using no oil and no seals or grease in the ball bearings - even the drag from lube can stall this millwatt power engine - however I have always run mine fully lubed including the grease and seals in the ball-bearings.

But I am impressed with now being able to run it so slowly - the video is not a slow-mo and was filmed in very subdued light so you can see the flame.

Hope you get something out of this.

Regards, Ken I
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Thanks Ken 1.
That is really helpful. And I am really impressed with the performance of your engine.
I was toying with the idea of making the displacer from a cigar tube (Aluminium pressing, very low mass!) and your idea of the spun end - correctly aligned - tells me all I need to know.
It was a Chinese kit, and they suggested a ball of wire wool for the displacer, but I could not see how that would act as a displacer with it being a thermal mass, porose to the air in the hot and cold end. But maybe it should work and it is my assembly that is useless?
Anyhow, I'll make the aluminium displacer like yours and re-set alignments, lose the oil drag and try again.
Just a few further thoughts about the design, which may shed some light on the unknowns? (but probably not!):
Re: Post 38 and a few before. My thought experiments - to figure out what is important so I can fix my model - lead me to think that the displacer volume should be as high as practical inside the heated chamber. To that end, the ball-end of the displacer is necessary to fill the glass test-tube internal shape. The clearance between glass tube and thimble - I reckon - could easily be reduced to 0.5mm - as the drawings show an OD of the displacer thimble at 13.5mm and the ID of the glass tube as 14.5mm. If it worked for Jeroen it should be OK. Ken (I think it was your comment?) you mentioned 0.9mm clearance all around the displacer. I can see that this is nearly double the connection CSA between the hot zone and cold zone, but I think the effect is maybe only a loss of a small amount of performance (because your engine works!) and the more significant factor is the displaced volume. You are only "losing" around 7% of the "pumped" hot gas - so a similar effect on the of power of the beast. While it would be interesting to know if a 1mm larger displacer would make any difference, the assembly/disassembly of the displacer suggests you should not bother. But for new engines, making the Displacer OD only 0.5~1mm below the ID of the glass tube seems like a good idea to me.
On thermodynamics - I have been pondering the effect of a larger thermal mass of the displacer, such as the filling of mine with wire-wool - as suggested by the Chinese manufacturer. But logically, I cannot see any reason "why?"... The Stirling cycle is all about rapid pulsations of pressure developed by heating a large volume of gas quickly, moving the gas to a cold zone and rapidly cooling the gas. To have a "hot displacer" - radiating heat - and transiting the cold zone is only going to pump heat into the cold zone without affecting gas pressure. - or even raising the cold zone temperature and overall pressure. So the increase of thermal inertia in the displacer seems like a bad idea to me....?
Of course, experts who have made many models - seen at shows, usually running - often use such things a s polystyrene foam as displacers, where temperature permits - probably for the low mass, but I suspect also for the low thermal inertia of the displacer? An insulator (as opposed to the brass thimble?) would enable the "hot-end" of displacer to remain "hotter" and the "old=end" remain colder, so the thermodynamics are improved by less "wasted" heat transfer from Hot to Cold ends. The only path for the heat transfer should be by gas motion, to maximise the internal pressure fluctuation, which in turn maximises the work on the power piston. To this end, a shiny surface of displacer would also help, as the "hot" part moving into the cold zone is radiating heat, but this radiation can be reduced significantly when polished and shiny. I suspect that if I make a displacer from an aluminium tube, then polishing the aluminium - and possibly filling with polyurethane foam? - will give me the "best" displacer.
That's all my ideas used up now,
Thanks for teaching me about these engines! Theory is good, but practical results prove it.

Not an expert in the field but my findings from some research.

Some random comments about the displacer that might help design decisions. Your ideas line up very well with the ideals mentioned below and your imagination will quickly take you to even better solutions. To set the stage for selection criteria that follows it, I'll define the role of the displacer.

  • The job of the displacer is a thermal switch. It is not a syringe that sucks in air, makes it hot and pushes it to the next guy to work with.
  • During the on state, it thermally connects the pressurized air to the heat exchanger. The gas heats up and and either the pressure increases or the volume increases (both happen, but which depends on if the power piston is pulling or pushing at that point in time). What does it look like when it is on? It moves out of the firing line so the heat exchanger can heat up the gas, not the displacer. Is only the gas in the displacer chamber influenced? No, because of the clearance around the displacer, the volume and pressure change is propagated throughout, reaching the power piston.
  • During the off state, it thermally disconnects the pressurized air from the heat exchanger. The off state is effectively the displacer occupying the area where the gas was and absorbing the heat, preventing the gas from heating up.

To perform this job well, the following ideal conditions are to be aimed for (which both tie back to the overall ideals around zero dead-space and zero friction):
  • All of the gas in the system needs to be disconnected from the heat exchanger when off. More deadspace in the displacer side, means not all gas is disconnected from the heat exchanger.
    • So having a curved displacer to fill the tube end is better (as you have deduced)
    • Smaller clearance around the displacer is better because it reduces deadspace but, smaller clearance increases the gas flow resistance meaning more energy will go into moving the gas away from and to the heat exchanger, effectively acting like increased friction between gas molecules. Net result is less RPM (or output power) for the same input power (same as friction on bearings).
    • One of the factors that influence the ideal clearance is the length of the displacer.
  • Displacer should not conduct any heat. If it does, the previous ideal cannot be achieved because it would become part of the heat exchanger (again, as you have indicated)
    • Because of temperatures involved, the least conductive material that can handle the heat and mechanical constraints should be aimed for. A solid silver (or diamond should you have the $$$) displacer is the worst choice, with a hollow silver displacer being far superior as the air inside does not conduct heat nearly as good. This ideal explains why plastic displacers are used and work in lower temperature engines. A low hanging fruit for hobby machinists is at least making it hollow before moving to exquisite materials like titanium or aerogel. Using steel wool is good from a thermal point of view but has other mechanical side effects (like friction if not contained in a cylinder) and in some instances acting like a regenerator could be more beneficial than it's drawbacks.
    • Filling the displacer with a vacuum and having the thinnest walls possible is probably the most achievable. Coming back to the steel wool, stainless has less thermal conductivity and fibreglass even less. Using foam might trip you when it heats up enough to melt.
    • Using brass vs aluminum is better because it has lower thermal conductivity. Depending on the rest of the engine, galling might be another reason and you already mentioned melting point of aluminum.
These engines are complex analytical beasts as you already know but I feel like understanding the influence of changes at a high level ends up in more enjoyable experimentation.

Hope this view is useful to someone.
Thanks for the explanation! Turned my guesswork on its head - I think?
I had always understood the displacer to be a simple way of pumping the gas to and fro the hot and cold chambers to give the required (timed) pressure rise and fall to power the piston. But knowing "how" and "what works well" from a material viewpoint wasn't right from my screed...
Thanks for the explanation.