First stirling engine - can't get running

[Nut]

Hello. I've just built my first Stirling engine based on the Grizzly H8101 plans. However I cannot get it to run. I've been over it with a fine tooth comb but still no cigar. The flywheel and crank will free wheel for 25 seconds with no pistons attached. If I attach the displacer and mount the displacer cylinder it will free wheel for about 15 seconds and if I add the power piston it free wheels for about 5 seconds. If I apply heat, and disconnect the power piston from the crank and spin the flywheel the power piston jumps up and down but no where near its stroke distance. The power piston is a graphite running in an aluminium bore. It's a good fit (pops on extraction and falls slowly in an enclosed system). The displacer piston is hollow aluminium running in a steel displacer cylinder with 1mm clearance and no rubbing. The displacer cylinder has 1mm thick walls.

The only disparity I can find is the internal working space in the displacer. The displacer is 40mm long and has a 20mm stroke. So 60mm overall and adhering to the general design rule of 2/3, 1/3 . The displacer cylinder however is drawn (and was fabricated) at 68 mm internal length. Could the extra 8mm air gap be enough to stop it and also account for why there is not much jump in the power piston? I'm in the process of machining an arbour and I will take 7mm off the displacer cylinder and try again.

Any imparted wisdom would be greatly appreciated. Thank you.

[Bumpkin]

I googled that plan and it showed a smaller version with a 15 mm displacer and a 13 mm power piston. With the single crank throw and rocking beam design it looks like the stroke is the same for both. If your bigger version has a similar displacement ratio it would take a very high temperature ratio to run, especially considering aluminum parts. Could you change the ratio of the rocking beam to shorten the power piston stroke?

Bumpkin

[Nut]

Sorry, I wasnt clear enough. The displacer piston is 15mm diameter and 40mm long. the power piston is 13mm diameter and 22mm long. They have an equal stroke. I did wonder about the power stroke as I was under the impression that general design rules state that the displacer has 1.5 times the swept volume of the power. The ratio in this one is 1.14. So with a little trigonometry I could reduce the power stroke to close in on the golden ratio. But I don't have the knowledge or experience to understand how significant that actually is.

[Tom Booth]

I liked this little engine, but it required a whole lot of heat to get it started and to keep it running.


https://youtu.be/R_QB5amihko?si=y2BddtnVRVqI3MD_


I liked that it was rugged, all metal construction and could actually take really high heat input.

I found, though, that it had a solid aluminum displacer, which I thought might be the reason for it being difficult to start and keep running. Aluminum conducts heat VERY rapidly and the displacer needs to be hot only at one end.

I replaced the aluminum displacer with one, identical in shape and size but made out of a wooden handle. Being non-heat conducting, I figured there would be better heat separation.

Well, that worked really well.

Here I'm trying to see just how little heat was necessary to keep the engine running, so I kept trimming down the wick on the candle, and even moved the candle so the tiny flame was hardly even touching the engine and is continued running nice and steady.


https://youtu.be/bQ44Rm40unA?si=b3Jw7_qLeE991I-J


It also still ran just as well with high heat as before, which is what burned the wood some.

As I think can be seen the candle isn't really even under the engine, just close to the end of the cylinder, and even kept running when the candle went out or was removed.

[Nut]

Thanks Tom. My displacer is hollow aluminium and perhaps that is the problem. I'll try a wooden one as you did and see if that works. I notice you shielded the displacer, that could also be worth trying. I've now shortened the displacer cylinderto match the piston and stroke but that made no difference. I've tried using an oxy torch but still no luck. Something is inherently wrong I just need to work out what. My aluminium body has slight porisity (pin holes) and I'm wondering if perhaps they are bleeding off pressure. My next step will be to try the new displacer piston, then make a new body and power cylinder out of steel (as opposed to the current aluminium) then reduce the power piston stroke to try and and get a swept volume ratio of 1:1.5. After that I'm out of ideas.

[Tom Booth]

If your engine body is at all porous, that should be, or perhaps, I should say, IS definitely a problem.

Maybe not if the pinholes don't go all the way through and are still air tight.

Is this a casting defect? Why would the engine body have pin holes.

Also, what I said regarding the displacer is also true in regard to the engine body.

If possible I try to avoid metal altogether. Any heat conducted through any part of the engine, other than the gas or air inside the engine, robs power. The heat mostly just "short circuits" through the metal.

Some metals, like Stainless steel are not too bad as they are relatively poor conductors of heat, but aluminum and copper both conduct heat like crazy.

Of course, a good heat conductor is needed for the heat input at the heat exchanger.

Better engine body material would be something like a very low heat transmitting ceramic.

I've seen some small all aluminum engines that will run but your at a disadvantage. If the aluminum is porous, that really won't do at all. The engine needs to be able to hold air under pressure when it's heated. Aluminum might be a little better than copper because it tends to throw heat off to the air rather than conducting to the cold side, but the heat is still lost.

I'm not exactly sure what you mean by "porosity" but there can't be any air leaks in the engine body anywhere.

[Nut]

Thanks Tom. I made a wooden displacer piston which interestingly is about 10% lighter than the hollow aluminium one. Sadly that did not solve the problem. I have started making two steel replacements for the aluminium pieces. Unfortunately I dont have access to any other materials.

The porosity in the aluminium is because I cast my own stock. This was some of the first stock I made and the most porous. The porosity is trapped hydrogen. I believe the engine is air tight but it seems that is the last unturned stone.

The kits that are sold by Grizzzly do work (according to youtube) and are predominantly an aluminium construct.

I'll keep pushing on. It must be close. Thanks again for your wisdom.

[Tom Booth]

It appears from the parts list that several parts of the engine are supposed to be Brass rather than solid aluminum.

This is not uncommon infact, the engine in the previous videos I posted above has brass cylinder sleeves.

Resize_20240217_110822_2510.jpg (227.75 KiB) Viewed 5542 times

https://manualzz.com/doc/23268919/grizz ... ner-manual


Brass, like Stainless steel generally has low heat conductivity, somewhat dependent on the composition of the alloy, but perhaps the engine body could be saved.

In my experience Brass is also somewhat self lubricating or relatively "slippery" compared with aluminum. I don't know how many engines I've seen with aluminum power cylinders that were completely ruined by friction. Some bit of dirt or carbon gets in there or it gets low on oil and time for a new engine block (or a sleeve). Even my small model engines that have graphite pistons don't last very long if the power cylinder is aluminum. The aluminum surface wears away at the graphite quite rapidly, or so it seems to me.

I've replaced some aluminum cylinder and graphite piston combination engines with new glass cylinders with graphite pistons with good results . They seal better, have much less friction and last, so far, indefinitely, compared to the aluminum

It's messy, just glued on with epoxy, but this engine ran way better with the glass cylinder, even though a bit smaller diameter than the aluminum cylinder.

Resize_20240217_112700_0191.jpg (138.19 KiB) Viewed 5541 times

I'm holding the old troublesome aluminum cylinder that was removed.

I did this mainly because the old piston wore out and the new glass cylinder and piston set was all I had, but overall, I like it much better and it seemed like a vast improvement.

The graphite/aluminum combination always seemed to have a lot of "drag"/friction.

[Nut]

You're a wealth of information Tom. Thank you. I'm making a new steel power cylinder. I think I need to invest in some larger brass stock. I only have smaller stock for bushes etc. I'm hoping that although the steel is far from perfect it will give me proof of concept and allow me to refine from there. At the moment all I want is for the thing to spin under its own power.

A secondary question: How much cooling does the power cylinder contribute to the whole ecosystem or is its function purely to convert pressure to !inear motion?

[Tom Booth]

...: How much cooling does the power cylinder contribute to the whole ecosystem or is its function purely to convert pressure to linear motion?

For a gas "heat" and pressure are basically synonymous. Gay-Lussac's Law.

When heat is supplied, the energy level of the gas increases, increasing the temperature.

The increase in temperature is the measure of the increase in energy. (The force with which the gas molecules strike the container walls, or a thermometer). Fundamentally, or on a molecular level, then, temperature and pressure result from or are, measures of the same thing, the energy level of the gas.

So, saying: " to convert pressure to linear motion" is basically the same as saying; to convert the "heat" or "internal thermal energy" of the gas to mechanical motion.

Not really the power cylinder, but the power piston is involved in converting "heat"/pressure into mechanical motion.

But the subject is fraught with conceptual difficulties, semantic traps and historical, scientific controversies regarding the nature of heat and energy conversion and not all are in agreement regarding such matters.

If by "cooling" you mean exclusively drawing off heat by conduction, (rather than conversion) you have again stepped into the middle of a controversy, especially if addressing me with the question.

Obviously with the design of the Grizzly H8101, the large cooling fins are intended for air cooling to remove heat. But your question is, apparently, how much cooling does that actually contribute, relative to the cooling fins on the displacer cylinder?

Again, the aluminum engine is going to conduct unwanted or unneeded heat across from the displacer cylinder to the power cylinder which could cause metal expansion, possibly seizing of the piston and generally short circuiting of the heat from the desired path, which would be through the working fluid, or gas in the engine for conversion to force to drive tbe piston, rather than through the aluminum body, which does not contribute to the desired energy conversion. You want the heat to expand the gas, not the metal, so, IMO not a good design necessitating the removal of such misdirected heat. IMO it would have been better to have had an engine body constructed of some non-heat conducting material and avoid the problem, resulting in a more efficient and more powerful engine. But, given that the aluminum is likely to conduct excess heat, the cooling fins become necessary so this wasted heat needs to be dissipated before it can do any damage, deforming the cylinder and possibly seizing the engine.

I might try (actually I will be) carving the entire engine body out of some refractory brick, like one of these:

Resize_20240219_055655_5805.jpg (63.61 KiB) Viewed 5505 times

or casting it out of ceramic, and there should then be no necessity for such cooling fins.

Now others might say that the cooling fins are necessary in order to remove the heat that is transported to the power cylinder by the working fluid or the gas itself. IMO this is wrong.

The gas does not heat up so much as expand and cool in the process of energy conversion, but I've been banned from several science and physics forums for suggesting as much, so you might also want to check with some of the more level headed forum members on the subject, who aren't building their engines out of brick.

[Nut]

Thank you so much for your input Tom. You have educated me no end. I understand the basic principles but once we delve deeper into the thermodynamics I'm lost. Although I still haven't got this engine working yet, I refuse to be beaten. I just today finished lapping a new power cylinder (mild steel) and have an excellent fit with the graphite piston. You have pointed me to a number avenues of investigation to which I'm very grateful. I'll get there.

[Nut]

I'm proud to report I now have a running engine. It will only run on a little hand held propane burner at this stage with the aluminium disp!acer piston. It runs much better if I put the wooden displacer piston in but it can't handle the heat and begins to burn. So the next step in the journey is to concentrate on maximising the temperature differential so it will run on an alcohol burner. Nonetheless, I'm chuffed that it is finally running.

[VincentG]

Glad your engine is finally running. Try an epoxy displacer made from JB Weld, it will handle the heat and has very low thermal conductivity.

[Nut]

Thank Vincent. Do you make them hollow some how or are they just a chunk of epoxy machined to suit?

[VincentG]

Yes actually I 3d print a hollow ring and set it in the middle of the epoxy and the 3d printed form. It's really not necessary for smaller displacers where weight is not a concern though. I use paraffin wax as a release agent for the main form as well. The ring tends to float to the top so pouring in two stages may be helpful, after letting the bottom pour set a bit. If you don't have access to a 3d printer, I'm sure you could make a hollow section from other materials.

epoxy displacer form.jpg (222.44 KiB) Viewed 5451 times

hollow epoxy displacer.jpg (166.55 KiB) Viewed 5452 times