RESONANT STIRLING ENGINE GENERATOR

[Raymax642]

Brilliant! You have eliminated a lot of friction power loss from moving connecting rods. Straight linear power flow is more efficient.

[iliacht5]

Have you tried to move the left piston while running. Could be just a fantasy but you may hit a resonant air volume.

Also how about moving the steel whool on right side of the flame. I know on every video of hundreds it is just that way but I think the purpose of it is absorb the heat while piston runs forward (and cool it down so some suction occurs) and bring it back while moving backword.

[solarheatengines]

Nice little engine. Have you measured the power output on the linear generator? I'm working on a linear generator for a free-piston engine and am having trouble getting more than about 20mw out of it.

[akinorhan5]

glass>warming>viscosity....

[Zappyguy111]

Very nice!

[tv205]

Have a read of Mark Shelton's book 'The Next Great Thing' there they describe the design of a 5kw linear stirling engine.. using a linear generator. The stroke of the engine believe it or not is 15mm...

[UserIsAnFBIAgent]

Could you use a capacitor to regulate the fluctuating voltage and current?

[KenMacMillan]

Looks like a fun little project.

[redneckhippiefreak]

Some pipe blowers could make you some borocilicate glass tubes for that thing iboro glass can handel the temp and frequency changes plus the thickness can be tuned for the heat sink check around the head shops for a artist or check with a supplyer some times you can get spicific mm.diameter and thickness

[UserIsAnFBIAgent]

AWESOME !!!! I think I've found my new science project.

[piespokladowy]

*****

[spanglerangle]

any plans?

[420840chi]

why does this happen? reply briefly . i'm doin a science fair. how does the pressure change that fast?

[uponmysoapbox1]

is it possible to make a stirling engine which could power something other than an LED light? im looking into free energy and im thinking sunlight with the help of a lens. obviously this example is too small to power anything larger?

[ncbookz]

if he uses swamp gas or biogas he has free power!

[mikerushford]

What is the rod and guide on the right end of the glass tube for?

Will the frequency of the motor change with heat or lengths?

[mowerofdoom]

Hi. The rod guide is there to reduce 'crab' loading on the piston (because the magnet at the other end is relatively heavy and tries to jam the piston). Frequency rises as the working air space is reduced but there doesn't appear to be a relationship between frequency and heat input. Heat input is linked to amplitude however. The hole in the brass is about 6 or 7 mm as I remember - it doesn't seem to be that critical though.

[mikerushford]

Nice adjustment. Can you tell us the size of the hole in the brass part between the hot and cold side of the motor, is this something that needs to be optimized?

[super6plx]

so cool! so friggin cool! it works just on a flame the size of 3 or 4 candles!

[anaxagoras23]

hola este video esta genial utiliza algun dispositivo electronico para causar la resosnancia o solo es con el ciclo stirling

[mowerofdoom]

esto es sólo el ciclo stirling que lo hace resonar. Gracias por el comentario. :-)

[jasondean88888]

@jasondean88888 Sorry about all the replies, it kept saying "error try again" so I did. Gave up and thought I'd try again. Little heat stroke sorry. Been testing a prototype home made passive solar water heater I want to use to run a stirling. It's up to 80% efficient at turning the sun into heated water right now. Haven't tested it's top temp potential yet. Thats today. Anyone know the minimum temp difference needed to run a stirling capabable of generating...say 5 watts? (I'll start small :)

[Traqr]

@jasondean88888 Hmmm... lots of (better) Stirling info on the Internet, but that's kind of the wrong question. Temperature differential only limits the theoretical efficiency - you probably need a bigger engine & heat source for 5W output with a low TD than with a high TD, but even that depends on what other losses & inefficiencies you create by building a HTD engine. Pressure, configuration, working gas, heat transfer (source and sink), regenerator / recuperator, etc. etc. Just start playing!

[Jeffaraujo1000]

what you used in the test tube? I mean the substance that dark!

[mowerofdoom]

Steel wool.

[jimmydorsey2]

I am interested in the stirling heat engine. I am thinking of a solar source for heat and a much larger wheel generator. Are there any sites/books that would specialize in power versus temperature difference for design of stirling engine that you know of?

[nicklockard]

Were you using borosilicate glass? Kymax does well with large temperature differentials. There are others as well.

[mowerofdoom]

Thanks for the comment. I've not heard of Kymax.

Generally I'm using cut down test tubes, but I've been wondering about quartz glass too. I'll do a bit of research into Kymax now you've mentioned it.

Cheers :-)

Geoff.

[yuqichn]

Hey sir! I'm a high school student in San Francisco who is trying to make a stifling engine generator, could you please point out some important parts please?

[mowerofdoom]

Hi. I'd say that because there are so few parts that they are all important! If you are building, I'd recommend keeping friction low, get good sealing (fine tolerances) on the piston and use an alcohol flame. Make as much of the engine adjustable as you can because you'll probably need it! (Stroke, position of stroke with respect to orifice, flywheel inertia maybe). Hope this helps. Geoff.

[steve03820]

eh.....but don't you get light from the flame that's being used as the original power source??

[mowerofdoom]

You're right! But it wouldn't be half as much fun :-)

[jasondean88888]

@steve03820 Yes, but you're not losing that light by creating more. Just an improvment by creating additional light from the waste heat created by the original light sorce.

[jasondean88888]

@steve03820 Yes, but you're not losing that light by creating more from the waste heat of the original light sorce, just an improvement of efficiency.

[imimicc]

hello, what piston and cylinder do you use?

[mowerofdoom]

It's a hard aluminium piston running in a glass bore. Graphite in glass would have been a better option. :-)

[clubcalichad20]

Wow ive never seen one of these. Whats going on in there? So not only are electricity and magnetism intertwined, but heat too? Thats crazy

[fernandesilyt]

Can I make a thermoacustic sitrling engine with a common test tube and a glass syringe, or have some "rule" for the engine work?

[mowerofdoom]

Hi. Yes, you can! I do not know the rules, but if you copy first, this will give you a better chance. Make sure you use borosilicate test tube and a syringe designed for autoclaving. Soda glass will not stand the heat. Good luck!

[ChangeWasALie]

Finally, someone made a Stirling engine do something.

Impressive.

[DragonTamer2345]

@ChangeWasALie Lol, my thoughts exactly.

Now it just needs to be balanced...

[theonethatgoofs]

this is wonderfull.keep it comming

[jtfunkymojo]

It runs pretty fast without the flywheel.

[josephdupont]

wow you are making electricity!!! It is great....

a capacitor would smooth it out.. more more more...

[josephdupont]

what about putting a magnet in that plunger?

a coil and you make electricty..

[AALiberatore]

Great.

I am looking build a thermoacoustic engine. Have you developed this farther? What did you use as regenerator?

[mowerofdoom]

Hi. I've made a few different versions and am still experimenting - I'll hopefully be posting more stuff soon (ish). The regenerator is fine steel wool.

[sniegol445]

6 volts is quite imperssing result

[cool70200]

pretty cool,

I want to build me some stirling engines, but I need to find the blueprints and materials

[knifehunter1900]

great job can you try a hho torch as a energy sourse and loop it so it can make the energy to power the hho engine free energy if ya can do it

[mrcss]

epick fail

[josephdupont]

bravo..very very interesting..how long have you had it run?

could you power it with a fresnel lens??

[firedude201234]

generator and tatoo needle gun haha great!!!!

[32937isready]

Could the electric output be enough to heat the front without the flame. If someone could do that, we would be free.

[mowerofdoom]

I think the answer to that one is no. :-)

[waditto404]

Awesome idea of an automatic desktop crack pipe that does the breathing for you!

[fortytwotheanswer]

Excellent

[hottermale]

Well done!

Can't wait for the next vid.

plz make these changes in next prototype.

1. Use of electic heater to measure input power.

2. plz measure output voltage and current.

so we can find out is it better/efficient than normal stirling engine or not?

thanks and Best luck!

[mowerofdoom]

Electric heater is a good idea - hopefully the next engine will be a 'research' engine - so that would be very useful. Better than a standard stirling - probably not, but we can hope!!!

Thanks for the interest :-)

[thetechnosmith]

Great work! Consider: Aluminum expands at a higher rate than glass, so of course there will be tolerance problems as temperature changes.

Suggestion: for a piston, cast graphite powder (used in lubricating locks) in epoxy. Make it graphite-rich, because though graphite changes very little with temperature, plastics expand with temperature far more than metals.

[richieT737]

Very nice work. I was wondering how you managed to get a very tight fit with low friction using the aluminium piston. I have tried and either got too much friction or the fit was too loose for oscillation to commence. Rich

[mowerofdoom]

I turned the piston in the lathe and left it slightly tight. The rest was polished off with metal polish by observing the small drag lines. I used a hard aluminium to avoid galling in the tube. I've had a few alunium pistons stick this way and they weld themselves in permanently!

Recently, I've used old carbon brushes. These can be finished by rubbing on paper, are lighter and provide less friction. The next best thing to graphite maybe. Cheers, Geoff

[djangobelgo]

This is great! Waiting for the metal one!!

[30GB]

lol same happened to me after filming my tube cracked see my videos for my projects on this lol

[42n8]

very cool... methinks the variation in output reflects a spring resonance problem.

[mowerofdoom]

Are you thinking that two resonant frequencies are mixed to give varying amplitude? I would love to know what you are thinking!! I agree that I am probably wrong about non-concentric magnets being the cause. I was then thinking it could be friction/tight spot as the piston rotated......

Thanks for the comment. Regards, Geoff.

[42n8]

Hi Geof, Althought a bit irrelevant since the machine is now dead but I note that Garret has picked up on the perturbation I mentioned. However...

[42n8]

The spring must return to a steady state during the pistion travel time. If it doesn't then it will cause a modulated output because of differing btravel lengths. When being struck the spring can be in one of three states. Expanding, Neutral or Contracting. If the spring is expanding it takes more energy to overcome the expansion force than it does to compress an already contracting spring

[mowerofdoom]

An interesting theory. I cannot say for sure, but I think that the spring would probably be in its low energy state when hit again. I'll check with a strobe when I run one of the other "free piston" engines next. Should be able to 'freeze' the motion and see what is happening.

Thanks for the comments! Geoff.

[ozykiss]

Giday Geoff, Did you figure out : 1./ Bore 2./ Stroke 3./ T kold 4./ T Hot 5./ Frequency 1 6./ Frequency 2 7./ A CRO picture would be good. 8./ Volts 9./ Amps

[ozykiss]

10./ How many cases of beer it would take to figure out the beat frequencies.

NOTES:

The beat frequency does occur in sync with the rotation.

The stroke increases when the brightness increases.

I think there may be something to your 'tight spot" theory.

Garret

[ozykiss]

I am guessing around 0.03 Watts ??

Garret

[mowerofdoom]

Hi Garret. Excellent observation on the piston amplitude - Hadn't noticed that before. Whether it is a tight spot, or whether it's a beat is still unclear. It could still be either. The only way to prove it would be to weight the piston and stop it rotating. Unfortunately, this engine met with a sad end so we will never know.

V&A, power etc. were briefly discussed early on in the comments - about 10mW I think. This is obviously very poor, but the engine was really only for 'proof of concept'.

[liveuk]

Proof of concept was 1800's when this was invented by the vigar lol not you

[mowerofdoom]

I'm not claiming that the principle (or whatever you'd like to call it) was either invented by me, nor that the original concept was mine. I just meant that I was proving to myself that this layout could be made to work. Just thought I should make that clear - don't want any misunderstandings :-)

Thanks for stopping by.

[dgalvarez]

Absolutely brilliant! Excellent work! I´m trying to make my own lamina stirling but it dont want to run :(

5strs!!

[1BustedMyth]

Nice !

[RichardBoSchroderDK]

from "your" design, i would like to build a large scale engine, heated by my woodpellets stove , cooled by centralheating returnwater , also with generator attached. It looks simple, but i quess its not. Any good advice for me ?

[mowerofdoom]

Hi. My advice would be not to use this type of engine! Using one with a displacer will make a huge difference to efficiency. I have not built any large engines so cannot comment further. Good luck!! Geoff.

[momodragon]

very nice looks better then having a flywheel this prolly has less friction and looks more energy effecint great work

[whotoinfinity]

Geoff... Beauty Motor Mano! Once this is posted I'll be subscribing.

Peace

Gill

[joliteal1]

? neodimium magnets for generator (more power or reduced oscillation?

?metal tube for increased heat transfer?

Al heat in must get out, Fins for the heat out area?

IT IS NICE!!

[electrician725]

Seems like the LED brightness variance was in sync with the flame brightness variation, or was that just the camera adjusting to the LED's???

Good video!

5 Donuts :D

[rabricci2]

Your resonant stirling engine generator is truly a cutting edge design. Your linear arrangement with the recoil spring and and piston incorporated with the coil was magnificent!! roland

[netlethe]

This is brilliant work .. thank you ... hope you'll join us.

Cheers,

Red.

[caerleon87]

Now i do not know anything about stirling engines, but presumably the thing blew up because of the thermal stresses in the glass, caused by the rather steep temperature gradient along the length of the thing. Could you have the glass tube in two halves with an insulator between the two bits??? But anyway, ABSOLUTELY AMAZING!!! I am well impressed with that!!!!

[emorag]

this is great!

[DannysCam]

dude you ROCK

[GetMeThere1]

Lovely! Thanks for the full and detailed commentary.

[mstuffbox]

WOW, I hope you continue your work on this engine. science in action!!! has anyone tried using dry ice as a power source?

[Ermineboy123]

I am normally not interested in this sort of thing being only a kid but when my grandpa reccomended this video i suddly took an intrest Thanks

[mowerofdoom]

I am very glad; you have made my day by telling me this.

There is a saying that "Nothing is interesting if you're not interested...". I think this applies to most things in life.

Thanks for taking the time to post. Geoff.

[thoxbui]

If you are interested: at the end of your video, the engine is making a sound of 30.5 Hertz pulse. Assuming that each of the sound pulse is cause by the end of the stroke, then your engine is running at 15.15 cycles per second (or 909 rpm). Nice work.

[mowerofdoom]

Thanks for that. 15Hz sounds about right. My long term aim is to bump this frequency up to hopefully double that by reducing piston mass (amongst other things).

regards, Geoff.

[RetroTheory]

How much power was you producing 1.5-3v? I am in the process of designing an ltd Stirling engine w/ a generator any info is GREATLY appreciated

[mowerofdoom]

Hello. Here's the info... 3V open circuit - power 8mW very roughly Coil 24V 25.6ohm. Heated tube ID=19.24mm od=21.97mm Length variable. Piston tube ID=16.19mm OD=18.41mm Piston length 16.2mm Aluminium Rod - carbon fibre Vent holes in rod guide 4x4mm dia Orifice 6mm dia Orifice length 26.4mm stroke=10-15mm typically dead space (unswept distance in cold side) 15mm length If you need any specific info, you can message me if you wish. Kind regards, Geoff.

[ironmonk2007]

Concerning fittings for the orifice that you used in this design, did you machine them or were they purchase from a store?

[mowerofdoom]

They were machined from solid. The o-ring groove depth is the only critical dimension though (just to get the right pressure to seal).

[mjdevink]

Splendidly well done. Just about the only stirling engine on youtube that actually produces some useful work. I agree with your naming that it is "resonant" rather than thermoacoustic. If it was thermoacoustic it would be humming at about 800hz, given the length of your hot end.

Looking forward to seeing your version 3. Cheers.

[Rockinghorseshart]

Just wondered if you'd seen the TMG Stirling video on YT built from kitchen utensils and generating 5 watts? I didn't see it in your favourites. Wikipedia gives more info:

The Harwell TMG Stirling engine, an abbreviation for "Thermo-Mechanical Generator", was invented in 1967 by E. H. Cooke-Yarborough...

Good Luck with your projects!

[mowerofdoom]

Hi. Yes, I've seen the TMG video. It's certainly an interesting machine to think about. The builder of the youtube version was kind enough to send some information on the original harwell machine a couple of weeks ago.

Give it a couple more weeks and my wife will be asking where all the kitchen utensils have gone.... ;-)

Thanks for the post!

[patertjepeet]

Very very interesting and rather "easy to implement" concept. I think this has a high potential, also in terms of efficiency, due to lack of converting and up/down (or left/right) movement into rotation (giving some loss). Do you use neodyme magnets? I believe you can get more electrical power using a smaller form-factor electric generator. I'm *VERY* curious in the amount of electric power you can draw from your Stirling setup.

Regards

[mowerofdoom]

Hi. I share your thoughts on a potential use and am sure that this engine has more to give! The magnets are neodynium/boron/iron but the air gap between coil and magnet is large (so efficiency is not so good). Certainly a smaller, better design coil or coils are required as you suggest. I am continuing to learn about these engines and share your curiosity on how much power they can produce. We shall see!

[patertjepeet]

Btw, can this lamina flow stirling also be made using a full metal housing, instead of glass? It's easier (for me) to get metal pipes or so instead of heat resistant glass. On the other hand, I can imagine too much "heat flow" between hot and cold parts of the engine (but probably less when using proper silicon rings, like you do). Bye

[phdecora]

thanks for posting. this is really cool. would love to know more about construction techniques.

[mowerofdoom]

Hi. Thanks for the interest. If there's something specific you would like to know, please feel free to message me. There's also a good amound of information around the net on 'laminar flow' or 'thermal lag' engines.

[electrique527]

I believe this should be called a "resonant thermal lag engine".

I think these things are essential:

-The resonant frequency of the piston and the air column

-The delay in flow between the hot and cold sides through the hole

-The heat transfer on the hot and cold sides

I believe the first 2 are crucial and must match precisely, the 3rd probably determines efficiency and power.

[mowerofdoom]

Your comments provoke many thoughts!

I agree. Thermal lag engine is the most descriptive of all the things it has been called.

I think the mass/air spring movement is a sine wave, but the heating/cooling has its own skewed sine wave. Matching waveforms exactly is probably impossible - its a 'best fit' scenario. The not side has one heating rate, the cool side has a different rate & the hole introduces a delay. I think this is the reason why the piston drifts - unequal forces =>unequal leakage.

[mowerofdoom]

Efficiency may be related to how well matched the waves are (so the piston resonance is not fighting the forces from heating/cooling) and ,as you say, overall power is related to heat transfer.

It's certainly an interesting engine to analyse. I would love do do some mathematical modelling but it is very complex!

I do wonder whether having a cold side regenerator would help make heating and cooling rates more equal & make the wave symmetrical (for better matching to the 'spring/mass' waveform.)

[IronGoober]

Absolutely brilliant! Excellent work sir!

I don't have any suggestions for improvements because it seems you've already received quite a few...I just have to say, this is probably one of the best stirling videos I have seen. I really think everyone should be trying to generate power with their stirling models.

[gilbondfac]

bravo!! i like your different thermoacoustic motor but I try to buid it but not work ...please have you got some plan please!

have you see my stirling "jump stirling"

thanks!

[mowerofdoom]

I noticed your video not long after you posted it. It's certainly different! Someone commented that you should try to extract some power - why not have a go?!

Thanks for the post and video response. Geoff.

[electrique527]

Hi,

great work!

I love watching this video time and time again :)

Did you develop this little machine further?

- Put the magnet inside the piston, the coil outside around the tube. Seal the engine and increase inside pressure.

- Make a double acting machine in a U-shape, so the hot ends are next to each other. The piston can be in a short straight part between the legs of the U :)

Did you study NASA's free piston engines for space use?

Your engine is simpler... :)

Richard

[mowerofdoom]

Hi. Thanks for all the comments and suggestions! I'm currently struggling to develop a version 3 so will post another vid when something interesting happens. I like the U-shape idea. Some have suggested twins and V's too.

Have seen some NASA information - they seem to have the small advantage of reliability, efficiency, ruggedness and usefulness! :(

Geoff.

[lecorfec]

Good Evening, Your coil looks like a small "Foucault current" brake. If it's the case and I was wondering about electrical efficiency(if your magnets are "co-axial" oriented as well as the coil then the "cut field" is not optimal: most of the energy will be lost. If the magnet is moving on his magnetic axle then the electrical conversion efficiency will remain low.(it's my understanding but I am not a specialist.

Disc magnet with "radius magnetism" exist but i do not have any idea about coil!

[lecorfec]

Bravo! Merveilleuse Expérience!

The engine doesn't seem to be near to stall! It gives the impression that much more power can be harvest.

Do you use the back of the piston as a pneumatic spring?

I guess trimming both engine and generator to reach the highest electrical yield will be a challenge!

The second challenge : building the first flat twin.

Really impressive work!

[mowerofdoom]

Hi! You're right - the engine is not under much load and I am sure that there is more power available if designed carefully.

The back of the piston is is able to push air in and out through holes in the rod guide - the piston always tends to drift inwards, so rear cushioning has not been necessary. (I do wonder why it does that though).

Love the idea of a twin - the mind boggles at setting up such a beast!

[kheston]

Great stuff! Thanks for posting.

I'd be curious to see some efficiency metrics. Specifically, what power are you getting out of the engine for what fuel expenditure?

[PalmRunner]

Remarkable experiment! You've realized what I wanted to do with my engine long time ago. Actually I also successfully generated some electricity using stepper motor as a generator, but again enough to light only a LED.

I would suggest you to try connecting a resonant capacitor in parallel to the coil. You can play with different values. If sized properly the capacitor would introduce voltage to current diphasing, which may have an effect of "electronic" flywheel.

[mowerofdoom]

Thanks for the suggestion. I may be completely off track here, but does the capacitor and coil inductance make a tuned LC circuit? In which case I should aim to match the natural frequency of the engine to the magnet/coil/capacitor (?)

Thanks! I will start collecting some capacitors and try it out....

[PalmRunner]

Some time ago I have constructed a simple prony brake to measure the power of my Lamina. It came out with an average shaft power of ~130mW. With some cooling it reached 220mW for a short moment. The peak power was at about half the maximum engine speed (~1000 rpm). That's why I think you should tune the LC group to the half of your max. frequency. You'll need to find a way to measure the coil inductance and then it's easy to calculate C for a definite resonance frequency.

[PalmRunner]

From my experiments I found that without a flywheel the engine is very unstable. It stops as soon as it is loaded a little, just because you force it outside its resonance. With a heavy flywheel there is a much broader range of operating speeds. This means that the engine does not produce power during the complete piston stroke and needs some energy storage element. In your case this would be the tuned LC circuit, which will deliver energy during the time where the piston does not produce any.

[mowerofdoom]

I re-built the engine today after its little accident and also fitted a rectifier. I was getting 3V open circuit, 0.2V short circuit with a current of 40mA.

Very poor magnetic coupling combined with a weight limit on magnet size is the problem. Started on Mk.III today!

[PalmRunner]

The engine should be able to produce some (100--150)mW of shaft power. If your measurements are accurate you get only 8mW of electrical power (0.2V x 40mA), which is in my eyes too less. I guess you should optimize the generator (stronger magnet, better magnetic coupling) or simply the engine does not operate at its peak power frequency. In my case it was approx. half of the maximum frequency.

[electrique527]

The rectifier causes a voltage drop, which is considerable at these low voltages.

No need to add a rectifier, you can just measure AC directly.

You can use resistors as a load.

Also, to determine maximum power, make sure you measure at different loads. The power delivered is maximum for a certain load. If I remember correctly the impedances should match for maximum power output.

Power output is at minimum at low load and at short circuit. There is hope! ;)

[electrique527]

If the engine keeps running when shorting the output circuit, then it means that it has more power left... :)

Oh... BTW, how can you have 0.2 volts at short circuit? Thats not correct right? Also dont forget the power in the rectifier diodes: 0.6 volts x 40mA = 24 mWatts per diode!

[mowerofdoom]

You're right. I was measuring current & voltage using an array of patch leads which resembled spagetti! I must have had a bad connection with some resistance.

The reason for using a rectifier was because this is how I would envisage using the power; also because the AC wave may not be a sine wave and I do not have a true RMS meter.

The engine did slow at short circuit (or what I thought was short circuit) so yes, there's hope yet! The magnetic coupling was poor too.

[mowerofdoom]

On the subject of power I can see exactly what you are saying regarding the bridge and matching the load. I hadn't really thought about it before.

I've just finished another engine which I rushed into during a spare afternoon. I'm going to post it anyway but I've made some bad errors because I was making it up as I went along. Bad decision!

You're bringing some really interesting and useful info to the project. I do appreciate it very much. Kind regards, Geoff.

[PalmRunner]

Some more thoughts on the topic: Similar to the electrical load, the mechanical load should also be matched. Higher electrical load loads the engine also mechanically and it will slow down. This is equivalent to a generator with higher output resistance (less ideal). Therefore, the optimum electrical load should match to the output resistance of the system engine-generator and not only to the output resistance of the stand-alone generator.

[PalmRunner]

Recently I've also spent some time to experiment with my engine. I used stepper motor as a generator and variable resistor as a load. The maximum shaft power for my engine is at ~ half the maximum speed. So, I gradually increase the load (reduce R) until the engine reaches half the maximum rpm. Under these conditions I believe I get the maximum electrical power. In my case I measured 6V @ 820Ohm = 44mW. This was about the maximum I could extract as electrical output.

[PalmRunner]

The shaft power @ 1000 rpm is ~ 130mW, and I get only 44mW of electrical power (only ~30%). The conversion efficiency is not very good, but don't know how to improve it. Theoretically it should be possible to get some 100mW. Some improvement can be achieved by using Schottky diodes in the bridge rectifier. They have very small forward voltage - twice less than a regular diode (0.25-0.3V). I have question: Do you know some parameters of the coil you use: resistance, number of windings, size ...?

[mowerofdoom]

Hi. I tried to reply last night but nothing has appeared here.....so here it is again!

Schottky diodes - I was too impatient to order any and ripped appart the nearest PSU to get the parts - just to get something going, but it's definitely worth a go. The engine is now in pieces so I'm planning another at the moment. This will certainly include them.

To answer your question - I tried 2 coils from actuated valves. 1st: 240V 2.6K, 2nd: 24V 26.5ohm. The 2nd was the best by far.

continued....

[PalmRunner]

...> Therefore, in order to operate the generator at its maximum efficiency I have to load it with 400ohm. But 400ohm loads the engine too much and the speed drops below the optimum 1000rpm. I used another gear ratio, which allowed the engine to run at 1000rpm while loaded with 400ohm at the generator side. With this setup I was able to measure 5,6V @ 390ohm load, which corresponds to 80mW. Unfortunately the engine maintain this output only for a short moment :( ...>

[mowerofdoom]

cont'd....

So the 24V was the one I used. Sorry, I don't know wire size of No. of turns.

Maybe your stepper motor is not designed to run at 1000rpm - is there an inductance sort of thing going on? You're changing the field over the coils very quickly. Maybe the inductance is coming into effect. Eddy current losses maybe? Is the resistance of the windings included in the 820ohm? What about using a 'solar motor'? The coupling in these is very good and commutator friction losses are small.

[PalmRunner]

Actually I have 2 gears to reduce the speed, so the stepper runs at <300rpm. The stepper motor has 2 independent coils, 150ohm each. I have a Schottky rectifier on both, as the rectified outputs are connected together (working in parallel). The equivalent output active resistance is then 75ohm. But there is also reactive resistance due to the coil inductance. I thought it is neglectable, but I measured the complete output impedance to be ~400ohm.

[PalmRunner]

...> I think you should also try to match the load of your coil. The easiest way is first to measure the voltage output of the unloaded coil. The resistive load at which the output voltage drops to half of its maximum value is then the optimum load. This means that the loading resistor is equal to the coil output impedance.

[PalmRunner]

The current design of Lamina is desperately inefficient. It produces only 200mW of shaft power. Assuming that a small alcohol burner has about 50W, we get only 0,4% efficiency. In my understanding this is due to absence of a displacer. In a normal Stirling there is a 90deg. diphasing between the piston and the displacer. In Lamina a small diphasing is achieved through the orifice, but this is hardly controllable and perhaps less than 10deg. I think this determines the poor Lamina efficiency.

[mowerofdoom]

Thanks for all your comments. I think the tuned LC circuit idea is good, but may have practical issues. The resonant frequency of the engine (mass and air 'spring') will change with temperature so any tuned LC circuit will only be exactly matched at one temperature. I am not sure if this would have a large effect or not. Also ... cont'd.

[mowerofdoom]

I also wonder whether the phase difference gets closer to 90deg as freq increases. This is based on the assumption that the orifice introduces a lag which is not directly related to freq (?) so as frequency increases the lag becomes a greater %age of 1 cycle. I may be wrong.....

[mowerofdoom]

I am still trying understand what happens with a LC circuit tuned to half full speed. I assume that the LC side overcomes the mass/airspring freq and force a lower speed.

I will have to think and experiment more on this!

I am also pondering the effect of pulling power if V and I are out of phase and if I would just be better trying a heavier piston & more compression to boost the amount of heat used. I am not worried about efficiency!

Thanks for all your suggestions!

Kind regards, Geoff.

[PalmRunner]

You get power, because V and I are not completely out of phase, but only 90 deg. In fact, a single L has also 90 deg V to I diphasing. Actually, I am not sure if the resonance phenomenon itself brings some profit in this case. My thinking is that a LC circuit returns some energy back to the piston, resembling in this way a mechanical flywheel. This would allow the engine to operate at frequencies other than the natural engine resonance determined by the air spring and the piston mass.

[PalmRunner]

You are right; the engine needs to operate at almost constant frequency, which means constant temperature, but also constant load. The possible deviation of the resonance freq. will be determined by the quality factor of the LC group. Low Q factor allows higher operating range but would generate less voltage at the output. Another practical issue is that at this relatively low resonance frequency you may need a really huge capacitor, perhaps several mF.

[PalmRunner]

I also think so, but maybe with this diameter of the orifice the phase lag is so small that increasing the speed from 1000 to 2000 rpm would for example increase the lag from 5 to 10 deg, so the effect would be hardly visible. Reducing the orifice diameter would increase the lag, but would oppose to speed increase due to airflow restriction, so finally you don't get more lag.

[mowerofdoom]

There is one odd phomenon. When run, the piston tends to work its way out of the bore over a few seconds, or tends to move towards the orifice end - it all depends where in the bore I start it vibrating. Presumably this is due to a change in lag in one direction compared to the other. More variables to ponder over!

[electrique527]

hi,

I cant stop watching this thingy, it is so fascinating! :)

I think the moving of the piston shows average pressure of the working gas. If the pressure increases (the gas gets hotter on average) the piston will tend to move outwards. If it gets cooler with time then the piston will move inward with time ...

[PalmRunner]

You may also try a series resonance - put L, C and the load in series. The parallel resonance magnifies the current, while the series resonance magnifies the voltage. It won't be easy, but it's worth to try at least.

[PalmRunner]

Parallel LC tuned at half full speed would probably force a lower speed, because at the resonance freq. it has maximum impedance (min load). This reminds me that a series resonance would not work, because it has minimum impedance (max load) at resonance so the engine will try to avoid exactly this freq.

[mowerofdoom]

Another fantastic range of thoughts on the subject! Thanks. (I'm replying here to all your recent comments.) The LC aspect is of great interest and I'm hoping to put some rough maths around it soon to find out exactly how large this capacitor/coil will have to be to do anything. Also, I think loading (parallel R)affects Q and need to have a look at this too.

I note your comments on lag and tend to agree - wouldn't it be nice to be able to measure it? Hmmmm.

[electrique527]

Hi,

there is also he lag from warming and cooling the gas.

In this type of engine, there is a lot of dead space, gas that is not heated/cooled. That lowers efficency. The hole / orifice introduces a lot of flow losses, again bad for efficiency... but you need the small hole for the engine to work :(

[electrician725]

Dood, are you the dyslexic french version of ME???

hehee, JK, please hit me BACK!

-Rob

[mowerofdoom]

Hi Rob. Er. Not sure I follow. Anyway. I think the sync must be coincidence. The brightness seemed to vary in time with the piston rotating. Thanks for the Donuts! Geoff.

[electrician725]

The above reply was to Electrique527, kinda the reverse of my screenname as you can see...

Youtube's message service screwed up I guess!