Stirling engine question on best method for generator

[large_ghostman]

Hi,

I apologise for the slightly misleading/wrong title, I couldn’t think of a quick and easy way to describe what information I am seeking.

It may be easier if I describe the situation and the experiences that have led to the question.

I have built several engines ((Stirling Alpha and a flame licker (I so hate that name)). They are small and have been built to generate small amounts of electricity, the overall project is for an exhibition on ‘power generation’. I chose to go with a old design of engine and then use this to drive a small dc motor as a generator.

The engines work fine but I soon found that the power output from them is tiny, so tiny that driving a dc motor simply coupled to the fly wheel struggles to keep the engine from stopping.

The electric produced is only to be in the range of around 5V and enough current to power say 10 leds, so I am looking for around 100mA from the set up.

Curiously the flame licker can handle this better than the Stirling engine, I think however this is more to do with the build quality of the engines rather than the design.

So I decided to do away with the motor acting as a generator and move to magnets mounted on the flywheel moving around the outside of several coils. This seems to work better but despite what I originally thought I cant decide which of the following would be better.

Is it better to have the magnets attached to the flywheel and the coil/coils mounted off the engine, or have several coils attached to the flywheel and the magnet mounted off the engine?

Apart from the coupling issue I found that coils moving around a fixed magnet gave better results, however to my thinking I would have thought that it was more efficient to have the magnets on the flywheel and use the extra weight/inertia to help the fly wheel.

I have played with both designs and experimented, time is now short for the final build and I need to decide which to go with. So from a mathematical perspective which should give better performance? I had originally thought that they would both perform the same but this isn’t the case, they do however work better than coupling a dc motor to the flywheel.

I am assuming that in making a alternator style generator (if that is correct?) then a slightly offset magnet on the flywheel that passes by a ring a small coils (8 coils) is the better option, can anyone help me work this out please? Yes I know I don’t come here much but where I normally go no one has any experience of these types of engines!

They have been great to build from scratch though.

 

[cowboybob]

LG!!!

SOOO good to see you again.

When did you break out?

By eliminating the commutator (fixed coils, rotating magnet(s) you've increased the durability of the generator. That, alone, increases efficiency if you take into account the maintenance/downtime costs.

 

[large_ghostman]

LG!!!

SOOO good to see you again.

When did you break out?

I moved platforms from pic to ARM so went to the dark side! I also got fed up with the way things were so left. Its a bit cheeky but I came back for advice as my experiment results dont fit what I am told should happen!
Knowing a few on here have actually played with these engines I thought I would give it a shot! Nice to see you though I hope your well

 

[Joe G]

do a google search on "hamster powered generators" https://www.otherpower.com/hamster.html, some interesting thoughts to think about

 

[large_ghostman]

Hi
Thank you for the link, I am not sure if I am happy or not? On the one hand it ties in with what I found and on the other it means a redesign and then machining a new flywheel for the magnets. It took a long time to get the weight and size of the flywheel right, in order to fit magnets I will have to make it lighter to compensate for the added magnet weight.
I am not complaining just not sure I can get this done in time if it goes wrong first attempt! my RPM's are good and after around 6 mins the flame licker reaches optimum temperature for around 11 mins before it starts to tail off, I found adding a oil dripper helps but you have to get the rate spot on.
Fascinating little engines, I am still not sure which I prefer, the Stirling or flame licker. I use a modified small pencil butane torch for the flame and it goes like the clappers! It has taken 3 months to build though

 

[Joe G]

I used to build the stirling engines, the low differential type. I was also going to add the small magnets to the flywheel, that way I wouldn't have to use a rotary connection (more friction). I still have my last one just for show and tell . And yes, they take a bit of time to build from scratch. mine has a 2mm shaft for the flywheel. They came from an old dc motor along with the nice bearings.
and use Neodymium magnets, they come in different grades of power just in case you didn't know

also just came to my mind, search on home-made bicycle light generators,

 

[large_ghostman]

I have enjoyed building the Alpha type, the flame licker is easier but there is something really cool about the Stirling, most of it I built from scratch using a cnc machine.

 

[crutschow]

For a similar coil and magnet the efficiency should be essentially the same whether the magnet is on the flywheel or the coil is.
But having to use slip-rings to get the power off the flywheel would eliminate the coil on the flywheel approach for me.

And why is added inertia a problem with the Stirling Engine?
Once the engine gets up to speed it should make little difference.

 

[large_ghostman]

For a similar coil and magnet the efficiency should be essentially the same whether the magnet is on the flywheel or the coil is.
But having to use slip-rings to get the power off the flywheel would eliminate the coil on the flywheel approach for me.

And why is added inertia a problem with the Stirling Engine?
Once the engine gets up to speed it should make little difference.

I agree they should both be EXACTLY the same in theory, but they dont work out in practice. Maybe its the friction on the slip rings (I didnt exactly do it like that though) or maybe its the added inertia works better as long as you can get a good weight balance.
I seriously dont know the reasons hence the question, I found that the weight of the flywheel mattered alot. Mostly as you pointed out in the starting of the engine, it proved much much harder to start and never really seemed to get going properly. Then again you seem to have to get everything balanced just right, this time I will remove the same amount of mass on the flywheel as the magnets add.
Using the magnets on the flywheel also allow a larger number of smaller diameter coils to be used, this is pure speculation as I havnt had chance to really test the difference out.
The flame licker is an entirely different beast and dosnt seem as sensitive to weight or balance, that seems to mostly require the timing of the valve to be spot on for best performance. After the exhibition I will continue to experiment with them, not because I think there is any useful energy to be had from them but mainly for the fun factor. I also want to try helium as the gas instead of air, they are great to build and run and I included them mainly for the interest they generate.
I am now looking for tiny magnets! I have tried to balance all the parts on the stirling as best I can, but there is still a slight vibration I cant pin down. I am starting to think this is down to the displacer as by default it isnt a tight fit so has some lateral movement.
My machine skills are still very new so there is room to improve! I wont have time to alter too much before it goes into the exhibition, my worst fear is doing something that means it dosnt work on the day! At the moment the engine runs so I am going to make another flywheel, that way if it goes tits up I have a fall back and can do without the electric generation.
I would like to have a go at building a vacuum engine with internal valve and a better flame nozzle so more flame enters the chamber. I have no idea how I will go about this yet! It might work better if I can make a tiny jet of flame and some kind of cowl to trap wasted heat and transfer that the hot end. Obviously it isnt going to make a noticeable difference but That isnt the point of doing something for learning is it?
oiling makes a big difference on both but too much and you get problems, so balancing that is difficult unless I can tie it too the RPM's in some way.
Sorry I am kinda thinking out loud a bit, thanks for the answers it has helped me decide which way too go. I will add magnets instead of adding more friction with slip rings. Anyone got a link so I can work out how many turns etc for the coils?

 

[Joe G]

I got my miniature magnets from https://www.kjmagnetics.com.

 

[large_ghostman]

Wow they have a great range! I am in the Uk but that dosnt stop me buying from them. I normally use ebay but for this I kind of need a certain size etc in order for it all to fit. There is no way I can get it here in time but I will order for later experiments, for now I will bodge something up using what is to hand.
I am not convinced most the you tube vids are correct with stroke length or timing, I have added a threaded means of adjusting the valve on the vacuum engine, very small amounts of adjustment have a large impact on performance.
I still have no idea why I am bothering with this so much! I have more than enough going on without tinkering with these things!!
But its highly addictive, I will try and get some pics of my engines I am proud of them and the amount of work that went into them.
I would really love a small model making lathe at home but just cant afford it, I have built a small furnace powered from a small bouncy castle air blower, it gets plenty hot enough to melt copper or aluminum, I now also have a vacuum pump and chamber for RTV silicon molding and herculite casting.
It is taking time to build up even simple tools but I am getting there. I want to build a model steam engine and a V8 flame licker that uses pulsed flame.
I will post updates and pictures of the current engines when I get a chance

 

[gary350]

You should experement with more efficient stirling engines. The easiest way to double or tripple the power of an existing engine is to increase the atmospheric pressure. If the pressure increases from 1 atm to 5 atm the power of the engine increases 5 times. You need to build V type engines with a sealed crank shaft other wise the whole engine needs to go inside of a pressure tank. Air works good but other types of gasses work better. I have built several stirling engines but I have only experemented with 100 psi air. A very tiny stirling engine can produce a lot more power for what your doing. A tiny 2 cylinder V engine 1.25" bore 1" stroke can make as much power as a gas weed eater engine. I have blue print drawings if your interested all I need is an email address, where to send them?

 

[large_ghostman]

Thanks alot I will pm you a email address! A V type engine is certainly on my wish list! I was going to do it with a open crank, so would love to see some plans for enclosed crank. I have seen a V8 version on youtube but it somehow didnt sound right. They are really fascinating engines, I had no idea you could get that kind of power out of them!
Because of the Gas Chromatograph machine and a couple of different welding machines I have access to Hydrogen ( a no no), argon,helium,nitrogen I can also make CO2 and Oxygen in good quantity.
I would love to try the helium, and also add a oiling device (this is for the making side of things more than anything).
I have access to a CNC machine (A3 bed) and pillar drill but no lathe as yet, I do keep looking for a cheapy to come up on ebay. I can get limited access to a Lathe but not that often .
Bloody Santa ignored my request for one

P.S
I love the sound of the last engine in your vids! I really do love the noise they make when well made

 

[large_ghostman]

Would you mind if I kind of move this over to a new thread in the lounge? I have been looking at some of your engines!!!
Its nor electronics so I will start another thread on engines

New thread here
https://www.electro-tech-online.com/threads/steam-engines.143511/