Designing for maximum watt output

[Wellington]

Ive decided to make a 12v stirling generator to insert into the side or top of a small wood stove. I would like it to have as much watt output for its size and will be making a 12v axial flux alternator to intergrate into a flywheel. Can someone please explain a diy method of pressurizing an engine and also tell me which design (alpha, beta gamma ringbom, free piston, piston free) is most suitable for maximum watt output? I'm assuming that pressurising an engine is the main factor that gives a stirling engine its torque? How many bar can you easily pressurise to without regular leek issues?
Some advice on how you choose a flywheel weight and material would also be helpful.

Thanks
.Wellington.

[Ian S C]

For a start I'll suggest that you find and down load(it's free) Andy Ross's book "Making Stirling Engines".
I'll put the advantage of pressurisation as simple as possible.(note temperature is absolute 273.15*K)At atmospheric pressure the max pressure you get by doubling the temperature is around 30 psi, now pump it up to 200 psi and double ambient temperature (say 20*C + 273.15*K) and you have something like 400 psi to work with. The engine must be totally enclosed, so that the pressure is on both sides of the power piston. The main problem is usually sealing the crankshaft.
I would suggest not going above 200psi, similar to the Philips motor, and stick to air at these pressures. At higher pressure the engineering gets quite complicated, and the gas needs to be inert, and hydrogen, and helium begin to be important.
Ian S C

[Wellington]

Thanks Ian. I'll read the Andy Ross download today. Useful info you've provided. I'm still undecided on which type of engine to build and am leaning to the simplicity of a beta design. This animation: http://tinyurl.com/zm4brlm seems like a very simple design with ultra low friction but I'm wondering if a linear alternator has the stroke length to provide decent wattage with just one magnet ring possible with a short stroke length. I'm also wondering how the magnets on this design attached to the power piston cup slide past the copper coils without the magnetic field sticking the magnets to the coils or the magnet cup channel walls and creating friction?. The magnets and coils appear to be surrounded by iron to focus the magnetic field intensity and the power piston shaft seems to be inserted into a piece of flexible tube to stabilize any horizontal shaft movement back to a central position. It seems the power piston cup is moved by the pressure wave created by the displacer and then is repelled backwards to hot end by the planar spring and/or the vacuum of cooling working gas?. You opinion on how this is working and your opinion on if this is a good design would greatly help. Its Not a newbee project I know but something to work towards. The reason I focus on this design is because it is geometrically simple and also the greenpowerscience channel pointed out that friction is the number one performance enemy of the stirling engine.
.Wellington.

[Ian S C]

If you do look toward the free piston motor with a linear alternator, the stroke should be very short so that the frequency is high. the alternator obviously generates an AC current, and ideally something near to 3000 strokes per min/50 Hz gives an easily managed power supply, my GAMMA type free piston motor runs at about a third of that, and after rectifying it requires a lot of smoothing if I run a radio off it.

[Wellington]

Nice looking project. How many watts output does it achieve?. Do you know if anyone has ever tried multiple levels of wire coils in a linear alternator stacked one on top of the other and what issues this might pose?
.Wellington.
PS. two other linear projects that look interesting:
http://tinyurl.com/j6vb2z6
http://tinyurl.com/hyhw84e

[Ian S C]

On each side of the magnet you will see what looks like a pair of transformers, they are! But with the I laminations removed, and leaving the Es in place, facing the magnet so that the magnet passes three poles per stroke. To increase voltage the two transformers are connected in series, to increase current, connect in parallel, it has a full wave rectifier. It only produces about a Watt, but it looks impressive. The bit up on the left side is a radiator for the water cooling. The power piston is sprung by using another magnet under the power magnet with like poles facing each other. Magnets from old microwave ovens, and large speakers. Inefficient, makes a bit of noise, it works!
Ian S C

[Wellington]

That's a real MacGyver masterpiece. I only wish I found stirling engines sooner in life. I remember dismissing stirlings years ago due to low torque but it would seem they can meat your needs if you can learn to be frugal with watts. I am researching how to build a 12v inverted stirling genset from junk but cannot find any info on how to design and make an axial flux alternator to charge a 12v battery. I assume I must first know the max rpm of the flywheel before I can calculate what strength magnets, which gauge wire, how many wire turns to use etc. I don't know if to make a dc motor or an ac alternator with bridge rectifier. I'm assuming the advantage with the alternator is the more volts you produce the more X Amps you have and watt output? but can a dc motor with the same quantity of magnets and wire give the same amount of power as an axial flux alternator made from scratch? Newbee questions I know but your guidance on AC/DC choice and How to design a 12v alternator/DC motor for a stirling would be much appreciated. I assume I will also need some kind of speed control mechanism to prevent meltdown?
Regards
Wellington.

[Ian S C]

I use a mixture of DC generators, and AC alternators, but I'v had best results from generators made from small DC motors. The voltage is controlled by the load on my machines, I have a wide range of pulleys, and belts so that I can alter the gear ratio between motor and generator. For what you are looking for, find a 12V motor from an old printer or something like that, you want one with reasonably low revs, because you will need to spin it up to it's full speed plus a little bit. For belts I use 3 mm heat joinable plastic belt material, and V pulleys made from scrap metal.
[img]

016 (640x480).jpg (136.67 KiB) Viewed 6975 times

[/img] This is an alternator made by modifying an old 6 pole shaded pole electric motor from an old fan. The new armature has two magnets from a microwave oven in a 6 pole claw type armature.
Ian S C

[Wellington]

Thanks Ian. And what MacGuiver solutions do you know for pressurising a stirling engine with air? Is there a simple way using a bycicle pump or something?
.Wellington.

[manvstaco]

Holy Cow Ian!! I love your engine! I'll send you a PM so I don't derail this thread.

[Wellington]

For a start I'll suggest that you find and down load(it's free) Andy Ross's book "Making Stirling Engines".
I'll put the advantage of pressurisation as simple as possible.(note temperature is absolute 273.15*K)At atmospheric pressure the max pressure you get by doubling the temperature is around 30 psi, now pump it up to 200 psi and double ambient temperature (say 20*C + 273.15*K) and you have something like 400 psi to work with. The engine must be totally enclosed, so that the pressure is on both sides of the power piston. The main problem is usually sealing the crankshaft.
I would suggest not going above 200psi, similar to the Philips motor, and stick to air at these pressures. At higher pressure the engineering gets quite complicated, and the gas needs to be inert, and hydrogen, and helium begin to be important.
Ian S C

Hi Ian. I was wondering how these pressure issues relate to the design of a free piston linear alternator?. Should I be looking first into pressurising a linear free piston design If my goal is to generate as many watts as possible OR is the speed of a linear pistons fwd and back motion more important than torque and horsepower (from pressure) to generate maximum watts???????. I realise HP Torque and Linear Speed all contribute to watt output but which is more beneficial in a free piston linear design with only magnetic cogging to overcome?????????????????????????????? I was put off from steam engines by the need for a potentially explosive boiler but now I'm starting to wonder if high pressure is unavoidable in stirlings too if im seeking a decent 1000Watt hrs per day goal.