Stirlingengineforum.com

Hi!

I'm making a 50W Solar Powered Gamma Stirling Engine, that will operate on a hot end temperature of 250 degree Centigrade. Can anyone please help me in designing the crank mechanism & connecting rods for the engine to transfer my power?

(I will be using an alternator to generate electrical power by coupling the crank shaft with the alternator)

Regards,
Osama

Osama, is your motor going to be 50 W, or do you hope to get 50 W from the alternator? Ian S C

Hey Ian!

I hope to get 50 W from the alternator.

Osama wrote:Hi!

I'm making a 50W Solar Powered Beta Stirling Engine, that will operate on a hot end temperature of 250 degree Centigrade. Can anyone please help me in designing the crank mechanism & connecting rods for the engine to transfer my power?

(I will be using an alternator to generate electrical power by coupling the crank shaft with the alternator)

Regards,
Osama

What type of alternator? How are you coupling it?

50 watts isn't a lot, so you can keep the mass of the crank assembly pretty low. I'd think very high quality bearing at each critical point is a must. Even in a larger engine that friction is a waste, so eliminate all you can. I'd go with a longer throw and lower RPM engine to build in reliability. Look at composites as carbon fiber and other exotics are extremely light and strong.

R

Hey R,

I haven't decided the alternator, but either be coupling it with the shaft or will be using a gear mechanism.

What I need to know is whether there is any sort of calculation for the diameter of the connecting rods?

Osama

You could work out the size by calculation, but I would use something like 1/2" dia, or what ever size of ball race you happen to have, 50 W is not a great amount of power, the motor on a portable fan, or similar has a shaft of about 1/4" dia.
Found a good spot for a solar hot air motor on the TV news last night, the windows of a sky scraper in London UK is concentrating the sun light on the street below, melting the plastic on cars, one a Jag actually started to melt(aluminium I suppose), and setting the carpet on fire in a shop over the road, and the seat of some ones bike started to smoulder. Some one stuffed up designing the building, it's not that the sun has changed it's position. Ian S C

http://www.visualphotos.com/image/1x778 ... ne_odeillo
Ian SC
The Britt that designed the building in London must not have done much traveling in France?

Thank you Ian! :) I will make sure to take your suggestion for the diameter.

I can't find any material to work out the size by calculation, so if you have any link to that, kindly share it. Thanks again!

It seems there are a few buildings in USA that do the same, it's all because of the curved glass walls. Ian S C

Isn't the rating of most adult bicycles far greater than 50 watts? Why not consider bike bearings/races and hubs/axles. They're already made. The offsets for the throw(s) could be made to "pinch" the main crank and slide away from, to towards, center and change the stroke length accordingly. Simple split plate, or pipe/tube. A couple tiny bolts n nuts and it's locked.

Again, 50 watts isn't that much. My first solar panel, a BP 75 watt 12V, is still putting out as if it were new. It's 11 years old. My point being that if you're gonna build this thing build it so that it's easy to repair, but rarely need it. I like machines that last a long time, and bike parts are gonna be around in piles forever. Hell, I bet 3 steel spokes would make a fairly stiff rod, if wound with fine wire.

As for using gears to drive your alternator I can't go with that. You engine needs to run independently, and even with clutching the gears are going to have to be either nylon/synthetic or lubricated in some way. Finding the right ratio to harvest the power from the crank to match the output of the alternator would be a bitch.

I'd use a very soft belt, maybe a leather strap, to drive a dual disc axial flux rare earth magnet alternator. (for 50 watts a simple DC brushed motor would work but for how long?) Pulleys/belts are easier to ratio than gears, are more quiet, CAN offer lower frictional losses, hate lube and can easily be clutched with a spring loaded idler pulley that has a travel stop both ways.

The reason you need the engine to run by itself is you need that to figure out everything else. Worry about the engine and then make the alternator match after it's spinning right along. You can experiment with load using a simple torque balance. When you know it is putting out at least 60 watts can you expect anywhere near 50 watts of electrical power. 75 watts at the crank would be better, as mechanical and electrical losses are unavoidable. Unless, of course, you have the budget of NASA and can use superconductor coils in your alternator.

The flywheel itself could serve as the disc for one of those alternators. If a series of reversing pole rare earth magnets were embedded in the flywheel and the coils moved nearer or farther away to engage the generation of power. This would take one hell of a lot of testing and ciphering to get right, but would probably be the best way. The field coils, built to move, could be rebuilt to match until a desirable/matching set was made, and then a duplicate could be made in reserve. As long as the magnets, and flywheel, don't take any damage they're not going to lose any strength in a human lifetime no matter how much power they generate. These magnets can't take heat though, so a hot flywheel would be a big no-no.

Hell, the crank from a bike, sans pedals already has the rear fork to mount on the cool end, the chain offset would make a good rod passage, and the seat post tube could be used to mount a little 50 watt alternator/motor. I can see it now, the "Bike Stirling". Why not?

Sorry to ramble... I'm off work today, and I've already done my chores!

R

A bike would be designed for a least 300 watts, an average human can do about 150 watts, and you need to design well over that.
For power transfer, use V pullies, and round heat joinable plastic belting, you can get that from 3 mm diameter, up to at least 12 mm. The pullies are easy enough to make on the lathe, make a reasonably large number of different sizes. Instead of a steel grub screw, I use a brass one while changing pullies often, so that it does not damage the shaft, when and if you find a permanant pully, you can then use a steel screw. You could use bike chain. Ian S C

Thank you everyone for your replies! Much appreciated :)

Sorry but i mistakenly wrote it as a beta type. I'm making a gamma type. Thanks anyways

Osama, don't worry about the type of motor, at least with a GAMMA motor you won't have to bore a hole through the piston, the simplest design of GAMMA is the V type, you only need one crank on the crankshaft, and you choose the cylinder bores to give the correct ratio. Ian S C

Thank you Ian for all the help! Your advice is quite valuable for a newbie! :)