design help and build log

[hephaestus]

happy new year!

Ok, I've been away... playing with the plans, kits and ideas learned here... getting my basic understanding of Stirling design...

But now its time to move forward, time to sh-t or get off the pot so to speak... means its time to put this knowledge to use...

The project goal - 500w output generator, trying to keep the footprint less than a 3000w gas genset. Some sketchup play says I might be able to get down into the Honda eu1000 range... We'll see...

Basic philosophy - pair of inline 4 engines, physically separating the hot and cold sides, cranks connected by a gearset - although chain worked on a paint can alpha nicely. Loose some efficiency with the extra drag, but gains in thermal efficiency offsets that in testing. Also allows gearing up the output rpms. Still contemplating ammonia absorption loop - for efficiency, have a neighbour who's a cooling tech who's working with me on this. Not as hard as I would have thought...

Engine management - arduino based, let it monitor physical conditions (cylinder temps, rpm, output) and adjust for efficiency. Generators will be a pair of RC brushless DC motors, one on each end of the main crank. They will also function as the starter motor. Still fighting with this a bit, was testing with RC brushless esc's but keep burning out the fets or diodes. Seems they really aren't designed to regen for longer periods. If all hell breaks loose, they'll also be the braking system, shorting the windings to stop the engine - this is partly why I want 2, redundancy and braking power...

Construction has begun, somewhat... got some 2", 1/4" wall cast pipe (sprinkler pipe) - that will be my cylinders. Working to smooth them out now, bored to smooth the weld, now working my way down to 800grit Emory to smooth the walls. Will be using steel plate as fins on the hot side, loop of copper tubing weaved through as heat input, possibly a second for the ammonia absorption loop. Cold side - aluminum sheets being soured again loop of copper if I do connect the ammonia absorption loop, all the fins will clamp onto the cylinders mechanically.

Pistons - still undecided, but I do have access to a 3d printer... think ABS or PLA would work? Otherwise, I'm sure I can come up with something.

Have an idea in my head for the cranks - mixture of standard machine shafts, plates of an undecided material, pondering skate bearings instead of pillow blocks to reduce friction... they seem to work well in various DIY applications...

I'm still puzzling over how much stroke, was thinking 1.75" I've got the cylinders cut at 6" right now, wanted them long - pondering filling the ends with copper tubes, as a heat exchanger in the cylinder, as the bore would be less efficient... heads will be simple plumbing caps threaded on and drilled/tapped for the crossover/regen.

Regen, haven't put a whole lot of thought into it, copper pipes vs steel wool, or a combination... have to figure out a fair bit before this comes into play anyway...

Ok oh all knowing ones - tear my plan to shreds, tell me where I need to make things better :)

[Ian S C]

hephaestus, happy new year to you.
Aim for 500W but don't be too sorry if you miss, look for 1W per cc, and you'll be doing OK.
Don't think you'll need braking, when all hell breaks loose, its too late.
Pistons, go for cast iron, they can be made fairly light, no rings.
Crankshaft, theres a number of ways of making them. Fabricated from mild steel shaft and MS plate is one way. The shaft wants to be 1/2" 12mm or so for that power, and skate bearings are 8 mm and not really up to much in quality.
Your cylinders puzzle me a bit, cast iron pipe does not have a welded seam, perhaps you have steel water pipe (horrible stuff to machine to a good finish), it will work OK with cast iron pistons, but not with steel.

How did you make an ALPHA motor from tin cans. Ian S C

[hephaestus]

Yup, partly why I'm shooting for 500w - 516cc total displacement...

The cast pipe confuses me, it looks sand cast, but it has a ridge the whole length, looks a lot like the hrew tube I usually work with... but it came from a sprinkler tech friend, he swears its cast...

Skate bearings are funny - some are top notch, some suck, but I've got access to tons of 1" shaft, if I really want to over build...

Braking - the idea is to set a nice red line, and not let it pass that... sure helps on these projects

Alpha was an online find - pair of rattlecans with epoxy piston, music wire crank... found it off one of this sites links, I'll see if I can find it again... have some pics over on the dead laptop, gotta see if I can recover that drive...

[Ian S C]

I suspect that if the pipe is cast, it would have a parting line down each side, the join of each side of the mold, but if you machine it most cast irons will form a black (messy) powdery swarf, not long stringy like most steels (the cast iron I'm working with at the moment sort of goes against that rule, stringy swarf, and not too messy black). I can't see cast iron being used anywere in a sprinkler system, maybe cast stainless steel, or bronze.
What do you have in the way of machinery, ie., lathe, mill/drill. Welding, or brazing gear.
The hot pistons should have a Heylandt Crown, also known as a hot cap, this is a light weight stainless steel cap attached to the piston, it resembles the displacer in a Beta, or Gamma motor. The hot end of the cylinder should be made of thin stainless, and with a 2" cylinder you may not need extra complication in the design of the heating, but make sure that your cooling system is up to the job.
For the regenerator, stainless steel foil, or stainless wire is what I would use, steel wool tends to break down, and bits get circulated around the motor. Stainless gauze is another one.
Do you have Andy Ross's book"Making Stirling Engines" its a free down load of 68 pages, it will give you some idea of what you can expect from a motor. Ian S C

[hephaestus]

Somewhere in between, get fragile stringy bits, not powder, not the usual steel swarf either...(Sprinkler as in fire protection, not as in watering your lawn). Not sure...

I have a little metal working lathe, and sherline, I'm more an arc welder, but I do have mig/tig... oxy/acy of course.

Yeah have any ross's book :)

[Ian S C]

I would love your welding gear, I'v got to get my TIG welding done by some one else, mind you even he finds some of my welds a bit tricky, I use stainless steel tube .007"/ .010" thick and weld an end onfor displacers, and their cylinders, he runs the welder at minimum power, and sharpens the electrodes to a needle point. I'v done gas welding, and stick welding, but I would like to learn TIG, and MIG.
If you have a little lathe you are proberbly working to the maximum sizes with 2" cylinders, take your time, and work carefully, you'll get there.
Steel water pipe has fairly ragged swafe, it quite low in carbon compared to mild steel, it means that as water pipe it can be bent for instalation. Ian S C

[hephaestus]

Ah mig, its just handy for quick sticks :) I hate it, bit its so quick and easy, takes almost no skill... tig is my nemesis, we have a love hate relationship...

Was playing with some carbon fibre last week, happened to knock out the remains of the cup... having a ridiculous thought... that was one heck of a slug that came out....since its not internal combustion, should I consider making pistons out of a composite? Can easily order up more cf weave, with a decent mold and some vacuum on it, that could be one light & strong piston...

[Ian S C]

My largest motor, a BETA type has a cylinder made of 2" galvanised water pipe. The only trouble I had when making it was that as I bored out the cylinder, the weld let go, and the pipe opened up with a 1/4" gap, so I ran a weld down the outside, and continued cleaning up the bore, and lapping it to a good finish. The motor has a light weight cast iron piston, no rings or packing. Ian S C