Low Temp stirling

[windstuffnow]

  Other than some tweaks it's all together and sealed.  Hasn't been run yet...

  I'll find out tomorrow if it's actually going to run.  I turn'd the flywheel to get it past the compression stroke and to my amazement the amount of force it takes to roll it over is pretty high.  I had calculated the power stroke forces but didn't even consider the initial start force... yikes.   I pulled the calculator out and it requires a minimum of 25 ft/lbs to push it through the first compression stroke.  I'm thinking the flywheel should have been a larger diameter.  

  The engine was built from off the shelf parts and scraps of raw materials.  The displacer drum was a 15 gallon and the water jackets made from a 30 gallon.  The power piston is simply an inner tube creating a diaphram type plunging system.  The flywheel was purchased at an old engine show some time back and was told it fit an old John Deere.   12 inches in diameter and weighs in at 60 lbs.  The engine and displacer with the cooling jackets in place is over 4ft tall and about 20 inches in diameter.  About 9 gallons of water in the hot side and 5 on the cold.

  If I can get the internal pressure up to 2psi it should do around 75 to 100 watts... if it runs at all.  It's going to take a heck of a swing to get it going, not sure my arm is up to it but I'll give it whatfor!  Here's hoping for a successful start...

.  

[motoman465]

Ed, you always come up with the neatest things.  I will be awaiting your next posting with much anticipation.  Looks great just standing still, you always have such a fine amount of workmanship in all your "toys"!  BTW, any progress on the Lenz 2.4?  I want to thank you for all that you do, you are a true inspiration!

Todd

[craig110]

Wow, nice!  What temperature difference will be required to get the 2psi level?  I've been looking for a decent sized stirling and your design just might fit the bill if it really does put out 75-100 watts.

Craig

[pepa]

great work as always Ed. i would not have though of the bellows power piston, can you give us another angle shot of the unit with your next post, thanks pepa.

[WXYZCIENCE]

Ed, What material are you using for the displacer? What is the water temperature difference between the hot and cold? Will you be trying any other gasses?
Joseph

[stop4stuff]

nice one ed... 25ft/lbs is a lot to get it going tho.

craig110, air at normal atmospheric pressure in a sealed system increases pressure by 0.5 psi per 10 degrees celcius increase, so 2psi difference = 40 C temp difference.

I really should get off my backside & build my heat difference engine design...

One quote about it "Aha! Each side moves very quickly from, for example, hot side empty to hot side full.  The temperature is constant while the volume changes.  Then the engine is held with constant volume by the ------ ------ while the temperature changes.  Then bang, the other ------ ------ ------!  And the cycle repeats! = Carnot cycle!  The theoretical maximum efficiency for a heat engine! I've got to check my physics again, but I think you've got a winner here!"

paul

[windstuffnow]

  No good news today... I ran out of time to get it up to temp.   I made it up to about 150* on the hot side and 60* on the cold, only a 90* difference.   The compression stroke is a bit more than I can handle to get a good swing on it.

  I think I'm going to re-design the power piston to reduce the compression a bit before my next attempt.  Also, I think the flywheel is way to small for this size arrangement.

  One other thing I realized, using the tube as a power piston, is the fact it doesn't have a vacuum return stroke like a solid piston and cylinder would normally have.   If there are any leaks in the displacer shaft there isn't any way for the motor to recapture the lost air and the tube will eventually go flat.   It would require a low pressure valve or "burp" valve to replenish the lost air.  Simple enough to "bandaid" but would also require a small resivour and pump to maintain the low pressure system.  

  WXYZcience;  I used 1 inch thick urethane foam for the displacer piston.  Cut 21 circles and glued them together.   I'm shooting for a 200*f hot side and 30* cold or about 170*f difference.

  Back to the drawing board...  Success is simply failures with all the bugs worked out...

.

[willib]

Ed how about a relief valve on the power piston to lesson the pressure to start it up

then close it once it gets going

[Garry]

Hi,

Could you use a rubber bellows for a shaft seal to the inner tube piston??

Garry

[RP]

Ed,

That's really nice work!  A few thoughts:

Rather than replace the flywheel, consider gearing it up.  Using a toothed belt for instance you could run the flywheel at maybe 4/1 over the crank speed and have most of the same advantages as a larger wheel.  If the flywheel came off of a tractor, it should be good to 2000rpm or so.

"but would also require a small resivour and pump to maintain the low pressure system."  

I didn't understand this.  I'm not up to speed on the Stirling Cycle but is there no "low pressure" stroke or location in the cycle where you could insert a check valve to atmosphere?

[windstuffnow]

Hi RP,

   The flywheel redrive might work but would require a different structure at this point to set it up.  

   Right now the tube is intact with the exception of the inlet tube, it's not actually attatched to the piston plunger.   If the pressure drops below 0 the tube goes flat.   The check valve would work fine if the tube was modified and attatched to the plunger to create a reverse draw on the check valve.   The tube sounded like a good idea when I thought of it but, like most idea's, there are a few problems to overcome to get it to work properly.   I'll most likely scrap that idea and modify the piston to something a bit more conventional.  I really don't like engineering "bandaids" to cure a problem, I would rather re-engineer the system without the problem.... I'll come up with something.  

Lots of fun!

.

   

[RP]

Ahh, now I get it.  

If you still like the flexible membrane piston, you could consider one of those air bladder truck springs.  They look similar to what you've got.  Of course you could cut the inner tube and bolt it with a metal ring to the end plates (1 ring each side actually).

The membrane does have zero blowby going for it.

[windstuffnow]

Hi RP,

   Cutting the tube and installing metal rings is what I'm considering right now.   Even though its still a weak negative pull it should offer a better advantage to add a relief valve.

   I never thought of the air springs from trucks, I believe they offer them as overload or helpers for small trucks as well.   We used to use them to lift cars on the alignment racks as well... pretty heavy duty.

   One of my concerns prior to building the stirling was the longivity of the tube before it would either crack or wear a hole in it...  we'll see.

.

[gizmo]

Hey Ed.

Friction is the killer with these low temp difference engines. I've built a few high temp and low temp stirlings and tried diaphram or bellows as the power piston, never got them to work, or a least until I fitted a normal piston/sleeve. The bellows my have no leak down, but a piston will out perform for this sort of application.

I do remember a university in Japan built a low temp stirling, 300 watts. The displacer was about 3 feet dia from memory, used hot and cold tap water. The power piston was a flat disc of HDPE or Polycarb with a O-ring aound the diameter.

Glenn

[windstuffnow]

Hi Glenn,

  Yea, I know, on the small ones a tiny fraction of a bind will keep the engine from running, doesn't take much.   This one is as friction free as can be.   Without the tube in place I can give the flywheel a push and it will make a couple rev's before it stops.  The inner tube used in that way was a really stupid idea as it turned out.   I made a few small diaphram type that turned out quite well and a couple that wouldn't run at all.  My quest for a simple way to make a large diameter power piston continues... I have one more trick up my sleeve for a large diaphram before I give up and move back to a nicely polished large diameter cylinder and precision machined piston with exotic seals.   Actually a couple ideas...

  I remember seeing that some time ago, don't remember where.  I used to subscribe to the Stirling news and have many years of these magazines piled up all over the place from back in the 70's and 80's.   It's fun to go back and see some of the stuff that was being worked on.

  I find it quite strange that no one has put together a larger unit.   Andy Ross and a couple others made some mighty attempts at some reasonable power but they all seem to go to the pressurized systems and high temp engines to keep them small.  Maybe I'm just kind of nostalgic but I like the larger engines with big flywheels poking along at 60 rpms or so.  The little engines are fun and easy to build but they really don't do anything.  The only choices are toys or big bucks for an exotic engine... nothing in between.  Time for that to change... there is plenty of waste heat sources including solar to get a couple hundred watts from at a reasonable price.  

  I have about 50 bux tied up in my unit right now and shouldn't have to spend much if any more to turn it into a good running engine...

  It's all apart and being modified and I'm hoping to have it back together next week sometime... running!

.  

[gizmo]

Hi Ed

Yes I think there is potential with these motors. A handfull of solar collector water heaters as the source of hot water, and a cooling pipe buried a few feet down in the cool earth to provide the cold water. It may be big and ugly, and need a kick start once the sun is up, but it could be a cheap form of solar power compared to solar panels, easy to scale up, and more important, easy to make.

It doesnt need to be efficient if its cheap.

Glenn

[craig110]

I agree with Glenn, Ed.  Keep on working on this!  Needing a manual kick-start in the morning (and who doesn't? ;-) is a fine tradeoff to achieve overall simplicity and cost-effectiveness.

Craig