Alpha Stirling Engine Project

Discussion on Stirling or "hot air" engines (all types)
Connor
Posts: 10
Joined: Mon Mar 16, 2015 10:49 am

Alpha Stirling Engine Project

Post by Connor » Mon Mar 16, 2015 12:42 pm

Hi, I'm new here. I've tried to take in as much as the forum activity as possible, but please let me know if I've posted wrongly.

I've machined an Alpha Stirling Engine for a college project, but I'm all out of ideas on getting it to work. Can anybody help me? please!
I've attached a photo.. I think.

It's based on the design of those I've seen on YouTube, which use two glass syringes as the pistons/cylinders.

Image

Here are some specs:
Bore diameter = 20mm
Outer cylinder diameter = 30mm
Cylinder length = 65mm
Stroke = 20mm
Piston Length = 57mm (based on the idea that it'll get less caught up on small cylinder wall imperfections).
Acrylic flywheel (x2) diameter = 100mm Thickness = 12mm
Inner diameter of regenerator = 9mm
Cylinder material = mild steel (from the machining shop so it has a lead content for easy machining)
Piston material = brass (chosen for graphite content)
lubrication = graphite powder
Heat source = Methylated spirits lamp
Flywheels run on abec 11 bearings, dried out and re lubed with light oil, as well as rubber seals removed to minimise friction.

The cylinder seals are good. When suspended vertically, with a finger over the exhaust, the pistons will fall through the cylinders under their own weight at an extremely low rate.
The compression/vacuum can be felt through the flywheel during attempt starts, which displays springy behaviour courtesy of the pistons.
When regenerator is detached, the flywheel will spin freely, pushing the cylinders in and out - no issues there.
Detaching one push rod and manually pushing one piston in and out, will easily turn the flywheel and simulate the engine running.
The weight of the flywheels seems substantial enough to easily move the pistons.

Initial teething problems were the following:
-Brass piston expansion in the hot cylinder, causing ceasing. Rectified by removing 0.05mm of material.
-air leakage around the cold cylinder piston & regenerator tube attachment points. Rectified by machining a new piston with a much better seal, as well as tightening wire around the regenerator tube attachment points for a better seal.
-Melting of the silicon hose where it attaches to the hot cylinder. Rectified by swapping the silicon hose for a section of car fuel line (unknown metal). This is the latest modification and is not pictured.

Today I heated the engine, trying to start it along the way, until the hot cylinder became purple, still nothing. Later followed by the brass piston ceasing again. Although it ceased, I'm pretty sure it's unnecessary for the cylinder to have to be purple hot (~300 degrees c) in order for it to run. I really hoped it'd work this time round.

It's not a thing of beauty as of yet, the plan is to make it mechanically functional first.

That's about it. Any tips, questions or help would be much appreciated. Thanks for reading :)

Aviator168
Posts: 308
Joined: Tue Feb 08, 2011 2:29 pm
Location: Brokeville, NY. USA

Re: Alpha Stirling Engine Project

Post by Aviator168 » Mon Mar 16, 2015 12:58 pm

So the displacement off each cylinder is about 6cc. What is the dead volume of the transfer tube?

Connor
Posts: 10
Joined: Mon Mar 16, 2015 10:49 am

Re: Alpha Stirling Engine Project

Post by Connor » Mon Mar 16, 2015 1:18 pm

Aviator168 wrote:So the displacement off each cylinder is about 6cc. What is the dead volume of the transfer tube?
I hadn't taken that into account. I believe it's also approximately 6cc, maybe just over. Is this too much?

Aviator168
Posts: 308
Joined: Tue Feb 08, 2011 2:29 pm
Location: Brokeville, NY. USA

Re: Alpha Stirling Engine Project

Post by Aviator168 » Mon Mar 16, 2015 2:33 pm

I wouldn't have TOTAL dead volume greater than 20% of displacement.
Look how small this tube is.

Connor
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Joined: Mon Mar 16, 2015 10:49 am

Re: Alpha Stirling Engine Project

Post by Connor » Mon Mar 16, 2015 3:06 pm

Aviator168 wrote:I wouldn't have TOTAL dead volume greater than 20% of displacement.
Look how small this tube is.
Ah right okay, I'll strip the current transfer tube off of it and install something much smaller. Not small enough though for it to restrict the flow however. Thanks for the tip!

Aviator168
Posts: 308
Joined: Tue Feb 08, 2011 2:29 pm
Location: Brokeville, NY. USA

Re: Alpha Stirling Engine Project

Post by Aviator168 » Mon Mar 16, 2015 4:24 pm

BTW. Why did you use copper for piston and ss for cylinder? You should use the other way around.

Ian S C
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Location: New Zealand

Re: Alpha Stirling Engine Project

Post by Ian S C » Tue Mar 17, 2015 1:40 am

Connor, start with the hot cylinder. (1) I'd take the out let out the side at TDC, The piston should have an extension on it, made in a similar fashion to a displacer, this is called a Heylandt Crown, the heating takes place at the top end of that, and the hot cap should be about .25 mm thick, more or less. It's quite all right to run the motor red hot, so the hot cap, and the Heylandt Crown are best made of stainless steel. The best metal for pistons is cast iron, the cylinders can be either cast iron (best), or steel.
(2) Pistons, these should be able to slide through their bores under their own weight, but when the end of the bore is blocked, they should slow to almost stop. If you use the ferrous metals there will be no problem with differential expansion. 30 mm would be plenty for the length of the pistons, and the hot piston would have about 30 mm to 40 mm extension with a clearance of about 1 mm around it.
(3) If you have not done it, it would be worth down loading Andy Ross's book "Making Stirling Engines".
Ian S C

Connor
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Joined: Mon Mar 16, 2015 10:49 am

Re: Alpha Stirling Engine Project

Post by Connor » Tue Mar 17, 2015 12:41 pm

Aviator168 wrote:BTW. Why did you use copper for piston and ss for cylinder? You should use the other way around.
Unfortunately there wasn't any square Brass bar that I could have used as the support, only cylindrical brass bar.

Connor
Posts: 10
Joined: Mon Mar 16, 2015 10:49 am

Re: Alpha Stirling Engine Project

Post by Connor » Tue Mar 17, 2015 1:01 pm

Ian S C wrote:Connor, start with the hot cylinder. (1) I'd take the out let out the side at TDC, The piston should have an extension on it, made in a similar fashion to a displacer, this is called a Heylandt Crown, the heating takes place at the top end of that, and the hot cap should be about .25 mm thick, more or less. It's quite all right to run the motor red hot, so the hot cap, and the Heylandt Crown are best made of stainless steel. The best metal for pistons is cast iron, the cylinders can be either cast iron (best), or steel.
(2) Pistons, these should be able to slide through their bores under their own weight, but when the end of the bore is blocked, they should slow to almost stop. If you use the ferrous metals there will be no problem with differential expansion. 30 mm would be plenty for the length of the pistons, and the hot piston would have about 30 mm to 40 mm extension with a clearance of about 1 mm around it.
(3) If you have not done it, it would be worth down loading Andy Ross's book "Making Stirling Engines".
Ian S C
Thanks Ian, It looks like I could have some redesigning/modifying to do. Before I do though, is there any likely hood of it working just by reducing the dead zone volume? I'm gradually running out of time to complete it. I hate to sound daft, but what does TDC stand for? Unfortunately there's no cast iron in the workshop. The pistons do behave the way you described though which is a plus, but I'll shorten them. I may as well remake the hot piston at least, but with steel instead to rule out any expansion differential.

If I'm not mistaken, are you refering to this kind of design? (Ignoring the 90 degree cylinder arrangement).

The book sounds good, I'll look into it!

Aviator168
Posts: 308
Joined: Tue Feb 08, 2011 2:29 pm
Location: Brokeville, NY. USA

Re: Alpha Stirling Engine Project

Post by Aviator168 » Tue Mar 17, 2015 4:01 pm

There is a chance your engine will work by going with a much smaller tube. Better yet, replace it with a metal tube.

TDC -- Top Dead Center. When the piston is at TDC, make sure there is no room left in the cylinder. Also, does your cold piston lag the hot piston by 90 degree?

Your design is a typical text book alpha stirling engine. Good for demonstration; but usually does not work or just barely. The one on your utube link is a better design and there are even better one on utube. Search "andy ross stirling" on utube.

Connor
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Joined: Mon Mar 16, 2015 10:49 am

Re: Alpha Stirling Engine Project

Post by Connor » Wed Mar 18, 2015 2:17 am

Aviator168 wrote:There is a chance your engine will work by going with a much smaller tube. Better yet, replace it with a metal tube.

TDC -- Top Dead Center. When the piston is at TDC, make sure there is no room left in the cylinder. Also, does your cold piston lag the hot piston by 90 degree?

Your design is a typical text book alpha stirling engine. Good for demonstration; but usually does not work or just barely. The one on your utube link is a better design and there are even better one on utube. Search "andy ross stirling" on utube.
Thanks. I'll be trying this first then, with some small diameter copper brake line. I'll re adjust the the pistons so that there is no space left at TDC, but with enough room so they're not hitting on the end of the cylinders. The cold piston does lag the hot by 90 degrees, the lag is also fully adjustable. Based on the successful videos I've seen on youtube, I'm fairly impressed with the perfomance of this style of engine and it'll comply with my project specification. Had this been a course higher than L3 Aeronautical Engineering, I would've made a more complex engine.

Ian S C
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Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Alpha Stirling Engine Project

Post by Ian S C » Wed Mar 18, 2015 3:00 am

Connor, best not to use steel against steel for cylinder and piston, cast iron is the only metal that can be run together without undue wear, or high friction. A polished steel bore, with a brass piston would be ok, or a brass tube with a steel piston.
Ian S C

Connor
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Joined: Mon Mar 16, 2015 10:49 am

Re: Alpha Stirling Engine Project

Post by Connor » Wed Mar 18, 2015 3:06 am

Ian S C wrote:Connor, best not to use steel against steel for cylinder and piston, cast iron is the only metal that can be run together without undue wear, or high friction. A polished steel bore, with a brass piston would be ok, or a brass tube with a steel piston.
Ian S C
Thanks Ian, in that case I'll have to keep my current brass/steel setup and refine the hot piston diameter to account for expansion. Both pistons and bores have been polished.

Ian S C
Posts: 2221
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Alpha Stirling Engine Project

Post by Ian S C » Wed Mar 18, 2015 3:11 am


Connor
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Joined: Mon Mar 16, 2015 10:49 am

Re: Alpha Stirling Engine Project

Post by Connor » Mon Mar 23, 2015 8:53 am

Update -

Fitted a much smaller capacity transfer tube as suggested. I also eliminated the dead zones at the end of the cylinders. It Still wont run, but feels closer & will go about a couple of rotations before stopping in a springy fashion.
The new transfer tube isn't so small that it's restrictive may i add. It's made out of a thin length of copper brake line, sealed in with expanding exhaust putty.

I may try cooling the cold side further with some ice to enhance the temperature differential, but other than that I'm once again out of ideas. I have an interview with British Aerospace soon and I'd like it to be working when I take it.

Does anyone else have any suggestions? (preferably modifications as opposed to redesigning ideas).

thanks
Image

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