First Stirling Engine Help

Discussion on Stirling or "hot air" engines (all types)
CamN
Posts: 6
Joined: Fri Jan 27, 2017 10:25 am

First Stirling Engine Help

Postby CamN » Fri Jan 27, 2017 10:36 am

Hi I've recently starting planning designs for a Beta Stirling Engine that I am building for my college course. I'd appreciate any help or opinions with the following 2 things:

1. Piston and cylinder design (Ratios etc.) (What I'm mostly worried about is if the stroke length needs to be higher due to piston diameter)
Cylinder OD = 2.0"
Cylinder ID = 1.750"
Full Cylinder Length = 5.5"
Wall Thickness = 0.250
Displacement Cylinder Portion =2.75"
Power Cylinder Portion = 2.5"
Piston Stroke Lengths = 1.5"
Diameter Power Piston = 1.750" (Theoretically)
Diameter Displacer Piston = 1.700"

2. Would an offset connecting rod to each piston give poor results? (As in not connected to direct center of piston head)

Ian S C
Posts: 1981
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: First Stirling Engine Help

Postby Ian S C » Sat Jan 28, 2017 2:20 am

With a BETA motor it is normal to have the same diameter cylinder for the power cylinder section and the displacer/hot cap. The ratio of 1.5 : 1 is achieved by lengthening the stroke of the displacer.
My first BETA motor has the power con rod off set with out any problems, but if you feel it would be better, there's nothing against having a crankshaft with 3 cranks, one each side for the power piston, and the displacer in the middle.
The length of the displacer in your motor should be about 5 1/4" long, about three times its diameter, and it should be about 1/16" from the end, and a similar distance from the power piston at the other end of the stroke.
This is the best I can do for now, a view looking toward the crankshaft, the displacer crank on the right, and the power crank on the left in the shadow. This motor has done many hundreds of hours of running, and now needs a new hot cap.
The flywheel on the right is 6" diameter, and the V pulley on the left is 4" diameter.
Ian S C
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CamN
Posts: 6
Joined: Fri Jan 27, 2017 10:25 am

Re: First Stirling Engine Help

Postby CamN » Sat Jan 28, 2017 7:02 am

Thanks Ian I appreciate the help. I have another idea though and am wondering what you think of it.

I'm trying to eliminate the need of a crankshaft by centering the flywheel with the cylinder and having 1 pin on either side (one for each piston con rod) therefore both will be offset. Is this still viable?

Ian S C
Posts: 1981
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: First Stirling Engine Help

Postby Ian S C » Sun Jan 29, 2017 2:18 am

Yes it would work, but you won't have a shaft that you can put a pulley on to drive something ie a generator, or something like this. The motor on the photo is an ALPHA, but the hacksaw was built to be run by the BETA in the above photo.
Ian S C
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CamN
Posts: 6
Joined: Fri Jan 27, 2017 10:25 am

Re: First Stirling Engine Help

Postby CamN » Fri Feb 10, 2017 4:12 pm

Hello,
I've gone through the design process and have finalized my design with CAD software but I am still considering a few things and need some suggestions.
I am on a budget of $300 and I am wondering what materials to make the following parts out of:

1) Cylinder (with fins)
2) Displacer Piston and Power Piston
3) Do other parts of a Beta engine need specific materials?

I have plenty of scrap aluminum and steel in the machine shop I work in, along with a large rod of aluminum that could be used for (1) or (2). I have also been considering buying a solid brass rod for the cylinder.

Thoughts? Opinions? I appreciate any help, thank you.

thanh-cuibap
Posts: 148
Joined: Tue Sep 06, 2016 8:09 am
Location: Việt Nam

Re: First Stirling Engine Help

Postby thanh-cuibap » Fri Feb 10, 2017 7:26 pm

Hi CamN !
my gamma striling engine
1) Cylinder (with fins) :Cylinder of displacer made of aluminium . for power cylinder : fisrt engine made of : aluminium , now : made of steel.
2) Displacer Piston and Power Piston : displacer : aluminium , power piston : fisrt engine made of : aluminium , now : made of brass - many people tell Cast iron : better !

Ian S C
Posts: 1981
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: First Stirling Engine Help

Postby Ian S C » Sat Feb 11, 2017 1:29 am

CamN, lucky you having money to build with.
(1) Power cylinder, the bore, steel or cast iron. The power piston cast iron. You can fit a water jacket on here for cooling, or fins for air cooling. The fins can be cut into the steel, cast iron, or aluminium fins fitted over the cylinder liner
(2) Displacer stainless steel (best),mild steel. Hot cap is best made of stainless steel, but can be made of mild steel.
(3)The bush through the piston for the displacer rod can be one of a number of materials, I'v used carbon impregnated Teflon, brass, bronze, and cast iron, my displacer rods are either stainless steel, or polished mild steel.
(4)Use a heavy type of metal for the flywheel, steel, cast iron , brass.
Ian S C

CamN
Posts: 6
Joined: Fri Jan 27, 2017 10:25 am

Re: First Stirling Engine Help

Postby CamN » Mon Feb 13, 2017 7:25 am

Thanks for the replies!

Along with those recommendations, is there any material for any part that is BAD to use?

Also (if it works) i've attached images of my cad design, and im wondering if there are any obvious issues. Base Plate is around 8x16 inches, cylinder OD (hot end) around 1.9 inches for reference. *Crankshaft is between flywheels
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Ian S C
Posts: 1981
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: First Stirling Engine Help

Postby Ian S C » Wed Feb 15, 2017 4:07 am

Bad materials: Aluminium cans for displacers, (a) they conduct heat too fast from the hot end to the cold end.(b)They don't stand too much heat.
Aluminium pistons and cylinders, in general aluminium, and its alloys should not be run together as a bearing surface it tries to weld its self together, and has high friction.
These are two displacers made from aluminium cans, both from the same motor(I'm a slow learner), the motor now has a stainless steel displacer(and a whole lot more modifications), its been running for a number of years with quite a bit more power.
Ian S C
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CamN
Posts: 6
Joined: Fri Jan 27, 2017 10:25 am

Re: First Stirling Engine Help

Postby CamN » Wed Feb 15, 2017 10:11 am

Is a hollowed out displacer always necessary? I've seen both solid and hollow across the internet.

Is it possibly a modification you would do later if the engine is running poorly and the flywheel cannot handle the displacer weight?

Ian S C
Posts: 1981
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: First Stirling Engine Help

Postby Ian S C » Thu Feb 16, 2017 2:07 am

FRICTION, more weight, more friction. What would you make a displacer the size of my melt downs out of, they are about 30 mm diameter x 90 mm long, what ever metal they could be made from is going to be heavy, and you need a metal with low heat conduction, iron, steel, stainless steel. Yes I'v seen small motors sold as kits with solid Aluminium diplacers, these could be improved by using thin walled stainless steel.
Ian S C

CamN
Posts: 6
Joined: Fri Jan 27, 2017 10:25 am

Re: First Stirling Engine Help

Postby CamN » Fri Feb 17, 2017 10:54 am

Oh that's right, trying to reduce all around friction is a good idea.

I'll likely have more questions along the way, but if not I will at least post the final product for others to see.

Thanks for the continued help,

CamN

Ian S C
Posts: 1981
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: First Stirling Engine Help

Postby Ian S C » Sun Feb 19, 2017 12:56 am

In some cases it is worth removing the seals from ball races, and flushing out the grease, and replacing it with very light oil, just a drop, in fact try it first without oil.
Ian S C


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