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Design criteria for Stirling cycle engine

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Picture of Design criteria for Stirling cycle engine

This Instructable  will describe a model Stirling cycle engine I built. More importantly, it will list design criteria, materials of construction, and tips so that you can build one of your own design.

This type engine is called a low temperature difference (LTD) Stirling engine, and there are several ways to build one, some of which are described in other Instructables on this site.

 
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Step 1: What is a Stirling cycle engine?

Picture of What is a Stirling cycle engine?

The Stirling cycle engine was invented by Robert Stirling in 1816, so it has been around for a while. It is a heat engine, and is based upon a cycle of heating, then cooling, of a gas (usually air) contained within the engine.  Since a Stirling engine is air tight, during the heating phase the air pressure inside increases, and during the cooling phase the pressure decreases.   A displacer connected to the crankshaft moves the internal air from hot side  to cold side of a cylinder. The change in pressure drives a power piston, which is also connected to the crankshaft. Since there are two stages, hot and cold, it is a two cycle engine

This diagram shows the concept of a traditional LTD Stirling cycle engine.  The components of the engine described in this Instructable  are arranged differently, but the concept is the same.

 

 

 

Step 2: Components

Picture of Components

Here are the components of the engine I built, which are similar to those shown in the previous diagram. I used copper for many of the components due to its excellent thermal conductivity. I used aluminum also; its thermal conductivity is good but not as good as copper. Both materials are very easy to work using common shop tools.

The steps that follow will describe  design details of the flywheel and  crankshaft,  displacer cylinder and displacer piston,  and the power piston.

 

shameem_782 months ago

This is great work. Have you tried using steel wool in the displacer instead of foam board and pipe? That way the displacer could also act as the regenerator.

http://www.instructables.com/id/Simple-coke-can-en...

I have been trying to build the 4 cyl swashplate engine described here - http://www.ohio.edu/mechanical/stirling/engines/engines.html - however time and skills are not my greatest friends and haven't made an y progress in a while.

Bill WW (author)  shameem_782 months ago
Thank you Shameem -

I have thought about using a regenerator, the steel wool is a good idea, will try.
I saw the swashplate when I looked thru some of your Youtube postings.
I have the same problem with time and skills - and often trying to do precision mechanical work with carpentry toold.

Bill
snoopindaweb3 months ago

Thanks for the treat.

Bill WW (author)  snoopindaweb3 months ago

Always glad to hear from a corrugator operator again.

Thanks for the comment.

A big A flute Thank You.
blueandmyboys5 months ago

can you get electricity from this in any way

Bill WW (author)  blueandmyboys5 months ago

Excellent question.

This particular engine does not have enough power to drive a generator. But yes, a Stirling engine if large enough, and if it has a great enough temperature difference, can generate electricity.

Thank you. I found a much cheaper to build stirling out of tin cans on YouTube. Its really nice man, I'll use a concept similar as yours but with wood to make a buggy
Bill WW (author)  blueandmyboys5 months ago

Oh, yes! My engine here is not the cheap or easy option!

I'm starting to make one that has to do with popsicle sticks. This is a photo of the beginning stage help me out if you can please sir.

WIN_20140201_235726.JPG
Very nice engine you have there! is is very well built.

I have a question. I'm from Europe, so we work with Newton/Meter.
How do you get the number of 0.1047?

I calculated this myself, and i'm getting 0.1033
From Wikipedia i get 1 Hp = 33000 ft*lbf /min.
That's 44.741,99235 Nm/ min

Then i get:
P (hp) = T (N/m) x f (rpm) / 7120
P (Watt) = T(N/m) x f (rpm) / 9.68

1/9,68 = 0.1033

am i doing something wrong? Maybe you can tell how you calculated the 0.1047 ?

Thank you!


Bill WW (author)  Triangle_Stirling6 months ago
Sorry for skipping over the derivation and just giving a "magic number".
The basic equation for calculating power from RPM and torque is:
P (Watts) = T*N* π /30 = T*N*0.10472
It can also shown as:
P (Watts) = T*N* π /30 = T*N/9.55

The above equations derive from the basic definition of mechanical power; the time rate that energy is produced: Joules per second, which is same as Newton-meters per second, which is same as Watts.
Bill WW (author) 7 months ago
Thank you.

I will likely have more on Stirling engines later.
heathbar648 months ago
Bill, thanks for sharing this. I've played a little with stirlings, but this is the most lucid and detailed info I have seen.
jott_18 months ago
Nice job.
Do you have an instructable for the dynamometer you used?
Bill WW (author)  jott_18 months ago
Thank you Jott.

Glad you asked about the dynamometer! I have the parts on my workbench right now for a generic dynamometer that can be used for small electric motors. I will be using it for fractional HP 120 volt motors, but it could be scaled up or down for other sizes.
jott_1 Bill WW8 months ago
I would be interested in seeing it when your done.
iceng8 months ago
Very Well Done !
Using minimal material components .... Kudos to you !

Years ago a friend in Reno looked at a heat engine
running on the twice daily 50° air temperature change
in relation to an easily accessible constant water table.

The difficulty was the volume of air that needed to be cycled
used more energy then the engine could put out :-(
Bill WW (author)  iceng8 months ago
Thanks for the comment. Your friend's heat engine is interesting. I can't help but think that there is some way to harness solar energy that will be more efficient than photo voltaic cells.
iceng Bill WW8 months ago
I agree and promote the solar Fresnel mirror steam approach for small works..
action pig8 months ago
This is very helpful, a friend and I are working through another stirling engine instructable. Thanks!
Bill WW (author)  action pig8 months ago
Appreciate your comment. If I can help with your project let me know.
dddddd8 months ago

I *liked* that torque measurement scheme, using a brake on the shaft, and measuring its effect as a change in force on a scale at the end of a moment arm.

I liked that graphite piston, too: lubrication and close tolerances at the same time.

Bill WW (author)  dddddd8 months ago
Thanks.
The graphite piston was a bit of a splurge, but it is a nice precision unit.
guitarpicker78 months ago
A thought:

the Stirling is not a 2-cycle engine by traditional definition. It is an EXTERNAL COMBUSTION engine.

I have seen many spinning fans in hot countries with little electric power. Seems odd to start a fire blow warm air, but it works!

Bill WW (author)  guitarpicker78 months ago
Correct, not a 2-cycle engine by traditional definition.
zawy8 months ago
I can focus an average of12 kilowatts for 6 hours a day (60 kWh per day) while tracking the Sun onto a 3x3 foot area for $200 in parts using aluminized mylar glued to six 4x8 insulation boards. See my heliostat instructable. Can someone build a sterling engine as the receiver? I'll make the Sun concentrator if you get the engine made. The target hot spot needs to be vertical 15 in front of the heliostat.. Non-low-E glass with little iron impurities (not green looking down the side) is needed for the front of the 3x3 black target (use Rustoleum high heat grill paint). I'll be updating my heliostat instructable in about 2 weeks to saw both units in operation with problems fixed and more specific videos on construction.
Kiteman8 months ago
There are some mechanisms I could watch all day...
Bill WW (author)  Kiteman8 months ago
Thanks Kiteman.
I could send you a much longer video.
Kiteman Bill WW8 months ago
Haha!
rimar20008 months ago
Magnific work, Bill!

I have some ideas for the day when I decide to make a Stirling motor (if that event come some day).

1) The shape of the chamber and displacer should be double tapered. This would facilitate almost fully displacement of the air, and consequently the conversion of heat into kinetic energy. If the displacer ends touch the end walls of the chamber, it get cold in the cold end and hot in the hot end. This would be a good thing.

2) The lenght of the chamber and displacer should be as large as possible. This would enhance the heat isolation of hot and cold end. Obviously, the limit is the common sense and the construction convenience.

3) The displacer should have a heat isolator too, at its middle point, to improve the difference of temperatures between its ends.

4) The crank should be modifiable, in order to try different schemas of angles and displacements of displacer and power piston. Maybe 90° is not the optimum angle, and maybe a little delay at one or both ends would improve the efficiency.

5) The fire should have a fan under it, to improve the heat production and transmision.

6) The cold end must have a fan too, to improve the cooling.

Possibly some of these ideas are a foolishness, but I fantasize with their experimentation. I have much more pending tasks than time available, which is a luck...
Bill WW (author)  rimar20008 months ago
Thank you Osvaldo; you comments are very good.
Sometimes I put ice on the cold end.
rimar2000 Bill WW8 months ago
Another idea I now remember was that the air chamber could have a valve to increase its pressure (more air, more power). This excedent pressure should be compensate by an equivalent on the open end of the power cylinder/piston (a spring?). The construction could be a little more complicated, but the advantage could be positive. In parallel you could use the valve to introduce other fluids into the chamber, i.e. alcohol, acetone, etc, in order to experiment how they affect the performance of the motor.
Bill WW (author) 8 months ago
Thanks for looking, Tesla.
Yes, the power I calculated is embarrassingly low. I have done that test several times and re-calculated, but still get 0.05 W.
There are other examples of model Stirling engines connected to a small generator, so your goal is not unrealistic. A greater delta T will definitely help; I was limited to around 350F max due having used solder in the construction of the displacer cylinder (duh!). Realize also that a larger power piston diameter and/or stroke will increase power IF you have aequate pressure from the displacer.
Look at the comments from Rimar2000 for additional suggestions.
Let me know if I can help further .
Good luck.
TeslaBoy8 months ago
Nice job! By far this is one of the best stirling engines I've seen instructions for. However your power output does seem to be quite low even for an engine of this size and precision. Do you think it's power output could be improved with a greater delta T value? I'm looking to have an output of around 10w.
Rodney77998 months ago
I was wondering if you could tell us the price you paid for the cylinder and piston from Airpot.
Bill WW (author)  Rodney77998 months ago
I wish I could - I bought a couple of sets a few years ago and do not have the price on this particular piston/cylinder set.
Here is the link to the site:
http://www.airpot.com/piston-cylinder-sets-stock-c-42_43_1023-l-en.html

If I come up with more info I will post it.