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I want to do a science fair on the effect of the working fluid of a stirling engine on the efficiency of the engine.
Is it possible to change the fluid?

Thanks in advance.

stuffandthings, I imagine you mean changing from air to some other gas, maybe hydrogen, or helium. Yes, but. If your engine is a simple atmospheric engine, the leakage on the low viscosity gasses might be too great. If your motor is sealed as in a pressurised motor, you should have no trouble, and it's only when the pressure gets up over 200 psi that a big difference is noted. Consider the Philips Bungalow motor, it used air at 200psi. Ian S C

Thanks.

could i buy a pressurized stirling engine (under 200 psi) in which i could replace the fluid?

also, I have one question...
does the molar heat capacity affect the efficiency of a stirling engine? and how?

the specific heat capacity, which is a factor in the molar heat capacity, doesn't seem to be the only factor, because hydrogen and helium have higher specific heat capacities than oxygen and nitrogen yet are better working fluids than air

Edit:
i did a bit of research, and it turns out the heat conductivity makes far more of an impact

btw i'm a highschool freshmen, don't hate on my MASSIVE (sarcasm) knowledge of physics/chemistry

In a nutshell, what properties of helium and hydrogen make them better working fluids than oxygen or nitrogen, the primary components of air

Sorry, you probably know about as much as I do about the properties of gasses. Gasses like hydrogen have lower viscosity, lower molecular weight, one of the main problems with hydrogen is that it will pass through (slowly) steel, and many other materials, specially when subjected to high pressure, helium is not so bad, and it is the second most effective gas to use. The gasses to try might be CO2, Argon, Nitrogen. Stay away from oxidising gasses.
I recommend that you try and get(library, E-book), "The Stirling Engine Manual" Vol. 2, by James G. Rizzo, this book has a few simple pressurised engines, and with a bit of help(maybe) you should be able to get something up and running. Ian S C

Thanks for the response, but i'll have to completely change my experiment
to test what i want to test for, i'd have to change one property of gas without changing another
the property i'm testing for is molar specific heat capacity
in this aspect, helium is actually VERY slightly better than hydrogen, but many other factors (viscosity, heat conductivity i think) make hydrogen better
so i will have to change the amount of any gas in a vacuum

stuffandthings wrote:Thanks for the response, but i'll have to completely change my experiment
to test what i want to test for, i'd have to change one property of gas without changing another
the property i'm testing for is molar specific heat capacity
in this aspect, helium is actually VERY slightly better than hydrogen, but many other factors (viscosity, heat conductivity i think) make hydrogen better
so i will have to change the amount of any gas in a vacuum

Hi,

I think you can do the tests using something like my gamma engine.


Just find a glass dome to enclose the whole upper half of the engine from the cooling plate up. Affix that dome and allow for a valve to change gases. I'd think hard about an electric heating element for precise input control and to lower explosion risk with hydrogen.

A pressure gauge to show the level of charge in side the dome.

You could measure RPM with a reflective sensor. Of course to bring power from inside the dome to the outside would require an internal generator or a magnetic coupler to pass torque through the dome.

This might be enough to give you reliable empirical results. Get a working engine and then figure out how to seal in the working gases.

R

I've needed to make yet ANOTHER change
so in essence, all i want to do is to pressurize a Stirling engine

could I do this by putting a Stirling engine into a container that can withstand pressure and then heating that container to raise pressure?
i would put both the heating source and the cooling source in the container. i would probably have to use some way to prevent the cooling source from heating significantly as i heated the pressure chamber, but I'd figure that out

but my main question is this:

I read that Carnot's efficiency limit to the efficiency of a Stirling engine states that the maximum efficiency is


Temperature of hot end - temperature of cold end
__________________________________________
temperature of hot end

as a result, even if the range is kept the same, changing the hot temperature would change the max efficiency

but does the Carnot efficiency actually scale with the performance of a Stirling engine?

if both hot and cold end temperatures of a Stirling engine were increased by 50 degrees Celsius, would the engine perform worse than before, or is that theoretical disadvantage lost?

should i keep a constant Carnot max efficiency or a constant temperature range?

by the way just a totally random question

theropod2 how much power can your gamma engine produce?

stuffandthings wrote:I've needed to make yet ANOTHER change
so in essence, all i want to do is to pressurize a Stirling engine

could I do this by putting a Stirling engine into a container that can withstand pressure and then heating that container to raise pressure?
i would put both the heating source and the cooling source in the container. i would probably have to use some way to prevent the cooling source from heating significantly as i heated the pressure chamber, but I'd figure that out

but my main question is this:

I read that Carnot's efficiency limit to the efficiency of a Stirling engine states that the maximum efficiency is


Temperature of hot end - temperature of cold end
__________________________________________
temperature of hot end

as a result, even if the range is kept the same, changing the hot temperature would change the max efficiency

but does the Carnot efficiency actually scale with the performance of a Stirling engine?

if both hot and cold end temperatures of a Stirling engine were increased by 50 degrees Celsius, would the engine perform worse than before, or is that theoretical disadvantage lost?

should i keep a constant Carnot max efficiency or a constant temperature range?

by the way just a totally random question

theropod2 how much power can your gamma engine produce?

Not much. I'd say maybe 10 watts, but I never run it hot, and the hot plate is much too large. I've pretty much let the engine set for months, but I did fire it up a couple weeks ago, and it ticked right along like always.

With my engine the displacer and hot end could be outside the bell/dome/pressure chamber. Just imagine a dome over the flywheel end. The cool end will still have 1/2 the surface exposed to air, so application and removal of heat should not be an issue. Just an idea.

R

A Stirling Engine was featured in "Model Engineer" magazine a number of years ago, that had an electric element wound around the hot cap, it used low voltage, I can't remember the Watts, not too much, probably less than 100W. Ian S C

Ian S C wrote:A Stirling Engine was featured in "Model Engineer" magazine a number of years ago, that had an electric element wound around the hot cap, it used low voltage, I can't remember the Watts, not too much, probably less than 100W. Ian S C

I remember reading paper on Stirling engines and the study used an electric heater to test the engines for efficient operation with a known input. This way other changes could count on input remaining constant and equal.

R

I know I've said this on another post, but
Is there any way to pressurize a Stirling engine by increasing ambient temperature?
If a Stirling engine isn't pressurized, it has atmospheric pressure. So if you were to put the Stirling engine inside a pressure vessel, would it have the pressure of the vessel?