Stirling Engines are difficult to analyse

Discussion on Stirling or "hot air" engines (all types)
Aviator168
Posts: 308
Joined: Tue Feb 08, 2011 2:29 pm
Location: Brokeville, NY. USA

Stirling Engines are difficult to analyse

Postby Aviator168 » Wed Dec 28, 2016 9:36 am

Stirling operation are very difficult to analyse. Take a beta or gamma type engine for example. Assuming the hot end temperature is twice that of the cold end. When the displacer is at the middle moving to either side, majority of the molecules are actually in the cold side. In fact, there are twice as many molecules in the cold side than in the hot side. Dalhousie university team report claims that dead space does not impact performance. I think that's ridiculous. Even when all the working fluid is moved to the hot side, there are considerable working fluid remaining in the cold side dead space as the pressure is same through out the system. As matter of fact, the higher the temperature difference between the cold and hot side, the more working fluid remains in the cold side. So when you think you are heating up majority of the working fluid, you are actually heating up much less of it, and more heat you put in it, the less you are heating up.

Just my 2c

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

Re: Stirling Engines are difficult to analyse

Postby Aviator168 » Wed Dec 28, 2016 12:58 pm

More analysis. It is paramount to reduce dead space and at the same time increase contact area. Increasing working pressure and hot end temperature won't help a lot if dead space is a large percentage of working volume.

A simple example. Say you have an alpha engine and the cooler/heater has dead space of 20% of volume, and you use some space age material that the hot gas temperature is 5 times that of the cold gas. So at the point of when all the gas in the hot cylinder; this is what happens.

P is the system pressure
V is the cooler dead space.
n1 is the gas molecule in the cooler dead space
n2 is the gas molecule in the hot cylinder space + heater dead space
T1 is the cooler gas temperature
T2 is the hot cylinder gas temperature -- T2 = 5*T1

So the following is true

P*V = n1*R*T1
P*5*V = n2*R*5T1

Divide the equations
(1/5) = n1/(5*n2)

n1 = n2

So the amount of gas in the dead volume is same amount in the hot cylinder. However, the reverse is not true. The system favors the colder cylinder. Conclusion. Make the Stirling engine a "cold" engine by raising the mean temperature and reduce the cold gas temperature.

Ian S C
Posts: 1970
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Stirling Engines are difficult to analyse

Postby Ian S C » Thu Dec 29, 2016 4:18 am

Dead space; some where I have an article by Prof. J.R. Senft from "Model Engineer" (can't find it just now), he experimented with a GAMMA motor, and found that even with a 6ft rubber tube joining the two cylinders there was no measurable difference in power.
Ian S C

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

Re: Stirling Engines are difficult to analyse

Postby Aviator168 » Thu Dec 29, 2016 7:08 am

Yes. I remember that; but I also like to see the machine's power output where he measure this on. Andy Ross's speed control is implemented with dead volume.

cbstirling2
Posts: 113
Joined: Sat Dec 17, 2016 9:35 pm

Re: Stirling Engines are difficult to analyse

Postby cbstirling2 » Mon Jan 02, 2017 6:56 pm

I too have seen the YouTube video where he talks about the dead volume to control the speed of his motor.
However in thermal acoustic engines, a lot of times there needs to be a vast amount of space to set up the oscillation. So this is just a thought, but on that gammaxengine when the professor used 6 feet of hose that is one thing, but if he had a branch in the with a considerable dead volume that is not linear nature so to speak, perhaps that's the issue. It all comes down to turbulence and laminar flow??
CBStirling2

cbstirling2
Posts: 113
Joined: Sat Dec 17, 2016 9:35 pm

Re: Stirling Engines are difficult to analyse

Postby cbstirling2 » Mon Jan 02, 2017 10:31 pm

Is there a URL to a Dalhousie university team report that claims that dead space does not impact performance?
CBStirling2

Trevor
Posts: 169
Joined: Sun Nov 06, 2016 4:07 am

Re: Stirling Engines are difficult to analyse

Postby Trevor » Tue Jan 03, 2017 1:22 am

Talking of dead space guys - I do not seem to loose any power from engines using a air operated water pump, that is, using the expansion and contraction of the air within the displacer cylinder. Wouldn't this be dead space even if I have allowed a little extra length for the displacer.
Trevor


Return to “Stirling and "Hot Air" Engine Forum”

Who is online

Users browsing this forum: No registered users and 64 guests