Pressurization

Discussion on Stirling or "hot air" engines (all types)
Aviator168
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Pressurization

Postby Aviator168 » Mon Feb 06, 2017 7:52 am

I am getting a lot of conflicting information about pressurization. The standard way for pressurization is to simulate a higher atmospheric pressure, but most people I have seen just pressurize the engine, not the crank case. So what is it?

Bumpkin
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Re: Pressurization

Postby Bumpkin » Mon Feb 06, 2017 12:03 pm

Pressurized Siemens-type Alphas or double-acting Gammas don't need sealed cases, but rod seal leaks may require constant replacement pressure. Diaphragm/roll-sock designs solve that problem but have their own limitations.

Just my opinion, but I'm not a fan of very high pressure. I don't see a Stirling competing successfully with I.C.s or P.V.s; so a truly useful Stirling needs to use an alternate low-grade heat source and co-generate to make up for its low efficiency. That necessitates that the exchange area both inside and outside of the engine take so much space that it seems silly to worry about minimizing the working volume.

This pressurized open-crank engine: http://www.starspin.com/stirlings/jimd6.html has been referenced here for years. It might make a good combination shop heater and power source, but I'm not sure I believe the power claims without a ripping-hot fire that would require constant tending.

Bumpkin

Aviator168
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Re: Pressurization

Postby Aviator168 » Mon Feb 06, 2017 5:15 pm

I was talking about this engine. The builder only pressurize the engine. The crank case is open to outside air.
https://www.youtube.com/watch?v=LLzFi9aZcBM&t=93s

https://www.youtube.com/watch?v=a5aket-HSYI&t=356s

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

Re: Pressurization

Postby Aviator168 » Mon Feb 06, 2017 7:37 pm

Just my opinion, but I'm not a fan of very high pressure. I don't see a Stirling competing successfully with I.C.s or P.V.s; so a truly useful Stirling needs to use an alternate low-grade heat source and co-generate to make up for its low efficiency. That necessitates that the exchange area both inside and outside of the engine take so much space that it seems silly to worry about minimizing the working volume.

Unfortunately, I do not share your view. I think stirling can compete with I.C. if it is done right in certain application. I do agree that heat exchange area both inside and outside is a challenge. However, if some compromises are made in certain areas, it can be done.

Bumpkin
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Re: Pressurization

Postby Bumpkin » Mon Feb 06, 2017 10:26 pm

I don't know how to interpret the text, but looking at the videos I can only imagine the extra pressure is in the interest of fun or science. As it is, the bores must leak enough that maintaining pressure probably takes more power than is produced. It probably wouldn't be very difficult to make a pressurized crankcase if he wanted to, but it's such a good looking engine it'd be a shame to cover up all of that pretty machinery.

Bumpkin

Aviator168
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Re: Pressurization

Postby Aviator168 » Tue Feb 07, 2017 9:32 am

I know the right way to do it is to pressurize the crank case and let the working fluid to get into the engine through a valve. But my question is what is the difference between pressurizing the crank case and pressurizing the engine directly. Pressurizing the engine directly and letting the crank case exposed seems to work better.

Ian S C
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Location: New Zealand

Re: Pressurization

Postby Ian S C » Thu Feb 09, 2017 2:26 am

In a pressurised engine before start up both the interior of the engine, and below the piston/pistons in an ALPHA motor is bought up to pressure. If the two sides of the engine were not equal the power piston would be at bottom dead center, and with the pressure above it you would not be able to turn the motor over. It is normal to join the two sides via a port in the cylinder that is opened by the power piston at the bottom of it's stroke. The pressure the motor is charged with becomes the ambient pressure in the atmosphere that it operates in.
Ian S C

cbstirling2
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Re: Pressurization

Postby cbstirling2 » Thu Feb 09, 2017 5:30 am

In an alpha engine is the port only on the hot piston or is it on both the hot and cold piston cylinders?
I guess I always assumed that an alpha motor unlike a beta motor, the hot and cold Pistons both make power. But if I extrapolating on what you said, are you suggesting that only the hot piston makes power in an alpha engine?
CBStirling2

Aviator168
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Re: Pressurization

Postby Aviator168 » Thu Feb 09, 2017 9:36 am

Both the hot and cold make power. Remember that the pressure change in one cylinder affects the WHOLE system.

Ian S C
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Re: Pressurization

Postby Ian S C » Fri Feb 10, 2017 2:51 am

Aviator168, I had a look at those vidios, couldn't under stand the ? French, but on looking at his motor(nice looking Ross Yoke), I saw that he only just got the needle off the stop to the first number(couldn't read the figures), but in pounds probably no more that 5psi. In the second video about half way through he turned the pressure up, probably only 3 or 4 lb, and the motor slowed. I think you'll find the pressure being applied here is just maintaining atmospheric, or just + a little.
Ian S C

Aviator168
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Re: Pressurization

Postby Aviator168 » Sat Feb 18, 2017 9:16 pm

OK. Have been thinking. There is really no need to pressurize the crank case for an alpha engine. I understand what Ian is saying. But the issue here is the operation is not that much difference with or without pressurizing the crank case. It is just like a IC with a super charger or a diesel with 50 bar compression rate.

Ian S C
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Location: New Zealand

Re: Pressurization

Postby Ian S C » Sun Feb 19, 2017 1:26 am

NO, the crankcase must be pressurised regardless of the type of motor.
Ian S C

PetePeterson
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Re: Pressurization

Postby PetePeterson » Sun Feb 19, 2017 10:07 am

Bumpkin,
The JD#6 needed tending every 15 to 20 minutes to keep the power fairly constant when I saw it running in 1999.
This could have been changed by adding a fuel feeder like a pellet-stove uses.

Aviator168,
I've tried to pressurize a few open crankcase engines, the limit was when the energy in the flywheel could no longer over-come the energy needed to compress the gas again. Now if those engines were multi-cylinder designs like an IC with super charger, the other cylinders would allow a higher internal pressure before power loss is seen. A pressurized crankcase would provide a higher power output at the same pressure.

---
Pete

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

Re: Pressurization

Postby Aviator168 » Sun Feb 19, 2017 11:23 am

I've tried to pressurize a few open crankcase engines, the limit was when the energy in the flywheel could no longer over-come the energy needed to compress the gas again.

That's the true reason to pressurize the crankcase.

Now if those engines were multi-cylinder designs like an IC with super charger, the other cylinders would allow a higher internal pressure before power loss is seen

But not all stirling engines are one cylinder. For a double acting four cylinder stirling, there is no crankcase pressure to speak of.

A pressurized crankcase would provide a higher power output at the same pressure.

The true reason for a higher power output is the increase the amount of work fluid in the engine. Not the increase in pressure of the crankcase.

Bumpkin
Posts: 123
Joined: Fri Mar 02, 2012 1:42 pm

Re: Pressurization

Postby Bumpkin » Sun Feb 19, 2017 12:31 pm

Hi Pete. Neat that you saw the Jim Dandy six operating. 15 or 20 minute feeding intervals wouldn't be too bad if you were right there working in the shop anyway. Burning home-made fuel beats buying pellets any day.

Aviator, you are probably a better machinist than me, but I'd still bet an open-crank pressurized Alpha would take more power to replace the blow-by than the engine could produce - and a sure bet if I was the machinist. :big smile:

Bumpkin


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