Engine pressurisation question

Discussion on Stirling or "hot air" engines (all types)
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Wellington
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Engine pressurisation question

Post by Wellington » Mon May 02, 2016 2:59 am

I'm a bit confused as to how a pressurized stirling works. At the top of this page is an animation of an alpha engine: https://en.wikipedia.org/wiki/Stirling_engine

If this engine was pressurized where in the engine is the elevated pressure? Is the Hellium in the regenerator and in the generator side of both pistons or is the helium in the crank case and/or heat end? A description would be helpful. Thanks
Wellington

Ian S C
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Re: Engine pressurisation question

Post by Ian S C » Mon May 02, 2016 3:41 am

In the ALPHA motor shown the crankshaft would be enclosed and the whole engine pressurised, sometimes there is no crankshaft protruding, instead a generator is built inside the motor making sealing a bit easier. For engines up to 200 or 300psi air is OK, cheaper, and easier to manage, 1200psi or so an inert gas such as Argon is useful. Hydrogen, and to a lesser extent Helium have the problem that because of there small mollicular size they tend to leak through steel, and in the process make the steel brittle, not good on a pressure vessel.
Ian S C

Wellington
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Re: Engine pressurisation question

Post by Wellington » Mon May 02, 2016 8:49 am

Ian S C wrote:In the ALPHA motor shown the crankshaft would be enclosed and the whole engine pressurised, sometimes there is no crankshaft protruding, instead a generator is built inside the motor making sealing a bit easier. For engines up to 200 or 300psi air is OK, cheaper, and easier to manage, 1200psi or so an inert gas such as Argon is useful. Hydrogen, and to a lesser extent Helium have the problem that because of there small mollicular size they tend to leak through steel, and in the process make the steel brittle, not good on a pressure vessel.
Ian S C
So what does pressurisation do exactly? how and why does it produce more power?.
Wellington.

Ian S C
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Re: Engine pressurisation question

Post by Ian S C » Mon May 02, 2016 8:13 pm

Unpressurised the working pressure is(about)30psi, and on heating can be double that, so the maximum pressure is 60 psi. Now put the pressure up to 200 psi, then heat it to double the temperature (Kelvin), and you are looking at 400 psi, it's also going to take more heat(not temperature) to heat the increased amount of gas, also the cooling system has got to be able to remove the extra heat.
Ian S C

Wellington
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Re: Engine pressurisation question

Post by Wellington » Mon May 02, 2016 9:28 pm

Ian S C wrote:Unpressurised the working pressure is(about)30psi, and on heating can be double that, so the maximum pressure is 60 psi. Now put the pressure up to 200 psi, then heat it to double the temperature (Kelvin), and you are looking at 400 psi, it's also going to take more heat(not temperature) to heat the increased amount of gas, also the cooling system has got to be able to remove the extra heat.
Ian S C
Thanks Ian. So can a simple laminar flow/thermal acoustic engine like this one be pressurised if it was big enough?:

Wellington.

Ian S C
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Re: Engine pressurisation question

Post by Ian S C » Tue May 03, 2016 3:29 am

Sorry I'm not sure about pressurising a laminar flow motor. It's probably possible, someone else may know, or have tried it.
Ian S C

Wellington
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Re: Engine pressurisation question

Post by Wellington » Sat May 07, 2016 11:10 am

Would I be right in saying that when you pressurize an engine you get more power but less rpm?
Wellington.

Josephkaisner
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Re: Engine pressurisation question

Post by Josephkaisner » Sat May 07, 2016 4:39 pm

In theory the crankcase and the swept space should have equal pressure if the engine is pressurized or not. If theory is right, then rpm should be decided by the pressurization's influence on engine power. So it seems that rpm and pressurization is not directly related.

Wellington
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Re: Engine pressurisation question

Post by Wellington » Sun May 08, 2016 12:31 am

Josephkaisner wrote:In theory the crankcase and the swept space should have equal pressure if the engine is pressurized or not. If theory is right, then rpm should be decided by the pressurization's influence on engine power. So it seems that rpm and pressurization is not directly related.
Thanks Joseph. I understand now that it can vary but in a general sense do you normally see a slowing of rpm when you increase pressure up to a given point? Wellington.

Ian S C
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Re: Engine pressurisation question

Post by Ian S C » Sun May 08, 2016 1:08 am

No, from what I read of these motors they tend to increase speed and power.
Ian S C

Josephkaisner
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Re: Engine pressurisation question

Post by Josephkaisner » Sun May 08, 2016 6:26 am

I agree with Ian.

Crazyguy
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Re: Engine pressurisation question

Post by Crazyguy » Mon May 16, 2016 2:39 pm

How long till the increased pressure leaks out. These engines with shafts and conn rods are not air tight.

Ian S C
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Re: Engine pressurisation question

Post by Ian S C » Mon May 16, 2016 8:02 pm

A lot of pressurised motors (including the Philips) have a small air pump to maintain pressure. Other than that, the design of air seals is a science in its self. The ideal way is to build a generator into the crankcase, the motor is started by running the generator as a motor from a battery, the battery then stays in the circuit and is charged by the generator, and gives a stable power supply.
Ian S C

Crazyguy
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Re: Engine pressurisation question

Post by Crazyguy » Sat May 21, 2016 4:12 am

Aha! Thanks, makes a lot of sense now.
So the only thing going from the pressurized vessel into atmospheric pressure is the wires?

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