Snifting valve

Discussion on Stirling or "hot air" engines (all types)
Aviator168
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Re: Snifting valve

Postby Aviator168 » Mon Dec 26, 2016 10:26 pm

In general, alpha stirling engines have to cold cylinder lagging the hot cylinder by 90*. Assuming the crank is at the bottom. Half stroke on the cold cylinder is also the point of maximum expansion when the piston is moving down and the point of minimum contraction when the piston is moving up. Or you can look at it this way. Half stroke on the cold cylinder is the beginning of the cooling cycle when the piston is moving down and the beginning of heating cycle when the piston is moving up.

Ian S C
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Re: Snifting valve

Postby Ian S C » Tue Dec 27, 2016 2:19 am

cbstirling2, I think your diagram should have the cranks at 90* rather than the 180* that is shown.
Ian S C

cbstirling2
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Re: Snifting valve

Postby cbstirling2 » Tue Dec 27, 2016 7:36 am

Ian S C wrote:cbstirling2, I think your diagram should have the cranks at 90* rather than the 180* that is shown.
Ian S C

My apologies for a confusing diagram. Or my apologies for being confused myself. I'm trying to figure this stuff out! ;)

On the diagram for the half Manson type cylinders labeled C and D are supposed to be the same cylinder is shown at different times. I left off the movement of the displacer. I was imagining a beta or gamma engine.

Ian, are you suggesting that in a beta or gamma engine bookcourt needs to be drilled at the point of the half stroke of the power piston? In the same way as an alpha design? ( instead of half stroke, perhaps I should say 90 degrees offset from the displacer...)
CBStirling2

Aviator168
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Re: Snifting valve

Postby Aviator168 » Tue Dec 27, 2016 11:56 am

Same thing as alpha. Snift in at minimum volume and snift out at maximum volume. For alpha engines, both of those points just happen to be at half stroke of the cold cylinder, and both pistons produce power.

Bumpkin
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Re: Snifting valve

Postby Bumpkin » Tue Dec 27, 2016 1:45 pm

"Same thing as alpha. Snift in at minimum volume and snift out at maximum volume."
Aviator, I respectfully disagree, but I'm always looking for new understanding. Why would you want to snift out? I believe the whole point of the snifter is to equalize at the highest volume to raise the average pressure in an otherwise atmospheric engine. It can notably increase the working pressure of high temperature ratio engines. As Ian mentioned, the one-way blowby rings or a simple check valve are simple tried and true methods. - Just my two cents based on absolutely no experience with high temperature engines.
Bumpkin

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

Re: Snifting valve

Postby Aviator168 » Tue Dec 27, 2016 5:18 pm

Why would you want to snift out? I believe the whole point of the snifter is to equalize at the highest volume to raise the average pressure in an otherwise atmospheric engine.

That's correct. Having a higher pressure in the cylinder than the crank case is not necessary a good thing in certain phase of a cycle. In the cooling phase, you want the pressure in the engine to be lower than atmospheric. The only way to ensure that is to reset the engine pressure at maximum volume. By the same token, the engine pressure is reset again to ensure it has a higher than atmospheric pressure when the working fluid is heated up. The power piston should be producing power in both directions.

cbstirling2
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Re: Snifting valve

Postby cbstirling2 » Tue Dec 27, 2016 9:06 pm

Bumpkin, I am trying to augment the ideas of the snifter valve and of Manson's engine.
On a Manson engine, the displacer and power piston are connected.
On tdc and BDC, there are ports to relieve excess pressure/vacuum.
Now I suspect those ports are needed since the displacer and power p are not offset by 90 degrees.

Thinking about what others have said, the ports are needed at the tdc and BDC of the displacer???
CBStirling2

Bumpkin
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Re: Snifting valve

Postby Bumpkin » Wed Dec 28, 2016 12:17 am

cbstirling, other than the Manson, I'd never heard or thought of ports for a snifter before this, but if I were to try it in a Beta or Gamma it would open for minimal degrees at BDC (max volume) of the power cylinder. I'm not sure it would give as much pressure boost as a check valve but it might be the best you could do with a port. On the other hand I can see Aviator's point that a snifter could eliminate the negative pressure/vacuum stroke unless there's a fairly air-tight crankcase to equalize the pressure gain on the other side of the piston. It feels like there's something I'm missing there, but for now that's all I can think of.
Bumpkin

cbstirling2
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Re: Snifting valve

Postby cbstirling2 » Wed Dec 28, 2016 9:11 pm

The reason why I was thinking about the Manson engine are due to my thoughts about the relationship in size between the power and displacer pistons.

What happens if you apply too much heat to a LTD stirling engine?
Does the "extra" pressure make the engine less efficient?
(Maybe it doesn't?) Just theories on my part...

So the ports idea of a Manson engine with a separately moving displacer may be more "self governing" and not so sensitive on power and displacer relationships...

From my research on a regular Manson engine, it was said they are 40% less efficient than a separately moving displacer engine.
CBStirling2

Ian S C
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Re: Snifting valve

Postby Ian S C » Fri Dec 30, 2016 1:43 am

If you put too much heat under a LTD motor like mine you get melt down, my one has a foam plastic displacer, and it doesn't take much to melt it.
I'v been reading the "Model Engineer" article by Andy Ross on his 35cc Rider Stirling Engine, And note that he used two snifter valves. The first in the sealed crankcase, and the second in the crown of the cold piston. Both of these valves are reed valves as you would find in a 2 stroke motor.
Ian S C

Bumpkin
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Re: Snifting valve

Postby Bumpkin » Fri Dec 30, 2016 4:17 am

Ian , in review it seems like a snifter in an open crankshaft engine doesn't make sense. Could the valve in the picture you posted in this thread possibly be something else like perhaps a condensation drain?
Bumpkin

Ian S C
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Re: Snifting valve

Postby Ian S C » Sat Dec 31, 2016 12:55 am

A snifter valve on an atmospheric motor is just to maintain pressure and make up for losses from leaks from the displacer gland, and power piston.
Ian S C

cbstirling2
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Re: Snifting valve

Postby cbstirling2 » Sat Dec 31, 2016 10:40 pm

Ian S C wrote:If you put too much heat under a LTD motor like mine you get melt down, my one has a foam plastic displacer, and it doesn't take much to melt it.
Ian S C


Let's say an Ltd made out of stainless. If you apply more heat than expected, doesn't the extra pressure slow the engine down??
CBStirling2

Ian S C
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Location: New Zealand

Re: Snifting valve

Postby Ian S C » Sun Jan 01, 2017 2:14 am

I think you would need to try it out with a graduated heat supply, and keep turning up the heat until the peak power was found. I suppose this would go for all engines, high or low temp.
Ian S C

Bumpkin
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Re: Snifting valve

Postby Bumpkin » Sun Jan 01, 2017 10:14 am

"A snifter valve on an atmospheric motor is just to maintain pressure and make up for losses from leaks from the displacer gland, and power piston."

Ian, please know I'm not trying to argue, I'm just curious.

I don't see how a snifter would help the pressure in an open crankshaft engine. I think that if the pressure on the back side of the piston stays at atmospheric (an open crankshaft) the pressure on the other side should vary higher and lower than atmospheric, and minor leaks would balance as the engine cycles through the pressure and vacuum strokes. A vacuum-relief snifter would boost pressure for the pressure stroke only as much as it would diminish vacuum for the vacuum stroke, and nothing would be gained. Maybe?
I looked up the definition for snifter valve and it could also be a condensation drain. I'm not saying that's what it is in that picture - just curious.

Bumpkin


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