Pressurization

[PetePeterson]

Aviator168,
"But not all stirling engines are one cylinder. For a double acting four cylinder stirling, there is no crankcase pressure to speak of."

With a double acting power cylinder the other side of the piston is Not at atmosphere. When it is compressing on one side, the other side is expanding thus helping the flywheel.

"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.
"
As to the last statement: my intent (I don't always say it right) was if the internal pressure of the engine stayed the same, and you then pressurize the crankcase, you would get more power. The amount of work fluid was still the same.

Another way to look at it: if you add pressure to just one side of a double acting cylinder if gets hard to compress that side, if you add the same pressure to the other side you can move the piston by hand more than before.

[Bumpkin]

Aviator, I forgot to mention another problem with a pressure imbalance above and below the piston. (beyond the normal power pulses) There is a greatly increased rod and main bearing load. I bumped into that while trying to think up a pressurized diaphragm type engine. A set of two opposed and linked solidly together in a push-me pull-you fashion might have worked, but it would have been a mess for what I wanted to do.

Bumpkin

[Aviator168]

I worked out all the numbers. Pressurizing the crank case would not increase the power of the engine. However, it does lighten up the loads of the bearings, cranks, and the connecting rods. It also reduce torsional vibrations greatly.

[Ian S C]

If you only pressurise one side, the power piston will be at bottom dead center, if you have 200 psi/15 bar, and try and turn the motor you just aint gonna do it.
Ian S C

[spinningmagnets]

I studied the Rider-Ericsson engine a while back. It is an alpha with two vertical / parallel cylinders, and the crankshaft is at the top of the engine. The crank side of the pistons are open to the ambient air.

It had an integral air-pump which could be adjusted to compress air at any degree of the flywheel position. If it was adjusted to add air to the interior at the moment of lowest pressure, it would be drawing the least amount of power away from the engines output. I am not aware of any other benefits that might occur if the air-pump was activated at a different position, but it is interesting that the designers felt that it was a useful option (would it run smoother at a different position?).

I seem to remember that the interior pressure at its lowest point was two atmospheres? (don't take my word for it), which was regulated by a relief valve on the air-pump. The air-pump also had an adjustable stroke, so when the leather seals on cylinders became worn (and thus leaked more) then the stroke could be increased to maintain the desired pressure.

I found it interesting that the leather seals were stationary on the tops of the cylinders, and the long "pipe-like" pistons entire length was a smooth surface. This meant that the cylinders could be simple pipe with no need to machine or hone the ID. Machining only the OD of the piston "pipe" was much easier.

[Ian S C]

With the early "pressurized" engines the basic idea of the pump was to replace lost air. to keep the internal pressure at, or just above atmospheric pressure.
Ian S C

[tonyanderson]

Hi, this is my first post, just joined yesterday.
I am nearing completion of a sterling engine based on Clarke V twin compressor. I am considering using mild pressurization say around 1 bar to start with. I was thinking about using a separate air supply for the crankcase and running it at a slightly lower pressure. The idea is to try to prevent too much oil going up the bores. I have fitted low tension piston rings and no oil control ring to free it up as much as possible.
I could post some photos if knew how! new to forums as well.
Tony

[Tom Booth]

Instead of "Quick reply", using "Full editor and preview" allows uploading images as attachments, then they can be placed online wherever the curser is positioned.

However, if I recall, there may be a waiting period. There is on many forums. I'm not sure about here. If you have images hosted somewhere, it may be possible to include an image that way, but that can be less permanent, if the image hosting service disappears or something.

[tonyanderson]

Many thanks Tom, I will give it a try.
Tony

[benjawolfe]

If you build an alpha sterling engine with a pressurised crank case and calendars you have a major problem.

Because the two cylinders are offset at 90 degrees to the crank shaft at some point both pistons will be traveling in and out at the same time, that is to say the travel of the pistons is not completely and perfectly alternating.

This means that with a sealed crank case the air/working fluid in the crank case will be compressing and decompressing at certain stages of the cycle therefore adding resistance to the operation of the sterling engine.

One solution would be to build two alpha sterling engines and offset the second sterling engine at 180 degrease on the crank shaft from the first sterling engine.
This would mean that any piston traveling in or out of a cylinder would have a counterpart traveling in the opposite direction this would minimise the resistance of the air/working fluid in the crank case on the two alpha sterling engines.

Also to maintain the seal on the system I would recommend a contactless magnetic coupling this would relieve the problem of trying to achieve an airtight crank shaft seal.

https://www.ktr.com/en/products/magnetic-couplings/

this website has information on magnetic couplings and might give you some ideas of how to build your own.
One of their products claims to have 1000nm of torque. I am shore if you want to put a load on your sterling engine a much lower torque of 149nm would be good enough that is about as much force as it takes to tighten your car wheel nuts to the correct tightness and that is a good amount of torque considering you are using a torque wrench for leverage.

So what pressure should you change your system to?

Well presuming increasing the pressure and therefore the density of your working fluid is actually going to improve efficiency then there are a number of considerations.

If you use both cylinders of the engine for power rather than just using one for the power stroke and the other as a displacer the advantage is that the hot cylinder will provide power when the air expands and the cold cylinder will provide power when the gas contracts.
Well that is my logic any way.

So when the air/ working fluid contracts what force can we expect to be exerted on the pistons?
Well presumably the maximum force we can expect would be that of a vacuum which means 14.7psi.
14.7psi is atmospheric pressure at sea level and is therefore the maximum force pushing on the outside of a space/container that has had all the air/working fluid removed from it.

What you need to remember is that this is the maximum amount of force you can achieve on the stroke when the air/working fluid in the engine is contracting however if the pressure in your crank case is for example twice that at 29.4psi and your pistons have a good seal you can expect the force on the piston on the contracting stroke to be greater than 14.7psi because the pressure in your crank case is now giving you a relative atmospheric pressure higher than that of the outside environment.

There is one problem thought because the pressure in your crank case is 29.4psi the air/working fluid on the expansion stroke must exceed 29.4psi in order to move the piston.

In other words in this way pressurising the system makes no difference, in fact considering you cannot achieve a perfect seal on any piston the pressure in the hole system including on both sides of the piston will be 29.4 psi.

So what douse my entire rambling mean?

Basically the only increase in efficacy you can expect to achieve from pressurising the system is that of having a more dense/concentrated working fluid within the whole system.

I still think it is worth building such a system and testing it unpressurised and then pressurised to see what differences there might be in efficiency.

[stephenz]

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

sorry for pulling up an old thread. I just posted a very similar question and your reply is something I considered as way to keep system running without pressure falling or rising over time.

do you have any illustration of how the 2 sides of the engine (effective volume and crank volume) are typically joined?

cheers.

[Bumpkin]

Welcome to the forum stephenz. Ian used to be one of our most prolific posters but he hasn’t been here for a few years. The timing sorta matches the pandemic but? I don’t know. He’s missed.
Hope you can resolve your question.
Bumpkin

[stephenz]

Thank you