Stirlingengineforum.com

Fooling around with various ideas lately, the Atkinson style extended expansion, entrainment, compression reduction and the venturi effect etc. I think I may have come up with something that might actually work. It works (theoretically) kind of like a turbocharger. Not just entrainment but boosted entrainment or semi-forced entrainment.

In a "Laminar Flow" engine (both in name and theory) the heated and expanding working fluid is made to pass through a venturi type nozzle or restriction that accelerates the fluid/gas into a narrow high velocity laminar stream.

The problem with this, IMO is that a laminar stream injected into a cylinder to drive a piston results in a vacuum which logically, to one degree or another works against expansion.


I've previously considered using this vacuum to entrain additional air.

This, theoretically would do two things, add more mass to the laminar flow and eliminate the vacuum.

The problem arraising from that potential power boost however is a cylinder charged with EXTRA air which would probably make compression impossible, the entrained air would have to be let back out in some way via a port or valve or something.

Thinking about the Atkinson differential engine with a second piston to vary the cylinder length to reduce compression OR to reduce compression by leaving the intake valve open (actually that came from the Miller cycle) ...

Maybe the "intake" ports for entrainment in association with a venturi could be "left open" to provide an outlet to reduce compression

Add to that a second piston and cylinder, only because there doesn't seem to be room to add a second opposed piston in the same cylinder.

It seems like all this could fairly easily be incorporated into a standard LTD type engine design.
My reasoning is, the "compression" for the "boost" piston is essentially non-existent as it is just feeding the vacuum created by the venturi effect.

On "expansion" of the "boost" piston, it is really just absorbing the force of the (unwanted) compression from the main power cylinder. That is, it acts as a kind of Miller/Atkinson "open intake valve" to reduce compression loses and a second "differential" compression reduction piston.

It may eliminate some or all of any high torque and high power resulting from an Otto cycle or Diesel (fire piston) type "heat of compression" boost, but Stirling engines are not particularly suited for such applications anyway. On the other hand, the boost from entrainment might more than compensated, since it's questionable if there is much "heat of compression" there to be lost in an LTD type Stirling especially.

Anyway, it looks like a relatively easy modification that could be tested by modifying a standard model LTD type engine.

I just had another idea that actually made me laugh when I thought of it

If you wanted to pressurize this thing, or even if you didn't,...

By sealing and connecting the top of the two cylinders, when the power piston was expanding, the "Boost" piston would be drawing a vacuum (above the power piston).

This would, in effect, or actually, eliminate the power piston having to work against atmospheric pressure, or even an "air spring" buffer pressure.

Instead, in effect, the top of the power piston would be forcing the Boost piston down which would force entrained air to push the bottom of the power piston itself.

Now I'm trying to decide if a boost piston is even needed, or even the chamber.

It seems the same effect could be produced just using the venturi with the entrainment ports left open. The entire atmosphere would be the "chamber". Unless, of course, you wanted to use pressurization or a helium or hydrogen working fluid or something, then the chamber could be used. But then, with the sealed chamber, does the secondary "boost" piston actually do anything or does it just add friction?

It may be that the boost piston timing could do something to initiate a positive air flow and/or a vacuum a little early, or give the whole thing a bit more reliable behavior. Difficult to say. Something to experiment with to see if it makes a difference, but in a way, with the sealed chamber, it looks like the back, upper side of the power piston acts as its own boost piston. Rather than pushing against outside atmosphere it's pushing against itself!! which is what made me laugh.

As the power piston moves up, it pushes the air back around through the chamber to be entrained and help boost itself!!! Ha! What a trick that would be. But would it actually work?


I think this may need an extra extra long cylinder for expansion.

Edit: in my drawing I forgot to close the bottom of the boost cylinder.

Now I'm wondering if that is actually necessary.

I think it would be, otherwise as heat is introduced in the displacer chamber the expanding gas would put pressure against the top of the power piston (or bottom of the boost piston) rather than going through the venturi.

I've been researching high velocity nozzle design and came across this video. The video is cor a commercial cleaning device but covers some nozzle configuration concepts and demonstrations that are quite interesting, particularly the blast from the nozzle demolishing a cinder block! Also the math related to the force involved in increasing velocity.

https://youtu.be/E_XOD1p-clc?si=rPgxpAnLUH53vur1

Unfortunately, not much is said about what nozzle configuration makes this possible.

Surprisingly, as far as the origin of the laminar flow engine goes, the "venturi nozzle" appears to have been nothing more than a means of holding O rings in place.

https://www.stirlingengines.org.uk/thermo/lamina.html

As far as I can find, nothing about what I've been calling a "narrow orifice" or "venturi" is even mentioned, other than as shown in the illustration as a simple means for holding the engine seals in position.

This is interesting:

https://howthingsfly.si.edu/aerodynamics/air-motion


The way I see it, heat is random motion, so the name of the game is to turn random motion into organized motion; or convert "heat" into "work".

Random motion of air molecules collectively cancel each other out. No matter the "pressure" in a volume of air, nothing happens. A tank of compressed air can sit in my shop for weeks or months and there is no motion, no mechanical work.

To get work from this disorganized chaotic cloud of air molecules the motion of the gas molecules needs to be arranged or directed in some organized way.

So I'm thinking that it is not so much the heating of the gas and the increase in pressure alone that causes a hot air engine to run and produce useful work but rather the action of the gas rushing towards and into the power cylinder in an organized manner that actually decreases pressure

To quote a passage from the above reference:

For a stream of air to speed up, some of the energy from the random motion of the air molecules must be converted into the energy of forward stream flow. The random motion of air molecules is what causes air pressure; so transferring energy from the random motion to the stream flow results in lower air pressure.

When moving air encounters an obstacle—a person, a tree, a wing—its path narrows as it flows around the object. Even so, the amount of air moving past any point at any given moment within the airflow is the same. For this to happen, the air must either compress or speed up where its flow narrows. ... So when you "squeeze" a stream of air, two things happen. The air speeds up, and as it speeds up, its pressure—the force of the air pressing against the side of the object—goes down. When the air slows back down, its pressure goes back up.

Why does the air speed up? Because of conservation of mass, which states that mass is neither created nor destroyed, no matter what physical changes may take place. This means that if the area in which the air is moving narrows or widens, then the air has to speed up or slow down to maintain a constant amount of air moving through the area.

It may have been simply a happy accident. In attempting to develop a "laminar flow" Stirling engine, as in the previously cited account ( https://www.stirlingengines.org.uk/thermo/lamina.html ) many things were tried.

Though the "narrow orifice" may have simply been a consequence of fashioning a seal for holding O rings in place, that was the design that worked, as a consequence of trial and error rather than planning or mathematical calculations.

For years I've seen mention of, and speculation on the actual purpose of this narrow passageway, that it somehow causes a delay or "thermal lag", that it helps dissipate heat as a heat sink. (Actually the narrowing of the passageway causes a cooling of the gas, not due to conduction, but due to conversion of heat (random molecular motion) into velocity (organized molecular motion) see for example: "Venturi Refrigeration Engine":

https://patents.google.com/patent/US20150135741A1/en

If anything then, it would seem that the inner walls of the orifice would tend to transfer heat to the gas passing through it rather than acting as a "sink" at all.

Perhaps, if it has ever been observed that adding "cooling fins" in this area as illustrated in the above venturi refrigerator, this could be because the vanes serve to help SUPPLY HEAT to the working fluid from the ambient air, perhaps helping to accelerate it.

It is interesting that in one of Blade Attila's experiments, in an effort to see if a laminar flow Stirling could be used as a heat pump, it was the power cylinder that produced a measurable drop in temperature rather than any part of the regenerator "stack" (which actually warmed up).


https://youtu.be/2CnNOY1OVhc?si=MQNpVUgIfvE57OIH

This venturi refrigeration can effect ordinary venturis, such as in an automotive carburator

Carb ice forms because the pressure drop in the venturi causes the air to "cool," and draw heat away from the surrounding metal of the carburetor venturi. Ice then can begin collecting on the cooled carburetor throat. This is the same principle that makes your refrigerator or air conditioner work.

https://www.aopa.org/training-and-safet ... etor-icing

I could see where, perhaps this venturi refrigeration effect is in part responsible for the pressure and temperature drop that allows the piston in a single cylinder thermoacoustic/laminar/thermal lag type engine to return to TDC so quickly and effectively in spite of the continued application of heat, so keeping that area cool or limiting heat in that area might (or might not) make the engine perform better.

On the other hand, with a load on the engine, I can see some possibility that supplying heat to the venturi could improve performance if the heat addition were compensated by additional work output

This engine, rather than having just a heat sink has the venturi area actually water cooled.

https://youtu.be/4AsnE9kwyDw?si=cVCEL-DHUtgPLCfC

The engine produces about 1amp at 12 volts or up to about 15 watts.

I can't help but wonder if cooling the venturi is helping or hurting or if in fact the ambient(?) temperature cooling water is cooling at all. If the venturi is causing a refrigerating effect below ambient, which is actually colder?

The video is apparently pieced together as there is sometimes a fish tank with a long tube connected to the engine and sometimes not. There is no mention of using ice water.

The problem, IMO is the narrow passage on these engines, call it thermoacoustic, laminar flow, thermal lag or whatever, has been universally assumed to be a "heat sink" that performs a necessary cooling function, because it has been assumed for centuries that a temperature difference, and therefore some sort of cooling SOMEPLACE in the engine is an absolute necessity.

It is difficult to design effectively or make improvements on an engine when it is not clearly understood how these single cylinder hot air engines actually operate in the first place.

Is the heat sink in the regenerator stack? The "venturi" or orifice? The power cylinder?

Is it a thermal lag? Thermoacoustic? Laminar Flow? So many different ideas and theories.

Well, I have another theory.

A heat engine works as it is supposed to, by CONVERTING heat into work.

I think that, until this is clearly understood, progress will be hampered by trying to apply methods of making improvements based on principles that are erroneous or defective.