Stirlingengineforum.com

Jack wrote: ↑Thu Dec 21, 2023 11:35 pm Fair enough, no absolute zero.

But in my thinking, because transferring every little bit of heat energy into work takes a long time

I don't think that is true at all.

Conducting or transferring heat to a "sink" can take a very long time. Heat is not "transferred" to work,

Conversion of heat into mechanical work is instantaneous.

if you're only bringing the fluid back to it's original size. That's why you would over expand it with a piston.

I don't know how you derive that conclusion.

If you liken heat to elevation, expansion of a gas by adding heat is like rolling a ball up a hill. (In opposition to atmospheric pressure or gravity).

Let go of the ball, or stop adding heat and the ball rolls back down the hill, or the piston returns to TDC.

Also, when the ball gets back down to the bottom of the hill, does it stop rolling? No, you've stored up "potential energy" and an object in motion remains in motion.

In other words, by heating and expanding the gas, you set up conditions for an oscillation. The return back to contraction/cold tends to have a kind of momentum so that the contraction is stronger than the expansion. Like a ball rolled up a hill. When allowed to roll back down, when it gets back down to the bottom of the hill it tends to keep on going.

I think this is because on expansion you store. "potential energy" then on contraction "gravity" or atmospheric pressure is helping.

At any rate, this "hard" return can be seen in several videos and I've seen it in my own experiments.

But finding the efficient amount of over expansion will be tricky.

"Over expansion" is not needed.

And I wonder if the engine in your video runs higher rpm because it doesn't have to lift up the displacer all the way.

I think that may be putting the cart before the horse

After all that is sorted out in still not convinced it will actually be more powerful than a cooled engine.

Well, I think this is all certainly a departure from the "established" theories of how a heat engine is supposed to work, that has dominated for the past 200 years.

My own tendency for extreme skepticism has kept me doing research and experiments for a decade rather than just building. I have not arrived at these conclusions easily or without opposition, and I have no desire to waste time, energy and money on something not based on experimental evidence.

Experimentally it can be observed that the piston returns to TDC "harder" than it went out with expansion, just like a ball rolls down hill much easier and faster than it can be pushed up a hill. I don't really need an iron clad theory to explain why that is, but it would help.

Well, here is a demonstration of what seems to be the principle involved:

That is, why the piston will return further, faster and seemingly with as much or more energy than it had during expansion.

Expansion is like pushing the bungy cord car. The return or "compression" stroke is like letting go and allowing it to roll back. It rolls back FURTHER than it was pushed in the first place.

A Bill Nye the science guy video on energy.

https://youtu.be/begl90R_gRo?si=UvjsWzhsj4djFEa6


This is actually a rather conservative view. What is rather radical IMO is the idea that "heat" is in some way different or special and for some unknown reason. behaves differently than all other forms of energy.

Experimentally it can be observed that the piston returns to TDC "harder" than it went out with expansion

This is the problem though. Until an atmospheric engine has a significantly stronger hot stroke than the return, we haven't made all that much power. And I have trouble seeing how just a blip of heat input at tdc will get there.

VincentG wrote: ↑Fri Dec 22, 2023 11:19 am

Experimentally it can be observed that the piston returns to TDC "harder" than it went out with expansion

This is the problem though. Until an atmospheric engine has a significantly stronger hot stroke than the return, we haven't made all that much power. And I have trouble seeing how just a blip of heat input at tdc will get there.

As I see it, adding an external load vs. no load is like pushing a cart up a hill either loaded down or empty.

The loaded down cart will take more energy to push up hill but will always roll back down. You can add as much heat as required at TDC to get up the hill again.

I'm not sure what you mean by "a blip of heat". The more work output, the more the working fluid cools and the more receptive it becomes to additional heat input.

I'm not sure what you mean by "a blip of heat"

I'm referring to the way a ringbom does not continue to heat the gas as the piston travels towards BDC, and also how I thought you idealized heat input.

edit-double post.

VincentG wrote: ↑Fri Dec 22, 2023 1:11 pm

I'm not sure what you mean by "a blip of heat"

I'm referring to the way a ringbom does not continue to heat the gas as the piston travels towards BDC, and also how I thought you idealized heat input.

I don't think I've ever particularly "idealized" heat input.

Mostly I think I've just tried to describe how a Stirling engine appears to actually operate. Mostly model engines under no-load.

But going way back to the begining of the "Stirling engine thermodynamics" thread I've argued that work output, due to applying a load, results in additional cooling. (Heat converted to work) as the temperature drops heat "flows" into the engine more quickly.

Gradually, more and more of a load can be applied, the engine does or is gradually able to do more and more work.

If the load is removed, the engine speeds up, the ∆T decreases. Heat builds up and the engine looses torque.

I mostly just got shot down for not knowing the "Stirling cycle" and not being able to support these "claims".

About. 90° before TDC (in a gamma/LTD type engine) the displacer "lifts" from, or uncovers the hot plate.

Years ago I thought that this was simply an "advance" because it takes time for heat to transfer from the hot plate into the working fluid.

The actual effect, however, seems to be that heat is introduced AS the piston is accelerating towards TDC and the COLD working fluid is being compressed.

At BDC atmospheric pressure drives the piston towards TDC. The pressure of atmosphere is converted to velocity of the piston.

So you get these two forces, the piston being driven towards TDC AND the cold "contracting" working fluid being rapidly heated when the displacer uncovers the heat source 90° before TDC.

So you have this cold working fluid receiving heat and then instantly it is compressed by the piston.

This combination of rapid heating followed by compression happening almost simultaneously creates a spike in temperature at TDC resulting in rapid expansion.

Now if there is a load on the engine, the expansion takes more energy. More hot air molecules have to hit the piston to drive it out the same distance. Heat is used up faster. Expansion-cooling begins earlier, the working fluid gets colder at BDC and so more heat can be taken in faster while returning to TDC. (But not until the next cycle)

In other words, as the engine does more work it is then, afterwards, able to take in additional heat to compensate.

If the load is removed the engine speeds up initially because expansion is now easier but there is less heat being converted to work. The working fluid does not get as cold and so less heat can be taken in on the return/compression stroke.

So, a Stirling engine is to one degree or another self-regulating. It takes in more heat as needed. Stops taking in heat when it isn't being used.

However, such adjustments are not instantaneous, but if the load is applied gradually, the engine torque will gradually increase. When the load is removed, engine torque will decrease.

I think it is interesting that this engine described by William Beale (SunPower founder & FPSE inventor) back in the 80's was of a Beta type design.
A few clips from his "UNDERSTANDING STIRLING ENGINES"

in 1980,... the U.s. Agency for International Development (USAID) funded the development of a simple Stirling engine specifically intended for manufacture and use in developing countries. The engine was designed, built, tested, and delivered to Bangladesh, and copies of it were built and put into operation there. This demonstrated the possibility of the engine's manufacture in simple machine shops of the type found in many regions of Africa, Asia, and Latin America....

A schematic of the crank-drive Stirling engine is shown in Figure 2, (above) while this engine is unusually large for the small amount of power (5 kilowatts) it produces, it is nevertheless very simple to make and operate....

Although the Stirling engine is an old machine, modern materials and design methods make it much more attractive than ever before. The crank-drive Stirling engine is definitely useful to anyone who has solid fuel. This type of Stirling engine can burn any local fuel as its source of heat to produce electricity, pump water, or perform tasks requiring mechanical power such as food processing. Very simple machines using atmospheric air as working fluid can be built from local materials such as metal containers. People who are inclined to try such designs have a good chance of success.

Because of its ease of operation, durability, local manufacturability, and the ability to use any local fuel as its heat source, the modern crank-drive Stirling engine is remarkably well suited for power generation in developing countries. Plans for this Stirling engine will be available from USAID in 1984 or 1985. Commercial production of the engine, or versions of it, is expected to begin in 1984.

I actually obtained a print copy of this publication back, not long after it was first published while working with a non-profit.

https://pdf.usaid.gov/pdf_docs/pnaas739.pdf

There seems to be no end to the controversy surrounding this Stirling engine project.

Apparently however, the much less powerful engine that became publicly known as the ST-5 was an Alpha type as evidenced from this forum post:

https://www.stirlingengine.com/forums/v ... 3449#p3449

A rather long post, so doing a text search for "alpha" will pinpoint the paragraph discussing specific changes that were made.

What stands out IMO as significant about this original (?) more powerful Beta ST-5 (?) engine is how when the piston and displacer are nestled together at TDC there is virtually no "dead air space" so it seems like the compression must have been very high.

In principle, I think something like this, a small LTD type engine of a Beta design might be worth experimenting with to start.


This engine appears to be of a similar construction.


https://youtu.be/SMMELH7CSNE?si=oPZqO2dQITU3aytk

What stands out in that VITA report, for me anyway is the line: "while this engine is unusually large for the small amount of power (5 kilowatts) it produces, it is nevertheless very simple to make and operate...."

Is that a misprint?

He's talking about an engine that could be knocked together in Bangladesh and other third world countries? In 1980? Maybe something some of us here could build, " from local materials such as metal containers".? Running on cow dung?

5000 Watts?

I don't know about others here but personally I'd get excited if I saw someone here build an engine powerful enough to charge a cell phone. That he calls 5 kilowatts a "small amount of power" seems incongruous, though, as gas powered generators on the market, most people would be familiar with go, something the size of a 55 gallon drum would perhaps seem enormous.

Well, we also have a forum post attesting to the fact that "we ran the engine on rice husks proving the output of 5 HP with a calibrated dynamometer. USAID signed off. The project went to phase two."

5 HP is more like 4kw but still a substantial amount of power IMO. Maybe the 5 kilowatts should have been 5 horsepower?

A sample chapter of “How I Built a 5 Hp. Stirling Engine” by Merrick Lockwood can be downloaded from the American Stirling website: https://www.stirlingengine.com/product/ ... ngine-book

Which I can only describe as entirely discouraging and definitely NOT something just about anyone could build, but the forum post relates "his book should be titled; 'How I screwed up a perfectly good Stirling Engine Design Project for the Third World".

I just don't know, but I still try to keep the idea alive that a 4 or 5 kilowatt Stirling that is "very simple to make" is actually possible. I'd actually be quite happy with a 500 watt engine. If it could run 24/7 on my wood stove all winter charging batteries to power an inverter, that is still a substantial amount of power.

200 watts? 100? 50?

How about something that can power a 5 watt phone charger?

Is anyone else experiencing some cognitive dissonance?

I may just have to try this now.

Getting a phone to light up and say "charging" is not necessarily going to charge it.

https://youtu.be/dOKnGouQX6s?si=7GD6Y7LQ9CjnFu_w

But, I can say that those particular models do seem to have potential for some power output.

Seems interesting too that the earliest version of Robert Stirling's engine appears also to have been a "Beta" type.
A single cylinder Ringbom Beta is also a possibility.

The majority of Stirling engine designs are inherently low compression with a lot of unavoidable "dead air space" due to the displacer needing room to move between hot and cold spaces.

During compression in a Beta style engine, the "cold space" is occupied by the power piston effectively eliminating the cold "dead air" space which would otherwise be a kind of permanent fixture robbing the engine of heat and compression.

I was thinking that perhaps there are other possible arrangements that could work the same way in principle but could be easier to manufacture.