My contribution to the ECE

[VincentG]

Hello all and thank you for providing this forum for us to discuss these fascinating engines. I have been lurking for quite a while here, gathering information as I can. I am a full time auto mechanic and hobby machinist. I believe the time will come when these engines will be a practical source of household power when petroleum fuels are either cost prohibitive or in limited supply, or in off grid applications.

For this reason, and for hobby as well, I am determined to develop a relatively easy to construct, wood fired Stirling engine that is reliably capable of at least 1000w continuous power output. I will use this thread to document my initial tests and small scale build processes. As a forewarning, I am not mathematically inclined and will rely on real world testing results to guide my design. Having access to a manual lathe, CNC mill and 3d printer I should be able to see this out to a working model.

I have considered all three types of Stirling engine, and I believe the Gama type to be easiest to construct, as well as capable of the highest power output at low RPM, which I believe is paramount to reliable operation. Of course if I find otherwise , I will pursue another type. Fundamentally, my testing and design will focus on what I believe is most critical for maximum power.

#1: Efficient and rapid transfer and removal of heat from the working gas
#2: Thermal separation of hot and cold sides
#3: Precise control of the displacer piston

In order to extract maximum power, it seems to me that the displacer piston must be controlled by either a camshaft or an electronically timed solenoid valve to allow proper timing and dwell. I think much testing needs to be done on the hot and cold sinks as well. I have a few ideas in mind already.

Progress and updates will be relatively slow as time allows. I hope if nothing else I can make a worth while contribution to the evolution of the Stirling engine, at least as it pertains to the average builder. -Vincent

[VincentG]

The first thing I'd like to confirm is my thoughts that a standard Stirling engine, especially when operated at high speed, is completely ineffective at producing meaningful pressure gradients. I had initially ordered a fast acting thermocouple for this test, and will certainly employ its use, but have since realized that pressure gradients is what we are after at the end of the day. I do plan on ordering a model Stirling engine to conduct further tests on. But for now lets focus on the hot and cold sinks.

This is the guage used for the first test, its a bit off zero but is still effective and very quick to respond. I will replace with a more accurate guage.



I have connected it to what I think we can agree is a relatively high surface area to volume steel tube(a pushrod). You can see when I apply heat via a MAP gas torch, how slow the pressure is to build to around 4psi where I removed the heat.
https://photos.app.goo.gl/DzFg96ggUxJSUEQJ7

I subsequently plunged the tube into ice water, where the pressure slowly dropped to 2psi. At that point the guage and rubber tube was still warm so I think it would have taken a long time to return to 0.
https://photos.app.goo.gl/se22QujwGiFPoFGw5

Interesting to consider how quickly a fire stick will heat air under compression enough for ignition. So we can assume a working Stirling engine with its compression and expansion strokes will alter air temperature in that way. However I don't believe that plays much of a role in power output or the results of this experiment. -Vincent

[VincentG]

Here is some great testing already done on the subject of heat exchangers and power output. Most interest to me is that overall efficiency can go down, while power output goes up. Given the effectively limitless output of a wood fire, that is acceptable in my eyes.
https://www.sciencedirect.com/science/a ... 0422007269

[VincentG]

I am retired to my room, whiskey in hand, and intently reading the discussions in this thread. viewtopic.php?f=1&t=5417&start=30

It may fall on deaf ears but I would like to document my thoughts throughout this process. At 35 years of age, I have grown up on message boards of various topics, and witnessed their decline as social media has taken over. Interesting as that may be, they do not serve us well to access information on technical subjects like this.

Of particular interest to me is the accidental discovery of the resonant free piston engine as seen here https://www.youtube.com/watch?v=DyPxNNJQo9M It is very similar to the simple free piston diaphragm engines constructed by TK on youtube.

The question arises, how does this engine operate without a flywheel to carry its momentum to the next cycle.

I am still looking into rapid response measuring instruments(affordable ones I might add) to study the cycles of the Stirling engine, but for now I will theorize. Forgive me if this has already been concluded here.

Starting the piston at TDC, the working gas is at full compression, which in and of itself adds heat. Then, more heat is added by the external source. Lets remember the rise in pressure takes a considerable amount of time, even an IC engine with its comparatively instantaneous ignition requires spark between 10 and 40 degrees before top dead center to produce max power. Back to the Stirling, from TDC the rise in temperature has increased pressure enough to send the piston hurling towards BDC. This is where I think it gets interesting. As Tom so often professes, in this configuration there may be no need at all for a cold side exchanger. The only exception, as seen in the video, is the heat transfer of the cylinder. But for the sake of the argument lets assume a perfect insulator is used for a cylinder.

With no crank shaft or bottoming stop, what is dictating BDC? And with no flywheel how does the cycle resume?

It seems so obvious to me that the role of the flywheel is assumed by to momentum of the piston going past the point of full expansion of the heated gas. The piston continues to travel towards BDC until the increase in volume lowers the temperature of the gas enough that it slingshots back towards TDC, where the same momentum carries it through the compression stroke.

The overextension towards BDC and TDC due to momentum is what must help change the temperature of the gas much quicker than any heat exchanger could ever hope to.

I think that is why the free piston may be the most efficient configuration of SE. Unfortunately to harness its power requires the use of a linear generator, which is much harder to reproduce as a hobbyist.

[Tom Booth]

The overextension towards BDC and TDC due to momentum is what must help change the temperature of the gas much quicker than any heat exchanger could ever hope to.

Cheers!

[Tom Booth]

I think you have the right idea, but "overextension" is, well...

Take this engine for example.

There really isn't a difference, with or without a flywheel.

https://youtu.be/HUWt3YrxoB4

Infact, as I did in this experiment, by increasing the throw, the flywheel can be used to force an overextension beyond what a "free piston" would do naturally.

https://youtu.be/P11q-BAhvqk

An interesting trick, though I'm not sure there is any advantage in terms of actual power output.

Probably optimal is the same as free piston or natural flywheel or no flywheel. Though the flywheel helps smooth things out.

[VincentG]

Ill clarify what I mean by overextension. It seems to me the free piston is an optimal design to assess the true nature of the cycle, over various frequencies, without the confines of a crankshaft.

I believe that the full power potential for a crank based engine is to clip the stroke just shy of the free pistons natural extremes. Though maybe its just past, or dead on, only testing will tell. Again it all boils down to the timing of the cycle. And it likely varies wildly with rpm.

[Tom Booth]

Ill clarify what I mean by overextension. It seems to me the free piston is an optimal design to assess the true nature of the cycle, over various frequencies, without the confines of a crankshaft.

I believe that the full power potential for a crank based engine is to clip the stroke just shy of the free pistons natural extremes. Though maybe its just past, or dead on, only testing will tell. Again it all boils down to the timing of the cycle. And it likely varies wildly with rpm.

An old timer model engine builder on this, or another forum, too long ago to remember, back when I was asking greenhorn questions, told me, that to get the throw of the piston he would leave the connecting rod disconnected, heat up the engine with whatever heat source was intended for use, then move the displacer manually and watch how far the piston traveled naturally.

He said to add a little bit more than that. The natural "free" distance, pluss a bit more.

So, just past the natural extreme was his rule of thumb.

He didn't give a reason. No mathematics. It's just what worked for him, trial and error, to get the engine to run well
.

[VincentG]

That was exactly my plan. It reminds me of this quote.

"Nobody can be so amusingly arrogant as a young man who has just discovered an old idea and thinks it is his own."

Sydney J. Harris


Of course I know I am following in footsteps before me. We are only trying to build apon their knowledge.

I think that stationary test will demonstrate the timing needed for nearly 0 rpm operation, and the key is to find the correct ratios/timing at a usable engine speed.

My LTD SE should hopefully arrive tonight.

[Tom Booth]

... the key is to find the correct ratios/timing at a usable engine speed.

My LTD SE should hopefully arrive tonight.

It would be nice if someone could figure out or devise some live means of making timing adjustments on a running Stirling engine as is standard on automotive IC engines (or used to be, before electronic ignition)

I've given it some thought from time to time but haven't come up with anything.

[dlaliberte]

Hey guys. (I've been reading all new posts the past few months... Still very interested in all this.)

Regarding the idea of making timing adjustments on a running engine, that would be a great way to enable doing many more experiments. But even if you have to restart the engine after a quick adjustment, that would help.

You may find that the ideal setting depends on changing conditions, such as the current temperature of the hot or cold sides, etc. In that case, there should be some automatic dynamic mechanism for controlling the setting based on feedback from a sensor. This can get pretty complex of course.

[Tom Booth]

Hey guys. (I've been reading all new posts the past few months... Still very interested in all this.)

Regarding the idea of making timing adjustments on a running engine, that would be a great way to enable doing many more experiments. But even if you have to restart the engine after a quick adjustment, that would help.

That's possible with the model engines I work with most often. Just a matter of loosening the screw on the displacers crankshaft journal and turning it one way or the other, relative to the power piston.

Resize_20230225_152246_6801.jpg (131.42 KiB) Viewed 7528 times

Still, not the same as the kind of live response you get setting the timing on a car engine at full throttle.

[VincentG]

Finally got my LTD Stirling in the mail. She runs well and should be a good test bed before I build a larger model. First order of business was to 3d print some nylon gears(1:8.3)to spin a generator at a reasonable speed while running the Stirling where I think peak torque will fall, around 60-100rpm

nylon gear picture.jpg (348.02 KiB) Viewed 7493 times

Here is the 3d printer I use. A Matterhackers Pulse Xe running glass reinforced nylon filament.

pulse xe picture.jpg (366.52 KiB) Viewed 7493 times

Still doing some research on efficient small scale generators. I'm looking to load the engine to quantify power output as I make changes to the engine. Any suggestions here would be much appreciated.

Just testing this motor for 10 minutes has taught me more about the Stirling cycle than I could have thought. Some things I have noticed....While spinning the engine over hot water with the working piston disconnected, I'm shocked how fast it reacts to the displacer position. I had thought there would be more thermal lag. When putting the engine under heavy load, you can see how underutilized the power strokes are. Initial testing seems to confirm my thoughts that a crank driven displacer has not only poor timing and lack of dwell, but more importantly, the displacer causes far too much temperature mixing during its lazy movement top to bottom and back. Some filming of a loaded engine at 60 frames per second should demonstrate this nicely.

I'm still working out the design, but a Ducati inspired desmodromic cam driven displacer should take care of these issues. Stay tuned. -Vincent

[Tom Booth]

Any suggestions here would be much appreciated.

I started out with that same type engine, but found it very difficult and frustrating to work with.

The one piece construction of the crank doesn't lend itself to making any real modifications or adjustments. The pressure fit to hold the crank in place allows the whole assembly to fall out easily.

I tried applying a little more heat than just a cup of coffee and the plastic/foam rubber parts melted.

The little thing really did show some promise, I thought, briefly, running like it might really be able to put out some power running like a champ on top of my wood stove, just before it melted down.

[VincentG]

Tom, I wish this forum allowed editing, to be clear I'm looking for suggestions for types of electric generator motors to power with the stirling.

I too am hopeful for this design. It shows an impressive amount of torque when on a fresh cup and after a cold soak. I realized quickly I will have to improve the cooling of the cold side greatly as it heat soaks in a matter of minutes. Although I'm sure that will improve just by putting the engine under load. A big advantage to the LTD design appears to be the nearly unrestricted and swirl inducing flow path between cylinders. As I type this I'm thinking it would be interesting to add some smoke into the cylinders and film the flow path at high speed. From there I could use the cnc mill to sculp the displacer/heat sinks to suite.