Model LTD max power effort

[VincentG]

I don't want to get bogged down in the lack of flywheel effect. There is certainly some rotating mass still at play. But there is also extra resistance from the spring based displacer system. I just thought it was interesting to observe.

Watch around the 2:30 mark in this video, where they explain the exhaust cycle of this IC engine. I think it directly correlates to the cold stroke in a SE and is similar to my cannon analogy in another thread here.

https://www.youtube.com/watch?v=9I0_3qFmPUM

[Tom Booth]

I wonder, when the engine is under a load and running slowly, so the heat input tends more towards isothermal, how many times might the same gas molecules be reheated during one expansion stroke?

I'm thinking that the gas when very hot and relatively "thin" and moving very quickly, following a single gas molecule, it could be heated, impact the piston, fall back down, be reheated again and hit the piston, etc. etc.

With each reheating the distance traveled is greater and greater.

Just my imagination running away with me now, but that seems like a likely scenario, rather than one continuous expansion, the molecules are in effect rapidly "boiling" so that each molecule of gas impacts the piston, either directly or indirectly many times each instant of time as the gas continues "expanding", or the piston keeps moving as a result of the repeated impacts.

The result of that would be, I think, more space between molecules. Fewer molecules doing more work. So in a sense the molecules are impacting the piston across a relative "void" so when the heat input stops, only the "void" is left, which offers but little resistance to "compression".

[VincentG]

As for the journey of gas molecules, I'm not willing to even speculate on that, but I have thought about putting some smoke in the engine to see what's happening.

This void of air is utilized well in the above video. Normal engine processes do not really take advantage of this for primary operation. A well designed cam and exhaust combination is known to significantly improve power on an ICE with this scavenging effect.

I've ordered a Nema 17 short body stepper motor to experiment with. It should make usable voltage in direct drive or even geared down from the engine a bit.

[Tom Booth]

(...)

As I have said, this engine is not at the zero point, so(theoretically) if you stop it after its been running for a while at the moment the piston is at TDC and the hot plate is covered(assuming 0 degrees advance), the internal pressure is still below ambient.

(...)

I can certainly agree with this as I've seen the same, or similar apparent "contraction" of the gas what I guess could be described as excessive contraction.

Running "free piston" in particular. The piston may be drawn inward further and further until it is finally bottoming out, hitting the lower inside end of the power cylinder.

That such a thing is even possible is somewhat perplexing while heat is being continuously applied, particularly in a laminar flow/thermal lag type engine with no displacer.

Like these recent experiments:

https://youtu.be/RODm1LCqwuk?si=u1kLq7ewrBB_uMd1

https://youtu.be/LG09AXAjpio?si=DSNWKDu7ZnjkZFEN

When the piston is perfectly centered the engine runs silently. But if there is a little too much or too little heat input to keep the oscillation perfectly centered in the cylinder the piston will hit one end or the other. (Or balance between heat input and "work" output).

What seems weird is that the majority of the time it is actually hitting the INNER end with "contraction", regardless of having absolutely no flywheel or revolving crankshaft of any kind. No displacer to move the gas to any "cold side", and I've eliminated any "sink" that the working fluid could possibly be contacting as a "cold sink".

[VincentG]

Today my nema 17 short body stepper motor came in so I did some testing with both coils in series. These things have a strong cogging effect and aren't the most efficient thing but it was cheap enough. I started with my 8:1 3d printed gears, with the small gear on the engine. Running over a tea candle and with ice on the cold end, I managed around .35 volts no load. The engine was just able to chug along.

I've been meaning to advance the phase angle of the displacer to see how it likes some more timing. I can only describe the results as incredible! It's like the difference between a Toyota Camry and a hotrod. What before seemed like a tame little thing now rockets away with power. The flat plates now bulge out from the pressure swings, losing what must be large amounts of energy that should be going to the piston.

I incrementally advanced the displacer until it started to "pre-ignite", and in fact would run in both directions at that point. I then retarded the displacer to reach peak voltage, which was now .5 volts. The engine settled to its loaded rpm, but seemed to have ample torque. So I switched to some RC car gears, of around 4:1 ratio. Rpm seemed the same and voltage was now 1.1, and still plenty of torque to spare!

Now the engine has so much torque its impressive and when allowed to free spin it runs faster than ever but will then pre-ignite. I attribute this to the compression cycle happening too fast to be cooled, then when the displacer lifts it's like a detonation event in an ICE.

Here is the stepper with the 4:1 gear set.

stepper motor 4 to 1.jpg (159.93 KiB) Viewed 7815 times

Here is the phase angle I ended up with. The displacer crank throw is facing straight up.

phase angle.jpg (162.81 KiB) Viewed 7815 times

Unfortunately, the displacer chamber could not handle the pressure and the o-rings really blew out this time. The new concrete chamber can be seen in the background. Along with a new 1/2" thick top plate, the flexing should be eliminated. Power piston size will go from 5cc to 15cc and the heat exchangers will have much more surface area. I'll also try to make provisions for increasing the buffer pressure and or using a different gas.

[VincentG]

Just wanted to make it clear how misleading the current phase angle is. The standard phase angle is where the displacer leads the power piston by 90(but with no distinct events). With the addition of the spring drive, the displacer falls back to maybe 5 degrees advance, with distinct events.

Currently the displacer is advanced maybe 30 degrees. It's clear at this point I'll need to make a degree wheel to measure advance more accurately, since it seems just as critical as in an ICE.

[Tom Booth]

(...)
I can only describe the results as incredible! It's like the difference between a Toyota Camry and a hotrod. What before seemed like a tame little thing now rockets away with power. The flat plates now bulge out from the pressure swings,
...
voltage was now 1.1, and still plenty of torque to spare!
...
Now the engine has so much torque its impressive and when allowed to free spin it runs faster than ever ...when the displacer lifts it's like a detonation event in an ICE.
...
Unfortunately, the displacer chamber could not handle the pressure and the o-rings really blew out this time. ...

Sounds incredible!

I guess you didn't get a chance to make a video before the seals failed.

Are you sure it wasn't the heat from the tea candle? When I tried to run one of those brand/type engines on a wood stove just briefly, the plastic ring softened which took the pressure off of the seals so the engine leaked air and would no longer operate.

If you are making the engine out of cement, I guess that should take care of the problem either way.

I was trying to figure out the advance from your photo. I thought it looked like almost exactly 45° or just slightly over, maybe 50° almost right in between 0° and 90°

[VincentG]

I have run the engine on the tea candle many times and while it does get a touch hot there's been no issues until the pressure went up.

The engine spins counter clockwise as viewed from the picture so the phase angle is more like 120 degrees(90+30).

Yea unfortunately I was so caught up in voltage testing I didn't get video. I'll see if I have a spare housing that's still in good shape.

[Tom Booth]

.... It's clear at this point I'll need to make a degree wheel to measure advance more accurately, since it seems just as critical as in an ICE.

That has been another long held assumption about Stirling engines.

I remember somewhere, sometimes, when I first started learning about Stirling engines, knowing how critical timing is on IC engines generally, Not sure if it was here or another forum, but I said something about changing timing and got a rather strident reprimand., to the effect of hundreds of years of testing has shown that a 90° phase angle gives the best performance.

To some degree this inspired awe and wonder, how well worked out all these "rules" seemed to be. but on another level I remained a bit skeptical. If true, there had to be a reason.

Once I started working on my own model Stirling engines, one of the first "experiments" in putting the kits together was to try and find the best timing position.

I found it made very little if any difference if the timing was off of exactly 90° by a pretty wide margin. 90° just seemed a midway point between extremes where the engine would no longer run.

I theorized that the heat input and "output" was just too spread out and mixed due to the sinusoidal displacer action. All timing positions were just generally equally bad as there simply was no identifiable heat input "event" to actually use as a basis for "timing". Rather heat input was spread out over nearly the entire 360°'s of rotation.

I struggled to find some good reason why this might actually be beneficial in some way.

The theoretically "ideal" isothermal heat input is spread out over 180°'s. Even in an IC engine it takes time for the fuel to fully burn so heat generation does continue after ignition through the entire expansion stroke, to one degree or another. In that case though, you do end up with rather low efficiency and a lot of "waste heat" out the exhaust.

It seemed at a minimum, restricting heat input to the 180° expansion stroke by introducing a definite "dwell" so there would be no heat input during "compression" at all, should produce some beneficial effect

Restricting heat input a bit further to the begining of the expansion should give the engine some time to fully utilized the heat as the heat must take some time to propagate through the gas.

Further restricting heat input to a more definite "ignition-like" event could possibly prove beneficial in allowing the engine to actually cool down in preparation for "contraction" reducing "waste heat".

All mostly just a lot of theory until tested, so your experiments are quite valuable. I look forward to seeing more.

[VincentG]

Yup Tom you were right on all along. It's amazing how easy it is to corral some people's thoughts into a fixed box. The LTD in standard configuration is poetry in motion to me, it's almost incredible that it runs as well as it does. I'm lucky to be in a position where I have the equipment and ability, as well as the leisure time, to put some real effort into(hopefully) developing these things a bit further.

[VincentG]

While the engine is down I thought I'd do some tests on the steel plate exchangers. The plate was painted flat black to make the infrared accurate. The first picture is the max temp it would reach over the tea candle. Notice how poorly the steel conducts heat away from the source. The outer rim averaged 230F, while the center was up at 390F at times.

tea candle max temp steel plate.jpg (244.04 KiB) Viewed 7712 times

This next picture was after a small 7/8" aluminum disc was placed on the plate for 15 seconds. The surface temperature quickly lowers and does not fully recover for over a minute.

tea candle 15 seconds cooling steel plate.jpg (237.56 KiB) Viewed 7712 times

In operation, the engine likely drops the temperature of the plate even below this, and the same effect is happening at the cold plate. Steel is just the wrong choice here. Between the slow thermal transfer of the steel itself and the loss in transfer to the gas due to low surface area, I'd be surprised if the DTU(my acronym for delta temperature utilization), or the temperatures the gas is actually reaching, is even half of the ETD(external temperature delta). This is promising really, since the engine ran so strong even at that.

[Tom Booth]

Do you think the aluminum is dissipating heat to the air faster, or just distributing it through the plate faster/further?

The reddish area looks much more spread out in the second image.

Also, is this just a plate? With a candle under it, then the aluminum disk over it?

Also was the aluminum removed before the photo was taken?

Do you think the aluminum disk took heat away with it, or conducted it right through to the air, or was it still there when the image was recorded?

I'm assuming from your wording that the aluminum disk was removed before the second picture was taken.

Very interesting.

It might be worthwhile to test copper also if possible. In my experience copper seems to transport heat through itself very fast, but aluminum dissipates the heat more quickly.

[VincentG]

I should have been more clear. The aluminum slug was removed before the picture. The aluminum was just a way of sucking up some heat from the top of the plate to simulate operation. I could get the same result from just blowing on the steel plate as well. The red spot looks bigger only because the highest temperature zone has been removed, so the color scale refreshes. A fixed scale would be best, but most of these guns auto refresh unfortunately.

It's hard to say really how the plate reacts to the engine in real operation, but I would imagine that there is more heat conduction away from the hot plate when its running than just being in free air. The aluminum foil on the displacer is very effective at pulling heat from the steel plate as well.

Yes it was just a candle under the steel plate. Copper would work way better, I have some similar thickness copper that I will repeat the same test with. The other main benefit of copper over aluminum even is a much lower specific heat. So I think copper sometimes appears to stay hotter longer because it absorbs heat and raises in temperature much faster.

My understanding is that surface area is mostly the key factor to transferring heat to the air, it just becomes a question of whether or not the metal can keep up internally with the heat transfer rate. Interestingly, certain surface coatings can vent heat faster than others as infrared heat however. Black carbon is especially good at this, the effect being known as black body radiation.

[VincentG]

A quick update on this project. I had been working on scaling up the displacer chamber to a 4" ID but thought I'd give this engine one more effort. So the new short-term plan is actually shrinking the chamber to match the current 5cc power piston displacement. From roughly a 3.5" displacer diameter down to a 2.4".

That with a roughly .13" displacer stroke will give around 10cc of displacement for a 1 to 2(power piston to displacer) ratio. As of last, the displacer displacement was roughly 45cc so that's a large change. From there I can find the minimum external temperature delta to run at that ratio and do further testing before moving on to a larger engine.

Unfortunately it seems my 60L drum-based engine is a non-starter. A good thing overall as I now believe the displacer chamber can be much smaller for even large-scale engines at higher temperatures.

[Tom Booth]

... it seems my 60L drum-based engine is a non-starter. ...

I've been curious about that. Was there some specific problem?

Do you mean a "non starter" figuratively or it literally would not start?