Displacer 90° out of phase why?

[Tom Booth]

Given that a Stirling engine is a driven mechanical oscillator. I think that this MIT demonstration quite possibly reveals why a 90° phase angle on the displacer is necessary:

https://youtu.be/aZNnwQ8HJHU

[Tom Booth]

Again, 90° or π/2

Coincidence?

https://youtu.be/jewSVEBkI_s

[VincentG]

If you slow the second video down you will see that the "power stroke" of the actuator is actually just barely after "tdc". So not 90 degrees out of phase as the analog is suggesting.

[Tom Booth]

If you slow the second video down you will see that the "power stroke" of the actuator is actually just barely after "tdc". So not 90 degrees out of phase as the analog is suggesting.

You mean like the Essex engine?

Looks to me like the plunger pulls back (down) before the weight reaches the top.

Anyway with the potential illusions possible due to camera frame rate, I'll trust the instrument.

[Tom Booth]

This video doesn't directly mention 90° but is still quite interesting in terms of describing a Stirling engine as a mechanical oscillator.

https://youtu.be/u7k2wiY4Flo

[VincentG]

Show us a combustion piston engine whos timing is at 90 degrees before TDC.

[Tom Booth]

Show us a combustion piston engine whos timing is at 90 degrees before TDC.

Why?

A Stirling engine is not a 'combustion piston engine". Is it?

[Tom Booth]

I actually used to think very strongly that a Stirling engine would probably run much better if the timing could be adjusted more like an IC engine, closer to TDC, but after years of fooling around with the timing on Stirling engines, paying no real attention to the advance angle, just moving it a little one way or the other to find where it ran the best, "the best" always seemed to end up exactly at a 90° advance, which left me scratching my head, as it was not what I had expected.

[Jack]

Looking at these engines theoretically I wouldn't expect it to be the best angle either.
Did you test if there's any difference in the ideal angle with and without a regenerator?

[VincentG]

A Stirling engine is not a 'combustion piston engine

A Stirling engine can only dream of being like an IC engine.

The spring and the standard Stirling engine are both eventless wishy-washy cycles, where "timing" is a poor choice of words.

You said it yourself.

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".

[Tom Booth]

Looking at these engines theoretically I wouldn't expect it to be the best angle either.
Did you test if there's any difference in the ideal angle with and without a regenerator?

I can't say I noticed any definite change or difference between regenerator or no.

The effect, real or imagined, (it is just a rather new idea or theory at this point) seems to have more effect in high temperature than in low temperature engines.

That a Stirling engine is an oscillator, I think can be best seen in a free piston engine where the piston motion is not "controlled". Then it will follow its "natural frequency".

I only just came across these references to the 90° phase offset in a damped and driven oscillator recently wondering if it could be significant or just a coincidence.

The strongest example was a high temperature high compression (due to liquid lubricant) engine I spent a very long time (hours) trying to locate the best timing position for the best performance.

Previously this engine would barely light its LED and would stop if there was any resistance applied to the flywheel at all.

https://youtu.be/KBwdKPJKGvs

https://youtu.be/D6F_cDjrEEU

https://youtu.be/WuqTKv0oXn8

I made other changes and improvements, but it really surprised me that the timing location where the engine ran very well with liquid lubricant and with the greatest power and torque turned out to be very close, if not right on 90° displacer advancement.

Previously I could not get the engine to run at all with the liquid lubricant, so I thought it might require different timing. So I adjusted and adjusted the timing looking first, just to get it running with the liquid lubricant, then fine tuned it by moving just a hair this way or that.

The "new" timing position turned out to be extremely close to 90°. There was some sloppiness in the connecting rod bearings and such that needed to be compensated for, but I was really expecting the "sweet spot" to be much closer to TDC. Instead it seemed nearly dead on 90°.

Before the presumed timing change the engine did not want to run at all with the liquid lube on the power piston.

https://youtu.be/9OX19YRY9fM

Needless to say, a lot of this is speculation at this point and the exact 90° advance may be nothing, but I'm begining to think there is more to it, having something to do with this 90° driven oscillator offset.

More a possibility that might be worth further exploration than anything.

[Tom Booth]

A Stirling engine is not a 'combustion piston engine

A Stirling engine can only dream of being like an IC engine.

The spring and the standard Stirling engine are both eventless wishy-washy cycles, where "timing" is a poor choice of words.

You said it yourself.

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 ...

Well I do a lot of theorizing and idle speculation.

Having a "wide margin" above and below the traditional 90° offset does not negate or contradict the oscillation theory. LTD engines seem to have more leeway in that regard.

As I said, I've had strong expectations that timing closer to TDC like an IC engine should, theoretically, give better performance, but my attempts to make "improvements" ended up mostly confirming the traditional 90° offset than anything.

I'm saying now that my earlier theory may be wrong, and I think I may have finally found a reason for the 90° advance.

Maybe it is associated with this 90° "driven oscillator" phenomenon.

Maybe not.

A theory is just a theory, but I'm starting to really think that Stirling engines are fundamentally VERY different from IC engines in ways I never anticipated.

A "driven oscillation" is very different from and much more subtle than a brute force expansion.

[Tom Booth]

Unfortunately, the whole field of "oscillators" and "driven oscillation" is pretty obscure, and references to this 90° offset are not easy to come by.

I've been searching for additional information the past few days. Not much more available on the subject that I can find so far.

[VincentG]

Exploring all of the extremes is my approach, so I understand the theorizing on all fronts.

This video seems relevant, with the goal of the power piston being slave to the displacer. Though in my testing the phasing is closer to 0 degrees than 90. Made possible I believe by more rapid displacer movement from the spring delay.

https://youtube.com/shorts/AvZ_GvFu8xE? ... 9wedUtBNb4

[Tom Booth]

The 90° "sweet spot" in those "driven oscillator" demonstrations, where the oscillations rather suddenly go out of control crazy reminds me A LOT of the behavior of the "wet thermoacoustic" engines:

viewtopic.php?t=5483

I've seen this in my own experiments with these engines.

As they warm up to operating temperature, there is a period where the oscillations peek, then with further heating taper off

I'm thinking that may have something to do with the distance into the rice or gravel medium the heat has penetrated which determines the offset.

If that ideal thermal "offset" can be found, it may be possible to "tune" these "rice" engines better by somehow controlling the temperature differential midpoint in the medium to produce the strongest harmonic oscillations.