Free the piston, but not the displacer!

[VincentG]

Thanks Fool, I have read the design manual but don't recall much talk of Matt's example. I'll go back and give it a look.

[Fool]

I don't know. I don't know even if Martini has an engine. His book is a general, but excellent design manual. It's more concepts and analysis, than building reference. Matt is above me, maybe he can enlightened us a bit. I didn't know if you knew of the book or not. So threw the link in for general readers of this thread. It might help someone. I will go back and look again too.

[matt brown]

Currently, Martini's "Stirling Engine Design manual" (2nd ed) is available here:

https://ntrs.nasa.gov/api/citations/198 ... 022057.pdf

I've had this NASA copy for decades, but I've only scanned over it. I agree with Fool (more concepts than build ref) since the plethora of data tends to diffuse vs focus design issues. Don't miss this sniglet which is DOE buzz for more moola...

Martini book.png (181.01 KiB) Viewed 1642 times

This edition is now 40 yrs old, but still has tons of useful info, and many references (Finkelstein, Walker, etc) are golden and rarely seen these days.

Vincent - check out page 63 (this pdf) with PV overlay of 2:1 volume ratio Stirling vs Otto with 300-900k cycle.

[matt brown]

VincentG, I think it would be completely acceptable to drive a displacer with a small efficient electric motor, or solenoid, have a "free piston" linear generator to get electricity and use some of it to power the displacer. That is what the flywheel crank mechanism does anyway.

The displacer could be driven at different speeds, and strokes to vary the power out.

It could produce delay at each end, and shorter transit times.

It probably will be less efficient than a crank system, the above benefits hopefully out weighing this.

A while back, I think MikeB proposed discontinuous displacer motion in a few lines that made this sound like a nobrainer. However, even with the best intent, language can be very deceptive and obscure reality while eluding fantasy (Tom holds an advanced degree in this subject). Only an abstract ideal displacer would not tax output vs a real displacer which does. Indeed, a common power piston suffers friction that a common displacer does not, but this does not nix displacer motion issues. The power piston passes inertia and momentum in and out via a flywheel vs each displacer motion is a total loss.

Many modifications could be done to just software.

My thoughts seem to add springs to retain harmonic motion, which would tend to limit it to a designed speed.

It could be made to easily exchange springs.

Yes great. Probably learn in the process.

The linear generator concept grew from NASA and DOE funding where grantmeisters had a field day. Unfortunately, many laymen (and investors) bought into this scheme as potential DIY/CHP reality when it's clearly not. Prior micro computers, the early scheme was simply a constant frequency with variable amplitude for variable output. Now, any of you armchair rocket scientists who are scratching their heads wondering WTF...gimme max output, so nix this variable amplitude BS and simplify/cheapen design (and I'll order 2, 1 for my lakeside cabin). So much for amateur hour, since you would need to control consistent input to achieve consistent output with constant frequency and amplitude. One early NASA scheme was to use constant input where 'overload' input was discarded vs system shut down when design input wasn't achievable. The early NASA scheme was hot plate on one side of 'spacecraft' and cold plate on other side which allowed some wiggle room with input.


As electronics evolved, so did this linear generator BS, until it's now considered de facto SE scheme for comm'l design.

[matt brown]

Thanks Fool, I have read the design manual but don't recall much talk of Matt's example. I'll go back and give it a look.

I'll try to track that bugger down, but unsure if this was by Martini himself or not.

Meanwhile, you might get some tidbits from this old thread

viewtopic.php?t=2273

which led me me to this patent (note Martini)

https://patents.google.com/patent/US3484616A/en

[matt brown]

Vincent - I got nowhere on that "Martini" engine graphic, but I did stumble across another wacky patent of his

https://patents.google.com/patent/US351 ... oq=3513659

entitled "Stirling Cycle Amplifying Machine" - I'll be back to the future before Marty can feed Doc's Mr. Fusion or Spock can replace Scotty's dilithium crystals...

I spent all afternoon reading Stirling Engine Design Manual and found few nuggets amidst endless mumbo-jumbo (a total bummer after being blown away by Making Stirling Engines).

[VincentG]

Thanks Matt for the research. Too bad some of the old forum guys aren't still around.

The Stirling amplifier looks like what evolved into the Stirling cycle artificial heart machines.

I was also a bit let down by the design manual. The best part was the list of related references at the end which could keep you busy reading for a lifetime.

I'm not too worried about discontinuous displacer motion, the spring mechanism returns much of the energy.

[Fool]

Matt, as usual, most of the ideas from do-it-yourselfers here have been tried before, as you're pointed out here. Thanks. I'm leaning towards the idea that explorations such as mentioned by this thread will be very educational for the builder. The idea of driving a piston with a crank while the displacer flops in and out from pressure, AKA Ringbom, is old knowledge.

It makes perfect sense that driving a displacer by a crank while the piston flops in and out, would have been tried, as well, over the last two hundred years.

The power piston passes inertia and momentum in and out via a flywheel vs each displacer motion is a total loss.

You are talking harmonic motion. Sine wave plus the integral being cosine, plus the integral for that being the sine, on to infinity, so to speak.

A flywheel spun up moving just a displacer will have harmonic motion. The energy of stopping and starting the displacer will get stored and released on the flywheel. This will have the benefit of being able to measure, separately from the piston, the power needed for the displaced chamber. IMHO quite the learning tool.

The piston could have its own crank and flywheel system, or it could be suspended by springs or air springs.

A spring mass system is also a harmonic motion sinusoid. Energy of the mass being stored and released to and from the spring. A solenoid can be incorporated into a spring mass system. Or a linear motor, potentially driven under computer control and brushless. The "Free Piston" Stirlings capitalize on that fact. And NASA is using that method as well.

Is opening that can worm less? Heck no. Fun and educational? Yes.

Cary on humans.

[VincentG]

I view the driven displacer as a testing tool, and potentially a way to oscillate a very large mass floating on the chamber pressure it can generate.

Based on my testing, I'd bet I could oscillate almost any practical weight with the right chamber and piston using only tens of degrees F temperature delta.

For now though, it's just a way to test displacer performance. And any off the shelf Stirling engine I've tested doesn't fair well, save for the Essex.

[Fool]

Excellent.

[matt brown]

It makes perfect sense that driving a displacer by a crank while the piston flops in and out, would have been tried, as well, over the last two hundred years.

The power piston passes inertia and momentum in and out via a flywheel vs each displacer motion is a total loss.

You are talking harmonic motion. Sine wave plus the integral being cosine, plus the integral for that being the sine, on to infinity, so to speak.

A flywheel spun up moving just a displacer will have harmonic motion.

make that...MAY have harmonic motion

The energy of stopping and starting the displacer will get stored and released on the flywheel.

Nope...a crank driven 'displacer' (by definition) is always a driven member where the crank is 'dragging it around' (up and down) continuously. This whole drive vs driven issue is often ignored by ECE fanboys, but not by ICE gearheads. The FP displacer is an attempt to thwart this lost work, but this further complicates entire scheme and would take a long white board video to explain.

A spring mass system is also a harmonic motion sinusoid. Energy of the mass being stored and released to and from the spring. A solenoid can be incorporated into a spring mass system. Or a linear motor, potentially driven under computer control and brushless. The "Free Piston" Stirlings capitalize on that fact. And NASA is using that method as well.

Oscillations, harmonics, waves, etc tend to exemplify various steady states, not work processes. For some lame reason, if we consider an ICE Otto cycle akin a gas spring 'oscillation' with source at head of cycle and sink at tail, then net work is achieved via the 'amplitude' of source vs sink temperature. This is total BS...merely miming electronic buzz into heat engine theory. This type of fuzzy 'logic' is nothing more than an endless rabbit hole of distraction/s.

AFAIK Sunpower made the first dual FP SE and is worth studying as to what to avoid. All this government stuff (NASA, DOD, DOE, etc) is $billions trashed for nothing, except a sweet gig for guys that couldn't get a third shift job at a Dairy Queen (love that Ross Perot dig).