Regenerators.... Have I got it straight what they do?

[brian hughes]

I've been figuring out in my head how regenerators work, what they do, and what they don't do. Do I understand correctly, that they work by storing and releasing some of the heat energy, so that the engine takes smaller "bites" of heat per cycle? So they improve the efficiency, with a small penalty of dead space? But they don't, on their own, improve engine output power for a given displacement and temperature difference- my understanding would be: if an engine runs faster with a regenerator, there might be an issue with the heat flow into the hot zone, or out of the cold zone... or am I missing something?

[Tom Booth]

I've been figuring out in my head how regenerators work, what they do, and what they don't do. Do I understand correctly, that they work by storing and releasing some of the heat energy, so that the engine takes smaller "bites" of heat per cycle? So they improve the efficiency, with a small penalty of dead space? But they don't, on their own, improve engine output power for a given displacement and temperature difference- my understanding would be: if an engine runs faster with a regenerator, there might be an issue with the heat flow into the hot zone, or out of the cold zone... or am I missing something?

If you're asking me, your in for trouble.

I used to think the regenerator acted passively to store and release some heat. But after years of careful observation, I think, together with compression(heating) and expansion(cooling) of the working fluid and timing of the passage of the gas back and forth through the regenerator during such moments of compressive heating and expansive cooling, it actually acts as a "Maxwell's demon" actively moving heat, exactly like a heat pump, increasing the temperature differential and improving engine performance

To get a better idea how that is possible I would suggest studying the patents of Rudolf Vuilleumier.

In particular: Vuilleumier, Rudolph, Method and apparatus for inducing heat changes, US1275507 A. 1918

https://patents.google.com/patent/US1275507A/en

I would suggest downloading the PDF as the online OCR text is often corrupted.

[Bumpkin]

I think there will always be issues with heat flow in and out of a Stirling, and the minimal dead space of a well designed regenerator would almost always be worth it, for just the reasons you mentioned.
Bumpkin

[Tom Booth]

To illustrate, using an LTD type engine with regenerative displacer.

IMG_20220126_125928640.jpg (238.72 KiB) Viewed 1363 times

The top two images show compresive heating and expansive cooling of a volume of gas.

The bottom two images show how by releasing additional stored heat at just the right time, the regenerator increases the temperature of the gas already being compressed and heated by the action of the piston (adiabatic heating).

Then, with expansion and cooling, the regenerator may absorb any remaining heat in the gas.

I think this is why a Stirling engine quite often needs some manual turning by hand so this regenerative heat sequestering effect can help establish a temperature differential within the gas, which would otherwise tend towards uniformity. And why the engine, after initial start up, for a time continues to uncrease in speed and develop more and more power as the regenerative effect progressively augments and increases the temperature variations.

In other words, the regenerator helps make the hot gas hotter when it is compressing and heating and cooler when it is expanding and cooling.

[Alphax]

The regenerator removes heat from the working fluid (the air in the engine) as it moves towards the cold end, and returns the heat to the working fluid as it subsequently moves towards the hot end.

It has been reported by some experienced designers (I'm quoting Julian Wood) that in well designed engines the regenerator can be more than 90% efficient in this specific task. A lot therefore depends on how well designed the engine is as to how much the regenerator contributes to overall engine efficiency.

[Alphax]

When it comes to regenerators in practice, then size matters.

Regenerators on small, cheap, mass produced models are hardly beneficial and many work adequately without regenerators at all - in such engines the regeneration occurs at the hot end (hot cap) and displacer walls.

Regenerators on large, well designed engines with good low friction and gas tight pistons will dramatically increase torque and power. This is especially true on engines designed to run on hydrogen or helium as the working fluid, and even more so on pressurised engines.