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 4436 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.

[Tom Booth]

I've had recently what I feel is a new insight into the value of a regenerator.

I never thought it worth much as a means of storing excess "waste heat" that would otherwise be lost, on the basis that a well designed efficient engine shouldn't have any waste heat, but even so, it seems such "waste heat" would accumulate in the regenerator rather than being reused, it's function didn't really seem clear or make a lot of sense.

I just realized however that by temporarily sequestering heat the regenerator could cool the working fluid making compression easier.

Moreover, it could cool the gas .making compression easier in a manner that does not waste the heat as cooling by means of an outside sink would do, removing the heat permanently.

This is not really anything new, just a different emphasis.

It isn't so much just about storing heat to release during the power stroke or to recover waste heat alone, it is a means by which the working fluid can be cooled to make compression easier but without wasting the heat. It just gets it out of the way temporarily. Helping to increase the temperature differential by cooling the working fluid for the compression stroke without the penalty of removing valuable heat/energy permanently.

[spinningmagnets]

The way I understand it, one of the biggest problems with a hot air engine is how much heat you can get into the working gas, and how much you can get out. If you improve the heat absorption and shedding, you raise the power of the engine, or conversely keep the same power while using less fuel.

Let's imagine a Alpha with no regenerator. A chunk of air in one cylinder is pushed into another cylinder. In these cylinders, there is a certain amount of laminar flow.

What I mean is the air touching the walls moves slow, and the air in the middle is not touching the walls and it moves fast. Imagine air flowing through a pipe, and the air in a center of the pipe is moving fast and its not getting much heat from the walls.

It's not uncommon to achieve 1200 RPM. Divided by 60 seconds, and that's 20 cycles per second. When the amount of heat being absorbed and shed by the gas is limited, do not be impressed by high speeds that have little torque. Once attached to a load (*generating electricity) it will slow down a LOT.

Having some internal turbulence can help, but it can also.create drag and losses.

The heater must be significantly hotter than the regenerator, but if half the heating task is accomplished by the regenerator, then the limits of the heater are "less bad".

In addition, the cooler will likewise have a difficult task absorbing the heat from the gas, and if half the heat is absorbed by the regenerator, it's job is eased, with its performance increased.

Hot air engines existed before Stirlings regenerator. Adding a well designed regenerator can increase power, or keep the same power by using a smaller engine, which uses less fuel per hour.

[Fool]

Matt ran into the problem, in one of his schemes, that the temperature of the regenerator was significantly higher than the regenerator so was ineffective in that adiabatic temperature range of the cold plate. Conversely it didn't get high enough either, for the hot plate. Changed his theoretical efficiency from 85% to 63%. Just an FYI.