Engine Pressurization

[stephenz]

That's exactly what I was referring to with the general lack of informal data. Where are those temperatures measured?
So 0 degree is power piston TDC I presume?

Remind to put more TC's than needed when I built mine.

[Tom Booth]

That's exactly what I was referring to with the general lack of informal data. Where are those temperatures measured?
So 0 degree is power piston TDC I presume?

Remind to put more TC's than needed when I built mine.

I would guess 0° to more likely represent BDC (full expansion at the end of the power stroke) whereas TDC would, I assume, represent full compression, which would roughly corelate with the peak temperatures.

Where, how, or even IF the temperatures were taken is a good question and I share your frustration at the lack of more specific information. If only a simulation, the temperatures were perhaps only calculated based on (? I'm not really sure what, gas laws).

If there is any truth behind the "simulation" of this supposedly "real" engine, then it seems plausible that not just what Matt stated could be true, that the gas, ideally, is as cold as the cold side, but actually colder. Not due to any effect from the regenerator, but rather due to the simultaneous expansion of the gas just before and/or as it is shifted over to the cold cylinder (in an Alpha) or cold side ( in a Gamma).

There is some clue as to what the temperatures represent in the text:

"Figure 5 illustrates the adiabatic properties of a real heat exchanger. The straight lines represent the temperatures of the solid portion of the heat exchanger, and the curves are the gas temperatures of the respective spaces."

[stephenz]

Yeah sorry BDC of course.

There is a little bit of a delay between what-could-be-BDC and the lowest temperature which can't really explain other than by the effect of the cooler, but that would beg the question of where that temperature was measured (or plotted from - off the simulation data).

[Tom Booth]

Yeah sorry BDC of course.

There is a little bit of a delay between what-could-be-BDC and the lowest temperature which can't really explain other than by the effect of the cooler, but that would beg the question of where that temperature was measured (or plotted from - off the simulation data).

I'd tend to agree with the highlighted, logically, except that the text of that diagram states explicitly: "The gas temperature fluctuations are caused by the effects of compression and expansion..."

IMO this is similar to a vapor compression heat pump.

In a heat pump the working fluid (refrigerant gas) is compressed on the hot side, expanded on the cold side, which action "moves" the heat in the working fluid into or towards the hot side of the system and out of the cold side.

Though I'm not a fan of the flow analogy, a Stirling engine seems to be rather like a dam holding back or resisting the "flow" from the "reservoir", working against the natural flow of heat, only leaving open one small channel where a flow of energy can find a path through the dam. Which in a heat engine is the power piston.

Of course, quite often there are a lot of "leaks", unintentional or by design, including by way of a spillway or overflow. The cold side of the engine only serves as an overflow, not the intended outlet, which is the piston.

[stephenz]

It could also be that BDC was not 0.
Either way it's hard to tell without what the ambient air temperature was.

[Tom Booth]

It could also be that BDC was not 0.
Either way it's hard to tell without what the ambient air temperature was.

Good point.

If the scale is consistent, 300k would be off the bottom of the chart. I'd estimate the bottom of the chart starts at about 320°k (47°C 116°F)

So the lowest "cold" temperature indicated would, in that case be about 325°k (52°C 125°F)

The cold side of the "heat exchanger" (straight blue line), might be around 345°K (71°C 161°F)

So this "cold" side heat exchanger is apparently in excess of any possible "ambient" temperature.

[Tom Booth]

I haven't been following this discussion very closely at all, so, not sure about relevance to the thread topic. (Pressurization)

The gas, working fluid, pressurized or not is sealed inside the engine housing and basically "knows" nothing regarding the outside "ambient" world, just the interior materials. Inside engine walls, heat exchangers surfaces, regenerator matrix, piston(s) displacer...

Why the higher than ambient temperature of the cold heat exchanger?

Is that due to heat carried to it through the working fluid with its temperature fluctuating up and down? Heat conducted through the engine housing from the hot side? Is the effective "ambient" water in a cooling jacket?

Too many unknowns to form a conclusion unfortunately.

My point is only that heating and cooling that results from expansion and compression has an influence which apparently, sometimes crosses the presumed thermal boundaries of what is normally considered the fixed heat source and sink or extreme ends of the regenerator gradient.

[Tom Booth]

...
So, factoring that in, it is at least theoretically possible for the working fluid exiting the regenerator on the cold side to be colder than the regenerator. The "hysteresis" mentioned in the text perhaps.

"When the gas temperature deviates above and below the heat exchanger temperature, it causes thermodynamic losses known as "heat transfer losses" or "hysteresis losses"."


The cold side of the regenerator is actually warming the expanding and cooling working fluid passing through it.

Likewise, traveling the other way, the working fluid is compressed and hot. Potentially hotter than the hottest parts of the regenerator. The regenerator, at those moments would then be reducing engine efficiency. Absorbing heat when.ideally it should be releasing heat to help expand the working fluid.

Higher compression ratios would, I think, logically, exacerbate these "hysteresis" losses.

[stephenz]

I was not what the straight lines meant, do you know what they are for a fact or only suspecting?

It all depends if you're talking about the real cycle or the theoretical model with 4 distinct steps.

If you consider the model, well while there is no shuttling taking place in the regenerator, there is nothing coming in and out. I suspect the model also assumes a theoretical regenerator with no volume, hence no compression or expansion.

Now with the real cycle, not only there is volume, so there will be compression/expansion, but also there are steps overlaps causing some shuttling to occur while compression or expansion are happening. So assuming that scenario toward the end of the expansion steps where shuttling is starting to push fluid to the compression space, the fluid exiting the regenerator can have its temperature lower than regenerator (assuming it transfers most of its energy to the regenerator, whilst still being slightly expanded).


While I do see this possible, I don't think it's very relevant as 1. we're talking about what should be a small amount of fluid, and 2.the amount of expansion happening is small (power piston is in last few degrees before hitting BDC).


For any given heat source, the cooler temperature is the one parameter that should always be minimized regardless of everything else. The lower it is the more compression potential there is, the faster the engine will run and the higher the efficiency. Bridging the gap between the ambient temperature and the fluid temperature at its lowest point can't be overstated.

[Tom Booth]

I was not what the straight lines meant, do you know what they are for a fact or only suspecting?
...
...

The Wikipedia article describing the graph states: "The straight lines represent the temperatures of the solid portion of the heat exchanger, and the curves are the gas temperatures of the respective spaces." as I referenced previously. That is a direct quote.

[VincentG]

I like where you guys are going here. This is the type of first principles thinking that Elon Musk advocates. Hard to imagine a whole new century doesn't bring in some new levels of understanding.

[Tom Booth]

I like where you guys are going here. This is the type of first principles thinking that Elon Musk advocates. Hard to imagine a whole new century doesn't bring in some new levels of understanding.

Personally, I don't see why such "hysteresis" can't be avoided. The placement of the regenerator and power piston seems backwards to me in a majority of Stirling engines.

The air, when being heated and expanded has to move backwards through the regenerator to reach the power piston. This includes the NASA type engines.

Some examples:

Resize_20230518_142609_9160.jpg (91.16 KiB) Viewed 2024 times

Resize_20230518_142609_9401.jpg (47.83 KiB) Viewed 2024 times

Resize_20230518_142609_9327.jpg (80.32 KiB) Viewed 2024 times

The gas, heated, has to expand back past both the cold side of the regenerator as well as the cooler.

I suppose the idea is pressure is pressure, so the temperature of the gas at the piston is irrelevant. Maybe that is true, I don't know, but if the generally accepted kinetic theory of heat is true, then shouldn't the hottest individual gas molecules transfer the most power to the piston if they could deliver a direct impact ratber than being "diluted" by passing back through theze intentionally cold spaces?

I would at least try a different arrangement and see what effect it had. Maybe like this:

Polish_20230518_143133096.jpg (246.88 KiB) Viewed 2024 times

Or this:

Polish_20230518_143342650.jpg (144.07 KiB) Viewed 2024 times

Would that not make more sense and eliminate this hysteresis issue?

[stephenz]

I am pretty sure this (those last 2 sketches) would only cause the crank to spin in the other direction.

The only thing you are changing here is instead of +90 deg phase, you now have -90 deg phase, hence while I think this will work, I think it will only work when you spin the flywheel in the other direction.

[VincentG]

Tom I agree completely that the Nasa engine is the worst offender here. The power piston is ensuring that some hot air is drawn past the cold end at the worst time.

To me it makes the most sense to put the regenerator in the displacer and move the transfer port to dead center between the hot and cold sides. Note that this arrangement is easily achieved by the standard LTD layout, in addition to the (to me) all important benefit of completely shutting off each respective heat exchanger when the other is exposed. The importance of which I don't think can be overstated as shown by my videos demonstrating the relationship of displacer position to power piston travel.

It's ideal to think of a total system pressure change in motion but in reality I think the pressure(temperature) propagates outward from the heat exchanger similar to an internal combustion event.

Placing the regenerator in the displacer at least forces the majority of gas to flow through. In the standard location I agree that it's questionable as to whether there is really a flow of "air"or "energy".

[matt brown]

I like where you guys are going here. This is the type of first principles thinking that Elon Musk advocates. Hard to imagine a whole new century doesn't bring in some new levels of understanding.

The gas, heated, has to expand back past both the cold side of the regenerator as well as the cooler.

I suppose the idea is pressure is pressure, so the temperature of the gas at the piston is irrelevant. Maybe that is true, I don't know, but if the generally accepted kinetic theory of heat is true, then shouldn't the hottest individual gas molecules transfer the most power to the piston if they could deliver a direct impact rather than being "diluted" by passing back through these intentionally cold spaces?

Tom - xlnt post thruout, and I agree that their premise is "pressure is pressure" where the scheme has gas pressure rise & fall, but (ideally) isolates 2 gas temperature regions. Your call out is why I favor alpha designs and rarely scheme beta or gamma.

typical regen.png (174.39 KiB) Viewed 2000 times

Since Vincent mentioned Elon's first principles mantra, consider common beta above. Note that if ANY displacer is truely a 'displacer' than the gas flows around the displacer. So, in my mind, the only way for gas to flow thru this regenerator is for the 'displacer' to act as a positive displacement piston...