Thermodynamic work vs. real work

[VincentG]

Overall system work is zero. The center of mass has not changed. Work done on or by the gas is zero, zero delta Volume. Constant volume. It's similar to the two weights being connected by a rope, or rocking beam. The rope or beam does no work.

If you want the overall system can be broken down to the addition of two systems. One for each weight.

Thermodynamic energy transfer is called heat, not work. A cylinder and piston does work if it pushes with a force, for a distance. We call it pressure times volume change. Or P•∆V=W this is the same as F•d=W

I'm falling asleep. Am I making any sense? Do you want more?

Yea that makes sense.

The problem I see is that is discounting some things. One thing is that a mass has been set in motion, so some work must have been done.

The other is that if one were to need the mass elevated on only one end and only once, then work has also been done. You’ve simply traded the lower elevation of one mass for the raised elevation of another. If you had a surplus of rocks that needed to be buried, you could raise construction materials to build a building, for instance.

It is also more of an analogy to a system that has balanced forces and an abundance of energy. The hole that the rocks are buried in should not be considered like a “cold hole” but instead the path where the energy within the rocks can flow.

The energy in this case is moving through the gas, but there seems to be no thermodynamic path.

[Fool]

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One thought I had is, if energy were stored on a flywheel, some of it could be used to accelerate the masses, they would coast into position, then be decelerated by putting energy back on the flywheel. Conservation of energy. Lack of conservation of position.

My problem when thinking about these thought experiments is that reality creeps in. I mentally put friction into the mix. I would need to push down on one side a lot harder just to get it moving, breaking static friction loose. Keep pushing hard to overcome dynamic friction. The gas would begin to be compressed until the second piston broke free of its static friction. Since dynamic friction is lower than static, the second piston would spring upwards, and possibly bounce/oscillate, as the system moves into the new desired position. It might stop and go moving in jerky steps. When finally near the new position, there would be no need to slow down. There would be no residual motion to reabsorb. Just stop pushing. Let it coast to a stop. At that point the gas might be more but slightly, compressed or expanded, than it started. In essence the gas would be a spring.

There are many ways the system could be micro modeled. One could even look at the spring constant of the piston material and how the force was applied. At some point it becomes more reliable to just simplify the system. Pistons are very stiff compared to air-spring-couplings and the device applying the force might be more springy than the piston.

Yes, realty tends to complicate my thoughts process. Modeling boils down to calculating what is needed for understanding and construction. Engineers begin with what is a freebody diagram, and move on to black box with inputs and outputs when reactions by the box are known. Design engineering doesn't need to be perfect. 2+2 could equal anything from 2.95 to 3.05 .

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[matt brown]

Here's a graphic of a helium 300-600k Lenoir cycle with regen

Lenoir regen.png (14.82 KiB) Viewed 1187 times

Shown as closed cycle with multi-bar P, this could also be open cycle with 1 bar ambient Pmin. AFAIK this is the only cycle that can be gamed this way and should be in every thermo book as an example of an adiabatic process shoehorned into a common cycle. Note Q and W values where isobaric Wneg steals 2/3 Wpos whereby this cycle has ideal eff=.33 so Carnot keeps his crown, but not bad for meager 1.5 volume ratio. However, an xlnt example of common isobaric issues where a hidden tax sinks many schemes, especially when an isobaric process is on the compression half of a cycle.

Above has isobaric regen after expansion, but isochoric regen after expansion is also possible

MARS cycle.png (16.8 KiB) Viewed 1187 times

Hmmm, the "MARS" cycle (Monatomic Adiabatic Regenerated Stirling). Values at bottom have previous Lenoir followed by MARS in window. Note that ideal eff went from Lenoir .33 eff with isobaric regen (and compression) to MARS .46 eff with isochoric regen (and no compression).

If only those 2 central "DP" pistons could be a conv'l displacer...

Vincent - note this later version has zero point values when reducing all P values by 4, thus ambient compression 1-2.

[matt brown]

Overall system work is zero. The center of mass has not changed. Work done on or by the gas is zero, zero delta Volume. Constant volume. It's similar to the two weights being connected by a rope, or rocking beam. The rope or beam does no work.

If you want the overall system can be broken down to the addition of two systems. One for each weight.

I'm falling asleep. Am I making any sense? Do you want more?

The problem I see is that is discounting some things. One thing is that a mass has been set in motion, so some work must have been done.

Fool is correct that "overall system work is zero" but missed that summing the system as two subsystems (one for each weight) is not optional, but mandatory. Vincent, you're right, "some work must have been done", so the only recourse is summing both events via Wpos vs Wneg whereby when Wpos=Wneg then Wnet=0. Indeed, clumsy wording for a paired event, but getting the energy balance correct trumps ideal wording, and why math is the preferred language of science.

The other is that if one were to need the mass elevated on only one end and only once, then work has also been done. You’ve simply traded the lower elevation of one mass for the raised elevation of another.

When Wneg of lower mass is removed from the equation then Wpos=Wnet (an xlnt example of how an agenda can effect perception).

[VincentG]

Matt you are a wizard and what a great name.

As for the energy balance, let’s say there was water running off of a cliff into a piston that was allowed to travel down. It is connected to a long conduit and finally another piston.

Same thing as a hydro turbine but now the work is passing through the gas. I’ve never heard of a net zero output being considered for hydroelectric.

[matt brown]

As for the energy balance, let’s say there was water running off of a cliff into a piston that was allowed to travel down. It is connected to a long conduit and finally another piston.

Same thing as a hydro turbine but now the work is passing through the gas. I’ve never heard of a net zero output being considered for hydroelectric.

Check out the hydraulic ram for an old magic show...

hydraulic ram.png (413.9 KiB) Viewed 1155 times

https://www.youtube.com/watch?app=desktop&v=aUTjVovpKvA

[VincentG]

The ram pump is cool but not quite what I’m on about, at least I don’t think so. I didn’t mean to get wrapped up in the gravity side of things but instead the repetition of a non cyclical process.

It’s just like a hydraulic circuit or a rope with a pulley, or any mechanical connection. The work is being transferred through a solid or liquid mass. The difference with the gas is how it can be interacted with so easily on multiple levels.

So you could argue zero net work, but the building materials being erected a mile away from the cliff (hypothetical scenario for dramatic effect) would argue otherwise. The question is, what is the difference between the real work being done vs. the thermodynamic work in the gas.

[matt brown]

Comparing the posted values of Lenoir vs MARS graphics:

Lenoir has isobaric compression that requires 20J Wneg, whereby 30J Wpos yields .33 eff
MARS has isothermal compression that requires 16J Wneg, whereby 30J Wpos yields .46 eff

These cycles have extreme similarity, but MARS has 20% less Wneg (16J vs 20J) which leverages thru values to 37% more RELATIVE eff (.46 MARS vs .33 Lenoir) from the same 30J Wpos !!! So, the MARS wins hands down, but only IF YOU CAN SCHEME A VALID MECH with enough wiggle room for inconsistencies (real vs ideal values).

In the PV domain for ECE, "closing the plot" requires the cycle to return to the same start state values, even when some values are outside the engine (think open vs closed cycle). The reason why Otto and Stirling dominate engine dreams is solely due to wiggle room of values where the paired isochoric processes of each allow various Qin from the same Qout. However, there's more to this than a quick glance reveals, since both Otto and Stirling also have "paired" expansion and compression processes. Yes, Otto and Stirling have drastically different expansion and compression processes, but these processes have a proportional similarity within each cycle that nixes incompatibility as if by magic.

This MARS pitch has some paired values, but the inherent dissimilarity between the adiabatic expansion and isothermal compression processes drastically raises the bar on gaming such cycles. Simply having the same volumetric compression and expansion ratio is inadequate since MARS has dissimilar pressure ratios during compression and expansion. Indeed, hard to believe that such a cycle is even possible, but PVT values could likely be reduced to a ratio equation which would explain this more logically than my stab in the dark...

[matt brown]

The question is, what is the difference between the real work being done vs. the thermodynamic work in the gas.

These are drastically different types of work and even Fool has missed this. It's not like using a different scale like lbs vs kgs for the SAME weight.

If we measure each work as force then the major difference becomes obvious, but this sneaky approach obscures the reality that both work will require some type of change, so both are dynamic, not static.

"rock" work - calculated via weight x distance where weight is constant and commonly ignoring ALL gravitational effects (acceleration, height change, etc)

"thermo" work - calculated via pressure x volume where both vary

rock vs thermo work.png (6.05 KiB) Viewed 1145 times

Returning to the 100 lb vertical model, the rock work rate is constant since the Wpos force is constant. Meanwhile, the thermo work rate is variable since the Wpos declines with expansion.

(1) when the rock has ambient force on both sides then Wpos is directly related to the 100 lb force (weight)

(2) the engine requires unequal force on both sides where the force inside engine exceeds the force outside the engine, but the same rock work can be achieved directly via manipulating this pressure differential or indirectly via increasing piston size (fan favorite)

(3) the engine cycle is usually considered 'spent' when the inside pressure equals the outside pressure, but a closed cycle can increase cycle work via sub ambient expansion (gamma, etc)

(4) summing rock Wnet requires only crude math vs thermo Wnet often requires some fancy algebra

However, this graphic obscures the real magic behind our engines that is lost to history...the flywheel and no one knows who discovered it. The flywheel is often described as a means to "smooth out" an engine cycle, but this glosses over the real magic. The primary job of the flywheel is not to store work from expansion process of cycle for compression process of cycle. Nope, the flywheel's primary job is to store work from early in expansion process for later in expansion process, in effect converting variable expansion work into constant expansion work ('thermo' work into 'rock' work).

The 100 lb weight may look impressive, but don't forget the weight of all that air above it (down to zero lbs LOL) makes 100 lbs look whimpy. IOW increasing the displacement of this 100 lb weight doesn't effect rock work.

[VincentG]

Thanks Matt I’ll have to think that over for a bit.

[matt brown]

Vincent - I was working on a post relating rock vs thermo graphic to early steam engines and flame lickers when I think that I stumbled upon your work "dilemma". Words might escape me, but at least I know what I'm chasing. How did you arrive at this ??? I'd call your graphic pitch extreme and it likely evolved for clarity, but still everyone wondered WTF. I only managed while comparing other engines, I glanced at that work graphic and saw "a flash in the fog" that was just long and bright enough that I could sense direction.

[VincentG]

Vincent - I was working on a post relating rock vs thermo graphic to early steam engines and flame lickers when I think that I stumbled upon your work "dilemma". Words might escape me, but at least I know what I'm chasing. How did you arrive at this ??? I'd call your graphic pitch extreme and it likely evolved for clarity, but still everyone wondered WTF. I only managed while comparing other engines, I glanced at that work graphic and saw "a flash in the fog" that was just long and bright enough that I could sense direction.

Short answer, to use more Elon buzz, I'm really trying to grok this subject so it evolved from questions I had. I'm curious as to what you picked up on here, as it's often not what I was thinking but maybe more relevant.

I had a thought that hopefully helps more than confuses the subject even more, as this is not a single ended question.

This first point addresses a repetitive open process.

To represent the first point, imagine a 200lb man on a construction site. He is climbing up and down a ladder all day to work and move materials up to the second story. He must climb up to work either way, but now instead of climbing back down the ladder he lowers himself on a platform, which is counterbalanced by 190lbs of construction materials that are now elevated to the second story. He could have taken all 190lbs in one trip, or 19lbs at a time in 10 trips but instead the same work was accomplished without the required force of the single trip or the energy of 10 trips.

While the material could have been lifted easily with the mechanical advantage of a block and tackle, this way is much faster and requires no extra work as he will have to climb back up to perform more useful work anyway.


The second point is to relate the rope, block, and tackle, to the gas.

If he were instead lowered down on a piston while lifting materials on the other piston, the force he exerts on the gas while lifting 190lbs of materials is less than if he were balanced against 200lbs of materials and able to perform no work.


The third point is that work must be put into the system initially. The mass of the pistons and the pressure inside the chamber must be enough to make the mass of the man insignificant so that when the system has a real workload (man and materials), the initial adiabatic compression is also insignificant in comparison to the real work being done.

[matt brown]

It's all a matter of how force is summed and the direction of work can be fuzzy (what you what to achieve will effect work definition)

pt 1 - man going up is Wneg=200lbs prior materials going up is Wpos=190 lbs while man going down is Wpos=10 lbs
Note cycle attribute here where man starts and returns to same 'point' vs materials do not. The hallmark of work is what changed in the end vs what events occurred (tho path of events alters eff). At a glance, 390 lbs went up and only 200 lbs came down, so yeah, this could appear as over unity.

The man going up and down 10x exerts more work from a human standpoint since (OK if) he can't recover spent energy going up ladder while going down ladder. If man could, then energy would balance between 1 or 10 trips and more trips would only waste more time.

pt 2 and 3 - need more specs, but this appears that 2 pistons/cylinders are connected to a reservoir/chamber

My flash in the fog relates to exactly this type of summing exercise. Consider old world walking beam steam engine with vertical cylinder where steam enters at bottom and drives piston up (sound familiar) whereupon cooling cylinder, cylinder drops pressure and piston descends with the COMBINED force of piston and air weight. Note here that exhaust pressure is ambient regardless of temperature. Watt's condenser lowered exhaust pressure to near vacuum whereby more work was produced for a given sized engine (maximizing the paltry Pmax limits due to early metallurgy). So, watt's 'gimmick' was a nobrainer for work gain, but eff gain was more complicated: (1) more work per cycle from the same steam mass lowered Qin per cycle as if by magic (2) separating the hot cylinder from the cold condenser was a major free lunch (3) after the crank and flywheel appeared, speeds, temps, and output increased until (with higher pressure than early days) someone discovered early "cutoff" of steam inlet for adiabatic expansion vs previous isobaric expansion, where output per cycle decreased but eff per cycle increased (and all this prior 1800).

My flash came thinking condenser vs no condenser. With no condenser (think open cycle) and isobaric steam, the steam pressure fights ambient pressure during intake and exhaust, but with condenser (think closed cycle) and isobaric steam, the steam only fights ambient pressure during intake. In effect, the condenser became a cold hole for steam engines by virtue of vacuum, not temperature. During the early days of steam, boiler pressures were so low that ambient pressure represented a massive backwork ratio when open cycle (just look up early PSIG values). Meanwhile, I was comparing this to flick lickers when I started pondering typical vs atypical forces and wondering if we'd missed something along the way.

[VincentG]

pt 1 - man going up is Wneg=200lbs prior materials going up is Wpos=190 lbs while man going down is Wpos=10 lbs
Note cycle attribute here where man starts and returns to same 'point' vs materials do not. The hallmark of work is what changed in the end vs what events occurred (tho path of events alters eff). At a glance, 390 lbs went up and only 200 lbs came down, so yeah, this could appear as over unity.

The man going up and down 10x exerts more work from a human standpoint since (OK if) he can't recover spent energy going up ladder while going down ladder. If man could, then energy would balance between 1 or 10 trips and more trips would only waste more time.

Matt it's funny that is where you ended up. Over unity is not what I was implying as it was more of a force multiplier that turned a single 390lb trip into two trips of less weight. I suppose the question is this; The man needed to climb up to perform work on the second story and was able to lift the materials with no extra effort on his part using gravity. Sure he ended up on the ground, but that was inevitable anyway. The cycle will repeat. Can we relate the trips up and down to heat needing to flow to a sink anyway, and can we relate gravity to the atmosphere. Further (as related to point 3), can we tie in a gaseous system where the internal(and opposing external) pressure and force is high enough compared to the workload that the thermodynamic work is substantially less significant.

[Fool]

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Wow! You guys are funny. LOL

For future reference the following:

https://m.youtube.com/watch?v=cf0_KQQeTjc

Perhaps not all of our mistakes are so painful.

The first error in physics is to make the erroneous assumption that just because it tries you out to push on a wall, all day, that means you are doing work. LOL. It is difficult in a physics problem/model to quantify human effort. Even just standing there tires out a human. Going up a ladder is easy to quantify 200# the height of the ladder one g.

However, the human doesn't get the energy back when coming down a ladder. That is tiring too. So what do we do? Simplify the problem by removing the 'human work'.

Let me simplify it more by using a teeter totter. You two seem to like 190# of bricks being raised by a 10# extra difference in balance. Seems a bit much, but I can work with it for a thought experiment.

Work in this is potential energy. Mgh.

On one end we put 190# of bricks. On the other we put a 200# weight. Bricks go up weight goes down.

Efficiency Wout/Win=190gh/200gh or about 0.95 = 95%.

For every load up, 5% more must come down. But wait a minute!

Mgh=.5MV^2 how fast are the bricks and weight and teeter totter moving when the bricks get to the top? Ten pounds are accelerating 390# plus the inertia of the teeter totter beam. I bet the energy in that movement is the missing 5%. If, when it hits the bottom, it could be used to raise 10# back up, the mechanism would be 100% efficient. Of course it also depends on how efficient the machine is that elevates the 200 pound weight back up.

If more beams were added to the system, like spokes of a wheel, it could continuously lift more bricks up for every 200# sent down.

Speaking of a wheel, isn't it just a bunch of weight counter balanced by both sides being symmetrical? Yes, and it won't spin unless spun. It won't stop unless stopped. F=Ma and you can't push on a rope or suck on a hydraulic system, LOL.

A piston in a U-tube will not move until pushed, nor will it stop unless pushed in the stopping direction.

Heating a gas is different. A gas piston depends on the path on a PV diagram. Adiabatic paths give and take less work than isothermal paths. Transferring between adiabatic lines absorbs, or releases heat energy. How those processes are done, path followed, effects efficiency for a complete cycle.

Comparing it to a single brick elevating process is not equal. It must be compared to repetitive cycles, for efficiency to be meaningful.

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