Kelvin-Planck 2nd Law of Thermo & a Syringe Steam Engine

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Twigg
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Summary:

This DIY hobby steam engine looks at first glance like an engine with a single hot reservoir. That can't be true. What's going on here?
So I found this steam engine on the youtubes:
Consider the system not including the candles, only including the can that makes up the boiler shell and the piston, flywheel, etc. The boiler can acts as a hot reservoir; however, the Kelvin-Planck statement of the 2nd law says you can't have an engine put out net work per cycle with only a hot reservoir. This little steam gizmo is certainly doing work. What gives?

My best guess is that a tiny amount steam is condensing inside the syringe during the flywheel-driven compression, dropping the pressure in the cylinder so that the overall work is positive. The steam condensing would give off heat to the walls of the syringe, and that heat flow would act as the cold reservoir. I just can't seem to use maths to predict how much vapor condenses, because to do that I need to know the pressure and temperature (so I can do energy and entropy balance inside the syringe control volume).

Two questions:
1) what is the "cold reservoir" in this machine? (is it condensation in the syringe? something else?)
2) how would you analyze this engine if you were trying to calculate the efficiency knowing only say: the heat power in the boiler can, the piston volumes at top and bottom dead center, and the RPMs?

P.S. No, I'm not trying to build this piece of junk. I am aware that a few mm slip of the solder on that boiler joint makes the difference between an engine and a pipe bomb. This is purely a physics question. Just getting that out of the way.
 
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  • #2
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1) what is the "cold reservoir" in this machine? (is it condensation in the syringe? something else?)
If that this not a fake than the air is the heat sink.
 
  • #3
Twigg
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DrStupid,

I did some back-of-the-envelope maths to see if heat exchange between the steam inside the syringe and the atmosphere looks feasible. Short version is that it does look feasible, and I'm satisfied I have the answer to this first question. Thanks!

Long version:
I measured a rotational frequency of 7 Hz (counted it on a 0.25x speed). The syringe is a 20mL syringe and the stroke is about 15mL, with a roughly 15mm OD, and based on google it's probably made of polypropylene and I'm guessing the walls are ~1mm thick. Also, the video shows the flywheel is 4 CDs thick. The connecting rod looks to be connected to the flywheel about 30mm off-center. I'm also going to guess each candle puts out about 40W of heat (based on google), of which lets say 20W (total guess) is transferred to the can, so ~40W of net heat into the can.

I calculated the work by calculating the mass of the flywheel from the dimensions of a CD and density of glass (times 4 CDs), using the coefficient of kinetic friction 0.2 for lubricated steel-on-steel (probably skateboard bearings inside that motor), and calculating the force of friction ##F = 0.2 \times 141g \times 9.8m/s^2 = 0.28N## and the work required to keep the flywheel spinning per cycle ##W = 2\pi \times 30mm \times 0.28N = 52mJ##.

The heat received in the boiler per cycle is ##Q_{in} = 40W / 7Hz = 5.71J##, so the heat that must be transferred from the syringe to the environment is ##5.66J##. The thermal conductivity of polypropylene is ##0.11\frac{W}{m*K}## and we know the syringe dimensions, so we know ##\dot{Q}=\frac{0.11W/(m*K) \times Area}{thickness} \times \Delta T##. The length of the stroke is given from the volume and diameter ##15mL / (\pi \times (7.5mm)^2) = 85mm##, and the average length of the steam-filled region is half the total stroke, ##43mm##. So the total surface area of the syringe is ##43mm \times 2\pi \times 7.5mm = 20cm^2##. Now, calculating the temperature drop: ##\Delta T = \frac{5.66J \times 7Hz \times 1mm}{0.11W/(m*K) \times 20cm^2} = 180 K##.

180K is only a factor of 2 larger than what I'd expect between saturated steam and atmospheric, which could easily just be me estimating the candle heat wrong.
 
  • #4
russ_watters
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It isn't clear to me what the syringe is doing, but with an open cycle steam engine, you don't necessarily need condensation, you can just release the steam to atmosphere after it pushes the piston. But I can't tell how the piston (syringe) in this cases is working; how it is releasing the pressure for the "up" stroke.
 
  • #5
Twigg
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The first thing I thought was that the video's poster had drilled an exhaust in the side of the syringe, but watching the video it doesn't look like they did. That's what made me think condensation: some steam condenses during forced compression by the flywheel, dropping the pressure of the remaining gas. That or heat transfer through the walls of the syringe, as per the lengthy ramble in my last post. Either one would fix the entropy balance.
 
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  • #6
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@Twigg,

I’m skeptical.

The surface of the boiler is at least 30 times as large compared to the syringe. That means less than 1/30 of the 40 W can go into the boiler – even if the thermal transmittance would be equal. Thus we are below 1.3 W thermal power.

As the boiler cannot be under high pressure it has a maximum temperature of 373 K. Assuming at least 273 K for the environment results in a Carnot efficiency of around 1/4 – even if the cold junction would have environmental temperature. Thus we are below 0.35 W mechanical power.

Even with this very optimistic assumptions there is not enough power left to keep the flywheel spinning (which requires 0.36 W according to your estimation) and to power the LED (which would need about 0.04 W with 100 % efficiency of the generator).

I would say that the syringe piston is powered by the motor and not the other way around.
 
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  • #7
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It isn't clear to me what the syringe is doing, but with an open cycle steam engine, you don't necessarily need condensation, you can just release the steam to atmosphere after it pushes the piston.
This is unlikely because there are no valves. It rather looks like a close system (apart from leaks which would reduce the efficiency). It most probably works as condensing engine (if it works).
 
  • #8
Ibix
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how it is releasing the pressure for the "up" stroke.
Compare the piston syringe to the one the guy uses to fill the tank - looks to me like he removed the rubber seal from the plunger. That's why it moves so easily. Could the steam leaking out around the plunger be enough?
 
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  • #9
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The video shows him removing the seal. So there's a lot of blow-by in the syringe.

After seeing this, I think I will pick up a hot glue gun next time I'm at the crafts store. Not sure how I made it through life so far without one of those.
 
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  • #10
Ibix
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The video shows him removing the seal. So there's a lot of blow-by in the syring
I thought I watched all the syringe bit - must have skipped that by accident.
 
  • #11
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Compare the piston syringe to the one the guy uses to fill the tank - looks to me like he removed the rubber seal from the plunger. That's why it moves so easily. Could the steam leaking out around the plunger be enough?
I'm not sure how a leak that is not controlled by a valve would result in a cycling operation. Maybe the pipe between the boiler and the syringe results in a phase shift between the pressure curves. But I do not know if that would be sufficient to work as a steam engine.
 
  • #12
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I just opened the video in youtube, where you can see the comments. Quite a few "I built this and it doesn't work" and "fake" etc.

I didn't see any "I built it and it works"
 
  • #13
Ibix
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I'm not sure how a leak that is not controlled by a valve would result in a cycling operation. Maybe the pipe between the boiler and the syringe results in a phase shift between the pressure curves. But I do not know if that would be sufficient to work as a steam engine.
That leak is the only place steam can be coming out, so if it's not faked (and the flywheel hides enough space that faking it would be trivial) it seems likely to be relevant. I noticed he used three or four CDs to make a flywheel - I wondered if the downstroke might be powered by the flywheel? Or does that violate energy conservation? I'm very wary of applying my ideal gas thermodynamics to steam, which is very non-ideal.
 
  • #14
Twigg
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@DrStupid

That's a good point. There's no bleepin way this engine would get anywhere near Carnot efficiency with all that blow-by. I'm sold on that argument. Now I suddenly feel like I understand why most of the "working" examples of steam engines on the youtubes are wired to motors (ahem, "generators").
 
  • #15
Twigg
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That leak is the only place steam can be coming out, so if it's not faked (and the flywheel hides enough space that faking it would be trivial) it seems likely to be relevant. I noticed he used three or four CDs to make a flywheel - I wondered if the downstroke might be powered by the flywheel? Or does that violate energy conservation? I'm very wary of applying my ideal gas thermodynamics to steam, which is very non-ideal.
You need a higher average pressure during your upstroke than the average pressure during your downstroke to do work. The leak is present on both both strokes, so it doesn't bias the upstroke. If the leak somehow only leaked on the downstroke, it could work. And at these pressures, I doubt you need to worry beyond the ideal gas law.
 
  • #16
hutchphd
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If the leak somehow only leaked on the downstroke, it could work.
Seems to me you have that backwards. How does this scheme make the flywheel spin?
Seems very unlikely to me overall.
 
  • #17
Twigg
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I might've mixed up up and downstroke. I meant if the leak worked during compression (when the flywheel does work on the gas).
 
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  • #18
anorlunda
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The first thing I thought was that the video's poster had drilled an exhaust in the side of the syringe, but watching the video it doesn't look like they did.
If there is so much doubt about what is actually seen in the video, how productive can it be to speculate?
 
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  • #19
hutchphd
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I don't think the issue is what is seen on the video. It is pretty explicit. I think the question is how can it possibly work. So the answer is what is "unseen" purposely on the video. This is a scam, by my reckoning.
 
  • #20
256bits
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Well, it is seen around 00:30 that the thing starts up running from a BDC position ie at the start of the "compression" stroke. There doesn't seem to be any movement of the hand to give it a spin.
If someone can explain that they would make millions, and turn the world of compression engines upside down.
 
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  • #21
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Well, it is seen around 00:30 that the thing starts up running from a BDC position ie at the start of the "compression" stroke.
This behaviour is to be expected for the case that the device is driven by condension of steam instead of steam expansion (or by an electric motor). That's how a Newcomen atmospheric engine works.
 
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  • #22
Lord Jestocost
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Summary:: This DIY hobby steam engine looks at first glance like an engine with a single hot reservoir. That can't be true. What's going on here?
It's a clumsy fake!
To prove that a steam engine works there is absolutely no need to connect some of its movable parts to an electric motor.
 
  • #23
256bits
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This behaviour is to be expected for the case that the device is driven by condension of steam instead of steam expansion (or by an electric motor). That's how a Newcomen atmospheric engine works.
I don't see any valve to close off the steam flow in the contraption.

If the thing does work, the only way I can see is by something similar to a pop pop boat, or a geyser, where the steam production is intermittent. Steam flows into the cylinder pushing the piston out, the boiler temperature drops, less steam is produced, the pressure drops for the upstroke, and the flywheel can return the piston to TDC for a completion of the cycle.
But, the system would have to be somewhat fine tuned for that to happen - too much heat and the steam flows continuously keeping the pressure up against the piston face hindering the return. Too little heat there is not enough pressure produced to push the piston tot give enough rotational velocity to the flywheel to return the piston.
Whether that can be accomplished in practice I do not really know, as it is just a surmise.
 
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  • #24
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I don't see any valve to close off the steam flow in the contraption.
That's why I assumed from the beginning that this is a fake.

If the thing does work, the only way I can see is by something similar to a pop pop boat, or a geyser, where the steam production is intermittent. Steam flows into the cylinder pushing the piston out, the boiler temperature drops, less steam is produced, the pressure drops for the upstroke, and the flywheel can return the piston to TDC for a completion of the cycle.
Or steam is sucked into the cylinder by the piston that is pulled out by the flywheel, the boiler temperature drops, less steam is produced, the steam in the cylinder condenses, the pressure drops, and the piston is pushed back by the outer air pressure for a completion of the cycle.
 
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  • #25
256bits
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That's why I assumed from the beginning that this is a fake.



Or steam is sucked into the cylinder by the piston that is pulled out by the flywheel, the boiler temperature drops, less steam is produced, the steam in the cylinder condenses, the pressure drops, and the piston is pushed back by the outer air pressure for a completion of the cycle.
I think we should patent our ideas before anyone else does - the valveless engine.
 
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