hybrid steam/stirling engine

Discussion on Stirling or "hot air" engines (all types)
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hybrid steam/stirling engine

Post by tmk » Tue May 29, 2007 11:36 pm

This idea just occurred to me.. a stirling operates on heat difference causing pressure changes in a sealed chamber due to air expansion. But the expansion of air is only so much. The expansion of liquid->gas is much greater.

what if we had our hot side at say 150 C and added a small amount of water.. presumably it would vaporize, which creates a much larger expansion than just heating air.. if the displacer then moved the steam to the cool side, it could condense and drip back down to be reheated.

seems like you'd get more pressure for the heat, but i suppose the temperatures would have to be somewhat controlled so that the water condenses and boils.



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Post by boydhouse » Wed May 30, 2007 5:59 pm

I wonder how long it takes to convert water to steam and then back again.

Would that be a process that is too long for a cycle in an engine?

Just a thought.

I like your idea. Sort of a half steam and half stirling engine.


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Post by tmk » Wed May 30, 2007 9:57 pm

Thanks, here's my thinking:

say quantity of water per cycle is very small, like a few drops. if the hot side is hot enough, it should vaporize pretty much on contact. I wouldn't have a lake on the hot side, as that wouldn't really be productive.

I would envision a reservoir of water from which a drop or two would be sprayed or otherwise placed on the hot side each cycle. I can't really think of a great way of doing this right now, but I'm sure something will come :)

The water would condense on the cold side and run back down into the reservoir where it could be used again.

As long as we have enough water in the reservoir to make it through a bunch of cycles, eventually the air will saturate and the pressure will rise so the condensation will speed up. Keep in mind that as the pressure in the engine increases, and the air saturation with water increases, it will be harder to boil the water, but it will also condense more readily. A colder cold side would help though.

The only thing I'm not sure of is if it takes a few cycles for the steam to start condensing, the pressure in the engine might get too high. I suppose if the flywheel is heavy enough and the cylinder tight enough, it would compress the gas which would help the vapor condense. A loose piston might not be such a bad thing :)

The key though, is that the conversion from water -> gas and from gas -> water each convert a lot of heat to pressure, which in theory should make it more efficient.

It would basically be a cyclic heat pipe which we extract work from, instead of letting it wick back down to the hot side

Last edited by tmk on Wed May 30, 2007 10:38 pm, edited 1 time in total.

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math time

Post by tmk » Wed May 30, 2007 10:21 pm

From the heat pump article above:
Water, for instance, expands 1600 times when it vaporizes at 1 atmosphere.
Some math:

Working gas volume: V1
Cold side: 25 C (298.15 kelvin)
Hot side: 150 C (423.15 kelvin)
Thermal expansion (charles law): V1/T1 = V2/T2

So without water the normal expansion due to temperature difference in the best case is

V2 = V1 * T2 / T1

V2 = V1 * 423.15 / 298.15

V2 = 1.42 * V1

Let's say our engine has a working gas volume of 80cc's. That would be about the size of the 'boydhouse' spray can chamber stirling engine.

V2 = 1.42 * 80 = 113.5 cc

So without the water, the volume difference in the chamber between cold and hot will be 33.5 cc's (V2 - V1). Since the pressure will increase in the can, you won't get quite that much since charles law only applies at constant pressure, but it's an OK start.

so let's add a few drops of water..

Various sources say 20 drops of water = 1mL = 1cc. Therefore each drop is .05 cc, and 2 drops is .1 cc.

If we use 2 drops of water (2 are evaporated and the same 2 are condensed), and given water expands 1600x, that means the .1 cc becomes 160 cc.

let's say that the air also heats up and expands (if we have enough btu/cycle), and add the 33.5 cc's we had before.

So in the end, we get V1 = 80cc, V2 = 160 + 33.5 = 193.5cc

So our 80cc engine just expanded 2.418 times! vs 1.4 times for heat alone.

We could probably push a piston the size of the displacer with that :) No wonder steam engines were popular for so long.

Of course extracting work will increase the pressure in the engine which will increase the heat needed to vaporize the water, so it probably won't be quite that much, but still. wow.

Feel free to correct my math if i made a mistake

Last edited by tmk on Sun Jun 03, 2007 10:12 pm, edited 1 time in total.

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.. and since i like pictures

Post by tmk » Wed May 30, 2007 11:16 pm

here's a picture of what one might look like


blue stuff is water
orange is the hot side
flywheel not shown, but this is a crankshaft arrangement

the piston on the left side would move down and displace the water (maybe it would have a spring on it and float, so that it won't eat up more power than necessary), which would then squirt out through that little hole onto the hot side.

It would genearlly be in sync with the power piston, but could lead or follow slightly depending on how fast the water evaporates.


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hmm, does this make sense?

Post by tmk » Sun Jun 03, 2007 10:46 pm

Hey so regarding this, i looked into it a bit more, and it seems that although the numbers are more impressive, it actually requires more energy per unit expansion to evaporate water vs heating air.

It took 1.472 Joules to expand the water by 1cc, whereas it took .378 Joules to expand the air by 1 cc. That means the water is ~3.9 times less efficient per unit heat

I guess it may still make sense to use water from a power/volume standpoint, but not from an efficiency one?

Of course, all my math is based on rather simplistic formulas, and with variable pressure and volume/extracting work out of the pressure, my equations may be totally invalid :)

Here's my math, someone correct me if it's obviously wrong :)

Using the example above.

80cc volume chamber
air specific heat: 1.012
water specific heat: 4.1813
1 L of air weighs ~ 1.25g
1 cc of water weighs 1g
1000cc's per L

weight of 80cc's of air:
(80cc/1000) * 1.25 g/L = .1g

energy to heat air from 25C to 150C (+ 125C):
125C * .1g * 1.012 = 12.65 Joules

The air expanded 33.5cc's, so it takes 12.65/33.5 = .378 Joules per cc to expand the air. Since the engine works on air expansion, we can use this as a rough measure of efficiency

Say the pool of water is at a relatively hot 80C, and it boils at 100C
2 drops = .1 cc = .1g

energy to heat water from 80C to 100C (+20C) is
20C * .1g * 4.1813 = 8.363 Joules

but that just heats it up. to actually boil it, you need:
Q = mλ
λ = latent heat of of vaporization
for water, this is 2272 joules/gram

.1g * 2272 J/g = 227.2 Joules

227.2 + 8.363 = 235.563 Joules

water expanded 160 cc's, so J/cc = 235.563/160 = 1.472 Joules/cc

Air was .378 joules/cc, so water is ~3.9 times less efficient. surprise surprise


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Re: hybrid steam/stirling engine

Post by tibsim » Sun Nov 11, 2012 2:34 pm

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Re: hybrid steam/stirling engine

Post by fullofhotair » Wed Dec 12, 2012 6:51 am

Maybe this isnt such a bad idea. I just read a short abstract from a Chinese scientic paper. They claim high thermal efficiency with a composite working fluid.One is a phase change conponent. I dont think it was water though. Website:
http://www.sciencedirect.com/science/ar ... 1101000382

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Re: hybrid steam/stirling engine

Post by spinningmagnets » Tue Dec 18, 2012 9:05 am

Organic Rankine Cycle (ORC) is very similar to steam cycle, but it uses a lower-temperature phase-change fluid, like freon, butane, toluene. Another interesting technology that may prove useful in a hybrid system is a "fluidyne", this is where a changing gas pressure (provided by a Gamma?) acts on liquid pistons.


Re: hybrid steam/stirling engine

Post by brittlee » Tue Dec 18, 2012 11:42 pm

Excellent post. I want to thank you for this informative read, I really appreciate sharing this great post. Keep up your work. Thanks for this very useful

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Re: hmm, does this make sense?

Post by Ante » Wed Dec 19, 2012 12:31 am

I was checking your post because something was suspicious to me
tmk wrote: water expanded 160 cc's, so J/cc = 235.563/160 = 1.472 Joules/cc
Air was .378 joules/cc, so water is ~3.9 times less efficient. surprise surprise
So according to [ http://www.engineeringtoolbox.com/satur ... d_457.html ]

at absolute pressure 1 bar steam specific volume is 1.69 m^3/kg (specific volume of water is negligible)
that is 0.00169 m^3/g
and that is 1690 cc/g

so 235.5/1690= 0.139 Joules/cc
air / steam = 0.378 / 0.139 =2.7 steam is almost 3 times efficient.

But we are not here because of stream engines :)


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Re: hybrid steam/stirling engine

Post by Ian S C » Wed Dec 19, 2012 2:38 am

A steam engine with a boiler made of a coil of tube can be opperated at very high pressures, and after going through the engine the steam can be condensed (cooling towers at the power station), the water is then fed back into the boiler, this way a steam engine becomes a closed circuit, but it's a long way from a "Stirling Engine", which is a "Hot Air Engine".

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Re: hybrid steam/stirling engine

Post by tibsim » Fri Jan 11, 2013 4:05 pm

Gamma striling with air and steam working flud:

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Re: hybrid steam/stirling engine

Post by clark5901 » Tue Jan 22, 2013 10:42 pm

Thanks for the nice information in this post dear...

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Re: hybrid steam/stirling engine

Post by Ian S C » Wed Jan 23, 2013 3:49 am

tibsim, what is the vidio proving ? Ian S C

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