General Category > Chatterbox
Talking Thermodynamics
MJM460:
I suggested starting this thread in response to a little side track on Chris's Lombard Hauler thread. It seemed to spark some interest so here goes. I am sure there are others with some knowledge of thermodynamics, so feel free to jump in to the discussion, especially if you disagree. The thoughts here are offered in the spirit of sharing knowledge, rather than wasting it, and enhancing our understanding of the marvellous machines we make.
First I will try and address the questions already raised while they are fresh, then I will post again the information in Chris's thread to keep it all together. Then we will see where it leads.
Flyboy Jim, pressure does work on the gas in a piston when the piston moves. There are two basic cases. In a cylinder with the inlet valve open, with adequate inlet piping etc so that the pressure remains constant. Then work (W) = Pressure (P) times volume change (v). Note that it does not matter whether the gas is air or steam or any other gas. A pressure of 15 psi exerts the same force on the piston what ever gas is involved.
The second case is a closed piston, i.e. The inlet and exhaust ports are both closed. Then when the piston moves, the volume increases, (expansion) and this results in a falling pressure. The formula for work done is more complex and involves the inlet pressure, the pressure ratio and a factor which for an ideal gas and perfect engine turns out to involve the specific heat of the gas. This factor is slightly different for air and steam. The maths is definitely not mental arithmetic, but easy enough with a scientific calculator or a spreadsheet such as excel. However the result is only a 2% difference which is not significant compared with other factors such as friction and heat gain or loss. We all know a steam cylinder loses heat, lagging is applied to reduce this heat loss which reduces the steam pressure faster than expansion alone.
I mention the possibility of heat gain because when the gas expands, it's temperature falls. Steam is still hot compared with the atmosphere, but if air starts at atmospheric temperature and then gets cooler, then the cylinder gains heat, and the pressure falls less quickly than due to expansion alone. I have seem the exhaust pipe of a small industrial turbine ice up when the turbine was run on air instead of steam.
RonGinger, the expansion of water to steam is indeed around 1700:1 however that expansion occurs in the boiler. We are talking about expansion of the gas in the cylinder, and only from the point where the inlet valve closes, for example around 50% of stroke, depending on the notching of the valve gear, to the point where the exhaust valve opens, ideally this occurs very close to the bottom of the stroke, and used for the example, but may be quite different in a real engine. It turns out that the actual cut off point makes little difference. My 2:1 is based on expansion in the cylinder from about 50% stroke to bottom dead centre.
Jo, I cannot argue with your observation. In my experience, when observation appears contradictory to theory, it usually means that there are more than one thing happening, and the one with the biggest influence may not be the suggested theory. It is then important to look more closely at what is going on. For example, the heat gain or loss mentioned above, or thermal expansion at steam temperatures might be changing clearances. Could you please tell us more about the differences you see? The point is often mentioned and there will be a lot of learning in that for us all.
Paul, your question is the big one that brings us to the purpose of this thread, how do they really work? This post is already too long, I will start another.
MJM460
Jo:
Lost me :noidea:, lets see if I can explain things in one sentence:
When gas expands it gets colder - which chills the container (cylinder) so when using air at room temperature to run an engine the result is that the engine will be chilled and things will get tighter/harder to turn over, if you use steam it will warm the engine and things will expand, making it easier to turn over the engine.
So if you want to wear out your steam engine fast - run it on compressed air ::).
Jo
Jasonb:
Jo, won't the piston also see a similar temperature change to the cylinder and the clearances stay the same assuming similar materials for both.
Gas_mantle:
--- Quote from: Jo on May 11, 2017, 12:54:42 PM ---
When gas expands it gets colder - which chills the container (cylinder) so when using air at room temperature to run an engine the result is that the engine will be chilled and things will get tighter/harder to turn over
So if you want to wear out your steam engine fast - run it on compressed air ::).
Jo
--- End quote ---
Surely though by using air it won't be at room temperature as it heats up during compression, the subsequent heat lost after expansion in the cylinder is simply returning things back to room temperature isn't it ?
I'm no expert but I'd expect the cylinder to remain at room temperature (albeit with other factors like friction affecting the result)
Jo:
Ok I tried to make it too simple :facepalm2: back to the theory....
Steam contains far more energy than air at the same pressure, this pressure is a symptom of the available heat energy in the operating gas. Were the gas is admitted into a cylinder through the whole of the stoke there would be very little difference in performance between steam or air (or even water). As soon as you cut off the supply stroke the gas expands while delivering work, so it draws on its heat content to provide the energy to do that work.
When compressed air expands the only heat it can acquire comes from a fall in its own temperature (which causes a reduction in specific volume) and from the cylinder walls. But when steam expands it can draw all the energy from within, and that reservoir of energy is considerable as it also includes the latent heat of evaporation of the water and if the spent steam is still a gas when it exits the engine this unused energy will have also warmed the engine.
So what does this mean in practise?
If you put your hand on a steam engine running on air you will notice that all of the engine will have over time cooled down, i.e. bearings, fits, covers/cylinders, piston etc. this will all have reduced clearances, in this situation lubrication is critical. If you put your hand on a Steam engine you will find it quickly heats up, this means the clearances are increasing, lubrication is still important but not as critical as running on air.
For this reason if you intend on running an engine on air rather than steam you alter the clearances (and the valve timing ::)) to take it into account.
Jo
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