This is a cross post from physics where this question didn't get an answer - it seems relevant here too.

People often express an idea that steam engines would be more efficient if they used a working fluid with a lower latent heat than water. I always thought that was a misconception because (so I thought) steam engines use the Rankine cycle, which is a closed thermodynamic cycle, so you get back all that energy when the fluid is condensed. A high latent heat means you can achieve a higher temperature difference between hot and cold reservoirs (so I thought), which only makes the cycle more efficient.

However, reading about the history of steam locomotives, it seems that while early designs like Watt's did use condensers, the later ones don't, they just vent steam into the atmosphere at above atmospheric pressure, avoiding the weight and complexity of a condenser.

This makes me unsure whether water's high latent heat is a good thing or a bad thing for steam engines. If they used a closed cycle then a high latent heat would either be a good thing or make no difference, but since they use an open cycle this might not be the case.

I realise there are all sorts of other reasons why a working fluid besides water could be impractical, especially if you're venting it into the atmosphere on every cycle, but let's ignore those - if there was another fluid, exactly like water except that it had a somewhat lower latent heat, would a steam engine using this fluid be more or less efficient than one using water? (Assuming both engines are optimised for their respective working fluid but both use the same operating principle, venting the fluid on each cycle.)

Just as on physics I see there are some previous questions about alternative working fluids for steam engines, such as Why use steam water and not other fuild in steam engines? and Why use steam instead of just hot air?, but I didn't see one that addresses the specific question of thermodynamic efficiency as a function of latent heat in an engine without a condenser.

  • $\begingroup$ Run the calculations with water as the working fluid and then the "other" fluid. Soon you will see. $\endgroup$
    – Solar Mike
    Apr 27 at 13:06
  • $\begingroup$ @SolarMike if I knew exactly which calculations to run I'd have done that already! $\endgroup$
    – N. Virgo
    Apr 27 at 13:07
  • $\begingroup$ Do a search for steam and dryness fraction. $\endgroup$
    – Solar Mike
    Apr 27 at 13:08
  • $\begingroup$ And reading this will tell you anyway: douglas-self.com/MUSEUM/POWER/oddfluid/oddfluid.htm $\endgroup$
    – Solar Mike
    Apr 27 at 13:09
  • $\begingroup$ "immediately helpful"? no, probably requires effort on your part. $\endgroup$
    – Solar Mike
    Apr 27 at 13:10

2 Answers 2


No, because the efficiency is not driven by the fluid but the temperature limits.

See http://www.douglas-self.com/MUSEUM/POWER/oddfluid/oddfluid.htm

  • 1
    $\begingroup$ That's true for closed cycles, but since steam locomotives vent the used steam instead of condensing it, it doesn't apply to them. The question starts by saying exactly that. $\endgroup$
    – N. Virgo
    Apr 27 at 13:15

Steam engines would use less fuel on start up if they had a liquid that turned to steam at say 70 Celsius, instead of 100. But this would only help during start up to heat the boiler initially from outdoor temperature to 100 Celsius. The same as it would take less fuel to initially heat the boiler on start up if you were in a hot climate say 30 Celsius, compared to starting up a boiler in a cold climate say 0 Celsius. But once the water is hot and the locomotive was moving you would not have more efficiency. The tiny bit of energy saved at start up would not make it worthwhile because as you pointed out, "there are all sorts of other reasons why a working fluid besides water could be impractical, especially if you're venting it into the atmosphere on every cycle".

Not venting wouldn't be a big problem. Steam ships typically did not vent the stream. Some increased efficiency by piping the steam from the first set of high pressure pistons, into larger lower pressure pistons. Sometimes using it 3 times. All connected to the propeller shaft. Then they eventually returned it to the boilers. Not only because warm water uses less energy to boil, but because ships at sea couldn't refill fresh water as easily as trains on land. They didn't want to use salt water in their boilers.

So you could reuse the liquid. But there would be a myriad of other problems. #1 being Cost and availability of the liquid. You would need to find something cheap, readily available, non flammable, non toxic, non corrosive, and in the end it wouldn't help much at all. A more important problem would be in places like Siberia, a different liquid might keep your boiler from freezing up when not in use. But in those places lubricating oil will freeze, so you need to keep your engine warm regardless.

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