Would steam locomotives be more efficient if the working fluid had a lower latent heat?

Question

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.)

There are some previous questions about other working fluids for steam engines, such as Why is steam a good working fluid for heat engines? and Could it be possible to create a pentane steam engine?, but neither mentions the issue that locomotives don't use a closed cycle, and neither has an answer that addresses this.

Answer 1

High latent heat is not necessarily beneficial. Neither to an open cycle nor to a closed cycle.

Lets assume two different working fluids evaporating at the same temperature, but one has a "high" latent heat and the other one has a "low" latent heat.

High latent heat in power cycles means high loss of exergy, because the working fluid's temperature remains constant during the phase change.

The exhaust gas has a lower heat capacity than the working fluid. In order to supply enough heat for the phase change, the temperature of the exhaust gas entering the heat exchanger needs to be higher for the working fluid with high latent heat than for the working fluid with low latent heat. However, the temperatures of the working fluids after evaporation are same. Thus, the former case used higher grade heat than the latter case. This is counter-productive, because you need the high grade heat to reach the highest possible temperature with your working fluid to achieve the best performance of your cycle.