When liquid water changes to vapor, the volume increase is over a thousand fold. Once it is in vapor form, it obeys the Boyles Law like any other gas. As temperature continues to increase so does the volume.
pV=mRT
p= observed pressure
V= total volume
m= mass
R= gas constant (gas specific)
If phase change of water makes little pressure difference, the steam engine will never work. The volume of the combustion chamber varies with crank angle, the pressure increase will vary with temperature, volume and position of the piston.
It is thermodynamic topic, a separate study of of Mechanical Engineering. Once you have familiar with the first and second law of thermodynamics, you can then apply it to the engine dynamics (Mechanical engineering).
Here is a good link to make the above clearer.
http://www.ohio.edu/mechanical/therm...Chapter2a.html
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