Question

If a steam engine produces work, how does the internal energy of the machine change?

Answer 1

A heat engine typically uses energy provided in the form of heat to do work and then exhausts the heat which cannot be used to do work.   Since the internal energy of an ideal gas depends upon its temperature, the PV diagram along with the temperatures calculated from the ideal gas law determine the changes in the internal energy of the gas so that the amount of heat added can be evaluated from the first law of thermodynamics.