Reversible Or Irreversibile If we know the path on P-V diagram, are we definitely sure that the process, in general, is reversible ? For example, we consider that the engine cycles are reversible, is it because we know the path?
 A: No, any path on a PV diagram can be reversible but isn't necessarily so. A position in PV space gives you some, but not all, the information that defines a state.
A: In an irreversible deformation of a fluid (gas or liquid), the force per unit area exerted by the fluid on different parts of the system interface with the surroundings (which you would probably refer to as the pressure, and more advanced students would refer to as the normal compressive stress, with includes viscous stresses) varies from location to location on the boundary.  So there would be no single value that could be considered the "pressure."  So, no single path on a PV diagram could represent an irreversible deformation of a fluid.
Moreover, in an irreversible deformation, the "pressure" could not be calculated from the ideal gas law (or other equation of state for the fluid); the equation of state gives the right answer only for a thermodynamic equilibrium state of the fluid (or a reversible process path, which consists of a continuous sequence of thermodynamic equilibrium states).
