If 100% efficiency is unattainable, what is the max possible efficiency which can be attained and what factors promote the attainment of this max value? In trying to answer these questions, thermodynamics has invented & used the concept of reversibility, absolute temperature and entropy.
Reversible Process:
-for a system is defined as a process which once having taken place, can be reversed and leaves no change in either the system or surroundings. Only ideal processes can do this and restore both system and surroundings to their initial states. Hence an ideal process must be a reversible process.
Example:
1) Frictionless relative motion
2) Extension and compression of a spring
3) Frictionless adiabatic expansion or compression of fluid.
4) Polytropic expansion or compression etc.,
i) There should be no friction
ii) There should be no heat transfer across finite temperature difference.
iii) Both the system and surrounding be stored to original state after the process is reversed.
Any process which is not reversible is irreversible.
Example: Movement of solids with friction, A flow of viscous fluid in pipes and passages mixing of two different substances, A combustion process.
Every quasistatic process is reversible, because a quasistatic process is of an infinite succession of equilibrium states.