In reality, there are no truly reversible processes. All real thermodynamic processes are somehow irreversible. They are not done infinitely slowly. Therefore, heat engines must have lower efficiencies than limits on their efficiency due to the inherent irreversibility of the heat engine cycle they use. However, for analysis purposes, one uses reversible processes to make the analysis simpler, and to determine maximum thermal efficiencies.
For example the Carnot cycle is considered as a cycle that consist of reversible processes:
- Reversible isothermal expansion of the gas
- Isentropic (reversible adiabatic) expansion of the gas
- Reversible isothermal compression of the gas
- Isentropic (reversible adiabatic) compression of the gas
Since the cycle is reversible, there is no increase in entropy during the cycle and entropy is conserved. During the cycle, an arbitrary amount of entropy ΔS is extracted from the hot reservoir, and deposited in the cold reservoir.
One way to make real processes approximate reversible process is to carry out the process in a series of small or infinitesimal steps or infinitely slowly, so that the process can be considered as a series of equilibrium states. For example, heat transfer may be considered reversible if it occurs due to a small temperature difference between the system and its surroundings.
A perfectly reversible process is not possible in reality because it would require an infinite time and infinitesamally small steps. Although not practical for real processes, this method is beneficial for thermodynamic studies since the rate at which processes occur is not important.
What Carnot’s principle states about reversible processes:
- No engine can be more efficient than a reversible engine (a Carnot heat engine) operating between the same high temperature and low temperature reservoirs.
- The efficiencies of all reversible engines (Carnot heat engines) operating between the same constant temperature reservoirs are the same, regardless of the working substance employed or the operation details.