Which is the most efficient cycle?

Hint:In order to answer this question, we should first understand the meaning of efficiency. Efficiency is the measurement of how much work or energy we can conserve in a process. It means comparing the output of the energy to the input of the energy in the system given. It is usually the output of energy divided by the energy input and then expressed in the form of percentage.

Complete answer:
Carnot engines are considered as the most efficient as they are reversible and all reversible engines will have the same efficiency. This engine is a theoretical thermodynamic cycle which was proposed by Leonard Carnot. It calculates the maximum possible efficiency that a heat engine can possess during the process of conversion of heat into work and also work into heat, working between two reservoirs.

According to the theorem proposed by Carnot, we can say that any system working between ${T_1}$(hot reservoir) and ${T_2}$ (cold reservoir) can never have more efficiency than the Carnot engine operating between the same reservoirs. The efficiency of this type of engine is not dependent on the nature of the working substance. It is dependent only on the temperature of the hot and the temperature of cold reservoirs.

Therefore, the most efficient cycle is the Carnot engine.

Note: It is important to note that a Carnot cycle can be defined as an ideal reversible closed thermodynamic cycle. There are four successive operations involved as follows: isothermal compression, isothermal expansion, adiabatic expansion, and adiabatic compression. The expansion and compression of the substance can be done up to the required point and back to the initial state during these operations.