Thermal efficiency and work output in four three-stage heat engine models

We propose and analyze four reversible thermodynamic cycles composed of three distinct processes each: one isothermal, one adiabatic, and a third either isochoric or isobaric. Labeled A–T–V, T–V–A, P–A–T and T–P–A, these models are collectively referred to as Tri-stage Heat Engine Models (THEMs). The proposed THEMs are built exclusively from the four elementary thermodynamic processes commonly explored in undergraduate textbooks. These models offer distinct configurations for connecting isothermal and adiabatic transformations into closed cycles. We demonstrate that both the thermal efficiency and net work output of each cycle, when normalized by the energy of the highest-energy state, depend solely on the compression ratio and the adiabatic index. This allows a direct and consistent comparison between the proposed cycles, the Otto cycle, and the Carnot cycle (at its maximum work condition). This work extends the analytical approach presented in our previous study, providing a pedagogically relevant framework for analyzing thermal efficiency and mechanical output under uniform compression constraints.