Imagine a steam engine where instead of a steel piston there is an event horizon, and the cylinder is curved space itself. This metaphor has gained mathematical flesh: physicists have for the first time constructed a complete set of reversible heat cycles with a Schwarzschild black hole confined in a spherical cavity. For decades, the cosmological constant was a stumbling block: in extended thermodynamics, pressure was tied to it, and changing it felt like altering the laws of physics mid-cycle. The solution — shift from the global games of the Universe to the local walls of a container. Now we don't touch fundamental constants, but vary the cavity volume, making the horizon move like a piston.

The key was the quasilocal approach, inheriting ideas from Jacob Bekenstein and Stephen Hawking in the Brown–York formalism. At the cavity boundary, surface pressure and gravitational redshift are computed: this sets the Tolman temperature of the working substance. The thermodynamic volume naturally coincides with the area of the cavity itself, and the entropy with the horizon area according to the law \(S = \frac{\pi r_h^2}{G}\). Then the first law sounds familiar: dE = T dS − P dV. But the work here is purely gravitational: by changing the cavity volume, we press on space itself, forcing the horizon to contract or expand.

For the five classical cycles (Carnot, Otto, Diesel, Brayton, Stirling) exact efficiencies have been obtained. Numerical simulation revealed a strict hierarchy: the Stirling cycle with regeneration leads. Its efficiency of about 0.48 almost touches the Carnot limit \(\eta = 1 - \frac{T_c}{T_h} = 0.5\), and as the hot source temperature rises, the gap shrinks as 1/Th. But contact is never made — the non-ideality of the heat capacity is to blame. The heat rejected during the cold stage cannot be fully returned to heating without losses, and the regenerator always needs external topping up. If space were flat, the cycle would reach perfection, but the curvature almost whispers of the quantum nature of the horizon.

The constructed gravitational engine is no desktop toy. Its efficiency serves as a probe of the horizon's microstructure, sensitively responding to the slightest modifications of gravity. Unlike holograms in anti-de Sitter space, there is no scale invariance here: efficiency becomes a tool to test theories of quantum gravity via heat engines. Next in line are rotating and charged holes, as well as an analysis of irreversibility: how fast does such a cosmic piston work? Could the singularity play the role of a hidden energy reservoir? The answers will bring us closer to understanding how quantum thermodynamics operates in the strongest fields of the Universe. And perhaps one day we will hear the whisper of gravitational qubits in the beat of this engine.