Universal features in the efficiency at maximal work of hot quantum Otto engines

Classically, external power optimization over the coupling times of a heat engine to its baths leads to universal features in the efficiency. Here we study internal work optimization over the energy levels of a multilevel quantum Otto engine, and find similar universal features. It is shown that in the ultra-hot regime the efficiency is determined solely by the energy level optimization constraint, and is independent of the engine's details. Constraints on the energy levels naturally appear due to physical limitations or design goals. For some constraints the results significantly differ from the classical universality.