  We study a quantum Otto engine at finite time, where the working substance is
composed of a two-level system interacting with a harmonic oscillator,
described by the quantum Rabi model. We obtain the limit cycle and calculate
the total work extracted, efficiency, and power of the engine by numerically
solving the master equation describing the open system dynamics. We relate the
total work extracted and the efficiency at maximum power with the quantum
correlations embedded in the working substance, which we consider through
entanglement of formation and quantum discord. Interestingly, we find that the
engine can overcome the Curzon-Ahlborn efficiency when the working substance is
in the ultrastrong coupling regime. This high-efficiency regime roughly
coincides with the cases where the entanglement in the working substance
experiences the greatest reduction in the hot isochoric stage. Our results
highlight the efficiency performance of correlated working substances for
quantum heat engines.
