  Relativistic quantum systems exhibit unique features not present at lower
energies, such as the existence of both particles and antiparticles, and
restrictions placed on the system dynamics due to the light cone. In order to
understand what impact these relativistic phenomena have on the performance of
quantum thermal machines we analyze a quantum Otto engine with a working medium
of a relativistic particle in an oscillator potential evolving under Dirac or
Klein-Gordon dynamics. We examine both the low-temperature, non-relativistic
and high-temperature, relativistic limits of the dynamics and find that the
relativistic engine operates with higher work output, but an effectively
reduced compression ratio, leading to significantly smaller efficiency than its
non-relativistic counterpart. Using the framework of endoreversible
thermodynamics we determine the efficiency at maximum power of the relativistic
engine, and find it to be equivalent to the Curzon-Ahlborn efficiency.
