Active engines: Thermodynamics moves forward

The study of thermal heat engines was pivotal to establishing the principles of
equilibrium thermodynamics, with implications far wider than only engine optimization. For
nonequilibrium systems, which by definition dissipate energy even at rest, how to best convert
such dissipation into useful work is still largely an outstanding question, with similar potential
to illuminate general physical principles. We review recent theoretical progress in studying the
performances of engines operating with active matter, where particles are driven by individual
self-propulsion. We distinguish two main classes, either autonomous engines exploiting a particle
current, or cyclic engines applying periodic transformation to the system, and present the strategies
put forward so far for optimization. We delineate the limitations of previous studies, and
propose some future perspectives, with a view to building a consistent thermodynamic framework
far from equilibrium.