  From the steam engine to current nano-devices, the design of efficient
thermal machines has been instrumental in modern societies. In its essence a
thermal engine can be thought as a working substance, in contact with two or
more baths, undergoing a cyclic transformation. What happens if the working
substance is on the verge of a phase transition? Already in 1902 the latent
heat was identified as a key to improve the efficiency of steam engines Despite
this early observation, the impact of phase transitions on the performance of
thermal machines has not been addressed. By combining the tools of
non-equilibrium and quantum thermodynamics with finite-size-scaling and
information theory, we unveil an unnoticed mechanism, triggered by the vicinity
to a phase transition, to boost the performance of an engine. This result sheds
new light on the so called power-efficiency dilemma and could be used to
realise powerful and, at the same time, efficient engines. Specific
implementations with trapped ions and superconducting nano-circuits will be
discussed.
