  Nanotechnology has not only provided us the possibility of developing quantum
machines but also noncanonical power sources able to drive them. Here we focus
on studying the performance of quantum machines driven by arbitrary
combinations of equilibrium reservoirs and a form of engineered reservoirs
consisting of noninteracting particles but whose distribution functions are
nonthermal. We provide the expressions for calculating the maximum efficiency
of those machines without needing any knowledge of how the nonequilibrium
reservoirs were actually made. The formulas require the calculation of a
quantity that we term entropy current, which we also derive. We illustrate our
methodology through a solvable toy model where heat "spontaneously" flows
against the temperature gradient.
