  Real quantum heat engines lack the separation of time and length scales that
is characteristic for classical engines. They must be understood as open
quantum systems in non-equilibrium with time-controlled coupling to thermal
reservoirs as integral part. Here, we present a systematic approach to describe
a broad class of engines and protocols beyond conventional weak coupling
treatments starting from a microscopic modeling. For the four stroke Otto
engine the full dynamical range down to low temperatures is explored and the
crucial role of the work associated with the coupling/decoupling to/from
reservoirs in the energy balance is revealed. Quantum correlations turn out to
be instrumental to enhance the efficiency which opens new ways for optimal
control techniques.
