  Quantum heat engines (QHE) are thermal machines where the working substance
is quantum. In the extreme case the working medium can be a single particle or
a few level quantum system. The study of QHE has shown a remarkable similarity
with the standard thermodynamical models, thus raising the issue what is
quantum in quantum thermodynamics. Our main result is thermodynamical
equivalence of all engine type in the quantum regime of small action. They have
the same power, the same heat, the same efficiency, and they even have the same
relaxation rates and relaxation modes. Furthermore, it is shown that QHE have
quantum-thermodynamic signature, i.e thermodynamic measurements can confirm the
presence of quantum coherence in the device. The coherent work extraction
mechanism enables power outputs that greatly exceed the power of stochastic
(dephased) engines.
