  We put forward four schemes of coupled-qubit quantum Otto machine, a
generalization of the single-qubit quantum Otto machine, based on work and heat
transfer between an internal system consisting of a coupled pair of qubits and
an external environment consisting of two heat baths and two work storages. The
four schemes of our model are defined by the positions of attaching the heat
baths, which play a key role in the power of the coupled-qubit engine. Firstly,
for the single-qubit heat engine, we find a maximum-power relation, and the
fact that its efficiency at the maximum power is equal to the Otto efficiency,
which is greater than the Curzon-Ahlborn efficiency. Second, we compare the
coupled-qubit engines to the single-qubit one from the point of view of
achieving the maximum power based on the same energy-level change for work
production, and find that the coupling between the two qubits can lead to
greater powers but the system efficiency at the maximum power is lower than the
single-qubit system's efficiency and the Curzon-Ahlborn efficiency.
