  Recently, Zhang {\em et al.} [PRA, {\bf 75}, 062102 (2007)] extended Kieu's
interesting work on the quantum Otto engine [PRL, {\bf 93}, 140403 (2004)] by
considering as working substance a bipartite quantum system $AB$ composed of
subsystems $A$ and $B$. In this paper, we express the net work done $W_{AB}$ by
such an engine explicitly in terms of the macroscopic bath temperatures and
information theoretic quantities associated with the microscopic quantum states
of the working substance. This allows us to gain insights into the dependence
of positive $W_{AB}$ on the quantum properties of the states. We illustrate
with a two-qubit XY chain as the working substance. Inspired by the expression,
we propose a plausible formula for the work derivable from the subsystems. We
show that there is a critical entanglement beyond which it is impossible to
draw positive work locally from the individual subsystems while $W_{AB}$ is
positive. This could be another interesting manifestation of quantum
nonlocality.
