  We study the performance of a quantum Otto heat engine with two spins coupled
by a Heisenberg interaction, taking into account not only the mean values of
work and efficiency but also their fluctuations. We first show that, for this
system, the output work and its fluctuations are directly related to the
magnetization and magnetic susceptibility of the system at equilibrium with
either heat bath. We analyze the regions where the work extraction can be done
with low relative fluctuation for a given range of temperatures, while still
achieving an efficiency higher than that of a single spin system heat engine.
In particular, we find that, due to the presence of `idle' levels, an increase
in the inter-spin coupling can either increase or decrease fluctuations,
depending on the other parameters. In all cases, however, we find that the
relative fluctuations in work or efficiency remain large, implying that this
microscopic engine is not very reliable as a source of work.
