  We study the performances of an imperfect quantum many-body Otto engine based
on free-fermion systems. Starting from the thermodynamic definitions of heat
and work along ideal isothermal, adiabatic, and isochoric transformations, we
generalize these expressions in the case when the hypotheses of ideality are
relaxed (i.e., nonperfect thermalization with the external baths, as well as
nonperfect quantum adiabaticity in the unitary dynamic protocols). These
results are used to evaluate the work and the power delivered by an imperfect
quantum many-body heat engine in a finite time, whose working substance is
constituted by a quantum Ising chain in a transverse field: We discuss the
emerging optimal working points as functions of the various model parameters.
