  We conduct numerical simulations for an autonomous information engine
comprising a set of coupled double quantum dots using a simple model. The
steady-state entropy production rate in each component, heat and electron
transfer rates are calculated via the probability distribution of the four
electronic states from the master transition-rate equations. We define an
information-engine efficiency based on the entropy change of the reservoir,
implicating power generators that employ the environmental order as a new
energy resource. We acquire device-design principles, toward the realization of
corresponding practical energy converters, including that (1) higher energy
levels of the detector-side reservoir than those of the detector dot provide
significantly higher work production rates by faster states' circulation, (2)
the efficiency is strongly dependent on the relative temperatures of the
detector and system sides and becomes high in a particular Coulomb-interaction
strength region between the quantum dots, and (3) the efficiency depends little
on the system dot's energy level relative to its reservoir but largely on the
antisymmetric relative amplitudes of the electronic tunneling rates.
