  The characterization and control of quantum effects in the performance of
thermodynamic tasks may open new avenues for small thermal machines working in
the nanoscale. We study the impact of coherence in the energy basis in the
operation of a small thermal machine which can act either as a heat engine or
as a refrigerator. We show that input coherence may enhance the machine
performance and allow it to operate in otherwise forbidden regimes. Moreover,
our results also indicate that, in some cases, coherence may also be
detrimental, rendering optimization of particular models a crucial task for
benefiting from coherence-induced enhancements.
