  In a recent Letter [EPL, 118 (2017) 40003], Polettini and Esposito claimed
that it is theoretically possible for a thermodynamic machine to achieve Carnot
efficiency at divergent power output through the use of infinitely-fast
processes. It appears however that this assertion is misleading as it is not
supported by their derivations as demonstrated below. In this Comment, we first
show that there is a confusion regarding the notion of optimal efficiency. We
then analyze the quantum dot engine described in Ref. [EPL, 118 (2017) 40003]
and demonstrate that Carnot efficiency is recovered only for vanishing output
power. Moreover, a discussion on the use of infinite thermodynamical forces to
reach Carnot efficiency is also presented in the appendix.
