  We revisit and analyze the thermodynamic efficiency of the
Feynman-Smoluchowski (FS) ratchet, a classical thought experiment describing an
autonomous heat-work converter. Starting from the full kinetics of the FS
ratchet and deriving the exact forms of the hidden dissipations resulting from
coarse-graining, we restate the historical controversy over its thermodynamic
efficiency. The existence of hidden entropy productions implies that the
standard framework of stochastic thermodynamics applied to the coarse-grained
descriptions fails in capturing the dissipative feature of the system. In
response to this problem, we explore an extended framework of stochastic
thermodynamics to reconstruct the hidden entropy production from the
coarse-grained dynamics. The approach serves as a key example of how we can
systematically address the problem of thermodynamic efficiency in a
multi-variable fluctuating non-equilibrium system.
