  For a gas confined between surfaces held at different temperatures the
velocity distribution shows a significant deviation from the Maxwell
distribution when the mean free path of the molecules is comparable to or
larger than the channel dimensions. If one of the surfaces is suitably
structured, this non-equilibrium distribution can be exploited for momentum
transfer in tangential direction between the two surfaces. This opens up the
possibility to extract work from the system which operates as a heat engine.
Since both surfaces are held at constant temperatures, the mode of momentum
transfer is different from thermal creep flow that has gained more attention so
far. This situation is studied in the limit of free-molecular flow for the case
that an unstructured surface is allowed to move tangentially with respect to a
structured surface. Parameter studies are conducted, and configurations with
maximum thermodynamic efficiency are identified. Overall, it is shown that
significant efficiencies can be obtained by tangential momentum transfer
between structured surfaces.
