  Even microscopic engines have hitherto been defined to require macroscopic
elements such as heat reservoirs, but here we observe that what makes engines
useful is energy transfer across a large ratio of dynamical time scales
("downconversion"), and that small, closed dynamical systems which could
perform steady downconversion ("Hamiltonian daemons") would fulfill the
practical requirements of autonomous microscopic engines. We show that such
daemons are possible, and obey mechanical constraints comparable to, but
different from, the laws of thermodynamics.
