  It is shown that in theories of gravity where the cosmological constant is
considered a thermodynamic variable, it is natural to use black holes as heat
engines. Two examples are presented in detail using AdS charged black holes as
the working substance. We notice that for static black holes, the maximally
efficient traditional Carnot engine is also a Stirling engine. The case of
negative cosmological constant supplies a natural realization of these engines
in terms of the field theory description of the fluids to which they are
holographically dual. We first propose a precise picture of how the traditional
thermodynamic dictionary of holography is extended when the cosmological
constant is dynamical and then conjecture that the engine cycles can be
performed by using renormalization group flow. We speculate about the existence
of a natural dual field theory counterpart to the gravitational thermodynamic
volume.
