Working substance
The matter — gas, vapour, or fluid — that an engine repeatedly expands and compresses to convert heat into work.
Definition
The working substance is the material medium inside a heat engine whose expansion and compression actually do the pushing: the steam in a steam engine, the air–fuel mixture in a petrol engine, the refrigerant in a heat pump. Heat flows into and out of the working substance, and its changes in volume against a piston or turbine blade deliver the work.
Carnot's deepest and most surprising conclusion was that, at the reversible limit, the choice of working substance does not matter: the maximum efficiency depends only on the reservoir temperatures. This independence let thermodynamics make universal statements about engines without specifying what was inside them.
In practice the working substance is chosen for engineering reasons — water for its abundance and large latent heat, particular refrigerants for their boiling points and stability — but none of these choices can push efficiency past the Carnot ceiling.
History
Eighteenth- and nineteenth-century engineers experimented with steam, air, and other fluids in search of a more efficient medium; Carnot's theorem of 1824 showed that for an ideal reversible engine the search was futile, since the ceiling is substance-independent.