In
thermodynamics, a
heat engine is a system that converts heat or
thermal energy to
mechanical energy, which can then be used to do
mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state temperature. A heat "source" generates thermal energy that brings the working substance to the high temperature state. The working substance generates work in the "
working body" of the engine while
transferring heat to the colder "
sink" until it reaches a low temperature state. During this process some of the thermal energy is converted into
work by exploiting the properties of the working substance. The working substance can be any system with a non-zero
heat capacity, but it usually is a gas or liquid.