= Steam engine
A <steam engine> is a <heat engine> that converts part of the energy carried by <steam> into mechanical <work>. In a reciprocating design, expanding <steam> pushes a <piston>; valves admit and exhaust the working fluid so that the motion can repeat. A crank can convert the reciprocation into rotation. A <steam turbine> instead extracts <work> from a continuously flowing stream.
The boiler supplies <heat> to water, but the engine's useful output depends on pressure changes during expansion and exhaust. For an idealized quasi-static <piston>, boundary <work> is $W=\int p\,dV$. The net <work> over a cycle is the signed enclosed area on a <pressure-volume diagram>. Some input <heat> must be rejected: turning boiler heating into motion does not circumvent the <Second law of thermodynamics>.
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