Elmer Ambrose Sperry. (Library of Congress)

A steam engine is a device that converts a portion of the heat energy absorbed by a liquid, after it has vaporized, into mechanical work.

For a classic reciprocating steam engine, pressurized steam from a boiler is admitted into cylinder, driving a piston attached to a connecting rod and crankshaft. Steam admission is typically cut off in midstroke. This cutoff steam continues to expand against the piston until the stroke ends, converting more heat energy into work.

The most efficient performance is achieved when the engine expands steam to achieve the greatest absolute difference between steam admission and exhaust temperatures. Superheatersare devices in the boiler that raise the temperature of steam above that of the water from which it was generated. Efficiency is improved because this added temperature, or superheat, permits greater expansion (and thus temperature drop) before the steam...