compound engine

compound engine, a development from the single cylinder reciprocating marine engine in which the steam, after leaving the first cylinder, was passed through a second low-pressure cylinder of larger diameter before being drawn off to a condenser to be changed back to boiler feed water. This second use of the steam added to the thrust produced by the engine results in a higher engine efficiency for the same amount of steam. Although the principle of compounding an engine was patented as early as 1781 by Jonathan Hornblower, a contemporary of James Watt, it was not until the 1850s, when higher boiler pressures were introduced in marine boilers, that the compound engine, designed by John Elder and Charles Randolph, became practicable at sea. As steam expands in a cylinder the temperature falls, and the greater the expansion the lower the temperature of the cylinder wall. When steam is admitted to the other side of the piston for the next stroke this steam encounters the cool cylinder wall and some of the steam condenses. This results in less steam being available to do useful work. To avoid this, steam was expanded in two states so that the temperature drop in each stage (cylinder) was less, resulting in reduced condensation and higher efficiency. See also steam propulsion; triple expansion engine.