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Compound locomotive
A compound locomotive is a steam locomotive which is powered by a compound engine, a type of steam engine where steam is expanded in two or more phases. Colloquially referred to as a compound engine, the locomotive is only one application of the compounding principle.
Compounding became popular for railway locomotives from the early 1880s and by the 1890s were becoming common. Large numbers were constructed, mostly two- and four-cylinder compounds, in France, Germany, Austria, Hungary, and the United States. It declined in popularity due to maintenance issues and because superheating provided similar efficiencies at lower cost. Nonetheless, compound Mallets were built by the Norfolk and Western Railway right up to 1952.
==Introduction==
The usual arrangement on a compound engine is that the steam is first expanded in one or two high-pressure ''(HP)'' cylinder(s), then having given up heat and losing pressure, it exhausts into a larger volume low-pressure ''(LP)'' cylinder, (or two, - or more), thus extending the cycle. Thus the cylinders can be said to work in "series" as opposed to the normal arrangement of a simple-expansion locomotive where they work in "parallel", the steam being expanded just once in any one cylinder. In order to balance piston thrusts of a compound, the HP:LP cylinder volume ratio has to be carefully determined, usually by increasing the LP cylinder diameter and/or by lengthening the stroke. In non-condensing engines, the HP:LP volume ratio is usually 1:2¼. On geared locomotives, cylinder volumes can be kept more or less identical by increasing LP piston speed. Strictly speaking ''compound'' only stands for double-expansion, but the term is loosely applied to any multiple-expansion engine. The instances where railway locomotives have employed triple-expansion with steam going from high pressure ''(HP)'' cylinder, to medium pressure ''(MP)'' cylinder and finally to a low pressure ''(LP)'' one are rare, but triple expansion engines were very common in steamships in the late 19th/early 20th centuries.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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