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.

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.

For railway locomotive applications the main benefit sought from compounding is economy in fuel and water consumption plus high power/weight ratio due to temperature and pressure drop taking place over a longer cycle, this resulting in increased efficiency; additional perceived advantages include more even torque and in many cases, superior riding qualities with consequent less wear on the track. In situations where heavy grades and low axle loads were combined, the ordinary compound locomotive was often deemed to be the most viable solution. However, in the past, optimum performance demanded constant attention and expert handling, which usually made single-manning advisable, thus reducing a locomotive's availability. At the end of the steam age, this particular problem was being addressed by Andre Chapelon, and Livio Dante Porta also actively pursued this line of research. Successful design of a compound locomotive demands a firm grasp of thermo- and fluid dynamics; that such has frequently not been the case is why many productions in the past have been far from optimal. This is especially true of locomotives built in the early years of the 20th century. The problem not only affected compounds, but was dramatic in their case due to the long steam cycle which made them particularly sensitive to temperature-drop and condensation of the steam during its lengthy passage. In rebuilding older locomotives from 1929 onwards, Chapelon was able to inexpensively obtain what seemed almost "magical" improvements in power and economy by improving flow through the steam circuit, at the same time putting in a larger superheater in order to increase the initial steam temperature so that cooling took longer. Subsequently, in order to maintain a more constant temperature throughout the cycle, Chapelon successfully applied re-superheating between HP and LP stages plus steam-jacketed cylinders to a "test-bed" freight locomotive, 160 A1 (tested 1948-51). Resuperheating was also a feature of L.D. Porta's prototype 4-8-0 rebuild: 'Presidente Peron'/'Argentina' (tested around the same time in Argentina from 1949). Proponents of simple expansion argue that use of early cut-off in the cylinder thus expanding small quantities of steam at each piston stroke obviates the need for the complication and initial expense of compounding and indeed multi-cylinder single expansion – this is an ongoing debate.

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