Mallet (locomotive)

The Mallet locomotive is a type of articulated steam railway locomotive, invented by the Swiss engineer Anatole Mallet (1837–1919).

The essence of his idea combines articulation of the locomotive and compound steam use. The articulation was achieved by supporting the front of the locomotive on a bogie frame (called a Bissell truck); the compound steam system fed steam at boiler pressure to high-pressure cylinders for the main driving wheels. As the steam was exhausted from those cylinders, it was passed into a low-pressure receiver and was then sent to low-pressure cylinders to power the driving wheels on the Bissell truck.

Steam under pressure is converted into mechanical energy more efficiently if it is used in a compound engine; in such an engine steam from a boiler is used in high-pressure (HP) cylinders and then under reduced pressure in a second set of cylinders. The lower-pressure steam occupies a larger volume and the low-pressure (LP) cylinders are larger than the high-pressure cylinders.

A third stage (triple expansion) may be employed. Compounding was proposed by the British engineer Jonathan Hornblower in 1781.

The American engineer W. S. Hudson patented a system of compounding for railway locomotives in 1873 in which he proposed an intermediate receiver surrounded by hot gas from the fire, so that the low-pressure steam is partly superheated.

Mallet himself proposed cross-compounding in which a conventional steam locomotive configuration would have one high-pressure cylinder and one low-pressure cylinder.

Mallet patented such a system in 1874, and in 1876 the first locomotive on his principle was built: an 0-4-2T for the Bayonne and Biarritz Railway, and several others followed for railways in mainland Europe

The London and North Western Railway locomotive engineer F W Webb adopted the idea and converted some existing locomotives in 1879, followed by de Glehn and others in the 1880s and several American engineers in the 1890s which included some vertical boiler railcar applications.

Mallet found typical main line railways unwilling to adopt his particular ideas and, in 1884, he started to propose compounding combined with articulation; on lightly engineered secondary lines this could give greater power to locomotives whose axle load and size was limited. His patent 162876 in France specified four cylinders, two large and two small, with one pair of cylinders acting on two or three fixed axles, and the other pair acting on axles mounted in a swivelling truck

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