Compounding
Steam under pressure is converted into mechanical energy more efficiently if it is used in a compound engine; in such an engine, steam direct from a boiler is used in high pressure (hp)cylinders, and then, under reduced pressure, used again 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") can 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 proposes 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.
Read more about this topic: Mallet Locomotive