Noun
compound engine (plural compound engines)
A condensing engine in which the mechanical action of the steam begins in one cylinder and ends in a larger cylinder.
A compound engine, which connected two or more engines was described. Source: Internet
In the compound engine, high-pressure steam from the boiler expands in a high-pressure (HP) cylinder and then enters one or more subsequent lower-pressure (LP) cylinders. Source: Internet
It is a logical extension of the compound engine (described above) to split the expansion into yet more stages to increase efficiency. Source: Internet