Compound steam engine
![Cutaway of triple expansion compound steam engine, 1888](/Images/godic/202501/05/TMW_677_-_Triple_expansion_compound_steam_engine3401.jpg")
![A Robey horizontal 'cross-compound' steam engine: the smaller, high-pressure cylinder is on the left; the larger, low-pressure cylinder on the right](/Images/godic/202501/05/Cross-compound_Robey_steam_engine,_close-up_of_cylinders,_Bolton_museum3401.jpg")
![An animation of a double-acting inverted triple-expansion marine engine.High-pressure steam (red) enters from the boiler and passes through the engine, exhausting as low-pressure steam (blue) to the condenser.](/Images/godic/202501/05/Triple_expansion_engine_animation3402.gif")
![The Coldharbour Mill Pollit and Wigzell cross-compound engine, which drives the rope race seen in the background, transmitting power to line shafts on all five levels of the mill.](/Images/godic/202501/05/Coldharbour_Mill_steam_engine_-_geograph.org.uk_-_6820883402.jpg")
A compound steam engine unit is a type of steam engine where steam is expanded in two or more stages. A typical arrangement for a compound engine is that the steam is first expanded in a high-pressure (HP) cylinder, then having given up heat and losing pressure, it exhausts directly into one or more larger volume low-pressure (LP) cylinders. Multiple-expansion engines employ additional cylinders, of progressively lower pressure, to extract further energy from the steam.