Expansion valve (steam engine)

An expansion valve is a device in steam engine valve gear that improves engine efficiency. It operates by closing off the supply of steam early, before the piston has travelled through its full stroke. This cut-off allows the steam to then expand within the cylinder. This expanding steam is still sufficient to drive the piston, even though its pressure decreases as it expands. As less steam is supplied in the shorter time for which the valve is open, use of the expansion valve reduces the steam consumed and thus the fuel required. The engine (on 1875 figures) may deliver two-thirds of the work, for only one-third of the steam.

An expansion valve is a secondary valve within a steam engine. They represent an intermediate step between steam engines with non-expansive working and later valve gears that could provide for expansion by controlling the motion of a single valve.

Expansion valves were used for stationary engines and marine engines. They were not used for locomotives, although expansive working was achieved by the use of the later variable expansion valve gears.