Piston valve (steam engine)

For other applications of piston valves, see Piston valve.
Diagram of cylinder and piston valve. The valve is next opened by moving it to the right, allowing the clear space in the middle of the valve to align with the channel in the cylinder above it.

Piston valves are one form of valve used to control the flow of steam within a steam engine or locomotive. They control the admission of steam into the cylinders and its subsequent exhausting, enabling a locomotive to move under its own power. The valve consists of two piston heads on a common spindle moving inside a steam chest, which is essentially a mini-cylinder located either above or below the main cylinders of the locomotive.