regulator
A device that controls the supply of steam to the cylinders of a steam locomotive.
regulator: the throttle that feeds steam to the engine
A regulator is the primary valve on a steam locomotive that controls how much steam flows from the boiler to the cylinders. It sits between the boiler and the steam pipes leading to the valve gear, acting as the driver's throttle. By opening or closing the regulator handle, the engineman adjusts steam pressure and volume delivered to the cylinders, which directly controls the locomotive's power output and speed. Without it, steam would rush uncontrolled into the cylinders and the engine would become unmanageable.
Most locomotive regulators are slide valves or poppet valves mounted inside a casting called the regulator chest, located on top of or alongside the boiler. Early designs used simple flat slide valves that moved horizontally; later designs employed poppet valves with springs and more precise control. The regulator handle extends down into the cab where the engineman can operate it by hand or, on some engines, via mechanical linkage connected to a wheel. Pressure gauges show the boiler pressure, but the regulator's position is the engineman's direct command to the engine.
Regulators must seal completely when closed to prevent back-flow and maintain boiler pressure during idle periods. Over time, the valve faces wear from the constant thermal cycling and erosion caused by steam passing over them. Leaking regulators waste fuel and reduce available steam pressure. Rebuilding involves lapping the valve seat and stem to restore a tight fit, a job that requires precision work on the lathe. Some locomotives had twin regulators on dual-boiler or compound systems, requiring coordinated operation.
The term comes from the verb regulate, meaning to control by rule or measure. In locomotive terminology, it occupies a different function than the reverser, which controls the direction of motion by changing the valve gear timing, or the brake valve, which applies brakes. Modern diesel and electric locomotives have no regulator in this sense because their engines do not depend on a centralized steam supply to drive reciprocating cylinders.