Cutoff/en

Cutoff is a control device, typically a lever or a wheel, that sets how long during a cylinder cycle the steam will be admitted to the cylinders. When the cutoff is pushed all the way forward, the steam is admitted almost all the time during a cycle, whereas when it's near the center position, the steam is admitted only for a small fraction of the cycle.

With the regulator open, the vehicle’s chest is fed steam at a relatively constant rate from the boiler. However, the chest’s exhaustion rate to the cylinders increases with the vehicle’s speed, because the cylinder cycle becomes more frequent. At a certain speed, with the cutoff set to full forward, the chest no longer receives enough steam to overcome the exhaustion and the vehicle stops accelerating. However, by gradually pulling back on the cutoff the faster the vehicle goes, the driver is able to give the chest increasingly more time to pressurize, making it possible to reach greater pressures and speeds.

Apart from determining the intervals in which steam is admitted to the cylinders, the cutoff control also determines the direction of the locomotive movement, just like the reverser does on other powertrain types. Pulling the cutoff all the way backwards sends full steam in reverse. Cutoff must never be put in the opposite direction while in motion, as this could suck ash particulates and water into the cylinders, causing damage.

Cutoff setting also affects airflow. Being in the furthest position gets more air drawn into the firebox than when the cutoff is closer to the center position.

See also: Regulator, Reverser, Cylinder Cocks, Boiler