Cutoff (steam engine)

A late cutoff delivers full steam pressure to move the piston through its entire stroke, for maximum start-up forces.

An early cutoff has greater thermodynamic efficiency but results in a lower mean effective pressure so less average force on the piston and is used for running the engine at higher speeds.

Late cutoff is used to provide maximum torque to the shaft at the expense of efficiency and is used to start the engine under load.

The lower pressure is used to avoid "wheel spin" that frequently occurs when starting a train onto motion.

After establishing train moderate speed, the cutoff is reduced, and higher boiler pressure is admitted into the cylinders, which results in much better engine efficiency.

Schematic indicator diagram of pressure in a steam locomotive cylinder. The pressure in the cylinder declines after cutoff as the steam pushes the piston down its bore.