Cylinder Cocks

Cylinders are steam engine components that house pistons that propel the vehicle. Typically, there are two cylinders in the front of the vehicle, one on each side. They are alternately fed steam from the steam chest , with every full rotation of the driving wheels, in the amount set with the cutoff .

Steam in cylinders pushes the pistons inside, mechanically linked to the driving wheels, propelling the vehicle. The used steam is then expelled through a blastpipe, where it mixes with smoke from the firebox in the smokebox, and together they are exhausted to the atmosphere through a chimney.

When steam is no longer fed to the cylinders, during coasting or being stationary, for example, it quickly cools down. This is because the cylinders are separated from the hot boiler and the firebox . Naturally, the remaining steam in the cylinders quickly condenses back into water. If enough water collects in cylinders, they will start producing a sloshy sound with each gear cycle. This is dangerous, because unlike steam, water isn't compressible, and if pushed by a piston it can break the cylinder and cause catastrophic powertrain failure .

To prevent this, steam engines come equipped with cylinder cocks. They are usually operated by a lever from inside the cab , and allow water to be expelled from the cylinders. However, since steam escapes through the cocks too, they should only remain open for a short time, typically just a few seconds, when the vehicle begins moving from a standstill.

Cylinders can also fill with water if the boiler is overfilled , or from running the vehicle with the cutoff set in the opposite direction.