The engine is a three-lobe rotor design. As the rotor rotates on the eccentric crank each lobe defines a variable volume chamber. In the following diagrams we follow the engine through its cycles, Also note that crank rotates 3 times the rate of the rotor. For every 90 of crank rotation the rotor rotates 30 . In the illustration to the left color is used to illustrate the engine cycles.
Bright red is the live throttled steam from the boiler.
The darker red is the steam expanding after the inlet valve closes. Steam engines use cutoff, the practice of admitting steam for part of the stroke and expanding over the remaining portion, to control the torque of the engine. A long cutoff is used, and necessary, for starting from a stop or hard pulling. Cutting back on the cutoff gives better economy. Green is used to indicate the exhaust steam leaving the engine.