How does cutoff expansion work in steam engines?

In a steam engine, cutoff is the precise moment when the inlet valve closes, stopping the entry of fresh steam from the boiler. Once closed, the trapped steam expands adiabatically inside the cylinder, pushing the piston while its pressure drops. This technique balances engine power and thermodynamic efficiency depending on whether an early or late cutoff is applied.

What we know

• Cutoff is the point in the piston stroke at which the inlet valve is closed to stop steam from entering the cylinder. [2]
• After the inlet valve closes, the steam trapped in the cylinder expands adiabatically, causing the steam pressure to drop. [2]
• A late cutoff delivers full steam pressure through the entire stroke for maximum start-up force, but reduces engine efficiency. [2]
• An early cutoff offers greater thermodynamic efficiency and uses less steam and fuel, but results in a lower mean effective pressure. [2, 3]

Where to go next

• how it workedHow does a reversing gear control locomotive cutoff?
• whoWhat role did Boulton and Watt play in steam engine development?
• compared withHow do expansion valves improve steam engine efficiency?
• the bigger pictureWhat is the Rankine cycle in thermodynamic analysis?
• a case in pointHow did Old Bess pioneer early cutoff technology?
• an unexpected connectionHow did an 18th-century beam engine pioneer expansive working?Old Bess, an early beam engine built in 1777, holds historical significance as the first engine designed specifically to work with an early cutoff to harness steam expansion for greater efficiency.

Sources

• [1]Cutoff (Wikidata Q1158775) · CC0 1.0
• [2]Cutoff (steam engine) · CC BY-SA 4.0
• [3]Expansion valve (steam engine) · CC BY-SA 4.0
• [4]Steam engine · CC BY-SA 4.0
• [5]Old Bess (beam engine) · CC BY-SA 4.0