Definition
The length of time, measured in degrees of crankshaft rotation, that an intake or exhaust valve in a reciprocating engine remains open during one complete cycle of operation.
Plain English
How long a valve stays open, measured by how far the crankshaft turns while it is open rather than by seconds.
Context Anchor
Seen in piston-engine maintenance, valve timing diagrams, and engine performance discussions.
Derivation
Duration comes from the Latin durare, meaning 'to last.' Here it refers to how long the valve lasts in the open position, expressed in crankshaft degrees rather than time, because engine speed varies but the geometry of the cam and crank does not.
Why Pilots Care
Valve duration directly influences cylinder filling, power output, and engine smoothness; incorrect timing reduces performance or risks valve damage.
Intuition Check
Do not read duration here as clock time. In engine valve timing, duration is measured in degrees of engine rotation, not seconds.
Example Sentence 1
The mechanic checked the valve duration against the engine's overhaul manual to confirm the camshaft was within specification.
Example Sentence 2
Mechanics check valve duration against specifications when reassembling an engine after an overhaul.