Definition
The relationship between the opening and closing of the intake and exhaust valves in a reciprocating engine and the position of the piston in its cylinder. Valve timing is expressed in degrees of crankshaft rotation before or after top dead center (TDC) or bottom dead center (BDC), and it determines how efficiently the engine breathes — drawing in the fuel-air mixture and expelling exhaust gases.
Plain English
The exact moments, measured against piston position, when the engine's intake and exhaust valves open and close. Get this right and the engine breathes well; get it wrong and the engine runs poorly or not at all.
Context Anchor
Seen in aircraft piston engine maintenance, engine overhaul, troubleshooting, and manufacturer engine specifications.
Derivation
“Valve” comes from a Latin word meaning a folding door or door leaf. That fits the engine meaning because a valve acts like a controlled door, opening and closing the path into or out of a cylinder. “Timing” means setting when something happens.
Why Pilots Care
Correct valve timing ensures efficient combustion, maximum power, and prevents valves from striking the pistons.
Intuition Check
Valve timing does not mean clock time. It means the valve opens and closes at a specific point in the engine’s movement.
Example Sentence 1
After the cylinder overhaul, the mechanic carefully checked the valve timing to make sure each valve opened and closed at the correct point in the piston's travel.
Example Sentence 2
Incorrect valve timing caused a noticeable power loss on the takeoff roll.