Valve timing

Valve timing is the schedule of intake and exhaust valve opening and closing in a reciprocating engine. It determines when gases enter and leave each cylinder relative to piston motion.

Four-stroke engine: An internal-combustion engine cycle with intake, compression, power, and exhaust strokes. Its four piston strokes provide the reference cycle for scheduling valve events.
Variable valve timing: A system that changes valve-event timing while an engine operates. It adjusts the schedule to suit changing speed and load.
Camshaft: A rotating shaft with lobes that actuate engine valves through a valve train. Its lobe positions and shapes set the basic valve events.
Fixed valve timing: A valve schedule whose timing remains mechanically fixed relative to crankshaft rotation. It cannot shift events to accommodate different engine speeds and loads.
Valve overlap: The interval near the end of the exhaust stroke when intake and exhaust valves are open simultaneously. Overlap is the timing interval that links outgoing exhaust flow with incoming charge.
Variable valve lift: A system that changes how far engine valves open, often alongside changes to timing. Changing lift modifies gas flow without relying only on event timing.
Crankshaft: A rotating shaft that converts reciprocating piston motion into torque. Valve timing is normally measured by crankshaft angle, even when a camshaft drives the valves.
Desmodromic valve: A valve mechanism that uses positive mechanical actuation to open and close a valve. It contrasts with conventional spring-return valvetrains, though both require timed valve events.
Valve lift: The distance a valve moves away from its seat when opened. Lift combines with opening duration to determine how much gas can pass through a valve.
Cam phasing: A method of advancing or retarding a camshaft relative to the crankshaft. It shifts valve opening and closing events while largely preserving their duration.

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