Editing Poppet valve (section)

=== Actuation method ===
{{see also|Camshaft}}
Early engines in the 1890s and 1900s used an "automatic" intake valve, which was opened by the vacuum in the combustion chamber and closed by a light spring. The exhaust valve had to be mechanically driven to open it against the pressure in the cylinder. Use of automatic valves simplified the mechanism, but [[valve float]] limited the speed at which the engine could run, and by about 1905 mechanically operated inlet valves were increasingly adopted for vehicle engines.

Mechanical operation is usually by pressing on the end of the valve stem, with a spring generally being used to return the valve to the closed position. At high engine speeds ([[revolutions per minute|RPM]]), the weight of the [[valvetrain]] means the valve spring cannot close the valve as quickly enough, leading to [[valve float]] or ''valve bounce''. [[Desmodromic valve]]s use a second rocker arm to mechanically close the valves (instead of using valve springs) and are sometimes used to avoid valve float in engines that operate at high RPM.

In most mass-produced engines, the [[camshaft]](s) control the opening of the valves, via several intermediate mechanisms (such as [[pushrod]]s, [[roller rocker]]s and [[valve lifter]]s). The shape of the cams on the camshaft influence the [[valve lift]] and determine the timing of when the valves open.