Editing Variable valve timing (section)

===Steam engines===
The history of the search for a method of variable valve opening duration goes back to the age of [[steam engine]]s when the valve opening duration was referred to as "steam [[Cutoff (steam engine)|cut-off]]”.  The [[Stephenson valve gear]], as used on early steam locomotives, supported variable [[cutoff (steam engine)|cutoff]], that is, changes to the time at which the admission of steam to the cylinders is cut off during the power stroke.

Early approaches to variable cutoff coupled variations in admission cutoff with variations in exhaust cutoff.  Admission and exhaust cutoff were decoupled with the development of the [[Corliss valve]].  These were widely used in constant speed variable load stationary engines, with admission cutoff, and therefore torque, mechanically controlled by a [[centrifugal governor]] and [[trip valve]]s.

As [[poppet valve]]s came into use, a simplified valve gear using a [[camshaft]] came into use.  With such engines, variable cutoff could be achieved with variable profile cams that were shifted along the camshaft by the governor.<ref>{{cite web|url=http://www.practicalmachinist.com/vb/showthread.php?t=115316 |title=Variable Valve Timing - 1886 - Practical Machinist|publisher=Practical Machinist|access-date=2010-04-04}}.  This discusses the valve gear of the [[:File:Geograph 3146566 Wills Brickworks engine - Westonzoyland.jpg|W & F Wills Patent Brickworks engine]].</ref> The [[Léon Serpollet|Serpollet]] steamcars produced very hot high pressure steam, requiring poppet valves, and these used a patented sliding camshaft mechanism, which not only varied the inlet valve cut-off but allowed the engine to be reversed.<ref>Improvements in Valve Gear for Single-Acting Steam Engines, UK Patent 190005128, 1900, {{cite web|url=https://worldwide.espacenet.com/publicationDetails/biblio?DB=EPODOC&II=20&ND=3&adjacent=true&locale=en_ep&FT=D&date=19010302&CC=GB&NR=190005128A&KC=A#|title = Espacenet}}</ref>