Editing Poppet valve (section)

== Usage in internal combustion engines ==
[[File:4StrokeEngine Ortho 3D Small.gif|thumb|right|upright|Poppet valves in a typical [[overhead camshaft engine]] ]]
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Poppet valves are used in most [[piston engine]]s to control the flow of intake and exhaust gasses through the [[cylinder head]] and into the [[combustion chamber]]. The side of the poppet valve which sits inside the combustion chamber is a flat disk, while the other side tapers from the disk shape to a thin cylindrical rod called a "valve stem".

=== Materials and durability ===
In a typical modern mass-production engines, the valves are solid and made from steel [[alloy]]s. However some engines use hollow valves filled with [[sodium]], to improve [[heat transfer]].

Many modern engines use an aluminium cylinder head. Although this provides better heat transfer, it requires steel [[valve seat]] inserts to be used; in older [[cast iron]] cylinder heads, the valve seats are often part of the cylinder head. A gap of {{convert|0.4|-|0.6|mm|in|3|abbr=on}} is present around the valve stem, therefore a valve stem [[oil seal]] is used to prevent oil being drawn into the intake manifold and combustion chamber. Typically, a rubber lip-type seal is used. A common symptom of worn valve guides and/or defective oil seals is a puff of blue smoke from the exhaust pipe at times of increased intake [[manifold vacuum]], such as when the throttle is abruptly closed.

Historically, valves had two major issues, both of which have been solved by improvements in modern [[metallurgy]]. The first was that in early internal combustion engines, high wear rates of valves meant that a [[valve job]] to regrind the valves was required at regular intervals. Secondly, [[Tetraethyllead#In motor fuel|lead additives]] had been used in petrol (gasoline) since the 1920s, to prevent [[engine knocking]] and provide lubrication for the valves. Modern materials for the valves (such as stainless steel) and valve seats (such as [[Stellite]] and [[inconel]]) allowed for leaded petrol to be phased out in many industrialised countries by the mid-1990s.

===Sodium Cooled Exhaust Valves===
Exhaust valves are subject to very high temperatures and in extreme high performance applications may be [[sodium]] cooled. The valve is hollow and filled with sodium, which melts at a relatively low temperature and, in its liquid state, convects heat away from the hot valve head to the stem where it may be conducted to the cylinder head. Common in second world war piston engines, now only found in high performance engines.<ref>{{cite web |last1=Cameron |first1=Kevin |title=Some Facts About Sodium-Filled Valves |url=https://www.cycleworld.com/story/bikes/some-facts-about-sodium-filled-valves/ |website=Cycle World |publisher=Octane Media, LLC |access-date=9 July 2023 |date=22 April 2020}}</ref>

=== 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.

=== Number and location of valves ===
{{multiple image
 | align    = right
 | direction= vertical
 | width    = 180
 | image1   = Side-valve_engine_with_Ricardo%27s_turbulent_head_01.png
 | caption1 = Flathead engine (valve shown in light blue)
 | image2   = Dolomite Sprint Valves Section.jpg
 | caption2 = Overhead camshaft engine
}}
{{see also|Multi-valve}}

Early [[flathead engine]]s (also called ''L-head engines'') saw the valves located beside to the cylinder(s), in an "upside down" orientation parallel to the cylinder.<ref>{{Cite web |url=http://www.fsoc.co.uk/ |title=fsoc |website=fsoc |access-date=24 April 2018 |url-status=live |archive-url=https://web.archive.org/web/20180318113101/https://www.fsoc.co.uk/ |archive-date=18 March 2018 }}</ref> Although this design made for simplified and cheap construction, the twisting path of the intake and exhaust gasses had major drawbacks for the airflow, which limited engine RPM<ref>{{Cite web |url = http://www.rucenterprises.com/Clinton/A_Handy_Guide_To_Clinton_Engines.pdf |title = A Handy Guide to Clinton Engines |date = 1956 |access-date = October 2, 2015 |page = 2 |quote = R. P. M. 2200&nbsp;— 3600 |url-status = live|archive-url = https://web.archive.org/web/20151003194636/http://www.rucenterprises.com/Clinton/A_Handy_Guide_To_Clinton_Engines.pdf |archive-date = October 3, 2015 }}</ref> and could cause the engine block to overheat under sustained heavy load. The flathead design evolved into [[IOE engine|intake over exhaust (IOE) engine]], used in many early motorcycles and several cars. In an IOE engine, the intake valves were located directly above the cylinder (like the later ''overhead valve engines''), however the exhaust valve remains beside the cylinder in an upside down orientation.

These designs were largely replaced by the [[overhead valve engine|overhead valve (OHV) engine]] between 1904 until late-1960s/early-to-mid 1970s, whereby the intake and exhaust valves are both located directly above the cylinder (with the camshaft located at the bottom of the engine). In turn, OHV engines were largely replaced by the [[overhead camshaft engine|overhead camshaft (OHC) engines]] between 1950s until 1980s. The location of the valves is broadly the same between OHV and OHC engines, however OHC engines saw the camshaft located to the top of the engine with the valves and OHC engines often have more valves per cylinder. Most OHC engines have an extra intake and an extra exhaust valve per cylinder (four-valve cylinder head), compared with the design of two valves per cylinder used by most OHV engines. However some OHC engines have used three or five valves per cylinder.
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