Editing Poppet valve (section)

=== 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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