Editing Flathead engine (section)

==Disadvantages==
The main disadvantages of a sidevalve engine are poor gas flow, poor combustion chamber shape, and low compression ratio, all of which result in a low-revving engine with low power output<ref>{{cite web | url=https://stevemckelvie.wordpress.com/2012/07/12/a-critique-of-the-flathead-or-side-valve-engine-design/| title= A critique of the flathead or side valve engine | access-date = 2015-08-22| date= 13 July 2012 }}</ref> and low efficiency.<ref name="Kremser 1942_50">H. Kremser (author): Der Aufbau schnellaufender Verbrennungskraftmaschinen, in Hans List (ed): Die Verbrennungskraftmaschine, volume 11, Springer, Wien 1942, {{ISBN|978-3-7091-9755-4}}, p. 50</ref> Because sidevalve engines do not burn the fuel efficiently, they suffer from high hydrocarbon emissions.<ref>Richard van Basshuysen, Fred Schäfer: Handbuch Verbrennungsmotor. 8. Auflage, Springer, Wiesbaden 2017, {{ISBN|978-3-658-10901-1}}, Chapter 10, p. 534</ref>

Sidevalve engines can only be used for engines operating on the [[otto cycle|Otto principle]]. The combustion chamber shape is unsuitable for [[Diesel engine]]s,<ref name="Pischinger 1948_14">Anton Pischinger (author): Die Steuerung der Verbrennungskraftmaschinen, in Hans List (ed): Die Verbrennungskraftmaschine, volume 9, Springer, Wien 1948, {{ISBN|978-3-211-80075-1}}, p. 14</ref> which require a high [[compression ratio]] for [[Combustion|ignition]] to occur.

In a sidevalve engine, intake and exhaust gases follow a circuitous route, with low volumetric efficiency, or "poor breathing", not least because the exhaust gases interfere with the incoming charge. Because the exhaust follows a lengthy path to leave the engine, there is a tendency for the engine to [[Thermal shock|overheat]].  (Note: this is true for V-type flathead engines but less of an issue for inline engines which typically have the intake and exhaust ports on the same side of the engine block.) Although a sidevalve engine can safely operate at high speed, its [[Volumetric efficiency#Internal combustion engines|volumetric efficiency]] swiftly deteriorates, so that high power outputs are not feasible at speed. High volumetric efficiency was less important for early cars because their engines rarely sustained extended high speeds, but designers seeking higher power outputs had to abandon the sidevalve.  A compromise used by the [[Willys MB|Willys Jeep]], [[Rover Company|Rover]], [[Land Rover]], and [[Rolls-Royce Limited|Rolls-Royce]] in the 1950s was the [[F-head engine|"F-head"]] (or "intake-over-exhaust" valving), which has one sidevalve and one overhead valve per cylinder.<ref>''Road and Track'', some time in the 1960s</ref>

The flathead's elongated combustion chamber is prone to [[Engine knocking|preignition]] (or "knocking")  if compression ratio is increased, but improvements such as [[laser ignition]] or microwave enhanced ignition might help prevent knocking.<ref>{{cite conference |url=http://enu.kz/repository/2009/AIAA-2009-223.pdf |title=Microwave Enhanced Ignition Process for Fuel Mixture at Elevated Pressure of 1MPa |first1=Yuji |last1=Ikeda |first2=Atsushi |last2=Nishiyama |first3=Masashi |last3=Kaneko |date=5–8 January 2009 |conference=47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition |publisher=American Institute of Aeronautics and Astronautics |archive-url=https://web.archive.org/web/20140725103130/http://enu.kz/repository/2009/AIAA-2009-223.pdf|archive-date=2014-07-25|access-date=2014-07-03 |quote=With plasma-enhanced combustion, a large flame kernel formed and the flame propagation speed increased. In the single-cylinder engine, the combustion stability improved and the microwave-enhanced ignition increased the lean limit from 19.3 to 24.1. |url-status=dead |page=1 }}</ref> Turbulence grooves may increase [[Combustion chamber#Swirl|swirl]] inside the combustion chamber, thus increasing torque, especially at low rpm. Better mixing of the fuel/air charge improves combustion and helps to prevent knocking.<ref>{{cite web |url=http://www.popsci.com/cars/article/2004-09/obsession-mr-singhs-search-holy-grail?nopaging=1 |title=Obsession: Mr. Singh's Search for the Holy Grail |author=Graeber, Charles |date=23 September 2004 |website=Popular Science |publisher=Bonnier |access-date=2014-07-03 |quote=In November 2002 Singh actually received one such permission from a manufacturer to test his modification on its engines. The manufacturer was Briggs and Stratton, and the engines were two 149cc side valves.}}</ref><ref>{{cite tech report |url=http://pesn.com/2005/10/13/9600187_Design_to_Improve_Turbulence_in_Combustion_Chambers/ARAI_Test_Report.pdf |url-status=dead |archive-url=https://web.archive.org/web/20161007034551/http://pesn.com/2005/10/13/9600187_Design_to_Improve_Turbulence_in_Combustion_Chambers/ARAI_Test_Report.pdf |archive-date=2016-10-07 |first=V. G. |last=Pirangute |author2=N.V.Marathe |title=Full throttle performance |number=PUS/2407/Garuda/52(d) |institution=ARAI |date=14 January 2002 }} The test report reveals that fuel consumption and temperatures decreased at low engine speed while torque increased.</ref><ref>{{cite web |url=https://www.youtube.com/watch?v=PzMgPZxD7Iw&t=90  |archive-url=https://ghostarchive.org/varchive/youtube/20211212/PzMgPZxD7Iw| archive-date=2021-12-12 |url-status=live|title=Engine modify by Somender Singh |author=amrelweekil |date=14 September 2009 |website=YouTube |at=Grooved flathead at 1:31–1:38 |access-date=2014-04-09 }}{{cbignore}}</ref><ref>Patent {{patent|US|6237579}} Somender Singh: "Design to improve turbulence in combustion chambers"</ref>

An advance in flathead technology resulted from experimentation in the 1920s by [[Sir Harry Ricardo]], who improved their efficiency after studying the gas-flow characteristics of sidevalve engines.<ref>''The internal-combustion engine'' by Harry Ralph Ricardo, Blackie and Son Limited.</ref><ref name="Kremser 1942_50" />{{clarify|date=April 2018}}

The difficulty in designing a high-compression-ratio flathead means that most tend to be spark-ignition designs, and flathead diesels are virtually unknown.