Editing Flathead engine

{{Short description|Type of four-stroke engine}}
{{Use dmy dates|date=December 2015}}
[[File:Single-cylinder T-head engine (Autocar Handbook, 13th ed, 1935).jpg|thumb|A crossflow T-head sidevalve engine]]
[[File:Side-valve engine v2.png|thumb|The usual L-head arrangement]]
[[File:Side-valve engine v3.png|thumb|Pop-up pistons may be used to increase compression ratio]]
[[File:Side-valve engine with Ricardo's turbulent head 01.png|thumb|Flathead with Ricardo's turbulent head]]

A '''flathead engine''', also known as a '''sidevalve engine'''<ref name=AmericanRodder1994>''American Rodder'', 6/94, pp.45 & 93.</ref><ref>(As the cylinder cross-section has the shape of an inverted L, other names such as "L-block" or "L-head" are also used)</ref> or '''valve-in-block engine''', is an [[internal combustion engine]] with its [[poppet valve]]s contained within the [[Cam-in-block|engine block]], instead of in the [[cylinder head]], as in an [[overhead valve engine]].

Flatheads were widely used internationally by automobile manufacturers from the late 1890s until the mid-1960s<ref>{{cite web | url=https://www.autoweek.com/car-life/a1713546/what-was-final-year-new-flathead-powered-american-car/ | title=What Was the Final Year for a New Flathead-Powered American Car? | work=Autoweek | date=20 December 2018 }}</ref> but were replaced by more efficient overhead valve and [[overhead camshaft engine]]s. They are currently experiencing a revival in low-revving [[aviation engine|aero-engines]] such as the [[D-Motor]].<ref>{{Cite web |url=http://lightningautogyro.com/gallery.aspx |title=D-Motor image |access-date=29 April 2018 |archive-url=https://web.archive.org/web/20180225114628/http://www.lightningautogyro.com/gallery.aspx |archive-date=25 February 2018 |url-status=dead |df=dmy-all }}</ref>

==The side-valve design==
The valve gear comprises a camshaft sited low in the cylinder block which operates the [[poppet valve|poppet]] [[valves]] via [[tappet]]s and short pushrods (or sometimes with no pushrods at all). The flathead system obviates the need for further [[valvetrain]] components such as lengthy pushrods, rocker arms, overhead valves or [[overhead camshaft]]s.<ref>An exception is the [[Indian Motocycle Manufacturing Company|Indian]] which employs both rocker arms and pushrods to transmit motion from the cam lobes to the valve stems.</ref> The sidevalves are typically adjacent, sited on one side of the cylinder(s), though some flatheads employ the less common [[Crossflow cylinder head|"crossflow"]] [[T-head engine|"T-head"]] variant. In a T-head engine, the exhaust gases leave on the opposite side of the cylinder from the intake valve.

The sidevalve engine's combustion chamber is not above the piston (as in an OHV (overhead valve) engine) but to the side, above the valves. The spark plug may be sited over the piston (as in an OHV engine) or above the valves; but aircraft designs with [[Dual ignition|two plugs per cylinder]] may use either or both positions.<ref>The D-motor flathead aero-engines have both spark pugs above the valves.</ref>

"Pop-up pistons" may be used with compatible heads to increase compression ratio and improve the combustion chamber's shape to prevent [[Engine knocking|knocking]].<ref>{{cite web |url=http://www.hotrod.com/techarticles/engine/hdrp_0511_ford_flathead_engine/viewall.html |title=Ford Flathead V8 – The Flathead Guide of Death |author= Davis, Marlan |date=29 September 2006 |website=Hotrod.com |publisher=Hot Rod Magazine |at=Combustion Chamber |access-date=2014-04-08 |quote=Trying to gain back compression ratio by using popup pistons may improve airflow provided proper attention is paid to the transfer area and overall piston-to-combustion chamber interface. The best balance has been the subject of debate for over 60 years. Currently the most popular approach is running a big popup piston, but with a scallop on the side adjacent to the valves to keep the transfer area clear between the valves and the cylinder bore. Recommended bottom-line street-gas-friendly compression ratios are between 7.5–8:1 on naturally aspirated engines and 6.5–7.0:1 with a blower.}}</ref> "Pop-up" pistons are so called because, at [[top dead centre]], they protrude above the top of the cylinder block.

==Advantages==
The advantages of a sidevalve engine include: simplicity, reliability, low part count, low cost, low weight, compactness, responsive low-speed power, low mechanical engine noise, and insensitivity to low-octane fuel. The absence of a complicated valvetrain allows a compact engine that is cheap to manufacture, since the cylinder head may be little more than a simple metal casting. These advantages explain why side valve engines were used for [[car|passenger cars]] for many years, while [[Overhead valve engine|OHV]] designs came to be specified only for high-performance applications such as [[aircraft]], [[luxury car]]s, [[sports car]]s, and some [[sport motorcycle|motorcycles]].{{citation needed|date=October 2021}}

At top dead centre, the piston gets very close to the flat portion of the cylinder head above, and the resultant [[Squish (piston engine)|squish]] [[turbulence]] produces excellent fuel/air mixing. A feature of the sidevalve design (particularly beneficial for an aero-engine) is that if a valve should seize in its guide and remain partially open, the piston would not be damaged, and the engine would continue operating safely on its other cylinders.{{citation needed|date=October 2021}}

==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.

==History and applications==
The sidevalve arrangement was especially common in the United States and used for motor vehicle engines, even for engines with high specific power output.<ref name="Pischinger 1948_14" /> Sidevalve designs are still common for many [[small engine|small]] [[Single-cylinder engine|single-cylinder]] or [[Straight-twin engine|twin-cylinde]]r engines, such as [[lawnmower]]s, [[cultivator|rotavators]], [[two-wheel tractor]]s and other [[subsistence agriculture|basic farm machinery]].{{citation needed|date=January 2019}}<!--regular off-the-shelf lawn mowers (Honda for instance) have OHC engines-->

===Flathead cars===
Multicylinder flathead engines were used for cars such as the [[Ford Model T engine|Ford Model T]] and [[Ford Model A (1927–31)|Ford Model A]], the [[Ford flathead V8 engine]] and the [[Ford Sidevalve engine]]. [[Cadillac]] produced [[Cadillac V16 engine#Series 90|V-16 flathead engines]] for their [[Cadillac V-16#1938.E2.80.931940|Series 90 luxury cars]] from 1938 to 1940.<ref>{{cite web | url = http://www.hemmings.com/hcc/stories/2007/02/01/hmn_feature6.html | title = Reignmaker – 1939 Cadillac Series 39-90 | last = LaChance | first = David | date = February 2007 | website = Hemmings Motor News | publisher = American City Business Journals | access-date = 17 November 2015 | quote = Mechanically, the Series 90 cars shared the advances of the Series 75. The V-8 car's three-speed manual transmission was deemed up to the task of handing the torque of the V-16, in part because the larger engine delivered its impulses so smoothly.}}</ref> [[Packard]] and [[Pontiac straight-8 engine|Pontiac]] produced flathead inline 8-cylinder engines until 1954. Also in the British [[Morris Eight]], and [[Morris Minor]] series I.

After [[WWII]], flathead designs began to be superseded by OHV (overhead valve) designs. Flatheads were no longer common in [[automobile|cars]], but they continued in more rudimentary vehicles such as [[off-road vehicles|off-road]] [[Jeep|military Jeeps]]. In US [[custom car]] and [[hot rod]] circles, restored examples of early Ford flathead V8s are still seen.<ref name=AmericanRodder1994/><ref>''Street Rodder'', 1/85, p.72.</ref>

===Flathead aero-engines===
The simplicity, lightness, compactness and reliability might seem ideal for an [[aircraft engine|aero-engine]], but because of their low efficiency, early flathead engines were deemed unsuitable. Two notable exceptions were the American [[Aeronca E-107]] opposed twin [[aero engine]] of 1930 and the [[Continental A40]] flat four of 1931, which became one of the most popular light aircraft engines of the 1930s. Two modern flatheads are the Belgian [[D-Motor]] [[flat-four]]s and [[flat-six]]es.<ref>Although very small and compact, the D-Motor flat-six displaces nearly 4 litres.</ref> These are extremely [[oversquare]] and compact aero-engines with direct drive to a propeller.<ref>{{cite web |url=http://d-motor1.vpweb.be/Recent-information.html |title=Kapelstraat 198 8540 Deerlijk – Recent information |publisher=D-motor.eu |access-date=2011-12-06 |url-status=dead |archive-url=https://web.archive.org/web/20120328094443/http://d-motor1.vpweb.be/Recent-information.html |archive-date=March 28, 2012 |df=mdy-all }}</ref><ref name="WDLA15">Tacke, Willi; Marino Boric; et al: ''World Directory of Light Aviation 2015-16'', pages 256-257. Flying Pages Europe SARL, 2015. {{ISSN|1368-485X}}</ref>

===Flathead motorcycles===
{{main|Flathead motorcycles}}

Flathead designs have been used on a number of early pre-war motorcycles, in particular US V-twins such as [[Harley-Davidson]] and [[Indian Motocycle Manufacturing Company|Indian]], some British singles, [[History of BMW motorcycles|BMW]] [[flat twin]]s and Russian copies thereof.<ref>For example, some Dnepr and Ural used flathead designs that BMW had licensed to the Soviets.</ref> The [[Cleveland Motorcycle Manufacturing Company]] produced a T-head four-cylinder in-line motorcycle engine in the 1920s.
<gallery>
Image:1915 Cadillac Model 51 V8 4stroke engine photo2.JPG|1915 Cadillac flathead engine block
Image:flathead.jpg|Harley-Davidson flathead
Image:Indian Chief Black Hawk 80 cubic inch 1950.jpg|Indian Chief Black Hawk
Image:BMW motorbike R 12 (aka).jpg|BMW R12
Image:Cleveland Model 4-45 1927.jpg|Cleveland Model 4-45
</gallery>

==See also==
* [[Heron cylinder head|Heron Head]]

==Notes==
{{Reflist}}

==External links==
*[http://www.flatheads-forever.com/ Ford Flathead Engine]
*[http://www.factoryfat.com/hdmotorhistory.html Images of each style of Harley-Davidson engine]
*[http://www.harleykmodel.com Harley-Davidson K-model motorcycles]

{{Automobile configuration}}

{{DEFAULTSORT:Flathead Engine}}
[[Category:Cam-in-block valvetrain configurations]]
[[Category:Engine technology]]
[[Category:Engine valvetrain configurations]]
[[Category:Harley-Davidson engines]]
[[Category:Motorcycle engines]]
[[Category:Piston engines]]