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A crossflow T-head sidevalve engine
The usual L-head arrangement
Pop-up pistons may be used to increase compression ratio
Flathead with Ricardo's turbulent head

A flathead engine, otherwise sidevalve engine,[1][2] is an internal combustion engine with its poppet valves contained within the engine block, instead of in the cylinder head, as in an overhead valve engine.

Flatheads are an early design concept that has mostly fallen into disuse, but they are currently experiencing a revival in low-revving aero-engines such as the D-Motor.[3]


The side-valve designEdit

The valve gear comprises a camshaft sited low in the cylinder block which operates the poppet valves via tappets 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 camshafts.[4] The sidevalves are typically adjacent, sited on one side of the cylinder(s), though some flatheads employ the less common "crossflow" "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 two plugs per cylinder may use either or both positions.[5]

"Pop-up pistons" may be used with compatible heads to increase compression ratio and improve the combustion chamber's shape to prevent knocking.[6] "Pop-up" pistons are so called because, at tdc, they protrude above the top of the cylinder block.


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 economy cars, trucks, and agricultural engines for many years, while OHV designs came to be specified only for high-performance applications such as aircraft, luxury cars, sports cars, and some motorcycles.

At top dead centre, the piston gets very close to the flat portion of the cylinder head above, and the resultant 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.


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[7] and low efficiency.[8] Because sidevalve engines do not burn the fuel correctly, they suffer from high hydrocarbon emissions.[9]

Sidevalve engines can only be used for engines operating on the Otto principle. The combustion chamber shape is unsuitable for Diesel engines.[10]

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 overheat. Although a sidevalve engine can safely operate at high speed, its 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 Jeep, Rover, Landrover, and Rolls-Royce in the 1950s was the "F-head" (or "intake-over-exhaust" valving), which has one sidevalve and one overhead valve per cylinder.[11]

The flathead's elongated combustion chamber is prone to preignition (or "knocking") if compression ratio is increased, but improvements such as laser ignition or microwave enhanced ignition might help prevent knocking.[12] Turbulence grooves may increase 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.[13][14][15][16]

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.[17][8][clarification needed]

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 applicationsEdit

The sidevalve arrangement was especially common in the United States and used for motor vehicle engines, even for engines with high specific power output.[10] Sidevalve designs are still common for many small single-cylinder or twin-cylinder engines, such as lawnmowers, rotavators, two-wheel tractors and other basic farm machinery.[citation needed]

Flathead carsEdit

Multicylinder flathead engines were used for cars such as the Ford Model T, the Ford flathead V8 engine and the Ford Sidevalve engine. Cadillac produced V-16 flathead engines for their Series 90 luxury cars from 1938–1940.[18] After WWII, flathead designs began to be superseded by OHV (overhead valve) designs. Flatheads were no longer common in cars, but they continued in more rudimentary vehicles such as off-road military Jeeps. In US custom car and hot rod circles, restored examples of early Ford flathead V8s are still seen.[1][19]

Flathead aero-enginesEdit

The simplicity, lightness, compactness and reliability might seem ideal for an 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-fours and flat-sixes.[20] These are extremely oversquare and compact aero-engines with direct drive to a propeller.[21][22]

Flathead motorcyclesEdit

Flathead designs have been used on a number of early pre-war motorcycles, in particular US V-twins such as Harley-Davidson and Indian, some British singles, BMW flat twins and Russian copies thereof.[23] The Cleveland Motorcycle Manufacturing Company produced a T-head four-cylinder in-line motorcycle engine in the 1920s.

See alsoEdit


  1. ^ a b American Rodder, 6/94, pp.45 & 93.
  2. ^ (As the cylinder cross-section has the shape of an inverted L, other names such as "L-block" or "L-head" are also used).
  3. ^ "D-Motor image". Archived from the original on 25 February 2018. Retrieved 29 April 2018. Cite uses deprecated parameter |dead-url= (help)
  4. ^ An exception is the Indian which employs both rocker arms and pushrods to transmit motion from the cam lobes to the valve stems.
  5. ^ The D-motor flathead aero-engines have both spark pugs above the valves.
  6. ^ Davis, Marlan (29 September 2006). "Ford Flathead V8 – The Flathead Guide of Death". Hot Rod Magazine. Combustion Chamber. Retrieved 8 April 2014. 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.
  7. ^ "A critique of the flathead or side valve engine". 13 July 2012. Retrieved 22 August 2015.
  8. ^ a b 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
  9. ^ Richard van Basshuysen, Fred Schäfer: Handbuch Verbrennungsmotor. 8. Auflage, Springer, Wiesbaden 2017, ISBN 978-3-658-10901-1, Chapter 10, p. 534
  10. ^ a b 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
  11. ^ Road and Track, some time in the 1960s
  12. ^ Ikeda, Yuji; Nishiyama, Atsushi; Kaneko, Masashi (5–8 January 2009). Microwave Enhanced Ignition Process for Fuel Mixture at Elevated Pressure of 1MPa (PDF). 47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition. American Institute of Aeronautics and Astronautics. p. 1. Archived from the original (PDF) on 25 July 2014. Retrieved 3 July 2014. 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. Cite uses deprecated parameter |dead-url= (help)
  13. ^ Graeber, Charles (23 September 2004). "Obsession: Mr. Singh's Search for the Holy Grail". Popular Science. Bonnier. Retrieved 3 July 2014. 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.
  14. ^ Pirangute, V. G.; N.V.Marathe (14 January 2002). Full throttle performance (PDF) (Technical report). ARAI. PUS/2407/Garuda/52(d). Archived from the original (PDF) on 7 October 2016. Cite uses deprecated parameter |dead-url= (help) The test report reveals that fuel consumption and temperatures decreased at low engine speed while torque increased.
  15. ^ amrelweekil (14 September 2009). "Engine modify by Somender Singh". YouTube. Grooved flathead at 1:31–1:38. Retrieved 9 April 2014.
  16. ^ Patent US 6237579  Somender Singh: "Design to improve turbulence in combustion chambers"
  17. ^ The internal-combustion engine by Harry Ralph Ricardo, Blackie and Son Limited.
  18. ^ LaChance, David (February 2007). "Reignmaker – 1939 Cadillac Series 39-90". Hemmings Motor News. American City Business Journals. Retrieved 17 November 2015. 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.
  19. ^ "Street Rodder, 1/85, p.72.
  20. ^ Although very small and compact, the D-Motor flat-six displaces nearly 4 litres.
  21. ^ "Kapelstraat 198 8540 Deerlijk – Recent information". Archived from the original on March 28, 2012. Retrieved December 6, 2011. Cite uses deprecated parameter |deadurl= (help)
  22. ^ Tacke, Willi; Marino Boric; et al: World Directory of Light Aviation 2015-16, pages 256-257. Flying Pages Europe SARL, 2015. ISSN 1368-485X
  23. ^ For example, some Dnepr and Ural used flathead designs that BMW had licensed to the Soviets.

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