Flathead engine

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A crossflow T-head sidevalve engine Single-cylinder T-head engine (Autocar Handbook, 13th ed, 1935).jpg
A crossflow T-head sidevalve engine
The usual L-head arrangement Side-valve engine v2.png
The usual L-head arrangement
Pop-up pistons may be used to increase compression ratio Side-valve engine v3.png
Pop-up pistons may be used to increase compression ratio
Flathead with Ricardo's turbulent head Side-valve engine with Ricardo's turbulent head 01.png
Flathead with Ricardo's turbulent head

A flathead engine, also known as a sidevalve engine [1] [2] or valve-in-block engine 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 were widely used internationally by automobile manufactures from the late 1890s until the mid-1950s but were replaced by more efficient overhead valve and overhead camshaft engines. They are currently experiencing a revival in low-revving aero-engines such as the D-Motor. [3]

The side-valve design

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 top dead centre, 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 passenger cars 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.[ citation needed ]

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.[ citation needed ]


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 efficiently, 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. (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 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 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. [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 cars

Multicylinder flathead engines were used for cars such as the Ford Model T and Ford Model A, 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-engines

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 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, 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 also


  1. 1 2 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.
  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". Hotrod.com. 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. 1 2 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. 1 2 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.
  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. 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. Archived from the original on 12 December 2021. 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". D-motor.eu. Archived from the original on March 28, 2012. Retrieved December 6, 2011.
  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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A stratified charge engine describes a certain type of internal combustion engine, usually spark ignition (SI) engine that can be used in trucks, automobiles, portable and stationary equipment. The term "stratified charge" refers to the working fluids and fuel vapors entering the cylinder. Usually the fuel is injected into the cylinder or enters as a fuel rich vapor where a spark or other means are used to initiate ignition where the fuel rich zone interacts with the air to promote complete combustion. A stratified charge can allow for slightly higher compression ratios without "knock," and leaner air/fuel ratio than in conventional internal combustion engines.

Four-stroke engine Internal combustion engine type

A four-strokeengine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:

  1. Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure into the cylinder through its downward motion. The piston is moving down as air is being sucked in by the downward motion against the piston.
  2. Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
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In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket. In most engines, the head also provides space for the passages that feed air and fuel to the cylinder, and that allow the exhaust to escape. The head can also be a place to mount the valves, spark plugs, and fuel injectors.

Hemispherical combustion chamber

A hemispherical combustion chamber is a type of combustion chamber in a reciprocating internal combustion engine with a domed cylinder head in the approximate shape of a hemisphere. An engine featuring this type of hemispherical chamber is known as a hemi engine.

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Overhead valve engine Type of piston engine

An overhead valve (OHV) engine is a piston engine whose valves are located in the cylinder head above the combustion chamber. This contrasts with earlier flathead engines, where the valves were located below the combustion chamber in the engine block.

Multi-valve Type of car engine

In automotive engineering a multi-valve or multivalve engine is one where each cylinder has more than two valves. A multi-valve engine has better breathing and may be able to operate at higher revolutions per minute (RPM) than a two-valve engine, delivering more power.

Ford flathead V8 engine Motor vehicle engine

The Ford flathead V8 is a V8 engine with a flat cylinder head designed by the Ford Motor Company and built by Ford and various licensees. During the engine's first decade of production, when overhead-valve engines were used by only a small minority of makes, it was usually known simply as the Ford V‑8, and the first car model in which it was installed, the Model 18, was often called simply the "Ford V-8", after its new engine. Although the V8 configuration was not new when the Ford V8 was introduced in 1932, the latter was a market first in the respect that it made an 8-cylinder affordable and a V engine affordable to the emerging mass market consumer for the first time. It was the first independently designed and built V8 engine produced by Ford for mass production, and it ranks as one of the company's most important developments. A fascination with ever-more-powerful engines was perhaps the most salient aspect of the American car and truck market for a half century, from 1923 until 1973. The engine was intended to be used for big passenger cars and trucks; it was installed in such until 1953, making the engine's 21-year production run for the U.S. consumer market longer than the 19-year run of the Ford Model T engine for that market. The engine was on Ward's list of the 10 best engines of the 20th century. It was a staple of hot rodders in the 1950s, and it remains famous in the classic car hobbies even today, despite the huge variety of other popular V8s that followed.

Lean-burn refers to the burning of fuel with an excess of air in an internal combustion engine. In lean-burn engines the air:fuel ratio may be as lean as 65:1. The air / fuel ratio needed to stoichiometrically combust gasoline, by contrast, is 14.64:1. The excess of air in a lean-burn engine emits far less hydrocarbons. High air–fuel ratios can also be used to reduce losses caused by other engine power management systems such as throttling losses.

Valvetrain Mechanical system

A valvetrain or valve train is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture into the combustion chamber, while the exhaust valves control the flow of spent exhaust gasses out of the combustion chamber once combustion is completed.

The cam-in-block valvetrain layout of piston engines is one where the camshaft is placed within the cylinder block, usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine. This contrasts with an overhead camshaft (OHC) design which places the camshafts within the cylinder head and drives the valves directly or through short rocker arms.

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T-head engine

A T-head engine is an early type of internal combustion engine that became obsolete after World War I. It is a sidevalve engine that is distinguished from the much more common L-head by its placement of the valves. The intake valves are on one side of the engine block and the exhaust valves on the other. Seen from the end of the crankshaft, in cutaway view, the cylinder and combustion chamber resembles a T - hence the name "T-head". An L-head has all valves at the same side.

Squish (piston engine)

Squish is an effect in internal combustion engines which creates sudden turbulence of the air-fuel mixture as the piston approaches top dead centre (TDC).

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