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An overhead camshaft (OHC) engine is a
piston engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common feat ...
where the
camshaft A camshaft is a shaft that contains a row of pointed cams, in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves), mechanically controlled ignition systems ...
is located in the
cylinder head In an internal combustion engine, the cylinder head (often abbreviated to simply "head") sits above the cylinders and forms the roof of the combustion chamber. In sidevalve engines, the head is a simple sheet of metal; whereas in more modern o ...
above the
combustion chamber A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process. Intern ...
. This contrasts with earlier
overhead valve engines An overhead valve (OHV) engine, sometimes called a ''pushrod 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 be ...
(OHV), where the camshaft is located below the combustion chamber in the engine block. ''Single overhead camshaft'' (SOHC) engines have one camshaft per bank of cylinders. ''Dual overhead camshaft'' (DOHC, also known as "twin-cam".) engines have two camshafts per bank. The first production car to use a DOHC engine was built in 1910. Use of DOHC engines slowly increased from the 1940s, leading to many automobiles by the early 2000s using DOHC engines.


Design

In an OHC engine, the camshaft is located at the top of the engine, above the
combustion chamber A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process. Intern ...
. This contrasts the earlier
overhead valve engine An overhead valve (OHV) engine, sometimes called a ''pushrod 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 be ...
(OHV) and
flathead engine A flathead engine, also known as a sidevalve engine''American Rodder'', 6/94, pp.45 & 93. 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, ...
configurations, where the camshaft is located down in the engine block. The valves in both OHC and OHV engines are located above the combustion chamber; however an OHV engine requires
pushrod 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 (or air alone for direct-injected engines) ...
s and
rocker arm In the context of an internal combustion engine, a rocker arm is a valvetrain component that typically transfers the motion of a pushrod to the corresponding intake/exhaust valve. Rocker arms in automobiles are typically made from stamped steel ...
s to transfer the motion from the camshaft up to the valves, whereas an OHC engine has the valves directly actuated by the camshaft. Compared with OHV engines with the same number of valves, there are fewer reciprocating components and less valvetrain inertia in an OHC engine. This reduced inertia in OHC engines results in less valve float at higher engine speeds (RPM). A downside is that the system used to drive the camshaft (usually a timing chain in modern engines) is more complex in an OHC engine. A disadvantage of OHC engines is that during engine repairs which require the removal of the cylinder head the camshaft engine timing needs to be reset. The other main advantage of OHC engines is that there is greater flexibility to optimise the size, location and shape of the intake and exhaust ports, since there are no pushrods that need to be avoided. This improves the gas flow through the engine, increasing power output and
fuel efficiency Fuel efficiency is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical potential energy contained in a carrier (fuel) into kinetic energy or work. Overall fuel efficiency may vary per device ...
.


Single overhead camshaft (SOHC)

The oldest configuration of overhead camshaft engine is the ''single overhead camshaft'' (SOHC) design. A SOHC engine has one camshaft per bank of cylinders, therefore a
straight engine The straight or inline engine is an internal combustion engine with all cylinders aligned in one row and having no offset. Usually found in four, six and eight cylinder configurations, they have been used in automobiles, locomotives and aircraf ...
has a total of one camshaft and a
V engine A V engine, sometimes called a Vee engine, is a common configuration for internal combustion engines. It consists of two cylinder banks—usually with the same number of cylinders in each bank—connected to a common crankshaft. These cylinder ...
or
flat engine A flat engine is a piston engine where the cylinders are located on either side of a central crankshaft. Flat engines are also known as horizontally opposed engines, however this is distinct from the less common opposed-piston engine design, ...
has a total of two camshafts (one for each cylinder bank). Most SOHC engines have two valves per cylinder. Motion of the camshaft is usually transferred to the valves either directly (using a tappet) or indirectly via a
rocker arm In the context of an internal combustion engine, a rocker arm is a valvetrain component that typically transfers the motion of a pushrod to the corresponding intake/exhaust valve. Rocker arms in automobiles are typically made from stamped steel ...
.


Double overhead camshaft (DOHC)

A ''double overhead cam'', ''dual overhead cam'', or ''twin-cam'' engine has two camshafts per bank of the cylinder head, one for the intake valves and another for the exhaust valves. Therefore there are two camshafts for a straight engine and a total of four camshafts for a V engine or a flat engine. A
V engine A V engine, sometimes called a Vee engine, is a common configuration for internal combustion engines. It consists of two cylinder banks—usually with the same number of cylinders in each bank—connected to a common crankshaft. These cylinder ...
or
flat engine A flat engine is a piston engine where the cylinders are located on either side of a central crankshaft. Flat engines are also known as horizontally opposed engines, however this is distinct from the less common opposed-piston engine design, ...
requires four camshafts to function as a DOHC engine, since have two camshafts in total would result in only a single camshaft per cylinder bank for these engine layouts. Some V engines with four camshafts have been marketed as "quad-cam" engines, however technically "quad-cam" would require four camshafts per cyinder bank (i.e. eight camshafts in total), therefore these engines are merely ''double overhead camshaft'' engines. To further confuse the terminology, some SOHC flat-twin and V-twin motorcycle engines (for example by Harley-Davidson, Indian, Riley Motors, and Triumph) have been incorrectly marketed with as "twin-cam" engines. Most DOHC engines have four valves per cylinder. The camshaft usually operates the valves directly via a bucket
tappet A tappet is most commonly a component in an internal combustion engine which converts the rotating motion of the camshaft into linear motion of the valves, either directly or indirectly. An earlier use of the term was for part of the valve gea ...
. A DOHC design permits a wider angle between intake and exhaust valves than in SOHC engines, which improves the air-fuel mixture's flow through the engine. A further benefit is that the spark plug can be placed at the optimum location, which in turn improves combustion efficiency.


Components


Timing belt / timing chain

The rotation of a camshaft is driven by a
crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecti ...
. Many 21st century engines use a toothed ''timing belt'' made from rubber and kevlar to drive the camshaft. Timing belts are inexpensive, produce minimal noise and have no need for lubrication. A disadvantage of timing belts is the need for regular replacement of the belt; recommended belt life typically varies between approximately . If the timing belt is not replaced in time and fails and the engine is an interference engine, major engine damage is possible. The first known automotive application of timing belts to drive overhead camshafts was the 1953 Devin-Panhard racing specials built for the SCCA H-modified racing series in the United States. These engines were based on Panhard OHV flat-twin engines, which were converted to SOHC engines using components from Norton motorcycle engines. The first production car to use a timing belt was the 1962 Glas 1004 compact coupe. Another camshaft drive method commonly used on modern engines is a ''timing chain'', constructed from one or two rows of metal
roller chains Roller chain or bush roller chain is the type of chain drive most commonly used for transmission of mechanical power on many kinds of domestic, industrial and agricultural machinery, including conveyors, wire- and tube-drawing machines, prin ...
. By the early 1960s most production automobile overhead camshaft designs used chains to drive the camshaft(s). Timing chains do not usually require replacement at regular intervals, however the disadvantage is that they are noisier than timing belts.


Gear train

A gear train system between the crankshaft and the camshaft is commonly used in diesel overhead camshaft engines used in heavy trucks. Gear trains are not commonly used in engines for light trucks or automobiles.


Other camshaft drive systems

Several OHC engines up until the 1950s used a shaft with bevel gears to drive the camshaft. Examples include the 1908–1911 Maudslay 25/30, the Bentley 3 Litre, the 1929-1932
MG Midget The MG Midget is a small two-seater sports car produced by MG from 1961 to 1979. It revived a name that had been used on earlier models such as the MG M-type, MG D-type, MG J-type and MG T-type. MG Midget MkI (1961–64) The first version, ...
, the 1925-1948 Velocette K series, the 1931-1957
Norton International The Norton International or Cammy Norton is a Norton Motors Ltd overhead cam (OHC) motorcycle built between 1931 and 1957. More than a ''TT replica'' sports roadster, the OHC Model 30, was 500 cc and the OHC Model 40 was 350 cc. Dur ...
and the 1947-1962 Norton Manx. In more recent times, the 1950-1974 Ducati Single, 1973-1980 Ducati L-twin engine, 1999-2007
Kawasaki W650 The Kawasaki W650 is a retro standard motorcycle marketed by Kawasaki for model years 1999-2007 and superseded by the Kawasaki W800. The "W" in "W650" refers to Kawasaki's W1, W2 and W3 models, manufactured between 1967 and 1975. The "650 ...
and 2011-2016
Kawasaki W800 The Kawasaki W800 is a parallel twin motorcycle produced by Kawasaki from 2011 to 2016, and then since 2019. The W800 is a retro style model that emulates the Kawasaki W series, three models that were produced from 1967 to 1975, and which in tur ...
motorcycle engines have used bevel shafts. The
Crosley Crosley was a small, independent American manufacturer of subcompact cars, bordering on microcars. At first called the Crosley Corporation and later Crosley Motors Incorporated, the Cincinnati, Ohio, firm was active from 1939 to 1952, int ...
four cylinder was the last automotive engine to use the shaft tower design to drive the camshaft, from 1946 to 1952; the rights to the Crosley engine format were bought by a few different companies, including
General Tire Continental Tire the Americas, LLC, d.b.a. General Tire, is an American manufacturer of tires for motor vehicles. Founded in 1915 in Akron, Ohio by William Francis O'Neil, Winfred E. Fouse, Charles J. Jahant, Robert Iredell, & H.B. Pushee as ...
in 1952, followed by
Fageol Fageol Motors was a United States manufacturer of buses, trucks and farm tractors. History The company was founded in 1916, in Oakland, California, by Rollie, William, Frank and Claude Fageol, to manufacture motor trucks, farm tractors a ...
in 1955, Crofton in 1959, Homelite in 1961, and Fisher Pierce in 1966, after Crosley closed the automotive factory doors, and they continued to produce the same engine for several more years. A camshaft drive using three sets of cranks and rods in parallel was used in the 1920–1923
Leyland Eight The Leyland Eight was a luxury car produced by Leyland Motors from 1920 to 1923. The car was designed by the chief engineer of Leyland Motors, J.G. Parry-Thomas and his assistant Reid Railton, and was intended to be the finest car availabl ...
luxury car built in the United Kingdom. A similar system was used in the 1926-1930 Bentley Speed Six and the 1930-1932
Bentley 8 Litre The Bentley 8 Litre was a large inline 6-cylinder super-luxury car made in various configurations by Bentley Motors Limited at Cricklewood, London. Announced 15 September 1930, it was also the last completely new model by Bentley before the c ...
. A two-rod system with counterweights at both ends was used by many models of the 1958-1973 NSU Prinz.


History


1900–1914

Among the first overhead camshaft engines were the 1902 Maudslay SOHC engine built in the United Kingdom and the 1903 Marr Auto Car SOHC engine built in the United States. The first DOHC engine was a Peugeot inline-four racing engine which powered the car that won the
1912 French Grand Prix The 1912 French Grand Prix was a Grand Prix motor race held at Dieppe on 25–26 June 1912. The Race The race was run over two days with the drivers completing ten laps on each day and their times being aggregated to produce the winner (simila ...
. Another Peugeot with a DOHC engine won the
1913 French Grand Prix The 1913 French Grand Prix was a Grand Prix motor race held at Amiens on 12 July 1913. The race The restriction on Grand Prix cars for 1913 included an minimum weight and an maximum weight, as well as a fuel consumption limit. The buildup t ...
, followed by the ''Mercedes-Benz 18/100 GP'' with an SOHC engine winning the 1914 French Grand Prix. The
Isotta Fraschini Tipo KM The Isotta Fraschini Tipo KM is a luxury car produced between 1910-1914 in Italy. Only 50 were built.Motorbase1913 Isotta Fraschini 100-120 hp Tipo KM 4 Four-Seat Torpedo Tourer - Auction Lot - Motorbase, accessdate: 19. July 2018 Many of those 50 ...
— built in Italy from 1910–1914— was one of the first production cars to use an SOHC engine.


World War I

During World War I, both the Allied and
Central Powers The Central Powers, also known as the Central Empires,german: Mittelmächte; hu, Központi hatalmak; tr, İttifak Devletleri / ; bg, Централни сили, translit=Tsentralni sili was one of the two main coalitions that fought in W ...
; specifically those of the
German Empire The German Empire (),Herbert Tuttle wrote in September 1881 that the term "Reich" does not literally connote an empire as has been commonly assumed by English-speaking people. The term literally denotes an empire – particularly a hereditary ...
's '' Luftstreitkräfte'' air forces, sought to quickly apply the overhead camshaft technology of motor racing engines to military aircraft engines. The SOHC engine from the ''Mercedes 18/100 GP'' car (which won the 1914 French Grand Prix) became the starting point for both Mercedes' and Rolls-Royce's aircraft engines. Mercedes created a series of six-cylinder engines which culminated in the Mercedes D.III. Rolls-Royce reversed-engineered the Mercedes cylinder head design based on a racing car left in England at the beginning of the war, leading to the Rolls-Royce Eagle V12 engine. Other SOHC designs included the Spanish
Hispano-Suiza 8 The Hispano-Suiza 8 was a water-cooled V8 SOHC aero engine introduced by Hispano-Suiza in 1914, and was the most commonly used liquid-cooled engine in the aircraft of the Entente Powers during the First World War. The original Hispano-Suiza ...
V8 engine (with a fully enclosed-drivetrain), the American
Liberty L-12 The Liberty L-12 is an American water-cooled 45° V-12 aircraft engine displacing and making designed for a high power-to-weight ratio and ease of mass production. It saw wide use in aero applications, and, once marinized, in marine use both ...
V12 engine, which closely followed the later Mercedes D.IIIa design's partly-exposed SOHC valvetrain design; and the
Max Friz Max Friz (October 1, 1883 – June 9, 1966) was a German mechanical engineer specializing in engine design. He was the key contributor of engine design and innovation that led to the founding of Bayerische Motoren Werke AG (BMW) in 1917. Early ...
-designed; German
BMW IIIa BMW IIIa was an inline six-cylinder SOHC valvetrain, water-cooled aircraft engine, the first-ever engine produced by Bayerische Flugzeugwerke AG, who, at the time, were exclusively an aircraft engine manufacturer. Its success laid the foundatio ...
straight-six engine. The DOHC
Napier Lion The Napier Lion is a 12-cylinder, petrol-fueled 'broad arrow' W12 configuration aircraft engine built by D. Napier & Son from 1917 until the 1930s. A number of advanced features made it the most powerful engine of its day and kept it in produ ...
W12 engine was built in Great Britain beginning in 1918. Most of these engines used a shaft to transfer drive from the crankshaft up to the camshaft at the top of the engine. Large aircraft engines— particularly air-cooled engines— experienced considerable thermal expansion, causing the height of the cylinder block to vary during operating conditions. This expansion caused difficulties for pushrod engines, so an overhead camshaft engine using a shaft drive with sliding spline was the easiest way to allow for this expansion. These bevel shafts were usually in an external tube outside the block, and were known as "tower shafts". Hispano Suiza 8 A Brussel.jpg , 1914–1918 Hispano-Suiza 8A SOHC aircraft engine Hispano-suiza-V8 220PS.jpg , 1914–1918 Hispano-Suiza 8Be SOHC aircraft engine with "tower shafts" at the rear of each cylinder bank Later Mercedes D III Valvetrain.jpg , Later production (1917-18) Mercedes D.III upper valvetrain details sketch, its design features copied by the BMW III and the Allied
Liberty L-12 The Liberty L-12 is an American water-cooled 45° V-12 aircraft engine displacing and making designed for a high power-to-weight ratio and ease of mass production. It saw wide use in aero applications, and, once marinized, in marine use both ...
engines Liberty L-12-2.jpg , Detail closeup of a
Liberty L-12 The Liberty L-12 is an American water-cooled 45° V-12 aircraft engine displacing and making designed for a high power-to-weight ratio and ease of mass production. It saw wide use in aero applications, and, once marinized, in marine use both ...
's upper valvetrain, showing the similarity to the later-production Mercedes design


1919–1944

An early American overhead camshaft production engine was the SOHC straight-eight engine used in the 1921–1926
Duesenberg Model A The Duesenberg Model A was the first automobile in series production to have hydraulic brakes and the first automobile in series production in the United States with a straight-eight engine. Officially known as the Duesenberg Straight Eight, t ...
luxury car. In 1926, the
Sunbeam 3 litre Super Sports The Sunbeam 3-litre is a sports car introduced by Sunbeam in October 1925The Motor Show IV, Sunbeam Cars. ''The Times'', Tuesday, Oct 13, 1925; pg. 8; Issue 44091 at the London Motor Show, and was offered from 1926 until 1930. It was seen at ...
became the first production car to use a DOHC engine. In the United States, Duesenberg added DOHC engines (alongside their existing SOHC engines) with the 1928 release of the Duesenberg Model J, which was powered by a DOHC straight-eight engine. The 1931–1935 Stutz DV32 was another early American luxury car to use a DOHC engine. Also in the United States, the DOHC
Offenhauser The Offenhauser Racing Engine, or Offy, is a racing engine design that dominated American open wheel racing for more than 50 years and is still popular among vintage sprint and midget car racers. History The Offenhauser engine, familiarly ...
racing engine was introduced in 1933. This inline-four engine dominated North American open-wheel racing from 1934 until the 1970s. Other early SOHC automotive engines were the 1920–1923 Wolseley Ten, the 1928-1931 MG 18/80, the 1926–1935 Singer Junior and the 1928–1929 Alfa Romeo 6C Sport. Early overhead camshaft motorcycles included the 1925–1949 Velocette K Series and the 1927–1939
Norton CS1 The CS1 was a Norton motorcycle between 1927 and 1939. Originally built as a TT racer, and Norton's first design of an overhead cam engine, it proved successful as a ''TT Replica'' road bike. After the early 1930s redesign of Norton's cammy m ...
.


1945–present

The 1946–1948 Crosley CC Four was arguably the first American mass-produced car to use an SOHC engine. This small mass-production engine powered the winner of the 1950
12 Hours of Sebring The 12 Hours of Sebring is an annual motorsport endurance race for sports cars held at Sebring International Raceway, on the site of the former Hendricks Army Airfield World War II air base in Sebring, Florida, US. The event is the second rou ...
. Use of a DOHC configuration gradually increased after World War II, beginning with sports cars. Iconic DOHC engines of this period include the 1948–1959 Lagonda straight-six engine, the 1949–1992 Jaguar XK straight-six engine and the 1954–1994 Alfa Romeo Twin Cam inline-four engine. The 1966-2000 Fiat Twin Cam inline-four engine was one of the first DOHC engines to use a toothed timing belt instead of a timing chain. In the 1980s, the need for increased performance while reducing fuel consumption and exhaust emissions saw increasing use of DOHC engines in mainstream vehicles, beginning with Japanese manufacturers. By the mid-2000s, most automotive engines used a DOHC layout.


See also

* Cam-in-block * Camless *
Overhead valve engine An overhead valve (OHV) engine, sometimes called a ''pushrod 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 be ...
*
Variable valve timing In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combination with var ...


Footnotes


References

{{DEFAULTSORT:Overhead Camshaft Engine valvetrain configurations