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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 featu ...
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 ov ...
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. Interna ...
. 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 bel ...
(OHV), where the camshaft is located below the combustion chamber in the
engine block In an internal combustion engine, the engine block is the structure which contains the cylinders and other components. In an early automotive engine, the engine block consisted of just the cylinder block, to which a separate crankcase was atta ...
. ''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. Interna ...
. 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 bel ...
(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, as ...
configurations, where the camshaft is located down in the
engine block In an internal combustion engine, the engine block is the structure which contains the cylinders and other components. In an early automotive engine, the engine block consisted of just the cylinder block, to which a separate crankcase was atta ...
. 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 Valve float is an adverse condition which can occur at high engine speeds when the poppet valves in an internal combustion engine valvetrain do not properly follow the closure phase of the cam lobe profile. This reduces engine efficiency and perform ...
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, wh ...
.


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 aircraft ...
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 gear ...
. 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 connecting ...
. 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 An interference engine is a type of 4-stroke internal combustion piston engine in which one or more valves in the fully open position extends into any area through which the piston may travel. By contrast, in a non-interference engine, the piston ...
, 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 The Glas 1004 is a small two-door, four-seater automobile produced by Hans Glas GmbH at Dingolfing. It was first exhibited in public, in coupé form, at the Frankfurt Motor Show in September 1961. Volume production of the 1004 coupé started in M ...
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, pri ...
. 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 A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission ...
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 gear Bevel gears are gears where the axes of the two shafts intersect and the tooth-bearing faces of the gears themselves are conically shaped. Bevel gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at oth ...
s to drive the camshaft. Examples include the 1908–1911 Maudslay 25/30, the
Bentley 3 Litre The Bentley 3 Litre was a car chassis manufactured by Bentley. The company's first, it was developed from 1919 and made available to customers' coachbuilders from 1921 to 1929. The Bentley was very much larger than the 1368 cc Bugattis that d ...
, 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 Motorcycle Company, Norton Motors Ltd Overhead camshaft, overhead cam (OHC) motorcycle built between 1931 and 1957. More than a ''TT replica'' sports roadster, the OHC Model 30, was 500  ...
and the 1947-1962
Norton Manx The Norton Manx or Manx Norton is a British racing motorcycle that was made from 1947 to 1962 by Norton Motors Ltd. Norton had contested every Isle of Man TT race from the inaugural 1907 event through into the 1970s, a feat unrivalled by any ...
. In more recent times, the 1950-1974 Ducati Single, 1973-1980
Ducati L-twin engine The L-twin is a naturally aspirated two-cylinder petrol engine by Ducati. It uses a 90-degree layout and 180-degree firing order as is mounted with one cylinder horizontal. The next new Ducati engine to appear after the Ducati Apollo was the 90 ...
, 1999-2007 Kawasaki W650 and 2011-2016 Kawasaki W800 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, inter ...
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 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 and au ...
in 1955, Crofton in 1959,
Homelite Homelite Corporation is an American power equipment manufacturer, i.e. (chainsaws, leaf blowers, trimmers), that became notable for being one of the largest post-World War II manufacturers of portable electrical generators and professional a ...
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 luxury car built in the United Kingdom. A similar system was used in the 1926-1930
Bentley Speed Six The Bentley 6½ Litre and the high-performance Bentley Speed Six were rolling chassis in production from 1926 to 1930. The Speed Six, introduced in 1928, would become the most successful racing Bentley. Two Bentley Speed Sixes became known as th ...
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 The NSU Prinz (Prince) is an automobile which was produced in West Germany by the NSU Motorenwerke AG from 1958 to 1973. NSU Prinz I, II & III The first post-war NSU car, the Prinz I, was launched at the Frankfurt Motor Show in September 1957 a ...
.


History


1900–1914

Among the first overhead camshaft engines were the 1902
Maudslay Henry Maudslay ( pronunciation and spelling) (22 August 1771 – 14 February 1831) was an English machine tool innovator, tool and die maker, and inventor. He is considered a founding father of machine tool technology. His inventions were an ...
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. Another Peugeot with a DOHC engine won the 1913 French Grand Prix, followed by the ''Mercedes-Benz 18/100 GP'' with an SOHC engine winning the 1914 French Grand Prix. The Isotta Fraschini Tipo KM— 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 An alliance is a relationship among people, groups, or states that have joined together for mutual benefit or to achieve some common purpose, whether or not explicit agreement has been worked out among them. Members of an alliance are called ...
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 ...
; 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 The ''Deutsche Luftstreitkräfte'' (, German Air Force)—known before October 1916 as (Flyer Troops)—was the air arm of the Imperial German Army. In English-language sources it is usually referred to as the Imperial German Air Service, alt ...
'' 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 The Rolls-Royce Eagle was the first aircraft engine to be developed by Rolls-Royce Limited. Introduced in 1915 to meet British military requirements during World War I, it was used to power the Handley Page Type O bombers and a number of oth ...
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 Marinisation (also m ...
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 foundati ...
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 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 8A ...
SOHC aircraft engine Hispano-suiza-V8 220PS.jpg , 1914–1918
Hispano-Suiza 8Be 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 8A ...
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 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 ...
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 Marinisation (also m ...
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 Marinisation (also m ...
'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 luxury car. In 1926, the Sunbeam 3 litre Super Sports 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 The Duesenberg Model J is a luxury automobile made by Duesenberg. Intended to compete with the most luxurious and powerful cars in the world, it was introduced in 1928, the year before the stock market crash that led to the Great Depression. The M ...
, 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 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 Wolseley Ten is a light car which was produced by Wolseley Motors Limited in 1939 and from 1945 to 1948 . The ten horsepower class of cars was an important part of the market in 1930s Britain and Wolseley entered the sector with their 10/4 ...
, 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. 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 The Jaguar XK is a two-door 2+2 grand tourer manufactured and marketed by British automobile manufacturer Jaguar Cars from 1996–2014 in hatchback coupé and convertible bodystyles, across two generations. The XK was introduced at the Ge ...
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 A cam-in-block engine is where the camshaft is located in the engine block. Types of cam-in-block engines are: * F-Head Engine * Flathead engine * Overhead valve engine (the only type where the valves are above the combustion chamber) * T-head eng ...
*
Camless A camless or free-valve piston engine is an engine that has poppet valves operated by means of electromagnetic, hydraulic, or pneumatic actuators instead of conventional cams. Actuators can be used to both open and close valves, or to open valves ...
*
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 bel ...
*
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 varia ...


Footnotes


References

{{DEFAULTSORT:Overhead Camshaft Engine valvetrain configurations