Overhead camshaft
Overview
 
Overhead cam valvetrain
Valvetrain
Valvetrain is an all-encompassing term used to describe the mechanisms and parts which control the operation of the valves. A traditional reciprocating internal combustion engine uses valves to control air and fuel flow into and out of the cylinders, facilitating combustion.-Layout:Valvetrain: The...

 configurations place the engine camshaft
Camshaft
A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...

 within the cylinder head
Cylinder head
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...

s, above the combustion chamber
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...

s, and drive the valves
Poppet valve
A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...

 or lifter
Tappet
A tappet is the noise made by a worn cam follower, however the term is widely used to represent the cam follower itself. In mechanical engineering it is a projection which imparts a linear motion to some other component within an assembly...

s in a more direct manner compared to overhead valve
Overhead valve
An overhead valve engine, also informally called pushrod engine or I-head engine, is a type of piston engine that places the camshaft within the cylinder block , and uses pushrods or rods to actuate rocker arms above the cylinder...

s (OHV) and pushrods. Compared to OHV pushrod (or I-Head) systems with the same number of valves the reciprocating components of the OHC system are fewer and have a lower total mass. Though the system that drives the cams may become more complex, most engine manufacturers easily accept that added complexity in trade for better engine performance and greater design flexibility.
Encyclopedia
Overhead cam valvetrain
Valvetrain
Valvetrain is an all-encompassing term used to describe the mechanisms and parts which control the operation of the valves. A traditional reciprocating internal combustion engine uses valves to control air and fuel flow into and out of the cylinders, facilitating combustion.-Layout:Valvetrain: The...

 configurations place the engine camshaft
Camshaft
A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...

 within the cylinder head
Cylinder head
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...

s, above the combustion chamber
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...

s, and drive the valves
Poppet valve
A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...

 or lifter
Tappet
A tappet is the noise made by a worn cam follower, however the term is widely used to represent the cam follower itself. In mechanical engineering it is a projection which imparts a linear motion to some other component within an assembly...

s in a more direct manner compared to overhead valve
Overhead valve
An overhead valve engine, also informally called pushrod engine or I-head engine, is a type of piston engine that places the camshaft within the cylinder block , and uses pushrods or rods to actuate rocker arms above the cylinder...

s (OHV) and pushrods. Compared to OHV pushrod (or I-Head) systems with the same number of valves the reciprocating components of the OHC system are fewer and have a lower total mass. Though the system that drives the cams may become more complex, most engine manufacturers easily accept that added complexity in trade for better engine performance and greater design flexibility. Another performance advantage is gained as a result of the better optimized port configurations made possible with overhead camshaft designs. With no intrusive pushrods the overhead camshaft cylinder head design can use straighter ports of more advantageous crossection and length.

The OHC system can be driven using the same methods as an OHV system, which include using a rubber/kevlar toothed timing belt
Timing belt
A timing belt, or cam belt , is a part of an internal combustion engine that controls the timing of the engine's valves. Some engines, such as the flat-4 Volkswagen air-cooled engine, and the straight-6 Toyota F engine use timing gears...

, chain
Roller chain
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, printing presses, cars, motorcycles, and simple machines like...

, or in less common cases, gears.

In conjunction with multiple (3, 4 or 5) valves per cylinder, many OHC engines today employ variable valve timing
Variable valve timing
In internal combustion engines, variable valve timing , also known as Variable valve actuation , is a generalized term used to describe any mechanism or method that can alter the shape or timing of a valve lift event within an internal combustion engine...

 to improve efficiency and power. OHC also inherently allows for greater engine speeds over comparable cam-in-block designs, as a result of having lower valvetrain mass.

There are two overhead camshaft layouts: single overhead camshaft ("SOHC"), and double (or dual) overhead camshaft ("DOHC").

Single overhead camshaft

Single overhead camshaft (SOHC) is a design in which one camshaft is placed within the cylinder head
Cylinder head
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 an inline engine this means there is one camshaft in the head, while in a V engine
V engine
A V engine, or Vee engine is a common configuration for an internal combustion engine. The cylinders and pistons are aligned, in two separate planes or 'banks', so that they appear to be in a "V" when viewed along the axis of the crankshaft...

 or a horizontally-opposed engine (boxer; flat engine
Flat engine
A flat engine is an internal combustion engine with multiple pistons that move in a horizontal plane. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is sometimes known as the boxer, or horizontally opposed engine. The concept was patented in 1896...

) there are two camshafts: one per cylinder bank.

The SOHC design has less reciprocating mass than a comparable pushrod design. This allows for higher engine speeds, which in turn will increase power output for a given torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....

. The cam operates the valves directly or through a rocker arm
Rocker arm
Generally referred to within the internal combustion engine of automotive, marine, motorcycle and reciprocating aviation engines, the rocker arm is a reciprocating lever that conveys radial movement from the cam lobe into linear movement at the poppet valve to open it...

, as opposed to overhead valve pushrod engines which have tappets, long pushrods, and rocker arms to transfer the movement of the lobes on the camshaft in the engine block to the valves in the cylinder head
Cylinder head
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 the early era of the liquid-cooled aircraft engine
Aircraft engine
An aircraft engine is the component of the propulsion system for an aircraft that generates mechanical power. Aircraft engines are almost always either lightweight piston engines or gas turbines...

 field, single overhead cam format engines were in existence during the First World War, for both the Allies
Allies of World War I
The Entente Powers were the countries at war with the Central Powers during World War I. The members of the Triple Entente were the United Kingdom, France, and the Russian Empire; Italy entered the war on their side in 1915...

 and the Central Powers
Central Powers
The Central Powers were one of the two warring factions in World War I , composed of the German Empire, the Austro-Hungarian Empire, the Ottoman Empire, and the Kingdom of Bulgaria...

. The Hispano-Suiza 8
Hispano-Suiza 8
The Hispano-Suiza 8 was a water-cooled V8 SOHC aero engine introduced by Hispano-Suiza in 1914 and used by a number of Allied aircraft during the First World War...

 V8 engine, designed by Marc Birkigt
Marc Birkigt
Marc Birkigt was a Swiss engineer who moved to Barcelona, Spain when he was hired as an engineer by Emilio de la Cuadra, founder of Hispano-Suiza automobiles. He created the Dewoitine company along with Émile Dewoitine. Birkigt was nominated for the Car Engineer of the Century prize for the...

 in the Allied camp, and the series of Mercedes inline-6 aviation engines, culminating in the Mercedes D.III
Mercedes D.III
The Mercedes D.III, or F1466 as it was known internally, was a six-cylinder, liquid cooled inline aircraft engine built by Daimler and used on a wide variety of German aircraft during World War I. The initial versions were introduced in 1914 at 160 hp, but a series of changes improved this to...

 for the German Empire
German Empire
The German Empire refers to Germany during the "Second Reich" period from the unification of Germany and proclamation of Wilhelm I as German Emperor on 18 January 1871, to 1918, when it became a federal republic after defeat in World War I and the abdication of the Emperor, Wilhelm II.The German...

, both used rotary shaft-driven single overhead camshaft valve drive systems, and were among the most prominent aviation powerplants of the First World War era. The late-war Liberty L-12
Liberty L-12
The Liberty L-12 was a 27 litre water-cooled 45° V-12 aircraft engine of 400 horsepower designed both for a high power-to-weight ratio and for ease of mass production.-History:...

 V12 configuration American aviation engine also used the general Mercedes D-series single overhead camshaft design, based primarily on the later D.IIIa's drive system from rocker box to valvestem.

SOHC designs offer reduced complexity compared to pushrod designs when used for multi-valve heads in which each cylinder has more than two valves. An example of an SOHC design using shim and bucket valve adjustment was the engine installed in the Hillman Imp
Hillman Imp
The Hillman Imp is a compact, rear-engined saloon car that was manufactured under the Hillman marque by the Rootes Group from 1963 to 1976...

 (4 cylinder, 8 valve); a small, early 1960s 2-door saloon car with a rear mounted alloy engine based on the Coventry Climax FWMA
Coventry Climax
Coventry Climax was a British forklift truck, fire pump, and speciality engine manufacturer.-History:The company was started in 1903 as Lee Stroyer, but two years later, following the departure of Stroyer, it was relocated to Paynes Lane, Coventry, and renamed to Coventry-Simplex by H...

 race engines. Exhaust and inlet manifolds were both on the same side of the engine block (thus not a crossflow cylinder head
Crossflow cylinder head
A crossflow cylinder head is a cylinder head that features the intake and exhaust ports on opposite sides. The gases can be thought to flow across the head...

 design). This did, however, offer excellent access to the spark plugs.

In the early 1980s, Toyota and Volkswagen
Volkswagen
Volkswagen is a German automobile manufacturer and is the original and biggest-selling marque of the Volkswagen Group, which now also owns the Audi, Bentley, Bugatti, Lamborghini, SEAT, and Škoda marques and the truck manufacturer Scania.Volkswagen means "people's car" in German, where it is...

 also used a directly actuated, SOHC parallel valve configuration with two valves for each cylinder. The Toyota system used hydraulic tappet
Tappet
A tappet is the noise made by a worn cam follower, however the term is widely used to represent the cam follower itself. In mechanical engineering it is a projection which imparts a linear motion to some other component within an assembly...

s while the Volkswagen system used bucket tappet
Tappet
A tappet is the noise made by a worn cam follower, however the term is widely used to represent the cam follower itself. In mechanical engineering it is a projection which imparts a linear motion to some other component within an assembly...

s with shims for valve lash adjustment. Of all valvetrain systems, this is the least complex configuration possible.

Double overhead camshaft

A double overhead camshaft (also known as dual overhead camshaft) valve-train layout is characterized by two camshafts located within the cylinder head, one operating the intake valves and one operating the exhaust valves. Some engines have more than one bank of cylinder heads (ie: V6, V8 where 2 cylinder banks meet to form a 'V') and these have two camshafts in total, but they remain SOHC, unless each side has two camshafts. The term "twin cam" is imprecise, but will normally refer to a DOHC engine. Some manufacturers still managed to use a SOHC in 4-valve layouts. Honda, for instance, with the later half of the D16 family, utilizes the 4-valve per cylinder, SOHC layout to reduce overall costs.
Also not all DOHC engines are multivalve engines—DOHC was common in two valve per cylinder heads for decades before multivalve heads appeared. Today, however, DOHC is synonymous with multi-valve heads since almost all DOHC engines have between three and five valves per cylinder.

History

Among the early pioneers of DOHC were Isotta Fraschini's Giustino Cattaneo, Austro-Daimler
Austro-Daimler
Austro-Daimler was an Austrian automaker company, from 1899 until 1934. It was subsidiary of the German Daimler Motoren Gesellschaft .-Early history:...

's Ferdinand Porsche
Ferdinand Porsche
Ferdinand Porsche was an Austrian automotive engineer and honorary Doctor of Engineering. He is best known for creating the first hybrid vehicle , the Volkswagen Beetle, and the Mercedes-Benz SS/SSK, as well as the first of many Porsche automobiles...

 Stephen Tomczak (in the Prinz Heinrich), and W. O. Bentley
W. O. Bentley
Walter Owen Bentley, MBE engineer; designer of aero engines, designer and racer of motor cars, founder of Bentley Motors Limited in Cricklewood near London.He was known as "W.O." without any need to add the word Bentley....

 (in 1919); Sunbeam built small numbers of racing models between 1921 and 1923 and introduced one of the world's first production twin cams in 1924 - the Sunbeam 3 litre Super Sports
Sunbeam 3 litre Super Sports
The Sunbeam 3-litre is a heavy sports car introduced by Sunbeam in October 1924 at the London Motor Show, and was offered until 1930. It was seen at the time and subsequently as the retort of Louis Hervé Coatalen, Sunbeam's energetic chief engineer, to the Bentley 3 Litre which by then was...

, an example of which came 2nd at LeMans in 1925. The first DOHC engines were either two- or four-valve per cylinder racing car designs from companies like Fiat
Fiat
FIAT, an acronym for Fabbrica Italiana Automobili Torino , is an Italian automobile manufacturer, engine manufacturer, financial, and industrial group based in Turin in the Italian region of Piedmont. Fiat was founded in 1899 by a group of investors including Giovanni Agnelli...

 (1912), Peugeot
Peugeot
Peugeot is a major French car brand, part of PSA Peugeot Citroën, the second largest carmaker based in Europe.The family business that precedes the current Peugeot company was founded in 1810, and manufactured coffee mills and bicycles. On 20 November 1858, Emile Peugeot applied for the lion...

 Grand Prix (1913, 4 valve), Alfa Romeo
Alfa Romeo
Alfa Romeo Automobiles S.p.A. is an Italian manufacturer of cars. Founded as A.L.F.A. on June 24, 1910, in Milan, the company has been involved in car racing since 1911, and has a reputation for building expensive sports cars...

 Grand Prix
Alfa Romeo Grand Prix
A.L.F.A. 40/60 GP or GP was a fully working early racing car prototype made by the company now called Alfa Romeo. Only one example was built in 1914, which was later modified in 1921. This was creation of Giuseppe Merosi and was first Alfa Romeo DOHC engine. It had also four valves per cylinder,...

 (1914, 4 valve) and 6C
Alfa Romeo 6C
The Alfa Romeo 6C name was used on road, race and sports cars made between 1925–1954 by Alfa Romeo. 6C refers to a straight 6 engine. Bodies for these cars were made by coachbuilders such as James Young, Zagato, Touring, Castagna, and Pininfarina...

 (1928), Maserati
Maserati
Maserati is an Italian luxury car manufacturer established on December 1, 1914, in Bologna. The company's headquarters is now in Modena, and its emblem is a trident. It has been owned by the Italian car giant Fiat S.p.A. since 1993...

 Tipo 26 (1926), Bugatti
Bugatti
Automobiles E. Bugatti was a French car manufacturer founded in 1909 in Molsheim, Alsace, as a manufacturer of high-performance automobiles by Italian-born Ettore Bugatti....

 Type 51
Bugatti Type 51
The Type 51 series succeeded the famous Type 35 as Bugatti's premier racing car for the 1930s. Unlike the dominant Type 35s of the prior decade, the Type 51 were unable to compete with the government-supported German and Italian offerings.-Type 51:The original Type 51 bowed in 1931...

 (1931).

When DOHC technology was introduced in mainstream vehicles, it was common for it to be heavily advertised. While used at first in limited production and sports cars such as the 1925 Sunbeam 3 litre, Alfa Romeo is one of the twin cam's greatest proponents. 6C Sport, the first Alfa Romeo road car using a DOHC engine, was introduced in 1928. Ever since this, it has been a trademark of all Alfa Romeo engines.

Fiat
Fiat
FIAT, an acronym for Fabbrica Italiana Automobili Torino , is an Italian automobile manufacturer, engine manufacturer, financial, and industrial group based in Turin in the Italian region of Piedmont. Fiat was founded in 1899 by a group of investors including Giovanni Agnelli...

 was one of the first car companies to use a belt-driven DOHC engines across their complete product line, in the mid-1960s., Jaguar's XK6
Jaguar XK6 engine
The renowned Jaguar XK dual overhead camshaft inline 6-cylinder engine was introduced in 1949 and continued in manufacture until 1992. It was produced in five displacements for Jaguar passenger cars, with other sizes being made by Jaguar and privateers for racing...

 DOHC engine was displayed in the Jaguar XK120
Jaguar XK120
The Jaguar XK120 is a sports car which was manufactured by Jaguar between 1948 and 1954. It was Jaguar's first sports car since the SS 100, which ceased production in 1940.-History:...

 at the London Motor Show in 1948 and used across the entire Jaguar range through the late 1940s, 1950 and 1960s. By the late 1970s, Toyota was the best seller of DOHC engines.

More than two overhead camshafts are not known to have been tried in a production engine. However MotoCzysz
Motoczysz
MotoCzysz is a Portland, Oregon, based motorcycle company that intends to compete in the MotoGP. The C1 prototype has an engine with perfect balance and does not need a balance shaft...

 has designed a motorcycle engine with a triple overhead camshaft configuration with the intake ports descending through the head to two central intake ports between two outside exhaust camshafts actuating one of two exhaust valve per cylinder each.

In inline
Inline engine
Inline engine may refer to:* In general use it refers to any type of straight engine;* In aviation use it refers to any non-radial reciprocating cylinder engine: Inline engine ,...

 piston aero engines, DOHCs have been used for many engines. The 1917 Napier Lion
Napier Lion
The Napier Lion was a 12-cylinder broad arrow configuration aircraft engine built by Napier & Son starting in 1917, and ending in the 1930s. A number of advanced features made it the most powerful engine of its day, and kept it in production long after contemporary designs had stopped production...

, for example, had them.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK