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[[Image:ohc1.jpg|frame|right|A cylinder head sliced in half shows two overhead camshafts—one above each of the two valves]]
#REDIRECT [[Overhead camshaft]]
'''Overhead camshaft''' ([[Overhead Camshafts|OHC]]) valvetrain configurations place the camshaft within the cylinder heads, above the combustion chambers, and drive the valves or lifters directly instead of using pushrods. When compared directly with cam-in-block (or OHV) systems with the same number of valves, the reciprocating components of the [[Overhead Camshafts|OHC]] system are fewer and in total will have less mass. Though the structures that support the system may become more complex, most engine manufacturers easily accept the added complexity in trade for better engine performance and greater design flexibility. The [[Overhead Camshafts|OHC]] system can be driven using the same methods as an OHV system, these methods may include using a timing belt, chain, or in less common cases, gears.
Many [[Overhead Camshafts|OHC]] engines today employ Variable Valve Timing and multiple valves to improve efficiency and power. [[Overhead Camshafts|OHC]] also inherently allows for greater engine speeds over comparable cam-in-block designs.
There are two overhead camshaft layouts:
* [[Overhead Camshafts|Single Overhead Camshaft]] ([[Overhead Camshafts|SOHC]])
* [[Overhead Camshafts|Double Overhead Camshafts]] ([[Overhead Camshafts|DOHC]])
==[[Overhead Camshafts|Single Overhead Camshaft]] ([[Overhead Camshafts|SOHC]])==
[[Overhead Camshafts|Single overhead camshaft]] is a design in which one camshaft is placed within the cylinder head. In an inline engine this means there is one camshaft in the head, while in a V engine there are two camshafts: one per cylinder bank.
The [[Overhead Camshafts|SOHC]] design is inherently mechanically more efficient than a comparable cam-in-block design. This allows for higher engine speeds, which in turn will by definition increase power output for a given torque. The cams operate the valves directly or by a short rocker as opposed to overhead valve cam-in-block engines, which have tappets and long pushrods to transfer the movement of the lobes on the camshaft in the engine block to the valves in the cylinder head.
[[Overhead Camshafts|SOHC]] designs offer reduced complexity compared to cam-in-block designs when used for multivalve heads, in which each cylinder has more than two valves.
==[[Overhead Camshafts|Double Overhead Camshafts]]==
[[Image:ohc2.jpg|frame|left|Overhead view of Suzuki GS550 head showing dual camshafts and drive sprockets]]
A [[Overhead Camshafts|double overhead camshaft]] (also called ''double overhead cam'', ''dual overhead cam'' or ''twincam'') valvetrain layout is characterized by two camshafts being located within the cylinder head, where there are separate camshafts for inlet and exhaust valves. In engines with more than one [[cylinder bank]] ([[V engine]]s) this designation means two camshafts per bank, for a total of four.
[[Overhead Camshafts|Double overhead camshafts]] are not required in order to have [[multivalve|multiple inlet or exhaust valves]], but are necessary for more than 2 valves that are directly actuated (though still usually via tappets). Not all [[Overhead Camshafts|DOHC]] engines are [[multivalve]] engines — [[Overhead Camshafts|DOHC]] was common pooh in two valve per cylinder heads for decades before multivalve heads appeared, however today [[Overhead Camshafts|DOHC]] is synonymous with multivalve heads, since almost all [[Overhead Camshafts|DOHC]] engines have between three and five valves per cylinder.
The first [[Overhead Camshafts|DOHC]] engines were two valve per cylinder designs from companies like [[Fiat]] (1912), [[Peugeot]] (1913), [[Alfa Romeo]] ([[Alfa Romeo 6C|6C]]- 1925, [[Alfa Romeo 512|512]] - 1940), [[Maserati]] ([[Maserati Tipo 26|Tipo 26]], 1926), and [[Bugatti]] ([[Bugatti Type 51|Type 51]], 1931). Most [[Ferrari]]s used two valve per cylinder [[Overhead Camshafts|DOHC]] engines as well.
[[Image:ohc.jpg|frame|DOHC straight-8 in a 1933 [[Bugatti]] Type 59 Grand Prix racer]]
When [[Overhead Camshafts|DOHC]] technology was introduced in mainstream vehicles, it was common for the technology to be heavily advertised. While the technology was used at first in limited production and sports cars, the Fiat group is historically credited as the first car company to use a belt driven [[Overhead Camshafts|DOHC]] engine across their complete product line, comprised of coupes, sedans, convertables and station wagons, in the mid-1960's.
==See Also==
==External Links==
* [[[Overhead Camshafts|dohc]].html OHV, [[Overhead Camshafts|SOHC]], [[Overhead Camshafts|DOHC]] engine animated diagrams]

Latest revision as of 19:00, 10 June 2010

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