Overhead cam (OHC) valvetrain configurations place the engine camshaft within the cylinder heads, above the combustion chambers, and drive the valves or lifters in a more direct manner compared to overhead valves (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, chain, or in less common cases, gears. Early Ducati engines used shafts (with bevel gears) to drive the camshafts in their OHC engines.
In conjunction with multiple (3, 4 or 5) valves per cylinder, many OHC engines today employ variable valve timing 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. In an inline engine this means there is one camshaft in the head, while in a V engine or a horizontally-opposed engine (boxer; flat engine) 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. The cam operates the valves directly or through a rocker arm, 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. SOHC cylinder heads are generally less expensive to manufacture than DOHC cylinder heads. Timing belt replacement can be easier since there are fewer cam sprockets that need to be aligned during the replacement procedure.
In the early era of the liquid-cooled aircraft engine field, single overhead cam format engines were in existence during the First World War, for both the Allies and the Central Powers. The Hispano-Suiza 8 V8 engine, designed by Marc Birkigt in the Allied camp, and the series of Mercedes inline-6 aviation engines, culminating in the Mercedes D.III for the German Empire, 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 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 (4 cylinder, 8 valve); a small, early 1960s 2-door saloon car with a rear mounted alloy engine based on the Coventry Climax FWMA race engines. Exhaust and inlet manifolds were both on the same side of the engine block (thus not a crossflow cylinder head design). This did, however, offer excellent access to the spark plugs.
Dual overhead camshaft
A dual-overhead-camshaft (also known as double-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. This design reduces valvetrain inertia more than a SOHC engine, since the rocker arms are reduced in size or eliminated. A DOHC design permits a wider angle between intake and exhaust valves than SOHC engines. This allows for a less restricted airflow at high engine speeds. Some engines have more than one bank of cylinder heads (i.e. V6, V8 where 2 cylinder banks meet to form a 'V') with 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 use a SOHC in a Multi-valve design. Also, not all DOHC engines are Multi-valve engines. DOHC cylinder heads existed before Multi-valve heads appeared in the 1980's. Today, however, DOHC is sometimes confused with multi-valve heads, since almost all modern DOHC engines have between three and five valves per cylinder - but multivalve and DOHC are distinct features.
Among the early pioneers of DOHC were Isotta Fraschini's Giustino Cattaneo, Austro-Daimler's Ferdinand Porsche Stephen Tomczak (in the Prinz Heinrich), and W. O. 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, an example of which came second at LeMans in 1925. The first DOHC engines were either two- or four-valve per cylinder racing car designs from companies like Fiat (1912), Peugeot Grand Prix (1913, 4 valve), Alfa Romeo Grand Prix (1914, 4 valve) and 6C (1928), Maserati Tipo 26 (1926), Bugatti Type 51 (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, DOHC has been a trademark of most Alfa Romeo engines (some Alfa V6 engines are SOHC, not DOHC. Most Alfasud boxer engines were also SOHC).
Fiat was one of the first car companies to use belt-driven DOHC engines in some of their products in the mid-1960s.Template:Citation needed, Jaguar's XK6 DOHC engine was displayed in the Jaguar XK120 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.Template:Citation needed
More than two overhead camshafts are not known to have been tried in a production engine. However MotoCzysz 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 valves per cylinder each.
- ↑ Tan, Paul. "SOHC vs DOHC Valvetrains: A Comparison". Paultan.org. http://paultan.org/archives/2005/06/22/sohc-vs-dohc-valvetrains/. Retrieved 2010-11-07.
- ↑ Georgano, G. N. Cars: Early and Vintage, 1886-1930. (London: Grange-Universal, 1985).
- ↑ 3.0 3.1 Kevin Clemens. "An Echo of the PastThe history and evolution of twin-cam engines". europeancarweb.com. http://www.europeancarweb.com/features/0209ec_twin_cam_analysis/index.html. Retrieved 2009-06-18.
- ↑ "Motoczysz triple cam engine". Motoczysz.com. 2006-08-25. http://www.motoczysz.com/rss_news_view.php?id=66. Retrieved 2010-11-07.
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