Editing Overhead camshaft engine (section)

== Design ==

In an OHC engine, the camshaft is located at the top of the engine, above the [[combustion chamber]]. This contrasts the earlier [[overhead valve engine]] (OHV) and [[flathead engine]] 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]]s and [[rocker arm]]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).<ref name=Hillier/> 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, such as the 4-chain valvetrain of the Audi 3.2 or the 2 meter chain on Ford cammers. Another disadvantage of OHC engines is that during engine repairs where the removal of the [[cylinder head]] is required, the camshaft [[engine timing]] needs to be reset. In addition, an OHC engine has a large cylinder head to accommodate the camshaft or an extra set of valves to increase the [[volumetric efficiency]], so that with the same displacement as an OHV engine, the OHC engine will end up being the physically larger of the two mostly due to the enlarged cylinder head.

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.<ref name=Hillier/> This improves the gas flow through the engine, increasing power output and [[fuel efficiency]].

=== Single overhead camshaft (SOHC) {{anchor|SOHC}} ===
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[[File:Dolomite Sprint Valves Section.jpg|thumb |SOHC design (for a 1973 [[Triumph_slant-four_engine#Sprint_16-valve_engine|Triumph Dolomite Sprint]]) ]]

The oldest configuration of overhead camshaft engine is the ''single overhead camshaft'' (SOHC) design.<ref name=Hillier/> A SOHC engine has one camshaft per bank of cylinders, therefore a [[straight engine]] has a total of one camshaft and a [[V engine]] or [[flat engine]] has a total of two camshafts (one for each cylinder bank).

Most SOHC engines have 2 valves per cylinder (sometimes 3 or 4), 1 intake valve and one exhaust valve.{{efn-la|However a few engines, such as the 1973 [[Triumph_slant-four_engine#Sprint_16-valve_engine|Triumph Dolomite Sprint engine]] and [[Honda_J_engine | Honda J Series V6 Engine]] were/are a SOHC configuration with four valves per cylinder. This was achieved by the camshaft being located in the center of the cylinder head, with equal length rocker arms actuating the intake and exhaust valves.<ref>{{cite journal|last= Heseltine|first= Richard|title= Triumph Dolomite Sprint|journal= [[Motor Sport (magazine)|Motor Sport]]|date= June 2010|volume= 86|issue= 6|page= 122|editor-last= Roebuck|editor-first= Nigel|issn= 0027-2019|location= London, UK|url= https://www.motorsportmagazine.com/archive/article/june-2010/122/triumph-dolomite-sprint|access-date= 29 March 2015}}</ref> This arrangement was used to provide four valves per cylinder while minimising the valvetrain mass and minimising the overall engine size.<ref>{{Cite journal|last= Lewis|first= Jimmy|title= New for '02: Honda CR250R CRF450R|journal= [[Cycle World]]|page= 62|date= November 2001|volume= 40|issue= 11|editor-last= Edwards|editor-first= David|issn= 0011-4286|publisher= Hachette-Filipacchi Magazines|url= {{Google books|NKRYLxEd6uAC|Cycle World Magazine Jan-Jul 2001|page=RA12-PA62|plainurl=yes}}|access-date=2 January 2015}}</ref><ref>{{Cite web|url= http://powersports.honda.com/experience/articles/090111c0811a9fc2.aspx|title= How It Works: Honda Unicam® Engines|website= www.honda.com|date= 23 January 2013|access-date= 2 January 2015|url-status= dead|archive-url= https://web.archive.org/web/20140222215132/http://powersports.honda.com/experience/articles/090111c0811a9fc2.aspx|archive-date= 22 February 2014}}</ref><ref>{{Cite web |url= http://www.moto123.com/motorcycle-reviews/article,2010-honda-vfr1200a-first-ride.spy?artid=112508|title= 2010 Honda VFR1200A First Ride |website=www.moto123.com |access-date= 2 January 2015|date= 19 October 2009|url-status= live|archive-url= https://web.archive.org/web/20150102212534/http://www.moto123.com/motorcycle-reviews/article,2010-honda-vfr1200a-first-ride.spy?artid=112508|archive-date= 2 January 2015}}</ref>}} Motion of the camshaft is usually transferred to the valves either directly (using a tappet) or indirectly via a [[rocker arm]].<ref name=Hillier/>

=== {{anchor|DOHC|Dual overhead camshaft|Twin cam}} Dual overhead camshaft (DOHC) ===
[[File:DOHC-Zylinderkopf-Schnitt.jpg|thumb|DOHC design (for a V12 engine)]]
A ''dual overhead cam'', ''double overhead cam'', or ''twin-cam'' engine has two camshafts over each bank of the cylinder head,<ref name=Hillier/><ref name=Stoakes/> 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 or flat engine requires four camshafts to function as a DOHC engine, since having 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,<ref>{{cite web |title=Technically Interesting: Ford Indy DOHC V8 |url=https://bringatrailer.com/2018/04/26/technically-interesting-ford-indy-dohc-v8/ |website= Bring a Trailer |access-date=6 September 2022 |date=26 April 2018}}</ref> however technically "quad-cam" would require four camshafts per cylinder bank (i.e. eight camshafts in total), therefore these engines are merely ''dual overhead camshaft'' engines. 

Many DOHC engines have 4 valves per cylinder (sometimes 5, [[Audi]] or [[Volkswagen]] for instance).{{efn-la|However DOHC engines with two valves per cylinder include the [[Alfa Romeo Twin Cam engine]], the [[Jaguar XK6 engine]], the early [[Ford I4 DOHC engine]], 10V [[Volvo Modular engine]] and the [[Lotus-Ford Twin Cam|Lotus Ford Twin Cam engine]].}} The camshaft usually operates the valves directly via a bucket [[tappet]]. 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]]. Another newer benefit of DOHC engine design is the ability to independently change/phase the timing between each camshaft and the crankshaft. This affords better fuel economy by allowing a broader torque curve. Although each major manufacturer has their own trade name for their specific system of variable cam phasing systems, overall they are all classified as [[variable valve timing]].