View source for Overhead camshaft engine

{{Short description|Valvetrain configuration}}
{{Redirect|OHC||Ohc (disambiguation){{!}}Ohc}}
[[File:Head D15A3.JPG|thumb |OHC cylinder head (for a 1987 [[Honda D engine#D15A3|Honda D15A3]] engine)]]

An '''overhead camshaft''' ('''OHC''') '''engine''' is a  [[piston engine]] in which the [[camshaft]] is located in the [[cylinder head]] above the [[combustion chamber]].<ref name=Hillier>{{cite book|last1=Hillier|first1=V.A.W.|title=Fundamentals of Motor Vehicle Technology|type=Academic text-book|edition=6th|volume=Book 1|year=2012|orig-year=First published 1966|publisher=Nelson Thornes Ltd|others=In association with: ([[Institute of the Motor Industry|IMI]])|isbn=9781408515181|chapter=2}}</ref><ref name=Stoakes>{{cite book|last1=Stoakes|first1=Graham|last2=Sykes|first2=Eric|last3=Whittaker|first3=Catherine|title=Principles of Light Vehicle maintenance & repair|series=Heinmann Work-Based Learning|year=2011|publisher=Babcock International Group and Graham Stoakes|isbn=9780435048167|pages=208–209|chapter=3}}</ref> This contrasts with earlier [[overhead valve engines]] (OHV), where the camshaft is located below the combustion chamber in the [[engine block]].<ref>{{Cite news|url=https://www.samarins.com/glossary/dohc.html|title=OHV, OHC, SOHC and DOHC (twin cam) engine - Automotive illustrated glossary |website=www.samarins.com|access-date=2018-09-20|language=en-US}}</ref>

''Single overhead camshaft'' (SOHC) engines have one camshaft per [[cylinder bank|bank of cylinders]]. ''Dual overhead camshaft'' (DOHC, also known as "twin-cam"<ref>[[Harley-Davidson Twin Cam engine]], [[Fiat Twin Cam engine]], [[Alfa Romeo Twin Cam engine]], [[Quad 4 engine]], [[Lotus-Ford Twin Cam]] </ref>) 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]]. 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}} ===
<!-- This section is linked from [[Inline-four engine]] -->
[[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]].

== Components ==
=== Timing belt / timing chain ===
{{Main|Timing belt (camshaft)}}
[[File:Replacing a timing belt.jpg|thumb|upright=0.8 |Rubber timing belt during installation]]
The rotation of a camshaft is driven by a [[crankshaft]]. Many 21st century engines use a toothed ''timing belt'' made from rubber and kevlar to drive the camshaft.<ref name=Hillier/><ref name=dansmc.com>{{cite web|url=http://www.dansmc.com/camchain.htm|title=Dan's motorcycle 'Cam Drives'|website=www.dansmc.com|access-date=29 August 2012}}</ref> Timing belts are inexpensive, produce minimal noise and have no need for lubrication.<ref name="Decker">{{Cite journal|last= Decker|first= John|title= Saturday Mechanic: Replacing Your Timing Belt|journal=[[Popular Mechanics]]|date= June 1993|volume=  170|issue=  6|issn= 0032-4558|url= {{Google books|JOQDAAAAMBAJ|Popular Mechanics Jun 1993|page=93|plainurl=yes}}|access-date=1 March 2015|editor-last= Oldham|editor-first= Joe|publisher= Hearst|location= New York, NY US}}</ref>{{refpage|page=93}} A disadvantage of timing belts is the need for regular replacement of the belt;<ref name="Decker"/>{{refpage|page=94}} recommended belt life typically varies between approximately {{convert|50000|-|100000|km|mi|abbr=on}}.<ref name="Decker"/>{{refpage|pages=94-95}}<ref name="Dorries">{{Cite book|last= Dorries|first= Elisabeth H.|title= TechOne: Automotive Engine Repair |url= {{Google books|KsYFA0skuZ8C|TechOne: Automotive Engine Repair|page=250|plainurl=yes}}|access-date= 1 March 2015|publisher= Thompson Delmar Learning|location= Clifton Park, NY US|year= 2005|isbn= 1-4018-5941-0|lccn= 2004057974}}</ref>{{refpage|page=250}} If the timing belt is not replaced in time and fails and the engine is an [[interference engine]], major engine damage is possible.

The first known automotive application of timing belts to drive overhead camshafts was the 1953 [[Bill_Devin#Devin_Enterprises|Devin-Panhard]] racing specials built for the SCCA H-modified racing series in the United States.<ref name="Pace&Brinker">{{cite book|last1= Pace|first1= Harold W.|last2= Brinker|first2= Mark R.|title= Vintage American Road Racing Cars 1950-1969|year= 2004|page= 62|publisher= MotorBooks International|location= St. Paul MN US|isbn= 0-7603-1783-6|url={{Google books|oZempfWXoe4C|Vintage American Road Racing Cars 1950-1969 |plainurl=yes}}|access-date= 27 February 2015}}</ref>{{refpage|page=62}} These engines were based on Panhard OHV flat-twin engines, which were converted to SOHC engines using components from Norton motorcycle engines.<ref name="Pace&Brinker"/>{{refpage|page=62}} The first production car to use a timing belt was the 1962 [[Glas 1004]] compact coupe.<ref>{{cite book |last=Norbye |first=Jan P. |title=BMW - Bavaria's Driving Machines |year=1984 |publisher=Publications International |location=Skokie, IL |isbn=0-517-42464-9 |chapter=Expanding on Excellence: The 5-Series and 3-Series |page=[https://archive.org/details/bmwbavariasdrivi00norb/page/191 191] |chapter-url-access=registration |chapter-url=https://archive.org/details/bmwbavariasdrivi00norb/page/191 }}</ref>

Another camshaft drive method commonly used on modern engines is a ''timing chain'', constructed from one or two rows of metal [[roller chains]].<ref name=Hillier/><ref name=dansmc.com/> By the early 1960s most production automobile overhead camshaft designs used chains to drive the camshaft(s).<ref name="Boddy">{{cite journal|last=Boddy |first=William |author-link= Bill Boddy |url=https://www.motorsportmagazine.com/archive/article/january-1964/17/random-thoughts-about-ohc|title= Random Thoughts About O.H.C. |journal= [[Motor Sport (magazine)|Motor Sport]]|date= January 1964 |volume= XL|issue= 1|publisher= Teesdale Publishing|location= London, UK}}</ref>{{refpage|page=17}} Timing chains do not usually require replacement at regular intervals, however the disadvantage is that they are noisier than timing belts.<ref name="Dorries"/>{{refpage|page=253}}

===Gear train===
A [[gear train]] system between the crankshaft and the camshaft is commonly used in diesel overhead camshaft engines used in heavy trucks.<ref>{{cite book|last= Bennett|first= Sean|title= Modern Diesel Technology: Diesel Engines|pages= 88–89, 362|publisher= Cengage Learning|location= Stamford, CT US|isbn= 978-1-285-44296-9|url={{Google books|8OzKAgAAQBAJ|Modern Diesel Technology: Diesel Engines|page=88|plainurl=yes}}|access-date= 4 January 2015|quote= In most commercial diesels, OHCs are gear-driven.|date= 2014-01-01}}</ref> Gear trains are not commonly used in engines for light trucks or automobiles.<ref name=Hillier/>

=== Other camshaft drive systems ===
[[File:Norton No66, pic-003.JPG|thumb|Norton motorcycle engine with a [[bevel gear|bevel]] shaft-driven camshaft]]

Several OHC engines up until the 1950s used a shaft with [[bevel gear]]s to drive the camshaft. Examples include the 1908–1911 [[Maudslay Motor Company|Maudslay 25/30]],<ref>{{cite journal|last= Boddy|first= William|author-link= Bill Boddy|title= An Edwardian Overhead-Camshaft 25/30 Maudslay|journal= [[Motor Sport (magazine)|Motor Sport]]|page= 909|url= http://www.motorsportmagazine.com/archive/article/august-1972/68/edwardian-overhead-camshaft-2530-maudslay|access-date= 6 February 2015|date= August 1972|volume= XLVIII|issue= 8|editor-last= Boddy|editor-first= William|publisher= Teesdale Publishing|location= London, UK|url-status= live|archive-url= https://web.archive.org/web/20150206033639/http://www.motorsportmagazine.com/archive/article/august-1972/68/edwardian-overhead-camshaft-2530-maudslay|archive-date= 6 February 2015}}</ref><ref name="Culshaw">{{cite book |last1= Culshaw|first1= David|last2= Horrobin|first2= Peter|year= 2013|orig-year= 1974|chapter= Maudslay|title= The Complete Catalogue of British Cars 1895 - 1975|edition= e-book|location= Poundbury, Dorchester, UK|publisher= Veloce Publishing|page= 210|isbn= 978-1-845845-83-4}}</ref> the [[Bentley 3 Litre]],<ref>{{Cite book|last= Norbye|first= Jan P.|title= The complete handbook of automotive power trains|page= 318|publisher= Tab Books|year= 1981|isbn= 0-8306-2069-9|lccn= 79026958|url= {{Google books|YSpKEyATeM8C|The complete handbook of automotive power trains|page=318|plainurl=yes}}|access-date= 7 January 2015}}</ref> the 1917-? [[Liberty L-12]],<ref>{{Cite web|url=https://books.google.com/books?id=hylFAQAAIAAJ&pg=RA4-PA67|title=The Bulletin of the Airplane Engineering Department, U.S.A.|first=United States Bureau of Aircraft Production Airplane Engineering|last=Department|date=March 4, 1918|publisher=War Department, Bureau of Aircraft Production, Airplane Engineering Department|via=Google Books}}</ref> the 1929-1932 [[MG M-type|MG Midget]], the 1925-1948 [[Velocette#Velocette 'K' series|Velocette K series]],<ref>{{Cite journal|last= Cameron|first= Kevin|author-link= Kevin Cameron (journalist)|title= TDC: Little things|journal= [[Cycle World]]|date= March 2004|volume= 43|issue= 3|issn= 0011-4286|page= 14|url= {{Google books|VYB28ExdkSMC|Cycle World Magazine Jan 2004|page=RA2-PA22|plainurl=yes}}|access-date=7 January 2015}}</ref> the 1931-1957 [[Norton International]] and the 1947-1962 [[Norton Manx]].<ref>{{cite book|last= Wilson|first= Hugo|title= The Encyclopedia of the Motorcycle|url= https://archive.org/details/encyclopediaofmo00wils_1|url-access= registration|year= 1995|publisher= Dorling Kindersley|location= London, UK|isbn= 0-7513-0206-6|page= [https://archive.org/details/encyclopediaofmo00wils_1/page/144 144]|chapter= The A-Z of Motorcycles}}</ref> In more recent times, the 1950-1974 [[Ducati singles|Ducati Single]],<ref>{{cite book|last= Walker|first= Mick|author-link= Mick Walker (motorcycling)|title= Ducati Singles Restoration|chapter= 4 Engine|page= 48|year= 2003|orig-year= 1991|publisher= Motorbooks International|location= St. Paul, MN US|isbn= 0-7603-1734-8|url= {{Google books|JaXQtjcbmC4C|Ducati Singles Restoration|page=48|plainurl=yes}}|access-date= 4 January 2015|chapter-url={{Google books|JaXQtjcbmC4C|Ducati Singles Restoration|page=32|plainurl=yes}}}}</ref> 1973-1980 [[Ducati L-twin engine]], 1999-2007 [[Kawasaki W650]] and 2011-2016 [[Kawasaki W800]] motorcycle engines have used bevel shafts.<ref>{{Cite web|url=http://www.kawasaki.eu/en/products/sports/2015/w800_/overview?Uid=096BDlgLWQ0JWAldClhcXAsMWwoMCVlcXgteXwoLC1BaUQw|title= 2015 W800|website=www.kawasaki.eu |access-date=19 December 2019}}</ref><ref>{{cite news|last= Ash|first= Kevin|author-link= Kevin Ash|title= Kawasaki W800 review|newspaper= [[The Daily Telegraph|The Telegraph]]|date= 26 October 2011|url= https://www.telegraph.co.uk/motoring/motorbikes/8833767/Kawasaki-W800-review.html|url-status= live|archive-url= https://web.archive.org/web/20130621065158/http://www.telegraph.co.uk/motoring/motorbikes/8833767/Kawasaki-W800-review.html|archive-date=21 June 2013}}</ref> The [[Crosley]] 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]] in 1955, Crofton in 1959, [[Homelite]] 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.<ref>{{ cite patent | country = US | number = 1495620 A | status = patent | title = Internal Combustion Engine | pubdate =  | gdate = 1924-05-27 | fdate = 1921-03-15 | pridate =  | inventor =John Godfrey Parry Thomas | assign1 =  | assign2 =  | class = |ref=none }}</ref><ref>{{US patent|1495620}}</ref><ref name="WBMarch74">{{cite journal|title= How Did The Leyland Eight Rate?|last= Boddy|first= William|author-link= Bill Boddy|page= 230|journal= [[Motor Sport (magazine)|Motor Sport]]|volume= L|issue= 3|editor-last= Boddy|editor-first= William|date= March 1974|url= https://www.motorsportmagazine.com/archive/article/march-1974/38/how-did-leyland-eight-rate|access-date= 3 January 2015}}</ref> A similar system was used in the 1926-1930 [[Bentley Speed Six]] and the 1930-1932 [[Bentley 8 Litre]].<ref name="WBMarch74" /><ref>{{cite journal | last1 = Brooks | first1 = Philip C.  | editor1-last =  Carpenter | editor1-first = Rhonda | editor2-last = Iwalani | editor2-first = Kahikina | year = 2009 | title = The Mighty Sixes | journal = The International Club for Rolls-Royce & Bentley Owners Desk Diary 2010 | pages = 27, 32 | location = Tampa, FL USA | publisher = Faircount }}</ref> A two-rod system with counterweights at both ends was used by many models of the 1958-1973 [[NSU Prinz]].<ref name="Boddy"/>{{refpage|page=16-18}}
{{clear right}}

== History ==
=== 1900–1914 ===
Among the first overhead camshaft engines were the 1902 [[Maudslay Motor Company|Maudslay]] SOHC engine built in the United Kingdom<ref name="Culshaw"/>{{refpage|page=210}}<ref name="Boddy"/>{{refpage|page=906}}<ref>{{cite encyclopedia|title= Maudslay|last= Georgano|first= G. N.|author-link= G. N. Georgano|page= [https://archive.org/details/newencyclopediao0000unse_v2r4/page/407 407]|encyclopedia= The New Encyclopedia of Motorcars 1885 to the Present|editor-last= Georgano|editor-first= G. N.|year= 1982|orig-year= 1968|publisher= E. P. Dutton|location= New York|edition= Third|isbn= 0525932542|lccn= 81-71857|ref= none|url= https://archive.org/details/newencyclopediao0000unse_v2r4/page/407}}</ref> and the 1903 [[Marr (automobile)|Marr Auto Car]] SOHC engine built in the United States.<ref name="Marr Auto Car Company">{{cite web |url=http://www.marrautocar.com/Marr_Auto_Car_Company/Welcome.html |website=www.marrautocar.com |title=Marr Auto Car Company |url-status=dead |archive-url=https://web.archive.org/web/20180411094907/http://marrautocar.com/Marr_Auto_Car_Company/Welcome.html |archive-date=11 April 2018}}</ref><ref name=Kimes>{{cite book|last1=Kimes|first1=Beverly Rae|title=Walter L Marr: Buick's Amazing Engineer|year=2007|publisher=Racemaker Press|isbn=978-0976668343|page=40}}</ref> 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.<ref>{{cite web |title=1913 Isotta Fraschini 100-120 hp Tipo KM 4 Four-Seat Torpedo Tourer - Auction Lot |url=http://www.motorbase.com/auctionlot/profile/2008/08/15/316-1913-isotta-fraschini-100-120-hp-tipo-km-4-four-seat-torpedo-tourer/ |website=www.motorbase.com |access-date=29 December 2019}}</ref>

=== World War I ===
[[File:Napier Lion cambox.jpg|thumb|DOHC cylinder head of a 1917–1930 [[Napier Lion]] aircraft engine]]

During World War I, both the [[Allies of World War I|Allied]] and [[Central Powers]]; specifically those of the [[German Empire]]'s ''[[Luftstreitkräfte]]'' 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]] V12 engine. Other SOHC designs included the Spanish [[Hispano-Suiza 8]] V8 engine (with a fully enclosed-drivetrain), the American [[Liberty L-12]] V12 engine, which closely followed the later Mercedes D.IIIa design's partly-exposed SOHC valvetrain design; and the [[Max Friz]]-designed; German [[BMW IIIa]] straight-six engine. The DOHC [[Napier Lion]] 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".<ref>{{Cite book|last= Thorpe|first= Leslie Aaron|title= A text book on aviation: the new cadet system of ground school training|volume= 3|pages= 14–15|year= 1936|publisher= Aviation Press|url= {{Google books|aVluAAAAIAAJ|A text book on aviation: the new cadet system of ground school training, Volume 3|page=14|plainurl=yes}}|access-date=27 February 2015|quote= The overhead camshafts are driven by bevel gears and vertical shafts known as tower shafts.}}</ref>

<gallery mode="packed" heights="180px" style="text-align:left">
Hispano Suiza 8 A Brussel.jpg |1914–1918 [[Hispano-Suiza 8|Hispano-Suiza 8A]] SOHC aircraft engine
Hispano-suiza-V8 220PS.jpg |1914–1918 [[Hispano-Suiza 8|Hispano-Suiza 8Be]] 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]] and the Allied [[Liberty L-12]] engines
Liberty L-12-2.jpg |Detail closeup of a [[Liberty L-12]]'s upper valvetrain, showing the similarity to the later-production Mercedes design 
</gallery>

=== 1919–1944 ===
[[File:1933 Bugatti Type 59 Grand Prix engine.jpg|right|thumb |1933 [[Bugatti Type 59]] straight-eight grand prix racing engine ]]
An early American overhead camshaft production engine was the SOHC straight-eight engine used in the 1921–1926 [[Duesenberg Model A]] luxury car.<ref>{{cite book| last = Mueller| first = Mike| year = 2006| chapter = Chapter 6 - Chariot of the Gods Duesenberg Straight Eight| chapter-url ={{Google books|69qYnoohrMYC|American Horsepower: 100 Years of Great Car Engines|page= 49|plainurl= yes}}| title  = American Horsepower 100 Years of Great Car Engines| url ={{Google books|69qYnoohrMYC|American Horsepower: 100 Years of Great Car Engines|page=51|plainurl= yes}}| location = St. Paul, MN USA| publisher = Motorbooks| pages = 51| isbn = 978-0-7603-2327-4| lccn = 2006017040| access-date =2015-02-02}}</ref>

In 1926, the [[Sunbeam 3 litre Super Sports]] became the first production car to use a DOHC engine.<ref name=Autocar197710>{{cite journal| journal = [[Autocar (magazine)|Autocar]] |volume =  147 (nbr 4221)|title =Talking of sports cars: Sunbeam three-litre| pages = 69–71| date = 1 October 1977}}</ref><ref>{{cite book |last1=Georgano |first1=G.N. |title=Cars: Early and Vintage, 1886-1930 |date=1985 |publisher=Grange-Universal |location=London}}</ref> 

In the United States, Duesenberg added DOHC engines (alongside their existing SOHC engines) with the 1928 release of the [[Duesenberg Model J]], which was powered by a DOHC straight-eight engine. The 1931–1935 [[Stutz_Motor_Company#Stutz_Motor_Car_Company_of_America,_Inc.|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 [[List_of_Wolseley_automobiles#Inter-war_cars_1920_to_1940|Wolseley Ten]], the 1928-1931 [[MG 18/80]], the 1926–1935 [[Singer_Motors#Models|Singer Junior]] and the 1928–1929 [[Alfa_Romeo_6C#6C_1500_(1927–1929)|Alfa Romeo 6C Sport]]. Early overhead camshaft motorcycles included the 1925–1949 [[Velocette#Velocette_'K'_series|Velocette K Series]] and the 1927–1939 [[Norton CS1]].

=== 1945–present ===
[[File:1948 Crosley COBRA engine OHC.jpg|right|thumb|upright=0.8 |1946–1949 [[Crosley#Crosley_CoBra_(1946–1949)|Crosley CoBra]] SOHC engine]]

The 1946–1948 [[Crosley#Crosley_CC_Four_(1946–1948)|Crosley CC Four]] was arguably the first American mass-produced car to use an SOHC engine.<ref name="Simanaitis">{{Cite journal|last=Simanaitis|first=Dennis|title= Tech Tidbits|journal= [[Road & Track]]|date= January 1994|page= 121|editor-last= Bryant|editor-first= Thos L.|publisher= Hachette Filipacchi Magazines|location= Newport Beach, CA US|volume= 45|issue= 6|issn= 0035-7189}}</ref><ref>{{cite web |url=https://crosleyautoclub.com/EngineTree/Crosley_Eng_Tree.html |title=Crosley Engine Family Tree - Taylor Years |website=www.crosleyautoclub.com |access-date=19 December 2019 }}</ref><ref>{{cite web |url=https://crosleyautoclub.com/EngineTree/Crosley_Eng_Tree-1.html|title=Crosley Engine Family Tree - CoBra Years |website=www.crosleyautoclub.com |access-date=19 December 2019 }}</ref> This small mass-production engine powered the winner of the 1950 [[12 Hours of Sebring#Overall winners|12 Hours of Sebring]].<ref name="Simanaitis"/>{{refpage|page=121}}

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-6 engine|Lagonda straight-six engine]], the 1949–1992 [[Jaguar XK engine|Jaguar XK]] straight-six engine and the 1954–1994 [[Alfa Romeo Twin Cam engine|Alfa Romeo Twin Cam]] inline-four engine.<ref name=EuropeanCarWeb.com>{{cite web |url=http://www.europeancarweb.com/features/0209ec_twin_cam_analysis/index.html|title=An Echo of the Past — the history and evolution of twin-cam engines|date=February 2009|access-date=29 August 2012 |website=www.EuropeanCarWeb.com|publisher=European Car Magazine, Source Interlink Media|url-status=dead|archive-url=https://web.archive.org/web/20120303061544/http://www.europeancarweb.com/features/0209ec_twin_cam_analysis/index.html|archive-date=3 March 2012}}</ref><ref>{{Cite web|url= http://www.alfisti.co.uk/boxerpage.htm|title= Technical- Boxer History|website= www.alfisti.co.uk|url-status= live|archive-url= https://web.archive.org/web/20130329040043/http://www.alfisti.co.uk/boxerpage.htm|archive-date= 29 March 2013}}</ref> The 1966-2000 [[Fiat Twin Cam engine|Fiat Twin Cam]] inline-four engine was one of the first DOHC engines to use a toothed timing belt instead of a timing chain.<ref>{{cite web|title=Old Fiat ad with Aurelio Lampredi|url=https://i.kinja-img.com/gawker-media/image/upload/19f9mk0fozzvppng.png |website=www.kinja-img.com |access-date=31 January 2015}}</ref>

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.<ref name=EuropeanCarWeb.com/> By the mid-2000s, most automotive engines used a DOHC layout.{{citation needed|date=December 2020}}

== See also ==
* [[Cam-in-block]]
* [[Camless]]
* [[Overhead valve engine]]
* [[Variable valve timing]]
{{clear right}}

== Footnotes ==
{{reflist|group=lower-alpha}}

== References ==
{{Reflist|30em}}

{{Internal combustion engine}}
{{DEFAULTSORT:Overhead Camshaft}}
[[Category:Engine valvetrain configurations]]