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{{short description|Component of a cylinder of an internal combustion engine}}
{{multiple image
{{one source|date=November 2020}}
| align = right
[[Image:Cylinderhead.JPG|thumb|A 302/5.0L [[Ford Windsor engine|Ford Windsor V8]] cylinder head]]
| direction = vertical
[[File:Cylinder head of a Caterpillar engine.jpg|alt=Cylinder head of a Caterpillar engine|thumb|Cylinder head of a Caterpillar engine]]
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In an [[internal combustion engine]], the '''cylinder head''' (often informally abbreviated to just '''head''') sits above the [[cylinder (engine)|cylinder]]s on top of the [[cylinder block]].<ref name="Wright 2015 p. 310">{{cite book | last=Wright | first=G. | title=Fundamentals of Medium/Heavy Duty Diesel Engines | publisher=Jones & Bartlett Learning | year=2015 | isbn=978-1-284-06705-7 | url=https://books.google.com/books?id=s5_OCwAAQBAJ&pg=PA310 | access-date=2020-11-07 | page=310}}</ref> It closes in the top of the cylinder, forming the [[combustion chamber]]. This joint is sealed by a [[head gasket]]. In most engines, the head also provides space for the passages that feed [[Atmosphere of Earth|air]] and [[fuel]] to the cylinder, and that allow the [[Exhaust gas|exhaust]] to escape. The head can also be a place to mount the [[poppet valve|valves]], [[Spark plug|spark plugs]], and [[Fuel injection|fuel injectors]].▼
| image1 = K20 head.jpg
| caption1 = Side view of a [[Dual overhead camshaft|DOHC]] cylinder head (with the valves and camshafts installed)
| image2 = Cylinderhead.JPG
| caption2 = Underside of a [[Overhead valve engine|OHV]] cylinder head (with the valves installed)
}}
 
▲In ana [[internalReciprocating combustionengine|piston engine]], the '''cylinder head''' (often informally abbreviated to just '''head''') sits above the [[cylinder (engine)|cylindercylinders]]s on top of the [[cylinder block]].,<ref name="Wright 2015 p. 310">{{cite book | last=Wright | first=G. | title=Fundamentals of Medium/Heavy Duty Diesel Engines | publisher=Jones & Bartlett Learning | year=2015 | isbn=978-1-284-06705-7 | url=https://books.google.com/books?id=s5_OCwAAQBAJ&pg=PA310 | access-date=2020-11-07 | page=310}}</ref> It closes informing the toproof of the cylinder, forming the [[combustion chamber]]. This joint is sealed by aIn [[headsidevalve gasketengine]]. In most engines,s the head alsois providesa spacesimple forplate theof passagesmetal thatcontaining feedthe [[Atmosphere ofspark Earth|airplug]]s and possibly [[fuelAir-cooled engine|heat dissipation fins]]. toIn themore cylinder,modern and[[overhead thatvalve allowengine|overhead thevalve]] and [[Exhaustoverhead gascamshaft engine|exhaustoverhead camshaft]] toengines, escape. Thethe head canis alsoa bemore acomplicated placemetal toblock that also mountcontains the inlet and exhaust passages, and often [[poppetRadiator valve(engine cooling)|valvescoolant passages]], [[Spark plug|spark plugsValvetrain]] components, and [[FuelFuel_injection#Direct injection systems|fuel injectors]].
== Sidevalve engines ==▼
In a [[flathead engine|flathead]] or [[sidevalve engine|sidevalve]] engine, the mechanical parts of the [[Valvetrain|valve train]] are all contained within the block, and a 'poultice head' may be used that is essentially a simple metal plate bolted to the top of the block.
 
== Number of cylinder heads ==
Keeping all moving parts [[cam-in-block|within the block]] has an advantage for physically large engines in that the camshaft drive gear is small and so suffers less from the effects of thermal expansion in the cylinder block. With a chain drive to an overhead camshaft, the extra length of chain needed for an overhead cam design could give trouble from wear and slop in the chain without frequent maintenance.
A piston engine typically has one cylinder head per [[Engine_configuration#Multiple_cylinder_banks|bank of cylinders]].
 
Most modern engines with a [[straight engine|"straight" (inline) layout]] today use a single cylinder head that serves all the cylinders.
Early sidevalve engines were in use at a time of simple fuel chemistry, low [[octane rating]]s and so required low [[compression ratio]]s. This made their [[combustion chamber]] design less critical and there was less need to design their ports and airflow carefully.
 
Engines with a [[V engine|"V" layout]] or [[flat engine|"flat" layout]] typically use two cylinder heads (one for each [[cylinder bank]]), however a small number of 'narrow-angle' V engines (such as the Volkswagen [[VR5 engine|VR5]] and [[VR6 engine|VR6]] engines) use a single cylinder head spanning the two banks.
One difficulty experienced at this time was that the low compression ratio also implied a low expansion ratio during the power stroke.{{efn|The [[work (physics)|work]] done on the piston during the expansion stroke is limited by how much stroke there is in which to achieve it.}} Exhaust gases were thus still hot, hotter than a contemporary engine, and this led to frequent trouble with burnt exhaust valves.
 
Most [[radial engine]]s have one head for each cylinder, although this is usually of the [[monobloc engine#Cylinder head|monobloc]] form wherein the head is made as an integral part of the cylinder. This is also common for motorcycles, and such head/cylinder components are referred to as ''barrels''.
A major improvement to the sidevalve engine was the advent of [[Harry Ricardo|Ricardo]]'s [[Ricardo turbulent head|turbulent head]] design. This reduced the space within the combustion chamber and the ports, but by careful thought about the airflow paths within them it allowed a more efficient flow in and out of the chamber. Most importantly, it used turbulence within the chamber to thoroughly mix the fuel and air mixture. This, of itself, allowed the use of higher compression ratios and more efficient engine operation.
 
Some engines, particularly medium- and large-capacity [[diesel engine]]s built for industrial, marine, power generation, and heavy traction purposes (large [[truck]]strucks, [[locomotive]]s, [[heavy equipment]], etc.) have individual cylinder heads for each cylinder. This reduces repair costs as a single failed head on a single cylinder can be changed instead of a larger, much more expensive unit fitting all the cylinders. Such a design also allows engine manufacturers to easily produce a 'family' of engines of different layouts and/or cylinder numbers without requiring new cylinder head designs. ▼
The limit on sidevalve performance is ''not'' the gas flow through the valves, but rather the shape of the combustion chamber. With high speed engines and high compression, the limiting difficulty becomes that of achieving complete and efficient combustion, whilst also avoiding the problems of unwanted pre-detonation. The shape of a sidevalve combustion chamber, being inevitably wider than the cylinder to reach the valve ports, conflicts with achieving both an ideal shape for combustion{{efn|At the simplest level, a sphere approaches the ideal shape for combustion as it has the shortest paths across which to propagate the flame front. As one wall of this is the moving piston, hemispheres are more commonly chosen}} and also the small volume (and low height) needed for high compression. Modern, efficient engines thus tend towards the pent roof or hemi designs, where the valves are brought close into the center of the space.
 
== NotesDesign ==▼
Where fuel quality is low and octane rating is poor, compression ratios will be restricted. In these cases, the sidevalve engine still has much to offer. Particularly in the case of the developed [[IOE engine]] for a market with poor fuels, engines such as [[Rolls-Royce B series engine|Rolls-Royce B series]] or the [[IOE engine#Rover IOE engines|Land-Rover]] use a complicated arrangement of inclined valves, a cylinder head line at an angle to the bore and corresponding angled pistons to provide a compact combustion chamber approaching the near-hemispherical ideal. Such engines remained in production into the 1990s, only being finally replaced when the fuels available 'in the field' became more likely to be diesel than petrol.
{{refimprove|section|date=March 2022}}
 
==Engine/valvetrain configurations==
==Detail==
Internally, the cylinder head has passages called [[cylinder head porting|ports or tracts]] for the fuel/air mixture to travel to the inlet valves from the intake [[Inlet manifold|manifold]], and for [[exhaust gas]]es to travel from the exhaust valves to the exhaust manifold. In a [[Internal combustion engine cooling|water-cooled]] engine, the cylinder head also contains integral ducts and passages for the engines' coolant—usually a mixture of water and [[antifreeze]]—to facilitate the transfer of excess heat away from the head, and therefore the engine in general.
 
{| class="wikitable" style="text-align:centerleft"▼
In the overhead valve (OHV) design, the cylinder head contains the [[poppet valve]]s and the spark plugs, along with tracts or 'ports' for the inlet and exhaust gases. The operation of the valves is initiated by the engine's [[camshaft]], which is sited within the cylinder block, and its moment of operation is transmitted to the valves' [[pushrod]]s, and then [[rocker arm]]s mounted on a rocker shaft—the rocker arms and shaft also being located within the cylinder head.
 
In the overhead camshaft (OHC) design, the cylinder head contains the valves, spark plugs and inlet/exhaust tracts just like the OHV engine, but the camshaft is now also contained within the cylinder head. The camshaft may be seated centrally between each offset row of inlet and exhaust valves, and still also utilizing rocker arms (but without any pushrods), or the camshaft may be seated directly above the valves eliminating the rocker arms and utilizing 'bucket' [[Tappet#Overhead cam engines|tappets]].
 
==Implementation==
The number of cylinder heads in an engine is a function of the [[engine configuration]]. Almost all [[straight engine|inline (straight) engines]] today use a single cylinder head that serves all the cylinders. A [[V engine|V (or Vee)]] engine has two cylinder heads, one for each [[cylinder bank]] of the 'V'. For a few compact 'narrow-angle' V engines, such as the [[Volkswagen Group|Volkswagen]] [[VR6 engine|VR6]], the angle between the cylinder banks is so narrow that it uses a single head spanning the two banks. A [[flat engine]] (basically a V engine, where the angle between the cylinder banks is now 180°) has two heads. Most [[radial engine]]s have one head for each cylinder, although this is usually of the [[monobloc engine#Cylinder head|monobloc]] form wherein the head is made as an integral part of the cylinder. This is also common for motorcycles, and such head/cylinder components are referred to as ''barrels''.
 
▲Some engines, particularly medium- and large-capacity [[diesel engine]]s built for industrial, marine, power generation, and heavy traction purposes (large [[truck]]s, [[locomotive]]s, [[heavy equipment]], etc.) have individual cylinder heads for each cylinder. This reduces repair costs as a single failed head on a single cylinder can be changed instead of a larger, much more expensive unit fitting all the cylinders. Such a design also allows engine manufacturers to easily produce a 'family' of engines of different layouts and/or cylinder numbers without requiring new cylinder head designs.
 
The design of the cylinder head is key to the performance and efficiency of the internal combustion engine, as the shape of the combustion chamber, inlet passages and ports (and to a lesser extent the exhaust) determines a major portion of the [[volumetric efficiency]] and [[compression ratio]] of the engine.
 
▲{| class="wikitable" style="text-align:center"
|+ Automotive 4-Stroke Engine Head Designs—Valve and Camshaft Configurations
|-
! Engine type <br/> (in chronological order)!! [[Camshaft]] <br/> location !! Intake [[Poppet_valve#Usage_in_internal_combustion_engines|valves]] <br/> location !! Exhaust [[Poppet_valve#Usage_in_internal_combustion_engines|valves]] <br/> location
! Common Names !! Camshaft !! Intake Valves !! Exhaust Valves !! Notes
|-
| '''[[Sidevalve engine|Sidevalve]]<br/>(flathead)''' || Block || Block || Block
| '''Double Overhead Camshaft'''<br>DOHC, Twin-Cam, Cammer|| Head || Head || Head || Allows optimum positioning of the valves for a [[crossflow cylinder head]].<br>Double camshafts are used to allow direct actuation of well-placed valves, without rockers.<br>Widespread in modern car design
|-
| '''[[IOE engine|Inlet-Over-Exhaust over exhaust]]''' <br/> (IOE, F-Head,)''' Intake-Over-Exhaust|| Block || Head || Block || Always uncommon, obsolete for decades▼
| '''Single Overhead Camshaft'''<br>OHC, SOHC, Single-Cam, "Single-Jingle", Cammer|| Head || Head || Head || Widely used for cars in recent decades, but increasingly superseded by DOHC.<br>Sometimes utilizes rocker arms to actuate some valves, other times does not
|-
| '''[[Overhead valve engine|Overhead valve]] <br/> (OHV)''' || Block || Head || Head
| '''Overhead Valve''' <br>OHV, I-Head, Pushrod, Cam-In-Block|| Block || Head || Head || Still used in some large-displacement V8 engines, usually of American or British origin<br>Needs pushrods and rocker arms to actuate valves
|-
| '''[[Overhead camshaft engine|Overhead camshaft]] <br/> (OHC)''' || Head || Head || Head
| '''Sidevalve''' <br>Flathead, L-Head, T-Head|| Block || Block || Block || Once universal, now obsolete<br>Simplest possible configuration<br>Cams operate directly on the valves
|-
▲| '''[[IOE engine|Inlet-Over-Exhaust]]''' <br>IOE, F-Head, Intake-Over-Exhaust|| Block || Head || Block || Always uncommon, obsolete for decades
|}
 
▲=== Sidevalve engines ===
In a [[flathead engine|flathead]] (''sidevalve'') engine, all of the [[valvetrain]] components are [[cam-in-block|contained within the block]], therefore the head is usually a simple plate of metal bolted to the top of the [[engine block]]. Sidevalve engines were once universal but are now largely obsolete in automobiles, found almost exclusively in small engines such as lawnmowers, weed trimmers and chainsaws.
 
===Intake over exhaust (IOE) engines===
[[IOE engine|Intake Over Exhaust]] (IOE) engines combined elements of the sidevalve and overhead valve designs. Used extensively in American motorcycles in the early 1900s, the IOE engine remained in production in limited numbers until the 1990s. IOE engines are more efficient than sidevalve engines, but also more complex, larger and more expensive to manufacture.
 
=== Overhead engines (OHV & OHC)===
In an [[overhead valve engine|overhead valve]] (OHV) or [[overhead camshaft engine|overhead camshaft]] (OHC) engine, the cylinder head contains several airflow passages called ''ports''; intake ports deliver the fuel+air intake charge from the [[intake manifold]] to the [[combustion chamber]], and exhaust ports route combustion waste gases out the combustion chamber to the [[exhaust manifold]]. [[poppet valve|Valves]] open and close the ports, with the intakes offset fore-and-aft from the exhausts. The head also contains the [[spark plug]]s, and on [[Internal combustion engine cooling#Liquid cooling|water-cooled]] engines, the coolant passages.
 
==== Overhead valve (OHV) engines ====
A single [[camshaft]] located in the engine block uses [[pushrod]]s and [[rocker arm]]s to actuate all the valves.
 
OHV engines are typically more compact than equivalent OHC engines, and fewer parts mean cheaper production, but they have largely been replaced by OHC designs, except in some American V8 engines.
 
====Overhead camshaft (OHC) engines ====
An [[Overhead camshaft engine|overhead camshaft]] (OHC) engine locates the camshaft(s) in the cylinder head above the combustion chamber. Eliminating pushrods lessens valvetrain inertia and provides space for optimized port designs, both providing increased power potential.
 
In a [[Overhead camshaft engine#Single_overhead_camshaft_(SOHC)|single overhead camshaft]] (SOHC) engine, the camshaft may be seated centrally between valve rows, or directly above a single row of valves (replacing rocker arm actuation with [[Tappet#Overhead cam engines|tappets]]). SOHC engines were widely from the 1960s to 1990s. (eliminating pushrods but still utilizing rocker arms)
 
[[Overhead camshaft engine#Double_overhead_camshaft_(DOHC)|Double overhead camshaft]] (DOHC) engines seat a camshaft directly above each row of offset valves (intakes inboard, exhausts outboard).<ref>{{cite web |title=FORD DuraTec Engine 3D Simulation(18) - Dailymotion Video |url=https://www.dailymotion.com/video/xaafuy_ford-duratec-engine-3d-simulation-1_tech |website=www.dailymotion.com |access-date=27 March 2022 |language=en |date=27 August 2009}}</ref> DOHC designs allow optimal [[crossflow cylinder head|crossflow]] positioning of valves to provide higher-[[RPM]] operation. They are typically larger in size (especially width) than equivalent OHV or SOHC engines. Even though more components raise production costs, DOHC engines seen widespread use in automobile engines since the 1990s.
 
==Gallery==
<gallery mode="packed" heights="200px" perrow="3" style="text-align:left">
<gallery>
Image:DOHC-Zylinderkopf-Schnitt.jpg |A[[Double cylinderoverhead camshaft|DOHC]] head- sliced in half showing the intake and exhaust valves, intake and exhaust ports, coolant passages, cams, tappets and valvecutaway springs.view
Image:Head D15A3.JPG|A [[overhead camshaft#Single overhead camshaft|single overhead camshaft]] (SOHC) cylinder head from a [[Honda|SOHC]] [[Honda D engine#D15A3|Honda D15A3]]. engine
Image:Malossi 70cc Morini cylinder head.jpg|[[Flathead engine|Flathead]] head for a [[Moto Morini]] scooter (bottom top piece on left, top piece on right)
Image:K20 head.jpg|A [[overhead camshaft#Double overhead camshaft|double overhead camshaft]] (DOHC) cylinder head from a Honda [[Honda K engine#K20Z3|K20Z3]].
Image:Suzuki-GS550-DOHC.jpg|[[Double overhead camshaft|DOHC]] head for an air-cooled [[Suzuki GS series|Suzuki GS550]] engine
Image:Malossi 70cc Morini cylinder head.jpg|The bottom (left) and top (right) of a Malossi cylinder head for single-cylinder, [[two-stroke]] scooters. Hole in the middle for the spark plug, four holes for the cylinder bolt posts.
Image:SuzukiCylinder-GS550-DOHChead.jpg|[[Overhead view of an [[air-cooledvalve engine|air-cooledOHV]] cylinder head fromfor a [[Suzuki]]GMC [[Suzuki(General GSMotors seriesdivision)|GS550GMC]] showingvan dual(view camshaftsof underside, drivewith sprocketsvalves and coolingexhaust fins.manifold also visible)
Image:Cylinder-head.jpg|The cylinder head from a [[GMC (General Motors division)|GMC]] [[van]]. The [[poppet valve|valves]] and part of the [[exhaust manifold]] are visible.
</gallery>
 
== See also ==
* {{Commons category-inline | 1=Cylinder heads}}▼
* [[Crossflow cylinder head]]
* [[Reverse-flow cylinder head]]
* [[Head gasket]]
* [[Junk head]]
* [[Monobloc head]]
* [[Flathead engine]]
 
* [[T-head engine]]
▲== Notes ==
{{notelist}}
 
==References==
{{reflist}}
 
==External links==
{{Portal|Cars}}
▲* {{Commons category-inline | 1=Cylinder heads}}
* [http://www.dailymotion.com/video/xaafuy_ford-duratec-engine-3d-simulation-1_tech Assembly of Ford Duratec Engine 3D simulation]—video showing construction and operation of a four-cylinder internal combustion engine.
 
{{Internal combustion engine}}
{{Aircraft piston engine components}}
 
[[Category:Cylinder head| ]]
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