In an

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...

, the cylinder head (often abbreviated to simply "head") sits above the

cylinders A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infini ...

and forms the roof of the

combustion chamber A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process. Intern ...

. In sidevalve engines, the head is a simple sheet of metal; whereas in more modern

overhead valve An overhead valve (OHV) engine, sometimes called a ''pushrod engine'', is a piston engine whose valves are located in the cylinder head above the combustion chamber. This contrasts with earlier flathead engines, where the valves were located be ...

and

overhead camshaft An overhead camshaft (OHC) engine is a piston engine where the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion ...

engines, the cylinder head is a more complicated block often containing inlet and exhaust passages, coolant passages, valves,

camshaft A camshaft is a shaft that contains a row of pointed cams, in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves), mechanically controlled ignition systems ...

spark plug A spark plug (sometimes, in British English, a sparking plug, and, colloquially, a plug) is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air ...

s and fuel injectors. Most straight engines have a single cylinder head shared by all of the cylinders and most

V engine A V engine, sometimes called a Vee engine, is a common configuration for internal combustion engines. It consists of two cylinder banks—usually with the same number of cylinders in each bank—connected to a common crankshaft. These cylinder ...

s have two cylinder heads (one per bank of cylinders).

Design

A summary of engine designs is shown below, in chronological order for automobile usage.

Sidevalve engines

In a flathead (''sidevalve'') engine, all of the

valvetrain A valvetrain or valve train is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) ...

components are contained within the block, therefore the head is usually a simple sheet of metal bolted to the top of the

engine block In an internal combustion engine, the engine block is the structure which contains the cylinders and other components. In an early automotive engine, the engine block consisted of just the cylinder block, to which a separate crankcase was attac ...

. Sidevalve engines were once universal in automobiles but are now largely obsolete in automobiles, aside from small engines such as lawnmowers, weed trimmers and chainsaws. A later development called the intake over exhaust (IOE) engine, which 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 valve & overhead camshaft engines

In an

(OHV) or

(OHC) engine, a cylinder head consists of several passages (called ''ports''); some of which form the path for intake gasses from the intake manifold to the

, and the others are for exhaust gases to travel from combustion chamber to the exhaust manifold. The cylinder head also contains the valves and the

s. Specifically in an OHV engine, a single

is located within the

and uses

pushrod A valvetrain or valve train is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) ...

s and

rocker arm In the context of an internal combustion engine, a rocker arm is a valvetrain component that typically transfers the motion of a pushrod to the corresponding intake/exhaust valve. Rocker arms in automobiles are typically made from stamped steel, ...

s to actuate valves. OHV engines are typically more compact than equivalent DOHC engines, however they have largely been replaced by DOHC designs, except in some American V8 engines. In an

OHC design, the cylinder head contains the valves, spark plugs and inlet/exhaust tracts (as per an OHV engine), but the camshaft is now located in the cylinder head instead of the engine block. 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' tappets. OHC engines with a single camshaft per cylinder bank were widely used in automobiles in the 1960s to 1990s, with most designs using a rocker arm to actuate the valves on the opposite side of the engine to the camshaft. OHC engines with dual camshafts per cylinder bank (''DOHC'' engines) have become widespread in modern automobile engines since the 1990s. DOHC engines allow optimum positioning of the valves for a

crossflow cylinder head A crossflow cylinder head is a cylinder head that features the intake and exhaust ports on opposite sides. The gases can be thought to flow across the head. This is in contrast to reverse-flow cylinder head designs that have the ports on the sam ...

and direct actuation of valves (i.e. without rockers). They therefore generally allow for higher-RPM operations, however they are typically larger in size (especially width) than equivalent OHV or SOHC engines. For water-cooled OHV and OHC engines, the cylinder head also contains passages for the engine's coolant fluid, which is used to transfer heat away from the cylinder head.

Number of cylinder heads in an engine

Most modern engines with a "straight" (inline) layout today use a single cylinder head that serves all the cylinders. Engines with a "V" layout or "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 and VR6 engines use a single cylinder head spanning the two banks. Most

radial engine The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders "radiate" outward from a central crankcase like the spokes of a wheel. It resembles a stylized star when viewed from the front, and is ...

s have one head for each cylinder, although this is usually of the 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 The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a so-ca ...

s built for industrial, marine, power generation, and heavy traction purposes (large

truck A truck or lorry is a motor vehicle designed to transport cargo, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame constructi ...

s, locomotives,

heavy equipment Heavy equipment or heavy machinery refers to heavy-duty vehicles specially designed to execute construction tasks, most frequently involving earthwork operations or other large construction tasks. ''Heavy equipment'' usually comprises five e ...

, 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.

Gallery

Image:DOHC-Zylinderkopf-Schnitt.jpg ,

DOHC An overhead camshaft (OHC) engine is a piston engine where the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion ch ...

head- cutaway view Image:Head D15A3.JPG,

SOHC An overhead camshaft (OHC) engine is a piston engine where the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion ch ...

Honda D15A3 engine Image:Malossi 70cc Morini cylinder head.jpg, Flathead head for a Moto Morini scooter (bottom top piece on left, top piece on right) Image:Suzuki-GS550-DOHC.jpg,

head for an air-cooled Suzuki GS550 engine Image:Cylinder-head.jpg, OHV head for a GMC van (view of underside, with valves and exhaust manifold also visible)

References