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In an internal combustion engine, the cylinder head (often informally abbreviated to just head) sits above the cylinders on top of the cylinder block.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 air and fuel to the cylinder, and that allow the exhaust to escape. The head can also be a place to mount the valves, spark plugs, and fuel injectors.
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A summary of engine designs is shown below, in chronological order for automobile usage.
|Intake valves |
|Exhaust valves |
| Sidevalve |
| Inlet over exhaust |
| Overhead valve |
| Overhead camshaft |
In a flathead (sidevalve) engine, all of the valvetrain components are contained within the block, therefore the head is usually a simple sheet of metal bolted to the top of the engine block. 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.
In an overhead valve (OHV) or overhead camshaft (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 combustion chamber, 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 spark plugs.
Specifically in an OHV engine, a single camshaft is located within the engine block and uses pushrods and rocker arms 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 OHV engines are typically more compact than equivalent DOHC engines, however they have largely been replaced by DOHC designs, except in some American V8 engines.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 overhead camshaft 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 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.
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 engines 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 engines built for industrial, marine, power generation, and heavy traction purposes (large trucks, locomotives, 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.
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A poppet valve is a valve typically used to control the timing and quantity of gas or vapor flow into an engine.
The camshaft is a rotating object— usually made of metal— that contains pointed cams, which converts rotational motion to reciprocal motion. Camshafts are used in internal combustion engines, mechanically controlled ignition systems and early electric motor speed controllers. Camshafts in automobiles are made from steel or cast iron, and are a key factor in determining the RPM range of an engine's power band.
The engine configuration describes the fundamental operating principles by which internal combustion engines are categorized.
A hemispherical combustion chamber is a type of combustion chamber in a reciprocating internal combustion engine with a domed cylinder head in the approximate shape of a hemisphere. An engine featuring this type of hemispherical chamber is known as a hemi engine.
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 same side.
VTEC is a system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine, resulting in higher performance at high RPM, and lower fuel consumption at low RPM. The VTEC system uses two camshaft profiles and hydraulically selects between profiles. It was invented by Honda engineer Ikuo Kajitani. It is distinctly different from standard VVT systems which change only the valve timings and do not change the camshaft profile or valve lift in any way.
In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and camless systems. Increasingly strict emissions regulations are causing many automotive manufacturers to use VVT systems.
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 chamber in the engine block.
An overhead valve (OHV) 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 below the combustion chamber in the engine block.
In automotive engineering a multi-valve or multivalve engine is one where each cylinder has more than two valves. A multi-valve engine has better breathing and may be able to operate at higher revolutions per minute (RPM) than a two-valve engine, delivering more power.
The Pontiac straight-6 engine is a family of inline-six cylinder automobile engines produced by the Pontiac Division of General Motors Corporation in numerous versions beginning in 1926.
A flathead engine, also known as a sidevalve engine or valve-in-block engine is an internal combustion engine with its poppet valves contained within the engine block, instead of in the cylinder head, as in an overhead valve engine.
The BMC C-Series was a straight-6 automobile engine produced from 1954 to 1971. Unlike the Austin-designed A-Series and B-Series engines, it came from the Morris Engines drawing office in Coventry and therefore differed significantly in its layout and design from the two other designs which were closely related. This was due to the C-Series being in essence an enlarged overhead valve development of the earlier 2.2 L Straight-6 overhead camshaft engine used in the post-war Morris Six MS and Wolseley 6/80 from 1948 until 1954, which itself also formed the basis of a related 1.5 L 4-cylinder engine for the Morris Oxford MO in side-valve form and the Wolseley 4/50 in overhead camshaft form. Displacement was 2.6 to 2.9 L with an undersquare stroke of 88.9 mm (3.50 in), bored out to increase capacity.
A tappet is most commonly a component in an internal combustion engine which converts the rotating motion of the camshaft into linear motion of the valves, either directly or indirectly.
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 into the combustion chamber, while the exhaust valves control the flow of spent exhaust gasses out of the combustion chamber once combustion is completed.
The cam-in-block valvetrain layout of piston engines is one where the camshaft is placed within the cylinder block, usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine. This contrasts with an overhead camshaft (OHC) design which places the camshafts within the cylinder head and drives the valves directly or through short rocker arms.
The intake/inlet over exhaust, or "IOE" engine, known in the US as F-head, is a four-stroke internal combustion engine whose valvetrain comprises OHV inlet valves within the cylinder head and exhaust side-valves within the engine block.
The Mitsubishi 4A9 engine is the newest family range of all-alloy inline four-cylinder engines from Mitsubishi Motors, introduced in the 2004 version of their Mitsubishi Colt supermini, and built by DaimlerChrysler-owned MDC Power in Germany.