Cylinder head

Last updated
A 302/5.0L Ford Windsor V8 cylinder head Cylinderhead.JPG
A 302/5.0L Ford Windsor V8 cylinder head
Cylinder head of a Caterpillar engine Cylinder head of a Caterpillar engine.jpg
Cylinder head of a Caterpillar engine

In an internal combustion engine, the cylinder head (often informally abbreviated to just head) sits above the cylinders on top of the cylinder block. [1] 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.


Sidevalve engines

In a flathead or sidevalve engine, the mechanical parts of the 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.

Keeping all moving parts 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.

Early sidevalve engines were in use at a time of simple fuel chemistry, low octane ratings and so required low compression ratios. This made their combustion chamber design less critical and there was less need to design their ports and airflow carefully.

One difficulty experienced at this time was that the low compression ratio also implied a low expansion ratio during the power stroke. [lower-alpha 1] Exhaust gases were thus still hot, hotter than a contemporary engine, and this led to frequent trouble with burnt exhaust valves.

A major improvement to the sidevalve engine was the advent of Ricardo's 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.

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 [lower-alpha 2] 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.

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 or the 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.


Internally, the cylinder head has passages called ports or tracts for the fuel/air mixture to travel to the inlet valves from the intake manifold, and for exhaust gases to travel from the exhaust valves to the exhaust manifold. In a 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.

In the overhead valve (OHV) design, the cylinder head contains the poppet valves 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' pushrods, and then rocker arms 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' tappets.


The number of cylinder heads in an engine is a function of the engine configuration. Almost all inline (straight) engines today use a single cylinder head that serves all the cylinders. A 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 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 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.

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.

Automotive 4-Stroke Engine Head Designs—Valve and Camshaft Configurations
Common NamesCamshaftIntake ValvesExhaust ValvesNotes
Double Overhead Camshaft
DOHC, Twin-Cam, Cammer
HeadHeadHeadAllows optimum positioning of the valves for a crossflow cylinder head.
Double camshafts are used to allow direct actuation of well-placed valves, without rockers.
Widespread in modern car design
Single Overhead Camshaft
OHC, SOHC, Single-Cam, "Single-Jingle", Cammer
HeadHeadHeadWidely used for cars in recent decades, but increasingly superseded by DOHC.
Sometimes utilizes rocker arms to actuate some valves, other times does not
Overhead Valve
OHV, I-Head, Pushrod, Cam-In-Block
BlockHeadHeadStill used in some large-displacement V8 engines, usually of American or British origin
Needs pushrods and rocker arms to actuate valves
Flathead, L-Head, T-Head
BlockBlockBlockOnce universal, now obsolete
Simplest possible configuration
Cams operate directly on the valves
IOE, F-Head, Intake-Over-Exhaust
BlockHeadBlockAlways uncommon, obsolete for decades

See also


  1. The work done on the piston during the expansion stroke is limited by how much stroke there is in which to achieve it.
  2. 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

Related Research Articles

Two-stroke engine

A two-strokeengine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during only one crankshaft revolution. This is in contrast to a "four-stroke engine", which requires four strokes of the piston to complete a power cycle during two crankshaft revolutions. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust functions occurring at the same time.

Poppet valve Type of valve

A poppet valve is a valve typically used to control the timing and quantity of gas or vapor flow into an engine.

A stratified charge engine describes a certain type of internal combustion engine, usually spark ignition (SI) engine that can be used in trucks, automobiles, portable and stationary equipment. The term "stratified charge" refers to the working fluids and fuel vapors entering the cylinder. Usually the fuel is injected into the cylinder or enters as a fuel rich vapor where a spark or other means are used to initiate ignition where the fuel rich zone interacts with the air to promote complete combustion. A stratified charge can allow for slightly higher compression ratios without "knock," and leaner air/fuel ratio than in conventional internal combustion engines.

The engine configuration describes the fundamental operating principles by which internal combustion engines are categorized.

Engine tuning

Engine tuning is the adjustment or modification of the internal combustion engine or Engine Control Unit (ECU) to yield optimal performance and increase the engine's power output, economy, or durability. These goals may be mutually exclusive; an engine may be de-tuned with respect to output power in exchange for better economy or longer engine life due to lessened stress on engine components.

Hemispherical combustion chamber

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 combustion chamber is that 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.

Ford 335 engine

The Ford 335 engine family was a group of engines built by the Ford Motor Company between 1969 and 1982. The "335" designation reflected Ford management's decision to produce an engine of that size with room for expansion during its development. This engine family began production in late 1969 with a 351 cu in (5.8 L) engine, commonly called the 351C. It later expanded to include a 400 cu in (6.6 L) engine which used a taller version of the engine block, commonly referred to as a tall deck engine block, a 351 cu in (5.8 L) tall deck variant, called the 351M, and a 302 cu in (4.9 L) engine which was exclusive to Australia.

Overhead valve engine Type of piston engine

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.

Flathead engine

A flathead engine, otherwise sidevalve 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.

Lean-burn refers to the burning of fuel with an excess of air in an internal combustion engine. In lean-burn engines the air:fuel ratio may be as lean as 65:1. The air / fuel ratio needed to stoichiometrically combust gasoline, by contrast, is 14.64:1. The excess of air in a lean-burn engine emits far less hydrocarbons. High air–fuel ratios can also be used to reduce losses caused by other engine power management systems such as throttling losses.

Alfa Romeo Twin Spark engine

Alfa Romeo Twin Spark (TS) technology was used for the first time in the Alfa Romeo Grand Prix car in 1914. In the early 1960s it was used in their race cars to enable it to achieve a higher power output from its engines. And in the early and middle 1980s, Alfa Romeo incorporated this technology into their road cars to enhance their performance and to comply with stricter emission controls.

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.

IOE engine Type of combustion engines

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.

Model engine

A model engine is a small internal combustion engine typically used to power a radio-controlled aircraft, radio-controlled car, radio-controlled boat, free flight, control line aircraft, or ground-running tether car model.

T-head engine

A T-head engine is an early type of internal combustion engine that became obsolete after World War I. It is a sidevalve engine that is distinguished from the much more common L-head by its placement of the valves. The intake valves are on one side of the engine block and the exhaust valves on the other. Seen from the end of the crankshaft, in cutaway view, the cylinder and combustion chamber resembles a T - hence the name "T-head". An L-head has all valves at the same side.

Squish (piston engine)

Squish is an effect in internal combustion engines which creates sudden turbulence of the air-fuel mixture as the piston approaches top dead centre (TDC).

Internal combustion engine Engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber

An internal combustion engine (ICE) is a heat engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is applied typically to pistons, turbine blades, rotor or a nozzle. This force moves the component over a distance, transforming chemical energy into useful work. This replaced the external combustion engine for applications where weight or size of the engine is important.


  1. Wright, G. (2015). Fundamentals of Medium/Heavy Duty Diesel Engines. Jones & Bartlett Learning. p. 310. ISBN   978-1-284-06705-7 . Retrieved 2020-11-07.