In an
internal combustion engineThe internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
, the
cylinder head (often informally abbreviated to just
head) sits above the
cylinderA cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
s on top of the
cylinder blockA cylinder block is an integrated structure comprising the cylinder of a reciprocating engine and often some or all of their associated surrounding structures...
. It closes in the top of the cylinder, forming the
combustion chamberA combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...
. This joint is sealed by a
head gasketA head gasket is a gasket that sits between the engine block and cylinder head in an internal combustion engine. Its purpose is to seal the cylinders to ensure maximum compression and avoid leakage of coolant or engine oil into the cylinders; as such, it is the most critical sealing application in...
. 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
valveA poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...
s,
spark plugA spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...
s, and fuel injectors.
In a
flatheadA flathead engine is an internal combustion engine with valves placed in the engine block beside the piston, instead of in the cylinder head, as in an overhead valve engine...
or sidevalve engine, the mechanical parts of the valve train are all contained within the block, and the head is essentially a metal plate bolted to the top of the block; this simplification avoids the use of moving parts in the head and eases manufacture and repair, and accounts for the flathead engine's early success in production
automobileAn automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
s and continued success in
small engineA small engine is the general term for a wide range of internal combustion engines used to power lawn mowers, generators, concrete mixers and many other machines that require independent power sources. Most small engines are single-cylinder, V-twin, or flat-twin units...
s, such as lawnmowers. This design, however, requires the incoming air to
flowAn air flow bench is a device used for testing the internal aerodynamic qualities of an engine component and is related to the more familiar wind tunnel.Used primarily for testing the intake and exhaust ports of cylinder heads of internal combustion engines...
through a convoluted path, which limits the ability of the engine to perform at higher
revolutions per minuteRevolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
(rpm), leading to the adoption of the
overhead valveAn overhead valve engine, also informally called pushrod engine or I-head engine, is a type of piston engine that places the camshaft within the cylinder block , and uses pushrods or rods to actuate rocker arms above the cylinder...
(OHV) head design, and the subsequent
overhead camshaftOverhead cam valvetrain configurations place the engine camshaft within the cylinder heads, above the combustion chambers, and drive the valves or lifters in a more direct manner compared to overhead valves and pushrods...
(OHC) design.
Detail
Internally, the cylinder head has passages called
ports or tractsCylinder head porting refers to the process of modifying the intake and exhaust ports of an internal combustion engine to improve the quality and quantity of the air flow. Cylinder heads, as manufactured, are usually suboptimal due to design and manufacturing constraints...
for the fuel/air mixture to travel to the inlet valves from the intake
manifoldIn automotive engineering, an inlet manifold or intake manifold is the part of an engine that supplies the fuel/air mixture to the cylinders...
, for
exhaust gassesExhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...
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
antifreezeAntifreeze is a freeze preventive used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters....
- 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 valveA poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...
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
camshaftA camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...
, which is sited within the cylinder block, and its moment of operation is transmitted to the valves pushrods, and then
rocker armGenerally referred to within the internal combustion engine of automotive, marine, motorcycle and reciprocating aviation engines, the rocker arm is a reciprocating lever that conveys radial movement from the cam lobe into linear movement at the poppet valve to open it...
s mounted on a rocker shaft - the rocker arms and shaft also being located within the cylinder head.
In the 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' tapets.
Implementation
The number of cylinder heads in an engine is a function of the
engine configurationEngine configuration is an engineering term for the layout of the major components of a reciprocating piston internal combustion engine. These components are the cylinders and crankshafts in particular but also, sometimes, the camshaft....
. Almost all
inline (straight) enginesUsually found in four- and six-cylinder configurations, the straight engine, or inline engine is an internal-combustion engine with all cylinders aligned in one row, with no offset...
today use a single cylinder head that serves all the cylinders. A
V (or Vee)A V engine, or Vee engine is a common configuration for an internal combustion engine. The cylinders and pistons are aligned, in two separate planes or 'banks', so that they appear to be in a "V" when viewed along the axis of the crankshaft...
engine has two cylinder heads, one for each
cylinder bankInternal combustion piston engines are usually arranged so that the cylinders are in lines parallel to the crankshaft. Where they are in a single line, this is referred to as an inline or straight engine....
of the 'V'. For a few compact 'narrow angle' V engines, such as the
VolkswagenVolkswagen Group is a German multinational automobile manufacturing group. , Volkswagen was ranked as the world’s third largest motor vehicle manufacturer and Europe's largest....
VR6The VR6 engine is an internal combustion engine configuration, consisting of six cylinders. It was developed by the Volkswagen Group in the late 1980s, and evolutions of the original variant are still produced by them....
, the angle between the cylinder banks is so narrow that it uses a single head spanning the two banks. A
flat engineA flat engine is an internal combustion engine with multiple pistons that move in a horizontal plane. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is sometimes known as the boxer, or horizontally opposed engine. The concept was patented in 1896...
(basically a V engine, where the angle between the cylinder banks is now 180°) has two heads. Most
radial engineThe radial engine is a reciprocating type internal combustion engine configuration in which the cylinders point outward from a central crankshaft like the spokes on a wheel...
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 engineA diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
s built for industrial, marine, power generation, and heavy traction purposes (large
truckA truck or lorry is a motor vehicle designed to transport cargo. Trucks vary greatly in size, power, and configuration, with the smallest being mechanically similar to an automobile...
s,
locomotiveA locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...
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 efficiencyVolumetric efficiency in internal combustion engine design refers to the efficiency with which the engine can move the charge into and out of the cylinders. More specifically, volumetric efficiency is a ratio of what quantity of fuel and air actually enters the cylinder during induction to the...
and
compression ratioThe 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...
of the engine.
Automotive 4-Stroke Engine Head Designs - Valve and Camshaft Configurations
| Common Names | Camshaft | Intake Valves | Exhaust Valves | Notes |
Dual Overhead Cam
DOHC |
Head |
Head |
Head |
Allows 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...
.
Double camshafts are used to allow direct actuation of well-placed valves, without rockers.
Widespread in modern car design |
Overhead Cam
OHC |
Head |
Head |
Head |
Widely used for cars in recent decades, but increasingly superseded by DOHC. |
Overhead Valve
OHV, I-Head, Pushrod |
Block |
Head |
Head |
Still used in big V8 pushrod engines
Needs pushrods and rockers to actuate valves |
Sidevalve
Flathead, L-Head, T-Head |
Block |
Block |
Block |
Once universal, now obsolete
Simplest possible configuration
Cams operate directly on the valves |
Inlet-Over-Exhaust
IOE, F-Head |
Block |
Head |
Block |
Always rare, obsolete in cars
Common in motorcycles, especially Harley-Davidsons |
External links