In apiston engine, thecylinder head sits above thecylinders,[1] forming the roof of thecombustion chamber. Insidevalve engines the head is a simple plate of metal containing thespark plugs and possiblyheat dissipation fins. In more modernoverhead valve andoverhead camshaft engines, the head is a more complicated metal block that also contains the inlet and exhaust passages, and oftencoolant passages,Valvetrain components, andfuel injectors.
A piston engine typically has one cylinder head perbank of cylinders.
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 eachcylinder bank), however a small number of 'narrow-angle' V engines (such as the VolkswagenVR5 andVR6 engines) use a single cylinder head spanning the two banks.
Mostradial engines have one head for each cylinder, although this is usually of themonobloc 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 asbarrels.
Some engines, particularly medium- and large-capacitydiesel 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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| Engine type (in chronological order) | Camshaft location | Intakevalves location | Exhaustvalves location |
|---|---|---|---|
| Sidevalve (flathead) | Block | Block | Block |
| Inlet over exhaust (IOE) | Block | Head | Block |
| Overhead valve (OHV) | Block | Head | Head |
| Overhead camshaft (OHC) | Head | Head | Head |
In aflathead (sidevalve) engine, all of thevalvetrain components arecontained within the block, therefore the head is usually a simple plate of metal bolted to the top of theengine 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 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 anoverhead valve (OHV) oroverhead camshaft (OHC) engine, the cylinder head contains several airflow passages calledports; intake ports deliver the fuel+air intake charge from theintake manifold to thecombustion chamber, and exhaust ports route combustion waste gases out the combustion chamber to theexhaust manifold.Valves open and close the ports, with the intakes offset fore-and-aft from the exhausts. The head also contains thespark plugs, and onwater-cooled engines, the coolant passages.
A singlecamshaft located in the engine block usespushrods androcker arms 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.
Anoverhead 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 asingle 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 withtappets). SOHC engines were widely from the 1960s to 1990s. (eliminating pushrods but still utilizing rocker arms)
Double overhead camshaft (DOHC) engines seat a camshaft directly above each row of offset valves (intakes inboard, exhausts outboard).[2] DOHC designs allow optimalcrossflow 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.
