It is at the top of cylinders and consists of a platform
containing part of the combustion chamber and the location of the valves
and spark plugs. In a flathead engine, the mechanical
car parts of the
valve train are all contained within the block, and the head is
essentially a flat plate of metal bolted to the top of the cylinder bank
with a head gasket in between; these simplicity lead to ease of
manufacture and repair, and account for the flathead engine's early
success in production automobiles and continued success in small
engines, such as lawnmowers. This design, however, requires the incoming
air to flow through a convoluted path, which limits the ability of the
engine to perform at higher rpm, leading to the adoption of the overhead
valve head design.
In the overhead valve head, the top half of the cylinder head contains
the camshaft in an overhead cam engine, or another mechanism to transfer
rotational mechanics from the crankshaft to linear mechanics to operate
the valves. Internally the cylinder head has passages called ports for
the fuel/air mixture to travel to the inlet valves from the intake
manifold, for exhaust gases to travel from the exhaust valves to the
exhaust manifold, and for antifreeze to cool the head and engine.
The numbers of cylinders heads in an engine are function of the engine
configuration. A straight engine has only one cylinder head. A V engine
usually has two cylinder heads, one at each end of the V, although
Volkswagen, for instance, produces a V6 called the VR6, where the angle
between the cylinder banks is so narrow that it utilizes a single head.
A boxer engine has two heads.
The cylinder head is key to the performance of the internal combustion
engine, as the shape of the combustion chamber, inlet passages and ports
determines a major portion of the volumetric efficiency and compression
ratio of the engine.