helps retain compression in the cylinders. In the
The camshaft is supported in the cylinder block by
gasoline engine there are threaded holes in the cylinder
bushings that fit into machined holes in the block. On
head that lead into the combustion chamber. The spark
plugs are screwed into these holes.
are attached to the side of the cylinder block.
In the diesel engine, the cylinder head may be cast
in a single unit or may be cast for a single cylinder or
two or more cylinders. Separated head sections (usually
The cylinder head provides the combustion
covering 1, 2, or 3 cylinders in large engines) are easier
chamber for the engine cylinders. It is built to conform
to handle and can be readily removed. They do not warp
to the arrangement of the valves: L-head, I-head, or
easily, are cheaper to replace, and require less work for
other. Cylinder heads are usually cast from iron, from
iron alloyed with other metals, or from aluminum alloy.
Aluminum has the advantage of combining lightness
The L-head type of cylinder head shown in figure
3-21 is a comparatively simple casting. It contains
will tend to run cooler, other factors being equal. A high
water jackets for cooling and openings for spark plugs.
Pockets into which the valves operate are also provided.
because it assures that the heat of combustion will be
Each pocket serves as a part of the combustion
rapidly carried away, thus preventing the formation of
chamber. The fuel-air mixture is compressed in the
hot spots, which cause preignition (premature ignition
pocket as the piston reaches the end of the compression
of the fuel-air mixture in the cylinder).
stroke. Note that the pockets have a rather complex
curved surface. This shape has been carefully designed
In the water-cooled engine, the cylinder head (fig.
so that the fuel-air mixture, in being compressed, will
3-21) is bolted to the top of the cylinder block to close
be subjected to violent turbulence. This turbulence
the upper end of the cylinders. It contains passages,
assures uniform mixing of the fuel and air, thus
matching those of the cylinder block, that allow the
improving the combustion process.
cooling water to circulate in the head. The head also
Figure 3-21.--Cylinder head for L-head engine.