Figure 12-11.—Intake and exhaust manifolds.
matching those of the cylinder block, that allow
the cooling water to circulate in the head. The
head also helps keep compression in the cylinders.
The gasoline engine contains tapped holes in the
cylinder head that lead into the combustion
chamber. The spark plugs are inserted into these
tapped holes.
In the diesel engine the cylinder head may be
cast in a single unit, or it may be cast for a single
cylinder or two or more cylinders. Separated head
sections (usually covering one, two, or three
cylinders in large engines) are easy to handle and
can be removed.
The L-head type of cylinder head shown in
figure 12-10 is a comparatively simple casting. It
contains water jackets for cooling, openings for
spark plugs, and pockets into which the valves
operate. Each pocket serves as a part of the
combustion chamber. The fuel-air mixture is
compressed in the pocket as the piston reaches the
end of the compression stroke. Note that the
pockets have a rather complex curved surface.
This shape has been carefully designed so that the
fuel-air mixture, compressed, will be subjected to
violent turbulence. This turbulence ensures
uniform mixing of the fuel and air, thus improving
the combustion process.
The I-head (overhead-valve) type of cylinder
head contains not only valve and combustion
chamber pockets and water jackets for cooling
spark-plug openings, but it also contains and
supports
the
valves
and
valve-operating
mechanisms. In this type of cylinder head, the
water jackets must be large enough to cool not
only the top of the combustion chamber but also
the valve seats, valves, and valve-operating
mechanisms.
Crankcase
The crankcase is that part of the engine block
below the cylinders. It supports and encloses the
crankshaft and provides a reservoir for the
lubricating oil. Often times the crankcase contains
a place for mounting the oil pump, oil filter,
starting motor, and generator. The lower part of
the crankcase is the OIL PAN, which is bolted at
the bottom. The oil pan is made of pressed or cast
steel and holds from 4 to 9 quarts of oil, depending
on the engine design.
The crankcase also has mounting brackets that
support the entire engine on the vehicle frame.
These brackets are either an integral part of the
crankcase or
12-12
