will be trapped and will dilute the incoming fuel-air
mixture when the intake valves open. Since the piston
has little downward movement while in the rock
position, the exhaust valve can remain open during this
period and thereby permit a more complete scavenging
of the exhaust gases.
Ignition timing refers to the timing of the sparks at
the spark plug gap with relation to the piston position
during the compression and power strokes. The ignition
system is timed so that the sparks occurs before the
piston reaches top dead center on the compression
stroke. That gives the mixture enough time to ignite and
start burning. If this time were not provided, that is, if
the spark occurred at or after the piston reached top dead
center, then the pressure increase would not keep pace
with the piston movement.
At higher speeds, there is still less time for the fuel-
air mixture to ignite and bum. To make up for this lack
of time and thereby avoid power loss, the ignition
system includes an advance mechanism that functions
on speed.
CLASSIFICATION OF ENGINES
Engines for automotive and construction equipment
may be classified in several ways: type of fuel used, type
of cooling employed, or valve and cylinder arrange-
ment. They all operate on the internal combustion
principle. The application of basic principles of
construction to particular needs or systems of manu-
facture has caused certain designs to be recognized as
conventional.
The most common method of classification is based
on the type of fuel used; that is, whether the engine burns
gasoline or diesel fuel.
GASOLINE ENGINES
DIESEL ENGINES
Mechanically and in
VERSUS
overall appearance, gasoline
and diesel engines resemble one another. However,
many parts of the diesel engine are designed to be
somewhat heavier and stronger to withstand the higher
temperatures and pressures the engine generates. The
engines differ also in the fuel used, in the method of
introducing it into the cylinders, and in how the air-fuel
mixture is ignited. In the gasoline engine, we first mix
air and fuel in the carburetor. After this mixture is
compressed in the cylinders, it is ignited by an electrical
spark from the spark plugs. The source of the energy
producing the electrical spark may be a storage battery
or a high-tension magneto.
The diesel engine has no carburetor. Air alone enters
its cylinders, where it is compressed and reaches a high
temperature because of compression. The heat of
compression ignites the fuel injected into the cylinder
and causes the fuel-air mixture to burn. The diesel
engine needs no spark plugs; the very contact of the
diesel fuel with the hot air in the cylinder causes ignition.
In the gasoline engine the heat compression is not
enough to ignite the air-fuel mixture; therefore, spark
plugs are necessary.
ARRANGEMENT OF CYLINDERS
Engines are also classified according to the arrange-
ment of the cylinders. One classification is the in-line,
in which all cylinders are cast in a straight line above
the crankshaft, as in most trucks. Another is the V-type,
in which two banks of cylinders are mounted in a V
shape above the crankshaft, as in many passenger
vehicles. Another not-so-common arrangement is the
horizontally opposed engine whose cylinders mount in
two side rows, each opposite a central crankshaft. Buses
often have this type of engine.
The cylinders are numbered. The cylinder nearest
the front of an in-line engine is numbered 1. The others
are numbered 2, 3,4, and so forth, from the front to rear.
In V-type engines the numbering sequence varies with
the manufacturer.
The firing order (which is different from the
numbering order) of the cylinders is usually stamped on
the cylinder block or on the manufacturers nameplate.
VALVE ARRANGEMENT
The majority of internal combustion engines also
are classified according to the position and arrangement
of the intake and exhaust valves. This classification
depends on whether the valves are in the cylinder block
or in the cylinder head. Various arrangements have been
used; the most common are the L-head, I-head, and
F-head (fig. 12-8). The letter designation is used because
the shape of the combustion chamber resembles the
form of the letter identifying it.
L-Head
In the L-head engines, both valves are placed in the
block on the same side of the cylinder. The valve-
operating mechanism is located directly below the
valves, and one camshaft actuates both the intake and
exhaust valves.
12-8