Chapter 3 Engine Classifi cation, Parts Identifi cation 51
mixture. This has all happened in one-half revo-
lution of the crankshaft.
The spark plug fi res the cylinder charge and
drives the piston down. Several things happen as
the piston travels downward. The air-fuel mixture
in the crankcase is compressed and the reed valve
closes. The piston is also performing the power
stroke at this time. When the piston has trav-
eled down the cylinder far enough to expose the
exhaust port, the exhaust will fl ow out of the port.
As the piston continues to travel down, the intake
port is uncovered. As the crankcase air-fuel mix-
ture is compressed, it is forced to fl ow through the
now uncovered intake port into the cylinder.
The piston top is usually shaped to defl ect the
incoming air-fuel charge upward. This fi lls the top
of the cylinder with a fresh charge while simul-
taneously scavenging (cleaning out) the remain-
ing exhaust gases. This has all taken place in one
revolution of the crankshaft. Every time the piston
reaches the top of its travel, it will be driven down-
ward by a power stroke. Study Figure 3-2.
Advantages of the Two-Stroke Cycle Engine
The two-stroke cycle engine has several
advantages that make it popular for use in small
engine applications. By eliminating the poppet
valves, it also eliminates valve springs, cam fol-
lowers, camshaft, and gears. Since there is no valve
“float,” two-cycle engines can operate at very high
speeds. This enables the engine to be quite com-
pact and light. Machining costs are reduced.
Because the two-stroke engine fi res on every
stroke, more power per pound of engine weight
is available. However, it does not produce the
power of a four-stroke cycle engine of compara-
ble size. A two-stroke engine has poor volumet-
ric effi ciency (ability to intake fuel) and will lose
some power through compressing the crankcase
mixture. In order to completely scavenge the
cylinder, some of the intake air-fuel charge also
leaves the cylinder, resulting in higher emis-
sions and less fuel economy. A two-stroke cycle
engine that uses an exhaust valve and no mix-
ture in the crankcase will be shown in the sec-
tion on diesel engines later in this chapter.
Valve Location Classifi cations
Engines are also classified according to the
valve location. The various valve arrangements
are known as the L-head, T-head, F-head, and
I-head. The names are derived from the resem-
blance to letters of the alphabet. The I-head is
used in all modern cars and light trucks.
L-Head, T-Head, and F-Head Designs
These three head designs are no longer used
in automotive applications, but it is possible to
fi nd them in some stationary power plants or
small engines. The L-head engine has both valves
in the block and on the same side of the cylinder.
A line drawn through the cylinder and across to
the valves will produce the letter L in an inverted
(upside down) position. Figure 3-3 illustrates the
L-head design. This design may still be found on
some small engines and forklifts.
Reed
valve
Reed valve open Reed valve closed
Figure 3-2.
In the left-hand view, the piston is traveling upward and has
almost reached the top of its stroke. It has compressed the
air-fuel charge trapped above the piston. The spark plug has
just fi red and the mixture is starting to burn.
On its way upward, the piston has drawn a fresh fuel
mixture into the crankcase through a reed valve. The fuel
vapor contains suspended droplets of oil that lubricates
the surfaces of moving parts.
In the right-hand view, the air-fuel mixture has fi red and
the piston is traveling down. It has uncovered the intake
and exhaust ports, allowing burning gases to escape and
fresh charge to enter. The air-fuel mixture fl ows through
the intake port because the piston, while traveling down,
compressed the mixture in the crankcase.
In two-strokes, the engine has performed all the
necessary functions to enable it to receive a power stroke
for every crankshaft revolution.
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