TM 5-3820-245-14&P
SERIES 53 ENGINES
SECTION I
CHAPTER VII
DESCRIPTION
PRINCIPLES OF OPERATION
The diesel engine Is an internal combustion power unit,
The unidirectional flow of air toward the exhaust valves
in which the heat of fuel is converted into work in the
produces a scavenging effect, leaving the cylinders full of
cylinder of the engine.
clean air when the piston again covers the inlet ports.
In the diesel engine, air alone is compressed in the
As the piston continues on the upward stroke. the
cylinder; then, after the air has been compressed, a
exhaust valves close and the charge of fresh air is
charge of fuel is sprayed into the cylinder and Ignition is
subjected to compression as shown in Fig. 1
accomplished by the heat of compression.
(compression).
Shortly before the piston reaches its highest position, the
The Two-Cycle Principle
required amount of fuel is sprayed Into the combustion
chamber by the unit fuel Injector as shown in Fig. I
In the two-cycle engine, intake and exhaust take place
(power). The intense heat generated during the high
during part of the compression and power strokes
compression of the air ignites the fine fuel spray
respectively, as shown in Fig. 1. In contrast, a four-
immediately.
The combustion continues until the
cycle engine requires four piston strokes to complete an
injected fuel has been burned.
operating cycle; thus, during one half of its operation, the
four-cycle engine functions merely as an air pump.
The resulting pressure forces the piston downward on its
power stroke. The exhaust valves are again opened
A blower is provided to force air into the cylinders for
when the piston is about halfway down, allowing the
expelling the exhaust gases and to supply the cylinders
burned gases to escape into the exhaust manifold as
with fresh air for combustion. The cylinder wall contains
shown in Fig. I (exhaust). Shortly thereafter, the
a row of ports which are above the piston when it is at
downward moving piston uncovers the inlet ports and the
the bottom of its stroke. These ports admit the air from
cylinder is again swept with clean scavenging air. This
the blower into the cylinder as soon as the rim of the
entire combustion cycle is completed in each cylinder for
piston uncovers the ports as shown in Fig.
I
each revolution of the crankshaft, or, In other words, in
(scavenging).
two strokes; hence, it is a "two-stroke cycle ".
Figure 7-1 The Two-Stroke Cycle
PAGE 112
