TM 5-3895-346-14
PRINCIPLES OF OPERATION
ENGINE OVERHAUL
PRINCIPLES OF OPERATION
The diesel engine is an internal combustion power unit, in which the heat of fuel is converted into work in the
cylinder of the engine.
In the diesel engine, air alone is compressed in the cylinder; then, after the air has been compressed, a charge
of fuel is sprayed into the cylinder and ignition is accomplished by the heat of compression.
The Two-Cycle Principle
In the two-cycle engine, intake and exhaust take place during part of the compression and power strokes
respectively as shown in Fig. 1. In contrast, a four-cycle engine requires four piston strokes to complete an
operating cycle; thus, during one half of its operation, the four-cycle engine functions merely as an air pump. A
blower is provided to force air into the cylinders for expelling the exhaust gases and to supply the cylinders with
fresh air for combustion. The cylinder wall contains a row of ports which are above the piston when it is at the
bottom of its stroke. These ports admit the air from the blower into the cylinder as soon as the rim of the piston
uncovers the ports as shown in Fig. 1 (scavenging).
The unidirectional flow of air toward the exhaust valves produces a scavenging effect, leaving the cylinders full
of clean air when the piston again covers the inlet ports.
As the piston continues on the upward stroke, the exhaust valves close and the charge of fresh air is subjected
to compression as shown in Fig. 1 (compression).
Figure 1. - THE TWO STROKE CYCLE
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