DETROIT DIESEL 53
PRINCIPLES OF OPERATION
The unidirectional flow of air toward the exhaust valves
The diesel engine is an internal combustion power unit,
produces a scavenging effect, leaving the cylinders full
in which the heat of fuel is converted into work in the
of clean air when the piston again covers the inlet ports.
cylinder of the engine.
As the piston continues on the upward stroke, the
In the diesel engine, air alone is compressed in the
exhaust valves close and the charge of fresh air is
cylinder; then, after the air has been compressed, a
subjected to compression as shown in Fig. 1
charge of fuel is sprayed into the cylinder and ignition is
accomplished by the heat of compression.
Shortly before the piston reaches its highest position,
The Two-Cycle Principle
the required amount of fuel is sprayed into the
combustion chamber by the unit fuel injector as shown
In the two-cycle engine, intake and exhaust take place
in Fig. 1 (power). The intense heat generated. during
during part of the compression and power strokes
the high compression of the air ignites the fine fuel
respectively as shown in Fig. 1. In contrast, a four-
spray 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 half way 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. 1 (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 bottom of its stroke. These ports admit the air from
the cylinder is again swept with clean scavenging air.
the blower into the cylinder as soon as the rim of the
This entire combustion cycle is completed in each
piston uncovers the ports as shown in Fig.
cylinder for each revolution of the crankshaft, or, in
other words, in two strokes; hence, it is a "two-stroke
Fig. 1 - The Two Stroke Cycle
1972 General Motors Corp.