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
in which the heat of fuel is converted into work in the
full of clean air when the piston again covers the inlet
cylinder of the engine.
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.
The Two-Cycle Principle
Shortly before the piston reaches its highest position,
the required amount of fuel is sprayed
In the two-cycle engine, intake and exhaust take place
combustion chamber by the unit fuel injector as shown
during part of the compression and power strokes
in Fig. 1 (power). The intense heat generated during
respectively, as shown in Fig. 1. In contrast, a four-
the high compression of the air ignites the fine fuel
cycle engine requires four piston strokes to complete an
spray immediately. The combustion continues until the
operating cycle; thus, during one half of its operation,
injected fuel has been burned.
the four-cycle engine functions merely as an air pump.
The resulting pressure forces the piston downward on its
A blower is provided to force air into the cylinders for
power stroke. The exhaust valves are again opened
expelling the exhaust gases and to supply the cylinders
when the piston is about halfway down, allowing the
with fresh air for combustion. The cylinder wall contains
burned gases to escape into the exhaust manifold as
a row of ports which are above the piston when it is at
shown in Fig. 1 (exhaust). Shortly thereafter, the
the bottom of its stroke. These ports admit the air from
downward moving piston uncovers the inlet ports and
the blower into the cylinder as soon as the rim of the
the cylinder is again swept with clean scavenging air.
piston uncovers the ports as shown in Fig.
This entire combustion cycle is completed in each
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