PRINCIPLES OF OPERATION
The unidirectional flow of air toward the exhaust
The diesel engine is an internal combustion power
valves produces a scavenging effect, leaving the
unit, in which the heat of fuel is converted into work
cylinders full of clean air when the piston again covers
in the cylinder of the engine.
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
subjected t o c o m p r e s s i o n a s shown in Fig. 1
is accomplished by the heat of compression.
Shortly before the piston reaches its highest position,
The Two- Cycle principle
t h e 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
injected fuel has been burned.
a n operating cycle; thus, during one half of its
operation, the four-cycle engine functions merely as
The resulting pressure forces the piston downward on
an air pump.
its power stroke. The exhaust valves are again opened
when the piston is about halfway down, allowing the
burned gases to escape into the exhaust manifold as
A blower is provided to force air into the cylinders for
shown in Fig. 1 (exhaust). Shortly thereafter, the
expelling the exhaust gases and to supply the cylinders
downward moving piston uncovers the inlet ports and
with fresh air for combustion. The cylinder wall
the cylinder is again swept with clean scavenging air.
contains a row of ports which are above the piston
This entire combustion cycle is completed in each
when it is at the bottom of its stroke. These ports
cylinder for each revolution of the crankshaft, or, in
admit the air from the blower into the cylinder as soon
other words, in two strokes; hence, it is a "two-stroke
as the rim of the piston uncovers the ports as shown in
Fig. 1 (scavenging).
Fig. 1 - The Two-Stroke Cycle