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
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
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 pons.
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).
Shortly before the piston reaches its highest position, the required amount of fuel is sprayed into the combustion chamber
by the unit fuel injector as shown in Fig. 1 (power). The intense heat generated during the high compression of the air
ignites the fine fuel spray immediately. The combustion continues until the injected fuel has been burned.
The resulting pressure forces the piston downward on 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 shown in Fig. I (exhaust).
Shortly thereafter, the downward moving piston uncovers the inlet ports and the cylinder is again swept with clean
scavenging air. 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 cycle".
Fig. 1 - The Two-Stroke Cycle