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. 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 (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 (Compression).
Shortly before the piston reaches it highest position, the required amount of fuel is sprayed into the combustion chamber by the unit fuel injector (Power). The intense heat generated during the high compression of the air ignites the fine fuel spray immediately. The combustion continues until the fuel injected 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 half way down, allowing the burned gases to escape into the exhaust manifold (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".