The compression-ignition engine is an internal combustion engine whose ignition is triggered by compression of air in its cylinders, instead of sparks from a spark plug. This type of machine uses diesel as fuel as it does not require an ignition system. Here, air alone is first inducted into the cylinder; then, just as the piston goes up to the top of its compression stroke, fuel is injected directly into the engine cylinder, where it is vaporized and get ignited by the heat that is created by the compression of air in the cylinder.
A compression ignition engine is not self-speed limiting because the amount of air that is injected into the cylinder is always the same and to the maximum volume possible. Therefore, it is unchanged. There are a wide variety of CI engine designs and they are employed in a wide range of applications: trucks, cars, locomotives, and marine power generation. Diesel engines are heavier than gasoline engines because of the stronger and heavier materials used to withstand the greater dynamic forces which arise from the higher combustion pressures in their cylinders.
Like a spark-ignition machine, a compression ignition engine is also a reciprocating engine that is driven by pistons which move laterally in two directions. There are two types: a two-stroke and a four-stroke, or four cycle, engine. In the former type, there is a complete cycle of operation in every two strokes of a piston. In the four-stroke type, the first downstroke of the piston draws in air, which is compressed on the upstroke; the at the top of the stroke, a fuel is squirted in by the fuel injector. When the fuel is ignited, the quick expansion of hot gases, which arose out of the explosion, pushes the piston down in the working stroke. The next upstroke drives the gases out through the exhaust valve, completing the cycle.
Below, cut-away drawing of a four cylinder, naturally aspirated, indirect injection Volkswagen compression ignition engine of a car.
