Two Strokes
Two-stroke internal combustion engines are more simple mechanically than four-stroke engines, but more complex in thermodynamic and aerodynamic processes. In a two-stroke engine, the four "cycles" of internal combustion engine theory (intake, compression, ignition, exhaust) occur in one revolution, while in a four-stroke engine it occurs in two complete revolutions. In a two-stroke engine, more than one function occurs at any given time during the engine's operation.
- Intake begins when the piston is near the bottom dead center. Air is admitted to the cylinder through ports in the cylinder wall (there are no intake valves). All two-stroke Diesel engines require artificial aspiration to operate, and will either use a mechanically-driven blower or a hybrid turbo-supercharger to charge the cylinder with air. In the early phase of intake, the air charge is also used to force out any remaining combustion gases from the preceding power stroke, a process referred to as scavenging.
- As the piston rises, the intake charge of air is compressed. Near top dead center, fuel is injected, resulting in combustion due to the extremely high pressure and heat created by compression, which drives the piston downward. As the piston moves downward in the cylinder it will reach a point where the exhaust port is opened to expel the high-pressure combustion gasses. However, most current two-stroke diesel engines use top-mounted poppet valves and uniflow scavenging. Continued downward movement of the piston will expose the air intake ports in the cylinder wall, and the cycle will start again.
In most GM and EMD engines, very few parameters are variable and all the remaining ones are fixed. The scavenging ports are open from about 45 degrees before bottom dead center to about 45 degrees after bottom dead center (this parameter is necessarily symmetrical about bottom dead center) and the fuel is injected at 4 degrees before top dead center. The remaining parameters have to do with exhaust valve timing, and these are set to effect maximum exhaust scavenging and maximum intake charge air, and these two parameters are not necessarily symmetrical about top dead center. A single camshaft operates the valves and the Unit injector, using three lobes: two for exhaust valves (ether two valves on the smallest engines or four valves on the largest; and the third for the Unit injector.
Read more about this topic: Two-stroke Diesel Engine
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—Henry David Thoreau (1817–1862)