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Marine diesel engine working principle
- Nov 06, 2018 -

A fresh stream of air is drawn or pumped into the engine cylinders and then compressed by the moving piston to a very high pressure. When the air is compressed, its temperature rises so that it can ignite the fine mist of fuel injected into the cylinder. The combustion of the fuel adds more heat to the charged air, causing expansion and forcing the generator piston to work on the crankshaft, which in turn drives the propeller of the ship through other shafts.


The operation between two fuel injections is called a duty cycle. In a four-stroke diesel engine, this cycle needs to be accomplished by four different strokes of the piston, namely suction, compression, expansion and exhaust. If we combine suction and exhaust with compression and expansion, the four-stroke engine becomes a two-stroke generator.


The two stroke cycle begins with the piston rising from the bottom of its stroke (both bottom dead center), with the air inlet on the cylinder side open. At this point, the exhaust valve is also opened, fresh air is charged into the cylinder, and the exhaust gas remaining in the previous stroke is blown out through the open exhaust valve. The valve is blown out.


When the piston runs up to about one-fifth of its travel time, it closes the air intake and the exhaust valve closes, so both temperature and pressure rise to very high values.


When the piston reaches the top of its stroke (ie, top dead center), the fuel valve injects fine mist of fuel into the high temperature air in the cylinder, and the fuel burns immediately, and the heat causes the pressure to rise quickly. Thus, the expanded gas forces the piston to move downward during the power stroke.


When the piston moves down to a point halfway through the stroke, the exhaust valve opens and the high temperature gas begins to flow outward through the exhaust valve due to its own pressure, which is assisted by the fresh air entering through the intake port. The air inlet opens as the piston descends further. Then another cycle begins again.


In a two-stroke engine, the crankshaft makes one power stroke in one turn, while the four-stroke engine requires two turns of the crankshaft to do a power stroke, which is why a two-stroke engine can do about twice as many strokes in the same size. The reason why the engine does work. In current practical use, an engine with the same bore and the same speed, the two-stroke engine outputs about 80 percent more power than a four-stroke engine. This increase in engine power makes the two-stroke generator widely used as a large ship mainframe.