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How does the engine of a large ship start?
Mar 10, 2018

The engines of large ships are mainly diesel engines.

The work of a diesel engine is accomplished by the four processes of intake, compression, combustion expansion and exhaust. These four processes form a working cycle. The piston takes four processes to complete a working cycle of diesel engine called four-stroke diesel.

The first stroke - intake, its mission is to fill the cylinder with fresh air. When the intake stroke starts, the piston is located at the upper stop, and there is some exhaust gas in the combustion chamber in the cylinder. When the crankshaft rotates the elbow, the connecting rod causes the piston to move from the upper stop point to the lower point. Meanwhile, the drive mechanism connected with the crankshaft is used to open the inlet valve. With the downward movement of the piston, the volume of the piston above the cylinder increases gradually: the air pressure inside the cylinder is lower than that of the inlet pipe, so the air outside is continuously filling the cylinder. Due to the air through the intake pipe and in the process of inlet flow resistance when the inlet valve, so the intake stroke of gas pressure is lower than the atmospheric pressure, the value of 0.085 ~ 0.095 MPa, throughout the admission process, the gas pressure in the cylinder is roughly unchanged. When the piston moves downward near the next check point, rushed into the cylinder still has the very high speed airflow, inertia is very big, to take advantage of to improve your of inertia of the airflow, air intake valve in the piston over the next check point after closing. Although the piston goes up, the gas can still fill the cylinder due to the inertia of the air flow.

The second stroke -- compression. When the compression piston movement from the next check point between the check point, the function of the stroke has two, one is increase the temperature of the air, to prepare for the fuel ignition itself: the second is to create conditions for gas expansion work. When the piston upward, intake valve closed, the air in the cylinder is compressed, with the small volume, air pressure and temperature is rising, the pressure at the end of the compression is associated with the degree of compression of air and humidity, which is associated with compression ratio, the pressure and temperature at the end of the compression is commonly: Pc = 4 ~ 8 mpa, Tc = 750 ~ 950 k.

Third stroke - combustion expansion. At the start of the stroke, most of the fuel sprayed into the combustion chamber burns. The pressure and temperature of the gas rises sharply. The piston moves down under the action of high temperature and high pressure, and turns the crankshaft through the stem to make work. So this stroke is called work or work stroke. As the piston goes down, the volume of the cylinder increases, the pressure of the gas drops, and the working stroke ends when the piston moves to the bottom and the exhaust valve opens.

Fourth stroke - exhaust. The function of the exhaust stroke is to drain the expanded exhaust gas to fill the fresh air and prepare for the next round of air intake. When the working stroke piston moves to the next stop, the exhaust valve opens and the piston moves from the bottom to the bottom of the crankshaft and connecting rod, and the exhaust gas out of the cylinder. Due to the resistance of the exhaust system, the gas pressure in the cylinder is 0.025-0.035mpa higher than the atmospheric pressure at the beginning of the exhaust stroke, and its temperature is 1000-1200k. In order to reduce the drag of the piston motion, the exhaust valve is opened before the next stop. As soon as the exhaust valve is opened, the gas with a certain pressure will rush out of the cylinder, and the pressure in the cylinder will drop rapidly so that when the piston moves up, the exhaust gas in the cylinder will be driven out by the piston. In order to use the air flow inertia of the exhaust gas exhaust to clean, the exhaust valve is closed after the last point.


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