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  • Working principle of marine diesel engine


    Marine diesel engine has high thermal efficiency, good economy, easy to start and has great adaptability to all kinds of ships. It has been used as ship propulsion power soon after it came out. By the 1950s, diesel engine almost completely replaced steam engine in newly built ships. Marine diesel engine is the main power of civil ships, small and medium-sized ships and conventional submarines (see ship power plant). Marine diesel engine can be divided into main engine and auxiliary engine according to its function in ship. The main engine is used as the propulsion power of the ship, and the auxiliary engine is used to drive the generator, air compressor or water pump.
    Marine diesel engines are generally divided into high-speed, medium speed and low-speed diesel engines. The main performance indexes of three types of diesel engines are listed in the table.
    Working principle of two stroke diesel engine
    The diesel engine that completes one working cycle through two strokes of the piston is called two stroke diesel engine. The oil engine completes one working cycle with only one turn of the crankshaft. Compared with the four stroke diesel engine, it improves the work capacity and has great differences in the specific structure and working principle.
    Two stroke diesel engine and four stroke diesel engine have the same basic structure, but the main difference is in valve train. Two stroke diesel engine has no intake valve or even exhaust valve. Instead, scavenging port and exhaust port are set at the lower part of cylinder, or scavenging port and exhaust valve mechanism are set. A scavenging pump driven by moving parts and a scavenging box for storing pressure air are specially set up. The matching of piston and air port is used to complete the valve distribution, thus simplifying the structure of diesel engine. Figure is the working principle diagram of two stroke diesel engine. The scavenging pump is attached to one side of the diesel engine, and its rotor is driven by the diesel engine. The air is inhaled from the pump and discharged after compression. It is stored in a scavenging box with a large volume and maintains a certain pressure in it. The working principle of the two-stroke diesel engine is illustrated in this paper.
    Combustion expansion and exhaust stroke:
    The fuel ignites and burns in the combustion chamber to generate high temperature and high pressure gas. Driven by the gas, the piston moves downward from the top dead center to work externally. The piston goes down until the exhaust port is opened (at this time, the crank is at the point position, at this time, the work of gas expansion ends, and a large amount of exhaust gas in the cylinder is discharged freely by its own high pressure, and is discharged from the exhaust port to the exhaust pipe. When the pressure in the cylinder drops to close to the scavenging pressure (generally, the scavenging pressure in the scavenging box is 012, the downward piston opens scavenging port 3 (at this time, the crank is at point 4, the scavenging air enters the cylinder, and the exhaust gas in the cylinder is driven out of the cylinder through the exhaust port). The piston runs to BDC and the stroke ends, but the scavenging process continues until the exhaust port of the next stroke is closed (at this time, the crank is at the point position). ·4.342 Part 3 maintenance diagram of marine diesel engine schematic diagram of working principle of two stroke diesel engine scavenging and compression stroke: the piston moves upward from bottom dead center. Before the piston covers the scavenging port, the air stored in the scavenging box is supplied by the scavenging pump and enters the cylinder through the scavenging port. The residual exhaust gas in the cylinder is swept out of the cylinder through the exhaust port. The piston continues to move upward and gradually covers the scavenging port. When the scavenging port is completely closed (at this time, the crank is at the point position, the air stops filling, and the exhaust is still in progress. This stage is called "post exhaust stage". When the exhaust port is closed (the crank is at the point position, the air in the cylinder begins to be compressed. When compressed to the front of TDC, the fuel injector injects fuel into the cylinder, mixes with high temperature and high pressure air, and then ignites near TDC and ignites by itself. This stroke ends and forms a complete working cycle with the previous stroke. The indicator diagram of two-stroke diesel engine is shown in the figure, in which, is the injection starting point, is the piston top dead center, is the combustion end point. Compared with four stroke diesel engine, two-stroke diesel engine has some obvious advantages, but also has its own inherent disadvantages.
    Working principle of four stroke diesel engine
    The work of diesel engine is completed by four processes: suction, compression, work and exhaust, which constitute a working cycle. A four stroke diesel engine is called a four stroke diesel engine. Now we explain its working principle by comparing with the animation above.
    1、 Suction stroke
    The first stroke - suction, its task is to fill the cylinder with fresh air. When the suction stroke begins, the piston is at top dead center and there is still some exhaust gas left in the combustion chamber in the cylinder.
    When the crankshaft rotates the elbow, the connecting rod makes the piston move from the top dead center to the bottom dead center. At the same time, the transmission mechanism connected with the crankshaft makes the suction valve open.
    With the downward movement of the piston, the volume above the piston in the cylinder gradually increases: the air pressure in the cylinder is lower than that in the intake pipe, so the outside air is constantly charged into the cylinder.
    In the process of air intake, the change of gas pressure in the cylinder with the volume of the cylinder is shown in the animation. In the diagram, the ordinate represents the gas pressure P, and the abscissa represents the cylinder volume VH (or piston stroke s), which is called indicator diagram. The pressure curve in the figure shows the variation law of the gas pressure in the cylinder when the diesel engine is working. From the soil, we can see that the air intake is slightly higher than atmospheric pressure P0 due to the existence of residual exhaust gas. During the intake process, because of the flow resistance of air passing through the intake pipe and valve, the gas pressure in the intake stroke is lower than the atmospheric pressure, which is 0.085 ~ 0.095mpa. During the whole intake process, the gas pressure in the cylinder is roughly unchanged.
    When the piston moves downward close to the bottom dead center, the air flow into the cylinder still has a high speed and high inertia. In order to use the inertia of the air flow to improve the inflation rate, the intake valve

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