[Principle and Model of Marine Diesel Engine]Working Principle of Marine Diesel Engine Model
Working principle of marine diesel engine
1. Working principle of two-stroke diesel engine
A diesel engine that completes a working cycle through two strokes of the piston is called a two-stroke diesel engine. The engine completes a working cycle and the crankshaft only rotates once. Compared with a four-stroke diesel engine, it improves the working power. In the specific structure and working principle There are also big differences.
The basic structure of a two-stroke diesel engine is the same as that of a four-stroke diesel engine, and the main difference is in the valve train. Two-stroke diesel engines do not have an intake valve, and some do not even have an exhaust valve. Instead, a scavenging port and an exhaust port are provided at the lower part of the cylinder; or a scavenging port and an exhaust valve mechanism are provided. A scavenging pump driven by a moving part and a scavenging box storing pressurized air are specially arranged, and the air distribution is completed by the cooperation of the piston and the air port, thereby simplifying the structure of the diesel engine. The picture shows the working principle of a 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. Air is sucked in from the pump, compressed and discharged, and stored in a scavenging box with a large volume, where it maintains a certain pressure. The working principle of a two-stroke diesel engine is now explained with a diagram.
Combustion Expansion and Exhaust Stroke:
The fuel ignites and burns in the combustion chamber, producing high temperature and high pressure gas. Pushed by the gas, the piston moves downward from the top dead center to perform external work. The piston goes down until the exhaust port is opened (the crank is at the point position at this time, and the gas expansion work ends, and a large amount of exhaust gas in the cylinder is freely exhausted by its own high pressure, and is discharged from the exhaust port to the exhaust pipe. When the pressure in the cylinder drops When it is close to the scavenging pressure (generally the scavenging pressure in the scavenging box is 012, the downward piston opens the scavenging port 3 (the crank is at point 4 at this time, the scavenging air enters the cylinder, and the exhaust gas in the cylinder is exhausted). The air port is driven out of the cylinder. The piston runs to the bottom dead center, and the stroke ends, but the scavenging process continues until the exhaust port closes in the next stroke (the crank is at the point position at this time. 4 342 Chapter 3 Marine Diesel Engine Maintenance Fig. Schematic diagram of the working principle of a two-stroke diesel engine Scavenging and compression stroke: the piston moves upward from the 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 remaining exhaust gas is swept out of the cylinder by the air entering the cylinder through the exhaust port. The piston continues to move upward, gradually covering the scavenging port. When the scavenging port is completely closed (the crank is at the point position at this time, 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 at this time, the air in the cylinder begins to be compressed. When the compression reaches the point before top dead center, the fuel injector will inject the fuel The cylinder is mixed with high-temperature and high-pressure air, and then ignites near the top dead center, igniting and burning by itself. This stroke ends, and a complete working cycle is formed with the previous stroke. The dynamometer diagram of the two-stroke diesel engine is shown in the figure, among which, is The starting point of fuel injection is the top dead center of the piston and the end point of combustion. Compared with the four-stroke diesel engine, two-stroke diesel engines have some obvious advantages, but of course they also have inherent disadvantages.
2, four-stroke diesel engines Working principle of diesel engine
The work of diesel engine is completed by four processes of suction, compression, work and exhaust, which constitute a working cycle. A diesel engine that takes only one cycle to complete a working cycle is called a four-stroke diesel engine. Now compare the animation above to illustrate its working principle.
Intake stroke
The first stroke – intake, its task is to fill the cylinder with fresh air. When the intake stroke begins, the piston is at top dead center and there is some exhaust gas left in the combustion chamber inside the cylinder.
When the crankshaft rotates the elbow, the connecting rod moves the piston from the top dead center to the bottom dead center, and at the same time, the intake valve is opened by the transmission mechanism connected with the crankshaft.
As the piston moves The downward movement, the volume above the piston in the cylinder gradually increases: the air pressure in the cylinder is lower than the pressure in the intake pipe, so the outside air is continuously charged into the cylinder.
In the process of intake The change of gas pressure with the volume of the cylinder is shown in the animation. The ordinate in the figure represents the gas pressure P, and the abscissa represents the cylinder volume Vh (or the stroke of the piston S), which is called a dynamometer diagram. The pressure in the figure Curve table It shows the change rule of gas pressure in the cylinder when the diesel engine is working. From the soil, we can see that the air intake begins, slightly higher than the atmospheric pressure P0 due to the presence of residual exhaust gas. During the intake process, due to the flow resistance generated when the air passes through the intake pipe and the intake valve, the gas pressure of the intake stroke is lower than the atmospheric pressure, and its value is 0.085-0.095MPa. During the entire intake process, the gas pressure in the cylinder is remained roughly the same.
When the piston moves downwards close to the bottom dead center, the airflow rushing into the cylinder still has a high speed and great inertia. It closes after the bottom dead center. Although the piston goes up at this time, the gas can still fill the cylinder due to the inertia of the airflow.
Compression Stroke
Second Stroke – Compression. During compression, the piston moves from the bottom dead center to the top dead center. This stroke has two functions. One is to increase the temperature of the air to prepare the fuel for self-ignition; the other is to create conditions for the gas to expand and work. When the piston goes up and the intake valve is closed, the air in the cylinder is compressed. As the volume becomes smaller, the pressure and temperature of the air continue to rise. The pressure and humidity at the end of the compression are related to the degree of compression of the air, which is related to The compression ratio is related. Generally, the pressure and temperature at the end of compression are: Pc=4～8MPa, Tc=750～950K.
The self-ignition temperature of diesel is about 543-563K, and the temperature at the end of compression is much higher than that of diesel, which is enough to ensure that the fuel injected into the cylinder self-ignites and burns.
The diesel fuel injected into the cylinder does not ignite immediately, but only after physical and chemical changes. This period of time is about 0.001 to 0.005 seconds, which is called the ignition delay period. Therefore, the atomized fuel should be injected into the cylinder when the crank reaches the crank angle of 10-35° before the top dead center, and the highest combustion pressure is reached in the combustion chamber when the crank is 5-10° after the top dead center, forcing the The piston moves downward.
Combustion Expansion Stroke
Third Stroke – Doing work. At the beginning of this stroke, most of the fuel injected into the combustion chamber is burned. A lot of heat is released during combustion, so the pressure and temperature of the gas rise sharply, the piston moves downward under the action of high temperature and high pressure gas, and the crankshaft rotates through the connecting stalk to perform external work. So this stroke is also called work or work stroke.
As the piston descends, the volume of the cylinder increases, the gas pressure drops, and the working stroke ends when the piston travels to the bottom dead center and the exhaust valve opens.
In the animation, the pressure change of the working stroke, the rising part of this line represents the sharp increase in pressure when the fuel is burned in the cylinder, the highest point represents the highest combustion pressure Pz, the pressure and temperature at this point are:
Pz=6～15MPa, Tz=1800～2200K
The ratio of the highest combustion pressure to the compression end pressure (Pz/Pc) is called the pressure rise ratio during combustion, It is represented by λ. Depending on the type of diesel engine, the range of λ value at maximum power is as follows: λ=Pz/Pc=1.2～2.5.
Exhaust Stroke
Fourth Stroke – Exhaust. The function of the exhaust stroke is to discharge the expanded exhaust gas so that it can be filled with fresh air in preparation for the intake of the next cycle. When the working stroke piston moves to the vicinity of the bottom dead center, the exhaust valve opens, and the piston moves from the bottom dead center to the top dead center driven by the crankshaft and the connecting rod, and discharges the exhaust gas out of the cylinder. Due to the resistance of the exhaust system, at the beginning of the exhaust stroke, the gas pressure in the cylinder is 0.025-0.035MPa higher than the atmospheric pressure, and its temperature is Tb=1000-1200K. To reduce resistance to piston movement during exhaust, the exhaust valve opens just before bottom dead center. As soon as the exhaust valve is opened, the gas with a certain pressure will immediately rush out of the cylinder, and the pressure in the cylinder will drop rapidly, so that when the piston moves upward, the exhaust gas in the cylinder is discharged upward by the piston. In order to make use of the inertia of the airflow during exhaust to discharge the exhaust gas cleanly, the exhaust valve is closed after the top dead center.
In the animation, the exhaust stroke curve indicates that during the exhaust process, the gas pressure in the cylinder is almost constant, but slightly higher than atmospheric pressure. The pressure Pr at the end of the exhaust stroke is about 0.105-0.115MPa, and the temperature Pr of the residual exhaust gas is about 850-960K.
Because the intake and exhaust valves are opened early and closed late; so at the end of the exhaust stroke and the beginning of the intake stroke, when the piston is near the top dead center, there is a period of time for the intake and exhaust valves. The valve opens at the same time, and this period of time is represented by the crankshaft angle, which is called the valve overlap angle.
After the exhaust stroke is over, the intake stroke starts again, and the entire working cycle repeats according to the above process. Because the working cycle of this diesel engine is completed by four piston strokes, that is, two revolutions of the crankshaft, it is called a four-stroke diesel engine.
In the four strokes of a four-stroke diesel engine, only the third stroke, that is, the working stroke, generates power to do external work, and the other three strokes are the preparation process for consuming work. For this reason, a flywheel must be installed on a single-cylinder diesel engine, and the crankshaft can run continuously and evenly in four strokes by using the rotational inertia of the flywheel.
