How does a diesel engine work?

The work of a diesel engine is accomplished by four processes: intake, compression, combustion expansion and exhaust, which constitute a working cycle. The piston goes through four processes to complete a working cycle in a diesel engine called a four-stroke diesel engine. The work of a diesel engine is accomplished by four processes: intake, compression, combustion expansion and exhaust, which constitute a working cycle. The piston goes through four processes to complete a working cycle in a diesel engine called a four-stroke diesel engine.

一、Intake stroke

The first stroke-air intake, its task is to fill the cylinder with fresh air. When the intake stroke begins, the piston is at TDC and there is some exhaust gas left in the combustion chamber of 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 the drive mechanism associated with the crankshaft opens the intake valve. As the piston moves down, the volume above the piston in the cylinder increases: the air pressure in the cylinder is less than the pressure in the intake pipe, so the outside air is constantly filling the cylinder. During the intake process, the air pressure in the cylinder changes with the volume of the cylinder as shown in the animation. The vertical coordinate represents the gas pressure p, the horizontal coordinate represents the cylinder volume VH (or Piston Punch S) , this figure is called indicator diagram. The pressure curve in the figure shows the variation of the gas pressure in the cylinder when the diesel engine is working. From the soil we can see that the intake begins with the presence of residual exhaust gas, which is slightly higher than the atmospheric pressure P 0. In the process of air intake, because air through the intake pipe and the intake valve when the flow resistance, so the air intake stroke gas pressure is lower than the atmospheric pressure, its value is 0.085 ~ 0.095 mpa, in the whole process of air intake, the gas pressure in the cylinder remains approximately constant. When the piston moves down near the BDC, the air flow into the cylinder still has a high speed and great inertia, in order to use the inertia of the air flow to increase the charge, the intake valve is closed after the piston has passed the BDC. Although at this time the piston upward, but because of the inertia of the flow of air, gas can still fill the cylinder.

二、Compression stroke

The second stroke-compression. When compressed, the piston moves from the lower to the upper dead center. This stroke serves two purposes. One is to raise the temperature of the air in preparation for the self-ignition of the fuel. The other is to create the conditions for the expansion of the gas to work. When the piston goes up and the intake valve is closed, the air in the cylinder is compressed. As the volume gets smaller and smaller, the pressure and temperature of the air will get higher and higher, the pressure and humidity of the end-point of compression are related to the degree of compression of the air, that is, to the compression ratio. The pressure and temperature of the general end-point of compression are: PC = 4 ~ 8 mpa, TC = 750 ~ 950K. Diesel combustion temperature is about 543-563K, compression end point temperature than diesel combustion temperature is much higher, enough to ensure that the fuel injected into the cylinder self-ignition combustion. When diesel fuel is injected into the cylinder, it 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 ignition delay period. Therefore, the atomized fuel should be injected into the cylinder at the crank angle of 10 ~ 35 ° before the crank turns to the TDC, and the maximum combustion pressure should be reached in the combustion chamber at the crank angle of 5 ~ 10 ° after the TDC, force the piston down.

三、Combustion expansion stroke

The third stroke-combustion expansion. At the start of the stroke, most of the fuel injected into the combustion chamber burns. The combustion gives off a lot of heat, so the pressure and temperature of the gas will rise rapidly, the piston under the action of high temperature and high pressure gas downward movement, and through the connecting rod to make the crankshaft rotation, external work. So this stroke is also called work or work stroke. As the piston goes down, the cylinder volume increases, the pressure of the gas drops, and the working stroke ends when the piston goes to the bottom stop and the exhaust valve opens.

四、Exhaust stroke

Fourth stroke-exhaust. The function of the exhaust stroke is to discharge the expanded exhaust gas in order to fill with fresh air in preparation for the next cycle of intake. When the working stroke piston moves near the BDC, the exhaust valve opens and the piston, driven by the crankshaft and connecting rod, moves from the BDC to the BDC and expels the exhaust gas from the cylinder. At the beginning of the exhaust stroke, the gas pressure in the cylinder is 0.025-0.035 MPA higher than the atmospheric pressure, and the temperature TB = 1000-1200K. To reduce the resistance of the piston to movement during exhaust, the exhaust valve opens before the stop point.

As soon as the exhaust valve is opened, the gas with a certain pressure immediately rushes out of the cylinder, the pressure in the cylinder drops rapidly, so that when the piston moves up, the exhaust gas in the cylinder depends on the piston to go up. In order to make use of the inertia of the air flow when the exhaust gas is discharged clean, the exhaust valve is closed only after the top check point.