Analysis of stroke process of Cummins diesel generator set

Diesel generator sets generally use a four-stroke diesel engine as power to drive the motor. The work of the four-stroke diesel engine is completed by the four processes of intake, compression, combustion expansion and exhaust, which constitute a working cycle. The piston takes four processes to complete a working cycle.

1. 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 some exhaust gas remains 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.

With the downward movement of the piston, the volume above the piston in the cylinder gradually increases. At this time, the air pressure in the cylinder is lower than the pressure in the intake pipe, so the outside air can be charged into the cylinder. During this process, the gas pressure in the cylinder changes with the volume of the cylinder. Slightly above atmospheric pressure due to residual exhaust gas from the previous duty cycle. During the intake process, flow resistance is generated when the air passes through the intake pipe and the intake valve, so 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 in the cylinder The pressure remains roughly the same.

When the piston moves down close to the bottom dead center, the airflow rushing into the cylinder still has a high speed and great inertia. In order to use the inertia of the airflow to increase the air charge, the intake valve is closed after the piston passes the bottom dead center. . Although the piston goes up at this time, the gas can still be charged into the cylinder due to the inertia of the airflow.

2. 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 for the self-ignition of the fuel; 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 the 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 the self-ignition temperature of diesel, which is enough to ensure that the fuel injected into the cylinder ignites and burns by itself. Diesel injected into the cylinder does not ignite immediately, but only after physical and chemical changes. This period of time is about 0.001~0.005 seconds, which is the “Ignition Delay Period).

Therefore, the atomized fuel should be injected into the cylinder when the crank turns to 10~35° crank angle before top dead center, and when the crank is 5~10° after top dead center, it will reach 10°C in the combustion chamber to the highest combustion pressure, forcing the piston down.

3. Combustion Expansion Stroke
Third stroke (combustion). At the beginning of this stroke, most of the fuel injected into the combustion chamber is burned. When it burns, it releases a lot of heat, so the pressure and temperature of the gas.

It rises sharply, the piston moves downward under the action of high temperature and high pressure gas, and the crankshaft rotates through the connecting rod to perform external work. So this stroke is also called work or work stroke.

With the downward movement of the piston, the volume of the cylinder increases, the pressure of the gas drops, and the working stroke ends when the piston travels to the bottom dead center and the exhaust valve opens.

The pressure change of the working stroke The rising part of this line represents the sharp increase in the pressure of the fuel 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 3 6~15MPa, Tz=1800~2200K The ratio of the highest combustion pressure to the compression end pressure (Pz/Pc), which is called the pressure rise ratio during combustion, is expressed by . Depending on the type of diesel engine, the range of input value at maximum power is as follows: Input=Pz/Pc=1.2~2.5.

4. 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=1000~120OK. 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. The exhaust stroke curve shows that during the exhaust process, the gas pressure in the cylinder is almost constant, but slightly higher than the atmospheric pressure.

The pressure Fr 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. Since 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 will be a period of time when the intake and exhaust valves open at the same time. Time is expressed in crankshaft angle, called valve overlap angle.

After the exhaust stroke ends, the intake stroke starts again, and the entire working cycle repeats according to the above process. Since the working cycle of this diesel engine consists of four pistons

The stroke is completed by two revolutions of the crankshaft, so it is called a four-stroke diesel engine.
In the four strokes of the four-stroke diesel engine, only the third stroke, that is, the working impulse, generates power to do external work, and the other three strokes are the preparation process for consuming work.