Working Principle of Four Stroke Diesel Engine of Generator Set

In the thermal process, only in the expansion process of "working medium" can work be done, and we require that the generator set engine can continuously produce mechanical work, so the working medium must be expanded repeatedly. Therefore, the working medium must be restored to its original state and then expanded. Therefore, the diesel engine of the generator set must go through the four thermal processes of air intake, compression, expansion and exhaust before it can return to the initial state, so that the diesel engine can continuously generate mechanical work. Therefore, the four thermal processes mentioned above are called a working cycle.

If the piston of a diesel engine completes one working cycle after four strokes, the engine is called a four stroke diesel engine. If the piston completes two strokes and one working cycle, the diesel engine is called a two-stroke diesel engine. At present, diesel engines equipped with generator sets are all four stroke engines.

1. Intake stroke

The purpose of the intake stroke is to draw in fresh air in preparation for fuel combustion. To achieve air intake, a pressure difference shall be formed between the inside and outside of the cylinder. Therefore, in this stroke, the exhaust valve is closed, the inlet valve is opened, the piston moves from TDC to TDC, the volume of the cylinder above the piston gradually expands, the pressure decreases, and the gas pressure in the cylinder is about 68~93kPa lower than the atmospheric pressure. Under the action of atmospheric pressure, fresh air is drawn into the cylinder through the intake valve. When the piston reaches BDC, the intake valve is closed and the intake stroke is ended.

2. Compression stroke

The purpose of the compression stroke is to increase the pressure and temperature of the air in the cylinder and create conditions for fuel combustion. As the intake and exhaust valves have been closed, the air in the cylinder is compressed, and the pressure and temperature increase accordingly. The degree of increase depends on the degree of compression. Different diesel engines are slightly different. When the piston is close to TDC, the air pressure in the cylinder reaches (3000~5000) kPa and the temperature reaches 500~700 ℃, far exceeding the autoignition temperature of diesel.

3. Expansion stroke

At the end of the piston's upward travel, the fuel injector starts to inject diesel into the cylinder, mix it with air to form a combustible mixture, and spontaneously ignite immediately. At this time, the pressure in the cylinder rapidly rises to about 6000~9000kPa, and the temperature reaches (1800~2200) ℃. Under the thrust of high-temperature and high-pressure gas, the piston moves down to the dead center and drives the crankshaft to rotate. As the gas expansion piston descends, its pressure gradually decreases until the exhaust valve is opened.

4. Exhaust stroke

The purpose of the exhaust stroke is to remove exhaust gases from the cylinder. After the end of the power stroke, the gas in the cylinder has become exhaust gas, its temperature drops to (800~900) ℃, and its pressure drops to (294~392) kPa. At this time, the exhaust valve is opened, the inlet valve is still closed, and the piston moves from the bottom dead center to the top dead center. Under the action of the residual pressure in the cylinder and the piston thrust, the exhaust gas is discharged out of the cylinder. When the piston reaches TDC again, the exhaust process ends.

After the exhaust process of the generator set is completed, the exhaust valve is closed, and the inlet valve is opened again. Repeat the next cycle and work continuously.

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