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At the end of the four-stroke diesel engine's power stroke, the piston moves from the bottom dead center to the upper dead center under the inertia of the crankshaft. At this time, the exhaust valve is opened, and the intake valve is still closed: the exhaust gas pressure in the cylinder is different from the external atmospheric pressure and the displacement of the piston is upward, and the exhaust gas is quickly discharged from the exhaust valve. Due to the influence of the resistance of the exhaust system and the presence of the combustion chamber volume, the exhaust gas in the cylinder is not fully exhausted at the end of the exhaust, and the pressure is still higher than the atmospheric pressure.
The four-stroke diesel generator has four strokes of intake, compression, work and exhaust. The piston reciprocates four times in the cylinder. After the crankshaft rotates for two weeks (720 °C), a working cycle is completed. The above four strokes continue, and the diesel engine continuously performs external work. 
Each stroke of the four-stroke diesel generator piston completes a working process, and each working process can describe the process with a corresponding piston stroke.