The Stages of The Air Exchange Process in Four Stroke Diesel Engine

The working principle of a four stroke diesel engine is completed by four processes: intake, compression, combustion expansion, and exhaust, which constitute a working cycle. A four stroke diesel engine is a diesel engine in which the piston travels through four processes to complete one working cycle. The air exchange process of a four stroke diesel engine can be divided into three stages: free exhaust, forced exhaust, and scavenging.

(1) Free exhaust stage

The free exhaust stage is carried out in the later stage of the power stroke. At this point, the gas pressure has dropped to 0.3~0.4MPa, which has little effect on the expansion work. When the piston reaches the bottom dead center, the exhaust valve can be opened to use the pressure of the exhaust gas to exhaust outside the cylinder. Although this period is short, due to high exhaust pressure and high flow rate, the amount of exhaust gas discharged can reach over 60%. Free exhaust reduces the pressure inside the cylinder, reducing the resistance of the piston to return. At the same time, the exhaust valve opens prematurely. When the piston returns through the bottom dead center, the exhaust valve has a larger opening, making the exhaust unobstructed and improving the exhaust effect.

(2) Forced exhaust stage

The piston reaches the bottom dead center, the free exhaust stage ends, and the pressure inside the cylinder drops below 0.12MPa. At this point, the exhaust valve has already opened wide, and when the piston returns, the remaining exhaust gas is further discharged from the cylinder. At the end of the exhaust stroke, the exhaust valve will gradually close.

(3) Scavenge stage

The piston reaches the top dead center and the forced exhaust phase ends. The piston starts moving from the top dead center to the bottom dead center. At this point, the intake valve has opened. As the piston descends, the volume of the cylinder increases, and fresh air enters the cylinder, sweeping the remaining exhaust gas out of the cylinder.

Four stroke diesel engines are often used as automotive power equipment and are widely used in various aspects such as transportation machinery, agricultural machinery, engineering machinery, and diesel generator sets.

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