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How does a diesel engine work?

April 20, 2022
The diesel engine is developed and designed according to the advanced heavy-duty engine technology concept, and has the advantages of high reliability, good durability, good fuel economy, small size, high power, large torque, large torque reserve, and strong component versatility. According to the working principle of a diesel engine, the sequence of its working process in a working cycle is intake, compression, combustion, work, and exhaust.
A diesel engine is a machine that converts the chemical energy of fuel into mechanical energy. Diesel engines must go through the following four stages or processes to achieve energy conversion: the intake process, which inhales fresh air into the cylinder; the compression process, which compresses the air inhaled into the cylinder to increase its temperature and pressure; the expansion process, which injects fuel into the cylinder. In the gas in the cylinder that has been compressed and the temperature reaches the auto-ignition temperature of the fuel, the fuel is quickly mixed with the air and burns rapidly; in the exhaust process, the exhaust gas that has been burned and worked is discharged from the cylinder. Here are the detailed instructions:
The intake valve is opened, the exhaust valve is closed, the piston moves from the top dead center to the bottom dead center, the cylinder volume above the piston increases, and a vacuum degree is generated, and the pressure in the cylinder drops below the intake pressure. The gasoline atomized by the carburetor or gasoline injection device is mixed with air to form a flammable mixture, which is sucked into the cylinder by the intake port and intake valve. The intake process continues until the piston passes the bottom dead center and the intake valve closes. Then the upward piston begins to compress the gas.
The intake and exhaust valves are all closed, the combustible mixture in the cylinder is compressed, the temperature of the mixture increases, and the pressure rises. Before the piston approaches the top dead center, the pressure of the combustible gas mixture rises to about 0.6-1.2MPa, and the temperature can reach 330℃-430℃.
Do work
When the compression stroke is nearing the end, under the action of the high-pressure oil pump, the diesel oil is injected into the combustion chamber of the cylinder through the injector at a high pressure of about 10MPa, and it is immediately self-ignited and burned after being mixed with the air in a very short time. The pressure of the gas in the cylinder rises rapidly, the highest is 5000-5000kPa, and the highest temperature is 1800-2000K.
The exhaust of a diesel engine is basically the same as that of a gasoline engine, but the exhaust temperature is lower than that of a gasoline engine. Generally Tr=700-900K. For a single-cylinder engine, its rotational speed is uneven, the engine work is not stable, and the vibration is large. This is because only one of the four strokes does work, and the other three strokes are strokes that consume power to prepare for work. To solve this problem, the flywheel must have a large enough moment of inertia, which in turn leads to an increase in the overall engine mass and size.
Every time a diesel engine completes the above four processes, it is a working cycle. This is true whether it is a two-stroke diesel engine or a four-stroke diesel engine. For a two-stroke diesel engine, after completing the above four processes, the crankshaft rotates once (360°), and the piston runs up and down once each (that is, two piston strokes), so it is called a two-stroke diesel engine. For a four-stroke diesel engine, the crankshaft rotates twice (720°) to complete the above four processes, and the piston runs up and down twice each (that is, four piston strokes), so it is called a four-stroke diesel engine.