Working principle of diesel engine

1. Working principle of two-stroke diesel engine

1. Intake, compression stroke The piston moves upward from the bottom dead center, and the exhaust gas turbocharger sends fresh air with a certain pressure into the cylinder; when the piston runs to a certain time, it blocks the scavenging port, the cylinder becomes closed, the piston continues to move upward, and the pressure in the cylinder gradually increases.

2. Expansion, exhaust stroke When the piston runs to a certain moment before the top dead center, the fuel is sprayed into the combustion chamber in a good atomization state through the injector and mixed with the air to ignite and burn automatically. When the piston passes the top dead center, the gas pressure and temperature in the cylinder rise sharply. Under the action of high-temperature and high-pressure gas, the piston moves from the top dead center to the bottom dead center, and transmits power to the crankshaft through the connecting rod. The crankshaft outputs power in the form of rotation. As the piston moves downward, the cylinder volume gradually increases, and the pressure and temperature of the gas gradually decrease. Until the exhaust valve opens.

2. Working principle of four-stroke diesel engine

The working process of a diesel engine is actually the same as that of a gasoline engine. Each working cycle also goes through four strokes: intake, compression, power, and exhaust.

1. The working medium entering the cylinder during the intake stroke is pure air. Since the resistance of the diesel engine intake system is small, the intake end pressure is higher than that of the gasoline engine. The intake end temperature is lower than that of the gasoline engine. The task of the intake is to fill the cylinder with fresh air. When the intake stroke begins, the piston is at the top dead center, and there is still some exhaust gas in the combustion chamber in the cylinder. When the crankshaft rotates, the connecting rod moves the piston from the top dead center to the bottom dead center, and at the same time, the transmission mechanism connected to the crankshaft is used to open the intake valve. As the piston moves downward, the volume above the piston in the cylinder gradually increases: causing the air pressure in the cylinder to be lower than the pressure in the intake pipe, so the outside air is continuously filled into the cylinder.

2. Compression stroke Since the compressed working medium is pure air, the compression ratio of the diesel engine is higher than that of the gasoline engine (generally ε=16~22). The pressure at the end of compression is 3 000~5 000kPa, and the temperature at the end of compression is 750~1 000K, which greatly exceeds the auto-ignition temperature of diesel (about 520K).

3. Power stroke When the compression stroke is nearing the end, under the action of the high-pressure oil pump, the diesel is sprayed into the cylinder combustion chamber through the injector at a high pressure of about 10MPa. After mixing with the air in a very short time, it immediately ignites and burns. The pressure of the gas in the cylinder rises rapidly, up to 5000~9000kPa, and the highest temperature reaches 1800~2000K. Since the diesel engine ignites and burns by compression, the diesel engine is called a compression ignition engine.

4. Exhaust stroke The exhaust of the diesel engine is basically the same as that of the gasoline engine, but the exhaust temperature is lower than that of the gasoline engine. Generally, Tr=700~900K. For a single-cylinder engine, its speed is uneven, the engine works unsteadily, and the vibration is large. This is because only one of the four strokes is for power, and the other three strokes are for consuming power to prepare for power. In order to solve this problem, the flywheel must have a sufficiently large moment of inertia, which will lead to an increase in the mass and size of the entire engine. The use of a multi-cylinder engine can make up for the above shortcomings.