The working principle 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. However, since the fuel used in the diesel engine is diesel, its viscosity is higher than that of gasoline, it is not easy to evaporate, and its auto-ignition temperature is lower than that of gasoline, so the combustible mixture and ignition method are different from gasoline engines.
Diesel engines suck in pure air during the intake process. At the end of the compression stroke, the oil pressure of the diesel oil is increased to above 10MPa by the fuel injection pump, and then injected into the cylinder by the fuel injector, where it is mixed with the compressed high-temperature air in a short time to form a combustible mixture. Due to the high compression ratio of the diesel engine (generally 16-22), the pressure in the cylinder can reach 3.5-4.5MPa and the temperature can reach 750-1000K when the compression ends ), greatly exceeding the auto-ignition temperature of the diesel engine.
Therefore, after diesel is injected into the cylinder, it will react with diesel in a short time. The air pressure in the cylinder rises rapidly to 6-9MPa, and the temperature also rises to 2000-2500K. Driven by the high-pressure gas, the piston moves downward, driving the crankshaft to rotate and do work, and the exhaust gas is also discharged into the atmosphere through the exhaust pipe. Ordinary diesel engines are driven by the engine's camshaft, and diesel is delivered to the fuel chamber of each cylinder through a high-pressure oil pump. This fuel supply method changes with the engine speed, so it cannot achieve the best fuel supply at various speeds.
At present, the common rail injection system of electronically controlled diesel engine is more and more widely used, which can better solve this problem. The common rail injection fuel supply system consists of a high-pressure oil pump, a common oil pipe, an injector, an electronic control unit (ECU) and some pipeline pressure sensors. Each fuel injector in the system is connected to the common fuel supply pipe through its own high-pressure fuel pipe, and the common fuel supply pipe and fuel injector play the role of hydraulic energy storage. When working, the high-pressure fuel pump delivers fuel to the common fuel pipe at high pressure. The high-pressure fuel pump, pressure sensor, and ECU work in a closed loop to precisely control the oil pressure in the common fuel pipe, completely changing the phenomenon that the fuel supply pressure changes with the engine speed.
Its main features are as follows:
- The injection timing is completely separated from the fuel metering, and the injection pressure and injection process are controlled by the ECU in real time.
- The injection pressure, injection starting point and duration of each cylinder can be adjusted according to the working conditions of the engine to pursue the best control point of injection.
- High injection pressure and pre-injection of diesel can be realized.