What is the working principle of diesel engine?

The working principle of diesel engine is basically the same as that of gasoline engine, and it is also four-stroke, which does work through the reciprocating motion of piston. The main difference lies in the formation mode and ignition mode of combustible mixture. Table 10-6 shows the difference between diesel engines and gasoline engines. The diesel engine has no carburetor, and only air enters the cylinder block through the intake valve during the intake stroke. Because the compression ratio of diesel engine is higher than that of gasoline engine, the obtained high-pressure gas temperature is higher.

At the end of the compression stroke, the fuel is directly injected into the cylinder through the fuel injector, and the high-pressure air is mixed with the injected fuel, so that the fuel spontaneously ignites at high temperature and high pressure, which requires high pressure. The fuel injection speed and the mixing speed of fuel and air determine the combustion speed of fuel. In order to avoid too much fuel entering at one time, resulting in too high pressure in the cylinder, fuel can be continuously injected during the power stroke.

The gasoline engine changes the work done by the engine through the position of the throttle. At idle speed, the throttle is closed and only a small amount of air enters the cylinder block. When the output work is large, more air enters through the throttle. However, under different working conditions, the air quantity entering the diesel engine is constant. It determines the engine speed and output work by controlling the fuel injection quantity.

In a diesel engine, after fuel is injected into the combustion chamber, it cannot be completely mixed with air, and the combustion state in the center of the cylinder block is rich, and the combustion area cannot reach the cylinder wall. When the diesel engine is running at full load, the rich combustion area in the combustion chamber is large, and the fuel residue is large, which will produce a lot of black smoke.

Diesel engines can be divided into direct injection type and premixed type according to different structures. The central nozzle of a direct injection diesel engine injects fuel directly into the combustion chamber. The air and fuel in the combustion chamber are not fully mixed, and there are rich combustion zones and lean combustion zones. Premixed diesel engine is an indirect injection diesel engine. Fuel is not directly injected into the master cylinder, but a premixing cylinder is added in front of the master cylinder. Fuel burns here first, and the burning gas enters the master cylinder through the connecting hole. This working mode increases the turbulence of the mixed gas, makes the fuel and air mix more fully, and there is no over-concentrated area, thus reducing the formation of black smoke particles. Most light diesel engines are premixed. But when the gas expands into the cylinder through the connecting hole, it will lose some energy and reduce the efficiency of the engine.

2 Formation of pollutants emitted by diesel engines

The harmful components of diesel engine exhaust mainly include CO, HC, nitrogen oxides, sulfides, particulate matter (or particulate matter) and odor. Because the average air-fuel ratio of the mixture used by diesel engine is greater than the theoretical air-fuel ratio, its CO and HC emissions are obviously lower than those of gasoline engine, while the NO emissions of diesel engine are almost the same as those of gasoline engine, while the emissions of particulate matter and unpleasant gases are much higher than those of gasoline engine. Experiments show that the particulate emission of diesel engine can reach 10 times that of gasoline engine. The emission characteristics of diesel engine have a great relationship with the form of combustion chamber, especially the emission of direct injection diesel engine and indirect injection diesel engine is very different. The NO, CO, HC and smoke of swirl combustion chamber diesel engines are generally lower than those of direct injection diesel engines, especially the emission concentration of nitrogen oxides is generally lower than that of direct injection diesel engines 1/3 ~ 1/2.

The test results on direct injection diesel engines and swirl chamber diesel engines with the same structure but different combustion chambers show that the NO, CO, HC and smoke of direct injection diesel engines are higher than those of swirl chamber diesel engines, especially at high load and CO and HC at low load. However, the fuel consumption rate of swirl chamber diesel engine is higher than that of direct injection diesel engine.