Working principle of diesel engine

- May 17, 2017 -

The working process of diesel engine is the same as gasoline engine, each work cycle also undergoes gas intake, compression, work, exhaust four stroke. But because diesel engine fuel is diesel, its viscosity is larger than gasoline, not easy to evaporate, and its spontaneous combustion temperature is lower than gasoline, so the formation of combustible mixture and ignition methods are different from gasoline engines.

The diesel engine inhaled in the intake stroke is pure air. When the compression stroke is close to the end, the diesel fuel injection pump will increase the oil pressure to more than 10MPa, through the injector spray into the cylinder, in a short time with the compressed high temperature air mixture, forming a combustible mixture gas. Because the diesel engine compression ratio (generally 16-22), so the air pressure in the cylinder can reach 3.5-4.5MPa, at the same time temperature up to 750-1000K (while the gasoline engine at this time of mixing gas pressure will be 0.6-1.2MPa, temperature Tatsu 600-700K), much more than the diesel ignition temperature. Therefore, the diesel fuel injection into the cylinder, in a very short time after mixing with the air immediately ignition combustion. The air pressure in the cylinder rises sharply to the 6-9MPa, and the temperature rises to the 2000-2500K. The piston moves downward by the high-pressure gas and drives the crankshaft to rotate, and the exhaust gases are also discharged into the atmosphere by the exhaust pipes.

The ordinary diesel engine is driven by the engine camshaft, which is transported to the fuel tanks of the cylinders with the help of the high-pressure pump. This kind of oil supply way to change with the engine speed change, do not to the optimum amount of fuel supply at various speeds. The common rail injection system of electronically controlled diesel engines has been used to solve the problem.

Common rail injection system is composed of high-pressure pumps, public tubing, injector, electric control Unit (ECU) and some piping pressure sensors, each injector in the system is connected with the public supply tubing through the respective high-pressure tubing, and the public supply tubing plays a hydraulic pressure-accumulator effect on the injector. At work, high-pressure pumps to high pressure fuel to the public supply tubing, high-pressure pumps, pressure sensors and ECU composed of closed-loop work, the public oil pipes to achieve precise control of oil pressure, completely changed the fuel supply pressures with the engine speed changes. The main characteristics of the following three aspects:

1. The fuel injection is completely separated from the oil metering, and the fuel injection pressure and oil spray process are controlled by the ECU.

2. According to the engine work conditions to adjust the cylinder injection pressure, fuel injection start point, duration, so as to pursue the best fuel injection control point.

3. can achieve high fuel injection pressure, and can realize diesel injection.

Compared with gasoline engines, diesel engines have a low fuel consumption rate (average 30% lower than gasoline engines), and diesel prices are lower, so fuel economy is better; at the same time, the diesel engine speed is generally lower than the gasoline engine, the torque is larger than the gasoline engine, but the quality of the large, working noise, manufacturing and maintenance costs are high, while emissions are worse than gasoline engines. But with the development of modern technology, these shortcomings of diesel engines are gradually being overcome, and now some of the high-grade sedan has begun to use diesel engines.

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