Fuel injector

In fuel injection, fuel is atomized and distributed in the part of the combustion chamber where it is mixed with air. It is mainly composed of an injector and an injector body. Its installation position and angle on the cylinder head depend on the design of the combustion chamber. The spray characteristics of fuel injector include atomization particle size, oil mist distribution, oil beam direction, range and diffusion cone angle. These characteristics should meet the requirements of diesel engine combustion system, make the mixture formation and combustion perfect, and obtain higher power and thermal efficiency. Fuel injectors are divided into open and closed types. Open injector is rarely used because of its simple structure but poor atomization. Closed injector is widely used in various diesel engines.

The nozzle consists of a pair of precisely coupled parts, namely the needle valve and the needle valve body. The pressure regulating spring tightly presses the needle valve on the needle valve body to close it. When the fuel injection pump starts to pressurize the oil, a high-pressure wave is generated in the oil path, and the high-pressure wave propagates from the fuel injection pump end to the fuel nozzle end at the sound speed. The oil pressure acts on the upper conical surface of the needle valve, overcoming the spring force to lift the needle valve, so that the fuel flows into the pressure chamber and is ejected through the nozzle. Once the oil pump stops supplying oil, the oil pressure drops and the needle valve closes immediately under the action of spring force. Adjusting the tightness of the spring can change the opening pressure of the injector.

Fuel injectors are divided into hole type, needle type and throttle type according to the head shape. Injectors with holes below 4 are used for semi-open combustion chambers (see diesel engine); Porous injectors are mostly used in open combustion chambers. This kind of injector can spray a small cone angle and a long range per hole. The number of holes, the diameter of holes and the thickness of hole wall have influence on the spray characteristics, which depends on the model of diesel engine. Needle injector is used in separate combustion chamber and compound combustion chamber, such as swirl chamber and precombustion chamber. The shaft needle at the head of the needle valve passes through the nozzle hole to form an annular injection channel, which can make the sprayed oil mist take the shape of a hollow cone. The throttle type is similar to the pin type, except that the pin shape is changed to form the throttle when the needle valve is lifted, thus changing the fuel injection law and making the diesel engine work softly.

The nozzle end of the injector is in contact with gas, and the temperature is very high. Enhanced diesel engines usually use long nozzles, which make the precise matching parts between the needle valve and the needle valve body farther away from the combustion chamber and avoid the deformation of the coupling due to thermal expansion and seizure. Cooling nozzles are used in large high-strength diesel engines. Another pen injector is small in size, easy to be arranged on the cylinder head and less in contact with gas, which is suitable for small high-speed diesel engines.