Q：Common problems of diesel fuel injector nozzle

A：If the diesel engine injector has been used for a long time, the injector needle valve lift will increase, the injector injection pressure will be too high or too low, the fuel return pipe of the injector will be too much and the needle valve will be stuck, which will affect the normal operation of the diesel engine. By analyzing these faults, the professional knowledge of drivers can be improved.

The function of the fuel injector on the diesel engine is to inject the high-pressure diesel oil delivered by the injection pump into the combustion chamber with a certain injection pressure, injection quantity and angle in a mist spray to promote the good mixing of fuel and compressed air, so as to achieve a better combustion. The fuel injector and its needle valve are one of the key components of the diesel engine fuel system, and are also vulnerable parts. Among the three precision components of diesel fuel system, its working reliability is the worst and its average service life is the shortest. If the injector is used for too long, the following conditions will occur: the lift of the injector needle valve increases, which changes the fuel injection law and worsens the combustion of the diesel engine; The fuel injection pressure of the fuel injector is too high or too low, resulting in uneven operation of each cylinder of the diesel engine and power reduction; And the fuel return pipe of the fuel injector is too much and the needle valve is stuck, which affects the normal operation of the diesel engine, resulting in lower fuel injection pressure, poor atomization, oil leakage and dripping, and the diesel engine can not work well.

1. The needle valve is stuck. The needle valve is an important coupling on the injector. It is composed of the needle valve and the needle valve body. It is a high-precision coupling. It is ground in pairs and cannot be interchanged. The upper part of the outer circle of the needle valve is provided with a support shoulder. During assembly, the injector cap tightly presses the upper end face of the needle valve body and the lower end face of the injector body through the support shoulder. The needle valve head has two conical surfaces and an inverted cone shaped pin. The pin of the inverted cone is inserted into the spray hole of the needle valve body to form a circular gap of the inverted cone. When the needle valve is raised to inject oil, a positive cone atomized oil spray can be formed. If the needle valve is stuck in the open state, the diesel fuel injected from the nozzle cannot be atomized, resulting in incomplete combustion, and black smoke will also occur. In addition, unburned diesel will also wash the cylinder wall, dilute the engine oil, and accelerate the wear of piston rings and cylinder liners. If the needle valve is stuck when it is closed, it will produce a high-pressure knocking sound in the combustion system, and even damage the fuel injection pump plunger. There are five possible reasons for the stuck needle valve: ① Improper installation of the fuel injector causes the local temperature of the fuel injector to be too high and burnt out. ② The fuel injector is not maintained and adjusted regularly. ③ The diesel oil contains impurities or excessive moisture. ④ The conical surface of the needle valve of the fuel injection nozzle is not tightly sealed, and the fuel injection nozzle is burnt out when the diesel oil leaked to the end face of the fuel injection nozzle burns. ⑤ The working temperature of diesel engine is too high.

2 The lift of the injector needle valve increases the distance that the needle valve can rise from the closed position when the injector injects fuel, which is called the lift. The injector needle valve lift, injection pressure and the annular clearance between the needle valve and the valve seat determine the amount of fuel injection. In order to ensure accurate fuel injection and atomization quality, the needle valve lift must be accurate. If the lift of the needle valve increases, the flow section between the needle valve and the seating surface increases, the flow rate increases, and the time required for the needle valve to move from the highest position to the fuel cut-off position (seating) increases, and the injection duration increases, which changes the normal injection law. This will worsen the combustion process of the diesel engine, increase the thermal load, reduce the power and increase the fuel consumption. In addition, it will also cause overheating and carbon deposition of the fuel injector and aggravate the wear of the sealing cone. When the fuel injector is working, the impact part of the lower end face of the fuel injector body will be concave under the long-term impact wear of the needle valve shoulder end face; For the injector with adjustable needle valve lift (such as the injector of 4146 diesel engine), the lift may also be increased due to improper adjustment. Because the maintenance personnel do not pay attention to or even understand the change of the needle valve lift, which will affect the operation of the diesel engine, the inspection of the wear of the lower end face of the fuel injector body is often ignored during maintenance, so that the needle valve lift is too large, which affects the normal operation of the engine, and in serious cases, leads to a reduction in power.

3. The injection pressure of the fuel injector is too high or too low. The pressure when the fuel injector starts to inject is called the injection pressure, which is adjusted by turning the pressure regulating screw to change the preload of the pressure regulating spring. In order to ensure the normal combustion process, the injection pressure of various diesel engine fuel injectors has certain standards, which are clearly specified in the instructions of each engine, and should not be arbitrarily adjusted too high or too low. Too high or too low injection pressure of the fuel injector will cause uneven operation of each cylinder of the diesel engine, power reduction, and even early wear of combustion chamber, piston and other parts. Generally speaking, if the fuel injection pressure is adjusted too low, the atomization of fuel injection will be greatly deteriorated, and the phenomenon of oil dripping is easy to occur; At the same time, it will also make the starting point of fuel injection advance, and after the end point is late, the fuel injection duration will increase, the fuel injection volume will increase, and it is difficult to start. The adjustment of the injection pressure too high is also not good, causing the engine to produce a knocking sound during operation, and reducing the power, increasing the loss of diesel leakage, reducing the injection volume, shortening the injection duration, and increasing the injection rate, resulting in rough operation of the diesel engine, and sometimes bursting the high-pressure oil pipe. It can be seen that too high or too low injection pressure will not only reduce the power of the diesel engine and increase the fuel consumption rate, but also have a certain impact on the reliability and service life of the diesel engine. The injection pressure of the fuel injector must be adjusted according to regulations. The injection pressure can be adjusted to the upper limit of the specified value when the plunger coupling and needle valve coupling are in good technical condition, and to the lower limit of the specified value when the coupling is seriously worn. Generally, it is not allowed to change the injection pressure at will.

4. When the fuel injector returns too much oil, the diesel oil in the oil cavity of the needle valve body has a high pressure. Although the matching parts are precise, a small amount of diesel oil will leak into the cavity of the fuel injector body and flow back to the diesel filter or oil tank through the return pipe (this part of diesel oil also has the function of lubricating the needle valve). Therefore, a small amount of return oil in the return pipe is normal. However, if there is too much return oil, the cause should be found out. It is possible that the joint surface between the injector body and the needle valve body is damaged or not clean, and the contact is not tight, resulting in some diesel fuel directly leaking into the injector body cavity from the annular oil groove on the upper part of the needle valve body; It may also be that the guide surface of the needle valve and the needle valve body is seriously worn and the matching clearance is too large, which increases the diesel leakage. Once excessive oil return is found, repair or replace relevant parts immediately instead of plugging the oil return pipe. Otherwise, the leaking diesel oil cannot be discharged, which will cause the oil pressure in the injector body cavity to rise, and the resistance will increase when the oil needle rises, resulting in the high injection pressure. The normal fuel injection law is damaged, which makes the diesel engine unstable, exhaust smoke and knocking sound, further exacerbating the wear of the fuel injection pump and fuel injector.