The fuel injection nozzle is composed of fuel injector body, tappet, fuel injection nozzle coupling, pressure regulating spring, etc When the fuel injection pump is working, the high-pressure fuel enters the annular high-pressure oil chamber in the middle of the needle valve body from the fuel inlet pipe connector of the injector through the oil passage in the injector body and needle valve body. The oil pressure acts on the pressure bearing slope in the middle of the needle valve to form an upward axial thrust. When this thrust exceeds the pressure of the pressure regulating spring, the needle valve moves up, and the fuel is sprayed into the combustion chamber in an atomized state through the four orifices at the bottom of the valve body. When the oil supply is interrupted and the oil flow pressure drops, the needle valve returns quickly under the action of the pressure regulating spring. The sealing cone at the lower part of the needle valve presses against the sealing cone of the needle valve body and closes the injection hole, stopping fuel injection.

(1) Common faults and maintenance methods of fuel injection nozzle

The working conditions of the fuel injection nozzle are very bad, with high mechanical load and thermal load. In addition, improper installation and adjustment often lead to poor spray, oil dripping from the nozzle, carbon deposition, and even stuck needle valve.

① The fuel injection nozzle does not inject fuel. Check whether there is air in the oil circuit. If there is air, remove it in time; Whether the oil supply of the fuel transfer pump is normal; Whether the fuel system leaks oil; Check whether the injection needle valve is stuck in the needle valve hole. If necessary, disassemble and clean it for repair.

② The sealing cone is worn. Generally, the cone angle of the valve body is 590, while the cone angle of the needle valve is 600, and the width of the contact surface is 0.2-0.3 mma. If the fuel entering the fuel injection nozzle contains particles of mechanical impurities, the cone surface will be worn out with grooves. The contact belt will be widened, and the fuel will leak and drip from the seat surface, causing coke deposition, difficult starting, black smoke, power reduction, and increased fuel consumption. Clean and maintain frequently, and replace parts if necessary. Check the needle valve fit as shown in Figure 33

③ The needle valve guide surface is worn. The fitting clearance of the needle valve body is very small, about 0.002-0.004mm. If dust and mechanical impurities invade the surface of the two parts, the geometric accuracy and roughness of the guide surface will be damaged, and high pressure and transient high temperature will be generated on the contact surface, resulting in adhesive wear and seizure of the surface. In this case, replace it with a new one.

④ The pin and the orifice are worn. The main reason for the abrasion of needle valve shaft and valve body orifice is that the fuel is flushed with high pressure and speed. At the same time, the nozzle part of the axle needle extends into the combustion chamber to contact with the combustion gas, making the surface brittle, resulting in peeling or comb shaped grooves. The axle needle head changes from the original cylindrical shape to the conical shape, and the nozzle hole diameter is also worn and enlarged, losing the positive circle shape, all of which lead to the drop of the injection pressure, the deterioration of the spray quality and the shortening of the injection time, which should be replaced and repaired.

⑤ The tension of pressure regulating spring is insufficient. The injection pressure of the fuel injector is too low, the fuel atomization is poor and cannot be adjusted, and even the compressed gas in the cylinder pushes open the needle valve of the fuel injector and flows through the oil return pipe to the high-pressure oil pump, causing the diesel engine to shut down without pumping oil. In this regard, the pressure regulating spring should be renewed. If no new pressure regulating spring can be replaced, a small washer with appropriate thickness can be added between the original pressure regulating spring and the pressure regulating spring seat to increase the preload of the spring to solve the problem of insufficient elasticity.

Delphi automobile fuel injection nozzle

(2) Inspection and adjustment of fuel injection nozzle

① The abnormal operation of the fuel injection nozzle usually leads to the poor operation of the fuel injection nozzle parts. In addition to checking the use of other parts of the fuel injection nozzle one by one, the fuel injection nozzle parts should be overhauled. The fitting surface of the nozzle parts and the end face of the needle valve should be bright, free of damage and rust, and the sealing cone of the needle valve and needle valve body should be bright, free of pitting and scratches.

② The needle valve and needle valve body are made by pairing and grinding and cannot be interchanged, so they cannot be mixed during cleaning, and should be cleaned separately in pairs; Replace the needle valve when it is stuck in the needle valve body.

③ Carbon deposits on needle valve and needle valve body can only be removed with special tools, but special care should be taken not to damage the mating surface.

④ The fuel injection nozzle shall be installed on the tester, and the fuel shall be pressed evenly and slowly with the handle. At the same time, the adjusting screw shall be screwed until the fuel injection nozzle sprays at the specified pressure. Stop the oil pressure and observe whether the pointer of the pressure gauge drops (without dripping); The fuel injection pressure difference of each fuel injection nozzle of the same engine shall not exceed the specified value. If the pressure does not meet the requirements, turn the pressure regulating spring screw to adjust it to the specified value.

⑤ Inspection of spray quality: generally, there should be four spray at the nozzle, each of which is about 150-200mm long. The fuel spray cone from the nozzle should not deflect, and its cone should comply with the original manufacturer's regulations. When pressing oil slowly with the handle, the oil mist sprayed from the nozzle shall be fine, and the oil particles in the fog bundles of each oil hole shall be evenly distributed with the same length, and there shall be no visible oil mist splash, oil particle column or oil drop; The fuel shall be cut off in time with a clear sound. If the quality of spray is poor, replace the nozzle

⑥ Inspection of fuel injection nozzle failure on the engine: once the fuel injection nozzle fails, the running engine will produce black smoke, uneven operation of each cylinder, increased vibration, power reduction and other abnormal phenomena. The cylinder cutoff method can be used to find out the faulty fuel injection nozzle in turn. The method is: when the engine runs at idle speed, stop the fuel injection of each cylinder in turn. When the fuel supply of a cylinder stops, if the exhaust no longer emits black smoke and the engine speed does not change significantly, the fuel injection nozzle fails; If the engine speed drops significantly, the working condition becomes worse, and the fuel injection nozzle of the cylinder works normally.