Construction and Working Principle of Diesel Engine Injector

1. Function of Diesel Injector

A diesel injector is a device that sprays high - pressure fuel into the combustion chamber of a diesel engine. According to the requirements of different diesel engines, it atomizes the diesel from the high - pressure oil pump and sprays it into a specific position in the combustion chamber with a certain injection pressure, spray fineness, injection pattern, range, and spray cone angle to mix with air and burn.

2. Structure of Diesel Injector

Most diesel injectors used in automobiles are hole - type injectors. The basic structure is shown in the figure. The main part of the injector is a pair of precision mating parts, called the injector nozzle or injector head, which consists of a needle valve and a needle valve body.

3. Working Principle of Diesel Injector

The needle valve and the needle valve body are made of high - quality bearing steel. The clearance between their mutually - fitting sliding cylindrical surfaces is only 0.001mm - 0.0025mm, which is obtained through high - precision machining or grinding and matching. Different injector nozzle assemblies cannot be interchanged. If the clearance is too large, the injection pressure will drop and the spray quality will deteriorate; if the clearance is too small, the needle valve is likely to get stuck. The annular conical surface (pressure - bearing conical surface) in the middle of the needle valve is located in the annular oil chamber of the needle valve body. Its function is to bear the axial thrust generated by the oil pressure, causing the needle valve to rise. The conical surface (sealing conical surface) at the lower end of the needle valve cooperates with the needle valve body to seal the inner cavity of the injector.

There is a shoulder on the upper part of the needle valve. When the needle valve is closed, the distance h between the shoulder and the lower end face of the injector body is the maximum lift of the needle valve, which determines the amount of fuel injection. Generally, h = 0.4mm - 0.5mm. There are 1 - 2 locating pins at the joint between the needle valve body and the injector to prevent the needle valve body from rotating, which may cause the misalignment of the oil inlet hole.

When the injector is working, the high - pressure diesel from the fuel injection pump enters the oil inlet passage on the injector body through the oil pipe joint, and then enters the annular oil chamber in the middle of the needle valve body. It acts on the pressure - bearing conical surface of the needle valve, forming an upward axial thrust on the needle valve. Once this thrust is greater than the pre - pressure of the injector's pressure - regulating spring, the needle valve immediately moves upward, opening the injection hole, and the high - pressure diesel is then sprayed into the combustion chamber. When the fuel injection pump stops supplying fuel, the pressure in the high - pressure oil passage drops rapidly. The needle valve returns to its original position under the action of the pressure - regulating spring, closing the injection hole and stopping fuel injection.

A small amount of diesel entering the annular oil chamber of the needle valve body flows to the end of the pressure - regulating spring through the clearance between the mating surfaces of the injector nozzle assembly, enters the return oil pipe, and flows back to the filter for lubricating the injector nozzle assembly.

The opening pressure (injection pressure) of the needle valve depends on the pre - tightening force of the pressure - regulating spring. Different engines have different injection pressure requirements, which can be adjusted by the pressure - regulating screw. Screwing it in increases the pressure, while screwing it out decreases the pressure.

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