Made of high-quality bearing steel, its mutual sliding cylindrical surface gap is only 0.001mm-0.0025mm. It can be obtained through high-precision machining or grinding. Different fuel injectors are not interchangeable. If the gap is too large, the injection pressure will drop and the spray quality will deteriorate. If the gap is too small, the needle valve will easily get stuck.
The annular conical surface (pressure conical surface) in the middle of the needle valve is located in the annular oil chamber of the needle valve body, and its role is to withstand the axial thrust generated by the oil pressure, so that the needle valve rises. The conical surface (seal cone) at the lower end of the needle valve cooperates with the body of the needle valve to act as a seal for the inner chamber of the injector.
The upper part of the needle valve has a shoulder. When the needle valve is closed, the distance between the shoulder and the lower end surface of the injector body is the maximum lift of the needle valve. The size of the needle valve determines the amount of fuel injection, generally h=0.4mm-0.5mm. . There are 1-2 positioning pins at the junction of the needle valve body and the injector body to prevent the needle valve body from rotating, so as to avoid misalignment of the oil inlet hole.
When the injector is in operation, the high-pressure diesel from the fuel injection pump enters the oil inlet on the injector body via the oil pipe joint and enters the annular oil chamber in the middle of the needle valve body to act on the pressure cone of the needle valve. In the above, an upward axial thrust is formed on the needle valve.
Once this thrust is greater than the pre-pressure of the fuel injector pressure regulating spring, the needle valve immediately moves upwards to open the injection hole, and the high-pressure diesel fuel is then injected into the combustion chamber. When the fuel injection pump stops supplying oil, the pressure in the high pressure oil passage drops rapidly, the needle valve returns to the position under the action of the pressure regulating spring, the injection hole is closed, and the injection of fuel is stopped.
A small amount of diesel oil entering the annular oil chamber of the needle valve body flows through the gap between the mating surfaces of the injection nozzle assembly to the pressure regulating spring end, enters the return oil pipe, flows back to the filter, and is used to lubricate the fuel injection nozzle coupling.
The opening pressure (injection pressure) of the needle valve depends on the preload of the pressure regulating spring. Different engines have different fuel injection pressure requirements, which can be adjusted by adjusting screws, the pressure increases when screwed in, and the pressure decreases when unscrewing.