The electronically controlled fuel injector is the most crucial and complex component in the common rail fuel system. Its function is to, based on the control signals sent by the ECU, control the opening and closing of the solenoid valve to inject the fuel in the high-pressure fuel rail into the combustion chamber of the diesel engine at the optimal injection timing, injection volume and injection rate.
The structure of the electronically controlled fuel injectors of BOSCH and ECD-U2 is basically similar. Both are composed of fuel injectors, control pistons, control volume holes and control solenoid valves that are similar to those of traditional fuel injectors. Figure 5 shows the structure diagram of the electronically controlled fuel injector of BOSCH. When the solenoid valve is not powered, the solenoid valve closes the measurement hole A at the top of the control piston. The fuel pressure of the high-pressure oil rail acts on the control piston through the measurement hole Z, closing the nozzle. When the solenoid valve is energized, the measurement hole A is opened, the pressure in the control chamber drops rapidly, the control piston rises, and the fuel injector starts to spray fuel. When the solenoid valve is closed, the pressure in the control chamber rises, and the control piston descends to close the fuel injector and complete the fuel injection process.
The shape that has controlled the fuel injection rate needs to be reasonably optimized and designed to achieve the predetermined fuel injection shape. The volume of the control chamber determines the sensitivity of the needle valve when it opens. If the volume of the control chamber is too large, the needle valve cannot quickly cut off the fuel supply at the end of the fuel injection, resulting in poor atomization of the fuel in the later stage. The volume of the control chamber is too small to provide sufficient effective stroke for the needle valve, increasing the flow resistance during the injection process. Therefore, the volume of the control chamber should also be reasonably selected based on the maximum fuel injection volume of the model.
The size of the control orifice A and Z has a decisive influence on the opening and closing speed of the fuel injector and the fuel injection process. The three key structures of the double-hole valve body are the oil inlet hole, the oil return hole and the control chamber. Their structural dimensions have a significant impact on the fuel injection performance of the fuel injector. The difference in flow rates between the return fuel volume hole and the intake fuel volume hole and the volume of the control chamber determine the opening speed of the fuel injector needle valve, while the closing speed of the fuel injector needle valve is determined by the flow rate of the intake fuel volume hole and the volume of the control chamber. The design of the fuel inlet hole should ensure that the needle valve of the fuel injector has a sufficient closing speed to reduce the part of the fuel injector that atomizes poorly in the later stage of injection.
In addition, the minimum fuel injection pressure of the fuel injector depends on the flow rate of the return fuel volume hole and the intake fuel volume hole as well as the end face area of the control piston. After determining the structural dimensions of the fuel inlet hole, the fuel return hole and the control chamber, the stable and shortest fuel injection process for the fuel injector needle valve to be fully open is determined, and at the same time, the stable minimum fuel injection volume of the fuel injector is determined. Reducing the volume of the control chamber can make the response speed of the needle valve faster and minimize the impact of fuel temperature on the fuel injection volume of the fuel injector.
However, the volume of the control room cannot be reduced without limit. It should be able to ensure the lift of the fuel injector needle valve to fully open the needle valve. The two control holes determine the dynamic pressure in the control chamber, which in turn determines the movement law of the needle valve. By carefully adjusting the flow coefficients of these two holes, an ideal fuel injection law can be produced.
Due to the extremely high injection pressure of the high-pressure common rail injection system, the cross-sectional area of the nozzle holes of its fuel injectors is very small. For instance, the nozzle hole diameter of the fuel injectors of BOSCH Company is 0.169mm×6. Under such a small nozzle hole diameter and such a high injection pressure, the fuel flow is in an extremely unstable state, the spray cone Angle of the fuel beam becomes larger, and the fuel atomization is better However, the penetration distance has decreased. Therefore, the vortex intensity of the original diesel engine's intake air and the shape of the combustion chamber structure should be changed to ensure the best combustion process.
For the fuel injector solenoid valve, since the common rail system requires it to have sufficient opening speed, and considering that pre-injection is an important injection method to improve the performance of diesel engines, the response time of the control solenoid valve should be shortened even more.