Working principle of fuel injector
1. When the injector solenoid valve is not triggered, the small spring presses the ball valve under the pivot plate to the relief valve
On the oil hole, the oil drain hole is closed and a common rail high pressure is formed in the valve control chamber. Similarly, a common rail high pressure is also formed in the nozzle cavity. As a result, the needle valve is forced to enter the valve seat and isolate and seal the high-pressure channel from the combustion chamber, and the needle valve remains closed.  
2. When the solenoid valve is triggered, the pivot plate moves up, the ball valve opens and the oil drain hole is opened
At this time, the pressure in the control chamber decreases, and as a result, the pressure on the piston also decreases. Once the resultant force of the pressure on the piston and the nozzle spring falls below the pressure acting on the pressure cone of the needle valve of the fuel injection nozzle (the oil pressure here is still common rail high pressure), the needle valve will be opened and the fuel will be injected into the combustion chamber through the nozzle hole. This indirect control of the injector needle valve adopts a set of hydraulic pressure amplification system, because the force required to quickly open the needle valve cannot be directly generated by the solenoid valve. The so-called control function required to open the needle valve is to open the oil drain hole through the solenoid valve to reduce the pressure in the control chamber, so as to open the needle valve.  
3. Once the solenoid valve is powered off, it will not be triggered. The small spring force will push down the solenoid valve core and the ball
The valve closes the drain hole. After the oil drain hole is closed, the fuel enters the valve control chamber from the oil inlet hole to establish oil pressure. This pressure is the fuel rail pressure. This pressure acts on the end face of the plunger to generate downward pressure. In addition, the resultant force of the nozzle spring is greater than the pressure of the high-pressure fuel in the nozzle chamber on the conical surface of the needle valve, so that the nozzle needle valve is closed.  
4.Moreover, because of the high fuel pressure, leakage will occur at the needle valve and the control plunger, the leaked oil will flow into oil returning port.