What happens when the pressure of the fuel overcomes the pressure of the spring in a hydraulic injector?

When the pressure of the fuel exceeds the pressure of the spring in a hydraulic injector, it causes the nozzle needle to move upward. This movement occurs because the hydraulic system is designed to allow the pressurized fuel to overcome the spring's resistance. As the force from the fuel pressure surpasses the force exerted by the spring, the nozzle needle is pushed upward, allowing fuel to be injected into the combustion chamber. This is a critical action in the operation of injectors as it directly impacts the fuel delivery process, ensuring that the engine receives the necessary fuel for combustion.

The interaction between fuel pressure and spring pressure is fundamental to the controlling mechanism of the injector's operation, reflecting the principle of hydraulic actuation where pressure differences result in movement.