Question 62

The Correct Answer is B **Why option B ("spring force") is correct:** Differential needle valves (also known as nozzle valves) in conventional fuel injectors are designed to open when the fuel oil pressure acting beneath the valve overcomes a pre-set opposing force. This opposing force is provided by a strong, calibrated **spring** situated above the valve spindle. When the pressure from the fuel pump drops (i.e., when the injection event ends), the fuel pressure is no longer sufficient to compress the spring, and the spring immediately forces the needle valve downwards, closing the nozzle against the seat and cutting off the fuel supply sharply. **Why the other options are incorrect:** * **A) cam action:** Cam action is used to drive the plunger of a mechanical fuel pump (like a jerk pump) to generate the high pressure, but the cam does not directly open or close the needle valve within the injector nozzle itself. * **C) fuel oil pressure:** Fuel oil pressure is the force used to **open** the differential needle valve, not close it. The pressure must overcome the spring force to initiate injection. * **D) firing pressure:** Firing (combustion) pressure occurs inside the engine cylinder. While this pressure acts against the tip of the injector, the injection event is carefully timed to occur just before ignition. The valve closure mechanism must be immediate and reliable, and is governed by the internal spring force, not the external, fluctuating cylinder pressure.