Question 34 AEL01 - Assistant Engineer - Limited

The Correct Answer is A. **Explanation for A (To assist in a quick cutoff of fuel injection):** The delivery check valve (or delivery valve) is a specialized non-return valve located between the high-pressure pumping element (plunger/barrel) and the high-pressure fuel line leading to the injector. Its primary function is two-fold: 1. **Prevent Backflow:** It prevents the extremely high-pressure fuel oil in the pipe leading to the injector from flowing back into the pump barrel during the suction stroke or when the effective pumping stroke ends. 2. **Pressure Drop (Quick Cutoff):** Crucially, the valve incorporates a retraction piston (or displacement piston) which is slightly smaller in diameter than the valve body. As the pressure stroke ends and the valve snaps shut, this retraction piston momentarily lowers the pressure in the high-pressure fuel line by displacing a small, precise volume of fuel back into the pump chamber. This rapid pressure drop causes the injector nozzle needle valve to snap shut quickly and cleanly, ensuring a sharp end to the injection event and preventing secondary injections, dribbling, or carbon buildup on the nozzle tip. This action is essential for achieving a **quick cutoff of fuel injection.** **Why the other options are incorrect:** * **B) To allow oil backflow from the injector to the helix:** This is incorrect. The valve is specifically designed as a check valve (non-return) to *prevent* backflow into the pump barrel (where the helix is located). * **C) To meter the quantity of fuel delivered:** This is incorrect. Fuel metering (determining the quantity delivered per stroke) is achieved by the interaction between the angled edge (helix) of the plunger and the spill port (or regulating port) on the barrel, controlled by rotating the plunger. The delivery valve only manages the end of the injection event, not the quantity. * **D) To reduce fuel oil pressure between injection strokes:** While the retraction feature *does* reduce pressure (as described in A), its *purpose* is not merely to "reduce pressure." The pressure reduction is a means to achieve the required functional outcome: the quick, clean closing of the injector nozzle. The main pressure regulator in the system is the pump action itself, and the primary function of the valve is quick cutoff.