Question 19 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is B **Explanation for Option B (Correct):** The function of the injector nozzle tip orifices (holes) is to atomize the fuel and shape the spray pattern. If the spray pattern is observed to be distorted (i.e., not the specified symmetrical cone or pattern), it indicates that the flow of fuel through these small holes is obstructed, uneven, or damaged. Common causes include carbon buildup, dirt, or erosion/damage to the orifices. Since the test shows that the pressure performance (leakage, hold pressure, and popping pressure) is satisfactory, the mechanical operation (spring, spindle, needle valve) is working correctly up to the point of injection. Therefore, the problem lies specifically with the final fuel shaping component, which is the nozzle tip orifices. Maintenance requires either cleaning the existing tip to remove obstructions or replacing the entire tip if the orifices are physically damaged or eroded. **Explanation of Incorrect Options:** **A) The injector spindle and nozzle holder bore must be reconditioned or replaced.** The spindle (or push rod) and the holder bore primarily guide the motion of the needle valve and transfer force from the spring. Issues here usually affect the mechanical movement, potentially leading to slow response, sticking, or incorrect popping pressure/leakage. Since the popping pressure is within specification and there is no pre-pop leakage, the general mechanical function and sealing surfaces are likely acceptable. **C) The injector spring compression must be readjusted or a broken spring replaced.** The spring compression directly determines the popping pressure. If the popping pressure is measured and found to be "within specification," the spring tension is correct. A broken spring or incorrect tension would result in an incorrect popping pressure reading, which is contrary to the test observations. **D) The injector needle valve and seat must be reconditioned or replaced.** The needle valve and its seat are responsible for sealing the pressurized fuel chamber until the popping pressure is reached. Problems here would typically manifest as leakage (dribbling) prior to popping, or an inability to hold pressure just below the popping point. Since the test shows "no leakage prior to reaching the popping pressure" and the "pressure holds at just below the popping pressure," the needle valve and seat are seating effectively and sealing correctly.