Question 34 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is B. **Explanation of Correctness (Option B):** Over speed trip devices on marine diesel engines, particularly those using unit injectors (like certain older Detroit Diesel or EMD engines often found in harbor tugs), are designed to immediately cut fuel supply to prevent catastrophic engine failure. When the engine speed exceeds a set limit, a mechanical governor mechanism activates a trip linkage. This linkage rotates a **trip shaft**. The trip shaft is equipped with cams that interface directly with the unit injectors. By rotating the trip shaft, these cams force the unit injector racks (or followers) to a specific position. In the design described, this action forces the plungers of the unit injectors to the **bottom of their respective strokes**. When the plunger is held at the bottom of its stroke, it cannot reciprocate to build pressure and inject fuel, effectively cutting off all combustion and shutting down the engine immediately. **Explanation of Incorrect Options:** * **A) A trip shaft cam positions all the unit injectors plungers to the top of their respective strokes, preventing plunger reciprocation, which results in engine shutdown.** * This is incorrect. While the mechanism uses a trip shaft cam, holding the plunger at the **top** of its stroke is not the standard mechanism for achieving fuel cutoff via mechanical linkage on this type of unit injector. The design typically forces the plunger to the bottom (no-fuel position). * **C) The fuel control linkage controlling the unit injector racks goes to the no fuel position, which results in engine shutdown.** * This is partially correct but insufficient. While the **result** is the racks going to the no-fuel position, the description focuses on the standard governor linkage. The over speed trip is a distinct, separate safety system that mechanically **forces** the racks (or plungers) past the position the normal governor linkage controls. Option B provides the specific mechanical action (plunger positioning) that the trip shaft uses to ensure absolute and immediate fuel cutoff. * **D) A solenoid valve in the common fuel supply line to the unit injectors is closed, which results in engine shutdown.** * This describes a common method for emergency or routine engine stop (often via a remote fuel rack shutoff or electric fuel solenoid on the supply line), but it is generally *not* the primary method used by a dedicated mechanical **over speed trip device** on large marine diesel engines equipped with mechanical unit injectors. The mechanical trip shaft acting directly on the injectors is faster, more robust, and ensures immediate cutoff independent of electrical power.