Question 48 UFIV01 - Chief Engineer - UFIV

The Correct Answer is A **Explanation for Option A (Correct):** Option A is correct because it accurately describes the procedures for setting the timing and synchronization of mechanically operated unit injectors (like those commonly found on older large diesel engines, such as EMD, Detroit Diesel, or certain Caterpillar models). * **Injector Timing:** Timing dictates *when* injection begins relative to the piston position (crankshaft angle). In these mechanical unit injectors, the vertical position of the injector body relative to the camshaft lobe determines the starting point of the injection stroke. This vertical position is set by adjusting the height of the **cam follower** (or roller) or the rocker arm screw that actuates the injector plunger. Adjusting this height changes the effective lift, thus advancing or retarding the start of injection relative to the camshaft profile. * **Injector Synchronization (or Fuel Delivery/Metering):** Synchronization ensures that all cylinders receive the same amount of fuel at the same throttle setting, ensuring equal power output and smooth running. The amount of fuel delivered by a unit injector is controlled by the **control rack** (or linkage), which rotates the injector plunger to vary the effective stroke length. Synchronization involves ensuring that the racks on all injectors are set identically when the engine governor linkage is at a specific position (e.g., idle or full fuel stop). **Explanation of Incorrect Options:** * **B) Incorrect:** This option reverses the roles of the adjustments. Adjusting the control rack sets the fuel quantity (synchronization/metering), not the initial timing point relative to the camshaft. Adjusting the cam follower height sets the timing, not the fuel quantity synchronization. * **C) Incorrect:** This option incorrectly assigns both timing and synchronization to the control racks. The control rack controls fuel delivery/quantity (synchronization), but it has no direct influence on the fundamental timing point dictated by the camshaft lobe and follower setting. * **D) Incorrect:** This option incorrectly assigns both timing and synchronization to the cam follower heights. While the cam follower height sets the injection timing, it does not control the fuel quantity delivered (synchronization). Fuel quantity is controlled by the control rack.