Question 45 MODE01 - Chief MODU Engineer

The Correct Answer is B **Explanation for B (rack position):** In a unit injector (like those used in older mechanical diesel engines, such as Cummins PT or Detroit Diesel applications), the amount of fuel delivered during each injection cycle is precisely controlled by the effective stroke of the plunger. This effective stroke is modified by rotating the plunger or a control sleeve/spool valve assembly. This rotation is mechanically governed by a control **rack**. The position of the control rack determines the timing of the fuel cutoff port opening (spill timing) relative to the plunger movement, thus regulating the volume of fuel metered and injected into the cylinder. **Why the other options are incorrect:** * **A) camshaft:** The camshaft provides the *motive force* (via a lobe and rocker arm) to actuate the unit injector plunger, causing the injection event to happen at the correct time. It dictates *when* the injection occurs, but not the *quantity* of fuel delivered. * **C) main spring:** The main spring (or return spring) inside the unit injector is responsible for returning the injector plunger to its starting position after the injection event is completed. It plays no direct role in metering the amount of fuel delivered. * **D) engine speed:** Engine speed (RPM) is a *condition* that requires a specific amount of fuel to be delivered (more fuel needed at high RPM under load). However, engine speed is the *input* to the governor (or ECU), which then physically adjusts the **rack position** to achieve the necessary fuel quantity. The rack position is the physical control mechanism.