Question 21

The Correct Answer is C. ### Explanation for Option C (3) Option C refers to position **3** in the illustration, which depicts the fuel pump plunger at the point where fuel injection stops. In this type of helix-controlled fuel injection pump (often used in diesel engines for marine applications like winch drives): 1. **Injection Starts:** Injection begins when the top edge of the plunger covers the spill port (or inlet port) as the plunger travels upwards (position 1 to 2). 2. **Injection Ends:** Injection ends when the helical edge (or control helix/spill groove) on the plunger aligns with and uncovers the spill port (position 3). Once the helix uncovers the port, the high-pressure fuel trapped above the plunger is instantly released and spills back into the fuel gallery, causing the pressure to drop rapidly and stopping the flow of fuel to the injector nozzle. Therefore, position 3 represents the **end of injection**. 3. **End of Stroke:** Position 4 represents the plunger reaching the top of its stroke (TDC), well after injection has ceased. ### Explanation for Incorrect Options **A) 1:** Position 1 shows the plunger at the bottom of its stroke (BDC). The cylinder above the plunger is filling with fuel through the inlet ports. Injection has not yet begun. **B) 2:** Position 2 shows the plunger moving upwards, having covered the inlet/spill port, meaning compression has begun, and fuel is being forced through the delivery valve to the injector. This position corresponds to the period **during injection**, typically the beginning or middle of the injection event, but not the end. **D) 4:** Position 4 shows the plunger at the top of its maximum travel (TDC). Injection ended earlier, at position 3, when the spill helix opened the port. Position 4 represents the end of the pumping stroke, long after the effective delivery of fuel has been terminated.