Question 20 UFIV02 - Assistant Engineer - UFIV

The Correct Answer is A ### Explanation for why Option A is Correct: Unit Injectors (UIs) or Electronic Unit Injectors (EUIs) are mechanically driven by the engine's camshaft (or rockers) and use a plunger that moves up and down within a barrel to generate the extremely high pressure required for injection. 1. **Beginning of Injection (BOI):** The plunger moves **downward** (compression stroke) driven by the rocker arm. Before this movement, the fuel gallery/ports are open, allowing low-pressure fuel to fill the space above the plunger. Injection begins the moment the **upper edge (or upper helix)** of the downward-moving plunger covers the **upper fuel port**. Once both ports are covered, the fuel trapped above the plunger is rapidly compressed. 2. **Ending of Injection (EOI):** As the plunger continues its downward movement, the pressure increases until the spray nozzle opens. Injection continues until the **lower helix** on the plunger moves far enough to uncover the **lower fuel port (spill port)**. Once this port is uncovered, the highly compressed fuel rushes out (spills) into the low-pressure fuel gallery, causing the pressure above the plunger to drop instantly. This rapid depressurization causes the nozzle needle valve to close, thus ending the injection. Therefore, Option A accurately describes the mechanism: The plunger moves downward, injection starts by covering the upper port with the upper helix, and injection ends by uncovering the lower port with the lower helix. ### Explanation of Why Other Options are Incorrect: * **B) Injection begins when the lower fuel port is covered by the lower helix of the downward moving plunger, and injection ends when the upper fuel port is uncovered by the upper helix of the downward moving plunger.** * This reverses the start and end points of the injection process relative to the ports and helices. Injection begins when the top (upper) helix covers the fuel entry port, and ends when the bottom (lower) helix uncovers the spill port. * **C) Injection begins when the upper fuel port is covered by the upper helix of the upward moving plunger, and injection ends when the lower fuel port is uncovered by the lower helix of the upward moving plunger.** * This is incorrect because the injection phase (the high-pressure stroke) occurs when the plunger is moving **downward**, driven by the engine camshaft/rocker arm, not upward. * **D) Injection begins when the lower fuel port is covered by the lower helix of the upward moving plunger, and injection ends when the upper fuel port is uncovered by the upper helix of the upward moving plunger.** * This is incorrect for two reasons: the high-pressure injection stroke occurs during the **downward** movement of the plunger, and the sequence of port covering/uncovering is reversed relative to the standard unit injector design.