Question 68 3AE01 - Third Assistant Engineer

The Correct Answer is B **Explanation of Option B (Affect fuel oil metering):** Injection pump plungers are high-precision components designed to move within a closely fitted barrel. The clearance between the plunger and the barrel is extremely small (often measured in micrometers), which allows the plunger to pressurize the fuel effectively and prevent significant leakage (blow-by) back into the suction chamber. Surface irregularities, such as erosion, pitting, or scoring, on the plunger: 1. **Increase Leakage:** These irregularities create pathways for the high-pressure fuel to escape past the plunger into the lower pressure area (the pump housing or suction gallery). This is known as internal leakage or slippage. 2. **Reduce Effective Delivery:** Because some fuel leaks past the plunger instead of being compressed and delivered to the injector nozzle, the total volume of fuel delivered to the cylinder for a given plunger stroke is reduced. 3. **Affect Metering:** Fuel metering is the process of accurately controlling the quantity of fuel delivered per injection event. Increased and variable leakage due to surface damage directly compromises this accuracy, leading to poor control over the engine's power output and speed. Therefore, surface irregularities on the plunger significantly **affect fuel oil metering**. **Explanation of Other Options:** * **A) Increase ignition delay:** Ignition delay is primarily a function of fuel quality (cetane number), compression temperature, and pressure within the combustion chamber. While poor fuel metering (the result of the damaged plunger) can indirectly affect the timing and pressure of injection, the physical condition of the plunger itself does not directly govern the chemical time lag between fuel injection and auto-ignition (ignition delay). * **C) Affect engine performance at low speed only:** Damage to the plunger affects metering at all operating speeds. While the *impact* of poor metering might be more noticeable (e.g., rough running) at low speeds due to the smaller required fuel quantities, the *percentage* of leakage remains a problem across the entire operating range and will limit maximum power output as well. High injection pressures achieved at high speeds can even exacerbate leakage. * **D) Disappear due to fuel oil abrasion:** Surface irregularities like pitting and erosion are often caused by abrasive particles, corrosion, or cavitation. They represent material loss and surface damage. High-precision components are hardened, and while fuel oil does have some mild lubricating and scouring properties, it cannot "heal" or eliminate significant pitting or scoring. Abrasion will more likely **worsen** the existing damage over time, leading to eventual pump failure or mandatory replacement.