Question 41 UFIV01 - Chief Engineer - UFIV

The Correct Answer is D ### Explanation for Option D (Correct Answer) **Option D: The filter inlet pressure gradually rises, while the filter outlet pressure gradually drops.** The lube oil pump, being engine-driven and operating at a constant engine RPM, delivers a relatively constant volume of oil flow (Q) to the system, including the filter. 1. **Clogging Effect:** As the filter elements gradually clog, the resistance (R) to oil flow across the filter increases significantly. 2. **Upstream (Inlet) Pressure:** Since the pump is constantly trying to push a constant flow (Q) through an increasing resistance (R), the pressure immediately upstream of the filter (the inlet side) must increase according to the principle that pressure drop is proportional to flow rate times resistance ($\Delta P \propto Q \times R$). This rise in inlet pressure occurs until the pressure relief/bypass valve set point is reached (though the question specifies the bypass valve remains closed). 3. **Downstream (Outlet) Pressure:** The overall system pressure is maintained by the pressure control/regulating valve downstream of the main pump discharge (often located before the main bearings). As the filter resistance increases, the overall pressure drop ($\Delta P$) across the filter massively increases. This means that for a given inlet pressure, the pressure available immediately downstream of the filter (the outlet pressure) decreases. Crucially, the increasing pressure drop across the filter starves the downstream system, causing the pressure gauge located at the filter outlet to gradually drop. Therefore, the defining characteristic of a gradually clogging full-flow filter is the **widening differential pressure (DP)**: the inlet pressure rises, and the outlet pressure drops. *** ### Why Other Options Are Incorrect **A) The filter inlet AND outlet pressures BOTH gradually rise.** This is incorrect. While the inlet pressure rises due to increased resistance, the outlet pressure cannot rise simultaneously. The increase in resistance causes a greater pressure drop across the element, meaning the pressure downstream (outlet) must fall, assuming the flow rate remains constant. **B) The filter inlet pressure gradually drops, while the filter outlet pressure gradually rises.** This is completely opposite to the effect of clogging. A drop in inlet pressure would suggest a problem with the pump or a bypass upstream of the filter, not increased resistance from clogging. An increase in outlet pressure would imply *decreased* filter resistance, which contradicts the premise of a gradually clogging element. **C) The filter inlet AND outlet pressures BOTH gradually drop.** This is incorrect. A drop in both pressures typically indicates a fault with the lube oil pump (e.g., worn gears, excessive internal leakage, or cavitation) or an unrestricted bypass condition, leading to reduced overall system pressure. Clogging causes the inlet pressure to *rise*, not drop.