Question 12 OSE01 - Chief Engineer - OSV

The Correct Answer is B ### Explanation for why Option B is Correct Option B states that "As the lube oil temperature increases, the supply header pressure tends to decrease, and as the engine rpm increases, the supply header pressure tends to increase." This statement accurately reflects the principles of fluid dynamics and positive displacement pumping used in engine lubrication systems. **Part 1: Effect of Lube Oil Temperature on Pressure (Inverse Relationship)** * **Temperature and Viscosity:** Engine lube oil is a non-Newtonian fluid whose viscosity (resistance to flow) decreases significantly as its temperature rises. * **Viscous Resistance and Pressure Drop:** The supply header pressure is the result of the pump overcoming the resistance to flow (viscous drag) within the engine's galleries, bearings, and passages. When the viscosity decreases (due to increased temperature), the oil flows more easily, resulting in less resistance within the system. * **Relief Valve Action:** Most positive displacement pumps use a relief valve to maintain a set maximum pressure. Because the thinner, hotter oil leaks past the clearances and flows through the system with less resistance, the flow demand on the pump is met at a lower discharge pressure, causing the pressure to naturally drop below the relief valve set point or requiring less oil to be bypassed by the relief valve. Thus, an increase in temperature leads to a decrease in supply header pressure. **Part 2: Effect of Engine RPM on Pressure (Direct Relationship)** * **Engine-Driven Pump Speed:** Since the lube oil pump is directly driven by the engine (e.g., geared to the crankshaft), the speed of the pump is directly proportional to the engine RPM. * **Positive Displacement Pumps:** Lube oil pumps are typically positive displacement pumps (gear or screw pumps). For these pumps, the flow rate (volume of oil delivered per unit of time) is directly proportional to the rotational speed. * **Pressure Response:** As the engine RPM increases, the pump speed increases, delivering a significantly higher volume of oil into the lubrication system. This increased flow must overcome the resistance of the system. While a relief valve usually limits the *maximum* pressure, as RPM increases from idle toward operating speed, the flow increases rapidly, overcoming the system's resistance and causing the header pressure to climb until the maximum set point is reached or maintained. Therefore, an increase in engine RPM leads to an increase in supply header pressure (up to the relief valve limit). ### Explanation for why other options are incorrect **A) Incorrect:** This option incorrectly states the relationship for both variables. It is wrong because increasing temperature decreases pressure (not increases it), and increasing RPM increases pressure (not decreases it). **C) Incorrect:** This option correctly identifies that increasing temperature decreases pressure, but it incorrectly states that increasing engine RPM decreases the pressure. Since the pump is engine-driven, higher RPM generates higher flow and, thus, higher pressure (up to the relief valve setpoint). **D) Incorrect:** This option is incorrect because increasing lube oil temperature decreases its viscosity, leading to a decrease in header pressure (not an increase). It correctly identifies the RPM relationship, but the temperature relationship is wrong.