Question 40 UFIV01 - Chief Engineer - UFIV

The Correct Answer is A **Explanation of why option A is correct:** Option A accurately describes the relationship between lube oil temperature, engine speed (RPM), and the resulting supply header pressure in an engine utilizing an engine-driven pump. 1. **Effect of Lube Oil Temperature on Pressure (Inverse Relationship):** Lube oil viscosity decreases as its temperature increases. Since the main function of the oil pump is to overcome resistance (flow restriction) to maintain pressure, a less viscous (thinner) oil flows more easily through the system clearances and bearings. In a positive displacement pump system, this decrease in flow resistance means that less pressure is generated to maintain the flow rate, and more oil bypasses the system via the relief valve (if present), or simply leaks through clearances more easily, resulting in a **decrease** in supply header pressure. 2. **Effect of Engine RPM on Pressure (Direct Relationship):** When the lube oil pump is directly driven by the engine (engine-driven), the speed of the pump is directly proportional to the engine RPM. A higher pump speed moves a larger volume of oil per unit of time (increased capacity). This increased capacity generates higher flow rates and thus greater resistance within the lubricating system, leading to a **tendency to increase** the supply header pressure, provided the pressure relief valve has not already opened completely (or until the system hits the relief valve set point). Therefore, the combined effect is: Higher Temperature $\rightarrow$ Lower Pressure; Higher RPM $\rightarrow$ Higher Pressure. **Explanation of why the other options are incorrect:** * **B) As the lube oil temperature increases, the supply header pressure tends to increase, and as the engine rpm increases, the supply header pressure tends to decrease.** This is incorrect because: (1) Higher temperature decreases viscosity, which decreases pressure (not increases it). (2) Higher RPM increases pump speed and output, which increases pressure (not decreases it). * **C) 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 decrease.** This is incorrect because while the first part (Higher Temp $\rightarrow$ Lower Pressure) is true, the second part is false; higher RPM increases the pump speed and output, leading to an increase in pressure, not a decrease. * **D) As the lube oil temperature increases, the supply header pressure tends to increase, and as the engine rpm increases, the supply header pressure tends to increase.** This is incorrect because the first part is false; higher temperature decreases viscosity, which causes the pressure to decrease, not increase.