Question 13

The Correct Answer is A. **Explanation of why option A is correct:** Option A accurately describes the typical behavior of an engine's lube oil pressure system when the pump is engine-driven: 1. **Effect of Lube Oil Temperature:** When lube oil temperature increases, the oil's viscosity (thickness) decreases. Thinner oil flows more easily through clearances (bearings, journals, etc.) and leaks past the relief valve mechanisms. Since the resistance to flow decreases and leakage/flow increases for a given pump speed, the pressure required to maintain the flow against system resistance tends to drop. Therefore, **as temperature increases, pressure tends to decrease.** 2. **Effect of Engine RPM:** Since the lube oil pump is directly driven by the engine, the speed of the pump is directly proportional to the engine RPM. As the engine RPM increases, the pump spins faster, moving a greater volume of oil per unit of time (increased flow rate). Assuming the clearances and resistance in the system remain relatively constant (or increase minimally), the greater volume supplied by the faster pump forces the pressure to rise to overcome system resistance and return excess oil through the relief valve. Therefore, **as engine RPM increases, pressure tends to increase.** **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 increased temperature reduces viscosity, which lowers pressure (it does not increase pressure). Also, increased RPM increases pump speed and flow, which raises pressure (it does not decrease pressure). * **C) 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 increased temperature reduces viscosity, which lowers pressure (it does not increase pressure). The second part regarding RPM is correct, but the first part is fundamentally wrong. * **D) 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 (temperature increase leading to pressure decrease) is correct, the second part (RPM increase leading to pressure decrease) is wrong. Increased engine RPM increases pump speed and flow, which increases supply pressure.